7 Limayem
Content
Progress in Energy and Combustion Science 38 (2012) 449e 449e467
Contents lists available at SciVerse at SciVerse ScienceDirect
Progress in Energy and Combustion Science journal homepage: www.elsevier.com m// l o c a t e / p e c s
Review
Lignocellulosic biomass for bioethanol production: Current perspectives, potential issues and future prospects Alya Limayem a b, Steven C. Ricke a b ,
a b
,
,
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Department of Food Science, University of Arkansas, Fayetteville, AR 72704, USA Center for Food Safety, University of Arkansas, Fayetteville, AR 72704, USA
a r t i c l e
i n f o
Article history:
Received 4 April 2011 Accepted 25 October 2011 Available online 11 April 2012 Keywords:
Lignocellulosic feedstocks Bioethanol Fermentation Bioconversion Risk assessment
a b s t r a c t
During the most recent decades increased interest in fuel from biomass in the United States and worldwide has emerged each time petroleum derived gasoline registered well publicized spikes in price. The willingness of the U.S. government to face the issues of more heavily high-priced foreign oil and climate change has led to more investment on plant-derived sustainable biofuel sources. Biomass derived from corn has become one of the primary feedstocks for bioethanol production for the past several years in the U.S. sources. However,Consequently, the argumentindustrial of whether to use food as biofuel has ledmore to a search foron alternative non-food research efforts have become focused low-cost large-scale processes for lignocellulosic feedstocks originating mainly from agricultural and forest residues along with herbaceous materials and municipal wastes. Although cellulosic-derived biofuel is a promising technology, there are some obstacles that interfere with bioconversion processes reaching optimal optim al perfor performance mance associated with minimal capital investment investment.. This review summar summarizes izes current approaches on lignocellulosic-derived biofuel bioconversion and provides an overview on the major steps involved in cellulosic-based bioethanol processes and potential issues challenging these operations. Possible solutions and recoveries that could improve bioprocessing are also addressed. This includes the development develo pment of geneti genetically cally engineered strains and emerging pretrea pretreatment tment technolog technologies ies that might be more ef cient and economically feasible. Future prospects toward achieving better biofuel operational performance via systems approaches such as risk and life cycle assessment modeling are also discussed. 2012 Elsevier Ltd. All rights reserved.
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Int Introd roduct uction ion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 450 Histor Historica icall and cur curren rentt tren trends ds of biofue biofuell in the U.S. U.S. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .451 Lignoc Lignocell ellulo ulosic sic so sourc urces es and co compo mposit sition ion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .451 3. 3.1 1. Lig Lignoc nocell ellulo ulosic sic so sourc urces es . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 45 451 1 3. 3.1 1.1. For Fores estt woody woody fee feedst dstock ockss . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 45 451 1 3. 3.1. 1.2. 2. Agric Agricultu ultural ral resid residues, ues, herbace herbaceous ous and munic municipal ipal solid solid wastes wastes (MSW) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 451 3. 3.1 1.3. Marine Marine al algae gae . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 452 3.2 3.2.. Lig Lignoc nocell ellulo ulosic sic bi bioma omass ss com compo posit sition ion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 452 3.2 3.2..1. Hemic Hemicell ellulo ulose se . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 452 3.2 3.2.2. .2. Cellul Cellulose ose . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 452 3.2 3.2.3. .3. Lig Lignin nin . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 453 Pathw Pathway ayss of bioetha bioethanol nol produ producti ction on fro from m cellulo cellulosic sic fe feeds edstoc tocks ks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 453 4. 4.1 1. Pretr Pretreat eatmen mentt ove overvi rview ew . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 453 4.2 4.2.. Hydrol Hydrolys ysis is . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 455 4.3 4.3.. Fe Ferme rmenta ntatio tion n . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 456 4.4 4.4.. Se Separ parati ation/ on/dis distil tillat lation ion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 45 457 7
Corresponding author. Department of Food Science, University of Arkansas, 2450 N. Young Ave., Fayetteville, AR 72704, USA. Tel.: þ1 479 57 5755 6864; fax: þ1 479 575 575 6936. E-mail address: [email protected] [email protected] (S.C. (S.C. Ricke).
0360-1285/$ e see front matter 2012 Elsevier Ltd. All rights reserved. doi:10.1016/j.pecs.2012.03.002 doi:10.1016/j.pecs.2012.03.002
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A. Limayem, S.C. Ricke / Progress in Energy and Combustion Science 38 (201 (2012) 2) 449 449e e467
Current Current issu issues es and challenges challenges of lign lignocel ocellulos lulosic ic bioethanol bioethanol produ production ction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .457 5.1. 5.1. Overcomin Overcoming g recalc recalcitran itrance ce of lignocellu lignocellulosic losic mat material erialss . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 457 5.2. Pot Potentia entiall water availabi availability lity cha challeng llenges es for the biofue biofuell system . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 458 Current Current pros prospect pectss for systems systems approaches approaches to biomass biomass conversion conversion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 459 6.1. 6.1. Overall Overall analysis analysis of perfo performanc rmance: e: life cycle cycle assess assessment ment (LCA (LCA)) comparisons comparisons . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 459 6.2 6.2.. Op Optim timiza izatio tion n of the biofue biofuell process process ma main in steps steps . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 459 6.3. Cell Cellulol ulolytic ytic/ferm /fermenta entative tive micro microbial bial ecol ecology ogy e e iden identific tificatio ation n of indigeno indigenous us candi candidate datess . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 460 6.4. Fermentati Fermentation on opti optimizat mization ion e potential ntial genetical genetically ly modified modified organi organisms sms (GMO) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 461 e pote 6.5 6.5.. Mic Microb robial ial ri risk sk assess assessmen mentt (MRA (MRA)) mode modelin ling g . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 462 6.5 6.5..1. Concep Concepts ts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 462 6.5.2.. 6.5.2 Applicati Application on of risk assessment assessment in largelarge-scale scale fe ferment rmentation ation sys systems tems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 462 Con Conclu clusio sions ns e future ure pros prospec pects ts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 464 e fut Ac Ackno knowl wledg edgmen ments ts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 464 Refer Referenc ences es . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 464
The agreement implemented by Policy Energy Act (PEA) [1] followed by the Energy Independence and Security Act (EISA) reach 36 billio billion n gallons gallons (1 (136.2 36.277 L) of bioethanol bioethanol by [2] aims to reach the year year 2022. 2022. Rising Rising conce concern rn over over dep deplet leting ing fos fossil sil fue fuell and greenhouse gas limits has resulted in a high level of interest in non-conve non-c onvention ntional al fuel originating originating from bio-r bio-renew enewable able sources sources including inclu ding sugars, sugars, starches starches and lignocellu lignocellulosic losic materials materials [3e8] 8].. During the last decade, the production of ethanol from biomass materials received more attention in the United States (U.S.) and worldwide. In the U.S., bioethanol is primarily produced from corn starch feedstocks while in Brazil biofuel is mainly produced from sugarcane sugar cane juice and molas molasses. ses. To Togeth gether er,, these countries countries acco account unt for 89% of the current global bioethanol production [9] production [9].. Several Seve ral countries countries have initiated initiated new alternatives alternatives for gasol gasoline ine from renewable feedstocks [10]. [10]. In the North American hemisphere, bioethanol has been extracted from starch sources such as corn while in the South American hemisphere, biofuel has been largely provided from sugars including sugarcane and sugar beets [11]. While European countries are deploying extensive efforts to [11]. increase their 5% worldwide bioethanol production [12] production [12],, biodiesel produced in Europe primarily in France and Germany remains by far more substantial and accounts for approximately 56% of the global prod productio uction n mainly because because of the rising rising impor importance tance of diesel diesel enginess and feeds engine feedstock tockoppo opportuni rtunity ty cost costss [13]. [13]. Al Alth thou ough gh,, most most of th thee remaining countries countries in the world collectively account for only 5% of the global bioethanol production, China, Thailand as well as India are continuing to invest substantially in i n agricultural biotechnology and emerge as potential biofuel producers [14,15]. [14,15]. In the U.S., biofuel-derived from corn has emerged as one of the primary raw materials for bioethanol production [16] production [16].. According to the renew[9] statistics, the production of bioethanol able fuels association association [9] was historically unparalleled in the U.S. by year 2009 with nameplate capacity capacity reac reaching hing 10.9 10.9 billio billion n gallons gallons (41.26 (41.26 billionlitre billion litres) s) representing 55% of the worldwide production. In the year 2010 corn-base corn -based d ethanol ethanol operating operating productio productions ns gener generated ated a total total of 12.82 12.82 billion billion gallons gallons (48.52 (48.52 billion billion litres) litres) with the largest largest nameplate capacity in Iowa (28%) followed by Nebraska (13%) [17] (13%) [17].. Although corn-based and sugar based-ethanol are promising substitutes to gasoline production mainly in the transportation sector, they are not suf cient to replace a considerable portion of the one tri trillio llion n gallon gallonss of fossil fossil fuel fuel prese presently ntly cons consume umed d
a substantial renewable substrate for bioethanol production that do not compete with food production and animal feed. These cellulosic materials also contribute to environmental sustainability [22]. Additionally ally,, lignoc lignocellulo ellulosic sic bioma biomass ss can be supp supplied lied on [22]. Addition a large-scale basis from different low-cost raw materials such as municipal and industrial wastes, wood and agricultural residues [23] [23].. Currently the most promising and abundant cellulosic feedstocks derived from plant residues in the U.S., South America, Asia and Eur Europeare opeare from from corn corn stover stover,, su sugar garca cane ne bag bagass asse, e, rice rice and wheat wheat 27].. straws, respectively [24 respectively [24e27] Howeve How everr, lign lignoc ocellu ellulos losicic-bas based ed feeds feedstoc tockk is a recal recalcit citran rantt material that requires an intensive labor and high capital cost for processing [28]. [28]. Hen Hence, ce, these these pro proced cedur ures es curre currentl ntlyy are not econom eco nomica ically lly feasib feasible. le. Whe When n consid consideri ering ng enz enzyma ymatic tic or aci acidic dic decomposition of lignocellulosic structure, it must be taken into account that D-xylose is the second important sugar forming the hemicellulosic portion of the plant cell wall and constituting onethird of the sugars in the lignocellulosic feedstock [29] feedstock [29].. However However,, the pr prima imary ry ind indust ustria riall ye yeast ast use used d in bioeth bioethano anoll pr produ oducti ction, on, Saccharomyces Saccharomy ces cerevisiae conve converts rts only only hex hexos osee su sugar garss suc such h as glucose and is not able to co-ferment glucose and xylose [30] xylose [30].. There are four stages in the production of lignocellulosic-based ethanol: pretreatment, hydrolysis, fermentation and distillation. During the past decades, there have been substantial advances in genetic and enzymatic technologies that have helped to improve these steps of ethanol production and expand the capability of S. cerevisiae for fermenting different sugars simultaneously [31] [31].. Although there is a wide range of fungal and recombinant bacteria that are able to ferment xylose sugar, they are not all capable of adapting to fermentation-process conditions and some of them produce only low ethanol yields. Their tolerance to ethanol and [32,33].. Moreover, productivity still require further renements nements [32,33] cellulosic materials contain microbial contaminants that compete with wit h the fermen fermentin tingg ye yeast ast fornutrient fornutrientss and these these con contam tamina inants nts can produce toxic end-products. Both of these adverse conditions can create a considerable loss in ethanol yields [34,35] yields [34,35].. Additionally, pretr pretreat eatme ment nt pr proce ocesse ssess may res result ult in the format formation ion of toxic toxic components including primarily, acetic acid along with furfural, hy hydro droxym xymeth ethyl yl furfur furfural al and phenol phenolic ic com compon ponent entss [36,37]. [36,37]. However, in addition to the formation of fermentation inhibitors during biofuel production, there is occurrence of lignin side effects on enzyma enzymatic tic hy hydr droly olysis sis and cellul cellulase ase inhibi inhibito tors rs inc includ luding ing [38,39].. Lignin and derivative primarily phenolic-derived lignin [38,39]
worldwide each as year year [18] [18]. . Furthermore, the ethical concerns about the use of food fuel raw materials have encouraged research efforts effor ts to be more focused focused on the potential potential of inedible feedstock feedstock [19e21] 21].. Lignocellulosic biomass materials constitute alternatives [19 alternatives
effects extensively reviewed section. Thi Thissare revie rev iew w exami examines nes what whatinisa later cur curren rentl tlyy known known regard regarding ing recent rece nt technolo technologies gies and approache approachess that are used in derivedderivedlignoc lign ocellu ellulos losic ic biofue biofuell pro produ ducti ction. on. This This revie review w also also pr provi ovides des
1. Introduction
A. Limayem, S.C. Ricke / Progress in Energy and Combustion Science 38 (2012) 449 449e e467
a summary of the current bottlenecks and barriers that interfere with the lignocellulosic based-ethanol pathway and places the emphas emp hasis is on po poten tentia tiall iss issues ues cha challe llengi nging ng biote biotechn chnolo ologic gical al conversion and bioethanol performance. Speci c focus is directed to towar ward d descri describing bing cu curre rrent nt soluti solutions ons and po possi ssible ble syste systemat matic ic remedies reme dies that could could be adopted adopted to circumv circumvent ent lignocellu lignocellulosic losic-derive der ived d ethano ethanoll pr probl oblems ems and strat strategi egies es for the bioeth bioethano anoll industry indu stry to beco become me more economically economically feasible and therefore therefore commercially viable. Future prospects for the systematic optimization of lignocellulosic bioconversion are also addressed. 2. Hist Historical orical and curre current nt trends of biofue biofuell in the U.S.
Little attention attention was focused focused on bioet bioethanol hanol pro product duction ion in the U.S. U.S. before 1860 when Nicholas Otto initiated the use of ethanol as a fuel for engine combustion. As early as 1908, Henry Ford was already aware aware of the promising substitute to gasoline, ethanol. This led to the developme development nt of the Ford Model Model T capable capable of operating operating off of gasoline, ethanol or combinations of both [40] both [40].. At that time, the potentia pot entiall for fuel ethanol ethanol received received only moderate moderatecons consider ideration ation due to the dominance of low priced petroleum derived gasoline. Interest in ethanol from biomass such as corn starch emerged in the 1970s when the price of fossil fuel rose and methyl tertiary butyl ether (MTBE) used in gasoline was identi ed as an environmentall pollutant menta pollutant agent [41] [41].. Moreo Moreove verr, the wil willing lingnes nesss of the U.S. U.S. to stay independent from high-priced foreign oil, led the federal government to implement new research programs directed towar toward d the development of more sustainable alternative fuels originating fro from m renew renewabl ablee source sources. s. Be Betwe tween en 1980 and 1990, 990, there there wa wass a consider considerable able effort from the governmen governmentt to boost boost industria industriall efforts effor ts toward toward manufactu manufacturing ring fuel from biomass materials materials by adjusting adju sting tax-exem tax-exemptio ptions ns and encou encouragin ragingg bioethanol bioethanol research research and deve developm lopment ent programs programs.. Biofuel Biofuel producti production on grewexpo grew exponentia nentially lly from approximately approximately 200 million gallons (757 million litres) in 1982 1982 PEA [1] to 2.9 billion billion gallons gallons (10.9 (10.9 bbillion illionlitre litres) s) in 2003 [42] 2003 [42].. The PEA [1] implem imp lement ented ed in 2005 fol follow lowed ed by the EIS EISA A [2] in 2007 was accompanied by a partnership between the U.S. and Brazil, the world s largest biofuel producer at the time. In 2009, bioethanol-based production achieved an unprecedented increase (approximately (approximately 1111 billion gallons, 41 billion litres). In the year 2010, the U.S. became the world s leadi leading ng biofu biofuel el produce pro ducerr and exporte exporterr with 13.5 13.5 billion billion gallons gallons (5 (511 bbillion illionlitre litres) s) nameplate capacity. Almost 200 operational corn-based ethanol plants are currently operating in 29 states [42] most [42] most of them are located in the corn belt in the U.S. Midwest [12] [12].. It was also reported in 2010 that despite the global economic-burden, bioethanol production continues to expand rapidly and to contribute signicantly to the economic development of rural communities in the U.S. [42]. [42]. Although the price of most food products has inc increa reased sed,, co corn rn prices prices have have not subst substant antiall iallyy been been altere altered. d. However,, the debate of whether to use plants as a fuel feedstock or However as human food remains a controversial issue. This debate has led researchers to work on more acceptable sources containing lignocellulosic biomass that are derived mainly from agricultural residues,, industrial dues industrial wastes, forest bioma biomass ss and other other herbaceou herbaceouss materials [42] materials [42].. ’
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There are several groups of raw materials that are differentiated by their origin, composition and structure. In the U.S. most cultivated land constitutes around 35% of the forestland, approximately 27% graz grazed ed la land nd as we well ll as herb herbac aceo eous us and and 19% crop crop la land ndss per per [44,45]. Forestapproximately 2.25 billion acres (9.0 million km 2) [44,45]. land materials include mainly woody biomass namely, hardwoods and softwoods followed by sawdust, pruning and bark thinning residues while pasture and grassland encompass primarily agricultural residues that cover food or non-food crops and grasses such as switch grass and alfalfa [46]. [46]. Municipal and industrial wastes are also potential recyclable cellulosic materials that can originate either from residential or non-residential sources such as food wastes and paper mill sludge [46,47] [46,47].. Annual total tonnage available is summarized in Table in Table 11.. 3.1 3.1.1 .1.. Forest woody feedstocks
Forest woody feedstocks account for approximat approximately ely 370 million tons per year (30%) of lignocellulosic li gnocellulosic biomass in the U.S. [43] U.S. [43].. There are two types of woody materials that are classi ed into broad categories of either softwoods or hardwoods. Softwoods originate and unlike hardwoods, from conifers and gymnosperm trees [48] trees [48] and softwoods possess lower densities and grow faster. Gymnosperm trees, include mostly evergreen species such as pine, cedar, spruce, cypress, r, hem hemloc lockk and redwoo redwood d [49]. [49]. Hard Hardwoo woods ds are angiosperm angiosperm trees and are mostly deciduous [50] deciduous [50].. They are mainly found in the Northern Nor thern hemisphere hemisphere and in includ cludee tree treess such as popl poplar, ar, willow, willow, oak, cottonwood and aspen. In the U.S., hardwood species account for over 40% of the trees [51] trees [51].. The genus Populus (cottonwood) which includes 35 species is the most abundant fast-growing species suitable for bioethanol production. Populus deltoids species cover most of North America from the eastern to midwestern U.S., while Populus trichocarpa covers primarily the western U.S.[52] U.S. [52].. Unlike agricultural biomass, woody raw materials offer exible harvesting times and avoid long latency periods of storage [53] [53].. Additionally Additionally,, this study reported that woody feedstock possessed more lignin tha than n agr agricu icultu ltural ral resid residuesand uesand less less ash con conte tent nt (closeto (closeto zero).These zero).These unique characteristics of woody biomass including primarily high density and minimal ash content make woody raw material very attractiv attr activee to cost cost-effe -effective ctive transpor transportatio tation n in conj conjuncti unction on to its lower content in pentoses over agricultural biomass and more favorable for greater bioethanol conversion if recalcitrance is surmounted [53]. [53]. Forestry wastes such as sawdust from sawmills, slashes, wood chips and branches from dead trees have also been used as bioethanol feedstocks [43] feedstocks [43]..
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3.1 3.1.2. .2. Agricultural residues, herbaceous and municipal solid wastes (MSW)
Crops residues consist of an extensive variety of types. They are mostly comprised of agricultural wastes such as corn stover, corn stalks, rice and wheat straws as well as sugarcane bagasse [54]. [54]. Theree are appr Ther approxim oximately ately 350 e450 million million tons tons per year (1 (127 27 million millio n metric metric tons to 31 3177.5 million million metric metric tons) tons)harve harvested sted annually annually in the U.S. [42,43,54] U.S. [42,43,54] with with residues originating primarily from rice Table 1
Annual total tonnages of biomass for biofuel in the U.S. (U.S. Department of Energy Biomass Program, 2009) [54] 2009) [54]..
3. Ligno Lignocellu cellulosic losic source sourcess and composit composition ion
Biomass
3.1 3.1.. Lignocellulosic sources
Agricultural residues Forest resources Energy crops
Lignocellulosic Lignocellu losic material material constitut constitutes es the world world s larges largestt biobioethanol renewable resource. In the U.S. alone the production of biomass from lignocellulosic materials is estimated to be nearly 1.4 billion dry tons per year, 30% originating from forest biomass [43] biomass [43].. ’
Grains and corn Municipal and industrial wastes Others (i.e., oilseeds) Total
Million dry tons/year 428 370 377 87 58 48 1368
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and wheat straws as well as corn stalks being considered the bioethanol feedstocks with the most potential. Crop residues contain more hemicellulosic material than woody biomass (approximately 25e35%) [55] [55].. Aside Aside fro from m bei being ng an enviro environme nmenta ntally lly friend friendly ly process pro cess,, agricultur agricultural al residues residues help to avoid avoid reliance reliance on forestforestwoody biomass and thus reduce deforestation (non-sustainablecutting plants). Unlike trees, crop residues are characterized by a short-har short-harvest vest rotation rotation that rend renders ers them more more consisten consistently tly available to bioethanol production [25,26] production [25,26].. Switch grass is the primary herbaceous prairie grass and energy
More recently, marine algae biomass is regaining interest as a third generation biofuel feedstock due to the rapid biore neries ex expan pansio sion n leadin leadingg to a shorta shortage ge on cu curre rrent nt ene energycrop rgycropss design designate ated d for bioethanol and biodiesel industries. Aside from being potential bioethanol biomass, algae would also be a feedstock for other biofue biofuels ls inc includ luding ing mainly mainly,, biodie biodiesel sel and fuel fuel for avi aviati ation on in additi addition on to other possible applications involving bio-crude oils, bio-plastics and recover recovered ed livestoc livestockk co-p co-prod roducts ucts [60] [60].. Furt Furtherm hermore, ore, algae feedstock with its thin cellulose layer has a high carbohydrate composition making it capable of yielding 60 times more alcohol
crop that grows in the plains of the North American hemisphere, namely, Canada and the U.S. These perennial grasses are of interest due to their low-cost investment as well as abundance in the U.S., th their eir ab abili ility ty to re resis sistt di dise seas ases,and es,and theirhigh theirhigh yie yield ld of sugar sugar subs substr trat ates es peracre.Moreover peracre.Moreo ver,, switch switchgras grasss is lowmainte lowmaintenan nance cereq requiri uiring ng litt little le or no fertilization. fertilization. Miscanthus giganteus is another fast-growing grass that is a poten potential tially ly optima optimall candidat candidatee for bioe bioethan thanol ol produ producti ction. on. It is nativetoAsiaandisgrowninEuropeforcombustibleenergyuse [56]. [56]. In addition to cellulosic feedstocks, municipal and industrial solid wastess are waste are als alsoo a potential potential raw material material for biofuel prod production uction.. Their utilizat util ization ion limits limits env enviro ironme nmental ntal proble problems ms associ associate ated d with the disposal of garbage household, processing papers, food-processing by-products, black liquors and pulps [57] pulps [57].. Although over one billion tons of biomass per year would be potentially available to meet the 30% replacement of petroleumderived deriv ed gasoline in 2030 [43] 2030 [43],, the high cost of biomass could be a seriou seriouss hin hindr dranc ancee if po poten tentia tiall land landss and fee feedst dstock ockss are are not [57].. While woody biomass and managed and utilized ef ciently ciently [57] agricultural residues potential was overestimated in 2005, highyielding energy crops including primarily Miscanthus have started to regain regain consider considerable able interest interest compared compared to woody woody and agric agricultur ultural al residues because of their potential to cover 50 e70% of the total feedstock [57] feedstock [57].. According to this study, in addition to the possible one billion tons of various feedstocks that would be available, an additional cultivation of high yielding energy crops on Conservation Reserve Program (CRP) lands that are ef ciently managed would be the key option to meet a 30% petroleum-based gasoline displacement in 2030. However, a more recent research study concluded conc luded that bioethanol bioethanol production production has alread alreadyy reached reached the satur saturati ation on level level just just to cover cover the blendi blending ng lim limit it of 10% of bioeth bioethano anoll which could be a substantial obstacle for further increases to reach EISA (2007) projections [58,59] projections [58,59]..
than soybeans per acre of land [61] land [61].. It also provides 10 times more ethanol than corn per growing area ethanol area [62] [62].. Unlike corn and sugarcane, algae biomass does not compete directly with foods and does no nott requ require ire agr agricu icultu ltural ral land land or us usee of fresh fresh wa water ter to be cul cultiv tivate ated. d. It consumes a high level of CO2 during its growth, which makes it environmentally attractive as a CO2 sink sink [63] [63].. 3.2. Lignocellulosic biomass composition
Lignocellulosic Lignocellulo sic mater material ial can gener generally ally be divided divided into three main componen components: ts: cellu cellulose lose (30e50%), hemicellul hemicellulose ose (1 (155e35%) [64 e67] 67].. Cellulose and hemicelluloses make and lignin (10e20%) 20%) [64 up approximately 70% of the entire biomass and are tightly linked to the lignin component through covalent and hydrogenic bonds that make the structure structure highly robust robust and resistant resistant to any treattreatment [25,66,68] [25,66,68].. Potentia Potentiall lignoc lignocellulo ellulosic sic feed feedstoc stocks ks and their composition are summarized in Table in Table 22.. 3.2.1.. Hemicellulose 3.2.1
Hemicellulose is an amorphous and variable structure formed of het hetero eropol polymer ymerss incl includin udingg hexose hexosess (D-glucose, D-gal -galact actose ose and D-mannose) as well as pentose ( D-xylose and L -arabinose) -arabinose) and may contain sugar acids (uronic (uronic acids) namely, D -glucuronic, D-galacturonic and methylgalacturonic acids [69,70] [69,70].. Its backbone chain is primarily primar ily composed composed of xylan b (1 4)-linka 4)-linkages ges that include D-xylose [67].. Bra Branch nch (nearl (nea rlyy 90%) and L -arabinose -arabinose (appr (approximat oximately ely 10% 10%)) [67] frequencies vary depending on the nature and the source of feedstocks. The hemicelluloses of softwood are typically glucomannans while hardwood hemicellulose hemicellulose is more frequen frequently tly composed of xylans [69]. [69]. Alt Althoug hough h the most most abun abundant dant component component in hem hemii[71].. cellulo cell ulose, se, xyla xylan n compos compositio ition n stil stilll varies varies in each each feedst feedstock ock [71] Because of the diversity of its sugars, hemicellulose requires a wide range of enzymes to be completely hydrolyzed into free monomers. /
3.1 3.1.3. .3. Marine algae
Interest in algae as a potential biofuel feedstock has existed since 1978 in the U.S. and has recently received suppor supportt by the DOE Aquatic Program [54] Program [54].. Special focus was directed to assess several aspects of algae biomass including the estimation of its productivity per acre, water consumption and non-food feedstocks with respect to by- and co-products recovered during biofuel production. However, improving the ef ciency of algae feedstock and thus its dev develo elopm pment ent as a viab viable le and scalab scalable le source source commer commercia ciall enterprise remained limited during the 20th century.
3.2.2. Cellulose
Cellulose is a structural linear component of a plant s cell wall consisting of a long-chain of glucose monomers linked b (1 4)glycosidic bonds that can reach several thousand glucose units in length. The extensive hydrogen linkages among molecules lead to a crystalline and strong matrix structure [72] structure [72].. This cross-linkage of numerous hydroxyl hydroxyl groups constitutes the micro brils which give the molecule more strength and compactness. Although starchy materials require temperatures of only 60e70 C to be converted ’
/
Table 2
Potential lignocellulosic biomass source and composition (% dry weight). Raw material Agricultural residues Hardwood Softwood
Hemicelluloses 25e50 25e40 25e29
Cellulose 37e50 45e47 40e45
Grasses Waste papers fro from chemical pulps Newspaper Switch grass
35e50 12e 20 25e40 30e35
25e40 50e 70 40e55 40e45
a
Not present.
Lignin 5e15 20e25 30e60
Others (i.e., ash) 12e16 0.80 0.50
a
6e10 18e30 12 e
e e e e
References [14,54,63,189]
A. Limayem, S.C. Ricke / Progress in Energy and Combustion Science 38 (2012) 449 449e e467
from crystalline to amorphous texture, texture, cellulose requires 320 C as well as a pressure of 25 MPa to shift from a rigid crystalline structure to an amorphous structure in water [73] water [73].. Cellulose is the most prevalent organic polymer and is approximately 30% of the plant composition [54] composition [54].. Cotton, ax and chemical pulp represent the purest sources of cellulose (80 e95% and 60e80%, respectively) while soft and hardwoods contain approximately 45% cellulose [55,56,64]. [55,56,64]. 3.2.3. Lignin
Lignin is an aromatic and rigid biopolymer with a molecular weight of 10,000 Da bonded via covalent bonds to xylans (hemicellulose portion) conferring conferring rigidity and high level of compactness to the plant cell wall [66] wall [66].. Lignin is composed of three phenolic monomers of phenyl propionic alcohol namely, coumaryl, coniferyl and sinapyl alcohol. Forest woody biomass is primarily composed of cellulose and lignin polymers. Softwood barks have the highest level of lignin (30e60%) followed by the hardwood barks (30e55%) while grasses and agricultural residues contain the lowest level of lignin (10e30% and 3e15%, respectively) [55,64] respectively) [55,64].. Conversely, crop residues such as corn stover, rice and wheat straws are comprised mostly of a hemicellulosic heteropolymer that includes a large number num ber of 5-c 5-carb arbon on pe pento ntose se sugars sugars of primar primarily ily xyl xylose ose [74] [74].. Previo Pre vious usly ly,, little little int intere erest st has been been giv given en to lig lignin nin chemis chemistry try potential on hydrolysis. However, lignin components are gaining importance because of their dilution effect on the process once solids are added to a fed batch hydrolytic or fermentation bioreactor in addition to their structure and concentration effects that would affect potential hydrolysis [75] hydrolysis [75].. For instance, the adsorption of lig lignin nin to cellul cellulase asess requir requires es a hig higherenzym herenzymee loadin loadingg becaus becausee thi thiss binding generates a non-productive enzyme attachment and limits the acc access essibil ibility ity of cellul cellulose ose to cellul cellulase ase [76]. [76]. Fur Furtherm thermore, ore, phen phenolic olic groups are formed from the degradation of lignin. These components substantia substantially lly deactivat deactivatee cellulolyti cellulolyticc enzymes enzymes and hence inuence enzymatic hydrolysis. This negative impact caused by lignin lign in has led to int intere erest st in loweri lowering ng the lignin lignin neg negati ative ve effect effect.. Chen Chen demonstrate trated d that lignin modicat cation ion via et al al.. (2 (2006 006)) [76] demons genetically engineering practices targeting its biosynthetic pathwayss could way could consider considerably ably redu reduce ce lignin formation and impr improve ove ethanol yield. However, this could be somewhat problematic as lignin components serve as the major plant defense system to pathogen and insects and its modi cation could disrupt the plants [77].. Retaining the lignin could have bene ts as natural protection protection [77] Ladisch et al. [75] al. [75] have have demonstrated that lignin components, once recovered from biofuel process may be a potential energy selfsustaining source to retain bioreneries nancial solvency. solvency.
453
Unlikee the thermoche Unlik thermochemical mical route, route, bioch biochemica emicall conv conversio ersion n involves physical (i.e. size reduction) or/and thermo-chemical with [82].. Biochemical pretreatment possible biological pretreatment [82] is mainly mainly used to overcome overcome recalci recalcitrant trant materia materiall and incre increase ase surfac surfacee area area to op optim timize ize cellul cellulos osee access accessibi ibility lity to cellul cellulase asess [53,82,83]. [53,82,83]. The upstream operation is followed by enzymatic or acidic hydrolysis of cellulosic materials (cellulolysis) and conversion of hemicellulose into monomeric free sugars (sacc (sacchari harication) subsequent to biological fermentation where sugars are fermented into ethanol and then puried via distillation [79,81] distillation [79,81].. Concurrently, lignin, the most recalcitrant material of cell walls is combusted and Overall, all, bioch biochemica emicall converte conv erted d into elect electricity ricity and heat [80]. [80]. Over approache appr oachess includ includee four unitunit-oper operations ations namel namely, y, pre pretreat treatment ment,, hydroly hyd rolysis, sis, ferme fermentatio ntation n and distil distillation lation [84,85]. [84,85]. Cur Curre rentl ntlyy the biochemical route is the most commonly used process [86] process [86].. Fig. 1 adopted from Ladisch et al. [75] provides a ow diagram illustrating the major steps involved in biochemical bi ochemical process with lignin co-product co-produ ct recovery for a self-suf cient energy system. 4.1. 4.1. Pretreatment o overview verview
Lignocellulosic biomass can be transformed into bioethanol via two different different approach approaches, es, (i.e. biochemic biochemical al or thermoche thermochemical mical conversion) [78] conversion) [78].. Both routes involve degradation of the recalcitrant cell wall structure of lignocellulose into fragments of lignin, hemicellulo hemic ellulose se and cellu cellulose lose.. Each polysacc polysaccharid haridee is hydroly hydrolyzed zed into sugars that are converted into bioethanol subsequent followed by [79,80].. However, these conversion routes do a puricatio cation n process process [79,80] not fundamentally fundamentally follow follow similar similar tech techniqu niques es or pathways. pathways. The thermochemical process includes gasication of raw material at a high temperature of 800 C followed by a catalytic reaction.
Effective pretreatment is fundamental for optimal successful hydr hydroly olysis sis and downst downstrea ream m operat operation ionss [87] [87].. Pre Pretr treat eatmen mentt upstream operations include mainly physical, (i.e., biomass sizereduc reductio tion) n) and thermo thermoche chemic mical al pr proce ocesse ssess that that inv involv olvee the disrup disruptio tion n of the recalc recalcitr itrant ant mat mater erial ial of the biomas biomass. s. Thi Thiss upstre ups tream am op opera eratio tion n inc incre rease asess su subst bstra rate te por porosi osity ty wit with h lignin lignin redistribution. Therefore, Therefore, it enables maximal exposure of cellulases to cellulose surface area to reach an effective hydrolysis with minima min imall ene energy rgy consu consump mptio tion n and a max maxima imall sug sugar ar rec recov overy ery [53,82,83,88].. Fig. 2 illustrates 2 illustrates the major outcomes from pretreat[53,82,83,88] ment upstream processes subsequent to hydrolysis and fermentation operations. Zhu and Pan [53] Pan [53] concluded concluded that the pretreatment process pro cess of wood woodyy bioma biomass ss diffe differs rs subs substantia tantially lly from the agric agriculultural biomass due to differences in their chemical composition and physical properties. Unlike woody biomass, agricultural residues pretreatm pret reatment ent does not require require as much energy as recalcitr recalcitrant ant woody woo dy material material to reach size redu reductio ction n for further further enzym enzymatic atic saccharication. This study placed emphasis on the importance of the energy energy cons consump umption tion from the mechanical mechanical operatio operation n (size (size-reduction) primarily based on the estimation of woody biomass pretreatment energy ef ciency (hPretreatment ¼ Total sugar recovery (kg)/Total (kg)/To tal energy consumption (MJ)). In addition to sugar recovery and ethano ethanoll yie yield, ld, this this energyef energyef ciency ciency ratio ratio and mass mass bal balanc ancee wa wass deemed crucial for the complete estimation of pretreatment ef ciency [53,89 ciency [53,89e91] 91].. Toxic inhibitory level estimation has also been co cons nsid ider ered ed impo import rtan antt for for ev eval alua uati ting ng pr pret etre reat atme ment nt cost cost-effectiveness primarily when dilute acid is added. Costly detoxi cation steps could be a major hindrance to reach high-performance Over eral all, l, th thee rati ratioo incl includ udin ingg ener energy gy pretreatment [36,92]. [36,92]. Ov co consu nsump mptio tion n versussugar versussugar yield yield with with regardto regardto feed feed stock stock versat versatilit ilityy [53,89] as we well ll as toxic toxic inhib inhibito itors rs for formed med per lev level el of sug sugars ars recovered are of prime consideration on the estimation of the pretreatment ef ciency and cost effectiveness of the operation in an effort to reach optimal conditions [93] conditions [93].. Several pretreatment methods, namely, mechanical, chemical or microbiological have been used to remove the recalcitrant cell wall material of lignocellulosic biomass depending on the raw [93,94].. More recently, there has been material being extracted [93,94] co consi nsider derable able adv advanc anceme ement nt in develo developme pment nt of pr pretr etreat eatmen mentt
Appli Ap plicat cation hig high hs)levels lev elsasofhy heat hea t conver con verts ts raw raw mater mat ial CO int into synthe syn thesis sision gasof (sy (synga ngas) such such hydro droge gen, n, car carbonmono bonmonoxid xideeerial and 2o. In the presence of catalysts, the resulting syngas can be utilized by the microorganism Clostridium ljungdahlii to form ethanol and water can be further separated by distillation [81] distillation [81]..
processes processes [19,23,94 [19,23,94 96].. Table 96] 3 illustrates 3 illustrates some of the Although pretreatment methods that have been examined over the years. most of these treatments can liberate hemicellulose and cellulose from the cell wall, some of them remain economically unfeasible due to key technic technical al issu issues. es. Furthermor Furthermore, e, they are not all able to
’
4. Path Pathway wayss of bioethan bioethanol ol productio production n from cellu cellulosic losic feedstocks
e
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Fig. 1. Lignocell Lignocellulose ulose substrate conversion conversion steps for ethanol ethanol and copr coproduc oducts ts generatio generation. n. Lignin Lignin coproduc coproductt is retur returned ned for a self-e self-energy nergy suf cient system (ado (adopted pted from )). Refs. ([75,113] ([75,113])).
overcome the recalcitrant material found mainly in wood-based feedstocks. Typically, few treatments are endowed with ability to overcome feedstock versatility [97,98] versatility [97,98].. Unlike agriculture residues, forest and wood materials are high in lignin (approximately 29%) and cellulose (approximately 44%) [55] 44%) [55] which which renders them morereca more recalcitr lcitrant. ant. Agric Agricultur ultural al residues residues suchas such as cornstov corn stover er,, rice and wheat straws are mostly composed of hemicellulose (32%) and low levels of lignin (3e13%) conferring to them a less resistant texture but a higher level of pentose sugars rendering them less practical than woody recalcitrant material. The most prevalent prevalent treatment treatmentss include include acid hydrolysis, hydrolysis, hot water,, dilute acid pretreatment and lime [92,93,99 water lime [92,93,99e108] 108].. However, the conventional methods using acidic treatments (usually dilute
sulfuric acid with concentrations below 4 wt% and temperatures [109] are are always accompanied by formation of greater than 160 C) C) [109] toxic inhibitors such as furfural from xylose and hydroxymethyl furfur furfural al (HM (HMF) F) from from glu gluco cose se in add additio ition n to phenol phenolics ics and acetic acetic aci acid d [20,36,93,110]. [20,36,93,110]. Acetic acid resulting from dilute acid pretreatment of agricultural residues as well as herbaceous and hardwood hardwoodss is pH dependent and can reach a high concentration of approximately 10 g/L [20,36] that [20,36] that is more dif cult to separate and detoxify than HMF and furfural. Unlike dilute acid pretreatment, ammonia ber explosion (AFEX) treatments are suf cient to hydrolyze primarily agricultural residues such as corn stover and have not been associated with the formation of toxic products including HMF [97]. [97]. Given that woody feedstock is gaining increasing attention for its
A. Limayem, S.C. Ricke / Progress in Energy and Combustion Science 38 (2012) 449 449e e467
455
Major changes: Major targeted components:
Pretreatment: Lignocellulosic feedstock
- Mechanical and/or -Thermo-chemical and/or - Biological
- Lignin - Hemicelluloses - Cellulose
Energy consumption
- Lignin redistribution. - Increased porosity of the lignified cell-wall. - Size- reduction. - Increased surface area of hemicelluloses and cellulose for greater enzymes accessibility
% of toxic byproducts released depends on pretreatment type
Hydrolysis Fermentation Distillation
Bioethanol Fig. 2. Pretreatment upstream process: Major effects.
attractive attributes over low-lignin materials, organosolv along with steam explosion [111] explosion [111] and and sulte pretreatment to overcome recalcitrance (SPORL) [112] (SPORL) [112] have have become of prime interest for their ability to degrade high-lignin forest materials [53,112]. [53,112]. A recent study reported that steam explosion consumed the highest level of energy yielding the lowest pretreatment energy ef ciency ratio of 0.26 kg sugar sugar/MJ /MJ when compared compared to organoso organosolv lv (0.31 (0.31e0.40 kg sugar/MJ) suga r/MJ) and SPORL SPORL (0.35e043 kg sugar sugar/MJ) /MJ) [53]. [53]. Whil Whilee th thee organ organoso osolv lv tre treatm atment entss degrad degradee high-lig high-lignin nin woody woody biomas biomasss including inclu ding both softwood softwood and hardwood hardwood,, they produce produce cons consider iderable able qu quant antiti ities es of inhibit inhibitor orss nam namely ely furfur furfural al and HM HMFF, yie yield ld a low hemicellulosic sugar concentration and are also associated with a high capital investment [113] investment [113].. Consequently, SPORL remains the most attractive candidate for its exibility and ability to overcome both hardwood and softwood recalcitrance with the highest sugar recovery and lowest energy consumption [53] consumption [53].. 4.2. Hydr Hydrolys olysis is
acid hydroly hydrolysis, sis, conc concentra entrated ted acid hydroly hydrolysis sis is not followed followed by high co conce ncentr ntrati ations ons of inh inhibi ibitor torss and pr prod oduce ucess a hig high h yie yield ld of free free sug sugars ars (90%); however, however, it requires large quantities of acid as well as costly [117].. acid recycling, which makes it commercially less attractive attractive [117] Whilee acid pretr Whil pretreat eatmen mentt result resultss in a formati formation on of reactiv reactivee substrates when acid is used as a catalyst, acid hydrolysis causes signicant chemical dehydration of the monosaccharides formed such that aldehydes and other types of degradation products are generated [19] generated [19].. This particular issue has driven development of research to improve cellulolytic-enzymes and enzymatic hydrolysis. Effective pretreatment is fundamental to a successful enzy[118].. Dur During ing the pre pretre treatm atment ent pr proce ocess, ss, the matic hydr hydrolysi olysiss [118] lignocellulosic substrate enzymatic digestibility is improved with the increased porosity porosity of the substrate and cellulose accessibility to cellulases. cellulases. Trichoderma reesei is one of the most ef cient and productive prod uctive fungi used to prod produce uce industrial grade cellulolytic cellulolytic enzymes. The most common cellulase groups produced by T. reesei that cleave the b 1,4 glycosidic bonds are b-glucosidase, endoglucanases and exoglucanases [113] exoglucanases [113].. However, cellulase enzymes exposed to lignin and phenolic-derived lignin are subjected to adverse effects [36,37,119] effects [36,37,119] and and have demonstrated that phenolicderived lignin have the most inhibitory effects on cellulases. This study reported that a ratio of 4 mg to 1 mg peptides, reduced by half the concentration of cellulases (i.e. b-glucosidases) from T. reesei. This This stra strain in was also sho shown wn to be 10 to 10 fold fold mor moree sensit sen sitive ive to phe phenol nolics ics than than Aspergillus Aspergillus niger . In ad addi diti tion on to phenoli phe nolicc com compon ponent entss effe effect ct on cellula cellulases ses,, lign lignin in has also an adverse effect on cellulases. As mentioned previously, the lignin adverse effect has two aspects including non-productive adsorption and the limitation of the accessibility of cellulose to cellulase. Although Althou gh considerable considerable genetic modications (GMs) have been deployed to transform lignin effects, lignin has been shown to be a potent potential ial source source of self sus sustain taininging-ene energy rgy and addedadded-val value ue /
The su succe ccess ss of the hy hydr droly olysisstep sisstep is essent essential ial to the effect effectiv ivene eness ss of a pretreatmen pretreatmentt operation operation [80] [80].. During this reaction, the released polymer sugars, cellulose and hemicellulose are hydrolyzed into fre freee monome monomerr molecu molecules les re readi adily ly avail availabl ablee for fermen fermentat tation ion conversion to bioethanol [79]. [79]. There are two different types of hydrolysis processes that involve either acidic (sulfuric acid) or enzymatic reactions [114] reactions [114].. The acidic reaction can be divided into dilute or concentrated acid hydrolysis. Dilute hydrolysis (1 e3%) requires a high temperature of 200e240 C to disrupt cellulose [115].. It is followed followed by hexose hexose and and pentose pentose degrada degradation tion and crystals [115] crystals formation of high concentrations of toxic compounds including HMFF and phe HM phenol nolics ics detrim detriment ental al to an effecti effective ve saccha sacchari rication [19] [19].. The Madison wood-sugar process was developed in the 1940s to optimize alcohol yield and reduce inhibitory and toxic byproducts.
4 (0.5 wt%) that This uses sulfuric acidtemp H 2SOerature ouslyyprocess ousl to the bioma biomass ss at a high temperatu re of 1150 50e180ows C incontinua short period of time allowing for a greater sugar recovery [116] recovery [116].. Concentra trate ted d aci acid d hy hydro drolys lysis, is, the mo more re prev prevale alent nt method method,, has been been considered to be the most practical approach [102]. [102]. Unlike dilute
components. Consequently, several research studies of have determined practical approaches in eliminating inhibition cellulases [120] have demonwithout involving GM approaches. Lui et al. [120] strated that the application of metal components namely, Ca(II) and Mg(II) via ligninemetal complexation substantially enhanced
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Table 3
Pretreatment methods and key characteristics. Pretreatments Dilute acid (H2SO4, HCL (0.5e5%) 5%)
Key characteristics - Prac Practi ticcal an and d si simp mple le tec techn hniique. que. Do Does es not not requ equir iree ther therma mall ener energy gy.. - Effective hydrolyze of hemicelluloses with high sugar yield. - Generates toxic inhibitors - Requires recovery steps
References [79,93,103,105,106,194]
Hot water
- The majority of hemicelluloses can be dissolved. - No chemicals and toxic inhibitors. - Averag Averagee solid load. - Not successful with softwood.
[46,92,94e96,108,195,196]
Lime
- High total sugar yield including pentose and hexose sugars. - Effective against hardwood and agricultural residues. - High pressure and temperature hinder chemical operation. - Commercial scalability problem
[53,107,196]
Ammonia ber ber expa expans nsio ion n (A (AFE FEX) X)
- Effe Effeccti tive ve ag agai ains nstt ag agrric icu ult ltur ural al resi residu dues es main mainly ly cor orn n stove toverr wi with thou outt formation of toxic end-products. - Not suitable for high-lignin materials. - Ammonia recovery - No wastewaters
[19,118,122,147,149,183]
Ammo Ammoni niaa re reccycle ycle per perco cola lati tion on (A (ARP RP))
- Hi High gh re redi dist stri ribu buti tion on of lig igni nin n (85% (85%)) - Recycling ammonia - Theoretical yield is attained
[26,199,200]
St Stea eam m exp explo losi sion on wi with th catal atalys ystt
- Effe Effeccti tive ve ag agaain inst st agr griicu culltu tura rall re resi sidu dues es and and hard hardwo wood od.. - High hemicelluloses fractions removal - Not really effective with softwood
[106,122,201e203]
Organosolv
- High yield is enhanced by acid combination. - Effective against both hardwood and softwood.
[202,204]
-- Low hemicellulosic sugar concentration Formation of toxic inhibitors - Organic solvent requires recycling - High capital investment Sulte pretreatment top overcome recalcitrance (SPORL)
- Effective against high-lignin materials, both softwood and hardwood. - Highest pretreatment energy ef ciency - Minimum of inhibitors formation - Accommodate feedstocks versatility. - Steam explosion combined to SPORL in presence of catalyst becomes effective against softwood materials - Cost-effective.
[53,89,90,112,132,133,184]
Ozone
- Effectively remove lignin from a wide range of cellulosic material without generating inhibitors. - Expensive
[19]
Al Alkkal aliine we wett ox oxiidati dation on
- The The comb ombin inat atio ion n of ox oxyg ygen en,, wa wate terr, high high temp temper erat atur uree and alka alkalli reduc educee toxic inhibitors. - High deli deligni gnication and solubilization of cellulosic material - Low hydrolysis of oligomers
[97,202]
Fungal bioconversion
- Environmentally friendly - Low use of energy and chemical - Slow bioconversion
[181,206]
enzymatic hydrolysi enzymatic hydrolysis. s. Additionally Additionally,, Erickson Erickson et al. [121] have reported repor ted the importance importance of additives additives namely, namely, surfactants surfactants and bovine serum albumin (BSA) in blocking lignin interaction with reported ed that that the adv advers ersee cellula cell ulases. ses. Sewalt Sewalt et al. [119] have report effect of lignin on cellulases can be surmo surmounted unted by ammoniation ammoniation and various N compounds. Moreover, the enzymatic treatment can can be ac acco comp mplis lishe hed d si simu multa ltane neou ously sly with with th thee en engin ginee eere red d co-ferm co-ferment entatio ation n microb microbial ial proces processs known known as sim simulta ultaneo neous us saccharication and fermentation (SSF) [31,122] (SSF) [31,122].. This proc process ess has been bee n of interes interestt since since the late late 1970s 970s for its effectiv effectivene eness ss to minimiz min imizee cell cellulo ulolyti lyticc produc productt inhibiti inhibition on and subseq subsequen uently tly
Pretreatmentt and hydrolys Pretreatmen hydrolysis is proc processes esses are design designed ed to optimize optimize the fermentation process [80] [80].. This natural, biological pathway depending on the conditions and raw material used requires the presence pres ence of micro microorganis organisms ms to ferment sugar into alcohol, lactic aci acid d or other other end pro produ ducts cts [11,79] [11,79].. Moreover, Moreover, industrial yeasts such as S. cerevisiae have been used in alcohol production mostly in the
increase alcohol production [122] production [122].. Typically, separate hydrolysis and fermentation (SHF) processes involve the inhibition of the hydrolytic enzymes (cellulases) by saccharide products such as glucose and cellobiose. Unlike SHF, the SSF process combines hydrolysis and fermentation activities simultaneously and hence
brewery industries for thousands years. S. cerevisiae has also and beenwine utilized for corn-based and of sugar-based biofuel industries indus tries as the primary fermentative fermentative strain strain.. Once becoming acc access essible ible for enz enzyma ymatic tic or acidic acidic hy hydro drolys lysis, is, the pretre pretreate ated d cel cellulo lulosic sic slu slurry rry is sub subseq seque uently ntly conve converte rted d into fermen fermentabl tablee
keeps the concen keeps concentra tration tion of sacchar saccharide idess too low to cau cause se any any considerable cellulase inhibition [109] inhibition [109].. 4.3. Ferm Fermentat entation ion
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free sugars. The sugars are mixed with water to form a broth. Typica Typ ically lly,, dur during ing batc batch h fer fermen mentati tation on S. cerevi cerevisia siaee ferments hexose sugars, mainly glucose, into ethanol in a large tank via the EmbdeneMey Meyerh erhof of pathw pathway ay under under anaero anaerobic bic conditi conditions ons and contro con trolled lled temper temperatu ature. re. Yeas east-b t-base ased d ferment fermentatio ation n is always always accompanied by formation of CO2 by-products and supplemented by nitrogen to enhance the reaction. This conventional strain is optim optimal al at a temper temperatu ature re of appro approxim ximate ately ly 30 C and resis resists ts a high osmotic pressure in addition to its tolerance to low pH [123].. S. cerevisiae can levels of 4.0 as well as inhibitory products products [123] generate a high yield of ethanol (12.0e17.0% w/v; 90% of the theoretical) from hexose sugars [34,124] sugars [34,124].. Traditiona Trad itionally lly,, separate separate hydrolys hydrolysis is and fermentatio fermentation n (SHF) (SHF) sequential steps are used in bioethanol production. production. However, there is particular interest in targeting bioethanol production that can be derived from lignocellulosic biomass materials where both hexose and pentose sugars are available from the hemicellulose fraction. Despite its broad tolerance to stressful bioethanol process conditions, S. cerevisiae is not able to ferment sugars other than hexose. Unfortunately, lignocellulosic material includes a large proportion of hemice hemicellu llulos losic ic biomas biomasss that that contain containss mai mainly nly pen pentos tosee sugar sugarss such such as D-xylose [125] [125].. Moreov Moreover er,, an opt optimal imal fermentati fermentative ve microor microor-ganism should be tolerant to a high ethanol concentration and to chemical inhibitors formed during pretreatment and hydrolysis process. In response to this inability of S. cerevi cerevisiae siae to ferment pentose sugars, extensive efforts have been employed to develop genetically engineered microorganisms that are capable of fermenting pentose and hexose sugars simultaneously. An optimal fermentative microorganism should be able to utilize both hexose and pentose pentose simultaneo simultaneously usly with minimal minimal toxic toxic end-pro end-product ductss formation form ation.. Different Different techniqu techniques es including including SSF and conso consolidat lidated ed bioprocessing (CBP) have been developed to ensure the combination of hydrolysis (step 3) and fermentation (step 4) in one single reactor and thus, reduce product inhibition and operation costs. In addition to continuing downstream steps, CBP processing integrates both fermentation and cellulase formation in one fermentative/cellulolytic microorganism [75]. [75]. Howe Howeve verr, desp despit itee th thee extensive range of prokaryotic and eukaryotic microorganisms that have been shown to be able to produce ethanol from sugars, most of them remain limited in terms of sugars co-fermentatio co-fermentation, n, ethanol yield and tolerance to chemical inhibitors, high temperature and ethanol. In an effort to summarize relevant advantages and major limitations tatio ns of microbial microbial ferme fermentati ntative ve species, species, Ta Tabl blee 4 compares po poten tentia tiall micro microorg organi anisms sms for lig lignoc nocell ellulo ulosic sic-ba -based sed biofue biofuell fermentation including bacteria, yeasts and fungi that could be optimized and become potential avenues to enhance alcohol yield and product productivity ivity in large-scal large-scalee lignocellul lignocellulosicosic-based based ethanol ethanol fermentation.
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the top of the column, volatiles are separated as a distillate and residue is recovered at the bottom of the column. 5. Current issues and c challenges hallenges of lignoc lignocellulosic ellulosic bioethanol production 5.1. 5.1. Overcoming recalcitr recalcitrance ance of lignocellulosic materials
Although lignocellulosic biomass is a potential feedstock for biorenerie neries, s, its reca recalcitr lcitrant ant stru structur cturee and complexit complexityy rema remain in
Bioethanol obtained from a fermentation fermentation conversion conversion requires further furth er se separ parati ation on and puri purica catio tion n of ethano ethanoll from from wate waterr through a distillation process. Fractional distillation is a process implemen impl emented ted to separat separatee ethanol ethanol fro from m water water based based on their their different volatilities. This process consists simply of boiling the ethanol ewate waterr mixtu mixture re.. Becau Because se the boilin boilingg point point of wate waterr (100 C) is higher higher than than the eth ethano anol-b l-boil oiling ing point point (78.3 (78.3 C), ethanol will be converted converted to steam before water. water. Thus, water can
a major economic and technical obstacle to lignocellulosic-based biofuel production [127]. [127]. The resilience of lignocellulosic materials is due to their composition and physicochemical matrix. The organization of vascular, epicuticular waxes as well as the amount of sclerench sclerenchymato ymatous us and the comp complexi lexity ty of matri matrixx molecules molecules,, co contr ntribu ibute te to the com compac pactne tness ss and streng strength th of the cellul cellulosi osicc material [87] material [87].. Furthermore, lignocellulosic materials as discussed previously are composed principally of three components namely, cellulose, hemicellulose and lignin. Together the polysaccharides, cellulose and hemice hemicellu llulos loses es serve serve as ini initia tiall sub substr strate atess for sub subseq sequen uentt saccharication and fermentation. However, these components are encapsulated via a tight covalent and hydrogen link to the lignin seal [96] seal [96].. These tight bonds not only give the cell wall its compact structure but limit enzyme access to the surface area. Moreover, cellulose, a polymer of glucose molecules linked via b (1 4)glycosidic bonds confers to cellulose a crystalline and compact structure [66] structure [66].. Hemicellulose, the amorphous part of the cell wall, is composed of different different hexoses hexoses and pentose pentose sugars including including xylose and arabinose arab inose bond bonded ed through through xylans b (1 4)-lin 4)-linkage kages. s. These Thesevarie varieties ties of sugars polymers and linkages between molecules impose more complexities to the cell wall and therefore the hydrolysis process necessitates numerous cost-prohibitive enzymes to cleave polysaccharides entirely into fermentable sugar fragments. Additionall ally, y, compo componen nents ts inc includ luding ing primar primarily ily xyl xyloo-oli oligos gosacc acchar haride idess produced from hemicelluloses hydrolysis have been shown to be inhibitory inhibi tory to cellu cellulase lase enzymes enzymes [128] [128].. Altho Although ugh xylose xylose caus causes es a higher level of inhibition inhibiti on to cellulase enzymes than xylan, soluble xylo-oligomers are considered the most inhibitory to cellulase and substantially inuence enzymatic hydrolysis [129,130] hydrolysis [129,130].. Hence, the removal of these components in addition to organic acids and phenolics is desired in an attempt to achieve an ef cient cellulose conversion via enzymatic hydrolysis [75] hydrolysis [75].. Thus, a successful and low-co low -cost st ethano ethanoll biocon bioconve versi rsion on is closel closelyy rel relate ated d to the ef ciency ciency of the pretreatment step. Pretreatment which is mechanical and/ or thermo-ch therm o-chemica emical,l, and/ and/or or a biolo biological gical agent prim primarily arily involves involves redistribution of lignin and improving cellulose accessibility to enzymes by increasing the surface area that will be subjected to fur furthe therr hy hydr droly olysis sis.. An effect effective ive pr pretr etreat eatmen mentt also also req requir uires es a reduction of energy consumption consumption with minimum toxic inhibitory Howeve verr, in ad addi diti tion on to th thes esee product pro ductss formation formation [53,80]. [53,80]. Howe complexi comp lexities ties and differ difference encess betw between een comp component onentss within the lignocellulosic material, lignocellulose composition from each type of biomass varies depending on the origin and geographical location. Not all types of lignocellulosic feedstocks require the same pretreatment strategy. These heterogeneities have an important impact imp act on the cho choice ice of pre pretr treat eatmen ments ts and the do downs wnstre tream am processes [131] processes [131].. Currently, the SPORL treatment is of interest for its broad spectrum ability on acting in both softwood and strong
be separatedatvia a condensation procedu re. Typically, and ethanol distillate most largerecaptured a concentration of procedure 95% [23]. 95% [23] scale industries and bioreneries use a continuous distillation column system with multiple effects [126] effects [126].. Liquid mixtures are heated and allowed to ow continuously all along the column. At
hardwood materials materials [115,132] [115,132]. pretreatment highlignin lignin forest forest mat mater erial ial wit with h a. This lim limite ited d format formation iondegrades of hy hydro drolys lysis is [133].. Wang et al. (2009) [132] (2009) [132] have have demonstrated that inhibitors [133] inhibitors lignin redistribution and increased porosity and surface area were achieved in only 30 min and was followed by 10 h of enzymatic
4.4. Separation/ Separation/distillation distillation
/
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Table 4
Advantages and drawbacks of potential organisms in lignocellulosic-based bioethanol fermentation. Species
Characteristics Facultative anaerobic yeast
Saccharomyces cerevisiae
Candida shehatae
Zymomonas mobilis
Pichia stiplis
Pachysolen
Advantages - Naturally adapted to ethanol fermentation. - High alcohol yield (90%). - High tolerance to ethanol (up to 10% v/v) and chemical inhibitors. - Amenability to genetic modications
Kluveromyces marxianus
References [69] [143] [207] [80] [123] [208]
Micro-aerophilic yeast
- Ferment xylose
- Low tolerance to ethanol - Low yield of ethanol. ethanol. - Require micro-aerophilic conditions - Does not ferment ferment xylose at low pH
[69] [209] [94] [210]
Ethanologenic Gram-negative bacteria
- Ethanol yield surpasses S. cervesiae (97% of the theoretical), - High ethanol tole tolerance rance (up to 14% v/v) - High ethanol productivity (ve-fold more than S. cerevisiae volumetric productivity) - Amenability to genetic modication. - Does not require additional oxygen
- Not able to ferment xylose sugars. - Low tolerance to inhibitors - Neutral Neutral pH range
[211] [212] [69]
Facultative anaerobic yeast
- Best performance xylose fermentation. - Ethanol yield (82%). - Able to fer ferment ment most of cellulosic-material sugars including glucose, galactose and cellobiose. - Possess cellulase enzymes favorable to SSF process.
- Intolerant Intolerant to a high concen concentrati tration on of ethanol above 40 g/L - Does not ferment ferment xylose at low pH - Sensitive to chemical inhibitors. - Requires micro-aerophilic conditions to reach peak performance - Re-assimilates formed ethanol
[69] [213] [209] [214]
Aerobic fungus
- Ferment xylose
- Low yield of ethanol.
[209]
-- Require conditions Does notmicro-aerophilic ferment ferment xylose at low pH
[215]
tannophilus Esherichia coli
Drawbacks - Not able to ferment ferment xylose and arabinose sugars. - Not able to survive high temperature of enzyme hydrolysis.
Mesophilic Gram-negative bacteria.
- Abil Ability ity to use both pentose and hexose sugars. - Amenability for genetic modi cations
- Repression catabolism interfere to co-fermentation - Limited ethanol tolerance - Narrow pH and temperature growth range - Production of organic acids - Genetic Genetic stabili stability ty not proven yet - Low tolerance to inhibitors and ethanol
[80] [215] [33]
Ther Thermo moph phil ilcc ye yeas astt
- Able Able to gr grow ow at a hi high gh temp temper erat atur uree above 52 C - Suitable for SSF/CBP process - Reduces cooling cost - Reduces contamination - Ferments a broad spectrum of sugars. - Amenability to genetic modications
- Excess of sugars affect its alcohol yield - Low ethanol tolerance - Fermentation of xylose is poor and leads mainly to the formation of xylitol
[153] [109] [180]
- Resistance to of an70 extremely high C. temperature
- Low tolerance tolerance to ethanol
[217] [109,154,155] [95]
Thermophilic bacteria: Thermoanaerobacterium saccharolyticum Thermoanaerobacter ethanolicus Clostridium thermocellum
Extreme anaerobic bacteria
- Suitable for SSCombF/CBP Processing - Ferment a variety of sugars - Display cellulolytic activity - Amenability to genetic modication.
hy hydr droly olysis sis.. A small amount amount of 4% sod sodium ium bis bisulf ulfate ate wa wass added added to the solution under pH level of 2.0 e4.5 and at a temperature of 180 C. The entire conversion of cellulose to glucose sugar was accompanied by gener generation ation of low concentrati concentrations ons of inhibitors inhibitors (less than 20 mg/g). 5.2. Potential water availability challenges for the biofuel system
Althou Although gh the biofue biosustainability fuell wa water ter us use is anneries, import important ant co compo mponen nentt to consider for ofe biore limited information is available worldwide and in the U.S. on water requirements for the emerging agricultural practices and technologies that could [134].. While water availability impact water supplies and quality [134]
does not pose a serious constraint in several countries such as Brazil, Canada, Russia and some African nations, other countries inc includ luding ing China, China, Ind India, ia, Sou South th Afr Africa ica and Tur Turkey key are alread alreadyy encountering scarce water issues before even considering estimates of additional water consumption associated with biofuel production [135]. [135]. In the U.S., water availability could become an issue in the near future if appropriate and more effective agricultural water sustainability practices are not implemented. To date, U.S. ethanol isavailable only produced a pilot scale [134]. However, this levellignocellulosic-based and is not yet commercially [134]. at st stud udyy al also so repo report rted ed th that at ener energy gy corn corn-d -der eriv ived ed bi biof ofue uell has has already achieved an exponential growth requiring an increasing availability of water in the Great Plains and other arid regions of
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the country. Moreover, biofuel water availability is a very complex plex issue beca because use it varies varies by regions regions and type of crops crops [136] [136].. Wi With th the increasing awareness toward the adverse effects of biofuel system on the quality and availability of water, there has been a series of investigations led by the U.S. National Academy of Science Scien ce (NAS) (NAS) to determine determine current agricultural agricultural prac practices tices and their their impact impact on wa water ter res resour ourcesand cesand qu qualit alityy [136]. [136]. NAS NAS has rep repor orted ted that that the most most import important ant fac factor torss that that cause cause sub substa stanti ntial al wa water ter stress due to biofuel production is the expansion of energy crops such as corn in those areas of the U.S. Midwest that are already suscepti susc eptible ble to drou drought ght and hence require intensive intensive irrigation. irrigation. Although biofuel processing utilizes a signicant level of water, it does not consume as much water as biofuel crops. Furthermore, biofuel crops involve a substantial use of pesticides and herbicides in additi addition on to fer fertili tilizer zerss res result ulting ing in a surplu surpluss of nutrie nutrients nts including, nitrogen and phosphorus. This excess of nutrients used for corn and other energy crops was demonstrated to lead to an expansion of the dead zone in the Gulf of Mexico caused by oxygen depletion [137] depletion [137].. NAS envisions a solution that places the emphasis on increasing irrigation-ef ciency used by farmers as well as plant water recycling. However, Huffaker Huffaker [138] [138] suggests suggests that efforts should be directed toward improving water quality impact rather than water recycling and irrigation ef ciency. While further furt her expansion expansion of cellulosic cellulosic feedstock feedstock sources would be an attractive alternative within the next decade to mitigate water supplies and reduce fertilizer use geared toward intensive crop cultivation, a shortage of water resulting from inef cient water utilization during biofuel processing could also jeopardize biofuel water sustainability [134] sustainability [134].. “
”
6. Curre Current nt prospect prospectss for systems appro approaches aches to biomass conversion
Current research is continuing to deploy individual and specic effo effort rtss to towa ward rd ac achi hiev evin ingg opti optima mall so solu luti tion onss via via impr improv ovin ingg lignocellu ligno cellulosic losic-base -based d ethanol ethanol performan performance ce with a minimum minimum capital capital investment on energy consumption and water supplies. Future prospec pro spects ts for the optimizati optimization on of lignocellu lignocellulosic losic bioconver bioconversion sion must embrace a more systematic enhancement of bioethanol for all four major steps in bioethanol production. Pretreatment as a rst step is the most costly operation and accounts for approximately 33% of the total cost [139] cost [139] with with respect to the economic feasibility of each step as well as the consideration of microbial and chemical contaminations that can potentially reduce yields. Developin Deve lopingg geneticall geneticallyy modied fermentati fermentative ve and cellulolyt cellulolytic ic microorganisms enhanced by co-culture systems is desirable to increase ethanol yield and productivity under the stressful conditions associated with high production bioethanol-processes [140] bioethanol-processes [140].. SSF SSF as well well as simu simult ltan aneo eous us sacc saccha hari ricat cation ion and combin combined ed fermentatio ferme ntation n (SSCombF) (SSCombF) of the enzymatic enzymatic hydroly hydrolyzate zate,, glucose glucose with the hemicelluloses-derived sugars [120] and CBP are also consider cons idered ed to be cost-effe cost-effective ctive and offer promise promise in reducing reducing end-pro endproduct duct inhibi inhibition tion and operation operation numbers numbers [122,141]. [122,141]. However However,, an overall analysis of performance would provide a clear vision of the sy syste stem m condit condition ionss and all allow ow imp implem lement entati ation on of feasib feasible le preventive interventions aimed at enhancing biofuel production ef ciency.
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on biofue biofuell sys system temss as wel welll as the enviro environme nmenta ntall imp impact act.. LC LCA A methodolo method ologie giess are consi consider dered ed to be the ana analy lysis sis model model of choice choice for quan uantit titati ativel velyy compar comparing ing the enviro environme nmenta ntall impact impactss of eac each h biomass-based energy generating system. This approach primarily focuses on the estimation of direct impacts along with indirect and co co-pr -prod oduct uctss credit creditss inc includ luding ing the car carbon bon cy cycle cle as we well ll as gas emission, fossil fuel consumption, water consumption and generation of wastes involving energy utilization. Recent studies conducted by Mu et al. [81] [81] have have analyzed and compared biochemical and thermochemical conversion pathways based on LCA studies. They concluded that despite the equivalent alcohol productivity and energy ef ciency performance between the two routes, in the short run biochemical conversion is considere ered d to have have a more more fav favor orabl ablee env enviro ironme nmenta ntall per perfor forman mance ce than than the thermochemical route. LCA approaches rely on quantitative estimations of direct (chemical pollutant agents) and indirect (greenhouse gas emissions (GHG), fossil fuel intake, water consumption) impacts along with biomass contribution and co-product credits (electricity, mixed alcohol and heat). Assessments performed by legislators on the validity of the biomass-based energy, stipulated that a satisfactory alternative to petroleum petroleum gasoline should achieve at least 20% reduction in GHG. Biochemical conversion of cellulosic materials was able to achieve 50% reduction of GHG emission compared to a non-renewable fuel. The biochemical route also saved save d consump consumption tion of fossi fossill fuel reso resource urcess (1 (1..13 MJ/L) MJ/L)but but generated generated chemic che mical al releas releases es inc includ luding ing phosph phosphoru oruss and nit nitrog rogen en to the atmosphere causing additional eutrophication and acidication. While the biochemical route exhibited higher water consumption than the thermochemical process, it did yield a better short-term environmental performance on parameters such as GHG emissions and fossil fuel consumption. This in turn leads to a lower impact on the environment as it uses components such as lime, sulfuric acid and nutrients that can considerably inuence LCA estimates of fossil oil, water consumption and greenhouse gas emission. Much more detailed LCA comparisons between thermochemical and biochemical operations have been discussed else [81].. where [81] where 6.2. Opti Optimizat mization ion of the biofuel proce process ss main steps
6.1. 6.1. Overall analysis of performance: life cycle assessment (LCA) comparisons
To date, various approaches have been advanced to improve the four-steps of the bioethanol process. Pretreatment is considered the most costly operation and a major constraint tow toward ard achieving high-yield via low-cost capital [93] capital [93].. Therefore, an initial step for improvement is crucial to the success of downstream operations. There has been considerable advancement in pretreatment technology and several approaches are already available and successful depending on the characteristics of the respective lignocellulose biomass source. Feedstocks richer in lignin exhibit a high recalcitran trance ce and and resi resist stan ance ce,, th thus us requ requir irin ingg di diff ffer eren entt trea treatm tmen entt approaches from raw materials that have a higher quantity of amorphous hemicelluloses rich in pentose sugars [142] sugars [142].. Hence, the ine inevit vitabl ablee feedst feedstoc ockk versat versatilit ilityy and var variab iabilit ilityy has bec become ome a potential issue for bioethanol investors. Given that ethanol is a com commod modity ity produ product, ct, bioeth bioethano anoll pla plants nts wou would ld have have lim limite ited d choices choic es for ava available ilable feedsto feedstock. ck. This key issue has led research researchers ers to look for a pretreatment process able to deal with a variety of raw [53].. Moreover, the appropriate treatment is also corrematerials [53] materials lated lated to the manufa manufactu cturin ringg eco econom nomics ics as well well as lay-o lay-out ut and
As technologies emerge that improve various stages of biofuel production from biological sources, there is increasing need to compare overall performance with current operational systems to verify their validity in terms of water use and energy performance
possible poss ible investment investments. s. The selec selection tion of a suita suitable ble technological pret pretreat reatment ment relies primarily on environmental, economical and factors facto rs inclu including ding energy energy sav savings, ings, wastew wastewater ater,, recyclin recyclingg issu issues, es, substrate recovery along with a maximal solid loading yield and minimal use of chemicals [143] chemicals [143]..
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Traditionally, dilute acidic pretreatment is the most commonly used method in the bioethanol process. This upstream treatment is consid con sider ered ed to be the most most pr pract actica icall du duee to its effect effective ivenes nesss at a lowlowcost [102,144]. [102,144]. Howeve However, r, the for format mation ion of hig high h levels levels of to toxic xic inhibitors inhibit ors namely, namely, acetic acetic acid, HMF and pheno phenolic lic component componentss requiring an additional detoxication step have led researchers to focuss on better focu better alternativ alternatives. es. Phenolic Phenolic component componentss particular particularly ly phenolic hydroxyl groups can inuence cellulase enzyme activities [53] [53].. Co Conse nsequ quent ently ly,, it is import important ant to remov removee phenol phenolics ics if enzyma enzymatic tic hydrolysis is to be improved. Furthermore, according to Ladisch [75],, since toxic inhibitors such as aldehyde components et al. [75] considera consi derably bly inue uence nce microb microbial ial gro growth wth rat ratee and volume volumetri tricc productivity, selecting a fermentative culture from metabolically modied microorganisms would improve microbial resistance to inhibitors. Steam explosion in the presence of catalyst has gained considerable interest and researchers are examining the potentially high correlation between catalyst concentration and ethanol yield. Of the numerous techniques tested, Öhgren et al. [145] al. [145] con conrmed the effectiven effec tiveness ess of catalyzed catalyzed steam-explos steam-explosion ion by 3% (w/w) (w/w) sulfur sulfur dioxide (SO2) pretreatment accompanied by a cellulase and xylanase hydrolysis step at 45 C during 72 h. These operations yielded approxi appr oximatel matelyy 96% glucose glucose and 86% xylose xylose from residue residue corn stover feedstocks. The Consortium for Applied Fundamentals and Innovation [145] have also demonstrated the ef ciency of SO2 Innovation deltoids ids) as it ste steam am explo explosio sion n agains againstt po popla plarr hardw hardwoo oods ds (P. delto produced an 86.2% xylose yield with a nal ethanol concentration of 25.9 g/L. Although Although SO2 could be toxic to the environment and sulfur alone could pose potential harmful effects to some cellulolytic enzymes and distillation, a SO2 catalyst has been demonstrated to increase enzymes accessibility to the biomass owing to a more complete and rapid hemicellulose release [145,146] release [145,146].. Additionally, information is still lacking to con rm residual SO2 side effects once ethanol is used in motor vehicles. Moreover, Hu et al. (2008) [46] (2008) [46] reported reported that the acetic or uronic acid associated to autocatalysis effects from wood pretreatment could be a better alternative to sulfuric acid or SO2 catalysts. According to this study, despite optimal cellulases pH levels of 4.5e5, an impregnation of the biomass at room temperature with an appropriate dosage of acetic acid of 1 mM corresponding to a pH level of 3.9 is feasible. This acid impregnation followed by a pretreatment temper temperature ature at 200 C for 10 min would not require substantial toxic compound removal or adverse effects to cellulolytic enzymes. enzymes. Thus, acetic acid could be a potential alternative to dissociate the biomass. However, However, further furth er investigati investigations ons need to be performe performed d to validate these assumptions. AFEX has also been developed as another emerging economical pretreatment that limits inhibitor formation for agricultural residues such as corn stover [19,147,148] stover [19,147,148].. Moreover, extensive research continues to improve steam explosion with catalyst effectiveness against recalcitrant softwood materials. Zhu et al. [112] al. [112] developed developed a potential pretreatment SPORL to overcome the high recalcitrance of wo wood odyy biomas biomasss such such as softwo softwood od mat materi erial. al. This This appro approach ach produ pr oduced ced rea readil dilyy hydro hydrolyz lyzed ed sugars sugars and achiev achieved ed ex excel cellen lentt recovery of the hemicelluloses with minimal generation of inhibitors. Interestingly, 87.9% of the hexose and pentose sugars were recovered with the SPORL method when compared with overall saccharides recovered from dilute acid (56.7%) [133]. [133]. The short pretreatment time period associated with this approach permitted a low liquid-to liquid-to-woo -wood-r d-ratio atio leading leading to a grea greater ter pretreatm pretreatment ent
[53] [53]. . losion Moreover, SPORL tostbeand compleenergy ciency mentar men taryyef tociency ste steamam-exp explos ion whe when n using usingappears a cat cataly alyst thus thus improves its effectiveness against softwood biomass [133] biomass [133].. Different strategies including SHF, SSF as well as SSCombF have been extensiv extensively ely evaluated evaluated and subsequ subsequently ently implement implemented ed to
initiatee hy initiat hydr droly olysis sis of relea released sed sugar sugar polyme polymers. rs. The There re is som somee evidence that while these treatments have advantages there are disadvantages as well. Since optimal enzymatic hydrolysis is initiated at approxim approximately ately 50 C whi while le an op optim timal al fermen fermentat tation ion is enhanced at 35 C, the SHF operation appears to be more cost effect effective ive than than SSF [148] [148].. Howev However er,, the SSF pathw pathway ay has the advantage adva ntage of saving saving one step-costs step-costs in addition addition to its potentia potentiall to prevent cellulase inhibition by end-products such as glucose and cellobiose. From another perspective, SSCombF improves the SSF techniqu tech niquee by adding the co-f co-ferme ermentatio ntation n pro process cess as it allow allowss saccharicatio cation n along with simultaneo simultaneous us sugar sugar co-fermen co-fermentatio tations ns in a single reactor. 6.3. Cellulolytic/fermentativ Cellulolytic/fermentativee microbial eco ecology logy e identi cation cation of indigenous candidates
Alth Althoough extens tensiive rese resear arch ch ha hass bee been de devo vote ted d to lignocellulosic-based biofuel conversion conversion [147] [147],, less information has been provided on the microbial ecology and natural occurrence of viable microora in cellulosic biomaterial as well as its derived residues. Typically, an in-depth knowledge and understanding of the ecology of the indigenous candidates could yield potential microorg micr oorganism anismss usefu usefull for micr microbial obially-ba ly-based sed fermentatio fermentation n and cel cellul luloly olytic tic hy hydro drolys lysis is in biofue biofuell pro produ ducti ction. on. Ho Howev wever er,, mos mostt research rese arch efforts have focused on fores forestry try and agric agricultu ultural ral soil microbial characteristics reecting microbial diversity associated with these ecosystems, since there is a mutual and close relationship between the soil-microora and plant roots [150] roots [150].. Cellulosiccontai con tainin ningg soil soil con consis sists ts of a wid widee ran range ge of mic micro roorg organi anisms sms including bacteria, lament lamentous ous fungi and wild yeasts. Synergism among these microorg microorganism anismss is fund fundament amental al to the ecological ecological balance constituting the biomass ecosystem [151] ecosystem [151].. The nature of microorg micr oorganism anismss as well as the frequenc frequencyy and abund abundance ance vary depending on the ecological factors such as geographical location, climate, climat e, soil and viable forms. Bacterial Bacterial population populationss in norm normal al fertile agricultural soil can reach 10e100 million colony-forming units (CFU)/g [150] (CFU)/g [150].. Yeasts in soil can range from a few to greater than a 1000 cells per gram. In southwestern Slovakia, 111 yeast strains were isolated from 60 different agricultural soil samples. Among the wide range of collected strains 4 genera namely, Cryptococcus, Candida, Metschnikowia and Sporobolomyces were considered to be the most predominant [151] predominant [151].. This study revealed that the number of yeasts collected from agricultural soil was ten times tim es lower lower tha than n ye yeast astss iso isolat lated ed from from forestsoil forestsoil sin since ce less less fun fungic gicide ide and tillage were used in the nearby forest. Of the numero numerous us mic microo roorga rganis nisms ms collec collected ted from from biomas biomasss ecosy eco syste stems, ms, only only a fewstrai fewstrains ns have have pr prov oven en to be of int inter erest est for their their ethanologenic or cellulolytic abilities in bioethanol bioconversion. In northeastern Brazil, genera such as Candida, Pichia and Dekkera were isolated from sugarcane sugarcane molasses. molasses. Despite their over overall all fermentative ability, these genera yielded low ethanol concentracerevisia isiaee and produced tions tions in compar compariso ison n to S. cerev produced acetic acetic acid which was inhibitory to the fermentative yeast [152] [152].. However, some natural ethanologenic yeast species such as Pichia stipilis, Pachysolen tannophilius, Kluyveromyces Kluyveromyces marxianus and Candida shehatate appeared to have promise in replacing S. cerevisiae in [140].. Nev Neverthe ertheless, less, lignocellulosic-based ethanol fermentation [140] these wild yeasts still require further development to survive bioethanoll fermentatio ethano fermentation n cond conditions itions and yield an optimal optimal ethano ethanoll concentr conc entration. ation. The comp competiti etitive ve exclusio exclusion n as well as rep repress ression ion catabolism (competitive of isms hexose sugar tra transp nsport ort)) among amo ng these theseinhibition mic micro roor organ ganism s inand thepentose bioeth bioethano anolic lic ecosystem render addition of a selective agent to not be of particular value for improving yield performance [131] performance [131].. However, selective temperatures with thermophilic yeasts including K. marxianus
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or bacteria such as Clostridium cellulolyticum and Thermoanaerobacterium saccharolyti saccharolyticum cum may serve serve as alter alternat native ivess if the these se microo mic roorga rganis nisms ms are used used as the major major fermen fermentat tative ive and cel cellul luloly olytic tic agents agen ts at high temperat temperature ure operation operationss (approx (approximat imately ely 50 C) 156].. Furtherm Furthermore, ore, indigeno indigenous us groups groups of meso mesophilic philic and [153e156] thermophilic-ethanologenic bacteria such as Zymomonas mobilis and Bacillus stearothermo stearothermophilus philus have have prove proven n to be promi promisin singg candidates to convert sugars into ethanol [140] [140];; however, they remain decient as optimal ethanol producers in comparison with S. cerev cerevisiae isiae in ter terms ms of res resis istan tance ce to hig high h alc alcoho oholl co conce ncentr ntrati ation on and chemical inhibitors. While a selection of indigenous bacteria and yeasts that possess fermentative abilities is possible, fungi isolated from agricultural residues and forest woods also possess attractive lignocellulolytic properties for initiation of the pretreatment step. In 1976, almost 14,000 cellulolytic fungi were collected from plant cell walls [157] walls [157].. Only a few fungal isolates isolates were selected selected for addit additional ional research research and further categorized into three groups, namely white-, soft- and brown-rott fungi. Brown-rot brown-ro Brown-rot fungi primarily hydrolyze the cellulose polymer, while white- and soft-rot fungi are able to degrade most of the lignin, hemicellulose and cellulose. White rot fungi such as Basidomycetes (e.g. Phanerochaete chrysosporiu chrysosporium m RP78) are indigenous to the northern part of the world. P. chrysosporium is considere ered d amo among ng the mos mostt attra attracti ctive ve altern alternati ative ve fungi fungi for biomas biomasss processing due to their physico-chemical abilities to non-selectively break down lignin recalcitrant material from the cell wall while whi le lib libera eratin tingg cellul cellulose ose and hemice hemicellu llulos lose. e. These These fungi fungi are [158].. thermo-tolerant and can survive a temperature of 40 C [158] Chrysosporium is also known as a wood-decaying fungus for its unique oxidative system system and has been shown to be effective on the pre-treatment of cotton stalks [159]. [159]. Phlebia radiata, as well as Phlebia oridensis and Daedalea avida belong to Basidomycetes species and are capable of selectively degrading lignin in wheat Trichoderma viride, Trichostraws and cellulosic residues [160]. [160]. Trichoderma derma emersoni along with T. reesei ( Ascomyctes) and A. niger are are also attractive for their cellulolytic properties, tolerance to low pH and high temperature in addition to their ability to release largescale cellulase enzymes [158] enzymes [158].. T. viride grows rapidly at a wide pH range of 2.5e5.0 red reducing ucing potentia potentiall contaminat contamination ion from other microorganisms [129,162] microorganisms [129,162].. Mushrooms including Volvariella species also possess hydrolytic capabilities. They have been isolated mostly from rice straws in Asian or African countries. Lentinus edodes has also been used in Japan and China to digest lignied residue residues. s. Asid Asidee from from their ability to degra degrade de lig lignoc nocell ellulo ulosic sic biomat biomateri erial, al, some some whitewhite-ro rott fungi fungi belonging to the genus Pleurotus are able to convert waste into protein for human and animal consumption [163,164] consumption [163,164].. Clostridium thermocellum, an anaerobic thermophilic microorganism, is among the rare bacteria that possess cellulolytic properties in addition to its ability to ferment sugar polymers into ethanol [162] ethanol [162].. Several physiological attributes make this microorganism a promising candidate. It has a selective growth temperature of 50 C during the fermentation process and can convert cellulose polymer directly into ethanol yielding 0.3 g/g ethanol per converted cellulose at a high temperature of approximately 60 C [165,166]. [165,166]. C. thermocellum has been considered among the more promising promising thermophil thermophilic ic microorga microorganisms nisms suitable for SSF and CBP [141] CBP [141].. 6.4. Fermentation optimization e potential genetically modi ed ed
461
modied stain, designed on the basis of expressing the same gene for P. stipilis xylose reductase (Ps-XR) is not only capable of cofermenting saccharides but can also generate less HMF products (3 times less than the initial industrial strain) [169] strain) [169].. As mentioned previously, CBP is also a promising approach in combining both hydro hydrolys lysis is and fermen fermentat tation ion oper operati ations ons in one sin single gle ve vesse ssel.l. Additiona Add itionally lly,, CBP bioproce bioprocessing ssing enables enables genetically genetically-mod -modiied microorga micr oorganisms nisms that are able to produc producee cellu cellulase lase enzyme to ferment sugars in one step and thus prevent further investment in costly cellulolytic enzymes [141] enzymes [141].. Furthermore, Ladisch et al. [75] al. [75] have reported that CBP could be combined with the pretreatment operation to generate lignin that could be used as a boiler fuel and g. 1). provide suf cient energy to run the process (see Fi (see Fig. Howev However er,, fermen fermentat tative ive micro microor organ ganism ismss must must be thermo thermo-tolerant tole rant to surv survive ive the high temperatu temperatures res of SSF/ SSF/SSCo SSCombF/ mbF/CBP CBP processes.. These processes can also be accompanied by a biological processes treatment trea tment step that utilizes cellulolyt cellulolytic ic fungi which require require high te tempe mperat ratur uree and lowpH. Fu Furth rtherm ermor ore, e, Ku Kumaret maret al. [109] suggested examining thermophilic anaerobic bacteria and yeasts such as T. saccharolyticum, Thermoanaerobacter ethanolicus, C. thermocellum and K. marxianus IMB3 for their potential to utilize a wide range of feedstocks at high temperatures above 65 C. These thermophilic bacteria are able to ferment both hexose and pentose sugars in addition addit ion to their ability to produce produce cellulase cellulase enzymes enzymes and avoid avoid the addition of commercial enzymes. Kumar et al. [109] have also reporte rep orted d that Thermoanaerobacter BG1L1 BG1L1 had the potentia potentiall to ferment corn stove feedstocks at 70 C within an undetoxied bio biomas masss in a con contin tinuo uous us react reactor or sys syste tem. m. Thi Thiss thermo thermophi philic lic fermentation yielded 0.39e0.42 g/g (ethanol per sugar consumed) and nearly 89e98% xylose was utilized utilized despite the low tolerance to ethanol reported by Claassen et al. [124] al. [124].. Ethanol fermentation at high temperature continues to be an emerging technology as it allows selection for microorganisms by temperature and does not require cooling costs and cellulase addition [170] [170].. Recently, the thermo-tolerant yeast, K. marxianus has been documented as an att attrac ractiv tivee can candid didatedue atedue to its abi abilit lityy to co-fer co-fermen mentt both both hex hexoseand oseand pe pento ntose se sug sugars ars and sur surviv vivee hig high h inc incuba ubatio tion n tem tempe perat ratur ures es of [171].. Moreove Moreover, r, K. marxianus was geneticall geneticallyy modiedto 42e45 C [171] Aspergillus aculeatus cellulolytic exhibit T. reesei and Aspergillus cellulolytic activities allowing direct conversion of cellulosic b-glucan into ethanol at 48 C under under continuou continuouss conditions conditions,, yieldi yielding ng 0.4 0.477 g/g ethanol; ethanol; 92.2% from the theoretical yield and making it an ideal GMO for CBP [171].. processing [171] processing The ind indust ustria riall po poten tentia tiall for S. cerev cerevisiae isiae fermentatio fermentation n has already been proven for rst gener generation ation large large-scal -scalee bioet bioethanol hanol prod product uction ion.. The gen geneti eticc imp impro rovem vement ent of the conven conventio tional nal fermentati ferme ntative ve strain strain is gaining gaining increasing increasing research research interest interest since this strain strain is alread alreadyy the most most op optim timally ally adapt adapted ed to bioeth bioethano anoll fermentatio ferme ntation n condition conditions. s. To date, CBP for biofu biofuel el ferme fermentatio ntation n using genetically modied S. cerevisiae is an emerging technology tha thatt has bee been n dev develo eloped ped in sever several al stu studie diess [172e174] 174].. The These se studie studiess demonstrate that in addition to its co-fermentative genetic exibility, S. cerevisiae can also be genetically engineered to express cellulolyt cellu lolytic ic and hemic hemicellulo elluloytic ytic heterologo heterologous us enzym enzymes. es. van Zyl et al. [173] dem demons onstra tratedthis tedthis type type of mod modiicat cation ion of S. cerevi cerevisiae siae by reassembling all existing components of a minicellulosome on its membrane surface from the thermophilic microorganism C. cellulolyticum via heterologous expression expression of a chimeric protein scaffold underr phosphogl unde phosphoglycer ycerate ate kinas kinasee 1 (PGK 1) regulation regulation.. The successfu successfull functionality of cohesin and dockerin from C. cellulolyticum cellu
organisms (GMO)
Advances in genetic engineering have been made to alter the conventional yeast, S. cerevisiae s capability to ferment glucose and pentose sugars simultaneously [167,168] simultaneously [167,168].. A S. cerevisiae TMB3400 ’
S. cerevisiae proved that this genetic modi cation based losomein on a minicellulosome minicelluloso me model may may be an attractive option to the CBP process in hydrolyzing and fermenting substrates in a single step. Unlike T. reesei, recombinant S. cerevisiae is not able to simultaneously neou sly control control cellu cellulolyti lolyticc enzym enzymee exp express ression ion to effec effectivel tivelyy
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reported the effectiveness hydrolyze cellulose. Yamada et al. [175] al. [175] reported of a cocktail d -integration approach approach that consists of the insertion of a hig high h cel cellul lulase ase act activi ivitie tiess bas based ed cas casset sette te int intoo the yeast yeast chr chromo omosom somee to optimize its cellulase expression ratio. Z. mobilis is also among the more attract attractive ive ethalonogenic ethalonogenic bacteria candidates due to its high ethanol yield production and resistance to temperatures in the range of 40 C (2.5 fold higher than S. cerevisiae) [176] [176].. Numer Numerou ouss ge genes nes have have been been int introd roduc uced ed and heterologous expression has been incorporated into Z. mobilis to extend extend its effective effectiveness ness toward toward other other substrate substratess namel namely, y, xylose xylose and arabinose since this strain is only able to ferment glucose [177] glucose [177].. Furthermore, the insertion of bb -glucosidase gene into Z. mobilis to [176,178,179].. also convert cellobiose can be used in the SSF process process [176,178,179] Currently Curr ently,, commerci commercial al companies companies (DuP (DuPont ont Danisco Danisco Cellulosic Cellulosic Ethanol (DDCE) and Butalco) have assayed genetically engineered Z. mobilis and S. cerevisiae potential for their high ethanol yield performance and adaptability [180] adaptability [180].. Enhancing large-scale low-cost ethanol bioprocessing by biological pretreatment involving fungi (e.g. T. reesei and a Basidiomyctes) that exhibit lignocellulolytic properties at low pH levels and high temperatures is also a promising added-value treatment to SSF ethanol bioconversion. While fungi bioconversion activities have been demonstrated to be slow, optimization of potential lignocellulolytic fungi has been demonstrated possible via mutagenesis, heterologous gene expression and co-culturing [181] co-culturing [181].. Although some of the emerging strategies and methods have proven to be promising under different circumstances, some of these technologies remain biomaterial-type and cost dependent. For example, Talebnia et al. [143] al. [143] have have concluded that the most suitable pretreatment for wheat straw material was steam explosion since it required a shorter reaction time, lower chemicals and high solid solubilization. However, this study also demonstrated that steam explosion operation exhibited a high level of inuence on the downstream operations and its success depended on the fra framew mework ork of the ent entire ire process. process. Thus far far,, Binod Binod et al. [182] hypothesized that an environmentally friendly biological conversion approach using thermo-tolerant stains such as Clostridium phytofermentumss and Basidomycetes in SSF/CBP processings would phytofermentum be the future method of choice for rice straw feedstock if slow bioconversion is to be overcome. Furthermore, Lau and Dale [183] Dale [183] have have demonstrated the effectiveness of AFEX against corn stover feedstock via SSF process, using the 424 A (LN-ST) strain of S. S. cerevisiae, designed by Ho et al. [168].. This pretreatmen pretreatmentt achie achieved ved an ethano ethanoll concentr concentration ation of [168] 40. 40.00 g/L (5.1 vol vol/v /vol% ol%)) withou withoutt adding adding nutrie nutrients nts or requir requiring ing washing and detoxication steps. The Consortium for Applied and Innovation [173] Innovation [173] team team selected by the Department of Energy (DOE) of ce of the Biomass program has demonstrated a higher recalcitrance of poplar wood in comparison with corn stover. Optimal performance was achieved by a more severe treatment involving mainly mai nly SO2 steam steam ex explo plosio sion n or lime lime associ associate ated d with with the co co-fermenting yeast strain 424 A (LN-ST) of S. cerevisiae. However, a large portion of these studies focused more on sugar yield with minimal attention given to mass balance and energy estimates crucial cruc ial for a complete complete evaluation evaluation of pretreat pretreatment ment ef ciency. Zhu and Pan [53] Pan [53] conducted conducted an in depth study on the impact of the energyy consump energ consumption tion from woody feedstock feedstock on estimating estimating the effectiven effec tiveness ess of potential potential pretreat pretreatments ments.. They establishe established d the benchm ben chmar arkk based based pr prima imaril rilyy on the energy energy consum consumpti ption on for comparing the performance of the more attractive lignocellulosic
co-product potential from softwood. Zhu et al. [89] [89] con conrmed the effectiveness of SPORL pretreatment prior to a disc-milling operation on Lodgepole pine softwood in terms of pretreatment energy ef ciency of 0.26 kg of sugar/MJ, an ethanol yield of 276 L/ton softwood (using thermo-tolerant, S. cerevisiae D5A), and an energy output of 4.55 GJ/ton wood correlated to the mass balance. Recent studies stud ies published published by Tian et al. [184] identi [184] identied the benets from SPORL technique over dilute acid (DA) pretreatment used for the least resistant woody biomass, aspen (Populus tremuloides). This study study revea revealed led that that SPO SPORL RL pr pretr etreat eatmen mentt exhib exhibite ited d a hig higher her substrate enzymatic digestibility (SED) than DA and was favorable to th thee hi high gh etha ethano noll yi yiel eld d SSF SSF pr proc oces ess. s. Ti Tian an et al al.. [184] also concluded that SPORL pretreatment with 10% higher sugar and bioethanol yield as well as a higher ethanol and sugar production energy ef ciency 395 kg/GJ over 339 kg/GJ for DA, remained one of the most most att attrac ractiv tivee altern alternati ative vess for lowand hig high h recalc recalcitr itrant ant wo wood odyy mater mat erial. ial. Olofss Olofsson on et al. [131] used used raw raw spr spruce uce mat materi erial al to demonstrate the importance of adopting a controlled feeding of cellulase enzymes to prevent the competitive inhibition of sugars transport trans port (glucose (glucose over xylose). This stud studyy demonstr demonstrated ated that controlled contr olled-cell -cellulase ulaseaddit addition ion increased increased the total total xylos xylosee uptake uptake from 40 to 80%. Overall, sustained efforts are still required to improve bioconversion technology toward reaching the best performance possible to deal with lignocellulosic feedstock variability. Improvement in each of these prospects represents individual stepss toward step toward implementi implementing ng successf successful ul cost-effec cost-effective tive lignocellulignocellulosiclosic-bas based ed bioeth bioethano anoll op opera eratio tions. ns. Ho Howev wever er,, to acc accom ompli plish sh substantial improvement will require more of a comprehensive systems approach that simultaneously accounts for all inputs and outputs during the entire operation regardless of changes in any of these individual steps.
biomass SPORL, organosolv steam explosionpretreatments with catalyst. including, They demonstrated that SPORLand pretreatment overall was the most advantageous and commercially scalable able to su sugar gar re recov covery ery along along with with to total tal energy energy co consu nsump mptio tion n (physical and thermo-chemical) in addition to the returned lignin
food in today s global market still early in assessing development.products For biofermenters, MRA would beare a useful tool in the exposure risk of using antibiotics to control large-scale microbial contamination by evaluating major st steps eps from the plant source source to the dis distill tillati ation on nal pr proce ocess ss for po poten tential tial ge gener nerati ation on and
6.5. Microbial risk a assessment ssessment (MRA) mod modeling eling 6.5.1.. Concepts 6.5.1
The use of GMOs presents another challenge to the bioethanol industry. Introduction of such organisms into large-scale fermentat tation ion op oper erati ations ons ope opens ns up the po possi ssibili bility ty of enviro environme nmenta ntall dissemination and potential exposure risks to public health. Likewise, industrial operations using antibiotics to control microbial contaminants in industrial scale fermenters or as strain markers would woul d generate generate and rele release ase antibio antibiotic tic resistant resistant organisms organisms and offer another anoth er pot potentia entiall envir environme onmental ntal public public health risk [35,185] [35,185].MRAis .MRAis a comprehensive approach that can provide guidance for reducing potential microbial microbial public health exposure by estimating the risk risk of microbial dissemination over all steps in a microbial-based process process such as bioethanol formation. MRA is an emerging systematic and science-based method generally used to provide a qualitative and quantitative evaluation of the probability of occurrence of adverse health effects originating from microbial hazard contamination in food products [186] products [186].. It is based on four major steps namely, hazard identicatio cation, n, hazar hazard d chara characteri cterizatio zation n (resp (response onseedose asse assessssment) followed by exposure assessment and risk characterization [186] [186].. Currently MRA is the primary science-based tool of Codex on which the World Trade Organization (WTO) uses Alimentarius on Alimentarius to describe food safety and risk estimation of food products [187] products [187].. 6.5.2. Application of risk assessment in large-scale large-scale fermentation systems
Applications using MRA to certify the safety and equivalence of ’
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GMOs: Genet Genetically ically modied organisms; CTs: Contaminants including antibiotic Fig. 3. Hypothet Hypothetical ical MRA Model of Biofuel Biofuel Sourc Source-to e-to-dist -distillati illation on Syste System m (F (FAO, AO, 2005) 2005) [190]. [190]. GMOs: resistance organisms.
dissemination of antibiotic resistant organisms [188] organisms [188].. Fig. 33 illus illustrates a hypothetical model system of MRA for biomass processing based bas ed on the method methodolo ology gy ado adopt pted ed by Fo Food od and Agricu Agricultu lture re
suppressing suppres sing micr microbial obial diss disseminat emination ion via selec selective tive cost cost-effe -effective ctive control measures that does not cause damage to the ecosystem is of primary concern [185] concern [185]..
[189] of the United this representation, Organization [189] Organization the MRA MRA co conce ncept pt ofwas app applie lied dNations. to theIn lig lignoc nocell ellulo ulosic sic-ba -based sed biofuel biofu el operation operation from harvest-to-d harvest-to-distil istillation lation in an attempt attempt to design des ign a model model descri describin bingg tra transp nspare arentl ntlyy dynami dynamicc microb microbial ial contamination. Detecting microbial problems at an early stage and
Rapid agricultural inre, the 198 980s 0s has hadevelopment s led to th thee of em emer erge genc ncee ofbiotechnology GM GMOs Os.. Ther Therefo efore , itearly has has increased incre ased public concern concern on their potentia potentiall hazard hazardss including including pathogenic microbial mutations and the long-term proliferation of harmfu har mfull genes genes in the env enviro ironme nment nt that that cou could ld have have a seriou seriouss
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consequence on public health and the respective environments [190]. The awareness of the possible impact that could originate [190]. from large scale GMO applications has encouraged work primarily fro from m the To Toxic xic Substa Substance ncess Contro Controll Act (TSCA (TSCA)) on a pragma pragmatic tic science-based methods such as MRA combined to biotechnology risk assessment (BRA) to predict the probability of occurrence of adverse outcomes in the environment from large scale GMOs based applications[191] applications[191].. Thus, greater control could be performed to improve improve public health and ensure ensure comprehen comprehensive sive environmental safety.
[2] EISA. Energy independence and security act. Federal and State incentives and Laws, http://www.afdc.energy.gov/afdc/laws/eisa Laws, http://www.afdc.energy.gov/afdc/laws/eisa;; 2007. [3] Sheehan J, Cambreco Cambreco V, Duf eld J, Garbo Garboski ski M, Shapouri H. An overview of biodiesel and petroleum diesel life cycles. A report by US Department of Agriculture and Energy 1998; 1e35. [4] Caledria K, Jain AK, Hoffert MI. Climate Climate sensitivity uncertainty and the need for energy without CO2 emission. Science 2003;299:2052 e4. [5] Demain AL, Newcomb M, Wu JHD. Cellulase, clostridia and ethanol. M Microbiol icrobiol Mol Biol Rev 2005;69:124e54. [6] Hill J, Nelson E, Tilman D, Polasky S, Tiffany D. Environmental, economic economic and energetic costs and bene ts of biodiesel and ethanol biofuels. PNAS 2006; 103(30):11206e10. [7] Ragauskas Ragauskas AJ, William Williamss CK, Davison Davison BH, Britovsek Britovsek G, Cairney J, Ecker Eckertt CA,
Cellulosic-based biofuel is a potential alternative over foodderived derive d bioethan bioethanol ol ori origina ginating ting mai mainly nly from from cor cornst nstarc arch h and sugarcane provided by the world s large producers U.S. and Brazil, respectively. Pretreatment, the most costly step is of particular concer con cern n due to the hig high h rec recalc alcitr itranc ancee of lignoc lignocellu ellulos losic ic raw materi mat erials. als. Given Given that lign lignoce ocellul llulosi osicc feedst feedstoc ockk is a versat versatile ile material and bioethanol is a commodity product, it has been deemed imperative to design a general pretreatment combination that would be effective against a wide range of cellulosic material and hence deal with feedstock variability. For instance, researchers have shown that pretreatments involving steam explosion with either catalyst or lime are potential candidates to agricultural residues, herbaceous materials and hardwoods. The inability of
et al. The path forward for biofuels and biomaterials. Science 2006;311: 484e9. [8] Current state and prospects. Appl Microbiol Microbiol Biotechnol 2006;69:627e42. [9] RFA. US fuel ethanol ethanol indus industry try bioreneries and capacity. Washington, DC: Renewable Fuels Association, http://www.ethanolrfa.o http://www.ethanolrfa.org/industr rg/industry/locations/ y/locations/;; 2010. [10] Goldemberg Goldemberg J. Ethanol for a sustai sustainable nable energy future. Scienc Sciencee 2007;3 2007;315: 15: 808e10. [11] Wheals Wheals AE, Basso LC, Alves DMG, Amorim Amorim HV. Fuel ethanol after after 25 years. Trends Biotechnol 1999;17:482e7. [12] Gnansounou Gnansounou E. Production Production and use of lignocellu lignocellulosic losic bioetha bioethanol nol in Europe Europe:: current situation and perspectives. Bioresour Technol 2010;101:4842 e50. [13] EU. Directive on the promotion of the use of energy from renewable sources. Of cial J Eur Union; June, 2009. [14] Swart JAA, Jiang Jiang J, Ho P. Risk perceptions perceptions and GM crops: the case of China. Tailoring Biotechnol Soc Sci Technol 2008;33:11e28. [15 [15]] Licht FO. World World fuel fuel ethanol ethanol pro produc ductio tion, n, <http://www.ethanolrfa.org/ industry/statistics/>; 2008. [16] DOE. Biomass multi-year multi-year program program plan. Of ce of the Biomass Program. US DOE. At website, http://www1.eere.energy.gov/biomass/pdfs/algal_biofuels_ website, http://www1.eere.energy.gov/biomass/pdfs/algal_biofuels_ roadmap.pdf ; 2009. [17 [17]] Of cia ciall Neb Nebras raska ka governm government ent web websit site. e. Was Washin hington gton,, DC/Lin DC/Lincol coln, n, NE:
steam explosion explosi combined combined with cataly st to degrade degr adethesoftwood softwo od materials can on be compensated by catalyst the low-cost and energy ef cient SPORL SPORL pretreatm pretreatment ent approach. approach. Emerging Emerging technologies technologies including SSCombF and CBP represent potential improvements as they reduce operation steps as well as chemical inhibitors and can be enhanced by lignin, energy-self-sustaining co-products. These processes are typically associated with thermophilic and cellulolytic microorganisms including organsisms such as T. reesei along with P. P. chrysosporium, K. marxianus marxianus and C. cellulo cellulolyticu lyticum m with some of them possessing fermentative abilities in addition to their hydrolytic properties. However, some companies such as DDCE (DuPont (DuPo nt Danisco Danisco Cellulosic Cellulosic Ethanol) and Butalco Butalco prefer prefer using genetically genet ically engineered engineered convention conventional al strains, strains, S. cerevisiae cerevisiae and ethanologenic Z. mobil mobilis is for their higher alcohol tolerance and yield. In conjunction to rapid molecular biology techniques, mathematical modeling including MRA and biotechnology risk assess-
Renewable Fuels Association/Nebraska Energy Of ce, <http://www.neo.ne. gov/statshtml/121_200912.htm >; 2009. [18 [18]] Bell Bell JL, Atteld PV. Break Breakthroug through h in yeast for making bio-ethanol bio-ethanol from ligncellul lignc ellulosics. osics. Sydney, NSW21 NSW2109, 09, Australia: Australia: Microbioge Microbiogen n Pty LTD, Macquarie University Campus; 2006. [19] Sun Y, Cheng J. Hydrol Hydrolysis ysis of lignocellul lignocellulosic osic materia materials ls for ethanol producproduction: a review. Bioresour Technol 2002;83:1e11. [20] Taherzadeh, MJ. Ethanol from lignocellulose: physiological effects of inhibitors and fermentation strategies. Chemical reaction engineering. Chalmers University of Technology. Göteborg, Sweden. 1999. Doctoral thesis Nr. 1247. [21] Food & Watch. The rush to ethanol: not all biofuels are created equal; 2007. Washington, DC. [22] Demirbas Demirbas A. Energy and environmental environmental issues relating to greenh greenhouse ouse gas emissions in Turkey. Energy Convers Manage 2003;44:201 e13. [23] Cardona CA, Sanchez OJ. Fuel ethanol production: process design trends and integration opportunities. Bioresour Technol 2007;98:2415 e57. [24] Kadam KL, McMillan JD. Availa Availability bility of corn stover as a sustainable feedstock for bioethanol production. Bioresour Technol 2003;88:17e25. [25] Knauf M, Moniruzzam Moniruzzaman an M. Lignocell Lignocellulosic ulosic biomass processing. processing. Persp Int Sugar J 2004;106:147e50. bioethanol production production from wasted crops [26] Kim S, Dale BE. Global potential bioethanol and crops residues. Biomass Bioenerg 2005;29:361 e75.
ment (BRA) (BRA) can be used to ensure ensure greater greater pre predicta dictability bility for limiting limiting antibiotic antibio tic resi resistant stant micro microora and GMO dissem dissemina inatio tion n during during operation oper ation.. While technologi technological cal accomplis accomplishment hmentss and multiple multiple research coalition efforts are still progressing, an ef cient combination of the most advanced systems analysis and economical tec techni hniqu ques es de desig signed ned to cope cope with with feedst feedstoc ockk versat versatilit ilityy and commodity should emerge as the option of choice in an attempt to achieve optimal second-generation biofuel performance.
[27] hemicelluloses Cheng KK, Cai BY, Zhang JA,for Ling HZ, Zhou YJ, Ge JP,byetacid al. Sugarcane Su garcane process. bagasse hydrolysate ethanol production recovery Biochem Eng J 2008;38:105 e9. [28] Himmel ME, Ding SY, Johnson DK, Adney WS, Nimlos MR, Brady JW, et al. Bio Biomas masss recalc recalcitr itranc ance: e: engineer engineering ing plants plants and enzyme enzymess for bio biofue fuels ls production. Science 2007;315:804e9. [29] Lachke Lachke A. Biofuel from D-xylose e the second most abundant sugar. Resonance 2002;5:50e6. [30] Ho NWY, Chang SF. Cl Cloning oning of yeast xylulokinase gene by complementation complementation of E. E. coli and yeast mutations. Enzyme Microbiol Technol 1989;11:417 e21. [31] Cao NJ, Krishnan MS, Du JX, Gong CS, Ho NWY. Ethanol production production from corn cob pretreated by the ammonia steeping process using genetically engineered yeast. Biotechnol Lett 1996;118:1013 e8. [32] Jeffries TW. Emergi Emerging ng technology for fermenting D -xylose. Trends Biotechnol 1985;3:208e12. [33] Weber C, Boles E. Sugar-hungry Sugar-hungry yeast to boost biofue biofuell productio production. n. Science News. Science Daily 2010;92:881e2. [34 [34]] Bayroc Bayrockk D, Ingled Ingledew ew WM. Chang Changes es in steady steady sta state te on introd introduct uction ion of a Lactobacillus contaminant to a continuous culture ethanol fermentation. J Ind Microbiol Biotechnol 2001;27:39e45. [35] Muthaiyan Muthaiyan A, Limay Limayem em A, Ricke SC. Antimicrobi Antimicrobial al strategies for limit limiting ing bac bacter terial ial contam contamina inants nts in fuel fuel bio bioetha ethanol nol fermen fermentat tation. ion. Pro Progg Energy Energy
7. Concl Conclusions usions e future prospects
’
Acknowledgments Acknowled gments
This review was partially supported by grants from the South Central Sun Grant (U.S. Department of Transportation) program, Novozyme North America, Inc., Franklinton, NC, and the Institute of Food Science and Engineering, University of Arkansas, Fayetteville, AR. References [1] PEA. Policy energy energy act. Public Law, <http://www.gpo.gov/fdsys/pkg/PLAW109publ58/content-detail.html>; 2005.
2011;37:351 e70. [36] Combust Larsson S,Sci Palmqvist E, Hahn-Hagerdal B, Tengborg C, Stenberg K, Zacchi G, et al. The generation of fermentation inhibitors during dilute acid hydrolysis of softwood. Enzyme Microbiol Technol 1999;24:151e9. [37] Ximenes Ximenes E, Kim Y, Mosier N, Dien B, Ladisch Ladisch M. Inhibition Inhibition of cellu cellulases lases by phenols. Enzyme Microbiol Technol 2010;46:170e6.
A. Limayem, S.C. Ricke / Progress in Energy and Combustion Science 38 (2012) 449 449e e467
465
[38] Kim Y, Ximenes E, Mosier NS, Ladisch MR. Soluble inhibitors/deactivators of cellulase enzyme from lignocellulosic biomass. Enzyme Microbiol Technol 2011;48:408e15. [39] Ximenes E, Kim Y, Mosier N, Dien B, Ladisch M. Deactivation of cellulases by phenols. Enzyme Microbiol Technol 2011;48:54 e60. [40] Bernton H, Kovarik B, Skla Sklarr S. The forbidden fuel: Power alc alcohol ohol in the 20th century (New Haven, CT: W.B. Grif n, 1982). 1982, p274. Bibl. Index 8185112. 19.95 ISBN 0-941726-00-2. [41] Kovar Kovarik ik B, Kettering CF. Fuel alcohol alcohol:: Energy and environment in a hungry world.. London: International world International Institute for Envir Environment onment and Devel Development, opment, 1982. The Development Development of Tetraethyl Lead in the Context of Technologic Technological al Alternatives, Society of Automotive Engineers, Fuels & Lubricants Division, Historical Colloquium, Baltimore, MD 1994.
[70] Saha BD. Hemicellulose bioconversion. J Ind Microbiol Biotechnol 2003;30: 279e91. [71] Aspin Aspinall all GO. Chemistry of cell wall polysacchar polysaccharides. ides. In: Preiss J, editor. editor. The biochemistry of plants (a comprehensive treatise). Carbohydrates structure and function, vol. 3. NY: Academic Press; 1980. p. 473e500. [72] Ebringerova A, Hromadkova Z, Heinze T. Hemicellulose Adv Polym Sci 2005; 186:1e67. [73] Deguchi S, Mukai SA, Tsudome M, M, Horikoshi K. Facile generation of fullerene nanoparticles by hand-grinding. Adv Mater 2006;18:729 e32. [74] Foody BE, Foody KJ. Development Development of an integr integrated ated system for produ producing cing ethanol from biomass. In: Klass DL, editor. Energy from biomass and waste. Chicago: Institute of Gas Technology; 1991. p. 1225 e43. [75] Ladisch MR, Mosier NS, NS, Kim Y, Ximenes E, Hogsett D. Converting cellu cellulose lose to
[42 [42]] RFA. Ethanol Ethanol indust industry ry outloo outlook: k: cli climat matee of opport opportuni unity, ty, http://www. ethanolrfa.org/page/-/objects/pdf/outlook/RFAoutlook2010_n.pdf? nocdn¼1; 2010. [43] Perlack RD, Wright L, Turhollow LA, Graham RL, Stokes B, Erbach DC. Biomass as feedstock for a bioenergy bioenergy and biopr bioproducts oducts industr industry: y: the technical feasibility of a billion-ton annual supply. Oak Ridge National Laboratory Report ORNL/TM-2005/66. Oak Ridge, TN: US Dept. of Energy; 2005. [44] Searc Searchinger hinger T, Heimli Heimlich ch R, Houghto Houghton n RA, Dong F, Elobeid A, Fabiosa J, et al. Use of U.S. croplands croplands for biofuels increas increases es greenhouse greenhouse gases through emissions from land-use change. Science 2008;319:1238 e40. [45] WorldFact Book. C Central entral Intelligence Agency, < https://www.cia.gov/library/ publications/the-world-factbook/geos/us.html>; 2011. [46 [46]] Hu G, Hei Heitma tmann nn JA, Rojas Rojas OJ. Fee Feedst dstock ock pre pretre treatm atments ents str strate ategie giess for producing ethanol from wood, bark, and forest residues. BioResources 2008; 3:270e94. [47] Cardo Cardona na CA, Quintero JA, Paz IC. Production Production of bioeth bioethanol anol from sugarca sugarcane ne bagasse: status and perspectives. Bioresour Technol 2009;101:4754 e66. [48] Hoadley RB. Understanding wood: a craftsman s guide to wood technolog technology. y. 2nd ed. Newtown, CT: Taunton Press; 2000. [49 [49]] Boone Boone RS, Kozlik Kozlik CJ, Bois CJPJ, CJPJ, Wen Wenger gertt PPJEM. PPJEM. Dry kiln schedu schedules les for commercial woods temperate and tropical. Gen. Tech. Rep. FPL_GTR_57. Madison, WI: U.S. Department of Agriculture, Forest Service, Forest Products
biofuels. SBE special supplement biofuels. CEP 2010;106(3):56e63. [76] Chen F, Srinivasa RMS, Temple S, Jackson L, Shadle G, Dixon RA. Multi-site Multi-site geneticc modulation geneti modulation of monolignol monolignol biosynthesis biosynthesis suggest suggestss new route routess for formation of of syringyl lignin and wall-bound ferulic acid in alfalfa ( Medicago sativa L.). Plant J 2006;48:113e24. [77] Li X, Weng JK, Chappl Chapplee C. Impro Improvement vement of biomass through lignin modication. Plant J 2008;54:569e81. [78] Demirbas A. Progress and recent recent trends in biofuels. Prog Energy Combust Sci 2007;33:1e18. [79 [79]] Chandel Chandel AK, Chan Chan E, Rudrav Rudravara aram m R, Nar Narasu asu ML, Rao LV, Ravind Ravindra ra P. Economics and environmental impact of bioethanol production technolo an appraisal. Biotechnol Mol Biol Rev 2007;2:14 e32. gies: an gies: [80] Gamage J, Howard L, Zisheng Z. Bioethanol production from lignocellulosic biomass. J Biobased Mater Bioenerg 2010;4:3 e11. [81] Mu D, Seager T, Suresh Rao P, Zhao F. Compar Comparative ative life cycl cyclee assessment of lignocellul lignoc ellulosic osic ethanol produ production: ction: biochemical biochemical versus versus thermochemica thermochemicall conversion. Environ Manage 2010;46:565e78. [82] Yang B, Wyman CE. The key to unlocking low-cost low-cost cellulosic ethanol. Biofuels Bioprod Bioref 2008;2:26e40. [83] Zhu JY, Wang GS, Pan XJ, Gleisner R. Specic surface to evaluate the ef ciencies of milling and pretreatment of wood for enzymatic sacchari cation. Chem Eng Sci 2009;64:474e85.
Laboratory; 1988. [50] Markwardt LJ, Wil Wilson son TRC. Strength and related properties of woods grown in the United States. Tech. Bull. 479. Washington, DC: U.S. Department of Agriculture, Forest Service. U.S. Government Printing Of ce; 1935. [51 [51]] Am Ameri erican can Har Hardwo dwood od Exp Export ort Counci Councill (AH (AHEC). EC). <http://www.ahec.org/ hardwoods/species.html>. 2002. [52 [52]] Ken Kenned nedyy JHE. Cottonw Cottonwood, ood, an Americ American an wood. wood. Was Washing hington, ton, DC: U.S U.S.. Department of Agriculture, Forest Service; 1985. [53] Zhu JY, Pan HJ. Woody biomass treatment for ccellulosic ellulosic ethanol production: technology and energy consumption evaluation. Bioresour Technol 2010; 101:4992e5002. [54] U.S. Depart Department ment of Energy Biomass Program. Program. <http://www1.eere.energy. gov/biomass/pdfs/biomass_deep_dive_pir.pdf >. 2009. [55] Demir Demirbas bas A. Bioeth Bioethanol anol from cellulosic cellulosic materials: materials: a renewable motor fuel from biomass. Energy Source 2005;27:327 e37. [56] Lugar RG, Woolsey RJ. The new petroleum. For Foreign eign Affairs 1999;1:88e102. [57] Khanna M. A billion tons of biomass a viable goal but at high price. Am J Agric Econ, <http://news.illinois.edu/news/11/0216biomass_MadhuKhanna. html>; 2011. [58] Green Car Congre Congress. ss. Study concludes concludes U.S. will not meet renewa renewable ble fuel targets targe ts with ethanol as prim primary ary biofuel, <http://www.greencarcongress. com/2011/01/tyner-20100108.html>; 2011.
[84] Sánchez ÓJ, Card Cardona ona CA. Trends Trends in biotechnolo biotechnological gical product production ion of fuel ethanol from different feedstocks. Bioresour Technol 2008;99:5270 e95. [85] Spata Spatari ri S, Bagley DM, MacLean MacLean HL. Life cycle evaluation evaluation of emergi emerging ng lignocellulosic cellu losic ethanol conver conversion sion technologies. technologies. Bioresour Technol 2010;101: 654e67. [86] Fehrenbacher K. Logen suspends U.S. U.S. cellulosic ethanol plant plans [accessed Oct ctob ober er 22], 2], <http:earth2tech.com/2008/06/04/iogen-suspends-uscellulosic-ethanol-plant-plans-/>; 2009. [87 [87]] Wyman CE. What What is (and is not) vital vital to hydrol hydrolysa ysate te to ethanol. ethanol. In: Himmel ME, Baker JO, Overend RP, editors. Enzymatic conversion of biomass for fuels produ production. ction. ACS symposium series, vol. 566. Washington, Washington, DC: American Chemical Society; 1994. p. 411 e37. [88] Zheng Y, Pan Z, Zhang R. Overview Overview of bioma biomass ss pretreat pretreatment ment for cellulosi cellulosicc ethanol production. Int J Agric Biol Eng 2009;2:51 e68. [89] Zhu JY, Zhu W, OBryan P, Dien BS, Tian S, Gleisner Gleisner R, et al. Ethanol production from SPORL-pretreat SPORL-pretreated ed lodge pole pine: Prelimina Preliminary ry evalu evaluation ation of mass balance and process energy ef ciency. Appl Microbiol Biotechnol 2010;86: 1355e65. [90] Zhu JY, Xuejun P, Ronald S, Zalesny Jr . Pretreatme Pretreatment nt of woody biomass biomass for biofuel production: energy ef ciency, technologies, and recalcitrance. Appl Microbiol Biotechnol 2010;87:847e57. [91] Hsu TA, Ladisch MR, Tsao GT. Alcohol from cellulose. cellulose. Chem Technol 1980;
[59] goa U.S.lsDepar Department tment of Energy. USDA regional roadm meeting meeting the biofuel biofuel goals of the ren renewa ewable ble fuels sta standa ndard rd byroadmap 2022, 2022,ap <tohttp://www.usda.gov/ documents/USDA_Biofuels_Report_6232010.pdf >; 2010. [60 [60]] Emerg Emerging ing mar market kets, s, ALGAE ALGAE 2020, 2020, <http://www.emerging-markets.com/ algae/>; 2011. [61] Rodol L, Zitelli GC, Bassi N, Padovani G, Biondi N, Bionini G, et al. Microalgae for oil: strai strain n selection, selection, inducti induction on of lipi lipid d synthesis synthesis and outdoor outdoor mass cultivation culti vation in a low-c low-cost ost photo-biorea photo-bioreactor. ctor. Biotechnol Bioeng 2009;102: 100e12. [62] Ferre Ferrell J, Sarisky-Reed Sarisky-Reed V. Nation National al algal biofuels biofuels technology roadmap. roadmap. US Department of Energy (DOE). Of ce of Energy and Renewable Energy; 2010. Of ce of the Biomass Program. [63] Harel A. Noritech Noritech seaweed biotechnolog biotechnologyy Inc. Algae Worl World d Confer Conference. ence. Rotterdam, NL. 2009. [64] Pettersen RC. The chemical composition of wood. In: Rowell RM, editor. The chemistry of solid wood. Advances in chemistry series, vol. 207. Washington, DC: American Chemical Society; 1984. p. 115 e6. [65] Badger PC. In: Jannick J, Whipsekey A, editors. Trends in new crops and new uses. Alexandria, VA: ASHS Press; 2000. p. 17e21. [66] Mielenz JR. Ethanol production from biomass: technology and commercialization status. Curr Opin Microbiol 2001;4:324e5. [67] Girio FM, Fonseca C, Carvalhe Carvalheiro iro F, Duar Duarte te LC, Marques Marques S, BogelBogel-Luka Lukasic sic R.
10(5):315 e9. [92] Laser M, Schulman D, Allen SG, Lichwa J, Antal MJ, Lynd LR. A comparison of liquid liquid hot water water and steam pretre pretreatm atment entss of sug sugar ar cane cane bagass bagassee for bioconversion to ethanol. Bioresour Technol 2002;81:33e44. [93] Wyman CE, Dale BE, Elander RT, Holtza Holtzapple pple M, Ladisch MR, Lee YY. Coordinated development of leading biomass pretreatment technologies. Bioresour Technol 2005;96:1959e66. [94 [94]] Baner Banerjee jee S, Mud Mudlia liarr S, Sen R, Bal Balendu endu BG, Chakra Chakrabar barti ti T, Pandy Pandy RA. Commerciali Commer cializing zing lignoc lignocellul ellulosic osic bioethanol: bioethanol: technology technology bottle bottlenecks necks and possible remedies. Biofuel Bioprod Biorg 2009;4:77 e93. [95] Lynd LR, Weimer PJ, van Zyl WH, Pretorius IS. Microbial Microbial cellulose utiliza utiliza-tion: fundamentals and biotechnology. Microbiol Mol Biol Rev 2002;66: 506e77. [96] Mosier N, Wyman C, Dale BE, Elander R, Lee YY, Holtzapple M, et al. Features of promising technologies for pretreatment of lignocellulosic biomass. Bioresour Technol 2005;96:673e86. [97] Klink Klinkee HB, Thomsen AB, Ahring BK. Inhibition Inhibition of ethanol ethanol-pro -producin ducingg yeast and bacteria by degradation products produced during pre-treatment of biomass. Appl Microbiol Biotechnol 2004;66:10e26. [98] Olsson L, Hahn-Hägerdal B. Fermentation of lignocellulosic hydrolysates for ethanol production. Enzyme Microbiol Technol 1996;18:312 e31. [99 [99]] Mok WSL, WSL, Antal Antal MJ. Uncatal Uncatalyzed yzed solvolysis solvolysis of whole whole biom biomass ass hemihemi-
Hemicellulose Bioresour Technol 2010;101:4775 e800. [68] Edye LA, Doher Doherty ty WOS. Fractionation Fracti onation of a lignocellulosic lignocellu losic material. material. PCT Int Appl 2008;25. [69] McMillan McMillan JD. Pretr Pretreatment eatment of lignoc lignocellul ellulosic osic biomass. In: Himmel ME, Baker JO, Overend RP, editors. Enzymatic conversion of biomass for fuel production. Washington, D.C: American Chemical Society; 1993. p. 292e323.
cellulose 1157e61. by hot compressed liquid water. Ind Eng Chem Res 1992;31: [100] Kohlm Kohlmann ann KL, Westgate PJ, Velayudhan A, Weil J, Sarik Sarikaya aya A, Brewer MA, et al. Enzyme conver conversion sion of lignocellul lignocellulosic osic plant materials materials for resource resource recovery in a controlled ecological life support system. Adv Space Res 1996; 18(1e2):251e65.
’
466
A. Limayem, S.C. Ricke / Progress in Energy and Combustion Science 38 (201 (2012) 2) 449 449e e467
[101] Weil JR, Brewer M, Hendrick Hendrickson son R, Sari Sarikaya kaya A, Ladisch MR. Continuous Continuous pH monitoring during pretreatment of yellow poplar wood sawdust by pressure cooking in water. Appl Biochem Biotechnol 1997;68:21e40. [102] Torget R, Walter P, Himmel M, Grohmann K. Dilute acid pretreatment pretreatment of short rotation woody and herbaceous crops. Appl Biochem Biotechnol 1991; 24e25:115e26. [103] Yasuda S, Murase N. Chemical sstructures tructures of sulfuric-acid lignin. 12. 12. Reaction of lignin models with carbohydrates in 72-percent H2SO4. Holzforschung 1995;49:418e22. [104] Schell DJ, Dowe N, Ibsen KN, Riley CJ, Ruth MF, Lumpk Lumpkin in RE. Contaminant occurrence, identication and control in a pilot-scale corn ber to ethanol conversion process. Bioresour Technol 2007;98:2942 e9. [105] Gamez S, Ramirez JA, Garrote G, Vazqu Vazquez ez M. Manuf Manufactur acturee of ferment fermentable able
[131] [132] [133] [134]
on the Society for Industrial Microbiology: 32nd Symposium on biotechnology for fuels and chemicals, Clear Water Beach, FL. 2010. Olofsson K, Bertlisson Bertlisson M, Liden G. A short review on SSF e an interesting process option from lignocellulosic lignocellulosic feedstocks. Biotechnol Biofuels 2008;1: 1e7. Wang GS, Pan XJ, Zhu JY, Gleisner R. Sul te pretreatment to overcome recalcitrance of lignocellulose (SPORL) for robust enzymatic sacchari cation of hardwoods. Biotechnol Prog 2009;25:1086e93. Shuai L, Yang Q, Zhu J, Lu FC, Weimer PJ. Ralph J Pan XL. Comparative study of SPORL SPORL and dil dilute ute-ac -acid id pre pretre treatm atments ents of spr spruce uce cellul cellulosi osicc ethanol ethanol Bioresour Technol 2010;101:3106e14. production. Bioresour production. Keeny D, Muller M. Waste use by ethanol plant plantss potential chall challenges. enges. Minneapolis, MN: Institute for Agriculture and Trade Policy; 2006. p. 7.
sugar solutions from sugar cane bagasse hydrolyzed with phosphoric acid at atmospheric pressure. J Agric Food Chem 2004;52:4172 e7. Lloyd TA, Wyman Wyman CE. Combin Combined ed sug sugar ar yields yields for dilute dilute sul sulfur furic ic acid acid pre pretre treatm atment ent of cor corn n stover stover fol follow lowed ed by enz enzyma ymatic tic hyd hydrol rolysi ysiss of the remaining solids. Bioresour Technol 2005;96:1967 e77. Kim S, Holtza Holtzappl pplee MT. Delign Deligniication cation kineti kinetics cs of cor corn n sto stover ver in lime lime pretreatment. Bioresour Technol 2006;97:778 e85. Bunnel Bunnelll K, Martin E, Lau C, Pelkk Pelkkii M, Patterson DW, Claus Clausen en EC, et al. Hot water and dilute acid pretreatment from high and low speci c gravity Populus sp. Bioengineeri Bioengineering ng Resea Research rch Lab. Unive University rsity of Arkansas. Arkansas. Poster Biofuels conference, Tampa, FL. 2010. Kumar S, Singh SP, Mishra IM, Adhikari DK. Rec Recent ent advances in production of bioethanol bioetha nol from lignocellulosic lignocellulosic biomass. Chem Eng Techno Technoll 2009;32: 2009;32: 517e26. Antal MJ, Mok WSL, Richards GN. Ki Kinetics netics studies of the reactions of ketoses and aldoses in water at high temperature. 1. Mechanism of formation of 5(hydroxymethyl)-2 furaldehyde from D -fructose and sucrose. Carbohydr Res 1990;199:91e109. Ohgren K, Bura R, Saddl Saddler er J, Zacch Zacchii G. Effect of hemic hemicellul ellulose ose and lignin removal on enzymatic hydrolysis of steam pretreated corn stover. Bioresour Technol 2007;98(13):2503e10. Zhu JY, Pan XJ, Wang GS, Gleisner R. Sulte pretreatment (SPORL) for robust
[135] Berndes Berndes G. Bioenergy and wate waterr e the implications of large-scale bioenergy production produ ction for water use and supply. Global Environ Change 2002;12 2002;12:: 253e71. [136] National Research Council (NRC). Water implication implication of biofuels production in the United States. Washington DC. USA: National Academy of Science Press; 2007. [137] Jackson H. U.S. corn boom has downside for Gulf. Associated Press; 2007. [138] Huffaker R. Protecting water resources in biofuels biofuels production. Water Policy J 2010;12:129e34. [139] Tomas-Pejo E, Olivia Olivia JM, Ballesteros Ballesteros M. Realistic approach approach for full-scale full-scale bioethanol bioetha nol production from lignocellul lignocelluloses. oses. Rev J Sci Ind Res 2008;67: 874e84. [140] Chen YCB. Initial investigat investigation ion of xylose fermentation fermentation for lignoc lignocellul ellulosic osic bioethanol production. Thesis. Auburn University, AL. 2009. [141] Lynd LR, Zyl WH, McBride JE, Laser M. Consolidated bioprocessing bioprocessing of cellulosic biomass: an update. Curr Opin Biotechnol 2005;16:577 e83. [142] Mabee WE, Gregg DJ, Arato C, Berlin A, Bura R, Gilkes N, et al. Updates Updates on softwood-to-ethanol process development. Appl Biochem Biotechnol 2006; 129:55e70. [143] Talebnia F, Karakashev D. AngelidikaI. AngelidikaI. Production of bioethanol from wheat straw. An overview on pretreatment, hydrolysis and fermentation. Biores Technol 2010;101:4744e53.
enzymatic saccharication of spruce and red pine. Bioresour Technol 2009; 100:2411e8. Eggeman T, Elander RT. Proce Process ss and economi economicc analy analysis sis of pretr pretreatment eatmentss technologies. Bioresour Technol 2005;96:2019 e25. Nazhad MM, Ramos LP, Paszner L, Saddler JN. Structural constraints affecting the initial enzymatic hydrolysis of recycled paper. Enzyme Microbiol Technol 1995;17:66e74. Xiang Q, Lee YY, Pettersson Pettersson PO, Torget RW. Heterogeneous Heterogeneous aspects of acid hydrolysis of aa -cellulose. Appl Biochem Biotechnol 2003;105/108:505 e14. Harris EE, Beglinger E. The Madi Madison son wood-sugar process. Report no R1617. Madison, Wisconsin: US Department of Agriculture, Forest Products Laboratory; 1946. Hamelinck CN, Hooijdonk G, Faaij APC. Ethanol from lignocellulosic biomass: techno-economic performance in short-, middle- and long-term. Biomass Bioenerg 2005;28:384e410. Hendri Hendriks ks ATWM, Zeeman G. Pretreatments Pretreatments to enhance the digest digestibili ibility ty of lignocellulosic biomass. Bioresour Technol 2009;100:10e8. Sewalt VJH, Glasser WG, B Beauchemin eauchemin KA. Lignin impact on ber degradation. 3. Reversal of inhibition of enzymatic hydrolysis by chemical modication of lignin and by additives. J Agric Food Chem 1997;45:1823 e8. Liu H, Zhu JY, Fu S. Effects Effects of ligninlignin-meta metall comple complexat xation ion on enz enzyma ymatic tic hydrolysis of cellulose. J Agric Food Chem 2010;58:7233e8.
[14 [144] 4] Tengb Tengborg org C, Stenber Stenbergg K, Galbe Galbe M, Zachhi Zachhi G, Larsson Larsson S, Palmqvi Palmqvist st E. Comparison of SO2 and H2SO4 impregnation of softwood prior to steam pretreatment on ethanol production. Appl Biochem Biotechnol 1998;70 e72: 3e15. [145] Wyman CE, Dale BE, Elander RT, Holtzapple Holtzapple M, Ladis Ladisch ch MR, Lee YY, et al. CAFI, Consortium for Applied Fundamentals and Innovation. Comparative Sugar Recovery and Fermentation Data Following Pretreatment of Poplar Wood by Leading Technologies. Biotechnol Prog 2009;25:333 e9. [14 [146] 6] Morjan Morjanoff off PJ, Gray PP. Optimi Optimizat zation ion of ste steam am explos explosion ion as method method for increasing susceptibility of sugarcane bagasse to enzymatic sacchari cation. Biotechnol Bioeng 1987;29:733e41. [147] Lynd LR. Overview and evaluation of fuel ethanol from cellulosic cellulosic biomass: technology, technol ogy, economics economics,, the environment, and policy. Annu Rev Energy Environ 1996;21:403e65. [148] Kadar Z, Szengye lZ, Reczey Reczey K. Simultan Simultaneous eous sacchar sacchariication and fermentation (SSF) of industrial wastes for the production of ethanol industrial crops and products. Ind Crop Prod 2004;20:103 e10. [149] Dale BE, Henk LL, Shiang M. Fermentation of lignocellulosic materials treated by ammonia freeze-explosion. Dev Ind Microbiol 1984;26:223 e33. [150] Waksman SA. Soil microbiology. Annu Rev Biochem 1936;5:561 1936;5:561e84. [151] Slaivikova E, Vadkertiova R. The diversity of yeast in agricultur agricultural al soil. J Basic Microbiol 2003;5:430e6.
[121] matic Erick Erickson son T, Borjesson J, Tjerneld F. Mechanism Mechanis of surfactant surfa effect2002;31: in enzyhydrolysis of lignoc lignocellul elluloses. oses. EnzymemMicr Microbiol obiolctant Technol 2002; 31: 353e64. [122] Bisaria Bisaria VS, Ghose TK. Biodegrad Biodegradation ation of cellu cellulosic losic materia materials: ls: substrate, substrate, microorganisms, enzymes and products. Enzyme Microbiol Technol 1981;3: 90e104. [123] Hahn-H Hahn-Hägerda ägerdall B, Karhumaa Karhumaa HBK, Fonseca C, Spencer-Ma Spencer-Martins rtins I, GorwaGrauslund MF. Toward industrial pentose-fermenting yeast strains. Appl Microbiol Biotechnol 2007;74:937e53. [124] Claas Claassen sen PAM, Lopez C, Sijtsma AM, Weusthuis Weusthuis L, Van Lier RA, Van Niel JB, et al. Utilization of biomass for the supply of energy carriers. Appl Microbiol Biotechnol 1999;52:741e55. [125] Marti Martin n C, Galbe Wahlb Wahlbom om M, Hagerdal Hagerdal BH, Jonsson Jonsson JL. Ethanol production production from enzymatic hydrolysate hydrolysatess of sugarcane sugarcane bagasses bagasses using recombina recombinant nt xylose e utilizing Saccharomyces Saccharomyces cerevisisae. Enzyme Microbi Microbiol ol Technol 2002;31(2):274e82. [126] Kent NL, Evers AD. Malting, brew brewing ing and distilling. distilling. In: Kent s technology of cereals. 4th ed., vol. 4. Cambridge: Woodhead Publishing; 1994. p. 218 e32. [127] Zhang M, Eddy MC, Deanda K, Finkelste Finkelstein in M, Picataggio Picataggio S. Metabolic engineering of a pentose metabolism pathway in ethanologenic Zymomonas mobilis. Science 1995;267:240e3. [128] Yang B, Wyman CE. Effect of xylan and lignin removal removal by batch and ow
[152] Simões Basílio ACM, de Araújoand PRL,identi de Morais Mora is JOF, da Silva Filho EA, de Morai Moraiss MA, DA. Detection cation of wild yeast contaminants of the industrial fuel ethanol fermentation process. Curr Microbiol 2008;56:322 e6. [153] Banat IM, Nigam P, Marc Marchant hant R. Isola Isolation tion of thermotolera thermotolerant, nt, fermentative fermentative yeasts growing growing at 52 C and producing ethanol at 45 C and 52 C. World J Microbiol Biotechnol 1992;8:259e63. [154] Georgieva T, Mikkelsen M, Ahring B. Ethanol production from wet exploded wheat straw hydrolysate by thermophilic anaerobic bacterium Thermoa BG1L1 in a continuous immobilized reactor. Appl Biochem Bionaerobacter BG1L1 technol 2008;145:99e110. [155] Shaw AJ, Jennery FE, Adams MWW, Lynd LR. End-product End-product pathway pathwayss in the xyl xylose ose ferment fermenting ing bacter bacterium ium Thermoanaerobacterium saccharolyticum. Enzyme Microbiol Technol 2008;42:453 e8. [156] Tomas-Pejo E, Oliva JM, Gonzalez A, Ballesteros I, Ballesteros Ballesteros M. Bioethanol production from wheat straw by the thermotolerant yeast Kluyveromyces marxianus CECT 10875 in a simultaneous sacchari cation and fermentation fed-batch process. Fuel 2009;88:2142e7. [157] Mandels M, Sternberg D. Recent advances in cellulase cellulase technology. Ferment Technol 1976;54:267e86. [158] Hofsten BV, Hofsten HV. Ultrastructure Ultrastructure of the thermotolerant basidomycete possibly suitable for production of food protein. Appl Microbiol 1974;27: 1142e8.
through pretreatment on2004;86:88 the enzymatic water. Biotechnol Bioeng e95. digestibility of corn stover with [129] Qing Q, Yang B, Wyman CE. Xylooligomers are strong inhibitors of cellulose hydrolysis by enzymes. Bioresour Technol 2010;101:9624e30. [130] Ximene Ximeness E, Kim Y, Felix S, Mosier NS, Ladisch Ladisch MR. Inhibition of cellulolytic cellulolytic enzymes due to products of hemicelluloses hydrolysis. A Special Conference
[159] Shi J, Chinn SharmaSharma-Shivap Shivappa pa Phanerochaete RR. Microbial Microbialchrysosporium pretreatment pretreatment. of cotton stalks by solidMS, state cultivation of Bioresour Technol 2008;99:6556e64. [160] Arora DS, Chander MKGP. Involvement Involvement of ligni lignin n perox peroxidase idase,, manganese manganese peroxidase and laccase in degradation and selective ligninolysis of wheat straw. Int Bioterior Biodegrad 2002;50:115 e20.
[106] [10 6] [107] [10 7] [108]
[109] [110]
[111] [112] [113] [114] [115] [116] [117] [118] [119] [120] [12 0]
’
A. Limayem, S.C. Ricke / Progress in Energy and Combustion Science 38 (2012) 449 449e e467
467
[162] Cooney CL, Wise DL. Thermophili Thermophilicc anaerobic digesti digestion on of solid wastes for fuel gas production. Biotech Bioeng 1975;17:1119e35. [163] Chang ST, Hayes WA. The biology and and cultivation of edible mushrooms. New York, NY: Academic Press; 1976. [164] Zadrazel F. The ecology and indus industrial trial production production of Pleurotus ostreatus, P. P. orida orida, P. cornucopiae, and P. eryagii. Mushroom Sci 1976;9:621 e52. [165] Weimer PJ, Zeikus JD. Fermentation of cellulose and cellobiose by Clostridium thermocellum. Appl Environ Microbiol 1977;33:289 e97. [166] Zertuche L, Zal lRR. A study of producing ethanol from cellulose using image. Biotechnology 1982;24:57e8. [167] Ho NWY, Chen ZD. Stable recombinant recombinant yeasts capable of effective fermentation of both glucose and xylose. WO 97/42307, 1997. [168] Ho NWY, Chen ZD, Brainard AP. Genetically Genetically engineered Saccharomyces yeast capable of effective cofermentation of glucose and xylose. Appl Environ Microbiol 1998;64:1852e9. [169] Almaida Almaida JRM, Modig T, Roder A, Liden G, Gorwa Gorwagraus grauslund lund MF. Pichia stiplis xylose reductase helps detoxifying lignocellulosic hydrolysate by reducing 5hydroxymethyl-furfural (HMF). Biotechnol Biofuels 2008;11:1 e12. [170] Fonesca Fonesca GG, Heinz Heinzle le E, Wittmann Wittmann C, Gombert Gombert AK. The yeast Kluyveromyces marxianus and its biotec biotechnologi hnological cal potential. potential. Appl Microbiol Biotechnol 2008;79:339e54. [171] Yanase S, Hasunuma T, Yamada R, Tanaka Tanaka T, Ogino C, Fukuda H, et al. Direct Direct ethanol production from cellulosic materials at high temperature using the thermotolera thermo tolerant nt yeast Kluyveromyces Kluyveromyces marxianus marxianus displaying displaying cellulolyti cellulolyticc enzymes. Appl Microbiol Biotechnol 2010;88:381 e8. [172] Den Haan R, Rose SH, Lynd LR, van Zyl WH. Hydrolysis and fermentatio fermentation n of amorphous cellulose by recombinant Saccharomyces cerevisiae. Metab Eng 2007;9:87e94. [173] van Zyl WH, Lynd LR, Den Haan R, McBride JE. Consolida Consolidated ted bioprocessing bioprocessing for bioethanol production using Saccharomyces cerevisiae. Adv Biochem Eng Biotechnol 2007;108:205e35. [174] Lilly Lilly M, Fierobe Fierobe HP, van Zyl WH, Volsc Volschenk henk H. Heterologou Heterologouss expressio expression n of a Clostridium minicellulosome in Saccharomyces cerevisiae. FEMS. Yeast Res 2009;9:1236e49. [175] Yamada Yamada R, Taniguchi Taniguchi N, Tanaka T, Ogino C, Fukuda H, Kondo A. Cocktai Cocktaill d -
[186] Haas CN, Rose JB, Gerba CP. Quantitativ Quantitativee micro microbial bial risk assessment. assessment. New York, NY: John Wiley; 1999. [187] Reij MW, Van Schothorst Schothorst M. Critical Critical notes on micro microbiolo biological gical risk assessment of food. Brazilian J Microbiol 2000;31:1 e8. [188] Snary EL, Kelly LA, Davison HC, Teale CJ, Wooldridge Wooldridge M. Antimicrobial Antimicrobial resistance: a microbial risk assessment perspective. J Antimicrob Chemother 2004;53:906e10. [189] FAO. Fazil AM. A primer primer on risk assessment assessment modelling: modelling: focus on seafood products. Fisheries Technical Paper. No. 462. Rome, 2005, p. 56. [190] Krimsky S, Golding D. Social theories of risk. Westport, CN: Praeger; 1992. [191] Congre Congressiona ssionall Research Research Service. Service. The Toxic Substances Substances Control Act (TSCA) (TSCA):: implem implementa entation tion and new challe challenges nges,, <http://www.gmaonline.org/lemanager/Chemicals/CRS_Paper_on_TSCA.pdf >; 2008. [194] Schell DJ, Farmer J, Newman M, McMillan JD. Dilute-sulfuric acid pretreatpretreatment of corn stover in pilot-scale reactor: investigation of yields, kinetics and enzymatic digestibilities of solids. Appl Biochem Biotechnol 2003;105: 69e85. [195] Ladisch MR, Kohlmann K, Westgate P, Weil J, Yang Y. Processes for treating cellulosic material. US Patent 1998;5:846. [196] Weil J, Westgate PJ, Kohlmann KL, Ladisch MR. Cellulose pretreatments of lignocell lignocellulosi ulosicc substrate substrates. s. Enzyme Enzyme Micr Microbiol obiol Technol Technol 1994;16(1 1994;16(11): 1): 1002 e4. [199] Drapcho CM, Nhuan NP, Wla Wlaker ker TH. Biofuel engineering process technology. New York, NY: McGraw Hill Companies, Inc; 2008. p. 371. [200] Gupta R, Lee YY. Pretre Pretreatment atment of hybrid poplar by aqueous ammonia. Bio Prog 2009;25:357e64. technol Prog technol [201] Galbe M, Zacchi G. A review of the production production of ethanol from softwood. softwood. Appl Microbiol Biotechnol 2002;59:618e28. [202] Monavari S, Galbe M, Za Zacchi cchi G. Impact of impregnation time and chip size on sugar yield in pretr pretreatment eatment of softwoo softwood d for ethanol produ production. ction. Bioreso Bioresour ur Technol 2009;100:6312e6. [203] Bura R, Chandra R, Saddler J. Inuence of xylan on the enzymatic hydrolysis of steamsteam-pretr pretreated eated corn stover and hybrid poplar poplar.. Biotechnol Biotechnol Prog 2009; 25(2):315e22. Special Issue. [204] Pan XJ, Xie D, Gilkes N, Gregg DJ, Saddler JN. Strategies Strategies to enhanc enhancee the
integration: a novel method to construct cellulolytic enzyme expression ratio-optimized yeast strains. Appl Microbiol Biotechnol 2010;85:1491 e8. Die Dien n BS, Cotta Cotta MA, Jef Jeffer feries ies TW. Bacteria Bacteria engine engineere ered d for fuel fuel ethanol ethanol production: current status. Appl Microbiol Biotechnol 2003;63:258 e66. Deanda Deanda K, Zhang M, Eddy C, Picat Picataggio aggio S. Developme Development nt of an arabinos arabinoseefermenting Zymomonas mobilis strain by metabolic pathway engineering. Appl Environ Microbiol 1996;62:4465e70. Yanase H, Nozaki K, Okamoto K. Ethanol production from cellulosic cellulosic materials by genetic genetically ally enginee engineered red Zymomonas mobilis. Biotec Biotechnol hnol Lett 2005;27: 2005;27: 259e63. Rebros Rebros M, Rosenberg Rosenberg M, Grososvá Z, Krist Kristooková L, Paluch M, Sipöcz M. Ethanol production from starch hydrolyzates using Zymomonas mobilis and glucoamyla gluco amylase se entrap entrapped ped in polyvi polyvinylal nylalcohol cohol hydrogel. hydrogel. Appl Biochem Biotechnol; 2009:158561e70. Weber C, Farwick Farwick A, Benisch F, Brat D, Dietz H, Subtil T, et al. Trends Trends and challenges in the microbial production of lignocellulosic bioalcohol fuels. Appl Microbiol Biotechnol 2010;87:1303e15. Dashtban M, Schraft H, Qin W. Fungal bioconversion of lignocellulosic resiresidues; opportunities & perspectives. Int J Biol Sci 2010;5:578 e95. Binod P, Sindhu R, Singha Singhania nia RT, Vikram S, Devi L, Nagal Nagalakshmi akshmi S, et al. Bioethanol Bioeth anol production production from rice straw: straw: an overview. overview. Bioresour Technol 2009;101:4767e74.
enzymatic enzyma tic hydrolysis of pretreated pretreated softwood with high resid residual ual lignin content. Appl Biochem Biotechnol 2005;121:1069e79. Nguyen DL, Naotsugu N, Tamikazi K. Effect of radiati radiation on and fungal treatment on lignocelluloses and their biological activity. Radiat Phys Chem 2000;59: 393e8. Jorgensen H. Effect of nutrients on fermentation of pretreated wheat straw at very high dry matter content by Saccharomyces cerevisiae. Appl Biochem Biotechnol 2009;153:44e57. Rogers PL, Jeon YJ, Lee KJ, Lawfo Lawford rd HG. Zymomonas mobilis for fuel ethanol and higher value products. Springer-Verlag Berlin. Adv Biochem Eng Biotechnol 2007;108:263e88. Zaldi Zaldivar var J, Nielsen Nielsen J, Olsso Olsson n L. Fuel ethanol production production from lignocell lignocellulose: ulose: a challenge for metabolic engineering and process integration. Appl Microbiol Biotechnol 2001;56:17e34. Ligthe Ligthelm lm ME, Prior BA, du Preez JC. The oxygen requiremen requirements ts of yeasts for the fermentation fermentation of D D -xylose and D -glucose to ethanol. Appl Microbiol Biotechnol 1988;28:63e8. Herre Herrero ro AA. End product inhibition inhibition in anaerobic ferment fermentation. ation. Trends Biotechnol 1983;1:49e53. Balat M, Balat HO. Progress in bioethanol processing. Prog Energy Combust Sci 2008;34:551e73. Nigam JN. Ethanol production fr from om wheat straw hemicellulose hydrolysate
[17 [176] 6] [177] [178] [179]
[180] [181] [182]
[183] Lau MW, Dale BE. Cellulos Cellulosic ic ethanol production from AFEX-treated AFEX-trea ted06(5): corn stover using Saccharomyces Saccharomyces cerevisiae 424A(LNH 424A(LNH-ST). ST). PNAS 2008;106(5): 2008;1 1368e73. [184] Tian S, Zhu W, Gleisner R, Pan XJ, Zhu JY. Comparis Comparisons ons of SPORL and dilute acid pretreatment for sugar and ethanol production from aspen. Biotechnol Prog 2011;27:419e27. [185] Muthaiyan Muthaiyan A, Rick Rickee SC. Current perspec perspectives tives on detect detection ion of micro microbial bial contaminati contam ination on in bioethanol bioethanol fermentors. fermentors. Bioresour Bioresour Technol Technol 2010;101: 2010;101: 5033e42.
[206] [207] [208] [209] [210] [211] [212] [213]
Pichia stipitis. J Biotechnol 2001;87:17e27. [214] by Jeffries TW, Grigoriev IV, Grimwood J, Laplaza JM, Aerts A, Salamov A, et al. Genome Genome sequenc sequencee of the lignoce lignocellu llulos loseses-bio biocon conver verting ting and xyl xylose ose-fermenting yeast Pichia stipilis. Nat Biotechnol 2007;25:319e26. [215] Zayed G, Meyer O. The single-batch bi bioconversion oconversion of wheat straw toethanol employing the fungus Trichoderma viride and the yeast Pachysolentanno phylus. Appl Microbiol Biotechnol 1996;45:551e5. [217] Zeikus JG, Ben-Bassat AHL, Ng TK, Lamed RJ. Thermophilic ethanol fermentations. Basic Life Sci 1981;18:441 e61.
