Overview of technical barriers and implementation of cellulosic ethanol in the U.S.

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Energy 66 (2014) 13e19

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Overview of technical barriers and implementation of cellulosicethanol in the U.S.

Tae Hoon Kim a, Tae Hyun Kim b, *

a Department of Applied Chemical Engineering, Dankook University, Cheonan, Chungnam 330-714, Republic of Koreab Department of Environmental Engineering, Kongju National University, 1223-24 Cheonandae-Ro, Budae-dong, Cheonan, Chungnam 330-717,Republic of Korea

a r t i c l e i n f o

Article history:Received 26 January 2013Received in revised form12 July 2013Accepted 6 August 2013Available online 10 October 2013

Keywords:Ethanol industryBiofuelsBiorefineryCellulosic ethanolCommercializationPretreatment

* Corresponding author. Tel.: þ82 41 521 9426; faxE-mail addresses: [email protected], thkim@k

0360-5442/$ e see front matter � 2013 Elsevier Ltd.http://dx.doi.org/10.1016/j.energy.2013.08.008

a b s t r a c t

There is mounting concern about the buildup of carbon dioxide (CO2) and other so-called greenhousegases in the atmosphere. In general, bioethanol production requires minimal fossil fuel input in theconversion step, and ethanol is considered a promising alternative fuel to petroleum-derived products. Itis anticipated that ethanol production with second-generation biomass, i.e. lignocellulosic materials, willbe possible on a large scale in the near future. Latest efforts have been focused on overcoming technicalchallenges in bioconversion, particularly pretreatment, and finding the solutions required to implementbiorefinery on a large scale. This paper introduces and reviews the current status of research, and of theethanol industry in the U.S. In addition, other important concepts in biofuels, cellulosic ethanol, andbiorefinery in general are reviewed, and the key technical issues in bioconversion of cellulosic ethanol,such as pretreatment and factors affecting bioconversion of biomass are also discussed.

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1. Introduction

The economy of the United States is the world’s largest nationaleconomy. Its nominal GDP was estimated to be over $15.7 trillion in2012 [1]. The U.S.’s present economy is mostly dependent on fossilenergy sources including oil, coal, natural gas, and so on. The UnitedStates was ranked second in the world after China in terms of CO2emitted as of 2011; China and the U.S. emitted 23.3% and 18.2% of theglobal total, respectively [2]. The amount of CO2 released has resultedin growing pressure by the international community for the U.S. tojoin the mandatory reduction programs. Under the Kyoto Protocol of1997, which is a working agreement of the UNFCCC (United NationsFramework Convention on Climate Change), Annex I countries mustreduce their aggregate emissions of greenhouse gases by at least fivepercent from 1990 levels during the 2008e2012 period.

It is also anticipated that the future energy system in the US willbe more dependent on the various alternative energy technologiesincluding solar, wind, tidal, geothermal, fuel cells, and biofuels frombiomass. In particular, according to the U.S. EISA (Energy Inde-pendence and Security Act), the consumption of biofuels, such as

: þ82 41 552 0380.ongju.ac.kr (T.H. Kim).

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biomass-based diesel and advanced biofuels, will increase onehundred-fold during the next 10 years [3].

A biorefinery is a new concept in which fuels, power, andchemicals can be produced from biomass in the same way thattoday’s petroleum refineries produce multiple fuels and productsfrom petroleum. Biorefinery technologies have been generallyidentified as the most promising route to the creation of a newdomestic bio-based industry, to the development of a sustainableindustrial society, to energy independence, and to the effectivemanagement of greenhouse gas emissions [4e7].

There is no doubt that renewable energy will be the mostimportant product of the future bio-based economy. Amongvarious alternative or renewable energies, ethanol production fromrenewable resources is now being accepted across the world as avisible substitute for traditional petroleum based fuels. In partic-ular, cellulosic ethanol, in other words bioethanol, can be producedfrom inexpensive and abundant lignocellulosic biomass. Thislignocellulosic biomass is the most available potential feedstock forthe production of bioethanol, which is currently the most widelyused liquid biofuel alternative to fossil fuels [8].

Use of bioethanol has many advantages in terms of the environ-ment, national energy security, and rural economic improvement.Currently, a lot of research and work towards commercialization arebeing done to meet rapidly growing energy demands. Ethanol is

T.H. Kim, T.H. Kim / Energy 66 (2014) 13e1914

presently being promoted as a promising alternative fuel for trans-portation; the use of fossil fuels has contributed to the buildup ofcarbondioxide in theatmosphere, howeverethanol isa clean-burningfuel that makes a significantly lower contribution to global warmingthananyother fossil fuels because the carbondioxideproducedby thecombustionofethanol isconsumedbyplantgrowth, thusmaintainingthe natural carbon cycle balance.

This paper introduces and reviews the current status ofresearch, of technical barriers in bioconversion, and of the ethanolindustry in the U.S., which is one of the leading countries in bio-refinery, specifically ethanol research and industry, and has vastcropland available for the planting of biomass, as well as abundantbiomass resources.

Fig. 2. Corn ethanol industry overview in the U.S. Source: Renewable Fuel Associationhttp://www.ethanolrfa.org/ [9].

2. Feedstock and present ethanol industry

In general, biomass can be divided into first generation cropssuch as sugars from sugar cane or sugar beets, and starch from corn,rice, wheat, and so on, and second generation energy sources suchas various lignocelluloses and algae including macro- and micro-algae (Fig. 1). Corn grain is the primary feedstock used for theproduction of fuel ethanol in the United States, producing about13.3 billion gallons of corn ethanol in 2012 [9]. Therefore, ethanolproduction is concentrated in the Midwest corn-belt states.Meanwhile, the RFA (Renewable Fuel Association) reported thatthere were only 50 ethanol plants in 2000, a number which hadincreased to 198 by 2011 (Fig. 2). Fig. 2 also indicates that 76 plantswere under construction in 2005, while this number droppedsharply to seven in 2011. It collectively suggests that the ethanoldemand in the US appears to be the saturation point and govern-ment’ corn-starch ethanol mandates are only three years or 12%away from reaching their maximum [10].

Various cellulosic biomass resources are widely distributedacross the United States. The NREL (National Renewable EnergyLaboratory) provided information on the biomass resources avail-able in the United States, which include agricultural residues, woodresidues, and dedicated energy crops. According to the biomassresource distribution map, less biomass resources are available inthe West, while greater inventories are available in the Mid-west,East, and South [11].

According to other reports, there are about 120 million dry tonsof cellulosic biomass, mainly corn stover, corn fiber, wheat, rye, and

Fig. 1. Various biomass

barley straw, that can be sustainably harvested today, and which isenough to produce about five billion gallons of ethanol, roughlyequal to half what the U.S. is making today from corn starch. On theother hand, more than one billion tons of biomass could potentiallybe harvested if dedicated energy crops could be developed, grown,and harvested sustainably. If this, alongwith sustainable harvestingof forestry and paper mill wastes were achieved, the U.S. couldsupply about 60 billion gallons of fuel ethanol; 30% of U.S. liquidtransportation fuel needs [12e15].

Achieving the cellulosic biofuel targets set forth in the EnergyIndependence and Security Act (EISA) of 2007 will require a verylarge increase in harvested cellulosic biomass feedstocks fromagricultural, forest, and other resources. It is estimated that by2022, nearly 180 million dry tons of biomass will be neededannually to produce the 16 billion gallons of cellulosic biofuelscalled for by EISA [16].

3. Advanced biofuels

Biofuels including bioethanol were typically categorized intofirst, second, and third generation biofuels. First generation biofuelis made from oil, sugars and starch, which can easily be convertedto diesel, ethanol, and butanol using conventional technology.

and classification.

Fig. 3. The 2007 Energy Independence and Security Act required aggressive increase inadvanced biofuels. Source: 2007 Energy Independence and Security Act [19].

T.H. Kim, T.H. Kim / Energy 66 (2014) 13e19 15

Second generation biofuels, mainly cellulosic ethanol, are madefrom lignocellulosic biomass such as forestry residues, agriculturalresidues, and dedicated energy crops, and they are much moredifficult to convert than oil, sugars, and starch.

Third generation biofuels are much more similar in compositionand fuel value to petroleum. In general, these so called ‘drop-infuels’ are replacements for gasoline, diesel, and jet fuel producedfrom various sustainable sources such as cellulose, municipalwaste, or algae; for example, Mascoma Corporation (Waltham, MA,USA) suggested consolidated bioprocessing which utilizes devel-opment of organisms that break down biomass and produce bio-fuels with no added enzymes or pretreatment, and LS9 Inc. (SouthSan Francisco, CA, USA), Amyris Inc. (Emeryville, CA, USA) and GevoInc. (Englewood, CO, USA) used synthetic biology by developmentof new organisms with new pathway to produce renewable pe-troleums, in other word third generation biofuels, directly frommultiple feedstocks.

Meanwhile, characterizing biofuels by genealogy is confusing andnot always meaningful. RFS2 (Renewable Fuel Standard 2) from theEPA (Environmental Protection Agency) and the Energy Indepen-dence and Security Act of 2007, suggested that advanced biofuels ismore appropriate to describe second and third generation biofuels[17]. In this paper, conventional and advanced biofuels (bioethanol)are used to describe biofuels in terms of their performance.

RFS2 lays the foundation for achieving significant reductions ofgreenhouse gas emissions from the use of renewable fuels, forreducing imported petroleum, and for encouraging the develop-ment and expansion of US’s renewable fuels sector. EISA of 2007established specific lifecycle GHG (greenhouse gas) emissionthresholds for each renewable fuel, requiring a percentageimprovement compared to lifecycle GHG emission for gasoline ordiesele(1) any renewable fuel: 20% reduction; (2) advanced biofuel(or biomass-based diesel): 50% reduction; (3) cellulosic biofuel:60% reduction.

In 2005, Congress enacted the Renewable Fuel Standard as partof the Energy Policy Act and amended it in the 2007 EISA. The aimof the RFS is to encourage development of biofuels, lower depen-dence on foreign oil, and reduce greenhouse gas emissions [18]. TheRFS called for 36 billion gallons of renewable fuel to be blendedwith traditional fuels by 2022, and the vast bulk of this will comefrom corn ethanol. It also called for the volume requirement ofcellulosic biofuels for the year 2012 to be 0.5 billion gallons, and foran increase in advanced biofuels to 16 billion gallons by the year2022 [19] (Fig. 3). Specifically, the law mandates that the UnitedStates must produce 16 billion gallons of cellulosic biofuels by 2022,along with 15 billion gallons of conventional corn-based ethanol, 1

billion gallons of biodiesel, and 4 billion gallons of advancedbiofuels.

In 2007, the U.S. Department of Energy selected six cellulosicethanol projects to receive up to $385million in grants. This projectwas established to satisfy two major goals of the US government:(1) to make ethanol out of nonfood biomass, including billions ofpounds of agricultural waste, at a cost competitive with gasoline by2012, and (2) to increase the use of renewable and alternative fuels.For this project, more than $1.2 billion was to be invested to buildsix biorefinery facilities across the U.S. However, only one is underconstruction as of 2011.

The lesson learned was that in order for commercialization ofbiofuel production to be successful, many hurdles, including securingfeedstock and financing construction, will need to be overcome [20].In the US, the supply of advanced biofuels (2nd generation) wasdetermined to be behind schedule, and this is due to limited feed-stock supply and to the economic cost of conversion technologies.Overall, the main reason is that the initial costs for most advancedbiofuels will be very high; (1) plant capital costs are 5e10 times asmuch as corn ethanol plants, (2) production costs are 2e5 timesmore expensive than corn ethanol, and (3) not enough low-costfeedstock is available.

4. Biorefinery and sustainability

The term ‘sustainability’ is rooted in the term ‘sustainabledevelopment’ [21,22]. The concept suggests that it is possible toachieve economic growth and industrialization without environ-mental damage. The core of mainstream sustainability thinking hasbecome the idea of three dimensions, environmental, social andeconomic sustainability [23]. In addition, the NREL (NationalRenewable Energy Laboratory in Golden City, CO, USA) defined thebiorefinery as “a facility that integrates biomass conversion pro-cesses and equipment to produce fuels, power, and chemicals frombiomass.” The biorefinery concept is analogous to today’s petro-leum refineries, which produce multiple fuels and products frompetroleum [11].

Biorefinery technologies will include several different conver-sion processes and different sized installations, due to a variety offeedstocks, and different local circumstances. Optimization andhigh efficiency are the keys to making biorefineries sustainable andeconomically viable [24,25]. Although biorefinery has great po-tential for replacement of petroleum-derived products, the in-dustry is still in a nascent state. To date, most of the industrialbiorefineries have been focused on the production of bio-basedproducts from corn grain, ethanol, animal feeds, corn syrup, andother chemical building blocks. On the other hand, numerousresearch efforts have been focused on the use of second-generationbiomass such as lignocellulosic biomass, and the consensus amongresearchers is that these will probably be ready for large-scalecommercial production within ten years.

In order to facilitate the commercialization of lignocellulosicintegrated biorefineries, industry must employ the best technolo-gies for harvest, storage, transportation, pretreatment, saccharifi-cation, and fermentation. The development of technology in theaforementioned sectors will push the industrialization of bio-refineries and in particular the mass production of cellulosicethanol on a commercial scale.

5. Key technical issues in commercialization of advancedbioethanol

The biological conversion of ethanol from lignocellulosic biomasscan be achieved by three major stepsepretreatment, enzymatic hy-drolysis and sugar fermentation. The OBP’s (Office of the Biomass

T.H. Kim, T.H. Kim / Energy 66 (2014) 13e1916

Program) roadmap from the Biomass to Biofuels Workshop in 2006,identified barriers to realizing the potential of cellulosic biofuels, andalso highlighted related high-level topical areas [26] (Table 1). In thisreport, key scientific milestones to achieving progress toward OBPgoals were identified by workshop participants, and included fourtopical areas; (1) feedstocks for biofuels, (2) biomass (feedstocks)deconstruction to sugars, (3) sugar fermentation to ethanol, and (4)consolidated processing.

The commercialization of cellulosic ethanol production dependssignificantly on the improvement of technologies in biomassdeconstruction to sugar. This is because cellulosic ethanol priceswill depend heavily on the cost of the cellulase and hemicelluloseenzymes used to break down the cellulosic biomass into ferment-able sugars. Reducing enzyme costs increases the market potentialof biofuels [27]. However, all four topical areas are to be consideredequally important. In 2010, a report indicated that enzyme costswere estimated at about $0.50 per gallon, and significant work wasunderway to reduce this to about $0.10 per gallon [28].

The goal, either reduction of total enzyme cost used or reductionof total enzymes used in the process, can possibly be achieved bymeans of improving pretreatment technology and improvingenzyme efficiency. A general approach for the conversion ofbiomass into ethanol is to break down the cellulose and hemicel-lulose chains, comprising two-thirds to three-quarters of thelignocellulosic biomass, by chemical pretreatment and enzymesaccharification, into their component sugars, and then fermentthose sugars into ethanol.

5.1. Pretreatment

Lignocellulosic biomass consists primarily of three differenttypes of polymersecellulose, hemicellulose and lignin, which aretightly associated with each other [29,30]. The lignin-hemicelluloseassociation shields the cell wall polysaccharides from enzyme hy-drolysis, and thus a pretreatment process is required to permitsaccharification. Cellulose and hemicellulose portions in lignocel-lulosic biomass are carbohydrates, which can be converted intofuels and chemical by fermentation. In order to produce fuels andchemicals from lignocellulosic biomass, cellulose and hemicellu-lose in this feedstock should be hydrolyzed to produce variousmonomeric sugars either by acid or enzymes.

Table 1DOE-OBP Research milestones.

Topic Technolog

FeedstocksDevelop sustainable technologies

to supply biomass to biorefineries

Better comand structsugars pro

Biomass (feedstock) deconstruction to sugarsDevelop biochemical conversion

technologies to produce low-costsugars from lignocellulosic biomass

Pretreatm

Enzyme hyto sugars

Sugar fermentation to ethanolDevelop technologies to produce

fuels, chemicals, and power frombiobased sugars and chemicalbuilding blocks.

Cofermentsugars

Consolidated processingReduce process steps and

complexity by integratingmultiple processes in singlereactor

Enzyme prhydrolysiscofermentcombinedreactor

Acid hydrolysis of lignocellulosic biomass can depolymerizecellulose and hemicellulose into monomers without pretreatment,but the acid must be recovered for environmental and economicreasons. It has been challenging to develop a low-cost acid recoveryprocess to meet these needs [31,32]. Only biological conversionusing enzymes and microbes is widely accepted as a mean ofcellulosic ethanol and biochemical production.

Lignocellulosic biomass has a recalcitrant nature, which makesit resistant to enzymatic hydrolysis. In the bioconversion processesof lignocellulosic biomass, pretreatment is the most essential stepto improve the microbial conversion yield of fermentation from alignocellulosic biomass. Although the corn ethanol industry iscurrently producing ethanol at a cheap price, production of ethanolfrom lignocellulosic biomass is a very different system than thatused for corn grain because carbohydrates in lignocellulosicbiomass are much more difficult to solubilize than the starch ingrain [33]. Moreover, lignocellulosic material is very resistant toenzymatic break down, requiring pretreatment in order to open upthe rigid structure and to enhance the susceptibility of the biomassto the enzyme [34].

The common effects of pretreatment include the following: (1)Decrease of lignin, hemicellulose, and extraneous components, (2)Increase of surface area, porosity, and pore size, (3) Reduction of thecrystallinity of cellulose, and (4) Enhancement of the accessibility ofenzymes to the cellulosic substrate.

5.2. Factors affecting biological conversion

There are many factors affecting biological conversion oflignocellulosic biomass. Lignocellulosic material is not a naturallydigestible material, since it is provided with many chemical/phys-ical barriers that inhibit the enzyme hydrolysis of lignocellulose[35]. As described earlier, the biological conversion of lignocellu-losic biomass to fuels and chemicals can be achieved by pretreat-ment, enzymatic hydrolysis and fermentation. Pretreatment is oneof the key elements in the bioconversion of this non-digestiblebiomass. It is required for efficient enzymatic hydrolysis ofbiomass because of the physical and chemical barriers that inhibitthe accessibility of enzymes to the cellulose substrate [36].

A number of factors have been suggested to affect enzymatichydrolysis. Among the known chemical barriers are lignin [37e39],

y goals

positionures forduction

DomesticationBetter agronomicsSustainability

ent Reduced severityReduced wasteHigher sugar yieldsReduced inhibitorsReduction in non-fermentable sugars

drolysis Higher specific activity & thermal toleranceReduced product inhibitionBroader substrate rangeCellulases and cellulosomes

ation of C-5 and C-6 sugar microbesRobust process toleranceResistance to inhibitorsMarketable by-products

oduction,, andationin one

Production of hydrolytic enzymes,fermentation of needed productsProcess tolerance & stable integratedtraits; All processes combined in asingle microbe or stable culture

Fig. 4. Factors to be considered in designing of pretreatment method.

Table 2Various pretreatment technologies and representative reaction conditions (source [52,68e70]).

Methods Reaction condition Features

Dilute acid(co-current)

130e200 �C, high pressure (3e15 atm),0.1e3.0% sulfuric acid, 2e30 min,solid loading 10e40%

Most of hemicellulose is solubilized. 20e40% delignification.

Steam explosion Acid impregnation with H2SO4, SO2,(160e270 �C), 1e30 min

70e90% hemicellulose dissolution. 20e40% delignification.

Hot water(pH controlled)

160e190 �C, 20e60 min, high pressure(6e14 atm), solid loading 5e30%

Most of hemicellulose is solubilized. 30e40% delignification.

Ammonia recyclepercolation (ARP)

150e170 �C, 10e20 min, 10e15% ammonia,high pressure (9e17 atm), solid loading 15e30%

50% hemicellulose dissolution. 70e95% delignification.

Soaking in aqueousammonia (SAA)

30e90 �C, 4-72 h, 5e30% ammonia,solid loading 15e30%

10e20% hemicellulose dissolution. 60e70% delignification.Very mild reaction cond. Simple process scheme.

Low moistureanhydrousammonia(LMAA)

Ambiente80 �C, w72 h, ammonia gas,no washing, moisture w50%

No washing is required; low liquid loading(only w50% moisture is enough).Mild reaction condition.

AFEX 70e90 �C, w5 min, high pressure (15e20 atm),anhydrous ammonia, (solid loading 60e90%)

Hemicellulose remains in solid form. No lignin removal.Popcorn effect.

Lime (with orwithout air)

Lime 25e130 �C, 1 he2 months, 0.05e0.15 gCa(OH)2 water)/g of biomass (solid loading 5e20%)

20e40% hemicellulose dissolution, form. 60e80%delignification, de-acetylation.Low energy intensity.

T.H. Kim, T.H. Kim / Energy 66 (2014) 13e19 17

hemicellulose [40], and the acetyl group [37,41]. The physical factorsof biomass, such as crystallinity [38,40], surface area [42,43], particlesize [44], pore size [45], and degree of polymerization [46], have alsobeen known to influence enzymatic hydrolysis. Among the manyfactors, lignin has been considered the major impediment.

5.3. Various pretreatment technologies

Enzymatic hydrolysis can produce high yields of relatively pureglucose syrups without generating glucose degradation products,and utility costs are low since the hydrolysis occurs under mildreaction conditions [47]. Themethodology used to achieve this goalvaries widely depending on the specific application: treatmentswith various types of acids, alkaline treatments, steam treatment,and simple thermal-mechanical treatments. The net effect of thepretreatment also varies widely in terms of its physical andchemical characteristics.

As summarized in the Table 2, there have been numerous at-tempts to enhance the enzymatic reactionegrinding/milling[46,48], steam/steam explosion [40,49], hot water/autohydrolysis[50,51], acid treatment [42,45,52], alkali treatment [53,54], andother methods [55,56].

The various pretreatments are usually designed to makebiomass more susceptible to enzyme hydrolysis by removing ligninand/or hemicellulose, reducing the cellulose crystalline structure,and increasing the pore size and surface area. Many kinds ofchemical and physical pretreatment methods have been used toenhance the enzymatic reaction. Considerable attention has beendevoted to agents that will cause swelling of the cellulose anddisrupt the crystalline structure by cleavage of hydrogen bonds incellulose. In chemical pretreatments, there are two ways in whichthis may occur; (1) intercrystalline swelling caused by uptake ofwater and chemicals between the crystal units, which causes areversible volume change of up to about 30 percent, and (2)intracrystalline swelling, which involves the penetration of thecrystalline structure and can lead to unlimited swelling or completesolution of the cellulose.

Sodium hydroxide, amines, and anhydrous ammonia have beenused for intercrystalline swelling. In Europe, during World War II,high concentrations (70e75 percent) of sulfuric acid or fuminghydrochloric acid and metal chelating solvents were used forintracrystalline swelling.

Most of the chemical pretreatment methods for improving enzy-matic digestibility generate hydrolysates containing a mixture of

hexose and pentose sugars and lignin. Usually the hydrolysate fromthe pretreatment/fractionation process requires detoxification,because the microorganisms used in the subsequent fermen-tation step [57] poorly withstand the inhibitory environment oflignocellulose-hydrolysates [58e64]. Detoxification of the hydroly-sates will increase the cost of pentose (xylose) fermentation. This isa disadvantage that most of chemical treatment methods mustovercome.

5.4. An example of recent progress; low moisture anhydrousammonia pretreatment

To date, various pretreatment methods have been suggested toenhance the enzymatic digestibility and fermentability of lignocel-lulosic biomass. Although a few of them may be effective, severalcost barriers which prohibit scale-up still exist, including (1) highchemical input and (2) excessive water use [65]. In order to solvethese problems, a simpler pretreatment method using anhydrousammonia has been developed [66]. In order to eliminate the addi-tional water washing step and to improve the cost-effectiveness ofammonia pretreatment processes, the LMAA (low moisture anhy-drous ammonia) pretreatmentmethodwas recently suggested [66].

As shown in Fig. 5, this method comprises four steps for ethanolproduction; ammoniation, pretreatment, evaporation for excessiveammonia recovery, and SSF (simultaneous saccharification and

Fig. 5. Bioconversion process of biomass into ethanol using low moisture anhydrous ammonia (LMAA) pretreatment.

T.H. Kim, T.H. Kim / Energy 66 (2014) 13e1918

fermentation). In the first step, ammoniation, biomass with 30e70% moisture was put into contact with anhydrous ammonia in asimple reactor under mild conditions (near ambient conditions).After the ammoniation step, biomass was subjected to a simplepretreatment step at moderate temperatures (40e120 �C) for 48e144 h, which can be carried out in a simple closed plastic bag orcontainer. In the subsequent SSF step, treated biomass is saccha-rified and converted into ethanol without a post-washing step.Although this method enhanced bioconversion yield of the agri-cultural residues, this alkaline method has not been proven to beeffective for woody biomass such as hardwood or softwood.

Themain features of thismethod include the fact that LMAAwasdesigned to reduce energy input and ammonia consumptionsignificantly. Pretreatment of biomass with low moisture usinggaseous ammonia leads to short exposure time and can be carriedout under ambient conditions, allowing low capital costs to beprojected. In addition, it has been speculated that ammoniation canalso supply assimilable nitrogen (up to 1.2 weight percent (wt.%) ofdry biomass) for microbial growth in the fermentor using thetreated biomass as substrate [67]. Furthermore, corn stover cantypically be harvested once a year in the fall, but not all of theharvested corn stover can be converted into the desired products inthe biorefinery within a few weeks of harvesting. Long-term stor-age of the feedstock with minimal destruction of the availablecarbohydrates, for example by microbial biodegradation, is highlydesirable. Ammonia has been known to be effective against bac-teria. It was reported that the antimicrobial effect of ammoniatingallowed long-term storage of biomass with minimal biodegrada-tion of carbohydrates [68]. Thus, it is suspected that the LMAApretreatment method can also be integrated with long-term stor-age of the biomass feedstock while awaiting processing.

6. Integration of pretreatment into the biomass conversionprocess

It is important to look for adequate and abundant sources oflocal lignocellulosic biomass from which ethanol can be producedat a reasonable cost with available resources. If the feedstock isselected, the pretreatment method should be selected by consid-ering the biomass characteristics and the overall bioconversionscheme. In order to design the pretreatment method as a criticalpart of the integrated bioconversion process, many factors shouldbe considered, as shown in Fig. 4.

Some of pretreatment methods such as acid, hot-water, andsteam explosion, designed to improve enzymatic digestibility,generate hydrolysates containing a mixture of sugars and lignin,

while alkaline pretreatment methods such as ARP (ammoniarecycle percolation), SAA (soaking in aqueous ammonia), AFEX(ammonia fiber explosion) and lime pretreatment solubilize mostlignin and/or part of hemicellulose [69e71]. Soluble lignin presentin the pretreatment liquid is known to inhibit the enzymatic hy-drolysis and bioconversion processes [37e40,43].

Hydrolysates of common pretreatment processes, especiallypretreatments using various acids and high temperature, also containvarious other toxic compounds that create an inhibitory environmentin which microorganisms cannot sustain the viability required forefficient bioconversion [58e64]. In addition to toxic compounds,various chemical reagents used for pretreatment should be removedor recovered. In order to utilize these soluble sugars, the contami-nated hydrolysates must be cleaned and detoxified before they aresubjected to microbial fermentation. Pretreatment is a component ofthe integrated biomass conversionprocess that has not been tested atthe commercial scale and still requires further development. It is oneof the most important aspects of the overall process, not only tech-nologically but also economically [69].

7. Conclusion

For several decades, ethanol has been promoted as a promisingalternative fuel for transportation. At present, corn grain and sugarcane are the primary feedstocks for conventional fuel ethanolproduction. However, shifting from edible food crop to non-foodmaterials will offer great opportunities for the future biorefineryindustry because the conventional fuel ethanol market is almostsaturated. It is anticipated that ethanol production with second-generation biomass on a large scale will be possible in the nearfuture. A strong focus on the implementation of cellulosic ethanolproduction and biorefinery will lie on effective deconstruction andeconomic conversion process of lignocellulosic biomass.

Acknowledgments

This research was supported by “Basic Science Research Pro-gram through the National Research Foundation of Korea (NRF)funded by theMinistry of Education (No. 2013R1A1A2010001)” and“Human Resources Development program (No. 20134030200230)of the Korea Institute of Energy Technology Evaluation and Plan-ning (KETEP) grant funded by the Korea government Ministry ofTrade, Industry and Energy”.

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