Wet Explosion: a Universal and Efficient Pretreatment Processfor Lignocellulosic Biorefineries

Rajib Biswas & Hinrich Uellendahl & Birgitte K. Ahring

# Springer Science+Business Media New York 2015

Abstract Lignocellulosic biomass resources especially agri-cultural and forests residues, perennial crops, farmwastes, andthe organic fraction of municipal solid waste hold significantpotential for the widespread production of sustainable fuels,chemicals, and bioproducts worldwide. For biochemical con-version processes, deconstruction of lignocellulosic biomassinto its components (cellulose, hemicellulose, and lignin) forfurther microbial conversion has been a major challenge dueto the recalcitrant nature of lignocellulose. Thus pretreatmentis prerequisite for efficient hydrolysis of lignocellulose andcost for such treatment is currently about one third of theoverall processing costs in a cellulosic biorefinery. Thus, thedevelopment of a more efficient and cost-effective pretreat-ment method is crucial for the commercialization of lignocel-lulosic biorefineries. Wet explosion (WEx), a thermochemicalpretreatment method with additional features of oxygen addi-tion and explosive decompression, can be adjusted to differentbiomass feedstock and to subsequent bio-catalytic and micro-bial processes. The WEx pretreatment method has been suc-cessfully applied in combination with both microbial fermen-tation and anaerobic digestion processes using both agricul-tural and forest residues as well as manure fibers. Steam ex-plosion, represents a related process to WEx pretreatmentwhere high pressure is used but no oxygen is added. This

process has been tested in demonstration scale while WEx ison its way to commercialization. Presented here is a summaryof the basic concepts and parameters involved in WExpretreatment.

Keywords Wet explosion pretreatment . Lignocellulosicbiorefineries . Biomass feedstock . Enzymatic hydrolysis .

Anaerobic digestion

Introduction

First-generation biofuels such as bioethanol produced fromsugar- or starch-based feedstocks such as corn, sugarcane,wheat [1], and biodiesel produced from renewable lipid feed-stocks include mainly soybean, rapeseed, and palm oils [2],have experienced unprecedented growth over the last decadesas means to reduce the dependency on oil supply from polit-ically unstable regions of the world. In addition, biogas pro-duced by the anaerobic digestion process has become anotherimportant biofuel for the production of renewable natural gasand electricity, especially in Europe, with rising productionfrom both organic residues and energy crops [3, 4].

To eliminate the increasing controversy over land use whenproducing food-based feedstock for the first-generationbiofuels, non-food lignocellulosic resources such as agricultur-al and forest residues, perennial crops, marine biomass, andorganic fractions of farm and municipal waste have been rec-ognized as a more sustainable option [5–7]. These abundantlignocellulosic biomass resources are accessible, and relativelycheap (typically $20 to $100 per dry ton) as well as having alower carbon footprint. As a result, the establishment of a ro-bust bio-based industry has driven considerable research in thearea during the past years with the aim of finding economic

R. Biswas : B. K. Ahring (*)Bioproducts, Sciences and Engineering Laboratory (BSEL),Washington State University, 2710 Crimson Way,Richland 99354, WA, USAe-mail: bka@tricity.wsu.eduURL: http://www.tricity.wsu.edu/bsel/

H. UellendahlSection for Sustainable Biotechnology, Aalborg UniversityCopenhagen, Copenhagen, Denmark

Bioenerg. Res.DOI 10.1007/s12155-015-9590-5

ways to utilize lignocellulosic biomass [8, 9]. Still, improve-ments in the conversion processes are needed for large-scaleeconomic implementation of biorefining technologies [10–12].

Pretreatment is an indispensable process step in biochemicalconversion of lignocellulose needed for the deconstruction ofthe complex structure of cell wall, made of chemical bondsbetween cellulose, hemicellulose and lignin [10], in order tofacilitate enzymatic hydrolysis of the polysaccharides into fer-mentable sugars. However, high capital and operational costsof pretreatment processes are often claimed to be the mainreasons for the slow penetration of industrial application oflignocellulosic biorefineries [13, 14]. Physical, thermal, chem-ical, and biological pretreatment methods and their combina-tions have been previously reviewed on their ability to fraction-ate lignocellulose [15–18]. Of the various pretreatmentmethods, thermochemical pretreatment has been consideredas one of the most cost-efficient methods [12, 19] and has beenapplied in demo/large-scale facilities such as the Inbicon plantin Denmark and the Beta Renewables’ Crescentino plantin Italy [20, 21]. Recently, new plants such as POET-DSM’s Project Liberty cellulosic ethanol plant inEmmetsburg, Iowa (http://poet-dsm.com/liberty) andAbengoa Bioenergy’s cellulosic ethanol plant inHugoton, Kansas (http://www.abengoabioenergy.com)have further come on-line.

Wet explosion (WEx), a thermochemical pretreatment thatcan either be applied as steam explosion without the oxidizingagents or with oxidizing agents [22–24], and the process hasshown its potential as an effective pretreatment method for awide variety of lignocellulosic biomass. Benefits of WEx in-clude relatively simple technical setup, relatively low energyinput, lower risk of inhibitory degradation products formation,and no need for other chemicals besides oxygen, which can beapplied as air. In addition, the pretreatment technology is feed-stock neutral, i.e., applicable to various types of ligno-cellulosic feedstocks including softwood and hardwood,and suitable for application in different biochemical pro-cesses such as yeast, fungal or bacterial systems. Thisreview provides an overview of recent development ofthermochemical pretreatment of lignocellulosic biomass,focusing on wet explosion. The mechanisms of the dif-ferent process parameters involved in WEx and theiradjustment for different biomass and processes arereviewed. Additionally, wet explosion will be comparedto other promising pretreatment methods, and the feasi-bility and limitations of scale-up will be discussed.

Structure and Composition of Lignocellulose

Lignocellulose consists of cellulose, hemicellulose and lignin(Fig. 1). Depending on the respective biomass, cellulose ac-counts for 40–50 %, hemicellulose 20–40 %, and lignin 10–

40 % of the biomass dry weight [25]. Cellulose is a linearpolymer composed of glucose subunits (C6nH10n+2O5n+1, n=degree of polymerization of glucose) linked by β-1,4-glyco-sidic bonds forming cellobiose molecules. Its crystalline struc-ture is strengthened by strong intra- and inter-molecule hydro-gen bonds within elemental fibrils and by van der Waalsforces. Hemicellulose consists of five- and six-carbon sugarswhere xylose is the most common type. The other sugar com-ponents of hemicellulose are glucose, mannose, arabinose,and galactose with lower levels of rhamnose, fucose, anduronic acids. Hemicellulose is highly branched with the pres-ence of acetyl groups, lacking the crystalline structure of cel-lulose. These heteropolymers bind bundles with cellulose fi-brils and enhance the stability of the cell wall [26]. The degreeof polymerization (DP) of cellulose and hemicellulose rangefrom 100 to 20,000 and 50 to 300, respectively.

The third major component of lignocellulose, lignin, is acomplex hydrocarbon polymer consisting primarily of threephenylpropane units, both aliphatic and aromatic constituents[10], such as p-hydroxyphenyl (H), guaiacyl (G), and sinapyl(S) units linked by aryl-ether or C–C bonds (Fig. 1). Lignin ishydrophobic and amorphous and provides structural strengthby cross-linking between the phenylpropane units and cellu-lose–hemicellulose fibrils that makes the structure recalcitrantto microbial degradation. Being a thermoplastic polymerexhibiting glass transition and melting temperature of around90 and 170 °C, respectively [27], lignin is the most recalcitrantcomponent of the plant cell wall. Thus removal and/or disrup-tion of its chemical bonds is necessary in order to improve thebioavailability of cellulose and hemicellulose for enzymaticpenetration and improved activity [26]. The chemicalcompositions of a few potential lignocellulosic feedstocksfor biorefineries are shown in Table 1, standard NREL labo-ratory analytical procedures or it’s slight modification havemostly been used for the analysis. The compositional differ-ences largely impact conversion efficiency of lignocellulose,for example, feedstocks with increased lignin content havebeen shown to have lower pretreatment efficiency and cellu-lose enzymatic digestibility [45]. The proportion of cellulose,hemicellulose, and lignin and their characteristics depend up-on factors such as biomass type, location, maturity at harvest,climate, growing conditions, and storage time after harvest[29, 46], even within species and particularly between soft-wood and hardwoods [47].

Criteria of a Suitable Pretreatment Method

The structural and compositional properties of lignocellu-lose such as cellulose crystallinity, degree of polymeriza-tion, lignin content, and degree of hemicellulose acetyla-tion are affected by different pretreatments to differentdegrees. A large research effort has been spent to develop

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pretreatment strategies including physical, chemical [48,49], hydrothermal [48, 50], thermochemical [30, 51, 52],and biological [53] processes in recent years. The prima-ry purpose of pretreatment is to increase the accessibilityof cellulose and hemicellulose for subsequent hydrolysisby cleaving or opening their bonds to lignin, which canfurther simplify downstream processing. Therefore, anideal pretreatment features high carbohydrate recovery;no or low chemical use; produces no or very limitedamount of degradation products, cost-efficient for indus-trial application, and feedstock neutral, i.e., applicable toa wide variety of biomass feedstock regardless the chem-ical composition of biomass.

Furthermore, an efficient pretreatment method can reducethe amount of enzymes required for high substrate enzymaticdigestibility (SED) of the carbohydrates [54, 55]. Therefore,the pretreatment must be effective enough to provide satisfac-tory enzymatic cellulose conversion (often refers to >75 %cellulose conversion) using least amount of enzymes. Thebiomass material obtained after pretreatment and/or enzymatichydrolysis (hydrolysate) undergoes downstream microbialconversion into products such as ethanol, acids and otherchemicals. Increase in pretreatment severity may lead to theformation of degradation compounds such as HMF and furfu-ral, and lignin products known to inhibit subsequent enzymat-ic saccharification and microbial growth [56, 57]. The

High temperature and pressure

Oxidizing agents

Residence time

Explosive decompression

Crystalline cellulose

Hemicellulose

Lignin

[ [

{Glucose

n-3

[ [

n-3

[[n-3

{{{

Hexose

Pentose

{

{

Hydrogen bond

OH

OH

OH

OH

O

OH

OH

O O

p-Coumaryl alcohol Coniferyl alcohol Sinapyl alcohol

Untreated cell wall

H

G

SH G

S

H

G

S

H

G

H

G

G

S

S

H

S

G

G

H

Fig. 1 Effect of wet explosion pretreatment on the cell wall structure oflignocellulose with its constituents (cellulose, hemicellulose, and lignin)as affected by temperature, pressure, oxidizing agents, residence time,and explosive decompression. Typically, the pretreatment solubilizes

hemicellulose and partially dissolves lignin as low-molecular lignin com-pounds in the aqueous phase while crystalline cellulose and non-degradedlignin remain in the solid fraction (modified after Rubin [10])

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presence of those compounds in the hydrolysate at higherconcentrations can be detrimental for the biochemical processin terms of yields and productivity [58]. However, for thethermochemical conversion process, the compounds such asHMF and fufural can be regarded as desired products, depend-ing on the process design/final target. The effects of eachpretreatment condition may vary widely depending on thebiomass type. In general, the major limitations of a pretreat-ment method include:

1. High capital and operational costs, high energy and/orchemicals input.

2. Material losses due to degradation—thus low mass recov-ery, slow conversion process, limited applicability to cer-tain feedstocks (for instance softwood, hardwood etc.).

3. Hazardous chemicals use, necessity of chemical recoveryand recycling, corrosion problems—requiring expensiveequipment and maintenance, environmental issues asso-ciated with the use of chemicals such as ammonia.

4. Formation of excessive degradation products (inhibitors),detoxification can be necessary in prior to microbialfermentation.

The Wet Explosion Process

In general, the WEx pretreatment method, first patented in2004 [22], includes oxidation and physical disruption, andpartial degradation of the biomass [30, 59, 60]. The severityof WEx can be adjusted by three parameters which are tem-perature, residence time, and the amount of oxygen added to

the reactor. Batch-operated WEx pretreatment is performed ina hermetically closed stainless steel reactor that withstandshigh pressure and equipped with a mixer where biomass isexposed to temperatures of 140–210 °C, and pressures of 0–3.5 MPa. Upon reaching the target temperature, oxygen ispurged into the reactor and biomass is treated for a period oftime (5–120 min). The reaction is terminated by opening dis-charge valve to a flash tank initiating sudden decompressionof the pretreated material [24, 30]. Wet oxidation, on the otherhand, does not have the feature of sudden decompression dur-ing the termination of reaction. Another similar process issteam explosion, in which saturated steam is purged into thereactor without any oxygen addition. This process often re-quires much higher temperature for satisfactory pretreatment[19] and degradation products will be formed to a greaterextent due to the higher temperature used [61]. Hemicelluloseis the polymer that is solubilized first with increasing tempera-ture, releasing also the chemically bound cellulose. The solu-bilization of hemicellulose liberates acids by oxidation of theacidic hemicellulose components and by de-esterification of theacetyl groups on hemicellulose [44]. Addition of an oxidizingagent like gaseous oxygen, air, or hydrogen peroxide (H2O2)[59, 62, 63] results in decomposition of lignin with furtherrelease of cellulose and hemicellulose (Fig. 2). As oxidizingagent, oxygen/air is preferable due to low process operationalcost, and it does not require any additional step (s) such as post-treatment detoxification or neutralization or recovery of thechemical used compared to other inorganic salts, acids, or basepretreatment methods that often involves toxic, corrosive, andhazardous chemicals [64, 65].

The WEx can further be useful for both the anaerobic di-gestion process besides processes involving enzymatic hydro-lysis and microbial fermentation for production of a variety ofmolecules as shown in Fig. 2. The process parameters can beoptimized for different scenarios such as maximum carbohy-drate digestibility, maximum cellulose digestibility, maximumdigestibility with minimum degradation products, and soforth. Generally, a balance between higher digestibility ofthe biomass components and lower formation of unwantedinhibitory by-products are considered during process optimi-zation [66]. In order to establish the WEx process as a cost-efficient technology and suitable for large-scale implementa-tion, current research and development activity has centeredon identifying the optimal process parameters for differentbiomass resources, and on the development of a continuousprocess that could handle biomass inputs of up to 45 % (drymatter basis) [67, 68]. Previous studies of the pretreatment incombination with biogas production revealed that the ef-ficiency of the WEx pretreatment largely depends on thelignin content of the lignocellulosic biomass [59, 69]. Theprocess further improves the enzymatic hydrolysis with asugar recovery up to 95 % with high enzyme efficiency[24, 30].

Table 1 Chemical composition of selected lignocellulosic biomass(dry weight % basis)

Lignocellulosicbiomass

Cellulose,%

Hemicellulose,%

Lignin,%

Reference

Corn stover 37–42 20–28 18–22 [28]

Sugarcane bagasse 26–50 24–34 10–26 [29–31]

Wheat straw 31–44 22–24 16–24 [32–34]

Hardwood stems 40–45 18–40 18–28 [31, 34, 35]

Softwood stems 34–50 21–35 28–35 [34–36]

Rice straw 32–41 15–24 10–18 [33, 37, 38]

Barley straw 33–40 20–35 8–17 [34, 39, 40]

Switch grass 33–46 22–32 12–23 [35, 41, 42]

Energy crops 43–45 24–31 19–12 [41]

Grasses (average)a 25–40 25–50 10–30 [31]

Manure solid fibers 8–27 12–22 2–13 [43]

Municipal organicwaste

21–64 5–22 3–28 [44]

a e. g., reed canary grass, smooth brome grass, tall fescue, etc

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Effects of Wet Explosion Pretreatment on Lignocellulose

Effects of Temperature

For wet explosion, temperature is probably the most importantfactor [44] for the disruption of the lignocellulose matrixfollowed by the concentration of the oxidizing agent. Whilethe solubilization of hemicellulose starts at 110 °C, the cellu-losic crystallinity remains unchanged up to 170 °C, providedthat no addition of oxidative or catalytic agents. Typically, theeffect of temperature is more pronounced when an oxidativeagent is added [30]. At temperatures above 150 °C, organicacids are released from the acetylated side-chains, leading toautohydrolysis [70]. Exothermic degradation of sugars from

cellulose and hemicellulose initiates above 195 °C, due to thexylose ring of hemicellulose [71], although water-mediateddegradation may occur at a lower temperature. Thus the bio-mass pretreatment regimes involving temperatures above160–180 °C typically induces the formation of degradationproducts at a higher degree [72, 73]. With increasing temper-atures and extended residence time degradation products suchas organic acids, 5-hydroxymethylfurfural, furfural are formedthat are known to be inhibitors in subsequent biological pro-cesses [58, 74]. This is especially true for nitrogenous com-pounds that are converted into NOx during the oxidative treat-ment, while formation of ammonia is observed at tempera-tures below 200 °C even with no oxygen is added [75, 76].Ammonia can be produced as a stable end-product by

Flash tank

WE

x r

eact

or

Oxid

igin

g a

gen

t (e

.g. O

2)

(A)

Mono-culture Co-culture

Enzymatic hydrolysis unit Microbial fermentation units

(C)

HO

O

OH

OH

HO

HO

HO

OH

OH

OH

O

HO

OH

OH

OH

O

HO

O

OH

OHHO

HO

HO

O

OH

OH

HO

HO

H3C OH

H3C OH

H3C OH

H3C OH

H3C

OH

O–

O

H3C

OH

O–

O

H3C

OH

O–

O

(B)

CO O H2

CH3

OH

O

H3 C

OH

O

CH3

OH

O

Heat

Electricity

Fuel

Organic fertilizer

H3C OH

Ethanol

O OH

Formic acid

H3C

OH

OH

O

Lactic acid

H3C

OH

O

Propionic acid

HO

O

OH

OSuccinic acid

OH

O

OH

O

Fumaric acid

OH

HO

O

OH

O

Malic acid

HO

OH

OH

OHOH

Xylitol

CH 3

HO

Propanol

CH3

HO

Butanol

CH3

H2C

H2C

Isoprene

(D)

HO

OH

OH

OH

OH

OH

Sorbitol

O

HO

3-hydroxypropionaldehyde

Agricultural residues

Woody biomass

Organic waste

Marine biomass

Fig. 2 Wet explosion as pretreatment in biorefineries for the productionof different biobased products and platform chemicals. a Wet explosionpretreatment units; b anaerobic digestion process; c enzymatic hydrolysis

and microbial fermentation units; d a list of top value-added chemicalsfrom lignocellulosic biomass

Bioenerg. Res.

oxidation of cyanide and amine-containing compounds [75].Temperature will further affect the pH of the lignocellulosicmaterial in reactor during the pretreatment at an elevated tem-perature. For example, the pH of pure water itself is nearly5.0 at a temperature of 200 °C [77]. Formation of carboxylicacids (mainly succinic, glycolic, formic, and acetic acids) oc-curs under severe pretreatment conditions with high oxygenaddition as a result of oxidation of sugars, phenols, and othercompounds [44, 78]. Other sugar degradation products likefurfural and hydroxymethyl furfural (HMF) is further derivedfrom the degradation of pentoses and hexoses at higher tem-peratures, respectively [72].

Autohydrolysis Mechanism

During hydrothermal pretreatment at temperatures of 150–230 °C, biomass undergoes autohydrolysis by releasing aceticacid from the esterified form of arabinoxylans, resulting insoluble oligomers due to the depolymerization of hemicellu-lose polysaccharides [29, 70, 79]. The autohydrolysis processis catalyzed by hydronium ions (H3O

+) coming from waterauto-ionization and also from the acetic acid and uronic acidsin a later stage, leading to a pH drop and improving reactionkinetics of the hydrolysis [80]. During autohydrolysis, solubi-lization of sugars and phenolic compounds occurs due to thecleavage of lignin-carbohydrate bonds [81, 82]. Cleavage ofacetyl and uronic group is enhanced with increasing tempera-ture and pressure which liberates acetic and uronic acid. Attemperatures below 150 °C, autohydrolysis does not modifycellulose and lignin substantially, but major fractions of hemi-cellulose can be converted to xylo-oligosaccharides. Theproducts of autohydrolysis are, therefore, a mixture of oligo-saccharides, monosaccharides, acetic acids, furan derivatives(mainly furfural and HMF), and some phenolic compoundsfrom lignin.

Effects of pH

The pH applied in the thermochemical pretreatment directlyinfluences the solubilization of the biomass components [83,84]. For pretreatments performed under neutral pH conditions,the degradation products of biomass autohydrolysis tend tolower the pH of the biomass slurry to pH (3–4.5) at the endof the process, depending on the amount of the acid chainsreleased under the applied pretreatment conditions. The influ-ence of temperature is more pronounced if the pretreatment isperformed under initial acidic condition, improving the solu-bilization of hemicellulose in particular, leaving the ligninmostly insoluble. In the contrary, lignin solubility increasesunder alkaline conditions due to disruption of the lignin struc-ture followed by saponification of intermolecular ester bondsthat crosslink hemicelluloses and other components, resultingin increased material porosity [83]. Thermochemical

pretreatment of organic waste, for example, showed an increasein COD (chemical oxygen demand) solubilization from 36.9 %at pH=8 to 76.1 % at pH=13 [84]. Separation of structurallinkages between carbohydrate and lignin results in decreaseddegree of polymerization and crystallinity. The influence of thepH on the severity of the pretreatment is described by the fol-lowing equation for combined severity [85]:

Combined severity CSð Þ ¼ log Roð Þ – pH ð1Þ

Where,

log Roð Þ ¼ log t � exp T – T re fð Þ=14:7½ �f g ð2Þ

Here, Ro is the severity factor as a function of treatmenttime (t, in minute) and temperature (T, in degree Celsius)where Tref=100 °C at which no solubilization occurs.

Other investigations showed, however, that the influence ofthe pH is temperature dependent, for example during hydro-thermal liquefaction of biomass at a temperature of 200 °Cand above [41]. The formation of unwanted byproducts is alsovery dependent on pH. While the formation of degradationproducts such as 5-hydroxymethyl furfural is favored underacidic conditions, alkaline conditions result mainly in frag-mentation products such as glycolaldehyde and glyceralde-hyde. Moreover, the use of an acid catalyst revealed to triplethe yield of 5-hydroxymethylfurfural [41].

Effects of Residence Time

When the pretreatment has reached the desired treatment con-ditions, i.e., temperature and pressure, the biomass remains inthe reactor for an amount of time, which is referred to theresidence time. Residence time has earlier been an integratedpart for the equation (Eq. 1) to describe the severity of apretreatment [86].

In general, high-temperature-activated hydrolysis is fast(<60 min) and the residence time is typically in the range of2 to 60 min depending on the other conditions applied. Thepretreatment at higher temperatures requires comparativelyshorter residence times for achieving the same effect due tothe increase in reaction rate. Lower reaction rates at lowertreatment temperatures may be compensated by longer resi-dence times. For example, in steam explosion pretreatment ofsweet sorghum bagasse, a 36 % higher cellulose conversionwas observed at 190 °C when the treatment time was in-creased from 5 to 10min and 39% improvement was obtainedwhen temperature was further increased from 190 to 200 °Cwith 5 min residence time [87]. With addition of oxygen, thesame pretreatment efficiency can be reached for lower tem-peratures and lower residence time. Arvaniti et al. [88], forexample, found higher yields and recoveries for wet oxidationof rape straw for 2- to 3-min residence time at 205–210 °C

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with 1.2 MPa of O2 than pretreatment for 15 min at 195 °C.However, the effect of residence time is more pronouncedunder high temperature in terms of hemicellulose-derived sug-ar recovery in the hydrolysate [89]. Extending the pretreat-ment time beyond 30 min was reported to be disadvantageousdue to sugar decomposition reactions [90].

Effects of Oxygen Addition

Delignification of lignin-rich biomass is greatly improvedwhenan oxidizing agent such as compressed atmospheric air, oxy-gen, or hydrogen peroxide (H2O2) is added during pretreatment[91]. Cellulose is relatively resistant to oxidizing agents [27]and is only partially solubilized under severe conditions. Nearlycomplete solubilization of the hemicellulose fraction [92] andpartial solubilization of lignin occurs when an oxidizing agentis added during pretreatment at high temperatures. Compared tothe addition of acid the addition of an oxidizing agent offers theadvantage of partial hydrolysis of the lignin [93].

At temperatures above 170 °C, oxygen addition results inan exothermic process with reduced heating requirements forthe pretreatment process [94]. This exothermic oxidation re-action follows Arrhenius dependence; therefore, the reactionrate increases with increasing temperature [75]. Alternativesto pure oxygen are atmospheric air and H2O2, although oxy-gen has proven to be most efficient in lignocellulose convert-ibility [95] and cost competitive compared to H2O2 [95].Oxidation reactions are two to three times faster in woodybiomass solubilization when oxygen was added instead ofjust air [44], reflecting the influence of higher oxygen con-centration. Further, oxygen also catalyzes the formation oforganic acids and CO2 from liberated sugars and lignin.This indicates a requirement for optimal concentrationsfor the addition of oxygen in the pretreatment of specificbiomass for a certain downstream process in which thedesired effect is highest.

Thus, the use of proper temperature, oxidizing agent, andextended time for the pretreatment of lignocellulose eliminatesthe requirement of external acid/chemicals. The use of chemicalsuch as acid in the pretreatment could potentially enhance theformation of degradation products including organic acids andfuran derivatives [72, 80]. In the contrary, optimization of pro-cess parameters in the WEx pretreatment for specific biomasstype can potentially enhance the hydrolysis and at the sametime reduce the formation of degradation products that can beinhibitory for subsequent biological processes.

Tailoring the Pretreatment to the Subsequent ConversionProcess

As mentioned earlier, the adjustment of the pretreatment pa-rameters is dependent not only on the specific biomass to be

pretreated but also on the subsequent biological processes. Anefficient hydrolysis of the polysaccharides into sugarmonomers (intermediate platform) can be used for avariety of products and chemicals in biorefineries drivenby enzymatic processes (Fig. 2c). In ethanol fermenta-tion processes, for example, the pretreatment is carriedout initially to achieve a higher bioavailability of thecarbohydrates for a successful enzymatic hydrolysis,followed by yeast fermentation of the hydrolyzed sugarmonomers into ethanol. In particular, the efficiency ofthe pretreatment largely depends on the recovery of thedifferent fractions after pretreatment, enzymatic convert-ibility, enzyme dosage and the creation of potential in-hibitors for the enzymatic hydrolysis and fermentation.The maximum allowed inhibitor concentration in thepretreated hydrolysate for the subsequent biological pro-cess can be used as a parameter to adjust the pretreat-ment severity. On the other hand, when the pretreatmentis applied to boost hydrolysis in anaerobic digestionprocess, the wet exploded biomass directly undergoessingle/multiple step anaerobic digestion process wherehydrolytic enzymes, fermentat ive bacteria, andmethanogens are present for further degradation. In or-der to achieve higher biogas yields from lignocellulosicbiomass, pretreatment is useful to overcome the rate-limiting factor due to incomplete substrate hydrolysis.Thereby, the performance of the pretreatment isreflected in the increase in biogas yield due to improveddestruction of volatile solids, i.e., improved hydrolysiswhile the complex process will adapt to the inhibitorsformed during pretreatment.

Effect on Enzymatic Digestibility

Besides the reaction conditions such as pH and temper-ature, intimate contact of cellulases to cellulose and cel-lulose crystallinity are the important factors in success-ful enzymatic hydrolysis [55]. The efficiency of the pre-treatment on enzymatic hydrolysis depends on the typeof enzymes mixture used (e.g., cellobiohydrolases, beta‐glucosidases, cellulase/xylanase), the enzyme activity,and the concentration of enzymes as enzyme proteinadded per unit of dry matter/cellulose. The effects ofselected thermochemical pretreatments with similaritiesto WEx on enzymatic hydrolysis are listed in Table 2.The basis of this comparison includes the recovery afterpretreatment, enzyme dosage, and enzymatic convertibil-ity of the lignocellulosic feedstock. Wide ranges of tem-perature, residence time, oxidizing agents have beenused in the pretreatment methods (Table 2). Enzymaticconvertibility was assessed although numbers of differ-ent equations were used to calculate the enzymatic con-vertibility. Kristensen et al. [99] introduced a correction

Bioenerg. Res.

Tab

le2

Processconditionsforwetexplosion,wetoxidationor

steam

explosionpretreatmento

fvariouslig

nocellu

losicfeedstocks

andits

effectson

enzymaticconvertib

ility

Pretreatm

enttype

Biomass

Conditio

nsapplied

Chemicalused

Inhibitorsin

the

hydrolysate

Recoveryafterpretreatment

DM,enzym

edosage

Enzym

aticconvertib

ility

Reference

Wetexplosion

Sugarcane

bagasse

185°C

,10min,

16%

ofDM

O2,0.6MPa

Acetic

acid,furfural,

HMFat5.3,1.7,

0.3g/L,respectively

Neglig

ibleloss

8%

and22

mg

enzyme

protein(EP)/g

cellu

lose

a

87%

cellu

lose

[30]

Wetexplosion

Loblolly

pine

170°C

,22min,

25%

ofDM

O2,0.72MPa

Acetic

acid,furfural,

HMFat3,1.5,

0.5g/L,respectively

Neglig

ibleloss

25%,60mgEP/g

cellu

lose

b96

%cellu

lose

andnearly

100%

hemicel.

[24]

Wetexplosion

Wheatstraw

180–185°C

,15

min,14%

ofDM

O2,1.2–1.8MPa

Furfuralat0.2g/L

93%

cellu

lose,

72%

hemicel.

14%,10FP

U/g

cellu

lose

b

70%

cellu

lose,

68%

hemicel.

[95]

Wetexplosion

Wheatstraw

180–185°C

,15

min,14%

ofDM

H2O2,35%

(v/v)

Furfuralat0

.3g/L

>90

%cellu

lose,

>55

%hemicel.

5%,20FPU/g

cellu

lose

b69

%cellu

lose,

55%

hemicel.

[95]

Wetexplosion

Miscanthusc

170°C

,5min,

15%

ofDM

Air,<

1.8MPa

Nodataavailable

61%

glucose,

95%

xylose

7%,45FPU/g

DM

and55

FBG/g

DM

b56

%glucose,

32%

xylose

[63]

Wetexplosion

Winterrye

straw

195°C

,15min,

6%

ofDM

O2,1.2MPa

and

Na 2CO3,2

g/L

Carboxilic

acids,

<5g/L

87%

cellu

lose,

66%

hemicel.

2%,30FPU/g

DM

d49

%cellu

lose,

11%

hemicel.

[96]

Wetexplosion

Oilseedrape

straw

195°C

,15min,

6%

ofDM

O2,1.2MPa

and

Na 2CO3,2

g/L

Carboxilic

acids,

<7g/L

89%

cellu

lose,

60%

hemicel.

2%,30FPU/g

DM

d58

%cellu

lose,

10%

hemicel.

[96]

Wetexplosion

Faba

bean

straw

195°C

,15min,

6%

ofDM

O2,1.2MPa

and

Na 2CO3,2

g/L

Carboxilic

acids,

<6g/L

92%

cellu

lose,

69%

hemicel.

2%,30FPU/g

DM

d43

%cellu

lose,

10%

hemicel.

[96]

Wetoxidation

Sugarcane

bagasse

195°C

,15min,

5.3%

ofDM

O2,1.2MPa

and

Na 2CO3,0.2%

(w/w)

Formicacid,acetic

acid,phenols,furfural,

HMFat2.1,3.2,2.7,

0.2,0g/L,respectively

100%

cellu

lose,

around

90%

hemicellulose

2%,25FPU/g

DM

a,b

57.4%

aand54.3%

b

cellu

lose,44.8%

a

and74.9

%bxylan

[61]

Wetoxidation

Rapestraw

195°C

,15min,

6%

ofDM

O2,1.2MPa

Formic,acetic

acid,

furfural,phenolsat1.6,

1.9,0.3,1.7g/L,

respectiv

ely

96%

cellu

lose,

77%

hemicellulose,

58%

lignin

2%,30FPU/g

DM

d45

%cellu

lose

[88]

Wetoxidation

Rapestraw

205°C

,3min,

6%

ofDM

O2,1.2MPa

Formicacid,acetic

acid,

furfural,phenolsat1.0,

1.1,0.2,1.5g/L,

respectiv

ely

>95

%cellu

lose,100

%hemicellulose,

83%

lignin

2%,30FPU/g

DM

d60

%cellu

lose

[88]

Wetoxidation

Sugarcane

bagasse

195°C

,15min,

5.5%

ofDM

O2,1.2MPa

and

Na 2CO3,0.2%

(w/w)

Carboxilic

acids,furfural,

HMF,phenolsat6.2,

0.2,0,3.9g/100g

bagasse,respectiv

ely

>92

%cellu

lose,

48%

hemicellulose

2%,25FPU/g

DM

a,b

74.9%

aand68.9%

b

cellu

lose,nearly

44%

aand75

%b

xylan

[78]

Wetoxidation

Cloverand

ryegrass

mixtures

195°C

,10min,

6.25

%of

DM

O2,1.2MPa

Formic,acetic,glycolic

acid,furfural,HMF

at1.4,1.7,0.7,0.3,

0.2g/L,respectively

>94

%cellu

lose

2%,25FPU/g

DM

a,b

93.6%

aand88.5%

b

cellu

lose,66.1%

a

and93.3

%bxylan

[97]

Wetoxidation

Wheatstrawe

190°C

,250

L/h

H2O2,0.5%

Nodada

available

100%

cellu

lose,

80%

hemicellulose

2%,30FPU/g

DM

dAround60

%cellu

lose,40%

hemicellulose

[98]

Bioenerg. Res.

factor as given in Eq. (3) to determine the sugar yieldsfor high solids hydrolysis.

Yield %ð Þ ¼ GluEH; L� �þ 1:0526 � CelEH; L

� �

1:111 � FCelluloseRB � ini: sol½ �� 100% � f Correction ð3Þ

Where, fCorrection is the yield correction factor for high solidhydrolysis, [GluEH, L] and [CelEH, L] are the glucose and cel-lobiose concentrations (in gram per liter) in the liquid fractionafter enzymatic hydrolysis. FCelluloseRB is the fraction of cellu-lose in the raw biomass and [ini. sol] refers to the initial solidsconcentration (in gram per liter) used for enzymatic hydroly-sis. To determine the enzymatic efficiency, EE (in percent) andGlucose yield (in percent), more simplified equations [Eqs. (4)and (5)] were used by others [30, 100].

EE %ð Þ ¼ Glucose½ �; g=Lþ Xylose½ �; g=LSolids � Glucan% � 1:111ð Þ þ Solids � Xylan% � 1:136ð Þ; g

.L� 100%

ð4Þ

Glucose yield %ð Þ

¼ Glucose½ �; g=LSolids � Glucan%� 1:111ð Þ; g=L � 100% ð5Þ

In which [Glucose] and [Xylose] represent the concentra-tions of the respective sugars after enzymatic hydrolysis;1.111 and 1.136 are the coefficient of glucose and xyloseobtained from glucan and xylan, respectively.

Soluble inhibitors including organic acids (acetic, formicand, levulinic acid) and furan derivatives (HMF and furfural)formed during thermochemical pretreatment are also knownto restrict enzymatic hydrolysis as well as subsequent fermen-tation [57, 101]. Although the purpose of pretreatment is tomake substrate amenable to enzymes for improved hydrolysisof the polysaccharides by breaking the condensed ligninlayers, creation of phenolic compounds during the thermo-chemical pretreatment is likely to occur due to lignin break-down. Studies suggest that vanillin, syringaldehyde, trans-cinnamic acid, and hydroxybenzoic acid significantly inhibitcellulose enzymes, especially beta-glucosidase, while thepolumeric phenol tannic acid was identified as a major inhib-itor and deactivator for all of the enzyme activities [102, 103].The chemical composition such as lignin/hemicellulose andtheir decomposition during the pretreatment at higher severityare the key factors forming soluble inhibitors that are nega-tively correlated with enzyme digestibility [104, 105]. Theconcentration of soluble inhibitors of enzyme activities andmicrobial growth such as polyphenols derived from lignindegradation during biomass pretreatment are clear targets forT

able2

(contin

ued)

Pretreatmenttype

Biomass

Conditio

nsapplied

Chemicalused

Inhibitorsin

the

hydrolysate

Recoveryafterpretreatment

DM,enzym

edosage

Enzym

aticconvertib

ility

Reference

Wetoxidation

Wheatstraw

185°C

,15min,

6%

ofDM

Na 2CO3,6.5g/L

Formic,acetic,

glycolic,oxalic,m

aleic

acid

at1.5,2.1,1.1,

0.7,0.2g/L,respectively

95.8%

cellu

lose,58.0%

hemicellulose,76.0%

overall

1%,1,500

NCU/m

Land250CBU/g

DM

d66

%cellu

lose

[79]

Steam

explosion

Sugarcane

bagasse

205°C

,10min,

93.9%

ofDM

Saturated

steam,

4.0MPa

Formic,acetic

acid,

phenols,furfural,H

MF

at0.4,1.7,2.1,0.6,

0.2g/L,respectively

87%

cellu

lose,around

80%

hemicellulose

2%,25FPU/g

DM

a,b

48.9%

aand39.0%

b

cellu

lose,38.8%

a

and52.3%

b,xylan

[61]

aEnzym

atichydrolysiswas

carriedouto

nthewashedpretreated

solid

fractio

nbEnzym

atichydrolysiswas

carriedouto

ntheslurry

ofthepretreated

biom

ass

cThe

biom

asswas

soaked

ina0.75

%(v/v)H2SO4solutio

n(10%

DM)at100°C

for14

hdEnzym

atichydrolysiswas

carriedouto

ntheseparatedsolid

fractio

nof

thepretreated

biom

ass

eThe

cutstraw

(1-to

6-cm

pieces)was

presoakedin

water

at80–90°C

for6min

Bioenerg. Res.

Tab

le3

Processconditionsforwetexplosion,wetoxidationor

steam

explosionpretreatmento

fvariouslig

nocellu

losicfeedstocks

andits

effectson

methane

yields

inanaerobicdigestion(A

D)

Pretreatm

ent

type

Feedstock

Conditio

nsapplied

Catalyst/agent

ADconditions

Digestersize,

feed/in

oculum

Effectsof

pretreatment

Com

ments

Reference

Wetexplosion

Digestedmanure

fibers

165°C

,10min,

12%

ofDM

O2,0.6

MPa

Mesophilic,38°C

117mL,0.5

gVS/25mL

+129%

CH4

Nodataon

CSTR

[23]

Wetexplosion

Feedlot

manure

170°C

,25min,

15%

ofDM

O2,0.4

MPa

Therm

ophilic,

55°C

40L,2.22g

VS/L/Day

+357%

CH4

Dataon

CST

Ronly

[59]

Wetexplosion

Wheatstraw

180°C

,5min,

10%

ofDM

H2O2,6

g/100g

DM

Therm

ophilic,

55°C

117mL,0.5

gDM/20mL

−11%

CH4

Noyieldincrease

inCSTR

[108]

Wetoxidation

Yardwaste

185°C

,15min

O2,1.2

MPaand

Na 2CO3,2

g/L

Therm

ophilic,

55°C

,28days

100mL,0.5

gVSS/60

mL

+99

%CH4

Realized

5days

lagperiod

[109]

Wetoxidation

Foodwaste

185°C

,10min

O2,1.2

MPaand

Na 2CO3,2

g/L

Therm

ophilic,

55°C

,28days

100mL,0.5

gVSS/60

mL

+7%

CH4

Realized

5days

lagperiod

[109]

Wetoxidation

Digestedbiow

aste

185°C

,15min

O2,1.2

MPaand

Na 2CO3,2

g/L

Therm

ophilic,

55°C

,28days

100mL,0.5

gVSS/60

mL

+72

%CH4

1.5tim

eshigher

yields

inbatch

than

CSTR

[109]

Wetoxidation

Willow

180°C

,5min,

15%

ofDM

H2O2,6

%Therm

ophilic,

55°C

,60days

NAa

+80

%CH4

Nodataon

CST

R[110]

Wetoxidation

Digestedfibers

170°C

,NAa ,5%

ofDM

H2O2,0.1

g/g-DM

Therm

ophilic,

55°C

,16days

117mL,1

gVS/20mL

+42

%CH4

Assum

ed5%

VS

loss

[69]

Wetoxidation

Cow

manure

170°C

,NAa ,8%

ofDM

H2O2,0.1

g/g-DM

Therm

ophilic,

55°C

,51days

117mL,1

gVS/20mL

+26

%CH4

Assum

ed5%

VS

loss

[69]

Wetoxidation

Pig

manure

170°C

,NAa ,3%

ofDM

H2O2,0.1

g/g-DM

Therm

ophilic,

55°C

,51days

117mL,1

gVS/20mL

−28%

CH4

Assum

ed5%

VS

loss

[69]

Wetoxidation

Manure-sludge

mixture

170°C

,NAa ,5%

ofDM

H2O2,0.1

g/g-DM

Therm

ophilic,

55°C

,59days

117mL,1

gVS/20mL

+14

%CH4

Assum

ed5%

VS

loss

[69]

Wetoxidation

Fibersof

pig

manure

170°C

,NAa ,14.5

%of

DM

H2O2,0.1

g/g-DM

Therm

ophilic,

55°C

,58days

117mL,1

gVS/20mL

−10%

CH4

Assum

ed5%

VS

loss

[69]

Wetoxidation

Chicken

feathers

140°C

,60min,

10gCarbon/L

air,2.0MPa

Mesophilic,

36°C

,70days

160mL,N

Aa

+195%

CH4

Yield

asmLCH4/g

VSS

[111]

Wetoxidation

Chicken

feathers

200°C

,60min,

10gCarbon/L

air,2.0MPa

Mesophilic,

36°C

,70days

160mL,N

Aa

+50

%CH4

Yield

asmLCH4/g

VSS

[111]

Wetoxidation

Kraftpulp

solid

s200°C

,60min,

10gCarbon/L

air,2.0MPa

Mesophilic,

36°C

,70days

160mL,N

Aa

+100%

CH4

Yield

asmLCH4/g

VSS

[111]

Wetoxidation

New

spaper

waste

190°C

,60min,

20g/L

1.76

gO2

Mesophilic,

35°C

,60days

2Lworking

volume,9.96

TCOD(g/L)

88%

ofcellu

lose

was

degraded

59%

Conversion

efficiency

[112]

Wetoxidation

Winterryestraw

195°C

,15min,

6%

ofDM

O2,1.2

MPaand

Na 2CO3,2

g/L

Mesophilic,

42°C

,67days

100mL,

0.3gDM/20mL

+34

%CH4,

g/100gDM

96%

oftheoretical

yield

[96]

Therm

al(autoclave)

Digestedsolid

fractio

ns120°C

,30min

None

Mesophilic,

40°C

,56days

2L,ISRb=2

(VSbasis)

+115%

CH4

Nodataon

CST

R[113]

Therm

al(autoclave)

Solid

fractio

nof

manures

140°C

,40min

None

Therm

ophilic,

55°C

,10days

118mL,10gwet

solid

/26mL

+57

%CH4

Only7%

yield

increase

inCSTR

[114]

Steam

explosion

Wheatstraw

Steam,2.0

MPa

+14

%CH4

Nodataon

CST

R[115]

Bioenerg. Res.

reduction in the pretreatment process. High solid pretreatmentalong with higher severity is a primary cause of reduced en-zyme activity due to formation of higher concentration of thesoluble inhibitors. However, low severity pretreatment at highsolid concentrationmay limit the inhibitor concentration whileimproving the enzymatic hydrolysis process. Rana et al. [24]performed wet explosion pretreatment of Loblolly pine at170 °C for 22 min at 25 % solid concentration with0.72 MPa O2 pressure and enhanced overall yields ofmonosaccharaides (up to 96 % cellulose and nearly 100 %hemicellulose conversion). The level of inhibitors concentra-tion did not seem to impede the enzymatic hydrolysis even at25 % dry matter (DM). However, most of the lab-scale enzy-matic hydrolysis of wet exploded biomass has been carriedout at solids concentrations of 1–8 %, substantially lower thanthe concentration of pretreated solids needed for economicoperation of a biorefinery. Nevertheless, cellulose and hemi-cellulose yields in enzymatic hydrolysis are reported between56–100 % and 38–100 %, respectively, depending on the bio-mass and pretreatment parameters used (Table 2).

Effect on Methane Yield

Substantial research efforts have been made in recent years toimprove the anaerobic digestibility of lignocellulosic feed-stock by applying different pretreatment methods [23, 106,107]. Results presented in literature on testing enhancementmethod such as thermal and thermochemical pretreatmentmethods are showed in Table 3.

Unlike the sugar-based bioethanol processes, hydrolyticenzymes are present in the anaerobic digestion (AD) mixedcultures, where they are excreted by hydrolytic microorgan-ism. Consequently, degradation of organic polymers such ascellulose, hemicellulose, proteins, and lipids will occur evenwithout adding any external enzymes to the process. In ADprocesses, the hydrolysis of organic polymers is typically car-ried out by bacterial extracellular enzymes (hydrolases) [116]as well as physicochemical reactions. Vavilin et al. [117] de-scribed a two-phase model for hydrolysis kinetics during AD.In the first phase, hydrolytic bacteria cover the surface ofsolids and enzymes are released cleaving the polymers intomonomers. Both hydrolytic, acidogenic, and acetogenic bac-teria utilize these hydrolysis products for growth and convertsthem into intermediate products such as volatile fatty acids,CO2, and H2 which are further converted by acetoclastic andhydrogenotrophic methanogens to produce CH4 and CO2

[118, 119].The overall degradation process is dependent on the proper

performance of each microbial group in the AD pathway. Themethanogens are often more the most sensitive microbes inAD to any inhibitors (e.g., changes in pH, O2 contamination,inhibitory compounds such as ammonia) compared to anyother groups of microorganisms. Several factors related toT

able3

(contin

ued)

Pretreatm

ent

type

Feedstock

Conditio

nsapplied

Catalyst/agent

ADconditions

Digestersize,

feed/in

oculum

Effectsof

pretreatment

Com

ments

Reference

160°C

,10min,

22%

ofDM

Mesophilic,

37.5

°C250mL,ISR

b=3

(DM

basis)

Steam

explosion

Wheatstraw

180°C

,15min,

22%

ofDM

Steam,2.0

MPa

Mesophilic,

37.5

°C250mL,ISR

b=3

(DM

basis)

+20

%CH4

Nodataon

CST

R[115]

aNoavailabledata

bInoculum

tosubstrateratio

Bioenerg. Res.

the inhibition of microbial consortia have been described else-where [74]. Formation of by-products under harsh thermo-chemical pretreatment conditions may inhibit microbial pro-cesses of the different stages in the AD pathway resulting in alower methane yields or even complete cessation of methaneproduction. The production of organic acids during pretreat-ment, however, does not lead to inhibition of the AD process,but rather to an increase in methane yield since organic acidsserve as substrate for acetoclastic methanogens after the pro-cess has been adapted to the new conditions. Also the releaseof C5 sugars will directly boost the methane yield since thesecan be readily converted by fermentative bacteria. Therefore,the adjustment of the pretreatment parameters in combinationwith AD may lead to other optimal values for the same bio-mass than when applying the pretreatment in combination

with enzymatic hydrolysis and ethanol fermentation.Furthermore, the pretreatment should only be applied specif-ically to those fractions of the feedstock that cannot readily bedegraded by the microbial consortia of AD. This can for ex-ample be achieved when the pretreatment is carried out on theundigested fibrous fraction of the substrate after an initial di-gestion [23]. This concept can be useful to reduce the volumeof the substrate to be pretreated and, thereby reducing energydemand and costs for the pretreatment in an AD plant.

Accordingly, Lissens et al. [109] found only a 7 % increasein methane yields as an effect of wet oxidation pretreatment ofraw food waste (Table 3), while a 72 % increase was achievedwith digested biowaste composed of green waste along withsource-sorted household waste. Uellendahl et al. [69] ob-served a 10 % decrease in methane yield after wet oxidation

Table 4 Advantages and disadvantages of selected pretreatment methods for lignocellulosic biomass

Pretreatment Advantages Disadvantages

Steam explosion ● No chemical, recycling or environmental cost [19]● Requires comparatively low energy input● Low operational cost [120, 121]

● Incomplete destruction of lignin-carbohydratematrix [17, 19]

● Partial solubilization of hemicellulose● Creates inhibitors at high temperature

Wet explosion(WEx)

● Solubilizes hemicellulose and lignin under lower temperature than steamexplosion, no chemical recovery required

● Higher enzymatic efficiency in hydrolysis● Uses high solids, suitable for wide variety of biomass

● Requires optimization to limit the formation ofinhibitors at high temperature

● Moderate capital cost, cost for oxygen/air/H2O2/other oxidizing agents

● Has not been applied beyond pilot-scaleinvestigation

Liquid hot water(LHW)

● Uses water as solvent, requires no chemical or recovery [18]● Uses lower temperature than steam explosion, forms no/less inhibitors [19]● Cost-effective for large-scale operation

● Dilutes solubilized products at a lowerconcentration [19]

● Less capability in removing lignin [17]● Involves large volume of water, thus energyintensive [16]

Dilute acid (DA) ● Higher reaction rate [19]● Solubilizes cellulose and hemicellulose, alters lignin structure [15]● Increases accessible surface area [15]

● Corrosive, expensive, and hazardous chemical use,required alkali to neutralize

● Forms inhibitors at higher concentrations [17, 89]● High equipment and operational cost [16]

Alkali ● Increases surface area, swells structure● Disrupts lignin structure [122]● Improves enzymatic hydrolysis

● High process operation cost● Low-quality lignin as byproduct [123]● Additional costs due to pH adjustment

AFEX/ARP ● Efficient and selective delignification method, alters hemicellulose whiledecrystallizing the cellulose [19]

● Minimizes the formation of inhibitors● Ammonia in cheaper than sulfuric acid

● Effective for biomass with low lignin content, noteffective for softwood [19]

● Very high capital cost and ammonia recovery cost[16]

● Environmental issues with ammonia

Ionic liquids(ILs)

● Very effective dissolution of cellulose● Formation of no/least inhibitors [124]● Synthesizes value added organics

● High recyclability is essential for economicviability [125]

● Still in the exploratory stage

Organosolv ● Accelerates delignification by disassociation of hydrogen ion [19]● Uses comparatively lower temperature● High-quality reactive lignin and cellulose and an aqueous hemicellulosestream [19]

● Solvent must be recycled to be economic● Any residual solvent may cause inhibition formicrobial growth [89]

● Explosion hazards, environmental and health andsafety concerns [19]

Biological ● Produces biomass-degrading ezymes, thus reduces enzymes cost [53]● Uses mild conditions and low energy● Environmentally sustainable application

● Slow rate, can take several weeks/months● Inefficient for industrial application● Requires favorable growth conditions for microbes

which can be tedious [19]

Bioenerg. Res.

of fibers separated from raw pig manure at 170 °C with addi-tion of H2O2 (0.1 g/g-DM), while the methane yield increasedby 42 % when treating digested manure fibers under the samecondition . Similarly, Biswas et al. [23] achieved a 129 %increase in CH4 yield on digested fibers using wet explosionpretreatment at 180 °C for 10min using 0.6MPa O2. A similarinvestigation on digested solid fractions was conducted byMenardo et al. [113] using autoclavation at 120 °C for30 min and the methane yield was increased by 115 %.Most recently, it has been shown that the oxygen-assisted WEx pretreatment promotes lignin solubility,thus enhancing the conversion of lignin in AD process.Ahring et al. [59] found 44.5 % conversion of ligninwhen feedlot manure was pretreated at 170 °C for25 min using 0.4 MPa oxygen in WEx pretreatment. Itwas evident that formation of low molecular lignin com-pounds, especially aliphatic compounds, occurred duringthe pretreatment that have been converted in subsequentAD process.

Comparison of WEx Pretreatment with Other MostCommon Pretreatment Methods

Although many pretreatment methods are promising for in-dustrial application and have been extensively investigated,generally, they are only useful on some specific biomassesand for a specific biological process. Formation of inhibitorycompounds during pretreatment is increased by the use ofchemicals and the requirement of detoxification prior to thebiological process. Furthermore, high energy consumption,low mass recovery together with the need for recovery of thechemicals added to the pretreatment—all need to be addressedto ensure positive process economy. A summary of the advan-tages and disadvantages of different pretreatments are shownin Table 4. Hydrothermal pretreatments such as steam explo-sion, liquid hot water are effective and environmentallysound; however, the need for higher process temperaturesleading to higher energy input and has a low efficiency forattacking the lignin matrix. The WEx pretreatment on theother hand, requires less energy input due to the exothermicreaction taken place in the process and has moderate capitalcost. The use of oxygen or other oxidizing agents have aninfluence on the operational cost. However, the lower concen-trations of inhibitory products such as HMF, furfural, andorganic acids as well as the high performance rate of the pro-cess will offset these extra costs.

Conclusions and Perspectives

If lignocellulosic feedstock is going to play a dominant role infuture biorefineries, an efficient pretreatment method will be

the key for tapping the potential of the abundant renewableresources. Wet explosion, a thermochemical pretreatmentmethod where temperature, residence time, and addition ofan oxidizing agent can be adjusted to the biomass compositionand to the subsequent microbial processes, has shown its po-tential to achieve this goal. The advantages of using wet ex-plosion pretreatment method include:

& High efficiency for enhancing the enzymatic hydrolysis ofbiomass at high dry matter concentrations for ethanol andother sugar-based fermentations as part of biorefineries;

& No detoxification of the hydrolysate is necessary, due tolow/tolerable formation of degradation products for sub-sequent microbial processes;

& Applicable to a wide range of feedstocks and subsequentfermentation processes;

& Suitable for industrial scale-up, environmental friendlyprocess.

Adjustment of the optimal pretreatment conditions for wetexplosion is essential for each biomass in order to maximizethe product yields and minimize by-product formation. Afteroptimization, the process has shown a major potential for useon any biomass including woody materials in addition to ag-ricultural residues.

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