Results of acid-catalyzed SE of selected biomass.
A major challenge to commercial production of cellulosic ethanol pertains to the cost-effective breakdown of the complex and recalcitrant structure of lignocellulose into its components by pretreatment methods—physical, chemical, physico-chemical, biological and various combinations thereof. The type and conditions of a pretreatment impacts both upstream processes such as size reduction as well as downstream processes such as enzymatic hydrolysis and enzyme loadings, and as such the choice of a pretreatment method for a specific biomass (or mix of materials) is influenced by several factors such as carbohydrate preservation and digestibility, sugar and ethanol yields, energy consumption, equipment and solvent costs, lignin removal and quality, formation of sugar/lignin degradation products, waste production, and water usage, among others. This chapter reviews both well-known and emerging physico-chemical methods of biomass fractionation with regards to process description and applications, advantages and disadvantages, as well as recent innovations employed to improve sugar yields, environmental sustainability and process economics.
- physico-chemical pretreatment
Pretreatment and enzymatic hydrolysis present the most practical challenges (technical, environmental and economic) in the attempt to commercialize cellulosic bioethanol. Pretreatment is costly since it represents about 20% of total cost . However, without pretreatment, enzymatic degradation of native biomass is generally below 20% yield , making pretreatment a crucial process of bioethanol production. In general, the selection of a pretreatment method for a material or mix of feedstocks is influenced by factors that include carbohydrate preservation and digestibility, sugar and ethanol yields, energy consumption, equipment and solvent costs, lignin removal and quality, formation of degradation products from sugars and lignin, waste production, and water usage.
Pretreatment may be categorized as physical (e.g., size reduction, autoclaving, irradiation, popping, ultrasonication, steaming and extrusion), chemical (use of acids, alkali, solvents, etc.), biological (white-rot fungi, brown rot fungi, etc.), and physico-chemical which combine both physical and chemical processes to ensure the digestibility of the lignocellulosic material. Physical pretreatment usually demands high-energy consumption (e.g., side reduction) and is undertaken before chemical or biological pretreatment. While chemical methods offer benefits such as efficient fractionation of biomass and good sugar yields during enzymatic hydrolysis, their environmental impacts are higher than physical and biological methods due to biomass degradation into enzyme-inhibiting compounds, corrosion of reactors, solvent recycling issues, and generation of waste [3, 4].
The well-known physico-chemical pretreatment include liquid-hot water (LHW), steam explosion (SE), ammonium fiber explosion (AFEX), soaking in aqueous ammonia (SAA) and irradiation-chemical method. This chapter reviews and present novel findings as well as process innovations in physico-chemical processing of recalcitrant biomass to sugars and ethanol.
2. Liquid hot-water (LHW) pretreatment
This hydrothermal process involves cooking of biomass in liquid water at high temperatures (150–240°C) and short times (≤ 50 min). Pretreatment causes pressurized water to rupture and penetrate the cell structure, resulting in fractionation of biomass into two product streams—liquid hydroxylate containing hemicelluloses sugars, minerals, and degradation products such as furfural and acetic acid, and a solid fraction comprising most of the cellulose and lignin and some residual hemicellulose. Pretreated solid substrates have increased surface area and pore volume, and consist of separated individual cellulose fibers, with large particles of repolymerized lignin on the surfaces of the cellulose matrix . The harshness of the process is described by a severity factor (
Generally, the carbohydrate content of pretreated substrates increases with temperature until a maximum temperature is reached where further temperature increase result in substantial degradation. Thus, high temperatures greater than 230°C disrupt pretreated particles and reduce the surface area and pore volume, which in turn limit enzymatic digestibility .
Based on the work of Mosier et al. , it is observed that at reaction conditions of 200–230°C and ≤ 15 min, biomass dissolution ranged from 40 to 60%, comprising 4–22, 35–60, and 100% of cellulose, lignin, and hemicellulose, respectively. Much of the hemicellulose dissolve into poly- and oligosaccharides even at high severities which is beneficial since the minimization of monomer formation reduces the chance of further degradation into aldehydes (2-furaldehyde, 5-hydroxymethylfurfural, etc.), which are known inhibitors of enzymatic hydrolysis. However, oligomers also possess inhibitory tendencies to cellulase activity . It has been established that digestibility of pretreated substrates depended more on partial removal of hemicellulose and relocalization of lignin than the modification of crystallinity and rupture of the cell wall [5, 8].
LHW pretreatment has been applied to pretreat various feedstocks including agricultural residues, woods and industrial waste. Archambault-Leger et al.  applied both batch and flowthrough pretreatment to corn stover, bagasse, and poplar and observed higher hemicellulose recovery, removal of non-carbohydrate carbon, and glucan conversion under simultaneous saccharification and fermentation (SSF) in the flowthrough reactor. Some authors have included additives to positively influence pretreatment outcomes. For example, by adding AlCl3 to pretreatment solution of microcrystalline cellulose, low concentration of degradation products and high glucose yields were obtained . Optimum glucose yield of 80% was also observed when solid carbon dioxide was used in hydrothermal fractionation of
Combinations of LHW and other methods have also been employed to overcome inherent drawbacks and to improve sugar yields. Low degradation products and higher sugar yields—xylose (91.62%) and glucose (88.12%)—was observed when LHW treatment (180°C, 20 min) of
At demonstration and industrial scale, one notable application of LWH is in the Integrated Biomass Utilization System (IBUS) platform where biomass is converted into ethanol, C5 molasses, and lignin pellets using uncatalyzed steam in an energy efficient manner underscored by high dry matter content in all process routes . It was developed as a three-stage, pilot-scale process for treating wheat straw—by soaking at 80°C for 20 min, hemicellulose recovery at 170–180°C for 7.5–15 min, and cellulose hydrolysis at 195°C for 3 min. Under these conditions both ethanol production and lignin recovery for power production are maximized . High glucose yield required the avoidance of water addition to the third stage while high hemicellulose yield (83%) required water addition. It was improved by Petersen and team  who used a two-stage procedure to achieve high cellulose recovery (over 90%) on wheat straw: soaking of biomass at 80°C for 5–10 min followed by pretreating at temperatures and residence times ranging from 185 to 198°C and 6–12 min respectively. The optimum pretreatment temperature was observed at 195°C at which cellulose and hemicellulose recovery reached 93–94 and 70% respectively at lower water/biomass ratio compared to the three-stage process. However, the two-step process was found to present economic challenges in the recovery of C5 sugars after the first pretreatment in a commercial-scale plant, prompting Inbicon to settle for a simpler, one-stage treatment processs . Currently, the Inbicon demonstration plant, which is based in Kalundborg (Denmark), processes about 4 tonnes straw/h and at yields greater than 198 L ethanol/tonne of wheat straw.
2.3. Positive attributes and drawbacks
LHW offers improved digestibility of cellulose by enzymes due to the solubilization of hemicelluloses and avoidance of inhibitors. Compared to steam explosion, LHW gives lower concentrations of solubilized hemicellulose and lignin products due to higher water input as well as higher pentosan recovery. Generally, catalysts/chemicals are avoided resulting in no/low neutralization demands and byproduct/precipitate generation, with additional benefits such as reduced risk of reactor corrosion and explosion. Reactor cost is lower compared to methods such as AFEX . The effect of particle size reduction on hydrolysis is low, thus, large biomass flowrates can be handled effectively.
There are however drawbacks in LHW related to hemicellulose fractionation into large fractions of oligomers, and xylose yields are generally low, which affect sugar and ethanol yields. There is a risk of sugar degradation into byproducts such as carboxylic acids and furans at severe conditions [19, 20]. A major cost involved in LHW pertains to high energy used to generate saturated liquid water. Consequently, solid loadings are restricted to about 20% .
3. Steam explosion (SP)
In steam explosion, biomass is exposed to saturated steam at high pressure (0.5–4.8 MPa) for a maximum period of 60 min followed by sudden reduction of pressure to atmospheric or lower, resulting in explosive decompression of biomass into component fiber and fiber bundles. The explosion is triggered by evaporation within biomass cells and sudden drop of pressure around the biomass. Exploded materials experience increase in water retention and pore size and specific surface area. Consequently, the bulk density is decreased. To improve penetration efficiency and swelling, biomass is pre-soaked before pretreatment. While the buffering effects of free moisture reduce heat transfer and increase energy demand, bound moisture softens fibers and increase pretreatment efficiency . Thus, by carefully regulating water content of feedstock, substantial gains in sugar yield can be obtained during enzymatic hydrolysis, with collateral benefits in reduced energy demand .
The pretreated solids comprise unhydrolyzed cellulose, chemically-transformed lignin, and residual hemicelluloses. The liquid hydrolysate, on the other hand, contains solubilized hemicelluloses in oligomeric forms, with concentrations of monomers usually exceeding similar situations under LHW. Hemicellulose is hydrolyzed via the breakdown of both glycosidic and hemicellulose-lignin bonds. Hydrolysis of parts (acetyl groups and uronic acid substitutions) of hemicelluloses—via the catalytic actions of protons generated from the autoionization of water—occurs to form acetic and other acids which enhance further fractionation of hemicellulose , and trigger the release of carbonium ions from benzyl alcohol structures in lignin which cause the breakdown of some of the β-O-4 structures in lignin leading to reduced molecular weight . Simultaneously, condensation reactions may take place in the presence of electron-rich carbon atoms, resulting in lignin repolymerization [25, 26], with the composition affected by pretreatment severity .
The process is affected by temperature, reaction time, material size, moisture content and efficient mixing of biomass. The explosion mechanism and time which are independent of the severity factor are also known to affect yields . Increasing reaction time and temperature decreases the degree of polymerization of cellulose . Though severe conditions contribute to reduction in crystallinity and increase in moisture retention, they do not necessarily lead to increased hydrolysis rates due to possibility of thermal degradation of cellulose. Similarly, xylose recovery is reduced for longer pretreatment times due to formation of degradation products. Further, severe conditions increase the intensity of repolymerization and condensation reactions from byproducts of lignin, hemicellulose, and extractives leading to increased molecular weights of lignin . This development reduces substrate amenability to enzymatic hydrolysis caused by the covering of cellulose surface with the repolymerised lignin-like materials (pseudo-lignin). The problem of lignin repolymerization was overcome by Li et al.  who used a carbonium ion scavenger (2-napththol) to achieve solubilize lignin, resulting in improved recovery (91%) as against 51% for steam pretreated aspen wood without the additive.
SE has been applied in combination with additives and pretreatment methods to improve yields and overall process economics. The major variations include the use of acids and bases as catalysts.
3.3. Acid-catalyzed steam explosion (ACSE)
In this process, SE is undertaken after the biomass is soaked with dilute acid or impregnated with SO2 or CO2 at low or atmospheric pressures for 0.5–25 h depending on the temperature (5–100°C). It favors solubilization of hemicelluloses into monomer units, making substrates more reactive while improving enzymatic hydrolysis of cellulose. Compared to dilute acid, SO2 impregnates biomass substrates better and more uniformly but requires harsher conditions to remove hemicellulose . Both SO2- and CO2-based SE create the formation of pores of different sizes and shapes in the outer region of the cell wall of pretreated substrates, with the effect more noticeable in SO2-based applications due to its higher combined severities under similar conditions . Though CO2 has a lower solubility compared to SO2, CO2 is highly available, less toxic and corrosive, and thus safer to apply.
A major positive attribute about ACSE is that most glucan and lignin are untouched and remain in solid form after pretreatment  though lignin presence hinders enzymatic hydrolysis . Nonetheless, high sugar yields are generally obtained. Yields obtained by some investigators are given in Table 1.
|Agent/catalyst||T (°C), t (min)||Biomass||Observation||Reference|
|CO2||205, 15||Sugar cane bagasse and leaves||High glucose yield of 86.6%|||
|220, 5||High glucose yield of 97.2%|
|SO2||190, 5||Sugarcane bagasse||Moderately high glucose yield of 79.7%|
|Sugarcane leaves||High glucose yield of 91.9%|
|SO2||205–225, 5–10||Spruce, pine, birch and aspen||High fractionation efficiency of alkaline extractable lignin for hard woods, but low for softwoods.|||
|H2SO4||185, 2||Rice straw||Overall saccharification yield of 73% in a pilot plant|||
|H2SO4||190, 10||Wheat straw||Glucose and xylose yields of 102 and 96% of theoretical. Ethanol yield of 67% based on glucose content of raw material in SSF.|||
|Acetic/ethanol||180–225, 3–60||Wheat straw||Sugar yield after enzymatic conversion was found higher than treatment without additive, with maximum yield of 264 g/kg DS obtained for ethanol/SE.|||
The main disadvantages include the toxicity of SO2 in SO2-catalyzed applications and the unavoidable release of degradation products. The acidic nature of pretreatment requires expensive reactors that can withstand corrosion. SO2 may be costly and as such on-site production could be an alternative for improving the financial viability . The efficient use of co-products such as lignin and hemicellulose in process integration improves the economic health of the process considerably.
3.4. Alkaline-catalyzed steam explosion
Alkaline-catalyzed SE has received less attention compared to acid-based SE. The alkaline solution improves delignification of biomass, giving higher enzymatic degradability. Park et al.  pretreated
3.5. Double-stage pretreatment involving SE
The major target of the two-step process is to achieve higher delignification and increase biomass digestibility. In many cases, significant increase in glucose yields relative to SE application only, have been observed as outlined in Table 2.
|First stage||Second stage||Biomass||Results||Reference|
|SE||Organosolv||Poplar||Improved lignin removal; over 98% recovery of cellulose; glucan digestibility >88%|||
|SE||O2 in alkaline solution||Douglas-fir||84% removal of lignin left in exploded substrates|||
|SE||H2O2 + stabilizers||Douglas-fir||Effective lignin removal|||
|SE||Laccase||Wheat straw||Effective removal of lignin phenols; high ethanol yields||[43, 44]|
|SE||Fungi||Wheat straw||75% of lignin degraded|||
|SE||WO||Pine||96% cellulose yield; ~100% hemicellulose yield|||
|Dilute acid||SE||Rice straw||Reduced inhibitor formation; enhanced xylose yield degradability|||
|SE||Alkaline||Sugarcane straw||Enzymatic conversion of 85% in an industrial (SE) reactor|||
3.6. Industrial application
SE is among leading pretreatment methods in terms of cost effectiveness and has been implemented at demonstration (e.g., BioGasol plant in Denmark; Green Plains’s plants in USA) and industrial scale (e.g., Crescentino, Italy; Raízen and Iogen’s plant in São Paulo, Brazil).
3.7. Positive attributes and drawbacks
SE is among the most cost-effective methods for and agricultural residues and hardwoods since it does not require external catalysts. It offers the possibility of pretreatment at high solids loading due to the high-energy content of steam and low water requirements which reduce capital expenditure. Moreover, excessive dilution of sugars in pretreated liquor is reduced while the downstream processing of waste solution is minimized or eliminated. Another advantage relates to the possibility of using large biomass sizes which can lead to lower energy intensity. Though particles smaller than 2 cm are usually used, a recent study using larger biomass size (2.5 cm) was found to improve saccharification yield and overall process economics more than smaller sizes (0.5–1 cm); however, smaller particles recorded higher pretreated sugar recovery . Corrosion is reduced due to the non-usage/low-use of chemicals.
Despite the advantages, there are inherent drawbacks associated with SE. The formation of inhibitory products, especially furan derivatives, weak acids and phenolic compounds, negatively affect enzymatic hydrolysis and fermentation . Severe conditions cause increased degradation of cellulose and hemicellulose. There is also a risk of condensation and precipitation of soluble lignin components which leads to reduced digestibility of the biomass substrates [41, 51], while disrupting the lignin structure. SE is less effective on softwood and unexploded materials are common. Further, pretreatment at high temperatures and pressures creates additional challenges in material handling, reactor operation, energy management and heat recovery . Thus, scaling-up is a challenge since large volumes of biomass must be heated to high temperatures in short times.
4. Ammonium fiber explosion (AFEX)
In AFEX, liquid (anhydrous) ammonia at moderate-to-high temperatures (60–200°C) and pressures (6.5–45 bar) is mixed with moist biomass for about 5–30 min, followed by a sudden drop in pressure to atmospheric. Ammonia is usually fed at less than 2 kg/kg of dry biomass. AFEX leads to the removal of lignin and some hemicelluloses, in addition to the decrystallization of cellulose, partly due to the strong affinity of ammonia for such biomass components. According to Chundawat et al. , pretreatment causes morphological and physicochemical changes to cell walls of the material, by creating nanoscale network of interconnected tunnels within the cell wall structure through the cleaving of lignin-carbohydrate ester bonds, and the partial removal and subsequent deposition of extractives on cell wall surfaces, leading to enhanced enzymatic access to cellulose. Further, Maillard reactions between ammonia and carbonyl-based aldehydic groups give rise to several intermediate products .
AFEX is generally affected by the moisture content and particle size of biomass, ammonia loading and process conditions including temperature and residence time. Higher temperatures cause more ammonia to flash causing greater disruption of the fibrous structure. Both glucan and xylan conversion (at fixed temperature and ammonia loading) was found to increase with moisture content of switchgrass . In another study, particle size reduction increased the conversion of cellulose and xylan during pretreatment of corn stover .
AFEX has been widely applied to various class of lignocellulosic materials. Some results obtained from AFEX pretreatment of some biomass are given in Table 3.
|Reaction conditions||NH3 loading, g/g dry mass||Biomass||Results||Reference|
|102°C, 30 min, 2.24 MPa||2a||Agave bagasse||~100% carbohydrate preservation; 42.5 g glucose and xylose/100 g native biomass|||
|40–110°C, 1.4 MPa||1||Rice and wheat straw, sorghum and maize stovers||60–85% glucose recovery, 50–85% xylose recovery|||
|165.1°C, 69.8 min, 14.3% NH3, 2.2 MPa of CO2||Rice straw||93.6% glucose yield; 97% theoretical ethanol yield|||
|170°C, 10 min||5||Giant weed||94.2% glucan conversion; 84.4% xylan conversion|||
|150°C for 30 min||1.5||Switchgrass||98% xylose yield|||
|70°C, 350–430 psi, 14–18 min||0.8||Dry distillers’ grains||90% cellulose conversion to glucose|||
4.3. Positive attributes and drawbacks
AFEX is a dry-to-dry process since no liquid stream is produced, making it potentially less costly compared to steam explosion  and dilute acid methods . The process is simple as it reduces requirements of post-pretreatment washing, stream separation and nutrient supplementation, and produces intermediates that are of value in developing advanced bioproducts. Reaction temperatures are moderate and energy requirements are low. Large solids (up to 5 cm) can be fractionated with good yields. Moreover, desired solid loadings are easily obtained, and high solid loadings are easier to implement due to low water demands. High glucose and xylose yields are both obtained under similar process conditions which simplify the optimization of process parameters. Moreover, except for some phenolic fragments of lignin and cell wall extractives that may form on the surface of pretreated solids, no enzyme-inhibitors are produced . AFEX give high sugar yields at low enzyme loadings of 1–10 FPU cellulase/g of dry biomass . Klason lignin and carbohydrates are preserved and pretreated substrates possess high fermentability. Recently, process improvements bordering on ammonia loading and recovery, ammonia recycle concentration, and enzyme loadings have been developed and shown to reduce the cost of operation of AFEX-based biorefinery .
Its main demerit is its unsuitability for handling materials with high lignin content such as wood. Much of the hemicellulose is fractionated to oligomers making it more challenging during fermentation. High pressures are usually required due to high ammonia loadings and high vapor pressure of ammonia. Moreover, ammonia is expensive and recovery of all feed ammonia for reuse is challenging. Safety issues arising from the corrosive and toxic nature of ammonia present additional challenges in process operation at industrial level. Compared to soaking in aqueous ammonia (SAA), AFEX requires expensive reactors and equipment.
5. Soaking in aqueous ammonia (SAA)
SAA involves treatment of biomass with aqueous ammonia (5–50%w/w) at low temperatures (25–90°C) under ambient pressure in a batch reactor. Pretreatment is undertaken for residence times ranging from about 1 h to 3 months. Pretreatment efficiency is depended on variables such as temperature, reaction time and ammonia concentration. Lignin dissolves in the aqueous solution without appreciable decrease in the carbohydrate content, and high levels of solubilization are observed with high temperatures and times. In addition, severe conditions also cause release of acetyl groups, hemicelluloses, extractives and ash into pretreatment liquor . In other aqueous ammonia treatment, moderate temperatures (≥100°C) are used to achieve high delignification of biomass using pressure vessels . Higher temperatures are compensated using lower reaction times.
Chen et al.  used aqueous ammonia to pretreat silvergrass, napiergrass and rice straw at room temperature, resulting in over 90% of cellulose recovery in 4 weeks. On destarched barley hull, SAA pretreatment (15w/w NH3, 75°C, 48 h) produced zero glucan loss and 83% saccharification yield using 15 FPU/g-glucan; and with the addition of a xylanase in simultaneous saccharification and co-fermentation (SSCF), a high ethanol yield of 89.4% of the maximum theoretical was obtained . High ethanol concentration and yields from SAA-pretreated corn stover followed the use of a two-phase SSF involving pentose and hexose conversion with the help of
Two-stage processes targeting separate removal of hemicelluloses and lignin have also been investigated. Kim et al.  employed acetic acid medium to remove hemicelluloses followed by aqueous ammonia at elevated temperatures. Results obtained from other studies are given in Table 4.
|Biomass||Optimal pretreatment||DL, %||X/H, %||Hydrolysis||Yield, %||Reference|
|Rice straw||27 wt% NH3, 25°C, 2 wk||42||71||44–49|||
|Rice straw||21 wt% NH3, 69°C, 10 h||60.6a||15 FPU/g-glucan, 30 CBU/g-glucan||71.1||83.1|||
|15 wt% NH3, 130°C, 325 psig, 20 min||No acid treatment||69.8||77||50°C, 15 FPU/g-glucan, 15 CBU/g-glucan||83.2|||
|+ acid treatment||90.8|
|60°C, 15 wt% NH3, 24 h||PBI: 3 kGy, 45 MeV||50°C, 60 FPU/g-glucan, 10 CBU/g-glucan||90|||
|Corn fiber (destarched)||15 wt% NH3, 65°C, 8 h||76–78||50°C, 72 h, 15.57 FPU/g-glucan, 30 CBU/g-glucan||85.4|||
|Corn stover||29.5 wt% NH3, 10–60 days, RT||56–74||85||50°C, 72 h, 15 FPU/g-glucan, 30 CBU/g-glucan||86–89||73–77|||
|15 wt% NH3, 60°C, 12 h||62||85||15 FPU/g-glucan||85||77|||
|50 wt% NH3, 30°C, 4 weeks||55||15 FPU/g-glucan, 30 CBU/g-glucan||86.5||73|||
|15 wt%NH3, 69°C, 12 h||>80||84|||
|15 wt% NH3, 60°C, 8 h||Hot water, 10 min||68||50°C, 24 h, 15 FPU/g-glucan, 30 CBU/g-glucan||96|||
|12.5 wt% NH3, 60°C, 24 h, O2||+ TiO2, UV||70||50°C, 24 h, 15 FPU/g-glucan, 30 CBU/g-glucan||85|||
|+ ZnO, UV||82|
|Switchgrass||29.5 wt% NH3, 10 days, RT||40–50||50||72|||
|30 wt% NH3, 5 days (pilot-scale)||Aseptic conditions||73|||
|15 wt% NH3, 40°C/24 h, 60°C/8 h||40.8–46.9||50°C, 72 h, 22–25 FPU/g-glucan, 44–50 CBU/g-glucan, + xylanase||>85|||
|15 wt% NH3, 120°C, 24 h||No H2O2||65||15 FPU/g-glucan, 30 CBU/g-glucan||53.7|||
|+ 5% H2O2||77||74.3|
|Oil palm trunk||80°C, 8 h and 7 wt% NH3||40–50||50°C, 96 h, 60 FPU/g-glucan||95.4||78.3|||
|Oil palm empty fruit bunch||60°C, 12 h, and 21 wt% NH3||40.9||60 FPU/g-glucan, 96 h||41.4||65.6|||
|Miscanthus||150°C/30 wt% NH3, 180°C/10 wt% NH3, 1 h (not optimum)||>65||39.3–77.1||50°C, 96 h, 20 FPU/g-glucan||53.4|||
|News paper||4 wt% NH3 + 2 wt% H2O2, 40°C, 3 h||50°C, 72 h, 60 FPU/g-glucan||90|||
5.3. Positive attributes and drawbacks
SAA retains most of the hemicelluloses in the solid, eliminating the need to separately process hemicellulose and cellulose sugars. It leads to efficient delignification, producing low levels of enzyme inhibitory compounds. The reactor configuration is simpler and less costly, while ammonia recovery is easier compared to AFEX . It can be adapted to small-scale production. Further, neutralized salts from liquid hydrolysates could be used as nutrient source in fermentation.
There are few disadvantages associated with SAA pretreatment. Since pretreated solids contain high fractions of hemicellulose, a high demand for C5 conversion enzymes is needed to produce xylose and other pentose monomers . Post-treatment washing usually result in carbohydrate losses.
6. Irradiation-chemical pretreatment
In irradiation-chemical pretreatment, the biomass is typically soaked in a solvent (water, acid, or alkali) before undergoing irradiation via microwaves, gamma radiation, proton and electron beam, or radio frequency. In some cases, irradiation is performed before the chemical or other pretreatment, with advantages that include solubilization of lignin and hemicellulose, minimization of cellulose degradation, use of lower doses of chemical and less severe conditions. Further, undertaking irradiation before milling of biomass can reduce energy consumption (from size reduction) significantly .
Dielectric heating of biomass causes more energy absorption by the more polar part which creates a hot spot, resulting in generation of high internal steam pressure that induces an explosive effect, disrupting the biomass structure [94, 95]. The disruption is underpinned by radiolytic reactions that cause release of free radicals, triggering cross-linking and chain scission . Cross-linking reactions are believed to happen within the cellulose structure and as such when they predominate over chain scission reactions, sugar yields are not affected.
In general, pretreatment results in degradation of hemicellulose and lignin, and the alteration of cellulose structure. There is an increase in the specific surface area and a reduction in the degree of polymerization , as well as a change in the crystallinity of cellulose to amorphous pattern . In general, higher radiation intensities and lower biomass moisture content lead to higher rates of increase in final temperatures; however longer radiation time causes higher average final temperature and lower rate of temperature increase . Increases in irradiation strength have been found to affect hemicellulose more than lignin or cellulose [96, 100].
Microwave-assisted pretreatment has been applied to various materials. In a comparative study of the efficacy of mild sulfuric acid (5% v/v) application in combination with various heating modes—hot plate (100°C, 30 min), autoclave (121°C, 30 min), and microwave (200°C, 700 W, 15 min) on the biodegradability of garden biomass, microwave heat treatment was found to produce 53.95% cellulose recovery, leading to reducing sugar yield of 46.97%, which was about 10% higher than the other two modes . Application of microwaves on alkali pretreated wheat straw  and coconut husk fiber  was found to produce higher ethanol concentration and yield than substrates that did not receive any radiation. In another study, yields of 25.3, 21.2, and 46.5 g/100 g biomass, respectively, was obtained during radio frequency-assisted NaOH pretreatment (27.12 MHz, 0.20–0.25 g NaOH/g biomass; 90°C) of switchgrass at solids content of 20% . In an investigation to ascertain the effects of microwave chemical pretreatment on sweet sorghum bagasse (12% moisture, 1–2 mm), lime was found to enhance lignin removal, with sugar yields reaching 23.2 g/100 g biomass (38% of theoretical yield) for lime concentration of 0.1 g/10 ml of water. Microwave has also been used in conjunction with eutectic solvent, with enhanced lignin and hemicellulose removal and improved cellulose digestibility .
Under electron beam application, Karthika et al.  obtained 79% sugar yield from the saccharification (30 FPU/g-biomass, 144 h) of a hybrid grass exposed to 250 kGy of radiation, while Bak et al.  realized 52.1% from rice straw when it was exposed to 80 kGy and saccharified using 60 FPU/g-glucan for 132 h. Prior removal of hemicellulose using dilute acid and alkaline before irradiation exposes cellulase to enzymatic action during hydrolysis, and culminates in higher sugar yields . Electron beam has also been applied together with other physico-chemical methods such as SE with good results . The main challenge regarding the use of electron beam pertains to its low energy and as such some interest are focusing on proton beam.
6.3. Positive attributes and drawbacks
The mode of heating is uniform, energy efficient and offers rapid processing of biomass. Pretreatment is performed at low temperatures and at shorter period. It has the potential to be used for effective isolation of hemicelluloses. Irradiation generates no/low levels of inhibitors and by carefully controlling the chemical pretreatment, inhibitor levels are reduced.
Irradiation-chemical methods do not come without disadvantages. Microwave-assisted pretreatment comes with the risk of causing extensive degradation of hemicelluloses and contamination of dissolved lignin at severe conditions, releasing toxic compounds that inhibit enzymatic hydrolysis. Hu and team  argue that practical issues with scaling-up is more of a challenge in microwave than in radio frequency which can be used on large quantities of biomass, and at relatively high solids loading (20–50%) with uniform temperature profile when combined with chemical methods.
Among the three main stages of cellulosic ethanol production, namely, pretreatment, hydrolysis and fermentation, pretreatment presents the most practical and economic challenges in the attempt to produce ethanol at industrial-scale due its influence on both upstream and downstream processes. Thus, emerging and promising pretreatment methods that rely on physico-chemical fractionation of biomass are discussed, with prominence given to process description, advantages, drawbacks, and innovations employed to counteract inherent technical, economic and environmental challenges. The methods reviewed include liquid hot-water (LHW), steam explosion, ammonium fiber explosion (AFEX), soaking in aqueous ammonia (SAA), and irradiation-based pretreatment. Size reduction operations have been well integrated with other chemical and physico-chemical methods at the pilot and demonstration levels though energy consumption remains the rain challenge and as such research is shifting in favor of relatively low-energy methods such as wet disc milling as well as post-pretreatment size reduction. Irradiation-based methods have also shown promise at the industrial-level as demonstrated by burgeoning research interest around the world. With regards to physico-chemical methods, steam explosion and LHW-based methods have already been developed for industrial application.
The authors appreciate the support of DANIDA under the development research project (DFC journal no. 10-018RISØ) “Biofuels production from lignocellulosic materials (2GBIONRG).”
Yang B, Wyman CE. Pretreatment: the key to unlocking low-cost cellulosic ethanol. Biofuels, Bioproducts and Biorefining. 2008; 2:26-40
Mosier N, Wyman C, Dale B, Elander E, Lee YY, Holtzapple M, Ladisch M. Features of promising technologies for pretreatment of lignocellulosic biomass. Bioresource Technology. 2005; 96:673-686
Chen H, Liu J, Chang X, Chen D, Xue Y, Liu P, Lin H, Han S. A review on the pretreatment of lignocellulose for high-value chemicals. Fuel Processing Technology. 2017; 160:196-206
Bensah EC, Mensah M. Chemical pretreatment methods for the production of cellulosic ethanol: Technologies and innovations. International Journal of Chemical Engineering. 2013; 2013:Article ID 719607. DOI: 10.1155/2013/719607
Xiao L-P, Shi Z-J, Xu F, Sun R-C. Hydrothermal treatment and enzymatic hydrolysis of Tamarix ramosissima: Evaluation of the process as a conversion method in a biorefinery concept. Bioresource Technology. 2013; 135:73-81
Overend RP, Chornet E. Fractionation of lignocellulosics by steam–aqueous pretreatments. Philosophical Transactions of the Royal Society of London. Series A. 1987; 321:523-536
Qing Q, Wyman CE. Supplementation with xylanase and b-xylosidase to reduce xylo-oligomer and xylan inhibition of enzymatic hydrolysis of cellulose and pretreated corn stover. Biotechnology for Biofuels. 2011; 4:18
Yu Q, Liu J, Zhuang X, Yuan Z, Wang W, Qi W, Wang Q, et al. Liquid hot water pretreatment of energy grasses and its influence of physico-chemical changes on enzymatic digestibility. Bioresource Technology. 2016; 199:265-270
Archambault-Leger V, Shao X, Lynd LR. Integrated analysis of hydrothermal flow-through pretreatment. Biotechnology for Biofuels. 2012; 5:49
Ma Y, Ji W, Zhu X, Tian L, Wan X. Effect of extremely low AlCl3 on hydrolysis of cellu-lose in high temperature liquid water. Biomass and Bioenergy. 2012; 39:106-111
Matsushita Y, Yamauchi K, Takabe K, et al. Enzymatic saccharification of Eucalyptus bark using hydrothermal pre-treatment with carbon dioxide. Bioresource Technology. 2010; 101:4936-4939
Liu J, Li R, Shuai L, You J, Zhao Y, Chen L, et al. Comparison of liquid hot water (LHW) and high boiling alcohol/water (HBAW) pretreatments for improving enzymatic saccharification of cellulose in bamboo. Industrial Crops and Products. 2017a; 107:139-148
Weiqi W, Shubin W, Liguo L. Combination of liquid hot water pretreatment and wet disk milling to improve the efficiency of the enzymatic hydrolysis of eucalyptus. Technology. 2013; 128:725-730
Imman S, Arnthong J, Burapatana V, Champreda V, Laosiripojana N. Influence of alka-line catalyst addition on compressed liquid hot water pretreatment of rice straw. Chemical Engineering Journal. 2015; 278:85-91
Larsen J, Haven MØ, Thirup L. Inbicon makes lignocellulosic ethanol a commercial reality. Biomass and Bioenergy. 2012; 46:36-45
Thomsen MJ, Thygesen A, Thomsen AB. Hydrothermal treatment of wheat straw at pilot plant scale using a three-step reactor system aiming at high hemicellulose recovery, high cellulose digestibility and low lignin hydrolysis. Bioresource Technology. 2008; 99:4221-4228
Petersen MØ, Larsen J, Thomsen MH. Optimization of hydrothermal pretreatment of wheat straw for production of bioethanol at low water consumption without addition of chemicals. Biomass and Bioenergy. 2009; 33:834-840
Tao L, Aden A, Elander RT, Pallapolu VR, Lee YY, Garlock RJ, et al. Process and technoeconomic analysis of leading pretreatment technologies for lignocellulosic ethanol production using switchgrass. Bioresource Technology. 2011; 102:11105-11114
Negro MJ, Manzanares P, Ballesteros I, Oliva JM, Cabañas A, Ballesteros M. Hydrothermal pretreatment conditions to enhance ethanol production from poplar biomass. Applied Biochemistry and Biotechnology. 2003; 105:87-100
Thomsen S, Jensen M, Schmidt J. Production of 2nd generation bioethanol from lucerne – optimization of hydrothermal pretreatment. BioResources. 2012; 7(2):1582-1593
Modenbach AA, Nokes SE. The use of high-solids loadings in biomass pretreatment—A review. Biotechnology and Bioengineering. 2012; 109(6)
Sui W, Chen H. Effects of water states on steam explosion of lignocellulosic biomass. Bioresource Technology. 2016; 199:155-163
Sui W, Chen H. Water transfer in steam explosion process of corn stalk. Industrial Crops and Products. 2015; 76:977-986
Egüés I, Sanchez C, Mondragon I, Labidi J. Effect of alkaline and autohydrolysis processes on the purity of obtained hemicelluloses from corn stalks. Bioresource Technology. 2012; 103:239-248
Li J, Gellerstedt G, Toven K. Steam explosion lignins; their extraction, structure and potential as feedstock for biodiesel and chemicals. Bioresource Technology. 2009; 100:2556-2561
Sassner P, Mårtensson C-G, Galbe M, Zacchi G. Steam pretreatment of H2SO4-impregnated Salixfor the production of bioethanol. Bioresource Technology. 2008; 99:137-145
Maniet G, Schmetz Q, Jacquet N, Temmerman M, Gofflot S, Richel A. Effect of steam explosion treatment on chemical composition and characteristic of organosolv fescue lignin. Industrial Crops and Products. 2017; 99:79-85
Yu Z, Zhang B, Yu F, Xu G, Song A. A real explosion: The requirement of steam explosion pretreatment. Bioresource Technology. 2012; 121:335-341
Bobleter O. Hydrothermal degradation of polymers derived from plants. Progress in Polymer Science. 1994; 19:797-841
Martin-Sampedro R, Capanema EA, Hoeger I, Villar JC, Rojas OJ. Lignin changes after steam explosion and laccase-mediator treatment of eucalyptus wood chips. Journal of Agricultural and Food Chemistry. 2011; 59:8761-8769
Li J, Henriksson G, Gellerstedt G. Lignin depolymerization/repolymerization and its critical role for delignification of aspen wood by steam explosion. Bioresource Technology. 2007; 98:3061-3068
Panagiotopoulos IA, Chandra RP, Saddler JN. A two-stage pretreatment approach to maximise sugar yield and enhance reactive lignin recovery from Poplar wood chips. Bioresource Technology. 2013; 130:570-577
Corrales RCNR, Mendes FMT. Structural evaluation of sugar cane bagasse steam pretreated in the presence of CO2 and SO2. Biotechnology for Biofuels. 2012; 5:36
Sipos B, Kreuger E, Svensson S-E, Réczey K, et al. Steam pretreatment of dry and ensiled industrial hemp for ethanol production. Biomass and Bioenergy. 2010; 34:1721-1731
Zhang X, Yuan Q, Cheng G. Deconstruction of corncob by steam explosion pretreatment: Correlations between sugar conversion and recalcitrant structures. Carbohydrate Polymers. 2017; 156:351-356
Ferreira-Leitão V, Perrone CC, Rodrigues J, et al. Aesnea rachpproach to the utilisation of CO2 as impregnating agent in steam pretreatment of sugar cane bagasse and leaves for ethanol production. Biotechnology for Biofuels. 2010; 3:7
Chen W-H, Tsai C-C, Lin C-F, Tsai P-Y, Hwang W-S. Pilot-scale study on the acid-catalyzed steam explosion of rice straw using a continuous pretreatment system. Bioresource Technology. 2013; 128:297-304
Linde M, Jakobsson E-L, Galbe M, Zacchi G. Steam pretreatment of dilute H2SO4-impregnated wheat straw and SSF with low yeast and enzyme loadings for bioethanol production. Biomass and Bioenergy. 2008; 32:326-332
Zabihi S, Alinia R, Esmaeilzadeh F, Kalajahi JF. Pretreatment of wheat straw using steam, steam/acetic acid and steam/ethanol and its enzymatic hydrolysis for sugar production. Biomass and Bioenergy. 2010; 105:288-297
Park J-Y, Kang M, Kim JS, Lee J-P, Choi W-I, Lee J-S. Enhancement of enzymatic digestibility of Eucalyptus grandispretreated by NaOH catalyzed steam explosion. Bioresource Technology. 2012; 123:707-712
Pan X, Zhang X, Gregg DJ, Saddler JN. Enhanced enzymatic hydrolysis of steam-exploded Douglas fir wood by alkali-oxygen post-treatment. Applied Biochemistry and Biotechnology. 2004:113-116
Cullis IF, Mansfield SD. Optimized delignification of wood-derived lignocellulosics for improved enzymatic hydrolysis. Biotechnology and Bioengineering. 2010; 106(6):884-893
Jurado M, Prieto A, Martínez-Alcalá A, Martínez ÁT, Martínez MJ. Laccase detoxification of steam-exploded wheat straw for second generation bioethanol. Bioresource Technology. 2009; 100:6378-6384
Moreno AD, Ibarra D, Ballesteros I, et al. Comparing cell viability and ethanol fermentation of the thermotolerant yeast Kluyveromyces marxianusand Saccharomyces cerevisiaeon steam-exploded biomass treated with laccase. Bioresource Technology. 2013(135):239-245
Zhang L-H, Li D, Wang L-J, Wang T-P, et al. Effect of steam explosion on biodegradation of lignin in wheat straw. Bioresource Technology. 2008; 99:8512-8515
Rana D, Rana V, Ahring BK. Producing high sugar concentrations from loblolly pine using wet explosion pretreatment. Bioresource Technology. 2012; 121:61-67
Chen W-H, Pen B-L, Yu C-T, Hwang W-S. Pretreatment efficiency and structural characterization of rice straw by an integrated process of dilute-acid and steam explosion for bioethanol production. Bioresource Technology. 2011; 102:2916-2924
Oliveira FMV, Pinheiro IO, Souto-Maior AM, et al. Industrial-scale steam explosion pretreatment of sugarcane straw for enzymatic hydrolysis of cellulose for production of second generation ethanol and value-added products. Bioresource Technology. 2013; 130:168-173
Liu Z-H, Qin L, Pang F, Jin M-J, Li B-Z, Kang Y, et al. Effects of biomass particle size on steam explosion pretreatment performance for improving the enzyme digestibility of corn stover. Industrial Crops and Products. 2013; 44:176-184
Alvira P, Tomás-Pejó E, Ballesteros M, Negro MJ. Pretreatment technologies for an efficient bioethanol production process based on enzymatic hydrolysis: A review. Bioresource Technology. 2010; 101:4851-4861
Hendriks ATWM, Zeeman G. Pretreatments to enhance the digestibility of lignocellulosic biomass. Bioresource Technology. 2009; 100:10-18
Ibbett R, Gaddipati S, Davies S, Hill S, Tucker G. The mechanisms of hydrothermal deconstruction of lignocellulose: New insights from thermal–analytical and complementary studies. Bioresource Technology. 2011; 102:9272-9278
Chundawat SPS, Donohoe BS, Sousa LD, et al. Multi-scale visualization and characterization of lignocellulosic plant cell wall deconstruction during thermochemical pretreatment. Energy & Environmental Sciences. 2011; 4:973
Chundawat SPS, Vismeh R, Sharma LN, et al. Multifaceted characterization of cell wall decomposition products formed during ammonia fiber expansion (AFEX) and dilute acid based pretreatments. Bioresource Technology. 2010; 101:8429-8438
Alizadeh H, Teymoum F, Gilbert TI, Dale BE. Pretreatment of switchgrass by ammonia fiber explosion (AFEX). Applied Biochemistry and Biotechnology. 2005; 121-124:1133-1142
Shao X, Jin M, Guseva A, Liu C, et al. Conversion for Avicel and AFEX pretreated corn stover by Clostridium thermocellumand simultaneous saccharification and fermentation: Insights into microbial conversion of pretreated cellulosic biomass. Bioresource Technology. 2011; 102(17):8040-8045
Perez-Pimienta J, Flores-Gómez CA, Ruiz HA, Sathitsuksanoh N, et al. Evaluation of agave bagasse recalcitrance using AFEXTM, autohydrolysis, and ionic liquid pretreatments. Bioresource Technology. 2016; 211:216-223
Blümmel M, Teymouri F, Moore J, Nielson C, Videto J, et al. Ammonia fiber expansion (AFEX) as spin off technology from 2nd generation biofuel for upgrading cereal straws and stovers for livestock feed. Animal Feed Science and Technology. 2018; 236:178-186
Cha Y-L, Yang J, Ahn J-W, Moon Y-H, Yoon Y-M, et al. The optimized CO2-added ammonia explosion pretreatment for bioethanol production from rice straw. Bioprocess and Biosystems Engineering. 2014; 37(9):1907-1915
Zhao C, Cao Y, Ma Z, Shao Q. Optimization of liquid ammonia pretreatment conditions for maximizing sugar release from giant reed ( Arundo donax L.). Biomass and Bioenergy. 2017; 98:61-69
Garlock RJ, Balan V, Dale BE. Comparative material balances around pretreatment technologies for the conversion of switchgrass to soluble sugars. Bioresource Technology. 2011; 102:11063-11071
Kim Y, Hendrickson R, Mosier NS, Ladisch MR, Bals B, Balan V, et al. Enzyme hydrolysis and ethanol fermentation of liquid hot water and AFEX pretreated distillers’ grains at high-solids loadings. Bioresource Technology. 2008; 99:5206-5215
Moniruzzaman M, Dale BE, Hespell RB, Bothast RJ. Enzymatic hydrolysis of high-moisture corn fiber pretreated by AFEX and recovery and recycling of the enzyme complex. Applied Biochemistry and Biotechnology. 1997; 67:113-126
Li B-Z, Balan V, Yuan Y-J, Dale BE. Process optimization to convert forage and sweet sorghum bagasse to ethanol based on ammonia fiber expansion (AFEX) pretreatment. Bioresource Technology. 2010; 101:1285-1292
Sendich EN, Laser M, Kim S, et al. Recent process improvements for the ammonia fiber expansion (AFEX) process and resulting reductions in minimum ethanol selling price. Bioresource Technology. 2008; 99:8429-8435
Wang J, Xin D, Hou X, Wub J, Fan X, Li K, Zhang J. Structural properties and hydrolysabilities of Chinese Pennisetum and Hybrid Pennisetum: Effect of aqueous ammonia pretreatment. Bioresource Technology. 2016; 199:211-219
Zulkiple N, Maskat MY, Hassan O. Pretreatment of oil palm empty fruit fiber (OPEFB) with aquaeous ammonia for high production of sugar. Procedia Chemistry. 2016; 18:155-161
Chen B-Y, Chen S-W, Wang H-T. Use of different alkaline pretreatments and enzyme models to improve low-cost cellulosic biomass conversion. Biomass and Bioenergy. 2012; 39:182-191
Kim TH, Taylor F, Hicks KB. Bioethanol production from barley hull using SAA (soaking in aqueous ammonia) pretreatment. Bioresource Technology. 2008; 99:5694-5702
Li X, Kim TH, Nghiem NP. Bioethanol production from corn stover using aqueous ammonia pretreatment and two-phase simultaneous saccharification and fermentation (TPSSF). Bioresource Technology. 2010b; 101:5910-5916
Cao S, Aita GM. Enzymatic hydrolysis and ethanol yields of combined surfactant and dilute ammonia treated sugarcane bagasse. Bioresource Technology. 2013; 131:357-364
Raj K, Krishnan C. High sugar yields from sugarcane ( Saccharum officinarum) bagasse using low-temperature aqueous ammonia pretreatment and laccase-mediator assisted enzymatic hydrolysis. Industrial Crops and Products. 2018; 111:673-683
Nahar N, Pryor SW. Effects of reduced severity ammonia pretreatment on pelleted corn stover. Industrial Crops and Products. 2017; 109:163-172
Kim DY, Kim YS, Kim TH, Oh KK. Two-stage, acetic acid-aqueous ammonia, fractionation of empty fruit bunches for increased lignocellulosic biomass utilization. Bio-resource Technology. 2016; 199:121-112
Phitsuwan P, Permsriburasuk C, Waeonukul R, Pason P, Tachaapaikoon C, Ratana-khanokchai K. Evaluation of fuel ethanol production from aqueous ammonia-treated rice straw via simultaneous saccharification and fermentation. Biomass and Bioenergy. 2016; 93:150-157
Ko JK, Bak JS, Jung MW, Lee HJ, Choi I-G, Kim TH, et al. Ethanol production from rice straw using optimized aqueous-ammonia soaking pretreatment and simultaneous saccharification and fermentation processes. Bioresource Technology. 2009; 100:4374-4380
Kim J-W, Kim KS, Lee JS, Park SM, Cho H-Y, Park JC, et al. Two-stage pretreatment of rice straw using aqueous ammonia and dilute acid. Bioresource Technology. 2011; 102:8992-8999
Kim SB, Kim JS, Lee JH, Kang SW, Park C, Kim SW. Pretreatment of rice straw by proton beam irradiation for efficient enzyme digestibility. Applied Biochemistry and Biotechnology. 2011; 164:1183-1191
Nghiem NP, Montanti J, Johnston DB, Drapcho C. Fractionation of corn fiber treated by soaking in aqueous ammonia (SAA) for isolation of hemicellulose B and production of C5 sugars by enzyme hydrolysis. Applied Biochemistry and Biotechnology. 2011; 164:1390-1404
Kim TH, Lee YY. Pretreatment of corn stover by soaking in aqueous ammonia. Applied Biochemistry and Biotechnology. 2005; 121-124:1119-1132
Kim TH, Lee YY. Pretreatment of corn stover by soaking in aqueous ammonia at moderate temperatures. Applied Biochemistry and Biotechnology. 2007;(81):136-140
Li X, Kim TH. Low-liquid pretreatment of corn stover with aqueous ammonia. Bio-resource Technology. 2011; 102:4779-4786
Yoo CG, Lee C-W, Kim TH. Two-stage fractionation of corn stover using aqueous ammonia and hot water. Applied Biochemistry and Biotechnology. 2011; 164:729-740
Yoo CG, Wang C, Yu C, Kim TH. Enhancement of enzymatic hydrolysis and Klason lignin removal of corn stover using photocatalyst-assisted ammonia pretreatment. Applied Biochemistry and Biotechnology. 2013; 169(5):1648-1658
Isci A, Himmelsbach JN, Pometto AL III, Raman DR, Anex RP. Aqueous ammonia soaking of switchgrass followed by simultaneous saccharification and fermentation. Applied Biochemistry and Biotechnology. 2008; 144:69-77
Isci A, Himmelsbach JN, Strohl J, Pometto AL III, Raman DR, Anex RP. Pilot-scale fermentation of aqueous-ammonia-soaked switchgrass. Applied Biochemistry and Bio-technology. 2009; 157:453-462
Pryor SW, Karki B, Nahar N. Effect of hemicellulase addition during enzymatic hydrolysis of switchgrass pretreated by soaking in aqueous ammonia. Bioresource Technology. 2012; 123:620-626
Gupta R, Lee YY. Investigation of biomass degradation mechanism in pretreatment of switchgrass by aqueous ammonia and sodium hydroxide. Bioresource Technology. 2010; 101:8185-8191
Jung YH, Kim IJ, Kim JJ, Oh KK, Han J-I, Choi I-G, et al. Ethanol production from oil palm trunks treated with aqueous ammonia and cellulase. Bioresource Technology. 2011; 102:7307-7312
Jung YH, Kim IJ, Han J-I, Choi I-G, Kim KH. Aqueous ammonia pretreatment of oil palm empty fruit bunches for ethanol production. Bioresource Technology. 2011; 102:9806-9809
Liu Z, Padmanabhan S, Cheng K, et al. Aqueous-ammonia delignification of miscanthus followed by enzymatic hydrolysis to sugars. Bioresource Technology. 2012(135):23-29
Kim SB, Moon NK. Enzymatic digestibility of used newspaper treated with aqueous ammonia–hydrogen peroxide solution. Applied Biochemistry and Biotechnology. 2003:105-108
Liu Y, Guo L, Wang L, Zhan W, Zhou H. Irradiation pretreatment facilitates the achievement of high total sugars concentration from lignocellulose biomass. Bioresource Technology. 2017; 232:270-277
Hu Z, Wang Y, Wen Z. Alkali (NaOH) pretreatment of switchgrass by radio frequency-based dielectric heating. Applied Biochemistry and Biotechnology. 2008; 148:71-81
Li X-J, Lu K-Y, Lin L-Y, Zhou Y-D, Cai Z-Y, Fu F. Fundamental characteristics of microwave explosion pretreatment of wood. I. Properties of temperature development. Forestry Studies in China. 2010; 12(1):9-13
Yin Y, Wang J. Enhancement of enzymatic hydrolysis of wheat straw by gamma irradiation–alkaline pretreatment. Radiation Physics and Chemistry. 2016; 123:63-67
Sarkar N, Ghosh SK, Bannerjee S, Aikat K. Bioethanol production from agricultural wastes: An overview. Renewable Energy. 2012; 37:19-27
Lee B-M, Jeun J-P, Kang P-H. Enhanced enzymatic hydrolysis of kenaf core using irradiation and dilute acid. Radiation Physics and Chemistry. 2017; 130:216-220
Li X, Zhou Y, Yan Y, Cai Z, Feng F. A single cell model for pretreatment of wood by microwave explosion. Holzforschung. 2010;(64):633-637
Sung YJ, Shin S-J. Compositional changes in industrial hemp biomass ( Cannabis sativa L.) induced by electron beam irradiation pretreatment. Biomass and Bioenergy. 2011; 35:3267-3270
Gabhane J, William SPMP, Vaidya AN, Mahapatra K, Chakrabarti T. Influence of heating source on the efficacy of lignocellulosic pretreatment—A cellulosic ethanol perspective. Biomass and Bioenergy. 2011; 35:96-102
Zhu S, Wu Y, Yu Z, Zhang X, et al. Production of ethanol from microwave-assisted alkali pretreated wheat straw. Process Biochemistry. 2006; 41:869-873
Ding TY, Hii SL, Ong LGA. Comparison of pretreatment strategies for conversion of coconut husk fiber to fermentable sugars. BioResources. 2012; 7(2):1540-1547
Chen Z, Wan C. Ultrafast fractionation of lignocellulosic biomass by microwave-assisted deep eutectic solvent pretreatment. Bioresource Technology. 2018; 250:532-537
Karthika K, Arun AB, Rekha PD. Enzymatic hydrolysis and characterization of lignocellulosic biomass exposed to electron beam irradiation. Carbohydrate Polymers. 2012; 90:1038-1045
Bak JS, Ko JK, Han YH, Lee BC, Choi I-G, Kim KH. Improved enzymatic hydrolysis yield of rice straw using electron beam irradiation pretreatment. Bioresource Technology. 2009; 100:1285-1290
Karthika K, Arun AB, Melo JS, Mittal KC, Kumar M, Rekha PD. Hydrolysis of acid and alkali presoaked lignocellulosic biomass exposed to electron beam irradiation. Bioresource Technology. 2013; 129:646-649
Leskinen T, Kelley SS, Argyropoulos DS. E-beam irradiation & steam explosion as biomass pretreatment, and the complex role of lignin in substrate recalcitrance. Biomass and Bioenergy. 2017; 103:21-28