Acid-catalyzed Steam Explosion Research Articles

Leveraging machine learning for acid catalyzed steam explosion pretreatment: Towards supporting fermentation by the trade-off between glucose and inhibitors

Pretreatment is essential for enhancing the sugar release from lignocellulose. Acid-catalyzed steam explosion (ACSE), a widely-used pretreatment method, still faces challenges, including inhibitors accumulation, which can be overcome by modeling. Here, artificial neural network models were constructed for sulfuric acid-based ACSE to predict sugars and inhibitors, from 12 variables regarding lignocellulose, acid, and steam explosion. Two expanding applications were demonstrated. Firstly, a constraint-based optimization strategy can provide the optimal ACSE condition for fermentation by considering glucose and the synergistic effect of inhibitors on microbial growth simultaneously. Compared to published works, the strategy led to 94% glucose with 22% inhibitors for corn stover, and 100% glucose with 13% inhibitors for wheat straw. Secondly, transfer learning was employed to model phosphoric acid-based ACSE with high accuracy (MSE 0.004) and low data requirement (∼30% of sulfuric acid-based ACSE). The proposed models and applications offer an effective optimization strategy for ACSE and other pretreatment methods for the following fermentation.

Wheat bran fractionation: Effect of steam explosion and hydrotropic extraction conditions on the recovery of sugars and lignin

Wheat bran, a major by-product of wheat processing, contains several valuable components that can serve as a potential feedstock for chemical production. This study presents a multiple-stage fractionation method, involving steam explosion and hydrotropic extraction, which were investigated with the goal of separating the main structural bran components: starch, protein, hemicelluloses, lignin, and cellulose. With prior de-starching and de-proteination, acid-catalysed steam explosion (STEX1: 218 °C, 8 min, 0.05 wt% H2SO4) solubilised up to 97% of the arabinoxylans. Higher monosaccharide yields arising from hemicelluloses were observed in the de-starched and de-proteinated bran than in untreated bran prior to steam treatment. Subsequent treatment using the 40% w/w hydrotrope solution, sodium xylene sulfonate (SXS), at 170 °C for 1 h (HEX1) showed lignin recovery of up to 42.6% from the steam-exploded bran, which was higher than 25.1% lignin recovery from the de-starched, de-proteinated and steam exploded bran. The lignin recovery values were slightly underestimated as some lignin ends up in the wash, which proved difficult to quantify. Finally, glucose monomer release during cellulose hydrolysis occurred most rapidly in the bran material that had undergone prior treatments of de-starching, de-proteination, steam explosion and hydrotropic treatment, with complete hydrolysis in less than 24 h for the de-starched, de-proteinated, steam exploded for both STEX1 and STEX3 (219 °C, 0.10 wt%, 14 min) and hydrotropically treated (HEX1). The selective release of wheat bran components in separate stages of the wheat-bran fractionating scheme provides a platform for further valorisation of each component into commercially relevant products. Structural elucidation of the recovered wheat bran lignin using 2D HSQC NMR revealed that oxidised syringyl (S′) units increased from 13.2 units per 100 C2 aromatic group (Ar) in bran without prior treatment, to 16.9 units/100 Ar in steam exploded bran and 18.2 units/100 Ar in de-starched, de-proteinated and steam exploded bran. The change in guaicyl (G) unit frequencies as well as the oxidised G units (G′) remained low across samples. As less oxidation is desired for further lignin valorisation, the results herein offer insights on future work for utilising wheat bran lignin.

Improving sugar extraction from brewers’ spent grain using sequential deproteinization and acid-catalyzed steam explosion in a biorefinery context

Brewers’ spent grain (BSG) is a complex biomass composed of sugars and lignin, with a high protein content. In this work, a BSG fractionation process was evaluated to improve sugar extraction. The developed process involved a sequential combination of enzymatic deproteinization and acid-catalyzed steam explosion (SE) pretreatment. Temperature and time effects on the SE pretreatment of deproteinized BSG (BSG-D) were experimentally studied. The deproteinization yield of BSG was 63.9% and, in the liquid after SE, up to 49.8% of xylose was recovered using BSG-D. The maximum conditions of the SE were evaluated at 173.5 °C for 15.5 min with acid-catalyzed H2SO4 (0.5% w/v) in the study region. Under these best scenario, 30% and 93% of xylose and arabinose, respectively, were recovered as monomeric sugars in the liquid; from the solid, up to 72.2% glucose was recovered using enzymatic hydrolysis (EH).

Enhancing Biobutanol Production from biomass willow by pre-removal of water extracts or bark

Aiming to understand the importance of debarking on the controlled utilization of phenolic-rich willow biomass, biobutanol was produced from it by using Clostridium acetobutylicum. Acid-catalysed steam explosion and enzymatic hydrolysis (EH) were investigated before the acetone-butanol-ethanol (ABE) fermentation. The hydrolysable sugar yield and ABE fermentation efficiency were found to decline progressively from willow wood (WW) to HWE WB (hot water extracted willow biomass), WB (willow biomass) and the WW + HWE (willow wood plus the artificial willow bark water extracts), indicating that the pre-removal of water extracts or the bark can significantly improve ABE yield. Notably, the ABE productivity of WW achieved 12.7 g/L at the solvent yield of 31%, and the butanol concentration (i.e. 8.5 g/L) generated by WW is relatively high among the reported lignocellulosic-derived biomass. Additionally, it is hypothesized that under acidic conditions and high temperatures the fructose present in willow water extracts form hydroxymethylfurfural during steam explosion, which then spontaneously condenses with phenolic substances of willow bark to form a solid furanic precipitate. The formed furanic precipitates play inhibitory role in the enzymatic hydrolysis and are thereby deleterious to the ABE fermentation.

Acid-catalyzed steam explosion for high enzymatic saccharification and low inhibitor release from lignocellulosic cardoon stalks

Cardoon is low-input biomass which can be efficiently exploited in the modern biorefinery for multi-purpose uses. In this work, cardoon lignocellulosic stalks underwent to acid-catalyzed steam explosion. A design of experiments approach was employed to assess the effect of the process variables (acid concentration, temperature and reaction time) on xylan recovery after direct hydrolysis and cellulose digestibility in the enzymatic hydrolysis step. According to the statistical model generated, the optimal conditions to maximize overall monosaccharides yield were 166 °C, 1.45% (w/w) and 10 min. A monosaccharides production and inhibitors formation of 36.76 g/100 g raw material (close to 70% of the maximal theoretical yield) and 1.80 g/100 g raw material, respectively, were achieved. The scale-up of the hydrolysis step showed superior performance of the washed solid fraction than to the whole slurry, resulting in cellulose conversion to glucose by 76% vs 58%. By employing the life cycle assessment approach, it was possible to determine the environmental impact in terms of the global warming potential (3.18 kgCO2eq/kg of fermentable carbohydrates) and cumulative energy demand (43.34 MJ/kg of fermentable carbohydrates). The low-inhibitor and high-sugar hydrolysate can be fermented to biochemicals and biofuels without any detoxification process.

Conversion of cardoon crop residues into single cell oils by Lipomyces tetrasporus and Cutaneotrichosporon curvatus: process optimizations to overcome the microbial inhibition of lignocellulosic hydrolysates

In the present work, the production of Single Cell Oils (SCOs) from undetoxified cardoon hydrolysates (UCH) was investigated. The raw material was pretreated by acid catalyzed steam explosion at 195 °C for 7.5 min and hydrolysed at high dry matter content to achieve a final hydrolysate containing 90 g L-1 glucose and 9 g L-1 xylose. The different ability of Cutaneotrichosporon curvatus and Lipomyces tetrasporus to overcome the toxic effect of the biomass degradation by-products, mainly furan derivatives, in different growth phases was investigated in different set-ups conditions including four hydrolysates concentrations, (glucose and xylose respectively 30 + 3, 45 + 4, 60 + 6, and 90 + 9 g L-1) and two fermentation modes, batch and fed batch. The results indicated that the inoculum age corresponded to a different metabolism of the furanic aldehydes only in L. tetrasporus. C. curvatus was severely inhibited by UCH in any growth phase. L. tetrasporus cells in the stationary phase (SP) grew in UCH at any concentration, while cells in the exponential phase (EP) grew only at the highest dilution rate. The maximum lipids yields and lipid cell content of 20% and 47% respectively (7.1 g L-1 lipids) were achieved in batch mode using SP-inocula and the highest dilution ratio. The dominant fatty acid was oleic acid (59%). The scale-up to the 2 L bioreactor, in fed-batch mode, increased the lipids concentration up to 12.7 g L-1 and the productivity up to 0.135 g L-1 h-1.

Physical techniques shed light on the differences in sugarcane bagasse structure subjected to steam explosion pretreatments at equivalent combined severity factors

Significant improvements in pretreatments and a deeper understanding of changes in the physical structure and chemical composition of pretreated lignocellulosic materials are required to guarantee the technical and economic sustainability of second-generation ethanol technologies. In present work, several biophysical techniques were applied to characterize sugarcane bagasse samples that have been subjected to autocatalytic, phosphoric acid and sulfuric acid catalyzed steam explosion under equivalent combined severity factors (CSF). Confocal laser scanning microscopy and fluorescence lifetime imaging studies demonstrated unequal changes in lignin distribution in the plant cell wall that could be nicely correlated with substrate susceptibility to enzymatic hydrolysis. Furthermore, solid state nuclear magnetic resonance corroborated these observations by showing the differences in biomass composition primarily associated with hemicellulose removal, whereas X-ray diffraction analysis revealed changes in crystallinity indexes and average crystallite sizes. Thus, the impact of pretreatment temperature on the physical structure and chemical composition of CSF-equivalent steam-exploded sugarcane bagasse was identified as the key factor for developing substrate accessibility. This calls for caution in using combined severity factor as a chief parameter for comparison of different pretreatment conditions.

Fractionation of Cellulose-Rich Products from an Empty Fruit Bunch (EFB) by Means of Steam Explosion Followed by Organosolv Treatment

In this study an empty fruit bunch (EFB) was subjected to a two-step pretreatment to defragment cellulose-rich fractions as well as lignin polymers from its cell walls. First pretreatment: acid-catalyzed steam explosion (ACSE) pretreatment of EFB was conducted under the temperature range of 180–220 °C and residence time of 5–20 min. The ACSE-treated EFB was further placed into the reactor containing 50% aq. ethanol and NaOH as a catalyst and heated at a temperature of 160 °C for 120 min for the second pretreatment: alkali-catalyzed organosolv treatment (ACO). The mass balance and properties of treated EFB were affected by the residence time. The lowest yield of a solid fraction was obtained when the residence time was kept at 15 min. Xylose drastically decreased, especially under the ACSE pretreatment. However, the crystallinity of cellulose increased by increasing the severity factor of the pretreatment and was 47.8% and 57% udner the most severe conditions. The organosolv lignin fractions also showed the presence of 14 major peaks via their pyrolysis-GC analysis. From here, it can be suggested that this kind of pretreatment can indeed be one potential option for lignocellulosic pretreatment.

Production of Carbohydrates from Cardoon Pre-Treated by Acid-Catalyzed Steam Explosion and Enzymatic Hydrolysis

Cardoon (Cynara cardunculus) is a promising crop from which to obtain oilseeds and lignocellulosic biomass. Acid-catalyzed steam explosion is a thermochemical process that can efficiently pre-treat lignocellulosic biomass. The drawback is the production of a high number of carbohydrate degradation products in the liquid fraction that could inhibit microbial growth. In this work, the lignocellulosic biomass of cardoon, gathered from a dedicated field, were used as the raw material for the production of fermentable monosaccharides by employing acid-catalyzed steam explosion. The raw material was pre-soaked with a dilute 1% (w/w) sulfuric acid solution and then subjected to steam explosion under three different severity conditions. The recovered slurry was separated into solid and liquid fractions, which were individually characterized to determine total carbohydrate and inhibitor concentrations. The slurry and the washed solid fraction underwent enzymatic hydrolysis to release glucose and pentose monosaccharides. By conducting the pre-treatment at 175 °C for 35 min and hydrolyzing the obtained slurry, a yield of 33.17 g of monosaccharides/100 g of cardoon was achieved. At the same conditions, 4.39 g of inhibitors/100 g of cardoon were produced.

Cellulosic sugars from biomass: Effect of acid presoaking on pretreatment efficiency and operating cost estimation for sugar production

A room temperature acid presoaking process with high mixing performance was developed to enhance the acid-catalyzed steam explosion pretreatment efficiency. The impact of the operational parameters of presoaking, such as presoaking time, acid concentration and acid solution recycling, on the acid-catalyzed pretreatment of rice straw, bagasse and wood chip was studied. Bench-scale investigations show that the maximum saccharification yields after acid-catalyzed steam explosion of rice straw, bagasse and wood chip were 90.8%, 81.1% and 80.3%, corresponding to 535, 536 and 537 kg sugar/ton of dry raw material, respectively. According to the results, the estimated operating costs of sugar production from high to low are 0.388, 0.378 and 0.314 US $/kg sugar for rice straw, sugarcane bagasse and wood chip, respectively. The results indicate that the recyclable and continuous pilot-scale presoaking system with reliable acid uptake control shows high potential for producing sugar from different lignocellulosic biomass.

Extraction and characterization of lignin from corncob residue after acid-catalyzed steam explosion pretreatment

Corncob residue after acid catalyzed steam explosion (CRSE) is rich in cellulose and lignin. However, the structure of the lignin present in the CRSE has not been studied in detail and this hinders its application. An enzymatic mild acidolysis lignin (EMAL) extraction is adopted to process the CRSE where the extraction conditions are modified to improve lignin recovery and purity. Extraction severity factor is calculated and correlations between it and lignin recovery, lignin purity and total phenolic groups are evaluated. The best extraction condition produces lignin with a purity of 99.0% and a yield of 57.3%. A comparative study between lignin samples extracted under the best condition from the CRSE and untreated corncob (CC) is conducted. The structures of extracted lignin samples are investigated by using 31P NMR, 2D-HSQC NMR, FTIR and GPC. Although SE pretreatment caused degradation of lignin, the abundance per 100 aromatic units of β-O-4′ linkages is 46.0, which is higher than most technical lignins. The molecular weight of CRSE EMAL is found to be higher than that of CC EMAL due to condensation reactions. Antioxidant property of CC EMAL and CRSE EMAL is measured and compared. The data shows that CRSE EMAL possesses better antioxidant activity (IC50 = 128 μg/mL) than CC EMAL (IC50 = 238 μg/mL), which makes CRSE EMAL an ideal additive for polymer composites.

Steam Explosion and Vibrating Membrane Filtration to Improve the Processing Cost of Microalgae Cell Disruption and Fractionation

The aim of this study is to explore an innovative downstream route for microalgae processing to reduce cost production. Experiments have been carried out on cell disruption and fractionation stages to recover lipids, sugars, and proteins. Steam explosion and dynamic membrane filtration were used as unit operations. The species tested were Nannochloropsis gaditana, Chlorella sorokiniana, and Dunaliella tertiolecta with different cell wall characteristics. Acid-catalysed steam explosion permitted cell disruption, as well as the hydrolysis of carbohydrates and partial hydrolysis of proteins. This permitted a better access to non-polar solvents for lipid extraction. Dynamic filtration was used to moderate the impact of fouling. Filtration enabled two streams: A permeate containing water and monosaccharides and a low-volume retentate containing the lipids and proteins. The necessary volume of solvent to extract the lipids is thus much lower. An estimation of operational costs of both steam explosion and membrane filtration was performed. The results show that the steam explosion operation cost varies between 0.005 $/kg and 0.014 $/kg of microalgae dry sample, depending on the cost of fuel. Membrane filtration cost in fractionation was estimated at 0.12 $/kg of microalgae dry sample.

The potential of agricultural banana waste for bioethanol production

Banana is one of the most important fruit crops around the world. After harvesting, it generates large amounts of lignocellulosic residues that could be used for second generation ethanol production. Optimal operating conditions of saccharification and fermentation processes, of the two main agricultural wastes of banana crop: pseudostem and rachis, for bioethanol production were selected in order attain conditions transferable to industry (optimizing resources and time). Both materials were previously pretreated with acid-catalyzed steam explosion. Full factorial experimental design was used in order to determine enzymatic hydrolysis process conditions to obtain glucose concentrations between 75 and 100 g.L−1. Optimal enzymatic hydrolysis conditions for rachis and pseudostem were 17.6% of solid loading and 16.0 FPU.g−1 glucan of enzyme dosage and 15.1% of solid loading and 14.9 FPU.g−1 glucan of enzyme dosage, respectively. The most suitable configuration to attain the highest volumetric ethanol productivity for rachis was 8 h prehydrolysis followed by a Simultaneous Saccharification and Fermentation (PSSF 8 h). Meanwhile, for pseudostem, higher ethanol production was obtained in Simultaneous Saccharification and Fermentation (SSF) configuration. At optimized conditions, pseudostem reached 112 L.t−1 of ethanol, and rachis 103 L.t−1. These conditions led to achieve ethanol production process at high solid loading, low enzyme dosage, low yeast inoculum, no mineral salts supplementation and maximum ethanol productivities.

Hemicellulose extraction from slash pine sawdust by steam explosion with sulfuric acid

One of the main obstacles to the development of advanced forest biorefineries is the optimal fractionation of the plant biomass. The aim of this work was to design a two-stage process for the fractionation of slash pine sawdust: alkaline deresination for extractives removal and sulfuric acid-catalyzed steam explosion (SE) for hemicellulose extraction. SE was carried out in a high-pressure steam reactor under different conditions of temperature (180 °C–200 °C), time (5 min–10 min) and acid concentration (1:100 H2SO4 - 3:100 H2SO4 (on mass of dry wood)) according to a factorial design. Extractives, lignin, carbohydrates, and degradation by-products were quantified. The effect of pretreatment on enzymatic hydrolysis (EH) was also evaluated. SE was very efficient for hemicelluloses extraction (90% at 200 °C with 3:100 H2SO4 (on mass of dry wood) for 5 min) with a high glucan recovery. EH yield increased 6-fold after SE. There was a trade-off between maximizing carbohydrate recovery, hemicellulose extraction, and cellulose digestibility.

Process development for producing a food-grade glucose solution from rice straws

Abstract In this paper, processes for producing a food-grade glucose solution through enzymatic hydrolysis of cellulose-rich solids obtained from rice straws are presented. The rice straws were pretreated by acid-catalyzed steam explosion, and the reaction efficiency, toxicity control, and process economic feasibility were studied. Mass transfer resistance to the hydrolysis reaction was reduced by grinding with glass beads. A higher glucose concentration could be obtained by feeding more cellulose in the hydrolysis reaction; however, this also resulted in the production of undesired byproducts. Thus, a soaking process for the cellulose solids in water was developed to effectively reduce the generation of byproducts in the hydrolysis reaction. The resulting food-grade glucose solution can provide 414 kilocalories per liter, and could be used during a food-shortage crisis in the future. The current production cost is estimated to be 0.82 USD·L − 1 .

Pilot-scale steam explosion pretreatment with 2-naphthol to overcome high softwood recalcitrance

BackgroundSteam explosion pretreatment has been examined in many studies for enhancing the enzymatic digestibility of lignocellulosic biomass and is currently the most common pretreatment method in commercial biorefineries. It is however not effective for overcoming the extremely high recalcitrance of softwood to biochemical conversion. Recent fundamental research in small-scale liquid hot water pretreatment has shown, though, that the addition of a carbocation scavenger like 2-naphthol can prevent lignin repolymerization and thus enhance the enzymatic digestibility of softwood cellulose. This work studies the technical application potential of this approach in a larger steam explosion pilot plant for surmounting softwood recalcitrance.ResultsThe addition of 35.36 g 2-naphthol to the steam explosion pretreatment of 1.5 kg spruce wood chips allowed to considerably enhance the enzymatic cellulose digestibility. Different ways of adding the solid 2-naphthol to steam pretreatment were tested. Mixing with the biomass before pretreatment could enhance digestibility by up to 55% compared to control experiments. Impregnation of the biomass with 2-naphthol was yet more effective. Acetone and ethanol were tested to dissolve 2-naphthol and impregnate the biomass. The solvents were then removed again by evaporation before the pretreatment. The impregnation allowed to enhance digestibility by up to 179 and 192%, respectively. A comparison to prevalent acid-catalyzed steam explosion pretreatments for softwood revealed that the scavenger approach allows for obtaining exceptionally high yields in enzymatic hydrolysis. The biomass impregnation with 2-naphthol even renders a complete enzymatic cellulose conversion possible, which is remarkable for a softwood pretreatment not removing lignin. Steam pretreatment experiments without explosive decompression revealed that the enhancing effects of the explosion and the scavenger complement each other well. The explosion enhances the accessibility of the cellulose while the use of the scavenger reduces particularly the deactivation of enzymes.ConclusionsThis is the first study to show that a carbocation scavenger in steam pretreatment can enhance the enzymatic digestibility of lignocellulosic biomass. The approach opens up a novel possibility for overcoming the high softwood recalcitrance in a process that does not require an acid catalyst or the removal of lignin from the biomass.

Deconstruction of corncob by steam explosion pretreatment: Correlations between sugar conversion and recalcitrant structures

In this work, acid-catalyzed steam explosion was carried out as a pretreatment to hydrolyze hemicellulose and increase the enzymatic digestibility of corncob. Pretreatment conditions were varied to achieve structural alterations in a wide range: type of acids (sulfuric acid and oxalic acid), acid concentration (0.1–1.5wt.%) and pressure (1.0–1.8MPa). The pretreated residues were analyzed by chemical analysis, surface area measurement and x-ray diffraction. Biomass and cellulose crystallinity, lignin content and specific surface area were obtained and their correlations with sugar conversion were compared. The results suggested that these parameters were coupled together and they explained in part the diversity of the literature data that improves understanding of steam explosion pretreatment.

Bioethanol production from pretreated Miscanthus floridulus biomass by simultaneous saccharification and fermentation

The production of ethanol from the fast-growing perennial C4 grass Miscanthus floridulus by simultaneous saccharification and fermentation (SSF) was investigated. M. floridulus biomass was composed of 36.3% glucan, 22.8% hemicellulose, and 21.3% lignin (based on dried mass). Prior to SSF, harvested stems of M. floridulus were pretreated separately by alkali treatment at room temperature, alkali treatment at 90 °C, steam explosion, and acid-catalyzed steam explosion. The delignification rates were determined to be 73.7%, 61.5%, 42.7%, and 63.5%, respectively, by these four methods, and the hemicellulose removal rates were 51.5%, 85.1%, 70.5%, and 97.3%, respectively. SSF of residual solids after various pretreatments was performed with dried yeast (Saccharomyces cerevisiae) and cellulases (Accellerase 1000) by using 10% water-insoluble solids (WIS) of the pretreated M. floridulus as the substrate. The ethanol yields from 72-h SSF of M. floridulus biomass after these pretreatments were 48.9 ± 3.5, 78.4 ± 1.0, 46.4 ± 0.1, and 69.0 ± 0.1% (w/w), respectively, while the ethanol concentrations after 72-h SSF were determined to be 15.4 ± 1.1, 27.5 ± 0.3, 13.9 ± 0.1, and 30.8 ± 0.1 g/L, respectively. Overall, the highest amount of ethanol (0.124 g/g-dried raw material) was generated from dried raw material of M. floridulus after alkaline pretreatment at 90 °C. The acid-catalyzed steam explosion pretreatment also resulted in a high ethanol yield (0.122 g/g-dried raw material). Pretreatment resulting in high lignin and hemicellulose removal rates could make biomass more accessible to enzyme hydrolysis and lead to higher ethanol production.

Intermittent pressurized operation of steam explosion pretreatment system

An acid catalyzed steam explosion pretreatment system was developed to process wood chips for biomass conversion. The degree of mixing in the pressurizing process was found to be a crucial factor to saccharification efficiency. With a low load fill (100l, half load) in the pressurized vessel, ribbon blender provided enough mixing power for wood chips. The temperature difference in the reactor was found to be less than 15°C, the total saccharification efficiency can be up to 63%. At high load fill condition (200l, full load), the temperature difference under regular mixing was found to be quite large (20°C) with a low saccharification efficiency of 38%. To increase the temperature uniformity, an intermittent pressure pulsation method was used. The pressure pulses helped the vertical mixing and a total saccharification efficiency up to 65% can be reached in the improved pretreatment process.

Difference analysis of the enzymatic hydrolysis performance of acid-catalyzed steam-exploded corn stover before and after washing with water.

The difference in the enzymatic hydrolysis yield of acid-catalyzed steam-exploded corn stover (ASC) before and after washing with water reached approximately 15% under the same conditions. The reasons for the difference in the yield between ASC and washed ASC (wASC) were determined through the analysis of the composition of ASC prehydrolyzate and sugar concentration of enzymatic hydrolyzate. Salts produced by neutralization (CaSO4, Na2SO4, K2SO4, and (NH4)2SO4), sugars (polysaccharides, oligosaccharides, and monosaccharides), sugar-degradation products (weak acids and furans), and lignin-degradation products (ethyl acetate extracts and nine main lignin-degradation products) were back-added to wASC. Results showed that these products, except furans, exerted negative effect on enzymatic hydrolysis. According to the characteristics of acid-catalyzed steam explosion pretreatment, the five sugar-degradation products' mixture and salts [Na2SO4, (NH4)2SO4] showed minimal negative inhibition effect on enzymatic hydrolysis. By contrast, furans demonstrated a promotion effect. Moreover, soluble sugars, such as 13g/L xylose (decreased by 6.38%), 5g/L cellobiose (5.36%), 10g/L glucose (3.67%), as well as lignin-degradation products, and ethyl acetate extracts (4.87%), exhibited evident inhibition effect on enzymatic hydrolysis. Therefore, removal of soluble sugars and lignin-degradation products could effectively promote the enzymatic hydrolysis performance.