Structural properties of pretreated biomass from different acid pretreatments and their effects on simultaneous saccharification and ethanol fermentation

Distinct polymer extraction and cellulose DP reduction for complete cellulose hydrolysis under mild chemical pretreatments in sugarcane

In this study, liquid hot water (LHW) and chemical (H₂SO₄, NaOH, CaO) pretreatments were performed in Saccharum species including sugarcane bagasse. In comparison, the LHW and CaO pretreatments significantly enhanced biomass enzymatic hydrolysis, leading to much high bioethanol yield obtained at 19% (% dry matter) with an almost complete hexoses-ethanol conversion in the desirable So5 bagasse sample. Despite the LHW and CaO are distinctive for extracting hemicellulose and lignin, both pretreatments largely reduced cellulose degree of polymerization for enhanced lignocellulose enzymatic saccharification. Further chemical analysis indicated that the pretreated So5 sample had much lower cellulose crystalline index, hemicellulosic Xyl/Ara and lignin S/H ratio than those of other biomass samples, which explained that the So5 had the highest bioethanol yield among Saccharum species. Therefore, a mechanism model was proposed to elucidate how mild pretreatments could enhance biomass enzymatic saccharification for a high bioethanol production in the desirable sugarcane bagasse.

Enzyme adsorption properties on dilute acid pretreated biomass by low vacuum-scanning electron microscopy and structural analysis of lignin

In this study, enzyme adsorption properties were investigated as a function of the structural change of lignin in dilute acid pretreated biomass using oxalic and sulfuric acid catalysts under the same reaction conditions. Although the contents of glucan and lignin in the dilute acid pretreated biomass were similar regardless of the catalysts used, the enzymatic hydrolysis efficiency and degree of enzyme adsorption differed considerably. The highest efficiencies were 87.79% and 96.49% for the oxalic acid and sulfuric acid catalysts, respectively. The reasons for this observation were investigated by low vacuum-scanning electron microscopy and the structural analysis of lignin. In the oxalic acid pretreated biomass, the enzyme was irreversibly adsorbed onto the lignin. The oxalic acid pretreated biomass possessed a higher content of G-type lignin than did the sulfuric acid pretreated biomass. This type of lignin has a high affinity for the enzyme, inducing irreversible enzyme adsorption onto the biomass.

Chemical pretreatment of lignocellulosic agroindustrial waste for methane production

This study investigates the effect of different chemical pretreatments on the solubilization and the degradability of different solid agroindustrial waste, namely winery waste, cotton gin waste, olive pomace and juice industry waste. Eight different reagents were investigated, i.e. sodium hydroxide (NaOH), sodium bicarbonate (NaHCO₃), sodium chloride (NaCl), citric acid (H₃Cit), acetic acid (AcOH), hydrogen peroxide (H₂O₂), acetone (Me₂CO) and ethanol (EtOH), under three condition sets resulting in treatments of varying intensity, depending on process duration, reagent dosage and temperature. Results indicated that chemical pretreatment under more severe conditions is more effective on the solubilization of lignocellulosic substrates, such as those of the present study and among the investigated reagents, H₃Cit, H₂O₂ and EtOH appeared to be the most effective to this regard. At the same time, although chemical pretreatment in general did not improve the methane potential of the substrates, moderate to high severity conditions were found to generally be the most satisfactory in terms of methane production from pretreated materials. In fact, moderate severity treatments using EtOH for winery waste, H₃Cit for olive pomace and H₂O₂ for juice industry waste and a high severity treatment with EtOH for cotton gin waste, resulted in maximum specific methane yield values. Ultimately, the impact of pretreatment parameters on the different substrates seems to be dependent on their characteristics, in combination with the specific mode of action of each reagent. The overall energy balance of such a system could probably be improved by using lower operating powers and higher solid to liquid ratios.

Genetic modification of plant cell walls to enhance biomass yield and biofuel production in bioenergy crops

Plant cell walls represent an enormous biomass resource for the generation of biofuels and chemicals. As lignocellulose property principally determines biomass recalcitrance, the genetic modification of plant cell walls has been posed as a powerful solution. Here, we review recent progress in understanding the effects of distinct cell wall polymers (cellulose, hemicelluloses, lignin, pectin, wall proteins) on the enzymatic digestibility of biomass under various physical and chemical pretreatments in herbaceous grasses, major agronomic crops and fast-growing trees. We also compare the main factors of wall polymer features, including cellulose crystallinity (CrI), hemicellulosic Xyl/Ara ratio, monolignol proportion and uronic acid level. Furthermore, the review presents the main gene candidates, such as CesA, GH9, GH10, GT61, GT43 etc., for potential genetic cell wall modification towards enhancing both biomass yield and enzymatic saccharification in genetic mutants and transgenic plants. Regarding cell wall modification, it proposes a novel groove-like cell wall model that highlights to increase amorphous regions (density and depth) of the native cellulose microfibrils, providing a general strategy for bioenergy crop breeding and biofuel processing technology.

Improvement of gaseous energy recovery from sugarcane bagasse by dark fermentation followed by biomethanation process

The aim of the present study was to enhance the gaseous energy recovery from sugarcane bagasse. The two stage (biohydrogen and biomethanation) batch process was considered under mesophilic condition. Alkali pretreatment (ALP) was used to remove lignin from sugarcane bagasse. This enhanced the enzymatic digestibility of bagasse to a great extent. The maximum lignin removal of 60% w/w was achieved at 0.25 N NaOH concentration (50°C, 30 min). The enzymatic hydrolysis efficiency was increased to about 2.6-folds with alkali pretreated sugarcane bagasse as compared to untreated one. The maximum hydrogen and methane yields from the treated sugarcane bagasse by biohydrogen and biomethanation processes were 93.4 mL/g-VS and 221.8 mL/g-VS respectively. This process resulted in significant increase in energy conversion efficiency (44.8%) as compared to single stage hydrogen production process (5.4%).

Evaluation of microwave-assisted pretreatment of lignocellulosic biomass immersed in alkaline glycerol for fermentable sugars production

A pretreatment with microwave irradiation was applied to enhance enzyme hydrolysis of corn straw and rice husk immersed in water, aqueous glycerol or alkaline glycerol. Native and pretreated solids underwent enzyme hydrolysis using the extract obtained from the fermentation of Myceliophthora heterothallica, comparing its efficiency with that of the commercial cellulose cocktail Celluclast®. The highest saccharification yields, for both corn straw and rice husk, were attained when biomass was pretreated in alkaline glycerol, method that has not been previously reported in literature. Moreover, FTIR, TG and SEM analysis revealed a more significant modification in the structure of corn straw subjected to this pretreatment. Highest global yields were attained with the crude enzyme extract, which might be the result of its content in a great variety of hydrolytic enzymes, as revealed zymogram analysis. Moreover, its hydrolysis efficiency can be improved by its supplementation with commercial β-glucosidase.

Enhanced bioethanol production from yellow poplar by deacetylation and oxalic acid pretreatment without detoxification

In order to produce ethanol from yellow poplar, deacetylation was performed using sodium hydroxide (NaOH). Optimal deacetylation conditions were determined by a response surface methodology. The highest acetic acid concentration obtained was 7.06 g/L when deacetylation was performed at 60 °C for 80 min with 0.8% NaOH. Acetic acid was recovered by electrodialysis from the deacetylated hydrolysate. The oxalic acid pretreatment of deacetylated biomass was carried out and the hydrolysate directly used for ethanol production without detoxification. Ethanol yields ranged from 0.34 to 0.47 g/g and the highest ethanol yield was obtained when pretreatment was carried out at 150 °C with 50 mM oxalic acid. The highest ethanol concentration obtained from pretreated biomass was 27.21 g/L at 170 °C, using a 50 mM of oxalic acid for the simultaneous saccharification and fermentation (SSF). Overall, 20.31 g of ethanol was obtained by hydrolysate and SSF from 100 g of deacetylated yellow poplar.

Comparison of dilute acid and sulfite pretreatments on Acacia confusa for biofuel application and the influence of its extractives

Chemical components of lignocellulosic biomass may impede biofuel processing efficiency. To understand whether the heartwood of Acacia confusa is suitable for biofuel application, extractive-free heartwood of A. confusa was subjected to dilute acid (DA) or sulfite pretreatments. Sugar recoveries were used to evaluate the performance of different pretreatments. Cell wall properties, such as 4-O-alkylated lignin structures, S/G ratios, and xylan contents, of the pretreated samples showed significant correlations with the enzymatic saccharification of glucan. The 4% bisulfite-pretreated samples produced higher total sugar recoveries than DA-treated samples. The highest total sugar recoveries from DA and sulfite pretreatment were 52.0% (170 °C for 20 min) and 65.3% (4% NaHSO3 and 1% H2SO4), respectively. The results also demonstrated that the existence of extractives in the heartwood of A. confusa hindered the sugar recoveries from both the pretreatments and enzymatic saccharification. Total sugar recoveries were reduced 11.7-17.7% in heartwood samples with extractives.

Recyclability of an ionic liquid for biomass pretreatment

This study investigated the possibility of reusing an ionic liquid for the pretreatment of biomass. The effects of lignin and water content in a pretreatment solvent on pretreatment products were examined, along with the recyclability of an ionic liquid for pretreatment. It was discovered that the presence of lignin and water within a pretreatment solvent resulted in a far less effective pretreatment process. 1-Ethyl-3-methylimidazolium acetate/ethanolamine (60/40 vol%) presents more promising properties than EMIM-AC, providing a small decrease in sugar conversion and also a small increase of lignin deposition with an increasing lignin amount in the pretreatment solvent. Deteriorations of the ionic liquid were observed from considerably low sugar conversions and lignin extraction after using the 5th and 7th batch, respectively. Furthermore, the changes of ionic liquid properties and lignin accumulation in ionic liquid were determined by analyzing their thermal decomposition behavior (TGA) and chemical functional groups (FTIR and (1)H NMR).

Ethanol production from industrial hemp: effect of combined dilute acid/steam pretreatment and economic aspects

In the present study, combined steam (140-180°C) and dilute-acid pre-hydrolysis (0.0-2.0%) were applied to industrial hemp (Cannabis sativa L.), as pretreatment for lignocellulosic bioethanol production. The influence of the pretreatment conditions and cultivation type on the hydrolysis and ethanol yields was also evaluated. Pretreatment with 1% sulfuric acid at 180°C resulted in the highest glucose yield (73-74%) and ethanol yield of 75-79% (0.38-0.40 g-ethanol/g-glucose). Taking into account the costs of biomass processing, from field to ethanol facility storage, the field-dried hemp pretreated at the optimal conditions showed positive economic results. The type of hemp cultivation (organic or conventional) did not influence significantly the effectiveness of the pretreatment as well as subsequent enzymatic hydrolysis and ethanol fermentation.

Effect of dilute acid pretreatment on the conversion of barley straw with grains to fermentable sugars

This study investigated the effects of pretreatment conditions, dilute sulfuric acid concentration and treatment time, on the carbohydrate solubility of a mixture of barley straw and grain. The conditions were expressed as combined severity (CS) to evaluate sugar recovery from pretreated samples. Enzymatic hydrolysates from the lignocellulose pretreatment residues were also included to the results. CS was positively correlating with glucose recovery in all conditions, but in higher acid concentrations CS did not predict xylose recovery. It appeared that the residual xylan better indicate the xylose release. An optimal fermentable sugar yield from the mixture of barley straw and grain was obtained by maintaining the CS at around 1.38, corresponding to an overall glucose yield of 96% and a xylose yield of 57%.