Pretreatment of Lignocellulosic Biomass

Valorization of crop residues and animal wastes: Anaerobic co-digestion technology

To switch the over-reliance on fossil-based resources, curb environmental quality deterioration, and promote the use of renewable fuels, much attention has recently been directed toward the implementation of sustainable and environmentally benign 'waste-to-energy' technology exploiting a clean, inexhaustible, carbon-neutral, and renewable energy source, namely agricultural biomass. From this perspective, anaerobic co-digestion (AcoD) technology emerges as a potent and plausible approach to attain sustainable energy development, foster environmental sustainability, and, most importantly, circumvent the key challenges associated with mono-digestion. This review article provides a comprehensive overview of AcoD as a biochemical valorization pathway of crop residues and livestock manure for biogas production. Furthermore, this manuscript aims to assess the different biotic and abiotic parameters affecting co-digestion efficiency and present recent advancements in pretreatment technologies designed to enhance feedstock biodegradability and conversion rate. It can be concluded that the substantial quantities of crop residues and animal waste generated annually from agricultural practices represent valuable bioenergy resources that can contribute to meeting global targets for affordable renewable energy. Nevertheless, extensive and multidisciplinary research is needed to evolve the industrial-scale implementation of AcoD technology of livestock waste and crop residues, particularly when a pretreatment phase is included, and bridge the gap between small-scale studies and real-world applications.

Effect of the Combining Corn Steep Liquor and Urea Pre-treatment on Biodegradation and Hydrolysis of Rice Straw

A novel pre-treatment using corn steep liquor (CSL) and urea was developed to enhance the enzymatic saccharification and degradability of rice straw (RS). We used RS (1) without (Con) or with additives of (2) 5% urea (U), (3) 9% CSL and 2.5% urea (CU), and (4) 9% CSL and 5% urea (C5U). The result showed that the water-soluble carbohydrate (WSC) conversion of RS reached 69.32% after C5U pre-treatment. Scanning electron microscopy (SEM), Fourier transform infrared (FTIR) spectroscopy, and X-ray diffraction analysis (XRD) confirmed that the surface of pre-treated RS exposed more cellulose and hemicellulose due to the disruption of the resistant structure of lignocellulose. Pre-treated RS significantly decreased neutral detergent fiber (NDF) and acid detergent fiber (ADF) contents and increased crude protein (CP) content, microbial colonization, and induction of Carnobacterium and Staphylococcus attachment. Altogether, we concluded that pre-treatment of a combination of CSL and urea has the potential to improve the nutritive value of RS.

Using low carbon footprint high-pressure carbon dioxide in bioconversion of aspen branch waste for sustainable bioethanol production

An innovative approach was developed by incorporating high-pressure CO₂ into the separate hydrolysis-fermentation of aspen leftover branches, aiming to enhance the bioethanol production efficiency. The high-pressure CO₂ significantly increased the 72-h enzymatic hydrolysis yield of converting aspen into glucose from 53.8% to 82.9%. The hydrolysis process was performed with low enzyme loading (10 FPU g^-1 glucan) with the aim of reducing the cost of fuel bioethanol production. The ethanol yield from fermentation of the hydrolyzed glucose using yeast (Saccharomyces cerevisiae) was 8.7 g L^-1, showing increment of 10% compared with the glucose control. Techno-economic analysis indicated that the energy consumption of fuel bioethanol production from aspen branch chips was reduced by 35% and the production cost was cut 44% to 0.615 USD L^-1, when 68 atm CO₂ was introduced into the process. These results furtherly emphasized the low carbon footprint of this sustainable energy production approach.

Ensiling characteristics, physicochemical structure and enzymatic hydrolysis of steam-exploded hippophae: Effects of calcium oxide, cellulase and Tween

To find a comprehensive way to enhance the utilizability of steam-exploded hippophae, calcium oxide (CaO) preimpregnation, cellulase-added storage and saccharification with addition of Tween 20 were investigated in this study. Both CaO preimpregnation and cellulase addition promoted the ensiling fermentation of anaerobically stored steam-exploded hippophae indicated by lower cellulose proportion and higher organic acids content, but led to the decrease of saccharification yield by 11.83% and 46.77-51.22%, respectively. When taking into account of organic acids being utilizable energy source, storing with addition of cellulase enhanced the utilizability of the materials in whole. Moreover, the addition of Tween 20 enhanced saccharification yield of the steam-exploded hippophae by 26.69-45.25%. Additionally, FTIR and XRD spectra clearly illustrated the structural alteration during storage. It is concluded that storing with addition of cellulase and hydrolyzing with addition of Tween 20 can enhance the utilizability of steam-exploded hippophae.

Effect of microwave-assisted ionic liquid/acidic ionic liquid pretreatment on the morphology, structure, and enhanced delignification of rice straw

In the present study, was investigated an environmentally friendly method for pretreating lignocellulosic rice straw (RS) by using 1-butyl-3-methylimidazolium chloride ([Bmim]Cl) as an ionic liquid (IL) and 1-butyl-3-methylimidazolium hydrogen sulfate ([Bmim]HSO₄) as an acidic-IL (Acidic-IL) under microwave irradiation (microwave-[Bmim]Cl and microwave-[Bmim]HSO₄). The conversion of lignocellulosic biomass into simple sugars requires both efficient pretreatment and hydrolysis enzymes to produce biofuels and specialty chemicals. Therefore, the applied [Bmim]Cl, [Bmim]HSO₄, microwave-[Bmim]Cl, and microwave-[Bmim]HSO₄ to improve hydrolysis yields. Structural analyses of the pretreated solids were performed to understand the synergistic effects of [Bmim]Cl, and [Bmim]HSO₄ pretreatment under microwave irradiation (microwave-[Bmim]Cl and microwave-[Bmim]HSO₄) on the efficiencies of enzymatic hydrolyses. The results of a chemical composition analysis of untreated and all pretreated RS samples by using the difference pretreatment methods showed that significant lignin removal was achieved using microwave-[Bmim]Cl (57.02 ± 1.24%), followed by [Bmim]Cl only (41.01 ± 2.67%), microwave-[Bmim]HSO₄ (20.77 ± 1.79%), and [Bmim]HSO₄-only (16.88 ± 1.14%). The highest glucan yield and xylan conversion achieved through the enzymatic saccharification of microwave-[Bmim]Cl-regenerated cellulose was consistent with the observations obtained from a structural analysis, which indicated a more disrupted, amorphous structure, with lowered crystallinity index (CrI) and lateral order index (LOI) of cellulose polymers. Thus results demonstrated that the pretreatment of lignocellulosic biomass with [Bmim]Cl under microwave irradiation has potential as an alternative method for pretreating lignocellulosic materials.

Combination of steam explosion pretreatment and anaerobic alkalization treatment to improve enzymatic hydrolysis of Hippophae rhamnoides

The optimum condition of steam explosion pretreatment was screened for hippophae, and anaerobic calcium oxide (CaO) alkalization was further used to improve its enzymatic hydrolysis. Steam-exploded hippophae reached the lowest pH value (4.01) and the maximal hemicellulose removal (77.16%) at pressure 1.5 MPa and residence time 20 min. Lignocellulosic fractions of hippophae was remarkably reduced by CaO alkalization or steam explosion treatment, and enzymatic sugar yield was increased from 66 mg/g DM (untreated material) to 270 and 300 mg/g DM, respectively. The sequent pretreatment of steam explosion and CaO alkalization achieved a sugar yield of 330 mg/g DM, where 2% CaO loading rate was high enough. Besides, SEM, FTIR, and XRD analyses validated structural and physicochemical changes of hippophae. In conclusion, the sequent pretreatment of steam explosion at pressure 1.5 MPa for 20 min and anaerobic CaO alkalization at 2% loading rate could remarkably improve enzymatic hydrolysis of hippophae.

Sustainable and efficient sugar production from wheat straw by pretreatment with biogas digestate

The use of liquid fraction of biogas digestate (LFBD) instead of fresh water (hydrothermal) for the pretreatment of wheat straw was evaluated to improve the yield of released sugars during the following hydrolysis step. The pretreatments were conducted at temperatures of 130, 160, and 190 °C for 30 and 60 min. In most of the cases, pretreatment using LFBD led to higher glucose yields and higher total sugars concentrations, compared to those obtained after applying hydrothermal pretreatments. The increase of temperature resulted in an increase in sugars during the enzymatic hydrolysis. The highest yields of glucose (about 59%) were observed after treatments at 190 °C for 60 min, independently of which type of pretreatment was applied and at 190 °C for 30 min using LFBD. Treatment, with LFBD at 190 °C and for 60 min, resulted in glucose and xylose concentrations of 7.36 g L-1 and 2.41 g L-1, respectively, after the subsequent hydrolysis for 48 h. However, the FTIR analysis indicated that the crystallinity index remained rather constant after treatment. Both FTIR and compositional analysis showed that the removal of hemicelluloses was the main effect of the pretreatment.