Lignocellulose biohydrogen towards net zero emission: A review on recent developments

Biohydrogen fermentation from pretreated biomass in lignocellulose biorefinery: Effects of inhibitory byproducts and recent progress in mitigation strategies

Lignocellulosic biomass (LCB) is expected to play a critical role in achieving the goal of biomass-to-bioenergy conversion because of its wide distribution and low price. Biomass fermentation is a promising method for the sustainable generation of biohydrogen (bioH2) from the renewable feedstock. Due to the inherent resistant structure of biomass, LCB needs to be pretreated to improve its digestibility and utilization. However, certain intermediates by-products generated during the pretreatment process, such as phenolic compounds, furan derivatives, and aldehydes, have been identified as potent inhibitors of subsequent anaerobic fermentation due to their disruptive effects on the physiological and metabolic functions of hydrogen-producing microbiota. To counteract the negative effects of these inhibitors on bio-H2 fermentation, various detoxification strategies for LCB hydrolysates have been explored. This review presents a comprehensive analysis of fermentation-inhibitory by-products commonly generated by modern pretreatment protocols and their negative impacts on biohydrogen fermentation. Furthermore, the underlying mechanisms of inhibition upon hydrogen-producing microbes and their impacts on microbial community dynamics are exhibited. State-of-the-art strategies for detoxifying pretreated LCB have been also discussed, along with alternative pretreatment strategies designed to minimize or eliminate the formation of inhibitory by-products. Additionally, this review addresses the significant gap in the economic viability assessments of these processes, offering a detailed evaluation of both the technological and economic feasibility of biomass fermentation. Given the limitations of previous studies, strategies for cost-effective pretreatment and detoxification should be developed in the future to overcome the inhibition of fermentation inhibitors in the bioconversion of biomass to hydrogen.

The outlooks and key challenges in renewable biomass feedstock utilization for value-added platform chemical via bioprocesses

The conversion of renewable biomass feedstock into value-added products via bioprocessing platforms has become attractive because of environmental and health concerns. Process performance and cost competitiveness are major factors in the bioprocess design to produce desirable products from biomass feedstock. Proper pretreatment allows delignification and hemicellulose removal from the liquid fraction, allowing cellulose to be readily hydrolyzed to monomeric sugars. Several industrial products are produced via sugar fermentation using either naturally isolated or genetically modified microbes. Microbial platforms play an important role in the synthesis of several products, including drop-in chemicals, as-in products, and novel compounds. The key elements in developing a fermentation platform are medium formulation, sterilization, and active cells for inoculation. Downstream bioproduct recovery may seem like a straightforward chemical process, but is more complex, wherein cost competitiveness versus recovery performance becomes a challenge. This review summarizes the prospects for utilizing renewable biomass for bioprocessing.

Characteristics of hydrogen energy yield in steam gasification of coffee residues

Coffee residues (CRs) were gasified using a laboratory-scale fluidized bed gasifier with an air/steam mixture as the carrier gas. The gasification was conducted at an equivalence ratio (ER) of 0.3, and different operation temperatures (700, 800, and 900 °C) and steam-to-biomass (S/B) ratios (0, 0.75, and 1.5) were applied. Increasing temperature without steam boosted H2 and CO concentrations in producer gas, raising lower heating value (LHV) and cold gas efficiency (CGE) through endothermic reactions like Boudouard, tar cracking, and water-gas formation. At 900 °C, gas had LHV of 3.76 MJ/Nm3 and CGE of 22.47%. It was elevating temperature from 700 to 900 °C and S/B ratio to 1.5 raised H2 and CO concentrations from 2.04 to 8.60% and from 9.56 to 11.8%, respectively. This also increased LHV from 2.23 to 3.89 MJ/Nm3 and CGE from 11.28 to 25.08%. The steam gasification reaction was found to increase the H2 concentration and was thus considered effective in converting CRs to syngas and increasing energy production. Overall, the study successfully demonstrated the feasibility of steam gasification as a means of converting coffee residues to syngas and increasing energy production. The results also highlighted the importance of operating temperature and S/B ratio in improving the gasification process.

Optimization of biogas production from straw wastes by different pretreatments: Progress, challenges, and prospects

Lignocellulosic biomass (LCB) presents a promising feedstock for carbon management due to enormous potential for achieving carbon neutrality and delivering substantial environmental and economic benefit. Bioenergy derived from LCB accounts for about 10.3 % of the global total energy supply. The generation of bioenergy through anaerobic digestion (AD) in combination with carbon capture and storage, particularly for methane production, provides a cost-effective solution to mitigate greenhouse gas emissions, while concurrently facilitating bioenergy production and the recovery of high-value products during LCB conversion. However, the inherent recalcitrant polymer crystal structure of lignocellulose impedes the accessibility of anaerobic bacteria, necessitating lignocellulosic residue pretreatment before AD or microbial chain elongation. This paper seeks to explore recent advances in pretreatment methods for LCB biogas production, including pulsed electric field (PEF), electron beam irradiation (EBI), freezing-thawing pretreatment, microaerobic pretreatment, and nanomaterials-based pretreatment, and provide a comprehensive overview of the performance, benefits, and drawbacks of the traditional and improved treatment methods. In particular, physical-chemical pretreatment emerges as a flexible and effective option for methane production from straw wastes. The burgeoning field of nanomaterials has provoked progress in the development of artificial enzyme mimetics and enzyme immobilization techniques, compensating for the intrinsic defect of natural enzyme. However, various complex factors, such as economic effectiveness, environmental impact, and operational feasibility, influence the implementation of LCB pretreatment processes. Techno-economic analysis (TEA), life cycle assessment (LCA), and artificial intelligence technologies provide efficient means for evaluating and selecting pretreatment methods. This paper addresses current issues and development priorities for the achievement of the appropriate and sustainable utilization of LCB in light of evolving economic and environmentally friendly social development demands, thereby providing theoretical basis and technical guidance for improving LCB biogas production of AD systems.

Selective separation of hemicellulose from poplar by hydrothermal pretreatment with ferric chloride and pH buffer

As an environmentally friendly lignocellulosic biomass separation technology, hydrothermal pretreatment (HP) has a strong application prospect. However, the low separation efficiency is a main factor limiting its application. In this study, the poplar components were separated using HP with ferric chloride and pH buffer (HFB). The optimal conditions were ferric chloride concentration of 0.10 M, reaction temperature of 150 °C, reaction time of 15 min and pH 1.9. The separation of hemicellulose was increased 34.03 % to 77.02 %. The pH buffering resulted in the highest cellulose and lignin retention yields compared to ferric chloride pretreatment (FC). The high efficiency separation of hemicellulose via HFB pretreatment inhibited the degradation of xylose. The hydrolysate was effectively reused for five times. The fiber crystallinity index reached 60.05 %, and the highest C/O ratio was obtained. The results provide theoretical support for improving the efficiency of HP and promoting its application.

Sustainable valorization of pitaya (Hylocereus spp.) peel in a semi-continuous high-pressure hydrothermal process to recover value-added products

This study evaluated the use of a semi-continuous high-pressure hydrothermal process for the recovery of value-added products from pitaya peel. The process was carried out at 15 MPa, a water flow rate of 2 mL/min, a solvent-to-feed ratio of 60 g water/g pitaya peel, and temperatures ranging from 40 to 210 °C. The results show that extraction temperatures (between 40 and 80 °C) promoted the recovery of betacyanin (1.52 mg/g), malic acid (25.6 mg/g), and citric acid (25.98 mg/g). The major phenolic compounds obtained were p-coumaric acid (144.63 ± 0.42 µg/g), protocatechuic acid (91.43 ± 0.32 µg/g), and piperonylic acid (74.2 ± 0.31 µg/g). The hydrolysis temperatures (between 150 and 210 °C) could produce sugars (18.09 mg/g). However, the hydrolysis process at temperatures above 180 °C generated Maillard reaction products, which increased the total phenolic compounds and antioxidant activity of the hydrolysates. Finally, the use of semi-continuous high-pressure hydrothermal process can be a sustainable and promising approach for the recovery of value-added compounds from pitaya peel, advocating a circular economy approach in the agri-food industry.

Effect of nano-metal doped calcium peroxide on biomass pretreatment and green hydrogen production from rice straw

In this study, calcium peroxide was modified and doped with metal-based nanoparticles (NP) to enhance the efficiency of pretreatment and biohydrogen generation from RS. The findings revealed that the addition of MnO2-CaO2 NPs (at a dosage of 0.02 g/g TS of RS) had a synergistic effect on the breakdown of biomass and the production of biohydrogen. This enhancement resulted in a maximum hydrogen yield (HY) of 58 mL/g TS, accompanied by increased concentrations of acetic acid (2117 mg/L) and butyric acid (1325 mg/L). In contrast, RS that underwent pretreatment without the use of chemicals or NP exhibited a lower HY of 28 mL/g TS, along with the lowest concentrations of acetic acid (1062 mg/L) and butyric acid (697 mg/L). The outcome showed that supplementation of NP stimulated the pretreatment of RS and improved the formation of acetic and butyric acid through the regulation of metabolic pathways during acidogenic fermentation.

Green synthesis of sulfonated cellulose/polyether block amide/polyethylene glycol diacrylate (SC/PEBAX/PEGDA) composite membrane by gamma radiation and sulfonation techniques for battery application

Sulfonated cellulose (SC) was successfully prepared through a two-step process of gamma radiation and subsequently sulfonation with potassium metabisulfite of microcrystalline cellulose extracted from sugarcane bagasse. The effect of gamma radiation dose on cellulose showed an increment of oxidation degree, which was evidenced by the intensity ratio of I1718 (carbonyl)/ I2892 (aliphatic) from FTIR analysis. The obtained SC was introduced into polyether block amide/polyethylene glycol diacrylate (PEBAX/PEGDA) polymer matrix as a reinforcement and hydrophilic filler for improving electrolyte affinity and thermal stability of its composite membrane. The increase of SC in PEBAX/PEGDA composite membranes resulted in enhancement of hydrophilicity, electrolyte uptake, and thermal stability compared to pristine composite membranes. However, the excess SC content in the composite membrane exhibited the low physical properties, caused by negligible dispersion on the surface membrane. With the optimum 2.0 wt% SC in PEBAX/PEGDA, the porosity, contact angle and electrolyte uptake capacity was found to be 64.0 %, 12.8° and 37.5 %, respectively. 2.0 wt% SC/PEBAX/PEGDA showed the outstanding thermal stability with negligible shrinkage <10 % at 150 °C whereas pristine PEBAX/PEGDA showed the shrinkage of 29 %. The obtained SC/PEBAX/PEGDA composite membrane is considered as a potential candidate to replace the commercial polyolefin-based separator in lithium-ion batteries.

Processing of Biomass Prior to Hydrogen Fermentation and Post-Fermentative Broth Management

Using bioconversion and simultaneous value-added product generation requires purification of the gaseous and the liquid streams before, during, and after the bioconversion process. The effect of diversified process parameters on the efficiency of biohydrogen generation via biological processes is a broad object of research. Biomass-based raw materials are often applied in investigations regarding biohydrogen generation using dark fermentation and photo fermentation microorganisms. The literature lacks information regarding model mixtures of lignocellulose and starch-based biomass, while the research is carried out based on a single type of raw material. The utilization of lignocellulosic and starch biomasses as the substrates for bioconversion processes requires the decomposition of lignocellulosic polymers into hexoses and pentoses. Among the components of lignocelluloses, mainly lignin is responsible for biomass recalcitrance. The natural carbohydrate-lignin shields must be disrupted to enable lignin removal before biomass hydrolysis and fermentation. The matrix of chemical compounds resulting from this kind of pretreatment may significantly affect the efficiency of biotransformation processes. Therefore, the actual state of knowledge on the factors affecting the culture of dark fermentation and photo fermentation microorganisms and their adaptation to fermentation of hydrolysates obtained from biomass requires to be monitored and a state of the art regarding this topic shall become a contribution to the field of bioconversion processes and the management of liquid streams after fermentation. The future research direction should be recognized as striving to simplification of the procedure, applying the assumptions of the circular economy and the responsible generation of liquid and gas streams that can be used and purified without large energy expenditure. The optimization of pre-treatment steps is crucial for the latter stages of the procedure.