Liquefaction of lignocellulosic biomass for methane production: A review

Recent advances in co-digestion conjugates for anaerobic digestion of food waste

Anaerobic digestion (AD) is a biological process that breaks down organic waste materials, such as food waste (FW) that produces biogas and digestate. The biogas can be utilized as biofuel, and digestate could be applied as fertilizer. However, AD of FW alone has limitations on optimal degradation, digester stability and biogas yield. Co-digestion of FW along with other organic wastes such as animal manure, agricultural residue, sewage sludge and industrial organic waste, has shown substantial improvement in degradation process with increased biogas yield. The inadequacies in FW for optimum AD, like low carbon-to-nitrogen ratio (C/N ratio), lack of trace elements and irregular particle sizes, can be nullified by adding appropriate co-digestion conjugates. This review aims to describe the characteristic inadequacies of FW and examines the effect on mesophilic co-digestion of FW with animal manure, waste sludge and agricultural wastes for biogas production optimization. A critical review on the impact of pretreatment and co-digestion to enrich the methane (CH4) content in biogas has been performed. The review also examines the microbial community shift due to co-digestion, which is critical for the stability of an anaerobic digester. Finally, it discusses the prospects and challenges for the widespread application of the co-digestion technique as an effective organic waste management practice.

Coupling hydropyrolysis and vapor-phase catalytic hydrotreatment to produce biomethane from pine sawdust

This study investigated hydropyrolysis and subsequent vapor-phase hydrotreatment over a NiAl2O4 catalyst to produce biomethane (CH4) from pine sawdust. The non-catalytic pressurized hydropyrolysis generated tar, CO2, and CO as the primary products. However, using a NiAl2O4 catalyst in the second-stage reactor significantly increased the formation of CH4 and reduced CO and CO2 in gas products. The catalyst also fully converted tar intermediates to produce CH4, resulting in a maximum carbon yield of 77.7% with 97.8% selectivity. The temperature plays a crucial role in CH4 generation, with both its yield and selectivity showing a positive correlation with the reaction temperature. Increasing the reaction pressure from 0.2 to 1.2 MPa notably inhibited the production of CH4, leading to a shift towards cycloalkanes due to a competitive reaction. This tandem approach shows great potential as an innovative technique for producing alternative fuels from biomass wastes.

Valorization of strawberry extrudate waste: Recovery of phenolic compounds by direct-hydrothermal treatment and subsequent methane production by mesophilic semi-continuous anaerobic digestion

Strawberry extrudate (SE) is an underused by-product from strawberry industry. Recovery of the phenolic compounds present in SE would represent a very interesting valorisation option. Two main challenges need to be solved, firstly, the solubilisation and recovery of the phenolic compounds contained in SE, and, after that, the stabilisation of the resulted de-phenolized SE. The present research evaluates the potential of a biorefinery process combining a hydrothermal pre-treatment, followed by a phenolic extraction process and, finally, the anaerobic digestion of the remaining SE for producing energy that will contribute to compensate the energy requirements of the whole system. Following the hydrothermal pre-treatment at 170 °C for 60 min, an extraction of 0.6 ± 0.1 g of gallic acid per kilogram of SE was achieved using an adsorbent resin, representing a recovery rate of 64 %. Long-term semi-continuous anaerobic digestion of de-phenolized SE was evaluated at different organic loading rates to evaluate the stability of the process. The anaerobic digestion of pre-treated SE achieved a stable methane production value of 243 ± 34 mL CH4·g volatile solids-1·d-1 at an organic loading rate (ORL) of 1.25 g volatile solids·L-1·d-1. During the operation at this ORL, the control parameters including pH, alkalinity, soluble chemical organic demand (sCOD), and volatile fatty acid (VFA) remained stable and consistently constant. Specifically, the VFA in the reactor during this stable period achieved a value of 102 ± 128 mg O2/L. Also, an economic balance showed that the minimal price of the generated phenolic extract for having benefited from the proposed biorefinery system was 0.812 €·(g of gallic acid equivalents)-1, a price within the range of phenolic compounds used in the food industry.

Rice straw for energy and value-added products in China: a review

The rise of global waste and the decline of fossil fuels are calling for recycling waste into energy and materials. For example, rice straw, a by-product of rice cultivation, can be converted into biogas and by-products with added value, e.g., biofertilizer, yet processing rice straw is limited by the low energy content, high ash and silica, low nitrogen, high moisture, and high-quality variability. Here, we review the recycling of rice straw with focus on the global and Chinese energy situations, conversion of rice straw into energy and gas, biogas digestate management, cogeneration, biogas upgrading, bioeconomy, and life cycle assessment. The quality of rice straw can be improved by pretreatments, such as baling, ensiling, and co-digestion of rice straw with other feedstocks. The biogas digestate can be used to fertilize soils. The average annual potential energy of collectable rice straw, with a lower heating value of 15.35 megajoule/kilogram, over the past ten years (2013-2022) could reach 2.41 × 109 megajoule.

Additional ratios of hydrolysates from lignocellulosic digestate at different hydrothermal temperatures influencing anaerobic digestion performance

Hydrothermal treatment (HT) is envisaged as a promising technology to treat the lignocellulosic biomass. HT temperature is an important parameter influencing the hydrolysate compositions such as organic compounds and potential inhibitors, and therefore affect the subsequential anaerobic digestion (AD) process. Herein, HT-AD was employed to treat the wheat straw-derived digestate. The HT temperature of 190 °C was proved to be the best performance with a higehst reducing sugar yield (45.05 mg g^-1) in the hydrolysate and a highest methane yield (120.8 mL g_TS^-1) from the AD of the hydrolysate, which was 42.5% higher than the methane yield in the control without the hydrolysate addition (84.8 mL g_TS^-1). 3-Furaldehyde was the dominant organic in the hydrolysates. The HT temperature of 210 °C led to the presence of AD inhibitory moieties (e.g., phenols and furans) in the hydrolysate, resulting in a low methane yield. Although the treatments with the addition of 100% hydrolysate outperformed those of 50% hydrolysate in the methane yields in the late stage, the latter had higher methane yields in the first stage, suggesting that the additional ratios of hydrolysates should be carefully considered in AD, especially the detrimental effects of inhibitors and adaptability issues of AD consortia. The MiSeq sequencing showed that the hydrolysis/acidogenesis was dominant in the first stage, while methanogenesis became dominant in the late stage with the acetoclastic/hydrogenotrophic methanogens (Methanosarcina and Methanobacterium) enriched in the hydrolysate-feeding reactors. These findings demonstrated that a integration of HT-AD was a promising approach for the digestate valorization and to reduce the potential carbon emission from waste treatments.

Hydrothermal treatment enhances energy recovery from pig manure digestate and improves the properties of residues

Energy recovery from biowaste is of high significance for a sustainable society. Herein, hydrothermal treatment (HT) was applied to valorize pig manure digestate. The effects of hydrothermal operational parameters, including temperature (130-250 °C), residence time (15-90 min), and total solid (TS) concentration (10%-20%), on reducing sugar yield were investigated in this study. Among them, hydrothermal temperature was identified as the most important factor influencing reducing sugar yield, followed by the TS concentration and time. The optimal hydrothermal conditions for the pig manure digestate were 175.6 °C, 35.4 min and a TS concentration of 10% with a reduced sugar yield of 9.81 mg gTS^-1. The addition of hydrolysate could enhance methane production by 21.6-50.4% from the anaerobic digestion of pig manure than that without the hydrolysate addition. After HT, the hygienic quality, including fecal coliform number and ascaris egg mortality, was improved in the residual digestate. Antibiotics such as sulfamonomethoxine, oxytetracycline, doxycycline and sulfaclodazine in the pig manure digestate were decomposed during HT and decreased environmental risk. These findings indicated that the hydrothermal process might be an effective technique to recover energy from the digestate of livestock and poultry manure and to improve the residual digestate for subsequent utilization.

Subcritical water pretreatment enhanced methane-rich biogas production from the anaerobic digestion of brewer's spent grains

ABSTRACTThis study evaluated the effectiveness of a semi-continuous flow-through subcritical water hydrolysis (SWH) pretreatment of brewer's spent grains (BSG) for subsequent application in the anaerobic digestion (AD) process. BSG pretreatment was conducted at 160 °C and 15 MPa with a flow rate of 10 mL water min-1 and 15 g water g-1 BSG. The results revealed that SWH attacked the hemicellulose structure, releasing arabinose (46.54 mg g-1) and xylose (39.90 mg g-1) sugars, and proteins (34.89 mg g-1). The start-up of anaerobic reactors using pretreated BSG (747.71 L CH4 kg-1 TVS) increased the methane yield compared with the reactor without pretreatment (53.21 L CH4 kg-1 TVS). For the process with pretreatment, the generation of electricity (134 kWh t-1 BSG) and heat (604 MJ t-1) are responsible for the mitigation of 43.90 kg CO2 eq t-1 BSG. The adoption of SWH as an eco-friendly pretreatment of biomass for AD could be a technological route to increase methane-rich biogas and bioenergy production, supporting the circular economy transition by reducing the carbon footprint of the beer industry.

Enabling the complete valorization of hybrid Pennisetum: Directly using alkaline black liquor for preparing UV-shielding biodegradable films

The conversion of lignocellulosic biomass into various high-value chemicals has been a rapid expanding research topic in industry and agriculture. Among them, alkaline removal and utilization of lignin are important for the accelerated degradation of biomass. Modern biorefinery has been focusing the vision on the advancement of economical, green, and environmentally friendly processes. Therefore, it is indispensable to develop cost-effective and simple biomass conversion technologies to obtain high-value products. In this study, the black liquor (BL) obtained from the alkaline pretreatment of biomass was added to polyvinyl alcohol (PVA) solution and used to prepare degradable ultraviolet (UV) shielding films, achieving direct and efficient utilization of the aqueous phase from alkaline pretreatment. This method avoids the extraction step of lignin fraction from black liquor, which can be directly utilized as the raw materials of films preparation. In addition, the direct use of alkaline BL results in films with similar UV-shielding properties, higher physical strength, and similar thermal stability compared with films made by commercial alkaline lignin. Therefore, this strategy is proposed for alkaline-pretreated biorefineries as a simple way to convert waste BL into valuable products and partially recover unconsumed sodium hydroxide to achieve as much integration of biomass and near zero-waste biorefineries as possible.

Emerging challenges for the agro-industrial food waste utilization: A review on food waste biorefinery

Modernization and industrialization has undoubtedly revolutionized the food and agro-industrial sector leading to the drastic increase in their productivity and marketing thereby accelerating the amount of agro-industrial food waste generated. In the past few decades the potential of these agro-industrial food waste to serve as bio refineries for the extraction of commercially viable products like organic acids, biochemical and biofuels was largely discussed and explored over the conventional method of disposing in landfills. The sustainable development of such strategies largely depends on understanding the techno economic challenges and planning for future strategies to overcome these hurdles. This review work presents a comprehensive outlook on the complex nature of agro-industrial food waste and pretreatment methods for their valorization into commercially viable products along with the challenges in the commercialization of food waste bio refineries that need critical attention to popularize the concept of circular bio economy.

Inhibitor formation and detoxification during lignocellulose biorefinery: A review

For lignocellulose biorefinery, pretreatment is needed to maximize the cellulose accessibility, frequently generating excess inhibitory substances to decline the efficiency of the subsequent fermentation processes. This mini-review updates the current research efforts to detoxify the adverse impacts of generated inhibitors on the performance of biomass biorefinery. The lignocellulose pretreatment processes are first reviewed. The generation of inhibitors, furans, furfural, phenols, formic acid, and acetic acid, from the lignocellulose, with their action mechanisms, are listed. Then the detoxification processes are reviewed, from which the biological detoxification processes are noted as promising and worth further study. The challenges and prospects for applying biological detoxification in lignocellulose biorefinery are outlined. Integrated studies considering the entire biorefinery should be performed on a case-by-case basis.

Design and operation of a fixed-bed pyrolysis-gasification-combustion pilot plant for rural solid waste disposal

This paper is to explore the use of rural solid waste (RSW) for pyrolysis-gasification-combustion in pilot plant scale aiming at sustainable management of rural waste in remote areas. Based on the experimental data obtained during pilot scale operation, the temperature in the furnace needs to be kept at least at 600 °C through analyzing the pyrolysis weight loss of the main combustibles in the RSW. Besides, the effects of the air supply method and ventilation rate on the pilot plant performance were explored. Results indicate that the active air supply method positively contributes to the performance of the pilot plant. The plant processed 10 t RSW/d, producing 12.82 g/Nm³ of tar with 1.75 % of ash. This study confirms the feasibility of the pilot plant for RSW disposal and provides theoretical support for the optimization of pilot plant operation.

Biochemical Methane Potential of a Biorefinery’s Process-Wastewater and its Components at Different Concentrations and Temperatures

A sustainable circular bioeconomy requires the side streams and byproducts of biorefineries to be assimilated into bioprocesses to produce value-added products. The present study endeavored to utilize such a byproduct generated during the synthesis of 5-hydroxymethylfurfural as a potential feedstock for biogas production. For this purpose, biochemical methane potential tests for the full process-wastewater, its components (5-hydroxymethylfurfural, furfural, levulinic acid, and glycolic acid), together with furfural’s metabolites (furfuryl alcohol and furoic acid), and phenols (syringaldehyde, vanillin, and phenol), were conducted at mesophilic and thermophilic temperatures to assess their biodegradability and gas production kinetics. 0.1, 0.2, 0.3, and 0.4 g COD of the test components were added separately into assays containing 35 mL of inoculum. At their lowest concentrations, the test components, other than the process-wastewater, exhibited a stimulatory effect on methane production at 37 °C, whereas their increased concentrations returned a lower mean specific methane yield at either temperature. For similar component loads, the mesophilic assays outperformed the thermophilic assays for the mean measured specific methane yields. Components that impaired the anaerobic process with their elevated concentrations were phenol, vanillin, and 5-hydroxymethylfurfural. Poor degradation of the process-wastewater was deduced to be linked to the considerable share of 5-hydroxymethylfurfural in the process-wastewater governing its overall characteristics. With excessive recalcitrant components, it is recommended to use such waste streams and byproducts as a substrate for biogas plants operating at moderate temperatures, but at low rates.

Ethanol-Assisted Hydrothermal Liquefaction of Poplar Using Fe-Co/Al2O3 as Catalyst

Although the conversion of lignocellulosic biomass into bio-oil with high yield/quality through hydrothermal liquefaction (HTL) is promising, it still faces many challenges. In this study, a Fex-Co(1-x)/Al2O3 catalyst was prepared with the coprecipitation method and low-content ethanol was used as the cosolvent for the HTL of poplar. The results showed that the Fex-Co(1-x)/Al2O3 catalyst significantly promoted the yield and energy recovery rate (ERR) of bio-oil compared with the control (10% ethanol content). At 260 °C for 30 min, 60Fe-40Co/Al2O3 had the best catalytic effect, achieving the highest bio-oil yield (67.35%) and ERR (93.07%). As a multifunctional bimetallic catalyst, Fex-Co(1-x)/Al2O3 could not only increase the degree of hydrogenation deoxidization of the product but also promote the diversity of phenolic compounds gained from lignin. The bio-oil obtained from HTL with Fex-Co(1-x)/Al2O3 as catalyst contained lower heterocyclic nitrogen, promoting the transfer of more bio-oil components to substances with lower boiling point.

Lignocellulosic composition based thermal kinetic study of Mangiferaindica Lam, Artocarpus Heterophyllus Lam and Syzygium Jambolana seeds

In the present study, pyrolysis of mangifera indica L., Artocarpus heterophyllus L. and jambolana seeds have been performed using thermogravimetric analysis. These biomasses have enriched lignocellulosic composition of hemicellulose (5-10%) and lignin (1-3%) which are unexplored. The TGA analysis was performed at various heating rates of 10, 15, 20, 25 and 30 °C/min from 25 to 600 °C. Kinetic investigation of the pyrolysis method using TGA statistics has been done using iso-conversional models of Flynn-Wall-Ozawa, Kissinger-Akahira-Sunose, Vyazovkin and Vyazovkin AIC. The apparent activation energies value ranged from 179.86 to 226.31 kJ/mol in the fractional conversion range of 0.1 to 0.7.

Determination of kinetic triplet, thermal degradation behaviour and thermodynamic properties for pyrolysis of a lignocellulosic biomass

Kinetic triplet, thermal degradation behaviour and thermodynamic properties of peanut shells were determined on the basis of non-isothermal thermogravimetric experiments conducted at three different heating rates under N₂ atmosphere. A single differential peak was observed for the devolatilization stage. The kinetic triplet of devolatilization stage was determined using Coats-Redfern and a combined method consisting the utilization of isoconversional and Criado methods. Kinetic validation revealed that the kinetic triplet determined using the combined method described the experimental values more precisely. The reaction mechanism ascertained by the combined method was D5-D3 combination. The E_a value was strong function of conversion, and computed using isoconversional methods (Boswell, Flynn-Wall-Ozawa, Starink, Tang) between 169 and 268 kJ/mol. Entalphy, entrophy and Gibbs energy changes were computed in 164-259 kJ/mol, -37-141 J/(mol.K) and 173-187 kJ/mol ranges, respectively. The comprehensive pyrolysis index values were also calculated, and shown to increase with increasing heating rate.

Anaerobic digestion beyond biogas

Anaerobic digestion (AD) is a matured technology for waste (water) remediation/stabilization and bioenergy generation in the form of biogas. AD technology has several inherent benefits ranging from generating renewable energy, remediating waste (water), and reducing greenhouse gas emission to improving health/hygiene and the overall socio-economic status of rural communities in developing nations. In recent years, there has been a paradigm shift in applications of AD technology beyond biogas. This special issue (SI) entitled, "Anaerobic Digestion Beyond Biogas (ADBB-2021)," was conceptualized to incorporate some of the recent advances in AD in which the emphasis is beyond biogas, such as anaerobic biorefinery, chain elongation, treatment of micropollutants, toxicity and system stability, digestate as biofertilizer, bio-electrochemical systems, innovative bioreactors, carbon sequestration, biogas upgrading, microbiomes, waste (water) remediation, residues/waste pre-treatment, promoter addition, and modeling, process control, and automation, among others. This VSI: ADBB-2021 contains 53 manuscripts (14 critical reviews and 39 research). The key findings of each manuscript are briefly summarized here, which can serve as a valuable resource for AD researchers to learn of major advances in AD technology and identify future research directions.