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Biohythane (a mixture of hydrogen and methane) may play a significant role in a future decarbonised energy system. The production of biohythane can be achieved by sequential dark hydrogen fermentation and anaerobic digestion. However, the technology readiness level of biohythane can be limited by many process constraints negatively affecting its commercial feasibility. Here, a pilot experiment on fermentative hythane production from cassava stillage residue (CSR) incorporating dilute acid pretreatment and enzymolysis was undertaken. The production of hydrogen and methane was 72.0 ± 10.7 and 295.4 ± 28.5 mL/g volatile solid, respectively. Different scenarios for techno-economic analysis were developed in terms of the dried/wet form of CSR and total solids content during fermentation. Results suggested that hythane from CSR was not economically feasible with a high production cost (1.39–2.33 €/m3). There was a trade-off relationship between the increase in methane yield through pretreatment and the associated cost. • Pilot-scale biohythane production from cassava stillage residue was studied. • Four scenarios for techno-economic analysis of hythane production were developed. • The production cost was not economically viable varying between 1.39 and 2.33 €/m3. • Profit was affected by total solid content, pretreatment and hydrogen production. [ABSTRACT FROM AUTHOR]

The effect of an organic loading rate (OLR) and a hydraulic retention time (HRT) was evaluated by comparing the single-stage and two-stage anaerobic digestion processes. Wastes from the food processing industry (FPW) and the organic fraction of the municipal solid waste (OFMSW) were used as substrates. The OLR was increased at each step from 2 gVS/l/d to 14 gVS/l/d, and the HRT was decreased from 10 days to 3 days. The highest theoretical methane yield achieved in the single-stage process was about 84% for the FPW during an OLR of 3 gVS/l/d at a HRT of 7 days and 67% for the OFMSW at an OLR of 2 gVS/l/d and a HRT of 10 days. The single-stage process could not handle a further increase in the OLR and a decrease in the HRT; thus, the process was stopped. A more stable operation was observed at higher OLRs and lower HRTs in the two-stage system. The OLR could be increased to 8 gVS/l/d for the FPW and to 12 gVS/l/d for the OFMSW, operating at a HRT of 3 days. The results show a conclusion of 26% and 65% less reactor volume for the two-stage process compared to the single-stage process for the FPW and the OFMSW, respectively. [ABSTRACT FROM AUTHOR]

A novel process simulation model (PSM) was developed for biogas production in anaerobic digesters using Aspen Plus®. The PSM is a library model of anaerobic digestion, which predicts the biogas production from any substrate at any given process condition. A total of 46 reactions were used in the model, which include inhibitions, rate-kinetics, pH, ammonia, volume, loading rate, and retention time. The hydrolysis reactions were based on the extent of the reaction, while the acidogenic, acetogenic, and methanogenic reactions were based on the kinetics. The PSM was validated against a variety of lab and industrial data on anaerobic digestion. The P -value after statistical analysis was found to be 0.701, which showed that there was no significant difference between discrete validations and processing conditions. The sensitivity analysis for a ±10% change in composition of substrate and extent of reaction results in 5.285% higher value than the experimental value. The model is available at http://hdl.handle.net/2320/12358 (Rajendran et al., 2013b). [ABSTRACT FROM AUTHOR]

The effect of recirculation in increasing organic loading rate (OLR) and decreasing hydraulic retention time (HRT) in a semi-continuous two-stage anaerobic digestion system using stirred tank reactor (CSTR) and an upflow anaerobic sludge bed (UASB) was evaluated. Two-parallel processes were in operation for 100 days, one with recirculation (closed system) and the other without recirculation (open system). For this purpose, two structurally different carbohydrate-based substrates were used; starch and cotton. The digestion of starch and cotton in the closed system resulted in production of 91% and 80% of the theoretical methane yield during the first 60 days. In contrast, in the open system the methane yield was decreased to 82% and 56% of the theoretical value, for starch and cotton, respectively. The OLR could successfully be increased to 4 gVS/L/day for cotton and 10 gVS/L/day for starch. It is concluded that the recirculation supports the microorganisms for effective hydrolysis of polyhydrocarbons in CSTR and to preserve the nutrients in the system at higher OLRs, thereby improving the overall performance and stability of the process. [ABSTRACT FROM AUTHOR]

Two-stage biogas production is reported to overcome the drawbacks of productivity in anaerobic digestion (AD). Recent publications indicate an increase in methane yield between 10 and 30% via two-stage AD. However, the industrial acceptance is minimal due to their reliability and operational issues. This paper critically reviews the two-stage AD for biogas production. Some of the research gaps identified in two-stage AD include lack of techno-economic analysis to show the industry about the feasibility of this process. There is a clear trade-off between the increase in the methane yield vs. the cost it takes to build the second digester. Practically, building a second digester is not economically feasible due to economies of scale. Other technical challenges include the recirculation leads to ammonia accumulation in the system, and disturbance in syntrophic relationships of microbes between the two-stages. Techno-economic analysis suggests that two stage AD could be about 3% expensive than a single stage AD. Further detailed analysis is required to show clear evidence about the economics and feasibility of two stage AD. The parasitic energy demand of the two-stage system will be higher than a single stage AD due to the reason that two reactors are involved for mixing or maintaining temperature. Most of the two-stage AD, operates at a different temperature and hence the energy demand will be different for different reactors. Some of the problem in the literature includes assessing the stage wise OLR, HRT data, and TS/VS balance before and after the process. To address these issues, further work is necessary to standardize the way two-stage experiments are carried out including the parameters that are necessary to be measured for reproducibility. • Two-stage AD increases the methane yield between 10% and 30%. • Mass transfer and pilot studies are required for commercialization. • Economics of two-stage processes needs to be verified. • Trade-offs of two-stage AD: methane yield vs. energy balance vs. cost. • Two-stage is capital intensive than a single stage process. [ABSTRACT FROM AUTHOR]

Techno-economic analysis and life cycle assessments are crucial for any processes to be sustainable using the tri-fold metrics including technical feasibility, economic viability, and environmental sustainability. Anaerobic digestion is portrayed as one of the mature technologies for handling solid waste management and bioenergy generation. Nonetheless, a clear assessment of the tri-fold sustainability metrics is not available yet and this review attempts to address this knowledge gap. Important problems in techno-economic analysis and life cycle assessments such as assumptions used, extrapolation of research data, robustness and reproducibility of results, the openness of materials were discussed. Anaerobic digestion helps in treating organic wastes that could be used for different purposes including electricity, vehicle fuel, natural gas substituent, heating, and cooking fuel. However, sustainability in terms of technology, economics and environment remains the question for it to be industrialized. Display Omitted [ABSTRACT FROM AUTHOR]

[Display omitted] • Biological pretreatment of food waste enhances biogas yield by 3.5 times. • Membrane-based biogas upgradation becomes sustainable when the CH 4 leakage is < 4 %. • Cryogenic separation of biogas generates CO 2 of purity 99.92% as a byproduct. • Hydrogenotrophic pathways can aid in enhancing the CUF of renewable energy plants. Food waste is primarily generated in marketplaces, agricultural fields, hotels, food manufacturers units, and halls. Food waste have a major impact on food security, quality and safety, economic development, and cause environment pollution. The improper disposal of food waste without proper treatments leads to generation of new diseases, unpleasant odour, air, water, and soil pollution. Nevertheless, food waste is a good substrate which can be disintegrated by digestion process because it exhibits more water contents and biodegradability. The conversion of food waste into biomethane is an appreciable solution in food waste management steps. This manuscript reviews the physico-chemical properties of food waste, various pretreatment methods of food waste to enhance the efficiency of anaerobic digestion (AD) process used to produce biomethane and discussed the impact of operational factors on biomethane production. Subsequently, the need for a biomethane upgradation using physical, chemical, and biological purification approaches was reviewed. In order to improve the efficiency of the anaerobic digestion (AD) process to a large-scale industrial level, the challenges and possible future developments needed to enhance biomethane generation from food waste were also reviewed significantly. [ABSTRACT FROM AUTHOR]

[Display omitted] • Process simulation and a techno-economic analysis for the AD of vinasses was assessed. • Deterministic and stochastic modelling were used to assess the economic performance. • Power generation showed the best economic performance in the Cuban scenarios. • High incentive values are required to get profit in biomethane alternatives. • Minimum milling capacities of 10,800 t cane /d are required for biomethane projects. This paper presents a process simulation model in Aspen Plus® and a techno-economic assessment for the anaerobic digestion of Cuban sugarcane vinasses considering three scenarios for biogas application: electricity production (S_1), biomethane as vehicle fuel (S_2), and biomethane for gas grid injection (S_3). From the simulation model, non-significant differences (p_value ≥ 0.1779) between experimental and simulation results were found. S_1 showed the best economic performance among the assessed biogas applications. From the sensitivity analysis, the mean electricity price leading to a net present value of zero for S_1 was 90 USD/MWh, while for S_2 and S_3 the mean incentive required was 0.33 USD/m3 biomethane and 0.67 USD/m3 biomethane , respectively. The uncertainty analysis showed a chance for investment failure in S_1 less than 10%, whereas for S_2 and S_3 it ranged between 31–37%. The minimum scale required (milling and distillery capacities, ethanol yield) for getting profits from biomethane projects was targeted at 10,800 t cane /day, 108 m3 ethanol /d at 10 L ethanol /t cane , respectively. To this end, Cuban plants should significantly increase their average capacities; otherwise, a centralized biomethane production by limiting the number of biomethane plants to one or two per province could be implemented. [ABSTRACT FROM AUTHOR]

[Display omitted] • Supervised ML models were used to predict biogas yield and methane composition. • 5 different ML algorithms were considered including SVM, DT, ET, GPR, and KNN. • Critical parameters that affect the BGP prediction were OLR and HRT, and C/N ratio. • SVM predicted the best accuracy with an R2 value of 0.91. • No significant difference was found between 2-datasets (p = 0.377). Biogas production through anaerobic digestion (AD) is one of the complex non-linear biological processes, wherein understanding its dynamics plays a crucial role towards process control and optimization. In this work, a machine learning based biogas predictive model was developed for high solid systems using algorithms, including SVM, ET, DT, GPR, and KNN and two different datasets (Dataset-1:10, Dataset-2:5 inputs). Support Vector Machine had the highest accuracy (R2) of all the algorithms at 91 % (Dataset-1) and 87 % (Dataset-2), respectively. The statistical analysis showed that there was no significant difference (p = 0.377) across the datasets, wherein with less inputs, accurate results could be predicted. In case of biogas yield, the critical factors which affect the model predictions include loading rate and retention time. The developed high solid machine learning model shows the possibility of integrating Artificial Intelligence to optimize and control AD process, thus contributing to a generic model for enhancing the overall performance of the biogas plant. [ABSTRACT FROM AUTHOR]

Total livestock emissions account for up to 14.5% of man-made greenhouse gas emissions. Counteractive measures, such as circular economy concepts and negative emission technologies are necessary to limit global warming below 1.5 °C. Possible treatment options for organic manure include anaerobic digestion, combustion, gasification, hydrothermal liquefaction and composting. The choice of treatment varies depending on the economics, the requirement of a specific product, and sociocultural factors. Commercialization of these treatments needs a blend of appropriate technology, feasible economics, policy support and agreeable socio-cultural conditions. Key findings of this study include the following: 1. Increasing scientific awareness about manure management and treatment; 2. Building a sustainable cooperative model to commercialize technologies; 3. Creating a market for manure recycling products; 4. The role of policy in supporting technologies and consumers; and 5. The codigestion of substrates for better efficacy. Current trends show minimal actions in place as opposed to the high-rate of acceleration that is necessary. • Improper disposal of organic manure could cause unwanted ecological damage. • Recent biorefinery models to recover energy and nutrients are addressed. • Biological treatment of organic manure remains the predominant recycling approach. • Limited knowledge of the potential of biorefinery approaches still exist. • Successful implementation of biorefineries requires policy and social support. [ABSTRACT FROM AUTHOR]

[Display omitted] • Bio-hydrogen can be generated at a minimum cost of $1.2/kg by gasifying biomass. • Centralized bio-H 2 generation is preferred when production quantity is large. • Bio-H 2 generation by gasifying eucalyptus wood produced least carbon foot of −1.6 kg CO 2 eq/kg H 2. • Bio-H 2 produced by an electrolyser is feasible only when the cost of electricity is < €60/MWh. Hydrogen is considered as the fuel of the future not only because of its high energy density but also due to its zero-carbon emission potential during combustion. However, to achieve sustainable growth, the hydrogen generation process must be techno-economically feasible and have minimum carbon footprint. The techno-economic analysis (TEA) of various hydrogen generation process aids in identifying the effective bio-hydrogen generation process at minimal cost thereby aiding in faster dissemination of the system by attracting investors. Among the various techniques available for bio-hydrogen production, gasification was found to be most economical ($1.2/kg H 2) followed by anaerobic digestion process ($1.25/kg H 2). Meanwhile, after carrying out the life cycle analysis (LCA) of the different bio-hydrogen generation process, it was found that generation of bio-hydrogen by gasification of eucalyptus wood produced least carbon foot of −1.6 kg CO 2eq. /kg H 2. Thus, the TEA and LCA of different biohydrogen production process also helps to identify the bottlenecks haunting the penetration of hydrogen in energy market which can be overcome by framing effective policies by the governing agencies. [ABSTRACT FROM AUTHOR]

Anaerobic digestion (AD) has been in use for many decades. To date, it has been primarily aimed at treating organic wastes, mainly manures and wastewater sludge, and industrial wastewaters. However, with the current advancements, a more open mind is required to look beyond these somewhat restricted original applications of AD. Biorefineries are such concepts, where multiple products including chemicals, fuels, polymers etc. are produced from organic feedstocks. The anaerobic biorefinery concept is now gaining increased attention, utilizing AD as the final disposal step. This review aims at evaluating the potential significance of anaerobic biorefineries, including types of feedstocks, uses for the produced energy, as well as sustainable applications of the generated residual digestate. A comprehensive analysis of various types of anaerobic biorefineries has been developed, including both large-scale and household level applications. Finally, future directives are highlighted showing how anaerobic biorefinery concept could impact the bioeconomy in the near future. [ABSTRACT FROM AUTHOR]

Biomethane from anaerobic digestion of agricultural feedstock is a versatile energy vector for decarbonising agriculture, heavy transport and heat. To lower costs and increase the emission-savings potential, photosynthetic biogas upgrading, cogenerating microalgae with biomethane is investigated here. In a first-of-its-kind work, this paper reports the enviro-economic performance and the marginal (CO 2) abatement cost (MAC) of a polygeneration plant co-producing energy (biomethane), food (Spirulina powder) and bio-fertiliser (digestate) from agricultural feedstock using photosynthetic biogas upgrading at small, medium, and industrial scales. A negative MAC at industrial scale (3 MW biomethane), highlighted the environmental and economic benefit (net present value > 11.5 million€ and internal rate of return >40%) of the process as a low-carbon technology over conventional biomethane production processes at a biomethane sale price of 3 c€/kWh (comparable to natural gas). The operational expenditure, including the cost of the Spirulina cultivation medium and the plant capacity factor had the highest influence on its profitability. Replacing beef as a complete food with Spirulina powder maximised the emission savings rather than replacing beef protein with Spirulina protein. Economic allocation as opposed to energy allocation ensured that the levelised cost and specific greenhouse gas emissions of biomethane (<5 c€/kWh; < 3.5 gCO 2-eq /MJ), Spirulina powder (<68 €/kg; < 4 kgCO 2-eq /kg) and digestate (<5.60 €/tonne; < 0.41 kgCO 2-eq /kg-nitrogen) are better than market-available alternatives across all scales. Trading emission savings from biomethane in the European Union emission trading system should allow the financial viability of smaller-scale processes by 2030. [Display omitted] • Co-generating biomethane, food and biofertiliser is a viable biorefinery process. • At 3 MW-biomethane scale, a negative marginal abatement cost was modelled. • Levelised costs of biomethane of as low as 2.73 c€/kWh were obtained. • For 300 kW scale, the process was profitable with a biomethane tariff of 7 c€/kWh. • Replacing beef as complete food with Spirulina powder maximised CO 2 savings. [ABSTRACT FROM AUTHOR]

Using renewable biogas from the anaerobic digestion of distillery by-products as a low-carbon heat source can decarbonize the distillery process and support the distillery industry to transition to a more sustainable production process. This study investigated the anaerobic digestion performance of different types of whiskey by-products and the effects of acid pretreatment on the digestion of solid by-products. Results of biomethane potential assays showed that the methane yield from the unprocessed by-products was 330 mL/g volatile solids (VS) from draff, 495 mL/g VS from thin stillage, and 503 mL/g VS from thick stillage. For the processed by-products the specific methane yield was 370 mL/g VS from cake maize, 382 mL/g VS from wet distillers' grains with solubles (WDGS), and 545 mL/g VS from syrup. Acid pretreatment (1% H 2 SO 4 at 135 °C for 15 min) did not significantly improve the methane yield from solid by-products (such as draff and WDGS) but reduced the digestion time by 54.5% for cake maize. The microbial community analysis revealed that methane production from the untreated and acid-pretreated solid by-products (draff and WDGS) was mainly through the hydrogenotrophic methanogenesis pathway. The gross thermal energy in the form of methane produced from 100 tonnes of mixed unprocessed by-products (draff, thin stillage, and thick stillage) was calculated as 24.4 MW th h equivalent to 60.6% of the thermal energy consumed in whiskey production, which effected the same percentage of CO 2 emissions reduction. Graphic abstract: Illustration of low carbon emission circular whiskey economy using on-site renewable biogas produced from distillery by-products. [Display omitted] • Biomethane potential of different whiskey by-products was assessed. • Acid pretreatment significantly reduced the digestion time of solid byproducts. • Moist grains had a higher energy conversion efficiency of 68% than draff of 54%. • Digesting 100-ton of mixed draff, thin/thick stillage achieved 24.4 MW th h of biogas. • 24.4 MW th h of biogas could reduce 60.6% of CO 2eq emissions in whiskey production. [ABSTRACT FROM AUTHOR]

In the transition to a climate neutral future, the transportation sector needs to be sustainably decarbonized. Producing advanced fuels (such as biomethane) and bio-based valorised products (such as pyrochar) may offer a solution to significantly reduce greenhouse gas (GHG) emissions associated with energy and agricultural circular economy systems. Biological and thermochemical bioenergy technologies, together with power to gas (P2G) systems can generate green renewable gas, which is essential to reduce the GHG footprint of industry. However, each technology faces challenges with respect to sustainability and conversion efficiency. Here this study identifies an optimal pathway, leading to a sustainable bioenergy system where the carbon released in the fuel is offset by the GHG savings of the circular bio-based system. It provides a state-of-the-art review of individual technologies and proposes a bespoke circular cascading bio-based system with anaerobic digestion as the key platform, integrating electro-fuels via P2G systems and value-added pyrochar via pyrolysis of solid digestate. The mass and energy analysis suggests that a reduction of 11% in digestate mass flow with the production of pyrochar, bio-oil and syngas and an increase of 70% in biomethane production with the utilization of curtailed or constrained electricity can be achieved in the proposed bio-based system, enabling a 70% increase in net energy output as compared with a conventional biomethane system. However, the carbon footprint of the electricity from which the hydrogen is sourced is shown to be a critical parameter in assessing the GHG balance of the bespoke system. Image 1 • A circular cascading bio-based system for advanced biomethane production is proposed. • The benefits of biomass pretreatment by deep eutectic solvents are shown. • Biochar produced from digestate can be used to enhance biogas upgrading and production. • Final net energy output of the proposed system can be potentially improved by 70%. [ABSTRACT FROM AUTHOR]