• Energy Research

Abstract 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.

Grass represents a major renewable feedstock in temperate climate zones, but its efficient utilization is challenging in biorefineries and advanced biofuels due to its structural recalcitrance. Here hydrothermal hydrolysis (100–180°C, for up to 40 min duration) was investigated to improve sugar yields from grass silage. The optimal conditions (140°C for 20 min duration) showed the highest sugar yield of 0.29 g/g volatile solid (VS) of grass silage. Further increasing the temperature to 180°C favored degradation of sugars (such as glucose, xylose) to by-products (such as furfural, hydroxymethylfurfural). A first-order reaction model confirmed a two-step reaction with the first step hydrolysis and the second step degradation. An energy balance calculation indicated that pre-treatment at 140°C required an energy input of 16.5 kJ/g VS, which could be significantly reduced to 5.1 kJ/g VS through efficient heat recovery. This research assists in understanding of the hydrolysis mechanism and provides a practical solution to produce grass-based sugars for further advanced biofuel and biorefinery applications.

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 proces ...

Abstract 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.

Abstract The levelized cost of energy of biomethane from food waste was assessed at 87 €/MWh, (87 c/L dieselequiv). Allowing for gate fees the incentive required for financial viability was 0.13 €/m3 (13 €/MWh). For context, various successful renewable energy policies were analysed across the EU including photovoltaics and biogas in Germany and electric vehicles in Norway. The schemes were compared with an incentive applied (or required) per tCO2 avoided. For Ireland, this study predicts that biomethane needs a financial subsidy of less than 180 €/tCO2 avoided, while most successful EU systems offer incentivisation levels less than 260 €/tCO2 avoided. In terms of incentives per tCO2 avoided Electric Vehicles (EV) stand out. When including all incentives such as grants and avoided parking costs, EVs can receive a sixteen-fold higher incentive as compared to biomethane based on tCO2 emissions avoided. The rationale for this high incentive and supporting policy is based on the requirement to initiate a new infrastructure that would not otherwise happen without intervention of a government incentivising decarbonised transport and clean air. Biomethane as a transport fuel requires a very significant change in infrastructure, including the provision of compressed natural gas service stations and natural gas vehicles. Initially (as for other successful renewable energy systems) larger incentives would be required to allow initiation of the industry, but these subsidies can be reduced over time. Biomethane as a transport fuel offers similar rewards as for electric vehicles, decarbonised transport and clean air along with energy security, renewable energy, indigenous jobs and supporting greening of agriculture.

Abstract 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/m3biomethane and 0.67 USD/m3biomethane, 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 tcane/day, 108 m3ethanol/d at 10 Lethanol/tcane, 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 Over the last decades, microalgae have gained a commendable role in the rising field of biofuel production as they do not compete with food supply, reduce greenhouse gases emission, and mitigate CO2. Specifically, thermochemical processing of microalgae yields products, which can be used both for energy and other industrial purposes, depending on the algal strain, processing method and operative conditions. Algae are converted into various high-value products, including nutraceuticals, colourants, food supplements, char, bio-crude, electricity, heat, transportation fuel, and bio-oil. Therefore, microalgae are believed to be a strategic resource for the upcoming years and their utilization is meaningful for many industrial sectors. In this framework, this review addresses the various thermochemical processing of microalgae to various biofuels and their industrial significance. The obstacles in various thermochemical conversion methods have been critically flagged, in order to enable researchers to choose the optimal method for fuel production. Furthermore, light is shed on cultivation systems to generate rapidly microalgal biomass for thermochemical processing. Eventually, all recent literature advancements concerning microalgae cultivation and thermochemical processing are critically surveyed, and summarized.