Energy storage is vital for the energy transition, enabling reliable power grids based on intermittent renewables. Reversible solid oxide cell (rSOC) technology is promising for seasonal energy storage. The novel finding from this work is that optimised air recirculation for rSOC in endothermic electrolyser mode leads to efficiency being nearly independent of current density. Thereby the operating region of highest efficiency is expanded from the thermoneutral point to the entire endothermic region, leading to highly efficient part-load operation. Air recirculation increases fuel cell mode efficiency too, particularly at higher loads. This widens the efficient operating window in both modes. These findings emerge from a thermodynamic study of an rSOC-based energy storage system with ammonia as fuel. A process design is developed and optimised for efficiency, supported with detailed exergy analysis. First, ammonia synthesis subsystem integrated with the rSOC system in electrolyser mode is optimised. Second, rSOC outlet air recirculation is optimised for high system efficiency. Finally, rSOC operating points are optimised for highest round-trip efficiency. We find the least exergy destruction for the ammonia synthesis subsystem at 170 bar synthesis pressure and 30 °C condensation temperature (without needing refrigeration). The overall system achieves round-trip efficiencies up to 60.3%.

This study is particularly aimed at investigating the influence of hydrogen chloride traces in biogas on direct internal reforming in solid oxide fuel cells (SOFCs). The experiments are performed with simulated biogas containing methane to carbon dioxide ratio of 3:2, the usual average proportion in biogas. To the best of our knowledge, there are no reported studies that investigated the effect of hydrogen chloride on direct internal reforming by clearly establishing the effect of reforming with outlet gas composition measurements. The experiments at SOFC operating temperature of 850 °C reveals no negative effect on reforming or cell performance, with 4, 8, and 12 ppm(v) of hydrogen chloride in biogas. At 800 °C, there is no visible performance degradation, but a negligible amount of methane (∼ 1%) is detected in the anode off gas. Both the reforming and electrochemical performance are marginally affected at 750 °C. Further, post-test analyses (FESEM-EDS, XRD) of the used SOFC reveals no damage to the cell at microstructure level or chlorine poisoning. All the experiments are performed in the context of utilizing the biogas generated from sewage treatment plants in an SOFC system. The reported level of chlorine traces in biogas generated from sewage sludge is < 10 ppm(v) and hence the limit set for experiments is at par with this value.

This article is the first of a two-part series presenting the thermodynamic evaluation and techno-economics of developing negative-emission power plants. The aim of this research is to evaluate the potential of biochar co-production in negative-emission power plants based on biomass-fed integrated gasification solid oxide fuel cell systems with carbon capture and storage (BIGFC/CCS) units. The influence of two gasification agents, namely, air and steam-oxygen, on the proposed system is investigated. In Part I, we present the thermodynamic models. A sensitivity analysis is carried out to investigate the system response to stepwise increase in biochar co-production (up to 10% by weight). Providing a secondary oxy-combustor in the steam-oxygen gasification case has been shown to be a solution to meet the heat requirements of the allothermal gasification process. A comprehensive exergy analysis indicated significant efficiency improvement for the steam-oxygen gasification case. The results show that the biomass steam-oxygen gasification yields the higher electrical exergy efficiency (48.3%) and combined heat and power (CHP) exergy efficiency (54.6%) for the similar rates of biochar co-production. The specific power output per unit of CO₂ stored is 2.65 MW/(kg/s) and 3.58 MW/(kg/s) for the air and steam-oxygen gasification cases, respectively, when the biochar is co-produced at 10% by weight for the given biomass flow of 20 kg/s. Moreover, the total CO₂ stored due to the proposed system is calculated as 133.9 t/h, and it is estimated to remove 1.17 Mt of CO₂ from the atmosphere annually (when the biochar-based carbon storage is also considered). The models are used for the techno-economic analysis presented in Part II of the series.

Support for the adoption of climate change mitigation measures in low-income regions depends on how such activities contribute to generating household income and gaining confidence from the local community. The planning of mitigation measures or pro-environmental activities need to consider the cost of deployment, customization of activities according to local conditions, and socio-cultural background and perceptions of people. This paper analyses the incentive induced “agroforestry” or “planting trees in farmland” as part of the Carbon Neutral Programme supported by the Government of Kerala in Meenangadi Grama Panchayath, Wayanad district. An increase in tree cover is proposed as a strategy for increasing carbon sequestration. Planting more trees in farmland (except grain cultivated areas) along with crops, according to farmers, may reduce crop yield and discourage farmers’ participation. The Government of Kerala put forward the concept of a tree banking/tree incentive program to attract farmers to expand tree cover. A survey was conducted among 100 individuals from the Meenangadi Grama Panchayath to assess the perceptions and concerns of farmers about the proposed “Agroforestry”/Tree Banking program. The sample size was chosen from the population assuming a 9.98% error tolerance. Tree Banking Programme designed to encourage farmers to plant trees has gained public interest, and the study also documented the factors influencing the willingness of farmers for planting trees. The study revealed that the majority of the individuals (93% of the survey participants) residing in the region are interested in supporting the activities for climate change mitigation. Financial incentives announced under tree banking generated interest among farmers. 89% of the survey participants consider the incentive scheme to be an attractive option, as it can compensate for the short-term loss in crop productivity. However, farmers were very selective in choosing the tree species to be planted on their farms. Incentivization helps to make sure that a large proportion of the planted saplings will grow into mature trees. Overall, it can be concluded that afforestation in the form of agroforestry could be potentially attractive to the farmers and contribute towards achieving carbon neutrality for tropical agricultural areas.

Small-scale electrical power generation (<100 kW) from biogas plants to provide off-grid electricity is of growing interest. Currently, gas engines are used to meet this demand. Alternatively, more efficient small-scale solid oxide fuel cells (SOFCs) can be used to enhance electricity generation from small-scale biogas plants. Most electricity generators require a constant gas supply and high gas quality in terms of absence of impurities like H ₂S. Therefore, to efficiently use the biogas from existing decentralized anaerobic digesters for electricity production, higher quality and stable biogas flow must be guaranteed. The installation of a biogas upgrading and buffer system could be considered; however, the cost implication could be high at a small scale as compared to locally available alterna-tives such as co-digestion and improved digester operation. Therefore, this study initially describes relevant literature related to feedstock pre-treatment, co-digestion and user operational practices of small-scale digesters, which theoretically could lead to major improvements of anaerobic digestion process efficiency. The theoretical preamble is then coupled to the results of a field study, which demonstrated that many locally available resources and user practices constitute frugal innovations with potential to improve biogas quality and digester performance in off-grid settings.

Tar compounds have been defined as Achilles’ heel in biomass gasification. Catalytic reforming solves problems caused by tar by converting them into H₂ and CO. Most of the research has focused on secondary and tertiary tar reforming while some information on primary tar can be derived from bio-oil reforming. However, these studies use humidified N₂, Ar or He as gas carrier. Therefore, in this work, three catalysts are compared for reforming 40 g/Nm³ acetic acid as main primary tar compound from biomass updraft gasification using simulated biosyngas as gas carrier. The catalysts were tested over a 72 h period between 680 and 750 °C with a gas composition of 35.0 vol% H₂O, 2.3 vol% CO, 19.5 vol% CO₂, 3.6 vol% CH₄, 24.0 vol% H₂ and 15.6 vol% N₂. Olivine completely converted acetic acid, but a considerable amount of carbonaceous deposits was found on the catalyst and the catalytic activity decreased over time with 0.2 g/Nm³ hydroxyacetone measured in the last day of testing. Dolomite showed promising performances by completely converting acetic acid and accumulating carbonaceous deposits only in the low temperature part of the catalyst bed. The carbonaceous deposits could be suppressed increasing the steam content to 50.1 vol% and the temperature to 900 °C. However, the catalyst became excessively brittle. The metal-based catalyst out-performed the naturally-occurring catalysts by completely converting acetic acid with almost no carbonaceous deposits accumulation. These results are expected to help the further development of tar reformers, and the commercialisation of biomass updraft gasifiers based systems.

Anaerobic Digestion (AD) is used worldwide for treating organic waste and wastewater. Biogas produced can be converted using conventional energy conversion devices to provide energy efficient, integrated waste solutions. Typically, the electrical conversion-efficiency of these devices is 30–40% and is lowered due to biogas utilization instead of high pure refined natural gas. The Solid Oxide Fuel Cell (SOFC) as an alternative device offers high (50–60%) electrical efficiency with low emissions (CO2, NOx) and high temperature residual heat. The high quality residual heat from SOFCs could be used to improve biogas production through thermal pre-treatment of the substrate for anaerobic digestion. This work discusses the advantages and challenges of integrated AD-SOFC systems against the most recent scientific and practical developments in the AD and SOFC domain. First, the biogas production process and its influence on the composition and level of contaminants in biogas are explained. Subsequently, the potential of various biogas cleaning techniques is discussed in order to remove contaminants that threaten stable and long-term SOFC operation. Since SOFCs utilize H2 and/or CO as fuel, possibilities for internal and external reforming are explained in detail. Special attention is given to biogas dry reforming in which CO2 naturally present in the biogas is utilized effectively in the reforming process. A detailed discussion on the choice of SOFC materials is presented, with a focus on biogas internal reforming. Various integrated SOFC system models with multiple configurations are also reviewed indicating the overall efficiencies. Some biogas SOFC pilot-plants are described and discussed to conclude with the techno-economic aspects of biogas SOFC systems.

This article presents a detailed thermodynamic case study based on the Willem-Alexander Centrale (WAC) power plant in the Netherlands towards retrofitting SOFCs in existing IGCC power plants with a focus on near future implementation. Two systems with high percentage (up to 70%) biomass co-gasification (based on previously validated steady state models) are discussed: (I) a SOFC retrofitted IGCC system with partial oxy-fuel combustion CO₂ capture (II) a redesigned highly efficient integrated gasification fuel cell (IGFC) system with full oxy-fuel CO₂ capture. It is concluded that existing IGCC power plants could be operated without major plant modifications and relatively high electrical efficiencies of more than 40% (LHV) by retrofitting SOFCs and partial oxy-combustion CO₂ capture. In order to apply full scale CO₂ capture, major process modification and redesign needs to be carried out, particularly in the gas turbine unit and heat recovery steam generator (HRSG). A detailed exergy analysis has also been presented for both the systems indicating significant efficiency improvement with the utilization of SOFCs. Additional discussions have also been presented on carbon deposition in SOFCs and biomass CO₂ neutrality. It is suggested that scaling up of the SOFC stack module be carried out gradually, synchronous with latest technology development. The thermodynamic analysis and results presented in this article are also helpful to further evaluate design challenges in retrofitted IGCC power plant systems for near future implementation, gas turbine part load behaviour, to devise appropriate engineering solutions and for techno-economic evaluations.