Biogas, generated from small scale digesters, is a traditional energy source for satisfying the thermal energy demand in off-grid communities. Recent developments in small scale solid oxide fuel cells (SOFCs) technology and progress in research and development of dry reforming, opens perspectives to couple small scale SOFCs with already existing digesters to meet both thermal and electrical energy demand, enabling power access to off-grid communities. However, one of the major challenges for SOFC integration to small scale digesters is the effect of biogas impurities, such as H₂S, on the performance of SOFCs. Previous work has shown that local operational practices could influence the biogas quality and particularly the H₂S content in the biogas. The here presented research expanded on the use of cow urine instead of water as solvent in manure digestion as a potential operational strategy that enables in-situ reduction of H₂S in the evolving biogas. This research investigated the following hypotheses: 1) urine addition results in a high pH that favours HS⁻ over H₂S, 2) given the presence of metal elements in the cow urine, insoluble metal sulphides are being formed, reducing the biogas H₂S content. The research was carried out by measuring cow urine composition of various samples, assessing the effects of different urine/water/manure mixtures on the evolving biogas-H₂S concentration, and verifying the experimental findings with phreeqC equilibrium speciation. Bio-kinetic modelling, using the anaerobic digestion model nr 1 (ADM1), was subsequently performed to explore the influence of different feed compositions on the H₂S content in the biogas. It was observed that addition of cow urine in all experiments resulted in an elevated pH of the reactor compared to water dilution, yet both experiments I and II-2 showed an increased maximum H₂S content when urine dilution was applied, compared to water dilution. Cow urine and cow dung characterisation in terms of metals and S, showed that experiment II-1 had the highest Fe:S ratio of 1:0.3–1:0.9. Equilibrium modelling confirmed that despite the positive urine-induced pH effect, the measured Fe:S ratios could indeed be decisive, as with an Fe;S ratio of 1:6 and 1:0.5, the H₂S production at equilibrium was 61 and 10 mL/ kg of solution, respectively. Furthermore, it was predicted through bio-kinetic modelling that inconsistency in feedstock composition may result in temporary H₂S peaks exceeding 400 ppm. Overall, results showed that if a cow urine/manure mixture is characterised by a total metal:S ratio exceeding 1:0.5 and total S content of less than 0.5 mM, then hydrolysed cow urine addition presents an interesting in-situ H₂S cleaning strategy for biogas-SOFC applications.

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.

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.

Internal dry reforming of methane is envisaged as a possibility to reduce on capital and operation costs of biogas fuelled solid oxide fuel cells (SOFCs) system by using the CO₂ present in the biogas. Due to envisaged internal dry reforming, the requirement for biogas upgrading becomes obsolete, thereby simplifying the system complexity and increasing its technology readiness level. However, impurities prevailing in biogas such as H₂S have been reported in literature as one of the parameters which affect the internal reforming process in SOFCs. This research has been carried out to investigate the effects of H₂S on internal dry reforming of methane on nickel-scandia-stabilised zirconia (Ni-ScSZ) electrolyte supported SOFCs. Results showed that at 800°C and a CH₄:CO₂ ratio of 2:3, H₂S at concentrations as low as 0.125 ppm affects both the catalytic and electric performance of a SOFC. At 0.125 ppm H₂S concentration, the CH₄ reforming process is affected and it is reduced from over 95% to below 10% in 10 h. Therefore, future biogas SOFC cost reduction seems to become a trade-off between biogas upgrading for CO₂ removal and biogas cleaning of impurities to facilitate efficient internal dry reforming.

Biogas is known as a traditional energy source for off-grid population throughout the world. And currently small-scale solid oxide fuel cell (SOFC) systems are being promoted for off-grid energy supply. Also, electricity demand is increasing at a high rate due to the ever-increasing population and technological revolution. Therefore, promotion of off-grid energy supply needs to be refocused. The small scale biogas-SOFC is an envisaged modern energy system which can meet both the thermal and electrical energy demand for off-grid population more efficiently (60% at 800 °C) than currently available technologies. However, it has been observed that cleaning of biogas could increase the system capital cost by 6–7% and >40% of the overall annual system operating cost. Cost-effective gas cleaning is therefore important for economic feasibility of the biogas-SOFC energy system. This review focuses on technical and economic challenges of current commercial and laboratory scale biogas cleaning technologies. Special focus is directed towards cost mitigation strategies for gas cleaning such as combined in-situ bioreactor upgrading and application of cost-effective sorbents. The results are useful to advance implementation of biogas-SOFC systems in off-grid applications in developing as well as developed world.