In-Situ H2S reduction in biogas fuel for solid oxide fuel cell applications by using cow urine

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 H 2 S, on the performance of SOFCs. Previous work has shown that local operational practices could influence the biogas quality and particularly the H 2 S 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 H 2 S in the evolving biogas. This research investigated the following hypotheses: 1) urine addition results in a high pH that favours HS − over H 2 S, 2) given the presence of metal elements in the cow urine, insoluble metal sulphides are being formed, reducing the biogas H 2 S 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-H 2 S 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 H 2 S 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 H 2 S 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 H 2 S 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 H 2 S 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 H 2 S cleaning strategy for biogas-SOFC applications. • The standardised bio-methane potential test was complemented with a method for biochemical gaseous H 2 S production potential and showed higher H 2 S production with lower Fe:S ratio • Biochemical gaseous H 2 S potential field experiments showed variable H 2 S content in biogas with different Ugandan cow manure and urine • Geochemical speciation modelling showed relationship between pH, metals in urine and H 2 S content of manure biogas • Bio-kinetic modelling combined with geochemical speciation modelling revealed that feeding composition and patterns influenced the rate of H 2 S production.