Thermodynamic analysis of syngas production and sulfur capturing from a mixture of methane and hydrogen sulfide using a solar thermochemical redox cycle

Syngas production and sulfur capturing from a mixture of methane and H₂S via a novel two-step solar thermochemical cycle based on metal oxide/metal sulfide redox reactions is thermodynamically analyzed. Fe₂O₃/FeS is used as a model metal oxide/metal sulfide pair for this study. During the reduction step, Fe₂O₃ is reduced to FeS using a mixture of CH₄ and H₂S. In this process, syngas (CO + H₂) is produced in the gas phase. In the oxidation step, FeS is oxidized using air to obtain Fe₂O₃ in the solid phase and SO₂ and unreacted N₂ in the gas phase. The produced SO₂ can be used to generate sulfuric acid. Favorable operating conditions for the redox cycle are determined using an open system, thermodynamic model. For the reduction step, T = 1200 K at 1 bar pressure provides complete conversion of Fe₂O₃ to FeS. At these conditions, gas phase products contain mainly H₂ and CO. Carbon formation is also predicted by the model between 580 and 1150 K temperatures. Thermodynamic analysis shows complete conversion of FeS to Fe₂O₃ during the oxidation step at T ≥ 600 K and 1 bar pressure. Effect of higher system pressures over the Fe₂O₃ reduction process is also determined using the model. With the increase in system pressure, carbon formation decreases. At 10 bar system pressure, no carbon formation is predicted by the thermodynamic model. A thermodynamic process model is also developed to assess the energetic feasibility of the complete process. Concentrated solar power can be used to provide the necessary energy for the endothermic Fe₂O₃ reduction process. Effect of concentration ratio and heat recuperation from exhaust gases over solar to fuel energy efficiency is studied. Energy efficiencies of the complete process at various oxidation temperatures are also determined using the thermodynamic process model. An overall system efficiency of 46.9% can be achieved for heat exchanger effectiveness of 0.9 and concentration ratio of 1000 when reduction and oxidation reactions are performed at 1200 K temperature.