Process simulation study on the production of green methanol from low-concentration mine gas coupled with solar thermal energy and hydrogen supplementation

Coal mining is conventionally accompanied by the large emission of greenhouse gas, among which low-concentration mine gas has a relatively low comprehensive utilization rate due to its intrinsic difficulty. Its direct release into the atmosphere can lead to non-negligible greenhouse effects, and converting it into methanol product is currently one of the popular pathways for high-value utilization. This study employs Aspen Plus simulation platform to establish an integrated relay process for producing green methanol from low-concentration mine gas. The above process mainly involves the internal reforming of CO₂, H₂O, and CH₄ components to derive syngas, followed by methanol synthesis unit, which creatively integrates photothermal energy and hydrogen supplementation. In the internal reforming unit, molten salt is used for supplemental heating to save traditional fossil fuels, while green hydrogen from water electrolysis is supplied in the methanol synthesis unit for the adjustment of H₂/CO ratio. Simulation results indicate that the optimal reaction temperature and pressure for internal reforming are 850 ℃ and 10 bar, respectively, achieving a satisfying methane conversion rate of 99.5%. In the meantime, the molten salt can provide 34.86% of the heat required for the internal reforming unit, equivalent to reducing CO₂ emissions from methane fuel combustion by 0.033 kg/s. For the subsequent methanol synthesis unit, the optimal reaction temperature and pressure are 260 ℃ and 35 bar, respectively, resulting in a carbon oxides conversion rate of 98.7% and a methanol yield of 90.6%. The results also showed that the green hydrogen flow rate supplied by water electrolysis under these conditions is calculated as 165 m³/h. This study proposes a clean conversion pathway for low-concentration mine gas while integrating renewable energy sources, offering practical, long-term energy-saving and carbon-reduction benefits.