Advanced exergy analysis of deep UCG to hydrogen production

Deep underground coal gasification (UCG) for making hydrogen can not only utilize the rich deep coal resources in China and convert difficult-to-mine or uneconomical deep coal resources into hydrogen, but also is a potential low-cost hydrogen production route. Based on the world’s only kilometer-scale deep UCG experimental data and combined with Aspen Plus process simulation, this study analyzes the energy utilization of hydrogen production through deep UCG using the advanced exergy analysis method. In comparison with the commercialized Lurgi surface coal gasification route, the energy consumption levels of the two hydrogen production routes were compared using the cumulative exergy consumption per unit of hydrogen output as an indicator. The research results show that under the hydrogen production capacity of 1.2 billion Nm³/a, the total exergy losses from raw materials to products in deep UCG for hydrogen production are 451.79 MW. Advanced exergy analysis can effectively quantify the exergy losses that can be avoided in the process, and the calculations indicate that 39.9% of these losses are unavoidable. The distribution of exergy destruction indicates that avoidable endogenous exergy destruction (E_\mathrmdest,k^\mathrmAV,EN ) and avoidable exogenous exergy destruction (E_\mathrmdest,k^\mathrmAV,EX ) of the methane reforming unit are 96.63 MW and 81.58 MW respectively, suggesting that the methane reforming unit has the greatest potential for energy efficiency improvement. By utilizing the heat from converted gas and flue gas as by-product steam, E_\mathrmdest,k^\mathrmAV,EN can be reduced by 38.5%. The E_\mathrmdest,k^\mathrmAV,EN and E_\mathrmdest,k^\mathrmAV,EX of the UCG unit are 4.38 MW and 62.73 MW respectively, indicating that the focus of reducing its exergy losses should be on improving the efficiency of other units to reduce avoidable exogenous exergy destruction. The remaining units have relatively small improvement potential and can be disregarded. When 1 kg of hydrogen is produced, the CExC of UCG-H₂ is 376.1 MJ, which is only 83.6% of that of SCG-H₂ (449.72 MJ). This indicates that the deep UCG for hydrogen production can significantly reduce the energy consumption level. Sensitivity analysis shows that the difference in CExC between the two routes increases with the expansion of production scale. The research results can provide a scientific basis for the process optimization and quantitative assessment of technical feasibility for hydrogen production with deep UCG.