Exergy analysis in the assessment of hydrogen production from UCG

The demand for H-2 increases rapidly with the gradual recognition of the potential of H-2 as an important secondary energy. At present, coal gasification is the main way to obtain hydrogen on a large scale and at a low cost in China. The underground coal gasification (UCG), as a kind of in-situ utilization technology that can exploit the unreachable deep coal resources, could become an alternative H-2 production pathway. This paper presents comparative study of energy utilization and resource consumption in H-2 production by UCG and typical surface coal gasification (SCG) technology, namely Lurgi fixed bed gasification, with 1.2 billion Nm3/a throughput of H-2 as example, to offer corresponding data support. The efficiency and the amount of resources consumed in constructing and operating each coal-to-hydrogen system under different conditions have been researched from exergetic point of view, which is not reported in existing literatures. In this paper, the exergy efficiency is calculated to be 40.48% and 40.98% for hydrogen production using UCG and SCG. The result indicates the competitiveness of UCG in the field of hydrogen production comparing with widely used coal gasification technology. The resource consumption is measured by cumulative exergy consumption (CExC), which is 8.17E+10 MJ and 6.57E+10 MJ for H-2 production from UCG and SCG. The result shows that although the H-2 production from UCG has higher CExC, it can significantly reduce the resource consumption of equipment comparing with H-2 production from SCG, indicating its advantage in total investment. It is found that the exergy efficiency increases with the rise in H2O-to-O-2 and O-2-to-CO2 ratio, while the value of CExC decreases with the appreciation of H2O-to-O-2 ratio yet increases as the O-2-to-CO2 ratio rises. In addition, the sensitivity analysis of production capacity reveals that the exergy efficiency gap and CExC gap between hydrogen production by UCG and SCG diminishes at smaller scale production capacities, showing that UCG is more suitable for small-scale hydrogen production. (C) 2020 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.