Carbon emission accounting and carbon reduction analysis for deep coal underground gasification to hydrogen

Innovating coal development and utilization technology to reduce carbon emissions during the whole life cycle of coal from production to utilization is a coal-based clean energy route that meets the characteristics of China's energy endowments. Underground gasification is one of the potential methods of deep coal in-situ mining, coupled with CCS/CCUS (Carbon Capture and Storage/Carbon Capture, Utilization and Storage) deep coal underground gasification hydrogen production technology route can not only utilize the rich deep coal resources, but also is expected to become an ideal of coal-based low-cost hydrogen production route. Based on the world's only kilometer-level deep coal underground gasification test data, combined with Aspen Plus process simulation, the carbon emission accounting and carbon emission reduction potential analysis of deep coal underground gasification hydrogen production were carried out. Compared with the commercial ground Lurgi fixed bed pressurized gasification hydrogen production route, the life cycle assessment method is used to establish the whole life cycle carbon emission calculation model of the two processes, compared the life cycle carbon emissions of the two hydrogen production routes.The CCS/CCUS path and carbon emission reduction potential of the underground gasification of deep coal are further evaluated.The results show that the carbon emissions during the life cycle of underground gasification of deep coal and ground gasification of Lurgi coal are 3.29×10⁹ t CO₂-eq and 3.93×10⁶ t CO₂-eq in the case of hydrogen production capacity of 1.2 billion Nm³/a, respectively, and the carbon dioxide directly emitted into the atmosphere in the form of exhaust gas is 2.09×10⁶ t and 2.24×10⁶ t, respectively. The main carbon emitters in the hydrogen production phase are exhaust gases, including exhaust gases from acid gas removal units and flue gases from methane reforming units. The high methane content characteristics brought about by the underground gasification process of deep coal lead to the CO₂ emissions contributed by the flue gas of the methane reforming unit to 8.84 kg when producing 1 kg of hydrogen.In terms of indirect emissions, deep coal underground gasification directly uses liquid water as a gasifier without consuming external steam, so the carbon emissions caused by steam consumption are lower than that of ground gasification. The upstream of Lurgi coal surface gasification includes the coal mining and processing and coal transportation stages, and although the carbon emissions in these two phases are only 6.7%, but still bring 2.63×10⁵ t CO₂-eq. If the deep coal underground gasification cavity is used for CO₂ geological storage under the premise of geological safety risk assessment, the storage capacity can reach 61.8% of the total carbon emissions, and if the urea device is supported by 470,000 tons / year, the remaining CO₂ can be effectively used,to form a near-zero emission deep coal underground gasification hydrogen production and urea co-production technical route. The research results provide a scientific basis for quantitative evaluation of carbon emissions for hydrogen production from deep coal underground gasification.