Pure hydrogen gas production in a coal supercritical water gasification system with CO2 as transporting medium

Supercritical water gasification (SCWG) technology was a clean coal conversion and utilization technology, which could decompose low-grade coal into gaseous products and make efficient and clean use of them. However, system with H2O transporting had much energy consumption on heating transporting medium and side reactions at heating process would weaken the gasification effect. Considering the advantages of low critical point and low specific heat capacity of CO2, and the precedent of CO2 as a transport medium in non-supercritical systems, CO2 was expected to become a transport medium in supercritical water gasification systems and to achieve the goal of improving the gasification effect of heating process and reducing energy consumption. A supercritical water coal gasification system of was established with lignite as raw material in this paper. Production yields of gasification with CO2 as the transporting medium at different working conditions were investigated. CO2 and H2O were used as the transporting medium of the system, contrastively. Compared with H2O-transporting system, production of H2 and CO2 in CO2-transporting system decreased while that of CH4 and CO increased as well. Furthermore, both energy and exergy efficiency of CO2-transporting system were higher than that of H2O-transporting system. The result proved the superiority of CO2 as the transport medium of supercritical water gasification system.