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.