Synthetic natural gas production by sorption enhanced steam hydrogasification based processes for improving CH4 yield and mitigating CO2 emissions

Abstract

Two new processes for synthetic natural gas (SNG) production based on the sorption enhanced steam hydrogasification (SE-SHR) are proposed. Experimental work was conducted on the gasification of coal and biomass feedstocks with a fixed quantity of quicklime as the sorbent. Results showed that SE-SHR can significantly increase H2 yield beyond the initial H2 input and minimize CO2 production. The SE-SHR parametric study was carried out by using lignite as a typical feedstock. By varying hydrogen to carbon molar ratio (H2/C) and steam to carbon molar ratio (Steam/C), the product gas composition and yield during SE-SHR and conventional steam hydrogasification (SHR) were obtained. The increase of H2/C enhanced the productions of H2 and CH4 and decreased the productions of CO and CO2. On the other hand, due to the domination of steam gasification reaction, the yields of H2, CO and CO2 were increased with the increase of Steam/C. The new SNG processes include SE-SHR coupled with water gas shift (WGS) and SE-SHR coupled with methanation. The simulation of individual WGS or methanation unit based on the preliminary bench-scale data was conducted using Aspen Plus software to obtain the final SNG composition. Results showed that the SE-SHR-Methanation process results in high CH4 production with self-sustained H2 supply and near zero CO2 emissions. The optimum gasification conditions for this process using lignite were H2/C of 1.08 and Steam/C of 2.22. Finally, a preliminary pilot-scale simulation of SE-SHR-Methanation process was conducted based on the predetermined optimum condition to evaluate the overall material and energy balance. It showed that carbon conversion efficiency to CH4 was as high as 39.8%.