Demonstration of a high-efficiency carbon dioxide capture and methanation system with heat/material integration for power-to-gas and zero carbon dioxide emissions in flue gasses

Abstract

Carbon dioxide (CO2) capture and methanation are important methods for realizing both power-to-gas and zero CO2 emissions. However, the technology needs to be improved to solve implementation barriers such as energy efficiency, compactness, economics, and CO2 source flexibility, especially for CO2 derived from fuel combustion.The present report describes the development of a high-efficiency integrated system to separate CO2 from flue gas with a high recovery rate and convert it to high-quality CH4 using in-system heat/material integration.A carbon recycling system with a throughput of 5.1-kW CH4 (LHV basis) was integrated with CO2 adsorbers using zeolite and catalytic methanation reactors. System efficiency, defined as the energy ratio of output CH4 and H2 against the sum of input H2 and power for system operation, was then evaluated using exhaust gas from a combustion furnace with a CO2 concentration of 9%. In general, a decrease in the energy required for system operations, such as gas treatment, CO2 separation, and auxiliary machines, improves efficiency. Two challenges were encountered during the study: reduction in CO2 separation energy by implementation of heat/material management using a H2 reactant as a sweep gas and the reaction heat of methanation, and a reduction in gas compression energy by conducting the methanation reaction under the lowest possible pressure. The results indicated that the CO2 conversion rate was greater than 99% under a pressure of 225 kPa-G, and the only energy consumed was by the oil pump for cooling the reactors. Due to the use of H2 sweep gas and methanation heat recovery, the CO2 recovery rate was also greater than 99%, and the additional energy consumption (1.8 GJ/t-CO2) was due to vacuum pumps. In addition, the system achieved an efficiency greater than 70% (LHV basis), while maintaining both a CO2 recovery rate and a conversion of greater than 99%. Because the energy for dehydration of the exhaust gas was the most significant portion of the total system operational energy, further improvement of the system efficiency is thought to be possible by reducing the energy required for dehydration.