Dynamic operation of two-stage CO2 methanation reactor: Start-up by H2 combustion and load change by independent control of heat-carrier flow rate

Abstract

This study suggested and evaluated methods for attaining the fast start-up and load change of a two-stage CO₂ methanation reactor. For start-up, catalysts inside the reactor need to be heated from the cold state to a temperature exceeding the methanation activation temperature (>150 °C). The proposed method combusts H₂ using methanation catalysts as the H₂ combustion catalyst. For load change (a change in the CH₄ production rate), the flow rates of the feed gases (H₂ and CO₂) need to be varied from the current value to the target value. The proposed method determines the ramp rate of the heat-carrier flow rates independently of the feed gas flow rates. Both methods were experimentally evaluated using a previously developed 6-kW two-stage CO₂ methanation reactor. The start-up was conducted from room temperature (25 °C) and the load was varied between 60 % and 100 %. For comparison, conventional start-up using a 3.5-kW heat-carrier heater was also conducted. The results show that the proposed start-up method could light off two-stage methanation in as little as 4 min while the conventional method took 26 min. Also, the proposed load change method realized a 2-s feed gas flow rate change without any reaction blow-outs.