Identification of key control variables for a methanation unit through system analysis using experimental results

The operation of methanation units, whether standalone or integrated with low or high temperature electrolysis, relies on the objective function to be maximized. In the cases of standalone operation or low temperature water electrolysis integration, the objective function is typically the higher heating value (HHV) based efficiency. The co-generation efficiency which includes the steam produced from the reaction's heat should be optimized for applications such as the coupling with steam electrolysis: solid-oxide electrolyser (SOE). This last case, assuming a methanation unit with heat recovery and direct steam production by the cooling system, was simulated while assuming both an equilibrium reactor, and a reactor with direct, previously published, measurements of conversion and heat loss. The simulations aimed to (1) identify the effect of the connectivity in the heat recovery system, and (2) diagnose the trade-offs between the HHV efficiency and co-generation efficiency as the control parameters are modified. Dividing the product gases to pre-heat both the reactant and cooling water provided the highest efficiencies. Assuming this configuration along with the direct conversion and heat loss measurements, an optimal water pressure of 20.73 bar A was identified to maximize the HHV efficiency (72.37 %, up from 71.91 % at 12 bar A). Yet, the co-generation improved by 2.09 % from 12 to 23 bar A. However, the trend suggests possible optimal water pressures for the co-generation efficiency at higher pressures than the upper bound of the simulation. Similarly, the reaction pressure would need to be increased beyond the simulated range to reach the possible optimal pressures. Conversely, between the minimum and maximum reactant flow rates, the HHV efficiency decreases by 0.6 % due to the drop in conversion while the co-generation improves by 1.75 % because the reduction in the specific heat loss surpasses the decrease in conversion.