Dynamic modelling of methanol synthesis from electrolytic hydrogen and captured carbon dioxide

Abstract

Power-to-methanol, in which methanol can be produced from captured CO2 and electrolytic H2, can be seen as a promising solution for CO2 emission reduction leading to diminish global warming. However, the fluctuation in the amount of H2 generated from renewable electricity via water electrolysis leads (1) to expensive, high-pressure storage requirement, which is not suitable for long-term storage and industrial scaling up, and (2) to a variable methanol production integrating with power and H2 generation. Therefore, building a dynamic model for methanol production plays a crucial role to understand the dynamic characteristics of the process at various feed stream disturbances, consequently resulting in the development of a robust control structure for methanol synthesis.

This thesis is a part of the Business Finland funded HYGCELL project. The results in this thesis are compared to other Power-to-hydrogen-to-products processes in dynamic environment to figure out the most feasible value chain and its development potential. In this thesis, a dynamic model of crude methanol synthesis through CO2 hydrogenation was built to assess the stability of control structures and the efficiencies of the process during feed stream disturbances. To be specific, a dynamic model in Aspen plus dynamics was built based on a steady-state model in Aspen plus (corresponding to ~25,000 tons MeOH/year) with the input data from publications. The investigation in terms of energy efficiency and dynamic characteristics during load changes was carried out.

The results pointed out that the flow rate of the outputs (crude methanol and purging gases) and heat duties of the heat exchangers, can follow up and agree with the changes in H2 feed rate. During loading changes, components composition at the reactor inlet fluctuates insignificantly, and the energy efficiency slightly drops from ~88.5 % to 86.75 % with the ramping rate of 50 %/ hour. The minimum loading based on Aspen plus dynamics is 18.8 % of the maximum capacity. However, for feasible operation of heat exchanger (reactor preheater), the process should be operated at over 24.6 % loading levels. The highest ramping rate that the model can handle is 50 % changes (decrease and increase) per 0.117 hours. A part of real-time electrolytic H2 data based on solar electricity in the range between 91 kmol/h (~31.1 % loading) and 292.5 kmol/h (full-load mode) within three hours was implemented and the validity of the model still remained in continuously fluctuating of H2 feeding condition.