Power-to-Gas: Process analysis and control strategies for dynamic catalytic methanation system

The methanation process, or Sabatier process, allows carbon dioxide and carbon monoxide to be hydrogenated into methane, which can be subsequently injected (once the gas grid specifications have been respected) into the gas network infrastructures already present in Europe. This process can be effectively adopted to convert captured CO2 streams from power plants or hard to abate plants by using green hydrogen from renewable-driven water electrolysis. The technical aspects of concern in the methanation process are certainly the strong exothermicity of the process, with consequent possible generation of hotspots along the entire catalyst bed and the management of reaction heat through thermal recovery. These peculiar aspects influence the choice of construction materials and geometry of the methanation reactor, operating parameters, cooling system, type of catalyst/support and the initial conditions of the feed. In particular, the generation of hotspots influences the local kinetic reaction in the methanation reactor, the diffusional limits of hydrogen and carbon dioxide inside the catalyst and the chemical-physical characteristics of the catalyst bed. The present work reports a first scale-up of the Sabatier process, developed in Aspen Plus simulation environment, with the following characteristics: reactor size to process 925 Nm3/h, (750 hydrogen and 175 carbon dioxide), at 15 bar and 250 degrees C. After the implementation in the steady state, a dynamic simulation is performed to carry out a transient study of the entire plant. In particular, the attention is focused on the variation in the hydrogen load, produced by electrolysis from the energy surplus from renewable sources. Considering the variable nature of the power flow supplied to the electrolyser and therefore any shutdown and cold start-up phases of this equipment, the following load variation scenarios for the Power to Gas (PtG) system are simulated: -5%, +5% and -30 % molar flow rate of incoming hydrogen, compared to steady conditions used in the preliminary design of the equipment. The study highlights how the cooled reactor configuration is more performing and characterized by a lower number of reactors in series (2 reactors) when compared to the adiabatic configuration (5 reactors). Furthermore, in the cooled reactor configuration the control system is able to respond more quickly to load variations compared to that designed for the adiabatic case, although the latter is mostly adopted from the industrial viewpoint due to temperature control issues. Moreover, the control system manages to respond to load variations by bringing the values of interest (gas grid residual concentration of hydrogen and carbon dioxide, Wobbe index) into the target intervals.