It may become attractive in the future to operate fuel-related chemical reactions, with H₂ as reactant, under flexible load conditions when H₂ is produced through H₂O electrolysis using fluctuating renewable energy. In this way, the size of the H₂ storage device connected to the electrolysis cell can be reduced and, consequently, its investment costs. The design of a flexible reactor depends on the characteristics of the catalyst and the chemical reaction. CO₂ hydrogenation over Fe catalysts to short-chain hydrocarbons, which can be used to adjust the heating value of substitute natural gas, is investigated as example reaction. A specific reactor design for flexible operation with gas recycling is a consequence of the constraints resulting from product partial pressures and reaction temperature. The experimental and mathematical methods developed can be applied to other fuel-synthesis processes.