Thermocatalytic hydrogen production by integrated multi-stage steam methane fuel processing with an exhaust gas recirculation loop in a high-temperature fuel cell power plant

Abstract

This study investigates thermocatalytic low-carbon hydrogen production in an integrated steam methane fuel processing system with exhaust fuel recirculation, specifically designed for residential fuel cell applications. The steam methane fuel processor comprises a combustor, preheater, steam methane reformer, water–gas shift reactor, and preferential oxidizer in the streamwise direction. Fundamental kinetic models for thermocatalytic hydrogen production were developed by considering catalyst particulate packing arrangements, thermofluidic characteristics, and chemical kinetics. These numerical models were then applied to evaluate hydrogen productivity in conventional steam-reforming reactors, independent of their geometric designs. The models were successfully validated against experimental data and subsequently integrated into extensive three-dimensional numerical analyses of transport phenomena and chemical reactions within a 5 kWe hydrogen production system. This system featured a top-fired combustor positioned at the center of a cylindrical fuel processor. The external hydrogen production model was further applied to a solid oxide fuel cell (SOFC) system with an anode off-fuel recirculation loop using a model-in-the-loop simulation. Numerical results indicated that the majority of hydrogen was generated via the steam methane reforming reaction, accompanied by a high-temperature water–gas shift side reaction. The remaining high-temperature water–gas shift and preferential oxidation reactions enhanced hydrogen purity. Additionally, the temperature increase caused by fuel recirculation shifted chemical reactions to mitigate temperature variations, consistent with Le Châtelier's principle. The optimal steam-to-carbon ratio was determined to be 2.4 by hydrogen production efficiency. Under this condition a fuel and air utilization ratio of 75 and 45.5 %, the integrated fuel processor achieved a system efficiency of 47.58 %, with a thermal energy consumption of 0.78 kcal. Notably, the recirculated anode exhaust gases in SOFC system can fully cover the steam demand for hydrogen production, including the thermal energy required for water preheating and vaporization.