Effect of flow configuration on the operation of coupled combustor/reformer microdevices for hydrogen production

The effect of flow configuration, namely co- vs. counter-current flow, on the operation of multifunctional microdevices for hydrogen production is analyzed using two-dimensional computational fluid dynamics simulations. Stoichiometric propane/air gaseous combustion and ammonia reforming on Ru occur in alternate parallel channels (600 and 300μm wide, respectively) separated by a thermally conducting wall (300μm thick). It is shown that complete conversion of ammonia can be achieved at millisecond residence times in both flow configurations. The power generated, based on 100% utilization of the hydrogen produced, is in the range of 8–60 W per cm height of the device depending on flow rates. A proper balance of the flow rates of the combustion and reforming streams is, however, crucial in achieving this. For either configuration, the maximum power generated is determined by extinction at large reforming stream flow rates. Materials stability, resulting from high temperatures generated at low reforming stream flow rates, determines the lower power limit for a given flow rate of combustible mixture. The two flow configurations are contrasted based on multiple performance criteria, such as device temperature, power exchanged, conversions, and net hydrogen production by constructing operation maps. They are found to be practically equivalent for highly conductive materials. Using properly balanced flow rates, the co-current configuration expands the operation window to medium as well as low thermal conductivity materials as compared to the counter-current configuration that shows a slightly superior performance but in a rather narrow regime of high ammonia flow rates and high thermal conductivity materials.