Ammonia Decomposition Coupled with Methane Combustion in Catalytic Microreactors for Hydrogen Production

The ammonia decomposition over ruthenium thermally coupled with the catalytic combustion of methane-air mixtures over platinum in catalytic microreactors for hydrogen production was studied numerically, using a two-dimensional computational fluid dynamics model that included detailed chemistry and transport. The effect of flow configuration on the operation characteristics was studied in catalytic microreactors consisting of alternating combustion and decomposition channels separated by a thermally conducting wall. Different performance measures were evaluated to assess the operability of the reactor. It was shown that the high temperatures generated through catalytic combustion result in high conversion in short contact times and thus to compact reactors. Complete conversion of ammonia can be obtained at the micro-scale in both flow configurations. A proper balance of the flow rates of the decomposition and combustion streams is crucial in achieving this. For a given flow rate of combustible mixture, material stability determines the lower power limit, caused by high temperatures generated at low decomposition stream flow rates. In contrast, the maximum power generated is determined by extinction at large decomposition stream flow rates. The two flow configurations were contrasted based on multiple performance criteria, such as reactor temperature, conversion, power exchanged, and hydrogen yield by constructing operating regime. They were found to be practically equivalent for highly conductive materials. Using properly balanced flow rates, the co-current flow configuration expands the operating regime to low and moderate thermal conductivity materials as compared to the counter-current flow configuration that exhibits a slightly superior performance but in a rather narrow operating regime of highly conductive materials and high ammonia flow rates.