Operating strategies for thermally coupled combustion-decomposition catalytic microreactors for hydrogen production

Abstract

Aiming towards COx-free hydrogen production for fuel cell applications, the catalytic decomposition of ammonia over ruthenium coupled with the steady catalytic combustion of fuel-lean propane-air mixtures in multifunctional microreactors consisting of alternating endothermic and exothermic reaction channels was investigated numerically. Comparisons with methane-air systems were also made. A two-dimensional CFD (computational fluid dynamics) model was developed, and numerical studies were performed to evaluate the effect of main design parameters and to provide guidelines for optimal design. Operating strategies were provided, and operating diagrams were constructed by different operating lines. Furthermore, the attainable operating region delimited from the breakthrough limit, material stability limit, self-sustained operation limit, and maximum power output limit was mapped out. It is shown that ammonia decomposition at millisecond contact times is feasible. High-conductivity materials are preferable due to their rather wide operating region, whereas low-conductivity materials result in slightly increased ammonia conversion at the expense of hot spot (temperature peak) formation. Sufficiently high ammonia flow velocities serve a dual purpose by improving hydrogen yields and reducing reactor temperatures. Higher hydrocarbons not only can extend the flow velocity operating region but also can improve the hydrogen yield; however, at high ammonia conversions, the choice of hydrocarbon fuel is immaterial when using suitable compositions to maintain the same energy input. The co-current flow configuration should be preferred due to its rather wide operating region and the improved thermal coupling, although co- and counter-current systems are essentially equivalent for highly conductive materials. Finally, channel gap sizes should be small enough to minimize mass-transfer effects.