Catalytic combustion of NH₃ in Pt-coated microchannels: A numerical study on the surface-gas chemistry coupling and its impact on product selectivity

Catalytic combustion has been proposed to overcome the obstacles when burning ammonia as a carbon-free fuel, such as difficult ignition, narrow flammability limits and combustion instability. To this direction, ignition of gas-phase combustion of ammonia in catalytic reactors, surface-gas chemistry coupling and, especially, the resulting impact on product selectivity, turn to be the key questions. As such, this work numerically investigated catalytic and coupled catalytic-gaseous combustion of premixed ammonia/oxygen/nitrogen mixtures in catalytic microchannels coated with platinum (Pt), which is one of the most widely used catalysts in combustion and emission control systems. NH₃ conversion and N₂ selectivity were examined for a wide range of operating conditions, including pressure (1 to 5 bar), equivalence ratio (0.1 to 0.5), channel height (0.5 to 3.0 mm) and catalyst temperature (800 to 1300 K). The roles of gas-phase ignition and interplays between the catalytic and the gaseous reaction pathways were further clarified. Results showed that gas-phase combustion could be ignited in the very narrow catalytic microchannels and was promoted by higher wall temperature, equivalence ratio and pressure. Under all investigated conditions, significant amounts of NO were produced from the catalytic pathway, which however could be substantially consumed by the gaseous pathway via the reaction of NO with NH₂. The practical implications are that NO emissions from NH₃ catalytic burners can be readily suppressed via proper reactor design and the selection of operating conditions. Hence, this work provides key information for the design and optimization of NH₃ catalytic combustion systems.