Numerical modeling and experimental testing of the hydrogen ultralean combustion on an adiabatic catalytic monolith in diverse working conditions

The catalytic combustion of ultralean air-hydrogen mixtures is a technique for generating thermal power without emissions at the stack with inherently safe systems. The technology utilizes mixtures beyond flammability limits and conducts combustion in catalyst-coated monoliths. This work develops a numerical model of an adiabatic channel of a catalytic combustor and provides data for its validation obtained with an experimental test rig operating with ultralean stoichiometry. The focus is on the superadiabatic temperature hotspot over the walls of the monolith as well as the fuel conversion in the device. Both these factors are numerically estimated and experimentally measured under various operational conditions by varying the inlet flow rate and the inlet hydrogen fraction. Furthermore, the numerical model is utilized to evaluate the impact of the heat conduction coefficient and of the channel diameter of the monolith on the parameters presented above. Finally, the start-up time is measured and compared against the numerical estimation. A 30 mm long, 400 CPSI, Pt- and Pd-based catalytic honeycomb monolith with square channels is exploited. The results reveal that despite significant adiabatic hotspots, the highest catalytic surface temperature measured is 692 K, with an adiabatic combustion temperature of 543 K, and with greater than 99% H2 conversion. The numerical and experimental results show that a catalytic combustor can be reliably operated with hydrogen outside the flammability limits and that the process can cold-start without external ignition.