CFD Analysis of the Impact of Ammonia Cracking on NOx Emissions in Turbulent Combustion

This paper investigates the effects of NH3/H2/N2 fuel blends due to ammonia cracking on NOx emissions when burned. Combustion simulations were run in Ansys Fluent using the Stagni combustion mechanism due to its relatively recent development and proven accuracy in modeling ammonia flames. Three simulation sweeps were conducted: 1) pure NH3 over a range of equivalence ratios, 2) incremental cracking of NH3 at the equivalence ratio corresponding to peak NOx emissions for pure NH3 combustion, which was found to be 0.8, and 3) fully cracked NH3 over a range of equivalence ratios. Because of pure ammonia’s low flammability and tendency to be difficult to ignite, preheated fuel and air alongside a high swirl combustor were used to better ensure complete combustion for every simulation point in all three sweeps. The results of these simulations show that NOx emissions at the outlet of the chamber increase substantially from the pure NH3 cases when the cracking percentage is in the range of 0% to 60%. Past this point, however, the amount of NOx emissions craters by at least an order of magnitude when the cracking percentage reaches 100%. This indicates that fully cracking the NH3 fuel before burning it can negate the excessively high NOx emissions that ammonia combustion is notoriously known for.