Rush-to-equilibrium concept for minimizing reactive nitrogen emissions in ammonia combustion

Ammonia is a zero-carbon fuel that has been receiving increasing attention for power generation and even transportation. Compared to hydrogen, ammonia's volumetric energy density is higher, is not as explosive, and has well established transport and storage technologies. However, ammonia has poor flammability and flame stability characteristics and more reactive nitrogen emissions (nitrogen oxides, nitrous oxide) than hydrocarbon fuels, at least with traditional combustion processes. Partially cracking ammonia addresses its flammability and stability issues, through on-board catalysts or autothermal crackers, into a mixture of ammonia, hydrogen, and nitrogen. However, reactive nitrogen emissions remain a challenge, and mechanisms of their emissions are fundamentally different in ammonia and hydrocarbon combustion. While rich-quench-lean ammonia combustion strategies have shown promise, the largest contributions to reactive nitrogen emissions are the unrelaxed emissions in the fuel-rich first stage due to overshoot of thermodynamic equilibrium within the reaction zone of premixed flames coupled with finite residence times available for relaxation to equilibrium. This work introduces a rush-to-equilibrium concept for partially cracked ammonia combustion, which aims to reduce the unrelaxed reactive nitrogen emissions in finite residence times by accelerating the approach to equilibrium. In the concept, a flow particle is subjected to a decaying mixing rate as it transits the premixed flame. This plays an important role in mitigating the mixing effects that prevents the flow particle approach to equilibrium, and promoting the chemistry effects to push the particle toward equilibrium, all while considering residence times typical of gas turbines for power generation. Evaluated with a state-of-the-art combustion model at gas turbine conditions, the concept shows the potential for a reduction in reactive nitrogen emissions by an order of magnitude at even modest mixing rate decay rates. It is also shown that the concept works irrespective of cracking extent, pressure, temperature, etc. A brief discussion of practical feasibility reveals reasonable geometric and flow parameters characteristic of modern gas turbine combustors for power generation. Novelty and Significance Statement A novel rush-to-equilibrium combustion concept is proposed with the aim of reducing reactive nitrogen emissions, which include nitrogen oxides and nitrous oxide, from partially cracked ammonia combustion at gas turbine conditions. Reactive nitrogen emissions are elevated in partially cracked ammonia combustion systems because insufficient residence time is available to reach thermodynamic equilibrium. A concept is proposed to address this issue, leveraging decaying mixing rates, without modifying typical gas turbine for power generation residence times by accelerating the approach to thermodynamic equilibrium. The new concept is demonstrated of being capable of significantly reducing reactive nitrogen emissions (by a factor of an order of magnitude). Finally, implementation of the new concept is showed to be practically feasible.