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

TL;DR

This study addresses the challenge of reactive nitrogen emissions in ammonia-based AHN combustion by introducing a rush-to-equilibrium concept that uses a temporally decaying mixing rate within a premixed flame manifold framework to accelerate the approach to thermodynamic equilibrium under finite residence times. The method leverages a 1D equilibrium/non-equilibrium manifold model, tracked in a Lagrangian sense, and implemented with AHN and methane kinetic mechanisms to compare emissions pathways. Results indicate emissions reductions on the order of an order of magnitude can be achieved at typical gas-turbine residence times, with NO and N2O decreases highly sensitive to cracking extent, pressure, and temperature; practical diffuser-based geometries are shown to be feasible for implementation. Overall, the rush-to-equilibrium concept provides a viable pathway to lower RN emissions in AHN combustion without extending residence times, guiding future design and optimization of diffuser-shaped combustors for ammonia-based power and propulsion.

Abstract

Ammonia (NH3) is a zero-carbon fuel that has been receiving increasing attention for power generation and even transportation. Compared to H2, NH3's volumetric energy density is higher, is not as explosive, and has well established transport and storage technologies. Yet, NH3 has poor flammability and flame stability characteristics and more reactive nitrogen (RN: NOx, N2O) emissions than hydrocarbon fuels, at least with traditional combustion processes. Partially cracking NH3 (into a NH3-H2-N2 mixture, AHN) addresses its flammability and stability issues. RN emissions remain a challenge, and mechanisms of their emissions are fundamentally different in NH3 and hydrocarbon combustion. While rich-quench-lean NH3 combustion strategies have shown promise, the largest contributions to RN emissions are the unrelaxed emissions in the fuel-rich 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 AHN combustion, which aims to reduce the unrelaxed RN 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 mitigates the mixing effects that prevents the particle approach to equilibrium, and promotes the chemistry effects to push the particle toward equilibrium, all while considering finite residence times. Evaluated with a state-of-the-art combustion model at gas turbine conditions, the concept shows the potential to reduce RN emissions by an order of magnitude, and that works irrespective of cracking extent, pressure, temperature, etc. A brief discussion of possible practical implementation reveals reasonable geometric and flow parameters characteristic of modern gas turbine combustors.

Figures (14)

• Figure 1: Temperature at thermodynamic equilibrium as a function of the equivalence ratio $\phi$. Blue lines: AHN with different cracking percentages $\alpha$. Red line: methane ($\rm CH_{4}$).
• Figure 2: $\rm NO$ mass fraction at thermodynamic equilibrium as a function of the equivalence ratio $\phi$, similar to Fig. \ref{['fig:T_vs_phi']}.
• Figure 3: $\rm N_{2}O$ mass fraction at thermodynamic equilibrium as a function of the equivalence ratio $\phi$, similar to Fig. \ref{['fig:T_vs_phi']}.
• Figure 4: Temperature at a finite residence time $t=10\rm\:ms$ as a function of the equivalence ratio $\phi$, at gas turbine conditions and for different cracking percentages $\alpha$. Line styles are the same as Fig.\ref{['fig:T_vs_phi']}.
• Figure 5: $\rm NO$ mass fraction at a finite residence time $t=10\rm\:ms$ as a function of the equivalence ratio $\phi$, similar to Fig. \ref{['fig:T_multi_a_multi_h2p_vs_phi']}.