Improvement of pure ammonia combustion performance using the catalytic pre-cracking method

Abstract

As a storage and transportation medium for hydrogen and a clean fuel with zero carbon emissions, ammonia (${\mathrm{NH}}_3$) plays an important role in promoting hydrogen energy economy and renewable energy utilization. However, ${\mathrm{NH}}_3$ faces issues of low combustion intensity and the difficulties of ignition when used as a fuel. To address these problems, a novel combustion method with high temperature resistance and strong activity catalyst for ${\mathrm{NH}}_3$ pre-cracking is proposed in this paper. The cracking product, hydrogen, has a higher combustion rate, lower ignition temperature, and higher combustion intensity, which can improve the combustion characteristics of pure ${\mathrm{NH}}_3$. Firstly, Computational Fluid Dynamics was used to simulate the whole process of catalytic cracking and combustion of ${\mathrm{NH}}_3$. An Eulerian multiphase flow model with a granular phase was employed to simulate the catalyst particles and a porous medium to simulate the support carrier for the catalyst particles. Secondly, the Langmuir-Hinshelwood model was built using user-defined functions (UDF) to describe the reaction kinetic rates of the adsorption, cracking, and desorption processes of ${\mathrm{NH}}_3$ on the surface of Ni/${\mathrm{Al}}_2{O}_3$ catalyst. Then, species indexing in the flow field was implemented using UDF to couple the catalytic reaction rate with the surface coverage concentration to improve the simulation accuracy and reliability. Finally, the simulation results revealed that the catalytic pre-cracking combustion method can significantly improve the thermal efficiency and stability of ${\mathrm{NH}}_3$ combustion.