Thermal performances investigation on an ammonia-fuelled heat-recirculating micro-combustor with reduced chemical mechanism

Abstract

This study addresses the challenges in utilizing ammonia as a CO2-free fuel. However, the drawbacks of ammonia are—low burning speed and nitrogen oxide emissions. To enhance ammonia's flammability in atmospheric micro-combustion, hydrogen is blended in the fuel. First, a simplified chemical reaction mechanism of ammonia consisting of 44-step reactions and 19 species specifically for ammonia is developed and validated using experimental data to reduce computational cost and time. Then, we proposed a heat-recirculating micro-combustor fueled by premixed hydrogen/ammonia/air. Five key parameters are identical to numerically studying the thermal performance, entropy generation, and NO emissions. The present findings confirm that higher inlet velocity boosts thermal performance (power output achieving 15.8 W at 7 m/s) and NO emissions peak at 3 m/s (0.0169). Unity equivalence ratio optimizes thermal performance, rich-fuel combustion reduces NO emissions. At the stoichiometric ratio, the power output is 8.34 W, with the highest NO emissions at 0.9 (0.168). Hydrogen blending has a small effect on the performance (8.5 W at ξm = 0.7, 0.51 W above ξm = 0.4), but effectively reduces NO emissions (ξm = 0.7 NO emissions around 30 % lower than ξm = 0.4). Furthermore, changing the material from steel to Corundum enhances power output by approximately 6 %, while longer heat recirculation improves thermal performance.