A numerical parametric analysis for NO_x reduction and combustion performance of ammonia fuels blended with direct and cracked hydrogen

Abstract

Ammonia possesses significant potential in clean energy applications. However, its inherently low flammability somehow confronts a major challenge for its direct applications, necessitating the utilization of effective strategies to enhance its combustion. Among these, the combustion of hydrogen-ammonia mixtures has emerged as a promising approach for improving the combustion characteristics of ammonia. Nevertheless, the presence of nitrogen, a by-product of in-line ammonia cracking, causes the two fuel systems—one with cracked hydrogen and one with direct hydrogen—to behave differently during combustion. In this study, a chemical kinetic analysis was conducted to study the combustion characteristics of hydrogen-ammonia mixtures from different sources. Key combustion parameters—including laminar burning velocity, ignition delay time, and NOx emissions—were evaluated across a wide range of conditions: equivalence ratios (0.7–1.4), hydrogen blending ratios (0–0.3), oxygen content of oxidizer (0.21–0.71), and inlet temperatures (298–2000 K) and pressures (1–60 atm). Sensitivity analysis was also performed to identify the important reactions involved. The optimal hydrogen blending ratio was determined to be 30 vol% based on its enhancement of laminar burning velocity and its favorable NOx emission levels, benchmarked against a baseline methane flame. Four key fundamental reactions exerted significant influence on both flame propagation and ignition. A two-stage combustion strategy demonstrated considerable advantages, with the lowest NOx emission intensity observed when the overall equivalence ratio was 0.65 and the ratio of secondary to primary oxygen flow was maintained between 75 and 80 %. These findings provide guidance for developing efficient, low-emission combustion strategies for hydrogen-ammonia fuels.</p