Laminar flame stability analysis of ammonia-methane and ammonia-hydrogen dual-fuel combustion

Abstract

In recent years, dual-fuel combustion of ammonia blended with reactive fuels like methane or hydrogen has gained attention for reduced carbon emissions. This study investigates the stability of NH3-CH4 and NH3-H2 laminar propagating flames, utilizing experimental and numerical methods. It covers an extensive span of equivalence ratios (0.7–1.6), fuel compositions, and a variety of initial temperatures (300–600 K), as well as pressures ranging from 1 to 5 bar·NH3-CH4 blends exhibit stability across all equivalence ratios, with Markstein lengths of 0.02–2.49. In contrast, NH3-H2 flames display significant variations in Markstein lengths, ranging from 3.73 to −3.36 at an equivalence ratio of 0.7, particularly destabilizing fuel-lean flames due to heightened preferential-diffusional instabilities. Increasing ammonia content stabilizes fuel-rich flames but destabilizes fuel-lean ones. As ammonia content rises, both NH3-CH4 and NH3-H2 flames become more hydrodynamically stable due to flame thickening, although this can exacerbate flame instabilities when preferential-diffusional instabilities dominate. Additionally, preheating destabilizes both ammonia-methane and ammonia-hydrogen flames hydrodynamically, while pressure affects NH3-CH4 and NH3-H2 mixtures oppositely, destabilizing the former and stabilizing the latter due to reduced flame thickness. The addition of diluents stabilizes NH3-CH4 flames but destabilizes NH3-H2 flames, influencing Le and Ze numbers. Diluent addition significantly reduces the flame temperature gradient (up to 64 %), resulting in thicker flames and enhanced hydrodynamic stabilities. This research provides insights into the complexities of ammonia-blended dual-fuel combustion and its impact on flame stability under various conditions.