Combustion Characteristics of NH3/H2 Blends in a Micro-combustor with a Bluff Body

Ammonia (NH3) is a carbon-neutral fuel that offers a promising option for reducing greenhouse gas emissions. Nonetheless, its use in practical combustion scenarios is hindered by its slow flame speed, low energy content, and NOx emissions. Incorporation of hydrogen (H2) can significantly address these disadvantages. In this study, effects of equivalence ratio (ϕ = 0.6, 0.8, 1.0) and hydrogen blending ratio (NH3:H2 = 70:30, 60:40, 50:50) on the flame blow-off limit and NO emission of a micro-combustor with a bluff body were numerically studied. The results indicate that: (1) When NH3:H2 = 70:30, flame is unable to be sustained in this micro-combustor. (2) Compared to ϕ = 0.6 and 0.8, increasing the hydrogen blending ratio is more effective to expand blow-off limit at ϕ = 1.0. (3) The NO mass fraction in flue gas increases monotonically with the increase of inlet velocity at ϕ = 0.6, but varies non-monotonically at ϕ = 0.8 and 1.0. Analysis reveals that the monotonic trend at ϕ = 0.6 is mainly attributed to the relatively low flame temperatures, whereas the non-monotonic trend at ϕ = 0.8 and 1.0 are the result of a decreasing flame temperature and an increasing supply of NH3 and N2. (4) For fixed velocity and hydrogen blending ratio, the NO mass fraction at ϕ = 0.8 is higher than that at ϕ = 0.6 and 1.0. As ϕ increases from 0.6 to 0.8, the flame temperature increases significantly, which prompts NO production; however, as ϕ further increases from 0.8 to 1.0, the N2 supply decreases and less NO is produced.