Comparative effects of H2 and NH3 on hydrocarbon fuels laminar combustion characteristics and pollutant emissions: Status and challenges

Zero-carbon fuels, such as hydrogen (H2) and ammonia (NH3), can effectively reduce greenhouse gas and promote carbon neutrality. Pure H2 and NH3 combustion have shortcomings of laminar burning velocities (LBVs) and NOx emissions, while can reduce CO2. This review summarizes various experimental and numerical data from blending H2 and NH3 with small molecule and macromolecular hydrocarbon fuels. The results show that H2 promotes LBVs of hydrocarbon fuels significantly than the inhibitory effects of NH3. At Phi = 1.1, there is linear relationship between LBVs and blending ratio in CH4/H2(or NH3)/air flames. The chemical effects of H2 promotes soot nucleation while NH3 inhibits soot formation through chemical and physical effects. CO2 and NOx emissions are also discussed in different temperatures, pressures and blending ratios. Specially, dual and ternary hydrocarbon fuels are deeply analyzed to approach the use of practical fuels. Moderate or intense low-oxygen dilution (MILD) combustion and H2-NH3 mixed combustion are potential methods to solve NOx emissions. The advantages of H2 enhancing flammability and NH3 inhibiting soot formation are more concerned. Finally, the challenges in the current high-pressure and NOx emissions combustion are identified, and an outlook for the future is put forward.