Synergistic effects of CO2 dilution and H2 addition on the laminar combustion characteristics of NH3/CH4 blends at high temperature and pressure

Synergistic effects of carbon dioxide (CO2) and hydrogen (H2) can improve the combustion and emissions characteristics of ammonia (NH3)/methane (CH4) blends. This study experimentally and kinetically investigates the interaction effects between CO2 dilution and H2 addition on flame propagation and instability mechanisms of NH3/CH4 blends at high temperatures and pressures. The results showed that both CO2 and H2 significantly decrease the thermal expansion ratio of NH3/CH4 blends. In contrast to CO2, H2 reduces flame thickness, which is hardly improved by CO2 dilution under lean-burn condition, leading to strong hydrodynamic instability. Due to the distinct effects of CO2 and H2 on the effective Lewis number, the transition from lean to rich combustion can be categorized into four regions. A new correlation based on CO2 and H2 mole fractions was proposed to predict laminar burning velocity (SL) over a wide range of conditions. The synergistic effects of CO2 and H2 can improve combustion and emission performance of NH3/CH4 blends under lean-burn and stoichiometric conditions. Considering flame instability, and temperature/pressure dependencies, an equivalence ratio range of 0.8–0.9 under high-temperature and low-pressure conditions is identified as the optimal range for improving the overall performance of NH3/CH4 blends through CO2 dilution and H2 addition.