Visualization and simulation study of ammonia blending with hydrogen as combustion application in lean-burn condition

Abstract

Ammonia (NH3), as one of the two zero-carbon fuels, has attracted lots of attention. However, its poor properties, including slow laminar burning velocity (LBV), high ignition energy, and low flammability, make it unfavorable for use in engines. On the other hand, hydrogen (H2), another zero-carbon fuel, has a much higher LBV and flammability. Adding a certain amount of hydrogen to ammonia mixture has the potential to improve the combustion process and extend the lean-burn limit of ammonia. This paper investigates the combustion and emission improvements of ammonia blended with a small amount of hydrogen under lean-burn conditions, both experimentally and numerically. A new NH3/H2 chemical reaction mechanism was developed to accurately predict ignition delay time (IDT), LBV and NO generation under a wide range of initial conditions. The flame propagation schlieren and NO chemiluminescence measurements with a homogenous NH3/H2 mixture ignited by a spark plug in constant volume combustion chamber (CVCC) were conducted. The 3D numerical simulations in CVCC, coupled with the new mechanism, were validated with the experimental results. The hydrogen energy fraction up to 30 % (X30) and ER down to 0.4 (ER0.4) were studied in experiments and simulations. The results showed that, in comparison to pure ammonia (X0) at ER1.0, lean-burn conditions can be extended to ER0.6 and ER0.4 for X10 and X30, respectively, while still achieving a shorter combustion duration. NO is initially increased and then decreased with decreasing ER, regardless of hydrogen addition. Comparatively, X30ER0.4 can achieve relatively low NOx emissions, and maintaining an equivalent combustion rate with a low combustion temperature.