Study of combustion and NO chemical reaction mechanism in ammonia blended with DME

Abstract

Ammonia (NH3) has been supposed to be the potential alternative energy source towards carbon neutrality. However, it has several unfavorable combustion characteristics, such as the slow combustion rate, poor combustion stability, high ignition energy and so on. As a renewable fuel, dimethyl ether (DME) is also considered to be a favorable carbon–neutral fuel, which can improve the combustion process of ammonia. To investigate the combustion and emission mechanism for the NH3/DME dual-fuel combustion, a detailed chemical reaction mechanism with 221 species and 1597 reactions was developed based on the previous relevant work. The laminar burning velocity (LBV) and ignition delay time (IDT) of ammonia, DME and NH3/DME combustion under a wide range of initial conditions were validated in 0-D and 1-D models. The process of NO formation and reduction reaction was also studied under the laminar burning combustion and ignition model, and it was found that fuel NOx still dominates the total NOx production in NH3/DME combustion, and the DME addition in ammonia up to 50% promotes the NO formation reaction, producing more NO. In order to analyze the effect of DME addition on NO formation mechanism, the analyses of chemical reaction pathway, sensitivity analysis on NO, typical active radical evolution behaviors and rate of production (ROP) were conducted in detail. In comparison with D50 (50%NH3/50%DME), D25(75%NH3/25%DME) with higher ammonia content generates more NH and NH2 radicals, and therefore, promoting the NO reduction reaction. Lower DME content in D25 also generates less H/O/OH active radicals, which inhibits the generation of NO. It is reduced by 9% approximately in total at equivalence ratio of 0.7. In addition, CH3 generated from DME could also partly promote the conversion of HNO and NO2 to NO, slightly increasing the NO production.