Dimethyl Carbonate/Methyl Octanoate/Methyl Decanoate Additions on Laminar Burning Velocity and Instability of Ammonia Composite Combustion

The applications of co-combustion with oxygen-containing carbon-neutral fuels and ammonia are highlighted by the investigators, but the primary flame characteristics of ammonia–ester composite combustion have not been adequately investigated. In this study, the laminar burning speed and flame instability of ammonia co-combustion with ester fuels with different molecular chain lengths (dimethyl carbonate, methyl caprylate, and methyl decanoate) were investigated in a constant-volume combustion bomb by using a high-speed Schlieren technique, and the intrinsic mechanism of ammonia–ester composite combustion has been analyzed by combining with chemical kinetic modeling. The results of this study demonstrate that ester addition can efficiently enhance the laminar burning velocity of ammonia, and the ester molecular chain length has a significant influence on laminar burning speed improvement compared to the molecular oxygen content. Large-molecule ester additions have a greater enhancement effect on NH3–H2 cracking and low/high-temperature exothermic reactions than small-molecule esters. Small-molecule and large-molecule esters promote flame propagation in the initial stage of flame nucleation and in the middle and late stages of flame development, respectively. Direct cleavage of small-molecule esters enhances the stretching stability of ammonia laminar flame compared to that of large-molecule bond breaking, while large-molecule esters can improve the thermo-mass diffusion instability of ammonia flame compared to that of small-molecule esters. Comprehensive research on ammonia–ester laminar flame characteristics in this study reveals that when dimethyl carbonate, methyl caprylate, and methyl decanoate are added at 43.1–72.5%, 33.0–88.2%, and 30.0–81.1%, respectively, they can realize the fast and steady combustion of ammonia, which is of important significance for ammonia energy popularization.