Experimental support for a new NOx formation route via an HNNO intermediate

Achieving minimal levels of nitrogen oxides (NOx) during combustion is a major constraint in the design of advanced high-efficiency engines. NOx can be formed during combustion of any fuel—including those without fuel-bound nitrogen—in air, where radicals can attack molecular nitrogen (N2) present in air to break the strong NN bond to ultimately form NOx. Paramount to the goal of minimizing NOx formation is knowledge of the fundamental routes by which the strong NN bond in N2 can be broken. Historically, there have been four known routes for breaking the strong NN bond in N2 to ultimately form NOx. We have recently posited that another route—mediated by an HNNO intermediate—may also play a role, particularly at the high pressures and low peak temperatures relevant to high-efficiency, low-NOx engines. Our previous theoretical and modeling studies show HNNO to be a major product of the N2O + H reaction at high pressures and low temperatures; once formed, HNNO is likely to react with radicals in barrierless reactions that would occur quickly and with high NOx yields. In the present paper, we report measurements of H2, O2, H2O, N2O, NO, NOx, and NH3 in jet-stirred reactor experiments for an H2/O2/N2O/NO/N2/Ar mixture that specifically target HNNO pathways. Importantly, we observe significant formation of NO and NH3—both of which provide signatures of the HNNO mechanism that are not predicted by previous models without it. Flame simulations using a new sub-model describing pressure-dependent formation and consumption of HNNO show these pathways to be among the most prominent formation routes at high pressures and low peak temperatures. However, exact quantification of the role of HNNO in NOx formation and quantitative predictions of NOx in general require more accurate rate constants for both HNNO pathways and mixture rules for pressure-dependent reactions.