N yO x formation in lean premixed combustion of methane in a high-pressure jet-stirred reactor

Abstract

A novel high-pressure jet-stirred reactor (JSR) has been used to study the formation of NOx, N2O, and CO in lean premixed combustion of methane-air mixtures at pressures up to 20 bars. Experiments were carried out at residence times of 1.0, 1.5, and 2.0 ms, at an equivalence ratio of 0.55 and at temperatures of 1510 and 1550 °C. The measured NOx concentrations show a distinct negative trend with increasing pressure and clearly decrease with decreasing residence times at pressures <5 bars. At higher pressures, the influence of residence time is diminished. The NOx and N2O concentrations are slightly lower than the predictions of a perfectly stirred reactor (PSR) using four well-known reaction mechanisms of the C−H−O−N system. However, the measured CO levels are considerably lower than the PSR predictions. This is mainly attributed to spatial inhomogeneities of the JSR, confirmed by measured temperature and concentration profiles at atmospheric conditions. To take these effects into account, the JSR was modeled as a series of two PSRs and one PFR (plug flow reactor). This model predicts the measured NOx, N2O, and CO concentrations well with all but one reaction mechanism: for NOx concentrations, significant deviations occur between measurements and predictions using the Miller-Bowman mechanism. The influence of finite-rate micromixing was studied by computations of a partially stirred reactor (PaSR), which show a minor influence on NOx and CO levels: for the CO concentrations, these effects are negligible compared to the macromixing effects.