Investigation of NO x and CO formation in lean-premixed, methane/air, high-intensity, confined flames at elevated pressures

Abstract

The coupling between NOx formation chemistry and the mixing/transport environment is of critical importance to the design of lean-premixed gas turbine combustors but is incompletely understood. In the present research, this problem was addressed via the study of NOx formation in a high-pressure jet-stirred reactor operating on lean-premixed methane/air. These experiments focused on the effects of residence time (0.5–4.0 ms), pressure (3.0, 4.7, and 6.5 atm), and inlet temperature (344–573K). The combustion temperature varied from 1815±5 K at the lowest residence times to 1910±30K at the largest residence times. The NOx was lowest at intermediate residence times, reaching higher values at the extremes. Increasing pressure and inlet temperature tend to reduce NOx concentrations. Concentration profiling in the reactor suggests two general environments: (1) a highly non-equilibrium reaction zone defined by high CO concentrations, and (2) a postflame environment. The NOx formation was concentrated in the region of strongly non-equilibrium combustion chemistry. The Damkohler number was 0.06≤Da≤1, and the ratio of turbulent intensity to laminar burning velocity was 28≤u′/SL≤356, indicating the combustion occurs in the high-intensity, chemical rate-limiting regime. The results were interpreted using a two-environment, detailed chemistry model in which the size and structure of the flame environment were established by matching the measured data, and which were independently verified using turbulent flame velocity/thickness correlations. The modeling suggests NOx formation is controlled by both the specific conditions in the non-equilibrium zone and by the size of the zone. Since both these features are influenced by the experimental parameters, a highly nonlinear scenario emerges with implications for minimizing NOx via combustor design. The modeling also suggests the unique case of well-stirred combustion for NOx at elevated pressure is obtained at low residence time conditions.