Design of Inlet Conditions for High Pressure NOx Measurements in Lean Premixed Combustors

Abstract

Semi–empirical characteristic time models (CTMs) for NOx emissions index (EI) and lean blowoff are used in the design of an inlet condition matrix for measurement of NOxEI from a lean premixed combustor. Such models relate either NOxEI or the weak extinction limit to times representing relevant physical and chemical processes in the combustor. Lean premixed (LP) natural gas/air combustion is considered for the following conditions: inlet temperature, 300–800 K; combustor pressure, 1–30 atm; and equivalence ratio, 0.5–0.7. The NOx model is used to determine combinations of inlet conditions corresponding to greatest NOx sensitivity. A dependence of NOx emissions on pressure is included in the model. Emissions of oxides of nitrogen are found to he most sensitive to variations in inlet temperature and combustor pressure, in the 560–800 K and 20–30 atm ranges, respectively, while sensitivity to variations in equivalence ratio is substantial over the entire range considered. Thus it is found that operating conditions for high thermal efficiency in LP turbine combustors conflict with the goal of lowering NOx emissions, a result consistent with thermal NOx from conventional, diffusion flame combustors. A lean blowoff model is used to estimate the lowest equivalence ratio at which a flame can he held, as well as to determine whether a flame can be stabilised at the operating conditions suggested by the NOx sensitivity analysis. The results suggest a nominal lower limit on equivalence ratio of 0.4, and that a flame can be held for most of the combinations of inlet conditions suggested by the NOx sensitivity analysis. Autoignition of the fuel/air mixture is also considered in relation to the location and/or design of the premixing system. The current NOx CTM is applied to LP natural gas fired data from the literature. A model modification, thought to better represent the fluid mechanics relevant to LP NOx formation, is applied, and its implications discussed.

Characteristic Time Modeling of NOx Emissions and Combustion Efficiency for a Staged Combustion Ramjet

“…A number of issues remain unresolved, including extrapolation of the characteristic time model to the combustor inlet conditions used by Hautman (1995), particularly at the higher flight Mach numbers; two-dimensional versus axisymmetric length scales; and methane and/or natural gas fuel that was modeled here using overall pre-exponential factors and activation energies for nc, thermal NO XEI, and lean premixed NO xEI based on correlations established with data from propane and higher hydrocarbon fuels. Although a lean premixed NO model for methane combustion is under development (McDonald and Mellor, 1995), replacing the technical grade methane used in the experiments by Hautman (1995) with propane and otherwise repeating some of the tests could confirm the assumption required for the modeling that other than for heating values the fuels are equivalent at the given operating conditions.…”

Effects of Unmixedness in Piloted–Lean Premixed Gas Turbine Combustors

“…Finally , the assumed Gaussian PDF can be compared to the computed fuel/air mixing results of Hamer and Roby (1995) who also used STAR-CD to investigate fuel/air mixing downstream of the fuel injector designed by them for the tests discussed b y McDonald and Mellor (1995). Fuel travels throu gh a single center manifold and is distributed through holes angled at 45 degrees upstream in each of the fuel pegs.…”