A Mathematical Modeling and Validation Study of NOx Emissions in Metal Processing Systems

Abstract

The model is based on separate calculations of prompt and thermal NOx and is accomplished using CFD code for flow, temperature and concentration fields in the combustion system. Thermal NOx is calculated with the Zeldovich model. The prompt NOx is considered to be independent of residence time and is computed with detailed kinetic data based GRI-Mech version 2.11 and CHEMKIN code by assuming that every computational cell is a perfectly stirred reactor. Three main parameters are considered to be critical in NOx production: 1) air equivalence ratio, 2) temperature, and 3) mixture dilution with combustion products. All of these parameters and methane oxidation reaction rates are readily available in every cell of the CFD domain. Once NOx emission index is computed by GRI-Mech in every cell, NOx reaction rate is easily evaluated by multiplying it with the methane oxidation reaction rate. The NOx concentration field is then calculated using known CFD transport parameters. Comparison of model predictions with measurements is made for a wide range of industrial natural gas-fired burners installed in combustion chambers and furnaces. The flame temperatures were in the range of 1 400-2 100 K, velocities were in the range of 10-200 m/s, burner nozzles were in the range of diameters 5-550 mm, and the combustion chamber or furnace internal equivalent size in the range of 0.05-3.5 m. Good agreement has been obtained in most cases.