Semiempirical correlations of NOx emissions from utility combustion turbines with inert injection

Semiempirical models describe the dominant subprocesses involved in pollutant emissions by assigning specific times to the fuel evaporation, chemistry, and turbulent mixing. Linear ratios of these times with model constants established by correlating data From combustors with different geometries, inlet conditions, fuels, and fuel injectors can then be employed to make a priori predictions. In this work, NOx emissions from two heavy-duty, dual-fuel (natural gas and fuel oil no. 2) diffusion-flame combustors designated A and B are examined. The data are first predicted and then correlated using the existing semiempirical characteristic time model. Heterogeneous effects are found to be significant, in contrast to previous results with aircraft engine combustors. Inert injection for NOx control is modeled as thermal, and two limiting cases are proposed that bound the measured data. An empirically selected effective inert injection Blame temperature was substituted for the stoichiometric flame temperature used to estimate the thermal NO formation rate in the model. This procedure correlated all of the measured NO(x)EI with a standard deviation of 1.02 g NO2/kg fuel that results from a curvature in the emissions index vs load data for combustor B. Removing the curvature empirically improves the combined correlation to a standard deviation of 0.28 g/kg (approximately 3.2 parts per million volume dry at 15% O-2).