Scaling of NOx emissions from a laboratory-scale mild combustion furnace

Abstract

A systematic experimental campaign has been carried out to investigate the scaling of NOx emissions from a moderate or intense low-oxygen dilution (MILD) combustion furnace operating with a parallel jet burner system in which the reactants and the exhaust ports are all mounted on the same wall. Its maximum capacity was 20 kW from the fuel and 3.3 kW from air preheat, with a turndown ratio of 1:3. The burner system was configured to achieve high dilution of the incoming reactants. A comprehensive data set comprising 191 global measurements of temperature and exhaust gas emissions is presented, together with temperature contours on the furnace centerline plane. It was found that, although heat extraction, air preheat, excess air, firing rate, dilution, and fuel type all affect global NOx emissions, they do not control NOx scaling. The combined effects of these global parameters can be ultimately characterized by a furnace temperature and a global residence time. A temperature-time scaling approach, previously reported for open jet diffusion flames, proved to be a useful tool for comparison of NOx emissions from highly diluted furnace environments regardless of the furnace/burner geometries. Regression-based predictions found the characteristic temperature to correlate with 85% of the data with an accuracy of only ±50%. The leading-order approach also showed that the jet exit Froude number is of limited value for NOx scaling in the MILD regime. Because of the weak dependence on temperature observed in the data and the moderate magnitude of the measured temperatures, it is deduced that the prompt-NO and/or N2O-intermediate pathways are of significance comparable to that of the thermal-NO pathway. The analysis also suggests that NOx formation is controlled neither by kinetics nor by mixing, and hence the conditions inside this furnace approach or span the range in which Damköhler numbers are of order unity, Da = O (1). © 2008 The Combustion Institute.