Summary

Flameless oxidation (FO) is proving to be an effective method for the reduction of thermal nitrogen oxides (NOx) emissions and for improving combustion efficiency in high temperature thermal processes. The mathematical modelling of this combustion concept and its application to industry combustor design and analysis is scarce. This paper reports a straightforward combustion model that enables the prediction of the general combustion and NOx pollutant characteristics of the partially premixed and diffusion flame conditions pertaining to FO, which can be conveniently deployed in a computational fluid dynamics (CFD) technique. The methodology is successfully validated against three experimental studies. It is found that an extinction criterion is necessary for good simulation of the ‘classical’ FO condition where the separated fuel and oxidant combustor entry streams have high momenta. Where a pre-combustor is deployed, allowing FO to be achieved at lower injection momenta, a simple global reaction serves. Radiative losses from the reaction zone are larger than for conventional flames because of the high recirculation ratio of high emissivity product gases. These losses significantly reduce the combustion temperature and so the NOx.