NOx formation in turbulent lean-premixed combustion with minimum heat losses

Abstract

The formation of NOx in turbulent combustion of lean-premixed methane/air mixtures and hydrogen/air mixtures was investigated at various levels of turbulent intensity using an impinging jet reactor. Much attention was paid to minimizing heat losses from the combustion field to the ambient. Experiments were carried out at atmospheric pressure and an inlet temperature of 323 K. The equivalence ratio ranged from 0.5 to 1.0 for CH4/air mixtures and from 0.6 to 0.7 for H2/air mixtures. Detailed results obtained at combustion gas temperatures (Tg) of 1390–2200 K with complete combustion are presented. It was found that a change in the level of turbulence intensity did not result in a change in NOx emissions. At Tg>1700 K. The NOx formation was contributed primarily by the thermal mechanism. The NOx emissions were proportional to the reactor residence time and sensitive to the combustion gas temperature. The NOx concentration for CH4/air can be expressed as follows: [NOx=τ(5.96×107) exp(−31,500/Tg) (ppm) where τ is the mean residence time in ms and Tg in K. At Tg<1700 K, however, the NOx concentrations were independent of residence time and were only slightly sensitive to Tg. The NOx concentrations for CH4/air mixtures were in agreement with those for H2/air mixtures. This implies that the NOx was formed by a mechanism without hydrocarbon-nitrogen path. The nitrous oxide mechanism is expected to be an important mechanism for NOx formation in the present impinging jet reactor.