(690b) Detailed Chemistry Investigation of the Effect of SO2 On NOx Formation in Fossil Fuel Burners

Environmental pollution from thermal power plants operating on fossil fuels is a matter of increasing concern. The pollutants include particulates, NOx, SOx and greenhouse gases such as CO₂. A large amount of modeling effort has been spent over the last several decades in predicting the rates of formation of these pollutants in conventional boilers. Detailed models are available for the prediction of NOx through different mechanisms. Similar models are also available for the prediction of SOx formation. However, the interaction between the two is often neglected. Since there is increasing presence of sulphurous compounds in fossil fuels (new coal finds in Assam in India contain large amount of sulphur) and biomass, a proper modeling of SOX-NOX interaction is necessary to correctly account for NOx formation in burners.

Studies by Durie et al. (1971) and Wendt and Ekmann (1975) show that the presence of SOx in the fuel may have a considerable effect on the rate of formation of NOx. Durie et al. observed the effect of SO₂ on the recombination of H and OH species in a fuel-rich propane-oxygen-nitrogen flame and proposed a three-step mechanism in which SO₂ plays the role of a catalyst in converting H to H₂ and OH to H₂O. Wendt and Ekman studied the effect of fuel sulphur on NO emissions in premixed flames and observed significant inhibition of formation of NOx by either SO₂ or H₂S doping. These observations have been confirmed by later researchers through theoretical and experimental studies. Recently, Glarborg et al. (2006) studied the interaction of SO₂ with the radical pool and observed that the interaction is more complex than previously assumed and involved HOSO and SO as well, and additionally HSO, SH and S radicals at high temperatures. They proposed a revised mechanism with new rate constants which agreed with a range of experimental results.

In the present study, the interaction is investigated in a two-stage process: a flame structure analysis is carried out with laminar flamelet model with SO₂-doped methane/air flames using the Smooke mechanism (1991) together with the sulphur chemistry of Lindstedt et al. (2006); followed by the investigation of a swirl-stabilized SO₂ -doped methane/air flame in the BERL burner which includes the detailed chemistry as well as the flow, turbulence and radiative heat transfer effects with Eddy Dissipation Concept (EDC) model. The radical concentrations obtained from these calculations have then been used to determine the NOx concentration with and without SO₂ doping. The results show agreement not only at the radical concentrations level but also in the prediction of NOx formation rates in the burner. The present framework of calculation can therefore be used to include the effect of SOx on NOx in CFD simulations of burners.