OCR Text |
Show INTRODUCTION Reburning is an NOx control technology which uses fuel to reduce NO.1-4 Thus, the main heat release zone can be operated normally to achieve optimum combustion conditions without regard for NOx control. A schematic of the reburning process is shown in Figure 1. With reburning, a fraction of the fuel is injected above the main heat release zone. Hydrocarbon radicals from combustion of reburning fuel react with nitric oxide to form molecular nitrogen. This process occurs best in the absence of oxygen. Thus sufficient reburning fuel, between 15 and 20 percent of the total heat input, must be added to produce an oxygen deficient reburning zone. Subsequently, air is provided to combust fuel fragments which remain at the exit of this zone. Since reduced nitrogen species NH3 and HCN are also present, air addition allows a further NOx reduction. Previous studies have shown that 60 percent reduction in NOx emissions can be achieved with natural gas reburning. 5 This is considerably less than the limits imposed by thermodynamics on NO formation under fuel-rich conditions. Figure 2 is a plot of equilibrium levels of total fixed nitrogen species (HCN, NH3, and NO) as a function of stoichiometry and temperature. Except for high temperatures (greater than 31000 F) and extremely rich conditions (SR<0.5), the equilibrium levels are less than 10 ppm. Equilibrium considerations would dictate operation of a combustor at moderate temperatures under rich conditions such as the reburning zone or low temperatures near stoichiometric conditions. The purpose of the studies described in this paper was to define the chemical constraints which prevent 85 percent NOx reduction with reburning. Tests were conducted in a bench scale tunnel furnace to independently examine the formation of N2 in the reburning zone and the conversion of XN species in the burnout zone. Detailed measurements were performed at the reburning zone exit to characterize unburned hydrocarbons, CO, NO, NH3 and HCN. The influence of the concentrations of reactive species was examined as were temperature effects for both the reburning and burnout zone. 1 |