OCR Text |
Show 10/79 Fuel-nitrogen species can react in the flame by two parallel reaction paths: RN + 0X + NO (1) RN + NO •> N2 (2) where RN is some fuel-nitrogen intermediate, and Ox is some oxygen-containing species. Under locally fuel-lean conditions (stoichiometric ratio, SR>J..O) Path 1 is favored, while under substoichiometric or fuel-rich conditions (SR<1.0) Path 2 is significant. For this reason burner parameters that increase the fuel/air mixing rate also increase the fuel-nitrogen conversion to N0X. This is shown in Figure 16 where increased radial injection of the fuel, thus enhanced fuel/air mixing rates, increased the conversion of fuel-nitrogen (ammonia) to fuel N0X.1 3 In a study of residual oil combustion8 narrow angles of fuel injection were suggested to reduce fuel N0X by reducing fuel/air mixing and providing an initially fuel-rich combustion zone. Besides affecting the local oxygen concentration in the flame by changing fuel/air mixing rates, air swirl (and other burner parameters controlling mixing) affects the local flame temperature. Although fuel-nitrogen conversion to N0X is not very temperature dependent, the rates of vaporization of liquid fuels are significantly dependent on flame temperatures and this affects the release of fuel-nitrogen species,8 especially those of the more refractory type. This in turn affects the local stoichiometry around the individual oil droplets. Related to this, studies have indicated that oil atomizer types5 have a significant effect on fuel-NOx formation because the different atomizers produce different droplet size distributions. Staged combustion is known to reduce both thermal- and fuel-NOx emissions.7*12 As mentioned previously, however, staging increases flame lengths, reduces combustion intensities, and lowers flame temperatures. Flame stability, fouling, corrosion, and soot (smoke) are also potential problems with staged combustion.10 The unburned fuel, including carbon monoxide, hydrocarbons, and soot, must be burned out in the subsequent stages or zones.12 The overall excess air level may have to be increased over unstaged combustion to facilitate burnout resulting in decreased thermal efficiency. c 6-6 I N S T I T U T E OF GAS T E C H N O L O GY |