OCR Text |
Show 10/79 temperature will increase thermal-NOx levels. Very high-excess air combined with good fuel/air mixing can result in reduced thermal NO if this lowers peak flame temperatures.1 Fuel/air mixing rates can be increased by a) increasing air swirl up to a point, b) increasing the difference between the fuel and air momentum, c) increasing the divergence angle of the fuel nozzle, and d) increasing the level of oil atomization. Also, flame retention devices (for example, bluff bodies in the flame-flow field) used to promote flame stabilization over a wide range of operating conditions can promote thermal-NOx formation.^ Air swirl affects the fuel/air mixing rate by increasing the shear between the fuel and air streams and producing reduced-pressure zones. At high levels of swirl, recirculation flows are formed due to this reduced pressure that cause combustion products to be recirculated back into the combustion zone. These recirculation zones often help stabilize the flame and may increase or decrease thermal N0X depending on the temperature and magnitude of the returning flow.8 If relatively cool combustion products are recirculated they can dilute the fuel/air mixing, reduce the peak flame temperatures, and reduce thermal-NOx formation. This is a form of "internal" flue-gas-recirculation.9 Figure 4 shows the types of flames produced by different swirl levels. Type I flames have moderate flue/air mixing combined with a toroidal recirculation zone around the fuel jet. Type II flames have very good fuel/air mixing and high peak flame temperatures. They occur when high swirl is combined with low fuel momentum. Type III flames occur when there is no swirl or very low swirl with high fuel momentum. For this type of flame there can be good fuel/air mixing at the base of the flame with high peak flame temperatures. The Type I flames with moderate mixing and good recirculation give the lowest NO emissions while, the Type III flames with little or no recirculation at the base of the flame give the highest NO emissions. Internal flue gas recirculation is less effective if the furnace walls are very hot and the recirculated combustion products are thus still quite hot.12 6-2 I N S T I T U T E OF G A S T E C H N O L O GY |