OCR Text |
Show stack is mixed with the effluent from the first stage to reduce the temperature to 1400- 1600° F, a level which minimizes thermal N O , formation. After the cooling step, makeup air is added to provide the second combustion stage which is operated under "oxidizing " conditions at 1800-2000° F to convert the excess "combustibles" to normal products of combustion, carbon dioxide (CO^), and water vapor. Figure 1. Staged. Thermal Incinerator System This approach was introduced in the early 1970s and demonstrated that N O , reductions to levels in the range of 200-250 ppmv could be readily achieved. Although this two-stage method has been successfully applied to a number of applications requiring N O , reduction, it has several drawbacks. One is its limitation on the destruction of air toxics and C O that can be achieved. The other is the higher levels of excess fuel required to achieve adequate N O , reduction. Two vapor streams are currently being vented to the two-stage incinerator. The compositions and flowrates of each are shown in Table I. W h e n the two streams are combined, there is sufficient oxygen to provide stable burning in the vortex burner of the incinerator. The unit requires about 6.81 million Btu per hour of auxiliary fuel to react with the oxygen ( Q ) , hydrogen cyanide (HCN), C O , and NQ< in the two vent streams. In order to maximize N O , reduction, a minimum of 5 0 % excess fuel is fired in the burner, bringing the total firing rate up to 10.22 Btu/hr. This results in a combustibles (CO and Hj) concentration of about 12.5% which is the driving force for N Q , reduction. 3 111-16 |