OCR Text |
Show Table I Vent Stream Flows and Compositions Component N, o, CO, CO H,0 NO NO, HCN Flow (SCFM) Stream 1 17.50 1.00 14.00 7.50 25.00 20.00 15.00 250 ppm 250 Stream 2 65.40 18.80 7.56 0.73 7.50 0.01 0.01 1000 This high level of combustibles is advantageous for N O , reduction, but it can create some problems. First, the formation of C O and Hj by the dissociation of CH« is an endothermic reaction and tends to lower the operating temperature in the reaction zone. Also, since the rate of reaction of the combustibles with the N O , is a function of temperature (as the temperature is lowered the reaction rate decreases), the lower temperature resulting from the dissociation of excess fuel somewhat offsets the benefit of the higher concentration of the reducing reactants ( C O and Hj). Second, the combustibles that leave the reduction section must be converted to products of combustion in the oxidation section. As the level of combustibles increases, more air and recycle to provide "burn out" of the combustibles and control of the oxidation chamber temperature are required. These high flowrates can result in a low residence time in the oxidation zone that is not sufficient to achieve the required level of C O oxidation. The third potential problem with high excess fuel operation is the tendency to form H C N and NH3 under reducing conditions. As the temperature is lowered and the excess fuel is increased, formation of these byproducts is favored. This, coupled with low residence time in the oxidation section prevents adequate destruction of H C N and C O . The plant has experienced both high levels of C O (1300-1700 ppmv) and appreciable quantities of H C N (50-100 ppmv) in the stack gases which vent to the atmosphere. Although the high levels of H C N have been attributed to the fact that H C N (-200 ppmv) enters the staged, thermal incinerator in the vent streams and are probably not generated at these levels in the reduction chamber, the same conditions which lead to H C N formation also will lead to minimum H C N destruction. The high levels of C O in the stack are required to maintain N O , levels in the stack below 400 ppmv. 111-16 |