OCR Text |
Show Reduced Air Preheat (RAPH)- The normal combustion air temperature for units with air preheat is in the range of 450 to 650°F. If this air temperature is lowered by removing baskets from the air preheater or by diverting a portion of the combustion air around the air preheater, then the combustion temperature is lowered by a proportional amount which reduces the formation of thermal NO. The reductions in emissions which have been measured on steam generators vary from 10 ppm/100°F to 75 ppm/100°F. However, the combustion air preheat cannot be decreased without incurring severe fuel penalties, as the stack losses necessitate about a 2% increase in.fuel flow per 100°F drop in air temperature, neglecting any losses due to increase in air flow or hopper recirculation rate needed to maintain design steam temperature. In addition, bypass RAPH tests on steam boilers have shown that there are problems associated wtih thermal stratification, which could interfere with the effectiveness of the technique. These disadvantages make RAPH a relatively unattractive method for reducing NO emissions. Water Injection- Injecting water into the combustion air downstream of the air preheater reduces the combustion air temperature and the NO reduction effect is similar to FGR and RAPH. However, for the same reduction in combustion air temperature as reduced air preheat, water injection has been found to be about twice as effective in reducing NO emissions. This increased effectiveness is attributed to the additional mass and higher heat capacity due to the additional water vapor in the combustion air. Water injection has two disadvantages. There is loss in efficiency of about one-half that of reduced air preheat for equal reductions in NO emissions. There is also a tendency, when using water injection, towards an increase in the smoke threshold of the unit and a loss of burner flame scanner signals, particularly at high injection rates. 4-7 P-233 |