| OCR Text |
Show with the combustion air before it reacts with the fuel. This internal recirculation lowers the peak temperature and reduces nitric oxide emissions. The area along the firing face is perhaps most critical because it is this gas traveling along the wall, that is close enough to mix with the combustion air before it reacts with the fuel in the primary flame zone. The ratio of primary heat transfer surface to heat release by combustion can be increased by design changes such as increasing the boiler perimeter, adding division walls, or increasing the distance between the top and bottom row of burners. However, those design changes which increase the distance between burners without changing burner size may be the most effective. No satisfactory correlations between nitric oxide emissions and primary (flame basket volume) nor total furnace volume have been developed. In general, nitric oxide formation decreases as the heat release per heat transfer area decreases. However, when data are taken at different loads on the same unit to explore the effect of heat release to primary heat transfer surface, a number of other parameters are also changed (furnace volume to heat release ratio, throat velocity, and combustion air temperature). Obviously, changing furnace volume, area, or number of burner rows would constitute a major unit modification. 4-9 P-233 |
