OCR Text |
Show the nitrogen as N 0 2 , necessarily to the exclusion of N O , is minor, i.e. less than one percent of the quantity of N O molecules. The error for quantities of combustion air different from 110 percent, i.e. boiler outlet oxygen concentration different from two percent, is in the mixture species concentration change only, 1000 300 600 900 TEMPERATURE 8C Figure 7. ULTRA HIGH TEMPERATURE NOx. 1200 provided the mixture is not substoichiometric. An important objective in flame design is to cause the conversion of F B N to N O x to be as inefficient as possible. Inefficiencies of seventy percent, i.e. thirty percent yield of N O x from FBN, are now being realized. The Total NOx curve is most useful because it indicates the practical band of total N O x expected at the boiler outlet including all N O x mechanisms. The total N O x curve and the 100 % F B N conversion curves are not directly comparable because the 100 percent line is fuel derived N O x only. Low excess air burners are capable of substantially reducing F B N N O x yield. Properly designed flames produce the most inefficient conversion of F B N yields, reducing total N O x especially when the outlet oxygen is within the more steeply sloping region of the curves in Figure 6. The upper portion of the band is populated by many of the currently available low N O x , low excess air burners. Those burners equipped with flames designed by analysis are along the lower edge of the band. In practice, the measured N O x from flames having only a few percent oxygen exiting the reaction zone is almost totally N O . N 0 2 is typically only a few ppmv or less as the flue gas leaves the furnace. .01 .1 Figure 8. NOx DEPENDENCY on FBN. The data included in these plots encompasses a wide range of fuels. Typical values of F B N fuels illustrate yield efficiency. A high nitrogen oil containing 0.30 mass percent nitrogen with 100 percent atomic conversion in the furnace yields 390 ppmv NOx. A low nitrogen oil containing 0.02 mass percent nitrogen with 100 percent atomic conversion yields 26 ppmv N O x . Natural gas containing, for example, ten mole percent molecular nitrogen, but with concentrations of nitrogen atoms chemically bound to carbon and hydrogen not greater than a few parts per billion by volume, yields no N O x from fuel bound nitrogen. The addition of F B N to a fuel which previously contained no FBN, produces N O x equal to the F BN conversion efficiency times the atomic nitrogen content. Prior to the addition of FBN, the actual total N O x yield is the sum of the thermal and prompt NOx. The addition of F B N to the fuel results in some F B N N O x yield from the flame, but in a precisely designed flame it precludes a portion of the thermal and prompt N O x yield. The total yield of the thermal, prompt and F B N N O x is less than the sum of N O x yield from the fuel devoid of FBN, added to the actual inefficient quantity of F B N N O x yielded alone. The low temperature F B N N O x reactions preclude an increasing portion of the thermal N O x as F B N concentration is increased. |