OCR Text |
Show - 8 - The helium recirculation tests indicated that the choke walls were only partly effective in eliminating tertiary air recirculation into the rebum zone. From the tests, it was concluded that the helium was recirculated from the outside of the forward flow along the furnace walls. The fraction of recirculated tertiary air decreased from 63% to 17% as the injection point was moved downstream from Pipe "1" to Pipe "4". An effective rebum zone stoichiometry, Aeff, could be estimated from the theoretical reburn zone stoichiometry, ARB, and the fraction of tertiary air recirculated. The value of Aeff was equated to the air supplied in the primary combustion and rebum zones divided by the air required to bum the coal, 'l'. The air supplied was the sum of the primary and rebum air, cl>cARB, plus the recirculated tertiary air from the first and second tertiary air injection locations according to: (1) Therefore: (2) where Mi and Mf are the fractions of tertiary air recirculated from the initial and fmal tertiary air injection pipe. The relationship between Aeff and ARB is shown in Figure 6. Along the centerline, Aeff was insensitive to the tertiary air injection location, increasing from 0.78 to 0.96 as ARB increased from 0.75 to 0.95. In the bulk of the forward flow at 50 em horizontal position, Aeff was sensitive to tertiary air injection location. With tertiary air injected through Pipes" 1 ,2", Aeff increased from 0.89 to 1.01 as ARB increased from 0.75 to 0.95. Delayed tertiary air reduced the recirculation and Aeff which increased from 0.84 to 0.99 for air injected through Pipes "2,4". Since there was a linear relationship between ARB and Aefi for a given tertiary air arrangement, either stoichiometry could be used to evaluate the effect of process parameters and mixing rate on NOx emissions. 3.1 Reburn zone stoichiometry Primary stoichiometry, Ap, and reburn fuel fraction, fRB, had a strong influence on NOx reduction. In general, these two variables could be considered a single variable, the reburn zone stoichiometry, ARB, and directly related to the NOx emissions. When tertiary air was injected through Pipes "1,2", the NOx decreased from 1050 to 350 ppm as ARB decreased from 1.1 to 0.77, Figure 7. The primary zone stoichiometry was not a sensitive parameter as long as the rebum fuel fraction was adjusted to obtain a given ARB. In practical applications, the primary zone stoichiometry may be minimized to limit the fraction of rebum fuel necessary to obtain a given ARB. |