OCR Text |
Show -13- Conclusions This study has examined the effect on NOx and burnout with an Internally Fuel Staged Burner fired with a primary fuel (coal) and reburning fuels of natural gas and coal. The burners have been investigated using input parameters of the primary zone stoichiometry, Ap, Reburn fuel fraction, Rff, (hence ARB), reb urn fuel type, Reburn fuel velocity, Rfv, and tertiary air staging through a number of channels located on or around the burner and as overfired air. A review of the results has shown that internal fuel staging with the combination of air staging significantly reduces NOx, compared to the baseline condition with 100 % coal firing, whilst still maintaining acceptable levels of burnout. With Gottelborn or El Cerejon coal as the primary fuel in the AP26 burner, and natural gas as the reburn fuel, optimised conditions were found (stable flames with low NOx concentrations and burnout levels> 99 0/0) when Ap = 0.9, Rff was in the range 20 % < Rff < 30 % and Rfv was > 300 m/ s. With these settings and the tertiary air introduced as overfired air (location 4) NOx levels were reduced by ,..., 75 %. Varying the Reburn fuel velocity in the range 56 m/s < Rfv < 100 m/s with all coals as the primary fuel and natural gas or coal as the reburn fuel, had very little effect on overall NOx concentrations. NOx levels were however,... 30- 50 ppm lower when the Rfv was> 100 m/ s. Overall, the results show that using Gottelborn as both primary and reburn fuel, produces lower levels of NOx compared to similar conditions with El Cerejon coal. Varying Ap in the range 0.45 < Ap < 1.15 showed that NOx levels could be reduced by ,..., 40 % with tertiary air injected through the annulus but there was no significant difference when injected as overfired air. Using Middelburg as the primary and reburn fuel, resulted in an increase in NOx levels compared to the results with Gottelborn and El Cerejon coal. In order to optimise the burner for operation with a medium volatile coal as the primary fuel, it is proposed that a higher volatile fuel is used as the reburn fuel. In general, a high volatile reburn fuel results in lower NOx emissions due to preferential burning of the higher volatile fuel. As a means of improving the burnout characteristics and NOx reduction potential of the medium volatile coal, it may be necessary to fire the coal with a much finer grind. |