OCR Text |
Show 60 40 c o o •o _ 20 -20- -•- Ashfordby coal ~n~ Thoresby coal fuel rich pockets in the rebum zone which can improve NO reduction and prevent oxidation of nitrogen in the rebum fuel to N O. In the present system, mixing and reaction was completed within 450-500 ms. This is comparable with the residence time that needs to be available in an existing utility boiler for N O reduction by rebum. 200 400 600 800 1000 Primary zone exit N O concentration (ppm) 1200 Fig. 5. Effect of primary zone N O level on the overall NO reduction (SR1=1.01-1.03, SR2=0.94, RF=19%, Tpr=1573K) Figure 5 shows the typical effect of varying the primary zone combustion products NO concentration (measured at the exit of the zone) on the reduction in the baseline N O emission for two coals. The results show that, beyond approximately 600 ppm, the N O reduction efficiency remained constant. At very low N O concentration (<200ppm), there is a rapid decline in the N O reduction efficiency in the rebum zone, and in some cases this resulted in a net increase in the N O emission at the exit to the furnace (negative N O reduction percentage). W h e n the rebum fuel contains inherent fuel nitrogen, it is possible that when the N O concentration is low at the point at which the rebum fuel is introduced, the balance between NO-forming reactions (from fuel-nitrogen) and N O - reducing reactions (with fuel-nitrogen-derived species and other reductants) moves in favour of N O formation, so the N O reduction efficiency falls, leading eventually to the net production of N O if conditions are suitable. This may impose a practical limitation on the use of coal or oil rebum technology when combined with low-NOx burner systems that produce less than around 150-170 ppm N O . On a cautionary note, the effects of mixing at low primary N O concentrations may also be important and this is considered below. At higher primary N O concentration (>600ppm) the N O reduction efficiency remains constant with increasing primary N O . 10, 0.8- Figure 6 illustrates the sensitivity of the N O reduction to coal type. This was achieved by statistically correlating the observed NO reduction against selected coal rank indicating components at the same reburning conditions [3]. The relativity is a measure of h o w well each property agrees with the initial assumption that the N O reduction is linearly dependent on that property. The analysis emphasises volatile matter content as the most influential coal component that affects N Ox reduction in coal reburning. This is closely followed by hydrogen, which is not 0.6- > -_ 0.4- 0.2 00 Fig. 6. Relativity of coal properties towards NO reduction assuming N O reduction = a + bX where X is VM, H,C, O or N content. |