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Show I F R F D o c N o K 7 0 / y / 1 13 IJmuiden, April 1997 10- A F R C Spring meeting 1997 For these flames, N O x increased with the increase in sewage feed, particularly in the range of 0-10 % sewage. At higher fuel ratios and with the high volatile coal (Gottelborn), N O x decreased with minima emissions occurring at co-firing ratios of 20-30 %. With the medium volatile coal (Med Mac), N O x was observed to increase with the increase in the co-firing ratio inespective of the sewage velocity. This suggests that the devolatilisation rate of the sewage is higher than that of the coal and that the high-nitrogen containing volatiles are converted into N O x . At the 1 0 0 % sewage firing rate, where burner considerations only allowed the particles to be introduced through the centre of the burner at low velocity, N O x emissions are similar to those obtained with 1 0 0 % coal firing, (918mg/m3 0 ( 6 % 02)). This confirms the prognosis that sewage devolatilisation in an oxygen lean environment delays oxidation of the volatile nitrogen species, resulting in lower N O x , as the preferential reaction pathway is towards molecular nitrogen. 3.2.2 The effect of burner and furnace air and fuel staging As expected, air and fuel staging had the desired effect of decreasing N O x with both type I and type II flames. With furnace air staging, on type II flames, N O x could be reduced by - 80 % with no significant effect on burnout. Incorporating furnace air staging and external air staging with type I flames, Figure 3.7, shows that the lowest levels of N O x were achieved when X^, was ~ 0.8 (470 mg/m3 0 (6 % 02)). 1500 1250 ~ 1000 o (0 ~ 750 80% GB / 20% Sewage O z 500 250 o GB (annulus) / Sewage (centre -19 m/s) -o- GB (annulus) / Sewage (centre - 87 m/s) -A- GB (annulus) + Sewage (centre) - mixed • EAS GB (annulus) / Sewage (centre - 87 m/s] 0< «- 0.8 0.9 1.0 1.1 Primary Stoichiometry 1.2 Figure 3.7 The Effect of Air Staging on Type I Flames with Gottelborn Coal The results from introducing natural gas through the centre of the burner with a 50 %/50 % and 20 %/80 % sewage/coal flame are shown in Figure 3.8. The principle of operation was to penetrate the LRZ of the primary flame leading to a high heating rate of the reburn fuel and a high mixing intensity of the primary flame products with the reburn fuel at the LRZ closure. |