OCR Text |
Show -5- Fuel volatility has an impact on the availability of fuel ~ the rebU~:~:~ and therefore the evolution of radical species. A high volatIle coal w~ ti n nitrogen content has, in theory, the ability to maximise overall NOx re uc 0 levels. Fuel nitrogen has been shown to increase the level of reactive nitrogen in the rebum zone, and therefore has an adverse effect on the coals ability to reduce NOx· Results from the AP26 investigation Baseline flue gas emissions for type II and type I flames [la, 11] ~ith Gottelborn coal and secondary air swirl numbers (So') of 1.0 and 0.6 are gIven below. The type I flame was created by axially inserting the coal gun into the quarl. These baseline flames were produced at a coal firing rate of 100 0/0, and with all the combustion air introduced as secondary air through the moveable block swirler. This data provided a bench mark by which the performance of the IFSB could be directly compared. Type II Type II [So' 1.0] [So' 0.6] 0 2 0/0 2.96 2.94 NOx [ppm @ 3 % 02] 926 868 CO ppm 5 7 CO2 0/0 15.67 15.69 502 [ppm @ 3 % O2] 653 661 Gottelborn as primary fuel and Natural Gas as reburn fuel Effect of tertiary air position on NOx Type! [50'1.0] 2.99 505 5 15.67 664 Figure 5 shows the effect of the tertiary air position on NOx. Position a represents the NOx concentration without tertiary or rebum air, i.e. 'standard' burner with a typical type II classification flame. Positions 1 (annulus), 2 (internal ~ith 16 pipes), a~d 3 (external with 8 pipes) relate ~o the position of the tertiary aIr that forms an Integral part of the IFSB, refer to FIgure 1 for clarification, and 4 is the overfired air also shown in Figure 1. The performance of the IFSB relative to the 'standard' burner shows that injecting natural gas as reburn fuel through the centre of the primary coal fla results in a decrease in NOx irrespective of the location of the tertiary air. Fur: e , reductions were achieved when the tertiary air was located at position 3 or 4. ~r best performance was achieved when the tertiary air was injected as over fired .e resulting in a 78 % reduction in NOx. aIr Another contributory factor to these low NOx levels was the p . stoichiometry. Figure 5 sho~s the effect of the primary stoichiometry o~~d with the reburn fuel fraction (Rff) constant at 20 % of the total fuel' x resulting in a reburn fuel velocity (ruv) of 58 m/s. Over the range of ~put, stoichiometries studied it is evident that there is no significant differenc p.n~ary when the tertiary air was located at position 1 or 2. With the tertia em. Ox ry aIr at |