OCR Text |
Show Figure 11 shows a range of coal-particle trajectories that can be generated by the annular coal injector. With the annul ar coal injector at or within the burner throat and moderate momentum ratios (MR ~ 0.1), trajectories such as 1 and 2 shown in Figure 11 would predominate. This interaction would be expected to result in high NOx emissions as the devolatilisation products would react in an oxygen rich environment within a high mixing intensity region on the IRZ boundary. Also, when the injector is located within the burner throat, a degree of premixing between the coal and the air occurs. Progressive insertion of the coal injector would encourage coal penetration into the IRZ, resulting in predominantly Type 3 particle trajectories. For these trajectories, the devolatilisation occurs primarily within the IRZ where the oxygen-lean environment would promote preferential reaction paths from volatile nitrogen species to molecular N2. When coal particle penetration into the IRZ is promoted, the combustion the non-nitrogen containing volatile matter may also be restricted. Excessive coal injector insertion would cause complete IRZ penetration and Type 4 particle trajectories (Type 1 flames) . Complete penetration would cause a lower residence time for the devolatilisation products within the IRZ and the possibility of some "breakthrough" of volatile species to be oxidised downstream of the IRZ. In this case a higher NOx emission would result than if partial penetration was occurring due to the higher oxygen concentrations downstream of the IRZ. Thus, Type 3 particle paths are desired for minimum NOx from this burner. This behaviour is reflected in the data in Figure 10. Comparing Curves A and B illustrates the effect of increasing HR on the particle trajectories and NOx . With the injector at the burner throat, increasing HR would be expected to entrain more secondary air into the coal jet and result in particle trajectories that penetrate further into the IRZ. Although these two effects counteract each other, in terms of oxygen availability to the volatile N2, the air entrainment seems to dominate and NOx values are higher for Curve B. For high HI, Curve B, full penetration of the coal jet occurs at a lesser gun insertion than for the low HI case. Curve C from Figure 10 shows the case where HR is 0.22 with effectively matched primary to secondary air velocity ratios and reduced swirl level. With the coal injector situated within the burner throat, the mixing between the coal jet and the secondary air is reduced because of the reduced swirl, and therefore lower NOx emissions result. As IRZ penetration is promoted by injector insertion, NOx is reduced due to enhanced devolatilisation within the 8 |