OCR Text |
Show IRZ. The predominant effect of reducing swirl is to move the IRZ downstream and reduce the recirculating mass flow [19-21]. This phenomena would .enable greater coal injector insertion before complete IRZ penetration is achieved and NOx increases. When comparing flames where penetration of the IRZ occurs with the non-penetrating cases, the macromixing time scale of the secondary air with the volatile matter is different. With the coal particle trajectories predominantly on the IRZ boundary, Type 1 trajectories, the mixing between secondary air and volatile matter is a continuous process. When optimum penetration into the IRZ is realised, mixing of the secondary air with the volatile matter is delayed. A small portion of secondary air enters the IRZ by turbulent transport across the boundary, but the majority mixes with the residual volatile species on the IRZ boundary after the coal particles and subsequent devolatilisation products have exited the IRZ radially. In this context, the majority of devolatilisation products react under effectively oxygen deficient conditions within the IRZ to allow preferential reaction paths from volatile nitrogen to molecular N2 to occur. The behaviour of the oblique coal injector is shown in Curve D of Figure 10. The curve shows significantly lower NOx emissions for all coal injector locations when compared to the annular coal injection mode. The low momentum ratio and the gun design, resulted in the products spending a significantly longer time within the IRZ. It may be suggested that for the oblique coal injector complete IRZ penetration was not realised for any coal injector insertion studied. Figure 12 shows the relationship between NOx emissions and total combustible burnout, Tx, for the AASB firing Scotts Branch for both coal injection modes. Performance is compared to the results from the externally air staged burner [22, 23], and a significant improvement can be seen. Figures 13 and 14 show the performance of the AASB firing Heinrich Robert coal. NOx reduction is achievable in much the same manner as for Scotts Branch coal, indicating that optimisation of particle trajectories within the IRZ can reduce the degree of volatile nitrogen conversion to NOx. However, there is a coal type dependence. Under conditions of minimum IRZ penetration, the MVB coal generates lower NOx than the HVB coal, whereas with an optimum degree of IRZ penetration a lesser degree of NOx reduction is realised. The effect of coal type would be expected from arguments made previously. Scotts Branch coal has a higher volatile 9 |