OCR Text |
Show The measured furnace exit N O concentration was around 215ppm for the case shown in Fig. 9, which represents a 5 5 % reduction of the primary N O . Comparing the predicted N O profile assuming N O reduction by H C N and char only, shows a similar trend to the experimental observations, but the quantitative agreement is poor. Including N O reduction by CH (HCN+CH+char) produces better quantitative agreement but only and the rate data of Chan et al [19] are used for heterogeneous N O reduction. The under-prediction at the furnace exit may also result form the fact that N O reduction mechanisms involving C O and soot have been ignored in the present model. The sensitivity of the predictions to total volatile yield and devolatilisation rate are compared in Fig. 9 for Rv = 1.1 and 1.32, corresponding to low and high volatile yields respectively. The higher volatile yields result in increased C H production and consequently increased N O reduction. Assuming a faster devolatilistion rate (1.8 times the previous value for R v =1.32) improves the accuracy of the prediction near to the coal injection point, where the volatile effect is greatest. Rapid devolatilisation can increase the extent of N O reduction since the reductants evolved with the volatile gases are able to disperse faster into the surrounding gas and consequently into a higher N O concentration region. Overall, using a value of 1.1 for Rv gives good agreement for this coal except that the N O concentration towards the exit of in the burnout zone is over-predicted, a consequence of the relative simple char-burnout model and kinetics used for this zone. From the theoretical calculations, the relative contribution of each of the major NO-reducing mechanisms towards the total reduction can be assessed. Hydrocarbon reduction by CH fragments accounted for 7 0 % of the total N O reduction, the fuel-derived H C N around 10%, and char particles around 2 0 % when using the NO/char rates determined by Chan et al [19]. 450 E CL Q. | 300 "to i_ c: <D O c o o O 150 0 0.0 0.5 1.0 1.5 2.0 2.5 Axial distance, m Fig. 9 Sensitivity of predicted NO concentration profiles to NO reduction mechanism and model devolatilisation parameters for Pittsburgh No.8, SR1=1.03, SR2=0.89, SR3=1.08, RF=0.31,andTpr=1300C. J I i I , L |