OCR Text |
Show - with excess air levels greater than approximately 10%,all burnout levels are excellent and generally speaking independent of atomiser type or firing density; - below 10% excess air there is a clear fall off in burn-out with decreasing excess air but even at 5% excess air almost all results show total burn-out greater then 98%; - as was found previously, results were more consistent using the Y-jet atomiser. Measurements of total radiative heat flux at the furnace wall for a firing density of 0.36 MW/m3 are presented in figure 9, which includes results for a reference oil flame. Relatively speaking these results are very similar to these obtained at the low firing density in that: - compared to HFO,similar profile characteristics could be obtained with COD; - the most comparible results were COD fired with the internal mix atomiser and HFO fired with a Y-jet atomiser. 3.3 Effect of preheat With these tests, all results were obtained at a firing density of 0.36 MW/m3 using some combinations of two different sized burners and two atomiser types. Comparisons were made of 200 °C combustion air preheat against ambient temperature air. Figure 10 shows the benefits gained by using preheat in terms of total burn-out and with the 2 MW burner,indicates that at 5% excess air very acceptable burnout levels may be obtained with either atomiser type. Also shown on figure 10 is the result that the decrease in combustion efficiency due effectively to turndown may be well compensated by employing combustion air preheat. This is evidenced by the data obtained using a nominal 4 MW burner fired at 2.3 MW. 3.4 NOx emissions No attempt was made during the trials to explore the possibility of reducing NOx emissions or to study systematically, the effect of the combined combustion of pulverised coal and heavy fuel oil upon NOx emissions. However, measurements were made regularly of NOx concentration in the exhaust ducts of both furnaces and some results are presented here in figure 11. No real conclusions may be drawn from this information concerning the effect of the coal particle size. It is of interest to note that considering the average nitrogen content of the fuel was 0.8% (coal nitrogen 1.7%; oil nitrogen 0.2%), the conversion of fuel nitrogen in the coal appeared to be somewhat low. For example firing a pulverized coal of nitrogen content, 1.1%, in the same furnace at a lower overall firing rate (2.5 MW, 0.1 MW/m3) can give rise to flue gas NOx emission levels greater than 700 ppm (0% O2) 0^D" Increasing the firing density to 0.36 MW/m3 had the effect of increasing the NOx emissions significantly (on average by about 100% at 10% excess air), but there was no significant effect due to preheating the combustion air. 15-10 |