OCR Text |
Show - 9 - number of problems were identified which may be encountered in a utility application. In the condition where the sewage sludge was injected through the annulus on the outside of the coal annulus (unstaged), NOx emissions increase with increase sewage sludge, in the co-firing range of 0 - 50 o/u, as shown in Figure 8. This increase in NOx can be correlated to the fuel bOlmd nitrogen content of the sewage sludge which is of the range 3-4 % compared to a fuel nitrogen content of the coal of 1-1.5% . In the staged conditions the sewage sludge or the sewage sludge coal blend is partially injected through the centre of the burner. The effect of co-firing ratio on the NOx emission level is dependent on the injection method and injection velocity. The NOx emission level can be red uced from 1100-1800 ppm down to 500-600 ppm by burner fuel staging, as shown in Figure 8. The combination of burner fuel staging and air staging results in emission levels of 250- 300 ppm, as shovJn in Figure 9. Preliminary results have also shown that firing a premixed blend of coal with sewage sludge, can increase the NOx concentration by a factor of two, compared to a typical coal flan1e . Air and fuel staging had the desired effect of decreasing NOx with both the staged and unstaged flames. However, due to the ash content of the sewage sludge (-46 %), slagging and fouling of the tertiary air and gas injectors VJere encountered. At high co-firing ratios (50 0/0), the ra te of slag build-up on the burner head was significant Clnd in some cases caused a change in the flame shape |