OCR Text |
Show gained using gas alone; improvement was only obtained using gas/air atomization. Without swirl or with low swirl, the burner end of the furnace never heated up to the extent needed and flame stabilization was always a problem without gas atomization. Even with high swirl, gas atomization was always necessary on first changing from preheat conditions on oil to the slurry. With the slurry, however, the furnace temperature would rise further, and after about an hour it was possible to turn off the gas and maintain excellent slurry flames using air atomization alone. However, this clearly depends on there being sufficient refractory at the burner end and this is not always the case in water-cooled flame tubes in shell boilers. Development of acceptable CWS firing conditions started with the swirl burner at zero swirl. This was to provide a direct comparison with the same burner using oil under the same conditions (using the CB atomizer), and also with prior oil combustion experiments using the CB burner. To match their zero air swirl and small air angle, the small-angle ME nozzles were used, using only air atomization as installation of the tank array for supply of methane had been delayed. As indicated in Table 1, only poor flames were obtained, with poor atomization; even when the flames did not oscillate, the window of stability was so small that small changes in firing conditions (air rate or fuel rate) often resulted in blow-off. However, these experiments identified the most probably source of the problems as too low a temperature at the burner end of the furnace that could be corrected by an appropriate level of swirl. This is illustrated in Fig. 4A which compares the roof temperatures using the (variable swirl) burner with oil at zero swirl, and with CWS at high swirl. With zero swirl, the peak temperature is seen to be about 2/3 of the way down the furnace or about 2 m |