| OCR Text |
Show 7 quality were gained by insulating the cold metal surfaces of the quarl with refractory material and by moving the fuel gun laterally deeper into the combustion chamber so as to increase the view angle between the injected fuel and the flame and hot refractory surfaces of the combustion chamber. 3.3 Atomization A number of atomizers were used in the preliminary experiments. They included both the "internally mixed" types (the Peabody atomizer), and "externally mixed" types (the modified forms of the Atlantic Research and Sonicore nozzles). In the other two designs used (the 'Y'-jet and the IFRF atomizers) the fuel and atomizing medium impinge internally, but these nozzles cannot be classified as internally mixed. Drawings of all five of these nozzles are shown in Figure 6. Using a laser-diffraction measurement technique (Swithenbank et al., 1977) a series of laboratory experiments was carried out to measure the droplet size distribution of coal water mixture sprays produced with different atomizers at varying pressure ratios of fuel and atomizing fluid. The results of these studies showed that mean droplet size decreases with increasing atomizing fluid to fuel pressure ratio and with decreasing fuel input for a given atomizing fluid to fuel ratio. Details of this study have been reported elsewhere (Beer et al., 1983)• Vhen the atomizers were used in the hot furnace environment some of them (particularly the 'Y'-jet and the IFRF internally mixed atomizers) were found to block after a short period of operation. Apparently the walls of the conduits through which the CVS passes have to be "lubricated" by an annular boundary layer of water and if the water dries out the nozzle is liable to block. It should be noted that, due to the relatively small flow rate through the CRF nozzles, the experimental conditions were more critical than they would be in a utility boiler scale atomizer. Also it was found that at very high solids concentrations, around 70/6, slight variations in the concentration due to small amounts of water evaporation can greatly increase the apparent viscosity of the mixture and this effect may have contributed to blockage. It was also evident from the experimental observations that a favorable design for a CVS atomizer nozzle would have to be based on the unobstructed straight flow of the fuel up to the point at which it is impinged upon, either internally or preferably externally, by the jets of the atomizing agent (air or steam) flow. Vhile the design principle of the Atlantic Research Corp. (or the 'modified' Sonicore) atomizer is most attractive, particularly from the point of view of trouble-free operation without blockage, the version employed in the CRF tests (Figure 6d) yielded a spray that was unacceptably narrow. The high momentum narrow spray of fuel appeared to pass beyond the internal recirculation zone of the combustion air jet, making flame stabilization virtually impossible after natural gas assistance was shut off. Acceptable continuous atomization in the hot furnace was achieved by the Peabody internally mixed atomizer. The atomization quality was observed to improve when the pressure of the atomizing air was increased |
