| OCR Text |
Show coal-separation cyclone. Most of the now-pulverized coal falls from the cyclone into the secondary coal hopper. The remaining very fine coal is transported along with air into the coal baghouse, where it is caught in the filter bags. The transport air escapes to the atmosphere through the vent. Pulverized coal, which has entered the secondary hopper, is stored until ready for firing, when the weight-loss screw feeder and rotary air lock are turned on to inject coal into the primary air piping. (The weight-loss screw feeder is capable of feeding material at a prescribed feed rate using electronic sensing devices built into the system.) The weight-loss screw feeder then empties material into a rotary air lock from which it is pneumatically transferred to the combustor through the primary air line. A separate, primary air fan is used for this purpose. The combustor uses a balanced-draft system, maintaining a furnace pressure of about -0.1 inch H2O. The system requires an induced-draft fan for the flue gas and a forced-draft fan for the secondary air. An additional forced-draft fan is required for the primary air to transport the coal into the combustor. COMBUSTION TESTS All of the Eastern coals tested were pulverized to about 70 percent through 200 mesh and were fired at a heat release rate of 1.25 x 106 Btu/h (95 to 125 lb/h coal feed rate). Test durations ranged from about 9 to 16 hours, depending on the individual run. Excess air levels were in the range from 15 to 25 percent. Furnace exit temperatures were maintained at about 2000°F. Slagging probe tube temperatures were maintained in the range from 800 to 1000+°F, and fouling probe tubes were operated between 900 and 1100°F. Table 6 presents the time-averaged operating conditions. The type, degree, and stage of slagging varied throughout the combustor depending on the fuel ash chemistry, local surface and flue gas temperatures, local heat fluxes, and flow stream conditions. The slagging encountered during the firing tests has been categorized into five general types which occur in various zones throughout the unit. Figure 11 shows the locations of these types of slagging, and each is identified by the letters A through E. 33 |
