OCR Text |
Show vicinity of 45 ft/s exist in this section when firing a typical Eastern bituminous coal at 100 lb/h and 15 percent excess air. The final stage of heat recovery is accomplished in a two-pass, tubular air preheater (air heater) included in the system to conserve heat and allow for secondary air preheat. FLY ASH RECOVERY EQUIPMENT Upon leaving the air heater, the flue gases enter a single-stage cyclone. Following the cyclone is a baghouse installed to capture the remaining superfine particles to allow for material balance calculations on the ash and discharge of clean air. The baghouse is equipped with a pressurized, air-pulse cleaning system which operates in intervals of approximately 20 seconds. Both of these systems are equipped with sample extraction valves at their bases. FOULING PROBES Three fouling probe assemblies are used to collect deposits for examination and analyses. One assembly is located in the horizontal, high-temperature gas pass. Two are located in the cooler, vertical gas pass. All three assemblies can be removed at the termination of a test for deposit examination. In addition, the last two probe banks along the flow path are equipped with refractory access plugs for quick service. Each probe bank contains three in-line, 2-inch diameter test specimen tubes situated along the centerline of flow. The test tubes extend across the height of the duct and are spaced on 3-1/2-inch centers. Mono-fin wall tubes line the flue gas duct on either side of the test specimen tubes to shield the deposits formed on the test specimen tubes from the effects of radiation imposed by the hot refractory, to give thermal similitude in the flue gas stream, and to provide a heat sink for cooling the flue gas. A schematic of the probe and the tube arrangement is shown in Figure 9. The three test specimen tubes are made of stainless steel, as are superheater tubes in a utility boiler. The monofin wall tubes are made of carbon steel, since no sampling is performed on them, and thus, material simulation is irrelevant. Each tube is air cooled in a bayonet fashion to maintain outer surface temperatures at those experienced in a utility boiler. 29 |