OCR Text |
Show EXPERIMENTAL PROCEDURES AND RESULTS Operating Procedures and Conditions The initial method of feeding the coal into the primary stream was to use a venturi-type ejector placed at the end of the metering screw feeder. Difficulties were encountered, primarily with the finer grinds, because of the agglomerating properties of the fine ground coal. This system was then replaced by a small, high-pressure fan. The air inlet to the fan was placed at the end of the coal metering screw feeder, and the coal/primary air mixture passed through the blower, as shown in Figure 3. This system overcame most of the difficulties with feeding the very fine ground coal, but some transport problems continued (primarily low-frequency pulsations). Natural gas was fired in the furnace during preheating of the furnace and during periods when coal was not used to maintain the furnace near thermal equilibrium. To ignite the coal and to maintain a flame in the furnace during periods of irregular coal flow, which occurred at start-up and shutdown, natural gas was simultaneously fired with the coal. During each combustion trial, the flame was examined visually for pulsations and flame stability. After the nozzle position was adjusted to optimize flame geometry and stability, the input conditions were set. The goal of these adjustments was to achieve a good balance of minimum flame length with the desirable amount of excess air. For all tests the excess air was near 20% with 3.4% to 3.9% oxygen in the flue gas. After the desired furnace and flame conditions were reached, a detailed flue gas analysis was taken to determine flue oxygen and to check for evidence of incomplete combustion in the form of carbon monoxide. If the CO levels were in the range of 100 to 500 ppm, the furnace was then allowed to achieve thermal stability so that reliable data could be collected. After stability was reached, a complete gas analysis (02, C02, CO, NO NO ) of the flue products was taken. Care was taken to assure that the furnace was under slight positive pressure to eliminate the 20 |