Atmospheric fluidized-bed combustion coal feeding test program

Development of a coal feeding and distribution system is one of the major requirements for successful operation of large scale atmospheric fluidized-bed boilers. Since combustion efficiency, sulfur capture, and tube corrosion are strong functions of coal distribution in the bed, the reliability of the feed system is critical. One proposed coal feeding method involves underbed feed nozzles, and pneumatic conveying is a key element in this method. Several technical problems are inherent in such systems. One program aimed at resolving these problems involves testing a pneumatic transport system constructed to represent the branch feed lines in TVA's 20 MW AFBC pilot plant. The major objective is to determine the practical limits of such operating parameters as surface moisture, transport velocity, and coal loading. In addition, correlations are being developed to predict line pressure drop as a function of the conveying parameters. Pressure drop is important because it determines the feed system pumping costs and is a good indicator of changes in the solids flow regime. Preliminary results indicate that surface moisture is the most important factor in getting coal into the transport line. Coal with up to 6 wt % surface moisture can be fed with little difficulty, but highermore » moisture results in severe bridging and compaction in the feed hopper and feeder mechanism. Once the coal is in the transport line, moisture level does not appear to be as important. It has also been found that within the range of particle sizes studied, the percentage of fines has little effect. In fact, as long as saltation is not occurring, the conveying characteristics of a coal-air mixture seem to depend primarily on coal loading. In general, saltation and plugging will not occur as long as superficial air velocity is above 12.2 m/sec (approx. 40 ft/sec).« less