Effect of additives on sintering of lignites during CFB combustion

Fossil fuel lignites are used widely for power generation employing CFB combustion. In CFBC, agglomeration due to sintering, contributes to loose deposit formation. Rapid sintering causes choking and blocking, which lead to shut down of the unit. A breakthrough has been achieved by averting the chocking and blocking inside the CFB steam generator by shifting the sintering pattern of ash, either to a lower or to a higher temperature, from the CFB operating range of 800-900 degrees C. Lignite sintering is characterized using the heating microscope. The addition of chemical modifiers to the fuel, alters the ash chemistry, and reduces the rate and the quantum of shrinkage. Sintering is examined for eight samples; six Indian lignite samples, one southern hemisphere bituminous coal and anthracite. The effects of adding silica, calcium and alumina on the sintering tendency and the efficacy in reducing that tendency are brought out. With an economic quantity of 5-10% by weight of the selected additive, which are available in abundance at relatively cheap cost, peak sintering temperatures could be either increased by 30 degrees C or lowered by 60 degrees C. Reductions up to 30% in the rate of shrinkage (%/degrees C) and 10% in the quantum of shrinkage could be achieved. Thus, the sintering start and end temperatures could be managed well and the operating temperature of CFB maintained. This novel technical research was demonstrated on utility scale lignite fired 2 x 125 MWe CFB steam generators wherein the sintering issue and the blocking of dense phase at cyclone outlet faced were successfully resolved with the addition of silica additive and the units started operating successfully with high availability now.