Prediction of unburned carbon in bottom ash in terms of moisture content and sieve analysis of coal

Abstract

Pulverized fuel combustion should be efficient in terms of power generation, resulting in less unburned carbon (UBC) in ash. In this study, three types of samples from stockyard, coalfeeders to mills, and bottom ash (BA) were taken from a local thermal power plant for 12days. Proximate analysis of stockyard coal (sub-bituminous) resulted as 9.57% ash, 37.14% volatile matter, 52.91% fixed carbon, and a gross calorific value of 6640kcal/kg. Petrographic analysis of coal showed that more than 80% consisted of reactive macerals. Coal from coalfeeders was also analyzed in terms of moisture content and sieve analysis. BA samples were analyzed and UBC content of BA over a 12day period changes between 0.49 and 0.71% for Unit 1 and 0.26 to 0.69% for Unit 2. A relationship was seen between the moisture content and size distribution of coal from the mill inlet with UBC in the BA. A modified population model was used to estimate UBC as a function of size distribution, moisture content, mill environment constant, boiler environment constant, and residence time. A population model was used because the grinding and combustion operations are similar in terms of formation of new particles. It is seen that the model proposed is in agreement with the experimental results. In conclusion, in order to lower the amount of UBC in ash, the size distribution and moisture content of feed coal to mills should be stabilized.