In high capacity coal fired boilers, the total losses account to about 12 to 14%, i.e. 86 to 88% boiler efficiency. Roughly 50% of the losses can be tuned to the optimum and the other 50% is governed by fuel properties like hydrogen in fuel, moisture in fuel, and ambient air conditions.
Coal Boiler efficiency is mainly depended on the amount of losses in the system. In high capacity coal fired boilers the total losses account to about 12 to 14%. Roughly 50% of the losses are governed by fuel properties like hydrogen in fuel, moisture in fuel and ambient air conditions. The other 50% losses are carbon loss and dry gas loss.
The best efficiency in the boiler can be achieved if the losses are kept to the minimum. Since 50% of the losses are dependent on the fuel and ambient condition, the best efficiency can be achieved by properly tuning the other 50%, i.e. mainly carbon loss and dry gas loss.
Carbon loss is due to unburned carbon in fly ash and bottom ash. Normally the unburned in bottom ash in the pulverised fuel firing are higher sized particle and of higher specific gravity having the unburned macerals like Fusinite embedded in mineral matter and the low melting constituents encapsulating the reactive maceral. Fly ash of pulverised coal firing system is around 80 to 90 percent of the total ash removed. The normal types of unburned are inert macerals, cenospheres, and carbonaceous clay.
Factors affecting carbon loss are
Coal rank and quality
Coal Petrographic characteristics
Characteristics and quantum of carbonaceous shale
Presence of low melting inorganics in coal ash
Residence time available for combustion in furnace
Type and number of burners
Type of milling system and primary air control system
Fineness of pulverised coal - Percentage of coarser particles
Primary air to secondary air ratios
Excess air at the burner/furnace and distribution of air into the burner/furnace
Burner Tilt (If provided).