The efficiency of a coal-fired boiler depends on the loss of the boiler, 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.

Factors affecting carbon loss

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)

The factors having the greatest influence on carbon loss are the boiler type, volatile matter content in the coal, furnace heat loading (heat input/furnace volume), mass fraction of pulverised coal smaller than 75 mm and excess air

Factors affecting dry gas loss

Keep an optimal excess air level, generally about 20% in a coal fired boiler

Tune combustion of coal to the optimal condition

Understand the coal being fired

Proper distribution of combustion air at primary, secondary, and tertiary levels

Keep the required fineness of coal about 75% through 200 mesh and less than 2% on 50 mesh sieve

Keep the maximum mill outlet temperature to reduce air bypassing air pre-heater

Soot blowing the entire heat transfer surface at an optimal frequency

Minimize the air in leakage to the boiler

Dry gas loss is also influenced by the economizer feed water inlet temperature and the ambient air temperature, which change the heat recovery pattern of economizer & air-pre-heater