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
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