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Running a Coal Fired Boiler at the Best

Efficiency

Written by: Dr V T Sathyanathan • Edited by: Lamar Stonecypher Updated Apr 5, 2010 • Related Guides: Coal Fired Boilers | Fuel 

In high capacity pulverized coal fired boilers, the total losses account to about 12 to 14%, i.e. 86to 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 airconditions.

Boiler efficiency is mainly depended on the amount of losses in the system. In high capacitypulverized 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 beachieved by properly tuning the other 50%, i.e. mainly carbon loss and dry gas loss.

Carbon 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 gravityhaving 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 isaround 80 to 90 percent of the total ash removed. The normal types of unburned are inertmacerals, cenospheres, and carbonaceous clay.

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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

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  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.

Dry gas loss 

The dry gas loss accounts to about 40% of total losses. Dry gas loss is due to the heat carriedaway by the flue gases leaving the boiler. Today many of the boiler designers reduce this loss by

recovering the heat after low temperature superheater by larger economizer and air pre-heaters.

As designers go for lower and lower exit gas temperature the size of air pre-heaters go up.

Boiler tuning for optimal Dry gas

  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 economiser & air-pre-heater.

Minor losses 

There are a few minor losses in boiler which can also be tuned are the mill reject loss and the

carbon monoxide loss. The mill reject loss depends on the amount of pyrite in coal and the mill

operating condition. The rejects by the mill do have a heat value which is accounted as millreject loss. This can be minimized by properly maintaining the mill cleanaces and operating at

the optimal condition. The carbon-monoxide loss is due to combustion not being fully

completed. However small these losses are, the present environmental condition requires these

losses also to be kept to the minimum.

Other losses like loss due to moisture in fuel, air moisture loss, loss due to hydrogen in fuel, etc

also get affected by change in exit gas temperature, but the major impact on these losses is byfuel and ambient property. When dry gas loss and carbon loss are kept to the optimum level then

the boiler is operated with its best efficiency.