What Are The Differences Between CFB Boiler And PC Boiler
Circulating Fluidized Bed combustion is a comparatively new technology that has given boiler and power plant operators a greater flexibility in burning a wide range of fuels. While the pulverized coal-fired boiler (PC boiler) burns very fine coal powder or coal dust as fuel. In this paper, we will discuss the differences between the CFB boiler and PC boiler.
Coal size required in boiler
A pulverized coal boiler requires very fine coal particle size. The fuel burnt in pulverized coal boiler is coal that is pulverized in a coal grinding mill. Pulverized coal is often conveyed to the burners in boiler through pneumatic means (by forcing coal particles by using moving air) from the coal pulverizer mill.
But for a CFB boiler, it requires the coal fuel should be crushed to sizes less than 10 mm. The time energy and facility required to crush coal is much lesser than pulverizing coal.
Combustion temperature
Coal in PC boiler is combusted at temperatures of about 1600-1800 deg C. At such high temperatures, a notorious pollutant NOx is formed. SOx pollutants are carried away in flue gases, which condense to form highly corrosive sulfuric acid at temperatures around 140-160 deg C. Thus, the exhaust temperature has to be maintained above this temperature to avoid large-scale corrosion of boiler water tubes. This limits the boiler energy extraction efficiency and constitutes the largest heat loss from the boiler.
Coal in CFB boiler is mixed with sand and limestone/dolomite (to absorb SOx pollutant gases). Coal is combusted with the help of hot primary air from air pre-heater. However, coal doesn't combust violently resulting into flames, but rather the heat resulting from combustion is instantaneously exchanged with the water tubes running through the coal-bed-air suspension bed. Thus, the combustion of coal takes place at a relatively lower temperature of around 850-900 deg C, necessitating lower thermal insulation of the boiler compared to PC boiler.
High pressure primary air
PC firing uses around 30 % of the combustion air as high pressure primary air for drying and transporting fuel.
CFB uses higher pressure primary air which is 60 % of the combustion air for fluidizing.
Desulfurization method
In a PC boiler a Flue Gas Desulfurization unit is required for the reduction of Sulfur Dioxide.
In CFB boiler limestone addition in the furnace reduces the Sulfur Dioxide during combustion itself. This requires only a simple limestone storage and handling unit.
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