Pulverized coal-fired boiler
A pulverized coal-fired boiler is an industrial or utility boiler that generates thermal energy by burning pulverized coal (also known as powdered coal or coal dust since it is as fine as face powder in cosmetic makeup) that is blown into the firebox.
The basic idea of a firing system using pulverized fuel is to use the whole volume of the furnace for the combustion of solid fuels. Coal is ground to the size of a fine grain, mixed with air and burned in the flue gas flow. Biomass and other materials can also be added to the mixture. Coal contains mineral matter which is converted to ash during combustion. The ash is removed as bottom ash and fly ash. The bottom ash is removed at the furnace bottom.
This type of boiler dominates the electric power industry, providing steam to drive large turbines. Pulverized coal provides the thermal energy which produces about 50% of the world’s electric supply.
How it works
The concept of burning coal that has been pulverized into a fine powder stems from the belief that if the coal is made fine enough, it will burn almost as easily and efficiently as a gas. The feeding rate of coal according to the boiler demand and the amount of air available for drying and transporting the pulverized coal fuel is controlled by computers. Pieces of coal are crushed between balls or cylindrical rollers that move between two tracks or “races.” The raw coal is then fed into the pulverizer along with air heated to about 650 degrees F from the boiler. As the coal gets crushed by the rolling action, the hot air dries it and blows the usable fine coal powder out to be used as fuel. The powdered coal from the pulverizer is directly blown to a burner in the boiler. The burner mixes the powdered coal in the air suspension with additional pre-heated combustion air and forces it out of a nozzle similar in action to fuel being atomized by a fuel injector in modern cars. Under operating conditions, there is enough heat in the combustion zone to ignite all the incoming fuel.
CFB (Circulating Fluidised Bed) Boiler
The circulating fluidized bed (CFB) is a developing technology for coal combustion to achieve lower emission of pollutants. By using this technology, up to 95% of pollutants can be absorbed before being emitted to the atmosphere.
Coal 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, here, 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.
Advantages of CFB boiler
- Lower combustion temperature translates into lower thermal insulation needs of the boiler, directly resulting in savings.
- As most of the SOx gases are absorbed by the bed, flue gases can be cooled to lower temperatures than PC boiler, thereby increasing the amount of potential heat exchange.
- Unburnt coal particles are recycled back with the help of cyclone separators. These separators separate heavy unburnt coal particles from the lighter ash particles by centrifugal action. The unburnt coal is then fed back into the bed for combustion. This results in direct savings on fuel.
Source – Wikipedia