Mesh Air Grid
Fluidized bed boilers
Atmospheric fluidized bed combustion
Technology
When air or gas is passed through an inert bed of solid particles such as sand supported on a fine mesh or grid. The air initially will seek a path of least resistance and pass upwards through the sand. With further increase in the velocity, the air starts budding through the bed and particles attain a state of high turbulence. Under such conditions bed assumes the appearances of a fluid and exhibits the properties associated with a fluid and hence the name fluidized bed.
Mechanism of fluidized bed combustion
If the sand, in a fluidized state was heated to the ignition temperature of the fuel and fuel is injected continuously into the bed, the fuel will burn rapidly and attain a uniform temperature due to effective mixing . This in short is fluidized bed combustion.
While it is essential that the temperature of bed should be equal to the ignition temperature of fuel and it should never be allowed to approach ash fusion temperature to avoid melting of ash. This is achieved by extraction of heat from the bed by conductive and convective heat transfer through tubes immersed in the bed.
If the velocity is too low fluidization will not occur, and if the gas velocity becomes too high, the particles will be entrained in the gas stream and lost. Hence to sustain stable operation of the bed, it must be ensured that gas velocity is maintained between minimum fluidization and particles entrainment velocity.
Advantages of FBC
v Considerable reduction in boiler size is possible due to high heat transfer rate over a small heat transfer area immersed in the bed.
v Low combustion temperature of the order of 800 to 950º C facilitates burning of fuel with low ash fusion temperature. Prevents Nox formation, reduces high temperature corrosion and erosion and minimize accumulation of harmful deposits due to low volatilization of alkali components.
v High sulphur coals can be burnt efficiently without generation of sox by feeding lime tone continuously with fuel.
v High turbulence of the bed facilitates quick start up and shut down
v Full automation of start up operation using simple reliable equipments is possible
v Inherent high thermal storage characteristics can easily absorb fluctuation in fuel feed rate.
Pressurized fluidized be d combustion.
The advantage of operation fluidized combustion at the elevated pressure is reduction in steam generator size can be achieved and make possible the development of a coal fired combined cycle power plant. The development of pressurized fluidized bed combustion is still in research stage only. With help of pressurized hot gas coming out of the furnace is cleaned primarily by a cyclone like CFBC boiler and the gas is expanded in a turbine and the exhaust gas from turbine is further cooled by the heat exchanger.
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