Fluidization is a new method of mixing fuel and air for obtaining combustion. A fluidized bed may be defined as the bed of solid particles behaving as a fluid.
It operates on the following principle:
When an evenly distributed air is passed upward through a finely divided bed of solid particles at low velocity, the particles remain undisturbed but if the velocity is steadily increased, a stage is reached when the individual particles are suspended in the air stream. If the air velocity is further increased, the bed becomes highly turbulent and rapid mixing of particles occur which appear like formation of bubbles in a boiling liquid and a bed is said to be fluidized.
The velocity of air causing fluidization depends on a given particle size, range and density. In fluidized bed combustion, rapid mixing ensures uniformity of temperature. The main advantage of such a combustion system is that municipal waste, sewage plant sludge, biomass, agricultural waste (such as rice straw) and other high moisture fuels can be used for generating heat.
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A fluidized furnace has an enclosed space with a base having openings to admit air. Crushed coal, ash and either crushed dolomite or limestone is mixed in the bed furnace and high velocity combustion air is then passed through the bed, entering from the furnace bottom. With the steady increase in the velocity of air, a stage will be reached when the pressure drop across the bed becomes equal to the weight per unit cross section of the bed and this critical velocity is called the minimum fluidizing velocity.
With the further increase in velocity of air, the bed will expand and allow passage of additional air, in the form of bubbles. When the air velocity becomes 3-5 times the critical velocity, the bed resembles a violently boiling liquid. The evaporator tubes of boiler are directly immersed in the fluidized bed and the tubes, being in direct contact with the burning coal particles, produce very high heat transfer rates.
Because of this, the unit size is reduced and also very low combustion temperatures are produced. Vertical type fluidized bed combustion (FBC) boilers are available up to 6 tonnes per hour, whereas the horizontal type FBC boilers are available in a capacity of 8 to 60 tonnes per hour.
There are two basic systems of fluidized bed combustion namely atmospheric FBC and pressurized FBC. The pressurized FBC system has the advantages of high burning rates, reduced volume, increased coal loading in comparison to atmospheric FBC, but has the drawback of difficult controls and short plant life.
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The advantages of fluidized bed combustion are high thermal efficiency, easy ash removal system, short erection and commissioning period, fully automatic and safe operation, multi-fuel operation, reduced maintenance, uniformity of temperature, efficient operation at temperatures down to 150°C (well below the ash fusion temperature), reduced unit size and capital costs due to high heat transfer rates, reduced fouling and corrosion of tubes due to low combustion temperatures, reduced coal crushing cost etc.
The system can respond rapidly to changes in load demand, due to quick establishment of thermal equilibrium between air and fuel particles in the bed.
The operation of fluidized bed furnace at lower temperature helps in reducing air pollution. The low temperature operation also reduces the formation of nitrogen oxides. By adding either dolomite (a calcium-magnesium carbonate) or lime stone (calcium carbonate) to the furnace the discharge of sulphur oxides to the atmosphere can also be reduced if desired.
The major drawback of this system is that the fan power is considerably increased since the air is to be supplied at a high pressure for supporting the bed.