- Fluidized Bed Combustion Definition: Fluidized bed combustion (FBC) is a method of mixing fuel and air in a bed of solid particles, which behaves like a fluid when air flows through it.
- Types of Fluidized Bed Combustion: There are two types—vertical FBC, for smaller plants, and horizontal FBC, for larger plants with higher steam production.
- High Heat Transfer: Evaporator tubes in FBC systems are immersed in the fluidized bed, ensuring high heat transfer rates and efficient combustion.
- Advantages of FBC: FBC systems offer high thermal efficiency, easy ash removal, fast load response, and reduced pollution, making them a preferred choice for power generation.
- Drawback of FBC: The main drawback is the high operating cost due to the continuous need for high-pressure air supply, though this is offset by its efficiency.
Fluidization is a way to mix fuel and air to get combustion. A fluidized bed is defined as a bed of solid particles that behave like a fluid. It works by passing air evenly through the particles; at low air velocity, the particles stay still, but as the air flow increases, the particles become suspended.
When the air velocity increases further, the bed becomes turbulent, and the particles mix rapidly, forming bubbles like boiling liquid. This process is called fluidized bed combustion.
The velocity of air, causing fluidization depends on a number of parameter, like :-
- Size of fuel particles.
- Density of air fuel mixture.
Hence, these parameters are given due consideration, while manipulating with air flow velocity for desired rate of combustion. In fluidized bed combustion, rapid mixing ensures uniformity of temperature. The main advantage of fluidized bed combustion system is that municipal waste, sewage plant sludge, biomass, agricultural waste and other high moisture fuels can be used for heat generation.
A fluidized furnace has an enclosed space with a base that admits air. Crushed coal, ash, and crushed dolomite or limestone are mixed in the furnace bed. High-velocity air enters from the bottom and passes through the bed.
As the air velocity increases steadily, a point is reached where the pressure drop across the bed equals the bed’s weight per unit area. This critical velocity is called the minimum fluidizing velocity.
With further increase in velocity of air, the bed will begin to expand and allow passage of additional air, in the form of bubbles. When the air velocity becomes 3 to 5 times the critical velocity, the bed resembles to that of a violently boiling liquid. A pictorial representation of fluidized bed combustion is given in the figure below:
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 to a great extent, and also produces combustion with very high efficiency.
Types of Fluidized Bed Combustion
Fluidized bed combustion (FBC) can be in 2 variants, namely:
- Vertical type FBC: These are generally used in smaller plant, and has the capacity to produce steam of up to 6 tonnes per hour only. Their vertical shape reduces the overall dimension of the steam boiler, and is extremely efficient in plants, where space provision is limited.
- Horizontal type FBC: There are almost 10 times in capacity when compared to vertical type fluidized bed combustion. They can produce as much as 60 tonnes of steam per hour, and are placed horizontally with respect to the boiler tubes. The high capacity of the horizontal type Fluidized boilers coupled with their high efficiency, makes them an extremely desirable choice for the coal fired thermal power generating station.
Advantages and Dis-advantages of Fluidized Bed Combustion
FBC is being used exhaustively these days in all major power stations all over the globe, owing to numerous advantages that it offers over the other pre-dominant methods of combustion. Few of those are:
- High thermal efficiency.
- Easy ash removal system, to be transferred for made cement.
- Short commissioning and erection period.
- Fully automated and thus ensures safe operation, even at extreme temperatures.
- Efficient operation at temperatures down to 150oC (i.e. well below the ash fusion temperature).
- Reduced coal crushing etc. (pulverised coal is not a necessity here).
- 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.
Despite all these advantages, the main drawback of fluidized bed combustion is the high fan power needed. The air must be supplied continuously at high pressure to support the bed, raising the operating cost. However, this cost is offset by the high efficiency of FBC.
