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Based on: Industry Standards and Best Practices

Languages: English

Sample Transcript

Fluidized bed combustion is an efficient process for burning solid fuels. A variety of fuel types and qualities can be effectively burned with low emission. Both types, bubbling and circulating, rely on a large fluidized mass of inert bed material to support combustion and heat transfer. The solids enable low combustion temperatures and greatly improve heat transfer. Bubbling bed boilers maintain a dense fluidized bed with a fixed level in the bottom of the boiler, and circulating bed boilers distribute solids throughout. The solids greatly improve heat transfer and enable low combustion temperatures.

Fluidized Bed Boilers

Training Time: 42 minutes

Conventional solid fuel power boilers burn the fuel in a fixed bed. In a fluidized bed boiler, hot air is blown at a high velocity up through a bed of sand, limestone, and ash causing the solid particles to fluidize, or behave like a liquid. Fluidization provides good mixing and allows for effective combustion when fuel is added. Fluidized bed combustion (FBC) provides several advantages for burning solid fuels, including higher efficiency, lower emissions, and the ability to burn low quality (e.g. high moisture) fuels. This course will focus on the operation of bubbling and circulating fluidized bed boilers.

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Circulating fluidized bed boilers disperse particles throughout the furnace for improved heat transfer. The solids are removed from the flue gas and returned to the combustion zone.

Circulating fluidized bed boilers disperse particles throughout the furnace for improved heat transfer. The solids are removed from the flue gas and returned to the combustion zone.

In a bubbling bed boiler, primary combustion air is forced through nozzles in the bottom of the boiler at a velocity high enough to fluidize the solid particles in the boiler.

In a bubbling bed boiler, primary combustion air is forced through nozzles in the bottom of the boiler at a velocity high enough to fluidize the solid particles in the boiler.

Convection occurs by the circulation and mixing of fluids in the boiler.

Convection occurs by the circulation and mixing of fluids in the boiler.

Learning Objectives

  • Describe the advantages and disadvantages of fluidized bed boilers
  • Differentiate between bubbling and circulating fluidized bed boilers
  • Describe the operation of both fluidized bed boiler types
  • Identify and describe the function of the key components of both fluidized bed boiler types
  • Define conduction, convection, and radiation and describe how heat is transferred in boilers
  • Describe how fluidized bed boilers can reduce emissions
  • Describe safety guidelines for working around boilers
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Fluidized Bed Boilers FAQs

What is a fluidized bed boiler?
Fluidized bed boilers use a hot bed in the bottom of the bed as a combustion source. This bed is fluidized by blowing hot air up through the bed at a high velocity. Fluidization provides good mixing and allows for effective combustion when fuel is added.

What is the difference between a bubbling bed and a circulating bed boiler?
A bubbling bed forces air up through the bed material to the point that it resembles a vigorously boiling liquid. In a circulating bed boiler the air velocity is high enough to entrain the particles in the air and disperse them throughout the boiler.

Are fluidized bed boilers better than conventional grate boilers?
Unless the fuel has a high alkali content, which can cause bed agglomeration, a fluidized bed boiler is superior to conventional boilers. Fluidized bed boilers have high efficiencies and produce lower emissions.

What materials are used for fuel in fluidized bed boilers?
Fluidized bed boilers can burn a variety of solid fuels, including, coal, bark, sludge, wood chips, and tire derived fuel (TDF). Oil or gas can also be used as supplemental fuel or during startup and upset conditions.

What are the important fuel particle parameters?
Particle size, shape, and density are important fuel and bed material parameters. The ideal bed would be a mixture of perfect spheres of the same diameter and density. Although this is not possible, the particle size distribution and density must be stable to maintain a stable fluidized bed.

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