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FLUIDIZED BED BOILER - STRUCTURE AND OPERATING PRINCIPLES

Fluidized bed boilers are widely used in industrial production due to their cost-saving and emission-reduction capabilities. This is considered an effective solution both economically and environmentally.

What is a Fluidized Bed Boiler?

- A fluidized bed boiler is a technology that utilizes the fluidized bed combustion process to efficiently harness heat for industrial production. It is used as an emission reduction system and operates under high-pressure conditions, burning a variety of fuels, including coal, waste, and biomass. This type of boiler is called a fluidized bed boiler because the solid layers and fuel particles reach a fluidized state within the combustion chamber, exhibiting characteristics similar to the boiling of liquids.

- According to research, fluidized bed boilers have significantly lower levels of CO2, NOx, SO2, and other emissions compared to conventional boilers that only burn fossil fuels. This makes it an efficient and environmentally friendly method.

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Classification of Fluidized Bed Boilers

- AFBC (Atmospheric Fluidized Bed Combustion): A bubbling fluidized bed boiler.

- CFBC (Circulating Fluidized Bed Combustion): A circulating fluidized bed boiler, a high-velocity variation of the bubbling fluidized bed with speeds of up to 4-6 m/s.

- PFBC (Pressurized Fluidized Bed Boiler): A pressurized fluidized bed boiler, a special variation where the entire boiler operates at high pressure, around 16kg/cm2.

Components of a Fluidized Bed Boiler

- Fuel Feeding System: It includes conveyors, screws, and feeders that transport fuel into the combustion chamber. Additionally, sensors within the combustion chamber monitor and control the fuel supply and boiler efficiency.

- Combustion Chamber: The chamber contains a solid layer (usually consisting of sand, slag, limestone, coal, or dolomite) that forms the fluidized bed. This bed expands when the air pressure increases, creating a fluidized layer to facilitate the combustion process.

- Air Supply System: This system comprises fans and blowers, including primary and secondary fans. The primary fan preheats the fluidized bed by passing air through an air preheater and distributing it into the bottom of the combustion chamber. Various nozzles on the distribution plate evenly distribute air across the combustion bed. The secondary fan supplies additional oxygen to enhance combustion efficiency.

- Air Preheater: This critical component is installed within the fluidized bed boiler to maximize the utilization of heat from flue gas, making it easier for fuel to ignite. It helps save fuel and reduces the operational cost of the boiler.

- Ash Disposal System: A fluidized bed boiler typically generates both gaseous and solid waste. The gaseous waste includes SOx, NOx, COx, and other emissions, which have a significant density before being released into the environment. Hence, fluidized bed boilers prioritize the installation of exhaust gas treatment equipment. Solid waste consists of ash generated from the combustion of fuel. The ash disposal system includes cyclone separators, bag filters, electrostatic precipitators, wet scrubbers, and gas treatment equipment. These systems ensure that emissions meet environmental standards, with minimal ash carried into the tail end of the boiler.

- Steam Generation System: This includes essential components like water walls, steam pipes, superheaters, reheaters, economizers, and more. Water circulation is maintained from a feedwater tank, descending downpipes, and then heated to generate steam. Steam is utilized for various industrial processes or electricity generation through a steam turbine.

- Safety Control System: Fluidized bed boilers are equipped with automatic control technology and software to monitor and control the boiler's operations, ensuring safe and efficient performance.

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Operation Principle of a Fluidized Bed Boiler

- Fuel is transported into the combustion chamber through a conveyor system, with the fuel quantity regulated by the feeding system's operation speed.

- Water is supplied into the fluidized bed boiler and stored in a tank. It is then pumped into the fluidized bed through a network of water tubes.

- Within the combustion chamber, solid particles (fuel, ash, slag) begin to float and reach a fluidized state, similar to liquid boiling. Prior to combustion, a stream of primary air enters the combustion chamber from the chamber's bottom, significantly increasing the chamber's temperature.

- By supplying air to the combustion chamber, the combustion process reduces the size of fuel particles and mixes them with the layer of slag and sand. This forms a fluidized bed or a fuel cushion.

- The heat generated in this process is transferred to a network of pipes that heat water to produce steam. This steam is then used for various industrial processes or electricity generation.

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Advantages and Disadvantages of Fluidized Bed Boilers

- Advantages:

  • Fluidized bed boilers can burn various types of fuels, including fossil fuels, biomass, household waste, and industrial waste, resulting in cost savings.
  • They efficiently utilize fuel for optimal boiler performance.
  • Multiple fuels can be used within a single combustion chamber, allowing for flexibility without altering the boiler's structure.
  • Fluidized bed boilers incorporate industrial-standard filters to reduce dust emissions, achieving up to 99% dust removal. Emissions are minimal and typically range between 100-120 mg/Nm3.
  • These boilers operate at temperatures below 900°C, serving as effective emission reduction systems, eliminating the need for costly emission control equipment while meeting environmental standards.
  • They do not have slag deposits in the steam tubes within the combustion chamber, and the surface of the tubes is submerged in the fluidized bed, reducing ash buildup on the heat transfer surface behind the boiler.
  • Fluidized bed boilers have a long lifespan and are durable, resulting in low maintenance and repair costs.

- Disadvantages:

  • When burning coal and other solid fuels, fluidized bed boilers with high limestone content may require a heat exchange tube bundle to prevent overheating. These bundles tend to have low durability due to continuous wear and tear.
  • High-pressure fans used in fluidized bed boilers consume a significant amount of energy to maintain high air pressure.
  • Heat loss to the environment through the boiler's body results in a substantial loss of thermal energy.
  • Despite having bag filters, fluidized bed boilers burning solid fuels may not eliminate emissions, leading to slag buildup behind the boiler.

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Applications of Fluidized Bed Boilers

- Fluidized bed boilers are widely used in various industrial sectors where there is a demand for steam, heat, and electricity.

- They are employed in industries such as food processing, paper mills, textiles, pharmaceuticals, and more, allowing businesses to save up to 80% on fuel costs.

- In the power generation sector, fluidized bed boilers are used to create high-energy steam flows, which rotate a turbine to produce electricity.

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