Fluidized Bed Steam Boiler

How Do Fluidized Bed Boilers Work?

The working principle of a fluidized bed steam boiler is essentially an advanced combustion method used to burn solid fuel particles while suspended in a chamber. To provide the necessary oxygen for combustion, air jets are blown through a bed of particulate materials such as sand and limestone, creating a bubbling fluidized bed. This process enhances rapid and complete mixing of gas and solids, promotes fast heat transfer, and supports efficient chemical reactions within the bed. Fluidized bed boiler systems can burn a variety of low-grade solid fuels like coal, wood, and biomass with high efficiency, without requiring expensive fuel preparation processes such as pulverization. Moreover, new generation models of these boilers are more compact than equivalent conventional furnaces, offering significant advantages in terms of cost and operational flexibility. They reduce the amount of sulfur emitted as SO₂. Limestone is used during combustion to capture sulfur, which also improves heat transfer efficiency to the devices used to extract energy from the boiler (typically water tubes). The released heat comes into direct contact with the tubes and is transferred by conduction, increasing overall efficiency.

Advantages of Fluidized Bed Boilers

Limestone is used during combustion to capture sulfur, which allows more efficient heat transfer to the equipment used to extract energy from the boiler (typically water tubes). The heated residue that directly contacts the tubes (conductive heating) increases efficiency. As it allows coal plants to burn at lower temperatures, less NO₂ is released. However, combustion at low temperatures may lead to increased hydrocarbon emissions. Commercial fluidized bed boiler units operate with high efficiency, cost less than today’s traditional boiler units, and emit lower levels of SO₂ and NO₂. These benefits make them one of the most preferred steam boilers.

Disadvantages of Fluidized Bed Boilers

However, there are drawbacks such as tube erosion inside the boiler, uneven temperature distribution caused by blockages at the air inlet of the bed, and long start-up times that can extend up to 48 hours in some cases.

• While fluidized bed systems operate at a lower combustion temperature of 750 °C, conventional boilers operate at 850 °C.
• Fluidized bed steam boilers experience lower sintering (melting of ash).
• Lower NO₂ production due to reduced temperature.
• Lower SO₂ production thanks to absorption by limestone.
• Ten times more heat transfer than other combustion processes due to the burning particles, resulting in higher combustion efficiency.
• Requires less space due to high convective heat transfer coefficient.
• Isothermal bed combustion may occur as temperature remains constant in both freeboard and active bed regions.

Types of Boilers

Fluidized bed steam boilers are mainly divided into two groups: atmospheric systems and pressurized systems. Pressurized systems are further classified into two subgroups: bubbling fluidized bed and circulating fluidized bed boilers.

Atmospheric Fluidized Bed Boiler

Atmospheric fluidized bed steam boilers use limestone or dolomite to capture sulfur released during coal combustion. Air jets suspend the mixture of sorbent and burning coal, transforming the mixture into red-hot particles that flow like a liquid. These boilers operate at atmospheric pressure.

Pressurized Fluidized Bed Boiler

The pressurized fluidized bed boiler uses sorbent and air jets to suspend the mixture of sorbent and burning coal during combustion. However, these systems operate under high pressure and produce a high-pressure gas stream at temperatures suitable for driving a gas turbine. Steam generated from the heat in the fluidized bed is sent to a steam turbine, creating a highly efficient combined cycle system.

Advanced Pressurized Fluidized Bed Boiler

The new generation advanced pressurized fluidized bed boiler system increases gas turbine ignition temperature by using natural gas in addition to spent air from the fluidized bed combustor. This system uses natural gas, typically more expensive than coal. In more advanced second-generation pressurized boiler systems, a pressurized carbonizer is included to convert feed coal into fuel gas and char. The pressurized fluidized bed boiler burns the coal to produce steam for the steam turbine and to heat the combustion air. The fuel gas from the carbonizer burns in a topping combustor connected to the gas turbine and heats the gases up to the turbine’s nominal ignition temperature. Heat from the gas turbine exhaust is recovered to produce steam for a conventional steam turbine, enhancing combined cycle power output efficiency. These systems are also referred to as advanced circulating pressurized fluidized bed boiler systems. A circulating pressurized fluidized bed boiler system is entirely coal-fired. Gasification involves a pressurized circulating fluidized bed partial gasifier that feeds synthesis gas to the gas turbine’s topping combustor. The gas turbine exhaust provides combustion air for an atmospheric circulating fluidized bed combustor that burns coal from the partial gasifier.