- 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 occurs when an upward gas flow supports and moves a bed of solid particles. A fluidized bed then behaves in some ways like a boiling liquid, which gives close contact between fuel, bed material and combustion air. Below the minimum fluidization velocity, the particles remain a packed bed.
Above the minimum fluidization velocity, particles mix and the bed expands. Distinct gas bubbles form in a bubbling bed. At a higher velocity, many particles leave the furnace with the gas and must be separated and returned. Burning fuel in these conditions is called fluidized bed combustion.
The required gas velocity depends on several coupled properties, including:
- The size, shape and density of the fuel and bed particles.
- The gas density and viscosity, bed depth, temperature and distributor design.
Operators set the airflow high enough to fluidize the chosen material while maintaining stable combustion and acceptable particle carryover. In fluidized bed combustion, strong gas-solid mixing helps maintain a comparatively uniform bed temperature. An important advantage of fluidized bed combustion is fuel flexibility: suitably designed plants can use coal, biomass, agricultural residues, refuse-derived fuel or sewage sludge. Fuel preparation, moisture, ash behaviour and emissions controls still limit what a particular unit can accept.
A fluidized-bed furnace has an air-distribution plate or nozzles near its base. The bed may contain sand, fuel ash, limestone or dolomite, depending on the fuel and process. Fuel enters the hot bed while primary air passes upwards through the distributor.
As the airflow increases, the pressure drop across a packed bed rises. Fluidization starts when the upward drag balances the effective weight of the particles per unit area. The corresponding superficial gas velocity is the minimum fluidizing velocity.
Further airflow expands a bubbling bed and sends gas through it as bubbles. There is no universal multiple of minimum fluidization velocity because the operating range depends on the particles and combustor design. The figure below illustrates one arrangement:
Some bubbling-bed boilers place evaporator tubes in the dense bed, where moving particles provide high heat-transfer rates. Other designs use furnace walls, external heat exchangers or surfaces above the bed. The heat-transfer arrangement, fuel burnout and auxiliary loads determine overall boiler efficiency.
Types of Fluidized Bed Combustion
The two principal atmospheric fluidized-bed combustion (FBC) arrangements are bubbling beds and circulating beds. Pressurized FBC is a separate classification based on operating pressure.
- Bubbling fluidized bed combustion: The relatively low fluidization velocity keeps most solids in a dense bed with a defined upper surface. A bubbling-bed steam boiler may use in-bed heat-transfer surfaces and is often selected for heterogeneous biomass or waste fuels.
- Circulating fluidized bed combustion: The higher gas velocity carries bed material out of the furnace. A cyclone or another separator captures these solids and returns them to the combustion zone, increasing solids residence time. This arrangement is widely used for larger industrial boilers and a coal-fired thermal power generating station.
Advantages and Limitations of Fluidized Bed Combustion
FBC is used in power and industrial boilers when its fuel flexibility, temperature control or in-bed sulphur capture suits the project. Its main operating characteristics include:
- Strong gas-solid mixing and heat transfer can support high combustion efficiency, but net plant efficiency depends on the complete steam cycle and auxiliary power use.
- Bed ash and fly ash can be removed continuously. Reuse in cement or construction requires testing because fuel, sorbent and unburnt-carbon content affect ash quality.
- A properly designed bed can burn a wider range of solid fuels than many pulverized-fuel systems.
- Automated fuel, air, pressure and temperature controls support stable operation. They do not remove the need for interlocks, monitoring and trained operators.
- Typical coal and waste FBC bed temperatures are roughly 800 to 950oC, below those of many conventional coal furnaces and often below ash-softening conditions.
- Fuel generally needs sizing and screening, but it does not need the fine pulverization used in a pulverized-coal boiler.
- The bed’s thermal inventory helps keep temperature stable. Load response depends on fuel feed, airflow, heat-transfer controls and whether solids circulate, so rapid response is not automatic.
- Lower furnace temperatures generally suppress thermal nitrogen-oxide formation. Limestone or dolomite can capture part of the sulphur dioxide in the bed. Actual emissions still depend on fuel and operating conditions, and particulate matter requires effective collection equipment.
Fluidized-bed systems need fan power to overcome pressure loss through the distributor, bed and gas-cleaning equipment. Other limitations can include particle erosion, bed agglomeration, sorbent consumption, solids handling and maintenance of refractory or recycle equipment. These costs must be assessed against fuel flexibility, emissions controls and the measured efficiency of the complete plant.
