Fluidized-bed gasifiers suspend feedstock particles in an oxygen-rich gas so the resulting bed within the gasifier acts as a fluid. These gasifiers employ back-mixing, and efficiently mix feed coal particles with coal particles already undergoing gasification. To sustain fluidization, or suspension of coal particles within the gasifier, coal of small particles sizes (<6 mm) is normally used. Coal enters at the side of the reactor, while steam and oxidant enter near the bottom with enough velocity to fully suspend or fluidize the reactor bed. Due to the thorough mixing within the gasifier, a constant temperature is sustained in the reactor bed. The gasifiers normally operate at moderately high temperature to achieve an acceptable carbon conversion rate (e.g., 90-95%) and to decompose most of the tar, oils, phenols, and other liquid byproducts. However, the operating temperatures are usually less than the ash fusion temperature so as to avoid clinker formation and the possibility of de-fluidization of the bed. This, in turn means that fluidized-bed gasifiers are best suited to relatively reactive coals, low rank coals, and other fuels such as biomass.
Some char particles are entrained in the raw syngas as its leaves the top of the gasifier, but are recovered and recycled back to the reactor via a cyclone. Ash particles, removed below the bed, give up heat to the incoming steam and recycle gas. At startup, the bed is heated externally before the feedstock is introduced.
Characteristics
Fluidized-bed gasifiers may differ in ash conditions (dry or agglomerated/slagging) and in design configurations for improving char use. Also, depending on the degree of fluidization and bed height, these types of reactors sometimes are also named as circulating fluidized bed reactors, and/or transport reactors.
Fluidized-bed gasifiers display these characteristics:
Load flexibility and high heat transfer rates
Fuel flexibility, can gasify a wide range of feedstocks
Moderate oxidant and steam requirements
Uniform, moderately high temperature throughout the gasifier
Higher cold gas efficiency than entrained-bed gasifiers, but lower carbon conversion
