Biofuels from Lignocellulosic Biomass

Fluidized Bed Gasifiers

Fluidized bed gasifiers are most frequently used for syngas production.

They have a bed material which is chemically inert and does not participate in combustion. Mostly it is sand. The fluidized bed is formed when the gasification agent flows rapidly through the gasifier it swirls up the bed material and flows around the added fuel. The fuel particles are much smaller (up to 40 mm) than in fixed bed reactors and are completely mixed with the bed material. Fluidized-bed gasifiers do not form distinct temperature and reaction zones. The reactions taking place during thermochemical conversion run in parallel in the entire reactor. The tempertures are around 700 to 900°C. The process can be controlled more easily. Due to the intensive heat transfer from the bed material to the fuel particles, the temperature and residence time of the fuel in the reactor is reduced. This enables a high material conversion even with smaller reactor dimensions.

Tar contents in product gas from fluidized bed gasifiers are higher than in that from downdraft gasifiers, but significantly lower than for updraft gasifiers. But the particle content in the product gas is significantly higher than for fixed bed reactors, since fine-grained fuel, fine-grained ash or abraded bed material is entrained with the product gas during fluidized bed gasification.

Two different engineering approaches can be realized:

Stationary fluidized bed reactors

• Gasification agents are air, oxygen, steam or mixtures thereof
• Technology have been tested on a large scale (coal and peat, waste wood, bagasse, olive kernels)
• Not suitable for biomass with low ash softening temperatures (e.g. straw)
  

Circulating fluidized bed reactors:

• Gasification agents are air, oxygen, steam or mixtures
• High gas velocities cause the bed material to be discharged from the reactor
• Product gas has to be cleaned with cyclones and bed material is fed back into the reactor
• Technology commercially proven (co-firing, paper and pulp industry, power generation from biomass - Integrated Gasification Combined Cycle technology)