Fluidized Bed Gasifier

In the fluidized bed reactor, the fuel is fluidized in oxygen and steam or air. The ash is removed dry or as heavy agglomerates that defluidize. The temperatures are relatively low in dry ash gasifiers, so the fuel must be highly reactive; low-grade coals are particularly suitable. The agglomerating gasifiers have slightly higher temperatures, and are suitable for higher rank coals. Fuel throughput is higher than for the fixed bed, but not as high as for the entrained flow gasifier. The conversion efficiency can be rather low due to elutriation of carbonaceous material. Recycle or subsequent combustion of solids can be used to increase conversion. Fluidized bed gasifiers are most useful for fuels that form highly corrosive ash that would damage the walls of slagging gasifiers. Biomass fuels generally contain high levels of corrosive ash.

To meet the challenges of fuel sensitivity in gasification, the fluidized bed gasifier was created. Used for industrial scale gasification, this type of gasifier can accept a wide range of fuel types, sizes and moisture contents.

In a fluidized bed gasifier, a granular sand-like material is fluidized by the upward passage of air from a porous plate structure below the sand bed and preheated to 1200 F. The size of the “sand” particles is chosen such that the passage of air through the bed moves and agitates the “sand bed” giving it a fluid like character. The bed resembles a rapidly boiling pot of water. The boiling action assures good mixing of the fuel and air. The bed also acts a very efficient “thermal flywheel” smoothing out variations in the fuel supply heat values. In normal operations the bed is typically about 98 - 99% by weight 'sand' and only 1 - 2% fuel, thus the behavior of the bed (including the passage of air and gas through the bed) is determined by the character of the sand bed, and not by the character of the fuel particles.

Moisture content of fuels can reach as high as 50% with the right design, but the 25-30% range is more ideal. Because tars are created, many systems directly fire the syngas in a CHP set up instead of using the syngas to power internal combustion engines.