For a wide dispersion of fuel over the bed, spreader wheels are used (Figure 8.19). The spreader throws the fuel received from a screw or other type of metering feeder over a large area of the bed surface. Typically it comprises a pair of blades rotating at high speed; slightly opposite orientation of the blades helps throw the fuel over a larger lateral area. This is not a metering device. A major problem with the spreader is that it encourages segregation of particles in the bed.

Pneumatic Injection Feeder

A pneumatic injection feeder is not a metering device; rather, it helps feed already metered biomass into the reactor. This works well for gravity feeding, and it is especially suitable for fine solids. Pneumatic injection is preferred for less reactive fuels, which must reside in a gasifier bed longer for complete conversion. It transports dry fuel particles in an air stream at a velocity higher than their settling velocity. The fuel is typically fed from underneath a bubbling fluidized bed. The maximum velocity of air in the fuel transport lines may not
exceed 11—15 m/s to avoid line erosion. The air for transporting constitutes part of the air for gasification.

Splitting of the fuel-air mixture into multiple fuel lines is a major problem with pneumatic injection. A specially designed feed splitter, like the one dis­cussed in Basu (2006, p. 355), can be used.

In a underbed pneumatic system, air jets that carry solid particles with high momentum to enter into the fluidized bed, forming a plume that could punch through the bed. To avoid this, a cap sits at the top of the exit of each feeder nozzle. This cap reduces the momentum of the jets breaking into the freeboard of a bubbling fluidized bed. A highly erosive zone may be formed near each outlet nozzle of the feeders, which might corrode the tubes nearby.

Another innovative, but one that is less common, feed system uses pulsed air. Controlled-air pulses push the biomass into the gasifier, avoiding pyrolysis of feed in the gasifier feed line. A very small amount of air minimizes dilution of the product gas with nitrogen. The University of Western Ontario in Canada applied this design with success in a commercial mobile pyrolyzer.