The six main feeder types for biomass are: (1) gravity chute, (2) screw con­veyor, (3) pneumatic injection, (4) rotary spreader, (5) moving-hole feeder, and (6) belt feeder. These are broadly classified as traction, nontraction, and others as shown in Figure 8.16. In the traction type, there is linear motion of the surface carrying the fuel, as with a belt feeder or a moving-hole or drag-chain feeder. In the nontraction type, the motion is rotating and oscillatory screw feeders and rotary feeders belong to this group. Oscillatory feeders are of the vibratory or ram type. Other feeder types move the fuel by gravity or air pressure.

Gravity Chute

A gravity chute is a simple device in which fuel particles are dropped into the bed with the help of gravity. The pressure in the furnace needs to be at least slightly lower than the atmospheric pressure; otherwise, hot gas will blow back into the chute, creating operational hazards and possible choking of the feeder due to coking near its mouth.

FIGURE 8.16 Types of feeders used in biomass plants.

In spite of the excellent mixing capabilities of a fluidized bed, a fuel-rich zone is often created near the outlet of a chute feeder that is subjected to severe corrosion. Since the fuel is not well dispersed in gravity chute feeding, much of the volatile matter is released near the feeder outlet, which causes a reducing environment. To reduce this problem, the chute can be extended into the furnace. However, the extension needs insulation and some cooling air to avoid premature devolatilization of the feed passing through it. Additionally, a pres­sure surge might blow fine fuel particles back into the chute while reducing conditions might encourage corrosion. An air jet can help disperse the fine particles away from the fuel-rich zone.

A gravity feeder is not a metering device. It can neither control nor measure the feed rate of the fuel. For this, a separate metering device such as a screw feeder is required upstream of the chute.