8.4 BIOMASS FEEDERS

Based on the type of biomass, feeders can be divided into two broad groups: (1) those for harvested biomass and (2) those for nonharvested biomass.

Harvested fuels include long and slender plants like straw, grass, and bagasse, which carry considerable amounts of moisture. Examples of nonhar — vested fuels are wood chips, rice husk, shells, barks, and pruning. These fuels are not as long or as slender as harvested fuels, and some of them are actually granular in shape.

8.4.1 Feeding Systems for Harvested Fuel

Harvested biomass, such as straw and nonharvested hay, is pressed into bales in the field, and sometimes the bales are left in the field to dry (Figure 8.14).

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FIGURE 8.14 Tall grass is cut in the field, baled, and left in the field for drying in Nova Scotia. (Source: Photograph by the author.)

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Baling facilitates transportation and handling (Figure 8.15). Cranes are used to load the bales at a certain rate depending on the rate of fuel consumption. The bales are brought to the boiler house from storage by chain conveyors.

Whole bales are fed into a bale shredder and a rotary cutter chopper to reduce the straw to sizes adequate for feeding into a fluidized-bed gasifier or combustor. In the final leg, the chopped straw is fed into the furnace by one of several feeder types. Figure 8.15 shows a ram feeder, which pushes the straw into the furnace. In some cases, the straw falls into a double-screw stoker, which presses it into the furnace through a water-cooled tunnel.

8.4.2 Feeding Systems for Nonharvested Fuels

Wood and by-products from food-processing industries are generally granular in shape. Wood chips and bark may not be of the right size when delivered to

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the plant, so they need to be shredded to the desired size in a chopper. However, fuels like rice husk and coffee beans are of a fixed granular size and so do not need further chopping. Rice husk, a widely used biomass, is flaky and 2 to 10 mm x 1 to 3 mm in size. As such, it can be fed as it comes from the source, but it can be easily entrained in a fluidized bed. For this reason one can press it into pellets using either heat or a nominal binder in a press.

Feeders for nonharvested fuels are similar to those for conventional fuels like coal. Speed-controlled feeders take the fuel from the silo and drop mea­sured amounts of it into several conveyors. Each conveyor takes the fuel to an air-swept spout that feeds it into the furnace. If the moisture in the fuel is too high, augers are used to push the fuel into the furnace.

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