5.3 Stokers

All stokers are designed to feed solid fuel onto a grate where the fuel burns as primary air is passed up through it and over-fire air is also introduced to enhance combustion. The stoker is located in the furnace, and it is also designed to remove the residue of ash that remains after combustion.

Stoker firing consists of various systems having the following major features:

1. A fuel feed system

2. A grate assembly, either stationary or moving, that supports the burning fuel and admits the majority of combustion air to the fuel

3. An over-fire air system for the completion of combustion and reduction of emissions such as NO _x

4. An ash discharge system

Stokers can handle a vast variety of solid fuels. They handle all forms of coal, wood, bark, bagasse, rice hulls, peach pits, vineyard trimmings, coffee grounds, and municipal solid wastes (MSW). There are two general types of stoker systems:

1. Underfeed stokers: Both fuel and air supply are from under the grate

2. Overfeed stokers: The fuel is supplied from above the grate and air is supplied from below the grate.

For overfeed stokers, there are two types:

1. Mass feed stoker. Fuel is fed continuously to one end of the stoker grate, and it travels horizontally (or sometimes inclined for fuels such as MSW) across the grate as it burns. Ash is removed from the opposite end. Combustion air is introduced from below the grate and moves up through the burning...