Stoker systems have been used on small boilers for over a century. They use a lump coal feed.
There are a number of different types, including:
- underfeed stokers
- static grates
- chain grates, travelling grates and spreader stoker systems.
Underfeed and static grate systems are only used on very small plant, and mainly for steam raising for heating water rather than power generation.
Chain and travelling grate furnaces have similar characteristics. Coal lumps are fed continuously on to a moving grate or chain. Air is drawn through the grate, and through the bed of coal on top. As the coal enters, it is heated by radiation from the refractory arch inside. Moisture and volatile matter are driven off. The chain/grate moves the coal slowly into the region in which ignition is established, and the temperature in the coal bed rises. The carbon gradually burns off, leaving ash which drops off at the end into a receptacle, from which it is removed for disposal. The ash formed may have a carbon content as high as 4-5%.
In the spreader stoker arrangement, a high-speed rotor throws the coal into the furnace over a moving grate, to promote fuel distribution.
Such boilers have commonly used in sizes equivalent to 10-25 MWe, but emissions control tends to be uneconomic from such units, apart from the use of cyclones for particulates removal. Combustion is relatively unstable, so that there can be intermittent emissions of CO, NOx and organics.
Emissions and residues
The emissions of SO2 from a stoker boiler will depend on the sulphur content of the coal. Residues may have a carbon-in-ash content as high as 5% because of the relatively inefficient combustion, and the restricted access of oxygen to all the combustible material.