Coal Dust

Green Sand Moulding

Green sand is a mould sand that is called “green” because of its inherent moisture, which is burned off when the molten metal is introduced into the mould. Green sand is generally made up of chrome sand, coal dust, a binder like organic clay, and other ingredients, depending on the intended application. The addition of coal dust improves the overall quality of the casting by eliminating the risk of certain casting defects.

For instance, on the mould surface, coking of coal dust causes softening and expansion. As expansion occurs, the sand grains are adjusted and expansion-related defects are controlled. Hydrogen pin-holing is also controlled by the inclusion of coal dust in the foundry process.

When molten iron is poured into a green sand mould, reducing gases are released, preventing the formation of iron oxide during the intermediate phase. Soot and condensation are formed as a thin layer of lustrous carbon on the mould surface in the presence of a limited oxygen supply. This layer creates a refractory barrier between the molten iron and the mould. This method is used to produce an improved casting finish.

Coal dust used for iron casting must have a low ash content of around 11% with minimal sulphur and chloride, inherent moisture between 2% and 4%, and volatile content of around 32-38%.

Refractory Bricks

Refractory bricks are bricks made for the foundry industry. They are designed to tolerate extreme heat environments without sustaining physical or chemical damage. Coal dust is a major component in the production of refractory bricks.

The coal dust is mixed with a clay binder and water. This mixture is applied to the brick surfaces and is allowed to air dry for two hours or so, depending on the materials involved and their intended application. The coal-coated bricks are then passed through an initial firing temperature somewhere in the neighbourhood of 110° C before undergoing full firing in temperatures near 1050° C for several hours.

The coating of coal dust provides a thermal barrier that helps the refractory bricks perform well. Refractory coal dust reduces thermal conductivity on the surface of the brick, increases the strength and porosity of the bricks, improves the brick’s ability to endure thermal and corrosive environments, and improves the brick’s ability to endure thermal shock.

Blast Furnace Application

Blast furnaces used in the production of steel all over the world sometimes employ a PCI or pulverised coal injection. This method improves performance and saves on operational costs. The PCI method was developed in the 1800s, but it wasn’t until the 70s that the technique was adopted by steel and iron manufacturers. This was inspired by a dramatic increase in the demand and cost of coke in the 70s.

The PCI method requires coal dust to be introduced to the blast furnace with the conveying gas through the mid-bottom inlet. A blowpipe in the tuyere delivers the blast, which mixes with the conveying gas. The combined dust and air are channelled to the furnace, where combustion of coke and coal takes place. This combustion is what causes the solid iron ore to liquify.

Thermal Power Generation

Approximately 27% of the world’s population does not have access to electricity. Access to electricity is essential in the alleviation of poverty in these areas. Coal-fired power stations generally use coal dust because the service area allows combustion to happen more rapidly.

Coal dust is introduced to the combustion chamber of a boiler. The low-ash coal dust in the PCC or pulverised coal combustion system burns at high temperatures, producing hot gases. This heat converts the boiler’s water into steam.

The steam is funnelled into a turbine, the blades of which are rotated by the steam, resulting in high-speed rotation, which powers a generator. The steam is then condensed and channelled back into the boiler to be reused.