Coal Dust Applications
Coal dust is the powdered variety of coal created by pulverization or grinding of coal into fine and smooth grains. Coal has a brittle property which allows it to take on a powdered or pulverized form during mining, transportation or as a result of mechanical handling. Pulverizing or grinding coal before passing it through the combustion process allows for improved speed and efficiency of burning. Below are the few applications attributed to coal dust:
Iron and Steel Production
Iron and steel have become an essential part of our lives. From ships to cars and numerous household item, there isn’t a doubt that iron and one of its alloy, steel are vital in our everyday living. Even medical equipment is not left out, plus the pieces of machinery used in manufacturing many of the products have steel as a component material. There isn’t anyone who could under emphasize the importance of steel.
Coal is an important material needed in iron and steel production. Some statistics will assist in putting into context how essential coal is in manufacturing iron and steel. Approximately 64% of steel manufactured on a global basis were derived from iron produced in blast furnaces which use coal. In 2003, the quantity of crude steel produced on a worldwide scale was put at 965 million tons with about 543 Mt used in the manufacturing process.
The raw materials used in iron production from a blast furnace include iron ore, coke (made from coking coals) and a small amount of limestone. However, some blast furnaces use pulverized coal injection (PCI) method such that cost is saved and better performance is derived. The PCI method was developed initially in the 19th century, but it was not until the 1970s that iron and steel manufacturers adopted this technique. Apparently, an upsurge in the cost of coke due to the rise in global demand in addition to increased competition for this resource led manufacturers to turn their attention towards this method.
The idea behind the PCI method is quite simple. It involves the primary air, also referred to the “conveying gas”, carrying coal dust (pulverized coal) which is introduced through a lance into the tuyere (the mid-bottom inlet of the blast furnace). Subsequently, a blowpipe in the tuyere delivers secondary hot air (also termed the blast) and then mixes with the primary air which as mentioned earlier, conveys foundry coal dust. This mixture is channeled to the furnace, creating a balloon-like cavity, otherwise known as a raceway. This “raceway” propagates the combustion of coke and coal, liquefying the solid iron ore and releasing molten iron in the process.
Thermal Power Generation
In the present day, many people cannot imagine a life without electricity, especially those living in developed countries. Unfortunately, approximately 27% of the world’s population do not have access to electricity. It is important to know that improved access to electricity is important in poverty alleviation. Most coal-fired power stations use coal dust because the surface area is increased and thus, combustion takes place more rapidly.
Coal dust is passed into the combustion chamber in the boiler of these pulverized coal combustion (PCC) systems. Here, burning of low ash coal dust occurs at a high temperature, producing hot gases and heat energy which converts water (present within the tubes lining the boiler) into steam.
The steam moving under high pressure enters into a turbine having a few thousands of propeller blades. The blades are pushed by the steam, resulting in the high-speed rotation of the turbine shaft. At one end of the turbine shaft, a generator is fitted. This generator contains several coils wound together meticulously. Electricity generation occurs when these propeller blades are rotated rapidly with a strong magnetic field produced by the coils. Steam is condensed after it might have passed through the turbine and returns to the boilers for reuse.
Presently, coal contributes about thirty-nine percent of the world’s electricity supply. With the lower cost associated with using coal for electrification purposes, both developed and developing countries have found great use of this mineral.
Green Sand Molding Process
Greensand describes molding sand that is neither baked nor dried but possesses an inherent moistness. The raw sand in its ore form is processed such that the grain sizing is evenly distributed. Organic clays act as binders for these grains during the course of processing into molding sand.
The addition of foundry coal dust helps to ensure that the casting quality is excellent as sand expands when hot molten metal is emptied into the mold. The usage of other additives including pitch, cellulose, and silica are also allowed. The sand, along with additives and water are blended in a mullor, otherwise referred to as a mixer. The sand is deemed ready to make a mold when it has mixed with other substances in the mullor.
The pouring of molten iron into a green sand mold containing coal dust will cause the release of reducing gases following the application of heat and consequently prevents iron oxide formation during the intermediate phase of burn-on production. A limited oxygen supply causes the production of soots and condensation of a thin-layered lustrous carbon onto the mold surface resulting from the hydrocarbon gases produced on burning coal dust. The film of lustrous carbon serves as a refractory barrier between the molten iron and the molding sand, thereby improving casting finish.
In the final phase of the molding process, coking of coal dust starts at the mold surface, leading to its softening and expansion. The critical quartz sand expansion in the base silica sand occurs alongside the softening and coking of coal dust. Consequently, the sand grains are readjusted and the occurrence of expansion-type defects is regulated.
The coal dust used in foundries for iron casting requires low ash coal dust which must possess a minimal sulfur and chloride content, inherent moisture of about 2-4%, and volatile content of 30% or thereabout. Summarily, foundry coal dust decreases defects associated with expansion and hydrogen pin-holing. An improvement in the dimensional stability of molds is also due to the inclusion of this substance in greensand.
Refractory bricks are capable of enduring high temperature and are characterized by a low thermal conductivity which allows for greater efficiency. Applications that require high thermal, chemical or mechanical stress calls for the use of dense refractory bricks. However, kiln brick – a more porous refractory brick – is more suitable for less harsh situations. Kiln bricks are weaker than the dense ones, but they are advantageous in the sense that they are lightweight and better insulators.
One of the major substances involved in the production of refractory brick otherwise referred to as firebrick is coal dust. Open hearth furnaces, electric arc furnaces, metallurgy furnaces, cement rotary kilns, and glass kilns are constructed with firebricks made from refractory coal dust. As an additive, it is necessary that coal dust is mixed with clay and water. Subsequently, the mixture undergoes a firing process which involves air drying for 120 minutes at a temperature of 30oC and passed through a temperature of 110oC. In the last phase of firing, the mixture sample is fed into a furnace and heated to a temperature of 1050oC in 6 hours.
Specifically, coal dust provides the thermal insulation required by refractory bricks to perform when needed. Refractory coal dust has the following effect on the firebrick:
- Reduces thermal conductivity
- Increase the crushing strength and porosity of the refractory brick
- Enhance the firebrick’s ability to withstand thermal and corrosive factors
- Promotes the ability to withstand thermal shock.