From Earth to Energy: How Coal is Processed and Used

What is Coal?

Coal is a sedimentary deposit, composed mostly of carbon, making it readily combustible. Millions of years ago, organic material buried within the earth experienced different temperatures and pressure conditions and created fossil fuels. Fossil fuels are non-renewable energy sources, meaning there is a limited amount of these kinds of resources.

Types of Coal

There are four types of coal that are naturally occurring: lignite, subbituminous, bituminous and anthracite. Each type contains different amounts of carbon and can produce different amounts of heat energy. The varying heat and pressure combinations that formed these types of coal affect their quality and usage.

Lignite is the lowest rank of coal, with a carbon content of about 25-35%, so it contains low amounts of energy. Lignite has a low heating value and retains more moisture than other ranks of coal.

Because of its low heating value and higher moisture content, it is expensive to transport. It’s often used near where it was mined, like power stations, but is also utilized to create synthetic natural gas.

Sub-bituminous coal contains about 35-45% carbon and is used as fuel to generate electricity. About 47% of all coal produced in the United States is sub-bituminous coal. This coal is the most commonly used globally, as 30 percent of coal resources are sub-bituminous.

Bituminous coal has a carbon content of about 45-86%. Because of its carbon content, bituminous coal is used in electricity generation and steel production.

Lastly, anthracite, also called “hard coal,” has the highest carbon content, from 86-97%. With such a high heat value, anthracite is typically used in industrial settings, like steelmaking, wastewater treatment, glass manufacturing and battery recycling.

How is Coal Formed?

The majority of the world’s coal began developing around 300 million years ago, during the Carboniferous period. The environment of this period contained widespread swamp forests. Over time, plants and algae from these swamps were buried under mud and vegetation.

Undergoing millions of years of pressure and heat, the plant matter became coal in a chemical and physical process called coalification. The various combinations of heat and pressure created different kinds of coal with different amounts of carbon.

The higher the heat and pressure on organic matter, the higher the carbon content in the coal. This higher carbon content gives the coal a higher heating value.

Where is Coal Found?

Coal is found through geochemical and geophysical surveys and exploration drilling. If an area contains sufficient quality and amount of coal, it will become a mine.

These areas of coal formations are called “coal seams” or “coal beds.” There are coal seams found on every continent and over 1.06 trillion tonnes of coal reserves worldwide, with the largest reserves in the United States, Russia, Australia, China and India.

How is Coal Mined?

There are two methods that are primarily used when mining coal: surface mining and underground mining. The depth of the coal in the earth determines the mining method used to collect it. There are also economical factors that determine which mining technique is best for an operation.

Surface Mining

Surface mining is typically used when coal is found less than 200 feet underground.

Large machines expose coal seams by removing topsoil and layers of rock, which is called overburden. After mining the coal, the area can be covered in topsoil to plant grass and trees. Surface mining is less expensive than underground mining and is the source of two-thirds of coal produced in the United States.

There are four methods of surface mining techniques: contour strip mining, area strip mining, open-pit mining and auger mining.

Contour strip mining is used when coal seams are visibly outcropping on a hilly terrain. The overburden is removed, and benches are created around the hill. Removed overburden either fills a designated valley or is replaced on areas where coal has been recovered.

Area strip mining exposes an area of the coal seam with a trench on flat terrain. Parallel cuts are made and filled with overburden as coal is recovered.

Open-pit mining establishes benches in the overburden and coal seam. This method is typically used when thick coal seams are covered by thick or thin overburden or steeply pitched coal seams.

Auger mining removes coal by drilling auger holes in contour cuts and extracting the coal with cutting heads.

Underground Mining

Underground mining is used when coal is more than 200 feet below the surface. There are four methods for underground mining, including room-and-pillar mining, longwall mining, shortwall mining and thick-seam mining.

Room-and-pillar mining uses multiple parallel entries in the coal seam. These entries are connected by wider entries, creating a grid formation with thick pillars of coal. As coal is blasted from the grid, roof supports are added.

Longwall and shortwall mining extract coal in slices, allowing the coal to be removed in large sections and dumped onto a conveyer. Shields are used to support the roof as coal is mined and loaded on the conveyer.

How is Coal Processed?

Raw coal or run-of-mine (ROM) coal needs to be processed before it can be used. Processing coal not only provides a quality product but helps lower transportation costs and meet customer specifications.

Steps in coal preparation include crushing or breaking, screening or sizing, washing and classifying, dewatering and refuse and tailings management. The state of the ROM coal determines how it is processed. Some coal requires washing to remove high-sulfur content, while others are only crushed and sized.

Crushing/breaking: This step reduces coal into acceptable sizes for use. Crushing machines used in coal processing include Feeder Breakers, Rotary Breakers and Sizers.

Screening/sizing: The crushed coal is separated, or screened, into different sizes to create various product sizes. Vibratory screens are commonly used for coal screening/sizing.

Washing and classifying: Some ROM coal requires cleaning to create a final product. Washing equipment helps remove dirt, ash, sulfur and rock from ROM coal, increasing heating value.

Dewatering: Dewatering removes excess moisture from washed coal, decreasing hauling costs and increasing its heating value. Dewatering is also helpful for water recovery from coal tailings and fines.

Sampling: Coal must meet strict quality standards, and sampling is important to ensure consistent high-quality coal. Mechanical sampling equipment collects representative samples of the product for evaluation and analysis in the lab.

Refuse and tailings management: The nature of coal processing produces inevitable waste. Coarse refuse can form a tailings impoundment or go to a refuse pile. Fine solids in water, or tailings, can be separated for water reuse.

What is Coal Used For?

Coal is primarily used for electricity generation, but it is used in other applications as well. Coal is used as fuel and is used in cement production, aluminum production and steel production.

Coal-fired power plants burn coal to produce heat and convert water into high-pressure steam. The steam drives a turbine and produces electricity.

In steel production, heat must separate iron ore from other minerals. Coal itself was originally used to produce steel, but when burned, it released impurities that made the metal weak.

To prevent this, coal is baked for about 12 to 36 hours in an oven at around 2,000°F. This removes the impurities from the coal and creates coke. Coke melts the iron, separating impurities, resulting in strong and steel.

Byproducts of coal are found in numerous everyday products, like soap, dyes, plastics and fibers. Important products, like carbon fiber and activated carbon are derived from coal. Carbon fiber is a strong, yet lightweight material used in construction and car manufacturing.

What is the Future of Coal?

Coal is a heavily relied upon resource, supplying crucial sources of energy around the world. It is the most abundant source of fuel for producing electricity, providing over 36% of electricity worldwide. Electricity generated by coal plants is affordable and reliable.

Not only is coal itself a vital resource, but the coal industry is an important part of the economy. In 2022, coal mining provided 4.7 million jobs globally.

Coal will continue to be a key resource globally, providing stable sources of energy.