Coal is a sedimentary deposition primarily composed of carbon and is readily combustible. Coal is a black or brownish-black sedimentary rock with a high carbon and hydrocarbon content. 

It contains more than 50 percent carbonaceous material by weight and above 70 percent by volume. 

It is made up of the energy stored by plants remnants that have been compressed, hardened, chemically changed, and metamorphosed throughout geologic time by heat and pressure.

Because of pressure and heat, the plant remnants were transformed into coal. Since this process takes millions of years to form coal, it is a non-renewable source of energy. 

Coal
Coal | Image Credit – Wikimedia Commons

Coal is an ample natural resource that may be utilized for energy, as a chemical source for various synthetic compounds (such as colors, oils, waxes, medicines, and insecticides), and the creation of coke during metallurgical processes. Coal is a primary energy source for steam generation, used to generate electricity.

Furthermore, coal gasification and liquefaction produce gaseous and liquid fuels that are easy to transport (e.g., through the pipeline) and stored in tanks. 

Even though it has been known and utilized for thousands of years, people only used it in small amounts till the Industrial Revolution. 

Coal consumption surged to the next level only after the invention of the steam engine. After a massive increase in coal consumption in the early 2000s, coal use worldwide peaked in 2012. 

Since then, coal use has been steadily declining, primarily compensated by advances in renewable energy. It is extensively used in iron and steel production and other industrial operations.

Formation

Coalification is the process of turning plant residues into coal. The Earth had extensive forests in low-lying marsh zones at varying times in the geological past. 

Coalification began in these wetlands when dead plant matter was preserved from biological degradation and oxidation, generally by muddy or acidic water, and turned into peat. 

The vast peat bogs trapped carbon, and sediments subsequently buried them. The intense heat and pressure of underground burial induced the loss of water, methane, and carbon dioxide over long periods, and the proportion of carbon increased.

The type of coal produced was determined by the temperature and pressure levels reached. For example, lignite (commonly known as “brown coal”) was formed under moderate conditions, and sub-bituminous coal was formed under more severe conditions.

Temperature is far more essential than pressure and burial duration when it comes to coalification. 

Subbituminous coal can be formed at temperatures as low as 35 to 80 degrees Celsius (95 to 176 degrees Fahrenheit). 

In contrast, anthracite needs a temperature in the range of 180 to 245 degrees Celsius (356 to 473 degrees Fahrenheit). 

Coal formation
Coal formation | Image Credit – Kentucky Geological Survey

Peatification and coalification are the two primary stages of the coal formation process. In the initial phase, bacterial activity produces peat. This process is called peatification. The increase in temperature and pressure caused by burial forms the coal, which is coalification.

The following procedures are conducted to create coal:

  1. Plant stuff such as ferns, bushes, creepers, trees, and algae dies and collects on the surface of mires and wetlands. Bacteria break down the organic stuff, producing carbon dioxide and methane.
  2. The plant matter is buried and no more exposed to the other elements. Anaerobic microorganisms then decompose the substance. Thousands of years of burial and accumulation can result in several meters of partially decaying plant materials known as peat. 
  3. When peat is buried deeply, the growing pressure squeezes water out and other chemicals, resulting in lignite formation, the lowest quality coal.
  4. This low-quality lignite coal is changed into higher-quality “black coals” due to continued burying, which results in increased pressures and temperatures. 
  5. First, lignite transforms into sub-bituminous coal, bituminous coal, and anthracite coal of outstanding quality. 
  6. The quantity of moisture and other chemicals in the coal reduces as these conversions occur, and the coal becomes denser. An increase in carbon content accompanies this.

Uses

Coal consumption chart
Coal consumption chart | Image Credit – U.S. Energy Information Administration

Coal was once primarily utilized to heat buildings, power engines, and factories on railroads. Coal, on the other hand, now operates differently in society. 

Coal is now primarily used to generate electric power. More than 90% of coal produced in the United States is utilized to generate electricity. 

Coking coal is also used for steel production and industrial process heating. Small quantities are also used to create chemicals and specialty products, often made from waste from other processes. 

The following sections contain more information:

1. Generation of Electricity

Coal-Fired powerplant electricity generation
Coal-Fired powerplant electricity generation | Image Credit – Kentucky Geological Survey

The majority of coal mined is utilized to generate electricity. Railways, barges, and trucks transport coal to power plants once mined. 

A conveyor belt transports the coal to a pulverizer, pulverizing it to talcum powder fineness. The powdered coal is then forced into a boiler’s combustion chamber, where it is burnt at a temperature of roughly 2,550°F. 

Water-filled pipes surround the boiler room’s walls. Due to the tremendous heat, the water evaporates into superheated (2,000°F) high-pressure (1,000 psi) steam. The steam is sent via a turbine (which looks like a giant propeller) that is connected to a generator. 

The entering steam rotates the turbine, producing a magnetic field within the generator’s wire coils. 

Due to this, an electric current flows through the coils and out through the transmission cables of the power plant. 

After passing through the turbine chamber, the steam is cooled in cooling towers before rejoining the water/steam cycle.

2. Manufacture Coke and Steel

Manufacturing steel from coal
Manufacturing steel from coal | Image Credit – Kentucky Geological Survey

Even though a tiny amount is required, metallurgical coal is a significant raw material in the steel-making process. 

Steel is made by heating coal in an oven to temperatures as high as 2060 degrees Fahrenheit without using air until the majority of the volatile stuff is liberated. 

It softens, then liquefies, then re-solidifies into a rigid porous material known as “coke” during this process. 

Steel is made from a porous, carbon-rich substance called coke. Coke is combined with mixed iron ore and limestone to produce molten iron, subsequently processed and heated to produce steel.

3. Industry and Residential Markets

Many households in the early 20th century relied on coal for heating. Few homes in developed regions still use coal to heat their houses. 

Meanwhile, its use in boilers for enterprises, factories, schools, and other institutions has significantly decreased. 

Coal is still employed for heating and many other applications in some commercial and industrial boilers and ovens. 

Coal is used in the cement, glass, ceramics, and paper industries. Among these, the cement industry is the largest coal consumer. The cement manufacturing process consumes a lot of energy.

4. Specialty Products and Chemicals

The chemical sector and the manufacture of specialty products consume a relatively small percentage of the total coal produced (less than 1%). Coals used for chemical processes or specialty products have distinct quality features. 

Aromatic compounds are abundant in high-quality coals and coal-derived tars and gases from metallurgical coking of coals in the steel-making industry. Aromatic compounds are made up of carbon. 

Aromatic compounds are six carbon atoms linked together in hexagonal ring configurations. Aromatic and polycyclic compounds (more than one ring) are predominantly present in coal and metallurgical coking byproducts, making them valuable feedstocks for various carbon-based chemicals. 

Currently, coal is used to make the majority of chemicals. Carbon can also make most chemicals instead of coal and coke byproducts. However, some carbon compounds and chemicals are easier to make from coal.

5. Rare Earth Elements from Coal

Modern periodic table showing rare earth elements | Image Credits – Science Notes

Rare-earth elements (REEs) are present in coal and coal processing products. REEs are a class of elements (indicated in orange on the periodic table above) that are relatively unknown to general people. 

Lanthanide elements are among them. Rare earth elements are vital in our current technological world because they are employed in many items that people use every day, such as televisions and cellphones, despite their rarity. 

In coals and coal byproducts, REEs are found in trace levels. Micro concentrations of 100 ppm or less are used to determine trace quantities. 

Coal provides more than one-third of the world’s electricity. It is essential for lighting and powering homes businesses and providing energy for transportation and the production of steel and concrete.

Coal is a critical resource for addressing the modern world’s difficulties, particularly the increasing rise in energy use. 

Coal is substantially less expensive and easier to obtain than other fossil fuels, and its reserves are more evenly spread over the globe.

Types of Coal

Various forms of coal are all carbon-rich minerals and rocks. This fossil fuel generates around 25 percent of the overall world’s primary energy and 40 percent of a total of the world’s electricity. 

However, not every coal is created equal; it comes in different carbon content levels, determining its grade or rank. High-quality coal emits less smoke, burns for longer, and offers more energy.

Coal comes in four major types or “ranks.” Rank describes the stages of “coalification.” Coalification is a long, natural process in which buried plant matter transforms into a denser, drier, high carbon content, and harder substance. 

The rating is primarily determined by the following:

  • The carbon content of the coal. 
  • The amount of heat energy it can provide
  • The degree of heat and pressure that acted on the plants throughout geological time 

The four significant ranks are as follows:

1. Lignite

Lignite coal
Lignite coal | Image Credit – Flickr

It is also known as brown coal, signifying its color. Lignite has the least energy output of all coal grades and includes 25–35 percent carbon. 

Lignite coal resources are often young and have not been subjected to intense temperatures or pressures. 

Because lignite is crumbly and has a high moisture content, it has a low heating value. This type of coal emits more carbon dioxide than tougher black coals due to its high moisture and low carbon concentration. 

In addition to carbon, it contains other compounds like mercury and sulfur in high concentrations. 

This type of coal is generally used to generate electricity.  It is considered the lowest grade of coal due to its lower carbon content and low heating value.

2. Subbituminous

Sub-bituminous coal, often known as black lignite, is grey-black or dark brown coal. It varies from hard to soft as an intermediary step between low-quality lignite and higher-quality bituminous coal. 

Sub-bituminous coal possesses a carbon content ranging from 35 to 45 percent. Sub-bituminous coal is around 251 million years old. 

It is among the younger and one of the most frequent types of coal geologically. Sub-bituminous coal accounts for 30% of all coal deposits. 

Sub-bituminous coal is widely utilized in power plants because it is used to produce steam to generate electricity. Sub-bituminous coal can also be liquefied and turned into petroleum and natural gas.

3. Bituminous

Bituminous Coal
Bituminous Coal | Image Credit – Flickr

Bituminous coal is the most prevalent and the second-highest quality coal (after anthracite). It is typically found in older coal beds (about 300 million years). 

It has a carbon content ranging from 45 to 86 percent. Because of this coal’s energy density, it releases a large amount of energy when burned. 

This type of coal is suitable for steel and cement manufacture and power generation and coke production due to its high carbon and low moisture content.

Bituminous coal is divided into two categories. These two types are used in distinct ways. The two types of bituminous coal are: 

  • Thermal

Thermal coal, also known as steam coal, has a slightly higher carbon content than metallurgical coal and is of a slightly higher grade. Thermal coal is used in power plants to generate steam. This steam generated from thermal coal is used to generate electricity. Thermal coal was also used to power steam locomotives.

  • Metallurgical

Metallurgical coal is, also known as Coking coal,  is used in the manufacture of coke. The coke produced is used to make steel since it contains carbon.

4. Anthracite

Anthracite coal
Anthracite coal | Image Credit – Wikimedia Commons

The highest standards grade of coal is anthracite is a dark black kind of coal. It is incredibly tough, has a low moisture content, and contains 86-97% carbon. 

Additionally, anthracite is the earliest variety of coal, having evolved from buried vegetation 350 million years ago. Anthracite is formed over a lengthy period and at extremely high temperatures. 

Anthracite can be found in various home products, most notably as a constituent of charcoal briquettes. 

Anthracite is also utilized for space heating since it is one of the cleanest types of fuel to burn, producing less smoke than other varieties. 

Because of its clean-burning features, anthracite may burn for longer than wood, making it a good choice for home heating stoves. 

Anthracite can reach extremely high temperatures when burned, making it an excellent fuel for fast heating up, releasing massive amounts of energy, and burning very hot.

Type of CoalCarbon Content (Source)Geological Time (Source)Uses (Source)
Lignite25%-35%350 million years agoPower generation
Sub-bituminous35%-45%300 million years agoPower generation and industrial manufacturing process
Bituminous45%-86%100 million years agoManufacturing cement, iron, steel, electricity generation
Anthracite86%-97%60 million years agoSpace heating, As smokeless fuel in households and industries.

Table of comparison for different types of coal

Peat is a predecessor of coal. Peat is a mushy, organic substance made up of partially decomposed plant and mineral materials. 

Peat undergoes physical and chemical changes (coalification) when exposed to high pressure and heat.

Coal and Pollution

Coal has historically been a stable source of energy globally, but it comes at a high price due to its extreme pollution. 

The chemistry that allows coal to produce high energy is the breakdown of carbon molecules. This reaction is the exact reason that causes a slew of severe environmental effects and toxins endangering people’s health. Two of the most critical issues are air pollution and global warming.

Coal has several detrimental effects on the environment due to its extraction, processing, ignition, waste storage, and transportation. 

There are numerous effects of coal mining and coal firing. Here is an overview of some major and adverse impacts:

1. Water Pollution

Acid Mine Drainage in Spain
Acid Mine Drainage in Spain | Image Credit – Flickr

Water that has been polluted by mining activities and is usually related to coal mining is known as abandoned mine drainage. 

It’s a principal source of water pollution in locations where mining has already occurred. Abandoned mines have several difficulties that have an impact on water quality. They are listed below: 

  • acid mine drainage: It is the most prevalent
  • alkaline mine drainage: It occurs when calcite or dolomite is found
  • metal mine drainage: It occurs when high amounts of lead or other metals drain from  abandoned mines

Sulfuric acid is formed when surface water (rainwater, snowmelt, pond water) and shallow subsurface water react chemically with rocks containing sulfur-bearing minerals, resulting in acidic water. 

The development and transport of acidic water rich in heavy metals is known as acid mine drainage.

The resultant fluids could be highly hazardous, and when mixed with groundwater, surface water, or soil, they could injure humans, animals, and plants.

2. Air Pollution

Emissions in the industrial area due to coal firing
Emissions in the industrial area due to coal firing | Image Credit – Pixabay

Coal-fired power plant pollution is linked to cancer, asthma, heart and lung disease, neurological issues, global warming, acid rain, and other severe environmental and public health consequences. 

These powerplants emit various pollutants, which contribute to a wide range of harmful environmental and health implications. 

When coal is burned to produce power, a mixture of toxic substances is released into the atmosphere and the human body. 

The pollutants include Sulfur dioxide, nitrogen oxides, particulate particles (PM), mercury. Both Coal firing and coal mining have similar effects on the environment. 

Particulate materials and gases such as methane (CH4), sulfur dioxide (SO2), nitrogen oxides (NOx), and carbon monoxide (CO)contribute to air pollution from coal mines.

3. Global Warming and Climate Impacts

The most critical and long-term global impact of coal is climate change. Coal is mainly made up of carbon, which combines with oxygen in the air to form carbon dioxide when burned. 

Carbon dioxide is a heat-trapping gas that acts as a blanket when released into the atmosphere, warming the globe over normal levels.

Drought, sea-level rise, flooding, harsh weather, and species loss are consequences of global warming. 

The severity of those consequences is directly proportional to the quantity of carbon dioxide we emit, including from coal-fired power plants. 

Coal contributes to nearly one-quarter of all energy-related carbon emissions in the United States. 

Coal-fired power stations emit one-third of all CO2 emissions, about equal to all modes of transportation combined (cars, SUVs, trucks, buses, aircraft, ships, and trains).

4. Radioactive effect

Coal is a source of fuel that is used to generate electricity. In coal, trace amounts of uranium, thorium, and other naturally occurring radioactive elements are present. 

Combustion, the process of burning coal in coal-fired power plants, produces wastes containing small amounts of naturally occurring radioactive material (NORM). These wastes can contribute to radioactive pollution if discharged into the environment.

5. Emissions from burning coal

Coal combustion produces significant emissions:

SNEmission from coalEffects
1Sulfur dioxide (SO2)Contributes to acid rain, and respiratory illnesses
2Nitrogen oxides (NOx)Contributes to smog and respiratory illnesses
3ParticulatesContributes to smog, haze, and respiratory diseases and lung disease
4Carbon dioxide (CO2)Principal greenhouse gas (GHG)
5Heavy metals like mercuryRelated to neurological and developmental damage in humans and other organisms
6Fly ash and bottom ashResidues created when power plants burn coal

Table showing the emissions from coal and their respective effects

Solutions

Energy transition
Energy transition | Image Credit – Energy Tracker Asia

According to a source, Coal provides 26% of fundamental energy requirements and 37% of power generation, compared to 23% by natural gas. 

Coal is a highly used fossil fuel on the planet. However, each year, coal combustion releases about 15 billion tonnes of carbon dioxide (CO2) into the atmosphere, the majority of which comes from power generation.

Coal power plants have aided in the development of industries worldwide. Still, their greenhouse gas emissions must be decreased swiftly to cut global emissions and address a significant contribution to climate change. 

Coal combustion for various reasons, such as power generation, produces a range of byproducts. Clean Coal Technologies are employed to manage these byproducts. 

Clean coal technologies are a collection of emerging answers to late-twentieth-century environmental issues, such as global warming caused by carbon dioxide emissions into the atmosphere.

Some of them are:

  • Coal Washing has been a routine procedure in affluent countries to reduce emissions of sulfur dioxide and fly ash emissions.
  • Electrostatic precipitators and fabric filters are widely used devices that can remove 99 percent of fly ash from flue gases.
  • Flue gas desulfurization reduces sulfur dioxide emissions to the atmosphere by up to 97 percent, depending on the sulfur content of the coal.
  • Low-NOx burners can cut nitrogen oxide emissions by up to 40% in coal-fired power plants. 

These are the methods to reduce the adverse effects of using coal. But for long-term use, we must focus on energy transition. We should work on the methods to manage and consequently reduce emissions. Increasing energy efficiency and transitioning to renewable energy resources shall help us create a greener and healthier planet.

Suraksha Pal is an Industrial Engineer currently pursuing my master’s degree in Renewable Energy Engineering at the Institute of Engineering, Pulchowk Campus. She has a keen interest in Renewable Energy and is passionate about sustainable development. She loves to express her views on these subjects through articles and blogs.