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Definition of Coal – Classification and Facts

Anthracite coal

Coal still represents today a major source of energy and heat in the energy mix of almost all the countries of the planet, despite the fact that this old fossil fuel is very harmful for the environment and severely affects the public health.

Being a cheap source of power and heat, coal is massively used today in many developing countries because all these nations require cheap electricity to bring the level of their economy up to the level of the developed countries.

Coal Definition

Coal is a fossil fuel represented by a black or brownish-black combustible rock that was formed from the prehistoric vegetation that lived on our planet millions of years ago and was buried underneath many layers of soil.

The remains of the prehistoric vegetation (ferns, plants and trees) have been turn into coal after millions of years of heat and high pressure.

The prehistoric vegetation (exactly like the today’s vegetation) stored solar energy during their life on the planet using the photosynthesis process.

That energy provided by the Sun was conserved this way in green plants and can be found today in the sedimentary rock known as coal.

Coal classification

There are several types of coal available today:

Peat consist of partly decayed vegetation and maybe some mineral matter. This soft organic material is named peat and is not considered coal.

Peat needs to be placed under high pressure and heat to become coal after a very long period of time.

1. Lignite

Lignite is considered the youngest type of coal (its age is around 60 million years) even if we know that to become coal the remains of the prehistoric vegetation have been subjected to millions of years of high pressure and heat.

Lignite (known as brown coal) has a lower heat content, which means that it gives off pretty low amounts of energy compared to other types of coal.

However, the fact that lignite is located pretty close to the ground surface makes it very accessible.

Being a soft, brownish-black combustible rock, lignite has a pretty low carbon content between 60 and 70 percent (contains a high amount of moisture), an ash content between 6 and 19 percent, and a heat content of around 17 MJ/kg (megajoules per kilogram) or 7,000 to 8,500 BTU (British Thermal Units) per pound.

Despite the lower carbon content, lignite is used today as fuel for coal-fired power stations that are burning lignite for energy production (electricity).

About 80 percent of the lignite production is used for energy generation, while 13 percent is used to generate synthetic natural gas, and the rest of 7 percent is used to produce different fertilizer products.

2. Sub-bituminous Coal

Sub-bituminous coal is considered in the U.S. and Canada an intermediate coal between lignite and bituminous coal and is called black lignite.

However, in other countries sub-bituminous coal is considered to be the brown coal.

The color of this coal rock is grey-black or dark brown (its age is around 251 million years), and has a carbon content between 70 and 75 percent (10 to 25 percent moisture), ash content up to 10 percent, heat content between 18 and 23 MJ/kg, or 9,000 to 11,500 BTU per pound.

Sub-bituminous coal is primarily used as fuel to heat water, and the steam resulted is used to spin large and powerful turbines to generate electricity (steam-electric power plants).

This type of coal can be liquefied in order to produce petroleum and gas.

3. Bituminous Coal

Bituminous coal also known as black coal is a pretty soft coal rock that contains bitumen or asphalt (a tar-like substance).

In terms of quality, bituminous coal can be ranked between lignite and anthracite.

Being an organic sedimentary rock with an age of about 300 million years, bituminous coal has a carbon content between 75 and 85 percent, an ash content between 6 and 12 percent, and the heat content ranges from 23 to 35 MJ/kg or 11,500 to 17,500 BTU per pound.

In coal mining, the extraction of bituminous coal demands the highest safety procedures (attentive gas monitoring, proper ventilation and a very vigilant site management) because this type of coal releases the largest amounts of firedamp (a flammable gas specific to coal mines).

Bituminous coal is used today in the production of steel and cement, coke production, and also to generate electricity.

Bituminous coal has two subtypes: metallurgical coal and thermal coal.

Having a slightly higher carbon content, thermal coal (also known as steam coal) is mainly used for electricity generation, while metallurgical coal is used to produce coke, which is a source of carbon in the production of steel.

4. Anthracite Coal

Anthracite is the coal rock with the highest quality grade because it has the highest carbon content and the lowest moisture content.

Anthracite coal also called hard coal is the oldest type of coal rock (its age is around 350 million years), and its color features submetallic luster.

Anthracite has a carbon content between 92 and 98 percent, an ash content between 12 to 20 percent, and the highest heat content among all coals (between 26 and 33 MJ/kg or 13,000 to 16,500 BTU per pound).

Being the coal with the highest purity, anthracite burns cleaner and reaches higher temperatures than other types of coal.

Anthracite is used to produce charcoal briquettes for several household applications and burns cleaner than other types of coal.

It is also used in residential heating (heating stoves) due to its high energy density (33 MJ/kg) and reaches very high temperatures when burning (produces more heat than wood and burns a longer period of time).

Coal Facts

1. Coal consumption is still high in the U.S.

In terms of energy consumption, coal accounts for about 14 percent in the energy mix of the United States.

Globally, coal accounts for about 17% in the energy mix of the planet, and this share will further decrease in the future.

2. Anthracite is the coal with the highest heating value

Anthracite is a hard and black coal rock that shines like a piece of metal and burns with a smokeless blue flame reaching a very high temperature, and releasing huge amounts of heat, which makes it suited for residential heating.

3. Lignite is the coal with the lowest heating value

Lignite, sub-bituminous coal and bituminous coal are coal forms that are associated with sedimentary rock, while anthracite undergoes metamorphism being linked to metamorphic rocks.

4. Coal is the dirtiest fossil fuel

Due to the massive air, soil and water pollution produced by the energy sector that uses coal to generate electricity, coal is becoming less popular in the energy mix of the planet, and the energy gap created this way is covered by natural gas and renewable energy.

5. Coal as energy source

Coal mines are sending the extracted coal to power plants, where is crushed and burned to produce steam.
The steam is used to spin powerful turbines connected to massive electrical generators that will generate electricity.

What is Coke?

Coke is produced when we heat coal (bituminous coal) under controlled conditions in the absence of air.
This process will remove some of the volatile materials and will increase this way the carbon content of the rock.

Coke is mainly used for metal processing and for other uses when a hot burning flame is required.

7. Coal is used to produce a wide variety of products

Coal is also used in in the manufacturing process of plastics, synthetic rubber, linoleum, insecticides, some medicines, solvents, paints and many others.

8. Coal is used to produce gasesous and liquid fuels

Coal can be converted in gaseous fuels and fuels in liquid form.

9. United States is still a major producer of coal10. Coal is also used for heating

Using coal for residential heating (heating stoves) and to generate electricity in coal-fired power plants, releases a large amount of greenhouse gas emissions, which are severely affecting the public health and are destroying the environment.

11. Coal will become history in a couple of decades

Being an old and polluting source of power, coal has a short future ahead, because it is already replaced by cleaner forms of energy such as natural gas and by renewable energy sources such as solar, wind, hydro, biomass, geothermal, etc.

Coal Effects on the Environment

Burning coal generates a large amount of heat, but also an excessive amount of smoke and soot, sulfur oxides (SOx that are responsible for acid rains), nitrogen oxides (NOx), carbon monoxide, carbon dioxide and also some dangerous impurities (arsenic and mercury).

Releasing such a large amount of harmful emissions when burning, coal is considered a main contributor to the high level of air pollution produced on our planet.

Article written by:

I am a writer and reporter for the clean energy sector, I cover climate change issues, new clean technologies, sustainability and green cars. Danny Ovy

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