Helena Joyce
Coal is a fossil fuel that has been a reliable energy source for the US and other countries. However, burning coal releases harmful pollutants and heavy metals into the environment, contributing to air pollution and global warming. Coal is ranked based on its carbon content, moisture content, energy density, and age. Lignite or brown coal, the lowest grade of coal, has a low carbon content, high moisture content, and low energy density, resulting in higher carbon dioxide emissions than harder black coals. On the other hand, anthracite, the highest-ranked coal, has a high carbon content, low moisture content, and high energy density, making it one of the cleanest-burning types of coal. It produces less smoke and burns longer than wood, making it a popular choice for home heating stoves. While efforts are being made to reduce emissions through pollution controls and emerging technologies like carbon capture and storage, the tremendous environmental and public health costs associated with coal pollution cannot be ignored.
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What You'll Learn
- Lignite coal, or brown coal, is the lowest grade with the least carbon concentration and highest moisture content
- Sub-bituminous coal is an intermediate grade with a carbon content of 35-45%
- Bituminous coal is a middle-grade coal with a carbon content of 45-86%
- Anthracite is the highest grade of coal with a carbon content of 86-97%
- The oldest coals tend to have lower moisture content and produce less pollution
Lignite coal, or brown coal, is the lowest grade with the least carbon concentration and highest moisture content
Lignite coal, also known as brown coal, is the lowest grade of coal with the lowest carbon concentration and the highest moisture content. It is a soft, brownish-black coal with a carbon content of 25–35% on a dry, ash-free basis. However, its inherent moisture content can be as high as 75%, resulting in a typical carbon content of just 25-35% when the coal contains both inherent moisture and mineral matter. The high moisture content of lignite leads to higher carbon dioxide emissions per unit of energy produced, as well as higher capital and transport costs.
Lignite is formed from naturally compressed peat, which exhibits the lowest carbon content of any precursor to coal (less than 60%). As sediment piles on top of the partially decaying organic matter, the temperature and pressure increase, and peat slowly becomes lignite. This process, known as coalification, involves the buried plant matter changing into an ever denser, drier, more carbon-rich, and harder material. Lignite is the first step in this process, and as such, it has the lowest carbon concentration of any type of coal.
The low heat value of lignite, typically ranging from 10 to 20 MJ/kg, makes it inefficient for transportation and trade on the world market. Lignite-fired plants are designed to burn specifically lignite, and it is expensive to turn them into coal-fired plants. As a result, lignite is often burned in power stations near the mines, such as in Poland, Australia, and Texas.
While lignite produces less heat per unit of carbon dioxide and sulfur released compared to other types of coal, it is the most harmful type of coal to human health. This is due to the presence of toxic heavy metals and naturally occurring radioactive materials in the coal fly ash produced from its combustion.
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Sub-bituminous coal is an intermediate grade with a carbon content of 35-45%
Coal is a combustible black or brownish-black sedimentary rock with a high amount of carbon and hydrocarbons. Coal is classified as a non-renewable energy source because it takes millions of years to form. Coal is classified into four main types, or "ranks", with each rank depending on the types and amounts of carbon the coal contains and the amount of heat energy the coal can produce. The four ranks are anthracite, bituminous, sub-bituminous, and lignite.
Sub-bituminous coal represents a transitional stage between low-quality lignite and higher-quality bituminous coal. Lignite, or brown coal, is the lowest grade of coal, with the lowest concentration of carbon (25-35%). It has a low heating value and a high moisture content. It is crumbly and has a low energy density of 18 MJ/kg. Lignite is the youngest fossil fuel produced, at approximately 60 million years old. Its low carbon content results in more carbon dioxide emissions than harder black coals.
Bituminous coal, on the other hand, is the second-highest quality of coal, with a carbon content ranging from 45-86%. It is the most abundant type of coal and one of the longest-buried fossil fuels, at approximately 300 million years old. Its energy density is relatively high at 27 MJ/kg. The high carbon and low moisture content of bituminous coal make it ideal for electricity generation and steelmaking.
Anthracite is the highest-quality coal, with a carbon content of nearly 95%. It is very hard, has a low moisture content, and can reach very high temperatures when burned. It has a very high energy density of 33 MJ/kg, the highest of any type of coal. Anthracite is the cleanest type of coal to burn, producing less smoke than other types.
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Bituminous coal is a middle-grade coal with a carbon content of 45-86%
Coal is a sedimentary deposit composed predominantly of carbon and is readily combustible. It is used primarily as fuel to generate electric power. Coal is formed from compacted, hardened, chemically altered, and metamorphosed plant remains. The four major types of coal, or "ranks", are differentiated by their carbon content, energy density, moisture content, volatile content, and appearance. The four types of coal, ordered from lowest to highest quality, are:
Peat
Peat is a soft, crumbly, dark brown substance that is formed from generations of dead and partially decaying organic matter. It is the first step in the formation of coal and exhibits the lowest carbon content (less than 60%) and the lowest energy density of all coal types.
Lignite
Lignite, also known as brown coal, is the lowest grade of coal and has the lowest concentration of carbon, ranging from 25% to 70%. It is brown in colour and is the youngest fossil fuel produced, with an age of approximately 60 million years. Lignite has a low heating value, a high moisture content, and exhibits a low energy density. Its high moisture content and lower carbon content result in more carbon dioxide emissions than harder black coals.
Bituminous Coal
Anthracite
Anthracite is the highest rank of coal and is a hard, brittle, and black lustrous coal, often referred to as hard coal. It contains a high percentage of fixed carbon (86%-97%) and generally has the highest heating value of all coal ranks. Anthracite is the oldest type of coal, having formed from biomass that was buried 350 million years ago. It has a very high energy density and burns hotter and cleaner than other types of coal, producing less smoke.
While coal is a major source of energy, its combustion contributes significantly to air pollution and carbon dioxide emissions. Efforts to reduce pollution from coal include the use of scrubbers, electrostatic precipitators, and baghouses to capture and remove pollutants. Additionally, research is underway to address emissions through methods like carbon capture and sequestration.
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Anthracite is the highest grade of coal with a carbon content of 86-97%
Anthracite is the highest grade of coal, often referred to as "hard coal". It contains between 86% and 97% carbon, the highest percentage of all coal types, and generally has the highest heating value of all ranks of coal. It is hard, brittle, and black lustrous, with a high energy density of 33 MJ/kg, the highest of any type of coal. Anthracite is also one of the cleanest types of coal to burn, producing less smoke than other types. It burns with a short, blue, and smokeless flame, making less soot. This is due to its high percentage of fixed carbon and low percentage of volatile matter.
Anthracite is not as commonly mined as other ranks of coal, accounting for less than 1% of coal mined in the United States in 2022. It is mainly used by the metals industry and for domestic heating. Anthracite is more expensive than other types of coal, making it less suitable for power plant use.
The other types of coal are ranked as follows:
- Bituminous coal, which usually has a high heating value and is used in electricity generation and steel making.
- Sub-bituminous coal, which is an intermediate stage between low-quality lignite and higher-quality bituminous coal. It has a carbon content of 35%-45%.
- Lignite, which is also known as brown coal, is the lowest grade of coal with the lowest concentration of carbon, ranging from 25%-35%. It is crumbly with a high moisture content, contributing to its low heating value.
Higher quality coal, such as anthracite, produces less smoke, burns longer, and provides more energy than lower-quality coal.
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The oldest coals tend to have lower moisture content and produce less pollution
Coal is a sedimentary rock formed from peat by the action of pressure from rocks laid down later on top. The four major types of coal are anthracite, bituminous, sub-bituminous, and lignite. Anthracite is the oldest type of coal, having formed from biomass that was buried 350 million years ago. It is very hard, has a low moisture content, and a carbon content of nearly 95%. Bituminous coal is the second-highest quality of coal, with a carbon content that ranges from 76-86%. It is the most abundant type, and one of the longest-buried fossil fuels—with an age of approximately 300 million years. Sub-bituminous coal is an intermediate stage between low-quality lignite and higher-quality bituminous coal. It is approximately 251 million years old. Lignite or brown coal is the lowest quality and youngest fossil fuel produced, with an age of approximately 60 million years.
Lignite has a high moisture content, which can cause problems in transportation and storage due to its susceptibility to spontaneous combustion. The moisture content in coal is an important parameter in coal analysis as it is needed for determining the calorific (heating) value and handling properties of a coal. Anthracite's low moisture content means that it produces less smoke than other types of coal. It is used for space heating as it is one of the cleanest types of coal to burn. Its clean-burning properties allow anthracite to burn longer than wood, making it appealing to use in home heating stoves.
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Frequently asked questions
Anthracite, a dark black form of coal, is the cleanest type of coal and produces less smoke than other types. It has a low moisture content and a carbon content of nearly 95%.
Lignite, or brown coal, is the lowest grade of coal with the lowest concentration of carbon. It has a high moisture content, which results in more carbon dioxide emissions than harder black coals.
Coal power plants emit harmful pollutants such as carbon monoxide, arsenic, lead, cadmium, mercury, and other toxic heavy metals. These pollutants have been linked to asthma, cancer, cardiovascular issues, and neurological problems.
Coal is a significant contributor to air pollution and global warming due to the release of carbon dioxide (CO2) and other toxic pollutants. It is also linked to environmental issues such as acid rain and water pollution from coal ash storage and landfills.
Carbon capture and storage (CCS) technologies can help capture and store CO2 emissions from coal plants. Additionally, pollution control devices and scrubbers can reduce emissions of fly ash, sulfur dioxide, nitrogen oxides, and particulate matter.
