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Basics of Fossil Fuels

  • Fuels have energy stored in chemical bonds, which is released as heat during the combustion reaction

  • Fossil fuels are chemical compounds made of carbon and hydrogen and were formed over millions of years in the earth

  • Coal, crude oil, and natural gas are the three major categories of fossil fuels, and each has different considerations for their extraction and conversion


Coal is a form of solid rock made mostly out of carbon that is mined from the ground. Coal was formed when dead plants and animals were compressed by pressure and heat over millions of years. After mining, coal undergoes a minimal amount of purification before being combusted in a power plant. The purification process includes removing large impurities such as rock, and pulverizing it into a fine powder to maximize the efficiency of combustion.



Fuels are essentially chemical compounds that have energy stored in their chemical bonds. During a combustion reaction, these bonds are broken and the energy is released as heat. We then use that thermal energy to do some sort of work, whether that is powering an engine, or heating steam to turn a turbine in an electrical generator.


General example of a combustion reaction:

Hydrocarbon fuel + oxygen → carbon dioxide + water + heat + other pollutants (from impurities in the fuel)


Coal, oil, and natural gas are fossil fuels, which means that they are hydrocarbon compounds formed from dead plants and animals over millions of years compressed in the earth.

Crude oil

Crude oil or petroleum is another type of fossil fuel that was formed by dead plants and animals being pressurized over millions of years. Instead of taking a solid form like coal, petroleum is a liquid extracted from the ground by drilling wells through which the oil flows.


Crude oil is a mixture of many different chemical compounds made primarily out of the elements carbon and hydrogen, and not all crude oil has the same exact composition. Before use, crude oil typically is separated (or “refined”) into different components. Gasoline, kerosene and diesel fuels are separated in an oil refinery through a process of distillation, which isolates components based on the temperature at which they boil. The refining process also creates many other products from crude oil, including the chemical precursors to plastics and chemicals used in pharmaceuticals, lubricants, paint, fertilizers, cosmetics, etc.


Natural gas (methane)

Natural gas is a gaseous form of hydrocarbons. Its main component is methane, which has the chemical composition of one carbon atom and four hydrogen atoms. Other components can include ethane, propane, butane, and more. Natural gas is often found and released along with other fossil fuels in the ground, particularly crude oil. More recently, however, large reserves of natural gas have been developed using the process of hydraulic fracturing.

Hydraulic fracturing (or fracking) is a technique where a fluid, often water, is pumped into shale and other rock formations at high pressure. The high pressure fluid causes cracks to form in the rock. Previously inaccessible fossil fuels trapped in the rock, mostly petroleum and natural gas, flow through the cracks to a well for extraction. Increased fracking in the United States has increased the contribution of natural gas to the country’s energy portfolio, supplying 38% of the nation’s energy in 2021. Critics of fracking point to its potential to contaminate drinking water and evidence showing that fracking has increased seismic activity in local areas.


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Questions for deeper thinking

  • Why have various forms of fuels been used over history? What are the advantages and disadvantages of each?

  • Why are fossil fuels used so much and hard for us to quit?

  • How has fracking technology influenced the US energy portfolio in recent years?

Sources and further reading

Page last updated: November 20, 2022​

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