Fossil Fuels

Formed over millions of years, fossil fuels are essential to way we live and have greatly improved our standard of living. In 2019, 87% of the world’s primary energy is from fossil fuels. In this article I will give an overview of these fuel sources.

Coal, Crude Oil and Natural Gas


Millions of years ago, plants died and other plants died on these plants forming peat. With every additional layer of plant matter, the layers below were compressed and heated forming coal, a carbon rich and energy density material.

Coal was a critical fuel of the Industrial Revolution, being vital for steam engines and furnaces. Before coal, wood was the primary fuel used but over time the transition occured due to it being cheaper and more energy dense

Not all coal is the same, the two most important factors to classifying coal are its calorific value, how much energy can it release when burned, and its percent of carbon, a higher percent carbon is a cleaner burning fuel, other atoms hydrogen, sulfur, oxygen and nitrogen are common.

Type Produced
Bituminous 46%
Subbituminous 45%
Lignite 9%
Anthracite <1%

Coal can also be used to form coke, which improves its carbon purity. With the increased purity, it can be burned at higher temperatures which is important as a fuel and reducing agent in smelting iron ore.

Coal’s main use is the generation of electricity. This is done through boiling water into steam and spinning a turbine connected to a generator to create electricity. The overall thermodynamic efficiency is between 25% to 50%.

With newer technologies, the coal can be convert the coal to Syngas to a heat recovery steam generator. This powers a steam turbine and the waste heat can be used to heat a process or a building. This type of power plant could reach efficiencies up to 94%.


We can divide the end use into direct, either in a vehicle or in a manufacturing process and electricity where we make electricity first. Out of the three fossil fuels coal is the most difficult to transport and creates the most pollution. Therefore coal is popular in electricity generation outside of cities.

Direct 32%

Cost 3.9 USD/GJ
Specific Energy 0.030 GJ/kg
Energy Density 38 GJ/m^3
Emission Intensity 96 kgCO2e/GJ

Electricity Generation 68%

Cost 34.7 USD/GJ
Emission Intensity 265 kgCO2e/GJ

Crude Oil

Also known as Petroleum

Formed when zooplankton and algae died and buried underneath sedimentary rock and compressed and heated for millions of years. Crude oil is a combination of many different carbon chains. These carbon chains are separated using a process called fractional distillation, which seperates products by boiling point. Each chain’s has a different boiling point, for example methane has -161.5 degC and gasoline has a boiling point of 35 degC. These form products such as gasoline, diesel, methane, asphalt and chemicals to make plastic.

As the same with coal, not all crude oil is the same. Coal is usually classified to where it is found, for example “WTI”, “Bret” and “Oman”. The fuel in these large pockets all have similar characteristics.

When comparing these pockets, the two largest factors are its sulfur content and API gravity. API gravity is a function of density and is a measure on how heavy or light the crude oil is.

A high API is desired as more gasoline can be extracted from it and a low sulfur content is desired as less processing is needed to remove or capture the excess sulfur.


Most crude oil is refined into fuels that we use in a vehicle, gasoline, diesel and jet fuel. Very little is used in electrical generation, therefore I was not able to find a cost of generating electricity. For direct use, I am showing gasoline.

Direct 96%

Cost 14.0 USD/GJ
Specific Energy 0.046 GJ/kg
Energy Density 34 GJ/m^3
Emission Intensity 70 kgCO2e/GJ

Electricity Generation 4%

Natural Gas

Natural gas consists of mostly methane. Similar to crude oil and coal it is formed from decaying organic matter and therefore is found close to both coal and crude oil layers. When extracting oil in the 19th and 20th century it was common to burn off the unwanted natural gas.

Natural gas is very important and is mostly used directly. It is a low density fuel compared to crude oil and coal, so it has limitation were it can be used. It is relatively clean burning, making it useful domestic heating and electricity generation.

Natural gas is classified by how pure the methane is but is not a large a factor as seen in crude oil and coal. Processing the natural gas involves striping out sulfur, ethane, propane, butanes and natural gasoline so it is under limits set by the government.

When converting to electricity, most natural gas-fire electrical generation uses a combined-cycle gas turbines process with a conversion efficiency of 62%.


Natural gas is used both in direct consumption for heating, cooking and in electrical generation. It also has lower CO2 emissions than other fossil fuels making it more environmentally friendlily option in comparison.

Direct 58%

Cost 3.35 USD/GJ
Specific Energy 0.056 GJ/kg
Energy Density 22 GJ/m^3
Emission Intensity 56 kgCO2e/GJ

Electricity Generation 42%

Cost 27.46 USD/GJ
Emission Intensity 166 kgCO2e/GJ

I hope this was a useful introduction to the largest energy sources that we use. I always heard about renewable energy but feel like I didn’t understand the energy system until I studied fossil fuels. They all have their benefits and drawbacks but I believe they will be important in the future.

In this write up I made a lot of assumptions and each number depends on my factors.

Thank you for reading.