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Basics of Fuel Conversion

  • Fuels can be used directly by combustion for heating, cooking, and transportation

  • Energy from fossil fuels, namely coal and natural gas, can be converted to electricity via combustion in power plants

  • In fuel cells, energy from fuels is converted directly into electrical energy

Fuels, whether fossil fuels or renewable fuels, have energy stored in chemical bonds within molecules. When the bonds are broken, that energy can be used for various purposes.

Using fuels directly

Humans use fuels directly for energy in several ways. One of these is heating of both water and air, by using natural gas or heating oil in furnaces and hot water heaters, for example. Another is cooking, as anyone who has used a natural gas-powered stove or a propane grill can tell you. One of the largest direct uses of fuels is for transportation.

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In the United States, as of 2021, transportation accounts for 28% of all energy use. Of that, 94% is fossil fuels and 6% biofuels. Petroleum products (gasoline, diesel, and jet fuel) account for the large majority of all energy used in the transportation sector. These liquid fuels are relatively energy dense and are easily stored and transported, meaning that vehicles can travel long distances into remote areas with their tank of fuel.


Most vehicles operate today with internal combustion engines. In these engines, a crankshaft moves to compress fuel and oxygen inside the chamber. Once a spark plug ignites the combustion reaction, the energy stored in the chemical bonds is released as heat and is converted into kinetic energy required to push the piston downard and to propel the crankshaft around. This powers the rotation of the wheels of a land vehicle or the propellers of a water vehicle.

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Figure modified and annotated from source. Car and Driver Magazine

Power plants: converting fuels to electricity via combustion

Stationary power plants convert energy from fuels into electricity. The process happens by combusting the fuel, and using the heat energy to turn a turbine in an electrical generator.

 

As of 2020, coal-fired power plants generated ~40% of the world’s electricity. Coal-fired power plants use enormous amounts of coal: a large power plant requires 10,000 tonnes of coal per day, which is about the equivalent of a large train load every day (100 cars of 100 tonnes each). Learn more about coal and other fossil fuels in our Fossil Fuels module. In a coal-fired power plant, the coal is pulverized into a fine powder, which is then burned in a boiler to generate steam. The steam turns a turbine to generate electricity, then is condensed in cooling towers and recirculated through the system. 

 

Large quantities of water are needed to operate a coal-fired power plant. Water is used to remove coal impurities. This process reduces the amount of pollutants released into the atmosphere, but pollute the water and local ecosystems instead. Water also is used in the cooling process, some of which is released as steam from cooling towers.

 

Not all of the components of coal can be combusted in coal-fired power plants, and a significant amount is left as ash. Coal ash is one of the largest industrial waste products generated in the United States and around the world. The coal ash contains high concentrations of pollutants, including heavy metals like arsenic, lead, mercury, and uranium.

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Figure modified and annotated over this source.

In recent years, more power plants have been built that burn natural gas as their fuel. These plants are smaller, quicker to build, and quicker to turn on and off than coal-fired power plants. They operate with a combustion turbine, which operates similarly to an internal combustion engine. Air and fuel are compressed and then combusted. The energy from the heated gas expanding turns the turbine in an electrical generator. 

 

Natural gas power plants have high thermal efficiencies and produce fewer toxic pollutants like nitrogen oxides (NOx), sulfur oxides (SOx), and particulate matter than coal power plants. These are the main components of smog. However, they still generate significant amounts of greenhouse gasses, including carbon dioxide and escaped methane. See the climate change module for more information on greenhouse gasses.

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Source. Tennessee Valley Authority

Fuel cells: converting fuels to electricity directly

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Source. Fuel Cell and Hydrogen Energy Association

Individual cells are combined into stacks to increase the electricity output. Fuel cells have applications in transportation, industrial/commercial/residential buildings, and long-term energy storage as part of the electrical grid.

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The fuel cell industry is rapidly growing, but is still an active area of research. Researchers are exploring new materials for all the components with the goal of improving their efficiency, durability, and performance. Cost is another factor to improve, since most commercial fuel cells rely on platinum electrodes.

Personal Vehicle Fuel Cell

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Grid Level Fuel Cell

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Source.

Fuel cells convert fuels directly into electricity through a chemical reaction, not combustion. They are scalable, efficient, clean, and quiet. If hydrogen is used as the fuel, as it most commonly is, the only products are electricity, heat, and water. Unlike batteries, fuel cells will continue producing electricity as long as they are supplied with more fuel.


A fuel cell consists of two electrodes immersed in electrolyte and separated by a membrane. At the negative electrode (the anode), hydrogen is split into electrons and protons. The electrons go through an external circuit, creating a flow of electricity. The protons migrate through the electrolyte to the positive electrode (the cathode), where they unite with oxygen and the electrons to produce water and heat.

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Questions for deeper thinking

  • What are ways that we could better incorporate and think about waste and byproducts in conversations about fuel conversion?

  • What would it take to actually reduce the amounts of fossil fuels we as a society use?

Sources and further reading

Page last updated: August 31, 2022​

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