Combustion

Combustion is the chemical name for burning. It is a fast reaction between a fuel and oxygen that releases lots of energy as heat and light. Combustion has been one of humanitys most important tools for hundreds of thousands of years. It cooks our food, heats our homes, drives cars, planes and rockets, generates most of the worlds electricity, and powers many factories. Understanding combustion (what it needs to start, what it makes, and how to control it) is one of the most useful pieces of chemistry there is.

  • What it isFuel + oxygen -> heat + lightA fast oxidation reaction
  • Fire triangleFuel + oxygen + heatAll three needed to keep burning
  • Common productsCO2 + H2OWhen complete and clean
  • Incomplete combustionMakes CO + sootDangerous in poorly ventilated rooms
  • Hottest flameOxy-acetylene, 3,500 CUsed to cut steel
  • Energy from petrol46 MJ/kgEnough to lift a car 4 km

The fire triangle

For combustion to happen, three things must be present at the same time. Together they form what is called the fire triangle:

  • Fuel: a substance that can react with oxygen. Wood, paper, petrol, gas, candle wax, alcohol.
  • Oxygen: usually from the air (about 21 per cent of the air around you is oxygen).
  • Heat: enough energy to get the reaction started. After the reaction begins, the heat it releases keeps it going.

Take any one of these away and the fire goes out. That is how fire-fighting works:

  • A blanket cuts off oxygen.
  • Water cools the fuel below its burning temperature.
  • Moving the fuel away starves the fire.

What happens chemically

In a typical combustion reaction, the fuel (usually a hydrocarbon, made of hydrogen and carbon) reacts with oxygen from the air:

  • Fuel + oxygen -> carbon dioxide + water + lots of energy

For methane (the gas in your cooker):

  • CH4 + 2 O2 -> CO2 + 2 H2O

For propane (the gas in BBQ bottles):

  • C3H8 + 5 O2 -> 3 CO2 + 4 H2O

The result of a clean burn is just carbon dioxide and water, plus a lot of heat. The heat is what we use to cook, warm, drive and generate power.

Fact Fire is not actually a thing. It is a process. The bright flames you see are hot gases and tiny soot particles glowing because they are so hot. Different fuels produce different coloured flames. A candle flame is yellow because of glowing carbon. A gas burner flame is blue because the gas is more fully burnt. Sodium in salt makes orange flames, copper makes green, lithium makes red. Firework makers use these colours to create the dramatic effects you see in displays.

Complete and incomplete combustion

Combustion can be either complete or incomplete, depending on whether there is enough oxygen.

Complete combustion:

  • Plenty of oxygen
  • Products: carbon dioxide and water only
  • Clean blue flame, very hot
  • No smoke or soot

Incomplete combustion:

  • Not enough oxygen
  • Products include: carbon monoxide (CO), soot (carbon particles), unburnt fuel
  • Yellow or orange flame, cooler
  • Smoke and smell

Incomplete combustion is dangerous because it produces carbon monoxide, a colourless, odourless gas that is highly poisonous. People can die in their sleep if a faulty boiler or blocked chimney lets CO build up in a house. That is why every home with a gas appliance should have a carbon monoxide detector.

Different fuels

  • Wood: humans first controlled fire over 1 million years ago using wood. Still used today for cooking, heating and ambience.
  • Coal: powered the Industrial Revolution; now used mainly in power stations and steel making.
  • Natural gas (mostly methane): clean, easy to pipe; used for cooking, heating and electricity.
  • Petrol and diesel: liquid hydrocarbons distilled from crude oil; the main fuels for cars, lorries, ships and planes.
  • LPG (propane, butane): stored as a pressurised liquid; used in BBQs, camping stoves and rural heating.
  • Hydrogen: very clean (just produces water), but storing and transporting it is difficult. Used in some buses, trains and rockets.
  • Biofuels: made from plants like sugar cane or maize, or used cooking oil. Lower net carbon emissions than fossil fuels.
Did you know? The hottest flames humans can produce in industry come from oxy-acetylene torches, reaching around 3,500 degrees Celsius. They use pure acetylene gas burning in pure oxygen (rather than ordinary air) and are hot enough to melt steel. Welders use them to cut and join thick metal. The Space Shuttle main engines burned liquid hydrogen with liquid oxygen at over 3,300 degrees Celsius.

Combustion and climate change

Burning fossil fuels (coal, oil and natural gas) for energy releases carbon dioxide into the atmosphere. Over the last 200 years, humans have released so much CO2 that the average global temperature has risen by around 1.2 degrees Celsius and is still climbing. This is the main cause of climate change.

Reducing fossil fuel combustion (and switching to wind, solar, nuclear and other low-carbon sources) is one of the biggest scientific and political challenges of our time.

Try this Watch a candle carefully (with adult permission and supervision). Notice the layers in the flame: the dim blue zone near the wick (where fuel evaporates), the dark inner cone (where unburnt fuel rises), the bright yellow cone (where carbon particles glow as they burn) and the outer veil (where complete combustion finishes). Hold a cool spoon (with tongs, briefly) in the yellow cone: you will see black soot. Hold it in the very top of the flame: no soot. You are watching incomplete vs complete combustion right in front of you.
Deeper dive: how a car engine turns fire into motion

A modern car engine is one of the most impressive combustion machines ever built. Inside it, millions of controlled explosions per hour turn the chemical energy of petrol or diesel into motion.

The engine has 4 (or 6 or 8) cylinders, each with a piston that slides up and down. The cycle has 4 steps, repeated thousands of times per minute:

  1. Intake: the piston moves down, sucking in a fine mist of petrol and air through a valve.
  2. Compression: the piston moves up, squeezing the air-fuel mixture to a small fraction of its original volume.
  3. Power: a spark plug fires. The mixture burns very rapidly (almost an explosion), gases expand and shove the piston down with great force.
  4. Exhaust: the piston moves up again, pushing the burnt gases (CO2, water vapour, plus some pollutants) out through another valve.

The pistons in each cylinder all push on a shared crankshaft, which rotates and drives the wheels. A typical family car might do 2,000 of these cycles per minute in every cylinder while cruising on a motorway.

Diesel engines work slightly differently. Instead of using a spark, they squeeze the air so much that it heats up to over 500 degrees Celsius, and the fuel ignites as soon as it is injected. Diesels are more efficient but produce more nitrogen oxides and tiny soot particles, which is why cities are increasingly restricting them.

Even the best internal combustion engines waste most of the energy in the fuel. Around 60 per cent is lost as heat, 10 per cent runs the engines own systems, and only about 30 per cent actually drives the wheels. Electric motors waste much less and that, along with cleaner air, is why the world is shifting to electric cars.

For more, see oxidation and rusting and endothermic and exothermic.