Ignition Temperature: Why a Fuel Must Be Heated Before It Burns

#Why heating is necessary

Combustion is a chemical reaction, and chemical reactions need activation energy. At room temperature, the molecules of paper and the oxygen around them are in constant contact and nothing happens, because they lack the energy to break the existing bonds and start reacting.

Heating supplies that activation energy. Once the reaction starts, it releases more heat than it needs to keep going — which is why it sustains itself after the match is removed. Ignition temperature is simply the threshold at which that self-sustaining condition is reached.

This is also why kindling exists. A log will not reach its ignition temperature from a match, because its mass conducts heat away faster than a match supplies it. Thin kindling has little mass and heats quickly, and it then supplies enough sustained heat to bring the log up to temperature.

#Typical ignition temperatures

Substance	Approximate ignition temperature
White phosphorus	~30 °C
Petrol	~246–280 °C
Kerosene	~220–295 °C
Diesel	~210–260 °C
Paper	~230 °C
Wood	~300 °C
Coal	~400 °C
Methane	~537 °C
Hydrogen	~500–571 °C
Charcoal	~350 °C

White phosphorus is the extreme case: it ignites spontaneously in air at ordinary temperatures, which is why it is stored under water and is not a fuel.

#The petrol paradox

Petrol's ignition temperature is around 246 °C — higher than paper's. Yet petrol is far more dangerous. Why?

Because ignition temperature is not the whole story for liquids. What burns is the vapour, not the liquid, and the relevant measure is the flash point: the lowest temperature at which a liquid produces enough vapour to form an ignitable mixture with air.

Fuel	Flash point	Ignition temperature
Petrol	~−43 °C	~246 °C
Kerosene	~38–72 °C	~220 °C
Diesel	~52–96 °C	~210 °C

Petrol produces ignitable vapour at any ordinary temperature, so a spark near an open container finds a ready fuel-air mixture. Kerosene at room temperature does not produce enough vapour to ignite from a spark; it has to be heated first, or drawn up a wick where a small quantity is heated by the flame itself.

This is the entire reason kerosene appliances can exist for domestic use while petrol appliances cannot, and why putting petrol in a kerosene heater is catastrophic rather than merely inefficient.

#Why an ideal fuel has a moderate ignition temperature

Both extremes create problems:

Too low. The fuel may ignite accidentally from ambient heat, friction or a small spark. Storage and transport become hazardous. Spontaneous combustion in coal stockpiles and haystacks is a real phenomenon caused by slow oxidation raising internal temperature to the ignition point.

Too high. The fuel is hard to light and may need a supporting fuel to start. Pulverised coal in a power station requires oil or gas burners to light off, precisely for this reason.

A moderate value gives safe storage and practical ignition — one of the standard characteristics of a good fuel.

#Why not every combustible substance is a fuel

This is the natural follow-up question, and the answer is that combustibility is necessary but nowhere near sufficient. A practical fuel must also:

• Have a worthwhile calorific value — magnesium burns brilliantly but is far too valuable and reactive to burn for heat
• Be available in quantity at acceptable cost
• Be safe to store and handle — sodium burns but reacts violently with water
• Burn at a controllable rate — an explosive is combustible and useless as a fuel
• Produce acceptable by-products — many plastics burn readily but release dioxins and hydrogen chloride

So: all fuels are combustible, but not all combustible substances are fuels.

This is the mechanism behind two practical guides: why green wood is so hard to burn, and why kerosene rather than petrol is the liquid fuel used in homes.

#The compact answer

A fuel must be heated to its ignition temperature before it starts burning. Ignition temperature is the lowest temperature at which a substance catches fire and continues to burn. Below this temperature, the fuel will not burn even if oxygen is available. A good fuel should have a moderate ignition temperature — neither so low that it catches fire accidentally, nor so high that it is difficult to ignite.