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Fire Fuels: Solids, Liquids & Gases (Pyrolysis, Vapor Density & Flammable Range)
Why everything burns as a vapor, how solids, liquids and gases behave differently, and how flammable range and vapor density change your size-up.
- Flame burns gases. Solids pyrolyze, liquids vaporize, and gases are already ready to burn.
- Surface area, position and moisture control how fast a solid fuel burns.
- Flash point, specific gravity, solubility and volatility decide how a liquid fire behaves and which agent works.
- Vapor density tells you whether a gas rises or pools at your feet. Cold vapors can sink even if the gas is lighter than air at room temperature.
- A fuel only burns inside its flammable range. Too rich is not safe, because ventilation can pull a mixture back into range.
Everything burns as a gas
Fuel may be a solid, a liquid or a gas, but flame only burns gases. To start combustion, the fuel has to be turned into a vapor by heat. Solid fuels give off flammable gases by pyrolysis, the chemical breakdown of a material by heat. Liquids give off vapor by vaporization, the same process as water evaporating in the sun. Gases need no heat at all to be ready, and that is why gas fires are so hard to control.
Chemically, the fuel is the reducing agent in the reaction. Most common fuels contain carbon, hydrogen and sometimes oxygen. They fall into two big families: hydrocarbon-based fuels (gasoline, fuel oil, most plastics) and cellulose-based materials (wood, paper, cotton).
Solid fuels and pyrolysis
Three things control how fast a solid fuel burns: surface area compared with mass, position, and moisture. More surface area for a given mass heats faster and pyrolyzes faster, which is why sawdust and wood shavings burn so readily while a heavy timber resists. Orientation matters too. A vertical surface burns faster than a horizontal one, because flame contact and convection preheat the material above it. That is why fire races up a wall, a curtain or a stair.
Wood does not suddenly ignite at one temperature. It decomposes in stages as it heats. The stages below are commonly cited approximations and shift with the species and how fast it is heated.
| Temperature | What is happening |
|---|---|
| Around 200°C (390°F) | Water vapor, carbon dioxide and acids are released |
| 200 to 280°C (390 to 535°F) | Less water vapor, some carbon monoxide; still mostly absorbing heat |
| 280 to 500°C (535 to 930°F) | Heat-releasing reactions with flammable vapors and particles |
| Above 500°C (930°F) | Mostly charcoal remains |
Today’s fuel load burns differently
The fuel in a modern living room is not what it was 50 years ago. Natural wood, cotton and wool have been replaced by polyurethane foam, synthetic fibers and engineered wood, which release heat faster. UL’s Firefighter Safety Research Institute burned matched rooms to compare them. Every room furnished with modern products reached flashover in less than five minutes. The fastest room with legacy furniture took just over 29 minutes. You will see what that does to tactics in Phases of Fire.
Liquid fuels
Liquids take the shape of their container and, in a spill, spread to the lowest ground they can find: basements, trenches, storm drains. Four properties decide how hard a liquid fire is to fight.
- Flash point is the lowest temperature at which a liquid gives off enough vapor to ignite. A flammable liquid has a flash point below 100°F (37.8°C). A combustible liquid is at or above it. Gasoline flashes far below freezing, so on a cold night it still gives off ignitable vapor.
- Specific gravity compares a liquid’s weight with water (water = 1). Most flammable liquids are lighter than water and float on it, so water can spread the burning layer. There are exceptions: carbon disulfide is heavier than water and sinks.
- Solubility decides what foam works. Hydrocarbons such as gasoline do not mix with water. Alcohols and other polar solvents do, and can break down ordinary foam, so they call for alcohol-resistant foam.
- Volatility is how easily a liquid gives off vapor. The more volatile it is, the more vapor you have to worry about before the liquid itself catches.
Gas fuels and vapor density
Gases fill their container but have no volume of their own. What matters on scene is vapor density: how heavy the gas is compared with air (air = 1). A gas lighter than air rises and disperses. A gas heavier than air hugs the ground, follows terrain, and collects in low spots such as basements, sewers and pits.
Natural gas (mostly methane) rises. Propane has a vapor density of about 1.55 and sinks. A leak of one can be a ceiling problem and the other a floor problem, so it matters which one you are chasing. A common memory aid for lighter-than-air gases is HA HA MICEN: Hydrogen, Anhydrous ammonia, Helium, Acetylene, Methane, Illuminating gas, Carbon monoxide, Ethylene, Nitrogen. Several of these are only slightly lighter than air, and cold vapors, such as refrigerated ammonia, can sink. Do not count on “lighter than air” to keep a gas away from you.
Many fuel gases, propane and butane among them, are stored as liquids under pressure. When one leaks, the liquid boils off into a very large volume of vapor, roughly 270 times the liquid volume for propane. A small liquid leak can produce a large vapor cloud quickly.
Flammable range: the mixture has to be right
A fuel vapor will only burn when it is mixed with air in the right proportions. Too little fuel is too lean. Too much is too rich. In between is the flammable range, bounded by the lower explosive limit (LEL) and the upper explosive limit (UEL).
| Fuel | LEL | UEL | Vapor vs. air |
|---|---|---|---|
| Gasoline vapor | 1.4 | 7.6 | Heavier |
| Propane | 2.1 | 9.5 | Heavier (about 1.55) |
| Methane (natural gas) | 5.0 | 15.0 | Lighter |
| Hydrogen | 4.0 | 75 | Much lighter |
| Acetylene | 2.5 | 100 (can decompose) | Slightly lighter |
Limits are quoted at about 70°F (21°C) and shift with temperature, pressure and the oxygen concentration. Use the numbers on the safety data sheet or in the NIOSH Pocket Guide to Chemical Hazards, and trust your gas meter. A meter reading of 100% LEL means the gas has reached the lower limit: for methane that is about 5% by volume, so 10% LEL is only about 0.5% methane.
“Too rich” is not the same as safe. A space above the UEL can pass back through the flammable range as it is ventilated and diluted. The same principle explains why hot, fuel-rich smoke in a closed room is waiting for air, which is the heart of the backdraft problem you will meet later.
Kingman, Arizona (1973): a propane tank car and a short fuse
On July 5, 1973, workers at the Doxol Gas bulk plant in Kingman, Arizona, were unloading a propane rail car when a leak caught fire. The Kingman Fire Department responded and began applying water to cool the tank. The pressurized car then failed in a boiling liquid expanding vapor explosion, a BLEVE. Thirteen people died, most of them firefighters, and many more were hurt.
Kingman became one of the best-known cases in fire service training. The propane was a liquid under pressure, the flames were heating the tank, and when it failed it released a huge volume of vapor that ignited in a fireball. The lesson: size up the container before you size up the fire. A pressurized tank exposed to flame can fail with little warning, so distance, shielding, cooling from a protected position and early evacuation of the danger zone are not optional.
Fireground talk
- Fuel load
- The amount of combustible material in a space. High fuel loads mean faster, bigger fires.
- Smoke is fuel
- Smoke is made of unburned pyrolysis products. Hot smoke can ignite when it finds air.
- LEL meter
- A gas meter that reads as a percentage of the lower explosive limit, not percent of gas.
- BLEVE
- Boiling liquid expanding vapor explosion: a pressurized liquid container fails and the liquid flashes to vapor.
- Pooling
- Heavier-than-air vapors collecting in low spots like basements and pits.
Practice it in the simulators
Study questions
What is pyrolysis?
The chemical decomposition of a solid by heat, which releases the flammable gases that actually burn. It is how solid fuels become vapor.
What is the difference between a flammable liquid and a combustible liquid?
A flammable liquid has a flash point below 100°F (37.8°C). A combustible liquid has a flash point at or above that temperature.
Does propane rise or sink when it leaks?
It sinks. Propane vapor is about 1.55 times heavier than air and collects in low areas. Natural gas, which is mostly methane, is lighter than air and rises.
What does 10% LEL mean on a gas meter?
That the gas concentration is 10% of the way to the lower explosive limit. For methane, with an LEL of about 5% by volume, 10% LEL is roughly 0.5% methane in air.
Sources and further reading
- OSHA Chemical Database: Propane (NIOSH data: LEL 2.1%, UEL 9.5%, vapor density 1.55) — OSHA / NIOSH
- CAMEO Chemicals: Methane — NOAA
- Modern Residential Fires (Fire Safety Journal, Issue 1) — UL Firefighter Safety Research Institute
- 50 Years Later: The Kingman BLEVE — Firehouse
- NIOSH Pocket Guide to Chemical Hazards — CDC / NIOSH
- NFPA 30, Flammable and Combustible Liquids Code — NFPA (print standard)
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