Fuel & Heat

FUEL

Fuel is the reducing agent in a fire. Put simply, it breaks down during the chemical process into seperate parts. What was once wood becomes various gases and carbon deposits. That's as far as I'm going on the chemistry angle. As much as I'd like to type out chemical equations, I promised I wouldn't.

We always classify fuels as solids, liquids, or gases. These are called the fuel's physical state. Usually the presence (or lack) of heat will determine which state you'll find a fuel in. For example, if water were a fuel, it would be found as a solid below 32ºF(0ºC), or as a gas at 212ºF(100ºC).

Solid fuels will maintain their shape (yes, Jell-O is a solid as long as it's still in a cube!), and most will expand or contract in the presence of heat or cold, respectively. It's also common for them to become more flexible when heated, and brittle when they cool.

Combustion itself takes place on the surface of a fuel, where it can come into contact with oxygen and heat to create that lovely fire tetrahedron. Using a wooden log as an example, it's obvious that the center of the log cannot burn because it doesn't come into contact with oxygen until the outer layers burn away. The process of the fuel decomposing is known as pyrolysis. Molecules from the wooden log are reacting with the air and breaking apart (the visible result of which is a flame, and a smaller log).

One last thing on solid fuel. A figure known as the Surface to Mass Ratio (STMR) can be important. The higher the number, the more easily something will burn. Our "wooden log" example has a very poor surface to mass ratio because it is quite dense, and doesn't offer much surface area. A sheet of newspaper, however, has a very high surface to mass ratio. It weighs almost nothing, and offers a fair amount of surface area. It's the perfect kindling for your beach party bonfires.

Liquid fuels don't have a definite shape of their own, but are instead formed by their container. Most liquids are similar to solids in that they expand when heated, and contract when cooled. With enough heat, a liquid will vaporize into a gas. Of course, some will vaporize more easily than others. Water needs to be given a lot of heat before it turns to steam, but gasoline can vaporize at room temperature. Liquids do not typically compress well, which means that pressure doesn't have much of an effect on the volume they take up. Water, for example, will be the same volume at any temperature (until it begins to boil into steam).

We use a figure with liquids called the Surface to Volume Ratio (STVR). It's very similar to STMR in that the higher the number, the more fire you're gonna get at any given moment. Dumping out gasoline in the street will give you a high surface to volume ratio, whereas keeping it in a gas can will keep that number quite a bit lower. Makin' sense? Good. I'm moving on.

Gases are where it's at for combustion. They refuse to offer an independant shape or volume. The air we breathe is a mixture of many gases (78% Nitrogen, 21% Oxygen, 11% Other). This situation is optimal for the burning process because our fuel is mingling with oxygen without restriction.

Other than providing the necessary heat to begin the combustion process, the gaseous fuel must have a specific ratio of fuel to air to burn. Too much fuel will displace all of the oxygen (this is called rich), and too much air won't provide enough fuel (this is called lean). Knowing these ratios will be a big part of your job if you decide to go Haz Mat.

Sometimes an oxidizing agent will substitute for Oxygen in the burning process. On the molecular level, it's basically the same thing. Our fuel and heat are finding the oxygen it needs from a more complex compound with oxygen in its chemical makeup.

HEAT

By now you should see where this is going. Heat is usually what's missing when you want to start a fire. The good news is that once the chain reaction has begun, it will be readily available.

If our fuel is not a gas, the heat produced by the fire will begin to bring the fuel to a temperature where it becomes one. This isn't observable with a block of wood, but this can be easily noticed with gasoline. Although it begins as a liquid, even the slight amount of energy given by direct sunlight will vaporize gasoline (and consequently make it easy to smell).

We commonly produce enough heat to ignite a fuel in three different ways:

Mechanical Energy - By rubbing two objects together to cause friction. (1 Caveman + 2 Sticks = Fire!)

Chemical Energy - With chemical reactions that are exothermic. (I promised no chemistry. Moving on!)

Electrical Energy - With electrical current. A byproduct of electric current is heat, which can be more than enough to ignite a nearby fuel.