It may seem like I am joking. People put iron and steel pans on hot gas flames for cooking on a regular basis and they are not catching fire.
The idea that you can burn iron and produce excess heat seems ridiculous. In fact it isn’t, and if you ever read my story on how sugar can be used as rocket fuel, you may already have a clue.
However when it comes to burning metals, there is more interesting physics going on. Your iron pan does not catch fire because metals are very good conductors of heat. That means when a gas flame on the stove heats up a point on the pan, heat quickly flows from that point to the rest of the pan, quickly cooling the point down. It is thus very hard to heat a point on a big chunk of metal to ignition temperature, because the rest of the metal will cool down whatever point you try to heat up, by sucking up heat.
That is different from say wood or coal which does not conduct heat well.
Surface Area Exposed to Oxidizer
An oxidizer is an element which strongly attracts electrons, while a reducer is an element which easily gives away electrons. A typical oxidizer is oxygen, hence the name. Given high enough temperature a metal will act as a reducer, giving away its electrons to oxygen and forming a metal oxide such as iron oxide, commonly known as rust or iron ore.
This process is what we typically refer to as burning. When the metal oxide is formed that produced a lot of heat. When something gets hot enough it will start emitting light. That is why we can see a flame.
For this burning to happen, we thus don’t only need to get to high enough temperature but we also need there to be oxygen for the metal to react with. The metal atoms below the surface of a piece of metal cannot come in contact with oxygen and thus cannot burn.
Thus by grinding down metal to fine powder or create fine strains as in steel wool, you get something that can more easily burn. This is because of two factors:
- You increase surface contact with oxygen.
- There is less other metal to conduct heat to, and potentially cooling down the metal before it ignites.
The trick is thus to suspend the metal in air to get maximum contact with oxygen. That is why air filled with coal dust, sugar dust or saw dust can be so explosive. It burns far more rapidly when suspended in air than when in a pile on the ground.
Steel Cutting Example
I think a nice example of this is how you can cut steel beams with just oxygen. A welder can start with oxy-acetylene, meaning he has a tank of acetylene gas and another of oxygen which react to produce a hot flame. You apply this hot flame to a steel beam to cut it. It is the melting that cuts it. Once you start, you can turn off the acetylene and use pure oxygen, because hot iron will react with the oxygen and create iron oxide. This process is highly exothermic, meaning it releases a lot of heat. Thus released heat will keep the process of melting the steel going.
This high heat production is what makes iron suitable as a fuel. You get more heat out than you had to apply to ignite it.
Benefits of Metals as Fuel
The reason why you never hear about metals being used as fuel, is because we have been blessed with an abundance of cheap fossil fuels for hundreds of years.
Metal does not lie around in pure form ready to be burned and used for fuel. We have spend energy to make the metal. Thus it has never made economic sense to use metal as a fuel.
However the economic calculation changes dramatically when we look for possible compounds for storing energy. When you cannot use fossil fuels anymore due to concerns for global warming and need something to store the fluctuating power output from renewable energy sources such as wind and solar, then metals suddenly becomes an interesting alternative.
Metal unlike hydrocarbons, don’t release CO2 when burned, instead we get a metal-oxide which we can easily save and recycle for back to metal again using wind and solar power.