What is a fuel limited fire?

fire behavior Sep 02, 2022

Well once I thought I understood it, then I did not, and now I am mostly confused. But let me explain what I mean.

As a firefighter I was taught that a fuel limited fire is limited by the amount of fuel that is available. A different word would be to say that the fire is fuel controlled. Same same. But I like the word fuel limited better so I want to use that.

The first question that popped into my head was of course - What is the fuel actually putting a limit on?
Was it the physical size of the fire? The temperature? The HRR (heat release rate)? Already at this point I started to struggle with getting good answers, as most firefighters cannot explain the difference between temperature and HRR well.
But HRR is the power of the fire, the amount of energy that is being released every second, minute or hour. And HRR is the direct result of the combustion process, which of course requires fuel and oxygen. So if I limit the fuel, I limit the HRR. Great, now I get it!

But what fuel is actually being limited? What do we mean when we say fuel? Lets look at this short video of a possible fire development in a room with an opening.

During the first 10 seconds of the video we can see a sofa in the corner of a room, flames above it, and little to no visible smoke.
If I were to ask firefighters how much fuel this room contains right now, they would probably list the content of the room (all the furniture and stuff), and the linings of the room (wall, ceiling, floor materials etc). We can measure and specify the chemical energy stored in content and linings in Joules, or Btu's for the Americans. So there is an abundance of fuel for this fire to go to flashover for instance.
But if I were to ask the same firefighters if this fire is fuel limited, or oxygen limited, I think most firefighters would answer that it is fuel limited. So it sounds like this fire lacks fuel to increase in HRR, even though the room has an abundance of fuel. Not all fuel seems to be fuel, it is confusing to me.

The fuel that is chemically stored in solids and liquids is just potential fuel. It is not ready for combustion yet (besides coal which is the main exception from the rule). For starters, it would be less confusing if we specified that solids and liquids contain potential fuel, rather then being fuel ready for combustion. So I try to use the term “contain potential fuel” every time I talk about the potential energy in stuff.

So when we say that a fire is fuel limited, we are generally not talking about the potential fuel contained in the content and lining materials. Great, now I think I get it! But again, no.

If we look at the video between 10-20 seconds we can see the same sofa in the corner, a slightly bigger flame and smoke in the air. So what is smoke? Well, a lot of firefighters would say that smoke is fuel. That would mean that all fires with smoke are oxygen limited, because there is an abundance of fuel in the smoke in the air at that point.
But again, if I were to ask firefighters if this fire at this point is oxygen or fuel limited, most would still say that the fire is fuel limited. So smoke sounds like a third kind of fuel, that you can have an abundance of but still have a fuel limited fire.

When the flames grow larger, some of the offgasing products (some of them with the ability to combust) will not find a spot with high enough temperature and oxygen to be able to combust. Those products of combustion will be pushed up and become part of what we call smoke. The offgased products that does not burn directly at the fire (but still are able to oxidize), we may identify by using the term “unburned fuel”. So smoke is not just fuel, but visible smoke does probably contain more or less unburned fuel.

It means that we have the potential fuel in the materials, we have the offgased fuel from the materials and we have unburned fuel in the smoke. When we say that the fire is fuel limited, it seems like we are only referring to the fuel that is combusted directly at the fire. That is what is limiting the HRR. Clear as day! Or is it?

If we go forward in the video from 20 seconds to the end, we can see the fire transitioning to a flashover. And a flashover is always oxygen limited at the end. It is impossible for the fire to access that amount of oxygen for anything but a couple of seconds.

So at which point in time did this fire go from being fuel limited, to oxygen limited? Was it when the floor ignited? Was it when the smoke layer started to burn? Was it when the furniture around the fire ignited? Was it when the smoke layer became really dark? I can imagine being a rookie and trying to figure this out because I do not know. You tell me!


As a side note. Is this not the best flashover video you have seen? The pyrolysis of the furniture and floor, the ignition of the pyrolysis gases and coupled with the thermal imager view. It almost cost me a FLIR K55 but the thing is bullet proof. Worth the risk! :-) Enjoy

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