I used to think I understood it but now I am mostly confused. Let me explain.
As a firefighter, I was taught that a fuel-limited fire is limited by the amount of fuel available. Some firefighters and researchers also call it a fuel-controlled fire. I like to use the term fuel-limited.
What is the fuel limiting?
- Is it the size of the fire?
- the smoke generated?
- the temperature?
- or the HRR (heat release rate)?
The fuel is limiting the heat release rate.
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.
What fuel are we talking about?
But what fuel is actually limited? What do we mean when we say fuel?
Let’s look at this short video of a possible fire development in a room with an opening. At 10 seconds we can see a sofa in the corner of a room with 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 contents of the room (all the furniture and stuff). They would hopefully also list the linings of the room (the walls, ceiling, and floor).
We could measure and specify the chemical energy stored in all those materials in Joules (or Btu’s for Americans). The result would show an abundance of fuel for that fire to reach flashover.
However, if I were to ask the same firefighters if that fire is fuel-limited or oxygen-limited, I think most would answer it is fuel-limited. Therefore, it seems like this fire lacks fuel to increase in HRR, even though the room has an abundance of fuel.
Not all fuel appears to be fuel; it is confusing to me.
Materials contain potential fuel.
The fuel that is chemically stored in solids and liquids is potential fuel that is not yet ready for combustion, with pure coal being the main exception to this rule. To make materials ready for combustion they have to be made into a gas.
It would be less confusing to specify that materials “contain potential fuel”, instead of being fuel actually ready for combustion.
Therefore, when we refer to a fire being fuel-limited, we are generally not talking about the potential fuel in materials.
Off-gasing from materials contain fuel.
Inside and close to the flames the materials may be off-gasing. This often white smoke coming from the surfaces usually contain a lot of water vapor and other inert (non combustible) substances.
The other substances produced may be more or less able to oxidize (combust) if they find a good mixture with oxygen and enough heat to start the process. Thus, off-gasing from materials also contain fuel.
Some firefighters call this white smoke “pyrolysis smoke”, which is at least partly true. Some of the off-gasing smoke is probably also vaporized or decomposed liquids. But that is a different topic for another day.
Off-gasing fuel may not contribute to HRR.
But not all of the off-gasing fuel will get anywhere close to being combusted. The fuel may be created beside or above the flames and there may not be enough heat or oxygen for it to burn there. In that case it does not contribute to the HRR.
The amount of off-gasing fuel is therefor not directly tied to the amount of heat being generated by the fire. Even if we have a fuel-limited fire, we may have an abundance of off-gasing fuel being created.
So, off-gassing fuel is another type of fuel we need to consider when we think about fire behavior and burn regimes.
Smoke is fuel, at least some of it.
If we examine the video between 10-20 seconds, we can see the same sofa in the corner, slightly larger flames, and smoke in the air.
Inside and close to the flames the surface materials are off-gassing. Those substances produced will be further decomposed into new substances by the heat from the flames.
But some of those substance will not find a place with enough temperature and oxygen to fully combust. Instead they will be pushed up and become part of what we then call smoke.
Further away from the flames, off-gassing substances from materials may end up in the smoke without having been further decomposed by the heat from the fire.
We can call these substances “unburned fuel”, which would make at least some of the smoke into fuel. Although the biggest part of smoke is probably inert gases like nitrogen, water vapor and carbon dioxide.
Fuel-limited fire, with a lot of fuel in the smoke.
If smoke is at least partially fuel, it would mean that all fires with visible smoke are oxygen-limited. After all, there is an abundance of fuel in the smoke at that point.
However, if we were to ask the same firefighters whether this fire at 20 seconds is limited by oxygen or by fuel, most would still say that it’s fuel-limited.
Consequently, smoke sounds like yet another type of fuel. A fuel that we can have an abundance of, but still have a fuel-limited fire.
Fuel in the right form, place and time.
We have the potential fuel in the materials, we have the off-gased fuel from the materials and we have the unburned fuel in the smoke.
All these fuels can be in abundance in the room, and still we may call the fire fuel-limited. It is confusing right?
But the fuel has to be in the right form, in the right place and time to actually matter.
HRR is limited by the avaliable fuel.
When we say that the fire is fuel-limited, we are only referring to the fuel that the flame can actually heat and oxidize. We might call it available fuel.
Fuel further away from the flames make no difference unless the flames spread there. A smoke layer with unburned fuel is just potential heat before it ignites.
A flashover is always oxygen-limited.
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.
So, at some point we call the fire oxygen-limited instead. The HRR from an oxygen-limited fire is being limited by the amount of oxygen available.
When did this fire stop being fuel-limited?
So, at which point in time did this fire go from being fuel-limited, to oxygen-limited?
Was it …
- when the smoke layer started to form?
- when the smoke layer became really dark?
- when the smoke layer started to burn?
- when the furniture around the fire ignited?
- when the floor ignited (flashover)?
I can imagine being a rookie and trying to figure this out. I am a firenerd and I do not know exactly where.
And if you think it is easy you probably do not understand it at all.
Fuel-limited fires have 21 % oxygen in the inlet.
Technically, the most important factor is the oxygen concentration in the inlet airtrack to the fire.
When the oxygen concentration in the inlet drops below 21 %, the fire becomes more or less oxygen-limited. And at around 12 % of oxygen, flaming combustion usually stops. It is a sliding scale.
The definition of a fuel-limited fire could thus be that it has 21 % oxygen in the inlet airtrack. But that is just my own thought on the matter.
All fires are in reality oxygen-limited.
Personally, I think of all fires as more or less oxygen-limited. If I blow air on a campfire it will increase in HRR, even though it is outside with 21 % oxygen in the inlet.
And all complex fires will be locally oxygen-limited as the oxygen cannot find its way all the way into the center of the flames.
So, I simply assume that all fires I respond to will increase in HRR if it gains access to more oxygen. And my job is to suppress it as fast as I can and then ventilate.