Combustion In A Boiler

what is combustion in a boilerThis baby is going to be all about combustion. Mainly, this will be geared for combustion in a boiler but we are going to cover the definition, the different types of combustion such as perfect and incomplete, as well as the role that one of the boiler support systems, combustion air, plays.

Combustion sure seems like a simple thing to understand. What do you think of when you hear that word?

Something on fire or burning would probably pop into most peoples heads. Combustion is key to boiler operation and controlling the rate of combustion is just as important. First, let’s make sure we have a foundation to start from and define combustion.

What Is Combustion?

Combustion is a high temperature exothermic chemical reaction between a fuel and oxidant. This definition is from Wikipedia.

It is also defined as the rapid union of oxygen with an element or compound that results in the release of heat. This is from a technical manual.

Say again?

How about we just say it’s when oxygen combines with an element or compound and releases heat.

Heard of the Fire Triangle?

combustion in a boiler

The infamous fire triangle. Remove one side, no fire.

Of course you have right? Combustion needs three things – oxygen, fuel, and heat.

The oxygen comes from the atmosphere around us. The fuel can be anything combustible, things like wood or a fossil fuel.

Where does that initial heat come from though? In the boiler combustion process, it comes from the ignitor.

You have probably seen an ignitor on your barbeque grill. In your car the initial heat comes from… your spark plug. In your furnace, water heater, or oven, maybe a pilot light.

Combustion, aka fire, needs those three things in order to sustain itself. Eventually, or quickly, the heat generated from the combustion itself is what continues to contribute rather than an ignition source.

If you ever find yourself facing a fire, you can just remove one of the 3 components of the fire triangle. This should put out the fire, or end the combustion.

If we smother a fire, like put a blanket over it or cover it with sand, we are keeping any new oxygen from being introduced. If we remove the fuel, there would be nothing to burn or combust. Now you don’t have any reason to panic when facing a fire. Seriously, you remove ANY of the 1 of those 3 components of the fire triangle (they have allegedly added a 4th so I guess it’s not a triangle anymore,) and combustion CANNOT take place.

Combustion in Boilers

complete combustion

The motor that provides the air to the boiler for combustion.

Combustion is important to stationary engineers and boiler operators because the heat produced by combustion is what we use to generate steam in the boilers.

There are different labels, categories, or classes of combustion that come along with operating boilers.

There are 3 of them and they are:

  • Perfect
  • Complete
  • Incomplete

What do these different combustion terms mean?

Perfect Combustion

This is defined as combustion that happens when only the theoretical amount of air is used to burn the fuel. Or, when exactly the precise amount of air needed to burn every molecule of fuel is used, and only then. Not one molecule of air more.

What is the theoretical amount of air you ask? It is the amount of air needed to hit perfect combustion.

Boilers can’t ever hit perfect combustion, they are not 100% efficient. Perfect combustion can only be demonstrated in labs.

Complete Combustion

This is classified as when we burn all of a fuel using only the least amount of air ABOVE the theoretical amount of air required. This is what we are shooting for when operating and adjusting boilers.

In complete combustion, the fuel is burned at the highest possible efficiency while emitting the least amount of pollutants.

Incomplete Combustion

This class is the least desirable. We do not want incomplete combustion. This happens when we don’t burn all of the fuel. When you don’t burn all of the fuel, combustion efficiency is reduced and this causes pollutants to be sent out into the air like smoke or soot. As the soot builds up, the efficiency lowers even more.

The three items needed for combustion (the fire triangle or square now) are fuel, heat, and oxygen. We just talked about the fuels, let’s talk about the oxygen, or combustion air. Combustion air is also divided up into 3 classes.

Combustion Air

combustion air

The vanes of the blower/fan that supply the combustion air.

Air that is used in the combustion process usually falls into 1 of 3 types:

  • Primary
  • Secondary
  • Excess

Primary Air

Controls the combustion rate. Controlling the combustion rate dictates the quantity of fuel consumed.

Secondary Air

Is the air that determines the efficiency of the combustion which we talked about above (perfect, complete, etc.) Secondary air controls how completely the fuel is burned.

Excess Air

Is extra air provided to the boiler ABOVE the theoretical amount required to completely burn all of the fuel.

Complete Combustion Helpful Info

Achieving complete combustion should be your goal as a boiler operator.

Remember that complete combustion is the burning of all fuel using the smallest amount of air possible in order to achieve this.

In order to hit that target, you must pay attention to 4 more things:

  • The proper atomization of the fuel (more surface area to burn.)
  • The proper mixture or ratio of air to fuel.
  • Adequate time to complete the combustion process.
  • And, the temperature of the fuel.


Primary combustion air

A damper for the combustion air. Peeking through, you can see the fan.

Here is an acronym that will help you remember what is required for complete combustion, MATT. I have never heard of this before and apparently missed it during my multiple readings of Low Pressure Boilers. Here is how MATT breaks down.

M = mixture of air and fuel.

It is crucial to maintain the proper ration of air to fuel at all firing rates. High firing rates demand more air.

A = atomization.

Atomization, like your shower head, breaks up the fuel into droplets, or a mist, or finer particles.

Breaking it up like this creates more exposed surface area of the molecules. This improves the contact and mixing of the fuel and air.

T = temperature.

You just need to maintain the proper temperature of the fuel, air, etc. Too cold or too hot will not result in complete combustion.

T = time.

Time required to obtain complete combustion. To hit maximum efficiency, ideally all combustion is completed before the gases of combustion make their way to the heating surface.


We continued along the boiler, stationary engineer knowledge path with this post. It covered combustion primarily.

We defined combustion and then discussed the 3 components required for combustion to occur, heat, fuel, and oxygen.

Next we went over the 3 types of combustion, perfect, complete, and incomplete and then the 3 types of combustion air, primary, secondary, and excess.

Lastly, how we try and achieve complete combustion was discussed. Use the acronym MATT to help you remember that the proper mixture of fuel and air, the atomization of the fuel, time, and temperature are the 4 items you need to pay attention to when shooting for complete combustion in your boilers.

And that’s a wrap folks. For more helpful information like this, maybe you should check out my Low Pressure Boilers book review here. If you have a comment about what you just read, or anything else, use the form below. If you are more of a private person, drop the site a line via email. Until next time.