Stages of wood combustion: How does wood burn?

Wood is a very old and well-known fuel source. It has been used to heat homes and cook food for centuries. But how does wood actually burn?

Burning wood is a complex process that goes through several stages. Before we get into the stages of wood combustion, let’s first look at the anatomy of a tree.

A tree is mostly made up of cellulose and lignin. Cellulose is a complex sugar that gives plants its structure. Lignin is a polymer that helps to bind cellulose together.

These two substances are what make up the majority of a tree’s wood. And they’re also what makes it possible for a tree to burn. However, there is one more important ingredient in wood that we need to mention, water.

Water is found in the cells of a tree and it makes up to 70% of the weight of wood. This water is stored in the cells of the tree and helps to keep the tree healthy.

But when a tree is cut down to be used as fuel, this water now becomes a liability.

The water in the wood needs to be evaporated before the wood can start to burn. And this is where the stages of wood combustion come in.

When we talk about combustion, we’re talking about a chemical reaction between a fuel and an oxidizer. In the case of wood, the fuel is cellulose and lignin, and the oxidizer is the oxygen in the air.

When we cut down a tree and use it for firewood, we are essentially just burning cellulose and lignin. But these two substances don’t just burn randomly.

Instead, they go through a series of chemical reactions that can be divided into five stages

Drying

Drying is the first stage of wood combustion. In this stage, water is removed from the wood. This can be done through evaporation or by heating the wood.

Heating the wood speeds up the drying process. As the water is removed, the wood becomes more flammable.

So, it’s important to get rid of as much water as possible before moving on to the next stage. This can be done by letting the wood sit in the sun or by using a kiln.

However, drying wood is a very important step and shouldn’t be skipped. Here is why you should dry your wood before burning it.

The water content in the wood can cause a number of problems.

First, it will make the wood harder to light. The water will absorb heat instead of the wood, making it harder to reach the combustion temperature.

Second, wet wood produces a lot of smoke. This is because the water needs to be heated to a boiling point before it will evaporate. The smoke produced by wet wood can be a nuisance and is also bad for your health.

Third, wet wood produces creosote. Creosote is a flammable substance that builds up in your chimney and is the leading cause of chimney fires.

Therefore, it is essential that you dry your wood properly before you attempt to burn it.

Therefore, it is best to use dry wood that has been properly seasoned. Seasoned wood is wood that has been cut and allowed to dry for at least 6 months. Certain wood may take up to two years to fully season.

To reduce the amount of time it takes to season your wood, you can split it into smaller pieces.

Increasing the surface area will allow the water to evaporate more quickly and reduce the amount of time it takes to season the wood.

You can also speed up the process by storing the wood in a dry, sheltered place. Even better, if you can kiln dry your wood, that’s the best way to ensure that it is properly seasoned.

The ideal moisture content for firewood is about 20%. Most kilns will get the moisture content down to 10-15% without needing to wait months. Kiln drying is an energy-intensive process, but it ensures that the wood is dry and ready to burn.

But once the water is gone, the wood will much more flammable. This leads us to the next stage of wood combustion: pyrolysis.

Pyrolysis

Pyrolysis is the process of rapid heat decomposition of wood in the absence of oxygen. When wood is heated to high temperatures it cannot immediately combust.

Instead, it will go through a process of thermal decomposition where the cellulose and lignin are broken down into their component parts.

This process happens quite quickly and produces a number of gases, including carbon dioxide, water vapor, methane, hydrogen, and other hydrocarbons.

These gases in combination are typically flammable and feed the fire, leading to the next stage of combustion: gasification.

Gasification

Although all stages of combustion produce gas, it is only in the gasification stage that these gases are used to help the fire.

Gasification is the process of burning the gases produced in the pyrolysis stage. This is where the wood really starts to burn.

The gases produced in the pyrolysis stage are combusted, producing heat and light. This is the stage where the majority of the wood is burned.

The gases produced in the gasification stage are mostly carbon dioxide and water vapor.

However, there are also a number of other gases produced, including carbon monoxide, hydrogen, and other hydrocarbons.

While these gases are flammable, they don’t contribute much to the fire. Instead, they are mostly just wasted heat and contribute to air pollution.

This leads us to the next stage of wood combustion:

Complete Combustion

Complete combustion is the maturity stage of fire where there is enough oxygen present for all wood to be burned. Plus, the temperature is high enough that any unburned hydrocarbons are burned as well.

The gases produced in this stage are mostly carbon dioxide and water vapor. However, there may also be a small number of other gases present, depending on the wood and the fire conditions.

This stage of combustion offers the highest efficiency and lowest pollution. Because the wood reacts completely with the oxygen that is present means there are higher temperatures and fewer wasted gases.

Die down

Once the wood has been fully combusted, the fire will start to die down. This is because there is nothing left to burn. The gases produced in this stage are mostly carbon dioxide and water vapor.

The temperature of the fire will start to decrease as well since there is no longer any heat being produced by the combustion process.

Smoldering

Smoldering is the slow oxidation of the charcoal that is left behind after the wood has been completely combusted.

The charcoal will continue to react with the oxygen in the air, producing heat and light.

However, this process is much slower than the previous stages of combustion and will eventually come to a stop. Once the smoldering has stopped, all that is left is ash.