Cellulosic Fires: A Few Facts About Them

30thMarch 2021
By
CPD

In one of our recent blog posts, we talked about the two main types of fire that a blaze can be classified as. Unsurprisingly given most of us don’t work in petrochemical or industrial plants, the layperson is usually only acquainted with fires caused by materials and chemicals found at home and the office, and are blissfully unaware that fires which occur in industrial and more complex structures are completely different in nature. But in reality, the sources of ignition are more complicated than what we know. There are those that are hydrocarbon in nature (Industrial fires), and cellulosic fires which are those in offices and homes. But, do you understand the differences between cellulosic and hydrocarbon fires, above and beyond the fuel source? If not, you may want to read a previous and related post we posted here.

Zooming On In Cellulosic Fires

To get a better understanding of what cellulosic fires are, let’s address how they start, how they intensify, and finally how they cause damage to property and human life.
Then, we will present you with a few actionable solutions on containing and minimising this type of fire plus.

What is a cellulosic fire?

The term cellulosic fire was coined after its main trigger —cellulose, a substance popularly used to produce paper. Thus, in short, this type of fire is fuelled by materials containing high amounts of cellulose, such as  paper, timber, leaves, desks, cotton, wood, and the like.

Cellulose is an organic polymer which can easily be obtained from different parts of plants. Being a natural substance, straight cellulose and those products made with it, easily catch fire.

Although initially the source of a cellulosic fire may be easy to put out, once the burning intensifies, the flames become harder to stop and the damage more intense. The reason for this can be explained by the science of pyrolysis.

The Science of Pyrolysis

According to the International Union of Pure and Applied Chemistry in its Compendium of Chemical Terminology printed in 2009, pyrolysis happens when a breakdown of molecules occurs due to exposure to a high temperature.

Let’s take for example wood. When it is introduced to heat, a thermal decomposition happens that disintegrates cellulose and other molecules of the wood. The pyrolysis of cellulose, along with the presence of oxygen, leads to the eventual ignition of the wood. With the ignition in place and the thermal decomposition happening, the wood also releases gases which react with air, therefore, inciting the combustion even more.

Cellulosic Fire’s Ignition Temperature

The temperature at which a cellulosic fire starts depends not JUST on the type of material but also on a number of external and internal factors. How fast a substance flares up depends on its moisture content, thickness, air flow, surrounding materials and other factors. That said, it is safe to say that dried leaves burn faster than fresh leaves while old wood ignites more easily compared to newly-cut lumber.

Assuming that all other factors remain the same for all types of cellulosic substances, let’s take a quick look at the typical ignition temperatures of the following. For paper—one of the most common cellulose-laden items—it only takes  a heat of 233 degrees Celsius for it to catch fire and burn on its own; wood, 370 degrees Celsius; and fabric (rayon), below 226 degrees Celsius.

Once ignition starts, the flame temperature of a cellulosic fire can reach 500 degrees Celsius within five minutes and as it devours other flammable materials, can reach up to 1000 degrees Celsius over a period of between 95 and 120 minutes.

How Cellulosic Fire Spreads: The Role of Heat Release

While temperature has a bearing in the spread of flames, the intensity and speed of combustion also rely on the heat release of the ignition source. But once alight, temperature of the flames and the heat released are not equal,. because the latter also considers quantity or volume of the burning object.

Let’s use fibre as an example. One burning roll of fibre and 10 burning rolls of fibre may have the same temperature but the heat release rate of the 10 rolls is 10 times the heat released by one roll of burning fibre.
The heat released is usually transferred in three ways: by conduction, convection, and radiation. When a cellulose substance is wide or long enough or when similar materials are placed together closely, the heat is easily transferred by conduction. If there’s no air around, the cellulosic fire is easily contained and can be put out right away.

But if there’s a rise of hot masses of air during a fire—thus bringing the cool air down—the heat is transferred to nearby objects and the combustion spreads faster. This is the process of convection.

Cellulosic Fire Protection

To contain a cellulosic fire, make sure you have  fire retardants that carry the Class A mark on hand in the workplace/home. These are fire extinguishers designed to put out fires originating from ordinary combustibles such as wood and paper.

Of course, there are measures which you can take to protect the building structure from the heat of flame, but that is another topic all together. But rest assured, when it comes to protecting structural steelwork from cellulosic fire, calling a Permax team member is a great place to start.

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