How to build a bonfire – the science of Combustion

The centre piece of a Bonfire Night party is, of course, the bonfire:  setting fire to Guy Fawkes’s effigy at the start, warming chilly hands during the evening and toasting marshmallows on the embers at the end.  A bonfire is a wonderful amalgamation of biology, chemistry and physics.  This blog looks at a little of all three.

The fuel for a bonfire is wood. Wood’s main components are long organic molecules containing carbon, hydrogen and oxygen.  The molecules are cellulose, hemicellulose and lignin and they make up the structure of the wood. The tree from which the wood came evolved this structure to be very tall and free, standing tall in order to reach the sunlight above all the other plants, yet still allowing transport of water, minerals and dissolved sugars throughout its structure in phloem and xylem. Other minerals are present in wood in small quantities, notably calcium and potassium.

When a fire burns, oxygen from the air reacts with the large organic molecules in wood breaking them down.  This reaction starts at the surface of the wood and continues in the air just above it because some molecules (or fragments) will vaporise. Complete combustion would result in carbon dioxide and water as the only products, but the large size of the molecules in wood means that by-products such as soot, carbon monoxide and a cocktail of organic fragments of the original molecule are also formed. These organic molecules give wood-smoke its characteristic smell and wood-smoked food its distinctive flavour. The minerals in wood do not burn, ending up as ash left on the ground or wafted away in the smoke.

A spark or small flame is necessary to provide energy to start a bonfire. This energy is called the activation energy, and it is required to break some of the chemical bonds in the molecules of wood. When the new bonds form in the products (carbon dioxide and water), large amounts of energy are released. All the molecules formed during combustion are then extremely hot. This heat provides enough energy to overcome the activation energy required to break yet more bonds in unburnt wood, so the reaction becomes self-sustaining and flames will be seen.

The flames are mainly yellow because of the high temperatures present in the carbon atoms and carbon containing molecules produced during combustion. Electrons within the carbon atoms are excited, and move up to higher energy levels within the atoms. When these electrons de-excite they emit their energy in the form of yellow light. Any other colours present are due to similar electron transitions in minerals in the wood. Calcium and potassium compounds will give an orange and lilac tinge to the flames respectively.

So, how to build the perfect bonfire?  Well, start with small pieces of wood which have a large surface area and a low mass; they will burn more easily than large logs because they get hotter faster and have more air in contact with their surface. A good air flow to the bottom of the fire will provide a good supply of oxygen. Light the fire at the bottom so that convection currents from the hot gases rising will heat wood higher up (drying it out if necessary) and it will spontaneously ignite once it has enough energy.

From the biology of wood, through to the chemistry of combustion and the physics of flames, bonfires are wonderful.  Finally some biology to end with: if your fire has been ready for some days before you light it, do check there are no little creatures in residence before you strike the match.



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