What happens when we burn biomass?

Burning biomass is essentially a reversal of photosynthesis.

Photosynthesis is the process by which plants can reverse the disorder of entropy by using energy from the sun to combine water and carbon dioxide to produce sugar and oxygen. The plant then creates complex carbohydrates, such as starch and cellulose, from the sugars and combines them with other elements derived from nutrients to make compounds such as lignin, which is an important component of wood.

The biomass burning process

When biomass is burnt, oxygen combines with the carbon stored by the plant to release energy in the form of heat and light with the production of carbon dioxide and water, thereby increasing entropy again.

The burning process also leaves behind ash, char, and other residues such as tar. Wood ash is mostly calcium carbonate (CaCO₃) if the fire is below 750 degrees Celsius, or calcium oxide (CaO) if the temperature is hotter than 750 C. Other components of wood ash include potash, which is various salts of potassium, and trace elements of iron, manganese, zinc, copper and some heavy metals.

Char is the carbon that remains from burning as black lumps. The amount of char is increased when the wood is burnt without oxygen in pyrolysis and is called ‘biochar’.

Benefits of burning biomass

The alkali nature of the wood ash from calcium carbonate, and the important plant nutrients in the potash, together with the char, make wood ash a good fertilizer for agriculture or market gardening. It can also be used to make other products such as ‘lye’ (potassium hydroxide) for soap manufacture, or in ceramic glazes during the firing of pottery.

Biochar has been shown to significantly increase the fertility of soils and also acts as a long-term store of carbon when dug into the soil.

Tar is created when the wood is burnt at lower temperatures, usually when the wood is a bit wet. It is made up of phenols and related compounds. Tars can be used to create creosote for preventing rotting of wood and tarry smoke is used as a way of preserving meat and fish in ‘smokers’.

The residues left behind from burning depends on the type of biomass (the feedstock) burnt, the temperature at which it is burnt and the amount of oxygen available during combustion.

Taking your learning further

If you are interested in learning more about the burning of biomass, sign up for the full course – Renewable Energy: Achieving Sustainability through Bioenergy – today.