Cooking with Firewood

Introduction - What is Firewood?

A key characteristic of humankind is the ability to control fire and utilize it to prepare food. The oldest cooking fuel is firewood in the form of logs and branches from trees. Firewood (synonym: fuelwood) is defined by the Food and Agriculture Organization of the United Nations (FAO) as “wood in the rough (from trunks and branches of trees) to be used as fuel for purposes such as cooking, heating or power production." Firewood can be categorized into hardwood and softwood: in comparison to hardwoods, softwoods burn more quickly and generate less heat owing to a lower energy (carbon) content per volume. However, energy content per weight is similar for all hardwood and softwood; the moisture content of a firewood primarily determines its energy content. The drier the firewood, the less energy is required to evaporate the water, thus the more energy is available for heating or cooking purposes.

How does Firewood Burn?

Firewood cannot ‘burn’ directly. Firstly it needs to be transformed into woodgas (pyrolysis), which can then be combusted when mixed with a certain amount of oxygen and ignited. A temperature of over 300° C is required to start the pyrolysis process and create combustible ‘wood-gas’. In most cases, this ‘external’ heat is initially provided by a lighting material, such as kerosene or a match. Once the temperature begins to increase, the following processes occur:

• Drying: As the biomass heats up and approaches 100° C, excess moisture contained in the biomass changes from liquid into water vapour. Excess moisture evaporates into the atmosphere and leaves a solid dry biomass behind.

 

• Pyrolysis: At temperatures beyond 300° C, biomass starts to pyrolyse (translation: ‘break apart by fire’). Increased temperatures eventually cause a complete conversion of the biomass into volatile vapours and a solid residue called char. The vapours contain various carbon compounds with fuel value, referred to as ‘wood-gas’. Since the solid by-product of this process is char, mostly composed of pure carbon, the process is also termed ‘carbonisation’.

Drying and pyrolysis are both endothermic processes, meaning that they consume heat and do not create any useful surplus of heat. The speed of the process is determined by the amount of available heat input and the amount of heat required to first dry out the fuel before the temperature of the biomass can attain a level at which pyrolysis can start: using air-dried fuel (moisture content of 10% – 20%) is recommended in order to shorten the drying time and reduce the required heat input.