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.

Moisture Content of Firewood

The efficient burning of firewood is determined by the availability of oxygen and the temperature. The Rocket-Stove-Principle optimises both factors. However, even the best-designed stove will perform inefficiently when used with wet firewood, because moisture reduces the net usable energy output of a fuel. The figure below illustrates how energy is wasted in firewood with high moisture content. It takes up to 3.21 MJ of energy to evaporate one kg of water in the process of heating the firewood up to the needed pyrolysis temperature of around 400° C from an ambient temperature. This takes into account the fact that water vapour is also heated to the same temperature as the wood-gas. The energy required to heat the water vapour is, in turn, not available for cooking. It is preferable to avoid using wet fuel and allow the sun to dry it rather than unnecessarily waste energy on evaporating water. Besides the lower energy output, firewood with high moisture content is more difficult to light. Moreover, the moisture from the fuel cools the flames and evaporated steam mixed with the combustible gases act as a fire-extinguisher leading to incomplete combustion as well as smoke.

Global Firewood Production and Consumption

The existing research on firewood is limited, mainly due to the fact that the definitions and conversion rates for firewood vary, and that firewood is primarily produced and traded in the informal sector. Statistics are often only available on “woodfuel”, which is firewood and wood transformed into charcoal. Worldwide wood fuel totals amounted to around 1.86 million m³ in 2010. Asia is the region with the highest production of woodfuels, accounting for 771 million m³ or nearly 45% of global production. This is strongly driven by China and India which together consume one quarter of global woodfuel consumption. Due to substitution for other fuels, the Asian consumption of woodfuel has decreased by 3.2% from 2005 -2010. Wood fuel  consumption in Africa and Latin America continue to grow.

Presently, climate policies in Europe and North America are contributing to growing demand for wood fuel. Densified wood fuel (wood pellets, briquettes) and chips in particular are gaining in importance due to the increasing share of heating and electricity generation applications that utilise woodfuel. Consequently, Europe’s wood production increased from 125 million m³ in 2001 to nearly 160 million m³ in 2011.