Wood is one of the oldest energy sources. Rough wood and bark may be burned directly for fuel, or wood may be converted into charcoal by charring in a kiln from which air has been excluded.
According to the Food and Agriculture Organization of the United Nations, more than half of all the wood utilized in the world at the end of the twentieth century was used for energy production.
Wood provides for as much as 60 to 95 percent of the total energy needs of some developing countries, but it provides less than 5 percent of the total energy required in most developed countries.
As a rough estimate, around two billion people use wood for their cooking and heating. In some developing areas of the world, fuelwood demand is greater than the supply; particularly in parts of Africa, consumption significantly exceeds replacement of the stock of trees.
Wood fuel also finds some use in industry, as in the paper industry. Industries often burn waste material from other manufacturing processes.
Bark removed from raw logs, sawdust, planer shavings, sander dust, edges, and trim pieces may all be burned to generate power while disposing of the unwanted material. Small wood particles such as sawdust and shavings may be compressed to produce briquets or “logs” for use as fuel.
Increasing numbers of forests are being planted and cultivated for the sole purpose of energy production. Entire trees are chipped and burned for energy production at the end of a rotation. These forests may be known as forest plantations, tree farms, or energy forests.
This type of wood production and fuel use has the potential to reduce dependency on fossil fuels. Energy forests remove carbon from the atmosphere over their life span, then release this carbon in various forms during combustion for energy production.
Types of Combustion
The direct burning of wood occurs when the surface is intensively irradiated so that the temperature is raised to the point of spontaneous ignition, anywhere from 500 to more than 900 degrees Fahrenheit (260 to 500 degrees Celsius), depending on the conditions. More common is indirect combustion, inwhich thewood breaks down into gases, vapors, and mists, which mix with air and burn.
About 1.4 pounds (0.6 kilogram) of oxygen are required for the complete combustion of a pound of wood. At normal atmospheric concentrations, this implies that about 6 pounds (2.7 kilograms) of air are needed for the complete combustion of a pound of wood.
During combustion, gases such as carbon dioxide and carbon monoxide, water vapor, tars, and charcoal are produced, along with a variety of other hydrocarbons. Dry wood or bark and charcoal burn relatively cleanly; wetter wood produces a larger amount of emissions. Collectors may be used to remove particulate matter from industrial sources.
It is less feasible to reduce emissions from cooking stoves (either chemically or mechanically), how ever, and cooking stoves are a major source of human exposure to emissions from wood burning in much of the world.
Charcoal is lighter than wood and has a higher energy content. It takes approximately 3 pounds (1.4 kilograms) of wood to produce 1 pound (0.45 kilogram) of charcoal.
The exact conversion ratio depends on the tree species, the form of wood utilized, and the kiln technology used. Charcoal is more efficient to transport than wood, and it can be burned at higher temperatures.
It is used both for domestic purposes and, in some countries—Brazil is one—as an industrial fuel. In general, charcoal is considered a cleaner, less polluting fuel than wood in that its combustion produces fewer particulates.
Charcoalwas used extensively as an energy source for smelting and metalworking from prehistoric times into the Industrial Revolution, but coal eventually became the principal alternative energy source for these processes in areas where it was available. Today, petroleum and natural gas are major sources of energy for industrial processes.
The average recoverable heat energy from a pound of wood is about 8,500 British thermal units (Btu’s). The value ranges from 8,000 to 10,000 Btu’s per pound for different species.
In some efficient processes, 12,500 Btu’s can be recovered from a pound of charcoal. If wood with a high moisture content is burned, some of the energy produced by combustion is absorbedas themoisture evaporates, reducing the recoverable energy.
Impacts on Environment and Health
These uses are sustainable and have relatively little environmental impact in areas with low human population levels, but they may be associated with serious air pollution problems as well as widespread deforestation and erosion if they are the major sources of energy for a large or concentrated population.
In most of the areas that have deforestation problems, the problem is primarily attributable to changes in land use, particularly the opening of land for agriculture and grazing. Fuel wood is often recovered during such land-use changes, but the need for fuel wood production is often a secondary cause or by-product of deforestation.
Industrial power production that utilizes available technology to ensure high-temperature, virtually complete combustion minimizes hydrocarbon and particulate emissions and can be designed to meet most existing air quality standards.
Less efficient domestic combustion may be associated with unacceptable levels of human exposure to airborne particulates, carbon monoxide, and other hydrocarbons produced by incomplete combustion. The health effects of exposure to domestic wood fires are difficult to determine, since they often occur along with other factors known to increase health risks.