Wood and woody biomass is an ancient source of fuel; just picture a caveman throwing a log onto a fire, and you can see how closely human civilization is tied to our use of wood as energy.
Wood and wood products were in fact the world's primary source of energy until the mid-1800s, when coal and other fossil fuels became more widely used. Wood and woody biomass remain important sources of energy throughout the developing world, mainly for cooking and heating homes.
Sources of Wood Fuel
In its earliest form, wood was used as it fell in the forest, either as branches or whole trees. Gathering firewood was one of the most basic tasks performed by primitive people, and it is still a critical part of domestic life in much of the world.
Technology, however, has provided many other sources of woody biomass. Waste wood -- in the form of sawdust, bark, wood chips, waste from paper mill operations -- is now widely available as an energy source.
The development of wood pellets has opened up a large market for wood fuels. Wood pellets -- usually made of waste wood -- of a consistent size and density are an efficient way to use waste wood for heat or energy, and wood pellet stoves are gaining in popularity worldwide.
Is Wood a Good Fuel Source?
Determining the efficiency of wood and woody biomass as a fuel source depends on several factors. Different trees and plants, of course, produce different kinds of wood: Maple, for example, is much harder and denser than pine, and maple therefore burns at a slower, more efficient rate.
Wood also burns differently based on the moisture content of the wood -- in order to burn efficiently, most wood needs to be dried or "seasoned" for at least 6 months before it can be burned as fuel.
The efficiency of wood and woody biomass is measured as a percentage. And because there is such a wide variation in types of wood-burning equipment and in types of woody fuel, there's an equally wide variation in the efficiencies of different wood-fuel energy sources.
Wood-gasification boilers, for example, reportedly burn wood at around 90 percent, making them an impressively efficient source of wood heat. And most household pellets stoves are about 80-90 percent efficient, while stoves that burn cut wood and logs are about 60-80 percent efficient.
(How these efficiencies are determined, however, is something of a mystery. Neither the EPA nor any other governmental agency, it seems, is dedicated to rating efficiencies of various wood and biomass fuels. And if the manufacturer is the sole source of this information, well, caveat emptor.)
Wood and Woody Biomass: Green Energy?
One critical consideration in understanding the efficiency of wood and woody biomass is the overall impact costs: Even an ordinary wood-burning stove at 65 percent efficiency wood becomes immensely more efficient if the wood is harvested locally, e.g., on the homeowner's property. And a 95-percent efficient pellet stove with a heavily-processed pellet fuel source that must be shipped hundreds of miles loses a lot of overall efficiency by comparison.
It's also important to note the negative environmental impacts associated with using wood and woody biomass fuels. Burning wood creates smoke that contains greenhouse gases like carbon dioxide (CO2), so it does contribute somewhat to climate change and global warming. Additionally, wood smoke contains toxic compounds that have a serious impact on human health. Relying on wood as a fuel source has also been cited as a primary cause of deforestation.
But wood has one undeniably huge advantage over many other fuel sources: It's renewable. Trees and woody plants as a biomass fuel source can be grown, making the argument in favor of wood's environmental and economic impact much more robust.
Furthermore, researchers have found that as trees and plants grow, they absorb a great deal of CO2 in a process known as "carbon sequestration." Some analysts report that the amount of CO2 absorbed by growing plants is roughly equal to the amount of CO2 emitted during burning, making wood fuels essentially carbon-neutral. According to Oak Ridge National Laboratory, this is particularly true when the trees and plants are fast-growing, harvested at the appropriate age, and are used to replace fossil fuels.
