Wood is a concentrated form of stored sunlight (solar energy). This energy can be released and used as a fuel. Wood has always been an important source of energy for people. Today, wood is the most important source of renewable energy in the United States and a primary source of fuel for much of the world. Whether it is as simple as a campfire, or as sophisticated as producing ethanol, wood has a number of inherent advantages that ensure it will continue to be an important bio-fuel in the future.
"SP702-A-Woody Biofuels: Past, Present and Future," The University of Tennessee Agricultural Extension Service, SP702A-1M-12/07 R12-1015-006-012-08 08-0093, https://trace.tennessee.edu/utk_agexbiof/9