Amazingly wood is one of the few carbon nuetral fuels. Sure you release carbon dioxide when you burn it, but the amount of CO2 released is exactly the same amount that was stored by the tree when growing. Hence carbon nuetral.
This same amount of co2 would be released anyway by the natural decomposition of the tree if left to rot,
A common hardwood, red oak, has an energy content of 14.89 megajoules per kilogram (6,388 BTU per pound), and 10.423 megajoules recoverable if burned at 70% efficiency
The Sustainable Energy Development Office (SEDO), part of the Government of Western Australia states that the energy content of wood is 16.2 megajoules per kilogram (4.5 kWh/kg)
According to The Bioenergy Knowledge Centre, the energy content of wood is much more dependent on the moisture content than the species. The energy content (number of joules of heat produced) improves towards the total number of joules stored in the wood as it dries
As with any fire, burning wood fuel creates numerous by-products, some of which may be useful (heat and steam), and others that are undesirable, irritating or dangerous.
One by-product of wood burning is wood ash, which in moderate amounts is a fertilizer (mainly potash), contributing minerals, but is strongly alkaline as it contains potassium hydroxide (lye). Wood ash can also be used to manufacture soap.
Smoke, containing water vapor, carbon dioxide and other chemicals and aerosol particulates, can be an irritating (and potentially dangerous) by-product of partially burnt wood fuel. A major component of wood smoke is fine particles that may account for a large portion of particulate air pollution in some regions. During cooler months, wood heating accounts for as much as 60% of fine particles in Melbourne, Australia.
Slow combustion stoves increase efficiency of wood heaters burning logs, but also increase particulate production. Low pollution/slow combustion stoves are a current area of research. An alternative approach is to use pyrolysis to produce several useful biochemical byproducts, and clean burning charcoal, or to burn fuel extremely quickly inside a large thermal mass, such as a masonry heater. This has the effect of allowing the fuel to burn completely without producing particulates while maintaining the efficiency of the system.
In some of the most efficient burners, the temperature of the smoke is raised to a much higher temperature where the smoke will itself burn (e.g., 1,200 degrees for igniting carbon monoxide gas). This may result in significant reduction of smoke hazards while also providing additional heat from the process.