Gasification* is the cleanest, most efficient combustion method known.

 It has been used for decades where clean heat is required. Examples include the thousands of vehicles which were directly fuelled by Gasifiers during the Second World War, or the coal gas "works" which were common in cities all over the World before natural gas. These produced gas which combusted so clean it was used in chimney-less household appliances such as cookers and heaters, without adverse effects.  http://www.waterwide.com/gasi.htm

Thanks for link, http://www.green-trust.org/woodgas.htm