The process is known as 'pyrolization", and it essentially partially decomposes the wood into a burnable gas. It is carbon-neutral too as burning wood produces the same amount of carbon dioxide as it would rotting on the ground. It would of course produce more particulates, so a solution would have to be found for that. During the 1920's thought the 1950's it was a popular technology as gasoline was either expensive or in short supply. There were factory produced wood gas cars made by Volkswagen and Toyota, and GM and Ford offered factory retrofit kits. Today, these boilers are highly prized as they are of a very good quality as compared to most of the home brew solutions used today.