to pyrolysis zone 20 at pyrolysis zone 20 the primary air provides oxygen for sustaining sub stoichiometric combustion the heat released from this combustion heats the surrounding fuel to form wood gases or pyrolysis gases 19 made up of for example hydrogen carbon monoxide methane carbon dioxide some higher hydro carbons water vapor nitrogen and other gasses as is known depending on the type of solid fuel and other factors pf pyrolysis or other volatile gas in the flaming pyrolysis zone although not shown primary air 22 may be introduced into conduits 23 through any known means such as compressed air or simply a conduit at negative pressure relative to atmosphere the flow of primary air 22 may also be reversed to that indicated by the arrows and drawn by a negative pressure applied to conduits 23 this arrangement has the advantage of preheating the primary air and the depleted smoke filled air may be exhausted with the flue gasses as described herein below referring to fig3 there is shown the end of auger 13 and primary air conduits 23 in this particular embodiment this illustrates the point at which primary air enters the fuel supply area volatile pyrolysis gases 19 produced in the pyrolysis zone 20 are swept downstream into the reduction or char zone 21 where they are reduced and the char is consumed secondary air represented by arrow 24 is injected downstream of reduction zone 21 secondary air 24 may be introduced through a conduit 25 by any.