US Patent 7588028 Biomass burner

A burner comprising a fuel reservoir, a primary combustion zone located at the lower end of the fuel reservoir, a secondary combustion zone located underneath the primary combustion zone, a passage providing communication between the primary and secondary combustion zones, a flue for discharging products of combustion from the secondary combustion zone, the primary combustion zone having a peripheral wall incorporating air hole means to provide combustion air to the primary combustion zone, wherein the cross-sectional area of the air hole means is substantially equal to the combined minimum cross-sectional area of the passage and the minimum cross?sectional flow area of the flue.