Method for generating heat from waste fuel

A method of generating heat from waste having fixed carbon, volatiles, moisture and non-combustible material, such as inorganic matter and dirt, comprising:

• introducing the waste into the bottom of a first chamber in a pile,
• moving underfire combustion air upwardly through said pile for drying and gasifying the waste in the pile,
• oxidizing the fixed carbon of the waste in the pile in said first chamber to generate heat to form volatile gases rising to a first chamber discharge at a temperature less than the melting temperature of the non-combustible material so as not to form slag within the first chamber,
• adding primary swirl air in said first chamber in an amount less than stoichiometric to maintain the temperature slightly less than the melting point of the mineral matter,
• impeding the movement of non-combustible material and other particulate by diverting the passage of the upwardly rising volatile gases laterally outwardly at the discharge of the first combustion chamber,
• adding secondary combustion air to said volatile gases in a second chamber to complete the oxidation of the gases and increase the temperature of the gases above the melting temperature of the non-combustible material for generating heat in an area relatively free of non-combustible material whereby the temperature in the first chamber where non-combustible material is found is too low for forming slag and the quantity of non-combustible material in the second chamber is minimized where melting temperatures are found so that the heat generation occurs through complete combustion but with virtually no slag formation, said pile being conical and having a varying thickness, said step of moving combustion air upwardly through said conical pile including separating the air into inner and outer paths so that less air is moved through lower thickness of said pile to avoid the formation of blow holes.