Abstract: Techniques for increasing the thermal efficiency of high temperature combustion sources such as glass and other process furnaces are disclosed. These techniques include one or more of the steps of co-firing of a carbonaceous solid fuel or heavy fuel oil with the primary fuel in the primary combustion zone such that the co-fired fuel is cracked and thereby generates a high number density of carbonaceous particles sufficient to improve soot radiation; providing enhanced mixing of first burnout air with the flue gas in a first burnout zone; and promoting instability of boundary layer gases adjacent refractory packings.