Abstract: The burner system is adapted for retrofit into the combustion chambers of firetube boilers. The burner is comprised of a hollow shell molded from a porous ceramic fiber matrix. Fuel and air reactants flow outwardly through the fiber matrix shell and are combusted along a shallow reaction zone on the outer surface. Heat is transferred primarily by radiation to the walls of the combustion chamber at temperatures which result in relatively low NO.sub.x emissions and high combustion efficiencies as compared to boiler systems with conventional burners.