A method of heating a furnace process chamber with the combustion of reactants including combustion air and fuel gas, comprising: a) raising the process chamber to an elevated temperature equal to or greater than an auto-ignition temperature of the fuel gas by heating the process chamber in a preheat mode by directing gas into and out of the process chamber in alternating cycles when the temperature of the process chamber is below the autoignition temperature of the fuel gas, including firing cycles in which gas is directed into the process chamber, and exhaust cycles in which gas is directed out of the process chamber and into a regenerative bed, including: forming preheated combustion air by directing combustion air through a heated regenerative bed; directing a stream of the preheated combustion air into the process chamber in a condition unmixed with the fuel gas; forming a fuel rich mixture of the fuel gas and combustion air; and directing the fuel rich mixture from a reactant delivery device into the process chamber adjacent to the stream of preheated combustion air, whereby the reactant delivery device functions as a burner in the preheat mode: and b) following the preheat mode, heating the process chamber in an auto-ignition mode when the temperature of the process chamber is at or above the auto-ignition temperature of the fuel gas, including: forming preheated combustion air by directing combustion air through the heated regenerative bed; directing a stream of the preheated combustion air into the process chamber in a condition unmixed with fuel gas; directing the fuel gas from the reactant delivery device into the process chamber in a condition unmixed with combustion air, whereby the reactant delivery device functions as a fuel injector in the auto-ignition mode. a) raising the process chamber to an elevated temperature equal to or greater than an auto-ignition temperature of the fuel gas by heating the process chamber in a preheat mode by directing gas into and out of the process chamber in alternating cycles when the temperature of the process chamber is below the autoignition temperature of the fuel gas, including firing cycles in which gas is directed into the process chamber, and exhaust cycles in which gas is directed out of the process chamber and into a regenerative bed, including: forming preheated combustion air by directing combustion air through a heated regenerative bed; directing a stream of the preheated combustion air into the process chamber in a condition unmixed with the fuel gas; forming a fuel rich mixture of the fuel gas and combustion air; and directing the fuel rich mixture from a reactant delivery device into the process chamber adjacent to the stream of preheated combustion air, whereby the reactant delivery device functions as a burner in the preheat mode: and forming preheated combustion air by directing combustion air through a heated regenerative bed; directing a stream of the preheated combustion air into the process chamber in a condition unmixed with the fuel gas; forming a fuel rich mixture of the fuel gas and combustion air; and directing the fuel rich mixture from a reactant delivery device into the process chamber adjacent to the stream of preheated combustion air, whereby the reactant delivery device functions as a burner in the preheat mode: and b) following the preheat mode, heating the process chamber in an auto-ignition mode when the temperature of the process chamber is at or above the auto-ignition temperature of the fuel gas, including: forming preheated combustion air by directing combustion air through the heated regenerative bed; directing a stream of the preheated combustion air into the process chamber in a condition unmixed with fuel gas; directing the fuel gas from the reactant delivery device into the process chamber in a condition unmixed with combustion air, whereby the reactant delivery device functions as a fuel injector in the auto-ignition mode. forming preheated combustion air by directing combustion air through the heated regenerative bed; directing a stream of the preheated combustion air into the process chamber in a condition unmixed with fuel gas; directing the fuel gas from the reactant delivery device into the process chamber in a condition unmixed with combustion air, whereby the reactant delivery device functions as a fuel injector in the auto-ignition mode.