Steam generating system with NO.sub.x reduction

A hydrocarbon fired steam generator comprising:

• a primary source of combustion air which contains a substantial oxygen concentration;
• a mixing chamber which receives the combustion air provided by the primary source;
• a combustion chamber in downstream communication with the mixing chamber;
• a fuel inlet in communication with the combustion chamber;
• an exhaust section in downstream communication with the combustion chamber;
• a recirculation line establishing communication between the exhaust section and the mixing chamber to provide a secondary source of combustion air for communication with the primary source in the mixing chamber;
• means for sensing the oxygen concentration in the combined combustion air presented to the mixing chamber and generating a signal as a function thereof;
• means for controlling the flow rate of the secondary source of combustion air which is responsive to the signal from the means for sensing the oxygen concentration in the mixing chamber;
• a second means for sensing oxygen concentration which is in sensory communication with the exhaust gas and which generates a second control signal which is a function thereof;
• means for controlling the flow of the primary source of combustion air which is responsive to the second control signal;
• and
• a heat exchanger, comprising:
• a water inlet;
• a fluid circulation system in thermal communication with the combustion reaction and in communication with the water inlet at one end;
• and
• a steam outlet in communication with the other end of fluid circulation system;
• whereby energy from the combustion reaction is transferred to the water which is converted thereby from a liquid to a vapor phase.