Disclosed is a staged-air burner device capable of high energy efficiency, high flame stability, combusting multiple readily switchable fuels ranging from pure hydrogen, to any hydrogen/methane mixture, to pure methane, and generating a low level of NOx. The burner device can include: a primary air chamber receiving a primary air and a flue gas; a burner tube capable of receiving a fuel jet and drawing in the air-flue gas mixture from the primary air chamber; a burner tip discharging the fuel-air-flue gas mixture formed in the burner tube to a first combustion zone and a second combustion zone via center orifices and side orifices on the burner tip, respectively; and a staged air chamber receiving staged air and discharging it via staged air ports into a third combustion zone. Combustion of the fuel occurs in at least one of the first, second, and third combustion zones.

The present invention relates to a method of combusting a fuel gas with a stoichiometric or near stoichiometric amount of molecular oxygen in the presence of a controlled amount of a diluent to enhance the extent of combustion reactions in high heat transfer environment. The energy released is utilized to heat a fluid by direct contact with the flame. The diluent can be different from the fluid to be heated with respect to composition, temperature or pressure. The diluent can be same as or derived from the fluid to be heated.

A liquid fuel or hybrid fuel burner and method of operating the burner are described. The combustion air and/or NOx reducing medium are introduced at more than one location in the burner. A portion of the combustion air and the NOx reducing medium can be injected into the primary combustion zone through the NOx reducing medium conduit which surrounds the liquid fuel injector which injects atomized liquid fuel into the primary combustion zone. Another portion of the combustion air and the NOx reducing medium can be introduced into the primary combustion zone through a passage in the burner tile surrounding the NOx reducing medium conduit.