Low NOx level combustion method in a radiant tube burner and a burning apparatus used for the method

Claim 1

A low NOx level combustion method in a radiant tube burner, the method comprising flowing an inner air flow in an inner air flow conduit connected to a source of combustion air, and arranged centrally of an annular chamber in the radiant tube, the chamber being provided with a base portion connected to the inner air flow conduit and having a rim portion integrally formed at the peripheral edge thereof, said chamber forming in the radiant tube an annular fuel space bounded by the rim portion and the central inner air flow conduit, flowing a fuel gas flow through a plurality of fuel-gas conduits connected at a first end to a fuel gas source and arranged in a circle in the radiant tube so as to surround the inner air flow conduit, said plurality of fuel-gas conduits opening at a second end open into the base portion of said chamber to form fuel-gas openings, said fuel gas openings communicating with the annular fuel space therethrough, jetting the fuel-gas flow into a primary combustion region adjacent to the rim portion of said chamber and surrounding the inner air flow conduit through said fuel-gas openings arranged in said chamber at the second end of said fuel-gas conduits, the inner air flow conduit extending forwardly and downstream of said chamber in the flowing direction of the fuel gas and along a central axis of the radiant tube, jetting the inner air flow into a secondary combustion region through an open end of the inner air flow conduit, and jetting an outer air flow in communication with the combustion air source and surrounding the fuel gas flow into the primary combustion region through a passage formed between the radiant tube and the rim portion of said chamber, the fuel gas being primarily burnt by the outer air flow and then secondarily burnt by the inner air flow, and further comprising jetting the fuel gas flow by means of 3˜6 fuel gas openings arranged in said chamber, carrying out a primary combustion of the fuel gas by controlling the ratio of the amount of the inner air flow to the total amount of introduced air to 35-80%, and limiting the momentum ratio of the fuel gas flow to the outer air flow to not lower than 0.40, and further carrying out a secondary combustion of the primarily burnt fuel gas by setting the length of the inner air flow conduit such that the secondary combustion is carried out at a time substantially later than the time of the primary combustion, whereby a uniform and smooth two stage combustion is carried out.