Burner and process for operating gas turbines with minimal...

Power plants – Combustion products used as motive fluid

Reexamination Certificate

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C060S039110, C060S740000

Reexamination Certificate

active

06199364

ABSTRACT:

BACKGROUND OF THE INVENTION
This invention relates to a burner and process for operating gas turbines with minimal emissions of air pollutants, especially nitrogen oxides (NO
x
). More particularly, the burner and process permit operation of gas turbine combustors at high excess air and at elevated pressure.
The development of a compact burner that would fit in the castings of gas turbines and yield combustion products with a limited content of atmospheric pollutants [NO
x
, carbon monoxide (CO) and unburned hydrocarbons (UHC)] has long failed to deliver a commercially acceptable product. In 1981, U.S. Pat. No. 4,280,329 of Rackley et al disclosed a radiant surface burner in the form of a porous ceramic V-shaped element. Theoretically, the proposed burner was attractive but, practically, it had serious deficiencies, such as fragility, high pressure drop therethrough and limited heat flux. No advance in the art of radiant surface combustion for gas turbines has appeared since the Rackley et al proposal.
Efforts to minimize atmospheric pollutant emissions from the operation of gas turbines have been directed in different approaches. U.S. Pat. Nos. 4,339,924; 5,309,709 and 5,457,953 are illustrative of proposals involving complicated and costly apparatus. Catalytica Inc. Is promoting a catalytic combustor for gas turbines which reportedly (San Francisco Chronicle, Nov. 21, 1996) is undergoing evaluation. None of the proposals provide simple, compact apparatus and catalysts are expensive and have limited lives.
A principal object of this invention is to provide compact burners for gas turbines which feature surface-stabilized combustion conducted at high firing rates with high excess air to yield minimal polluting emissions.
Another important object is to provide burners for gas turbines which permit broad adjustment of heat flux.
A related object is to provide compact burners with low pressure drop and stable operation over a broad pressure range and excess air variation.
Still another object is to provide burners for gas turbines which have simple and durable construction.
A further primary object of the invention is to provide a method of operating gas turbines to yield combustion products with a very low content of atmospheric pollutants.
These and other features and advantages of the invention will be apparent from the description which follows.
SUMMARY OF THE INVENTION
Basically, the burner face used in this invention is a porous, low-conductivity material formed of metal or ceramic fibers and suitable for radiant surface combustion of a gaseous fuel-air mixture passed therethrough. A preferred burner face is a porous metal fiber mat which, when fired at atmospheric pressure, yields radiant surface combustion with interspersed portions or areas of increased porosity that provide blue flame combustion. Such a burner face is shown in FIG. 1 of U.S. Pat. No. 5,439,372 to Duret et al who disclose a rigid but porous mat of sintered metal fibers with interspersed bands or areas of perforations. One supplier of a porous metal fiber mat is N. V. Acotech S. A. of Zwevegem, Belgium. As shown by the patentees, bands of perforations are formed in the porous mat to provide blue flame combustion while the adjacent areas of the porous mat provide radiant surface combustion.
Another form of porous metal fiber mat sold by Acotech is a knitted fabric made with a yarn formed of metal fibers. While the yarn is porous, the interstices of the knitted fabric naturally provide uniformly interspersed spots of increased porosity. Hence, the knitted metal fiber fabric provides surface radiant combustion commingled with numerous spots of blue flames.
Still another form of porous burner face suitable for this invention is the perforated, ceramic fiber plate disclosed in U.S. Pat. No. 5,595,816 to Carswell having small perforations effective for radiant surface combustion, which is simply modified to have interspersed areas with larger perforations for blue flame combustion.
Another version of a perforated, ceramic or metal fiber plate adapted for this invention is one having uniform perforations that produce blue flame combustion, but such a plate is combined with an upstream configuration that limits flow to selected portions of the plate such that those portions operate with surface combustion in or near a radiant mode. One embodiment of this approach could simply involve another perforated plate, slightly spaced from the upstream side of the main plate. The perforations of the back-up plate are of a size and distribution that some of its perforations are aligned with perforations of the main plate so that the latter perforations support blue flame combustion. The unperforated portions of the back-up plate that are aligned with perforations of the main plate impede the flow of the fuel-air mixture to these perforations so that they yield surface combustion. The back-up plate need not be a low-conductivity plate like the main plate that is the burner face. In this case, the back-up plate obviously serves to diminish the flow of the fuel-air mixture through selected areas of the perforated, ceramic or metal fiber plate.
A perforated back-up plate may also be used with the various other forms of burner face previously described; usually the back-up plate helps to ensure uniform flow of the fuel-air mixture toward all of the burner face. With the knitted fabric formed of a metal fiber yarn, the back-up plate provides support for the fabric as well as uniform flow thereto. Hence, a perforated back-up plate can have a different function depending on the burner face with which it is combined. Inasmuch as the burner face will in most cases be cylindrical, as hereinafter described, the back-up plate that may also be cylindrical will hereafter be called perforated shell.
The complete burner of the invention has a porous fiber burner face attached across a plenum with an inlet for the injection of a gaseous fuel-air mixture, a perforated shell within the plenum behind the burner face, and a metal liner positioned to provide a compact combustion zone adjacent to the burner face. Such a burner has been successfully operated at high firing rates or high heat-flux and with high excess air to produce combustion gases containing not more than 5 ppm NO
x
and not more than 10 ppm CO and UHC, combined. Through the control of excess air, the burner is capable of delivering combustion gases containing not more than 2 ppm NO
x
and not more than 10 ppm CO and UHC, combined. All ppm (parts per million) values of NO
x
, CO and UHC mentioned in the specification and claims are values corrected to 15% O
2
, the gas turbine standard.
At the high surface firing rates required for burners that can be fitted in the casings of gas turbines, say at least about 500,000 BTU/hr/sf (British Thermal Units per hour per square foot) of burner face, the flames from the areas of increased porosity produce such intense non-surface radiation that the normal surface radiation from the areas of lower porosity disappears. However, the dual porosities make it possible to maintain surface-stabilized combustion, i.e., surface combustion stabilizing blue flames attached to the burner face. For brevity, burners having faces with dual porosities will be referred to as surface-stabilized burners.
Visually, flaming is so compact that a zone of strong infrared radiation seems suspended close to the burner face. The compactness of flaming is aided by the metal liner that confines combustion adjacent the burner face. Even though this surface-stabilized combustion is conducted with about 40% to 150% excess air depending on inlet temperature, the combustion products may contain as little as 2 ppm NO
x
and not more than 10 ppm CO and UHC, combined.
The aforesaid firing rate of at least about 500,000 BTU/hr/sf of burner face is for combustion at atmospheric pressure. Inasmuch as gas turbines operate at elevated pressures, the base firing rate must be multiplied by the pressure, expressed in atmospheres. For example, at an absolute pressure of 150 po

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