Braided burner for premixed gas-phase combustion

Abstract

A surface burner for gas combustion has a burner surface which is fabricated by intertwining or interweaving an elongated flexible element across a distinct burner frame. This fabrication method can be best referred to as braiding, but also plaiting, lacing or another comparable method.

Claims

1. A burner for pre-mixed gas-phase combustion, the burner comprising: an elongated flexible element made of multiple strands of ceramic yarn twisted together; and a frame comprising: a holder having a first surface, a second surface, an outer wall, and an inner wall defining an aperture; and structural elements comprising: a plurality of full-U-shaped arches, each full-U-shaped arch including two legs and each leg extending from the first surface of the base in a substantially perpendicular direction with respect to the first surface; and a single half-U-shaped arch having a leg and a distal end opposite the leg, the leg extending from the first surface of the base in a substantially perpendicular direction with respect to the first surface and the distal end being attached to an adjacent full-U-shaped arch of the plurality of full-U-shaped arches, the elongated flexible element being braided around the frame in a braiding pattern, such that the elongated flexible element is intertwined across the frame such that segments of the elongated element form curved and inclined flow channels of a variable cross section and openings between these segments on a burner surface, which provides a flame stabilization surface.

2. The burner according to claim 1, wherein the burner has the shape of a basket.

3. The burner according to claim 1, wherein the r plurality of full-U-shaped arches comprises an even number of full-U-shaped arches.

4. The burner according to claim 3, wherein the full-U-shaped arches do not cross each other.

5. A method for fabricating a burner for premixed gas-phase combustion, the method comprising: providing an elongated flexible element made of multiple strands of yarn twisted together; providing a frame comprising; a holder having a first surface, a second surface, an outer wall, and an inner wall defining an aperture; and structural elements comprising; a plurality of full-U-shaped arches, each full-U-shaped arch including two legs and each leg extending from the first surface of the base in a substantially perpendicular direction with respect to the first surface; and a single half-U-shaped arch having a leg and a distal end opposite the leg, the leg extending from the first surface of the base in a substantially perpendicular direction with respect to the first surface and the distal end being attached to an adjacent full-U-shaped arch of the plurality of full-U-shaped arches; and intertwining the elongated flexible element about the structural elements of the frame such that the elongated flexible element is braided around the frame in a braiding pattern and such that segments of the elongated flexible element form curved and inclined flow channels of a variable cross section between these segments on a burner surface, which provides a flame stabilization surface.

6. A burner for pre-mixed gas-phase combustion, the burner comprising: a frame comprising: a base having a first surface, a second surface, an outer wall, and an inner wall defining an aperture; and structural elements comprising: a plurality of full-U-shaped arches, each full-U-shaped arch including two legs and each leg extending from the first surface of the base in a substantially perpendicular direction with respect to the first surface; and a single half-U-shaped arch having a leg and a distal end opposite the leg, the leg extending from the first surface of the base in a substantially perpendicular direction with respect to the first surface; and an elongated flexible element comprising multiple twisted strands of material, the elongated flexible element being intertwined about the frame to form a burner surface comprising curved and inclined flow channels.

7. The burner of claim 6, wherein the distal end of the single half-U-shaped arch is attached to an adjacent full-U-shaped arch of the plurality of full-U-shaped arches.

8. The burner of claim 6, wherein the multiple twisted strands of material comprise ceramic material.

9. The burner of claim 6, wherein each of the structural elements has a thickness that is less than a thickness of the elongated flexible element.

10. The burner of claim 6, wherein the curved and inclined flow channels have a variable cross section and segments of the elongated flexible element form openings on the burner surface.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) The invention will be further elucidated below on the basis of one particular embodiment illustrated in drawings, as well as plots containing measurement results, namely:

(2) FIG. 1 shows a burner with ceramic fiber cord braided across a frame; and

(3) FIG. 2 shows the burner.

(4) FIG. 3 shows a plot of measured mole fractions of NOx and unburned species versus calculated adiabatic flame temperature; and

(5) FIG. 4 shows a plot of optimal and allowable mixture equivalence ratio versus inlet temperature.

(6) This embodiment of the invention is a burner fabricated and tested by the inventors. The burner in the invention is not limited to this embodiment.

DETAILED DESCRIPTION OF THE DRAWINGS

(7) In FIG. 1, an embodiment of the burner 1 is shown. The burner surface is formed by an elongated flexible element formed by a cord 9 of flexible material braided into a pattern resembling a basket or a mitre headgear. The cord 9 is made of the high-temperature material that prevents burner failure at high inlet temperatures. The cord is braided around a frame 3 in FIG. 2.

(8) FIG. 2 shows a holder ring 7 of the burner frame. The holder ring diameter is 30 mm. In the illustrated embodiment, the frame is made from an even number (four) of full-U arches 5 and one half-U arch 5c. Each full-U arch comprises a bridging section 5b and two leg sections 5a essentially parallel to each other. The full-U arches 5 and one half-U arch 5c produce an odd (nine) number of vertical leg sections required for a favorable braiding pattern. Alternatively, the U arches could have crossed to form a cupola center point at the top. The material of the U arches of the burner in the illustrated embodiment is ceramics.

(9) The braiding cord 9 in FIG. 1 is made from ceramics yarns, which are composed of ceramic fibers. It has the diameter of 2 mm in a non-stretched state. The surface porosity, size of openings between the cord segments 11, tortuosity of the flow channels formed between cord 9 segments and other surface/pattern parameters can be adjusted via a proper selection of the: 1) cord thickness; 2) frame parameters; 3) braiding pattern; and other available design parameters.

(10) The burner presented in FIG. 1 has the external surface of approximately 33 cm.sup.2. It is scaled for a thermal power range between single to more than 10 kWTh at room conditions.

(11) Working Principle

(12) The burner in FIG. 1 functions as follows: A premixed fuel-air mixture is supplied through the holder ring. The overall mixture flow is self divided over the burner surface into two parts: The larger flow portion passes with a higher speed between the cord segments (braids) and jets through the openings between the braids on the burner surface. The smaller portion filters through the fiber material of the braiding cord and burns on the cord surface. The high-speed jets produce conical flames. These flames are additionally stabilized by the surface combustion. The stabilization is improved by the tortuosity of the space available to the flow between the braids and the mutual inclination of the mixture jets and the flame cones. Due to such effective flame stabilization, the flow range between flame quenching and blow-off is very wide. The braiding ensures that each individual jet channel is formed almost as a nozzle with a throat. The latter ensures a high resistance of the burner surface against flashback. The cord fiber and braiding easily allow accommodating thermal and mechanical stresses. In this way, resistance to thermal expansion and thermal shock is ensured. High thermal resistance and oxidation resistance of the ceramic fiber allow operating the burner at very high surface/material temperatures.

(13) Typical Burner Performance

(14) Some experimentally measured performance figures for the burner in FIG. 1 are described below in the following plots:

(15) FIG. 3: Measured (corrected to zero oxygen) mole fractions of NOx and unburned species (CO+UHC) versus calculated adiabatic flame temperature (T.sub.ad). Experiments are conducted for various inlet temperatures (T22-T740correspond to 22-740 deg. C.), absolute pressures (p1-p3 in bar), flow rates (100-1000 Nl/min) and mixture equivalence ratios (0.28-0.95).

(16) FIG. 4: Optimal (between solid lines) and allowable (between dashed lines) mixture equivalence ratio versus inlet temperature at absolute pressure 1-3 bar. Markers represent experimental points.

(17) As can be seen from FIG. 3 and FIG. 4, the burner was tested for combustion of premixed methane-air mixture over variable: inlet temperature, pressure, flow rate and mixture equivalence ratio (actual fuel-to-air flow ratio divided by the stoichiometric ratio). The burner was installed inside a quartz tube (to provide optical observation) with a diameter of 110 mm and extended over 150 mm from the burner base. The inlet temperature and absolute total pressure varied between room temperature and atmospheric pressure and 740 C and 3 bar respectively. The mass flow rate and fuel-to-air equivalence ratio varied from 100 to 1000 Nl/min (2-20 g/s) and 0.28 to 0.95 (depending on the inlet temperature) respectively. The thermal input ranged from >4 to 32 kWTh.

(18) Combustion completeness was evaluated for the burner in FIG. 1 via measuring mole fractions of CO and unburned hydrocarbons (UHC). NOx was also measured in all tested cases. FIG. 3 shows an index of unburned species (IU) defined as: IU=[CO]+[UHC] (ppm) and NO.sub.x mole fractions at zero oxygen concentration versus adiabatic flame temperature T.sub.ad. The adiabatic flame temperature is calculated as a function of the inlet temperature and equivalence ratio at each given pressure.

(19) If one would adopt the limits of NOx <40 ppm and IU<100 ppm (at zero O.sub.2), then in the range of adiabatic flame temperatures between 1450 C and 1650 C both IU and NOx can be maintained below these limits. The right adiabatic temperature can be ensured by a proper adjustment of the mixture equivalence ratio as a function of the mixture inlet temperature. Between solid lines in the middle of FIG. 4, low-emission operation can be achieved. The upper and lower dashed lines indicate the allowable operating range. The markers in FIG. 4 represent experimental points. The experiments prove that the burner can also operate at high equivalence ratios. This will, however, result in higher adiabatic flame temperatures and high NOx. The flame temperatures up to the melting/oxidation temperature limit of the burner surface material are safe (in this example up to 1800 C): The burner cannot be destroyed even if the flame will closely approach or even partially submerge into the surface. The burner can be operated at even higher combustion temperatures. However, for these regimes, special attention should be paid to avoiding an overheating of the burner material.

(20) Application at Elevated Inlet Temperatures and Pressures

(21) FIGS. 3 and 4 demonstrate experimental evidence that the burner according to the invention has a broad applicability range stretching from atmospheric (room) conditions and up to elevated pressures and inlet temperatures, including very high inlet temperatures.

(22) Among other appliances, elevated pressures and inlet temperatures are encountered in burners for gas turbine combustion, as a result of flow compressor. The inlet temperature can be further increased in a gas-turbine recuperator, which recuperates exhaust heat into the compressed flow. Recuperators are used on various gas turbines and commonly used on micro turbines.

(23) Premixed gas turbine burners are susceptible to flashback. Compared to other premixed burners, the flashback problem is more acute in gas turbines due to a broad range of operating conditions with varying pressures, inlet temperatures, flow rates and equivalence ratios. It is very difficult to ensure that conditions for a flashback will not occur within such a variation of operating conditions. Combinations of burners and recuperators, as well as other heat exchanges, are also encountered in other applications, including high-efficiency furnaces, boilers, etc.

(24) High inlet temperatures further promote flashback. As the inlet flow is hot and lacks the cooling capacity, any upstream flame propagation typically leads to a very rapid burner failure.

(25) The burner according to the invention has a superior flashback resistance, as any upstream flame propagation is counteracted by flow streams accelerated though the intricately inclined flow channels between the cord braids that terminate into openings on the burner surface. Additionally, the suitability of high-temperature materials (such as ceramics, high-temperature alloys, quartz and glass fibers, etc.) for the burner cord greatly extends possibilities for operation at very high inlet temperatures with reduced risks of burner failure. These statements are proven by the flashback-free operation and retention of structural integrity of the tested burners (FIG. 1-FIG. 4), including low NOx, CO and UHC operation.

(26) Therefore, the burner in this patent is proven to be ideally suitablebut not limited toapplications at high inlet temperatures, such as in recuperated appliances, including gas turbines and micro gas turbines. The latter also feature elevated pressures.

(27) Although the present invention is elucidated above on the basis of the given drawings, it should be noted that this invention is not limited whatsoever to the embodiments shown in the drawings. The invention also extends to all embodiments deviating from the embodiments shown in the drawings within the context defined by the description and the claims.