WO2004013538A2

WO2004013538A2 - Porous burner for gas turbine applications

Info

Publication number: WO2004013538A2
Application number: PCT/US2003/024655
Authority: WO
Inventors: Janet L. Ellzey; William Mathis, Jr.
Original assignee: Board Of Regents, The University Of Texas System
Priority date: 2002-08-05
Filing date: 2003-08-05
Publication date: 2004-02-12
Also published as: AU2003256855A1; WO2004013538A3; AU2003256855A8

Abstract

A burner including a porous element (12) is disclosed. The porous element may include an upstream section (112) and a downstream section (212). The porous element of the burner may be of a thickness that facilitates heat recirculation from the porous element to the incoming reactants, such that the incoming reactants are preheated prior to entering the porous element. Preheating of the incoming reactants may increase the firing rate of the burner. The flame may be stabilized such that substantially all of the combustion occurs within the pores.

Description

TITLE: POROUS BURNER FOR GAS TURBINE APPLICATIONS

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a burner that may be used in gas turbine applications. More particularly, the invention relates to the development of a porous burner having low nitrogen oxide (NOx) emissions in the combustion product gases.

2. Brief Description of the Related Art

Combustors used in gas turbine systems typically rely on combustion technology that involves free flames. In recent years, new emissions standards have been proposed and/or implemented that reduce the allowable emissions from such systems. Conventional combustors may not be able to meet the new standards.

Burners containing porous elements have been developed that allow combustion to occur in a porous medium. As discussed in Howell, J.R., Hall, M.J., and Ellzey, J.L. (1996) "Combustion of Hydrocarbon Fuels within Porous Inert Media," Progress in Energy and Combustion Science Vol. 22, 121:145, which is incorporated by reference as if fully set forth herein, porous burners have received substantial attention in the last few years due to their potential to produce low emissions over a wide operating range.

Since a porous burner operates on premixed fuel and air, a major issue is flame stabilization. Hsu, P.-F., Evans, W.D., and Howell, J.R. (1993) "Experimental and Numerical Study of Premixed Combustion Within Nonhomogeneous Porous Ceramics," Combust. Sci. and Tech., vol. 90, pp.149-172 (hereinafter "Hsu"), which is incorporated by reference as if fully set forth herein, describes a methane/air burner consisting of two sections of reticulated ceramics, a large-pore section stacked on top of a small- pore section. Khanna, V., Goel, R. and Ellzey, J.L. (1994), "Measurements of Emissions and Radiation for Methane Combustion Within a Porous Burner," Combust. Sci. Tech. 99, pp. 133-142 (hereinafter "Khanna"); Rumminger , M.D., Dibble, R.W., Heberle, N.H., and Crosley, D.R. (1996) "Gas Temperature above a Porous Radiant Burner: Comparison of Measurements and Model Predictions," Twenty-Sixth Symposium (International) on Combustion, The Combustion Institute, pp. 1755-1762 (hereinafter "Rumminger"); and Viskanta, R., and Gore, J.P. (2000) "Overview of Cellular Ceramics Based Porous Radiant Burners for Supporting Combustion," Environ. Comb. Tech. 1, pp. 167-203 (hereinafter "Viskanta"), each of which are incorporated by reference as if fully set forth herein, also describe burners using the two-section design as a means of flame stabilization.

Porous burners may exhibit a wide range of firing rates, depending on the particular design of the burner. Porous burners can be broadly divided into two categories depending on whether the effective flame speeds within the burner are greater or less than the laminar flame speed for the same fuel/air mixture. Mital, R., Gore, J.P., and Viskanta, R. (1997) "A Study of the Structure of Submerged Reaction Zone in Porous Ceramic Radiant Burners," Comb. Flame 111, 175-184 (hereinafter "Mital"), which is incorporated by reference as if fully set forth herein, reports a burner with an effective flame speed of 15 to 25 centimeters/second (cm/s) at an equivalence ratio of 0.9. These values are lower than the corresponding laminar flame speed of 35 cm/s, as described in Egolfopoulos, F.N., Zhu, D.L., Law, C.K. (1990) "Experimental and Numerical Determination of Laminar Flame Speeds: Mixtures of C₂-hydrocarbons with Oxygen and Nitrogen.," Twenty-Third Symposium (International) on Combustion, The Combustion Institute, pp. 471-478 (hereinafter "Egolfopoulos"), which is incorporated by reference as if fully set forth herein. The firing rates of the Mital burner ranged from 157 to 473 kilowatts per square meter (kW/m²). Rumminger describes a similar burner and also reports that the effective flame speeds were lower than the laminar flame speeds. The firing rate of the Rumminger burner was 315 kW/ m² at an equivalence ratio of 0.9. Other sources have reported effective flame speeds in porous media that are greater than the flame speed for a free flame. Hsu describes enhanced effective flame speeds and stable operation below the lean flammability limit. Khanna describes a similar burner and reported that the lean operating limit was close to that for a free flame. Effective flame speeds were significantly higher than those for a free flame. For instance, at an equivalence ratio of 0.75, flames were stabilized at velocities ranging from 50 to 80 cm/s while the laminar flame speed for this condition is 25 cm/s, according to Egofoulpoulos. These velocities correspond to firing rates of about 1200 to 1720 kW/m².

A primary difference between the burners operating at velocities less than the laminar flame speed and those operating above it may be the thickness of the flame stabilization layer. The burners described by Rumminger and Mital had flame stabilization layers of approximately 0.3 to 1 cm, whereas the flame stabilization layers in the burners of Hsu and Khanna were 4.0 cm and 2.55 cm, respectively. The thickness of the flame stabilization layer may affect how much heat is recirculated versus how much is radiated to the surroundings. The radiation to the surroundings may be a greater portion of the total heat input for a thin layer than for a thick layer. This heat loss may result in a suppression of the flame temperature and a decrease in effective flame speed. Conversely, heat may be more effectively recirculated in a thick layer, resulting in a higher flame temperature and an enhanced flame speed.

As discussed in Takeno, T., and Sato, K., (1979) "An Excess Enthalpy Flame Theory," Combust. Science Tech. 20, 73-84; Kotani, Y., and Takeno, T. (1982) "An Experimental Study on Stability and Combustion Characteristics of an Excess Enthalpy Flame," 19^th Symposium (International) on Combustion/The Combustion Institute, pp. 1503-1509; and Same, S.B., Kulkarni, M.R., Peck, R.E. and Tong, T.W. (1990) "An Experimental and Theoretical Study of Porous Radiant Burner Performance," Twenty-Third Symposium (Int.) on Combustion, The Combustion Institute, Pittsburgh, PA, pp. 1011-1018, each of which are fully incorporated by reference as if fully set forth herein, with sufficient heat recirculation, the flame speeds may be higher than those for a free flame. This characteristic of enhanced flame speeds may facilitate high firing rates. As discussed in Babkin, V.S., Korzhavin, A.A., and Bunev, V.A. (1990) "Propagation of Premixed Gaseous Explosion Flames in Porous Media," Comb. Flame 87: 182, which is fully incorporated by reference as if fully set forth herein, the effective flame speed may be sensitive to various parameters such as the equivalence ratio, pressure, and pore geometry.

Combustors that use porous burners may offer several advantages over conventional devices using free flames. As discussed above, porous burners may allow the reduction of emissions of nitrogen oxide and other pollutants. In addition, temperature and velocity profiles from a porous burner may be more uniform than those in a conventional combustor. These characteristics may result in longer system life.

Despite potential advantages of porous burners, firing rates produced in devices such as those described in the above references may not be sufficient for gas turbine applications. In particular, combustors for gas turbines may require firing rates of about 3000 kW/m².

In addition, materials used in the devices such as the ones described in the above references may lack durability. For example, many burners have been constructed of reticulated porous ceramics such as partially stabilized zirconia (referred to herein as "PSZ") or cordierite. In some respects, reticulated porous ceramics are an attractive material for porous burners because they have porosities approaching 90% and consequently low pressure drop. However, many of the materials used to date may be subject to degradation after exposure to typical combustion temperatures and thermal cycling. It is therefore desirable that improved combustion devices be developed that are characterized by high firing rates and low emissions. It is further desired that the combustion devices have increased durability and greater life under combustion temperatures and thermal cycling.

SUMMARY OF THE INVENTION Herein we describe a burner including a porous element. The porous element may include an upstream section and a downstream section. The porous element of the burner may be of a thickness that facilitates heat recirculation from the porous element to the incoming reactants, such that the incoming reactants are preheated prior to entering the porous element. Preheating of the incoming reactants may increase the firing rate of the burner. The flame may be stabilized such that substantially all combustion occurs within the porous element. In some embodiments, the thickness of downstream section of the porous element length may be greater than about 1 centimeter. In one embodiment, the thickness of the downstream section may be about 5 centimeters in length.

The porous element may be of a high temperature ceramic material. For example, the porous element may be of a yttria stabilized zirconia/alumina composite, zirconia toughened mullite, or a metal. In an embodiment, the upstream section may have pores that are smaller than the pores in downstream section. The upstream section may serve as a flame arrestor for the burner.

In an embodiment, a length of a porous element or section may be selected to facilitate heat recirculation to the incoming reactants. In another embodiment, a ratio between (a) the length of a porous element or section; and

(b) a dimension related to the cross-sectional area of the porous element (including, but not limited to, the diameter of a plug, the cross-sectional area of a plug, or the width of a rectangular block) may be selected to facilitate heat recirculation to the incoming reactants.

In an embodiment, a porous element may be in the form of a hollow cylinder through which gas may flow radially. The porous element may include an inner annular section that just fits inside of an outer annular section.

During use, a gas may flow into a core region encompassed by the inner annular section. In an embodiment, a cylindrical porous element may be coupled at one end with a hemispheric porous element. Alternatively, the cylindrical portion may be coupled at one end with a planar porous element or with an inert cap to partially enclose core.

In an embodiment, a porous burner may be incorporated in a gas turbine system. In another embodiment, a porous burner may be used as a radiant burner. In still another embodiment, a porous burner may be used as a heater for a fuel reformer in a fuel cell system.

BRIEF DESCRIPTION OF THE DRAWINGS

Features and advantages of the methods and apparatus of the present invention will be more fully appreciated by reference to the following detailed description of presently preferred but nonetheless illustrative embodiments in accordance with the present invention when taken in conjunction with the accompanying drawings in which:

FIG. 1 depicts a cut-away view of a porous burner having a porous element with upstream and downstream sections;

FIG. 2 depicts a cross sectional view of an embodiment having a porous element having upstream and downstream sections of approximately equal length;

FIG. 3 depicts a cross sectional view of an embodiment having a porous element having upstream and downstream sections, wherein the upstream section is shorter than the downstream section;

FIG. 4 depicts a cross sectional end view of a burner having a porous element in the form of a hollow cylinder; FIG. 5 depicts a cross sectional side view of a burner having a porous element in the form of a hollow cylinder;

FIG. 6 depicts a schematic diagram of a gas turbine system that includes a porous burner;

FIG. 7 depicts a plot of temperature vs. distance for YZA burner at φ= 0.65;

FIG. 8 depicts a plot of temperature vs. distance for ZTM burner at φ=0.65; FIG. 9 depicts a plot of temperature vs. distance for the YZA burner at φ=0.65 at flow velocities below 70 cm sec;

FIG. 10 depicts a plot of temperature vs. distance for YZA burner φ=0.65 at flow velocities above 70 cm/sec;

FIG. 11 depicts a plot of temperature vs. distance for YZA burner at φ=0.70; FIG. 12 depicts a plot of temperature vs. distance for the YZA burner at φ=0.75;

FIG. 13 depicts a plot of unburned hydrocarbons at the exit of the YZA over a range of velocities and equivalence ratios;

FIG. 14 depicts a plot of CO at the exit of the YZA burner over a range a velocities and equivalence ratios;

FIG. 15 depicts a plot of NOx exhausted from the shorter YZA burner over a range a velocities and equivalence ratios.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

Herein we describe a burner including a porous element. The porous element may include an upstream section and a downstream section. The porous element of the burner may be of a thickness that facilitates heat recirculation from the porous element to the incoming reactants, such that the incoming reactants are preheated prior to entering the porous element. Preheating of the incoming reactants may increase the firing rate of the burner. The flame may be stabilized such that substantially all combustion occurs within the porous element.

FIG. 1 shows an embodiment of a porous burner. Burner 10 may include porous element 12. Porous element 12 may be at least partially enclosed in insulating sleeve 14. The porous element may have entrance 16 and exit 18. The cross section of porous element 12 may be a variety of shapes, including, but not limited to, circular, square, or rectangular. Entrance 16 and exit 18 may be planar or non-planar surfaces.

In an embodiment, porous element 12 may include upstream and downstream sections 112, 212. In the embodiment shown in FIG. 1, upstream and downstream sections 112, 212 may be cylindrical plugs. Upstream section 112 may be proximate to entrance 16 and may function as the upstream portion of the porous element during use. Downstream section 212 may be proximate to exit 18 and may function as the downstream portion of the porous element during use. Junction 20 may exist at the junction between upstream and downstream sections 112, 212.

In some embodiments, porous element 12 may be a high temperature ceramic material. For example, the porous element may be of a yttria stabilized zirconia/alumina composite (referred to herein as "YZA"). YZA may include Y₂0 , Zr0₂, and A1₂0₃. The YZA material may have a maximum use temperature of 1600 degrees Celsius and a bulk density of 0.93 grams/cubic centimeter (g/cc). The YZA material may have a high tolerance to thermal shock as well as being inert. As another example, porous element 12 may be of zirconia toughened mullite (referred to herein as "ZTM"). In other embodiments, porous element 12 may be comprised of a metal. In an embodiment, upstream section 112 may have pores that are smaller than the pores in downstream section 212. For example, upstream section 112 may have about 26 pores per centimeter (ppcm). Downstream section 212 may have about 4 pores per centimeter. Upstream section 112 may serve as a flame arrestor for burner 10. In an embodiment, insulating sleeve 14 may be coextensive in length with porous element 12. In another embodiment, insulating sleeve 14 may be longer than porous element 12 and may extend beyond either entrance 16, exit 18, or both.

In another embodiment, porous element 12 may consist of a single section. The section may have about 4 pores per centimeter.

In an embodiment, the length of downstream section 212 may be greater than about 1 centimeter in length. In some embodiments, the length of downstream section 212 may be between about 3 to 5 centimeters in length. FIG. 2 shows an embodiment of the burner in which the length of upstream section 112 of porous element

12 is about the same as the length of downstream section 212. For example, each of the upstream and downstream sections 112, 212 may be about 5 centimeters in length. The diameter of upstream sections 112, 212 may each be about 5 centimeters. Insulating sleeve 14 may be a tall hollow cylinder of alumina 10 centimeters in outer diameter. The length of insulating sleeve may be about 20 centimeters. FIG. 3 shows an embodiment of the burner similar to that shown in FIG. 2, but in which upstream section

112 is shorter than downstream section 212. For example, upstream section 112 may be about 2.5 centimeters in length and downstream section 212 may be about 5 centimeters in length.

During use, burner 10 may be coupled to fuel mixture by an inlet tube or plenum (not shown). The inlet tube or plenum may be coupled to a fuel supply and an air supply. During use, the burner may be positioned vertically, horizontally, or at various other orientations.

Suitable fuels may include, but are not limited to, methane and propane. A mixture of fuel and air may be introduced into porous element 12 at entrance 16 and a flame may be stabilized within the porous element. In embodiments where porous element 12 includes two sections of porous media, the flame may be stabilized in downstream section 212. During use, heat may be transported through the solid matrix of porous element 12 through radiation and conduction from the postflame zone to the incoming reactants. During combustion in the porous medium, the solid matrix may transport heat through radiation and conduction from the postflame zone to the incoming reactants. In a burner having a relatively "thick" downstream section, more heat may be recirculated and the maximum gas temperature may be increased in comparison to that of a burner with a relatively "thin" downstream section. The maximum flow through the burner may increase with increasing temperature. In an embodiment, a ratio between (a) the length of a porous element or section; and (b) a dimension related to the cross-sectional area of the porous element (including, but not limited to, the diameter of a plug, the cross-sectional area of a plug, or the width of a square) may be selected to facilitate heat recirculation to the incoming reactants. For example, the ratio of interest may be the ratio between the diameter of the downstream section of a cylindrical element and length of the downstream section. In the embodiment shown in FIG. 1, the ratio is about 1:1.

The porous element and sections thereof of the burner may be one or more of a variety of shapes. The thickness of each section may vary depending on geometry. As used herein, the "thickness" of an element or section may be the approximate mean distance from the surface that receives the flow of the gas mixture to the surface that expels the gas mixture. Thus, in sections of uniform length (such as the stacked plugs shown in FIG. 1) the thickness of each section is equal to its length. Alternatively, in a hollow cylinder, the thickness may be the difference between the outer radius of the hollow cylinder and the inner radius of the hollow cylinder. In a porous element having multiple sections, each having different non-planar geometry, the "thickness" may be the mean path length from the surfaces that receive the flow of the gas mixture to the surfaces that expel the gas mixture. In an embodiment, a porous element may be in the form of a hollow cylinder through which gas may flow radially. Referring to FIGS. 4 and 5, porous element 12 may include inner section 312 and outer section 412. Inner section 312 may be a hollow cylinder having an annular cross section. Outer section 412 may be a larger hollow cylinder section having an annular cross section. Inner and outer sections 312, 412 may be sized such that the outer diameter of inner section 312 is substantially equal to the inner diameter of outer section 412, so that inner section 312 fits inside of outer section 412. A mixture may flow into core region 30 to entrance 16 through pipe 32 having perforations (not shown) or other openings adapted to permit the flow of gas to entrance 16.

In the embodiment shown in FIGS. 4 and 5, cylindrical porous element 12 may be coupled at one end with hemispheric porous element 13. Alternatively, cylindrical porous element 12 may be coupled at one end with a planar porous element (not shown) or with an inert cap (not shown) to partially enclose core region 30. In an embodiment, a porous burner may be incorporated in a gas turbine system. FIG. 6 is a schematic diagram of such a system. Gas turbine system 50 may include compressor 52, turbine 54, and combustor 56. Combustor 56 may include porous burner 10.

In another embodiment, a porous burner may be used as a radiant burner. In still another embodiment, a porous burner may be used as a heater for a fuel reformer in a fuel cell system.

Examples:

Apparatus and General Procedure for All Examples:

For the following examples, the apparatus included a mixing chamber, a porous burner, a system of thermocouples, and the emissions measuring equipment.

The mixing chamber consisted of a 5-centimeter (cm) diameter cylinder with two opposing entrances and one exit. The two entrances were for methane and air and the exit was used for the mixed gas. Methane and air flows were controlled using Hastings mass flow meters calibrated at the beginning of each burner experiment.

Methane was supplied by a pressurized tank certified at 99.5% pure. The laboratory air was filtered prior to introduction into the experimental apparatus. The temperature measurements were made with thermocouples located in the walls of the insulation. The first two burners had two rows of eight thermocouples located at 1.27-cm intervals along the length of the two porous plugs. The first thermocouple was located at the entrance to the burner, while the fourth thermocouple was located at the interface between the two plugs. The third burner had one row of 12 thermocouples located at 0.635 cm intervals along the length of the two porous plugs. Type K and type B thermocouples were used in temperature measurements. The Type B was selected because of the high temperatures expected near the flame region and upstream of the flame, while the K-type provided more accurate measurements at temperatures below 1070 K. Therefore, the first two thermocouples were K-type and the rest of thermocouples were B-type. The thermocouples are wired into a data acquisition system and the voltages were converted into temperatures and displayed in a spreadsheet program.

At the exit of the burner, a stainless steel exhaust probe, 0.625 cm in diameter, collected a sample of exhaust from the centerline of the exit plane using a vacuum pump. The exhaust gas was chilled in an ice bath to remove the water vapor and then sent into gas analyzers to determine the content of unburned hydrocarbons, CO, and NOx in the exhaust. Initially, the air flow meter was set to a mass flow rate for a mixture velocity of 30 cm/sec at the inlet face of the burner and an equivalence ratio of 0.75. At this time the emissions analyzers were calibrated using a zero gas and a known quantity span gas to ensure that the equipment was producing accurate measurements. The emissions equipment was calibrated before each experiment.

Before starting the flow of methane, the methane mass flow meter was set to a mass flow rate for a mixture velocity of 30 cm/sec at the entrance to the burner and an equivalence ratio of 0.75. The methane mass flow meter was then activated and methane was allowed to flow through the burner. Once the velocities stabilized, the mixture was ignited at the exit of the burner using a standard 14 cm methane lighter. Once ignited, the mixture briefly stabilized on the top of the porous plug. After a period of approximately 5 minutes, the flame migrated down to the interface of the two plugs and stabilized itself. The mixture was held constant for 15 minutes to allow the burner to heat-up and then the mixture was changed to the desired velocity and equivalence ratio.

After the warm-up period, the flow was set for a velocity of 30 cm/sec and the equivalence ratio was reduced to 0.6. The burner was allowed to stabilize at this new condition and temperature measurements were taken at 5-second intervals. A minimum of a 20-minute stabilizing period was used in each test. Once data taking was complete, the fuel/air velocity was increased to 35 cm/sec while maintaining a constant equivalence ratio. The velocities were increased in intervals of 5 cm/sec or 10 cm/sec and the resulting temperatures and emissions were recorded. This procedure was followed until the velocity reached 70 cm/sec, which was the limit of the air mass flow meter. The burner was then shut-off and the larger flow meter was installed. At this point, the burner was ignited as described above, allowed to warm-up, and slowly increased to the desired equivalence ratio and 70 cm/sec. After the burner was determined to be in stable operation the flow was increased in increments of 5 cm/sec or 10 cm/sec and allowed to stabilize. This process was continued until the flame experienced blow-off, or the external flame become so large that it posed a fire hazard.

After the full range of velocities was recorded at an equivalence ratio of 0.6, the burner was shut off and allowed to cool for an hour. The start-up procedure was duplicated and the velocity was set to 30 cm sec but the equivalence ratio was increased to 0.65. The procedure was repeated for equivalence ratios of 0.7 and 0.75. Example 1:

For this example, a burner was constructed of reticulated porous YZA. The upstream and downstream sections of the burner were each 5.08 cm in length. The upstream section consisted of ceramic foam with 26 ppcm and a downstream section consisted of ceramic foam with 4 ppcm. The temperature profiles for the YZA burner at an equivalence ratio of 0.65 are shown in FIG. 7. The interface in this burner was at approximately 5.1 cm. The peak temperature occurred very near the interface for a range of conditions. This indicates that the two-section design may be an effective means of stabilizing the flame. The peak temperature increased with increasing inlet velocity due to the increased volumetric heat release and approached a maximum of approximately 1550K. The inlet of the burner between 0 and 2.5 cm was not significantly heated and was near ambient for all conditions. The small-pore media in the upstream section had a smaller radiative path length. The smaller radiative path length may have limited the radiative heat transfer upstream.

Example 2: For this example, a burner was constructed of reticulated porous zirconia toughened mullite ("ZTM"). The upstream and downstream sections of the burner were each 5.08 cm in length. The upstream section consisted of ceramic foam with 26 ppcm and the downstream section consisted of ceramic foam with 4 ppcm.

The temperature profiles for the ZTM burner at an equivalence ratio of 0.65 are presented in FIG. 8. As in the YZA burner of Example 1, the interface was at approximately 5.1 cm. For velocities between 30 and 70 cm/sec, the peak temperature is within the upstream smaller-pored section. The flame is not necessarily stable at these locations because the experiments were terminated once the flame propagated upstream of interface. At a velocity of 75 cm/sec, however, the flame stabilized downstream of the interface.

These results indicate that the interface in the ZTM burner did not stabilize the flame over a range of conditions tested.

Example 3:

For this example, a burner was constructed of YZA with a 2.54 cm upstream section (instead of 5.08 cm as in Examples 1 and 2) and a downstream section of 5.08 cm. The upstream section consisted of ceramic foam with

26 ppcm and the downstream section consisted of ceramic foam with 4 ppcm. FIG. 9 shows the temperature for the shortened YZA burner at an equivalence ratio of 0.65 at flow velocities at or below 70 cm/sec. The temperature profile indicates that the interface was an effective flame holder.

The data also indicate that the reduced length of the upstream porous plug (relative to Example 1) did not significantly affect the performance of the burner. Compared with Example 1, the maximum temperatures were similar in both burners as well as the location of the flame relative to the interface. All of the temperature profiles match very closely with one another with the exception of the 70 cm/sec plot. At this velocity, the flame stabilized closer to the exit of the burner than in Example 1.

The temperature profiles for this same equivalence ratio but at higher velocities are shown in FIG. 10. The combustion was visibly different at velocities above 70 cm/sec. A faint orange flame was observed exiting the insulation. The insulation extended 12 cm beyond the exit of the porous burner so the size of the external flame was greater than this length when it was initially observed. This flame increased in size and intensity as the flow velocity was increased. As the peak temperature in FIG. 10 indicates, the flame was no longer stabilized at the interface between the two porous media but was instead anchored within the downstream section. These data points were recorded after a minimum of 30 minutes of the temperature profile holding constant, i.e., the flame was not moving within the burner. With each increase in flow velocity, the flame reaction zone moved slowly out towards the exit of the burner until at a condition of φ=0.65, blow-off was observed at 195 cm/sec.

This process was repeated with higher equivalence ratios. FIG. 11 shows the temperature profiles for the YZA burner with a 2.54 cm upstream section and a 5.08 cm downstream section at an equivalence ratio of 0.70. At this higher equivalence ratio, the flame was located at the interface of the two media up to a flow velocity of 130 cm/sec where it started moving slightly toward the exit of the burner. At 140 cm/sec the profile is similar to that at 130 cm/sec, but the peak temperature was closer to the exit of the burner at 150 cm/sec. Flow velocities above 150 cm/sec were not attempted at this equivalence ratio because the flame was sufficiently large that it was beginning to interact with the surrounding enclosure. It was clear, however, that this equivalence ratio could support flames at flow velocities greater than 150 cm/sec.

FIG. 12 shows the temperature profiles for the YZA burner with a 2.54 cm upstream section and a 5.08 cm downstream section at an equivalence ratio of 0.75.

The firing rates at each equivalence ratio are shown in Table I. The firing rate varied between 680 and 3975 kW/m². The largest range occurred at an equivalence ratio of 0.65, where there was almost a factor of five variation between the flashback and the blowout limits.

Table I. Minimum and maximum firing rates for the YZA burner Emissions data for the velocities less than 70 cm/s are shown in FIGS. 13-15. The levels of unburned hydrocarbons were elevated at low velocities. The CO levels were between 4 and 16 parts per million (ppm) with no consistent trend observable. NOx levels were below 10 ppm. For the equivalence of 0.65, NOx levels are below 7 ppm. For higher velocities, the flame interacted with the probe, causing the CO levels to increase. At an equivalence ratio of 0.65, a quartz sleeve was placed on top of the burner so that the probe could be moved downstream of the reaction zone. CO varied between 5 and 15 ppm. NOx was below 8 ppm and unburned hydrocarbons were below 5 ppm. Further modifications and alternative embodiments of various aspects of the invention will be apparent to those skilled in the art in view of this description. Accordingly, this description is to be construed as illustrative only and is for the purpose of teaching those skilled in the art the general manner of carrying out the invention. It is to be understood that the forms of the invention shown and described herein are to be taken as the presently preferred embodiments. Elements and materials may be substituted for those illustrated and described herein, parts and processes may be reversed, and certain features of the invention may be utilized independently, all as would be apparent to one skilled in the art after having the benefit of this description of the invention. Changes may be made in the elements described herein without departing from the spirit and scope of the invention as described in the following claims.

Claims

What is claimed is:

1. A burner comprising a porous element, wherein the porous element comprises an entrance and an exit, wherein a thickness ofthe porous element is selected to facilitate recirculation of heat to incoming reactants.

2. The burner of claim 1, wherein a thickness of the porous element is greater than about 1 centimeter.

3. The burner of claim 1 , wherein a thickness of the porous element is about 5 centimeters.

4. The burner of claim 1, wherein the porous element further comprises an upstream section proximate to the entrance and a downstream section proximate to the exit.

5. The burner of claim 1, wherein the porous element further comprises an upstream section proximate to the entrance and a downstream section proximate to the exit, and wherein a thickness of the downstream section is selected to facilitate recirculation of heat to incoming reactants.

6. The burner of claim 1, wherein the porous element further comprises an upstream section proximate to the entrance and a downstream section proximate to the exit, and wherein a thickness of the downstream section is greater than about 1 centimeter.

7. The burner of claim 1, wherein the porous element further comprises an upstream section proximate to the entrance and a downstream section proximate to the exit, and wherein a thickness of the downstream section is about 5 centimeters.

8. The burner of claim 1, wherein the porous element further comprises an upstream section proximate to the entrance and a downstream section proximate to the exit, wherein the upstream section comprises pores that are smaller than those of the downstream section.

9. The burner of claim 1 , wherein the porous element further comprises a high temperature ceramic.

10. The burner of claim 1, wherein the porous element further comprises a composite, wherein the composite comprises yttria stabilized zirconia and alumina.

11. The burner of claim 1 , wherein the porous element further comprises a cylindrical plug.

12. The burner of claim 1, wherein the porous element further comprises a cylindrical plug, wherein a ratio of a length of the cylindrical plug to a diameter of the cylindrical plug is selected to facilitate recirculation of heat to incoming reactants of the burner.

13. The burner of claim 1, wherein the porous element further comprises a cylindrical plug, wherein the porous element further comprises an upstream section proximate to the entrance and a downstream section proximate to the exit, wherein a ratio of a length of the downstream section to a diameter of the cylindrical plug is selected to facilitate recirculation of heat to incoming reactants of the burner.

14. The burner of claim 1, wherein the porous element further comprises a hollow cylinder, wherein the entrance is an inner surface of the hollow cylinder.

15. The burner of claim 1, wherein the porous element further comprises a hollow hemisphere, wherein the entrance is an inner surface of the hemisphere.

16. A burner comprising a porous element, wherein the porous element comprises an entrance and an exit, wherein the burner is configured such that substantially all combustion occurs within the porous element during use.

17. A burner comprising a porous element, wherein the porous element comprises an entrance and an exit, wherein a ratio of a length of the porous element to a square root of a cross-sectional area of the porous element is selected to facilitate recirculation of heat to incoming reactants.

18. A gas turbine system comprising: a compressor; a turbine; and a combustor, wherein the combustor comprises a burner, wherein the burner comprises a porous element, wherein the porous element comprises an entrance and an exit, wherein a thickness of the porous element is selected to facilitate recirculation of heat to incoming reactants.