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The combustion of liquid fuels within a porous media radiant burner
ELSEVIER
The Combustion of Liquid Fuels within a Porous Media Radiant Burner Michele Kaplan Matthew J. Hall Center for Energy Studies, Department of Mechanical Engineering, lhe University of Texas at Austin, Austin, Texas
• The combustion of liquid fuels within the porous inert media (PIM) of a radiant burner was examined. Existing designs of porous radiant burners have typically relied on premixed mixtures of gaseous fuels and air. A radiant burner was built, and various design configurations were tested using the following types of porous ceramics: magnesia-stabilized zirconia, silicon carbide, and yttrium-stabilized zirconia. The fuel heptane was impinged on the combustion section using an oil spray nozzle with a fixed flow rate of approximately 0.025 lpm. The burner had an insulated combustion section that was 10.0 cm in diameter and consisted of several 2.5-cm-thick ceramics. Stable complete combustion was achieved for heptane at equivalence ratios of 0.57-0.67. Temperature measurements taken across the exit plane of the combustion section were typically within 50°C of each other, indicating radially uniform combustion. For an equivalence ratio of 0.64, axial temperature measurements taken down the side of the combustion section showed relatively low temperatures, 1170-1370°C. Very low emissions of both CO and NO x were measured for the range of stable equivalence ratios. Corrected for 3% oxygen, CO varied from 3 to 7 ppm and NO, varied from 15 to 20 ppm. Several variations of the burner were tested, including higher fuel flow rates, prevaporized fuel, and a quartz-enclosed ceramic section.
Keywords: porous media, combustion, liquid fuel, ceramic foam, radiant burner INTRODUCTION Infrared heating is widely used by industry for manufacturing processes. Some applications include paper and wood drying, powder coating, annealing, food browning and baking, and plastics curing and forming. The burning of liquid fuels within porous media has several potential advantages over combustion in an open flame. A conventional burner must be rather large and maintained at a high temperature for there to be sufficient time for the liquid fuel to completely evaporate and burn. A porous ceramic burner has a very high radiant output due to the high emissivity of the ceramic material. The porosity provides the fuel with a convoluted path through a homogeneously radiant field that is without cold boundaries, ensuring droplet vaporization and contributing to complete reaction. The burner, having a high volumetric energy output due to a high burning rate, can be compact. Liquid fuel combustion in porous ceramics has potentially low emissions of NO x and CO. Unburned hydrocarbon and CO emissions are low due to fuel preheating and the increased residence time of exhaust gases in a high-
temperature postcombustion region. N O , emissions are low due to droplet vaporization and mixing in the ceramic media. This can result in partially premixed lean combustion, which has lower flame temperatures than diffusion flames where combustion occurs at stoichiometric conditions. A potential application of a liquid fuel porous ceramic burner is the incineration of liquid hazardous waste. Liquid hazardous wastes are difficult to incinerate in conventional burners because they are often low energy content fuels and contain chlorinated species. It may be possible to burn these wastes efficiently in porous ceramic burners because of the higher volumetric heat release. Also, the soot formed in conventional incinerators often acts as condensation sites for hazardous materials, becoming hazardous itself when emitted to the atmosphere. This problem may be eliminated by porous ceramic burners, which can vaporize the liquid waste before combustion. The radiant preheating of the fuel-air mixture, which is inherent to its design, may also aid the combustion of mixed fuels containing less reactive species. Computer modeling
Address correspondence to Professor Matthew J. Hall, Department of Mechanical Engineering, The University of Texas at Austin, Austin, TX 78712-1063.
Experimental Thermal and Fluid Science 1995; 11:13 20 © Elsevier Science Inc., 1995 655 Avenue of the Americas, New York, NY 10010
0894-1777/95/$9.50 SSDI 0894-1777(94)00106-I
14 M. Kaplan and M. J. Hall of liquid droplet vaporization in a porous ceramic burner was done at the University of Texas at Austin [1]. The combustion of gaseous fuels within porous ceramic burners has been well researched at UT Austin [2-4], at Arizona State University [5-7], and at other institutions [8]. It has been found that in burners that use premixed gaseous (as opposed to liquid) fuel-oxidizer streams, the flames ignited within the PIM can be burned with leaner mixtures, higher flame speeds, and in some cases higher temperatures than can be produced in similar burners without combustion within the PIM sections [9-12]. Echigo [13] showed analytically and experimentally that heat recirculation from the flame to the unburned reactants could be accomplished through radiation from a porous solid. In later work, Yoshizawa et al. [14] showed that the temperature profiles and burning velocities were highly dependent on the radiative properties of the porous matrix. When a flame within a PIM burner is extinguished because of momentary interruption of fuel or oxidant flow or other reasons, combustion can be immediately restarted by simply restoring the flow of fuel and oxidant. The reason is that the specific heat of most PIM materials is sufficient to maintain the temperature of the PIM above the ignition temperature for some time after the flame has been extinguished. This fact may make PIM burners useful in cases of transient or unsteady conditions that may result from an upset condition in an incinerator. Combustion of liquid fuels was not previously attempted in these burners because of the belief that coking or plugging of the pores by carbonization of the fuel would make such fuels unsuitable. This was not found to be a problem. EXPERIMENTAL APPARATUS Several versions of the porous ceramic burner were constructed and tested for combustion stability. A porous ceramic section capable of being removed and altered was fit onto the experimental apparatus (Fig. 1). Several porous ceramics (three or four), each 2.5 cm thick, were stacked inside a quartz cylinder or an alumina insulation sleeve, which makes up the 10.0-cm-diameter combustion section. The alumina insulating cylinder used is type AL-30, made by Zircar Fibrous Ceramics. The alumina cylinder wall was about 1 cm thick. The combustion section was fixed to the downstream end of a 10.0-cm I.D. 7.5-cm-long quartz
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insulated porous ceramics quartz window
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cylinder that acts as a window for viewing the upstream end of the combustion section. The quartz was connected to the downstream end of a 10.0-cm-diameter stainless steel tube. Air entered the 52-cm-long tube at its upstream end through two 1.25-cm-diameter openings. The air was drawn from a compressed air main and was filtered for particles and water vapor. Airflow rates were measured with a calibrated rotameter. An oil burner spray nozzle was used to introduce the liquid fuel. The liquid fuel is contained in a pressurized tank. A nitrogen pressurized fuel system maintained between 600 and 700 kPa (90-100 psig) delivered the fuel to the nozzle. All of the ceramics were obtained from the Selee Corporation of Hendersonville, NC. Each ceramic is 2.5 cm thick and 10 cm in diameter. Ceramics having two pore sizes were used; the larger had 4 pores per cm (ppcm), and the smaller 10 ppcm. Three types of ceramics were examined: silicon carbide, yttrium-stabilized zirconia, and magnesia-stabilized zirconia, all having porosities of approximately 85%. The fuel spray nozzles, manufactured by the Monarch Co. for use with industrial burners, were designed for single fuel flow rates of No. 2 fuel oil of approximately 0.025 lpm (liter per minute) (0.4 gal/hr) and 0.032 lpm (0.5 gal/hr). They delivered a full cone spray pattern with a spray angle of 60 °. By calibrating with heptane it was determined that the nozzles can supply heptane at flow rates within 3% of the rated value when the fuel is pressurized at 600-700 kPa. All experiments performed during the design of the PIM burner were conducted with the 0.025-1pro nozzle. For heptane, this results in an energy release rate of 12.8 kW based on the lower heating value of heptane. According to the manufacturer, typical fuel droplet diameters are in the range of 50 to 100 /zm with the largest droplets less than 400 /zm. The diameter of the droplets entering the burner could significantly affect its performance. Larger droplets require greater time to evaporate. If the residence time within the burner were insufficient to vaporize the droplets completely, combustion would not be completed within the porous matrix, resulting in nonpremixed combustion downstream of the ceramic. This would give higher emissions of CO and perhaps NO x. Experiments have shown that after an initial unsteady period, the square of the droplet diameter decreases linearly with time, d~ -
d 2 = K(t
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to),
(1)
where d is the droplet diameter, t is the time, d o is the diameter at t = to, and K is the evaporation constant, which is independent of time [15]. For the operating conditions examined, the mean velocity of the flow through the ceramic was about 0.75 m/s. This gives a residence time within the upper ceramic section of about 130 ms. The burning rate constant for a free droplet of nheptane has been found to be 0.79 mm2/s [16]. Using this burning rate constant in Eq. 1, a droplet having an initial diameter of 50 /zm would have a combustion time of 3.2 ms; a droplet with an initial diameter of 320 /xm would have a combustion time equal to the residence time in the burner of 130 ms. Stability in the PIM burner section is defined by two parameters, flashback and blowoff. Flashback occurs when the flame propagates upstream through the ceramic section and into the mixing region. For liquid fuel combus-
Combustion of Liquid Fuels tion, the flame attaches to the tip of the fuel spray nozzle and burns in a diffusion flame. Biowoff occurs when the flame propagates downstream beyond the top ceramic surface and then extinguishes. One method of extending the stability range before flashback is to place a small-pore ceramic upstream of a larger pore ceramic in the combustion section. The reaction zone is stabilized at the interface. Two possible mechanisms may contribute to this increased stability. For a transition from a large-pore to small-pore ceramic, changes in the conduction and heat transfer coefficients are thought to be negligible compared with the reduction in radiative preheating through the small-pore ceramic. The optical path length in the smaller pore material is approximately one-half that of the larger pore material (about 0.8 vs. 1.5 /zm) [17]. Thus, as a flame propagates upstream through the burner to the smaller pore material, the unburned gas temperature is reduced. This causes the flame speed to slow, which stabilizes combustion. This has been shown to work in gaseous fuel premixed burners [3]. The second possible mechanism involves the fluid mechanics of the flow. According to turbulence measurements [18], the magnitudes of the turbulence intensities are smaller in the small-pore ceramic. Therefore, reaction rates and turbulent flame speeds should be lower in the smaller pore ceramic. Thus, as the flame reaches the small-pore ceramic, its speed decreases, and it does not propagate through the ceramic section. In addition to maintaining a stable flame, the PIM burner must also result in complete combustion of the fuel within the ceramic section. Initially, the combustion was evaluated by observing whether there were any flames extending outside of the ceramic section. If there were, the combustion section was redesigned. The burner performance was evaluated by measuring the carbon monoxide emissions as well. A negligible amount of CO is created when a fuel is burned completely. Therefore, the combustion is most complete when the CO emissions are lowest. If the emissions are very high, a substantial amount of fuel is not being burned. This may be due to the combustion section design, as complete combustion is highly dependent on mixing. The ceramic section of the PIM burner must be designed in a way that optimizes the range of equivalence ratios that support stable combustion while promoting complete combustion of the fuel. In the initial experiments, three ceramic pieces, each having a pore size of 4 ppcm, were stacked together to form a section of porous material 7.5 cm thick. The ceramics were placed in the alumina insulating cylinder. The spray nozzle was placed 20 cm below the ceramic, with the spray impinging directly onto the bottom ceramic. The airflow was varied over a wide range to search for a stable condition. Combustion was successfully initiated; however, the combustion could not be stabilized for periods longer than about 10 min without the flame either flashing back at lower airflows and igniting the spray or blowing out the downstream end of the ceramic at higher flow rates. There also was a substantial amount of unburned fuel exiting the ceramic section and combusting outside the burner. In an attempt to increase combustion stability, the bottom ceramic was replaced by one having a smaller pore size of 10 ppcm. This configuration is shown as burner 1 in Fig. 2. Decreasing the pore size of the
15
bottom ceramic should increase stability due to the change in turbulence intensity and optical path length of the ceramic, as discussed previously. This revision increased the combustion stability slightly, although the flame still could not be contained within the ceramic section. Another revision, shown as burner 2, was to place a fourth ceramic piece upstream of the original three with a 2.5-cm gap between the two sections (Fig. 2). It is believed that the bottom ceramic provides additional upstream droplet vaporization and mixing, which results in more rapid and complete combustion within the porous media. A 4-ppcm and a 10-ppcm ceramic were used alternately in this position, with the 4-ppcm ceramic vaporizing and mixing the reactants more completely. This is consistent with turbulence measurements for the ceramics [17], which found that the turbulence intensity is higher in larger pores than in smaller pores. The length of the space between the bottom ceramic and the combustion section was varied between 2.5 and 7.5 cm; this had no apparent effect on the nature of the combustion or on stability. These two revisions resulted in a narrow range of stable airflow rates, 310-345 lpm of air, which corresponds to a range of equivalence ratios of 0.60-0.63. During various test runs in this range, the fuel was not burned completely within the upper section. A visible blue flame would exit the downstream end of the combustion section. Also, even though the combustion was stable, the measured emissions varied greatly from run to run depending upon the height and location of the diffusion flame exiting the burner. In an attempt to obtain repeatable complete combustion in the upper section, an additional 4-ppcm ceramic was placed on top of the section, lengthening it to four consecutive 2.5-cm-thick pieces (burner 3 in Fig. 2). This was found to have no effect; fuel would still burn out of the top of the combustion section. When the top ceramic of burner 3 was replaced with a ceramic with a smaller pore size, 10 ppcm, complete combustion was achieved within the ceramic. This modification, shown as burner 4 in Fig. 2, resulted in a range of stable complete combustion airflow rates, 325-380 lpm, which correspond to equivalence ratios of 0.57-0.67. It is
Burner 1: Three ceramic pieces were tested, a small pore and two large pore sizes.
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16 M. Kaplan and M. J. Hall thought that the combustion is completed below the upper 10-ppcm ceramic because the ceramic acts as a radiative shield, since the smaller pore ceramic has a shorter optical path length. The radiant flux from the reaction zone through the 10-ppcm ceramic into the atmosphere is lower than through the 4-ppcm ceramic. This increases the combustion temperature and reaction rates, stopping the flame from propagating downstream. The distance between the spray nozzle and the upstream ceramic section was critical to maintaining stable operation. If the nozzle was close enough to the ceramic for the spray to impinge directly on the ceramic (about one cylinder diameter), the mixing was insufficient and stable operation was not possible. With the nozzle approximately two cylinder diameters upstream, the atomized spray was entrained more completely with the air. Two spray nozzles were tested, one with a 60° spray angle, the other with a 45 ° spray angle, both with a solid cone spray. The spray angle did not seem to affect the performance of the burner. The data in this paper were obtained using the 60° nozzle. No accumulation of soot or pore plugging was experienced in the experiments. During startup, some soot would condense on the lower surface of the ceramic in the combustion section, but this burned off as the ceramic heated up and steady-state conditions were achieved. Three ceramic materials were examined: magnesiastabilized zirconia, silicon carbide, and yttrium-stabilized zirconia. Magnesia-stabilized zirconia supported complete combustion; however, it was found to crack severely after only a few start-up/shut-down cycles. Silicon carbide had very good resistance to thermal cycling; however, it was determined that the silicon carbide composite melts when the temperature in the burner exceeds 1300°C. The yttrium-stabilized zirconia had good durability and resisted melting under all conditions examined. Surprisingly, however, complete combustion could not be obtained using the yttrium burner. Most of the fuel appeared to burn as a diffusion flame through and out of the top of the burner, almost as though there were no ceramic present. A likely explanation for this behavior is that the yttrium ceramic surface is very smooth and shiny, unlike the surfaces of the magnesia or silicon carbide, which are rough and slightly porous. When impinged on the yttrium, the fuel may roll off and burn as larger droplets. When impinged on the magnesia or silicon carbide, the fuel may wet the surface of the ceramic, then vaporize and burn. This allows for better mixing of fuel and air plus additional residence time in the combustion section. To further examine this phenomenon, the yttrium burner was tested using prevaporized heptane in air. The heptane was prevaporized by moving the spray nozzle to the base of the steel cylinder and stacking several ceramics of various pore sizes within the cylinder. These ceramics were necessary to vaporize the fuel. Using a He-Ne laser passed through the quartz section below the ceramics, very little scattering could be seen from the droplets entering the burner, indicating that the fuel was mostly, but not completely, prevaporized. Under these conditions, it was possible to stabilize combustion completely within the burner section. When the ceramics were removed and the nozzle raised to its previous position, combustion again could not be stabilized completely within the yttrium. These results support the theory that fuel droplets
wet the surface of the silicon carbide and magnesia-stabilized zirconia, but roll off the yttrium-stabilized zirconia. Temperatures were measured with 0.25-mm (0.010-in.)diameter type B thermocouples. A thermocouple probe was moved across diameters of the burner both along the exit plane and just upstream of the upper ceramic section. For the exit plane measurements an alumina block approximately 4 mm thick and 8 mm square was supported by the thermocouple wires passing through it; this block shielded the downstream side of the thermocouple bead, which was about 1 mm from the surface of the block. The thermocouple used to measure the temperatures upstream of the ceramic was not shielded because temperatures were low ( < 100°C). Temperatures were also measured in the axial direction along the wall between the ceramics and the alumina insulation for the upper section of the burner. This was done by moving the probe within a gap in the insulation next to the ceramics. Figure 3 shows the location for the radial and axial temperature measurements. All the experimental data were obtained using a magnesia-stabilized zirconia burner of the following configuration from upstream to downstream: 4 ppcm--2.5-cm space--10 p p c m - - 4 ppcm--4 ppcm--10 ppcm, shown as burner 4 in Fig. 2. RESULTS Emissions of CO and NO~ were measured for the various burner configurations. An exhaust sample was obtained in the following manner. A quartz cylinder was secured on top of the burner section to prevent entrainment of air into the quartz sampling probe, which was placed approximately 7 cm above the exit plane of the burner. The exhaust sample was cooled and passed through a desiccant. The dry sample was then directed to a Model 48H CO analyzer and a Model 10 chemiluminescent NO/NO~ analyzer, both made by Thermo Environmental Instruments Inc.
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Combustion of Liquid Fuels U n d e r the stable operating conditions when a luminous flame could be seen above the burner, indicating that reaction was not c o m p l e t e d within the porous media, emissions of CO were very high, in the range of 650-7000 p p m corrected for 3% 0 2. U n d e r certain stable operating conditions with the fuel b u r n e d completely within the porous media, the corrected emissions of C O were very low, 3 - 7 ppm. T h e corrected concentrations of N O x were approximately 15-20 p p m for complete combustion. The measured N O x concentrations were accurate within an uncertainty of + 2 ppm. This follows from the manufacturer's specified uncertainty for the analyzer, which is +_1% of full scale for the 100-ppm range, yielding an analyzer uncertainty of + 1 ppm; the calibration gas uncertainty was _+ 1 ppm from its certified concentration of 21 ppm. These uncertainties are consistent with the observed reproducibility of the data of about _+2 ppm. The C O concentrations were also accurate within an uncertainty of _+2 ppm. This analyzer also had a specified uncertainty of ± 1% of full scale, which is 50 ppm; thus the analyzer uncertainty was _+0.5 ppm, with a calibration gas uncertainty of _+ 1 p p m out of 23 ppm. This was again consistent with observed reproducibility. M e a n temperatures were reproducible to within _+ 15°C for a given run. The axial t e m p e r a t u r e m e a s u r e m e n t s were m a d e between the P I M and the insulating alumina wall, minimizing radiation errors. The exit plane m e a s u r e m e n t s were made with a shielded thermocouple. T h r e e variations of the PIM b u r n e r configuration that could affect the emissions concentrations and the stability limits were examined. First, burners using a spray nozzle with a flow rate of 0.032 lpm (0.5 g a l / h r ) and a 0.025-1pm (0.4-gal/hr) nozzle were compared. Both burners were enclosed in alumina insulation, with the nozzle placed 10 cm below the ceramic section. The stability limits were d e t e r m i n e d and emissions concentrations m e a s u r e d for the range of stable combustion. Then emissions concentrations were c o m p a r e d between burners having the combustion section enclosed in quartz and in alumina insulation, both using the 0.025-1pm nozzle. Finally, the emissions from a quartz-enclosed burner combusting a heptane spray were c o m p a r e d with emissions from a quartz burner combusting prevaporized heptane. The range of equivalence ratios that support stable combustion was d e t e r m i n e d for a P I M b u r n e r using a 0.032-1pm flow rate spray nozzle. Stable combustion occurred for equivalence ratios of 0.52-0.69, a slightly greater range than the stable range for a 0.025-1pro nozzle, which is 0.57-0.67. The C O and N O x emissions concentrations are c o m p a r e d in Fig. 4 for burners with the two fuel flow rates. The magnitudes of the emissions concentrations are similar for the two burners: 0 - 7 p p m for the CO and 15-25 p p m for the N O x. Also, the trends are the same; the emissions concentrations increase with increasing equivalence ratio. The PIM b u r n e r used in the experimentation up to this point enclosed the porous ceramics in an alumina insulating cylinder. A new P I M b u r n e r was constructed that enclosed the porous ceramics in a quartz cylinder, to get a general comparison of the combustion characteristics. Both burners used the 0.025-1pro fuel spray nozzle. F o r the quartz burner, stable combustion was achieved within the ceramic section for equivalence ratios in the same range as the alumina burner. The CO and NOx emissions
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concentrations for both the alumina and quartz burners are shown in Fig. 5. As seen in the figure, the CO and NO~ concentrations are lower in the quartz-enclosed ceramic burner than they are in the alumina burner. The reduced N O x is most likely due to the radiative heat loss in the quartz burner, since NOx formation is highly t e m p e r a t u r e - d e p e n d e n t . With the alumina burner, there is significant radiative heat loss only through the downstream exit of the ceramic section, so the t e m p e r a t u r e s remain higher, causing the N O r concentrations to be greater. The reason for the lower CO emissions in the quartz burner is not clear, but the results were reproducible. F o r both burners the emissions of CO and N O x were quite low. Due to increased radial heat loss, the exit plane t e m p e r a t u r e s of the quartz tubing burner were lower than those of the alumina-insulated burner; this would result in a lower radiant output from the exit plane of the quartz burner. The very low CO emissions for both burners imply that complete combustion was obtained within the ceramic for both configurations.
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18
M. Kaplan and M. J. Hall 15
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The final variation on the original PIM burner design compared the combustion of a heptane spray with the combustion of prevaporized heptane in a magnesia-stabilized zirconia burner. The fuel was prevaporized as described previously. Several porous ceramics of various pore sizes were stacked in the cylinder. The ceramics acted as vaporizers and also helped premix the fuel and air. For both the spray and prevaporized fuel burners, the combustion section was encased in a quartz cylinder rather than in the alumina insulation. The CO and NO x emissions concentrations are compared in Fig. 6 for the spray and prevaporized fuel conditions. As seen on the plot, the concentrations are in the same range for both fuel conditions; the CO varies from approximately 1 to 4 ppm and the NO x varies from approximately 11 to 15 ppm. These results support the idea that the bottom large-pore mixing ceramic in the original PIM burner design vaporizes much of the heptane spray and helps premix the heptane and air. The marginally lower NO X for the nonpremixed case may have resulted from slightly lower temperatures due to the heat of vaporization of the heptane droplets. Temperatures were measured across diameters of the burner both along the exit plane and just upstream of the upper ceramic section where reaction occurred. Temperatures were also measured in the axial direction along the wall between the porous ceramic and the alumina insulation. Across the exit plane the temperatures ranged from 1000 to 1300°C for heptane in air equivalence ratios of 0.57-0.67. Figure 7 shows two examples of the radial temperature distribution for equivalence ratios of 0.64 and 0.67 at the downstream surface of the combustion section. The temperatures for each equivalence ratio generally lie within 50°C of a median value, indicating radially uniform combustion at that equivalence ratio. As expected, the average temperature is higher for greater equivalence ratios. The temperatures measured just upstream of the ceramic combustion section (downstream of the ceramic mixing section) ranged from 50 to 100°C. The axial temperature distribution along the combustion section of the burner is shown in Fig. 8 for an
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Figure 6. Emissions concentrations vs. @ for PIM burners combusting prevaporized heptane and a heptane spray. Burners within a quartz cylinder.
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Figure 7. Radial temperature distribution across the exit plane of a PIM burner combusting heptane. equivalence ratio of 0.64. These temperatures were measured by moving the thermocouple probe within a gap in the insulation next to the ceramics for the upper section of the burner. The temperature range was 1190-1365°C. This figure shows that the highest temperatures occur in the middle two ceramics, the region from 2.5 to 7.5 cm, which agrees with the expectation that combustion is maintained in the two 4-ppcm ceramics by the top and bottom 10-ppcm ceramics. The temperature drop near the exit plane is due to radiative heat loss. An important aspect of the performance of the burner is the amount of premixed combustion and nonpremixed combustion taking place within the ceramic. This can have a dramatic effect on emissions, particularly those of NOz, as a larger fraction of nonpremixed combustion will result in greater production of thermal NO x. For premixed combustion to take place, fuel droplets must vaporize and mix with air before entering the reaction zone within the ceramic. The range of equivalence ratios over which stable operation was achieved, 0.57-0.67, is within the flammability limits of heptane (the lean limit being • = 0.53 and the rich limit q~ = 3.2 at 298 K) [19], suggesting that partially premixed combustion was possible. 1400
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10 exit p l a n e
Figure 8. Axial temperature distribution along the side of an alumina-insulated PIM burner combusting heptane at qb = 0.64.
Combustion of Liquid Fuels To examine the mixing, laser light scattering was employed to observe whether the droplets vaporized completely before entering the upper ceramic section. The burner constructed within the clear quartz cylinder was used with a 4-ppcm 2.5-cm-thick mixing ceramic located 2.5 cm upstream of this section. A He-Ne laser beam was passed through the walls of the quartz cylinder to visualize the light scattered from the fuel droplets. This was done both upstream and downstream of the 4-ppcm mixing ceramic. Upstream of the mixing ceramic, most of the scattered light appeared to be produced by large individual droplets homogeneously distributed along the entrance plane of tile mixing ceramic. At the downstream plane, even without combustion (i.e., prior to fuel ignition in the combustion section), the size of the droplets exiting the mixing ceramic was significantly reduced. The bright scattering from large individual droplets was no longer seen; rather, the scattered light was characteristic of a higher density of much smaller droplets. The droplets upstream of the mixing ceramic section were not affected by combustion. However, after stable steady-state operation had been achieved, radiation from the combustion section preheated the fuel droplets in the region. This further reduced the droplet size, as was observed by the reduction in intensity of the scattered light in the region between the two ceramic sections. Under no conditions did the droplets completely vaporize before entering the upper ceramic section. These observations suggest that although a large amount of droplet prevaporization of the droplets occurred, this vaporization was not complete. P R A C T I C A L USEFULNESS / S I G N I F I C A N C E This research was undertaken to explore the feasibility of combusting a liquid hydrocarbon fuel within an inert medium of high porosity. A commercial burner having low emissions would have many potential applications. These may include: 1. The incineration of hazardous liquid wastes 2. Industrial radiant burners for use in areas where natural gas is not available or where a liquid fuel is a by-product of an industrial process 3. Mobile heating; for example, in ships or as a heat source for a methanol reformer that would reform methanol into H 2 and CO o r C O 2 for fuel cells to power electric vehicles The research presented here elucidated several characteristics of a liquid-fueled porous media burner. With further development a commercial burner of this type may be possible. CONCLUSIONS Judging from the experiments performed, stable combustion of a liquid hydrocarbon fuel (heptane) within a porous ceramic burner is possible. Although yttrium-stabilized zirconia and silicon carbide ceramics have good resistance to thermal cycling, they also have limitations: the yttriumstabilized zirconia supports complete combustion only for prevaporized fuels; the silicon carbide melts at the combustion temperatures experienced. Magnesia-stabilized
19
zirconia, although it has poor resistance to thermal cycling, does support complete combustion. Using magnesia-stabilized zirconia ceramics encased in an alumina insulating cylinder, complete combustion was achieved at a fuel flow rate of 0.025 lpm for fuel/air equivalence ratios of 0.57-0.67 when using the ceramic configuration 4 p p c m - - s p a c e - - 1 0 p p c m - - 4 p p c m - 4 p p c m - - 1 0 ppcm. The 10-ppcm ceramic at the burner exit provides for complete combustion within the ceramic through thermal radiation feedback. The lower 4-ppcm mixing ceramic in conjunction with the open space downstream of it provides the mixing and partial droplet vaporization required for stable combustion. The combustion processes, as indicated by the temperature distributions, were radially uniform and occurred primarily in the two middle 4-ppcm ceramics. Emissions concentrations for the alumina-insulated PIM burner were very low at a heptane flow rate of 0.025 Ipm. Corrected for 3% oxygen, CO varied from 3 to 7 ppm and NO x from 15 to 20 ppm. Run with an increased heptane flow rate of 0.032 lpm, the corrected emissions for this burner were still low: CO varied from 1 to 4 ppm and NO x varied from 15 to 25 ppm. Of the many aspects of burner design that can affect performance, two were examined. The first was insulation of the combustion section of the burner. An alumina (insulating) cylinder was replaced with a quartz cylinder to determine the effects of radiative heat loss from this section. For the quartz-enclosed burner, the CO and NO x emissions were lower. Corrected for 3% oxygen, CO concentrations were below 3 ppm for the quartz burner, and NO x concentrations varied from 11 to 13 ppm. The lower NO x concentrations were due to lower temperatures resulting from radiative heat loss through the quartz. The second aspect examined was prevaporization of the fuel. A quartz-enclosed burner was used to combust first a nonvaporized heptane spray and then prevaporized heptane. The emissions for both CO and NOx were similar for the two conditions: the CO varied from approximately 1 to 4 ppm and the NO x varied from approximately 11 to 15 ppm. This suggests that a significant fraction of the nonprevaporized heptane spray eventually vaporizes and mixes sufficiently well with the air to react as a premixed flame. R E C O M M E N D A T I O N S AND F U T U R E R E S E A R C H NEEDS Much work remains to be done to further investigate the combustion of liquid fuels in porous media. In particular, a better understanding is needed of porous media with regard to the dynamics of the droplets and vaporization within such media. Also, the effects of solid-phase material properties and structure plus fluid-phase mixing and diffusion must be elucidated. The behavior of fuels other than heptane needs to be investigated, in particular, fuels having higher heats of vaporization and higher viscosities. Methods of flame stabilization need to be examined further, as well as the ability of the burners to be scaled both up and down in capacity. We thank David Haack of Selee Corporation for his suggestions and for supplying the ceramics used in this work. Support for this work was provided by the Texas Advanced Research Program and the Center for Energy Studies of the University of Texas at Austin.
20
M. Kaplan and M. J. Hall REFERENCES
I. Haack, D. P., Mathematical Analysis of Radiatively Enhanced Liquid Droplet Vaporization and Liquid Fuel Combustion Within a Porous Inert Media, MS Thesis, Dept. Mech. Eng., Univ. Texas, Austin, TX, May 1993. 2. Gocl, R., An Experimental Investigation of Radiant Thermal Efficiency and Emissions for Premixed Methane-Air Combustion Within a Two Stage Porous Media Burner, MS Thesis, Dept. Mech. Eng., Univ. Texas, Austin, TX, December 1993. 3. Hsu, P. F., Evans, D., and Howell, J. R., Experimental and Numerical Study of Premixed Combustion with Non-homogeneous Porous Ceramic, Combust. Sci. Technol. 90, 149 172, 1993. 4. Khanna, V., Goel, R., and Ellzey, J. L., Measurements of Emissions and Radiation for Methane Combustion Within a Porous Medium Burner, Presented at the 1992 fall meeting of thc Western States Section of The Combustion Institute, Berkeley, CA, Oct. 12 13, 1992. 5. Sathe, S. B., Peck, R. E., and Tong, T. W., A Numerical Analysis of Combustion and Heat Transfer in Porous Radiant Burners, HTD-Vol. 106, pp. 461 468, ASME, New York, 1989. 6. Sathe, S. B., Kulkarni, M. R., Peck, R. E., and Tong, T. W., An Experimental and Theoretical Study of Porous Radiant Burner Performance, Twenty-Third Symp. (Int.) on Combustion, pp. 1011-1//18, 1990. 7. Kulkarni, M. R., and Peck, R. E., Modeling Radiant Surface Burner Performance, Paper 93-1(16, Western States Section of the Combustion Institute, Menlo Park, CA, Oct. 18-19, 1993. 8. Yoshizawa, Y., Sakaki, K., and Echigo, R., Analytical Study of the Structure of Radiation Controlled Flames, Int. J. Heat Mass Transfer 31(2), 311 319, 1988. 9. Chen, Y.-K., Matthews, R. D., and Howell, J. R., The Effect of Radiation on Combustion Within Highly Porous Materials, in
10. Chen, Y.-K., Matthews, R. D., Lim, 1., Lu, Z., and Howell, J. R., Experimental and Theoretical Investigation of Combustion Within Porous Inert Media, Twenty-Second Int. Symp. Combustion, pp. 481 489, August 1988. 11. Evans, D., Lean Limit for Combustion of Methane-Air Flames in Porous Inert Media, MS Thesis, Dept. Mech. Eng., Univ. Texas at Austin, May 1991. 12. Hsu, P., Howell, J. R., and Matthews, R. D., A Numerical Investigation of Pre-mixed Combustion Within Porous Inert Media, Proceedings', Joint ASME/JSME Heat Transfer Conference, Reno, Vol. 4, pp. 225 231, March 1991. 13. Echigo, R., Effective Energy Conversion Method Between Gas Enthalpy and Thermal Radiation and Application to Industrial Furnaces, 7th Int. Heat Transfer Conf., Munich, Vol. 6, pp. 361-366, 1982. 14. Yoshizawa, Y., Sasaki, K., and Echigo, R., Analytical Study of the Structure of Radiation Controlled Flame, Int. J. tteat Mass" Transfi'r 31(2), 311-319, 1988. 15. Williams, F. A., Combustion Theory, 2nd ed., Addison-Wesley, Reading, MA, 1985. 16. Williams, A., Combustion of Liquid Fuel Sprays, Butterworths & Co., London, 199(/. 17. Hendricks, T. J., and Howell, J. R., Absorption/Scattering Coefficients and Scattering Phase Functions in Reticulated Porous Ceramics, 6th Annual AIAA/ASME Thermophys. Heat Transfer Conf., Colorado Spring, CO, June 1994. 18. Hall, M. J., and Hiatt, J. P., Exit Flows from Highly Porous Media, Phys. Fluids 6(2), 469-479, 1994. 19. Boltz, R. E., and Tuve, G. L. (Eds.), The CRC Handbook of Tables for Applied Engineering Science, 2nd ed., CRC Press, Boca Raton, FL, 1973.
Radiation, Phase Change Heat Transfer, and Thermal Systems (Y. Jaluria, V. P. Carey, W. A. Fiveland, and W. Yuen, Eds.), HTD-Vol. 81, pp. 35 42, ASME, New York, 1987.
Received June 6, 1994; revised November 1, 1994
The Combustion of Liquid Fuels within a Porous Media Radiant Burner Michele Kaplan Matthew J. Hall Center for Energy Studies, Department of Mechanical Engineering, lhe University of Texas at Austin, Austin, Texas
• The combustion of liquid fuels within the porous inert media (PIM) of a radiant burner was examined. Existing designs of porous radiant burners have typically relied on premixed mixtures of gaseous fuels and air. A radiant burner was built, and various design configurations were tested using the following types of porous ceramics: magnesia-stabilized zirconia, silicon carbide, and yttrium-stabilized zirconia. The fuel heptane was impinged on the combustion section using an oil spray nozzle with a fixed flow rate of approximately 0.025 lpm. The burner had an insulated combustion section that was 10.0 cm in diameter and consisted of several 2.5-cm-thick ceramics. Stable complete combustion was achieved for heptane at equivalence ratios of 0.57-0.67. Temperature measurements taken across the exit plane of the combustion section were typically within 50°C of each other, indicating radially uniform combustion. For an equivalence ratio of 0.64, axial temperature measurements taken down the side of the combustion section showed relatively low temperatures, 1170-1370°C. Very low emissions of both CO and NO x were measured for the range of stable equivalence ratios. Corrected for 3% oxygen, CO varied from 3 to 7 ppm and NO, varied from 15 to 20 ppm. Several variations of the burner were tested, including higher fuel flow rates, prevaporized fuel, and a quartz-enclosed ceramic section.
Keywords: porous media, combustion, liquid fuel, ceramic foam, radiant burner INTRODUCTION Infrared heating is widely used by industry for manufacturing processes. Some applications include paper and wood drying, powder coating, annealing, food browning and baking, and plastics curing and forming. The burning of liquid fuels within porous media has several potential advantages over combustion in an open flame. A conventional burner must be rather large and maintained at a high temperature for there to be sufficient time for the liquid fuel to completely evaporate and burn. A porous ceramic burner has a very high radiant output due to the high emissivity of the ceramic material. The porosity provides the fuel with a convoluted path through a homogeneously radiant field that is without cold boundaries, ensuring droplet vaporization and contributing to complete reaction. The burner, having a high volumetric energy output due to a high burning rate, can be compact. Liquid fuel combustion in porous ceramics has potentially low emissions of NO x and CO. Unburned hydrocarbon and CO emissions are low due to fuel preheating and the increased residence time of exhaust gases in a high-
temperature postcombustion region. N O , emissions are low due to droplet vaporization and mixing in the ceramic media. This can result in partially premixed lean combustion, which has lower flame temperatures than diffusion flames where combustion occurs at stoichiometric conditions. A potential application of a liquid fuel porous ceramic burner is the incineration of liquid hazardous waste. Liquid hazardous wastes are difficult to incinerate in conventional burners because they are often low energy content fuels and contain chlorinated species. It may be possible to burn these wastes efficiently in porous ceramic burners because of the higher volumetric heat release. Also, the soot formed in conventional incinerators often acts as condensation sites for hazardous materials, becoming hazardous itself when emitted to the atmosphere. This problem may be eliminated by porous ceramic burners, which can vaporize the liquid waste before combustion. The radiant preheating of the fuel-air mixture, which is inherent to its design, may also aid the combustion of mixed fuels containing less reactive species. Computer modeling
Address correspondence to Professor Matthew J. Hall, Department of Mechanical Engineering, The University of Texas at Austin, Austin, TX 78712-1063.
Experimental Thermal and Fluid Science 1995; 11:13 20 © Elsevier Science Inc., 1995 655 Avenue of the Americas, New York, NY 10010
0894-1777/95/$9.50 SSDI 0894-1777(94)00106-I
14 M. Kaplan and M. J. Hall of liquid droplet vaporization in a porous ceramic burner was done at the University of Texas at Austin [1]. The combustion of gaseous fuels within porous ceramic burners has been well researched at UT Austin [2-4], at Arizona State University [5-7], and at other institutions [8]. It has been found that in burners that use premixed gaseous (as opposed to liquid) fuel-oxidizer streams, the flames ignited within the PIM can be burned with leaner mixtures, higher flame speeds, and in some cases higher temperatures than can be produced in similar burners without combustion within the PIM sections [9-12]. Echigo [13] showed analytically and experimentally that heat recirculation from the flame to the unburned reactants could be accomplished through radiation from a porous solid. In later work, Yoshizawa et al. [14] showed that the temperature profiles and burning velocities were highly dependent on the radiative properties of the porous matrix. When a flame within a PIM burner is extinguished because of momentary interruption of fuel or oxidant flow or other reasons, combustion can be immediately restarted by simply restoring the flow of fuel and oxidant. The reason is that the specific heat of most PIM materials is sufficient to maintain the temperature of the PIM above the ignition temperature for some time after the flame has been extinguished. This fact may make PIM burners useful in cases of transient or unsteady conditions that may result from an upset condition in an incinerator. Combustion of liquid fuels was not previously attempted in these burners because of the belief that coking or plugging of the pores by carbonization of the fuel would make such fuels unsuitable. This was not found to be a problem. EXPERIMENTAL APPARATUS Several versions of the porous ceramic burner were constructed and tested for combustion stability. A porous ceramic section capable of being removed and altered was fit onto the experimental apparatus (Fig. 1). Several porous ceramics (three or four), each 2.5 cm thick, were stacked inside a quartz cylinder or an alumina insulation sleeve, which makes up the 10.0-cm-diameter combustion section. The alumina insulating cylinder used is type AL-30, made by Zircar Fibrous Ceramics. The alumina cylinder wall was about 1 cm thick. The combustion section was fixed to the downstream end of a 10.0-cm I.D. 7.5-cm-long quartz
...........................
~.,~r___ ~
insulated porous ceramics quartz window
from nitrogen tank from air comorcssor
cylinder that acts as a window for viewing the upstream end of the combustion section. The quartz was connected to the downstream end of a 10.0-cm-diameter stainless steel tube. Air entered the 52-cm-long tube at its upstream end through two 1.25-cm-diameter openings. The air was drawn from a compressed air main and was filtered for particles and water vapor. Airflow rates were measured with a calibrated rotameter. An oil burner spray nozzle was used to introduce the liquid fuel. The liquid fuel is contained in a pressurized tank. A nitrogen pressurized fuel system maintained between 600 and 700 kPa (90-100 psig) delivered the fuel to the nozzle. All of the ceramics were obtained from the Selee Corporation of Hendersonville, NC. Each ceramic is 2.5 cm thick and 10 cm in diameter. Ceramics having two pore sizes were used; the larger had 4 pores per cm (ppcm), and the smaller 10 ppcm. Three types of ceramics were examined: silicon carbide, yttrium-stabilized zirconia, and magnesia-stabilized zirconia, all having porosities of approximately 85%. The fuel spray nozzles, manufactured by the Monarch Co. for use with industrial burners, were designed for single fuel flow rates of No. 2 fuel oil of approximately 0.025 lpm (liter per minute) (0.4 gal/hr) and 0.032 lpm (0.5 gal/hr). They delivered a full cone spray pattern with a spray angle of 60 °. By calibrating with heptane it was determined that the nozzles can supply heptane at flow rates within 3% of the rated value when the fuel is pressurized at 600-700 kPa. All experiments performed during the design of the PIM burner were conducted with the 0.025-1pro nozzle. For heptane, this results in an energy release rate of 12.8 kW based on the lower heating value of heptane. According to the manufacturer, typical fuel droplet diameters are in the range of 50 to 100 /zm with the largest droplets less than 400 /zm. The diameter of the droplets entering the burner could significantly affect its performance. Larger droplets require greater time to evaporate. If the residence time within the burner were insufficient to vaporize the droplets completely, combustion would not be completed within the porous matrix, resulting in nonpremixed combustion downstream of the ceramic. This would give higher emissions of CO and perhaps NO x. Experiments have shown that after an initial unsteady period, the square of the droplet diameter decreases linearly with time, d~ -
d 2 = K(t
-
to),
(1)
where d is the droplet diameter, t is the time, d o is the diameter at t = to, and K is the evaporation constant, which is independent of time [15]. For the operating conditions examined, the mean velocity of the flow through the ceramic was about 0.75 m/s. This gives a residence time within the upper ceramic section of about 130 ms. The burning rate constant for a free droplet of nheptane has been found to be 0.79 mm2/s [16]. Using this burning rate constant in Eq. 1, a droplet having an initial diameter of 50 /zm would have a combustion time of 3.2 ms; a droplet with an initial diameter of 320 /xm would have a combustion time equal to the residence time in the burner of 130 ms. Stability in the PIM burner section is defined by two parameters, flashback and blowoff. Flashback occurs when the flame propagates upstream through the ceramic section and into the mixing region. For liquid fuel combus-
Combustion of Liquid Fuels tion, the flame attaches to the tip of the fuel spray nozzle and burns in a diffusion flame. Biowoff occurs when the flame propagates downstream beyond the top ceramic surface and then extinguishes. One method of extending the stability range before flashback is to place a small-pore ceramic upstream of a larger pore ceramic in the combustion section. The reaction zone is stabilized at the interface. Two possible mechanisms may contribute to this increased stability. For a transition from a large-pore to small-pore ceramic, changes in the conduction and heat transfer coefficients are thought to be negligible compared with the reduction in radiative preheating through the small-pore ceramic. The optical path length in the smaller pore material is approximately one-half that of the larger pore material (about 0.8 vs. 1.5 /zm) [17]. Thus, as a flame propagates upstream through the burner to the smaller pore material, the unburned gas temperature is reduced. This causes the flame speed to slow, which stabilizes combustion. This has been shown to work in gaseous fuel premixed burners [3]. The second possible mechanism involves the fluid mechanics of the flow. According to turbulence measurements [18], the magnitudes of the turbulence intensities are smaller in the small-pore ceramic. Therefore, reaction rates and turbulent flame speeds should be lower in the smaller pore ceramic. Thus, as the flame reaches the small-pore ceramic, its speed decreases, and it does not propagate through the ceramic section. In addition to maintaining a stable flame, the PIM burner must also result in complete combustion of the fuel within the ceramic section. Initially, the combustion was evaluated by observing whether there were any flames extending outside of the ceramic section. If there were, the combustion section was redesigned. The burner performance was evaluated by measuring the carbon monoxide emissions as well. A negligible amount of CO is created when a fuel is burned completely. Therefore, the combustion is most complete when the CO emissions are lowest. If the emissions are very high, a substantial amount of fuel is not being burned. This may be due to the combustion section design, as complete combustion is highly dependent on mixing. The ceramic section of the PIM burner must be designed in a way that optimizes the range of equivalence ratios that support stable combustion while promoting complete combustion of the fuel. In the initial experiments, three ceramic pieces, each having a pore size of 4 ppcm, were stacked together to form a section of porous material 7.5 cm thick. The ceramics were placed in the alumina insulating cylinder. The spray nozzle was placed 20 cm below the ceramic, with the spray impinging directly onto the bottom ceramic. The airflow was varied over a wide range to search for a stable condition. Combustion was successfully initiated; however, the combustion could not be stabilized for periods longer than about 10 min without the flame either flashing back at lower airflows and igniting the spray or blowing out the downstream end of the ceramic at higher flow rates. There also was a substantial amount of unburned fuel exiting the ceramic section and combusting outside the burner. In an attempt to increase combustion stability, the bottom ceramic was replaced by one having a smaller pore size of 10 ppcm. This configuration is shown as burner 1 in Fig. 2. Decreasing the pore size of the
15
bottom ceramic should increase stability due to the change in turbulence intensity and optical path length of the ceramic, as discussed previously. This revision increased the combustion stability slightly, although the flame still could not be contained within the ceramic section. Another revision, shown as burner 2, was to place a fourth ceramic piece upstream of the original three with a 2.5-cm gap between the two sections (Fig. 2). It is believed that the bottom ceramic provides additional upstream droplet vaporization and mixing, which results in more rapid and complete combustion within the porous media. A 4-ppcm and a 10-ppcm ceramic were used alternately in this position, with the 4-ppcm ceramic vaporizing and mixing the reactants more completely. This is consistent with turbulence measurements for the ceramics [17], which found that the turbulence intensity is higher in larger pores than in smaller pores. The length of the space between the bottom ceramic and the combustion section was varied between 2.5 and 7.5 cm; this had no apparent effect on the nature of the combustion or on stability. These two revisions resulted in a narrow range of stable airflow rates, 310-345 lpm of air, which corresponds to a range of equivalence ratios of 0.60-0.63. During various test runs in this range, the fuel was not burned completely within the upper section. A visible blue flame would exit the downstream end of the combustion section. Also, even though the combustion was stable, the measured emissions varied greatly from run to run depending upon the height and location of the diffusion flame exiting the burner. In an attempt to obtain repeatable complete combustion in the upper section, an additional 4-ppcm ceramic was placed on top of the section, lengthening it to four consecutive 2.5-cm-thick pieces (burner 3 in Fig. 2). This was found to have no effect; fuel would still burn out of the top of the combustion section. When the top ceramic of burner 3 was replaced with a ceramic with a smaller pore size, 10 ppcm, complete combustion was achieved within the ceramic. This modification, shown as burner 4 in Fig. 2, resulted in a range of stable complete combustion airflow rates, 325-380 lpm, which correspond to equivalence ratios of 0.57-0.67. It is
Burner 1: Three ceramic pieces were tested, a small pore and two large pore sizes.
4 ppcm
[
10 ppcm
I I
Burner 2: 4 ppcm An upstream large pore ceramic and a 10 opcm ~ space were added to increase mixing of .II the fuel. 4 ppc
[ [ Il IJ
| ......
ABurnerlarge3:poreceramic was placed on top 4 ppcm of the section to try to obtain complete combustion within the ceramics. 10 ppcm 4 ppc Burner 4: A small pore ceramic was placed on top of the section to obtain complete combustion within the ceramics.
L
10 ppcm ~ 4 ppcm 10 ppc 4 ppc
I
~
~
1
16 M. Kaplan and M. J. Hall thought that the combustion is completed below the upper 10-ppcm ceramic because the ceramic acts as a radiative shield, since the smaller pore ceramic has a shorter optical path length. The radiant flux from the reaction zone through the 10-ppcm ceramic into the atmosphere is lower than through the 4-ppcm ceramic. This increases the combustion temperature and reaction rates, stopping the flame from propagating downstream. The distance between the spray nozzle and the upstream ceramic section was critical to maintaining stable operation. If the nozzle was close enough to the ceramic for the spray to impinge directly on the ceramic (about one cylinder diameter), the mixing was insufficient and stable operation was not possible. With the nozzle approximately two cylinder diameters upstream, the atomized spray was entrained more completely with the air. Two spray nozzles were tested, one with a 60° spray angle, the other with a 45 ° spray angle, both with a solid cone spray. The spray angle did not seem to affect the performance of the burner. The data in this paper were obtained using the 60° nozzle. No accumulation of soot or pore plugging was experienced in the experiments. During startup, some soot would condense on the lower surface of the ceramic in the combustion section, but this burned off as the ceramic heated up and steady-state conditions were achieved. Three ceramic materials were examined: magnesiastabilized zirconia, silicon carbide, and yttrium-stabilized zirconia. Magnesia-stabilized zirconia supported complete combustion; however, it was found to crack severely after only a few start-up/shut-down cycles. Silicon carbide had very good resistance to thermal cycling; however, it was determined that the silicon carbide composite melts when the temperature in the burner exceeds 1300°C. The yttrium-stabilized zirconia had good durability and resisted melting under all conditions examined. Surprisingly, however, complete combustion could not be obtained using the yttrium burner. Most of the fuel appeared to burn as a diffusion flame through and out of the top of the burner, almost as though there were no ceramic present. A likely explanation for this behavior is that the yttrium ceramic surface is very smooth and shiny, unlike the surfaces of the magnesia or silicon carbide, which are rough and slightly porous. When impinged on the yttrium, the fuel may roll off and burn as larger droplets. When impinged on the magnesia or silicon carbide, the fuel may wet the surface of the ceramic, then vaporize and burn. This allows for better mixing of fuel and air plus additional residence time in the combustion section. To further examine this phenomenon, the yttrium burner was tested using prevaporized heptane in air. The heptane was prevaporized by moving the spray nozzle to the base of the steel cylinder and stacking several ceramics of various pore sizes within the cylinder. These ceramics were necessary to vaporize the fuel. Using a He-Ne laser passed through the quartz section below the ceramics, very little scattering could be seen from the droplets entering the burner, indicating that the fuel was mostly, but not completely, prevaporized. Under these conditions, it was possible to stabilize combustion completely within the burner section. When the ceramics were removed and the nozzle raised to its previous position, combustion again could not be stabilized completely within the yttrium. These results support the theory that fuel droplets
wet the surface of the silicon carbide and magnesia-stabilized zirconia, but roll off the yttrium-stabilized zirconia. Temperatures were measured with 0.25-mm (0.010-in.)diameter type B thermocouples. A thermocouple probe was moved across diameters of the burner both along the exit plane and just upstream of the upper ceramic section. For the exit plane measurements an alumina block approximately 4 mm thick and 8 mm square was supported by the thermocouple wires passing through it; this block shielded the downstream side of the thermocouple bead, which was about 1 mm from the surface of the block. The thermocouple used to measure the temperatures upstream of the ceramic was not shielded because temperatures were low ( < 100°C). Temperatures were also measured in the axial direction along the wall between the ceramics and the alumina insulation for the upper section of the burner. This was done by moving the probe within a gap in the insulation next to the ceramics. Figure 3 shows the location for the radial and axial temperature measurements. All the experimental data were obtained using a magnesia-stabilized zirconia burner of the following configuration from upstream to downstream: 4 ppcm--2.5-cm space--10 p p c m - - 4 ppcm--4 ppcm--10 ppcm, shown as burner 4 in Fig. 2. RESULTS Emissions of CO and NO~ were measured for the various burner configurations. An exhaust sample was obtained in the following manner. A quartz cylinder was secured on top of the burner section to prevent entrainment of air into the quartz sampling probe, which was placed approximately 7 cm above the exit plane of the burner. The exhaust sample was cooled and passed through a desiccant. The dry sample was then directed to a Model 48H CO analyzer and a Model 10 chemiluminescent NO/NO~ analyzer, both made by Thermo Environmental Instruments Inc.
porous ceramics
l d alumina insulation
points of measurement
ii~i~i!~i:~iiiii~iiiiii~i~ ~i~ :~~,~,~;~,~:~:~::~:~:~:~,~ i::ii::~N~i~i
porous ceramic at the exit plane
:i~ii#%i~%:~:~:~:~:~:":~:~:~ ,'~ *' ~: ~'~~: aluminainsulation
Figure 3. Thermocouple locations for axial and radial temperature measurements.
Combustion of Liquid Fuels U n d e r the stable operating conditions when a luminous flame could be seen above the burner, indicating that reaction was not c o m p l e t e d within the porous media, emissions of CO were very high, in the range of 650-7000 p p m corrected for 3% 0 2. U n d e r certain stable operating conditions with the fuel b u r n e d completely within the porous media, the corrected emissions of C O were very low, 3 - 7 ppm. T h e corrected concentrations of N O x were approximately 15-20 p p m for complete combustion. The measured N O x concentrations were accurate within an uncertainty of + 2 ppm. This follows from the manufacturer's specified uncertainty for the analyzer, which is +_1% of full scale for the 100-ppm range, yielding an analyzer uncertainty of + 1 ppm; the calibration gas uncertainty was _+ 1 ppm from its certified concentration of 21 ppm. These uncertainties are consistent with the observed reproducibility of the data of about _+2 ppm. The C O concentrations were also accurate within an uncertainty of _+2 ppm. This analyzer also had a specified uncertainty of ± 1% of full scale, which is 50 ppm; thus the analyzer uncertainty was _+0.5 ppm, with a calibration gas uncertainty of _+ 1 p p m out of 23 ppm. This was again consistent with observed reproducibility. M e a n temperatures were reproducible to within _+ 15°C for a given run. The axial t e m p e r a t u r e m e a s u r e m e n t s were m a d e between the P I M and the insulating alumina wall, minimizing radiation errors. The exit plane m e a s u r e m e n t s were made with a shielded thermocouple. T h r e e variations of the PIM b u r n e r configuration that could affect the emissions concentrations and the stability limits were examined. First, burners using a spray nozzle with a flow rate of 0.032 lpm (0.5 g a l / h r ) and a 0.025-1pm (0.4-gal/hr) nozzle were compared. Both burners were enclosed in alumina insulation, with the nozzle placed 10 cm below the ceramic section. The stability limits were d e t e r m i n e d and emissions concentrations m e a s u r e d for the range of stable combustion. Then emissions concentrations were c o m p a r e d between burners having the combustion section enclosed in quartz and in alumina insulation, both using the 0.025-1pm nozzle. Finally, the emissions from a quartz-enclosed burner combusting a heptane spray were c o m p a r e d with emissions from a quartz burner combusting prevaporized heptane. The range of equivalence ratios that support stable combustion was d e t e r m i n e d for a P I M b u r n e r using a 0.032-1pm flow rate spray nozzle. Stable combustion occurred for equivalence ratios of 0.52-0.69, a slightly greater range than the stable range for a 0.025-1pro nozzle, which is 0.57-0.67. The C O and N O x emissions concentrations are c o m p a r e d in Fig. 4 for burners with the two fuel flow rates. The magnitudes of the emissions concentrations are similar for the two burners: 0 - 7 p p m for the CO and 15-25 p p m for the N O x. Also, the trends are the same; the emissions concentrations increase with increasing equivalence ratio. The PIM b u r n e r used in the experimentation up to this point enclosed the porous ceramics in an alumina insulating cylinder. A new P I M b u r n e r was constructed that enclosed the porous ceramics in a quartz cylinder, to get a general comparison of the combustion characteristics. Both burners used the 0.025-1pro fuel spray nozzle. F o r the quartz burner, stable combustion was achieved within the ceramic section for equivalence ratios in the same range as the alumina burner. The CO and NOx emissions
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concentrations for both the alumina and quartz burners are shown in Fig. 5. As seen in the figure, the CO and NO~ concentrations are lower in the quartz-enclosed ceramic burner than they are in the alumina burner. The reduced N O x is most likely due to the radiative heat loss in the quartz burner, since NOx formation is highly t e m p e r a t u r e - d e p e n d e n t . With the alumina burner, there is significant radiative heat loss only through the downstream exit of the ceramic section, so the t e m p e r a t u r e s remain higher, causing the N O r concentrations to be greater. The reason for the lower CO emissions in the quartz burner is not clear, but the results were reproducible. F o r both burners the emissions of CO and N O x were quite low. Due to increased radial heat loss, the exit plane t e m p e r a t u r e s of the quartz tubing burner were lower than those of the alumina-insulated burner; this would result in a lower radiant output from the exit plane of the quartz burner. The very low CO emissions for both burners imply that complete combustion was obtained within the ceramic for both configurations.
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Figure 5. Emissions concentrations vs. qb for heptane PIM burners within a quartz cylinder and within alumina insulation. Fuel flow rate is 0.025 lpm.
18
M. Kaplan and M. J. Hall 15
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The final variation on the original PIM burner design compared the combustion of a heptane spray with the combustion of prevaporized heptane in a magnesia-stabilized zirconia burner. The fuel was prevaporized as described previously. Several porous ceramics of various pore sizes were stacked in the cylinder. The ceramics acted as vaporizers and also helped premix the fuel and air. For both the spray and prevaporized fuel burners, the combustion section was encased in a quartz cylinder rather than in the alumina insulation. The CO and NO x emissions concentrations are compared in Fig. 6 for the spray and prevaporized fuel conditions. As seen on the plot, the concentrations are in the same range for both fuel conditions; the CO varies from approximately 1 to 4 ppm and the NO x varies from approximately 11 to 15 ppm. These results support the idea that the bottom large-pore mixing ceramic in the original PIM burner design vaporizes much of the heptane spray and helps premix the heptane and air. The marginally lower NO X for the nonpremixed case may have resulted from slightly lower temperatures due to the heat of vaporization of the heptane droplets. Temperatures were measured across diameters of the burner both along the exit plane and just upstream of the upper ceramic section where reaction occurred. Temperatures were also measured in the axial direction along the wall between the porous ceramic and the alumina insulation. Across the exit plane the temperatures ranged from 1000 to 1300°C for heptane in air equivalence ratios of 0.57-0.67. Figure 7 shows two examples of the radial temperature distribution for equivalence ratios of 0.64 and 0.67 at the downstream surface of the combustion section. The temperatures for each equivalence ratio generally lie within 50°C of a median value, indicating radially uniform combustion at that equivalence ratio. As expected, the average temperature is higher for greater equivalence ratios. The temperatures measured just upstream of the ceramic combustion section (downstream of the ceramic mixing section) ranged from 50 to 100°C. The axial temperature distribution along the combustion section of the burner is shown in Fig. 8 for an
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Figure 6. Emissions concentrations vs. @ for PIM burners combusting prevaporized heptane and a heptane spray. Burners within a quartz cylinder.
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Figure 7. Radial temperature distribution across the exit plane of a PIM burner combusting heptane. equivalence ratio of 0.64. These temperatures were measured by moving the thermocouple probe within a gap in the insulation next to the ceramics for the upper section of the burner. The temperature range was 1190-1365°C. This figure shows that the highest temperatures occur in the middle two ceramics, the region from 2.5 to 7.5 cm, which agrees with the expectation that combustion is maintained in the two 4-ppcm ceramics by the top and bottom 10-ppcm ceramics. The temperature drop near the exit plane is due to radiative heat loss. An important aspect of the performance of the burner is the amount of premixed combustion and nonpremixed combustion taking place within the ceramic. This can have a dramatic effect on emissions, particularly those of NOz, as a larger fraction of nonpremixed combustion will result in greater production of thermal NO x. For premixed combustion to take place, fuel droplets must vaporize and mix with air before entering the reaction zone within the ceramic. The range of equivalence ratios over which stable operation was achieved, 0.57-0.67, is within the flammability limits of heptane (the lean limit being • = 0.53 and the rich limit q~ = 3.2 at 298 K) [19], suggesting that partially premixed combustion was possible. 1400
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Combustion of Liquid Fuels To examine the mixing, laser light scattering was employed to observe whether the droplets vaporized completely before entering the upper ceramic section. The burner constructed within the clear quartz cylinder was used with a 4-ppcm 2.5-cm-thick mixing ceramic located 2.5 cm upstream of this section. A He-Ne laser beam was passed through the walls of the quartz cylinder to visualize the light scattered from the fuel droplets. This was done both upstream and downstream of the 4-ppcm mixing ceramic. Upstream of the mixing ceramic, most of the scattered light appeared to be produced by large individual droplets homogeneously distributed along the entrance plane of tile mixing ceramic. At the downstream plane, even without combustion (i.e., prior to fuel ignition in the combustion section), the size of the droplets exiting the mixing ceramic was significantly reduced. The bright scattering from large individual droplets was no longer seen; rather, the scattered light was characteristic of a higher density of much smaller droplets. The droplets upstream of the mixing ceramic section were not affected by combustion. However, after stable steady-state operation had been achieved, radiation from the combustion section preheated the fuel droplets in the region. This further reduced the droplet size, as was observed by the reduction in intensity of the scattered light in the region between the two ceramic sections. Under no conditions did the droplets completely vaporize before entering the upper ceramic section. These observations suggest that although a large amount of droplet prevaporization of the droplets occurred, this vaporization was not complete. P R A C T I C A L USEFULNESS / S I G N I F I C A N C E This research was undertaken to explore the feasibility of combusting a liquid hydrocarbon fuel within an inert medium of high porosity. A commercial burner having low emissions would have many potential applications. These may include: 1. The incineration of hazardous liquid wastes 2. Industrial radiant burners for use in areas where natural gas is not available or where a liquid fuel is a by-product of an industrial process 3. Mobile heating; for example, in ships or as a heat source for a methanol reformer that would reform methanol into H 2 and CO o r C O 2 for fuel cells to power electric vehicles The research presented here elucidated several characteristics of a liquid-fueled porous media burner. With further development a commercial burner of this type may be possible. CONCLUSIONS Judging from the experiments performed, stable combustion of a liquid hydrocarbon fuel (heptane) within a porous ceramic burner is possible. Although yttrium-stabilized zirconia and silicon carbide ceramics have good resistance to thermal cycling, they also have limitations: the yttriumstabilized zirconia supports complete combustion only for prevaporized fuels; the silicon carbide melts at the combustion temperatures experienced. Magnesia-stabilized
19
zirconia, although it has poor resistance to thermal cycling, does support complete combustion. Using magnesia-stabilized zirconia ceramics encased in an alumina insulating cylinder, complete combustion was achieved at a fuel flow rate of 0.025 lpm for fuel/air equivalence ratios of 0.57-0.67 when using the ceramic configuration 4 p p c m - - s p a c e - - 1 0 p p c m - - 4 p p c m - 4 p p c m - - 1 0 ppcm. The 10-ppcm ceramic at the burner exit provides for complete combustion within the ceramic through thermal radiation feedback. The lower 4-ppcm mixing ceramic in conjunction with the open space downstream of it provides the mixing and partial droplet vaporization required for stable combustion. The combustion processes, as indicated by the temperature distributions, were radially uniform and occurred primarily in the two middle 4-ppcm ceramics. Emissions concentrations for the alumina-insulated PIM burner were very low at a heptane flow rate of 0.025 Ipm. Corrected for 3% oxygen, CO varied from 3 to 7 ppm and NO x from 15 to 20 ppm. Run with an increased heptane flow rate of 0.032 lpm, the corrected emissions for this burner were still low: CO varied from 1 to 4 ppm and NO x varied from 15 to 25 ppm. Of the many aspects of burner design that can affect performance, two were examined. The first was insulation of the combustion section of the burner. An alumina (insulating) cylinder was replaced with a quartz cylinder to determine the effects of radiative heat loss from this section. For the quartz-enclosed burner, the CO and NO x emissions were lower. Corrected for 3% oxygen, CO concentrations were below 3 ppm for the quartz burner, and NO x concentrations varied from 11 to 13 ppm. The lower NO x concentrations were due to lower temperatures resulting from radiative heat loss through the quartz. The second aspect examined was prevaporization of the fuel. A quartz-enclosed burner was used to combust first a nonvaporized heptane spray and then prevaporized heptane. The emissions for both CO and NOx were similar for the two conditions: the CO varied from approximately 1 to 4 ppm and the NO x varied from approximately 11 to 15 ppm. This suggests that a significant fraction of the nonprevaporized heptane spray eventually vaporizes and mixes sufficiently well with the air to react as a premixed flame. R E C O M M E N D A T I O N S AND F U T U R E R E S E A R C H NEEDS Much work remains to be done to further investigate the combustion of liquid fuels in porous media. In particular, a better understanding is needed of porous media with regard to the dynamics of the droplets and vaporization within such media. Also, the effects of solid-phase material properties and structure plus fluid-phase mixing and diffusion must be elucidated. The behavior of fuels other than heptane needs to be investigated, in particular, fuels having higher heats of vaporization and higher viscosities. Methods of flame stabilization need to be examined further, as well as the ability of the burners to be scaled both up and down in capacity. We thank David Haack of Selee Corporation for his suggestions and for supplying the ceramics used in this work. Support for this work was provided by the Texas Advanced Research Program and the Center for Energy Studies of the University of Texas at Austin.
20
M. Kaplan and M. J. Hall REFERENCES
I. Haack, D. P., Mathematical Analysis of Radiatively Enhanced Liquid Droplet Vaporization and Liquid Fuel Combustion Within a Porous Inert Media, MS Thesis, Dept. Mech. Eng., Univ. Texas, Austin, TX, May 1993. 2. Gocl, R., An Experimental Investigation of Radiant Thermal Efficiency and Emissions for Premixed Methane-Air Combustion Within a Two Stage Porous Media Burner, MS Thesis, Dept. Mech. Eng., Univ. Texas, Austin, TX, December 1993. 3. Hsu, P. F., Evans, D., and Howell, J. R., Experimental and Numerical Study of Premixed Combustion with Non-homogeneous Porous Ceramic, Combust. Sci. Technol. 90, 149 172, 1993. 4. Khanna, V., Goel, R., and Ellzey, J. L., Measurements of Emissions and Radiation for Methane Combustion Within a Porous Medium Burner, Presented at the 1992 fall meeting of thc Western States Section of The Combustion Institute, Berkeley, CA, Oct. 12 13, 1992. 5. Sathe, S. B., Peck, R. E., and Tong, T. W., A Numerical Analysis of Combustion and Heat Transfer in Porous Radiant Burners, HTD-Vol. 106, pp. 461 468, ASME, New York, 1989. 6. Sathe, S. B., Kulkarni, M. R., Peck, R. E., and Tong, T. W., An Experimental and Theoretical Study of Porous Radiant Burner Performance, Twenty-Third Symp. (Int.) on Combustion, pp. 1011-1//18, 1990. 7. Kulkarni, M. R., and Peck, R. E., Modeling Radiant Surface Burner Performance, Paper 93-1(16, Western States Section of the Combustion Institute, Menlo Park, CA, Oct. 18-19, 1993. 8. Yoshizawa, Y., Sakaki, K., and Echigo, R., Analytical Study of the Structure of Radiation Controlled Flames, Int. J. Heat Mass Transfer 31(2), 311 319, 1988. 9. Chen, Y.-K., Matthews, R. D., and Howell, J. R., The Effect of Radiation on Combustion Within Highly Porous Materials, in
10. Chen, Y.-K., Matthews, R. D., Lim, 1., Lu, Z., and Howell, J. R., Experimental and Theoretical Investigation of Combustion Within Porous Inert Media, Twenty-Second Int. Symp. Combustion, pp. 481 489, August 1988. 11. Evans, D., Lean Limit for Combustion of Methane-Air Flames in Porous Inert Media, MS Thesis, Dept. Mech. Eng., Univ. Texas at Austin, May 1991. 12. Hsu, P., Howell, J. R., and Matthews, R. D., A Numerical Investigation of Pre-mixed Combustion Within Porous Inert Media, Proceedings', Joint ASME/JSME Heat Transfer Conference, Reno, Vol. 4, pp. 225 231, March 1991. 13. Echigo, R., Effective Energy Conversion Method Between Gas Enthalpy and Thermal Radiation and Application to Industrial Furnaces, 7th Int. Heat Transfer Conf., Munich, Vol. 6, pp. 361-366, 1982. 14. Yoshizawa, Y., Sasaki, K., and Echigo, R., Analytical Study of the Structure of Radiation Controlled Flame, Int. J. tteat Mass" Transfi'r 31(2), 311-319, 1988. 15. Williams, F. A., Combustion Theory, 2nd ed., Addison-Wesley, Reading, MA, 1985. 16. Williams, A., Combustion of Liquid Fuel Sprays, Butterworths & Co., London, 199(/. 17. Hendricks, T. J., and Howell, J. R., Absorption/Scattering Coefficients and Scattering Phase Functions in Reticulated Porous Ceramics, 6th Annual AIAA/ASME Thermophys. Heat Transfer Conf., Colorado Spring, CO, June 1994. 18. Hall, M. J., and Hiatt, J. P., Exit Flows from Highly Porous Media, Phys. Fluids 6(2), 469-479, 1994. 19. Boltz, R. E., and Tuve, G. L. (Eds.), The CRC Handbook of Tables for Applied Engineering Science, 2nd ed., CRC Press, Boca Raton, FL, 1973.
Radiation, Phase Change Heat Transfer, and Thermal Systems (Y. Jaluria, V. P. Carey, W. A. Fiveland, and W. Yuen, Eds.), HTD-Vol. 81, pp. 35 42, ASME, New York, 1987.
Received June 6, 1994; revised November 1, 1994