Environ. Sci. Technol. 1996, 30, 3053-3060
Field Investigation of the Temperature Distribution in a Commercial Hazardous Waste Slagging Rotary Kiln JOHN M. VERANTH,† DACHUN GAO,‡ AND G E O F F R E Y D . S I L C O X * ,† Department of Chemical Engineering and Department of Mechanical Engineering, University of Utah, Salt Lake City, Utah 84112
Gas and bed temperatures were studied in a 4.4 m by 12 m, co-current flow, slagging rotary kiln at a commercial hazardous waste incinerator. The visual observations used by the kiln operators to control the process are described. These observations were quantified using thermocouples, radiation pyrometers, and phase-change indicators. The objectives were to estimate the peak bed temperature and compare this to measurements at the kiln exit. The maximum bed temperature occurs toward the middle of this type of kiln and not at the discharge. The slag melting temperature and test pellets with known melting points indicate that the peak bed temperature can be 100-300 K higher than the kiln exit temperature reported by the permanent instruments at this facility. Both broad-band radiation pyrometers and thermocouples give a qualitative temperature indication that can be used for process control, but the readings depend on the sensor locations relative to the incompletely mixed air and combustion products. Twocolor radiation pyrometer measurements of surface temperature near the kiln exit are higher than the actual temperature due to reflected radiation.
Introduction Measuring the temperature of the solids or slag in industrial kilns is difficult because the moving bed is inaccessible and the kiln bed is a hostile environment for instruments. However, gas temperature, measured using either a radiation pyrometer or a thermocouple mounted in a metal or ceramic well, is commonly used as the process control variable (1). These techniques are economical, reliable, and reproducible and respond quickly to operating adjustments. The heat transfer rate and the bed temperature are assumed to track the changes in gas temperature. * Corresponding author mailing address: Department of Chemical Engineering, Rm 3290 MEB, University of Utah, Salt Lake City, UT 84112; telephone: 801-581-8820; e-mail address:
[email protected]. † Department of Chemical Engineering. ‡ Department of Mechanical Engineering.
S0013-936X(96)00115-0 CCC: $12.00
1996 American Chemical Society
A database of publicly available field data, which includes metal emissions from various types of incinerators, is being compiled under an EPA contract (2). The relationship between the reported exit temperature and the actual temperature distribution along the kiln bed is needed when analyzing results obtained at different sites. A better understanding of the bed temperature is also needed to predict full-scale incinerator performance from the results of fundamental studies and pilot-scale experiments. The peak temperature of the solids or slag in a hazardous waste rotary kiln is, in general, not the same as the gas temperature obtained from a single-point, exit-plane measurement. Because vapor pressure, equilibrium constants, reaction rates, and transport properties are strong functions of temperature, the axial variation of bed temperature is an important variable affecting the desorption of organics, the partitioning of metals, and the physical characteristics of the residue. Previous studies on full-scale hazardous waste facilities emphasized exit-plane measurements during liquids-only operation (3) or made gas-phase measurements using long probes inserted into the kiln (4). This paper presents field data for the bed temperature in a typical commercial hazardous waste kiln operating under normal conditions. A future paper will compare these field measurements to wall and gas temperatures calculated using a commercially available CFD-based reacting-flow furnace model.
Facility Description The field investigation was conducted on a slagging rotary kiln at a commercial hazardous waste treatment facility. The configuration of the kiln and the secondary combustion chamber, shown in Figure 1, is similar to a number of industrial hazardous waste kilns operating in the United States and in Germany. Table 1 shows typical operating conditions for 1 day as summarized in the facility trial burn report. The kiln shell is 4.4 m in diameter by 12 m long and is lined with refractory brick. The kiln discharges into a rectangular secondary combustion chamber 5.3 m across by 17.4 m high. The plant thermal rating is 35 MW. The multi-fuel burner and the feed chute for solids are mounted on the stationary front wall of the kiln. The system is preheated using clean fuel oil. The burner at the kiln front wall and the two burners in the secondary combustion chamber are switched to a blend of liquid wastes once the operating temperature is reached. Containerized wastes, which may include inert solids, combustible solids, and small quantities of free liquids, are fed through a barrel conveyor system. Bulk solids, typically contaminated soil or combustible debris, are fed using an apron conveyor, a batch scale, and a pair of gates that form an air lock. Primary air enters through the multi-fuel burners. Secondary air enters through the feed chute, through a curved slot below the front wall, and through the kiln seals. The kiln is sloped from the front wall toward the discharge. The gas and solids flow co-currently from the front wall into the secondary combustion chamber. Solids residence time, calculated by an empirical formula (5), is about 1 h. As the solids move down the kiln, the noncombustible portion is melted into a viscous slag. The slag eventually drops from the kiln end into a water-filled tank
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thermocouples in ceramic wells mounted at the top of the secondary combustion chamber. The permanently installed radiation pyrometers are identical, broad-band instruments that respond to wavelengths between 1.0 and 5.0 µm, which includes the major emission lines from water and carbon dioxide. Emission, absorption, and scattering along the sight path through the gas and emission from the far wall of the furnace all affect the reading from this type of pyrometer. For historical reasons, pyrometer A is located below the kiln center line and is aimed at an angle into the kiln. Pyrometer B is located above the kiln center line and is aimed across the secondary combustion chamber perpendicular to the kiln axis. The electronically smoothed average of the A and B pyrometers is used as the reported kiln temperature.
FIGURE 1. Incinerator overview. Major process flows and the locations of test ports and of key permanent combustion monitoring instruments are indicated. Permanent instruments: RP, broad-band radiation pyrometersA on near wall and B on far wall. TI, thermocouple in ceramic well. O2, oxygen analyzer. Test locations: TC, Thermocouple access on near and far side walls and on rear wall of secondary combustion chamber. TW, thermocouple inserted through kiln wall. VP, viewport used for visual observations and for measurements with the portable two-color radiation pyrometer. Refer to Figure 3 for plan and end views. TABLE 1
Operating Conditions As Reported for 1 Day of the Facility Trial Burn kiln feed
average
units
kiln liquid waste containerized waste bulk solid waste
0.29 0.51 0.57
kg/s kg/s kg/s
process measurements
max
min
av
units
pyrometer A pyrometer B secondary combustion chamber temp secondary combustion chamber oxygen stack flow stack oxygen
1483 1517 1439
1289 1277 1381
1383 1416 1417
K K K
15.1
4.1
8.3
wet %
32.1
24.6
27.9 10.3
actual m3/s dry %
where it is quenched and removed by a chain conveyor. The gas exits the kiln and flows upward into the secondary combustion chamber where additional blended liquid waste is burned to raise the gas temperature. The secondary combustion chamber provides over 3 s of residence time at a minimum temperatue of 1372 K and 3% minimum oxygen to complete the destruction of organic materials prior to entering the gas cleanup train. Instrumentation used for combustion control includes continuous monitoring of fuel and waste mass flow, kiln temperature, and secondary combustion chamber negative pressure, temperature, and oxygen. Inlet air flow is not reported since the operating permit puts limits on secondary combustion chamber oxygen and on stack flow at the gas cleaning exit. Temperature measurement includes the A and B gas radiation pyrometers installed beyond the kiln exit and three
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Historically, the operation of industrial kilns has been an art learned by experience. Visual observations of the flame and the bed condition provide the information that the operators use to adjust flow rates, rotational speed, damper settings, and other conditions. The operators use four view ports and two television cameras to monitor the kiln. Figure 2 summarizes typical site-specific kiln operator observations of the flame and the slag. The liquid burner on the kiln front wall produces an intense flame that is typically 1-1.5 m in diameter and can extend half-way down the kiln. Flame shape is adjusted by the cone angle of the interchangeable liquid spray nozzle and by adjusting the air flow. The combustible material in the kiln bed produces a secondary flame that merges with the flame from the liquid burner. This bed flame shape depends on the mixing of volatile organics with the secondary air flow. The combustion is spread over a large volume and varies in size and position as new combustible material in the bed is exposed by the kiln rotation. The flames often extend beyond the end of the kiln and into the secondary combustion chamber. The operators control the interval between the waste containers and the size and frequency of bulk solids batches based on the burning behavior of the material being processed, subject to permit limits on maximum feed rates. The solids that fall from the feed chute land in the first 1-2 m of the kiln. Typically there is a short delay before ignition of each charge and then rapid initial burning followed by a period characterized by a roughly exponential decrease in the burning rate. Kiln rotation exposes unburned material resulting in smaller flare-ups during the burnout period. The gas temperature and gas composition measured at a different full-scale kiln while burning single containers containing toluene and xylene adsorbed on clay (4, 6) agrees with these qualitative observations. The refractory lining in the front wall area is generally covered with a hard rough slag. Melting, dripping, or sticking is seldom observed in this area during normal operation. Soils and non-combustible debris tend to be free-flowing near the front wall and move by tumbling and sliding. Farther from the front wall, the solids become partially melted, and frequent operator attention is needed to avoid forming a ring or dam of semi-solid material. Near the middle of the kiln, the bottom is covered by a viscous pool of molten slag mixed with lumps of higher melting-point material. Jets of flammable gas can be seen venting from the slag pool. Slag is carried up by the ascending wall and flows back down in a sheet with some slag dripping from the top of the kiln.
FIGURE 2. Visual observations of a commercial slagging hazardous waste kiln under normal operating conditions.
The pattern of slag flow and refreezing near the kiln exit is an indication of the axial and radial variation in gas and slag temperature. The operators actively control slag consistency and movement by adjusting flame position. At times the slag becomes very viscous near the discharge, and a ring of frozen slag forms around the end of the kiln. Operators attempt to remelt such rings by adjusting the flame, but removal often requires pounding with long heavy steel bars. Excessively fluid slag results in rapid transport down the kiln, and steam puffs occur when sudden surges of low-viscosity slag reach the end of the kiln and drop into the deslagger water. A desirable operating condition is when slag near the exit is carried up the ascending wall to the top where it melts and falls in drops and strings. This results in a steady discharge from the kiln as small pieces of slag fall past the end of the kiln and drop into the deslagger. The drops of slag that fall back onto the kiln bottom are observed to cool from bright yellow to red and refreeze within seconds. Visible smoke and particulate at the kiln exit are highly variable. When burning fuel oil, the flame is compact, the kiln exit gas is transparent, and one can see from the rear wall of the secondary combustion chamber to the feed chute on the front wall. When burning liquid waste, the flame is larger, and there can be visible smoke at the kiln exit. Containerized wastes cause flare-ups that create thick smoke mixed with the flame, and this prevents seeing across the width of the secondary combustion chamber into the kiln. Some solid wastes produce entrained dust and embers at the kiln discharge. The objectives of this field investigation were to quantify the thermal conditions associated with these empirical observations, to test several commercially available alternatives for monitoring kiln temperature, and to provide an estimate of the peak bed temperature under operating conditions.
Gas Temperature Measurements A series of experiments was conducted to compare the plant’s permanent instruments, pyrometers A and B, to temporary thermocouples in metal thermowells installed at accessible locations near the kiln exit. These tests were conducted on a total of 10 days. There was no special selection of feed materials during these tests, and the facility was in commercial operation.
FIGURE 3. Comparison of test thermocouple and permanent broadband pyrometer readings. This time trend is typical of the variation observed during normal operation. A & B average: electronically smoothed average of the two broadband infrared pyrometers. This signal is used for plant process control and for permit compliance records. TC, side wall: thermocouple in a steel thermowell and inserted 0.6 m inside the secondary combustion chamber side wall and below kiln center line. TC, rear wall: thermocouple in steel thermowell and inserted 0.6 m inside the secondary combustion chamber wall opposite the kiln discharge and near the kiln horizontal and vertical centerlines.
Thermocouple assemblies consisting of a type K junction (Omega wire type XC-K-20) mounted at the end of a closed steel pipe were positioned at the locations shown in Figures 1 and 3. Data were collected with either a hand-held readout (Fluke Model 51) or with a computer interface card (Omega TM-WB-AA1) and a Macintosh computer. The location and viewing direction of the permanent radiation pyrometers are also shown in Figures 1 and 3.
Bed Temperature Determinations Techniques used to measure the temperature of the slag inside the kiln included the following: thermocouples inserted through the kiln wall to the interface between the
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brick and the slag, direct-contact thermocouple measurements of slag surfaces, two-color radiation pyrometer measurements of slag surfaces that could be sighted from the viewports, slag melting point determinations, and melting point indicator pellet assemblies that were fed with the containerized waste. Stainless steel thermocouple wells were installed through the kiln steel shell and outer insulating brick and into the dense refractory brick lining. The facility management had authorized installing wells that extended to the inside face of the brick, but the wells actually ended approximately 20 mm from the hot face due to installation problems during the shutdown. Removable type K thermocouples were installed in the wells, and readings were taken manually using a hand-held readout during preheat and during normal operation. The normal kiln speed of one rotation in 3-4 min is less than the characteristic response time estimated for the brick and slag layer separating the gas from the metal well, so the readings are interpreted as an average value. One thermowell failed during kiln startup, and deposits prevented inserting the thermocouple junction to full depth. A second thermowell failed and filled part way with slag during the first month. Since this was a unique opportunity to make temperature measurements through the kiln wall, an attempt was made to salvage useful data from the failed wells. The temperature gradient in the dense inner brick lining was measured by positioning a junction at various distances into the well. The gradient was also calculated for the composite wall using published refractory properties. This calculation ignored both radiation inside the well and conduction along its thin metal wall. The measured gradient was 16 K/cm and the calculated gradient was 19 K/cm. A linear temperature profile was then used to extrapolate from the measurement at the tip of each well to the temperature at the brick hot face. A slag layer of variable thickness separates the inside surface of the brick from the flowing gas. The extrapolation uncertainty of 3 K/cm was added to the measurement standard deviation when calculating error bars for the two failed thermowells. The facility operators take readings of the external shell temperature every 4 h using a commercial total radiation pyrometer. The emissivity setting on this pyrometer is adjusted to match magnetic base, bimetallic thermometers that are placed directly on the kiln shell. The shell temperature data from the operator log sheets were collected and analyzed. A commercial two-color radiation pyrometer (Capintec Model 5U) was available at the site. This type of pyrometer can give a remote reading of the temperature of a solid or liquid surface with unknown emissivity using the ratio of the emissions at two wavelengths. The equations used to calculate the temperature assume a gray surface. There is also the implicit assumption that the radiation leaving the surface is emitted by the target surface and that it is not reflected radiation from a hotter surface or from the flame. A series of tests was conducted to determine the effect of reflected radiation on the temperature measured by the two-color pyrometer. The temperature of frozen slag located on the shelf directly opposite the kiln discharge was simultaneously measured using both a thermocouple in direct contact with the slag and with the two-color radiation pyrometer. A thermocouple junction was mounted on the underside of a radiation shield, and a long probe was used to place the junction directly on a slag
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surface. The pyrometer was aimed at the slag surface surrounding the contact probe. The radiation shield was 30 mm by 50 mm, and the iris on the pyrometer was adjusted to view a circle approximately 150 mm in diameter at the distance of the slag surface. The two-color pyrometer was used at the viewport on the kiln front wall and at the viewports on the sides and rear of the secondary combustion chamber to observe surface temperatures. One of the two-color pyrometer measurement locations on the inside of the kiln coincided with the axial position of the thermocouple inserted through the kiln lining 10.4 m from the feed end. The local temperature inside the kiln bed can be estimated from slag melting point data. Even though the slag is not a eutectic mixture and typically has a wide range between initial melting and fully fluid conditions, visual observation of flowing slag establishes a minimum value for local temperature. Samples of kiln slag were collected and sent to a coal analysis laboratory where the slag was pulverized and compressed into cones. The cones were placed in a muffle furnace and heated in accordance with the ASTM coal ash fusion test procedure, D1857. This procedure defines the initial deformation, softening, hemispherical and fluid temperatures as the furnace temperature at which the cone shape reaches specified height-to-width ratios. Determinations were made in both reducing and oxidizing atmospheres. A range for the peak bed temperature can be determined by examining materials with known melting points that were recovered after passing though the kiln. This method brackets the peak temperature reached by non-combustible solid materials fed into the kiln. Pellets with known melting points are commercially available (Tempil pellets) and are used to monitor temperature in welding and heat-treating applications. A series of indicator pellets with melting points ranging from 1144 to 1644 K (1600-2500 °F in 50 °F steps) were available. Pellets were inserted into a 150 mm long carbon steel pipe (3/4 in. nominal diameter) with threaded end caps, and the assembly was placed in the kiln using the container feed system. The pellets inside the pipe were physically separated by steel washers, and the ends of the pipe were cushioned with ceramic fiber. The possibility of confusing actual melting with pellets that had been dissolved by the deslagger water was ruled out by immersing a full set of pellets in hot water. The method was also validated by placing pellet assemblies in a pilotscale laboratory rotary kiln. This allowed checking for interaction between the different pellets by tumbling the assembly for a known time at a controlled temperature. Recovery of the pellet assemblies required seeing a small object in the 2 t or more of slag discharged per hour. Initial attempts were unsuccessful. For later tests, recovery was aided by welding the short pipes to pieces of light gage scrap steel, approximately 0.5 m by 0.5 m. These carrier plates are similar in size and thickness to metal drums, PCB-contaminated electrical components, and other metal objects routinely fed into the kiln. Initial data were opportunistic determinations made at conditions controlled by production. Later, a 4-day sequence of tests was authorized to compare the bed temperature and the measured gas temperature under conditions that were expected to create long and short flame patterns. The hypothesis was that a short flame would reduce the measured temperature at the exit-plane but would increase the peak temperature inside the kiln. For
TABLE 2
Comparison of Three Different Test Thermocouple Locations and Two Permanent Pyrometer Locationsa
a
sensor
location
mean K
SD
short thermocouple medium thermocouple long thermocouple radiation pyrometer A radiation pyrometer B
1 m inside SCC wall and above kiln axis 1.5 m inside SCC wall and adjacent to short TC curved but extended beyond inside of kiln lining below kiln center line; aimed diagonally toward kiln above kiln center line; aimed perpendicular to kiln axis
1085 1286 1293 1265 1337
30 20 23 29 20
Data were taken under normal plant operating conditions. N ) 44 readings; duration ) 140 min.
this 4-day series, readings were taken at multiple locations inside the kiln using the two-color pyrometer while simultaneously collecting gas temperature, wall thermocouple, and phase change data. All normal operating data were obtained using the data logging capability of the plant’s distributed control system.
Gas Temperature Experimental Results Comparison of radiation pyrometer and thermocouple measurements at the kiln exit are of interest since these are the most common methods of reporting the operating temperature of hazardous waste kilns. Table 2 summarizes results from simultaneous measurements using the A and B broad-band radiation pyrometers and three different thermocouple locations at the kiln exit. The short and medium thermocouples were separated by only 0.5 m, but the average temperature differs by 200 K. However, the A and B radiation pyrometers bracket the average temperatures measured by both the medium thermocouple and by the long thermocouple. On one day, gas temperatures were measured using a single thermocouple with a perforated radiation shield surrounding the junction. A long air-cooled support was used to position the junction at various distances into the secondary combustion chamber. Measurements were made using the ports above and below the kiln center line on both side walls as shown in Figure 1. All four locations showed lower temperature near the walls. The difference on each traverse between the first measurement, 0.6 m inside the secondary combustion chamber, and the farthest penetration ranged from 150 to 270 K. Figure 3 shows a typical temperature trend comparing the electronically smoothed average of the A and B broadband pyrometers with temporary thermocouples mounted 0.6 m inside the side wall and rear wall of the secondary combustion chamber. These data show the effect of flame pattern on the incompletely mixed gas flow at the kiln exit. The trend can be divided into two periods. During the period from zero to 100 min a visible flame extended beyond the end of the kiln. Operating adjustments were then made which caused a change in flame position. From 110 to 240 min the flame was shorter, but flare-ups caused the flame to occasionally extend into the secondary combustion chamber. With a long flame, the averages of the infrared pyrometers and the rear wall thermocouple differed by only 3 K, but the correlation coefficient was 0.60. With a short flame, the difference between the pyrometer and rear wall thermocouple increased to 71 K, but the correlation coefficient increased to 0.81. The interpretation is that radiation pyrometers respond to radiation sources along a sight path while thermocouples respond to the convective and radiative environment around the junction. During the first period, the flame
was in the sight path of the infrared pyrometers, and the combustion products were also reaching the rear wall thermocouple. This resulted in a relatively steady pyrometer signal and high temperature at the rear wall thermocouple. During the second period, the flame was shorter, and the infrared pyrometer was alternately sighting hot combustion products and cold secondary air, and the flame was not always reaching the rear wall. Coincident temperature increases in both of the thermocouple readings and in the pyrometer readings occurred whenever the flame extended beyond the end of the kiln. The exit gas temperature measurement is strongly dependent on the location of the sensor in the time-varying, incompletely mixed flow of combustion products and secondary air. Thermocouple and broad-band radiation pyrometer readings overlap, and there was no evidence of a consistent difference between the two sensor types. Generally, the readings taken below the kiln center line were lower than the readings above the center line for both pyrometers and thermocouples. However, the location of the plume of combustion products moves when the air flow or the solids burning pattern changes. Wide temperature fluctuations were seen with all sensors. Thermocouples mounted 1 m or less from the secondary combustion chamber wall average lower temperatures than longer thermocouples, which extend into the flowing gas more of the time. These observations are consistent with the results obtained at a different site (4), which showed differences of 200-400 K between the gas in the upper and the lower portion of an industrial hazardous waste kiln.
Bed Temperature Experimental Results The temperature at the interface between the inside of the brick and the slag layer was measured using the thermocouples inserted in wells through the kiln refractory lining. The upper bars in Figure 4 show the overall average and the range of daily means obtained on 7 days over a 2-month period following a kiln brick replacement. Differences between the daily means at each location are statistically significant and are attributed to changes in operating conditions. The thermocouple measurements show that the temperature near the middle of the kiln is higher than at the discharge. The temperature pattern measured at the inside of the refractory lining agrees with expectations based on the visual observations of slag flow shown in Figure 2. Since measurements were limited to four points, the axial location of the peak temperature was not determined. The external shell temperature profile, shown by the lower bars in Figure 4, provides another indication that the peak temperature occurs near the middle of the kiln. The data are the average ( one standard deviation of readings taken at 4-h intervals over a 3-day period after the kiln brick replacement. The external shell temperature at mid-
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FIGURE 4. Average axial pattern of kiln external shell and inside brick temperature. Inside temperature measurements are from test thermocouples inserted into wells installed in the brick lining of the kiln. Bars show the overall mean and the range of daily means plus the extrapolation uncertainty. Data are from 7 test days over a 2-month period. Outside temperatures were taken from the 4-h operator readings for three consecutive days. Bars show mean ( 1 SD.
kiln was about 45 K higher than at the discharge. Axial variation in brick thickness due to wear was ruled out as the cause of the external temperature profile by comparing 3-day average shell temperature data for periods before and after a kiln brick replacement. Variation in the thickness of the slag layer was ruled out as a cause of the shell temperature variation by taking data at the end of the preheat after the rebricking and before any solid feed had formed a slag layer. In each case, the peak shell temperature occurred near the middle and not at the discharge. Simultaneous measurements of a stationary slag surface near kiln discharge, made with the portable two-color ratio pyrometer and with a contact thermocouple, confirmed the expectation that reflected radiation causes the twocolor pyrometer to read higher than the actual temperature. In a series of four determinations, the contact readings averaged 1130 K and the two-color pyrometer averaged 1260 K. The difference of the means exceeds six standard deviations. Attempts to make similar measurements of the flowing slag inside the kiln were unsuccessful. The apparent temperature due to emission plus reflected radiation from the stationary slag surface was estimated. The calculation used wall and gas temperature data from this study and a simplified geometry to obtain the incident radiation on the target surface, which was assumed to be at the contact measurement temperature. The calculated apparent temperature was consistent with the two-color radiation pyrometer measurement of the surface temperature. There were 6 days when measurements were taken with the two-color pyrometer aimed at the same axial position as one of the thermocouples that was inserted through the kiln wall. The two-color measurement of the gas-to-slag interface and the thermocouple measurement of the slagto-brick interface differed by an average of 306 K with a range from 250 to 340 K. The difference is due to a combination of the actual temperature gradient in the slag layer and the elevated pyrometer reading due to reflected radiation. This large difference indicates that caution must be used in interpreting a measurement of slag exit temperature made by either of these methods. Because reflected radiation was shown to cause significant error, other data collected with the two-color pyrometer are not reported. The indicator pellets and the slag melting data obtained by the ASTM ash fusion test both indicate that the peak
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bed temperature is much higher than the reported average exit gas temperature for this facility. Table 3 summarizes the tests where the melting pellet assemblies were fed through the kiln. The assemblies used in the later tests were welded to carrier plates, but the recovered carrier plates were bent and oxidized almost beyond recognition. Finding them in the large amount of discharged slag was still difficult, and only a few assemblies were recovered. Table 4 details the condition of the pellets in the two assemblies recovered on a test day with long flame operating conditions. The lower bound for peak temperature was set at 1422 K since these pellets in both assemblies had partially melted. The temperature of 1478 K was considered the upper bound of peak temperature since this pellet was intact in both assemblies. On the two days with short flame operation, all pellets melted in all five recovered assemblies. This indicates that the peak temperature exceeded 1644 K under the short flame condition. Table 5 summarizes two of the seven ash fusion tests. The slag melting behavior changes over a wide range with the daily changes in the composition of the non-combustible portion of the kiln feed. Correlations based on the ratio of acidic elements (Al and Si) to alkali elements (Na, K, Mg, Ca, and Fe) have been developed by pyrometallurgists. These empirical rules provide some insight into the slag temperature-viscosity relationship. There is also a large difference between the data taken under reducing and oxidizing atmospheres. For coal ash, this difference is attributed to the balance between ferric and ferrous species. Reducing conditions are believed to exist in the kiln slag at this facility. The slag was previously analyzed and averaged 1.5 ( 1.0 wt % residual carbon. Also, flammable gases were observed being released from the slag pool during normal operation. For this field study, observation of any slag melting was interpreted as an indication that the local temperature was greater than the initial deformation temperature of the slag sample measured under reducing conditions using the ASTM procedure. An observation of the transition from viscous deformation of semi-solid slag to a free-flowing pool of slag was interpreted as an indication that the local temperature was at or above the fluid point of the slag measured under reducing conditions. Figure 5 summarizes the data for the A and B radiation pyrometers, two of the four thermocouples inserted through the wall, the melting pellets, and the slag fusion test obtained on two different days of the 4-day test series. Comparison of the data for long flame and short flame operation supports the hypothesis that a short flame decreases the temperature sensed by the radiation pyrometers at the exit and increases the peak temperature inside the kiln. The exit temperature bar in Figure 5 is the mean of the two permanent radiation pyrometers ( one standard deviation for the 3-h period when the other data were collected. The wall thermocouples at 4.1 and 9.1 m from the feed end are shown with an error bar that includes the experimental standard deviation plus the extrapolation uncertainty. The melting pellet data range is bounded by the highest rated pellet that showed melting and the lowest pellet that was definitely intact. The slag melting data show the range between the initial deformation and the fluid condition under both oxidizing and reducing atmospheres. The slag was free-flowing inside the kiln on both days, indicating that the temperature was at or above the fluid point.
TABLE 3
Melting Point Indicator Pellet Test Resultsa day and conditions
recovery
observations
laboratory validation no. 1
1/1
gas temperature was 1320 K; all pellets less than 1311 K (1900 °F) melted; 1367 K (2000 °F) pellet was cracked but not melted; all remaining pellets were intact
laboratory validation no. 2
2/2
gas temperature was 1400 K; all pellets less than 1394 K (2050 °F) melted; the 1422 K (2100 °F) pellet was glazed but recognizable; all remaining pellets were intact
day 1, long flame
0/5
pellet assemblies were not attached to steel plates
day 2, long flame
2/4
no. 1, all pellets less than 1450 K (2150 °F) were melted; remaining pellets were intact; no. 2, all pellets less than 1394 K (2050 °F) were melted; 1422 K (2100 °F) pellet showed possible melting; all remaining pellets were intact
day 3, short flame
3/5
nos. 1 and 2, all pellets including 1644 K (2500 F) were melted; ceramic fiber at ends was mixed with slag but was recognizable; no. 3, all pellets were melted; ceramic fiber at the ends was white and friable
day 4, short flame
2/10
no. 1, all pellets including 1644 K (2500 °F) were melted or vitrified; 1561 K (2350 °F) pellet was vitrified, but the number was still readable; ceramic fiber was soft; no. 2, all pellets were melted; ceramic fiber at the ends was hardened, but there was no slag inside the assembly
a Commercial pellets are marked with the rated temperature on the Fahrenheit scale. These melting pellet tests indicate that extremely high temperatures occur under short-flame conditions
TABLE 4
Condition of Temperature-Indicating Pellets in Two Assemblies Recovered on Day 2 of Test Programa pellet rating
observation
°F
K
end fiber 1800 1900 2000 2050 2100
assembly 1
assembly 2
1256 1311 1367 1394 1422
intact melted melted melted melted partial
intact melted melted partial partial readable
2150
1450
partial
intact
2200 2250 2350 2450 end fiber
1478 1506 1561 1617
intact intact intact intact intact
intact intact intact intact intact
a The kiln was operated with a long flame, and the average kiln exit temperature recorded by the permanent pyrometers was 1344 K for the period when the assemblies were in the kiln.
TABLE 5
Slag Fusion Temperatures (K) Measured in Reducing and Oxidizing Atmospheresa day 2
day 4
cone shape
reducing
oxidizing
reducing
oxidizing
initial deformation softening hemispherical fluid
1356 1359 1359 1461
1418 1424 1456 1552
1329 1329 1331 1414
1506 1551 1597 1689
a The test is an adaptation of ASTM Method D 1857 for fusibility of coal and coke ash.
Discussion The difference between the kiln temperature reported using the permanent plant instruments and the peak temperature inside the kiln is important when using thermophysical
FIGURE 5. Comparison of test operation with long and short flame conditions. The short flame results in an increase in the difference between the exit temperature reported by the permanent pyrometers and the peak bed temperature inside the kiln. Exit temperature bar indicates mean ( standard deviation. Wall thermocouple bars indicate mean ( (standard deviation + extrapolation uncertainty). Melting pellet bar indicates the range between the last melted pellet and the first unmelted pellet. Since all pellets melted on day 4, an upper bound was not determined. Slag melting bar indicates the range between initial deformation and fluid condition.
property data and laboratory-scale experiments to predict full-scale performance. Equilibrium curves for the fraction of a metal present as a condensed metal species at various temperatures and chlorine concentrations (7) were used to evaluate the importance of the temperature range shown for the short flame operation in Figure 5. At the reported exit gas temperature, the vapor pressure of barium, nickel, and beryllium is low, and the metals are predicted to be in a
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solid or liquid phase. However, at the peak temperature indicated by the slag melting point and indicator pellet data, a significant portion of these elements are predicted to be in the vapor phase. Current research on forming non-leachable incinerator residue is based on achieving a high-temperature reaction between metals and alumino silicates. Again, the difference between the exit gas temperature and the peak local temperature implied by Figure 5 will affect the predicted residue characteristics. Industrial kilns are designed to achieve high material throughput, which requires steep temperature and concentration gradients. This study indicates that, for this type of hazardous waste kiln, the exit region is thermally nonuniform, with the liquid-fed flame being the hottest radiation source and the wet deslagger being a major heat sink. Classical heat transfer analysis predicts that these temperature differences will affect both the axial temperature profile and the sensor readings. The field observations were qualitatively consistent with these predictions.
received from the National Science Foundation, the State of Utah, 25 industrial participants, and the U.S. Department of Energy.
Acknowledgments
Received for review February 8, 1996. Revised manuscript received May 30, 1996. Accepted May 31, 1996.X
The cooperation of the management and plant operators at the field site is greatly appreciated. Financial support for this work was provided by the Advanced Combustion Engineering Research Center. Funds for this center are
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Literature Cited (1) Monsanto Research Corp. Engineering Handbook for Hazardous Waste Incineration; U.S. Environmental Protection Agency, Office of Research and Development: Washington, DC, 1981; SW-889 PB81-248163. (2) Rizeq, R. G.; Clark, W.; Seeker, W. R. Analysis of Toxic Metals Emissions from Waste Combustion Devices. In Proceedings of the 1992 Incineration Conference, Albuquerque, NM; University of California: Irvine, 1993; p 625. (3) Cundy, V. A.; Lester, T. W.; Sterling, A. M.; et al. J. Air Pollut. Control Assoc. 1989, 39, 1073-1085. (4) Lester, T. W.; Cundy, V. A.; Sterling, A. M.; et al. Environ. Sci. Technol. 1991, 25, 1142-1152. (5) Sullivan, J. D.; Maier, C. G.; Ralston, O. C. Passage of Solid Particles Through Rotary Cylindrical Kilns; Bureau of Mines: Denver, CO, 1927; Technical Paper 384. (6) Cundy, V. A.; Lester, T. W.; Leger, C.; et al. J. Hazard. Mater. 1989, 22, 195-219. (7) Linak, W. P.; Wendt, J. O. L. Prog. Energy Combust. Sci. 1993, 19, 145-185.
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Abstract published in Advance ACS Abstracts, August 1, 1996.
