295
Energy & Fuels 1995,9, 295-301
A Facility for High-pressure Measurement of Reaction Rates of Millimeter-Sized Coal Kenneth J. Bateman, Geoffrey J. Germane, L. Douglas Smoot," and Craig N. Eatough Advanced Combustion Engineering Research Center, Brigham Young University, Provo, Utah 84602 Received July 7, 1994@
A study was undertaken to design, construct, characterize, and demonstrate a new facility for determination of reaction rates of large coal particles at elevated pressures. A cantilever balance attachment (CBA) was designed, fabricated, and utilized in conjunction with the existing highpressure controlled profile (HPCP) reactor. Large particle (8 mm diameter) combustion experiments of Utah HVBB coal at both atmospheric and elevated pressures were performed to demonstrate the facility's capabilities. Measurements were obtained of particle mass loss rate and surface temperature coupled with a video record for visual observation.
1. Introduction
Coal, as a source of energy, has been and will continue to be utilized for many years. Worldwide, there are an estimated 400 billion tons of recoverable coa1.l The United States has coal deposits located in most regions and produces about one billion tons of coal per year.2 Though the supply of coal is substantial, making clean and efficient use of coal as a source of energy into the 21st century presents many obstacles. For example, coal structure and mineral content can vary widely among coal deposits. These factors limit the use of coal in many current facilities by changing output efficiency and making it difficult to comply with increasingly strict environmental regulation^.^ A substantial research effort in several specific areas is needed to provide information required to meet changing demand^.^ Coal can be used in various sizes such as pulverized, micrometer-sized particles, or larger particles, several millimeters in diameter. Larger coal particles are currently being used in fixed-bed furnaces, fluidizedbed combustors, and coal gasifier^.^ Some studies have been performed to examine combustion of these large particles at atmospheric pressure; however, at elevated pressures, only two studies of limited scope were found in the published l i t e r a t ~ r e . ~Previous ,~ work at this Centers led to the development of a high-pressure Abstract published in Advance ACS Abstracts, January 15, 1995. U.S. Coal; 1985 Energy Information Administration, U.S.Department of Energy: Washington, DC, 1985; DOEEIA-03333 (85). (2) Smoot, L. D.; Smith, P. J. Coal Combustion and Gasification; Plenum Press: New York, 1985; p 7. (3)Boardman, R. D.; Smoot, L. D. Pollution Formation and Control. In Fundamentals of Coal Combustion for Clean and Efficient Use; Smoot, L. D., Ed.; Elsevier: New York, 1993; Chapter 6. (4) Smoot, L. D. Role of Combustion Research in the Fossil Energy Industry. Energy Fuels 1993, 7, 659-699. (5) Radulovic, P. T.; Smoot, L. D. Coal Processes and Technologies. In Fundamentals of Coal Combustion for Clean and Efficient Use; Smoot, L. D., Ed.; Elsevier: New York, 1993; Chapter 1. (6)Gardner, N.;Samuels, E.; Wilkes, K. Catalyzed Hydrogasification of Coal Chars. Adu. Chem. Ser. 1974,131, 217. (7) Sears, J. T.; Maxfield, E. A.; Tamhankar, S. S. Pressurized Thermobalance Apparatus for Use in Oxidizing Atmospheres at High Temperatures. Ind. Eng. Chem. Fundam. 1982,21,474-478. @
(1)Paull, M. K.; Pantos, E. Annual Outlook for
0887-0624/95/2509-0295$09.00/0
reactor for use with pulverized coal particles where the reactor wall temperature profile could be controlled through a series of wall heating elements. This reactor is the high-pressure controlled profile reactor (HPCP). The main objective of this study was to design, fabricate, and demonstrate a detachable apparatus that could be used in conjunction with the HPCP to measure the mass loss, temperature, and burnout time of large-diameter coal particles at high pressure.
2. Background Thermobalances or thermogravimetric analyzers (TGA) have traditionally been used in the polymer industry to determine mass loss with time. They can use delicate electrical circuits because they generally operate at atmospheric pressure. Brown et al.9 were among the first of several to modify a DuPont Model 950 TGA and mount it in a pressure vessel for use at high pressure. They were able to operate a t a maximum of 2.03 MPa (20 atm) with a maximum temperature of 623 K. Williams and WendlandtlO also used a similar system to perform high-pressure tests. They were able to reach higher temperatures (773 K)and pressures (50.7 MPa), and they operated with several different types of gases. Dobner et al.ll described a thermobalance that could be used at high pressure under corrosive environments. His group reported that a modified TGA would permit operation at up to 3.04 MPa and temperatures to 1373 K, with corrosive atmospheres, and steam partial pressures up to 2.03 MPa. The apparatus was 0.9 m long and 0.25 m inside diameter. Several problems were encountered, such as insufficient gas mixing, long ~
~
~~~~
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(8)Monson, C. R.; Germane, G. J . A High-pressure Drop-Tube Facility for Coal Combustion Studies. Energy Fuels 1993, 7,928-936. (9) Brown, H. A.; Penski, E. C.; Callahan, J. J . An Apparatus for High Pressure Thermogravimetry. Thermochim. Acta 1972, 3, 271276. (10) Williams, J. R.; Wendlandt, W. W. A High Pressure Thermobalance. Thermochim. Acta 1973, 7 , 253-260. (11)Dobner, S.; Kan, G.; Graff, R. A.; Squires, A. M. AThermobalance for High Pressure Process Studies. Thermochim. Acta 1976,16, 251-265.
0 1995 American Chemical Society
296 Energy & Fuels, Vol. 9, No. 2, 1995
heating times, and inconsistent temperature readings. They did report that, at 2.07 MPa and 1073 K, buoyancy effects were only 0.3% of the sample weight. An important similarity in all of the experiments that used this early high-pressure TGA device is the fact that they all had the gas flowing horizontally across the sample. Bae12described a device that used a Hewlett-Packard Model FTA-1-1TGA with a full scale range of 1g and worked with pressures up to 7.09 MPa and temperatures up to 873 K. The gas flowed upward. Buoyancy effects were resolved by calibration runs. Li and Rogan13 used a DuPont 951 TGA. The main distinction of their design was the spatial separation of the balance mechanism from the reaction zone. This protected the mechanism from corrosive gases while allowing the reaction zone to be heated externally. The mechanism required vertical flow and the particle was suspended from a long wire. They were able to reach temperatures of 1123K at pressures of 2.02 MPa while using corrosive gases. Suspending particles from a wire to provide spatial separation was recognized as a very effective method of protecting the mechanism. Forgac and Angus14 used this procedure for their experiments with good results. A Perkin-Elmer Model AM-1 Auto-balance was mounted to a water-cooled, 316 stainless steel outer shell, 0.4 m long and 0.27 m in diameter. The mechanism was operated statically with no gas flow, routinely at 5.07 MPa and 1433 K and as high as 7.09 MPa and 1633 K. They reported that insignificant amounts of sample were lost during the heat-up and cooling-off periods. The apparatus was equipped with a quartz window and a pyrometer was used to record temperatures. Improving upon the Forgac and Angus design, Sears et al.' found that they could heat the reactor space to the maximum temperature in about 20 min. The system could handle sample sizes as small as 0.03 g. A major improvement was the addition of ceramic heaters located inside the reactor space, separated from the reactor wall using high-temperature insulation. This allowed for maximum operating conditions while maintaining wall temperatures of only 423 K. The system used suspended particles and had a downward gas flow. Mass loss was recorded on a strip chart recorder. A pyrometer was available for taking particle temperature measurements. Treptau and Miller15combined several designs where the solid was sandwichedbetween alumina powder with thermocouples embedded a t two locations. The sample heaters were located on the inside of the pressure vessel, thus maintaining low wall temperatures. They also measured the entire pressure vessel to obtain mass loss, similar to Gardner et a1.,16 but to improve the sensitiv(12)Bae, J. H. A Simple Thermogravimetric Apparatus for Pressures up to 70 Atmospheres. Rev. Sci. Instrum. 1972,43, No.7,983985. (13)Li, K.;Rogan, F. H. AThermogravimetric System for Corrosive Environments at High Pressures and Temperatures. Thermochim. Acta 1978,26,185-190. (14)Forgac, J. M.; Angus, J. C. A Pressurized Thermobalance Apparatus for Use at Extreme Conditions. Ind. Eng. Chem. Fundam. 1979,18,416-418. (15)Treptau, M.H.; Miller, D. J. An Internally Heated Weighed Reactor Thermobalance for Gas Solid Reaction Studies. Ind. Eng. Chem. Res. 1987,26,2007-2011. (16)Gardner, N.C.; Leto, J. J.; Lee, S.; Angus, J. C. Thermogravimetric Measurements at High Pressures. NBS Spec. Pub. 1980,580, 235-250.
Bateman et al.
ity, they used a counterbalance. Results obtained using C02 and 0 2 showed that the desired f10 mg sensitivity for a 2 g carbon sample was adequately measured. Each test required 1-2 h to stabilize the balance and 2-2.5 h to stabilize the temperature. Ragland and Yang17 used the hanging particle approach and had upward air flow at atmospheric pressure. The electronic balance, with 1 mg resolution, measured mass loss 10 times a second and stored the values in a computer. The gas temperature was measured 2 cm away from the particle and varied from 900 to 1200 K. A window allowed photographs of the burning particles to be taken. They measured effects of oxygen concentration, Reynolds number, particle size, and coal type. Particles were manually filed to nearspherical shape. Tests revealed that aerodynamic drag was less than 2% of the initial weight. A horizontal flow TGA was designed by Huang and Scaroni18 for use at atmospheric pressure. The gas temperature was varied from 973 to 1173 K and the size of the coal particles tested ranged from 0.8 to 4.2 mm. A quartz tube was heated and the sample was inserted into the tube. A constant sample introduction time was used for all runs. A platinum sample pan was replaced by a sample basket made of 100 mesh stainless steel screen to improve air flow and reduce catalytic reaction with the platinum. Gardner et a1.6 performed a kinetic study of catalyzed and noncatalyzed coal char hydrogasification using a TGA. Tests were performed at pressures of 3.4 and 6.8 MPa and gas temperature of 1223 K. Gas temperature was measured by a thermocouple located 0.6 cm below the char particle. Results show an increase in char reaction rate with pressure for both the catalyzed and noncatalyzed chars. Sears et al.7 reported briefly on oxidation of a lignite in CO2 at 101 kPa and 790 kPa while focusing on design of the apparatus. Most of the articles detailing mass loss devices for atmospheric or elevated pressure testing were not used specifically for coal combustion, and if they were, only limited results were reported. Few articles were written with details on both the apparatus and the results. While each facility contained favorable design aspects, a facility capable of providing detailed large particle coal and char reactivity and temperature data at elevated pressure and temperature with visual access was not found. A facility capable of obtaining instantaneous mass and particle temperature measurements at elevated pressures with visual access to the burning particles could greatly enhance the understanding of the entire combustion process of coals and chars. 3. Test Apparatus 3.1. General Description. The apparatus described herein was patterned after a combination of several previous designs noted above. The design achieves high pressures over a wide temperature range, allows for operation with corrosive gases, has rapid gas mixing, enables variation of gas flow rates, has viewing windows, and can be used for a range of coal or char sizes. Information obtained from the high pressure TGA (17)Ragland, K.W.; Yang, J. Combustion of Millimeter-Sized Coal Particles in Convective Flow. Combust. Flame 1985,60, 285-297. Scaroni, A. W. Prediction and Measurement of the (18)Huang, G.; Combustion Time of Single Coal Particles. Fuel 1992,71, 159-164.
Measurement of Reaction Rates of Millimeter-Sized Coal
Energy & Fuels, Vol. 9, No. 2, 1995 297
f 1
I5.2cm
t
Po\iiiimcr
I 2.6 meters
Collertion I'rohc > L
Figure 1. High-pressure controlled profile (HPCP)reactor cross section.$
designs indicated that a spatial separation from the burning particles and the mass loss measuring device should exist, a constant ambient temperature should be maintained for the mass loss device, and a mesh platform should be used to support a particle during combustion. Previous designs also implied that the time required for one test was long (1h or longer); therefore, minimizing the time required to reach steady-state conditions between tests was sought. In an effort to maximize usage of existing equipment and to minimize cost, the existing HPCP was adapted for this study.s This reactor has several desirable features that provide for low-cost implementation for large coal combustion studies, including rapid gas mixing, calibrated flow meters, a pressurized combustion chamber, thermocouples for accurate gas and wall temperature profiles, a gas collection system, computeroperated stepper motor and heaters, and quartz viewing windows. The original design of the HPCP reactor (shown in Figure l ) , however, did not accommodate the use of large coal particles; therefore, a cantilever balance attachment (CBA) was designed to be attached to the HPCP reactor. With some minor modifications to the HPCP reactor, the CBA was used for continuous measurement of particle weight loss during combustion. 3.2. Cantilever Balance Attachment (CBA). The primary purpose of the CBA was to insert and remove a large coal particle and provide mass loss information of that coal particle during devolatilization and oxidation. The HPCP reactor had several fixed parameters that constrained the design of the CBA. The gas flow of the HPCP reactor was downward, the optical viewing ports were located a significance distance (ca. 1m) from the top of the hot reaction tube, and it had fixed maximum pressures (1.52 MPa) and fixed maximum temperature ( 1700 K). A schematic diagram of the CBA
I
Stcppr mnior
Figure 2. Schematic diagram of cantilever beam attachment (CRA).
is shown in Figure 2. The CBA is 86 cm long with 17 cm 0.d. The CBA is mounted on either side of the HPCP reactor at the location of the quartz windows (see Figure 1). A 15 to 8 cm reducer allowed the 15 cm outsidc. diameter of the CBA to be attached to the 8 cm pip2 flange used to support the quartz window. A 4 in threadolet was welded toward the front of the shell to provide access into the vessel. This access allowed for changing of a coal particle, which was located in a platinum mesh basket. Though possible catalytic effects of platinum were considered, the upper operating temperature range was beyond that recommended for stainless steel (1200 K). Further, a large part of the particle was above the basket and first encountered the hot gas. The threadolet was sealed with a 10 cm plug and Teflon tape during testing. Additions to the outside of the shell consisted of a fitting for a high-pressure gas line, two modified fittings for electrical feed lines, a pressure gauge, and four manual positioners. The 18.4 cm diameter by 1.3 cm thick steel plate was bolted to the vessel. An O-ring provided the necessary sea!. While in use, the outer shell was supported in place wit3 a custom-built, adjustable stand. A Transducer Techniques Model GS30 load cell with a range of 0-30 g and an accuracy of f0.015 g was selected to provide the continuously measured mass loss data. A 38 cm ceramic rod was cantilevered from a pivot arm which was rigidly attached to the force transducer (see Figure 2). A platinum mesh basket was attached with high-temperature epoxy to the other end of the cantilevered rod. The sensitivity of the load cell to changes in mass at the location of the platinum basket depends on the length of the cantilevered rod. With the apparatus described, the transducer sensitivity was equal to f0.0014g at the location of the platinum basket where the coal particle rests. The force transducer was mounted onto a transducer holder which traveled, by the aid of two bearings, on a 1.3 cm stainless steel rod. Translation of the transducer assembly was enabled by a threaded ball screw rotated by a stepper motor. A computer-controlled stepper motor turned the 1c v . ball screw at a rate of 50 cndmin. Other rates werz possible but caused excessive vibration. This moved tl.2 platform, and hence the force transducer, the cantilever rod, the platinum mesh, and the coal particle. This provided the motion into and away from the reaction tube. The maximum travel distance was 56 cm. This provided the minimum distance of 40 cm needed for ii ceramic rod to reach the center of the HPCP reactor with enough clearance for the bearings and also for the installation of limit switches. The limit switches were installed on both ends of the 1.3 cm stainless steel rod. The front limit switch (reducer end) positioned the
298 Energy & Fuels, Vol. 9, No. 2, 1995
particle inside the HPCP reactor, while the rear limit switch positioned the platinum mesh underneath the threadolet. Manual positioners were used to translate the cantilever rod vertically up and down at a rate of 1 mm per full rotation of the screw. Also, two manual positioners were used to locate the particle in the center of the reaction tube and to remove any tilt that occurred when the CBA was attached t o the HPCP reactor. The manual positioners rested on universal joint assemblies. Larger drawings of individual parts are given in Bateman.lg A custom, high-pressure access valve was designedlg and fabricated to seal the pressure. The valve was used t o seal the CBA from the elevated pressures of the HPCP reactor during the loading and removal of a coal particle through the threadolet opening. The valve allowed the HPCP reactor to maintain steady-state conditions during a set of similar runs. The valve was water-cooled and maintained the CBA at about ambient temperature during the entire testing time. To pressurize and depressurize the CBA, a stainless steel tee fitting was inserted into the secondary gas flow line (going to the preheater). A second line was run to the vent line of the HPCP reactor flow panel. The new panel had two valves that allowed pressurized gas into or away from the CBA. A type “ S thermocouple inside a Mullite tube20 was used to acquire gas temperature profiles inside the reaction tube. The probe was located in the center of the reaction tube approximately 1 cm above the large coal particles. The gas temperature was displayed with a monitor. 3.3. Alignment and Calibration. CBA realignment was necessary if the linear motion assembly, consisting of the stepper motor, the threaded ball screw, and both universal joints, had been removed from the shell assembly. Electrical connections could be made when the linear motion assembly was inside or outside the shell. Operation outside the shell allowed the limit switches to be set, the cantilever rod to be positioned, and other calibrations of the linear motion assembly to be made. The linear motion assembly was aligned outside the shell. Alignment was performed by placing the linear motion assembly on a flat table, supporting both universal joints, and using a square to make sure the stainless steel rod and the threaded ball screw were parallel. The cantilever rod was required to be parallel to the rod and shaft and to be aligned with the hole in the universal joint. When inserting the linear motion assembly into the shell assembly, care was taken not t o bump the electrical lines to prevent short circuits. With the cantilever rod removed from the force transducer and the shipping pins installed in the force transducer (to prevent breakage), the linear motion assembly was inserted. The connection between the computer and the stepper motor was made when the linear motion assembly was nearly inserted (within 5 cm). With the stepper motor connection made, the manual positioners were put into place. The addition of a sealing compound to the positioners helped seal (19) Bateman, K.J. Millimeter Sized Coal Particle Combustion at Elevated Pressure. M.S. Thesis, Department of Mechanical Engineering, Brigham Young University, Provo, UT, 1993. (20)Monson, C. R. Char Oxidation at Elevated Pressure. W.D. Dissertation, Department of Mechanical Engineering,Brigham Young University, Provo, UT, 1992.
Bateman et al. Table 1. Analysis of Utah, Blind Canyon, HVBB Test Coal Proximate Analysis (wt %, as received basis) moisture ash volatile matter fixed carbon cal value (kJkg) 8.7
8.2
37.2
45.9
26857
Ultimate Analysis (wt %, dry basis) carbon
hydrogen
nitrogen
sulfur
oxygen
72.9
4.8
1.12
0.51
11.7
pressure and kept the positioners rigidly in place. When the linear motion assembly was completely inserted, the set screw rigidly mounted the linear motion assembly into the shell. The force transducer was then connected to the computer display, and the wire was attached so as to be out of the way. With the linear motion assembly mounted inside the shell, the force transducer was moved to the front of the CBA. Careful removal of the shipping pins and insertion of the cantilever rod into the force transducer allowed for calibration of the force transducer. Calibration weights of 0.1 and 0.2 g were placed on the platinum mesh. The digital display showed the mass loss of each run. The 0.1 g sample usually read around 18 counts, and the 0.2 g read around 36 counts. 4. Facility Demonstration 4.1. Facility Capabilities. The HPCP reactor with the CBA attached is capable of providing reactivities of large (up to 8 mm diameter) coal particles in various gas compositions and velocities at pressures up to 1.52 MPa and gas temperatures up to 1700 K with visual access. The following presents a series of coal combustion tests performed in the facility t o demonstrate its utility as a powerful research tool. 4.2. Test Coal. A Utah, Blind Canyon, high-volatile B, bituminous coal, one of the eight coals in the Argonne National Laboratory Premium Coal Sample Bank,21was selected for initial tests to demonstrate the facility. An analysis of the coal is given in Table 1. Coal particles were prepared by fracturing lump coal. The outer edges of the pieces were then chipped off, using pliers, until the particles approximated spheres. The weight of each particle was about 0.20 g (approximately 8 mm diameter). An 8 mm diameter particle was the maximum size that could pass through the window of the HPCP reactor. Results for particles of 5.5 mm (half the mass of 8 mm particles) are also reported by Bateman et a1.22 Particle sizes are similar to those used by other investigators at atmospheric condition^.^^^^^ VorresZ1 and Smith et al.24provide substantial additional details on the properties of the test coal. 4.3. Test Program. A series of 23 combustion tests (described in Table 2) were performed to confirm the test procedure described below and to demonstrate the (21) Vorres, K. S. The Argonne Premium Coal Sample Program. Energy Fuels 1990,4 , 420-426. (22) Bateman, K.J.;Germane, G. J.; Smoot, L.D.; Blackham, A. U. Effect of Pressure on Oxidation Rates of MM-Sized Char. Fuel, submitted for publication. (23) Blackham, A.U.; Smoot, L. D.;Yousefi, P. Rates of Oxidation of Millimetre-Sized Char Particles: Simple Experiments. Fuel 1994, 73, 602-612. (24) Smith, L.IC;Smoot, L. D.;Flecther, T. H. Coal Characteristics, Structure and Reaction Rates. Fundamentals of Coal Combustion; Smoot,L.D., Ed.;Elsevier: The Netherlands, 1993;Chapter 3, pp 131293.
Measurement of Reaction Rates of Millimeter-Sized Coal Table 2. Test Program for Utah HVBB Coal in HPCP Reactor test no. of press. gas temp Reynolds particle approx mass (g) dia(mm) series tests (@a) (K) no. 0.2 8.0 900 126 1 8 101 0.2 8.0 2 8 507 900 126 0.2 8.0 3 6 757 900 126 4 1 507 1200 63 0.2 8.0
facility's capabilities. For three selected pressures, multiple tests were performed a t the same conditions to determine test reproducibility. Tests for repeated runs performed sequentially and on different reactor operation days were included for statistical analysis. Air flow rate was set at a Reynolds number (based on coal particle diameter) of 126 for these illustrative tests. The value was in the range of studies by Ragland and Yang17 and Blackham et al.23at atmospheric pressure. A single test at a gas temperature of 1200 K was performed to demonstrate the ability of the two-frequencypyrometer to provide particle temperature traces, and t o demonstrate operation at higher temperature and lower air flows. 4.4. Test Procedure. With the cantilever rod calibrated and backed into the shell, the CBA was carefully attached to the HPCP reactor, with highpressure access valve in between. When a light was directed through the threadolet opening, the position of the CBA was observed from the opposite quartz window, and alignment adjustments were made to the CBA. The bolts held the CBA in its aligned position. The water inlet and outlet hoses were then connected to the valve. The gas line from the flow panel was then connected t o the CBA. With the valve open, the cantilever rod was inserted into the HPCP reactor to align the platinum mesh with the center of the reaction tube. Adjustments were made with the manual positioners. With the cantilever out of the HPCP reactor, the valve was then shut and the desired temperatures were set. The heaters were controlled by a microcomputer which provided the ability to set the desired temperature for each heater zone.8 The heat-up process took approximately 4 h for low-temperature, atmospheric tests and approximately 12 h for high-temperature, elevated pressure tests. The gas temperature probe provided the temperature inside the HPCP reactor. Longer heat-up time was required for high-pressure tests due to the increased heat loss through the reactor walls at elevated pressure.20 A video camera was placed at a quartz window located 90" around the periphery of the HPCP reactor to visually record the particle combustion process. This was continuously displayed on a 15 cm monitor and also recorded on the VHS tape at 1/30 s frame speed. The camcorder was manually focused on the center of the reaction tube. A second camcorder recorded the monitor, the gas temperature display, and the mass loss display. Both camcorders recorded at 30 frame&. An optical two-frequency pyrometer was used to measure surface temperature through a quartz viewing window with results recorded on a strip chart recorder. The operational limits of the pyrometer made it possible t o only obtain surface temperature measurements with gas temperatures of 1200 K or higher. During devola-
Energy & Fuels, Vol. 9, No. 2, 1995 299
tilization, the particle surface could not be directly observed due to the volatiles cloud surrounding the particle. The coal particle was placed on the platinum mesh through the threadolet with a pair of long tweezers. The gas velocity was set using the manual valve on the HPCP reactor flow panel. The particle was inserted into the HPCP reactor by engaging the stepper motor. Both camcorders were turned on during the 75 s the particle took t o enter the reaction tube, thus allowing the entire combustion process to be recorded. With the pyrometer operating, the strip chart recorder was started. The limit-switch stopped the coal particle in the center on the reaction tube, allowing the preheated air to flow over the top of the coal particle. To calibrate the strip chart recorder, the temperatures displayed on the pyrometer monitor were periodically noted on the strip chart. When the coal particle was no longer visibly burning and the force transducer no longer showed a significant mass loss, the test was determined to be finished. The particle support basket was withdrawn from the HPCP reactor and the instrumentation was shut down. The coal ash was removed from the platinum mesh and stored. The entire process averaged 5 min for the atmospheric pressure runs and about 15 min for the elevated pressure runs. Video analysis provided an independent qualitative check on particle mass loss and temperature, as well as combustion visualization. Particle homogeneous ignition was defined as the moment that a gaseous flame was first visible, which was often very bright and always very sharp and could be measured to within 1/30 s. Char oxidation was defined as the moment the particle began to glow brightly, whether before devolatilization or after. Most commonly, char oxidation preceded devolatilization and char oxidation time was burnout time less devolatilization time. Burnout was defined as the point when the particle no longer glowed and mass remained constant. Burning time was defined as the total time from particle ignition through particle burnout. Occasionally, oxidation and burning times were uncertain due to excessive particle fragmentation prior t o completion of burnout. 5. Test Results Representative particle mass loss traces at 101 and 507 kPa recorded from CBA strain gauge measurements are shown in Figure 3. Both have near-logarithmic mass loss traces with mass loss rate decreasing with time. The 507 kPa case exhibited a higher mass loss rate than the atmospheric case as indicated by the steeper slope on the mass loss trace and shorter overall burning time. Video records of the burning particles generally show distinct zones of particle heat-up, homogeneous ignition and burning of volatiles, and heterogeneous char oxidation. Volatiles ignition occurred within the time space between consecutive video frames which were 1/30 s. The volatiles burned as small flamelets jetting out from pores or fissures in the structure of the coal particle. Video records of the tests indicated that the devolatilization periods were about 25 and 15 s for the 101 and 507 kPa cases, respectively. This corresponds to 5% of the total burning time for each case.
300 Energy & Fuels, Vol. 9,No. 2, 1995
0 ~
0.15
Et
Region 1 = devolatilization Region 2 = char oxidation
3
g
Bateman et al.
0.10
E
-,g iy
k
0.05
0 0
200
1(H)
300
4oc
500
Time (sec)
Figure 3. Traces of mass loss using CBA attachment to H P C P reactor with U t a h HVBB coal, 0.2 g, H P C P reactor temperat u r e 900 K, a i r Re = 126.
Determination of the end of devolatilization and beginning of char oxidation was less certain than volatiles ignition due to the gradual decline of the volatiles flamelets and onset of heterogeneous char oxidation but could generally be resolved to within about 1 s. No obvious change in mass loss rate was observed during the transition from devolatilization to char oxidation (see Figure 3). The end of char oxidation could be resolved to within about 2 s. Some particle shrinkage was noted with the final diameter of the ash particle being between 75 and 90% of the original particle diameter. One particle tested at 507 kPa exhibited substantial ash layer collapse and had a final diameter of only about 40% of the original diameter. Table 3 summarizes the total burning and char oxidation times from the large particle demonstration tests performed in the HPCP reactor. Total burning and char oxidation times for one of the eight 507 kPa tests were not obtained since the particle exploded during devolatilization. Some tests for each condition were performed on different days so that day-to-day variation in facility operation could be included in the statistical evaluation. The standard deviations for total burning and char oxidation times for the 101 and 757 kPa tests are about f 1 0 % of the mean values. Total burning and char oxidation times for one of the 507 kPa tests was significantly faster (177 and 162 s, respectively) than the others, resulting in higher standard deviations (about f 1 7 % of the mean times). Observations made from a video record of the burning particles showed a significant collapse and possible sloughing of the ash layer around the burning particle during char oxidation for the test with the exceptionally fast burning rate. The final-to-initial particle diameter ratio for this case was measured as 0.4 while the ratios for all other tests were between 0.7 and 0.9. Reduction of the resistance to oxygen diffusion created by the ash layer would cer-
tainly increase the particle burning rate, particularly in the diffusion-controlled regime. Eliminating the fast burning data point from the statistical analysis provides standard deviations for total burning and oxidation times of 9 s for each. However, each of the remaining tests was sequential and, therefore, does not contain day-to-day variation in facility operation. Since it was determined that the detection limits of the two-frequency optical pyrometer allowed measurement of particle surface temperature only when air temperatures were 1200 K or higher, a test was performed with an HPCP air temperature of 1200 K. The particle surface temperature trace obtained during this test is shown in Figure 4 along with segments from the video record showing various stages of combustion. Highly fluctuating temperatures were noted during devolatilization due to the coruscating flame. During the early stages of char oxidation the particle temperature was nearly constant at about 1380 K. After 100 s the measured temperature decreased to about 1220 K. This decrease may have been caused by the particle's ash layer dropping below the pyrometer's focal point and therefore more nearly measure the far reactor wall temperature. It was discovered that, a t elevated pressure, a higher reactor wall temperature was required to maintain a desired gas temperature than was required at atmospheric pressure. This is likely due to the observed increase in heat loss through the reactor walls with elevated pressure.20 The implication is that the increased particle reactivity observed with pressure may be partially attributed to the higher wall temperatures and therefore higher particle surface temperature. Therefore, the dependence of particle temperature on reactor wall temperature must be considered when interpreting results. 6. Conclusions The objective of this study was to design, characterize, and demonstrate a new facility for determination of reaction rates of large (8 mm diameter) coal particles at elevated pressures. To this end, a cantilever balance attachment was designed, fabricated, and utilized, in conjunction with the HPCP reactor, to perform coal combustion experiments of large particles at both atmospheric and elevated pressures. The facility allows particle reactivity measurements to be made in a selectable gas composition and flows with optical access at temperatures and pressures up t o 1700 K and 1.52 MPa, respectively. The unique high-pressure access valve significantly reduced the time required for oxidation tests compared with existing facilities. Single particle combustion tests of Utah HVBB coal, performed at 101 and 507 kPa and at a gas temperature of 900 K, demonstrated the ability to obtain meaningful reaction rate measurements using the facility, with standard deviations of about f 1 0 % of the mean combustion times. Much of this variation is due to natural
Table 3. Test Results for Utah,Blind Canyon (HVBB) Coal test series
no. of tests
press.
gas temp
(kPa)
(K)
Reynolds no.
mean burning time (9)
std dev burning time (s)
mean oxidn time (s)
std dev oxidn time (s)
1 2 3 4
8 8 6 1
101 507 757 507
900 900 900 1200
126 126 126 63
435 282 302 258
39 47 30
409 259 280 239
40 44 29
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Measurement of Reaction Rates of Millimeter-Sized Coal
Energy & Fuels, Vol. 9, No. 2,1995 301
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Time (s) Figure 4. Surface temperature of Utah HVBB coal, 0.2 g, 507 P a , HPCP reactor temperature 1200 K, air Re = 63.
variations among individual coal particle properties. Results show mass loss rate increasing with increasing pressure. A test at a gas temperature of 1200 K demonstrated the ability to simultaneously collect particle surface temperature and mass loss data. A video record of the combustion process allowed visual observations to be correlated with burning regimes of particle heat-up, devolatilization, and char oxidation, substantially enhancing the understanding of the combustion phenomena and measurements. Sudden homogeneous ignition of volatiles was noted. Volatiles burning occurred in small flamelets jetting from pores in the coal structure while particle glowing was characteristic of char oxidation. Surface ignition and particle explosion were also observed.
Acknowledgment. This study was funded by the Department of Energy, Morgantown Energy Technology Center (Dr. Norman Holcombe, project officer) through a joint contract with Advanced Fuels Research, Inc. The Advanced Combustion Engineering Research Center (ACERC) at BYU, sponsored by the National Science Foundation's Engineering Education and Research Centers Division (Dr. Tapan Mukherjee, project officer),also financially contributed to the construction of the facility and completion of the project. The U.S.EPA, the State of Utah, and over 30 industrial companies also participate in ACERC. EF940136G
