Energy & Fuels 1991,5,587-594
587
Combustion Characteristics of Carbonaceous Residues from Heavy Oil Fired Boilers P. Scott Northropt and George R. Gavalas Departments of Chemical Engineering and Environmental Engineering Science, California Institute of Technology, Pasadena, California 91125
Yiannis A. Levendis* Department of Mechanical Engineering, Northeastern University, Boston, Massachusetts 02115 Received October 26, 1990. Revised Manuscript Received February 21, 1991
The combustion of solid carbonaceous particles generated in residual oil-fired furnaces has been investigated. These particles are large ash-rich cenospheres that are not completely burned in the furnace and end up in the effluent gases, creating problems associated with both emissions and reduced operating efficiency. To study the reactivity of these chars, single-particle combustion experiments were conducted where the particle temperature and burnout times were measured by near-infrared two-color pyrometry. The results show that the particle temperatureburn time traces exhibited higher particle-to-particle variability than those of coal-derived char particles, but the overall burnout times were of the same order of magnitude as coal chars of similar size. Taking into account random particle-bparticle property variations, and using a particle combustion model, reaction rate parameters were estimated. Two limiting forms of the model were used for this estimation. The first assumed the particles to burn as shrinking cores, with all reaction concentrated very near the external particle surface. The second assumed the particles to burn at constant radius, with increasing void fraction. Using first-order kinetics, the shrinking core model gave an average apparent activation energy of about 18 kcal/(g mol), with a corresponding average preexponential factor of approximately 37 g/(cm2 s atm of OJ. These values are similar to the respective parameters estimated for lignite and bituminous chars burned under similar conditions, indicating that these particles can be burned at temperatures and residence times similar to those existing in coal-fired furnaces. The constant radius model gave an intrinsic activation energy of 40 kcal/(g mol), with a preexponential factor of 550 g/(cm2 s atm of 02). The constant radius model gave a slightly better fit to the experimental data.
Introduction In the combustion of heavy fuel oils, coke particles may be formed in regions of the furnace where the availability of oxidizer is inadequate, or when heat is transferred from the particle too quickly (premature extinction). The mechanisms and kinetics of coke particle formation are not well understood;’ however, coke particle formation has been observed to occur in the final stages of droplet burningS2 Particles formed in this manner are hard, porous shells that often contain a large central void and many blowholes2+ and are thus known as cenospheres. The particles often contain high concentrations of impuritie~.~ Typical coke particles comprise about 3% (by mass) of the original fuel droplet? but their volume is considerably larger because of their low density. The cenospheric coke particles generally have detrimental effects on oil-furnace operation. They cause erosion (by impinging on solid surfaces) and inhibition of heat transfer (by depositing on boiler pipes). Furthermore, the operational efficiency of the power plant is reduced somewhat, since as much as 3% of the fuel mass (associated with the cenospheres) could remain unburned. Finally, emissions of such particles, containing compounds of vanadium, nickel, sulfur, sodium, etc., pose environmental problems, such as “acid They are also a fire hazards, through self-ignition6 at the exhaust clean-up devices. Current address: Mobil Research and Development Corp., P.O. Box 819047, Dallas, T X 75381.
To eliminate the problems related to the carbonaceous cenospheres, recent studies have focused on identifying conditions that suppress particle generation. Experimental studies, conducted elsewhere,’ on the formation of these particles using isolated droplet techniques have shown that the formation is primarily dependent on the initial fuel properties, with little influence from droplet size, environmental temperature, and oxygen concentration. Thus, it is important to determine the conditions under which, once formed, these particles will burn completely in the combustion chamber or in an afterburner device. Studies on the oxidation behavior of oil coke particles have been reported in the literature, at both low8 and high temper(1) Flagan, R.C.; Seinfeld, J. H. Fundamentals of Air Pollution Engineering; Prentice Hall: Englewood Cliffs, NJ, 1988. (2)Marrone, N.J.; Kennedy I. M.; Dryer, F. L. Combust. Sci. Technol. 1984, 36, 149. (3)Clayton, R. M. On the Phyeical and Chemical Characteristics of Cenospheres from the Combustion of Heavy Fuel Oil. Presented at the Fall Meeting of the Western States Section/The Combustion Institute, October 22-23, 1984. (4)Gavalas, G. R.; Loewenberg, M.; Bell&, J.; Clayton, M. Structure and Oxidation of Carbonaceous Cenospheres. Presented a t the annual AIChE meeting, Chicago, November 1&15,1985. (5) Braide, K. M.; Isles, G. L.; Jordan, J. B.; Williams, A. J. Znat. Energy 1979,52, 115. ( 6 ) Holmes, R.;Purvis, M. R. I.; Street, P. J. Combust. Sci. Technol. 1990, 70, 135. (7) Huey, S.;Dryer, F. L.; Heilweil, 1.; Yettar, R. A. Some Obeervationa on the Oxidation Properties of Heavy Fuel Coke Particulates. Presented at the Fall Meeting of the Eastern Section of the Combustion Institute, December 5-8, 1990. (8) Tyler, R. J. Fuel 1986, 65, 235.
0887-0624/91/2505-0587$02.50/00 1991 American Chemical Society
588 Energy & Fuels, Vol. 5, No. 4, 1991
atures! The preaent investigation is aimed at investigating the high-temperature kinetics of oil coke. It complements previous studies by incorporating direct measurements on the particle surface temperatures and individual particle burnout times. To obtain information about the burnout kinetics of oil generated coke cenospheres, two-color pyrometry has been employed as the principal kinetic measurement. Porosimetry was used, in parallel to the pyrometry measurements, to assist in the interpretation of the data. The monitoring of temperature-time histories of single particles burning within the hot reactor tube is subject to significant error due to the low signal-to-noise ratio and the interference of radiation from the furnace walls reflected by the particles. This becomes a significant component of the overall radiation when particle temperatures are low. To reduce both of these errors, in addition to studies in air (21% OJ, high oxygen partial prwurea were used resulting in particle temperatures in excess of 2000 K. These high oxygen partial pressures and the associated particle temperatures exceed those prevailing in typical oil-fired furnaces. It is hoped, nevertheless, that the combustion rate parameters derived herein can be used to estimate burnout times under conditions representative of commercial operation. This project is part of a larger study aimed at evaluating multicomponent fuel-spray combustion and predicting the performance of furnaces and boilers fired with arbitrary liquid fuel~.~s'*~~ To this end, development of theoretical models has been undertaken and experiments have been conducted with the cenospheric oil coke particles described herein. Experimental Procedures Particle Characterization. The cenosphere samples under examinationhave been obtained at the exit of a utility boiler fired with No. 6 residual oil. Exact information on the operating conditions of the boiler, during the time that the samples were collected, was not available. However, it was reported that oil coke particles were always present at the exhaust of the furnace. Selected characterization experiments were conducted in the present study to obtain parameters pertinent to the deviation of reaction rates. For comparison purposes, the interested reader can f i d elsewhere3an extensive characterizationstudy of similar cenospheres, obtained earlier at the exhaust of the same boiler. Total surface areas were measured by N2 gas adsorption at 77 K by using a rapid pseudostatic technique with a custom-built a p p a r a t ~ . ' The ~ same equipment was used for skeletal density determination using helium gas. Sample quantities of 0.2 and 0.42 g were used for the surface area and helium density determinations, respectively. Apparent densities, porosities, pore volume, and pore surface distributions for pores with openings in the range between 6.4 nm and 7 Nm were determined by mercury intrusion using a Quantachrome Autoscan porosimeter capable of achieving pressures of 225 MPa. Optical and scanning electron microscopy (SEM)observations were made using a CamScan electron microscope. SEM micrographs of particle cross sectionswere also obtained from samples of chars cast in epoxy at atmospheric pressure. These casts were polished in a Buehler Minimet automatic polisher and subse(9) Maseier, P. F.; Bellan, J.; Kwack, E.; Shakkottai, P.; Haratad K.; Gavalas, G. R. "Spray Combustion Modelliig and Evaluation". DOEECUT Pro am,Annual Summary Reporta for CY 1987 and 1988 (JPL D-4857 ant&-5699). (10) Bellan, J.; Harstad, K. Combust. Sei. Technol. 1987,53, 76-87. (11) Bellan, J.; Harstad, K. Combust. Flame 1990, 79, 272-286. (12) Kwack, E.; Shakkottai,P.; Massier, P. F.; Back, L. H. Visual Observations of Particulate Structure in Heavy Fuel Oil Burner Experiments. Submitted for publication in the ASME J. Heat Transfer. (13) Northrop, P . S.;Gavalas, G. R.; Flagan, R. C. Langmuir 1987,3, 300.
Northrop et al.
quently gold-coated. Particles that had been sectioned near their center, i.e., those images with the largest diameters,were selected for analysis. Elemental analysis for carbon, hydrogen, and nitrogen was performed by a Perkin Elmer analyzer. Combustion Experiments. The particles were burned in a laminar-flow,drop-tube furnace that was externally heated by Kanthal Super 33 molybdenum disilicide heating elements, described elsewhere." The particles were injected through a water-cooled injector into a 5 cm i.d. alumina tube at the top of a 20 cm long hot zone. Furnace wall temperatures were measured by permanently installed thermocouples, as well as by a disappearing filament optical pyrometer. Gas temperatures were measured with a suction pyrometer at appropriate locationsalong the axis of the tube. Particle combustion temperatures were measured by a nearinfrared two-color (800 and loo0 nm) optical pyrometer." Observations were made from the top of the injector viewing downwards along the axis of the furnace (and the trajectory of the particles)against a cold background. Radiation emitted from burning particles was transmitted through a bifurcated optical fiber bundle to the pyrometer. Pyrometry measurements were conducted in a single-particle mode. Particle intensity-time profiles were recorded for the completeparticle combustion history from ignition to extinction in a microcomputer for many particles for various sets of furnace wall temperatures and oxygen concentrations. Using Planck's radiation law these two-color traces were converted to temperaturetime profiles. The pyrometer was calibrated at the temperature of melting ~1atinum.l~ Results and Discussion Particle Properties. Several samples collected from the same power plant were mixed together to obtain average bulk properties. The particles were subsequently sieved; two different size cuts, 38-45 and 53-74 pm, were selected for analysis. Optical microscopy revealed that these size cuts consisted of whole (unfragmented) spheroidal particles. The total N2 BET surface area of the smaller particles was found to be 6.7 m2/g, and that of the larger particles were 3.5 m2/g. The apparent density, as determined in mercury at 1 atm, was 0.63 g/cm3. At this pressure, mercury penetrates only pores which have openings larger than ca. 7 pm. Since the cenospheres occasionally have blowholes larger than 7 pm, the apparent density of the chars may actually be somewhat lower. A conservative estimate of 0.42 g/cm2 can be deduced from Clayton's finding^,^ after correcting for the interstitial space (assumed for spheres of equal size). The mercury density, based on the volume of solid which includes all pores having diameters less than 6.4 nm (64 A), was calculated to be 1.94 g/cm3. The average skeletal or helium density was found to be 2.02 g/cm3. The pore size distribution was calculated assuming cylindrical pores and using the Washburn equation:I6 rp = -2a cos alp, where rp is the pore radius, P is the intrusion pressure, u is the surface tension of mercury, and a is the contact angle, taken to be 140O. Pore surface area the volume of the 38-45-pm particles are plotted versus pore radius in Figure 1. As expected, most of the pore surface area resides in the smaller pores, while the majority of pore volume is due to the larger pores. Figure l a indicates the existence of transitional pores of diameters 6.4-20 nm. The upward trend at the left-hand corner of the pore size distribution suggests the presence of even smaller pores, which are not accessible to mercury at 225 MPa. Parts a and b of Figure 1 both indicate a large (14) Levendis, Y. A.; Flagan, R. C. Combust. Sci. Technol. 1987,53, 117. (15) Scholten, J. J. F. In Porous Carbon SoliB, Bond, R. L., Ed.; Academic Press: London, 1967.
Combustion Characteristics of Carbonaceous Residues I
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Figure 1. Pore surface area and volume distributions of the 38-45-pmparticles, obtained by mercury intrusion. concentration of macropores with sizes from one to a few micrometers which are also evident in the SEM observations. The total mercury surface area of pores in the range of 6.4 nm to 7 pm was calculated to be 4.5 m2/g for the particles of the smaller size cut. SEM micrographs of the exterior and the interior of the cenospheric particles are presented in Figure 2. A more detailed SEM characterization for similar particles of various size is given el~ewhere.~ It is observed that the particles are spongelike spheroids that usually have one or more large voids communicating with the outside through blowholes ranging from 1 to 10 pm. Elemental analysis revealed that the carbon content of the material was approximately 70%, while the hydrogen was 1.4% and the nitrogen was 1.0%. The remainder, unidentified in the present study, was primarily ash and oxygen. Combustion Behavior. Combustion experiments for each particle size cut were conducted at oxygen partial pressures of 0.5 and 0.21 atm (air). The furnace wall temperature was varied from 1000 to 1450 K, with corresponding gas temperatures being approximately 100 K lower than those of the wall. Measured particle temperatures ranged between 1800 and 3000 K. Selected particle temperature-time profiles are shown in Figures 3-5. Figure 3, a and b, depicts profiles for 53-74-pm particles burning in 0.5 atm of 0,with a furnace wall temperature of 1450 K. Burnout times and temperatures differed significantly among these highly inhomogeneous particles. The average burn times for the particles depicted here range from 15 to 22 ms, and the particle temperatures vary from 2000 to over 3000 K. Similar results were obtained from other particles burning under the same conditions. Moat of the traces indicated burnout times of about 18 ms and average temperatures of about 2400 K. Many of the
53-74-pm particle traces exhibited a shallow local maximum in temperature near mid burn time, and an absolute maximum near the end. Spikes in the temperature trace appear a t both the beginning and the end of the trace. Those appearing a t the beginning of the signal could be due to residual volatile release when the particle enters the hot zone of the furnace, if the temperatures to which they were previously subjected in the power plant furnace were lower than the temperatures of the drop-tube furnace used in this study. The spikes that appear at the end of the traces may be caused by fragmentation of the increasingly friable particles or by diminished signal to noise ratio near complete conversion. Combustion of the smaller particles (38-45 pm) under similar conditions (0.5 atm of 02,1450 K wall) resulted in shorter burnout times (8-12 ms), as expected. Most of the traces indicated monotonically increasing particle temperatures during combustion. Average particle temperatures were generally higher than those of the larger particles, as seen in Figure 3, b and c, probably as a result of enhanced oxygen diffusion to the smaller particles. Two examples of large particles (53-74 pm) burning in air (0.21 atm of 0,) at a wall temperature of 1450 K are depicted in Figure 4, a and b. The observed burnout times for this group of particles ranged from 26 to 33 ms, and the observed temperatures averaged 2000-2100 K, substantially lower than those in 50% OP The traces showed a variety of different shapes; most appeared to have shallow maxima just prior to burnout. Examples of traces for large particles burning in 50% O2at a furnace wall temperature of 1200 K are shown in Figure 5, a and b. The particle temperatures are high, reaching or exceeding 3000 K in some cases, and most particles seem to burn as hot or hotter than those burned at a furnace wall temperature of 1450 K. Burnout times are also significantly shorter, ranging from about 9 to 14 ms. The smaller particles also burned at high temperatures at wall temperature 1200 K, as seen in Figure 6, a and b. Burnout times for the 38-45-rm particles ranged from 6.5 to 8.5 ms. When the wall temperature was lowered to lo00 K, the temperature traces for the large particles were found to be either relatively flat or monotonically increasing (Figure 7, a and b). Surprisingly, the temperatures were often observed to exceed 3000 K. Burnout times were also short, being around 12 ms in most cases. The high particle temperatures observed at low furnace wall temperatures are surprising, but not inexplicable. The pyrometer “sees” only those particles that reach a certain minimum threshold temperature. It was not possible with our experimental setup to determine how many particles did not reach that minimum temperature; however, direct visual observations indicated that, at the lower furnace temperatures, a substantial number of particles did not ignite. Those particles that did not reach the threshold temperature underwent oxidation at a temperature close to that of the furnace wall. It is also possible that particles that ignited had already undergone oxidation a t lower temperatures for a significant period of time prior to ignition. At the highest wall temperature used, 1450 K, most of the particles appeared to ignite; however, only a fraction reached temperatures in excess of 3000 K. Similarly, in the low wall temperature runs, it is likely that only the most reactive particles ignited and were seen by the pyrometer.
Reaction Rate Calculations Reaction rate parameters for the combustion of the coke cenospheres were estimated with the aid of a kinetic com-
590 Energy & Fuels, Vol. 5, No. 4,1991
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d
Figure 2. SEM photographs of petroleum-derivedcenospheres: (a, b) particles and (c, d) polished sections through the particles.
bustion model16J7and a statistical method for analyzing the experimental timetemperature traces. This analysis was applied only to the 53-74-pm particles burning in the furnace at 1450 K, in order to ensure the ignited particles were representative of the sample. In addition, only particles burning below about 2500 K were considered for kinetic analysis. The model was applied to a total of 15 experimental traces. Model results showed that the oxygen concentration at the particle surface was generally 30-80% of its free stream value. Thus, the extemal mass-transfer resistance was significant, but not controlling. The combustion model is described in detail elsewhere.17P1 It was expanded in this work to accommodate the statistical analysis of the experimental traces. Briefly, the particles are assumed to have uniform temperature and bum in a quiescent ambient of known composition. In view of the high temperature of the burning particles the only reaction considered is the direct oxidation of carbon to form carbon monoxide:22C + (1/2)02 = CO. Stefan flow is accounted for, as is the temperature dependence of the (16) Gavalas, G. R. AZChE J. 1980,26,577. (17) Laemenberg, M.; Bellan, J.; Gavalas, G. R. Chem. Eng. Commun. 1987,58,89. (18) Young, B. C.; Smith, I. W. Symp. (Znt.) Combust. [Roc.], 28, 1981,1249-1255. (19) Smith, I. W. Combust. Flame 1971, 27,303. (20) Tyler, R. J.; Smith, I. W. Fuel 1976,54,99. (21) Sahu, R.; N o r t h p , P. S.;Flagan, R. C.; Gavalas, G. R Combust. Sei. Technol. 1988,60, 215. (22) Levendis, Y. A.; Nam, S.; W n b e r g , M.; Flagan, R C.; Ga&, G. R. Energy Fuels 1989,3,28. (23) Smith, I. W. Symp. (Znt.) Combust., [Roc.],29 1982,1045. (24) Northrop, P. S. Ph.D. Thesis, Caltech, 1988. (25) Laurendeau, N. M:Rog. Energy Combust. Sci. 1978, 4, 221.
transport properties in the gas. The reaction rate is assumed to be of the Arrhenius form, and reaction order with respect to oxygen is taken as unity. The coupled equations of heat and mass transfer in the gas phase are solved numerically. Two limiting forms of the model are considered. In the first form, all reaction is lumped at the extemal surface; Le., the particle burns in the shrinking core mode. The rate parameters estimated by this form of the model are apparent rate parameters. These parameters are based on the partial pressure of oxygen a t the surface of the particle; i.e., they were computed after molecular (boundary) diffusion was accounted for. The apparent activation energy (E,), void fraction, and initial particle radius are input into the model, along with the wall temperature, the free stream oxygen concentration, and other relevant parameters. The model is used to calculate a theoretical trace for that set of parameters. If the burnout time of the model trace is larger than that of the experimental trace being matched, a smaller value of the apparent frequency factor (A,) is input and a new model trace is calculated. Likewise, if the calculated burnout time is smaller than that of the experimental trace, a larger value of A, is used. The value of A, for which the model and experimental burnout times match is thus obtained by an iterative process; this value of A, is unique for a given set of parameters. The model trace obtained from this set of parameters is then compared to the smoothed experimental trace a t 20 points uniformly distributed in time. A residual is calculated as the s u m of squares of differences in model and experimental particle temperature at those 20 points. Thus, the relative goodness-of-fit of the model to the experimental trace is represented by a single number. The entire process is repeated for a range of initial
Energy & Fuels, Vol. 5, No. 4, 1991 591
Combustion Characteristics of Carbonaceous Residues 3 I h
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radius (25-45 pm) and void fraction (0.3-0.8) in order to find the set of model parameters yielding the smallest residual for a given apparent activation energy (10-24
Figure 4. Temperature-time profiles of two 53-74-pm particles burning in air with T, = 1450 K.
kcal). For each apparent activation energy the minimum residuals were summed for all 15 experimental traces. The plot of the normalized total of residuals versus activation energy is shown in Figure 8a. The average particle properties were obtained as a simple arithmetic average of values from the best set of parameters for the activation energy yielding the smallest sum of residuals. The second form of the model assumes the particle to burn at constant radius, with increasing void fraction. To carry out this analysis,effective" factors were dculated based on Thiele moduli neglecting, for simplicity, the changes in the surface area. The effective diffusivity is proportional to void fraction (porosity) which increases with burnout. The estimation procedure is similar to the one described above, except that the estimates are for the intrinsic rate parameters. The results are shown in Figure 8b. In summary, particle size, void fraction, and preexponential factor were considered variable from particle to particle and were adjusted by curve fitting. The activation energy, on the other hand, was regarded as unknown but common for all particles. The apparent activation energy from the shrinking core model that gave the minimum overall residual was 18 kcal/(g mol). In this regime Ei,= 2Ew = 36 kcal/(g mol). The initial radius and void fraction that correspond to the apparent activation energy of 18 kcal/mol were 30 f 5pm and 0.58 f 0.22, respectively. The preexponential factor was 37 f 27 g/(cm2 s atm of 02). The large variation in the preexponential factor reflects both variability among the particles and experimental error in the temperature measurements. The constant radius model gave an estimate of 40 kcal/(g mol) for intrinsic activation energy. This is ap-
Northrop et al.
592 Energy & Fuels, Vol. 5, No. 4, 1991
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Figure 6. Temperaturetime profiles of two 38-45-pm particles burning in 50% O2 at T, = 1200 K.
Figure 9c, where particle radius is plotted versus time. The traces of Figure 9 were calculated with void fraction 0.65, initial radius 30 pm, and preexponential factor 33.6 g/(cm2 s atm of 02).An additional case is shown in Figure 10, where the void fraction was 0.65,initial radius 27.5 pm, and preexponential factor 18.7g/(cm2 s atm of 02).For both cases the estimated apparent activation energy of 18 kcal/(g mol) was used. Figure 11, a and b, shows examples of fits using the constant radius assumption. This form of the model generated curves with maximum temperatures at the end. In view of the assumed constant radius, heat loss did not increase with conversion to cause extinction. However, increasing effective diffusivities facilitated oxygen transport to the interior of the particles, resulting in higher temperatures at the end. The trace of Figure 10a was calculated with initial void fraction 0.6,radius 30 pm, and preexponential factor 671.6 g/(cm2 s atm of 02).For the trace of Figure lob, the initial void fraction was 0.5, the radius 32.5pm, and the preexponential factor 490.5 g/(cm2 s atm of 02).The optimized intrinsic activation energy of 40 kcal/(g mol) was used for both traces. The effectiveness factor calculated for those two particles and many other particles was close to 0.2. It must be noted that the two limiting forms of the model used in the estimation of kinetic parameters are mutually exclusive and represent the two extremes in terms of the role of pore diffusion. Consequently, the 2:l ratio of the intrinsic activation energy estimated by the constant radius model to the apparent activation energy estimated by the shrinking core model must be considered somewhat fortuitous. Under these conditions there is partial oxygen penetration within the particle and combustion takes place at constant radius for a certain initial
Energy & Fuels, Vol. 5, No. 4, 1991 693
Combustion Characteristics of Carbonaceous Residues I
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Figure 9. (a) Comparison of experimental and calculated temperatures for a residual-oil coke particle, (b) calculated particle temperature versus radius, and (c) radius versus burnout time. The calculations were performed with the shrinking core model with E, = 18 kcal/(g mol) and void fraction 0.65, radius 30 pm, A, = 33.62 g/(cm28 atm of 02).
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period followed by a period in which both radius and density decrease." The constant radius form of the model yielded somewhat lower residual error, but the results
clearly indicate intermediate behavior. The estimates of activation energies and preexponential factors must be taken with caution in view of the known compensation between those two parameters and in view of the adjustment of particle radius and void fraction for each particle. Comparison with Coal Chars. This comparison has been made to asses the rates of combustion of the residual oil cenospheres in relation to those of coal chars, in particular cenospheric bituminous chars. Since coal (and coal char) combustion has been extensively examined in the literature,l*m~l~*it was thought useful to conduct such a comparison to put the present results into perspective. Similar comparisons between coal chars and petroleum coke have also been conducted in the past.= However, for such a comparison to be unambiguous the present results
594 Energy & Fuels, Vol. 5, No. 4, 1991 4
Northrop et al.
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has been previously employed for bituminous and lignite chars,?-1* with the exception that a single void fraction was used for all particles. The values of apparent activation energy and preexponential factor for particles that averaged 57 pm in radius were 17 kcal/mol and 107 g/(cm2 s atm of 02),respectively. The apparent activation energy estimated for the lignite char was 14 kcal/mol, and the preexponential factor was 21 g/(cm2 s atm of 02).The particles had an averge diameter of 45 pm and a high ash content (35%). Considering the estimated apparent rate parameters, it appears that the petroleum coke is somewhat less reactive than the lignite and bituminous chars studied earlier. This calculation is based on the external area of the particles. This result may be, at least partly, due to the small internal surface area of the oil coke (3.5 m2/g) compared to the coal chars (12.6 m2/g for bituminous and 285 m2/g for lignite). All these reported surface areas are initial values.
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Figure 10. Comparison of experimental and calculated temperatures for a residual-oil coke particle. The calculationswere performed with the shrinking core model with E, = 18 kcal/(! mol) and void fraction 0.65, radius 27.5 pm, A, = 18.67 g/(cm 8 atm of Oz).
Conclusions
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Figure 11. Comparison of experimental and calculated temperaturea for two midual-oil coke particles. The dculatione were performed with the constant radius model with Ei = 40 kcal/(q mol) and (a) void fraction 0.6, radius 30 pm, A, = 671.6g/(cm s atm of Oz);(b) void fraction 0.5, radius 32.5 pm, A, = 490.5 g/(cmz s atm of 02).
were contrasted with those of refs 19 and 22 that describe the behavior of both bituminous and lignite chars characterized and burned in the same facilities, under identical conditions. The analysis using the shrinking core model
Coke cenospheres obtained from incomplete combustion of heavy fuel oil in a utility boiler were burned in a laboratory drop-tube furnace. Temperature-time histories of particles burned under a variety of conditions were collected and analyzed. The results show considerable variability in both the physical properties and reactivity of individual particles. On the average, the coke cenospheres were less reactive than coal chars burned under similar conditions. This could be due to physical factors like low BET surface area and microporosity, or to chemical factors like mineral matter distribution and structure of the carbonaceous matrix. ~ ' ' ' In an atmosphere of 50% oxygen, cenosphere particles were observed to burn at temperatures of 2000-3000 K, even when furnace wall temperature was as low as 1000 K. However, direct visual observation indicated that only a fraction of the particles ignited when the wall temperature was low; the remaining particles appeared to oxidize a t temperatures close to that of the wall. Particles that did not ignite were not recorded by the pyrometer; therefore, the pyrometer only saw the most reactive particles under these conditions. The observation that only a fraction of the particles ignited was an indication of the strong variability in particle properties. Once ignited, the combustion behavior of the most reactive particles appeared to be largely independent of the furnace wall temperature in the range tested (from lo00 to 1450 K). Two limiting forms, shrinking core and constant radius, of a particle combustion model were used to analyze the temperature-time traces and estimate combustion rate parameters. The estimated value of the effectiveness factor, 0.2, indicates that the measurements should be more consistent with particles burning as shrinking cores, though the constant radius form gave somewhat lower residual errors.
Acknowledgment. This work was supported by DOE Office of Energy Utilization Research, ECUT program. Helpful technical discussions with P. F. Massier and R. C. Flagan are acknowledged.
