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Adsorptive and Optical Properties of Fly Ash from Coal and Petroleum Coke Co-firing J. Yu, I. Ku¨laots,† N. Sabanegh,† Y. Gao,† R. H. Hurt,*,† E. S. Suuberg,† and A. Mehta‡ Division of Engineering, Brown University, Providence, Rhode Island 02912, Electric Power Research Institute (EPRI), Palo Alto, CA 94304 Received September 9, 1999. Revised Manuscript Received February 3, 2000
Eight commercial fly ash samples including two from petroleum coke/coal co-firing were characterized to determine the origin, structure, and properties of the unburned carbon. Because of its relevance to ash utilization, the characterization included measurements of adsorptivity toward concrete surfactants, and its dependence on carbon form, particle size, and contact time. The coal/coke co-firing ashes are observed to possess unique optical and adsorptive properties which are related to the size, density, and internal porosity of the coke-derived residual char.
Introduction Fly ash from pulverized fuel fired power stations invariably contains carbon, primarily in the form of porous char particles that originate from the pyrolysis and partial oxidization of the parent solid fuel particles. This carbon can seriously affect the economic utilization and disposal of fly ash by a variety of mechanisms. For example, unburned carbon increases ash volume in landfills due to its low particle (apparent) density, which is of order 0.5 g/cm3 vs about 2 g/cm3 for the glassy mineral phase). Carbon can also destroy the value of ash as a useful additive in Portland cement concrete through discoloration, or through disruption of the micro-void network required to impart freeze/thaw resistance in concrete.1,2 This latter effect occurs because porous carbon particles adsorb specialty surfactants, or “air entraining admixtures,” (AEAs) used in concrete pastes, thus preventing the interfacial accumulation necessary to stabilize sub-millimeter air bubbles.1-6 Recent work has investigated the surfactant adsorption phenomenon, and has related the adsorptive activity of ashes to carbon content and carbon properties, including specific surface area (m2/g-carbon) and surface chemistry.3-7 More work is needed to identify all the * Corresponding author: Robert Hurt, Division of Engineering, Box D, Brown University, Providence, RI 02912. † Brown University. ‡ Electric Power Research Institute (EPRI). (1) Rixom, M. R.; Mailvaganam, N. P. Chemical Admixtures for Concrete, 2nd ed.; E. & F. N. Spon Ltd.: London, 1986. (2) Helmuth, R. Fly Ash in Cement and Concrete; Portland Cement Association: Chicago, 1987. (3) Freeman, E.; Gao, Y. M.; Hurt, R. H.; Suuberg, E. S. Fuel 1997, 76 (8), 761-765. (4) Gao, Y.; Shim, H.; Hurt, R. H.; Suuberg, E. M.; Yang, N. Y. C. Energy Fuels 1997, 11, 457-462. (5) Hachman, L.; Burnett, A.; Gao, Y.; Hurt, R.; Suuberg, E. TwentySeventh Symposium (Int.) on Combustion; The Combustion Institute: Pittsburgh, 1998; pp 2965-2971 (6) Hill, R. L.; Sarkar, S. L.; Rathbone, R. F.; Hower, J. C. In Proceedings of 12th International Symposium on Coal Combustion ByProduct (CCB) Management and Use, Vol. 1; American Coal Ash Association: Alexandria, VA, 1997; pp 23-31.
factors governing the adsorptive behavior of ash samples, especially the effects of particle size, time (adsorption dynamics), pore size distribution, and surface functional groups. There is also a real need to expand the adsorptive database to encompass a wide variety of ash samples generated from coals of various rank and type as well as alternative solid fuels from biomass and solid waste materials. One important alternative solid fuel is petroleum coke, a solid carbon byproduct from petroleum refining.8 Petroleum coke is typically a low-volatile, low-ash, highsulfur fuel that can be economically co-fired with high volatile coals that provide flame stability, especially in boilers already equipped with flue-gas desulfurization units. Several utility companies have recent experience with the full-scale co-firing of coal and petroleum coke and have reported elevated levels of unburned carbon.9 Petroleum cokes have been characterized in the past with respect to combustion and oxidation behavior,10-14 gasification behavior,15 pyrolysis behavior,16 and carbon (7) Ku¨laots, I.; Gao, Y.-M.; Hurt, R. H.; Suuberg, E. S. “The Role of Polar Surface and Mesoporosity in Adsorption of Organics by Fly Ash Carbons,” Prepr. Pap.sAm. Chem. Soc., Div. Fuel 1998, 43 (4). (8) Adams, H. A. Chapter 10 in Introduction to Carbon Technologies (H. Marsh, E. A. Heintz, F. Rodriguez-Reinoso Eds.) Universidad de Alicante: Alicante; 1997. (9) Jones, A. F.; Booth, R. C.; O’Connor, D. EPRI Technical Report TR-105491; Electric Power Research Institute: Palo Alto, 1995. (10) Young, B. C.; Smith, I. W. Eighteenth Symposium (Int.) on Combustion; The Combustion Institute: Pittsburgh, 1981; pp 12491255. (11) Tyler, R. J. Fuel 1986, 65, 235-240. (12) Walsh, D. E.; Green, G. J. Ind. Eng. Chem. Res. 1988, 27, 11151120. (13) Rodriguez, N. M.; Marsh, H.; Heintz, E. A.; Sherwood, R. D.; Baker, R. T. K. Carbon 1987, 25 (5), 629-635. (14) Wall, T. F.; Gururajan, V. S.; Lucas, J.; Gupta, R. P.; Zhang, D.; Smith, I. W.; Young, B. C. Twenty-Third Symposium (Int.) on Combustion; The Combustion Institute: Pittsburgh, 1990; pp 11771184. (15) Harris, D. J.; Smith, I. W. Twenty-Third Symposium (Int.) on Combustion; The Combustion Institute: Pittsburgh, 1990; pp 11851190. (16) Kocaefe, D.; Charette, A.; Castonguay, L. Fuel 1995, 74 (6), 791-799.
10.1021/ef9901950 CCC: $19.00 © 2000 American Chemical Society Published on Web 03/18/2000
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Table 1. Properties of Ash and Carbon Samples surfactant adsorptivity (mL solna)
surfactant adsorptivity (mL/g-C)
total areab m2/g-ash
specific areac m2/g-carb
sample
type
LOI
A
class C ash from commercial coal firing ash from commercial coal/coke co-firingd class F ash from commercial coal firing class F ash from commercial coal firing class F ash from commercial coal firing ash from commercial coal/coke co-firinge class F ash from commercial coal firing class F ash from commercial coal firing (enriched in carbon by partial separation)
0.6%
0
0
1.9
205
2.4%
0
0
1.2
18
2.5%
0.074
1.51
2.3
62
4%
0.052
0.67
2
31
14.6%
0.49
1.7
10.6
68
17.5%
0
0
33.6%
1.2
1.8
17.7
50
65.5%
2.4
1.8
35.3
53
B C D E F G H
1.3
4.0
a Per 2 g-ash, as in standard foam index test. b Nitrogen BET. c Calculated as (total area - (1-LOI)*inorganic area)/LOI, where LOI is the mass fraction of carbon, and the inorganic area is determined by BET after oxidative removal of carbon at 700 °C. The inorganic area was taken as 0.68 m2/g for the class C ash (sample A) and 0.74 for all others based on a large series of such measurements for 60 U.S. coal ashes. d 5 wt % petroleum coke in feed. e 10-15 wt % petroleum coke in feed.
structure and material properties.13,17 To the authors' knowledge there have been no published studies of the properties of ash derived from petroleum coke and coal co-firing. The objective of the present work is to investigate the adsorptive and optical properties of fly ash with emphasis on particle size effects, adsorption dynamics, and the characterization of ash samples from full-scale cofiring of low rank coal and petroleum coke. These cofired samples will be shown to exhibit unusual adsorptive and optical properties of relevance to ash utilization. Experimental Section Experiments were carried out on eight fly ash samples obtained from full-scale combustion systems including two from coal/coke co-firing, as well as on a commercial activated carbon, and a raw petroleum coke. Properties of the ash samples are given in Table 1. The raw coke was part of the commercial feed fuel used to generate ash F, and was identified as a green delayed petroleum coke with 12.0% volatile matter, 87.7% fixed carbon, and 0.37% ash (dry basis). The coke contained 5.3% sulfur (dry basis). The activated carbon was a wood-derived commercial product (obtained from Alfa Aesar) with an N2 BET surface area of 850 m2/gm. Most of the experimental procedures employed here have been described in earlier publications, including the estimation of carbon content by loss-on-ignition (LOI),5 the characterization of surfactant adsorptivity by the foam index test3 using the commercial air-entraining admixture Darex II from W. R. Grace. Nitrogen surface areas were obtained by standard multipoint BET analysis. The carbon particle size distributions were determined by first performing a dry-sieve size classification followed by measurement of the total mass and LOI of each size fraction. In addition, the coal/petroleum-coke ashes were microscopically examined under reflected visible light after mounting in acrylic, grinding, and fine polishing. An additional experiment was devised to study adsorption dynamics. The adsorbent to be tested (20 mg) was added to a prepared solution containing 0.5 mL of 10% Darex II and 25 mL of distilled water. The suspension was stirred and 5 mL aliquots (17) Lefrank, P. A.; Stefanelli, J. J.; Hoff, S. L. Carbon 1989, 27, 245.
were taken at 1,15,30, and 60 min and transferred to cuvettes through a filter syringe. The optical absorbance at 233 nm was measured and divided by the absorbance of an aliquot taken from a solution prepared without ash. The relative absorbance, A/Ao, is a measure of the concentration of UV-active compounds left in solution at time t, and was used to track the dynamics of the adsorption process.
Results Figure 1 shows the visual appearance of eight fly ash samples, A-H. The samples become progressively darker with increasing LOI, with the exception of samples B and F, generated during coal/petroleum-coke co-firing. Particularly striking is sample F, which is light in color despite its very high carbon content of 17%. Figure 2 reports the carbon particle size distributions in one of the co-firing ashes, sample F, and two selected coal ash samples, D and G. The carbon-containing particles in the coal/coke ash (F) are significantly coarser than those in either coal ash (D,G). Figure 3 shows typical carbon particle morphologies found in the coal/coke ash, sample F. Two principal particle types are observed; these are (3A): rounded, vesicular particles (typically smaller) with isotropic texture; and (3B,C): sharp-edged, dense, fissured particles with pronounced optical anisotropy. The optical anisotropy is seen clearly in the higher magnification micrograph (Figure 3C). For comparison, Powder River Basin coal chars and petroleum coke chars were prepared at 1000 °C in helium in a laboratory tube furnace from the raw feed materials, and examined by optical microscopy using the same preparation procedure. With these reference materials the assignment of origin in the commercial ash samples is unmistakablesthe particle morphological types in Figure 3A derive from Powder River Basin low rank coal, and those in Figure 3B,C from petroleum coke. On the basis of an optical point count, the petroleum coke residues are estimated to comprise about 88 vol % of the unburned carbon in sample F. Using typical densities for low rank coal chars prepared at high
Fly Ash from Coal and Petroleum Coke Co-firing
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Figure 1. Visual appearance of commercial fly ash samples with increasing carbon content (LOI).
Figure 2. Size distributions of residual carbon in samples D and G (coal ashes) and sample F (coal/petroleum coke ash). This distribution is calculated for each size fraction as 100% * mai LOIi/Σ (mai LOIi), where mai is the total mass of ash in size bin i, and LOIi is the mass fraction carbon in that size bin. For each sample the frequencies sum to 100% (of the unburned carbon present).
heating rates (∼0.5 g/cm3) and for petroleum coke chars (1.4 g/cm3 measured in this study by ASTM D167-93), it is estimated that the petroleum coke residues account for approximately 95 wt % of the carbon in sample F. Similar carbon particle morphologies were observed in the other co-firing sample, but no point count analyses were performed. Table 1 and Figure 4 show the trends in surfactant adsorptivity. There is a strong correlation between carbon content and adsorptivity, as has been seen
previously3,4 with the outstanding exception of the petroleum coke derived sample, F, which has no measurable activity. It is a noteworthy that 17 wt % carbon can be benign in concrete, while typical regional regulations in the United States disallow fly ashes with carbon contents exceeding only 4%. The very low adsorptivity of the coke residue correlates with its low specific surface area reported in Table 1. Figure 5 gives the specific surfactant adsorptivity as a function of particle size for one of the coal-derived ash samples, the raw petroleum coke, and the commercial activated carbon. The various size fractions were obtained by light grinding of an initial carbon-rich fraction of size 150-212 µm. The raw petroleum coke has a very low adsorptivity which is not altered by size reduction. The coal-derived residual carbon and the activated carbon both increase in adsorptivity as particle size decreases. Note that the standard “foam index test” used to measure these adsorptivities requires from 2 to 15 min to complete, depending upon the amount of surfactant added, which varies from sample to sample. Figure 6 shows typical time profiles of UV absorption in surfactant solutions containing either fly ash H or activated carbon. Increasing the intensity of mechanical agitation had no effect on these profiles, suggesting that the finite uptake rate is due to diffusion limitations within the particles, not to diffusion limitations in the particle boundary layers. It is not possible to interpret these profiles quantitatively due to possible differences between the active amphipathic agents and the UVvisible agents in these complex mixtures. It can be concluded, however, that dynamic adsorption processes are occurring over time scales of at least 1 hsa time that is comparable to the mixing and delivery time for concrete batches in the field and longer than the industry standard adsorption test (foam index test).
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Figure 3. Typical particle morphologies in coal/petroleum coke ash seen by optical microscopy on polished cross sections: (A) subbituminous coal-derived particle (B) petroleum coke-derived particle, (C) petroleum coke-derived particle at higher magnification showing optical anisotropy.
Fly Ash from Coal and Petroleum Coke Co-firing
Figure 4. Standard adsorptivity of the eight ash samples as a function of LOI, or carbon content.
Figure 5. Effect of size reduction on surfactant adsorptivity for coal-derived fly ash, sample D, raw petroleum coke, and a commercial, wood-derived activated carbon.
Figure 6. Dynamic surfactant adsorption tracked by UV spectrophotometry at 233 nm. Fly ash is sample “H”. Activated carbon particles have a mean diameter of 180 µm.
Discussion Petroleum coke residues are readily identifiable by light microscopy and are the predominate source of carbon in the co-firing sample F. Unburned petroleum coke is clearly responsible for this sample’s unusual properties; namely high LOI, light color, and extremely low surfactant activity. Although BET surface area is only a rough indicator of specific adsorptivity,5,7 the very
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low BET areas of the petroleum coke residues are adequate to explain their low adsorptivity (see Table 1). This low area is believed to be an intrinsic property of the fuel, rather than a function of combustion conditions. Previously, surface areas of petroleum cokes have been reported from 0.9 to 8 m2/g,11,12 much lower than typical coal derived chars. Petroleum cokes are typically dense and at the molecular scale highly ordered; this order gives rise to efficient molecular packing, limited microporosity, and low internal surface area. As high-temperature combustion is not an efficient method for activating carbons (for developing fine pore structure), the low surface areas seen here for two cokecontaining samples (B,F) may be expected to be the general trend for petroleum-coke-rich ashes from pulverized fuel firing. We next examine the cause of the anomolously light color of the co-fired ash, F. Considering the high optical absorbance of elemental carbon relative to most inorganic particles, and considering that optical absorption occurs here primarily in the geometric limit (dp . λ), the dominant parameter determining the reflectance of a thick ash bed with a fixed amount of carbon is expected to be the total external carbon particle surface per gm of ash, estimated as Σ (miCi/Fi) (6/dp). Here mi is the mass fraction of ash in size bin i, Ci the carbon content or LOI, F the carbon particle density, and dp the particle diameter. Using this scaling formula, the carbon area is estimated to be 50 cm2-external-carbonarea/g-ash for the co-firing sample F, which is comparable to the light-colored, low-carbon, coal-derived ash, D, (also 50 cm2/g), and much less than the dark, highcarbon, coal-derived ash, G (370 cm2/g). This comparison indicates that the combination of high carbon particle density and large carbon particle size is an adequate explanation for the light appearance of the high-carbon co-fired ash, F. From these results it is also possible to comment on the cause of the high carbon content in the co-firing sample, F. Reports from the field9 indicate that addition of 10-15 wt % petroleum coke to the baseline subbituminous coal increased fly ash LOI to 12-24% from a baseline value of 1%. This increase is believed to be the direct result of the fuel properties of the petroleum coke: volatile content, swelling behavior, char reactivity, and particle size distribution. To test this hypothesis, numerical sensitivity studies were carried out with the CBK model of char combustion kinetics18 in an idealized high-temperature furnace.20 The combustion of a typical subbituminous coal (76% C, 40% volatile matter, daf; 10% ash) with typical utility grind (70% < 200 mesh (18) Hurt, R. H.; Sun, J.-K.; Lunden, M. Combust. Flame 1998, 113, 181-197. (19) Niksa, S.; Muzio, L.; Fang, T.; Hurt, R. H.; Sun J.; Kornfeld, A.; Stallings, J.; Mehta, A. Fifth International Conference on Technologies and Combustion for a Clean Environment; Lisbon, 1999. (20) Carbon burnout is a complex function of fuel and furnace operating conditions. Predicting the dependence on operating conditions requires detailed furnace modeling (as a minimum), but recent work has demonstrated that simpler techniques can be applied to estimate fuel-specific effects. Reasonable estimates of the effect of fuel switching under constant combustion conditions have been reported using idealized, one-dimensional furnaces coupled to detailed fuel conversion models.19 The calculation presented here uses a onedimensional furnace with gas temperatures decreasing linearly with residence time from 2000 to 1500 K, and an initial oxygen concentration of 12 vol %. The fuel-specific kinetics and properties are estimated by the CBK model.18
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and 0.5% > 50 mesh) was simulated numerically in a one-dimensional furnace model, and the residence time was adjusted to yield an LOI of 1% (resulting in 1.5 s residence time). Adding 15 wt % of a typical petroleum coke (0.4% ash, 12% volatile matter, 1.4 g/cm3 char density) to this baseline case at constant fineness increased LOI to 8.5%. This factor of 8.5 in unburned carbon reflects the inherently poor burnout characteristics of coke. Further, there is evidence in the field samples that the coke particles did not grind as efficiently as the coal. First, the coke residues were relatively coarse (see Figure 2) and second, examination of the coarse fraction of pulverizer output samples showed fractional coke contents much higher than the nominal 10-15% co-firing ratio. Further parameter studies with CBK indicate that such coarse grinding of coke would elevate unburned carbon levels significantly above the values quoted above. The observed elevation in unburned carbon levels upon co-firing are thus consistent with the inherently poor burnout characteristics of petroleum coke (low volatile content, nonswelling behavior, and low-to-moderate intrinsic reactivity, and high char density) coupled in this particular case with large mean particle size. Finally, the results presented here demonstrate that the surfactant adsorptivity measured by the standard laboratory procedure is particle size dependent. This size dependence is a classic indicator of mass transfer resistance, and there is further evidence that the chief resistance lies primarily inside the porous carbon particles. A previous study noted the low adsorptivity
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of activated carbon per unit BET surface area;4 the present results suggest that this is primarily due to transport limitations rather than strict size exclusion in molecular-sized pores. We also conclude that the industry-standard foam index test is often a nonequilibrium measurement, in particular when cement is included in the test mixture, as this limits the time available for equilibration before hydration reactions begin to increase the mixture viscosity. Overall, the petroleum coke rich ash sample, F, despite its very high carbon content (17%) does not suffer from either of the two major disadvantages of most high carbon ashessdiscoloration or high surfactant adsorptivity. As alternative solid fuels find wider practical use, these results underscore the importance of assessing ash suitability for concrete based on specific tests of optical and surfactant activity, rather than the mass fraction of combustible matter as in current regulations. Acknowledgment. The authors acknowledge financial support from the Electric Power Research Institute (WO 8032-04 and WO 8032-11). We are also grateful for sample donations and/or technical contributions from Albert Lau (Houston Industries), Dave O’Connor (EPRI), Greg Keenen (American Electric Power), Peter Calvert (formerly of New England Power), and Jian-Kuan Sun (Brown University) for the assistance in combustion calculations. EF9901950
