Environ. Sci. Technol. 1988, 22, 322-327
Comparative Studies of Adsorption of Polycyclic Aromatic Hydrocarbons by Fly Ashes from the Combustion of Some Australian Coals Gary K . 4 . Low" and Graeme E. Batley CSIRO, Division of Energy Chemistry, Private Mail Bag 7, Menai, N.S.W. 2234. Australia
The adsorption of polycyclic aromatic hydrocarbons (PAHs) on nine fly ash samples from the combustion of Australian bituminous and brown coals was examined by a high-performance liquid chromatographic procedure. Examinations of a number of standard isotherms indicated that the adsorption data can be best represented by the Freundlich equation. Correlation analysis between the adsorption capacity of PAHs and physical and chemical characteristics of fly ashes indicated that the residual carbon content is the main regulating parameter. Introduction Fly ash generated during combustion of coal is trapped as fine dust in cyclonic and electrostatic precipitators and eventually is ponded as waste products. A considerable amount, however, escapes precipitation and is emitted to the environment either as an aerosol of respirable dimensions or as a fine solid that deposits around the emission source (1). Polycyclic aromatic hydrocarbons (PAHs), formed during the combustion process (2), adsorb onto the surfaces of fly ash particles in the cooler parts of the stack and exit to the environment as an integrated fly ash adsorbed PAH pollutant. Korfmacher et al. (3) have demonstrated that PAHs in this state can be preserved for a considerable time via surface-associated chemical processes. Enhanced toxicity has also been postulated (4-6) when combinations of PAHs and ash particles are inhaled. Goetz (5) showed that the increase in toxicity was due to the delivery of a high local concentration of adsorbed toxic compound at the site of impact in the lung. Norberg and Pershagen (6) have reported enhanced carcinogenicity of benzo[a]pyrenein particulate matter due to the synergistic effects of metallic oxides. Adsorption processes are are also cited as important means of transport of organic pollutants by porous media in aqueous systems (7-9). Means et al. (7) have studied the transport of hydrophobic molecules in water/sediment and water/soil systems. Calvet (8) reported adsorption and transport phenomena of organic pesticides on a number of porous media. Low and Batley (9) have demonstrated the effective removal of phenolic compounds and PAHs from aqueous industrial wastes by adsorbing these compounds on fly ashes. The adsorption of PAHs on fly ash particles, therefore, can have an important implication in the transport and fate of PAHs in the environment. Consequently, a realistic approach for predicting this behavior lies in an understanding of fly ash related adsorption processes. The exact mechanism for PAH adsorption on fly ash particles in a gaseous system is still uncertain. The Natusch and Taylor model (4)is based upon a simple reversible adsorption process that bears little or no relation to condensation, since it is recognized that the vapor pressure of PAHs is never high enough for condensation to take place. Eiceman and Vandiver (IO), on the other hand, demonstrated that PAH adsorption on fly ash particles is essentially controlled by the concentration of PAHs in the flue gas of the combustion stack. A t low concentrations, strong irreversible chemisorption is dominant, whereas vapor pressure controlled condensation is 322 Environ. Sci. Technol., Vol. 22, No. 3, 1988
the main regulating parameter at high PAH concentrations. To exclude any possible effect of the condensation process on the total adsorption, a high-performanceliquid chromatography (HPLC) procedure was reported by Low and Batley (11)to measure the PAH adsorption on fly ash in dry organic solvents. Adsorption of PAHs has been postulated to proceed by a location displacement mechanism similar to the Snyder-Soczewinski model for normal-phase HPLC. This study demonstrated that for a single fly ash sample, the extent of adsorption was regulated principally by the residual carbon content and the molecular size of the adsorbing PAH compound. Using the same procedure, we have investigated the adsorptive behavior of three PAHs of increasing ring size on nine fly ash samples from the combustion of Australian coals. These ashes possess widely differing physical and chemical characteristics, and correlation analysis between these parameters and the adsorption capacities was studied. Different adsorption isotherm equations that have previously been employed to describe adsorption processes on solid substrates (7,12,13) were also evaluated with the PAH adsorption data. Comparison of the plotted data and the isotherm equations was necessary to evaluate the capability of each equation to reflect the relative adsorption of PAHs on different fly ash substrates in a normal-phase HPLC system. Chemicals. Anthracene and chrysene were obtained from Sigma Chemical Co. (St. Louis, MO), and dibenz[a,c]anthracene was obtained from Aldrich Chemical Co. (Milwalkee, WI). Fly Ash Samples. Of the nine fly ash samples used for the adsorption studies, four (Hazelwood, Yallourn, Loy Yang 0130, and Loy Yang 2100) were produced from the combustion of Victorian brown coals, and the remainder were produced from New South Wales bituminous coals. The former samples provided by the staff of the State Electricity Commission of Victoria were sampled from below electrostatic precipitators at Latrobe Valley power stations. Fly ash samples from bituminous coals were obtained from the CSIRO pilot-scale combustion and precipitation facility (CSIRO, Division of Fossil Fuels, North Ryde, NSW, Australia). The fly ash from this plant is comparable with that obtained from a full-scale coalfired power station (14). The collection temperature of the ashes from the electrostatic precipitators was 120 "C in each case. Sample Characterization. Each fly ash sample was characterized for a number of physical and chemical properties (Table I). The total mercury intrusion volume, rather than the total pore area, was used for the comparison of differences in fly ash pore structures as this measurement gave a more consistent result. This intrusion volume is defined as mercury penetration into the packed bed of fly ash at a pressure of 50 000 psi and, therefore, consists of both the interstitial volume between fly ash particles and the pore volumes of fly ash particles. The cumulative intrusion volume for a mean pore diameter 51 nm is used as the total instrusion volume for the internal fly ash pores. When total surface area is determined by the mercury porosimetry method, inaccuracies arise from
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Figure 2. Apparatus for isotherm measurement. Initially, the whole system is equilibrated with the mobile solvent (without PAH) by alternatively switching from the dummy column to the fly ash column. A known concentration of PAH is then injected through the U6K injector and thorough mlxlng is achieved in the flow path of valve 1 to injector to solvent reservoir. The system is equilibrated with the new solution in the flow path of valve 1 to valve 2 to dummy column. Isotherm measurement starts immediately after valve 2 is switched onto the fly ash column.
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the assumption of cylindrical pore geometry. All mercury porosimetry data were obtained on an (Micromeritics Corp., Norcross, GA) autopore 9200. Table I classifies fly ash samples according to the method of Roy and Griffin (15). Figure 1 , parts a and b, shows the particle size distribution for fly ash samples from brown and bituminous coals, respectively. The measurementa were obtained from a Malvern particle-size analyzer, which uses laser diffraction. The physicochemical characterization of the fly ashes selected for this study revealed that the samples possessed a broad range of values for all parameters. Apparatus for Isotherm Measurements. Figure 2 schematically depicts the apparatus used for isotherm measurement. A model 6000 solvent-delivery system (Millipore-Waters, MA) was coupled to two two-way switching valves. Valve 1could be switched to either valve 2 or a Universal Liquid Chromatographic Injector Model U6K (Millipore-Waters). Similarly, the effluent from valve 1could be directed by valve 2 to either the fly ash column
or a dummy column (4 cm X 0.60 mm 0.d. and 0.1 mm i.d.; stainless steel tubing). The effluent from these columns was fed to a model 450 variable-wavelength UV detector (Millipore-Waters). The detector signal was recorded on a Sekonic Model SS250F dual-pen strip-chart recorder. The ash column was thermostated to 23 f 2 "C. The fly ash samples were packed into 3 cm X 4 mm i.d. Uptight short HPLC columns (Activon Scientifics, Sydney, Australia) with methylene chloride as the slurry medium in a Micromeritics column packer (Micromeritics Instrument Corp., Norcross, GA). Generally only 250-350 mg of ash were required. For adsorption of PAHs on fly ashes, the effluent from the detector was directed to waste, and this configuration is referred to henceforth as an open system. A closed system is one in which the effluent from the detector is fed directly back to the solvent reservoir, with the volume of the solvent in the reservoir being kept constant. Such a configuration was used for equilibration of the system when incremental changes of mobile solvent concentration were required. The calibrated solvent reservoir was tightly capped with drying tubes (calcium chloride-silica gel) to keep the mobile solvent from ambient moisture, and dry helium was bubbled through the solvent intermittently to displace any dissolved air. The operation of the apparatus is described fully in ref 11. Calculation of Isotherm Points by Frontal Analysis. Frontal analysis relates the rate of formation of the concentration front by a step increase in concentration of a solute in the mobile solvent to the value of the solute's adsorption at elevated concentration. To calculate the amount of the solute adsorbed by the ash column when the solute concentration of the mobile solvent is changed from C, to C b , the following equation is used:
where tR is the retention time of front, taken here as the time required to reach the point of inflection of the step profile, t ois the retention time for an unadsorbed sample Environ. Sci. Technol., Vol. 22, No. 3, 1988
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(tJ is the system dead volume), F is the flow of the mobile phase in the column, and W is the mass of adsorbent in the column. A single change of solvent concentration produces one discrete point on the adsorption isotherm. This operation can be repeated with successive additions of solute to yield additional points on the isotherm. The procedure for the determination of the system dead volume is given in ref 11. The equilibrium adsorption was determined for each fly ash sample with 10 consecutive concentrations of each PAH compound. Curve fitting of the Henry, Langmuir, and Freundlich equations was performed by the computer with least-squares linear regression analysis (LSR) of manipulated data and the linearized equation shown in Table I11 of ref 11. For the Weber-Matthews isotherm, the data were fitted by using parametric least-squares procedures with the estimated LSR values of the adjustable parameters in the Langmuir equation and an exponent parameter of unity as the initial values. The program also calculated the coefficient of determination (r2)and plotted the experimental curves and the LSR isotherms. The isotherm is denoted as Langmuir I when the LSR estimates of the adjustable parameters are obtained from the first form of the linearized equation and Langmuir I1 when the second form is used (Table 11). Results and Discussion Curve Fitting of Theoretical Isotherms to Experimental Data. The adsorption data for anthracene, chrysene, and dibenz[a,c]anthracene were used to obtain the best fit estimates of the parameters in the five isotherm equations. These estimates, along with sample r2 values, are summarized in Table 11. The coefficient of determination is only an approximate estimate of the suitability of an isotherm model to describe the experimental points. On the basis of the r2values, the rank of suitability of the isotherm equation is WeberMatthews, Freundlich, Langmuir I, Langmuir 11, and Henry. A value of rz less than 0.80 indicates that the LSR of the data on a particular isotherm equation is unsuitable. For values 10.90, the discrimination is not clear. Kinniburgh (16) has recently discussed a numer of shortcomings in using the LSR for the estimation of the parameters in the Langmuir and Freundlich isotherms. To discern more 324
Environ. Sci. Technol., Vol. 22, No. 3, 1988
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effectively the performance of each isotherm, it is necessary to compare the plots of the experimental and LSR isotherms. This is particularly well illustrated in Figure 3A for the adsorption of anthracene on Drayton I1 fly ash, where the 1.2 values for the LSR fittings of the Freundlich, Weber-Matthews, and Langmuir I isotherms are calculated to be 0.99,1.00, and 0.99 respectively. From the plots, it is clearly seen that the first two models give more precise fits. Similar behavior 'is also observed for chrysene (Figure 3B) and dibenz[a,c]anthracene (Figure 3C). Generally, Henry's equation grossly underestimates, and the Langmuir equations overestimate the adsorption. When the data points are LSR-fitted by Henry's law, the isotherms in Figure 3 indicate an intercept at C = 0, whereas theoretically the law assumes Q = 0 at C = 0. Henry's law is therefore clearly unsuitable. The Weber-Matthews model is better, but considerably deviates from the experimental points at both high and low PAH concentrations. For all nine fly ash samples, the Freundlich equation provides the best fit for the adsorption of three PAHs over the range of concentrations studied. As demonstrated both by the r2 values and by the comparison of the plots of the LSR isotherms, the Langmuir I isotherm performed relatively better than the Langmuir I1 isotherm. Walters (17) has recently demonstrated that the two linear forms of Langmuir isotherms have different ranges of applicability, the first form favoring high solution concentrations and the second form low concentrations. This is not obvious with the range of concentrations used in this work. Dowd and Riggs (18)favored the first linear form since it tended to exaggerate deviations from the fitted equation, highlighting outliers and guarding against an overoptimistic interpretation of the goodness of fit. It is noted, however, from Table I1 that the estimated values of the parameters of the two linear forms of the Langmuir equation with some minor exceptions gave values close to each other. Large differences between some values in the two sets of data can be attributed to the LSR method of estimating these parameters. Fitting the Langmuir isotherm by using the transformed forms of the original equation ignores the particular distribution of error implied and may lead to biased estimates of the isotherm parameters (16). The estimated parameter values for the Weber-Matthews equation closely resemble the estimated
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parameters of the Langmuir isotherm, with the exponent parameter m significantly less than unity. This implies that either the Freundlich or an intermediate isotherm, which incorporates features of both the Langmuir and the Freundlich equations, would adequately represent the adsorption data for the experimental system employed in this work. The Freundlich K z parameter is used in the remaining discussion to give a relative measure of the affinity of fly ashes for PAH adsorption. Effect of Solute Sizes on Adsorption. A number of attempts to relate adsorption to the physical and chemical properties of the adsorbing compounds have appeared in literature (5, 19, 20). Means et al. (7) reported that effective chain length may be more important than the absolute increase in molecular weight. Nkedl-Kizza et al. (19) demonstrated that the hydrocarbonaceous molecular surface area of the adsorbing compound is an important parameter in regulating adsorption on soil. Tsezos and Set0 (20) have linked the water solubility and octanol/ water partition coefficient of the tested organic materials to the adsorption by a microbial biomass. It should be pointed out, however, that these studies have dealt with adsorption only from aqueous solutions and their conclusions may differ from those obtained for adsorption from dry organic solvents. We have reported (25) that, for a single fly ash, adsorption is dependent on the topological size of the adsorbing molecule and its degree of branching. This is further reinforced by the data shown in Table I1 where adsorption increased with increasing molecular size of PAH compounds for all fly ash samples. With the Kz parameter values estimated from the Freundlich equation, the adsorption of anthracene, pyrene, and dibenz[a,c]anthracene yielded ranges of capacity values (K,) of 14-211,52-389, and 144-2407 for bituminous coal fly ashes and 51-483, 234-1149, and 337-1768 for brown coal fly ashes, respectively. Two general conclusions emerged from this data: asorption increases with increasing number of rings in the adsorbing compound, and brown coal fly ashes adsorb more strongly than fly ashes from bituminous coals. The dependence of adsorption on the size of the PAH molecules is not linear. For the fly ashes from brown coal, the adsorption increase from anthracene to chrysene is greater than from chrysene to dibenz[a,c]anthracene. This trend is less obvious for the data obtained from bituminous coal fly ashes. Adsorption Dependence on Residual Carbon Content. The adsorption of PAH molecules on fly ashes from brown coals appears to be linearly dependent on the residual carbon content. Correlations between the adsorption capacities of anthracene, chrysene, and dibenz[a,c]anthracene and the residual organic carbon content in the fly ashes gave correlation Coefficients of 0.95,0.97, and 0.81, respectively. Clearly, differences in physical and chemical properties other than the carbon content have no major bearing on adsorption. The reversal in adsorption capacity values for chrysene and dibenz[a,c]anthraceneon Yallourn fly ash cannot be easily explained, except to note that this sample contains an unusually high content of Fez03,has a large pore structure, and has a large range of particle sizes. Griest and Tomkins (21) have recently studied the contribution to the total adsorption of radio-labeled benzo[a]pyrene by the mineral, magnetic, and carbonaceous subfractions of a fly ash and concluded that the first two fractions contributed substantially less than the fraction with high carbonaceous content. On the other hand, Banerjee et al. (22) in the study of adsorption of organic 326 Environ. Sci. Technol., Vol. 22, NO. 3, 1988
contaminants on soils reported that the clay mineral content can influence adsorption on adsorbents with low carbon content. Wu and Gschwend (23)also postulated that the adsorption kinetics of hydrophobic chemicals in suspended sediments and soil particles is influenced by the hydrophobicity of the adsorbing solute, as well as the organic content, density, and porosity of the adsorbent. Physical accessibility of active carbon sites by the PAH molecules seems to be another plausible explanation (9). The dependence of adsorption on the residual carbon content is less convincing for the bituminous coal fly ashes as reflected by the correlation coefficients of 0.74,0.54, and 0.89 for the three PAH compounds. The notable outliers from the linear dependence are Drayton I and Grose Valley. When their values are removed from the pooled data for the bituminous coal fly ashes, strong linear dependence is obtained with correlation coefficients of 1.0, 1.0, and 0.91, respectively. The adsorption capacity is higher for Drayton I and lower for Grose Valley than would be expected if adsorption was linearly dependent on the residual carbon content. This is especially obvious for the adsorption of anthracene and chrysene. Higher adsorptivity for Drayton I fly ash may be partially attributed to its high Fez03 content and high porosity. The importance of pore differences is further implied by comparing the adsorption data for Drayton I and Drayton I1 fly ashes. These two samples have broad physical and chemical similarities except that Drayton I fly ash has a lower carbon content. High adsorptivity of the former can again be attributed to its high porosity. Contrary evidence for the sole dependence of adsorption on residual carbon content is the poor correlation obtained with the pooled data of the bituminous and brown coal fly ashes. For anthracene and chrysene, correlation coefficients of 0.64 and 0.43 are obtained, whereas for the large dibenz[a,c]anthracenemolecule, 0.80 is obtained. This is indicative that, as the size of adsorbing solute molecules increases, the residual carbon content becomes the controlling mechanism for adsorption. The dendrogram of the cluster analysis (24) of the pooled data also indicated that there is a weak correlation (0.82,0.78) between the adsorption capacities of anthracene and chrysene and Fe203content of the fly ashes. Generally, for the same or lower contents of residual carbon, higher adsorption is obtained with the brown coal fly ashes than with the bituminous coals. Matrix correlation and cluster analysis cannot assign the differences in adsorption to the difference in chemical and physical characteristics for the two groups of the samples. The physical differences in the nature of the carbon and the accessibility of the carbon sites in the ashes can be invoked to explain the differences in adsorption. For instance, when Loy Yang 0130 fly ash was further combusted at 600 "C for 24 h, a reduction in 25% of the total pore intrusion volume was noted, while a drop of 10% was observed when Drayton I1 fly ash was combusted under the same conditions. This indicated that the carbon particles in the fly ash from the brown coal are more porous than those in the fly ash from the bituminous coals. These findings concur with previously reported observations (25). Griest and Harris (26)have microscopically identified several different physical types of carbonaceous particles in stack ashes, ranging from uncombusted macerals of optically isotropic fusinite to optically anisotropic coked products derived from vitrinite and exinite. The latter displayed complex porous internal structures containing mineral inclusions. The accessibility of carbon sites can be influenced significantly during the formation of fly ashes. The formation
of these ashes from brown and bituminous coals is known to proceed via different processes (25, 27, 28). Relationship between Total Mercury Intrusion Volume and PAH Adsorption Capacities. For fly ashes from bituminous coals, strong negative correlation coefficients are obtained between the total mercury intrusion volumes and Freundlich K2 parameter of anthracene and chrysene (-0.93 and -0.95, respectively). That of dibenz[a,c]anthracene is -0.65, and when the two outliers, Drayton I and Grose Valley, are removed from the pooled data, a correlation coefficient of 1.0 is obtained. This implies that the larger the intrusion volume, the smaller is the adsorption. Since total intrusion volume is largely interstitial volume, in a packed bed system such as the one used in this work, a continuous flow of a large interstitial volume of organic solvent tends to reduce the adsorption of PAHs. No significant correlation was found between the Freundlich K2 parameters of PAHs and the volume of mercury intrusion attributed to the pore volume, except that positive correlation coefficients (0.65, 0.60, and 0.4) were obtained, indicative of a trend toward higher adsorption for a fly ash with larger pore structures. Low correlation coefficients between total intrusion volumes and Freundlich K 2 parameters of PAHs on the brown coal fly ashes exhibited a negative trend (-0.6, -0.3, and -0.28). The difference from the bituminous coal fly ashes is difficult to rationalize except to postulate that adsorption of PAHs in bituminous coal occurs principally on the surface of fly ash particles, whereas for the brown coal fly ashes, adsorption occurs both on the surface and through the pores of the particles.
Conclusions The adsorption of anthracene, pyrene, and dibenz[a,clanthracene has been studied in a continuous packed-bed system for nine fly ashes that exhibit a wide range of physicochemical properties. Although the adsorption capacities correlated significantly with the organic contents within the respective groups of brown coal and bituminous coal fly ashes, poor correlation coefficients were obtained between PAH adsorption capacities and the organic contents of the pooled data for all samples. This strongly indicates that a controlling mechanism other than carbon content is important. Correlation analysis clearly cannot discern the relationship between adsorption capacities and differences in physical and chemical characteristics of fly ashes.
Acknowledgments We acknowledge the experimental assistance of Kim Prescott. We also thank Chris Brockbank for helpful advice on the writing of many of the computer programming routines. Registry No. Anthracene, 120-12-7;chrysene, 218-01-9; dibenz[a,c]anthracene, 215-58-7.
Literature Cited (1) Commission of Energy and Environment Coal and the
Environment; HMSO: London, 1981; pp 48-50. (2) Cavalieri, E.; Rogan, E.; Roth, R. In Free Radicals and Cancer;Floyd, R. A., Ed.; Dgkker: New York, 1982; p 117. (3) Korfmacher, W. A,; Natusch, D. F. S.; Taylor, D. R.; Mamanlov, G.; Wehry; E. L. Science (Washington,D.C.) 1980, 207, 764. (4) Natusch, D. F. S.; Taylor, D. R. Environmental Effects of Western Coal Combustion, Part IV; U.S. Government Printing Office: Washington, DC, 1980; EPA600/3-80-094. ( 5 ) Goetz, A. Air Water Pollut. 1963, 7, 168. (6) Norberg, G. F.; Pershagen, G. Toxicol. Enuiron. Chem. 1984, 9, 63. (7) Means, J. C.; Wood, S. G.; Hassett, J. J.; Banwart, W. L. Environ. Sci. Technol. 1984, 18, 395. (8) Calvet, R. In Pollutants in Porous Media;Yaron, B., Dagon, G., Goldshmid, J., Eds.; Springer-Verlag: New York, 1984; p 141. (9) Batley, G. E.; Low, G. K.-C. Proceedings of the 3rd Australian Workshop on Oil Shale, Lucas Heights, NSW, Australia, May 15-16, 1985, p 301. (10) Eiceman, G. A.; Vandiver, V. J. Atmos. Enuiron. 1983,17, 461. (11) Low, G. K.-C.; Batley, G. E. J. Chromatogr. 1986,355,177. (12) Walters, R. W.; Luthy, R. G. Enuiron. Sci. Technol. 1984, 18, 395. (13) McKay, G.; Blair, H. S.; Gardiner, J. R. J . Appl. Polym. Sci. 1982, 27, 3043. (14) Paulson, C. A. J.; Potter, C. E.; Kahane, R. “New Ideas in Precipitation Technology from CSIRO Combustion Rig”; Institute of Fuel (Australian Membership), Symposium on Changing Technology of Electrostatic Precipitator, Nov 1974, Adelaide, Australia. (15) Roy, W. R.; Griffin, R. A. J. Environ. Qual. 1982,11,563. (16) Kinniburgh, D. G. Environ. Sci. Technol. 1986, 20, 895. (17) Walters, R. W. Ph.D. Thesis, Carnegie-Mellon University, 1981. (18) Dowd, J. E.; Riggs, D. S. J. Biol. Chem. 1965, 240, 863. (19) Nkedl-Kizza, P.; Rao, P. S. C.; Hornsby, A. G. Environ. Sci. Technol. 1985,19,975. (20) Tsezos, M.; Seto, W. Water Res. 1986, 20, 851. (21) Griest, W. H.; Tomkins, B. A. Enuiron. Sci. Technol. 1986, 20, 291. (22) Banerjee, P.; Piwani, M. D.; Ebeid, K. Chemosphere 1984, 14, 1057. (23) Wu, S.4.; Gschwend, P. M. Environ. Sci. Technol. 1986, 20, 717. (24) Clayton, E. In Archaeometry: An Australasian Perspective; Ambrose, W., Duerden, P., Eds.; Department of Prehistory, Qesearch School of Pacific Studies, Australian National University: Canberra, ACT, Australia, 1982; p 90. (25) Stacey, W. 0.; Jones, J. C. Fuel 1986, 65, 1171. (26) Griest, W. H.; Harris, L. A. Fuel 1985, 64, 821. (27) State Electricity Commission of Victoria (Australia), Annual Report, 1984-1985; Research and Development Department, State Electricity Commission: Victoria, Report G0/86/91, Australia, 1986; p 17. (28) Wibberley, L. J.; Wall, T. F. Presented a t the Conference on Coal Ash and Combustion System, University of Newcastle, Australia, May 23-24, 1986. Received for review May 11, 1987. Revised manuscript received September 29, 1987. Accepted October 19, 1987.
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