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Coal combustion fly ash characterization: Adsorption of nitrogen and water
OKI-6981/80/040-0445
Armospheric Environment Vol. 14, pp. 445-456. Pergamon Press Ltd. 1980. Printedin Great Britain.
COAL COMBUSTION FLY ASH CHARACTERIZATION ADSORPTION OF NITROGEN AND WATER*
SO2.00/0
:
S. J. ROTHENBERG Inhalation Toxicology Research Institute, Lovelace Biomedical and Environmental Research Institute, P.O. Box 5890, Albuquerque, NM 87115, U.S.A. (First received 18 May 1979 and injinalform
5 September 1979)
Abstract - Surface characteristics of several types of fly ash from coal combustors were studied by determining adsorption of nitrogen and water. Specific surface areas determined ranged from 2 m* g- ’ to 40 m2 g-l demonstrating that fly ash must be considered an “active” solid. Adsorption and desorption isotherms for water vapor and rates of adsorption and desorption of water were determined at 0, 100 and 200°C. All samples adsorbed more than a monolayer of water at 0°C and vapor pressures above 0.4 Torr suggesting that fly ash is covered by a complete monolayer of water under normal ambient atmospheric conditicpns. Some samples adsorbed measurable quantities of water over the entire temperature range 0-200°C. At 0°C and water vapor pressures greater than 2 Torr, samples demonstrated large increases in weight which could not be explained by multilayer formation. Hysteresis was observed on desorption at 0,20 and 100°C. Pore size distributions were calculated for four of the fly ash samples. ESCA analysis data are presented for five samples.
INTRODUCTION Coal combustors which are widely used for electric power generation will continue to be of significant importance in the future. Total global emissions of ash (termed fly ash) from the stacks of these combustors to the atmosphere were estimated to he N 40 million tons in 1968 (Seinfeld, 1975). Coarse fly ash particles (dia. over 10pm) remain airborne for from a few hours to days while submicron particles have a residence time in the atmosphere of weeks or even months (Seinfeld, 1975). Fly ash particles may adsorb gases or vapors, some of which are potentially toxic, from those present in the exhaust stack and the plume (Natusch, 1978). Particles may be. inhaled by man and subsequently may be deposited in the lung. Adsorbed materials may be slowly extracted by lung fluids and thereby may pass into the general circulation (Kanapilly, 1977). Adsorption properties of fly ash are determined by chemical composition and available surface or specific surface area. Only limited data on the specific surface areas of fly ash and other ambient air particulate samples are currently available (Corn, 1971; Kaakinen, 1975 ; Miguel, 1976). Properties of fly ash may depend on the specific type of combustion process which produced the particles, therefore measurements were made on samples from three different types of combustors. Adsorption properties may also depend on the coal feed stock burned,
* Research performed under Department of Energy Contract Number EY-76-C-04-1013 and also in part under an interagency agreement with the National Institute of Environmental Health Sciences. A.E. 14.4-D
consequently samples taken from an experimental atmospheric pressure fluidized bed combustor burning Montana Rosebud and Texas lignite were compared. Adsorbed water may play an important role in adsorption and oxidation of sulfur dioxide (Frieberg, 1976). Since gas-solid reactions are usually slow but reactions in solution are fast at ambient temperatures, presence of adsorbed water may decrease the activation energy required for reaction and increase the reaction rate by several orders of magnitude. Adsorption of water vapor may also prevent the adsorption of other species, which is called “competition” (Gregg and Sing, 1967; Gregg, 1961). Data on water adsorption by soils, clays, fibers and mixtures of oxide catalysts have long been available (Everett, 1958; Faraday Sot. Discuss., 1948) while comparable adsorption data for fly ash samples are sparse.
EXPERIMENTAL Specific surface areas of samples were determined using nitrogen as an adsorbate and calculated by the method of Brunauer, Emmet and Teller (1938) (BET method). The customary value for the cross sectional area of the nitrogen molecule (A,) was used; A,,, = 16.2 A* (Gregg and Sing, 1967). Since surface area measurements are sensitive to the specific techniques employed, precautions observed are described in detail. Changes in sample weight caused by the adsorption of water at 0, 100 and 200°C were determined as well as rates of adsorption and desorption of water. A vacuum microbalance (Cahn RG 2000) was used for all measurements (Fig. 1). Weighing precision was checked at intervals during this series of experiments. Weighings precise to f 0.2 pg were obtained under optimum conditions, normal precision was + 1 pg. Samples were outgassed to constant weight at 200°C and 2 x 10e5 Torr before adsorption isotherms were determined. Research grade nitrogen gas (Matheson Gas)
445
446
S.J. ROTHENBERG
THE CAtfN MICRO-BALANCE
B PUMP/NG
Micro-balance
LINES fdiogrommoticl
Beam
_
._Vocuum Bottle Tore
Weights
Fig. 1. The Cahn microbalance and pumping lines (diagrammatic).
was employed. All connections were metal since water vapor slowly diffuses through rubber hose. A one-liter flask was connected to the 2 inch (- 5 cm) spacer at the top of the pumping stack (Fig. l), and half filled with distilled water. Distilled water in the flask was degassed for at least 10 minutes under reduced pressure to remove dissolved air and carbon dioxide before water vapor was admitted to the sample. Coal
combustion
fly ash
Samples of baghouse ash were obtained from: (1) a stoker fed power plant burning Colorado coal, (2) a conventional power plant burning pulverized western coal and (3) an experimental fluidized bed combustor (FBC) at the Morgantown Energy TechnoIogy Center (METC) burning Texas lignite or Montana Rosebud subbituminous coal (Newton et al., 1979). Studies were also made on samples of NBS standard reference material Number 1633 -fly ash, a mixture of fly ash from bag houses or electrostatic precipitators of five different conventional coal combustion power stations. Ap proximately 500 mg of each ash sample was withdrawn from the bulk (up to 20 1)sample, placed in a small vial and shaken. One sample, 20-250 mg in weight (Table 1) was withdrawn from the vial and placed in the vacuum microbalance pan. A second sample, which was withdrawn from the same vial, was used in the determination of the chemical composition of the ash. Samples were also taken directly from the efihrent gas stream at METC, using a sampling probe which was alternately connected at four different points in the effluent clean up train (Carpenter et al., 1978). Chemical
composition
Chemical analyses of the major inorganic constituents of bulk samples from the FBC have been published elsewhere (Mazza, 1978; Mei, 1978). The FBC fly ash consisted of aiuminosilicate material, mixed with carbon (or unburned coal) and aerosol&d bed material (silica or limestone). The carbon/ash ratio varied with sampling probe position, combustion temperature and air/fuel ratio. The range obtained was l-11% carbon for Texas lignite combustion and 2.5-3.5x for Montana Rosebud. The narrower range determined for Montana Rosebud corresponded to the smaller range of combustion conditions studied during work with Montana Rosebud. Electron spectroscopy for chemical analysis (ESCA) studies of samples were determined by using a PSI 548. Surface elemental composition was calculated from ESCA peak heights using the method given by Campbell (1978), and sensitivity factors for an aluminum source taken from
Berthou (1975). Berthou estimated the uncertainty in sensitivity factors as + 30%. Data are summarized in Table 2. High resolution ESCA analysis showed that Ca, Fe, Mg may be present as sulphates which form hydrates at water vapor pressures above 3 Torr at 0°C. Scanning electron microscopy (SEM) pictures of the ash were obtained using a JEOL Model JFM35, energy dispersive X-ray analysis (EDXA) using a Kevex 5100 attachment to the SEM. The ESCA and EDXA data will be discussed in detail in a subsequent publication. Temperature
control
Samples were heated to 100,200 and 300°C using a small electric furnace controlled to It:5°C by a Love control unit (Model 49) actuated by an iron-constantan thermocouple. Studies at 20 and 50°C were performed using a water jacket around the hang-down tube (Fig. 1) through which water from a thermostatically controlled bath (+ 1°C) was circulated. Samples were maintained at 0°C using a mixture of ice and distilled water in a 1.5 1. Dewar flask. Determination
of nitrogen
isotherms
Each sample was heated under vacuum (outgassed) at 200°C for 16-20 h, until it reached a constant weight, Nitrogen was then admitted, outgassed weight was determined at room temperature and the sample was cooled in liquid nitrogen to determine the first point on the adsorption isotherm. Both adsorption and desorption isotherms were determined. The sample was allowed to warm to room temperature, nitrogen desorbed and the sample weight was then redetermined. Sample weight at the end of the experiment agreed to within + 5 fig with that at the start if the nitrogen used was free from adsorbable impurities (e.g., water, organics). Buoyancy corrections, which were about 0.5 &Torr for the largest sample studied, were determined using nitrogen and helium with S.D. k 207”.
of adsorptionof water at 200”C, 100°C The sample was outgassed overnight at 200°C the outgassed weight determined and water vapor admitted to the sample. The sample was again outgassed overnight at 2OO”C, allowed to cool to 100°C under vacuum (less than 3 x lo-’ Torr) and water vapor admitted. Blank corrections for thermomolecular flow effects (TMF) were determined at 100 and 200°C using the same empty balance pan and hangdown wire.
Determination
Determination of udsorption of water at 0°C Initial studies established that the procedures described
Combustor type
Several Several
Coal type
FBC FBC FBC FBC
Texas Texas Texas Texas
Lignite Lignite Lignite Lignite
-
23.5 23.5 58.9 31.2 48.1 26.7
45.3 53.9 53.9 49.2 113.3 41.6 35.4
110.6 214.1 214.1 214.1 214.1
Sample weight (mg)
23 22 244 137 215 120
508 552 416 75 342 69 207
296 129 45t 122 128
(P&T)
21 23 249 148 217 119
74 327 71 206
503 549
134
308 129
Monolayer weight second determination (desorption) ocg)
* The exhaust stream clean-up system consists of two cyclones followed by a bag house filter (Carpenter et al., 1978). t See Appendix 1.
Experimental Experimental Experimental Experimental
Montana Rosebud
Filter samples 9 Experimental FBC
10 11 12 13
Western Montana Rosebud Montana Rosebud Texas Lignite
Conventional Experimental FBC Experimental FBC Experimental FBC
5 6 I 8
Bag house and electrostatic precipitator samples 3 Stoker-fed Colorado 4 Stoker-fed Colorado
Standard samples 1 Conventional 2 Conventional
Sample number
Monolayer weight first determination (adsorption)
Table 1. Surface area data for coal fired fly ash
3.4 3.2 14.4 12.8 15.6 15.8
39.1 35.7 30.8 5.3 10.5 5.1 20.4
9.3 2.1 0.7 2.0 2.1
3.0 3.4 14.7 13.8 15.7 15.7
5.2 10.0 5.2 20.2
38.7 35.2
2.2
9.7 2.1
Specific surface area SS rn’g-’ m2 g-’ first second determination determination
-
Above combustor Between cyclones Before bag house After bag house
Before bag house
Sampling point*
448
S. J. ROTHENBERG
Table 2. Elemental composition of fly ash samples determined by ESCA peak height analysis
Sample
type
Sample number
0
C
Ca
S
NBS* 3 5 7 8 13
58.4 44.0 55.5 45.3 51.0 48.9
14.4 12.7 5.5 0.5 3.9 2.8
4.6 3.4 4.1 24.8 8.1 9.0
5.5 8.9 9.7 6.7 6.2 5.2
Conventional Stoker-fed Conventional FBC Montana Rosebud FBC Texas Lignite FBC Texas Lignite
-._.____
* Data taken from Campbell,
were outgassed overnight at 2OO”C, nitrogen admitted, samples cooled to 0°C under nitrogen and the outgassed sample weight at 0°C determined. Nitrogen was then pumped off and water vapor admitted to the sample and the adsorption isotherm was determined. The blank correction was determined. operating
precautions,
4.8 9.9 6.7 4.5 8.9 8.2
9.2 9.6 6.6 6.4 9.1 8.6
3.2 5.3 3.1 2.8 4.9 6.2
Mg
Ti
Na
~~ 4.9 2.4 4.5 4.9 5.1
3.1 2.3
5.9 2.8 3.1
1978.
above were not satisfactory for experiments at 0°C. It was determined that cooling the sample below 100°C under vacuum resulted in a gain in weight. Therefore, the samples
Standard procedures
Element weight (“/,) Si Al Fe
validation
of
outgassing
To avoid contamination of the nitrogen with moisture, the nitrogen bleed line (Fig. 1) was pumped down to 2 x 10-s Torr and the line flushed for several minutes with nitrogen before admitting nitrogen to the sample, and the tine was kept above atmospheric pressure throughout the experiment. The sample was outgassed at 200°C following determination of the nitrogen isotherms and the outgassed sample weight was then redetermined. Values obtained were in agreement (within f. 5 pg) with those previously obtained for the same sample after a single outgassing. Samples which were outgassed for more than 24 h demonstrated no additional weight changes greater than 10 pg after the first 20 h confirming that outgassing times used (16-20 h) were adequate and that the outgassed sample was in a reproducible state. RESULTS AND DISCUSSION
Surface area measurements Plots of the weight of nitrogen adsorbed were made for each sample, an example
vs pressure of which is
shown (Fig. 2). The weight adsorbed in a monolayer or the “knee point”, of the plot was determined. A good sharp “knee” was obtained in all cases, demonstrating that treatment of the data by the BET method was justified. The BET plots (Fig. 3) were therefore used to obtain a more precise value of the weight of a monolayer of nitrogen for each sample, and hence that of the specific surface area (S, m2 g- ‘). Values obtained
from desorption isotherms usually agreed within f 10% with those obtained from adsorption isotherms, as shown in Table 1. Specific surface areas of samples (Table 1) (2-40 mz g- ‘) fall within the range of “active solids” as defined by Gregg (1961). The range of results is slightly higher than those reported for fly ash samples by previous workers (l-6 m2 g- ‘) (Corn, 1971; Kaakinen, 1975 ; Miguel, 1976): and slightly lower than the values reported by Frey and Corn (1967) for diesel engine particles (28.1-50.0m* g-t). Results of successive determinations of surface area for a given sample agree. Replicate samples from the same bottle rarely gave identical results; normal sample to sample variation was about 20%. Two different samples of NBS fly ash gave results differing by a factor of five. The NBS sample appeared highly polydisperse. Values of the specific surface area (S) for samples taken when the FBC was burning Montana Rosebud coal had a range of 3.7-10.5 m* g-‘, while
Sample 8
--
/
I
1
I50
I00
200
P TORR
Fig. 2. Adsorption
isotherm
for nitrogen
at 77°K. Raw data.
449
Coal combustion fly ash characterization
I
200
4c IO
IO3 P/po
Fig. 3. BET plot derived from raw data. 0 Adsorption, 0 desorption.
when the FBC fuel was Texas lignite, the range was
12.8-20.4m2 g-r. This appears to be outside the normal range of sample to sample variation indicating that further study is required to define the manner in which S varies with fuel type and combustion conditions. No systematic change in S with sampling position (samples 10-13) was found. The range obtained for conventional pulverized coal power plant samples, 2-9.5 m2 g- ‘, is considerably lower than values for the Stoker fed power plant samples, and similar to the range for samples from the FBC burning Montana Rosebud. Mass median aerodynamic diameters (MMAD) for FBC samples were calculated from impactor data, and have been reported elsewhere (Newton et al., 1979). Impactor samples were obtained during the same sampling periods as those used to obtain samples 6-13. When Montana Rosebud was being burned, MMAD for samples obtained at the stack exit was 2.3 pm, us = 1.7, while for the Texas lignite values were 2.8 pm, bg = 1.8. Values of MMAD at other sampling ports were somewhat higher. Specific surface areas may be estimated from particle diameters, using methods given by Gregg and Sing (1967). If the particles contain no pores and have a ratio of actual to geometric area (roughness factor) equal to one then S = 1 m2 g- ‘. Measured specific surface areas (Table 1) correspond to a roughness factor ranging from 3-20, or to an internal surface area ranging from 6595% of total surface area, dependent upon whether a model with a shallow pits (solid particle, rough exterior) or deep holes (porous particle), is utilized. The intrinsic uncertainties encountered in calculations made using data for polydisperse samples with cg greater than N 1.3 are considerable. Further study on size-selected samples (a, < 1.2) is required before surface area
measurements can be reliably correlated with measured particle sixes. Scanning electron microscope photos of material from the FBC show irregular particles, whether Montana Rosebud or Texas lignite was being burned. An example is shown (Fig. 4a). Temperatures attained in the FBC are substantially lower than those attained in a conventional combustor and are less than the fusion temperature of fly ash. Thus, the fly ash does not contain the hollow smooth spherical particles so characteristic of conventional fly ash (Fig. 4b) (Newton et al., 1979). Examination of the shells of such particles demonstrates numerous holes within the shell (Fig. 4c). Only those holes which communicate with the exterior contribute to the measured surface area or to the measured pore volume (Gregg and Sing, 1967). Few of the holes shown in Fig. 4(c) communicate with the exterior. Higher resolution examination of the shells has not demonstrated a change in this pattern. Photographs which were chosen are representative of approx a hundred which were examined. Similar photographs were published by Fisher (1976, figs le, If); however, no significance was attached to the holes in the fly ash particle shells. Adsorption of water at loo”C, 200°C
Several samples adsorbed measurable amounts of water at 100 and 200°C (pressures 1-15 Torr) (Table 3). Adsorption at 0°C was over 1% sample weight and is discussed below. The blank correction or correction for thermomolecular flow forces (TMF) at 200°C varied between 10 and 50 pg over the pressure range 3-15 Torr. When the system was pumped down the TMF correction passed through a sharp maximum of 400 pg at 5 5 x 10m2 Torr, and became less than 5 pg at pressures of less than low4 Torr (6 pg/10m4 Torr up
450
S. J. ROTHENBERG
Fig. 4a.
Fig. 4b.
Coal combustion fly ash characterization
Fig. 4c. Fig. 4. SEM pictures of fly ash from the FBC and from conventional combustors. (a) FBC ash, (b) rentional combustor ash, (c) broken shells of conventional combustor ash particles. (Pictures taken by M. Sturm.)
452
s.
J.
ROTHENBERG
Table 3. Adsorption of moisture at lOO”C,200°C Weight moisture adsorbed, pg 200°C 100°C 200°C 10 Torr 5 Torr 10 Torr
Sample number
Sample weight (mg)
100°C 5 Torr
2 4
214.1 53.9
80
120
5
49.2 47.6 35.4 26.7
20 20 35 25
25 20 38 25
I 8 13
10* 30t
200°C 15 Torr 130 60*
30* 50t (15) (10)
* Adsorption isotherm. t Desorption isotherm. ( ) Values indicated are smaller than the uncertainties in thermomolecular correction, or in balance reading reproducibility at the time of the experiment.
to 10-j Torr). When a 200mg sample was used, adsorption at 200°C was much greater than the TMF correction. The TMF correction for most other samples was of the same order of magnitude as the adsorption measured. As discussed in Appendix 1, outgassing at 200°C removes physically adsorbed water but does not remove chemically bound surface hydroxyl groups (Kiselev, 1971; Peri, 1965). Some samples lost as much as 1OOpg when heated from 200 to 300°C which suggests that most samples adsorb appreciable amounts of water at 200°C. Further study is required to document this finding. Water has been shown to catalyse many well known reactions, such as that between hydrogen and oxygen (Baker, 1894) and has been postulated to play a significant role in the catalysed conversion of sulfur dioxide to sulfates (Frieberg, 1972, 1976). Results indicate that fly ash probably adsorbs or retains water under conditions encountered in the stack, plume or ambient air, and this may be of significance to the environment.
2500
I
2000
I
I
Sample
8,
O’C
flow
Adsorption of water at 0°C Five (or more) points on the adsorption isotherm were determined for four samples. Adsorption isotherms obtained are shown in Figs. 5-7. Hysteresis loops which are characteristic of porous materials (Everett, 1958) were obtained when desorption curves were determined (Fig. 6). Hysteresis is probably not the result of retention ofchemisorbed water since three of the criteria given by Gregg (1961) for chemisorption have not been satisfied ; (1) only a small fraction of the water is adsorbed at the low pressures characteristic of chemisorption, (2) width of the adsorption loops obtained is significantly in excess of the calculated monolayer capacities determined by nitrogen adsorption, (3) most of the adsorbed water can be removed by prolonged pumping at 0°C. Several different models have been used to explain observations similar to those presented here. Most models include both pore filling and hydrate formation, as discussed below, but the working hypothesis that fly-ash is porous provides a self-consistent explanation of all of the data presented here.
I
>
I
I
Sample 13, O°C
-
P TORR
P TORR
Fig. 5. Adsorption isotherms, 0’ C, water vapour. 0 First determination, 0 second determination.
453
Coal combustion fly ash characterization
PO
P TORR
PO
P
Fig. 6. Hysteresis loops obtained with water vapor as adsorbate. 0 Adsorption, 0 desorption.
Adsorption isotherms show marked “knee-points” (monolayer completion) at pressures less than 0.2 Torr (Fig. 7). Isotherms are linear between ~0.5 Torr and N 1.5 Torr, but their slope increases rapidly above =1.5 Torr. Increase in weight is the result of both multi-layer formation and pore filling. Adsorption isotherms have been used to obtain pore size distributions using the method of calculation given by Gregg and Sing (1967) with corrections for multilayer 500
,-
I
I
Sample
7
0
0%
4oc )-
300
formation taken from their data. The pressure at which a pore fills (Equation 2, Appendix 2) has been used in these calculations. Results are shown in Fig. 8. Hydrate formation may have caused significant error in data points above 30 A, increasing the apparent (or calculated) pore volume in this size range. Shapes of hysteresis loops can be used to determine the type of pores present in a material as discussed by deBoer (1958). The very steep desorption curve for sample 7 (pressure range 10-4-10-1 Torr) and the very small slope over the pressure range 10-l-3 Torr (Fig. 6), is typical of a material containing pores with narrow constrictions or of “ink-bottle” shaped pores of the type described by deBoer (1958). Behavior of the other samples was more complex, however the shape of the isotherm obtained for sample 5 (Fig. 6) is similar to that of isotherms obtained for systems containing capillaries that are formed between parallel plates (Gregg and Sing, 1967). The blank correction was appreciable at water vapor pressures greater than 2 Torr (Table 4). The data in Table 4 are the mean of three determinations. Rates of adsorption and desorption of water
CII i 2oc I-
IOC )-
00°
1.0
2.0
i
3.c
P TORR
Fig. 7. Low pressure adsorption curve, water vapor, 0°C.
Times required to ieach adsorption equilibrium at 0” and pressures over 2 Torr exceeded 3 h and at pressures over 3 Torr exceeded 1 h. Times required to reach desorption equilibrium at 0°C were many hours in some cases. Rates of moisture loss at 20 and 50°C also were low, that is less than 200 pg h- l, however samples did reach a constant weight when heated under vacuum for 16-20 h at 200°C. The primary environmental concern is the adsorption of water in the presence of air, not in a vacuum system. Therefore, some measurements were made using nitrogen/water vapor mixtures, 500 Torr nitrogen and l-4 Torr water vapor. Weight increases
S.
454
Oo
20
400
20
J.
ROTHENBERG
4o.o RADIUS
20
40
0
20
40
%
Fig. 8. Pore size distributions (for one gram of material).
corresponding to pore filling occurred. Times (days rather than hours) taken to reach equilibrium at 0°C and water vapor pressures over one Torr were much greater than those obtained using pure water vapor at pressures 1-4 Torr. This observation is similar to data cited by Gregg and Sing (1967), who discuss the influence of solvent vapor and inert gases on the wetting of surfaces and filling of pores. These data support the hypothesis that fly ash is porous. Rate data will be discussed in detail in a subsequent publication. Interpretation of results
Corn (1971) used a single hole model to explain data for environmental aerosol samples having S > 2 m2 g - ’ and surface area data for ily ash samples suggest that all samples contained holes and were porous. Raman spectroscopy data for airborne particles resemble data for activated charcoal, a porous solid (Rosen, 1977). Similar data for fly ash samples would be of interest. The method of generation of the carbonaceous fraction of fly ash, volatilization of the bulk of a solid particle, is one of the four classic methods of preparing an active solid described by Francis Bacon (1658). Single coal particles lose up to 90% initial weight on gasification or combustion without detectable change in size, corresponding to pore volume fractions of -9oo/, (Nuttall, 1976; Hedman, 1978; Roach, 1978). Data for the FBC samples and the Stoker fed combustor show a correlation between carbon content, color and specific surface area. White Montana Rosebud ash (sample 7) had a smaller specific surface area than the brown Texas lignite samples (lo-13), while black Stoker fed samples (3,4) had the highest specific surface areas determined. The conventional combustor data are anomalous, in particular, the black NBS samples had surface concentration of carbon greater than any other sample
determined (Table 2), and the lowest surface area values determined (2 m2 g- ‘). Isotherm shapes obtained (Figs. 5-7) do not resemble isotherms obtained for adsorption of water by charcoal (Gregg and Sing, 1967), indicating that both carbonaceous and aluminosilicate fractions of fly ash may contribute signi%antly to the adsorption measured. Hysteresis has been observed in the adsorption of water by many adsorbents, most of which were porous (Faraday Sot. Discuss., 1948,197l). Orr (1968) studied pure salt aerosols and observed narrow hysteresis loops, which he attributed to supersaturation of droplets on desorption. Winkler and Junge (1972) studied mixed salt particles deposited on foil. They attributed the hysteresis found (pressure range lO--~~ relative humidity) to both su~~turation and formation of pores between particles packed densely on the foil. Data similar to those presented here, including hysteresis loops extending to the pressure range 1O-‘-1O-o Ton, have been obtained by Kiselev (1971), who studied porous catalysts (0-1OfKYC) and by Fuller et al. in the study of both thorium oxide catalysts and moon fines returned by the Apollo missions (Fuller, Holmes and Gammage, 1965, 1973b, 1973, 1974, 1972). Data were attributed to slow transport in micro and ultrapores, condensation of hydroxyl groups to form oxide ions and water during d~o~tion at temperatures over 2iNYC, blocking of micropores by adsorbed water and leaching of material to form pores. With the exception of leaching (Appendix l), all of these effects may be important in the interaction of water and fly ash. Thus, proof of the hypothesis that fly ash is porous and determination of definitive pore size distributions requires the use of an absorbate such as helium or krypton which is inert to the different constituents of fly ash.
Tabk 4. Blank correction for adsorption water at 0°C P Torr
AWpg
0.5 15
1.0 15
1.5 1.5
2.0 15 -
2.5 20
3.0 30
3.5 80
3.8 225
4.0 450
Coal combustion fly ash characterization SUMMARY AND CONCLUSIONS
Fly ash sampies are “active” soiids with specific surface areas greater than 1 m2 g- ‘. All of the samples of fly ash examined adsorbed more than a monolayer of water at water vapor pressures above 0.4 Torr at 0°C. It is probable that fly ash is covered by a complete monolayer of water under most conditions of environmental interest. This monolayer may prevent adsorption of PAH by competition and may catalyse the adsorption of sulfur dioxide. Data obtained could be explained by the hypothesis that all the samples which were examined were porous. Pore size distri-
butions included pores in the micropore range. Filling and emptying of micropores by water is slow, and this characteristic has been demonstrated for many other adsorbates (Gregg and Sing, 1967; Kiselev, 1971; Thomas and Thomas, 1967). Rate data obtained (0-300X) for the interaction of water vapor and fly ash demonstrate that times taken to reach adsorption or desorption eQuilibrium normally exceed a minute,
and may exceed a day. Acknowledgements - I would like to thank my colleagues, especially H. C. Yeh and B. V. Mokler, for many helpful suggestions and for critical review of the manuscript.
REFERENCES Adam N. K. 11948) Princioles of nenetration of liquids into solids. Discuss. Faraday-Sot. 3,-5-l 1. Bacon F. (1658) in Op&cula curia posthuma by Rawley (16581.See Shaw. P.. The Phiiosoohical Works of the Hon. Robeit Boyle, 1st Ed., 1, 149 (1733). 3aker H. B. (1894) Influence of moisture on chemical change. J. Chem. Sot. 65,611~624. Berthou H. and Jorgensen C. K. (1975) Relative photoelectron signal intensities obtained with a magnesium x-ray source. Analyt. Chem. 47, 482-488. Bond R. L., Griffith M. and Maggs F. A. P. (1948) Some properties of water absorbed in the capillary structure of coal. Discuss. Faraday Sot. 3, 29-40. _ _ Brtmauer S.. Emmett P. H. and Teller E. 119381Adsorotion of gases in ‘multi-molecular layers. J. km. &em. Sot. 60, 309-3 19. Campbell J. A., Smith R. D. and Davis L. E. (1978) Application of X-ray photoelectron spectroscopy to the study of By ash. Appl. Spectros. 32, 316-319. Carpenter R. L., Weissman S. H., Newton G. J., Hanson R. L., Peele E. R., Mazza, M. H., Kovach J. J., Green D. A. and Grimm U. (1978) Characterization of aerosols produced by an experimental fluid&d bed coal combustor operated with sub-bituminous coal, LF-57, Inhalation Toxicology Research Institute, Albuquerque, NM. Cohan L. H. (1938) Sorption hysteresis and vapor pressure of concave surfaces. J. Am. C/tern. Sot. 60.433-435. Cohan L. H. (1944) Hysteresis and the &pillary theory of adsorption of vapors. J. Am. Chem. Sot. 66,98-105. Corn M., Montgomery T. L. and Esman N. A. (1971) Suspended particulate matter: seasonal variation in specifit surface areas and densities. Enuir. Sci. Technol. 2, W-158. deBoer J. H. (1958) Shape of Capillaries. Structure and Properties of Porous Moteriuls. Tenth Symposium of the Colston Research Society, Univ. Bristol, 24-27 March, 1958 (Edited by D. H. Everett and F. S. Stone), pp. 68-94. Academic Press, New York.
455
Everett D. H. and Stone F. S., eds. (1958) Structure and Properties of Porous Muter~als. Symposium of the Colston Research Society, Univ. 3risto1, 24-27 March, 1958. Academic Press, New York. Faraday Society Discussion (1947-1948) Number 3, Interaction of Water and Porous Materials. Aberdeen, MD, University Press. Faraday Society Discussion (1971) Number 52, Surface Chemistry of Oxides. Aberdeen, MD, University Press. Fisher G. L., Chang D. P. Y. and Brummer M. (1976) Fly ash collected from electrostatic precipitators : Mi~~rystaliine structures and the mystery of the spheres. Science 192, 553-555. Frey J. W. and Corn M. (1967) Physical and chemical characteristics of particulates in a diesel exhaust. Am. ind. Hyg. Ass. 1. 28, 468-478. Freiberg J. (1972) The catalytic oxidation of SOI to acid mist in dispersing plumes, PhD Thesis, Johns Hopkins University. Freiberg J. (1976) The iron catalysed oxidation of sulfur
dioxide to acid sulfate mist in dispersing plumes. Atmospheric Enoironment 10, 121-130. Fuller E. L., Holmes H. F. and Secoy C. H. (1965) Gravimetric adsorption studies of thorium dioxide surfaces with a vacuum microbalance. Vacuum ~icr5bu~unce Tech. 4, 109125. Gammage R. B., Fuller E. L. and Holmes H. F. (1972) Adsorption on porous thorium oxide modified by water. J., Colloid Interface Sci. 38, 91-96. G regg S. J. (1961) The Surface Chemistry of Solids. Reinhold, New York. Gregg S. J. and Sing K. S. W. (1967) Adsorption, Surfuce Area and Porosity. Academic Press, New York. Hedman P. O., Smoots L. D., Hanks R. W., Thurgood J. R. and Skinner F. D. (1978) The BYU rate resolution coal furnace and coal gasifier, Third Rocky Mountain Fuel Symposium, Albuquerque, NM, 10-11, February, 1978. Holmes H. F., Fuller E. L., Jr. and Gammage R. B. (1973) Alteration of an Apollo 12 sample by adsorption of water vapor. Earth Planet Sci. Lett. 19, 90-96. Holmes H. F. and Fuller E. L., Jr. (1974) Some surface properties of Apollo 17 soils. Proceedings of the Fifth Lunar Science Conference, Geochim. cosmochim. Acta, Suppl. 5, Vol. 3, pp. 2275-2285. Pergamon Press, New York. Holmes H. F., Fuller E. L., Jr. and Gammage R. B. (1973b) Interaction ofgases with lunar materials: Apollo f2,14 and 16 samples. Proceedings of the Fourth Lunar Science Conference, Geochim. cosmochim. Acta, Suppl. 4, Vol. 4, pp. 2413-2423. Pergamon Press, Oxford. Kaakinen J. W., Jorden R. M., Lawasani M. H. and West R. E. (1975) Trace element behavior in coal-fired power plant. En&. Sci. Technol. 9, 862-869. Kanapilly G. M. (1977) Alveolar microenvironment and its retationship to the retention and transport into blood of aerosols deposited in the alveoli. H&h Phys. 32, 89-100. Kiselev A. V. (1971) The effect of geometrical structure and the chemistry of oxide surfaces on their adsorption properties. Discussions of the Faraday Society, No. 52. Surface Chemistry of Oxides, pp, 1432. University Press, Aberdeen, MD. Mazza M. H., Green D. A., Paris M. W. and Newton G. J. (1978) Mineral characteriza~on of fluidized-Ed combustion aerosol ash Montana rosebud subbituminous coal. Morgantown Energy Research Center Report, MERC/ TRP 78/l. Mei J. S., Grimm U. and Halow J. S. (1978) Fluidized bed combustion test of low-quality fuels - Texas lignite and lignite refuse. Morgantown Energy Research Center Report, MERC/RI-78/3. Miguel A. H. (1976) Studies of the adsorption of environmen~i pollutants onto particulates. Thesis, University of Illinois.
456
S. J.
ROTHECNRERG
Natusch D. F. S. and Tomkins B. A. (1978) Theoretical consideration of the adsorption of polynuclear aromatic hydrocarbon vapor onto fly ash in a coahired power plant. In Carcinogenesis - A Comprehensive Survey, Vol. 3: Polynuclear Aromatic Hydrocarbons, (Edited by Jones P. W. and Freundenthal R. I.) pp. 145.. 153. Raven Press, New York. Newton G. J., Carpenter R. L., Yeh H. C., Weissman S. H., Hanson R. L. and Hobbs C. H. (1979) Sampling process streams for chemical and physical properties of respirable aerosols. Proc. First Life Sciences Symp. on Potentiul Health and Environmental Effects of Synthetic Fuel Technologies, held in Gatlingburg, TN, 25-28 September.
1978, ORNL CONF-780908, NTIS. Springfield, VA. Nuttall H. E., Stoddart W. G. and Chen W. J. (1976) Reactivity and kinetics of sub-bituminous New Mexico coal. Society of Petroleum Engineers of AIME, Paper Number SPE6G154, 51st Annual Conference of the Society of Petroleum Engineers, New Orleans. 3-6 October, 1976. Orr C., Jr., Hurd F. K. and Corbett W. J. (1958) Aerosol size and relative humidity. J. CoRoid Sci. 13, 472-482. Peri J. B. (1965) Infrared and gravimetric study of the surface hydration of gamma-alumina. J. Phys. Chem. 69,21 l-219. Roach G. (1978) An investigation into the transport properties and kinetic parameters of single coal particles during devolatilization, MS Thesis, University of New Mexico. Rosen H. and Novakov T. (1977) Raman scattering and the characterization of atmospheric aerosol particles. Nature, Lond. 266, 708-710.
Seinfeld J. H. (1975) Air Pollution: Physical and Chemical Fundamentals, McGraw-Hill, New York. Thomas J. M. and Thomas W. J. (1967) introduction to the Principles of Heterogenous Catalysis. Chapter IV, pp. 180-240. Academic Press, New York. Thomson W. (1871) On the equilibrium of vapour at a curved surface of liquid. Phil. Mag. 42, 448-452. Wenzel R.N. (1936) Resistance of solid surfaces to wetting by water. Ind. Engng. Chem. UI, 988-994. Winkler P. and Junge C. (1972) The growth of atmospheric aerosol particles as a function of the relative humidity. Part 1 : Method and measurements at different locations. J.
The adsorption of nitrogen was determined after outgassing at 200°C for sample (2), after accidentally contaminating the sample by using impure helium in a buoyancy determination, and twice more after repeating outgassing at 200°C. The first and last values obtained for the monolaver i capacity are in good agreement as shown in Table 1; however, the second value is low. This is an example of “competition“ discussed in the Introduction. The adsorbed impurities prevent adsorption of nitrogen. The data also illustrate the magnitude of the errors which can be caused by improper technique. APPENDIX 2: USE OF ADSORPTION CURVES TO CHARACTERIZEPORE SIZE DISTRIBUTIONS The simplest explanation of the effect of pores on the shape of adsorption and desorption curves is a treatment given by Thomson (Lord Kelvin) (1871) and modified by Cohan (1938, 1944). Kelvin deduced equations for the equilibrium vapor pressure over curved liquid surfaces and demonstrated that for filled capillaries (or pores) the equilibrium vapor pressure is given by: Kelvin Equation
(1) where P” is the equilibrium vapor pressure over a plane surface, P’ that over the capillary or pore, y surface tension of liquid filling the pore, V molar volume of the liquid, r capillary radius, R gas constant, T absolute temperature. The equation shows that a pore, once filled, will remain full at pressure P’ which is less than P”. A thin film ofliquid or multilayer forms within a pore as it is being filled. Cohan (1938) adapted Kelvin’s approach to derive the equilibrium vapor pressure of a cylindrical film, and hence the pressure at which a pore will fill : Cohan Equation
(2)
Rech. Atmos. 6, 617-638.
APPENDIX 1: RELIABILITYOF SURFACE AREA DETERMINATIONS Checks on both the outgassing procedure and nitrogen purity were made during the study reported here. These are the only major experimental sources of systematic error in vacuum microbalance determinations of surface area. The outgassing temperature selected (200°C) had been used by Kiselev (1971) amongst others (Gregg and Sing, 1967) and was considered sufficiently high to remove most adsorbed molecular water. A higher temperature would be required to remove all chemically bound surface hydroxyl groups (Kiselev, 1971; Peri, 1965). 200°C is insufficient to decompose surface carbonates, nitrates and sulfates. These may be formed (from surface oxides) in the stack, but can also be formed during storage of the samples in ambient air at room temperature. Measurements were made with nitrogen at 77°K and with water at 0, 100 and 200°C for one sample (13). The measurements at 0°C were then repeated. All the points fall on one smooth curve (Fig. 4). Treatment of the sample with water vapor 1-15 Torr, at 100°C 200°C had not detectably altered the sample. This behavior is very difITerent from that reported for moon fines by Holmes et al. (1973). They reported alteration of the sample by the exposure to water vapor, which they attributed to leaching of radiation damage tracks not previously exposed to water vapor.
The two equilibrium pressures, P’ of Kelvin and Pz of Cohan are different. This is the simplest explanation of hysteresis loops (Fig. 6), a characteristic of porous materials. Most samples have a range of pore sizes (pore size distribution) with capillaries of nonuniform cross-sectional area. Theoretical reasons have been advanced for considering that radii derived from the Kelvin equation and the desorption isotherm indicate the size of any constrictions in the pores; and that each part of a nonuniform pore will fill at the value of P2 given by the Cohan equation (Gregg and Sing, 1967; Cohan, 1944). Conversely, using the Cohan equation and the adsorption isotherm, pore size distributions can be derived. As discussed by Gregg and Sing (1967), Wenzel (1936), Adams (1948) and Bond (1948) the correct value of the contact angle to be used in these calculations is subject to uncertainties, and the use of the bulk properties of water (surface tension, density) is almost certainly incorrect. This leads to uncertainties in the pore radii derived, particularly in the range r < 10 A. It is to be expected that increases in weight caused by mesopore (r > 50 A) filling as well as increases in weight caused by the tilling of interstices between the particles in the balance pan will occur at pressures above 4 Torr. Since artifacts caused by the placing of many particles in the same balance pan are unavoidable, cannot be accurately corrected for, and arenot ofenvironmental interest, data for adsorption of water at 0°C and pressures over 4 Torr are not reported here.
Armospheric Environment Vol. 14, pp. 445-456. Pergamon Press Ltd. 1980. Printedin Great Britain.
COAL COMBUSTION FLY ASH CHARACTERIZATION ADSORPTION OF NITROGEN AND WATER*
SO2.00/0
:
S. J. ROTHENBERG Inhalation Toxicology Research Institute, Lovelace Biomedical and Environmental Research Institute, P.O. Box 5890, Albuquerque, NM 87115, U.S.A. (First received 18 May 1979 and injinalform
5 September 1979)
Abstract - Surface characteristics of several types of fly ash from coal combustors were studied by determining adsorption of nitrogen and water. Specific surface areas determined ranged from 2 m* g- ’ to 40 m2 g-l demonstrating that fly ash must be considered an “active” solid. Adsorption and desorption isotherms for water vapor and rates of adsorption and desorption of water were determined at 0, 100 and 200°C. All samples adsorbed more than a monolayer of water at 0°C and vapor pressures above 0.4 Torr suggesting that fly ash is covered by a complete monolayer of water under normal ambient atmospheric conditicpns. Some samples adsorbed measurable quantities of water over the entire temperature range 0-200°C. At 0°C and water vapor pressures greater than 2 Torr, samples demonstrated large increases in weight which could not be explained by multilayer formation. Hysteresis was observed on desorption at 0,20 and 100°C. Pore size distributions were calculated for four of the fly ash samples. ESCA analysis data are presented for five samples.
INTRODUCTION Coal combustors which are widely used for electric power generation will continue to be of significant importance in the future. Total global emissions of ash (termed fly ash) from the stacks of these combustors to the atmosphere were estimated to he N 40 million tons in 1968 (Seinfeld, 1975). Coarse fly ash particles (dia. over 10pm) remain airborne for from a few hours to days while submicron particles have a residence time in the atmosphere of weeks or even months (Seinfeld, 1975). Fly ash particles may adsorb gases or vapors, some of which are potentially toxic, from those present in the exhaust stack and the plume (Natusch, 1978). Particles may be. inhaled by man and subsequently may be deposited in the lung. Adsorbed materials may be slowly extracted by lung fluids and thereby may pass into the general circulation (Kanapilly, 1977). Adsorption properties of fly ash are determined by chemical composition and available surface or specific surface area. Only limited data on the specific surface areas of fly ash and other ambient air particulate samples are currently available (Corn, 1971; Kaakinen, 1975 ; Miguel, 1976). Properties of fly ash may depend on the specific type of combustion process which produced the particles, therefore measurements were made on samples from three different types of combustors. Adsorption properties may also depend on the coal feed stock burned,
* Research performed under Department of Energy Contract Number EY-76-C-04-1013 and also in part under an interagency agreement with the National Institute of Environmental Health Sciences. A.E. 14.4-D
consequently samples taken from an experimental atmospheric pressure fluidized bed combustor burning Montana Rosebud and Texas lignite were compared. Adsorbed water may play an important role in adsorption and oxidation of sulfur dioxide (Frieberg, 1976). Since gas-solid reactions are usually slow but reactions in solution are fast at ambient temperatures, presence of adsorbed water may decrease the activation energy required for reaction and increase the reaction rate by several orders of magnitude. Adsorption of water vapor may also prevent the adsorption of other species, which is called “competition” (Gregg and Sing, 1967; Gregg, 1961). Data on water adsorption by soils, clays, fibers and mixtures of oxide catalysts have long been available (Everett, 1958; Faraday Sot. Discuss., 1948) while comparable adsorption data for fly ash samples are sparse.
EXPERIMENTAL Specific surface areas of samples were determined using nitrogen as an adsorbate and calculated by the method of Brunauer, Emmet and Teller (1938) (BET method). The customary value for the cross sectional area of the nitrogen molecule (A,) was used; A,,, = 16.2 A* (Gregg and Sing, 1967). Since surface area measurements are sensitive to the specific techniques employed, precautions observed are described in detail. Changes in sample weight caused by the adsorption of water at 0, 100 and 200°C were determined as well as rates of adsorption and desorption of water. A vacuum microbalance (Cahn RG 2000) was used for all measurements (Fig. 1). Weighing precision was checked at intervals during this series of experiments. Weighings precise to f 0.2 pg were obtained under optimum conditions, normal precision was + 1 pg. Samples were outgassed to constant weight at 200°C and 2 x 10e5 Torr before adsorption isotherms were determined. Research grade nitrogen gas (Matheson Gas)
445
446
S.J. ROTHENBERG
THE CAtfN MICRO-BALANCE
B PUMP/NG
Micro-balance
LINES fdiogrommoticl
Beam
_
._Vocuum Bottle Tore
Weights
Fig. 1. The Cahn microbalance and pumping lines (diagrammatic).
was employed. All connections were metal since water vapor slowly diffuses through rubber hose. A one-liter flask was connected to the 2 inch (- 5 cm) spacer at the top of the pumping stack (Fig. l), and half filled with distilled water. Distilled water in the flask was degassed for at least 10 minutes under reduced pressure to remove dissolved air and carbon dioxide before water vapor was admitted to the sample. Coal
combustion
fly ash
Samples of baghouse ash were obtained from: (1) a stoker fed power plant burning Colorado coal, (2) a conventional power plant burning pulverized western coal and (3) an experimental fluidized bed combustor (FBC) at the Morgantown Energy TechnoIogy Center (METC) burning Texas lignite or Montana Rosebud subbituminous coal (Newton et al., 1979). Studies were also made on samples of NBS standard reference material Number 1633 -fly ash, a mixture of fly ash from bag houses or electrostatic precipitators of five different conventional coal combustion power stations. Ap proximately 500 mg of each ash sample was withdrawn from the bulk (up to 20 1)sample, placed in a small vial and shaken. One sample, 20-250 mg in weight (Table 1) was withdrawn from the vial and placed in the vacuum microbalance pan. A second sample, which was withdrawn from the same vial, was used in the determination of the chemical composition of the ash. Samples were also taken directly from the efihrent gas stream at METC, using a sampling probe which was alternately connected at four different points in the effluent clean up train (Carpenter et al., 1978). Chemical
composition
Chemical analyses of the major inorganic constituents of bulk samples from the FBC have been published elsewhere (Mazza, 1978; Mei, 1978). The FBC fly ash consisted of aiuminosilicate material, mixed with carbon (or unburned coal) and aerosol&d bed material (silica or limestone). The carbon/ash ratio varied with sampling probe position, combustion temperature and air/fuel ratio. The range obtained was l-11% carbon for Texas lignite combustion and 2.5-3.5x for Montana Rosebud. The narrower range determined for Montana Rosebud corresponded to the smaller range of combustion conditions studied during work with Montana Rosebud. Electron spectroscopy for chemical analysis (ESCA) studies of samples were determined by using a PSI 548. Surface elemental composition was calculated from ESCA peak heights using the method given by Campbell (1978), and sensitivity factors for an aluminum source taken from
Berthou (1975). Berthou estimated the uncertainty in sensitivity factors as + 30%. Data are summarized in Table 2. High resolution ESCA analysis showed that Ca, Fe, Mg may be present as sulphates which form hydrates at water vapor pressures above 3 Torr at 0°C. Scanning electron microscopy (SEM) pictures of the ash were obtained using a JEOL Model JFM35, energy dispersive X-ray analysis (EDXA) using a Kevex 5100 attachment to the SEM. The ESCA and EDXA data will be discussed in detail in a subsequent publication. Temperature
control
Samples were heated to 100,200 and 300°C using a small electric furnace controlled to It:5°C by a Love control unit (Model 49) actuated by an iron-constantan thermocouple. Studies at 20 and 50°C were performed using a water jacket around the hang-down tube (Fig. 1) through which water from a thermostatically controlled bath (+ 1°C) was circulated. Samples were maintained at 0°C using a mixture of ice and distilled water in a 1.5 1. Dewar flask. Determination
of nitrogen
isotherms
Each sample was heated under vacuum (outgassed) at 200°C for 16-20 h, until it reached a constant weight, Nitrogen was then admitted, outgassed weight was determined at room temperature and the sample was cooled in liquid nitrogen to determine the first point on the adsorption isotherm. Both adsorption and desorption isotherms were determined. The sample was allowed to warm to room temperature, nitrogen desorbed and the sample weight was then redetermined. Sample weight at the end of the experiment agreed to within + 5 fig with that at the start if the nitrogen used was free from adsorbable impurities (e.g., water, organics). Buoyancy corrections, which were about 0.5 &Torr for the largest sample studied, were determined using nitrogen and helium with S.D. k 207”.
of adsorptionof water at 200”C, 100°C The sample was outgassed overnight at 200°C the outgassed weight determined and water vapor admitted to the sample. The sample was again outgassed overnight at 2OO”C, allowed to cool to 100°C under vacuum (less than 3 x lo-’ Torr) and water vapor admitted. Blank corrections for thermomolecular flow effects (TMF) were determined at 100 and 200°C using the same empty balance pan and hangdown wire.
Determination
Determination of udsorption of water at 0°C Initial studies established that the procedures described
Combustor type
Several Several
Coal type
FBC FBC FBC FBC
Texas Texas Texas Texas
Lignite Lignite Lignite Lignite
-
23.5 23.5 58.9 31.2 48.1 26.7
45.3 53.9 53.9 49.2 113.3 41.6 35.4
110.6 214.1 214.1 214.1 214.1
Sample weight (mg)
23 22 244 137 215 120
508 552 416 75 342 69 207
296 129 45t 122 128
(P&T)
21 23 249 148 217 119
74 327 71 206
503 549
134
308 129
Monolayer weight second determination (desorption) ocg)
* The exhaust stream clean-up system consists of two cyclones followed by a bag house filter (Carpenter et al., 1978). t See Appendix 1.
Experimental Experimental Experimental Experimental
Montana Rosebud
Filter samples 9 Experimental FBC
10 11 12 13
Western Montana Rosebud Montana Rosebud Texas Lignite
Conventional Experimental FBC Experimental FBC Experimental FBC
5 6 I 8
Bag house and electrostatic precipitator samples 3 Stoker-fed Colorado 4 Stoker-fed Colorado
Standard samples 1 Conventional 2 Conventional
Sample number
Monolayer weight first determination (adsorption)
Table 1. Surface area data for coal fired fly ash
3.4 3.2 14.4 12.8 15.6 15.8
39.1 35.7 30.8 5.3 10.5 5.1 20.4
9.3 2.1 0.7 2.0 2.1
3.0 3.4 14.7 13.8 15.7 15.7
5.2 10.0 5.2 20.2
38.7 35.2
2.2
9.7 2.1
Specific surface area SS rn’g-’ m2 g-’ first second determination determination
-
Above combustor Between cyclones Before bag house After bag house
Before bag house
Sampling point*
448
S. J. ROTHENBERG
Table 2. Elemental composition of fly ash samples determined by ESCA peak height analysis
Sample
type
Sample number
0
C
Ca
S
NBS* 3 5 7 8 13
58.4 44.0 55.5 45.3 51.0 48.9
14.4 12.7 5.5 0.5 3.9 2.8
4.6 3.4 4.1 24.8 8.1 9.0
5.5 8.9 9.7 6.7 6.2 5.2
Conventional Stoker-fed Conventional FBC Montana Rosebud FBC Texas Lignite FBC Texas Lignite
-._.____
* Data taken from Campbell,
were outgassed overnight at 2OO”C, nitrogen admitted, samples cooled to 0°C under nitrogen and the outgassed sample weight at 0°C determined. Nitrogen was then pumped off and water vapor admitted to the sample and the adsorption isotherm was determined. The blank correction was determined. operating
precautions,
4.8 9.9 6.7 4.5 8.9 8.2
9.2 9.6 6.6 6.4 9.1 8.6
3.2 5.3 3.1 2.8 4.9 6.2
Mg
Ti
Na
~~ 4.9 2.4 4.5 4.9 5.1
3.1 2.3
5.9 2.8 3.1
1978.
above were not satisfactory for experiments at 0°C. It was determined that cooling the sample below 100°C under vacuum resulted in a gain in weight. Therefore, the samples
Standard procedures
Element weight (“/,) Si Al Fe
validation
of
outgassing
To avoid contamination of the nitrogen with moisture, the nitrogen bleed line (Fig. 1) was pumped down to 2 x 10-s Torr and the line flushed for several minutes with nitrogen before admitting nitrogen to the sample, and the tine was kept above atmospheric pressure throughout the experiment. The sample was outgassed at 200°C following determination of the nitrogen isotherms and the outgassed sample weight was then redetermined. Values obtained were in agreement (within f. 5 pg) with those previously obtained for the same sample after a single outgassing. Samples which were outgassed for more than 24 h demonstrated no additional weight changes greater than 10 pg after the first 20 h confirming that outgassing times used (16-20 h) were adequate and that the outgassed sample was in a reproducible state. RESULTS AND DISCUSSION
Surface area measurements Plots of the weight of nitrogen adsorbed were made for each sample, an example
vs pressure of which is
shown (Fig. 2). The weight adsorbed in a monolayer or the “knee point”, of the plot was determined. A good sharp “knee” was obtained in all cases, demonstrating that treatment of the data by the BET method was justified. The BET plots (Fig. 3) were therefore used to obtain a more precise value of the weight of a monolayer of nitrogen for each sample, and hence that of the specific surface area (S, m2 g- ‘). Values obtained
from desorption isotherms usually agreed within f 10% with those obtained from adsorption isotherms, as shown in Table 1. Specific surface areas of samples (Table 1) (2-40 mz g- ‘) fall within the range of “active solids” as defined by Gregg (1961). The range of results is slightly higher than those reported for fly ash samples by previous workers (l-6 m2 g- ‘) (Corn, 1971; Kaakinen, 1975 ; Miguel, 1976): and slightly lower than the values reported by Frey and Corn (1967) for diesel engine particles (28.1-50.0m* g-t). Results of successive determinations of surface area for a given sample agree. Replicate samples from the same bottle rarely gave identical results; normal sample to sample variation was about 20%. Two different samples of NBS fly ash gave results differing by a factor of five. The NBS sample appeared highly polydisperse. Values of the specific surface area (S) for samples taken when the FBC was burning Montana Rosebud coal had a range of 3.7-10.5 m* g-‘, while
Sample 8
--
/
I
1
I50
I00
200
P TORR
Fig. 2. Adsorption
isotherm
for nitrogen
at 77°K. Raw data.
449
Coal combustion fly ash characterization
I
200
4c IO
IO3 P/po
Fig. 3. BET plot derived from raw data. 0 Adsorption, 0 desorption.
when the FBC fuel was Texas lignite, the range was
12.8-20.4m2 g-r. This appears to be outside the normal range of sample to sample variation indicating that further study is required to define the manner in which S varies with fuel type and combustion conditions. No systematic change in S with sampling position (samples 10-13) was found. The range obtained for conventional pulverized coal power plant samples, 2-9.5 m2 g- ‘, is considerably lower than values for the Stoker fed power plant samples, and similar to the range for samples from the FBC burning Montana Rosebud. Mass median aerodynamic diameters (MMAD) for FBC samples were calculated from impactor data, and have been reported elsewhere (Newton et al., 1979). Impactor samples were obtained during the same sampling periods as those used to obtain samples 6-13. When Montana Rosebud was being burned, MMAD for samples obtained at the stack exit was 2.3 pm, us = 1.7, while for the Texas lignite values were 2.8 pm, bg = 1.8. Values of MMAD at other sampling ports were somewhat higher. Specific surface areas may be estimated from particle diameters, using methods given by Gregg and Sing (1967). If the particles contain no pores and have a ratio of actual to geometric area (roughness factor) equal to one then S = 1 m2 g- ‘. Measured specific surface areas (Table 1) correspond to a roughness factor ranging from 3-20, or to an internal surface area ranging from 6595% of total surface area, dependent upon whether a model with a shallow pits (solid particle, rough exterior) or deep holes (porous particle), is utilized. The intrinsic uncertainties encountered in calculations made using data for polydisperse samples with cg greater than N 1.3 are considerable. Further study on size-selected samples (a, < 1.2) is required before surface area
measurements can be reliably correlated with measured particle sixes. Scanning electron microscope photos of material from the FBC show irregular particles, whether Montana Rosebud or Texas lignite was being burned. An example is shown (Fig. 4a). Temperatures attained in the FBC are substantially lower than those attained in a conventional combustor and are less than the fusion temperature of fly ash. Thus, the fly ash does not contain the hollow smooth spherical particles so characteristic of conventional fly ash (Fig. 4b) (Newton et al., 1979). Examination of the shells of such particles demonstrates numerous holes within the shell (Fig. 4c). Only those holes which communicate with the exterior contribute to the measured surface area or to the measured pore volume (Gregg and Sing, 1967). Few of the holes shown in Fig. 4(c) communicate with the exterior. Higher resolution examination of the shells has not demonstrated a change in this pattern. Photographs which were chosen are representative of approx a hundred which were examined. Similar photographs were published by Fisher (1976, figs le, If); however, no significance was attached to the holes in the fly ash particle shells. Adsorption of water at loo”C, 200°C
Several samples adsorbed measurable amounts of water at 100 and 200°C (pressures 1-15 Torr) (Table 3). Adsorption at 0°C was over 1% sample weight and is discussed below. The blank correction or correction for thermomolecular flow forces (TMF) at 200°C varied between 10 and 50 pg over the pressure range 3-15 Torr. When the system was pumped down the TMF correction passed through a sharp maximum of 400 pg at 5 5 x 10m2 Torr, and became less than 5 pg at pressures of less than low4 Torr (6 pg/10m4 Torr up
450
S. J. ROTHENBERG
Fig. 4a.
Fig. 4b.
Coal combustion fly ash characterization
Fig. 4c. Fig. 4. SEM pictures of fly ash from the FBC and from conventional combustors. (a) FBC ash, (b) rentional combustor ash, (c) broken shells of conventional combustor ash particles. (Pictures taken by M. Sturm.)
452
s.
J.
ROTHENBERG
Table 3. Adsorption of moisture at lOO”C,200°C Weight moisture adsorbed, pg 200°C 100°C 200°C 10 Torr 5 Torr 10 Torr
Sample number
Sample weight (mg)
100°C 5 Torr
2 4
214.1 53.9
80
120
5
49.2 47.6 35.4 26.7
20 20 35 25
25 20 38 25
I 8 13
10* 30t
200°C 15 Torr 130 60*
30* 50t (15) (10)
* Adsorption isotherm. t Desorption isotherm. ( ) Values indicated are smaller than the uncertainties in thermomolecular correction, or in balance reading reproducibility at the time of the experiment.
to 10-j Torr). When a 200mg sample was used, adsorption at 200°C was much greater than the TMF correction. The TMF correction for most other samples was of the same order of magnitude as the adsorption measured. As discussed in Appendix 1, outgassing at 200°C removes physically adsorbed water but does not remove chemically bound surface hydroxyl groups (Kiselev, 1971; Peri, 1965). Some samples lost as much as 1OOpg when heated from 200 to 300°C which suggests that most samples adsorb appreciable amounts of water at 200°C. Further study is required to document this finding. Water has been shown to catalyse many well known reactions, such as that between hydrogen and oxygen (Baker, 1894) and has been postulated to play a significant role in the catalysed conversion of sulfur dioxide to sulfates (Frieberg, 1972, 1976). Results indicate that fly ash probably adsorbs or retains water under conditions encountered in the stack, plume or ambient air, and this may be of significance to the environment.
2500
I
2000
I
I
Sample
8,
O’C
flow
Adsorption of water at 0°C Five (or more) points on the adsorption isotherm were determined for four samples. Adsorption isotherms obtained are shown in Figs. 5-7. Hysteresis loops which are characteristic of porous materials (Everett, 1958) were obtained when desorption curves were determined (Fig. 6). Hysteresis is probably not the result of retention ofchemisorbed water since three of the criteria given by Gregg (1961) for chemisorption have not been satisfied ; (1) only a small fraction of the water is adsorbed at the low pressures characteristic of chemisorption, (2) width of the adsorption loops obtained is significantly in excess of the calculated monolayer capacities determined by nitrogen adsorption, (3) most of the adsorbed water can be removed by prolonged pumping at 0°C. Several different models have been used to explain observations similar to those presented here. Most models include both pore filling and hydrate formation, as discussed below, but the working hypothesis that fly-ash is porous provides a self-consistent explanation of all of the data presented here.
I
>
I
I
Sample 13, O°C
-
P TORR
P TORR
Fig. 5. Adsorption isotherms, 0’ C, water vapour. 0 First determination, 0 second determination.
453
Coal combustion fly ash characterization
PO
P TORR
PO
P
Fig. 6. Hysteresis loops obtained with water vapor as adsorbate. 0 Adsorption, 0 desorption.
Adsorption isotherms show marked “knee-points” (monolayer completion) at pressures less than 0.2 Torr (Fig. 7). Isotherms are linear between ~0.5 Torr and N 1.5 Torr, but their slope increases rapidly above =1.5 Torr. Increase in weight is the result of both multi-layer formation and pore filling. Adsorption isotherms have been used to obtain pore size distributions using the method of calculation given by Gregg and Sing (1967) with corrections for multilayer 500
,-
I
I
Sample
7
0
0%
4oc )-
300
formation taken from their data. The pressure at which a pore fills (Equation 2, Appendix 2) has been used in these calculations. Results are shown in Fig. 8. Hydrate formation may have caused significant error in data points above 30 A, increasing the apparent (or calculated) pore volume in this size range. Shapes of hysteresis loops can be used to determine the type of pores present in a material as discussed by deBoer (1958). The very steep desorption curve for sample 7 (pressure range 10-4-10-1 Torr) and the very small slope over the pressure range 10-l-3 Torr (Fig. 6), is typical of a material containing pores with narrow constrictions or of “ink-bottle” shaped pores of the type described by deBoer (1958). Behavior of the other samples was more complex, however the shape of the isotherm obtained for sample 5 (Fig. 6) is similar to that of isotherms obtained for systems containing capillaries that are formed between parallel plates (Gregg and Sing, 1967). The blank correction was appreciable at water vapor pressures greater than 2 Torr (Table 4). The data in Table 4 are the mean of three determinations. Rates of adsorption and desorption of water
CII i 2oc I-
IOC )-
00°
1.0
2.0
i
3.c
P TORR
Fig. 7. Low pressure adsorption curve, water vapor, 0°C.
Times required to ieach adsorption equilibrium at 0” and pressures over 2 Torr exceeded 3 h and at pressures over 3 Torr exceeded 1 h. Times required to reach desorption equilibrium at 0°C were many hours in some cases. Rates of moisture loss at 20 and 50°C also were low, that is less than 200 pg h- l, however samples did reach a constant weight when heated under vacuum for 16-20 h at 200°C. The primary environmental concern is the adsorption of water in the presence of air, not in a vacuum system. Therefore, some measurements were made using nitrogen/water vapor mixtures, 500 Torr nitrogen and l-4 Torr water vapor. Weight increases
S.
454
Oo
20
400
20
J.
ROTHENBERG
4o.o RADIUS
20
40
0
20
40
%
Fig. 8. Pore size distributions (for one gram of material).
corresponding to pore filling occurred. Times (days rather than hours) taken to reach equilibrium at 0°C and water vapor pressures over one Torr were much greater than those obtained using pure water vapor at pressures 1-4 Torr. This observation is similar to data cited by Gregg and Sing (1967), who discuss the influence of solvent vapor and inert gases on the wetting of surfaces and filling of pores. These data support the hypothesis that fly ash is porous. Rate data will be discussed in detail in a subsequent publication. Interpretation of results
Corn (1971) used a single hole model to explain data for environmental aerosol samples having S > 2 m2 g - ’ and surface area data for ily ash samples suggest that all samples contained holes and were porous. Raman spectroscopy data for airborne particles resemble data for activated charcoal, a porous solid (Rosen, 1977). Similar data for fly ash samples would be of interest. The method of generation of the carbonaceous fraction of fly ash, volatilization of the bulk of a solid particle, is one of the four classic methods of preparing an active solid described by Francis Bacon (1658). Single coal particles lose up to 90% initial weight on gasification or combustion without detectable change in size, corresponding to pore volume fractions of -9oo/, (Nuttall, 1976; Hedman, 1978; Roach, 1978). Data for the FBC samples and the Stoker fed combustor show a correlation between carbon content, color and specific surface area. White Montana Rosebud ash (sample 7) had a smaller specific surface area than the brown Texas lignite samples (lo-13), while black Stoker fed samples (3,4) had the highest specific surface areas determined. The conventional combustor data are anomalous, in particular, the black NBS samples had surface concentration of carbon greater than any other sample
determined (Table 2), and the lowest surface area values determined (2 m2 g- ‘). Isotherm shapes obtained (Figs. 5-7) do not resemble isotherms obtained for adsorption of water by charcoal (Gregg and Sing, 1967), indicating that both carbonaceous and aluminosilicate fractions of fly ash may contribute signi%antly to the adsorption measured. Hysteresis has been observed in the adsorption of water by many adsorbents, most of which were porous (Faraday Sot. Discuss., 1948,197l). Orr (1968) studied pure salt aerosols and observed narrow hysteresis loops, which he attributed to supersaturation of droplets on desorption. Winkler and Junge (1972) studied mixed salt particles deposited on foil. They attributed the hysteresis found (pressure range lO--~~ relative humidity) to both su~~turation and formation of pores between particles packed densely on the foil. Data similar to those presented here, including hysteresis loops extending to the pressure range 1O-‘-1O-o Ton, have been obtained by Kiselev (1971), who studied porous catalysts (0-1OfKYC) and by Fuller et al. in the study of both thorium oxide catalysts and moon fines returned by the Apollo missions (Fuller, Holmes and Gammage, 1965, 1973b, 1973, 1974, 1972). Data were attributed to slow transport in micro and ultrapores, condensation of hydroxyl groups to form oxide ions and water during d~o~tion at temperatures over 2iNYC, blocking of micropores by adsorbed water and leaching of material to form pores. With the exception of leaching (Appendix l), all of these effects may be important in the interaction of water and fly ash. Thus, proof of the hypothesis that fly ash is porous and determination of definitive pore size distributions requires the use of an absorbate such as helium or krypton which is inert to the different constituents of fly ash.
Tabk 4. Blank correction for adsorption water at 0°C P Torr
AWpg
0.5 15
1.0 15
1.5 1.5
2.0 15 -
2.5 20
3.0 30
3.5 80
3.8 225
4.0 450
Coal combustion fly ash characterization SUMMARY AND CONCLUSIONS
Fly ash sampies are “active” soiids with specific surface areas greater than 1 m2 g- ‘. All of the samples of fly ash examined adsorbed more than a monolayer of water at water vapor pressures above 0.4 Torr at 0°C. It is probable that fly ash is covered by a complete monolayer of water under most conditions of environmental interest. This monolayer may prevent adsorption of PAH by competition and may catalyse the adsorption of sulfur dioxide. Data obtained could be explained by the hypothesis that all the samples which were examined were porous. Pore size distri-
butions included pores in the micropore range. Filling and emptying of micropores by water is slow, and this characteristic has been demonstrated for many other adsorbates (Gregg and Sing, 1967; Kiselev, 1971; Thomas and Thomas, 1967). Rate data obtained (0-300X) for the interaction of water vapor and fly ash demonstrate that times taken to reach adsorption or desorption eQuilibrium normally exceed a minute,
and may exceed a day. Acknowledgements - I would like to thank my colleagues, especially H. C. Yeh and B. V. Mokler, for many helpful suggestions and for critical review of the manuscript.
REFERENCES Adam N. K. 11948) Princioles of nenetration of liquids into solids. Discuss. Faraday-Sot. 3,-5-l 1. Bacon F. (1658) in Op&cula curia posthuma by Rawley (16581.See Shaw. P.. The Phiiosoohical Works of the Hon. Robeit Boyle, 1st Ed., 1, 149 (1733). 3aker H. B. (1894) Influence of moisture on chemical change. J. Chem. Sot. 65,611~624. Berthou H. and Jorgensen C. K. (1975) Relative photoelectron signal intensities obtained with a magnesium x-ray source. Analyt. Chem. 47, 482-488. Bond R. L., Griffith M. and Maggs F. A. P. (1948) Some properties of water absorbed in the capillary structure of coal. Discuss. Faraday Sot. 3, 29-40. _ _ Brtmauer S.. Emmett P. H. and Teller E. 119381Adsorotion of gases in ‘multi-molecular layers. J. km. &em. Sot. 60, 309-3 19. Campbell J. A., Smith R. D. and Davis L. E. (1978) Application of X-ray photoelectron spectroscopy to the study of By ash. Appl. Spectros. 32, 316-319. Carpenter R. L., Weissman S. H., Newton G. J., Hanson R. L., Peele E. R., Mazza, M. H., Kovach J. J., Green D. A. and Grimm U. (1978) Characterization of aerosols produced by an experimental fluid&d bed coal combustor operated with sub-bituminous coal, LF-57, Inhalation Toxicology Research Institute, Albuquerque, NM. Cohan L. H. (1938) Sorption hysteresis and vapor pressure of concave surfaces. J. Am. C/tern. Sot. 60.433-435. Cohan L. H. (1944) Hysteresis and the &pillary theory of adsorption of vapors. J. Am. Chem. Sot. 66,98-105. Corn M., Montgomery T. L. and Esman N. A. (1971) Suspended particulate matter: seasonal variation in specifit surface areas and densities. Enuir. Sci. Technol. 2, W-158. deBoer J. H. (1958) Shape of Capillaries. Structure and Properties of Porous Moteriuls. Tenth Symposium of the Colston Research Society, Univ. Bristol, 24-27 March, 1958 (Edited by D. H. Everett and F. S. Stone), pp. 68-94. Academic Press, New York.
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Everett D. H. and Stone F. S., eds. (1958) Structure and Properties of Porous Muter~als. Symposium of the Colston Research Society, Univ. 3risto1, 24-27 March, 1958. Academic Press, New York. Faraday Society Discussion (1947-1948) Number 3, Interaction of Water and Porous Materials. Aberdeen, MD, University Press. Faraday Society Discussion (1971) Number 52, Surface Chemistry of Oxides. Aberdeen, MD, University Press. Fisher G. L., Chang D. P. Y. and Brummer M. (1976) Fly ash collected from electrostatic precipitators : Mi~~rystaliine structures and the mystery of the spheres. Science 192, 553-555. Frey J. W. and Corn M. (1967) Physical and chemical characteristics of particulates in a diesel exhaust. Am. ind. Hyg. Ass. 1. 28, 468-478. Freiberg J. (1972) The catalytic oxidation of SOI to acid mist in dispersing plumes, PhD Thesis, Johns Hopkins University. Freiberg J. (1976) The iron catalysed oxidation of sulfur
dioxide to acid sulfate mist in dispersing plumes. Atmospheric Enoironment 10, 121-130. Fuller E. L., Holmes H. F. and Secoy C. H. (1965) Gravimetric adsorption studies of thorium dioxide surfaces with a vacuum microbalance. Vacuum ~icr5bu~unce Tech. 4, 109125. Gammage R. B., Fuller E. L. and Holmes H. F. (1972) Adsorption on porous thorium oxide modified by water. J., Colloid Interface Sci. 38, 91-96. G regg S. J. (1961) The Surface Chemistry of Solids. Reinhold, New York. Gregg S. J. and Sing K. S. W. (1967) Adsorption, Surfuce Area and Porosity. Academic Press, New York. Hedman P. O., Smoots L. D., Hanks R. W., Thurgood J. R. and Skinner F. D. (1978) The BYU rate resolution coal furnace and coal gasifier, Third Rocky Mountain Fuel Symposium, Albuquerque, NM, 10-11, February, 1978. Holmes H. F., Fuller E. L., Jr. and Gammage R. B. (1973) Alteration of an Apollo 12 sample by adsorption of water vapor. Earth Planet Sci. Lett. 19, 90-96. Holmes H. F. and Fuller E. L., Jr. (1974) Some surface properties of Apollo 17 soils. Proceedings of the Fifth Lunar Science Conference, Geochim. cosmochim. Acta, Suppl. 5, Vol. 3, pp. 2275-2285. Pergamon Press, New York. Holmes H. F., Fuller E. L., Jr. and Gammage R. B. (1973b) Interaction ofgases with lunar materials: Apollo f2,14 and 16 samples. Proceedings of the Fourth Lunar Science Conference, Geochim. cosmochim. Acta, Suppl. 4, Vol. 4, pp. 2413-2423. Pergamon Press, Oxford. Kaakinen J. W., Jorden R. M., Lawasani M. H. and West R. E. (1975) Trace element behavior in coal-fired power plant. En&. Sci. Technol. 9, 862-869. Kanapilly G. M. (1977) Alveolar microenvironment and its retationship to the retention and transport into blood of aerosols deposited in the alveoli. H&h Phys. 32, 89-100. Kiselev A. V. (1971) The effect of geometrical structure and the chemistry of oxide surfaces on their adsorption properties. Discussions of the Faraday Society, No. 52. Surface Chemistry of Oxides, pp, 1432. University Press, Aberdeen, MD. Mazza M. H., Green D. A., Paris M. W. and Newton G. J. (1978) Mineral characteriza~on of fluidized-Ed combustion aerosol ash Montana rosebud subbituminous coal. Morgantown Energy Research Center Report, MERC/ TRP 78/l. Mei J. S., Grimm U. and Halow J. S. (1978) Fluidized bed combustion test of low-quality fuels - Texas lignite and lignite refuse. Morgantown Energy Research Center Report, MERC/RI-78/3. Miguel A. H. (1976) Studies of the adsorption of environmen~i pollutants onto particulates. Thesis, University of Illinois.
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Natusch D. F. S. and Tomkins B. A. (1978) Theoretical consideration of the adsorption of polynuclear aromatic hydrocarbon vapor onto fly ash in a coahired power plant. In Carcinogenesis - A Comprehensive Survey, Vol. 3: Polynuclear Aromatic Hydrocarbons, (Edited by Jones P. W. and Freundenthal R. I.) pp. 145.. 153. Raven Press, New York. Newton G. J., Carpenter R. L., Yeh H. C., Weissman S. H., Hanson R. L. and Hobbs C. H. (1979) Sampling process streams for chemical and physical properties of respirable aerosols. Proc. First Life Sciences Symp. on Potentiul Health and Environmental Effects of Synthetic Fuel Technologies, held in Gatlingburg, TN, 25-28 September.
1978, ORNL CONF-780908, NTIS. Springfield, VA. Nuttall H. E., Stoddart W. G. and Chen W. J. (1976) Reactivity and kinetics of sub-bituminous New Mexico coal. Society of Petroleum Engineers of AIME, Paper Number SPE6G154, 51st Annual Conference of the Society of Petroleum Engineers, New Orleans. 3-6 October, 1976. Orr C., Jr., Hurd F. K. and Corbett W. J. (1958) Aerosol size and relative humidity. J. CoRoid Sci. 13, 472-482. Peri J. B. (1965) Infrared and gravimetric study of the surface hydration of gamma-alumina. J. Phys. Chem. 69,21 l-219. Roach G. (1978) An investigation into the transport properties and kinetic parameters of single coal particles during devolatilization, MS Thesis, University of New Mexico. Rosen H. and Novakov T. (1977) Raman scattering and the characterization of atmospheric aerosol particles. Nature, Lond. 266, 708-710.
Seinfeld J. H. (1975) Air Pollution: Physical and Chemical Fundamentals, McGraw-Hill, New York. Thomas J. M. and Thomas W. J. (1967) introduction to the Principles of Heterogenous Catalysis. Chapter IV, pp. 180-240. Academic Press, New York. Thomson W. (1871) On the equilibrium of vapour at a curved surface of liquid. Phil. Mag. 42, 448-452. Wenzel R.N. (1936) Resistance of solid surfaces to wetting by water. Ind. Engng. Chem. UI, 988-994. Winkler P. and Junge C. (1972) The growth of atmospheric aerosol particles as a function of the relative humidity. Part 1 : Method and measurements at different locations. J.
The adsorption of nitrogen was determined after outgassing at 200°C for sample (2), after accidentally contaminating the sample by using impure helium in a buoyancy determination, and twice more after repeating outgassing at 200°C. The first and last values obtained for the monolaver i capacity are in good agreement as shown in Table 1; however, the second value is low. This is an example of “competition“ discussed in the Introduction. The adsorbed impurities prevent adsorption of nitrogen. The data also illustrate the magnitude of the errors which can be caused by improper technique. APPENDIX 2: USE OF ADSORPTION CURVES TO CHARACTERIZEPORE SIZE DISTRIBUTIONS The simplest explanation of the effect of pores on the shape of adsorption and desorption curves is a treatment given by Thomson (Lord Kelvin) (1871) and modified by Cohan (1938, 1944). Kelvin deduced equations for the equilibrium vapor pressure over curved liquid surfaces and demonstrated that for filled capillaries (or pores) the equilibrium vapor pressure is given by: Kelvin Equation
(1) where P” is the equilibrium vapor pressure over a plane surface, P’ that over the capillary or pore, y surface tension of liquid filling the pore, V molar volume of the liquid, r capillary radius, R gas constant, T absolute temperature. The equation shows that a pore, once filled, will remain full at pressure P’ which is less than P”. A thin film ofliquid or multilayer forms within a pore as it is being filled. Cohan (1938) adapted Kelvin’s approach to derive the equilibrium vapor pressure of a cylindrical film, and hence the pressure at which a pore will fill : Cohan Equation
(2)
Rech. Atmos. 6, 617-638.
APPENDIX 1: RELIABILITYOF SURFACE AREA DETERMINATIONS Checks on both the outgassing procedure and nitrogen purity were made during the study reported here. These are the only major experimental sources of systematic error in vacuum microbalance determinations of surface area. The outgassing temperature selected (200°C) had been used by Kiselev (1971) amongst others (Gregg and Sing, 1967) and was considered sufficiently high to remove most adsorbed molecular water. A higher temperature would be required to remove all chemically bound surface hydroxyl groups (Kiselev, 1971; Peri, 1965). 200°C is insufficient to decompose surface carbonates, nitrates and sulfates. These may be formed (from surface oxides) in the stack, but can also be formed during storage of the samples in ambient air at room temperature. Measurements were made with nitrogen at 77°K and with water at 0, 100 and 200°C for one sample (13). The measurements at 0°C were then repeated. All the points fall on one smooth curve (Fig. 4). Treatment of the sample with water vapor 1-15 Torr, at 100°C 200°C had not detectably altered the sample. This behavior is very difITerent from that reported for moon fines by Holmes et al. (1973). They reported alteration of the sample by the exposure to water vapor, which they attributed to leaching of radiation damage tracks not previously exposed to water vapor.
The two equilibrium pressures, P’ of Kelvin and Pz of Cohan are different. This is the simplest explanation of hysteresis loops (Fig. 6), a characteristic of porous materials. Most samples have a range of pore sizes (pore size distribution) with capillaries of nonuniform cross-sectional area. Theoretical reasons have been advanced for considering that radii derived from the Kelvin equation and the desorption isotherm indicate the size of any constrictions in the pores; and that each part of a nonuniform pore will fill at the value of P2 given by the Cohan equation (Gregg and Sing, 1967; Cohan, 1944). Conversely, using the Cohan equation and the adsorption isotherm, pore size distributions can be derived. As discussed by Gregg and Sing (1967), Wenzel (1936), Adams (1948) and Bond (1948) the correct value of the contact angle to be used in these calculations is subject to uncertainties, and the use of the bulk properties of water (surface tension, density) is almost certainly incorrect. This leads to uncertainties in the pore radii derived, particularly in the range r < 10 A. It is to be expected that increases in weight caused by mesopore (r > 50 A) filling as well as increases in weight caused by the tilling of interstices between the particles in the balance pan will occur at pressures above 4 Torr. Since artifacts caused by the placing of many particles in the same balance pan are unavoidable, cannot be accurately corrected for, and arenot ofenvironmental interest, data for adsorption of water at 0°C and pressures over 4 Torr are not reported here.