Compound Forms of Fossil Fuel Fly Ash Emissions William M. Henry* Battelle, Columbus Laboratories, 505 King Avenue, Columbus, Ohio 43201
Kenneth T. Knapp
U S . Environmental Protection Agency, Research Triangle Park, N.C. 2771 1 A methodology program for the determination of inorganic compounds present in particulate emissions from fossil fuel combustion processes is described. Samples collected from power plants burning oil and coal fuels of different compositions provided a typical range of fly ashes for the investigations. Elemental (catiodanion) determinations of these were used to guide the compound methodology work. Water extractions of t h e samples proved to be effective in the separation of soluble sulfate compounds from insoluble oxides and silicates, and this reduced the complexity of the compound forms remaining in the water-soluble and water-insoluble phases. A library of Fourier transform infrared reference spectra, prepared and stored for ready recall, was found to be essential for identification of compound species. Based on elemental compositional data, potentially hazardous substances are postulated to be in the fly ashes derived from fossil fuel combustion processes. Despite concerns about t h e environmental significance of the millions of tons of particulate matter emitted annually, little is known as to the specific chemical forms of these particulate emissions. For lack of better data, the current practice for assessing the health effects of fly ash is based largely on the quantities of the emission elements and not on their chemical forms. Certain chemical forms of metals are known t o be more toxic than others; soluble forms are generally regarded as having potentially more adverse effects than those tied u p as insoluble oxides or in glassy structures. Knowledge of the chemical forms of fly ash emissions also can contribute to the planning of control strategy and to interpreting and predicting chemical interactions in the atmosphere, downwind fallout rates and potential damage to vegetation, and deterioration of materials and structures. An extensive literature review ( I ) revealed that very little information is available on the chemical forms of inorganic compounds in fly ash emitted from oil-fired power plants. The principal purpose of the work described here is to identify the major constituents of these emissions and to compare the results with more readily available data on coal-fired emissions. Since the fossil fuel combustion particulate products are principally inorganic substances, they are usually identified as oxides. Such designations, when obtained simply by the conversion of determined elements to their stable oxide forms, can lead to an impression t h a t the fly ashes are simple insoluble unreactive pollutant products. This is far from reality in respect to oil-fired power plant particulate emissions and only partially representative of coal-fired emissions. Actually, products of fossil fuel combustion processes are chemically complex. In addition to carbon oxides, water, oxygen, and nitrogen-the principal products emitted from burning hydrocarbon fuels-many other contaminants are present in the combustion zone. These contaminants include oxides of sulfur and nitrogen, the many inorganic contaminants present in the fuel, plus any additives used to condition the ash or to promote combustion. These substances and operating parameters such as the air-to-fuel ratio, the combustion zone temperature, the time of residence in the burning zone, and the control mechanism and its efficiency all affect the quantity, size distribu450
Environmental Science & Technology
tion, and chemical forms of the particulate emissions. T h e reaction process is quite short, but, presumably, by the time the stack sampling port is reached, most of the condensible inorganic components have interacted completely with the nonparticulate acidic gases present to form unreactive oxides, silicate/glasses, sulfates, water, and free acids.
Experimental Sampling. Samples were collected from oil-fired and coal-fired power plants a t the 3.5- to 4-in. diameter sampling ports normally provided for source emission testing, beyond any control system. An E P A Method 5 glass-lined sampling train probe inserted into the center of t h e stack was used to draw the stack emission particulates into a large fine-mesh Teflon bag held in a stainless steel, cylindrical container. Both t h e probe and the container were held a t stack temperature during a 24-h collection period, but no special effort was made to sample isokinetically. T h e 25- to 50-g samples so collected, along with more aged fly ash samples plus the NBS SRM 1633 coal fly ash, were used in the identification work. Water Solubilities and Sulfate Contents. Because of the chemical complexities of fly ash emissions, it was found important t o carry out species separations in order to simplify the structural identification work. Separation on the basis of water solubility proved quite enlightening for this purpose. As shown in Table I, exclusive of the sootlike insoluble carbon present, the fly ashes collected from oil-fired power plant stacks were found to be from 66 to 100%water soluble, with S042- the only anion of more than trace concentration found present in the water-soluble phase, indicating that the water-soluble components are largely sulfates. X-ray diffraction and chemical analyses showed the insoluble phase of t h e oil-fired fly ashes to be metal oxides and uncombined carbon. T h e coal-fired fly ashes were found to contain less watersoluble material, with one exception. This is an FGD system and, again, sulfate was the only major anion present in the water-soluble phase. The insoluble phase consisted primarily of amorphous and glassy structures plus some metal oxides. Oil-Fired and Coal-Fired Fly Ashes Are Different. Residual fuel oils used in electrical generation combustion sources have low ash contents as compared to coal fuels and as such do not present the large-tonnage disposal problems of coal combustion byproducts. However, most oil-fired power plants operate without particulate control systems; thus, the quantities of fly ash emitted from their stacks can be significant. As illustrated by typical micrographs shown in Figures 1 and 2, fly ashes emitted from oil-fired power plants differ physically from those emitted from coal-fired power plants, being generally of a more open and porous structure. Coalfired fly ashes are closed, glassylike structures. Note the crystallites in the oil-fired fly ash shown in Figure 1. X-ray elemental analysis showed these crystallites to be principally magnesium, sulfur, and vanadium, which is in accord with the finding of large amounts of magnesium and vanadium sulfates in this oil fly ash. Trace metals present in fuel oils are considerably enriched in their ash products. For instance, a fuel oil having a thermal 0013-936X/80/09 14-0450$01 .OO/O
@ 1980 American Chemical Society
Table I. Water Solubility and Carbon and Sulfate Contents of Fly Ash Samples "0.
content
C, %
S0a2--%
water SOIubility, "A
Oil-Fired Fly Ashes 1
2
4
5
6
SRM 1633
i
2
3
4
5
total sample water-soluble water-Insoluble total sample water-soluble water-insoluble total sample water-soluble water-insoluble total sample water-soluble water-insoluble total sample water-soluble water-insoluble
12.4
36.9
0
36.0
12.4 69.0
0.9 12.0 12.0
0 69.0 21.5
0 21.5 1.5
0 1.5 14.5
0 14.5
58.0 (66.2)a
23.3 ( 7 5 . V
0 41.2 41.1 0.1 57.6 58.0
72.0 (91.7)=
98.5 (lOO)a
0 49.2 48.4 0.8
Coal-Fired Fly Ashes total sampleo 3.3 0.98 water-soluble 0.60 water-insoluble 0.38 total sample 1.7 3.05 water-soluble 0 2.13 water-insoluble . 1.7 0.92 total sample 7.0 6.9 water-soluble 0 5.8 water-insoluble 7.0 1.1 total sample 0.5 22.1 water-soluble 0 19.6 water-insoluble 0.5 2.5 total sample 0.1 50.6 water-soluble 0 50.2 water-insoluble 0.1 0.4 total sample 0.1 5.23 water-soluble 0 2.32 water-insoluble 0.1 2.9
83.0 (97.1)a
3.5
5.3
Figure 1. Oil fly ash no. 4 (1600X)
13.0
34.0
79.2
9.1
Percentage contents exclusive of the carbon. @ T h i sis a blend Of ashes obtained from electiostaticprecipitators or by a mechanical Collector.
ash of 0.1% gives a fuel-to-ash concentrational factor of IOOOX which, with a fuel oil content of 50 t o 400 ppm of vanadium, could result in a fly ash content of &IO% vanadium. Of course the fossil fuel combustion process does not result in as low an ash content as that obtained by thermal ashing a t 725 "C, which drives off all the carbon, water, and some sulfates, but oil-fired fly ash contents of greater than 12% vanadium are found. Since, as seen later, a large proportion of the vanadium is present as VOSO4, the vanadium products can represent a major portion of fly ash from fuel oil combustion. The higher ambient concentrations of vanadium found in the East and Southeast have been attributed to fuel oil consumption in those areas and are in contrast with the much lower concentrations found in the Midwest where coal is the predominant fossil fuel used (2). T h e other principal metals contained in fuel oils include Ni and Fe, present in 10- to 100-ppm amounts, and Ca, AI, Na, K, and Mg which are present in 1- to 10-ppm amounts. (Mg, when added to fuel oils as an ash conditioner, is of course in higher concentrations.) All of these metals are enriched about IOOOX in the fly ash combustion products. Coal fuels used for power production average ahout 1 6 1 5 %
Figure 2. Coal fly ash no. 2 (1600X)
ash content, resulting in much lower concentration factors for contaminants in coals than for those in oil fuels. Usually present in coal are Fe, AI, and Si a t about 1%concentrations and Ca, Mg, Ti, Na, and K in 0.02-0.5% concentrations. With a fuel-to-ash Concentration factor of about IOX, the matrices of coal fly ashes are primarily iron-aluminum-silicates or calcium-iron-aluminum-silicates, which account for 60-90% of most coal-fired fly ashes. Compound Identification Methodology. X-ray diffraction (XRD), Fourier transform infrared (FT-IR), and chemical-phase or valence-state analyses were the principal techVolume
14, Number 4. April 1980
451
Table II. Oil- and Coal-Fired Fly Ash Compositions, Major Constituents (Percent) no.
c0 nte nt
C
H
N
NO3-
NOZ-
"4'
S042-
S032-
36.9 36.0 0.9 12.0 12.0 0.15 41.2 41.1 0.1 57.6 58.6 0 49.2 48.4 0.8
