Environ. Sci. Technol. 1082, 16, 148-154
Analysis of Fly Ash Produced from Combustion of Refuse-Derived Fuel and Coal Mixtures Davld R. Taylor," Mlchael A. Tompklns,+ Sarah E. Klrton, Thad Mauney, and Davld F. S. Natusch
Department of Chemistry, Colorado State University, Fort Collins, Colorado 80523 Phlllp K. Hopke
Institute of Environmental Studies, University of Illlnols, Urbana, Illinois 61801 H Mixtures of coal and refuse-derived fuel (RDF) were
burned and the fly ash was collected and analyzed for concentration trends with respect to RDF/coal ratio and particle size. RDF contributes more Cs, Mn, Sb, and P b to the fly a9h while coal contributes greater amounts of As, Br, Fe, Hf, Ni, Sc, V, and the rare earths. Smaller particles in the RDF fly ash had higher concentrations of As, Cd, Ga, K, Na, Sb, and the rare earths. RDF fly ash contains four distinct morphologies, exhibits a high specific surface area, and does not resemble fly ash derived from a conventional coal-fired power plant. The morphology of the ash helps explain the high solubility of many species in the RDF-rich fractions. ~~
Introduction The use of municipal refuse as a fuel is currently receiving widespread attention. Factors such as rising costs, lack of available landfill sites, a growing interest in recycling, and ever increasing energy demands have made refuse an increasingly attractive alternative to coal as an energy source. Several authors (1,2) have pointed out that the energy content of refuse-derived fuel (RDF) is considerably less than that of coal. In order to compensate for this deficiency, the use of mixtures of coal and RDF is now being explored. These mixtures combine the higher energy output of coal with the ready availability of urban waste. The use of RDF constitutes an entirely new and not well-characterized source of environmental pollutants, notably potentially toxic trace metals. For example, both Cambell (3) and Law and Gordon ( 4 ) have discussed the sources of metals in emissions from municipal incinerators. These authors have mentioned printing inks as sources of lead and zinc; paints as sources of titanium, chromium, and lead; and plastic stabilizers as sources of tin and cadmium. If municipal solid waste (MSW) is used to form the RDF, the possibility of metal contamination is considerably greater (4, 5). High metal concentrations have been reported in particles derived from refuse combustion. Rohten (2) reported that lead emissions increased substantially when RDF was added to coal. Greenberg and co-workers (6)found elevated levels of Zn, Cd, Sb, and possibly Ag, In, and Sn when refuse was burned in two incinerators in Washington, D.C., and Alexandria, VA. Subsequent work by Greenberg et al. (7) showed that the emissions from these two incinerators and that of a third in Chicago did not differ appreciably in the concentrations of metals. All showed elevated levels of the above-mentioned metals. In view of
* Address correspondence to this author at the following address: Systems, Science and Software, P.O. Box 1620, La Jolla, CA 92038. Present address: Chromatrix, 560 Oakmead Parkway, Sunnyvale, CA 94086. 148 Environ. Sci. Technol., Vol. 16, No. 3, 1982
these results, further investigation of the RDF-coal system is warranted. In this investigation, a comprehensive study has been made of fly ash derived from the combustion of mixtures ranging from pure coal to pure RDF. This study was undertaken to address the following objectives: (1) to determine the morphological and compositional characteristics of RDF/coal ash, (2) to determine the elemental composition of RDF/coal fly ash with respect to (i) the ratio of RDF to coal (ii) the size of the particles, (3) to determine the extent that individual elements present in the fly ash can be mobilized into solution as a result of an aqueous leaching process, and (4) to determine the factors that appear to be responsible for the partitioning of elements present in RDF/coal fly ash as a function of size and RDF content. It was felt that these factors would both define the physicochemical nature of RDF/coal fly ash and provide data for comparison with those already reported (8-20) for the fly ash derived from conventional coal-fired power plants. It has been established, for example, that fly ash derived from conventional coal combustion is generally spherical in nature and consists of several well-defined particle types ( 1 2 , 18,ZO). It has also been shown that many elements exhibit a trend of increasing concentration with decreasing particle size (8-20) although not all elements are consistent in their behavior. In addn., theories have been advanced that the outer surface of the particles contains substantially greater concentrations of many elements than does the fly ash bulk (11,21) and that smaller particles are most likely to exhibit this phenomenon. As a result, toxic trace elements associated with the surface could potentially increase the health risk of inhaling small particles even further (22). Given these well-defined characteristics of coal fly ash, an investigation of RDF/coal fly ash mixtures permits comparisons in a number of areas. In addition to characterizing the RDF/coal ash on the basis of morphology, composition, size, and RDF/coal ratio, the mobility of the elements in solution was also examined. Since much of the ash derived from RDF/coal combustion is destined for landfills, leaching of trace elements in the ash by rainfall, water runoff, or groundwater may play a significant role in the eventual environmental impact of RDF/coal combustion. Finally, the study reported herein was designed to evaluate those factors which were responsible for any partitioning of trace elements with respect to either size or RDF content, since it is only through an understanding of that process that effective control procedures can be designed (8, 20).
Experimental Section Materials. All fly ash samples were collected from a small power plant located in Hagarstown, MD. The plant, located at the Maryland Corrections Institute, is used
0013-936X/82/0916-0148$01.25/0
0 1982 American Chemical Society
primarily to generate steam for heating and cooking purposes and is of the shaking grate stoker type. All mixtures were burned under similar operating conditions so that results would be directly comparable. The RDF burned in the study was obtained in pelletized form from the National Center for Resource Recovery, Inc., Washington, D.C., which had separated out all noncombustible material. The coal was of the Kentucky Swickley seam type. The RDF/coal mixture was burned in proportions of 1:0, 1:1, 1:2, and 0:l RDF/coal by volume which corresponded to 0%, 35%, 52%, and 100% RDF by weight, respectively. Apparatus. Samples were collected by using an inertial cascade impactor (MeterologyResearch Institute, Altedina, CA) which had normal aerodynamic equivalent cutoff diameters at 50% efficiency of 30,15,6, 2.4, 1.5,0.65, and 0.37 pm for its seven stages. The combustion temperature was approximately 2230 O F (1220 "C), while the collection temperature was 400 "F (204 "C). Particles were scrapped off of the various stages into Teflon-lined glass bottles by using a Teflon scrapper. Two collections were composited when the 1:0 RDF/coal mixture was burned, three for both the 1:l and 1:2 mixtures, and six for the 0 1 mixture. Particle morphologies were observed by using a Hitachi Model HHS-2R scanning electron microscope. The unit was equipped with a Kevex Model 5000A energy dispersive X-ray spectrometer (XES)which was utilized in individual particle analysis of the 2.4-pm subsample. Elemental analyses were conducted by using several techniques. Semiquantitative analysis of the 6-pm size fraction was undertaken by using dc arc emission spectrometry (DCAES) in which the spectra were recorded photographically on a Baird-atomic 3-m grating spectrometer, Model 6x-1. Selected fly ash samples were analyzed by instrumental neutron activation analysis (INAA) utilizing the Illinois Advanced Reactor Facility having a neutron flux at the sample of approximately 2 X 1OI2 neutrons/(s/cm2). Finally, the size fractions 6-0.65 pm were analzyed for 18 elements by using plasma emission spectrometry (PES). A Spectrometrics Spectroscan I11 equipped with a three-electrode dc plasma source and an Echelle monochromator was employed in these analyses. Anionic analyses were performed by using a Dionex Model 10 ion chromatograph (Dionex Corp., Sunnyvale, CA). Surface area measurements were made through the use of the Quantasorb Sorption System (Qauntachrome Corp., Glenvale, NY)using nitrogen as the adsorbate and helium as the carrier gas. Procedures. To study particle morphologies and make individual particle X-ray studies, we mounted samples on double-sided cellophane tape and coated them with carbon. In those cases where X-ray information was not required, the samples were coated with gold by using a standard vapor deposition technique so as to minimize charging effects on the sample. Because of a limited quantity of sample, the standard procedure for making surface area measurements by nitrogen adsorption had to be modified. Under the usual operating conditions, samples are outgassed at 300 "C to remove any adsorbed material before determining the surface area. Prolonged exposure to this temperature can result in loss of potentially volatile elements such as arsenic or cadmium. To prevent this, outgassing was carried out at room temperature to enable subsequent elemental analyses of the same samples. Comparison of surface area measurements made under both conditions indicated incomplete outgassing at room temperature as expected. However, the values obtained were proportional to those obtained at 300 "C when outgassing and adsorption were
~
Table I. Surface Areas (mz/g)of RDF/Cod Fly Ash for the Seven Size Fractions Collected by Using an MRI Stack Sampler % stage cutoff (Gm) 50% efficiency RDF/ 6 2.4 1.5 0.65 0.37 coal 30 1 5 0 50 67 100
4.96 7.26 2.7
5.5
15.3
5.73 9.02 9.61 17.9
8.38 11.4 20.8
10.2 9.90
10.1
23.0
30.0
carried out under identical conditions for all samples. Samples analyzed by the DCAES were diluted with National SP-2 spectroscopic graphite using a Spex Mixer-mill. Indium (400 pg/g) was added as an internal standard. Samples were excited by a 28-A dc arc for 30 s. The spectra so obtained were analyzed with a manual densitometer. Those samples analyzed by PES were first digested in an acid mixture consisting of 3.5 mL of aqua regia, 2.5 mL of 48% hydrofluoric acid, and 0.5 mL of deionized water. The resulting digest was neutralized by using approximately 2 g of boric acid to remove the excess HF in the form of boron trifluoride (11). Specific analysis for arsenic was performed by generation of the hydride which was then determined by conventional flame atomic absorption spectrometry in a modification of the method described by Braman et al. ( 2 3 ) . Twenty-seven elements were detectable by INAA using both short and long irradiations. Following removal from the reactor, the samples were analyzed by using a Ge/Li detector in conjunction with a 4096-channel multichannel analyzer. The results were transferred to magnetic tape and analyzed by using the PIDAQ program (24). Analyses performed by INAA have a precision (based on counting statistics) of less than 10% for most elements. The accuracy appears to be comparable. Precisions of 1-2% are associated with the results obtained for As, Mn, and Na. It should be noted, however, that because of the small amounts of sample available, the sampling statistics are likely to be the limiting factor in determining the precision and the accuracy of the overall measurements. Thus, it is possible that the analyses are not totally representative of the overall sample. In order to obtain information about the solubility characteristics of RDF/coal fly ash, the size fractions from 6 to 0.37 pm were agitated by using a Heat Systems Model W 200R sonicator cell disruptor for 2 h with 15 mL of triply distilled deionized water. Sample masses varied from 0.0019 to 0.230 g (previous studies have established that water-soluble material can be quantitatively extracted under these conditions) (14). Following sonication the samples were filtered through a 0.45-pm Millipore filter and analyzed by using plasma emission spectrometery and ion chromatography.
Results The fly ash derived from burning RDF/coal mixtures appears to contain four basic morphological types, regardless of either size or RDF/coal ratio. These types are shown in Figure 1 and include particles resembling "shredded sponge", "rolled paper", "paint chips", and spheres. Unlike fly ash derived from a conventional coal-fired power plant, the majority of the observed particles were not spherical, and there is little evidence of smooth solid spheres or of cenospheres or plerospheres (11, 1 8 , 2 0 ) . In contrast, most material appeared to be of the type shown in Figure 1A. Most of the spherical particles, as is shown in Figure lD, were not solid and were significantly smaller than the other particles types. Envlron. Sci. Technol., Vol. 16, No. 3, 1982
149
Table 11. Elemental Concentrations ( p g l g ) of RDFI Coal Fly Ash for Different Percentages of RDF in the 2.4.pm Cutoff Impactor Stage "01% RDF 0 50 67 100 element aluminum 84800 57900 101000 46400 60 160 350 17 antimony
A
arsenic barium
bromine cadmium" calcium cerium
cesium chromiumD cobalt
a
coppera dysprosium
europium
gallium hafnium iron lanthanum lead' lutetium magnesium
manganese molvhdenum
C
phosphorus
potassium rubidium scandium
selenium strontium tantalum thallium thorium uranium
vanadiuma ytterbium zinc
1560 710 850 580 50 150 3 20 25000 44000 93 86
