Environ. Sci. Technol. 1998, 32, 2250-2256
Characterization of Flue Gas Residues from Municipal Solid Waste Combustors L Y D I E L E F O R E S T I E R * ,†,‡ A N D G U Y L I B O U R E L †,§ CRPG-CNRS, BP 20, 54501 Vandœuvre-le`s-Nancy, France, ENSG, BP 40, 54501 Vandœuvre-le`s-Nancy, France, and Universite´ H. Poincare´, Nancy 1, BP 239, 54506 Vandœuvre-le`s-Nancy, France
Solid residues recovered from treatment of flue gas resulting from the combustion of municipal solid waste (MSW) are of particular concern because of ever-increasing worldwide production rates and their concentrations of potentially hazardous transition elements and heavy metals. Three main residue types have been studied in this study: electrostatic precipitator ashes, wet filter cakes, and semidry scrubber residues. Using a large number of residues from two French MSW combustion (MSWC) facilities, the aim of this work is to determine their chemistry and mineralogy in order to shed light on their potential toxicity. We find that pollutant concentrations are dependent not only on the composition of MSW but also on the size of particles and flue gas treatment process. Using a procedure based on leaching, grain-size, density, and magnetic separations, we present a detailed description of the mineralogy of MSWC solid residues. These residues consist of a very heterogeneous assemblage of glasses, metals, and other crystals in which polluting elements are distributed. The results of this characterization will therefore help to contribute to the development of adequate waste management strategies.
Introduction Since the 1960s, the worldwide production of municipal solid wastes (MSW) has increased dramatically (e.g., increase of 60 vol % in France). Today, approximately 20 million ton of MSW is produced each year in France, representing nearly 1 kg person-1 day-1 (1). The MSW production rate is even greater in other countries, being between 1 and 2 kg person-1 day-1 in Sweden, Switzerland, Denmark, Germany, the United States, and Canada (2). In many countries, combustion has become a common management strategy for treating MSW. It presents several advantages: (1) a 90% volume reduction, (2) a 60-75% mass reduction, (3) a destruction of pathogenic agents, and (4) a possible recovery of exothermic energy. The combustion of MSW produces by mass approximately 70% fumes, 27% bottom ashes, and 3% MSW combustion (MSWC) solid residues resulting from the treatment of flue gas (3). Due to the high furnace temperatures and the high volatility of transition elements and heavy metals, MSWC solid residues are potentially the most polluting byproducts * To whom correspondence should be addressed; e-mail: lydie@ crpg.cnrs-nancy.fr; telephone: +33 383594211; fax: +33 383511798. † CRPG-CNRS. ‡ ENSG. § Universite ´ H. Poincare´. 2250
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ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 32, NO. 15, 1998
of combustion. Indeed pollutant elements such as arsenic, cadmium, chromium, mercury, nickel, lead, and zinc have been described in such residues (2, 4-8). Release of such elements during storage can pollute water tables and endanger living organisms. For the human being, arsenic, hexavalent chromium, nickel and their compounds are carcinogenic (9), hexavalent chromium can cause mutations, and the absorption of 1-2 g of HgCl2 is fatal (10). Due to their potential toxicity, it is essential to determine the concentration and distribution of pollutants in MSWC solid residues. Moreover, a detailed knowledge of the chemistry and mineralogy of these wastes is a prerequisite for any stabilization-solidification process (SSP) such as those required by many countries before storage. In this paper, we present the major, trace, and pollutant element chemistry of solid residues generated by two different MSWC facilities in France, over two periods of 1 month. In addition, we present a detailed study of their mineralogy to improve our knowledge of these anthropogenic materials and to shed light on their potential toxicity.
Materials Combustion and Treatment of Flue Gas. The solid residues studied come from two French MSWC facilities. Facility 1, opened in 1986, is located in an urban area of 600 000 inhabitants in southeast France and has a nominal capacity rating of 576 ton/day (two parallel trains of 12 ton/h each). MSW is fed into the combustion chamber with a grate consisting of six rollers for each train. Facility 2 is located in northeast France and began operation in 1988. This facility is equipped with two parallel trains, each having a nominal capacity rating of 6 ton/h. The feed stream is composed of household waste collected from an area with 167 000 inhabitants. Facility 2 consists of a primary combustor with movable grates. In both facilities, the furnace temperature is set around 1200 °C, and there is an air inlet above the burning waste to ensure that combustion of MSW occurs in oxidizing conditions. The flue gas thus generated consists of not only different gaseous species containing H, C, S, N, Cl, and O, among which HCl, CO2, SOx, and NOx dominate, but also gaseous forms of metals and organic species as well as dust particles. This flue gas is treated by one of two processes. The wet process, used at facility 1, produces two solid residues: (i) electrostatic precipitator ash (ESP ash), collected from electrofiltration of flue gas between 250 and 400 °C and (ii) filter cakes (FC), which are produced by treatment of the downstream flue gas in a scrubber at temperatures below 80 °C. In the scrubber, lime and water are used to neutralize acid gases, and TMT 15 (trimercaptotriazine) is used to fix Hg by forming organic sulfides of Hg. At facility 2, a semidry process is used, during which flue gas, after cooling, is cleaned by injection of a lime slurry into the scrubber, generating a single semidry scrubber residue (SDSR). These solid residues are, in general, heterogeneous materials resulting from complex processes occurring during the incineration and the raw gas treatment. As demonstrated by previous characterizations (2, 11), ESP ashes contain original fuel materials that have been mechanically transferred out of the fuel bed on the grate into the flue gas as well as condensate species found on the surfaces of fly ash particles, which result from the condensation of volatile species during the cooling phase of the flue gas inside the boiler. Filter cakes or wet scrubber residues contain salts from the neutralization of acid gases, mercury-bearing compounds, and other volatile-rich metal compounds S0013-936X(98)00100-X CCC: $15.00
1998 American Chemical Society Published on Web 06/27/1998
depending on the particle slip of the filter. Since fine particles that mainly pass the filter are enriched in volatile metals (11, 12), filter cakes determine the efficiency of the process, and are therefore important to characterize. Finally, semidry scrubber residues contain original fly ashes, reaction products (salts), and surplus reagent (lime) (6). SDSR can be considered as diluted ESP ashes as well as diluted scrubber residues. Sampling. In this paper, we describe all three types of MSWC solid residues: 18 ESP ashes, 19 FC, and 8 SDSR. For each sample of ESP ash from facility 1, 2-3 kg was collected directly at the base of the electrostatic filter prior to the storage silo. Samples of FC from facility 1 (1 kg) were collected at the exit of the filter press. For SDSR, 2-3 kg of sample was collected at the exit of the scrubber prior to the storage silo. In all cases, the collection method ensured that samples represent MSWC solid residues as produced rather than timeaveraged compositions. MSWC residues were sampled during two distinct periods in 1993. Between January 25 and February 5, 1993, 10 ESP ashes, 10 FC, and 8 SDSR were sampled at a rate of 4 or 5 samples per week. Between September 28 and October 29, 1993, 8 ESP ashes and 9 FC were sampled at a rate of 2 samples per week.
Methods Chemistry. Each sample of ESP ash and SDSR was dried at 80 °C for 24 h, homogenized, and ground to a grain size less than 70 µm. Samples of FC were initially dried at 80 °C for 24 h, divided into centimetric fragments before further drying at 80 °C for 24 h, and then homogenized and ground to a grain size less than 70 µm. Major elements (Si, Al, Fe, Mn, Mg, Ca, Na, K, Ti, and P) were analyzed by inductively coupled plasma atomic emission spectroscopy (ICP-AES), whereas concentrations of trace elements (As-Zr) were obtained by inductively coupled plasma mass spectrometry (ICP-MS). The analytical methodology for both ICP methods is as follows: 300 mg of sample portions is fused in platinum crucibles with 900 mg of LiBO2 in an automated tunnel furnace; the fused melts are dissolved in dilute nitric acid; and the final solutions are analyzed by ICP-AES and ICP-MS (13, 14). Specific methods are used for the following elements: chlorine is analyzed by absorptiometry (15), fluorine by potentiometry using an ion-selective electrode (15), sulfur and carbon by impulsion coulometry, and mercury by atomic absorption. For S, 50-250 mg of sample is sintered at around 1600 °C in an induction furnace under the flow of oxygen, the produced SO2 is absorbed in a cell at constant pH (pH ) 5) containing Na2SO4 and H2O2 to form H2SO4. The OH- ions produced by electrolysis to buffer the pH at the initial value are measured by coulometry and are directly proportional to SO2 content. For C, 50-500 mg of sample is sintered at 1100 °C in a tubular furnace under the flow of oxygen, the produced CO2 is absorbed in a cell at constant pH (pH ) 10) containing a solution of Ba(ClO4)2, and the OH- ions are also measured by coulometry. For Hg, 50-1000 mg of sample is dissolved using H2SO4-HNO3KMnO4 attack and measured using the amalgam method: Hg vapor is produced by reduction with SnCl2 and then transported under flow of argon to be fixed on an Au-Pt gauze; this gauze, sintered at 600 °C, frees Hg, which is finally measured by atomic absorption in a quartz cell. For all methods, the relative uncertainties are 5-25% for contents in the order of 1 ppm, 2-10% for contents in the order of 10 ppm, and 2-5% for contents above 100 ppm. Mineralogy. A single X-ray diffraction (XRD) pattern cannot be used to characterize the complex mineralogy of MSWC solid residues (Figure 1a). So, we have developed a procedure where ashes were leached in order to remove soluble salts and then separated according to grain size, density, and magnetic properties (Figure 1). A total of 100
FIGURE 1. Experimental procedure used to determine the mineralogy of MSWC solid residues and X-ray diffractograms of a ESP ash: (a) unwashed, (b) washed, (c) 50-200 µm fraction with a density G < 2.9 g cm-3, and (d) 50-200 µm fraction with a density G > 2.9 g cm-3. The dashed line indicates the occurrence of amorphous phases. Hal, halite; Syl, sylvite; Qz, quartz; Cal, calcite; Anh, anhydrite; Mel, melilite; Al, aluminum metal; Feld, feldspar; Lar, larnite; Cor, corundum; Hem, hematite; Rut, rutile; Per, perovskite; Wol, wollastonite; Zn, zinc metal. g of sample was mixed with 500 mL of distilled water. After one night of stirring, this mixture was filtered with a Bu ¨ chner filter equipped with a vacuum pump, and the solid residue obtained was dried and weighed. The residue was then sieved with water using three sieves: 500, 200, and 50 µm. The intermediate size fractions (50-200 and 200-500 µm) were separated with bromoform, which has a density of 2.9 g cm-3. Due to clustering of particles, the fraction
