characterization of flue gas residues from municipal solid waste combustors

Characterization of Flue GasResidues from Municipal SolidWaste CombustorsL Y D I E L E F O R E S T I E R * , † , ‡ A N DG U Y L I B O U R E L † , §

CRPG-CNRS, BP 20, 54501 Vandœuvre-les-Nancy, France,ENSG, BP 40, 54501 Vandœuvre-les-Nancy, France, andUniversite H. Poincare, Nancy 1, BP 239, 54506Vandœuvre-les-Nancy, France

Solid residues recovered from treatment of flue gasresulting from the combustion of municipal solid waste(MSW) are of particular concern because of ever-increasingworldwide production rates and their concentrations ofpotentially hazardous transition elements and heavy metals.Three main residue types have been studied in thisstudy: electrostatic precipitator ashes, wet filter cakes,and semidry scrubber residues. Using a large number ofresidues from two French MSW combustion (MSWC)facilities, the aim of this work is to determine their chemistryand mineralogy in order to shed light on their potentialtoxicity. We find that pollutant concentrations are dependentnot only on the composition of MSW but also on thesize of particles and flue gas treatment process. Using aprocedure based on leaching, grain-size, density, andmagnetic separations, we present a detailed descriptionof the mineralogy of MSWC solid residues. These residuesconsist of a very heterogeneous assemblage of glasses,metals, and other crystals in which polluting elements aredistributed. The results of this characterization willtherefore help to contribute to the development of adequatewaste management strategies.

IntroductionSince the 1960s, the worldwide production of municipal solidwastes (MSW) has increased dramatically (e.g., increase of60 vol % in France). Today, approximately 20 million ton ofMSW is produced each year in France, representing nearly1 kg person-1 day-1 (1). The MSW production rate is evengreater in other countries, being between 1 and 2 kg person-1

day-1 in Sweden, Switzerland, Denmark, Germany, the UnitedStates, and Canada (2). In many countries, combustion hasbecome 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 pathogenicagents, and (4) a possible recovery of exothermic energy.The combustion of MSW produces by mass approximately70% fumes, 27% bottom ashes, and 3% MSW combustion(MSWC) solid residues resulting from the treatment of fluegas (3). Due to the high furnace temperatures and the highvolatility of transition elements and heavy metals, MSWCsolid residues are potentially the most polluting byproducts

of combustion. Indeed pollutant elements such as arsenic,cadmium, chromium, mercury, nickel, lead, and zinc havebeen described in such residues (2, 4-8). Release of suchelements during storage can pollute water tables andendanger living organisms. For the human being, arsenic,hexavalent chromium, nickel and their compounds arecarcinogenic (9), hexavalent chromium can cause mutations,and the absorption of 1-2 g of HgCl2 is fatal (10). Due totheir potential toxicity, it is essential to determine theconcentration and distribution of pollutants in MSWC solidresidues. Moreover, a detailed knowledge of the chemistryand mineralogy of these wastes is a prerequisite for anystabilization-solidification process (SSP) such as thoserequired by many countries before storage.

In this paper, we present the major, trace, and pollutantelement chemistry of solid residues generated by two differentMSWC facilities in France, over two periods of 1 month. Inaddition, we present a detailed study of their mineralogy toimprove our knowledge of these anthropogenic materialsand to shed light on their potential toxicity.

MaterialsCombustion and Treatment of Flue Gas. The solid residuesstudied come from two French MSWC facilities. Facility 1,opened in 1986, is located in an urban area of 600 000inhabitants in southeast France and has a nominal capacityrating of 576 ton/day (two parallel trains of 12 ton/h each).MSW is fed into the combustion chamber with a grateconsisting of six rollers for each train. Facility 2 is locatedin northeast France and began operation in 1988. This facilityis equipped with two parallel trains, each having a nominalcapacity rating of 6 ton/h. The feed stream is composed ofhousehold waste collected from an area with 167 000inhabitants. Facility 2 consists of a primary combustor withmovable grates. In both facilities, the furnace temperatureis set around 1200 °C, and there is an air inlet above theburning waste to ensure that combustion of MSW occurs inoxidizing conditions. The flue gas thus generated consistsof 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 wellas dust particles. This flue gas is treated by one of twoprocesses. The wet process, used at facility 1, produces twosolid residues: (i) electrostatic precipitator ash (ESP ash),collected from electrofiltration of flue gas between 250 and400 °C and (ii) filter cakes (FC), which are produced bytreatment of the downstream flue gas in a scrubber attemperatures below 80 °C. In the scrubber, lime and waterare used to neutralize acid gases, and TMT 15 (trimercap-totriazine) is used to fix Hg by forming organic sulfides ofHg. At facility 2, a semidry process is used, during whichflue gas, after cooling, is cleaned by injection of a lime slurryinto the scrubber, generating a single semidry scrubberresidue (SDSR).

These solid residues are, in general, heterogeneousmaterials resulting from complex processes occurring duringthe incineration and the raw gas treatment. As demonstratedby previous characterizations (2, 11), ESP ashes containoriginal fuel materials that have been mechanically trans-ferred out of the fuel bed on the grate into the flue gas as wellas condensate species found on the surfaces of fly ashparticles, which result from the condensation of volatilespecies during the cooling phase of the flue gas inside theboiler. Filter cakes or wet scrubber residues contain saltsfrom the neutralization of acid gases, mercury-bearingcompounds, and other volatile-rich metal compounds

* To whom correspondence should be addressed; e-mail: [email protected]; telephone: +33 383594211; fax: +33 383511798.

† CRPG-CNRS.‡ ENSG.§ Universite H. Poincare.

Environ. Sci. Technol. 1998, 32, 2250-2256

2250 9 ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 32, NO. 15, 1998 S0013-936X(98)00100-X CCC: $15.00 1998 American Chemical SocietyPublished on Web 06/27/1998

depending on the particle slip of the filter. Since fine particlesthat mainly pass the filter are enriched in volatile metals (11,12), filter cakes determine the efficiency of the process, andare therefore important to characterize. Finally, semidryscrubber residues contain original fly ashes, reaction products(salts), and surplus reagent (lime) (6). SDSR can be con-sidered as diluted ESP ashes as well as diluted scrubberresidues.

Sampling. In this paper, we describe all three types ofMSWC solid residues: 18 ESP ashes, 19 FC, and 8 SDSR. Foreach sample of ESP ash from facility 1, 2-3 kg was collecteddirectly at the base of the electrostatic filter prior to the storagesilo. Samples of FC from facility 1 (1 kg) were collected atthe exit of the filter press. For SDSR, 2-3 kg of sample wascollected at the exit of the scrubber prior to the storage silo.In all cases, the collection method ensured that samplesrepresent MSWC solid residues as produced rather than time-averaged compositions. MSWC residues were sampledduring two distinct periods in 1993. Between January 25and February 5, 1993, 10 ESP ashes, 10 FC, and 8 SDSR weresampled at a rate of 4 or 5 samples per week. BetweenSeptember 28 and October 29, 1993, 8 ESP ashes and 9 FCwere sampled at a rate of 2 samples per week.

MethodsChemistry. Each sample of ESP ash and SDSR was dried at80 °C for 24 h, homogenized, and ground to a grain size lessthan 70 µm. Samples of FC were initially dried at 80 °C for24 h, divided into centimetric fragments before further dryingat 80 °C for 24 h, and then homogenized and ground to agrain size less than 70 µm. Major elements (Si, Al, Fe, Mn,Mg, Ca, Na, K, Ti, and P) were analyzed by inductively coupledplasma atomic emission spectroscopy (ICP-AES), whereasconcentrations of trace elements (As-Zr) were obtained byinductively coupled plasma mass spectrometry (ICP-MS).The analytical methodology for both ICP methods is asfollows: 300 mg of sample portions is fused in platinumcrucibles with 900 mg of LiBO2 in an automated tunnelfurnace; 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 followingelements: chlorine is analyzed by absorptiometry (15),fluorine by potentiometry using an ion-selective electrode(15), sulfur and carbon by impulsion coulometry, andmercury by atomic absorption. For S, 50-250 mg of sampleis sintered at around 1600 °C in an induction furnace underthe flow of oxygen, the produced SO2 is absorbed in a cellat constant pH (pH ) 5) containing Na2SO4 and H2O2 to formH2SO4. The OH- ions produced by electrolysis to buffer thepH at the initial value are measured by coulometry and aredirectly proportional to SO2 content. For C, 50-500 mg ofsample is sintered at 1100 °C in a tubular furnace under theflow of oxygen, the produced CO2 is absorbed in a cell atconstant 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-HNO3-KMnO4 attack and measured using the amalgam method:Hg vapor is produced by reduction with SnCl2 and thentransported under flow of argon to be fixed on an Au-Ptgauze; this gauze, sintered at 600 °C, frees Hg, which is finallymeasured by atomic absorption in a quartz cell. For allmethods, the relative uncertainties are 5-25% for contentsin the order of 1 ppm, 2-10% for contents in the order of10 ppm, and 2-5% for contents above 100 ppm.

Mineralogy. A single X-ray diffraction (XRD) patterncannot be used to characterize the complex mineralogy ofMSWC solid residues (Figure 1a). So, we have developed aprocedure where ashes were leached in order to removesoluble salts and then separated according to grain size,density, and magnetic properties (Figure 1). A total of 100

g of sample was mixed with 500 mL of distilled water. Afterone night of stirring, this mixture was filtered with a Buchnerfilter equipped with a vacuum pump, and the solid residueobtained was dried and weighed. The residue was then sievedwith water using three sieves: 500, 200, and 50 µm. Theintermediate size fractions (50-200 and 200-500 µm) wereseparated with bromoform, which has a density of 2.9 g cm-3.Due to clustering of particles, the fraction <50 µm could notbe separated with bromoform. Close to 99 wt % of the 50-200 µm fraction has a density below 2.9 g cm-3. This lightfraction was then magnetically separated using a Frantzseparator. This procedure was applied to two ESP ashes,two SDSR, and two FC. Each fraction was analyzed by ICP-AES, ICP-MS, and wet-chemical methods. XRD was used toidentify crystalline phases of each ash fraction. A Jobin Yvon-Sigma 2080 diffractometer was used along with a copperX-ray source (38 kV, 20 mA, 800 W) on sample powder (<50µm). Scans were conducted from 2 to 32° at a rate of 0.5°of θ/min. The diffractogram was evaluated for possiblecrystalline phases using the database of pure species of ASTM(American Society for Testing Materials). Furthermore, theelectron microprobe was used to determine the phasecompositions (solid solutions, alloys, glasses, etc.) and the

FIGURE 1. Experimental procedure used to determine the mineralogyof MSWC solid residues and X-ray diffractograms of a ESP ash:(a) unwashed, (b) washed, (c) 50-200 µm fraction with a densityG < 2.9 g cm-3, and (d) 50-200 µm fraction with a density G > 2.9g cm-3. The dashed line indicates the occurrence of amorphousphases. 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, wol-lastonite; Zn, zinc metal.

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nature of minor phases (by stoichiometric constraints).Polished thin sections of each ash fraction were examinedusing a CAMECA SX-50 electron microprobe equipped withfour spectrometers and a wavelength-dispersive system(WDS). All WDS analyses were performed with a 15 kVaccelerating voltage and 10.0 nA beam curent. The followingelements were analyzed: Na, Mg, Al, Si, P, S, Cl, K, Ca, Ti,Cr, Fe, Ni, Cu, Zn, Ba, and Pb. Electron microprobestandardization was conducted using polished geologicalstandards prior to analyses. This characterization wascompleted by using an Hitachi S-2500 scanning electronmicroscopy (SEM), equipped with a Kevex 4850-S energydispersive spectrometer (EDS).

ResultsTexture. Macroscopically, ESP ashes and SDSR are powderyashes; SDSR are gray in color while ESP ashes are gray spottedby black and white particles. FC look like gray modelingclay. As shown in Table 1, 90% of the particles forming theseresidues are smaller than 200 µm and are generally finer inFC particles than ESP ash and SDSR. Charred particlesdominate the fraction above 200 µm, but millimetric un-burned particles are also present, especially in ESP ash,indicating that the combustion of MSW is incomplete.Characterization of the samples by SEM shows a wide rangeof particle shapes, including flakes, prisms, and needles, aswell as frequent spheres and sintered agglomerations of dust.Char and agglomerated spheres have also been observed byStuart and Kosson in fly ashes and lime spray drier scrubberresidues from municipal waste combustion (5). Sphericalparticles, partially or completely glassy, with or withoutbubbles, indicate that melting and degassing occur in thecombustion chamber. It is also noteworthy that micrometriccrystals are observable on the surface of particles. In ESPash, alkali chloride and calcium sulfate were recognized onthe surface of glass spherules, consistent with previousobservations (16, 17).

Chemistry and Pollutant Concentrations. Our analysesshow that MSWC solid residues consist of calcium, chlorine,silicon, and aluminum as major elements in addition to minoramounts of carbon, sulfur, sodium, potassium, iron, titanium,magnesium, fluorine, and phosphorus (Table 2). The calciumcontent of MSWC solid residues varies according to thetreatment process. SDSR samples contain the highestcalcium concentrations, between 22 and 30 wt %, due to theinjection of limewater. These samples also contain thehighest chlorine contents (15-20 wt %) and show theefficiency of lime to neutralize the acid gases such as HCl.In general, the high chlorine content can be considered asa major characteristic of MSWC solid residues, 40-50% ofCl being originating from the plastic fraction, mainly fromPVC (1). The low silicon and aluminum contents in SDSRsamples (mean of 5.6 wt % for Si and 3.6 wt % for Al) contrastwith the 13.2 wt % Si and 9.3 wt % Al of ESP ash samples.This is most likely due to a dilution effect linked to theinjection of limewater.

Our chemical characterization also reveals the presenceof numerous minor and trace elements (Table 2), some ofwhich are potential pollutants (e.g., As, Cd, Cr, Hg, Ni, Pb,and Zn). ICP-MS analyses indicate that their concentrationsvary from tens of ppm for As up to tens of thousands of ppm

for Zn (Table 2). Zinc has a mean value of nearly 1 wt % forESP ashes and SDSR and 1.5 wt % for FC. MSWC solidresidues also contain several thousand ppm of lead, with amaximum mean value of 7000 ppm for FC. Chromium andnickel contents vary between 40 and 1000 ppm and are bothconcentrated in ESP ashes. We find that FC are rich incadmium and mercury with maximum contents of 600 and1600 ppm, respectively. Among the remaining trace ele-ments, tin, copper, barium, and antimony have the highestconcentrations, up to 2800 ppm for Sn. Ba and Sb areconcentrated in ESP ashes, whereas Cu and Sn are concen-trated in FC.

Comparison of different samples of the same waste typeshows that chemical variability between winter and autumn1993 is weak. For each type of MSWC solid residue, elementalconcentrations for Cd, Cr, Hg, Pb, and Zn vary by factorsbetween 1.3 and 3.5. Our data for French MSWC solidresidues are comparable with the database presented by theInternational Ash Working Group (2). Indeed, contents ofthe major elements Ca, Cl, Si, and Al and also the minorelements Na, K, Mg, and S in our MSWC solid residues arein the range of those collected by the IAWG. For Cd,concentrations in IAWG residues are in the same order thanthose in all our MSWC solid residues. In ESP ashes from theFrench facility 1, the trace elements Cr, Sb, Cd, Sr, Ni, As, Co,V, and Mo are present in concentrations below 1000 ppm,in agreement with IAWG data. However, Pb contentspresented by IAWG (5300-26 000 ppm) are higher than thoseobserved in our samples (<5300 ppm).

Mineralogy. Toxicity of MSWC solid residues is depend-ent not only on the polluting element concentration but alsoon the speciation of the pollutant element(s) and nature ofthe host phases. For instance, hexavalent chromium is moretoxic than trivalent chromium (9), and zinc chloride (ZnCl2),is very soluble in water while zinc carbonate (ZnCO3), is nearlyinsoluble. Therefore, a detailed knowledge of the mineralogyof these wastes is required. XRD of ESP ashes (Figure 1a)reveals the dominance of alkali chlorides (NaCl, KCl), calciumsulfate, calcite, quartz, and amorphous phases. For SDSR,the same phases have been identified by XRD in addition tothe major phase CaCl2‚Ca(OH)2‚H2O. This occurrence ofchlorides and sulfates is also supported by the high con-centrations of Ca, Na, K, Cl, and S in leachates from regulatoryFrench X31-210 leaching test (with demineralized water atambient temperature and during 48 h) (8).

A simple mass-balance calculation shows that it is notpossible to reconcile the bulk chemistry of MSWC solidresidues with the chemical composition of the phasesidentified by XRD. To better characterize the mineralogy ofthese residues, we have used, as decribed above, a procedurebased on grain size, density, and magnetic separations.Leaching of untreated ESP ashes and SDSR removes thesoluble salts and hence their prominent contribution in XRD(e.g., Figure 1a). This allows the identification of additionalaluminosilicate and silicate minerals (e.g., Figure 1b). It isimportant to notice that leaching may form some newcrystalline phases that are not originally present in MSWCsolid residues, e.g., hydroxides. After leaching, the lightfractions (F < 2.9 g cm-3) reveal the presence of melilite,feldspars, and larnite (e.g., Figure 1c). The dense fractions(F > 2.9 g cm-3) are characterized by a mineralogy (e.g., Figure1d) dominated by oxides, such as corundum, hematite, rutile,perovskite, with small amounts of wollastonite, magnetite,alloys, and metallic phases: zinc, iron, aluminum. Themagnetic fraction consists essentially of hematite, magnetite,and spinel. Using this separation coupled with the use ofXRD, SEM, and electron microprobe, we have been able toidentify sulfates, carbonates, silicates, phosphates, fluorides,chlorides, oxides, hydroxides, pure metals, alloys, graphite,and glasses in the same ash, emphasizing the extreme

TABLE 1. Grain Size Distribution of MSWC Solid Residues

MSWC solid residue <50 µm 50-200 µm 200-500 µm >500 µm

ESP ash (%)a (n ) 2)b 35-40 60-65 2-5 <1FC (%) (n ) 2) 55-70 25-30 5-15 0SDSR (%) (n ) 2) 45-50 40-50 2-10 <0.5

a % ) mass fractions. b n ) number of samples.

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diversity of their mineralogy (Table 3). Electron microprobeanalyses show that glasses have a calcium-rich alumino-silicate composition (Figure 2), melilite is close to Åk57-Geh43, plagioclase to anorthite, and alkali feldspar to Ab69/Or31. Moreover, ICP-MS analyses of each size fraction reveal,particularly for ESP ash, that concentrations of As, Cd, Cr,Cu, Hg, Pb, Sb, Sn, and Zn are inversely correlated with grain

size, consistent with previous work (5, 17, 19-21). However,such size concentration correlations are not observed forother elements.

Accurate characterization of bulk and phase compositionsallows the calculation by mass balance of phase proportions.The following major elements were used: Si, Al, Fe, Mg, Ca,Na, K, Ti, P, Cl, S, and C. Phase proportions (Figure 2) could

TABLE 2. Mean, Standard Deviation, and Minimum and Maximum Values of Chemical Contents in ESP Ashes, FC, and SDSR

ESP ashes (n ) 18) FC (n ) 19) SDSR (n ) 8)

element mean std deva minb maxc mean std dev min max mean std dev min max

Si (wt %) 13.22 0.68 11.84 14.67 7.11 1.78 3.95 9.73 5.61 0.68 4.95 6.53Al (wt %) 9.34 0.58 8.22 10.45 3.89 1.48 2.06 6.58 3.57 0.30 3.18 3.92Fe (wt %) 1.21 0.17 1.01 1.69 1.74 0.44 0.78 2.36 0.84 0.07 0.73 0.93Mn (wt %) 0.08 0.02 0.05 0.12 0.07 0.02 0.03 0.11 0.09 0.02 0.06 0.12Mg (wt %) 1.81 0.09 1.60 1.97 3.92 0.73 2.72 5.37 0.98 0.08 0.87 1.10Ca (wt %) 16.85 1.08 14.13 18.26 16.64 1.72 13.63 19.98 27.43 2.26 22.21 29.68Na (wt %) 2.49 0.39 1.97 3.47 0.12 0.05 0.06 0.22 1.52 0.13 1.33 1.79K (wt %) 2.80 0.45 2.24 3.97 0.22 0.11 0.02 0.42 1.69 0.16 1.34 1.87Ti (wt %) 1.06 0.06 0.98 1.22 0.36 0.13 0.18 0.58 0.44 0.05 0.38 0.50P (wt %) 0.62 0.04 0.55 0.71 0.31 0.08 0.16 0.44 0.32 0.03 0.28 0.36Cl (wt %) 6.91 0.88 5.73 9.54 10.00 1.84 7.20 14.45 18.26 1.65 15.46 20.42F (wt %) 0.23 0.03 0.18 0.29 2.28 0.49 1.30 3.39 0.18 0.04 0.13 0.24S (wt %) 1.24 0.16 0.92 1.55 3.05 1.00 1.43 4.63 1.11 0.27 0.70 1.77C (wt %) 2.55 0.58 1.60 3.73 2.73 0.65 1.68 4.21 2.15 0.50 1.70 3.31As (ppm) 28 16 16 82 45 9 29 61 19 2 17 21Ba (ppm) 1482 145 1329 1881 162 103 53 417 804 103 689 1024Cd (ppm) 166 53 90 311 429 87 252 610 126 20 98 154Co (ppm) 28 22 15 110 9 2 6 13 10 1 8 12Cr (ppm) 549 146 367 1048 292 80 161 421 217 28 188 267Cu (ppm) 741 177 532 1274 1341 268 903 1841 434 39 388 511Hg (ppm) 19 6 10 35 907 253 465 1565 18 5 12.5 27Mo (ppm) 36 10 25 58 18 8 8 44 ndd nd nd ndNi (ppm) 96 45 55 240 72 16 49 111 52 9 42 68Pb (ppm) 2611 822 1705 5250 7256 1558 4390 9805 2780 316 2380 3180Rb (ppm) 74 7 63 94 12 6 5 27 48 17 24 81Sb (ppm) 720 500 174 1781 461 279 195 1389 177 7 172 182Sn (ppm) 863 246 635 1700 2090 395 1410 2800 814 106 640 985Sr (ppm) 388 28 342 460 97 25 68 162 337 41 279 296V (ppm) 40 5 31 50 109 26 55 157 38 14 23 57W (ppm) 31 28 11 127 12 5 6 22 6 1 5 6Zn (ppm) 7339 1610 5200 12500 14874 3265 9120 20100 8211 1048 6830 9750Zr (ppm) 154 25 120 200 65 17 35 94 61 4 52 64

a std dev ) standard deviation. b min ) minimum. c max ) maximum. d nd ) not determined.

TABLE 3. Mineralogy of MSWC Solid Residues

silicateschlorides, sulfates,

and carbonates oxidesmetallicphases

otherphases

glass rich in SiO2. CaO, andAl2O3

a-chalitea,b

NaClhematitea,b

Fe2O3

aluminiuma,b

Algraphitea,b

Cquartza-c

SiO2

sylvitea,b

KClmagnetitea,b

Fe3O4

zinca,b

Znportlanditeb

Ca(OH)2larnitea,b

Ca2SiO4

compounda,b

CaCl2Ca(OH)2H2Ospinela(Zn, Ni, Fe)Fe2O4

irona

Feotherhydroxidesb

wollastonitea

CaSiO3

lead chloridea,b

PbCl2rutilea,b

TiO2

complex alloysa

Fe-Al-Cu-Ni-Zn-Si-Cfluoritea-c

CaF2melilitea,b

Ca2(Mg, Al)(Al, Si)2O7

zinc chloridea

ZnCl2perovskitea,b

CaTiO3

chlorapatitea,b

Ca5(PO4)3Clalkali feldspara,b

(Na, K)AlSi3O8

anhydritea,b

CaSO4 BaTiO3a

whitlockitea,b

Ca3(PO4)2anorthitea,b

CaAl2Si2O8

gypsuma-c

CaSO4‚2H2Ocorunduma,b

Al2O3Ti-bearing pyroxenea,b

CaMgSi2O6-CaTiAl2O6

baritea,b

BaSO4

lead oxidea

PbOtitanitea,b

Ca(Ti, Al, Fe)SiO4(O, OH, F)

ettringitea,b

3CaO‚Al2O3‚3CaSO4‚32H2O

calcitea-c

CaCO3

a Phases identified in ESP ash. b Phases identified in FC. c Phases identified in SDSR.


only be estimated for ESP ash and SDSR samples becausedensity separation was unsucessful for FC samples due totheir high content of fine particles (Table 1) and the clusteringof these fines. This calculation shows that ESP ash samplescontain around 40 wt % glass and 60 wt % crystallized phases,comprising 30 wt % silicates; 15 wt % chlorine-bearing phases;5-10 wt % sulfates and carbonates; and around 5 wt % metals,oxides, graphite, and remaining phases. SDSR samplescontain approximately 20 wt % glass and 80 wt % crystallizedphases, comprising 10-15 wt % silicates; 50-60 wt %chlorine-bearing phases; 10-15 wt % sulfates and carbonates;and less than 5 wt % metals, oxides, graphite, and otherphases. Despite their different bulk chemistry and geo-graphical origin, it is of note that ESP ashes and SDSR havea similar mineralogy in terms of the major phases (Table 3).

The main difference lies in the high proportion of a Ca-richhydroxy chloride compound (45-50 wt %) in SDSR samplesformed due to lime injection in the semidry process. Thisdetailed characterization extends previous studies devotedto the mineralogy of MSWC solid residues (6, 7, 22, 23).

DiscussionThe complex mineralogy of MSWC solid residues results fromseveral processes, including vaporization, melting, crystal-lization, vitrification, condensation and precipitation, thatoccur during combustion and treatment of flue gas (8, 11,17, 20, 24-26). Such processes are well illustrated in the wetprocess. Electrofiltration at 250-400 °C preferentially recov-ers dust particles, which include inherited phases from MSW(e.g., quartz, feldspar) and phases synthesized at hightemperature (e.g., silicate glass, melilite) but also micrometricphases condensed on the surface of ash particles during thecooling of the flue gas between the furnace and theelectrofilters (e.g., alkali chlorides). Finally, reactions of theresidual flue gas with lime and water in the scrubber (<80°C) favor phase precipitation, e.g., ettringite, chlorides,sulfates in FC.

Among these processes, volatilization-condensationmechanisms are fundamentally important in controlling boththe concentration and crystal chemistry of pollutants. Sinceelectrofiltration removes larger fly ash particles but is lessefficient for vapor and finer particles, the concentration ratioof elements between ESP ash and FC is a reflection of thepartitioning of elements between particulate and gaseoushosts (Figure 3). Most lithophile and siderophile elementsenter in larger solid particles. Elements with high boilingtemperatures (W, Mo, Zr, Ti, Co, Al, Si, Ba, Sr) do not volatilizeduring combustion but melt or are found in phases inheritedfrom MSW. Among lithophile elements, alkalis (Na, K, Rb,Cs), and P undergo volatilization-condensation between thecombustion area and electrofilters. On the contrary, a majorfraction of chalcophile and atmophile elements remains inthe vapor (e.g., F) or enters in minute phases condensedafter electrofiltration (e.g., Cu, Fe, Sn). It is of note that ofthe lithophile elements, Mg has a peculiar behavior with a(ESP ash/FC) ratio less than 1. Two interpretations arepossible: either Mg, used as an additive in plastics, musthave been vaporized at least for a short time or alternativelylime added for the treatment of flue gas is not pure andcontains some Mg. Concerning the pollutant elements,Figure 3 shows that Pb, Zn, Cd, As, and Hg are concentratedin FC with other chalcophile and atmophile elements. Thissuggests that they are present principally in the gas thatcondenses upon cooling (250-80 °C). On the contrary, Crand Ni are concentrated in ESP ash together with other

FIGURE 2. Phase proportions of MSWC solid residues. (a) Proportionsof major and minor phases contained in ESP ash. (b) Proportionsof major and minor phases contained in SDSR. Comp, compoundCaCl2‚Ca(OH)2‚H2O. Average glass composition in ESP ash andstandard deviation (n ) 17): Si ) 16.75 ( 2.61 wt %, Al ) 9.79 (4.18 wt %, Fe ) 2.24 ( 2.17 wt %, Ca ) 20.93 ( 6.28 wt %, Na )1.04 ( 0.74 wt %, K ∼ 0.50 wt %, Ti ∼ 0.96 wt %, P ) 0.92 ( 0.52wt %, S ∼ 0.28 wt %, Zn ∼ 3200 ppm. Average glass compositionin SDSR and standard deviation (n ) 21): Si ) 16.85 ( 1.87 wt %,Al ) 9.53 ( 3.81 wt %, Fe ) 1.47 ( 1.40 wt %, Ca ) 22.21 ( 4.93wt %, Na ) 1.11 ( 0.74 wt %, K ∼ 0.33 wt %, Ti ∼ 0.72 wt %, P) 1.13 ( 0.78 wt %, S ∼ 0.50 wt %, Zn ∼ 2300 ppm. Note their closecomposition, despite their formation in different MSWC facilities.

FIGURE 3. Element distribution between ESP ashes and filter cakesFC. ESPash/FC, mean element concentration in ESP ash/meanelement concentration in FC. Elements are grouped by theirgeochemical behavior: lithophile, siderophile, chalcophile, andatmophile elements. In each group, elements are ranked followingtheir decreasing boiling temperature.


lithophile and siderophile elements, suggesting that theseelements preferentially form solid particles. Similarly, theobserved relationship in ESP ash between pollutant con-centration (As, Cd, Hg, Pb, and Zn) and particle size can beaccounted for by such condensation-adsorption mecha-nisms. These pollutant elements or their compounds, whoseboiling or sublimation temperatures are below the combus-tion temperature (up to 1500-1600 °C), must be volatilizedand then condensed on the surface of particles. Sincecondensation is largely a surface process, the amount ofcondensed phases and therefore the concentration of thesevolatile elements will increase as the surface area of particlesincreases. Such pollutant behavior during incineration ofMSW is very similar to that observed for coal ashes (20) andother fly ashes (5, 11, 17, 19, 21).

The distribution of polluting elements (As, Cd, Cr, Hg, Ni,Pb, Zn) between the diverse solid phases present in MSWCresidues has direct consequences for the toxicity of thesematerials. As-, Cd-, Cr-, and Ni-bearing phases are difficultto determine because of the low contents of these pollutantsin MSWC solid residues. However, zinc, the most abundantpolluting element in these residues, is found as pure metallicparticles and chloride as well as in alloys, spinels, andcalcium-bearing aluminosilicate glass. In the same way, leadhas a complex distribution (Table 3 and Figure 2), since itis found as soluble salts, as metal, and in silicates and/oroxides. Sandell et al. (27) also indicate the presence of leadin several types of particles with various compositions (e.g.,Pb-Cl-rich inclusions). Clearly, more work needs to be doneon the crystal chemistry of pollutants since it determinesboth leaching behavior and toxicity of MSWC solid residues.

An ideal flue gas treatment would produce a small massof solid residue, highly enriched in polluting elements.Incineration of 1 ton of MSW produces on average 25 kg ofESP ashes and 3 kg of FC for facility 1 and 35 kg of SDSR forfacility 2; that is to say that the semidry process produces1.25 times more solid residues by mass than the wet process.Hence, in terms of mass produced, the wet process is moreefficient. However, the relative efficiency with which pol-lutants from MSW are retained during the two processes isdifficult to evaluate, and more work has to be done in thisdirection. Indeed, no chemical data for the input flow areavailable, preventing mass balance calculations of singlepolluting elements in the two flue gas treatments. However,several features should be noted: the wet process retainsmuch more Hg, thanks to TMT15, while Cl retention is moreefficient in the semidry process, due to the lime surplus.

In conclusion, this work documents the chemistry andthe mineralogy of MSWC solid residues, in particular wetand semidry scrubber residues, poorly characterized up tonow. This study shows that pollutant concentrations inMSWC solid residues are dependent not only on thecomposition of MSW but also on the size of particles and theflue gas treatment process. We also show that due to theconcentrations of pollutants and their significant leachability(6, 7, 8, 28), MSWC solid residues must be consideredpotentially toxic and require further treatment. Knowledgeof the nature and proportions of phases contained in eachtype of MSWC residue (i.e., volatile vs refractory, glassy vscrystalline, soluble vs nonsoluble) is therefore essential forany stabilization-solidification processes, i.e., cement- ororganic-based processes or vitrification. For instance, thenature of chlorine-bearing phases determines the setting andthe durability of hydraulic binders (29). Similarly, meltingtemperatures depend on the proportion of refractory com-ponents. Therefore, vitrification of SDSR, rich in calcium,will require higher temperatures than that of ESP ash, beingmore costly and less efficient at retaining polluting elements.

AcknowledgmentsFinancial support was provided from the joint researchprogram CNRS-SITA “Analyse et inertage des dechets ul-times” and ECOTECH. Thanks are expressed to SARM (CRPG)for chemical analyses, Francois Lhote (CRPG) for XRD,Sandrine Barda (Universite Henri Poincare) for electronmicroprobe analyses, and William Brown, Mike Toplis, JeanCarignan, and Jerome Sterpenich for useful discussions. Inaddition, we gratefully thank Francis Mosnier (SITA), Jean-Yves Bottero (Cerege), and Jacques Yvon (LEM) for theirsupport of this work.

Literature Cited(1) ADEME. Les dechets en chiffre; Agence de l’Environnement et

de la Maıtrise de l’Energie: 1994.

(2) Chandler, A. J.; Eighmy, T. T.; Hartlen, J.; Hjelmar, O.; Kosson,D. S.; Sawell, S. E.; Van der Sloot, H. E.; Vehlow, J. An internationalperspective on characterization and management of residuesfrom municipal solid waste incineration; International AshWorking Group: 1994.

(3) Troesch, O. Elimination des residus d’epuration des fumees del’incineration des ordures menageres; G.I.E. Recherche du PoleEnergie-Chaleur: 1991.

(4) Pluss, A.; Ferrell, R. E. Hazard. Waste Hazard. Mater. 1991, 8,275-292.

(5) Stuart, B. J.; Kosson, D. S. Combust. Sci. Technol. 1994, 101,527-548.

(6) Eighmy, T. T.; King, W. N., III; Koehler, C. M.; Krzanowski, J. E.;Eusden, J. D., Jr.; Martin, J. R.; Sawell, S. E.; Owen, G. T. MunicipalWaste Combustion Conference; Air and Waste ManagementAssociation: Washington, DC, 1995; pp 683-708.

(7) Eighmy, T. T.; Eusden, J. D., Jr.; Krzanowski, J. E.; Domingo, D.S.; Stampfli, D.; Martin, J. R.; Erickson, P. M. Environ. Sci. Technol.1995, 29, 629-646.

(8) Le Forestier, L. Residus ultimes de l’incineration de dechetsmenagers. Caracterisation chimique et mineralogique, essaisde stabilisation par vitrification et comportement des verres ala lixiviation. Thesis, Institut National Polytechnique de Lorraine,Nancy, 1996.

(9) Picot, A. Risques chimiques dans les laboratoiressToxicologiedes produits chimiques; Centre National de la RechercheScientifique: 1988.

(10) Rousseaux, P. Les metaux lourds dans les ordures menageres:origines, formes chimiques, teneurs, Laboratoire de ChimiePhysique Appliquee et de l′Environnement, INSA, Lyon, 1988.

(11) Cahill, C. A.; Newland, L. W. Int. J. Environ. Anal. Chem. 1982,11, 227-239.

(12) Campbell, J. A.; Laul, J. C.; Nielson, K. K.; Smith, R. D. Anal.Chem. 1978, 50, 1032-1040.

(13) Govindaraju, K.; Mevelle, G. J. Anal. At. Spectrom. 1987, 2, 615.

(14) Govindaraju, K.; Potts, P. J.; Webb, P. C.; Watson, J. S. Geostand.Newsl. 1994, XVIII, 211.

(15) Vernet, M.; Marin, L.; Boulmier, S.; Lhomme, J.; Demange, J. C.Analusis 1987, 15, 490.

(16) Furuya, K.; Miyajima, Y.; Chiba, T.; Kikuchi, T. Environ. Sci.Technol. 1987, 21, 898.

(17) Ontiveros, J. L.; Clapp, T. L.; Kosson, D. S. Environ. Prog. 1989,8, 200-206.

(18) Ministere de l’Environnement. Arrete du 18 fevrier 1994modifiant l’arrete du 18 decembre 1992 relatif au stockage decertains dechets industriels speciaux ultimes et stabilises pourles installations existantes, J. Offi. Republique Francaise 1994,6117.

(19) Natusch, D. F. S.; Wallace, J. R.; Evans, C. A. Science 1974, 183,202.

(20) Klein, D. H.; Andren, A. W.; Carter, J. A.; Emery, J. F.; Feldman,C.; Fulkerson, W.; Lyon, W. S.; Ogle, J. C.; Talmi, Y.; Van Hook,R. I.; Bolton, N. Environ. Sci. Technol. 1975, 9, 973-979.

(21) Greenberg, R. R.; Zoller, W. H.; Gordon, G. E. Environ. Sci.Technol. 1978, 12, 566-573.

(22) Giordano, P. M.; Behel, A. D.; Lawrence, J. E.; Solleau, J. M.;Bradford, B. N. Environ. Sci. Technol. 1983, 17, 193.


(23) Kirby, C. S.; Rimstidt, J. D. Environ. Sci. Technol. 1993, 27, 652-660.

(24) Fernandez, M. A.; Martinez, L.; Segarra, M.; Garcia, J. C.; Esplell,F. Environ. Sci. Technol. 1992, 26, 1040-1047.

(25) Vehlow, J. UTA Int. 1996, 2, 144-159.(26) Jakob, A.; Stucki, S.; Struis, R. P. W. J. Environ. Sci. Technol.

1996, 30, 3275-3283.(27) Sandell, J. F.; Dewey, G. R.; Sutter, L. L.; Willemin, J. A. J. Environ.

Eng. 1996, 122, 34-40.(28) Hjelmar, O. Proceedings of Sardinia ’95, Fifth International

Landfill Symposium, 1995; pp 329-356.

(29) Capmas, A.; Sorrentino, F. Etude des mecanismes de solidification/stabilisation des dechets solides par liants hydrauliques. Cas desREFIOM, Environnement et Technique/Info dechets, No. 136;1994.

Received for review February 2, 1998. Revised manuscriptreceived May 7, 1998. Accepted May 21, 1998.

ES980100T


characterization of flue gas residues from municipal solid waste combustors

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