Characterization of Fly Ashes from Two ChineseMunicipal Solid Waste Incinerators

Min Li,* Song Hu, Jun Xiang, Lu-Shi Sun, Pei-Sheng Li, Sheng Su, andXue-Xin Sun

State Key Laboratory of Coal Combustion, Huazhong University of Science and Technology,430074 Wuhan, Hubei, P.R.C.

Received April 24, 2003

In China, the generation of municipal solid waste (MSW) incineration fly ashes is expected toincrease significantly in the future. Because of ever-increasing generation rates and theconcentrations of potentially hazardous heavy metals, these fly ashes are of particular concern.Thus, such issues have necessitated the study of the characterization of the fly ashes. A detailedcharacterization of two samples of fly ashes, collected from two types of national MSW incinerators,was carried out in terms of chemical composition, morphology, mineralogy, and leaching behavior.Results of chemical analysis and leaching tests suggested that they should be classified ashazardous wastes and should be properly managed. Therefore, the results of this characterizationwould contribute to the development of adequate waste management strategies.

1. Introduction

As incineration of MSW has lots of advantages suchas significant volume reduction (about 90%) and massreduction (about 70%), complete disinfection, energyrecovery, and so on;1-4 and many large- and medium-size cities in China have constructed, are constructing,and/or plan to install MSW incineration devices.

However, incineration of MSW not only producessignificant fumes, but also gives rise to considerableamounts of solid residues (bottom ash, grate sifting, heatrecovery ash, fly ash, and air pollution control (APC)residue) that are generated at different points in theprocess of MSW incineration. With improvements inAPC systems, modern MSW incineration plants emitpractically no pollutants into the atmosphere; thatresults in the World Health Organization’s statementthat it no longer considers the emissions from modern,well-operated, and well-maintained MSW incineratorsto be a hazard to human health or the environment.5However, the hazardous fractions in MSW are concen-trated in the solid residues, especially in fly ashes.Indeed, pollutant elements such as As, Cd, Cu, Cr, Hg,Ni, Pb, and Zn have been described in fly ashes.6,7

Release of such elements during storage will have apotentially negative impact on environmental quality,human health, and groundwater as well as on surfacewater resources.2,8

With the incineration method being widely adoptedto cope with MSW, the generation of MSW incinerationfly ashes is expected to increase in the future in China.The large quantity, coupled with the potential leach-ability of high metal concentrations in the residues, hasnecessitated the study of the chemical, morphological,mineralogical, and leaching properties of these solidresidues. Moreover, determining safe management al-ternatives for these fly ashes requires extensive char-acterization.

This paper describes a comprehensive study of twosamples of MSW incineration fly ashes generated by twodifferent types of national MSW incineration facilities,with the intention of elucidating their chemical, mor-phological, and mineralogical properties as related totheir effects on the environment and utilization. Inaddition, a leaching test to shed light on their potentialtoxicity is also presented.

2. Sampling

The fly ashes used in this study came from twodifferent types of national MSW incinerators, which aredesignated as incinerator A and incinerator B, respec-tively. Incinerator A handles 1.5 × 105 kg/day MSW,and incinerator B has a capacity of 1.0 × 105 kg/day. Inincinerator A, the feeding MSW is combusted in thegrates of the primary combustion chamber at about 800°C, then the produced flue gases are re-combusted inthe second combustion chamber at 1100 °C. The fluegases are cooled through heat exchangers, semi-dryscrubber, and electrostatic precipitators (ESP). Incin-erator B adopts the incineration method of controlledair oxidation (CAO), that is, MSW is pyrolyzed tocombustible gases in the primary combustion chamberat 600 °C with controlled air, and then at high temper-ature (about 1000 °C) the pyrolyzed combustible gases

* Corresponding author. Telephone: 86-27-87545526. Fax: 86-27-87545526. E-mail: limin_hust@hotmail.com.

(1) Wey, M. Y.; Ou, W. Y. J. Hazard. Mater. 2001, B82, 247.(2) Stegemann, J. A.; Schneider, J. Waste Manage. Res. 1995, 13,

149-165.(3) Kirby, C. S.; Rimstidt, J. D. Environ. Sci. Technol. 1993, 27, 652-

660.(4) Hjelmar, O. J. Hazard. Mater. 1996, 47, 345-368.(5) Chandler, A. J. Municipal solid waste incinerator residues; The

International Ash Working Group: Elsevier Science: Amsterdam,1997; p 974.

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10.1021/ef030092o CCC: $25.00 © 2003 American Chemical SocietyPublished on Web 09/12/2003

are incinerated; thus thermal energy is recovered toelectricity by a waste heat boiler. In incinerator B, noAPC system is installed. The schematic diagrams ofincinerator A and incinerator B are illustrated as Figure1 and Figure 2, respectively. The fly ashes are the ESPashes for incinerator A (designed as a) and the wasteheat boiler ashes for incinerator B (designed as b),respectively.

The two kinds of MSW incineration fly ashes weresampled in December, 2000, and the fly ashes collectedon site were stored in sealed bags for analysis. Beforeanalysis, the fly ashes were dried at 55 °C for 24 h, andpassed through an 80 mesh sieve (180 µm) for theanalyses.

3. Results and Discussion

Many studies have reported that the physical andchemical characterization of fly ashes depends on manyfactors, such as the composition of feeding MSW, thetype of incinerator, the air pollution control devices(APCDs), the operation conditions, and so on.9-12

3.1. Chemistry. Table 1 shows the ash compositionand LOI of fly ashes. In the fly ashes of incinerator A(Table 1, a), the components CaO, SO3, Cl, Na2O, andK2O account for a large percent, while there is a largeamount of SiO2, CaO, and SO3 in the fly ashes ofincinerator B (Table 1, b). Except for SiO2 and CaO, theincrement of other compositions contributes to thefactors discussed below.

When MSW is incinerated, less volatile elementsremain in the bottom ashes and grate siftings, whilemore volatile elements are captured as residues in theAPCDs, such as the ESP ashes (incinerator A), and thewaste heat boiler ashes (incinerator B). For example,Si enriches in the bottom ashes and grate siftings asregarding the elements of less volatility. While in thefly ashes of incinerator A, the content is only 8.57%.Moreover, in the fly ashes of incinerator B, the contentis higher, but only 13.35%. Other less volatile elementssuch as Fe, Mg, and Al all have the same tendency.However, Na and K, which belong to volatile elements,show enrichment in the fly ashes. In addition, forincinerator A, lime is added into the semi-dry scrubberto reduce pollution, resulting in the higher Cl and SO3content of the fly ashes. Forestier et al.13 also consideredthat the contents of Cl and SO3 of the fly ashes correlatewith the lime added into the scrubber. The highercontents of Cl (12.47%) and SO3 (15.36%) suggest thatthere is a higher efficiency for the addition of lime toneutralize the acid gases such as HCl and SO2 of fluegases. Unlike incinerator A, no measure is made in theprocess of flue gas scrubbing for incinerator B. Thus,the Cl content of the fly ashes is very low. The LOIs ofthe fly ashes are relatively low, about 3% for these twoincinerators, suggesting complete combustion.

Table 2 shows the concentration of heavy metals inthe fly ashes. For the heavy metals, a phenomenonsimilar in the ash composition participation also occurs.In Table 2, Cd, Pb, Hg, and As are the volatile heavymetals, resulting in the enrichment in the fly ashes.Moreover, the concentrations of some toxic heavy metalsin the fly ashes are very high, such as Cd (1596 mg/kg),Pb (6793 mg/kg), Hg (0.435 mg/kg), and As (295.1 mg/kg). If these ashes are handled improperly, the heavymetals in them will be released into the environment,and will pollute the soil and groundwater, which has a

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

(7) Eighmy, T. T. Environ. Sci. Technol. 1995, 29, 629-646.(8) Alba, N.; Gasso, S.; Lacorte, T.; Baldasano, J. M. J. Air Waste

Manage. Assoc. 1997, 47, 1170-1179.

(9) Sawell, E.; Chandler, A. J.; Eighmy, T. T. Biomass Bioenergy1995, 9, 377-386.

(10) Van der Sloot, H. A.; Kosson, D. S.; Hjelmar, O. Waste Manage.2001, 21, 753-765.

(11) Toshihisa, M.; Kazumasa, S. J. Hazard. Mater. 2000, 76, 263-283.

(12) Rebeiz, K. S.; Mielich, K. L. J. Energy Eng. 1995, 121, 2-13.(13) Forestier, L. L.; Libourel, G. Environ. Sci. Technol. 1998, 32,

2250-2256.

Table 1. Ash Composition and LOI of Fly Ashes (wt %)

sample SiO2 Al2O3 Fe2O3 CaO MgO Na2O K2O TiO2 P2O5 MnO SO3 Cl LOI*

a 8.57 3.90 2.58 13.90 3.16 14.00 8.77 0.76 2.81 0.12 15.36 12.47 3.12b 13.35 7.10 3.68 14.37 3.95 4.27 4.44 1.24 1.28 0.16 22.50 1.39 3.60

*LOI means loss on ignition.

Figure 1. Schematic diagram of incinerator A. 1: primarycombustion chamber; 2: feed-in hopper; 3: second combustionchamber; 4: rear flue; 5: semi-dry scrubber; 6: ESP.

Figure 2. Schematic diagram of incinerator B. 1: pyrolyzatechamber; 2: feed-in hopper; 3: second combustion chamber;4: transition section; 5: waste heat boiler.

Table 2. The Concentration of Heavy Metals in FlyAshes (mg/kg)

sample Cd Zn Cu Cr Ni Pb Hg As

a 289.7 5622 1286 366.2 74.85 4451 0.435 130.7b 1596 7118 2641 1657 554.9 6793 0.123 295.1

1488 Energy & Fuels, Vol. 17, No. 6, 2003 Li et al.

serious impact on the people’s health. In that case,before these MSW incineration fly ashes are directlylandfilled and utilized, the harmness treatment shouldbe done on them.

As described above, the MSW incineration processand APC treatment have important effects on theparticipation of the elements. In detail, when MSW isincinerated, less-volatile elements remain in the bottomashes and grate siftings, while volatile elements areeasy to volatilize at high temperature, and enter intothe gas phase, then enter the rear flue with the fluegases. During the above process, with the decrease ofthe flue gas temperature, the volatile elements condenseonto the fly ashes. The fly ashes, containing condensedvolatile elements, are captured by the filter (ESP orfabric filter), which causes the enrichment of volatileelements. Meanwhile, some minerals (such as lime, theCa-containing compound) are added into the flue gasscrubber to remove the acid gases of the flue gases, forinstance, HCl and SO2. That is the reason the concen-tration of the corresponding elements (such as Ca, Cl,and S) in the fly ashes increases significantly.

3.2. Morphology. It is clear that the ash particleproperties are linked to its leaching behavior. Forexample, the presence of a nonporous continuous outersurface and a dense particle interior may prevent heavymetal leachability from the ash.14 In this regard, thestudy of the morphology of the MSW incineration flyashes and its influence on the leachability of heavymetals is of practical importance.

Figure 3 illustrates SEM photography of the fly ashesof these two incinerators. From Figure 3, many ag-

glomerates are present in the fly ashes of incinerator A(Figure 3a), which are high-temperature sinteringproducts. However, in the fly ashes of incinerator B(Figure 3b), few agglomerates are observed, the reasonis that the incineration temperature of incinerator A ishigher than that of incinerator B. Furthermore, theparticle size distributions of the fly ashes in incineratorB are more uniform.

Figure 4 illustrates a more vivid SEM photographyof the fly ashes of these two incinerators. In Figure 4,the irregular shapes and concave surfaces of the flyashes suggest that the crystals of the fly ashes aregenerally better. This is probably due to the fact thatthe particles of fly ashes usually experience highertemperature over the pathway in the flue gas stream.15

One can also observe that the MSW incineration flyashes have various forms, unlike coal fly ash that iscomposed of spherical particles. Some MSW incinerationfly ash particles are spherical (Figure 4b), elongated(Figure 4a), and needled (Figure 4a) particles. Remond16

and Forestier13 also observed similar forms in theinvestigation of MSW incineration fly ash and flue gasresidues, respectively.

3.3. Mineralogy. Mineralogy is the main way tounderstand the coalescent status of elements in theashes. Toxicity of MSW incineration fly ashes is de-pendent not only on the polluting elements concentra-tion, but also on the speciation of the pollutant elementsand the nature of the host phases.13 Therefore, adetailed knowledge of the mineralogy of these fly ashesis required.

(14) Ramesh, A.; Kozinski, J. A. Environ. Pollut. 2001, 111, 255-262.

(15) Chang, N. B.; Wang, H. P.; Huang, W. L. Resour. Conserv.Recycl. 1999, 25, 255-270.

(16) Remond, S.; Pimienta, P.; Bentz, D. P. S. Cem. Concr. Res. 2002,32, 303-311.

Figure 3. SEM photograph of solid residues (× 50).

Figure 4. SEM photograph of solid residues (× 10 000).

Fly Ashes from Two Chinese MSW Incinerators Energy & Fuels, Vol. 17, No. 6, 2003 1489

Figure 5 shows the XRD analyses of the MSWincineration fly ashes. As shown in Figure 5, themineralogy of the fly ashes is very complex. Thiscomplex mineralogy results from several processes,including vaporization, melting, crystallization, vitrifi-cation, condensation, and precipitation, that occur dur-ing combustion and treatment of the flue gas.17

The respective quantities of the different compoundsidentified by XRD are estimated from the height of thecorresponding peaks in the XRD diagrams (Figure 5).Table 3 shows the main crystalline compounds and theirrespective quantities. The symbol “X” indicates therelative quantity. The more quantities, the more “X”symbols. From Table 3, the fly ashes of incinerator Aare characterized by a large quantity of KCl, NaCl, and/or CaS. Also in them, small amounts of cristobalite andCaCO3 are present. In the investigation of the mineral-ogy of MSW incineration ESP ashes by Forestier et al.13

and the mineralogy of MSW fly ashes by Mangialardiet al.,18 the presence and amounts of CaCO3, CaSO4,

NaCl, KCl, and SiO2 were observed. The CaSO4 contentof the fly ashes of incinerator B is especially high, andsmall amounts of hexahedral crystal R-SiO2 and CaSare also obtained.

Compounds determined by XRD are in good agree-ment with chemical analysis results. In this regard, thefly ashes from incinerator A (Figure 5a) have high CaOcontent, resulting from the lime added in the process offlue gas treatment, thus the presence of CaCO3 is found.In addition, large fractions of Na2O, K2O, and Cl arepresent in the fly ashes of incinerator A, correspond-ingly, a large amount of NaCl and KCl is found in theXRD diagram. In our results, the chlorides are mainlypresented in the forms of NaCl and KCl. However, it ispossible that their presence is in the form of CaCl2(calcium chloride) in small quantities. Furthermore, theheavy metals concerned and dangerous to the environ-ment are not showed in the XRD diagram. It is a factthat below a mass percentage of 5%, compounds aredifficult to detect by XRD.16 With high SO3 content ofthese two fly ashes (Table 1), a large quantity of CaSand CaSO4 is observed in Figure 5 for incinerator A andincinerator B, respectively.

The change of the formation of SiO2 should be noted.The fly ashes of incinerator B shows hexahedral crystalR-SiO2, which is a fuel component, while in the fly ashesof incinerator A, only cristobalite is present, probablyformed as a high-temperature condensation product.19

So different forms of SiO2 are observed. It is found thatthe flue gas temperature of incinerator A is about 1100°C, and although the designed flue gas temperature ofincinerator B is 1000 °C, in practical operation, the fluegas temperature stays at about 900 °C. This implies thattemperature is an important factor affecing the forma-tion of SiO2 in the incineration system.15

3.4. Leaching Test. The fly ashes from a MSWincinerator are subject to weathering and other naturalprocesses during storage, disposal, or utilization. Con-taminants can be leached from them and may bereleased to groundwater and surface waters. Differentleaching tests, therefore, have been developed to deter-mine the interaction of the ashes with the surroundingenvironment.

In our study, a Chinese national standard of “Testmethod standard for leaching toxicity of solid wastes -Horizontal vibration extraction procedure” (GB5086.1-1997) was used as the method for testing leaching. Theleaching behaviors of the fly ashes adopted by suchmethod are listed in Table 4. In Table 4, the Chinese

(17) Jakob, A.; Stucki, S. Environ. Sci. Technol. 1996, 30, 3275-3283.

(18) Mangialardi, T.; Paolini, A. E. J. Hazard. Mater. 1999, B70,53-70.

(19) Thipse, S. S.; Schoenitz, M. Fuel Process. Technol. 2002, 75,173-184.

Table 3. Main Compounds and the Relative Quantities in the Fly Ashes Detected by XRD

sample R-SiO2 CaCO3 CaSO4 CaS KCl NaCl cristobalite

a / X / XXXXX XXXXX XXXXX Xb XXX / XXXXX XXX / / /

Table 4. Leaching Tests of Fly Ashes and the Identification Standarda

sample pH Cd Cr Cu Ni Pb Zn As Hg

a 9.7 1.106 2.622 3.721 0.177 16.04 10.11 0.685 0.002b 8.4 5.465 10.97 6.634 0.969 22.79 11.61 1.368 <0.001Standard 0.3 10 50 10 3.0 50 1.5 0.05

a The concentration of heavy metals in the leachate is in mg/L.

Figure 5. XRD diagram of fly ashes. 1: R-quartz (R-SiO2); 2:calcite (CaCO3); 3: anhydrite (CaSO4); 4: oldhamite (CaS); 5:sylvite (KCl); 6: halite (NaCl) and/or CaS; 7: cristobalite(SiO2). Note: the content in the parentheses is the formula ofthe compound.

1490 Energy & Fuels, Vol. 17, No. 6, 2003 Li et al.

national standard of “Identification standard for haz-ardous wastes - Identification for extraction proceduretoxicity” (GB5085.3-1996) is also included. During theleaching process, the pH of the leachate increases asbasic metal salts dissolve. The equilibrium pH isdetermined by the amount of CaCO3, CaO, and Al2O3in the ashes, and the dissolution of CO2 in water.20.21

For the same reason described previously, there is agreat amount of CaO in the fly ashes of incinerator A(Table 1). CaO dissolves into the leachate when the flyashes are in contact with water, causing the pH toincrease and the resulting leachate to be basic. Forincinerator B, the leachate of the fly ashes is alkaline.

Table 4 also gives the concentration of heavy metalsin the leachate of the fly ashes. According to thestandard of GB 5085.3-1996, the concentration of Pband Cd in the leachates of the fly ashes of these twoincinerators is much higher than the standard. Fur-thermore, the concentration of Cr in the leachate of thefly ash of incinerator B is a little higher than thestandard. As for Cu and Zn, their concentrations aremuch lower than the standard, because of their highacid solubility.20 More seriously, the field leachateconcentration can be significantly higher than what isobserved in laboratory tests.22 As a result, the fly ashesof these two incinerators are both considered as hazard-ous wastes, which means proper treatment should bedone prior to landfill and disposal.

4. Conclusion

A detailed characterization of the fly ashes from twonational MSW incinerators was performed in this paper.Our conclusion confirmed that the physical and chemi-cal characterization of the fly ashes depends on factssuch as the composition of feeding MSW, the type ofincinerator, the APCDs, the operation condition, and soon. Chemical analysis showed that the volatile elementswere captured by the fly ashes, and thus resulted in theenrichment. Especially, the concentrations of some toxicheavy metals in the fly ashes were significantly high,which caused a potential hazard. The morphologyobservations found that the crystals of the fly asheswere generally better, because of the high temperatureflue gas stream. XRD analyses suggested that themineralogy of the fly ashes was very complex, and thecompounds identified by XRD were in good agreementwith chemical analysis results. The leaching testsperformed on the fly ashes further validated that thefly ashes of these two incinerators should be classifiedas hazardous wastes because of their high content ofleachable heavy metals. Therefore, they should betreated prior to landfill.

EF030092O

(20) Van Herck, P.; Van der Bruggen, B.; Vogels, G. Waste Manage.2000, 20, 203-210.

(21) Andac, M.; Glasser, F. P. Cem. Concr. Res. 1999, 29, 179-186.(22) Bagchi, A.; Sopcich, D. J. Environ. Eng. 1989, 115, 447-452.

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