Energy & Fuels 2003, 17, 1487-1491
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Characterization of Fly Ashes from Two Chinese Municipal Solid Waste Incinerators Min Li,* Song Hu, Jun Xiang, Lu-Shi Sun, Pei-Sheng Li, Sheng Su, and Xue-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 to increase significantly in the future. Because of ever-increasing generation rates and the concentrations 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 detailed characterization 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 as hazardous wastes and should be properly managed. Therefore, the results of this characterization would contribute to the development of adequate waste management strategies.
1. Introduction As incineration of MSW has lots of advantages such as significant volume reduction (about 90%) and mass reduction (about 70%), complete disinfection, energy recovery, and so on;1-4 and many large- and mediumsize cities in China have constructed, are constructing, and/or plan to install MSW incineration devices. However, incineration of MSW not only produces significant fumes, but also gives rise to considerable amounts of solid residues (bottom ash, grate sifting, heat recovery ash, fly ash, and air pollution control (APC) residue) that are generated at different points in the process of MSW incineration. With improvements in APC systems, modern MSW incineration plants emit practically no pollutants into the atmosphere; that results in the World Health Organization’s statement that it no longer considers the emissions from modern, well-operated, and well-maintained MSW incinerators to be a hazard to human health or the environment.5 However, the hazardous fractions in MSW are concentrated 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 a potentially negative impact on environmental quality, human health, and groundwater as well as on surface water resources.2,8 * Corresponding author. Telephone: 86-27-87545526. Fax: 86-2787545526. E-mail:
[email protected]. (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, 652660. (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.
With the incineration method being widely adopted to cope with MSW, the generation of MSW incineration fly ashes is expected to increase in the future in China. The large quantity, coupled with the potential leachability of high metal concentrations in the residues, has necessitated the study of the chemical, morphological, mineralogical, and leaching properties of these solid residues. Moreover, determining safe management alternatives for these fly ashes requires extensive characterization. This paper describes a comprehensive study of two samples of MSW incineration fly ashes generated by two different types of national MSW incineration facilities, with the intention of elucidating their chemical, morphological, and mineralogical properties as related to their effects on the environment and utilization. In addition, a leaching test to shed light on their potential toxicity is also presented. 2. Sampling The fly ashes used in this study came from two different types of national MSW incinerators, which are designated as incinerator A and incinerator B, respectively. Incinerator A handles 1.5 × 105 kg/day MSW, and incinerator B has a capacity of 1.0 × 105 kg/day. In incinerator A, the feeding MSW is combusted in the grates of the primary combustion chamber at about 800 °C, then the produced flue gases are re-combusted in the second combustion chamber at 1100 °C. The flue gases are cooled through heat exchangers, semi-dry scrubber, and electrostatic precipitators (ESP). Incinerator B adopts the incineration method of controlled air oxidation (CAO), that is, MSW is pyrolyzed to combustible gases in the primary combustion chamber at 600 °C with controlled air, and then at high temperature (about 1000 °C) the pyrolyzed combustible gases
10.1021/ef030092o CCC: $25.00 © 2003 American Chemical Society Published on Web 09/12/2003
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Table 1. Ash Composition and LOI of Fly Ashes (wt %) sample
SiO2
Al2O3
Fe2O3
CaO
MgO
Na2O
K 2O
TiO2
P2O5
MnO
SO3
Cl
LOI*
a b
8.57 13.35
3.90 7.10
2.58 3.68
13.90 14.37
3.16 3.95
14.00 4.27
8.77 4.44
0.76 1.24
2.81 1.28
0.12 0.16
15.36 22.50
12.47 1.39
3.12 3.60
*LOI means loss on ignition. Table 2. The Concentration of Heavy Metals in Fly Ashes (mg/kg) sample a b
Figure 1. Schematic diagram of incinerator A. 1: primary combustion chamber; 2: feed-in hopper; 3: second combustion chamber; 4: rear flue; 5: semi-dry scrubber; 6: ESP.
Figure 2. Schematic diagram of incinerator B. 1: pyrolyzate chamber; 2: feed-in hopper; 3: second combustion chamber; 4: transition section; 5: waste heat boiler.
are incinerated; thus thermal energy is recovered to electricity by a waste heat boiler. In incinerator B, no APC system is installed. The schematic diagrams of incinerator A and incinerator B are illustrated as Figure 1 and Figure 2, respectively. The fly ashes are the ESP ashes for incinerator A (designed as a) and the waste heat boiler ashes for incinerator B (designed as b), respectively. The two kinds of MSW incineration fly ashes were sampled in December, 2000, and the fly ashes collected on site were stored in sealed bags for analysis. Before analysis, the fly ashes were dried at 55 °C for 24 h, and passed through an 80 mesh sieve (180 µm) for the analyses. 3. Results and Discussion Many studies have reported that the physical and chemical characterization of fly ashes depends on many factors, such as the composition of feeding MSW, the type of incinerator, the air pollution control devices (APCDs), the operation conditions, and so on.9-12 (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.
Cd
Zn
Cu
Cr
Ni
Pb
Hg
As
289.7 5622 1286 366.2 74.85 4451 0.435 130.7 1596 7118 2641 1657 554.9 6793 0.123 295.1
3.1. Chemistry. Table 1 shows the ash composition and LOI of fly ashes. In the fly ashes of incinerator A (Table 1, a), the components CaO, SO3, Cl, Na2O, and K2O account for a large percent, while there is a large amount of SiO2, CaO, and SO3 in the fly ashes of incinerator B (Table 1, b). Except for SiO2 and CaO, the increment of other compositions contributes to the factors discussed below. When MSW is incinerated, less volatile elements remain in the bottom ashes and grate siftings, while more volatile elements are captured as residues in the APCDs, such as the ESP ashes (incinerator A), and the waste heat boiler ashes (incinerator B). For example, Si enriches in the bottom ashes and grate siftings as regarding the elements of less volatility. While in the fly ashes of incinerator A, the content is only 8.57%. Moreover, in the fly ashes of incinerator B, the content is higher, but only 13.35%. Other less volatile elements such 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, for incinerator A, lime is added into the semi-dry scrubber to reduce pollution, resulting in the higher Cl and SO3 content of the fly ashes. Forestier et al.13 also considered that the contents of Cl and SO3 of the fly ashes correlate with the lime added into the scrubber. The higher contents of Cl (12.47%) and SO3 (15.36%) suggest that there is a higher efficiency for the addition of lime to neutralize the acid gases such as HCl and SO2 of flue gases. Unlike incinerator A, no measure is made in the process of flue gas scrubbing for incinerator B. Thus, the Cl content of the fly ashes is very low. The LOIs of the fly ashes are relatively low, about 3% for these two incinerators, suggesting complete combustion. Table 2 shows the concentration of heavy metals in the fly ashes. For the heavy metals, a phenomenon similar in the ash composition participation also occurs. In Table 2, Cd, Pb, Hg, and As are the volatile heavy metals, resulting in the enrichment in the fly ashes. Moreover, the concentrations of some toxic heavy metals in 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 heavy metals in them will be released into the environment, and will pollute the soil and groundwater, which has a (9) Sawell, E.; Chandler, A. J.; Eighmy, T. T. Biomass Bioenergy 1995, 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, 263283. (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.
Fly Ashes from Two Chinese MSW Incinerators
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Figure 3. SEM photograph of solid residues (× 50).
Figure 4. SEM photograph of solid residues (× 10 000).
serious impact on the people’s health. In that case, before these MSW incineration fly ashes are directly landfilled and utilized, the harmness treatment should be done on them. As described above, the MSW incineration process and APC treatment have important effects on the participation of the elements. In detail, when MSW is incinerated, less-volatile elements remain in the bottom ashes and grate siftings, while volatile elements are easy to volatilize at high temperature, and enter into the gas phase, then enter the rear flue with the flue gases. During the above process, with the decrease of the flue gas temperature, the volatile elements condense onto the fly ashes. The fly ashes, containing condensed volatile elements, are captured by the filter (ESP or fabric filter), which causes the enrichment of volatile elements. Meanwhile, some minerals (such as lime, the Ca-containing compound) are added into the flue gas scrubber to remove the acid gases of the flue gases, for instance, HCl and SO2. That is the reason the concentration 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 particle properties are linked to its leaching behavior. For example, the presence of a nonporous continuous outer surface and a dense particle interior may prevent heavy metal leachability from the ash.14 In this regard, the study of the morphology of the MSW incineration fly ashes and its influence on the leachability of heavy metals is of practical importance. Figure 3 illustrates SEM photography of the fly ashes of these two incinerators. From Figure 3, many ag(14) Ramesh, A.; Kozinski, J. A. Environ. Pollut. 2001, 111, 255262.
glomerates are present in the fly ashes of incinerator A (Figure 3a), which are high-temperature sintering products. However, in the fly ashes of incinerator B (Figure 3b), few agglomerates are observed, the reason is that the incineration temperature of incinerator A is higher than that of incinerator B. Furthermore, the particle size distributions of the fly ashes in incinerator B are more uniform. Figure 4 illustrates a more vivid SEM photography of the fly ashes of these two incinerators. In Figure 4, the irregular shapes and concave surfaces of the fly ashes suggest that the crystals of the fly ashes are generally better. This is probably due to the fact that the particles of fly ashes usually experience higher temperature over the pathway in the flue gas stream.15 One can also observe that the MSW incineration fly ashes have various forms, unlike coal fly ash that is composed of spherical particles. Some MSW incineration fly ash particles are spherical (Figure 4b), elongated (Figure 4a), and needled (Figure 4a) particles. Re´mond16 and Forestier13 also observed similar forms in the investigation of MSW incineration fly ash and flue gas residues, respectively. 3.3. Mineralogy. Mineralogy is the main way to understand the coalescent status of elements in the ashes. Toxicity of MSW incineration fly ashes is dependent not only on the polluting elements concentration, but also on the speciation of the pollutant elements and the nature of the host phases.13 Therefore, a detailed knowledge of the mineralogy of these fly ashes is required. (15) Chang, N. B.; Wang, H. P.; Huang, W. L. Resour. Conserv. Recycl. 1999, 25, 255-270. (16) Re´mond, S.; Pimienta, P.; Bentz, D. P. S. Cem. Concr. Res. 2002, 32, 303-311.
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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 b
/ XXX
X /
/ XXXXX
XXXXX XXX
XXXXX /
XXXXX /
X /
Table 4. Leaching Tests of Fly Ashes and the Identification Standarda sample
pH
Cd
Cr
Cu
Ni
Pb
Zn
As
Hg
a b Standard
9.7 8.4
1.106 5.465 0.3
2.622 10.97 10
3.721 6.634 50
0.177 0.969 10
16.04 22.79 3.0
10.11 11.61 50
0.685 1.368 1.5
0.002
