Investigation of basic properties of fly ash from urban waste incinerators in China

Transcription

1 Journal of Environmental Sciences 19(27) Investigation of basic properties of fly ash from urban waste incinerators in China JIANG Jian-guo 1,, XU Xin 1, WANG Jun 1, YANG Shi-jian 2, ZHANG Yan 1 1. Department of Environmental Science and Engineering, Tsinghua University, Beijing 184, China. jianguoj@tsinghua.edu.cn 2. Shanghai Environment and Science Academy, Shanghai 2123, China Received 14 April 26; revised 28 June 26; accepted 19 July 26 Abstract Basic properties of fly ash samples from different urban waste combustion facilities in China were analyzed using as X-ray fluorescence (XRF), scanning electron microscopy (SEM), X-ray diffraction (XRD). The leaching toxicity procedure and some factors influencing heavy metals distribution in fly ash were further investigated. Experimental results indicate that the fly ash structures are complex and its properties are variable. The results of XRF and SEM revealed that the major elements (>1 mg/kg, listed in decreasing order of abundance) in fly ash are O, Ca, Cl, Si, S, K, Na, Al, Fe and Zn. These elements account for 93% to 97%, and the content of Cl ranges from 6.93% to %, while that of SiO 2 does from 4.48% to 24.84%. The minor elements (1 to 1 mg/kg) include Cr, Cu and Pb. Primary heavy metals in fly ash include Zn, Pb, Cr, Cu etc. According to standard leaching test, heavy metal leaching levels vary from to mg/l (Pb) and from.49 to mg/l (Zn), mostly exceeding the Chinese Identification Standard for hazardous wastes. Morphology of fly ash is irregular, with both amorphous structures and polycrystalline aggregates. Further research showed that heavy metals were volatilized at a high furnace temperature, condensed when cooling down during the post-furnace system and captured at air pollution control systems. Generally, heavy metals are mainly present in the forms of aerosol particulates or tiny particulates enriched on surfaces of fly ash particles. The properties of fly ash are greatly influenced by the treatment capacities of incinerators or the variation of waste retention time in chamber. Fly ash from combustors of larger capacities generally has higher contents of volatile component and higher leaching toxicity, while those of smaller capacities often produce fly ash containing higher levels of nonvolatile components and has lower toxicity. The content of heavy metals and leaching toxicity maybe have no convincing correlation, and high alkali content of CaO greatly contribute to leaching toxicity of heavy metal and acid neutralization capacity against acid rain. Key words: urban waste; incinerator; fly ash; heavy metals; leaching toxicity Introduction There is a great deal of interest in the use of combustion for treating urban waste. After the process of combustion, most heavy metals remain in combustion residues. The physicochemical properties of heavy metals may change greatly during the combustion process. Combustion residues of municipal waste are classified into two major parts bottom ash and fly ash. Bottom ash is usually harmless. However, fly ash is regarded as hazardous waste (Nie, 2), for it contains heavy metals with more leaching toxicity. In the Unite States, fly ash mixed with bottom ash is referred to as combined ash which can be used as an aggregate substitute in asphalt, Portland cement concrete, and as final cover or gas venting layers at municipal solid waste landfills. In Europe, for example Project supported by the Hi-Tech Research and Development Program (863) of China (No. 22AA6441) and the National Tenth-Five Year Program of China (No. 23BA64A-11-7). *Corresponding author. jianguoj@tsinghua.edu.cn. Netherlands, fly ash is utilized as a fine aggregate filler in asphalt. However, in China, fly ash is generally treated for secure disposal and forbidden for reusage. In order to treat and dispose fly ash properly, it is very important to understand the basic properties of fly ash including their morphology, chemical composition, mineral composition and, specially, leaching toxicity. However, it is a tough task to analyze urban waste fly ash, since the heavy metals contents are low and mainly in the forms of embedded in silicates or aluminosilicates, which are not easily insoluble. At the same time, aluminosilicates may interfere with the analysis of leachable heavy metals. It is reported by Eighmy (1995) that the analysis results may vary greatly with different analysis techniques and methods. In this article, the basic properties of fly ash were investigated systematically, which were intended to be a ground on which the subsequent processing methods would be selected and optimized.

2 No. 4 Investigation of basic properties of fly ash from urban waste incinerators in China Experiment and analysis 1.1 Sample collection Fly ash samples were taken from the baghouses (fabric filter) of six urban waste combustion facilities: Yuqiao (A), Yanqian (B), Nanshan (C), Dadi (D), Jiangqiao (E), Laohukeng (F). Of these, B and C were sampled twice. B1 and C3 were sampled in April 24, while B2 and C4 were sampled in Sept. 24 and Mar. 25, respectively. All of the five facilities are up-to-date incinerators. A, D and E are located in East China, B, C and F are located in South China. They are fed with raw municipal wastes, employing multiple-stage fire grates and slurry lime injection combined with baghouses. The treatment capacities of the six facilities are 1 t/d for A, B, E, 4 t/d for C, and 15 t/d for D, respectively. Except special explanation, samples of B1, C3 and F were used to analyze the basic properties of fly ash. Results from the other two samples were only used for properties comparison. 1.2 Basic properties of fly ash Chemical composition of fly ash X-ray fluorescence (XRF) was used to determine chemical compositions of the fly ash. The experimental results were shown in Table 1. It could be seen that fly ash is the major elements (>1 mg/kg, listed in decreasing order of abundance) in fly ash are O, Ca, Cl, Si, S, K, Na, Al, Fe and Zn. These elements account for 93% to 97%, and the content of Cl ranges from 6.93% to 29.18%, while that of SiO 2 does from 4.48% to 24.84%. The minor elements (1 to 1 mg/kg) include Cr, Cu and Pb. Primary heavy metals in fly ash include Zn, Pb, Cr, Cu etc. These results are similar with those reported by other studies (Evans, 2; Li, 24). High contents of CaO derive from the excessive lime solution injected into the scrubber to remove acidic gases. The high contents of CaO serve to maintain a high ph solution surroundings which benefits the stabilization and fixation of some hazardous heavy metals, such Cd, Cu, etc. High concentrations of Cl result from high contents of food wastes and plastics in urban waste. The high level of SO 3, Na 2 O, K 2 O indicate a favorable operating condition of emission control system. It could also be seen in Table 1 that different facilities produced different chemical compositions of fly ash. For example, sample D contains more Al 2 O 3, Fe 2 O 3 and SiO 2, which are less volatile, while sample B has higher level of volatile K 2 O, Cl, Na 2 O and SO 3. Table 2 illustrates the chemical compositions of fly ash from the same facility at different sampling time. It can be found that the components, especially heavy metals, vary greatly at different sampling time. Relatively, there are more volatile elements in sample B, while less in sample C Leaching toxicity of fly ash The leaching toxicity of heavy metals in fly ashes is the most important indicator to evaluate their environmental impact and provide technical basis for treatment, disposal and reclamation of fly ash. Leaching toxicity of heavy metals was determined according to the Chinese Standard Method for leaching toxicity of solid wastesthe rollover leaching procedure (GB ). Firstly, 75. g dried sample (<5 mm particle size) was mixed with 75 ml deionized water in a 1 ml polyethylene bottle, at a solid to liquid ratio of 1:1; secondly, the bottle containing the mixture was fixed in a rolling agitator and rolled at a speed of 3±2 r/min for 18 h continuously, then it was left for settle down for 3 min; thirdly, the soluble component was separated by vacuum filtration, using a filter membrane with.45 µm micropores. Finally, the filtrate was adjusted to a given volume, then the concentrations of heavy metals in it were determined by inductively coupled plasma (ICP). Experimental data are shown in Tables 3 and 4. It could be seen that leachate concentrations of A, B1, B2, C4, E and F exceed limits of the Identification Standard for hazardous wastes, and those of C3 and D are within the regulatory levels SEM observation of fly ash Urban waste incinerator fly ash is in the form of very small particles, of which the minimal size or quantity would be very harmful to the environment and human Table 1 Main chemical composition and heavy metals of fly ashes (%) Al 2 O 3 CaO Fe 2 O 3 K 2 O MgO Cl Na 2 O SiO 2 SO 3 Cr 2 O 3 CuO PbO ZnO A B C D E F Table 2 Main chemical composition and heavy metals of fly ashes (%) Al 2 O 3 CaO Fe 2 O 3 K 2 O Cl Na 2 O SiO 2 SO 3 CdO Cr 2 O 3 CuO PbO ZnO B B C C

3 46 JIANG Jian-guo et al. Table 3 Leaching toxicities of several fly ash samples from different combustion facilities (mg/l) A B C D E F I.S. Cd Cr Cu Pb Zn I.S.: Identification Standard for hazardous wastes (GB ). Table 4 Leaching toxicities of fly ashes from the same combustion facility at different sample time (mg/l) B1 B2 C3 C4 I.S. Cd Cr Cu Pb Zn I.S.: Identification Standard for hazardous wastes(gb ). beings. It is quite meaningful to explore microstructures of fly ash. The application of scanning electron microscopy (SEM) enabled us to observe fly ash particulate clearly which are on the scale of several micrometers in diameter. The JSM-631 scanning electron microscope was used in this experiment. The analysis results are shown in Fig.1. Fig.1 shows that fly ash was generally anomalous, in shapes of tablets, batting, platelets or globules. Large quantities of agglomerate network structures were also seen. Generally, they are irregular amorphous forms and polycrystalline aggregates, regular crystals are rarely observed. A regular spherical particle is present in Fig.1c. Its enlarged view is showed in Fig.1d. This particle is about 5 Vol µm in diameter, with a lot of irregular tinier particulates stuck to the surface, and many amorphous substances surrounded. The peculiarity that the tiny irregular fly ash particles is composed of more tiny irregular amorphous tinier particulates, which showed the structure of fly ash is even more complex XRD analysis of fly ash The identical or similar chemical components can make up different crystalline phases, and different crystalline phases of heavy metals will result in their different leaching toxicities and environmental impacts. X-ray diffraction (XRD) was used to analyze the crystalline phases of fly ash and was performed using a D/Max-RB rotating-target X-ray diffractometer. The scanning voltage was 4 kv, the 2θ ranged from 1 to 9, and the 2θ/θ coupling linkage method was applied for sequential scanning. XRD analysis (Fig.2) has indicated that fly ash in China is generally present in the form of crystalline phases including KCl, NaCl, SiO2, CaSO4, Fe3 Si and CaCO3, as well as a small quantity of solid solution phases, such as NaCaAlSi2 O7. The crystalline phases of heavy metals were below detection limits of XRD, probably because fly ash has very low contents of heavy metals, and they are usually present as complex compounds imbedded in aluminosilicates or silicates, or as amorphous phases. Even if there are certain quantities of crystalline phases, their structures are always too tiny to be detected by XRD. 2 Discussion Urban waste incineration ash may be environmental hazardous, primarily due to the fact that heavy metals in it would exist a range of toxic health effects. The main Fig. 1 SEM analysis results of fly ashes.

4 No. 4 Investigation of basic properties of fly ash from urban waste incinerators in China 461 Fig. 2 XRD analysis results of fly ashes. aim to study the fundamental properties of urban waste fly ash is to solve the environmental problem and the reclamation drawback caused by heavy metals. Therefore, it is necessary to understand the impact of the process of incineration on the properties of fly ash. After urban waste combustion, heavy metals in urban waste are redistributed between the bottom ash and fly ash. This heavy metal distribution was affected by a variety of complex and interrelated factors during incineration. The mechanism was described graphically by Fig Volatilization of heavy metals Only the volatilized heavy metals could leave the furnace and be part of fly ash. Their volatilities influence the final partitioning of heavy metals in bottom ash and fly ash. Some metals with high partial vapor pressures and low boiling points, such as Hg and Cd, are usually concentrated in fly ash, while other non-volatile species like Fe, Cu and Ni are mostly enriched in bottom ash, their presence in fly ash should be attributed to the carry-over of fly ash particles. As to the semi-volatile species, like Pb and Zn, they have approximately the same likelihood to be present in bottom ash and in fly ash. Due to the high levels of chlorine in food waste and plastics (especially PVC), which are about 3%5% (w/w) of urban waste, strong chlorination reaction will occur during combustion. The presence of chlorine and hydrochloric acid can react with the metals to produce chlorides with lower boiling points than the corresponding oxides or elementary substances, as shown in Table 5. The volatilities affected by chlorine depended on properties of different elements and compounds. Studies showed that chlorine has stronger influences on less volatile metals such as Fe and Ni. For instance, the nonvolatile Ni will become partly volatile, after reacting with chlorine to form chloride (Li et al., 24). In addition, if the gas atmosphere in combustion chamber is reductive, the nonvolatile metal oxides may be reduced to some new forms of volatile compounds, and become part of gaseous products. Subsequently, the oxidative phase of cooled fuel gases result in the new produced compounds to be oxidized again, and then condensed and become part of fly ash finally. Also, heavy metals could be carried over by fly ash particles. The amount of carried metals depends on the size, shape, density of particles and combustion condition. Fig. 3 Distribution of heavy metals in the process of incineration.

5 462 JIANG Jian-guo et al. Vol. 19 Table 5 Boiling points of heavy metals and their compounds (Belevi, 2) Volatile temperature ( C) Element Chloride Oxide Element Chloride Oxide Pb Cu Cd Ni Cr Zn Condensation of heavy metals As the gas is cooling down during the post-furnace system, the heavy metals condense, either through the formation of an aerosol of discrete heavy metal particles or through adsorption by fly ash particles. As a result of rapid cooling of the gases or rapid formation of a new and nonvolatile species, the incineration gases may be supersaturated, which followed the formation of aerosol. On the other hand, heterogeneous deposition onto fly ash particles occurs when surfaces are available for condensation and the partial pressure of an inorganic vapor species is low (Belevi, 2). In addition to dew points of the condensing metal species, the relative rates of homogeneous nucleation and heterogeneous deposition also depend on decrease rate of temperature of the metal-containing flue gases. It has also been reported that when cooling rates are higher than 6 K/s, homogeneous nucleation would occur even in the presence of existing particles (Evans, 2). The adsorbed metal may also react with aluminosilicate on the surface of fly ash particles. The overall deposition process was not only physical adsorption but also a complex combination of adsorption and chemical reaction. Therefore, the main existing forms of heavy metals in fly ash include: (1) heterogeneous deposition on surfaces of fly ash; (2) formation of an aerosol of fine heavy metal particles when the incineration gases become supersaturated. Following homogeneous nucleation and heterogeneous deposition, particles can grow by collision and coagulation. However, it is highly likely that homogeneous nucleation seldom occur for heavily loading of flue gas in waste incineration. 2.3 Capture of fly ash Air pollution control devices collect the particulate matter passing through the boiler with the gaseous products and remove the acid gases. As to the modern urban waste incineration facilities with suitable operation and maintenance, the quantities of pollutant emitted from the stacks to atmosphere is within an acceptable range, and resulted in negligible environmental impact. Modern urban waste combustion facilities employ dry or slurry lime injection to remove the acid gases produced by the combustion of sulfur, chlorine and fluorine compounds in the waste. The reagent sprayed into the scrubber could also adsorb and condense the heavy metals carried by flue gases. The metals in the combustion products may be in the forms of oxides, chlorides, sulfates and carbonates. Of these, most of the chlorides are highly soluble. The presence of chlorine and hydrochloric acid can also increase leachabilities of some heavy metal species. The smaller particles size of fly ash having the larger mass fraction of heavy metals. After incineration, the activity of heavy metals was boosted up and their leachabilities were elevated. Therefore, the granules less than 1 µm in the gas contain heavy metals with higher concentration and stronger activity. Particulates less than 1 µm in aerodynamic diameter can easily penetrate into the lungs, thus exist a significant toxic health effect. Therefore, effectiveness of emission controls in removing smaller particles is of special concern. Compared with fabric filters, electrostatic precipitators are less capable of controlling heavy metals. Even if their collection efficiencies for dust particles probably can reach those of fabric filters, the electrostatic precipitators are still less efficient in collecting the fine particulates with high heavy metal contents. 2.4 Effects of the capacity of combustor on chemical composition and leaching toxicity of fly ash Table 1 shows that fly ashes from combustors of different capacities contain different mass fraction of major elements. The percentage of nonvolatile substances such as Al 2 O 3, Fe 2 O 3 and SiO 2 in fly ash D from the 15 t/d facility was as high as 42.29%. In contrast, the handling capacity of A, B, E was 1 t/d, respectively. Their fly ash contained more volatile K 2 O, Cl, Na 2 O and SO 3. As to B, the facility in South China, such components account for a proportion of 65.38%. Compared with B, the 4 t/d C produced fly ash of lower contents of K 2 O, Cl, Na 2 O, SO 3 and higher levels of Al 2 O 3, Fe 2 O 3, SiO 2, in spite of the fact that C is also located in South China, the same area of B. Although the capacities of combustors have no appreciable effects on mass fraction of heavy metals in fly ash, but exert significant influences on leaching toxicity of heavy metals. Tables 3 and 4 showed that, leaching concentrations of heavy metals in fly ash A, B and E all exceeded limits of the Identification Standard for hazardous wastes. Data from C4 also went beyond the limits, while what of C3 fell within upper limits of the standard, and data of D was much lower than the limits. The combustors with larger capacity like A, B, E, are characterized of more stable furnace temperatures and longer retention time of urban waste in combustion chambers. These factors result in relatively more vaporization of metal species, Large quantities of volatile metals were enriched into the fly ash. It should be noticed that the metals were captured just after the processes of vaporization and condensation, thus they tend to be more active and soluble, which give rise to higher leaching concentrations. On the other hand, urban waste in the combustors with less capacity usually undergoes shorter retention time and lower extent of vaporization. Urban waste in the combustion chamber was agitated more intensely to

6 No. 4 Investigation of basic properties of fly ash from urban waste incinerators in China 463 ensure thorough combustion, therefore large amount of nonvolatile substances were carried over by combustion vapor and finally become a portion of fly ash. As a result, fly ash from less capacity incinerators usually contains heavy metals of lower activity because they primarily come from carry-over of fly ash. They are also characterized of lower leaching concentrations, despite the possibility of heavy metal contents in this fly ash is high. 2.5 Effects of chemical composition on leaching toxicity of fly ash Several factors contribute to leaching toxicity of heavy metals in fly ash as follows: Composition and morphometrics of heavy metal in fly ash fundamentally determine their leaching toxicity. Generally, high content of heavy metals likely means more leaching toxicity, but actually, the content of heavy metals and leaching toxicity maybe have no convincing correlation. Even though the leaching condition coincide completely, we may obtain different leaching results from two samples, for heavy metals enrich in the surface of one sample, while contain inside as stable compounds of the other. For example, A and B contains.4% and 1.36% Pb in weight, respectively, while the leaching toxicity of them are contrarily 67.3 mg/l and 11.8 mg/l, respectively. Consequently, both composition and morphology of heavy metal in fly ash influence the leaching toxicity. Leaching conditions are also greatly influence the leaching toxicity, such as ph of leachate, ratio of solid and liquid, leaching time, etc. Many studies have shown that ph of leachate is the crucial determinant among all of those factors above, for solubility of some heavy metals are primarily ph-dependent, such Pb, Zn, Cd, etc. Due to the excessive alkali content of CaO in lime which are injected to neutralize acid gas, there exist such a highly alkaline conditions that it has a significant impact on the leaching toxicity of heavy metals. Another benefit for high CaO content is that the high acid neutralization capacity (ANC) against acid rain, which is prevalent in south of China. 3 Conclusions The major elements (>1 mg/kg, listed in decreasing order of abundance) in fly ash are O, Ca, Cl, Si, S, K, Na, Al, Fe and Zn. These elements account for 93% to 97%, and the content of Cl ranges from 6.93% to 29.18%, while that of SiO 2 does from 4.48% to 24.84%. The minor elements (1 to 1 mg/kg) include Cr, Cu and Pb. Primary heavy metals in fly ash include Zn, Pb, Cr, Cu etc. Leaching concentrations of heavy metals in fly ash generally exceeded the limits of the Identification Standard for hazardous wastes. Of them, Pb, Zn and Cd significantly go beyond the standard limits. Generally, their leaching toxicity varies from to mg/l (Pb) and from.49 to mg/l (Zn). Fly ash species taken from different producing area or at different sample time are usually different in crystal species and abundances. Small irregular fly ash particles are always made up of amorphous tinier particulates. The fly ash particles with only several micrometers of difference in size often differ significantly from each other in their compositions. Heavy metals are present mainly in the forms of aerosol particulates or tiny particulates enriched on surfaces of fly ash particles. Fly ash from combustors of larger capacities generally has higher contents of volatile component and higher leaching toxicity, while those of smaller capacities often produce fly ash containing higher levels of nonvolatile components and has lower toxicity. The content of heavy metals and leaching toxicity maybe have no convincing correlation, and high alkali content of CaO greatly contribute to leaching toxicity of heavy metal and ANC against acid rain. References Belevi H, 2. Factors determining the element behavior in municipal solid waste incinerator: 1. Field study[j]. Environmental Science and Technology, 34: Chinese standards for hazardous waste distinguishtoxicity leaching distinguish (GB )[S]. Chinese standards for hazardous waste landfill control [S]. (GB ). Eighmy T T, Comprehensive approach toward understanding element speciation and leaching behavior in municipal solid waste incineration electrostatic precipitator ash[j]. Environ Sci and Technol, 29: Evans J, 2. Heavy metal adsorption onto fly-ash in waste incineration flue gas. Process safety and environmental protection[j]. Transactions of the Institution of Chemical Engineers, Part B, 78(1): 446. Hasselriis F, Analysis of heavy metal emission data from municipal waste combustion[j]. Journal of Hazardous Materials, 47: Jiang J G, Wang J, Xu X et al., 24, Heavy metal stabilization in municipal solid waste incineration flyash using heavy metal chelating agents[j]. Journal of Hazardous Materials, 113: Li M, 24. Characterization of solid residues from municipal solid waste incinerator[j]. Fuel, 83(2): Li J X, Yan J H, 24. Study on heavy metals characteristics of MSW fly ash[j]. Journal of Zhejiang University, 38(4): Nie Y F, 2. Treatment of wastesengineering and technical handbook[m]. Beijing: Chinese Chemical Industry Press.