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Impact of moisture on volatility of heavy metals in municipal solid waste incinerated in a laboratory scale simulated incinerator
Waste Management 24 (2004) 581–587 www.elsevier.com/locate/wasman
Impact of moisture on volatility of heavy metals in municipal solid waste incinerated in a laboratory scale simulated incinerator Zhao Youcai a
a,b,*
, S. Stucki b, Ch. Ludwig b, J. Wochele
b
The State Key Laboratory of Pollution Control and Resource Reuse, School of Environmental Science and Engineering, Tongji University, Shanghai 200092, China b Laboratory for Energy and Materials Cycles (LEM), Paul Scherrer Institut, 5232 Villigen PSI, Switzerland Accepted 2 January 2004
Abstract In this work, the impact of moisture on the volatility of heavy metals present in municipal solid wastes (MSW) in a laboratory scale simulated incinerator was studied, using synthetic waste consisting of 5.4 g of wood powder, 2.6 g lava, 1.9 polythene, 0.19 g polyvinyl chloride, and a given quantity of water and heavy metals represented by lead, zinc and copper in forms of metallic, chlorides and oxides. It is found that the presence of high moisture in MSW will greatly reduce the volatilization of heavy metals in MSW in the incineration process. The volatilization behavior of chlorides, oxides and the metallic species with respect to the effect of moistures is quite different. For copper, the presence of moisture in MSW depresses the volatilization of oxides, and increases that of chloride and the metallic species, while in contrast, the volatilization of both lead and zinc is always depressed by the presence of moisture in MSW, regardless of the chemical forms used. The chemical mechanisms, which govern the volatilization behaviors of different chemical forms in the incineration process, are proposed. Hydrolysis, dewatering of hydrolyzed species, sublimation, chemical transformation of less volatiles to more volatiles or reverse, may participate in and affect the volatilization of heavy metals in MSW. Ó 2004 Elsevier Ltd. All rights reserved.
1. Introduction Incineration can substantially reduce the volume of municipal solid wastes (MSW) incinerated and kill all the bacteria, but cannot eliminate any heavy metals at all (Jakob et al., 1995,Jakob, 1998; Amalandu and Dennis, 1989). Heavy metals in MSW may distribute among gaseous phase (exhaust gas), bottom ash, boiler ash, and filter ash during incineration in an incinerator (Hasan, 1994, 1998; Zhang et al., 2000). Usually, the boiler ash and filter ash are combined together and referred to as fly ash. Flue gas generated in incinerator of MSW can be properly treated by modern facilities installed in situ in the incineration plant (Hasan, 1994, 1998). Hence, the remaining potential pollution sources in the MSW *
Corresponding author. Tel.: +86-21-65-980041; fax: +86-21-65982684. E-mail address: [email protected] (Z. Youcai). 0956-053X/$ - see front matter Ó 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.wasman.2004.01.004
incineration process may be the bottom and fly ashes if these ashes are not properly treated before discharging into the environment. The most significant pollutants in the ashes are heavy metals, whose contents and their distribution among flue gases and bottom ash vary with the composition and moisture in the MSW incinerated, incineration temperature and time, etc. (Cheng-Fong and Huang, 1990; Serge Biollaz et al., 2001). Typically, 10–25% of MSW will change into bottom ash and 0.5–2% into fly ash in a modern incinerator, with much less quantity of fly ash than that of bottom ash (Youcai et al., 2002a; Stucki and Jakob, 1997). In general, the bottom ash may be considered nonhazardous and non-toxic, as the heavy metal contents in the ash may be lower than the regulated standard values in terms of leaching toxicity or absolute values (Tay, 1987, Tay and Goh, 1991). However, the contents of heavy metals in bottom ash seemingly increase in the developed nations as the MSW incinerated may contains
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higher ratios of wastes with heavy metals, such as paints, metal articles and goods, etc. (Jakob et al., 1995,Jakob, 1998). In some countries, such as Switzerland, the environmental requirements have become stricter and the heavy metal contents in the bottom ash have been found to exceed the constraints set by the relevant environmental regulations (Serge Biollaz et al., 2001). In this case, the heavy metals in the bottom ash should be removed before it is reused as construction materials or placed in a landfill. However, the quantity of bottom ash is relatively great and the heavy metals contents in it are quite low. It will be cost- and energy-intensive when trying to reduce the heavy metal content in the bottom ash by any methods such evaporation at high temperature or separation of heavy metals before the waste is incinerated. Transferring heavy metals in MSW as much as possible into the flue gas so that their content in the bottom ash can be reduced to the regulated level may solve this dilemma. The corresponding content of heavy metals in fly ash collected from flue gas and boiler may thus increase to a level possibly comparable with ores. Much effort has been done in this innovative concept (Jorg and Stuki, 1999; Jorg et al., 1998). Though the incineration process of MSW has been studied intensively, the effect of moisture on the volatility of heavy metals, nevertheless, has not been studied. According to the discharging standards set in most nations, the heavy metals in the bottom ash are always within the limits of standards. As the environmental constraints increase, more attentions are being drawn to the pollution arisen from the bottom ash because the heavy metals in the ash may have exceeded the newly regulated standards. Many developing countries, such as China, have used incineration technologies to treat their increasing quantity of MSW. The moisture in the MSW in these countries may be quite high, usually as high as 40–65%, depending on their living customs and levels, as well as the functions of collection facilities. Take China as an example. Contents of paper, plastic, glass, etc., may be low, as these recoverable wastes have been collected by scavengers, while those of high-moisture food origin wastes may be high, in MSW. Moreover, most collection tools are not waterproof. As a result, the moisture in MSW in the dry and arid north part of China may be lower than that in the rainy, hot and humid south part of China, especially in the rainy summer season (Youcai et al., 2000, 2001, 2002a,b). The changes of moisture in MSW may lead to transformation of chemical forms of heavy metals and even the incineration mechanism in the incineration process, and thus influence the volatilities of these metals. Consequently, the distribution of heavy metals in gaseous phase, bottom and filter ashes will be correspondingly changed. It can be reasonably predicted that
different incineration parameters should be applied for different MSW with varying moisture, meaning that the incineration process should be adjusted when the technologies used in the developed nations are transferred to the developing ones. The objectives of this work are to explore the impacts of moisture in MSW on the volatility of heavy metals so that the distribution of heavy metals in three media can be roughly predicted.
2. Experimental 2.1. Simulated laboratory scale incinerator A scaled-down version of an MSW grate incinerator that would fulfil detailed geometric similarity is difficult to realize. Accepting, however, some simplifications, it is possible to build a simple model in the form of a tubular furnace reactor: the grate is then replaced by a ceramic crucible, as a sample container and means of transport of the sample in the furnace. The incineration process is carried out as a batch experiment: the temperature profile that a waste sample experiences in a continuously operated incinerator is simulated by moving the crucible to the appropriate temperature zones of the tubular furnace for given residence times. Air feed to the furnace can be regulated according to the air needs applied to the different temperature zones. The structure of the simulated incinerator is described in several literatures and its scheme is also shown in Fig. 1. The similarity between the simulated furnace and the real grate incinerator has been well justified (Jorg and Stuki, 1999,Jorg et al., 1998). The temperatures in the entrance point and the highest point is around 50 and 950–1000 °C, respectively, and distance between the two points is 128 cm. The crucible with or without samples is manually pushed into the tube from the entrance point to the highest temperature point and pulled out in reverse way with constant rate at a given time so that a certain temperature profile in which the samples are incinerated can be obtained.
Furnace
Primary air Flue gas
Crucible
Moving rod
Secondary air Temp1
Temp2 Ref Temp
Data collection computer
Fig. 1. Schematic diagram of the laboratory scale simulated grate incinerator.
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2.2. Preparation of synthetic wastes
2.3. Moisture calculation
It has well been confirmed that the real waste can be reasonably represented by the synthetic wastes consisting of the following components: 5.4 g of wood powder, 2.6 g lava (a volcanic ash mainly consisted of silica), 1.9 g polythene, 0.19 g polyvinyl chloride (totally 10 g, with 5% moisture) (Jorg and Stuki, 1999, Jorg et al., 1998). Such a synthetic waste is always used to simulate the real wastes in this work, unless otherwise indicated. The contents of heavy metals in this waste is negligible, and a given quantity of water and heavy metals representative of lead, zinc and copper in forms of chlorides, oxides and metallic, are added to the waste so that the study of the impact of moisture on volatility of heavy metals becomes possible. These components are mixed as thoroughly as possible and stored in a sealed container with water in the bottom for overnight so that the water can be absorbed by and homogenized in the wastes as the real waste does, and then put in a rectangle crucible (10 4 cm) and pushed into the furnace along the quartz tube. Two temperature monitoring points with a distance of 9 cm are set within the two inner sides of crucible, which are referred to as Temp 1 and Temp 2 in the text. A fixed temperature monitoring point, referring to as Ref Temp (reference temperature), is set in the right side of the tube. The highest temperature point is located in the left side of the Ref Temp point. The flue gas generated will pass through the Ref Temp monitoring point. Hence, the Ref Temp can accurately reflect the temperature change of flue gas in the incineration process, and thus can be used for quantitative characterization of heat release from the incineration. The Temp1 and Temp2 are actually the temperature of flames or flue gas just above the surface of wastes, and will change correspondingly when the waste samples release or absorb heat while entering into the tube. Ref Temp, Temp1 and Temp2 will keep unchanged if no wastes are pushed into the incinerator. The bottom ash is collected carefully so that it can be removed from the crucible as completely as possible and weighted after the incineration test is finished and the crucible is pulled out and cooled down. Then the ash is ground to fine powder, from which around 300 mg is weighted and digested in concentrated HF, HNO3 and H3 BO3 in pressure microwave. In general, the sample can be dissolved completely in the solution. Nevertheless, a few black tar may not be dissolved if the wastes with higher moisture, such as over 40%, are not well incinerated. Three parallel digestions for each sample are carried out and their average values are provided in the text. The concentrations of heavy metals in the aqueous solution are determined by ICP. It is observed that the data of three parallel digestions coincide well.
The water (moisture) in the mixture of 5.4 g of wood powder, 2.6 g lava, 1.9 polythene, 0.19 g polyvinyl chloride is 0.5 g. Suppose x g of water is added to the mixture, then the moisture, m (wt%), is calculated as follows: m ¼ ððx þ 0:5Þ=ð10 þ xÞÞ 100
3. Results and discussions 3.1. Temperature profile used Figs. 2 and 3 show the typical temperature profiles obtained with 5% and 59% moisture in the samples. 0.5 min is spent for the samples to be pushed from the entrance point to the highest temperature zone and then stay at the zone for 9.5 min before pulling out in 1.5 min, which is used for all experiments unless otherwise indicated. The temperature of sample with 5% moisture can reach to the peak value and ignite for incineration in 2.5 min, while for the sample with 59% moisture at least 5 min should be taken to reach such an ignition point. Obviously, more time will be needed for drying of sample with higher moisture before it is ignited for incineration, which may affect the volatility of heavy metals in the wastes in the incineration process. For the samples with different moistures, the higher the moistures, the longer time will be needed to reach the peak temperature and ignition. 3.2. Dependence of bottom ash weight on the incineration time The longer incineration time, the less bottom ash will be obtained, as shown in Fig. 4. It is found that the bottom ash weight will decrease to 2.69 and 2.60 g after 10 and 22 min incineration. The objective of this work is to explore the volatility of heavy metals in the wastes in
Temperature (˚C)
1200 1000 800 600 400
Temp 1 temp 2 Ref Temp
200 0 0
2
4
6
8
10
12
14
16
Time (minute) Fig. 2. Incineration process of the synthetic waste containing oxides of Zn, Pb and Cu at moisture of 5%.
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Temperature (˚C)
1200 Temp 1 Temp 2 Ref Temp
1000 800 600 400 200 0 0
5
10
15
20
Time (minute)
Weight of bottom ash (g)
Fig. 3. Incineration process of the sythetic waste containing oxides of Zn, Pb and Cu at moisture of 59.5%. 10 9 8 7 6 5 4 3 2 0
5
10
15
20
25
Incineration time (minute) Fig. 4. Effect of incineration time on the weight of bottom ash for the synthetic wastes (10 g with 5% moisture) without the presence of heavy metals.
the incineration process; hence, the incineration time should be limited so that their relative volatility can be reasonably compared. The volatility of heavy metals in the wastes may be consistent if sufficient incineration time is applied, when only moisture is changed. In this work, a total time of 10 min are used for the experiments as described above, unless otherwise indicated.
the wastes. It can be seen that the volatilities are quite different for the three types of metals. The volatilities for lead chloride can be considered nearly unchanged at around 62%, and those for zinc chloride decrease rather significantly, from 68% to 51%, while those for copper chloride increase considerably, from 9% to 21%, when the moistures in the wastes increase from 5% to 69%. From viewpoints of the percentages of volatilities of three metals, copper chloride is always the lowest, and the lead the highest except for the first point in the figure. 3.4. Volatility of oxides of lead, zinc and copper The volatilities of oxides are much less than the counterparts of chlorides, as shown in Fig. 6, with the same metal contents in the wastes. Moreover, the volatilities of both oxides of lead and copper (as cuprous oxide) decrease, from 30% to 19% for Pb and from 9% to 6% for Cu, with a faster rate for lead oxide, while those of zinc oxide can be considered unchanged at around 15%, when the moistures in wastes increase from 5% to 69%. 3.5. Volatility of metallic lead, zinc and copper The volatilities of metallic forms of three metals are always lower than their counterparts of both chlorides and oxides as given in Fig. 7. The volatilities of lead and zinc seem to decrease slightly from around 24% to 20% for Pb and from 14% to 9% for Zn, with minor variation of data obtained, while those of copper increase slightly from around 10% to 13%, when the moisture in the waste increases from 5% to 69%. 3.6. Effect of metal contents on the volatility in forms of oxides
3.3. Volatility of chlorides of lead, zinc and copper Fig. 5 shows the relationship between heavy metal volatility in the forms of chlorides and the moistures in
Fig. 8 shows the relationship between the initial metal contents (in the form of oxides) in the wastes and volatilities of metals in the incineration process. Three types
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Volativity of metal (wt%)
Volativity of metal (wt%)
80 60 50 40
lead chloride zinc chloride copper chloride
30 20 10 0 0
20
40
60
80
Moisture (wt%) Fig. 5. Relationship between the content of moisture and volatility of chlorides of lead, zinc and copper in the incineration of synthetic wastes. Metal content in the waste (mg metal/g dry waste) Pb 10.4; Zn 21.2; Cu 10.1.
35
lead oxide
30
zinc oxide
25
cuprous oxide
20 15 10 5 0 0
20
40
60
80
Moisture (wt%) Fig. 6. Relationship between the content of moisture and volatility of oxides of lead, zinc and copper in the incineration of synthetic wastes. Metal content in the waste (mg metal/g dry waste) Pb 10.4; Zn 21.2; Cu 10.1.
Volativity of metal (wt%)
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Longer time of incineration cannot remarkably raise the volatilisation of heavy metals in the wastes.
30 25 20
3.8. Discussions
15 10
a Zn Cu
5 0 0
10
20
30
40
50
60
70
80
Moisture (wt%) Fig. 7. Relationship between moisture and volatility of metallic lead, zinc and copper in the incineration of synthetic wastes. Metal contents in the wastes (mg metal/g dry waste) Zn 19.8; Cu 5.4.
60
Volativity of metal (wt%)
585
50 Pb Zn Cu
40 30 20 10 0 0
5
10
15
20
Initial metal contents (mg metal/g dry waste) Fig. 8. Relationship between volatility of oxides of Pb, Zn and Cu and their initial contents in the incineration of synthetic wastes. Dry wastes 9.5 g; moisture 59.0%.
of metals behave in rather different volatility trend. The percent volatilities of lead and zinc oxides decrease, from 54% to 24% for Pb and from 22% to 13% for Zn, with a faster rate for the former and lower rate for the latter, when the initial Pb and Zn increase from 1.2 to 11 mg/g dry waste for Pb and from 3 to 20 mg/g dry waste for Zn. In contrast, the volatilities of cuprous oxide increase slightly from 1% to 6%, when the initial contents of metals in the wastes increase from 1.4 to 10 mg/g dry waste. 3.7. Effect of incineration time As expected, the volatilities of lead and zinc in the wastes increase slightly while those of copper keep unchanged, when the incineration time extends (Table 1). Table 1 Effect of incineration time on the volatilities of heavy metals in the wastes Incineration time (min)
Lead volatility (%)
Zinc volatility (%)
Copper volatility (%)
10 15 22
23.84 24.50 25.03
13.53 14.10 15.60
6.70 6.81 6.76
Heavy metals (as oxides) contents in the wastes (mg metal/g dry waste): Pb 10.4; Zn 21.2; Cu 10.1; Moisture 59%.
The average contents of zinc, lead and copper in the Swiss MSW are reported to be around 1.45 mg/g dry waste, 0.73 mg/g dry waste and 0.42 mg/g dry waste (Hasan, 1994). The corresponding values in MSW in China are slightly lower in dry base, i.e., around (in mg metal/g dry waste) 1.35 for Zn, 0.56 for Pb and 0.21 for Cu (Youcai et al., 2002a). Few researches on the heavy metals analysis and volatility for China MSW have been carried out. In this work, the Swiss MSW is taken an example. According to the literature report, typically around 41–56% of lead, 47–57% of Zn and 3–6% of Cu may be evaporated from Swiss MSW when it is incinerated in grate incinerators (Youcai et al., 2002a; Stucki and Jakob, 1997). The forms of heavy metals in MSW will certainly be very complex, and it is hard to calculate the contribution and distribution of each chemical species for said element to their volatility. However, a definite conclusion can be drawn from this work that lead is always the most volatile and copper is the least with zinc the middle among the three elements tested, which is completely consistent with the above mentioned evaporation sequences of Pb, Zn and Cu observed in a real incinerator. By rough estimation from Figs. 2–8 at a MSW moisture of 30–60%, the volatility is 60–70% for Pb, 50– 60% for Zn and 5–10% for Cu, as chlorides, and 20–25% for Pb, 15% for Zn and 5–8% for Cu, as oxides, and 25%, 12% and 11% for Pb, Zn and Cu as metallic, respectively. The values are higher for chlorides, and lower for oxides and metallic, than the real data obtained in the incinerator, indicating that the chemical species of metals should be the mixtures of chlorides, oxides and metallic and other forms. Table 2 and Fig. 9 shows the boiling and melting points of the metals or their compounds of interest, as well as the temperature ranges of incineration in the simulated incinerator as indicated between the two dotted lines (Youcai et al., 2002a). Metals of Ni, Cr and Fe, oxides of Zn, Ni, Cr, and Fe, would be relatively less volatized, as the melting points of these compounds and metals fall above the incineration temperature range. However, the melting points of all the chlorides and most sulfates, and metallic Hg, Zn, Cu, Pb and Cd, fall in or below this range, which imply that these substances would be more volatile under the incineration conditions. Hence, it may be proposed that the volatility of heavy metals from MSW is possibly originated from the evaporation of chlorides and sulfates of Zn, Ni, Cr, and Fe and chlorides, sulfate and oxides for the other metals, and metallic elements for Hg, Zn, Cu, Pb and Cd
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Table 2 The physical properties of metals and their compounds Element
Melting point (°C)
Boiling point (°C)
Oxides
Chlorides
Sulfates
M.P. 275 °C, B.P. 301 °C M.P.283 °C, sublimation under calcination M.P. 620 °C, decomposable at 993 °C M.P. 501 °C, B.P. 950 °C M.P. 570 °C, B.P. 960 °C M.P. 1001 °C M.P. 83 °C
Decomposable at the M.P.
Hg
)39
357
Decomposable above 400 °C
Zn
419
907
Cu
1083
2595
Volatilization at 1800 °C M.P. 1026 °C
Pb Cd Ni Cr
327 321 1555 1900
1744 767 2837 2480
M.P. 886 °C, B.P. 1516 °C Sublimation at 900 °C M.P 1980 °C M.P. 2435 °C, B.P.3000 °C
Fe
1535
3000
M.P. 1377 °C, decomposable at 3410 °C
m.p.. of Metals b.p. of metals m.p. of chlorides of metals m.p. of oxides of metals 3400 2900
T (°C)
2400 1900 1400 900 400 -100 Hg Zn Cu Pb Cd Ni
Cr
Fe
Fig. 9. The melting and boiling points of the metals and their compounds concerned.
concerned. Nevertheless, the metallic forms may be oxidized into oxides and thus reduce the possibility of evaporation in the incineration. From Fig. 5 it can be seen that the presence of high moisture in MSW will reduce the volatilization of zinc chlorides while increase that of lead and copper chlorides. Zinc chlorides may hydrolyse to form zinc hydroxide in the presence of high moisture in MSW, which is dewatered to form less volatile zinc oxide. Also, zinc chloride sublimes at temperature over its melting point of 283 °C. The hydrolysis and formation of oxide from zinc chloride would exert unfavourable effect on the sublimation and subsequently depress the evaporation of zinc in the incineration process. For lead chloride, the melting and boiling points are 501 and 950 °C, respectively, without sublimation and decomposition at high temperature. The melting of copper chloride is 620 °C and it decomposes at 993 °C, also without any sublimation at high temperature. Hence, it may be the property of sublimation for zinc chloride that leads to different evaporation behaviour compared with lead and copper chlorides.
M.P. 282 °C, B.P. 316 °C
Decomposable under calciantion Decomposable at 560 °C M.P. 1170 °C M.P. 1000 °C M.P. 31.5 °C Decomposable at high temperature Decomposable at high temperature
Different effects of moisture on the volatilities of Pb and Zn as a group and Cu as another group for both forms of metallic and oxidic species show that the evaporation mechanism for these two groups should be different. As shown in Table 2 and Fig. 9, Pb and Zn should be volatile completely at the incineration temperature used in the simulated incinerator if the incineration time would be sufficiently long and no metallic metals would be oxidized into oxides. However, the metallic forms will always be oxidized, more or less, to oxides, depending on the redox conditions in the incinerator, which will inevitably reduce the volatilities of metals in the wastes as the volatilities of oxides are much less than those of metallic forms. The presence of moistures in the wastes may facilitate the oxidization of metallic Pb and Zn and consequently the volatilities of these two metals decrease as the moistures increase in the wastes. The presence of moistures seems to facilitate the volatilisation of copper, both on forms of metallic and oxides, completely in reverse to those of Pb and Zn. From Table 2 and Fig. 9, it can be seen that the volatility of Cu species decreases roughly in the following sequence: sulfates > chlorides > oxides > metallic The presence of moisture should promote the formation of more volatile forms from metallic and oxides, e.g., from metallic copper to its sulfate and chloride, or even from metallic to oxide. Such a transformation may be rather incomplete and ineffective as the sulfate and chloride possibly formed in situ in the incineration process would decompose simultaneously because these compounds are not stable at high temperature, resulting in still low volatility of metallic copper and oxides. The facts that longer incineration would not substantially increase the volatilities of oxides (and possibly for the other forms) imply that the thermochemical reactions occur between the wastes (both inorganic and
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organic) and heavy metals at high temperatures should be one of important factors for the volatilization of the heavy metals in the wastes. Less and less organics or inorganics such as chlorides are available for the transformation of heavy metals in MSW into more volatile forms as the incineration time extends. It has been well proved that the presence of certain chemicals such as chlorides in forms of both chlorine and salts can improve the volatilities of heavy metals from fly ash and bottom ash (Stucki and Jakob, 1997). These phenomena should also occur in the incineration process of MSW, i.e., the presence of wastes may facilitate the volatilization of oxides, when the other conditions such as moisture contents are kept consistent (Table 1).
4. Conclusions Increase of moistures in MSW will reduce the volatilization of all chemical forms of lead and zinc and oxides of copper, while slightly increase the volatilization of chlorides and metallic form of copper, when the wastes are incinerated at a limited time of 10 min in a laboratory scale simulated incinerator. The chemical mechanisms governing the volatilization behaviors of different forms in the incineration process are proposed. Hydrolysis, dewatering of hydrolyzed species, sublimation, chemical transformation of less volatiles to more volatiles or reverse, are considered among the factors affecting in the volatilization of heavy metals in MSW.
Acknowledgements This work was partially supported by Swiss National Science Foundation and China National Natural Science Foundation (No. 59778016), China Outstanding Young Scientist Program, Shanghai Key Research Program, Doctoral Research Program of China Education Ministry, and Shanghai Science and Technology Commission as part of Sino–Swiss Bilateral Cooperation Program. The authors want to thank Mr. Albert and Mr. Lutz for their work on the analysis of heavy metals.
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Impact of moisture on volatility of heavy metals in municipal solid waste incinerated in a laboratory scale simulated incinerator Zhao Youcai a
a,b,*
, S. Stucki b, Ch. Ludwig b, J. Wochele
b
The State Key Laboratory of Pollution Control and Resource Reuse, School of Environmental Science and Engineering, Tongji University, Shanghai 200092, China b Laboratory for Energy and Materials Cycles (LEM), Paul Scherrer Institut, 5232 Villigen PSI, Switzerland Accepted 2 January 2004
Abstract In this work, the impact of moisture on the volatility of heavy metals present in municipal solid wastes (MSW) in a laboratory scale simulated incinerator was studied, using synthetic waste consisting of 5.4 g of wood powder, 2.6 g lava, 1.9 polythene, 0.19 g polyvinyl chloride, and a given quantity of water and heavy metals represented by lead, zinc and copper in forms of metallic, chlorides and oxides. It is found that the presence of high moisture in MSW will greatly reduce the volatilization of heavy metals in MSW in the incineration process. The volatilization behavior of chlorides, oxides and the metallic species with respect to the effect of moistures is quite different. For copper, the presence of moisture in MSW depresses the volatilization of oxides, and increases that of chloride and the metallic species, while in contrast, the volatilization of both lead and zinc is always depressed by the presence of moisture in MSW, regardless of the chemical forms used. The chemical mechanisms, which govern the volatilization behaviors of different chemical forms in the incineration process, are proposed. Hydrolysis, dewatering of hydrolyzed species, sublimation, chemical transformation of less volatiles to more volatiles or reverse, may participate in and affect the volatilization of heavy metals in MSW. Ó 2004 Elsevier Ltd. All rights reserved.
1. Introduction Incineration can substantially reduce the volume of municipal solid wastes (MSW) incinerated and kill all the bacteria, but cannot eliminate any heavy metals at all (Jakob et al., 1995,Jakob, 1998; Amalandu and Dennis, 1989). Heavy metals in MSW may distribute among gaseous phase (exhaust gas), bottom ash, boiler ash, and filter ash during incineration in an incinerator (Hasan, 1994, 1998; Zhang et al., 2000). Usually, the boiler ash and filter ash are combined together and referred to as fly ash. Flue gas generated in incinerator of MSW can be properly treated by modern facilities installed in situ in the incineration plant (Hasan, 1994, 1998). Hence, the remaining potential pollution sources in the MSW *
Corresponding author. Tel.: +86-21-65-980041; fax: +86-21-65982684. E-mail address: [email protected] (Z. Youcai). 0956-053X/$ - see front matter Ó 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.wasman.2004.01.004
incineration process may be the bottom and fly ashes if these ashes are not properly treated before discharging into the environment. The most significant pollutants in the ashes are heavy metals, whose contents and their distribution among flue gases and bottom ash vary with the composition and moisture in the MSW incinerated, incineration temperature and time, etc. (Cheng-Fong and Huang, 1990; Serge Biollaz et al., 2001). Typically, 10–25% of MSW will change into bottom ash and 0.5–2% into fly ash in a modern incinerator, with much less quantity of fly ash than that of bottom ash (Youcai et al., 2002a; Stucki and Jakob, 1997). In general, the bottom ash may be considered nonhazardous and non-toxic, as the heavy metal contents in the ash may be lower than the regulated standard values in terms of leaching toxicity or absolute values (Tay, 1987, Tay and Goh, 1991). However, the contents of heavy metals in bottom ash seemingly increase in the developed nations as the MSW incinerated may contains
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higher ratios of wastes with heavy metals, such as paints, metal articles and goods, etc. (Jakob et al., 1995,Jakob, 1998). In some countries, such as Switzerland, the environmental requirements have become stricter and the heavy metal contents in the bottom ash have been found to exceed the constraints set by the relevant environmental regulations (Serge Biollaz et al., 2001). In this case, the heavy metals in the bottom ash should be removed before it is reused as construction materials or placed in a landfill. However, the quantity of bottom ash is relatively great and the heavy metals contents in it are quite low. It will be cost- and energy-intensive when trying to reduce the heavy metal content in the bottom ash by any methods such evaporation at high temperature or separation of heavy metals before the waste is incinerated. Transferring heavy metals in MSW as much as possible into the flue gas so that their content in the bottom ash can be reduced to the regulated level may solve this dilemma. The corresponding content of heavy metals in fly ash collected from flue gas and boiler may thus increase to a level possibly comparable with ores. Much effort has been done in this innovative concept (Jorg and Stuki, 1999; Jorg et al., 1998). Though the incineration process of MSW has been studied intensively, the effect of moisture on the volatility of heavy metals, nevertheless, has not been studied. According to the discharging standards set in most nations, the heavy metals in the bottom ash are always within the limits of standards. As the environmental constraints increase, more attentions are being drawn to the pollution arisen from the bottom ash because the heavy metals in the ash may have exceeded the newly regulated standards. Many developing countries, such as China, have used incineration technologies to treat their increasing quantity of MSW. The moisture in the MSW in these countries may be quite high, usually as high as 40–65%, depending on their living customs and levels, as well as the functions of collection facilities. Take China as an example. Contents of paper, plastic, glass, etc., may be low, as these recoverable wastes have been collected by scavengers, while those of high-moisture food origin wastes may be high, in MSW. Moreover, most collection tools are not waterproof. As a result, the moisture in MSW in the dry and arid north part of China may be lower than that in the rainy, hot and humid south part of China, especially in the rainy summer season (Youcai et al., 2000, 2001, 2002a,b). The changes of moisture in MSW may lead to transformation of chemical forms of heavy metals and even the incineration mechanism in the incineration process, and thus influence the volatilities of these metals. Consequently, the distribution of heavy metals in gaseous phase, bottom and filter ashes will be correspondingly changed. It can be reasonably predicted that
different incineration parameters should be applied for different MSW with varying moisture, meaning that the incineration process should be adjusted when the technologies used in the developed nations are transferred to the developing ones. The objectives of this work are to explore the impacts of moisture in MSW on the volatility of heavy metals so that the distribution of heavy metals in three media can be roughly predicted.
2. Experimental 2.1. Simulated laboratory scale incinerator A scaled-down version of an MSW grate incinerator that would fulfil detailed geometric similarity is difficult to realize. Accepting, however, some simplifications, it is possible to build a simple model in the form of a tubular furnace reactor: the grate is then replaced by a ceramic crucible, as a sample container and means of transport of the sample in the furnace. The incineration process is carried out as a batch experiment: the temperature profile that a waste sample experiences in a continuously operated incinerator is simulated by moving the crucible to the appropriate temperature zones of the tubular furnace for given residence times. Air feed to the furnace can be regulated according to the air needs applied to the different temperature zones. The structure of the simulated incinerator is described in several literatures and its scheme is also shown in Fig. 1. The similarity between the simulated furnace and the real grate incinerator has been well justified (Jorg and Stuki, 1999,Jorg et al., 1998). The temperatures in the entrance point and the highest point is around 50 and 950–1000 °C, respectively, and distance between the two points is 128 cm. The crucible with or without samples is manually pushed into the tube from the entrance point to the highest temperature point and pulled out in reverse way with constant rate at a given time so that a certain temperature profile in which the samples are incinerated can be obtained.
Furnace
Primary air Flue gas
Crucible
Moving rod
Secondary air Temp1
Temp2 Ref Temp
Data collection computer
Fig. 1. Schematic diagram of the laboratory scale simulated grate incinerator.
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2.2. Preparation of synthetic wastes
2.3. Moisture calculation
It has well been confirmed that the real waste can be reasonably represented by the synthetic wastes consisting of the following components: 5.4 g of wood powder, 2.6 g lava (a volcanic ash mainly consisted of silica), 1.9 g polythene, 0.19 g polyvinyl chloride (totally 10 g, with 5% moisture) (Jorg and Stuki, 1999, Jorg et al., 1998). Such a synthetic waste is always used to simulate the real wastes in this work, unless otherwise indicated. The contents of heavy metals in this waste is negligible, and a given quantity of water and heavy metals representative of lead, zinc and copper in forms of chlorides, oxides and metallic, are added to the waste so that the study of the impact of moisture on volatility of heavy metals becomes possible. These components are mixed as thoroughly as possible and stored in a sealed container with water in the bottom for overnight so that the water can be absorbed by and homogenized in the wastes as the real waste does, and then put in a rectangle crucible (10 4 cm) and pushed into the furnace along the quartz tube. Two temperature monitoring points with a distance of 9 cm are set within the two inner sides of crucible, which are referred to as Temp 1 and Temp 2 in the text. A fixed temperature monitoring point, referring to as Ref Temp (reference temperature), is set in the right side of the tube. The highest temperature point is located in the left side of the Ref Temp point. The flue gas generated will pass through the Ref Temp monitoring point. Hence, the Ref Temp can accurately reflect the temperature change of flue gas in the incineration process, and thus can be used for quantitative characterization of heat release from the incineration. The Temp1 and Temp2 are actually the temperature of flames or flue gas just above the surface of wastes, and will change correspondingly when the waste samples release or absorb heat while entering into the tube. Ref Temp, Temp1 and Temp2 will keep unchanged if no wastes are pushed into the incinerator. The bottom ash is collected carefully so that it can be removed from the crucible as completely as possible and weighted after the incineration test is finished and the crucible is pulled out and cooled down. Then the ash is ground to fine powder, from which around 300 mg is weighted and digested in concentrated HF, HNO3 and H3 BO3 in pressure microwave. In general, the sample can be dissolved completely in the solution. Nevertheless, a few black tar may not be dissolved if the wastes with higher moisture, such as over 40%, are not well incinerated. Three parallel digestions for each sample are carried out and their average values are provided in the text. The concentrations of heavy metals in the aqueous solution are determined by ICP. It is observed that the data of three parallel digestions coincide well.
The water (moisture) in the mixture of 5.4 g of wood powder, 2.6 g lava, 1.9 polythene, 0.19 g polyvinyl chloride is 0.5 g. Suppose x g of water is added to the mixture, then the moisture, m (wt%), is calculated as follows: m ¼ ððx þ 0:5Þ=ð10 þ xÞÞ 100
3. Results and discussions 3.1. Temperature profile used Figs. 2 and 3 show the typical temperature profiles obtained with 5% and 59% moisture in the samples. 0.5 min is spent for the samples to be pushed from the entrance point to the highest temperature zone and then stay at the zone for 9.5 min before pulling out in 1.5 min, which is used for all experiments unless otherwise indicated. The temperature of sample with 5% moisture can reach to the peak value and ignite for incineration in 2.5 min, while for the sample with 59% moisture at least 5 min should be taken to reach such an ignition point. Obviously, more time will be needed for drying of sample with higher moisture before it is ignited for incineration, which may affect the volatility of heavy metals in the wastes in the incineration process. For the samples with different moistures, the higher the moistures, the longer time will be needed to reach the peak temperature and ignition. 3.2. Dependence of bottom ash weight on the incineration time The longer incineration time, the less bottom ash will be obtained, as shown in Fig. 4. It is found that the bottom ash weight will decrease to 2.69 and 2.60 g after 10 and 22 min incineration. The objective of this work is to explore the volatility of heavy metals in the wastes in
Temperature (˚C)
1200 1000 800 600 400
Temp 1 temp 2 Ref Temp
200 0 0
2
4
6
8
10
12
14
16
Time (minute) Fig. 2. Incineration process of the synthetic waste containing oxides of Zn, Pb and Cu at moisture of 5%.
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Temperature (˚C)
1200 Temp 1 Temp 2 Ref Temp
1000 800 600 400 200 0 0
5
10
15
20
Time (minute)
Weight of bottom ash (g)
Fig. 3. Incineration process of the sythetic waste containing oxides of Zn, Pb and Cu at moisture of 59.5%. 10 9 8 7 6 5 4 3 2 0
5
10
15
20
25
Incineration time (minute) Fig. 4. Effect of incineration time on the weight of bottom ash for the synthetic wastes (10 g with 5% moisture) without the presence of heavy metals.
the incineration process; hence, the incineration time should be limited so that their relative volatility can be reasonably compared. The volatility of heavy metals in the wastes may be consistent if sufficient incineration time is applied, when only moisture is changed. In this work, a total time of 10 min are used for the experiments as described above, unless otherwise indicated.
the wastes. It can be seen that the volatilities are quite different for the three types of metals. The volatilities for lead chloride can be considered nearly unchanged at around 62%, and those for zinc chloride decrease rather significantly, from 68% to 51%, while those for copper chloride increase considerably, from 9% to 21%, when the moistures in the wastes increase from 5% to 69%. From viewpoints of the percentages of volatilities of three metals, copper chloride is always the lowest, and the lead the highest except for the first point in the figure. 3.4. Volatility of oxides of lead, zinc and copper The volatilities of oxides are much less than the counterparts of chlorides, as shown in Fig. 6, with the same metal contents in the wastes. Moreover, the volatilities of both oxides of lead and copper (as cuprous oxide) decrease, from 30% to 19% for Pb and from 9% to 6% for Cu, with a faster rate for lead oxide, while those of zinc oxide can be considered unchanged at around 15%, when the moistures in wastes increase from 5% to 69%. 3.5. Volatility of metallic lead, zinc and copper The volatilities of metallic forms of three metals are always lower than their counterparts of both chlorides and oxides as given in Fig. 7. The volatilities of lead and zinc seem to decrease slightly from around 24% to 20% for Pb and from 14% to 9% for Zn, with minor variation of data obtained, while those of copper increase slightly from around 10% to 13%, when the moisture in the waste increases from 5% to 69%. 3.6. Effect of metal contents on the volatility in forms of oxides
3.3. Volatility of chlorides of lead, zinc and copper Fig. 5 shows the relationship between heavy metal volatility in the forms of chlorides and the moistures in
Fig. 8 shows the relationship between the initial metal contents (in the form of oxides) in the wastes and volatilities of metals in the incineration process. Three types
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Volativity of metal (wt%)
Volativity of metal (wt%)
80 60 50 40
lead chloride zinc chloride copper chloride
30 20 10 0 0
20
40
60
80
Moisture (wt%) Fig. 5. Relationship between the content of moisture and volatility of chlorides of lead, zinc and copper in the incineration of synthetic wastes. Metal content in the waste (mg metal/g dry waste) Pb 10.4; Zn 21.2; Cu 10.1.
35
lead oxide
30
zinc oxide
25
cuprous oxide
20 15 10 5 0 0
20
40
60
80
Moisture (wt%) Fig. 6. Relationship between the content of moisture and volatility of oxides of lead, zinc and copper in the incineration of synthetic wastes. Metal content in the waste (mg metal/g dry waste) Pb 10.4; Zn 21.2; Cu 10.1.
Volativity of metal (wt%)
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Longer time of incineration cannot remarkably raise the volatilisation of heavy metals in the wastes.
30 25 20
3.8. Discussions
15 10
a Zn Cu
5 0 0
10
20
30
40
50
60
70
80
Moisture (wt%) Fig. 7. Relationship between moisture and volatility of metallic lead, zinc and copper in the incineration of synthetic wastes. Metal contents in the wastes (mg metal/g dry waste) Zn 19.8; Cu 5.4.
60
Volativity of metal (wt%)
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50 Pb Zn Cu
40 30 20 10 0 0
5
10
15
20
Initial metal contents (mg metal/g dry waste) Fig. 8. Relationship between volatility of oxides of Pb, Zn and Cu and their initial contents in the incineration of synthetic wastes. Dry wastes 9.5 g; moisture 59.0%.
of metals behave in rather different volatility trend. The percent volatilities of lead and zinc oxides decrease, from 54% to 24% for Pb and from 22% to 13% for Zn, with a faster rate for the former and lower rate for the latter, when the initial Pb and Zn increase from 1.2 to 11 mg/g dry waste for Pb and from 3 to 20 mg/g dry waste for Zn. In contrast, the volatilities of cuprous oxide increase slightly from 1% to 6%, when the initial contents of metals in the wastes increase from 1.4 to 10 mg/g dry waste. 3.7. Effect of incineration time As expected, the volatilities of lead and zinc in the wastes increase slightly while those of copper keep unchanged, when the incineration time extends (Table 1). Table 1 Effect of incineration time on the volatilities of heavy metals in the wastes Incineration time (min)
Lead volatility (%)
Zinc volatility (%)
Copper volatility (%)
10 15 22
23.84 24.50 25.03
13.53 14.10 15.60
6.70 6.81 6.76
Heavy metals (as oxides) contents in the wastes (mg metal/g dry waste): Pb 10.4; Zn 21.2; Cu 10.1; Moisture 59%.
The average contents of zinc, lead and copper in the Swiss MSW are reported to be around 1.45 mg/g dry waste, 0.73 mg/g dry waste and 0.42 mg/g dry waste (Hasan, 1994). The corresponding values in MSW in China are slightly lower in dry base, i.e., around (in mg metal/g dry waste) 1.35 for Zn, 0.56 for Pb and 0.21 for Cu (Youcai et al., 2002a). Few researches on the heavy metals analysis and volatility for China MSW have been carried out. In this work, the Swiss MSW is taken an example. According to the literature report, typically around 41–56% of lead, 47–57% of Zn and 3–6% of Cu may be evaporated from Swiss MSW when it is incinerated in grate incinerators (Youcai et al., 2002a; Stucki and Jakob, 1997). The forms of heavy metals in MSW will certainly be very complex, and it is hard to calculate the contribution and distribution of each chemical species for said element to their volatility. However, a definite conclusion can be drawn from this work that lead is always the most volatile and copper is the least with zinc the middle among the three elements tested, which is completely consistent with the above mentioned evaporation sequences of Pb, Zn and Cu observed in a real incinerator. By rough estimation from Figs. 2–8 at a MSW moisture of 30–60%, the volatility is 60–70% for Pb, 50– 60% for Zn and 5–10% for Cu, as chlorides, and 20–25% for Pb, 15% for Zn and 5–8% for Cu, as oxides, and 25%, 12% and 11% for Pb, Zn and Cu as metallic, respectively. The values are higher for chlorides, and lower for oxides and metallic, than the real data obtained in the incinerator, indicating that the chemical species of metals should be the mixtures of chlorides, oxides and metallic and other forms. Table 2 and Fig. 9 shows the boiling and melting points of the metals or their compounds of interest, as well as the temperature ranges of incineration in the simulated incinerator as indicated between the two dotted lines (Youcai et al., 2002a). Metals of Ni, Cr and Fe, oxides of Zn, Ni, Cr, and Fe, would be relatively less volatized, as the melting points of these compounds and metals fall above the incineration temperature range. However, the melting points of all the chlorides and most sulfates, and metallic Hg, Zn, Cu, Pb and Cd, fall in or below this range, which imply that these substances would be more volatile under the incineration conditions. Hence, it may be proposed that the volatility of heavy metals from MSW is possibly originated from the evaporation of chlorides and sulfates of Zn, Ni, Cr, and Fe and chlorides, sulfate and oxides for the other metals, and metallic elements for Hg, Zn, Cu, Pb and Cd
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Table 2 The physical properties of metals and their compounds Element
Melting point (°C)
Boiling point (°C)
Oxides
Chlorides
Sulfates
M.P. 275 °C, B.P. 301 °C M.P.283 °C, sublimation under calcination M.P. 620 °C, decomposable at 993 °C M.P. 501 °C, B.P. 950 °C M.P. 570 °C, B.P. 960 °C M.P. 1001 °C M.P. 83 °C
Decomposable at the M.P.
Hg
)39
357
Decomposable above 400 °C
Zn
419
907
Cu
1083
2595
Volatilization at 1800 °C M.P. 1026 °C
Pb Cd Ni Cr
327 321 1555 1900
1744 767 2837 2480
M.P. 886 °C, B.P. 1516 °C Sublimation at 900 °C M.P 1980 °C M.P. 2435 °C, B.P.3000 °C
Fe
1535
3000
M.P. 1377 °C, decomposable at 3410 °C
m.p.. of Metals b.p. of metals m.p. of chlorides of metals m.p. of oxides of metals 3400 2900
T (°C)
2400 1900 1400 900 400 -100 Hg Zn Cu Pb Cd Ni
Cr
Fe
Fig. 9. The melting and boiling points of the metals and their compounds concerned.
concerned. Nevertheless, the metallic forms may be oxidized into oxides and thus reduce the possibility of evaporation in the incineration. From Fig. 5 it can be seen that the presence of high moisture in MSW will reduce the volatilization of zinc chlorides while increase that of lead and copper chlorides. Zinc chlorides may hydrolyse to form zinc hydroxide in the presence of high moisture in MSW, which is dewatered to form less volatile zinc oxide. Also, zinc chloride sublimes at temperature over its melting point of 283 °C. The hydrolysis and formation of oxide from zinc chloride would exert unfavourable effect on the sublimation and subsequently depress the evaporation of zinc in the incineration process. For lead chloride, the melting and boiling points are 501 and 950 °C, respectively, without sublimation and decomposition at high temperature. The melting of copper chloride is 620 °C and it decomposes at 993 °C, also without any sublimation at high temperature. Hence, it may be the property of sublimation for zinc chloride that leads to different evaporation behaviour compared with lead and copper chlorides.
M.P. 282 °C, B.P. 316 °C
Decomposable under calciantion Decomposable at 560 °C M.P. 1170 °C M.P. 1000 °C M.P. 31.5 °C Decomposable at high temperature Decomposable at high temperature
Different effects of moisture on the volatilities of Pb and Zn as a group and Cu as another group for both forms of metallic and oxidic species show that the evaporation mechanism for these two groups should be different. As shown in Table 2 and Fig. 9, Pb and Zn should be volatile completely at the incineration temperature used in the simulated incinerator if the incineration time would be sufficiently long and no metallic metals would be oxidized into oxides. However, the metallic forms will always be oxidized, more or less, to oxides, depending on the redox conditions in the incinerator, which will inevitably reduce the volatilities of metals in the wastes as the volatilities of oxides are much less than those of metallic forms. The presence of moistures in the wastes may facilitate the oxidization of metallic Pb and Zn and consequently the volatilities of these two metals decrease as the moistures increase in the wastes. The presence of moistures seems to facilitate the volatilisation of copper, both on forms of metallic and oxides, completely in reverse to those of Pb and Zn. From Table 2 and Fig. 9, it can be seen that the volatility of Cu species decreases roughly in the following sequence: sulfates > chlorides > oxides > metallic The presence of moisture should promote the formation of more volatile forms from metallic and oxides, e.g., from metallic copper to its sulfate and chloride, or even from metallic to oxide. Such a transformation may be rather incomplete and ineffective as the sulfate and chloride possibly formed in situ in the incineration process would decompose simultaneously because these compounds are not stable at high temperature, resulting in still low volatility of metallic copper and oxides. The facts that longer incineration would not substantially increase the volatilities of oxides (and possibly for the other forms) imply that the thermochemical reactions occur between the wastes (both inorganic and
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organic) and heavy metals at high temperatures should be one of important factors for the volatilization of the heavy metals in the wastes. Less and less organics or inorganics such as chlorides are available for the transformation of heavy metals in MSW into more volatile forms as the incineration time extends. It has been well proved that the presence of certain chemicals such as chlorides in forms of both chlorine and salts can improve the volatilities of heavy metals from fly ash and bottom ash (Stucki and Jakob, 1997). These phenomena should also occur in the incineration process of MSW, i.e., the presence of wastes may facilitate the volatilization of oxides, when the other conditions such as moisture contents are kept consistent (Table 1).
4. Conclusions Increase of moistures in MSW will reduce the volatilization of all chemical forms of lead and zinc and oxides of copper, while slightly increase the volatilization of chlorides and metallic form of copper, when the wastes are incinerated at a limited time of 10 min in a laboratory scale simulated incinerator. The chemical mechanisms governing the volatilization behaviors of different forms in the incineration process are proposed. Hydrolysis, dewatering of hydrolyzed species, sublimation, chemical transformation of less volatiles to more volatiles or reverse, are considered among the factors affecting in the volatilization of heavy metals in MSW.
Acknowledgements This work was partially supported by Swiss National Science Foundation and China National Natural Science Foundation (No. 59778016), China Outstanding Young Scientist Program, Shanghai Key Research Program, Doctoral Research Program of China Education Ministry, and Shanghai Science and Technology Commission as part of Sino–Swiss Bilateral Cooperation Program. The authors want to thank Mr. Albert and Mr. Lutz for their work on the analysis of heavy metals.
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