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The adsorption of heavy metals by different sorbents under various incineration conditions
~
Chemosphere,Vol. 37, No. 13, pp. 2617-2625, 1998 © 1998ElsevierScienceLtd.All rightsreserved 0045-6535/98/$ - see frontmatter
t Pergamon
PIh S0045-6535(98)00161-1
The Adsorption of Heavy Metals by Different Sorbents Under Various Incineration Conditions Jyh-Chemg Chen, Ming-Yen Wey*, Yao-Chi Lin Dep. of Environmental Engineering, Chung-Hsing University, Taichung, Taiwan, R.O.C. (Receivedin Germany20 February1998;accepted23 April1998)
ABSTRACT
The emission of heavy metals can be controlled by adding solid sorbents into the combustion chamber during incineration processes. The objective of this work was to experimentally study the adsorption efficiency of different sorbents for heavy metals under various incineration conditions. Each sorbent has its optimum operating temperature. Kaolinite and aluminum oxide have the best adsorption efficiency at 800°C, and bauxite is at 700°C. The adsorption efficiencies of the three sorbents for the four heavy metals all follow the sequence of Pb > Cu > Cr >Cd. The presence of inorganic chloride (NaC1) and sulfate ~2qa2SO4) increases the adsorption efficiency of the sorbents, but organic chloride PVC decreases the adsorption efficiency. ©1998 ElsevierScienceLtd. All rights reserved
INTRODUCTION
The emission of heavy metals from the incineration of solid wastes presents potential environmental and health hazards [1][2]. Heavy metal compounds are volatilized at high temperatures, and then form the metallic vapor or submicro particles. Current practices for metal emission control employ conventional air pollution control devices (APCDs) such as wet scrubber, baghouse, and other systems to collect metals in the flue gas. These APCDs, however, may not always be sufficiently effective in collecting these submicro paniculate matter. Although a wet scrubber could effectively remove small particles by collisions between particles and water drops and provides the possibility of simultaneously removing solub_e gaseous pollutants by the addition of chelating reagent, the particle-laden scrubbing liquid must be disposed of [3]-[5]. 2617
2618
Earlier studies about heavy metal control [6]-[8] indicated that it is difficult to control the volatilization of metals during high temperature processes. An alternative control technology for heavy metal emission is using solid sorbents to capture metals by physical deposition and chemical adsorption. In the available incineration systems, the fluidized bed incinerator appears to be suitable for this purpose, because it provides intensive mixing of sorbents and metal species.
Ho et al. [9] used different sorbents to capture metals during the combustion process. Their results showed that most silica aluminum species such as kaolinite, bauxite, limestone, and aluminum oxide can be used to adsorb heavy metals. The adsorption efficiency of sorbents for different metals is quite different. The adsorption efficiency of limestone for lead and cadmium .is better than silica sand and aluminum oxide. While Uberoi et al. [10] showed that bauxite and kaolinite are more effective sorbents for capturing cadmium and alkali metals than limestone and emathline. And kaolinite is the best sorbent to canture lead compounds. Punjak et al. [ll], Uberoi et al.[6], and Chen et al. [12] indicated that the adsorption efficiency of different sorbents is influenced by operating temperature, chloride content, kinds of sorbents, sorbent size, the amount of sorbent additive, and the air flow rate. Of these factors, operating temperature, chloride content, and sorbent size are the most important. When the operating temperature increases, metals are easier to volatilize, thus decreasing the capture efficiency of metals. As chlorides content increases, the products of heavy metals are more volatile, and thus cannot be easily captured by the sorbents [13].
Ho et al. [7] and Lee et al. [14] addressed some mechanisms of sorbents to capture metals during incineration. One mechanism is metaled ash capture. In the incineratiori process, some metals did not volatilize, and stayed in metaled ash to be captured by sorbents or deposited. This adsorption mechanism is affected by operating temperature. The second mechanism is vapor capture. When metals were heated, some volatile metals existed in the gas phase, some formed particles, and some were captured by sorbents. The third mechanism is particles capture. Most metal particles can be captured by sorbents through nucleation, coagulation, and sweeping. This is evident especially when the surface of sorbents is more sticky, such as a glassy surface or sorbents coated with a layer of sticky material.
It is feasible to use solid sorbents to control heavy metals during fluidized bed incineration. But the adsorption efficiency of sorbent in different incineration conditions and the effects of organic chloride, inorganic chloride, and sulfate on adsorption efficiency of sorbents are seldom discussed. Therefore, the main objective of this study focus on the most toxic heavy metals Pb, Cr, Cu, and Cd to study the effect of different operating conditions on the adsorption efficiency of different sorbents for those metals in a fluidized bed incinerator. The various operating conditions studied included the operating temperature, addition of organic chloride, inorganic chloride, and sulfate.
2619
EXPERIMENTAL
Preparation of synthetic feed wastes The synthetic solid wastes used in the experiments were composed of 1.00g/bag plastics, 1.00g/bag sawdust, and 1g/bag heavy metal solution (simulating general municipal solid waste compositions). Four investigated metals Cr, Cu, Pb, Cd (with nitrate) of 0.03g/bag were dissolved in the distilled water. The material described above were enclosed in a PE plastic bag of 0.3g. The weight of each bag of synthetic material was 3.0g. When investigating the effect of organic chloride, inorganic chloride, and sulfate, 0.15g/bag PVC, NaC1, and Na2SO4 were added in each bag. Table 1 lists the compositions of various synthetic feed waste. Table 1 The compositions of synthetic feed wastes Sawdust
PP*
PVC**
NaCI
NazSO4 PE*** bag Heavymetal solution**** Runl-9 1.00 0.55 0.15 0 0 0.30 1.00 Runl0-12 1.00 0.55 0 0.15 0 0.30 1.00 Runl3-15 1.00 0.70 0 0 0.15 0.30 1.00 Runl6-18 1.00 0.55 0 0 0 0.30 1.00 *PP(Poly-propylene) ** PVC(Polyvinyl chloride) *** PE(Polyethylene) (unit: g/bag) **** 0.03g ofCr, Cu, Pb, and Cd were contained in the solution
Apparatus The reactor for this experiment is a fluidized bed incinerator, consisting primarily of a feedstock feeder, main combustion chamber (30cm height) of both inside 10cm diameter. The chmnber was made of lnconel with a 3mm wall thickness. One thermocouple was used to determine the temperature in combustion chamber. Figure 1 illustrates the incineration system.
t s
I
!
1
Figure 1 Fliudized bed incinerator and air pollution control equipment 1.Air Compressor 2.Flowmeter 3.Combustion chamber 4.Electrical heater 5.Thermal feedback controller 6.Thermocouple 7.Feeder 8.Cyclones 9.Scrubber 10.Induced fan
2621 of 700°C, especially for lead. While aluminum oxide is the most effective sorbent for the four metals at 800°C. When the operating temperature is 900°C, the most effective sorbent for adsobing Cr and Cd is bauxite, for Cu is aluminum oxide, and for Pb is kaolinite. The differences of adsorption efficiency of different sorbent for the metals at various operating temperature are resulted from the different physical and chemical properties of sorbent, the major species of heavy metals formed at high temperature, and the relative reactions between the sorbent and metal compounds.
g
50
• kaolinite
~
40 30
"=
20
[] A1203
°At*
• bauxite
0
Cr
Pb
Cu
Cd
heavy metals Figure 2 The adsorption efficiency of the sorbents (kaolinite, bauxite, and aluminum oxide) for the four heavy metals at 700°C
5O 40
i kaolinite
3o
•=
• bauxite
20
[] A1203
=1
0 Cr
Pb
On
Cd
heavy metals Figure 3 The adsorption efficiency of the sorbents (kaolinite, bauxite, and aluminum oxide) for the four heavy metals at 800°C
2622 25 ~"
20 • kaolinite
g~
• bauxite
10
[] A1203
s
0 Cr
Pb
Cu
CA
heavy metals Figure 4 The adsorption efficiency of the sorbents (kaolinite, bauxite, and aluminum oxide) for the four heavy metals at 900°C
The ~dsorption efficiency of sorbent for different heavy metals
Figures 2--4 also indicated that the adsorption efficiency of the three sorbents for the four metals had the same trend of Pb > Cu > Cr > Cd at each operating temperature. This results is relative to the different physical properties (such as volatility) of the heavy metal species formed at high temperature and the different reaction potentials between sorbents and metal species.
The effect of operating temperature on the adsorption efficiency of heavy metals
The effect of operating temperature on the adsorption efficiency of the sorbents for the four heavy metals is also displayed in Figures 2-4. The results showed that kaolinite had the best adsorption efficiency at 800°C. This indicated that the adsorption of kaolinite for these three metals included chemical and physical reactions. Because physical adsorption is dominated at lower temperature of 700°C, and chemical adsorption is dominated at higher temperature of 900°C. When the temperature increased, the physical adsorption process of kaolinite for the three metals was inhibited, but the decreased temperature also decreased the chemical adsorption reaction.
From figures 2-4, the best adsorption efficiency of bauxite is occurred at 700°C. Thus physical adsorption reaction is the dominated mechanism of bauxite. The molecular attraction between the sorbent and metal compounds is favorable at lower temperature. For the aluminum oxide, the best operating temperature is 800°C. The adsorption mechanisms of aluminum oxide also included physical and,chemical reactions, that is the same as kaolinite,
The effect of feed waste compositions on the adsorption of heavy metals
Figures 5-8 showed the adsorption efficiency of the three sorbents for the four heavy metals at different feed waste compositions. They indicated that the addition of inorganic chloride (NaCI) and sulfate
2623 (Na2SO4) are helpful to increase the capture efficiency of the three sorbents for heavy metals, but the presence of organic chloride (PVC) decreased the adsorption efficiency. This results are consistent with that of Chen [12], the presence of organic chlorides increased the formation of volatile metal chlorides rather than metal oxides, the presence of inorganic chlorides and sulfates can inhibit the formation of volatile metal chlorides. 150 120 ~
90
°
60
NPP NPP+PVC O PP+NaCI :•PP+Na2SO4
3o
0 kaolinite
bauxite
A1203
sorbents Figure 5 The adsorption efficiency of the three sorbents for heavy metal Cr at different feed waste compositions
.~ o
~
120 90 ~
~
PP • PP+PVC [] PP+NaCI • PP+Na2SO4 •
d
N 60 30 0 kaolinite
bauxite
A1203
sorbents Figure 6 The adsorption efficiency of the three sorbents for heavy metal Pb at different feed waste compositions 200 150
•Pp • Pp+Pvc i--iPP+NaCI • PP+Na2SO4
°.~100 o 50
0 kaolinite
bau ~ite
A 1203
sorbents Figure 7 The adsorption efficiency of the three sorbents for heavy metal Cu at different feed waste compositions
2624 35 ~ .......................................................... • PP
21
• PP+PVC
~
[] PP+NaCI [] PP+Na2SO4
7 0
.....
kaolinite
bauxite
'
Al203
sorbents Figure 8 The adsorption efficiency of the three sorbents for heavy metal Cd at different feed waste compositions.
In addition, the adsorption efficiency of three sorbents for the heavy metals is varied with different feed waste compositions. When chlorides and sulfates are absent in the feed waste, bauxite is the best sorbent for metals Pb, Cr, and Cd, and kaolinite is the best for Cu. When organic chloride PVC is present in the feed waste, bauxite is the best sorbent for the four metals. But kaolinite is the best sorbent for Pb, Cr, and Cd, and aluminum oxide is the best for Cu when inorganic chloride NaCI is present in the feed waste. When sulfate Na2SO4 is present in the feed waste, aluminum oxide has the best adsorption efficiency for Cr, Cu, and Cd, and bauxite is the best for Pb. Therefore, different heavy metal species were formed at different feed waste compositions, which also influenced the adsorption mechanism and efficiency of sorbents for metal compounds.
CONCLUSIONS
The emission of heavy metals during incineration can be effective controlled by solid sorbents. The adsorption efficiency of different sorbents for the four heavy metals is quite different at different operating temperature. Bauxite is the best sorbent for the four metals at the temperature of 700°C, and aluminum oxide is the best at 800°C. Each sorbent has its optimum operating temperature. Kaolinite and aluminum oxi& have the best adsorption efficiency at 800°C, and bauxite is more effective at 700°C. The adsorption efficiencies of the three sorbents for the four heavy metals all follow the sequence ofPb > Cu > Cr >Cd. Different feed waste compositions also have significant influences on the adsorption efficiency of the three sorbents. The presence of inorganic chloride NaC1 and sulfate Na2SO4 increases the adsorption efficiency of the three sorbents, but the addition of organic chloride PVC decreases the adsorption efficiency. This is owing to the present PVC would increase the volatility of metal species and reduce the adsorption efficiency of sorbent. But the presence of inorganic chloride NaC1 and Na2SO4 could inhibit the formation of volatile metal chlorides.
2625
REFERENCE [ 1] A. N. Glenn, A review of trace element emissions from the combustion of refuse-derived fuel with coal. Environmental Progress 11(2), 140-144 (1992). [2] P. I.. William and O. L. Wendt, Toxic metal emissions from incineration: mechanisms and control, Prog. Energy. Combust. Sci. 19, 145-185 (1993). [3] R. C. Flagan and J. H. Seinfeld, Fundamentals of air pollution engineering, p. 480. Prentice-Hall, Inc., New Jersey (1988). [4] R. C. Jain and S, C. Young, Laboratory/bench scale testing and evaluation of A.P.T. dry plate scrubber, DOE-ET-15492-2030, Air Pollution Technology, Inc., San Diego, CA (1985). i5] M. Y. Wey, The simulation of flue gas scrubbing, Environ. Int. 20, 493-505 (1994). [6] M. Uberoi, High temperature removal of metal vapors by solid sorbents, Ph.D. Thesis, Department of Chemical Engineering, The University of Arizona (1990). [7] T. C. Ho, J. M. Chen, S. Shukla and J. R. Hopper, Metal capture during fluidized bed incineration of solid wastes, AICHE Symposium Series 276(86), 51-60 (1990). [8] T. C. Ho, T. Tan, C. Chen and J. R. Hopper, Characteristics of metal capture during fluidized bed incineration, AICHE Symposium Series 281 (87), 1 l 8-126 ( 1991 ). [9] Y. C. Ho, C. Chen, J. R. Hopper and D. A. Oberacker, Metal capture during fluidized bed incineration of wastes contaminated with lead chloride, Combustion Science and Technology 85, 101-116 (1992). ~10] M. Uberoi and F. Shadman, Sorbents for removal of lead compounds from hot flue gases, AICHE Journal 36(2), 307-309 (1990). [11] V. A. Punjak, M. Uberoi and F. Shadman, High temperature adsorption of alkali vapors on solid sorbents, AICHE Journal 35(7), 1186-1194 (1989). [12] J. C. Chen, M. Y. Wey and M. H. Yan, Theoretical and experimental study of metal capture during incineration process, Journal of Environmental Engineering ASCE 123(11), 1100-1106 (1997). [13] J. C. Chen and M. Y. Wey, The effect of operating conditions on the capture of metals with limestone during incineration, Environmental International 22(6), 743-752 (1996). [14] S. H. D. Lee and I. J. Johnson, Removal of gaseous alkali metal compounds from hot flue gas by particulate sorbents, J. Eng. Power 102, 397 (1980).
Chemosphere,Vol. 37, No. 13, pp. 2617-2625, 1998 © 1998ElsevierScienceLtd.All rightsreserved 0045-6535/98/$ - see frontmatter
t Pergamon
PIh S0045-6535(98)00161-1
The Adsorption of Heavy Metals by Different Sorbents Under Various Incineration Conditions Jyh-Chemg Chen, Ming-Yen Wey*, Yao-Chi Lin Dep. of Environmental Engineering, Chung-Hsing University, Taichung, Taiwan, R.O.C. (Receivedin Germany20 February1998;accepted23 April1998)
ABSTRACT
The emission of heavy metals can be controlled by adding solid sorbents into the combustion chamber during incineration processes. The objective of this work was to experimentally study the adsorption efficiency of different sorbents for heavy metals under various incineration conditions. Each sorbent has its optimum operating temperature. Kaolinite and aluminum oxide have the best adsorption efficiency at 800°C, and bauxite is at 700°C. The adsorption efficiencies of the three sorbents for the four heavy metals all follow the sequence of Pb > Cu > Cr >Cd. The presence of inorganic chloride (NaC1) and sulfate ~2qa2SO4) increases the adsorption efficiency of the sorbents, but organic chloride PVC decreases the adsorption efficiency. ©1998 ElsevierScienceLtd. All rights reserved
INTRODUCTION
The emission of heavy metals from the incineration of solid wastes presents potential environmental and health hazards [1][2]. Heavy metal compounds are volatilized at high temperatures, and then form the metallic vapor or submicro particles. Current practices for metal emission control employ conventional air pollution control devices (APCDs) such as wet scrubber, baghouse, and other systems to collect metals in the flue gas. These APCDs, however, may not always be sufficiently effective in collecting these submicro paniculate matter. Although a wet scrubber could effectively remove small particles by collisions between particles and water drops and provides the possibility of simultaneously removing solub_e gaseous pollutants by the addition of chelating reagent, the particle-laden scrubbing liquid must be disposed of [3]-[5]. 2617
2618
Earlier studies about heavy metal control [6]-[8] indicated that it is difficult to control the volatilization of metals during high temperature processes. An alternative control technology for heavy metal emission is using solid sorbents to capture metals by physical deposition and chemical adsorption. In the available incineration systems, the fluidized bed incinerator appears to be suitable for this purpose, because it provides intensive mixing of sorbents and metal species.
Ho et al. [9] used different sorbents to capture metals during the combustion process. Their results showed that most silica aluminum species such as kaolinite, bauxite, limestone, and aluminum oxide can be used to adsorb heavy metals. The adsorption efficiency of sorbents for different metals is quite different. The adsorption efficiency of limestone for lead and cadmium .is better than silica sand and aluminum oxide. While Uberoi et al. [10] showed that bauxite and kaolinite are more effective sorbents for capturing cadmium and alkali metals than limestone and emathline. And kaolinite is the best sorbent to canture lead compounds. Punjak et al. [ll], Uberoi et al.[6], and Chen et al. [12] indicated that the adsorption efficiency of different sorbents is influenced by operating temperature, chloride content, kinds of sorbents, sorbent size, the amount of sorbent additive, and the air flow rate. Of these factors, operating temperature, chloride content, and sorbent size are the most important. When the operating temperature increases, metals are easier to volatilize, thus decreasing the capture efficiency of metals. As chlorides content increases, the products of heavy metals are more volatile, and thus cannot be easily captured by the sorbents [13].
Ho et al. [7] and Lee et al. [14] addressed some mechanisms of sorbents to capture metals during incineration. One mechanism is metaled ash capture. In the incineratiori process, some metals did not volatilize, and stayed in metaled ash to be captured by sorbents or deposited. This adsorption mechanism is affected by operating temperature. The second mechanism is vapor capture. When metals were heated, some volatile metals existed in the gas phase, some formed particles, and some were captured by sorbents. The third mechanism is particles capture. Most metal particles can be captured by sorbents through nucleation, coagulation, and sweeping. This is evident especially when the surface of sorbents is more sticky, such as a glassy surface or sorbents coated with a layer of sticky material.
It is feasible to use solid sorbents to control heavy metals during fluidized bed incineration. But the adsorption efficiency of sorbent in different incineration conditions and the effects of organic chloride, inorganic chloride, and sulfate on adsorption efficiency of sorbents are seldom discussed. Therefore, the main objective of this study focus on the most toxic heavy metals Pb, Cr, Cu, and Cd to study the effect of different operating conditions on the adsorption efficiency of different sorbents for those metals in a fluidized bed incinerator. The various operating conditions studied included the operating temperature, addition of organic chloride, inorganic chloride, and sulfate.
2619
EXPERIMENTAL
Preparation of synthetic feed wastes The synthetic solid wastes used in the experiments were composed of 1.00g/bag plastics, 1.00g/bag sawdust, and 1g/bag heavy metal solution (simulating general municipal solid waste compositions). Four investigated metals Cr, Cu, Pb, Cd (with nitrate) of 0.03g/bag were dissolved in the distilled water. The material described above were enclosed in a PE plastic bag of 0.3g. The weight of each bag of synthetic material was 3.0g. When investigating the effect of organic chloride, inorganic chloride, and sulfate, 0.15g/bag PVC, NaC1, and Na2SO4 were added in each bag. Table 1 lists the compositions of various synthetic feed waste. Table 1 The compositions of synthetic feed wastes Sawdust
PP*
PVC**
NaCI
NazSO4 PE*** bag Heavymetal solution**** Runl-9 1.00 0.55 0.15 0 0 0.30 1.00 Runl0-12 1.00 0.55 0 0.15 0 0.30 1.00 Runl3-15 1.00 0.70 0 0 0.15 0.30 1.00 Runl6-18 1.00 0.55 0 0 0 0.30 1.00 *PP(Poly-propylene) ** PVC(Polyvinyl chloride) *** PE(Polyethylene) (unit: g/bag) **** 0.03g ofCr, Cu, Pb, and Cd were contained in the solution
Apparatus The reactor for this experiment is a fluidized bed incinerator, consisting primarily of a feedstock feeder, main combustion chamber (30cm height) of both inside 10cm diameter. The chmnber was made of lnconel with a 3mm wall thickness. One thermocouple was used to determine the temperature in combustion chamber. Figure 1 illustrates the incineration system.
t s
I
!
1
Figure 1 Fliudized bed incinerator and air pollution control equipment 1.Air Compressor 2.Flowmeter 3.Combustion chamber 4.Electrical heater 5.Thermal feedback controller 6.Thermocouple 7.Feeder 8.Cyclones 9.Scrubber 10.Induced fan
2621 of 700°C, especially for lead. While aluminum oxide is the most effective sorbent for the four metals at 800°C. When the operating temperature is 900°C, the most effective sorbent for adsobing Cr and Cd is bauxite, for Cu is aluminum oxide, and for Pb is kaolinite. The differences of adsorption efficiency of different sorbent for the metals at various operating temperature are resulted from the different physical and chemical properties of sorbent, the major species of heavy metals formed at high temperature, and the relative reactions between the sorbent and metal compounds.
g
50
• kaolinite
~
40 30
"=
20
[] A1203
°At*
• bauxite
0
Cr
Pb
Cu
Cd
heavy metals Figure 2 The adsorption efficiency of the sorbents (kaolinite, bauxite, and aluminum oxide) for the four heavy metals at 700°C
5O 40
i kaolinite
3o
•=
• bauxite
20
[] A1203
=1
0 Cr
Pb
On
Cd
heavy metals Figure 3 The adsorption efficiency of the sorbents (kaolinite, bauxite, and aluminum oxide) for the four heavy metals at 800°C
2622 25 ~"
20 • kaolinite
g~
• bauxite
10
[] A1203
s
0 Cr
Pb
Cu
CA
heavy metals Figure 4 The adsorption efficiency of the sorbents (kaolinite, bauxite, and aluminum oxide) for the four heavy metals at 900°C
The ~dsorption efficiency of sorbent for different heavy metals
Figures 2--4 also indicated that the adsorption efficiency of the three sorbents for the four metals had the same trend of Pb > Cu > Cr > Cd at each operating temperature. This results is relative to the different physical properties (such as volatility) of the heavy metal species formed at high temperature and the different reaction potentials between sorbents and metal species.
The effect of operating temperature on the adsorption efficiency of heavy metals
The effect of operating temperature on the adsorption efficiency of the sorbents for the four heavy metals is also displayed in Figures 2-4. The results showed that kaolinite had the best adsorption efficiency at 800°C. This indicated that the adsorption of kaolinite for these three metals included chemical and physical reactions. Because physical adsorption is dominated at lower temperature of 700°C, and chemical adsorption is dominated at higher temperature of 900°C. When the temperature increased, the physical adsorption process of kaolinite for the three metals was inhibited, but the decreased temperature also decreased the chemical adsorption reaction.
From figures 2-4, the best adsorption efficiency of bauxite is occurred at 700°C. Thus physical adsorption reaction is the dominated mechanism of bauxite. The molecular attraction between the sorbent and metal compounds is favorable at lower temperature. For the aluminum oxide, the best operating temperature is 800°C. The adsorption mechanisms of aluminum oxide also included physical and,chemical reactions, that is the same as kaolinite,
The effect of feed waste compositions on the adsorption of heavy metals
Figures 5-8 showed the adsorption efficiency of the three sorbents for the four heavy metals at different feed waste compositions. They indicated that the addition of inorganic chloride (NaCI) and sulfate
2623 (Na2SO4) are helpful to increase the capture efficiency of the three sorbents for heavy metals, but the presence of organic chloride (PVC) decreased the adsorption efficiency. This results are consistent with that of Chen [12], the presence of organic chlorides increased the formation of volatile metal chlorides rather than metal oxides, the presence of inorganic chlorides and sulfates can inhibit the formation of volatile metal chlorides. 150 120 ~
90
°
60
NPP NPP+PVC O PP+NaCI :•PP+Na2SO4
3o
0 kaolinite
bauxite
A1203
sorbents Figure 5 The adsorption efficiency of the three sorbents for heavy metal Cr at different feed waste compositions
.~ o
~
120 90 ~
~
PP • PP+PVC [] PP+NaCI • PP+Na2SO4 •
d
N 60 30 0 kaolinite
bauxite
A1203
sorbents Figure 6 The adsorption efficiency of the three sorbents for heavy metal Pb at different feed waste compositions 200 150
•Pp • Pp+Pvc i--iPP+NaCI • PP+Na2SO4
°.~100 o 50
0 kaolinite
bau ~ite
A 1203
sorbents Figure 7 The adsorption efficiency of the three sorbents for heavy metal Cu at different feed waste compositions
2624 35 ~ .......................................................... • PP
21
• PP+PVC
~
[] PP+NaCI [] PP+Na2SO4
7 0
.....
kaolinite
bauxite
'
Al203
sorbents Figure 8 The adsorption efficiency of the three sorbents for heavy metal Cd at different feed waste compositions.
In addition, the adsorption efficiency of three sorbents for the heavy metals is varied with different feed waste compositions. When chlorides and sulfates are absent in the feed waste, bauxite is the best sorbent for metals Pb, Cr, and Cd, and kaolinite is the best for Cu. When organic chloride PVC is present in the feed waste, bauxite is the best sorbent for the four metals. But kaolinite is the best sorbent for Pb, Cr, and Cd, and aluminum oxide is the best for Cu when inorganic chloride NaCI is present in the feed waste. When sulfate Na2SO4 is present in the feed waste, aluminum oxide has the best adsorption efficiency for Cr, Cu, and Cd, and bauxite is the best for Pb. Therefore, different heavy metal species were formed at different feed waste compositions, which also influenced the adsorption mechanism and efficiency of sorbents for metal compounds.
CONCLUSIONS
The emission of heavy metals during incineration can be effective controlled by solid sorbents. The adsorption efficiency of different sorbents for the four heavy metals is quite different at different operating temperature. Bauxite is the best sorbent for the four metals at the temperature of 700°C, and aluminum oxide is the best at 800°C. Each sorbent has its optimum operating temperature. Kaolinite and aluminum oxi& have the best adsorption efficiency at 800°C, and bauxite is more effective at 700°C. The adsorption efficiencies of the three sorbents for the four heavy metals all follow the sequence ofPb > Cu > Cr >Cd. Different feed waste compositions also have significant influences on the adsorption efficiency of the three sorbents. The presence of inorganic chloride NaC1 and sulfate Na2SO4 increases the adsorption efficiency of the three sorbents, but the addition of organic chloride PVC decreases the adsorption efficiency. This is owing to the present PVC would increase the volatility of metal species and reduce the adsorption efficiency of sorbent. But the presence of inorganic chloride NaC1 and Na2SO4 could inhibit the formation of volatile metal chlorides.
2625
REFERENCE [ 1] A. N. Glenn, A review of trace element emissions from the combustion of refuse-derived fuel with coal. Environmental Progress 11(2), 140-144 (1992). [2] P. I.. William and O. L. Wendt, Toxic metal emissions from incineration: mechanisms and control, Prog. Energy. Combust. Sci. 19, 145-185 (1993). [3] R. C. Flagan and J. H. Seinfeld, Fundamentals of air pollution engineering, p. 480. Prentice-Hall, Inc., New Jersey (1988). [4] R. C. Jain and S, C. Young, Laboratory/bench scale testing and evaluation of A.P.T. dry plate scrubber, DOE-ET-15492-2030, Air Pollution Technology, Inc., San Diego, CA (1985). i5] M. Y. Wey, The simulation of flue gas scrubbing, Environ. Int. 20, 493-505 (1994). [6] M. Uberoi, High temperature removal of metal vapors by solid sorbents, Ph.D. Thesis, Department of Chemical Engineering, The University of Arizona (1990). [7] T. C. Ho, J. M. Chen, S. Shukla and J. R. Hopper, Metal capture during fluidized bed incineration of solid wastes, AICHE Symposium Series 276(86), 51-60 (1990). [8] T. C. Ho, T. Tan, C. Chen and J. R. Hopper, Characteristics of metal capture during fluidized bed incineration, AICHE Symposium Series 281 (87), 1 l 8-126 ( 1991 ). [9] Y. C. Ho, C. Chen, J. R. Hopper and D. A. Oberacker, Metal capture during fluidized bed incineration of wastes contaminated with lead chloride, Combustion Science and Technology 85, 101-116 (1992). ~10] M. Uberoi and F. Shadman, Sorbents for removal of lead compounds from hot flue gases, AICHE Journal 36(2), 307-309 (1990). [11] V. A. Punjak, M. Uberoi and F. Shadman, High temperature adsorption of alkali vapors on solid sorbents, AICHE Journal 35(7), 1186-1194 (1989). [12] J. C. Chen, M. Y. Wey and M. H. Yan, Theoretical and experimental study of metal capture during incineration process, Journal of Environmental Engineering ASCE 123(11), 1100-1106 (1997). [13] J. C. Chen and M. Y. Wey, The effect of operating conditions on the capture of metals with limestone during incineration, Environmental International 22(6), 743-752 (1996). [14] S. H. D. Lee and I. J. Johnson, Removal of gaseous alkali metal compounds from hot flue gas by particulate sorbents, J. Eng. Power 102, 397 (1980).