Energy & Fuels 2008, 22, 3789–3797
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Partitioning and Emission Characteristics of Pb and Organics during Fluidized Bed Thermal Treatment of Municipal Solid Waste Incineration (MSWI) Fly Ash Jia-Hong Kuo,† Chiou-Liang Lin,‡ and Ming-Yen Wey*,† Department of EnVironmental Engineering, National Chung Hsing UniVersity, Taichung 402, Taiwan, Republic of China, and Department of CiVil and EnVironmental Engineering, National UniVersity of Kaohsiung, Kaohsiung 811, Taiwan, Republic of China ReceiVed June 25, 2008. ReVised Manuscript ReceiVed August 7, 2008
The failure of municipal solid waste incineration (MSWI) fly ash to meet regulatory standards set through the toxicity characteristic leaching procedure (TCLP) of lead (hereafter, Pb) has resulted in its classification as a hazardous material that is unsuitable for use. In this study, a fluidized bed is proposed as a thermal treatment unit to treat MSWI fly ash. The aim of the present work is to treat fly ash by using a fluidized bed with operating conditions, including temperature, pretreatment, and an additive in the fluidized bed. We also considered the partition of the heavy-metal Pb and the emission of organics (PAHs) in the process. Results indicated that the Pb existed mainly in the solid phase, that is, the fly ash and bed materials, under different conditions. The partitioning of Pb to the fly ash was increased from 42.88 to 68.20%, with the operating temperature increased from 700 to 900 °C. When a water-washing pretreatment was applied, the Pb partitioning to the fly ash was less than that when water-washing pretreatment was not applied. This was attributable to the chlorine in raw fly ash that was washed out. Subsequently, the partitioning to the bed materials increased with the addition of CaO to the captured Pb. The concentrations of emitted organics increased when the waterwashing pretreatment and additive were used, because of the condensation of organics in fly ash and the lower combustion efficiency. Moreover, the application of the water-washing process resulted in a negative correlation between Pb and the emitted organics during the thermal treatment. However, a positive correlation occurred with the addition of CaO in the fluidized bed.
Introduction The combustion of municipal solid waste (MSW) is beneficial because heat and power are produced and the volume of waste to be handled is reduced. However, the MSW incineration process creates hazardous solid wastes and results in large volumes of toxic gaseous emissions.1,2 Fly ash and bottom ash generated after MSW incineration are considered as secondary solid wastes. The leaching of heavy metals from fly ash in a landfill and its ensuing leakage into the environment have generated significant concern and considerable attention.3 The collected fly ash is classified as a hazardous waste because of its failure to pass the imposed standards for leaching concentrations of heavy metals through the toxicity characteristic leaching procedure (TCLP), especially for Pb. Figure 1 shows the TCLP test results of three heavy metals (Pb, Cr, and Cd) for fly ash from a MSWI in Taiwan from 2004 to 2007.4 The results illustrated that Pb was the major metallic species that exceeded the regulation. Therefore, the fly ash certainly requires some form of treatment to reduce the leachability of its potentially * To whom correspondence should be addressed. E-mail: mywey@ dragon.nchu.edu.tw. † National Chung Hsing University. ‡ National University of Kaohsiung. (1) Chen, J. C.; Wey, M. Y. EnViron. Int. 1996, 22, 743–752. (2) Kuo, J. H.; Tseng, H. H.; Rao, P. S.; Wey, M. Y. Appl. Therm. Eng. 2008, 28, 2305-2314. (3) Chan, C. C. Y.; Kirk, D. W.; Marsh, H. J. Hazard. Mater. 2000, 76, 103–111. (4) Taiwan Environmental Protection Agency (TEPA). http://www.epa. gov.tw, 2008.
toxic components. Table 1 presents the data of MSW and fly ash in Taiwan from 2002 to 2007. In the past 6 years, it has been observed that the average percentage of fly ash/MSW was 3.14%, and the amount of fly ash generated was 170 712.91 tons per year.4 Given these high concentrations, it is imperative that fly ash treatment should be given the ample consideration that it requires. Table 2 illustrates the various treatment methods for fly ash that have been previously investigated. It summarizes the methods of fly ash treatment, including thermal treatment, stabilization/solidification treatment, chemical treatment, and electrodialytic treatment, to name a few. An examination of Table 2 points to thermal treatment methods as the major options to treat fly ash. Thermal treatments treat fly ash by reducing the concentration of the heavy metals. To achieve this end, it vaporizes the volatile heavy metal compounds and separates the contaminants from the fly ash matrix.3 Thermal treatments, such as sintering,13 vitrification,7 and melting,8 were adopted to remove the hazardous metals present in the MSW fly ash at a high temperature. The ash obtained after a thermal treatment process can be reused as an additive in cement or concrete manufacture.5 Various devices were applied for the thermal treatment of fly ash, such as the experimental-scale quartz reactor,15 muffle furnace,16 crucibles in an electrical furnace,24 and pilot-scale (5) Mangialardi, T.; Paolini, A. E.; Polettini, A.; Sirini, P. J. Hazard. Mater. 1999, 70, 53–70. (6) Mangialardi, T. J. Hazard. Mater. 2001, B87, 225–239. (7) Park, Y. J.; Heo, J. J. Hazard. Mater. 2002, 91, 83–93.
10.1021/ef800504q CCC: $40.75 2008 American Chemical Society Published on Web 10/02/2008
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studies found that the fluidized bed can be used as a thermal treatment unit to treat sludge,27,28 clay,16,29 and MSW.14 The composition of fly ash produced during incineration mainly depended upon the waste characteristics, the operating conditions in the combustion chamber, and the type of devices used for gaseous emissions control.30 During the thermal treatment processes of fly ash, the emission of heavy metal, such as Pb, Cr, and Cd, could be affected by the condition of thermal treatment.23,25,31,32 and specific elements, such as S and Cl, because of the formation of low-boiling-point metal chlorides and sulfates.26,32,33 Table 3 lists the partitioning of Pb in waste incineration and combustion by a fluidized bed. Most of the Pb existed in bottom ash, but Pb could be shifted to the fly ash by adding chlorine. However, limestone was used to adsorb heavy metals during incineration, especially for Pb. Chen et al.25,37 contended in their studies that limestone was an efficient adsorbent to capture Pb in fluidized bed. Therefore, CaO as limestone was applied in this study, and the efficiency of CaO was also discussed. The aim of our work was to prove whether the fluidized bed incinerator could be used to treat fly ash. Various operating parameters such as water-washing pretreatment and adsorbent (CaO) were used to investigate the emission and partitioning of Pb and organics [polycyclic aromatic hydrocarbons (PAHs)] at different temperatures in the thermal treatment processes. The relationship of emission characteristics of Pb and PAHs was also discussed. Finally, the TCLP leaching test for untreated
Figure 1. TCLP leaching test for fly ash from MSWI in Taiwan during 2004-2007: (a) Pb, (b) Cr, and (c) Cd. Table 1. Amount of Fly Ash and MSW from MSW Incineration in Taiwan during 2002-2007a year
MSW treated by incineration (ton)
amount of fly ash (ton)
ratio (%)
2002 2003 2004 2005 2006 2007 average
5297131.06 5470736.00 5611504.81 5614930.10 5683032.82 4963813.14 5440191.32
189283.36 189612.23 168284.76 158838.91 168169.89 150088.30 170712.91
3.57 3.47 3.00 2.83 2.96 3.02 3.14
a Source: collected from the Taiwan Environmental Protection Agency (TEPA).4
rotary kiln.14,23 However, a fluidized bed incinerator has the following edge over other kinds of incinerators: high heat and mass transfer, good materials mixing, and excellent combustion efficiency.25 Previous studies on a fluidized bed incinerator also illustrated that a fluidized bed is able to control the emission of gas pollutants through waste incineration.25,26 Likewise, the same
(8) Jung, C. H.; Matsuto, T.; Tanaka, N. Waste Manage. 2005, 24, 301– 310. (9) Wang, S.; Zhu, Z. H. J. Hazard. Mater. 2005, 126, 91–95. (10) Ferreira, C.; Jensen, P.; Ottosen, L.; Ribeiro, A. Eng. Geol. 2005, 77, 339–347. (11) Zhang, F. S.; Itoh, H. J. Hazard. Mater. 2006, 136, 663–670. (12) Lin, K. L.; Chang, C. T. J. Hazard. Mater. 2006, 135, 296–302. (13) Wu, H. Y.; Ting, Y. P. Enzyme Microb. Technol. 2006, 38, 839– 847. (14) Wey, M. Y.; Liu, K. Y.; Tsai, T. H.; Chou, J. T. J. Hazard. Mater. 2006, B137, 981–989. (15) Rio, S.; Verwilghen, C.; Ramaroson, J.; Nzihou, A.; Sharrock, P. J. Hazard. Mater. 2007, 148, 521–528. (16) Singh, I. B.; Chaturvedi, K.; Morchhale, R. K.; Yegneswaran, A. H. J. Hazard. Mater. 2007, 141, 215–222. (17) Kuchar, D.; Fukuta, T.; Onyango, M. S.; Matsuda, H. Chemosphere 2007, 67, 1518–1525. (18) Reinik, J.; Heinmaa, I.; Mikkola, J. P.; Kirso, U. Fuel 2007, 86, 669–676. (19) Ka´roly, Z.; Mohai, I.; To´th, M.; We´ber, F.; Sze´pvo¨lgyi, J. J. Eur. Ceram. Soc. 2007, 27, 1721–1725. (20) Navarro, R.; Guzman, J.; Saucedo, I. Waste Manage. 2007, 27, 425–438. (21) Wilewska-Bien, M.; Lundberg, M.; Steenari, B. M.; Theliander, H. Waste Manage. 2007, 27, 1213–1224. (22) Bhattacharyya, S.; Rona, J.; Donahoe, D. P. Fuel 2008, in press. (23) Chou, J. D.; Wey, M. Y.; Chang, S. H. J. Hazard. Mater. 2008, 150, 27–36. (24) Peng, F.; Liang, K.; Hu, A.; Shao, H. Fuel 2004, 83, 1973–1977. (25) Chen, J. C.; Wey, M. Y.; Lin, Y. C. Chemosphere 1998, 37, 2617– 2625. (26) Chen, J. C.; Wey, M. Y.; Ou, W. Y. Sci. Total EnViron. 1999, 228, 67–77. (27) Toledo, J. M.; Corella, J.; Corella, L. M. J. Hazard. Mater. 2005, 126, 158–168. (28) Corella, J.; Toledo, J. M. J. Hazard. Mater. 2000, 80, 81–105. (29) Selc¸uk, N.; Gogebakan, Y.; Gogebakan, Z. J. Hazard. Mater. 2006, 137, 1698–1703. (30) Goodarzi, F. Fuel 2006, 85, 1418–1427. (31) Chen, J. C.; Wey, M. Y.; Liu, Z. S. J. EnViron. Eng. 2001, 127, 63–69. (32) Wey, M. Y.; Yu, L. J.; Jou, S. I. J. Hazard. Mater. 1998, 60, 259– 270. (33) Wang, K. S.; Chiang, K. Y.; Lin, S. M.; Tsai, C. C.; Sun, C. J. Chemosphere 1999, 38, 1833–1849.
Fluidized Bed Thermal Treatment of MSWI Fly Ash
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Table 2. Comparison of Previous Papers on Fly Ash Treatment authors
country
material
treatment type
fly ash (MSW) fly ash (MSW) fly ash (MSW) fly ash, bottom ash, landfill residue fly ash (power plant) fly ash (MSW) fly ash (MSW) fly ash, bottom ash (MSW) fly ash (MSW)
stabilization/solidification treatment thermal treatment sintering thermal treatment vitrification thermal treatment melting, gasification melting sonochemical treatment electrodialytic treatment hydrothermal treatment melt treatment metal extraction: (bioleaching, chemical leaching) thermal treatment thermal treatment
rotary kiln quartz tube
14 15
thermal treatment
muffle furnace
16
DC arc plasma device
17 18 19
Mangialardi et al., 1999 Mangialardi, 2001 Park and Heo, 2001 Jung et al., 2005
Italy Italy Korea Japan
Wang and Zhu, 2005 Ferreira et al., 2005 Zhang and Itoh, 2006 Lin and Chang, 2006 Wu and Ting, 2006
Australia Denmark China Taiwan Singapore
Wey et al., 2006 Rio et al., 2007
Taiwan France
Singh et al., 2007
India
Kuchar et al., 2007 Reinik et al., 2007 Ka´roly et al., 2007
Japan Estonia Hungary
fly ash (MSW) fly ash (industrial waste), modified fly ash fly ash (industrial hazardous waste), clay molten fly ash (MSW) fly ash (power plant boiler) fly ash (MSW)
Navarro et al., 2007
Mexico
oil fly ash (power plant)
Wilewska-Bien et al., 2007
Sweden
fly ash (MSW)
Bhattacharyya et al., 2008
U.S.A.
fly ash (electro power plant)
Chou, et al., 2008
Taiwan
fly ash, bottom ash (MSW)
sulfidation treatment hydrothermal alkaline treatment DC arc plasma reactor, subsequent heat treatment alkaline or acidic extraction, precipitation, solvent extraction water leaching, filtration washing, displacement washing chemical treatment: (ferrous sulfate treatment, ferrous sulfate + CaCO3 treatment) thermal treatment
instrument type
alumina crucible
electrodialytic cell
reference 5 6 7 8 9 10 11 12 13
20 21 22 rotary kiln
23
Table 3. Partitioning of Pb in Waste Incineration and Combustion by a Fluidized Bed from the Literature authors
incinerator type
Pb partitioning
reference
Wei et al., 1998
modified MSW
waste type
BFB
34
Chen et al., 1999
modified MSW
BFB
(1) without additive: sand bed, 83.12%; fly ash, 9.21% (2) with organic chlorine: sand bed, 70.21%; fly ash, 11.35% (3) with inorganic chlorine: sand bed, 80.21%; fly ash, 7.62% (1) without additive: sand bed, 81.11%; cyclone1, 6.85%; cyclone2, 2.06%; scrubber/liquid, 0.55%; scrubber/solid, 1.58%; flue gas, 2.78% (2) with limestone additive: sand bed, 82.8%; cyclone1, 3.67%; cyclone2, 0.85%; scrubber /liquid, 0.24%; scrubber/solid, 0.40%; flue gas, 0.12% fly ash, 26%; bottom ash, 70-82%; gas phase, 7-15% (1) with less Cl-: bottom ash, 22.2%; coarse fly ash, 43.8%; thimble filter, 0.2%; cake filter, 33.7%; flue gas, 0.2% (2) with excess Cl-: bottom ash, 19.6%; coarse fly ash, 55.2%; thimble filter, 1.8%; cake filter, 23.0%; flue gas, 0.4% (1) without limestone: cyclone ash, 36%; filter ash, 46%; bottom ash, 18 (2) with limestone: cyclone ash, 42%; filter ash, 4%; bottom ash, 54% bottom ash and scrubber cake, 20 kg/year; fly ash, 4026 kg/year; stack gas, 2.0 kg/year; effluent water, 0.65 kg/year
Lind et al., 1999 biomass Corella and Toledo, 2000 doped sludge
CFB BFB
Selc¸uk et al., 2006
FB
high ash content lignite
van de Velden et al., 2008 sludge
FB
fly ash, treated fly ash, and bed material was considered to evaluate the efficiency for fly ash thermal treatment. Experimental Section Experimental Apparatus. The fluidized bed incineration system in the experiments is shown in Figure 2. The reactor is a bubbling fluidized bed incinerator composed of a preheated chamber (50 cm long) and a main chamber (105 cm high) with an inner diameter of 10 cm. The reactor is made of stainless steel (3 mm thickness, AISI 310). The electrically resistant materials are packed with ceramic fibers to thermally insulate the system. The stainless-steel porous plate is a gas distributor with a 15% open area. Three thermocouples were used to measure the temperatures of the preheated chamber, the sand bed, and the freeboard chamber. Sample Preparation. The inlet waste was fly ash in polyethylene (PE) bags. Fly ash samples were obtained from an MSW incinerator in Taiwan. MSWI fly ash contained SiO2 (7.62%), Al2O3 (3.38%), CaO (35.81%), Cl- (17.17%), and heavy metal Pb (304.33 mg/ kg). Compositions of water-washing fly ash included SiO2 (15.15%), Al2O3 (7.29%), CaO (41.54%), Cl- (9.37%), and heavy metal Pb (1034.7 mg/kg). The compositions of MSWI fly ash were already
26
35 25
29 36
presented in previous works.14,38 Parts of the fly ash samples were washed with distilled water for 3 h, and the solid/liquid ratio was 1:10. After a water-washing process, the solid and liquid were separated, and the washed fly ash was dried for 48 h. The weight of the fly ash was 0.6 g. The total amount of inlet fly ash was 76.8 g for each run. Before the experiment, fly ash was stored and dried for 40 °C to ensure its stability. Experimental Procedure. Approximately 40% excess air (relative to the theoretical air) was used under different operating temperatures. A total of 70 L/min input air at room temperature was determined. When the sand bed temperature was in a steady state, then fly ash was fed into the chamber at the rate of 1 bag/ 20 s. The operation conditions of the experiment are presented in Table 4. The variable parameters were water-washing pretreatment and applied CaO (particle size of 0.59-0.7 mm) mixed with silica (34) Wei, M. C.; Wey, M. Y.; Hwang, J. H.; Chen, J. C. J. Hazard. Mater. 1998, 57, 145–154. (35) Lind, T.; Valmari, T.; Kauppinen, E. I.; Sfiris, G.; Nilsson, K.; Maenhaut, W. EnViron. Sci. Technol. 1999, 33, 496–502. (36) van de Velden, M.; Dewil, R.; Baeyens, J.; Josson, L.; Lanssens, P. J. Hazard. Mater. 2008, 151, 96–102. (37) Chen, J. C.; Wey, M. Y.; Yan, M. H. J. Air Waste Manage. Assoc. 1999, 49, 1116–1120. (38) Chang, F. Y.; Wey, M. Y. J. Hazard. Mater. 2006, 138, 594–603.
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Figure 2. Bubbling fluidized bed incinerator: (1) auto-feeder controller, (2) PID controller, (3) data collector, (4) autofeeder, (5) blower, (6) flowmeter, (7) thermocouple, (8) pressure detector, (9) preheater chamber, (10) sand bed, (11) electric resistance, (12) sampling place, (13) U manometer, (14) cyclone, (15) induced fan, and (16) bag house. Table 4. Operating Conditions for the Experiments fly temperature fly ash run ash (g) (°C) pretreatment 1
700
2 3 4
0.6
5 6 7 8 9
0.6a
800 900 700
0.6
800 900 700
water washing
composition of bed materials
pollutant sampling
silica sand
heavy metal (Pb), organic (PAHs)
silica sand
heavy metal (Pb), organic (PAHs)
silica sand + heavy metal (Pb), CaOb organic (PAHs)
800 900 a
Mass measured after the water-washing process. CaO/silica sand ) 1:16.
b
Mixing ratio:
sand (particle size of 0.7-0.84 mm, CaO/silica sand ) 1:16) at different operating temperatures. The total amount of bed materials was 2240 g. The sampling procedure followed the standard method of TEPA (NIEA A302.72). In the heavy metals analysis, the amount of Pb in the gas and solid phase was sampled at four operating times, namely, 10, 20, 30, and 40 min. The concentration of Pb in the gas phase was sampled by using the absorption agent [HNO3 (5%) + H2O2 (10%)], and a glass filter was used for Pb sampling in the solid phase. Sampling and Analysis. Flue gas containing organics and heavy metals was isokinetically sampled during incineration. The U.S. EPA modified method 5 was employed to sample the organics, while method 5 was used for the heavy metals. Wey et al. have described the sampling train method.39 GC/FID was able to detect PAHs with 2-6 benzene rings of organic compounds with semivolatility. When the experiment was finished and the temperature cooled down, all fluidization media (silica sand or silica sand + CaO) in the combustion chamber and fly ash were collected and analyzed to calculate the mass balance and fraction of Pb. To calculate Pb partitioning to bed materials, the Pb concentration in different particle size distribution was concerned. The bed materials were (39) Wey, M. Y.; Ou, W. Y.; Liu, Z. S.; Tseng, H. H.; Yang, W. Y.; Chiang, B. C. J. Hazard. Mater. 2001, B 82, 247–262.
separated by the standard Tyler sieve into seven parts of particle size distribution (>1.41, 1.41-1, 1-0.84, 0.84-0.7, 0.7-0.59, 0.59-0.5, and 1.41 mm) and fine (
