J Mater Cycles Waste Manag (2013) 15:202–209 DOI 10.1007/s10163-013-0115-z
ORIGINAL ARTICLE
Fate and behavior of selected heavy metals with mercury mass distribution in a fluidized bed sewage sludge incinerator Deepak Pudasainee • Yong-Chil Seo Jeong-Hun Kim • Ha-Na Jang
•
Received: 22 August 2011 / Accepted: 10 December 2012 / Published online: 22 January 2013 Ó Springer Japan 2013
Abstract In this paper, emission and distribution behavior of six heavy metals (As, Cd, Cr, Ni, Pb, and Hg), particulate matter and mass distribution of mercury within the different streams of a fluidized bed sewage sludge incinerator are presented. At the inlet of air pollution control devices (APCDs); Cd, Cr, Ni and Pb were mainly enriched in coarse particles; comparatively As content was higher in fine particles (\PM2.5). The concentration of heavy metals in total particulate matter and PM2.5, at the inlet of APCDs, were in the order of Cr [ Ni [ Pb [ As [ Cd. Mercury was almost always distributed in flue gas. Metals, other than mercury, were efficiently removed in APCDs and their concentrations in bottom ash, with fly ash being higher, whereas for that in wastewater, then waste sand was lesser. Overall mercury removal efficiency of APCDs was 98.6 %. More than 83.3 % of mercury was speciated into oxidized form at the inlet of APCDs, attributed by higher chlorine content in sludge. Mercury was mainly distributed in wastewater (78.4 %), wastewater from a spray dry reactor (16.8 %), fly ash in a hopper (3.4 %) and flue gas (1.4 %). This result is one of the first for data to be obtained; more experiments are required to control emission from such sources. D. Pudasainee Y.-C. Seo (&) J.-H. Kim Department of Environmental Engineering, YIEST, Yonsei University, Wonju, South Korea e-mail:
[email protected] Present Address: D. Pudasainee Karlsruhe Institute of Technology, Institute of Technical Chemistry, 76344 Eggenstein-Leopoldshafen, Germany H.-N. Jang Hansol EME, Environmental R&D Center, Seoul, South Korea
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Keywords FBC Heavy metals Mercury Mass distribution Sludge incineration
Introduction Sewage sludge is a residue left after wastewater treatment. Sewage sludge treatment has been one of the major environmental problems to manage and regulate considering its high moisture, toxic metals and organic micro-pollutants content. Sewage sludge generation rate has been increased influenced by the stringent effluent discharge limit, technological advancement and environmental consciousness among the authorities and citizens. Conventionally, sewage sludge after dewatering was mainly used in agriculture, disposed in landfills or use as a conventional fuel substitute for energy production. In the recent past, sewage sludge has also been treated in cement kilns as a substitute fuel [1]. In most developed countries, conventional methods of sludge treatment such as disposing of sludge in agricultural fields and landfill have been restricted considering its adverse effect in the environment. Likewise, the dumping of sewage sludge in the ocean has been prohibited by the London Protocol of 1996. Influenced by these trends various feasible technologies to treat sludge in an environmental friendly manner have been investigated. Currently, several sewage sludge combustion and co-combustion technologies with high thermal destruction efficiencies are in use [2, 3]. Converting sewage sludge into useful energy such as utilizing pyrolysis and gasification, combustion and co-combustion process are getting more interest [4–6]. Worldwide, various types of incineration technologies have been used for sewage sludge treatment. One of the widely used technologies is fluidization bed combustion
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(FBC), which has several advantages [7]. This includes the complete combustion at relatively lower temperature, and a longer residence time of sludge in the hot bed. Besides these advantages, sludge incineration releases numerous air pollutants: total particulate matter (TPM), NOx, N2O, SOx, CO2, heavy metals, mercury, dioxins and furans, into the atmosphere [2, 8–10]. Formation and emission of fine inhalable particles enriched with toxic metals is a major concern of sludge incineration. During incineration, heavy metals contained in the sludge undergo transformation and removal within the system (boiler, cooler, flue gas cleaning system) and the remainder is finally emitted into the atmosphere possessing several environmental problems. Literature data for heavy metals distribution in sludge incineration process is limited. In this paper, emission characteristics of TPM, PM2.5, heavy metals, including mercury from a commercial FBC sewage sludge incinerator, were investigated. Further, mass distribution of mercury in the incineration process and its fate in the environment are discussed.
Materials and methods Process configuration of the tested facility and input materials The typical commercial sewage sludge incineration facility in Korea was selected for TPM, heavy metals emission characteristics and mercury mass distribution study. The capacity of the tested facility was 160 ton/day (wet basis) which treated mainly the municipal sewage sludge generated from urban wastewater treatment mixed with a less portion of industrial sewage sludge. Ash generation in average was about 6093 kg/day. In a bubbling fluidized bed boiler, silica sand was used for sludge incineration. Sludge after the dewatering and drying process was injected in the furnace, the average temperature was about 900 °C. Dry air was injected into the incinerator. The
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generated steam was recycled for the sewage sludge drying process. The APCDs in the facility were a bag filter (BF) for particulate removal, a spray-dry reactor (SDR) and a wet scrubber (WS) for mainly acid gas removal. In the SDR, slacked lime slurry is injected to remove SO2 and HCl, with more than 70 % of reduction efficiency and additional removal of acidic gases to be achieved in the wet scrubber by NaOH solution injection. Figure 1 shows the schematic diagram of a FBC sewage sludge incineration process. The moisture content in sludge before the dewatering and drying process was about 83 % and became 72 % after drying using a disk type dryer. The elemental compositions (average value) of sewage sludge were as C (36.7 %), H (5.6 %), O (18.7 %), N (5.1 %), S (2.8 %), Cl (2304.2 ppm). Table 1 shows the average concentration of heavy metals in input materials. Heavy metals enter into wastewater from industrial and domestic activities which also retain sewage sludge. Depending upon the nature of the domestic use, industrial activities, living standard and so on, heavy metals in sewage sludge differs. The sources of heavy metals in wastewater and ultimately the sludge may vary, as for instances, Sorme and Lagerkvist [11] classified sources into eight groups as: households, drainage water, business (car wash, dentist), pipe sediments (metals deposited in pipe), chemicals (contain traces of some heavy metals) added in the treatment process to improve the treatment process, atmospheric deposition, traffic (brake linings, tires, asphalt, gasoline), and building materials. The Cu is associated with leaching of plumbing materials, Zn with body care items, galvanized material and car washing [11, 12]. Pb, Cr, Cd, were mainly contributed from car washing; Cd, Cu and Zn were from detergents and washing powders [13]. Gaseous pollutants, particulate matter, heavy metals and mercury measurements Gaseous pollutants (NOx, SOx, CO2, CO), O2 in flue gas were measured by gas analyzer (MRU Air fair, 95/3 CD).
Fig. 1 Schematic diagram of the process configuration of a FBC sludge incinerator (FBC fluidized bed combustor, B/F bag filter, SDR spray dry reactor)
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Table 1 Heavy metals in input materials Input materials
Table 2 Gaseous composition in flue gas at the inlet and outlet of APCDs
Analyte
O2 (%)
CO2 (%)
CO (ppm)
NOx (ppm)
SOx (ppm)
Avg. Max.
4.8 5.4
14.8 15.1
14.7 16.0
73.3 78.0
2102.0 2249.0
Min
4.4
14.4
13.0
68.0
1928.0
Avg.
8.2
11.8
31.1
39.5
20.0
Max.
9.0
12.5
46.0
47.0
16.0
Min.
7.3
11.0
24.0
34.0
28.4
As
Cd
Cr
Ni
Pb
Hg
Sampling point
Sewage sludge
2.4
0.8
1413.5
145.8
33.9
12.45
APCDs
Fluidized sand
ND
0.04
1.9
1.02
0.9
ND
Caustic soda
0.37
0.002
0.026
0.002
1.23
1.7
Water
0.024
ND
ND
0.004
0.005
0.02
Unit: solid: mg/kg; liquid: mg/L ND Not detected
The TPM was sampled by the Korean standard method for air pollution, which is similar to the US EPA method 5 [14]. The Korean method uses the thimble-type filter, whereas the US EPA Method 5 uses the circle-type filter. The PM2.5 was sampled using a PM2-K Cyclone kit (Apex instruments). Selected heavy metals sampling and analysis were carried out at each ingoing stream (sludge, fluidized sand, caustic soda, water) and outgoing stream (fly ash, waste sand, SDR wastewater, WS wastewater). Sampling time at each sampling point was more than an hour, and sampling volume was more than 1.5 Sm3. The onsite measurements were carried out for a week. Heavy metals in TPM were analyzed by the inductively coupled plasma/ mass spectroscopy (ICP/MS) method (Varian Co. Ltd., Ultra mass 700), after pre-treatment of samples according to EPA method 3050B [15]. Mercury concentrations in all the ingoing and outgoing streams plus mercury speciation and concentration in flue gas at inlet APCDs and the stack emission were determined. Mercury sampling in flue gas and analysis was conducted according to the Ontario Hydro method (ASTM D 6784). Liquid and solid samples were collected and were analyzed by US EPA method 7470A and US EPA 7471. A Cold Vapor Atomic Absorption Spectroscopy (CVAAS) mercury analyzer was used to analyze mercury speciation in flue gas and total mercury concentration in liquid and solid samples. The feed rate of sludge, caustic soda, process water and production rate of fly ash, wastewater in SDR and WS were obtained from the facility. Mercury mass at each ingoing and outgoing stream was determined and the mass distributions of mercury in different streams were estimated.
Results and discussion Gaseous pollutants Table 2 presents gaseous pollutants concentration in flue gas at the inlet and outlet of APCDs of the tested facility; minimum, maximum and the average values are presented.
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Inlet
Stack
The O2 concentration at the stack emission ranged 7.3–9.0 %, and the CO concentration ranged 24.0–46.0 ppm. Average CO2 concentration at the inlet APCDs and the stack were 14.8 and 11.8 %, respectively. Average NOx concentrations at the inlet and outlet were 73.3 ppm and 39.5 ppm, respectively. The SOx concentration at the inlet and stack were 2102 and 20 ppm, respectively. The CO2, NOx and SOx are emitted due to the oxidation of carbon, nitrogen and sulfur present in the sludge. Given that their content varies with sludge types, CO2, NOx and SOx emission concentration can vary significantly. Particulate matter and heavy metals emission, other than mercury Sludge incineration emits TPM into the environment. Table 3 presents the concentrations of TPM and heavy metals in flue gas emission at the inlet of APCDs and the stack. Heavy metals concentrations in TPM and PM2.5 and the removal efficiency in APCDs are presented. The concentration of heavy metals in TPM was nearly twice than in PM2.5 (Table 3). This indicates that PM was formed mechanically by the generation of unburned products, such as non-volatile organics. The emission concentration of TPM, As, Cd, Cr, Pb was corrected for 12 % oxygen concentration, as required by the Korean regulation. The TPM and PM2.5 concentrations at the inlet APCDs were 12371.5 and 2564.6 mg/Sm3, respectively. Particulates mainly originated from sludge and to a lesser extent from fragmented bed materials and fuel. The TPM and PM2.5 concentrations in the flue gas emission were 1.3 and 1.0 mg/Sm3, respectively. Figure 2 shows the distribution of TPM and heavy metals at the inlet of APCDs. More than 82 % of TPM was composed of particles larger than 2.5 lm. This might be due to the generation of particles by the reaction of lime (injected at the downstream of the
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Table 3 PM and selected heavy metals concentration at inlet APCDs and stack emission, with removal efficiency in APCDs TPM (mg/Sm3) TPM
As (lg/Sm3) PM2.5
Cd (lg/Sm3)
TPM
PM2.5
TPM
PM2.5
Inlet APCDs
12371.5
2564.6
110.0
72.6
81.3
38.4
Stack
1.3
1.0
0.9
ND
ND
ND
Efficiency (%)
99.9
99.9
99.2
100
100
100
Cr (lg/Sm3)
Inlet APCDs
Ni (lg/Sm3)
Pb (lg/Sm3)
TPM
PM2.5
TPM
PM2.5
TPM
PM2.5
134054.8
50710.5
19872.9
8495.3
4702.0
1903.9
Stack
5.6
1.8
2.3
ND
7.6
ND
Efficiency (%)
99.9
99.9
99.9
100
99.8
100
TPM total particulate matter, PM2.5 particulate matter less than 2.5 l
Table 4 Heavy metals concentration in by-products By-products
Analyte As
Cd
Cr
Ni
Pb
Hg
Bottom ash
24.48
12.47
23193.62
1826.72
806.14
1.63
Fly ash
12.22
8.014
12370.38
1899.30
442.27
0.047
Waste sand
12.52
13.55
8981.82
1431.78
183.75
0.03
SDR wastewater
0.24
ND
0.09
0.019
0.088
1.534
WS wastewater
0.019
ND
ND
0.007
0.003
0.099
Unit: solid: mg/kg; liquid: mg/L SDR spray dry reactor, WS wet scrubber
Fig. 2 Distribution of PM and heavy metals in different sized particles at the inlet of APCDs
boiler for SO2 and HCl removal from flue gas) with acid gases. The share of PM2.5 in TPM in stack emission was 77 %. This is comparable data (i.e., 10.0–61.9 %) that we measured in coal-fired power plants with APCDs (selective catalytic reactor ? cold side electrostatic precipitator ? wet flue gas desulphurization) [16]. More than 99.9 % of the TPM was removed in APCDs. Most of the coarse particles were removed in APCDs and the share of PM2.5 increased from 20.7 % (before APCDs) to 77 % in stack emission. The TPM was mainly removed in BF. Generally, BF has a higher particulate removal efficiency. In addition, some portion of particulates remaining in flue gas after BF was also removed in SDR and WS. The emission concentrations of heavy metals in flue gas were related to their concentration in sludge. Among the tested metals, Cr concentration in sludge incinerated was the highest (1413.5 mg/kg) and Cd was the least (0.82 mg/ kg) (Table 4). Similarly, Cr concentration in flue gas was the highest and Cd was not detected in the stack emission.
The Cr was enriched in TPM and PM2.5 with concentration at 134.0 and 50.7 mg/Sm3, respectively. The Ni, As and Pb were not detected in PM2.5 particles in the stack emission. At the inlet of APCDs, concentration of heavy metals in TPM and PM2.5 were in the order of Cr [ Ni [ Pb [ As [ Cd. Metals were dominantly distributed in particles larger than PM2.5 and less enriched in fine particles. At the inlet of APCDs; Cd, Cr, Ni and Pb were mainly enriched in coarse particles whereas, As content was higher in fine particles (\PM2.5). Depending on volatilization, trace elements in coal ashes are classified into three classes [17, 18]. The Hg lies in Class III, which easily passes through APCDs and is distributed in gaseous phase. The Ni, Pb, Cd, and Cr fall in class II, which are mostly removed in the APCDs. Most of the non-volatile elements are collected with fly ash or bottom ash whereas volatile elements, such as Hg, are likely to leave a boiler with flue gases [19]. Intermediate volatile metals such as As and Se are emitted to the atmosphere to a larger extent [20]. Metals distribution and speciation during combustion are affected by several potential factors such as reactors
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configuration, temperature, chlorine, sulphur, water content, feeding conditions, oxygen availability, and element constituents (Al, Ca, Fe, K, Mg, Na, P and Si) [21]. Metals distribution during incineration is a complex process; some species like Ni and Cr interact with other ash constituent, which affect metals enrichment behavior [22]. More experiments and study on detail mechanisms are necessary to further explain metals enrichment behavior in byproducts. Trace heavy metals behave differently in different combustion environment, thus, regardless of the general discussion presented above, the behavior of these species during sludge incineration is incompletely understood and additional investigations are required.
already been removed in SDR and their concentration decreased in WS wastewater. Almost all Cd were efficiently removed in BF and therefore were not detected in wastewater. Since oxidized mercury (Hg2?) is soluble in water, it was removed in SDR solution; consequently, in the output stream mercury was mainly distributed in SDR wastewater (134.3 mg/L). As most of the mercury had been removed in the SDR, only a less amount was left downstream and mercury concentration in WS wastewater was lesser (4.54 mg/L). Metals removal in wastewater might be related to scrubber solution properties, such as the presence of sulphide in the scrubber solution, etc. Mercury emission and speciation
Heavy metals in input materials and by-products Table 4 shows the average concentration of heavy metals at outgoing streams in the tested facility. Heavy metals except mercury in flue gas can be attached in fly ash and removed particulate removal devices such as BF. The increase in fly ash carbon content increases the sorption capacity. The concentrations of Cr and Ni in sludge and, similarly, their concentration in ash and the emission concentration in flue gas were dominant. The Cd and As concentration in sludge, ash and emission into the atmosphere were the least concentrated. Fluidized sand used to heat the fluidized bed; caustic soda used to remove acid gases and process water used in the scrubber contained lesser metals concentration. In the outlet stream, the metals concentration in ash was the highest, followed by waste sand and wastewater. The concentrations of heavy metals in ash, waste sand, and wastewater were related to their concentration in sewage sludge. Metals were efficiently removed ([99.2 %) in BF, so their concentration decreased in wastewater. Metals had
Table 5 Mercury concentration and speciation at each sampling point
Sample
Hg speciation
Before APCDs lg/Sm
1
2
Average
3
Drying air
%
lg/Sm
3
Stack %
lg/Sm3
%
Hgp
1.05
0.39
0.21
40.54
0.59
13.13
Hg0
33.22
12.43
0.23
42.97
3.10
68.32
Hg2?
233.04
87.18
0.09
16.49
0.84
Total Hg
267.31
100
0.53
100
4.53
18.55 100
Hgp
0.36
0.09
0.23
30.47
0.10
2.53
Hg0
41.83
10.84
0.41
53.50
3.22
79.62
Hg2?
343.96
89.16
0.12
16.02
1.03
Total Hg
385.79
100
0.76
100
4.35
25.44 100
Hgp
0.70
0.22
0.22
34.62
0.35
8.09
Hg0
37.53
11.49
0.32
49.17
3.16
73.28
288.50 326.73
88.30 100
0.10 0.64
16.21 100
0.94 4.44
21.69 100
Hg2? Total Hg
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The major input of mercury in the sludge incinerator was from sludge (35.15 mg/kg). Sewage sludge contains a variable quantity of mercury; mercury contained in sludge is vaporized during incineration and is speciated into Hg0 in a hot zone at the boiler outlet. As flue gas cools passing through the heat exchanger and cleaning system, mercury is oxidized forming several species such as HgO, HgCl2, Hg2Cl2 and HgSO4 [23]. This further depends on the availability of Hg, S, Cl, and flue gas temperature, etc. Table 5 presents mercury emission concentration and the speciation at the inlet and outlet of APCDs. Mercury contained in sludge is released into flue gas during its incineration. Mercury emission concentration, on average was 326.7 and 4.4 lg/Sm3 at inlet APCDs and the stack emission, respectively. The mercury emission concentration is higher than the emission from coal-fired power plants at inlet APCDs (23.4 lg/Sm3) and the stack emission (2.7 lg/Sm3); municipal waste incinerators at inlet APCDs (69.7 lg/Sm3) and the stack emission (5.7 lg/Sm3) in Korea [10, 24]. However, this value is lower than the
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emission from industrial waste incinerators at inlet APCDs (428.2 lg/Sm3) and the stack emission (214.4 lg/Sm3) and hospital medical and infectious waste incinerators at inlet APCDs (475.9 lg/Sm3) and the stack emission (51.5 lg/ Sm3) [10]. Mercury concentration was less in air used to dry sludge, where mercury was mainly speciated into Hg0. At the outlet of APCDs, particulate mercury (Hgp) ranged from 2.5 to 13.1 %. It is considered that Hg2?reacts with fly ash and has a tendency to attach in fine particulates. The Hg2? is soluble in water, most of the mercury in flue gas was removed in wet APCDs, leaving behind Hg0 as dominant (72.3 %) in stack emission (Fig. 3). Overall mercury removal efficiency of APCDs was 98.6 %. This removal efficiency value is relatively higher than those obtained in coal-fired power plants and hazardous waste incinerators [25–27]. The tested facility was characterized by (i) higher mercury concentration at the inlet APCDs (ii) higher removal of mercury in APCDs (iii) higher portion of mercury speciated into Hg2? at the inlet APCDs. The Hg2? in an average amount was 88.3 % (at inlet APCDs) and 21.7 % (at stack emission). In the combustion zone, Hg compounds readily vaporize and exist in gaseous form after the boiler. Due to the instability of mercury compounds in the gaseous form at higher temperature, more often at above 700 °C, the compounds decompose to form Hg0 [28, 29]. At downstream of the boiler, the speciation of mercury changes: mercury is oxidized by interaction with other species. However, at downstream as the flue gas cools down, Hg0 react with other components such as HCl, SO2, H2O and fly ash forming an Hg2? species such as HgCl2, HgO [30, 31]. The higher portion of Hg2? in the boiler outlet is related to the higher content of Cl in sludge (2304.2 ppm) [32]. The Cl content in sludge was several times higher than that in coal (7.0–628 ppm) used in the
Fig. 3 Mercury speciation at each sampling point. The particulate bound mercury at the inlet of APCDs is 0.22 %, due to the scale of figure it is not seen in the figure
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Korean coal-fired power plants [33], which might have increased the portion of Hg2? in flue gas. Mass distribution of mercury Mercury in the tested incineration system entered mainly from sludge feeding (99.87 %) and to a lesser extent from process water (0.12 %) and caustic soda injection (0.01 %). Mercury entering into the system is removed through: fly ash in the hopper, SDR wastewater, WS wastewater and flue gas emission from the stack. The sludge, process water, and caustic soda injection rate were taken from the facility. Also the generation of fly ash, wastewater and effluent during the emission test were considered for the mass balance study. With the measured mercury concentration and flow rates taken from the facility operation data, at each ingoing and outgoing streams, mercury mass distributions were estimated. With several tests at identical conditions, average mercury input and output rates and mass distribution were estimated. Overall mercury recovery was achieved with in/out mass balance of 72.4–106 %. The in/out mass differences in the system were normalized as presented in Fig. 4. Mercury was mainly distributed in wastewater 78.4 %, SDR wastewater 16.8 %, fly ash in the hopper 3.4 % and flue gas 1.4 %. Unlike coal-fired power plants, Cl concentration in the sludge treated was very high and ultimately resulting in more portions of Hg2? in flue gas and higher removal efficiency in SDR and WS. It is interesting that mercury was less distributed in fly ash in the present study. This is because of the higher flue gas temperature at the inlet BF
Fig. 4 Normalized mass distribution of mercury in different streams of a sewage sludge incinerator
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(average: 190 °C) and less unburned carbon in ash. The tested facility was having more efficient APCDs: BF, SDR, WS, ultimately a lower fraction of mercury was distributed in the stack emission. This calls attention to mercury management in wastewater and effluents to be handled with due care. Mass distribution of mercury in facilities differs with APCDs configuration, process design, flue gas chemistry, operating temperature, input materials composition, etc. Considering the trace concentration and volatile nature of mercury in combustion facilities, more data evaluating the effect of such factors in mercury mass distribution are needed before generalization of the results.
Conclusion Emission of primary air pollutants (TPM, CO, NOx, SOx, CO2), heavy metals including mercury emission characteristics from a commercial FBC incinerator were investigated with the following conclusions. 1.
2.
3.
4.
At the inlet of APCDs, coarse PM was dominant; more than 80 % of TPM was composed of particles larger than 2.5 lm. Most of the coarse particles were removed in APCDs and the share of PM2.5 increased in stack emission compared to the inlet of APCDs. Metals were efficiently removed ([99.2 %) in APCDs. At the inlet of APCDs, concentration of heavy metals in TPM and PM2.5 were in the order Cr [ Ni [ Pb [ As [ Cd. The Cr was enriched in TPM and PM2.5 with concentrations 134.0 and 50.7 mg/Sm3, respectively. Metals were dominantly distributed in particles larger than PM2.5 and less enriched in fine particles. The emission concentration of heavy metals in flue gas was related to its concentration in sludge. The concentration of tested heavy metals in bottom ash and fly ash were higher than in the sludge. Mercury was almost always distributed in flue gas with the least in bottom ash and fly ash. The Pb was enriched in bottom ash (RE [ 0.9). Metals, other than mercury, were efficiently removed in BF and their concentrations in other byproducts (bottom ash, fly ash) were higher, whereas that in wastewater and waste sand were less. On the other hand, mercury was mainly distributed in WS wastewater and less in SDR wastewater downstream. In flue gas, more than 83.3 % of mercury was speciated into the oxidized form at the inlet of APCDs, related to higher Cl content in sludge. The Hg0 was highest at the stack emission; Hgp was less. Mercury emission concentrations, in average, were 326.7 and 4.4 lg/Sm3 at inlet APCDs and the stack emission,
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5.
respectively. Overall mercury removal efficiency of APCDs was 98.6 %. Mass distribution of mercury in the sewage sludge incineration process was estimated. Mercury was mainly distributed in wastewater (78.4 %), SDR waste water (16.8 %), fly ash in the hopper (3.4 %), flue gas (1.4 %) as reported firstly in the process. Due to the higher portion of Hg2? available in flue gas, the removal efficiency was relatively higher compared to other combustion facilities such as coal-fired power plants.
In view of the fact that, heavy metals behave differently even in the similar combustion environment depending on fuel constituents, flue gas components and so on, additional studies incorporating their association with other parameters are required. More long term studies are needed before generalization of the result and findings about its wider implication. Acknowledgments This study was supported by the Korea Ministry of Environment under the human resource development project for energy from waste and recycling. This research has also been financially supported by the R&D project of New & Renewable Energy, ‘‘Fluidization Combustion Technology of Sludge and RDF using Mixture of Recirculating Flue-gas and Oxygen,’’ funded by the Korea Institute of Energy Technology Evaluation and Planning (KETEP).
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