Fs from municipal solid waste incinerator by dual bag filter (DBF) system

Fuel 86 (2007) 813–819 www.fuelfirst.com

Removal characteristics of PCDDs/Fs from municipal solid waste incinerator by dual bag filter (DBF) system Byung-Hwan Kim a, Seungmoon Lee b, Sanjeev Maken b, Ho-Jun Song b, Jin-Won Park b,*, Byoungryul Min b b

a Institute of Daewoo E&C Co., 60 Songjuk-dong, Jangan-gu, Suwon-city, Republic of Korea Department of Chemical Engineering, Yonsei University, 134 Shinchon-dong, Seoul 120-749, Republic of Korea

Received 10 February 2006; received in revised form 5 September 2006; accepted 13 September 2006 Available online 10 October 2006

Abstract A dual bag filter (DBF) system was developed for the removal of dioxins (PCDDs/Fs) emitted from a municipal solid waste incinerator (MSWI). A 2000 N m3/h capacity DBF pilot plant was designed, manufactured, and operated with actual MSWI flue gases. The result showed that pressure drop of the filter bag is the most important variable in PCDDs/Fs removal by the DBF system. Removal efficiency of PCDDs/Fs decreased as pressure drop increased in the first bag filter of the DBF system. On the contrary, removal efficiency of PCDDs/Fs increased as the pressure drop of the second bag filter increased. Pressure drop ranges for the most effective operation in the filter bag were 150–200 mm H2O and 170–200 mm H2O for first and second bag filter, respectively. The emission of PCDDs/Fs after removal by the DBF system was below 0.05 ng-TEQ/ N m3, when pressure drop of the second bag filter was operated near 200 mm H2O. Activated carbon consumption was also less in case of DBF (40 mg/N m3) as compared to SBF which discharged about 100 mg/N m3.  2006 Elsevier Ltd. All rights reserved. Keywords: Pilot plant; Pressure drop; Removal of dioxins

1. Introduction Polychlorinated dibenzo-p-dioxins (PCDDs) and dibenzofurans (PCDFs) are among the most hazardous environmental pollutants. Their presence in vegetation, human, and marine life have been reported in literature [1,2]. Incineration of municipal solid waste (MSW) represents a major source of the environmental burden of these compounds, and attention has been focused on the best way of controlling and regulating such emissions [3,4]. Dioxin removal were achieved by various methods such as adsorption on activated carbon (AC) in carbon fibers, packed beds, moving beds or entrained flow techniques [5], adsorption on packed beds containing hydrophobic zeolites [6], sepiolite [7], and even carbon nanotubes [8], catalytic destruction/oxidation [9] coupled with deNOx in *

Corresponding author. Tel.: +82 2 364 1807; fax: +82 2 312 6401. E-mail address: [email protected] (J.-W. Park).

0016-2361/$ - see front matter  2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.fuel.2006.09.007

monolith reactors, and catalytic removal and destruction in catalytic bag filters [9–14]. Among them, activated carbon adsorption with a bag filter system is widely employed in Korea because of easy handling, effectiveness, and low capital cost [15,16]. However, the conventional activated carbon adsorption with a single bag filter system can result in significant powder AC loss because of the low utilization efficiency (less than 3%) of activated carbon (>150 mg/N m3). Also, it is hard to meet the Korean emission standard of PCDDs/Fs (0.1 ng-TEQ/N m3, 12% O2 basis) without additional facilities such as SCR. But, high capital and operating costs are always problem for SCR process. Therefore, it is possible to reduce the PCDDs/Fs emission without using the SCR when the activated carbon method is modified to be effective. For this purpose, the dual bag filter (DBF) system was investigated. In this research, 2000 N m3/h capacity pilot plant of a spray drying absorber (SDA)/DBF system was used to study the removal characteristics of PCDDs/Fs

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from flue gases of actual MSWI (200 tonnes/day). Also, the removal characteristics of PCDDs/Fs were compared with those of a conventional SDA/single bag filter (SBF) system.

Fresh AC : 100

Fresh AC : 100

Single BF

Flue Gas

Treated Gass

Contact AC : 100

AC Feeder

2. Experimental Discharged Di rg AC : 100

(AC Unit : mg/Nm3)

2.1. Dual bag filter (DBF) system Feeding AC : 840

The investigated DBF system was composed of two bag filter housings connected in series as shown in Fig. 1. The DBF system was located after the SDA which removes acid gases such as HCl and SO2 by sprayed lime slurry (Ca(OH)2). In the first bag filter of the DBF, fly ash and particle fraction of PCDDs/Fs were attached on to the surface and removed by filtration. But, gaseous fraction of PCDDs/Fs escapes the first bag filter. In the second bag filter of the system, special type of powder AC was injected in the middle of entrance duct of the second filter bag, and this adsorbed gaseous PCDDs/Fs present in the gas stream. As shown in Fig. 1, discharged AC on the bottom of the second bag filter housing was separated from the discharged fly ash on the bottom of the first bag filter housing in the system, which makes the DBF system more useful by recycling AC. Fig. 2 shows the activated carbon contacting types and material balances of a general SBF system and the investigated DBF system. Compare with a SBF system, the DBF system consumed 40% less AC with higher PCDDs/Fs removal efficiency. For the SBF system, AC feeding rate was 100 mg/N m3, and AC contacting amount with treating flue gas was 100 mg/N m3. But for the DBF system, fresh AC feeding rate was only 40 mg/N m3, and contacting amount of AC with treating flue gas was as high as 840 mg/N m3. Table 1 shows design parameters for the pilot scale of the DBF system. A pilot plant of capacity 2000 N m3/h was capable of treating actual MSWI flue gas. Gas-to-cloth ratio and filtration area of first and second bag filter housStack

ACI Incinerator 200 ton/day

EP 45,000 Nm3/ h

Fresh AC

WS

BF

Recycling AC

SDA 2,000 Nm3/h Gas Flow AC Flow Waste disposal

SCR

2nd Bag 1st Bag S1 Filter S2 Filter

ID-fan S3

Waste disposal (Ash)

Fig. 1. Flow diagram of the pilot plant of the SDA/dual bag filter (DBF) system (2000 N m3/h) in the MSW incinerator.

1st

2 BF

BF Contact AC : 840

Flue Gas

Fresh AC : 40 nd

Treated Gas AC Feeder

Discharged AC : 40

Recycled AC : 800 0

Fig. 2. Activated carbon contacting types and material balances of (a) a general single bag filter system and (b) a dual bag filter system.

Table 1 Design parameters of the DBF pilot scale test for MSWI flue gas Item

First filter

Second filter

Volume of treated gasa Gas-to-cloth ratio Area of filtration (Af) Area of unit filter (Au) Number of filter bag Linear gas velocityb Pressure drop Pulse airc Inlet particle contents Outlet dust contents

2200 N m3/h 1 m/min 63.5 m2 1.0 m2 64 e/a 0.5 m/s 200 mm H2O 55 N m3/h 5000 mg/N m3 15 mg/N m3

2200 N m3/h 1 m/min 62 m2 1.0 m2 64 e/a 0.6 m/s 200 mm H2O 55 N m3/h 5000 mg/N m3 15 mg/N m3

a b c

Safety factor: 1.1. Specific gravity of fly ash: 0.2–0.4. 2.5% of gas.

ings were 1 m/min for each bag filter, 63.5 and 62 m2, respectively. The linear gas velocity was different with each filter housing because the particle size of powder AC was larger than that of fly ash. Linear gas velocity was calculated by dividing actual flow rate of flue gas by cross section area of filter housing. Linear gas velocity of the first bag filter was 0.5 m/s in order to collect discharged fly ash in the bottom hopper. Linear velocity of the second bag filter was 0.6 m/s to increase the amount of attached AC on the filter surface after pulse discharge. 2.2. Pilot plant description The pilot plant was connected to the boiler outlet of a real stoker grate MSWI with 200 tonnes/day capacity, and average value of total exhaust gas was 45,000 N m3/h. As shown in Fig. 1, air pollution control (APC) facilities were composed of electric precipitator (EP), wet scrubber (WS), activated carbon injector (ACI), single bag filter (SBF), heat exchanger, and selective catalytic reduction (SCR). Three sampling ports S1–S3 were used to investi-

B.-H. Kim et al. / Fuel 86 (2007) 813–819 Table 2 Surface area (m2/g) and other characteristics of the powder activated carbon

Single-point BET Multi-point BET Langmuir surface area Meso pore area DR-method micro pore area Cumulative adsorption surface area Cumulative desorption surface area Total pore volume (cm3/g) ˚) Average pore radius (A Numerical weight mean (lg) Volume weightt mean (lg) Particle size range (lm)

678.8 651.5 1380.0 124.9 982.4 310.5 319.9 0.513 15.8 16.1 42.5 6–160

906.3 885.1 2031.0 225.5 1316.0 586.0 561.3 0.771 17.4 11 64.8 4–330

685.7 665.6 1221.0 80.6 999.9 426.5 413.7 0.435 13.1 12.4 52.5 5–230

3500 3000

150

3

Type Y

Gas Flow ( Nm / hr )

Type N

4000

o

Type S

4500 200

Temperature ( C )

Characteristic

815

2500 2000

100

1500 SDA Inlet SDA Outlet 1st BAG Outlet 2nd BAG Outlet Gas Vol.

50

0 0

200

400

600

800

1000 500 0 1000

Time (min)

Fig. 3. Operating conditions of temperature of the SDA/DBF pilot plant.

gate the characteristics of the operating conditions and the flowing gases through the pilot plant. As a whole, pilot plant was composed of a SDA reactor for acid gas treatment and the DBF system. Incinerated MSW had 53% moisture and its density was 0.3 tonnes/m3. It contained 46% food, 25% synthetic resin, 19.32% paper, 3.4% textile, 1.5% wood and about 5% non-combustible. The safety factor of volume of treating gas was 1.1 which means 10% was added on the treating gas for safe operation of the plant. Also another 2000 N m3/h capacity pilot plant of a SDA/ SBF system was tested at the site before the test of the SDA/DBF system. 2.3. Analysis A computerized control system was adapted for operational data. Temperature, gas flow rate, and first and second filter pressure drop were reported to the main PC through a RS232C communication cable and stored for regular intervals. K-Type thermocouple, PDTB-M500 (ULFA Technology Co. Ltd.), and TP-27 S (Green Sensor Co.) were used for temperature, pressure, and flow rate measurements, respectively. PCDDs/Fs were pretreated according to the Korean Standard Methods of Pollution Test [17], and high resolution GC/MS (JMS-700T, JEOL) were used for the analysis [18]. Three types of powder AC were used in this experiment, and the characteristics of each AC were analyzed by the Korea Testing Laboratory (KTL) and are shown in Table 2. The materials of filter bag of the DBF, provided by Clean Air Technology Co., were Teflon laminated glass fiber and polypropylene sulfide (PPS). 3. Results 3.1. Pilot plant operation Operational characteristics of 2000 N m3/h capacity pilot plant were investigated with actual exhaust gas from the MSWI. Measurements of temperature and treating

gas flow rate were carried out every 25 min for 13 h at the sampling ports (Fig. 1, S1–S3). As shown in Fig. 3, steady state was reached in about 600 min, and inlet gas temperature (SDA outlet) of the pilot plant reached to 180 C. Temperature was the most important parameter in the process management, and the optimum temperature range was critical in the removal of PCDDs/Fs. As the removal efficiency of the DBF system depends on the adsorption and filtration, the low temperature makes the adsorption of gas phase PCDDs/Fs on powder AC more effective. However, water vapor in the flue gas condensed below the acid dew point (less than 140 C), which might cause process damages by the increase of pressure drop of the filter bag. Thus, preheating was needed to avoid such condensation, and the temperature must be kept from 145 to 160 C under continuous operation. In steady state operation, the flue gas flow rate was 2200 N m3/h, and the average temperatures of the inlet and outlet of the SDA were 193 and 185 C while the first and second filter outlet of the DBF were 180 and 170 C, respectively. 3.2. The removal of PCDDs/Fs by a SBF system In a SBF system, fly ash and particulate phase PCDDs/ Fs attached on to the surface of the fly ash were removed by one bag filter simultaneously. But gaseous fraction of PCDD/F passed through the filter bag, so it was needed to adsorb the gaseous fraction of PCDDs/Fs by a special type of activated carbon in the filter bag. Characteristics of PCDDs/Fs removal by a conventional SBF system were necessary for finding the useful adsorbent and operating condition of the DBF system. Three types of powder activated carbon (geometric mean diameter is 11–16 lm) were used in this experiment (Table 2), and removal efficiency of PCDDs/Fs according to the concentration of type N was also tested. Amount of moisture and oxygen at the outlet of the SBF were 18% and 12%, respectively, and isokinetic sampling parameters were 101–107. Amount of sample varied from 1 to 2 N m3 according to the appropriate experimental purpose.

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3.2.1. Inlet concentration of PCDDs/Fs The test results of PCDDs/Fs at the inlet of the SDA/ SBF system were shown in Table 3 (Tests A1–A2). The inlet concentration of PCDDs/Fs varied from 3.88 to 5.01 ng-TEQ/N m3. According to congener distributions of inlet gas (shown in Fig. 4), PeCDFs showed the highest concentration, and the substituted concentration ratio of dioxin/furan was 0.21. This result was very similar to that reported by Kim et al. from other Korean incineration facilities [19]. 3.2.2. Effect of the types of activated carbon Effect of three types of powder AC on the removal of PCDDs/Fs in the SBF system was reviewed. Tests A3– A7, A8–A9, and A10–A11 in the Table 3 represent test results of PCDDs/Fs emissions at the out-let of the SBF system with the powder AC of types N, S, and Y, respectively. After passing through about 100 mg/N m3 of activated carbon injection, normally applied in the commercial SBF system, average PCDDs/Fs concentrations were 0.13, 0.22, and 0.06 ng-TEQ/N m3 for type N, S, and Y, respectively (mean value of Tests A4–A5, A8–A9, and A10–A11 in Table 3). Thus, type Y showed the highest PCDDs/Fs removal efficiency among three types of AC. However, type Y was sodium impregnated AC in a laboratory scale apparatus, and thus sample preparation for the pilot plant operation was limited. Next to type Y, type N showed high PCDDs/Fs removal efficiency. Therefore, type N was used throughout this experiment and also in DBF. 3.2.3. Effect of the feeding rate of activated carbon Removal efficiency of PCDDs/Fs decreased as AC content increased in case of type S and Y (Table 3). Many experimental parameters including inlet contents of dioxins and fly ash loading with AC on the filter bag influenced the test results. It required more tests to find an effect of the feeding rate of AC on the PCDDs/Fs removal efficiency using type N (Tests A3–A7 in Table 3). In the SBF system, PCDDs/Fs removal efficiency increased in the beginning as the concentration of AC increased as shown in Fig. 5. However, no considerable improvement was detected when higher than 150 mg/N m3 of AC was used. However, removal efficiency decreased when AC feeding rate was

higher than 300 mg/N m3. This is because the initial increase in injected AC in input of filter increase the pressure drops across the filter due to adsorption and hence dioxin decreases. But beyond that 300 mg/N m3, dioxin has increased dramatically. It might be possible that micro fly ash particles that hold dioxin pass though the filter. Similar trend in pressure drop with injected AC was also observed in first bag of DBF (Tests B8–B13 in Table 4). Similar results were also reported in literature [19–22]. Thus, 150–300 mg/N m3 of activated carbon feeding rate was the minimum value for the general SBF system in MSWI, which normally exhaust 3–5 ng-TEQ/N m3 of PCDDs/Fs at the inlet of the air pollution control device. However, conventional SBF system could not remove PCDDs/Fs below 0.1 ng-TEQ/N m3, permissible emission standards of Korea, without additional process such as SCR. Also, after the pulse cleaning of filter bag, injected activated carbon was mixed with dust such as fly ash and dry product of the SDA at the bottom of the bag filter housing. It interferes with discharged activated carbon recycling, and also the adsorption efficiency was very low (less than 3%). Consequently, the usage of the AC was very poor with conventional SBF system. 3.3. Characteristics of PCDDs/Fs removal by the DBF system In the dual bag filter (DBF) system, solid particles such as fly ash and particulate phase PCDDs/Fs were removed by the first bag filter, and gaseous fraction of PCDDs/Fs was removed by the second bag filter by activated carbon adsorption. Powder AC of type N was used in this DBF. Characteristics of PCDDs/Fs removal by the DBF system were tested by the pilot plant operating with actual MSWI flue gas and the measurements of dioxins were made (Table 4). 3.3.1. Effect of bag filter materials Two types of the DBF filter materials tested in this study were Teflon membrane (glass fiber/Teflon laminated) and poly propylene sulfide (PPS). Removal efficiencies of PCDDs/Fs for Teflon Membrane and PPS were 93% and 80%, respectively, when 150 mg/N m3 of Type N activated

Table 3 Test results of PCDDs/Fs with various activated carbons at the SBF pilot plant using MSWI flue gas Test no.

Sampling location

A1 A2 A3 A4 A5 A6 A7 A8 A9 A10 A11

Inlet Inlet Outlet Outlet Outlet Outlet Outlet Outlet Outlet Outlet Outlet

AC

Type Type Type Type Type Type Type Type Type

N N N N N S S Y Y

Feeding AC (mg/N m3)

O2 (%)

Isokinetic index (%)

Dioxins (ng-TEQ/N m3)

26.9 93.4 136.2 411.1 506.7 67.5 95.0 86.5 100.6

9.18 9.54 12.12 13.23 12.10 12.54 12.26 12.81 12.77 11.70 12.24

101.04 106.03 103.62 102.75 103.19 104.10 102.52 102.25 101.74 103.78 104.59

3.881 5.009 0.365 0.180 0.081 0.085 0.197 0.160 0.284 0.045 0.075

B.-H. Kim et al. / Fuel 86 (2007) 813–819

817 1.4

100

Conc . ng/ Nm3 TEQ ng/ Nm3

90

1.2

80

Conc. (ng/Nm3)

60

0.8

50 0.6

40 30

TEQ (ng/Nm3)

1.0

70

0.4

20 0.2 10

OCDD

1,2,3,7,8,9-HpCDD

1,2,3,4,6,7,8-HpCDD

1,2,3,6,7,8-HxCDD

1,2,3,4,7,8-HxCDD

1,2,3,7,8-PeCDD

OCDF

2,3,7,8-TCDD

1,2,3,4,7,8,9-HpCDF

1,2,3,4,6,7,8-HpCDF

1,2,3,7,8,9-HxCDF

2,3,4,6,7,8-HxCDF

1,2,3,6,7,8-HxCDF

1,2,3,4,7,8-HxCDF

2,3,4,7,8-PeCDF

2,3,7,8-TCDF

0.0

1,2,3,7,8-PeCDF

0

Fig. 4. The characteristics of PCDD/F congener distribution of inlet flue gas for test A1.

0.40

Dioxins (ng-TEQ/Nm 3 )

0.35 0.30 0.25 0.20 0.15 0.10 0.05 0.00 0

100

200

300

400

500

600

700

AC Injection (mg/Nm3 )

Fig. 5. Effect of the feeding rate of activated carbon on the dioxin removal in the SBF system (Type N, average inlet dioxins is 4.4 ng-TEQ/N m3).

carbon was applied. Therefore, Teflon laminated glass fiber was used for further study. Emission rates of PCDDs/Fs were closely related to the dust emissions passing through the filter materials. Thus, it was essential to control the dust concentration below 10 mg/N m3 at 200 mm H2O after the filtration. 3.3.2. Effect of pressure drop of the first bag filter Pressure drop increasing materials was quite different for the SBF and the DBF system. In the conventional SBF system, pressure increased due to the filter cake formed by the AC and solid particle. In the DBF system, powder AC was injected in front of the second bag filter while fly ash and other solid particle were collected in the first bag filter. Effects of pressure drop of the first bag filter on the PCDDs/Fs removal efficiency were measured (Tests

B4–B7 in Table 4). The concentrations of PCDDs/Fs were 0.331, 0.782, 0.814, and 1.274 ng-TEQ/N m3 at the average pressure drop of 82, 94, 185, and 225 mm H2O, respectively. Emitted PCDDs/Fs at the outlet of the first bag filter increased (Fig. 6) as the pressure drop of the first bag filter increased. It was thought that higher pressure drop makes fine dust particles passing through the first filter bag [23]. Increase of fly ash particles in the first bag filter means the increase of the PCDDs/Fs attached on the particle because of the average mean diameter of fly ash, 0.8 lm. As shown in Fig. 6, dust concentration at the outlet of first bag filter was increased very slowly up to 150 mm H2O, but it increased rapidly when higher pressure, about 200 mm H2O, was applied. Thus, operating pressure drop of the first bag filter of the DBF system must be cautiously observed, and pressure should not be over 200 mm H2O. 3.3.3. Effect of pressure drop of the second bag filter PCDDs/Fs concentrations emitted from the second bag filter were observed with pressure drop of second bag filter in the DBF system. The pressure drop of the second bag filter was increased with the increase in amount of added AC as shown in Fig. 7. Amounts of activated carbon used were 0, 0.5, 1, 20, and 40 kg in order to increase pressure drop of second bag filter from 15 to 215 mm H2O , and the concentration of PCDDs/Fs at the end of the second bag filter were 0.188, 0.061, 0.055, 0.046, and 0.039 ng-TEQ/N m3, respectively (Table 4 and Fig. 6). Thus PCDDs/Fs concentration at the outlet of second bag filter decreased as pressure drop increased. The result was opposite to what we observed in the pressure drop effects of the first bag filter. The pressure drop is caused by addition of AC. Once the required pressure drop is achieved, initially 840 mg/N m3 of AC was

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B.-H. Kim et al. / Fuel 86 (2007) 813–819

Table 4 Test results of PCDDs/Fs for the change of filter bag pressure drop at the DBF pilot plant using MSWI flue gas Test no.

Sample location

B1 B2 B3 B4 B5 B6 B7 B8 B9 B10 B11 B12 B13

S1 S1 S1 S2 S2 S2 S2 S3 S3 S3 S3 S3 S3

AC Type N (kg)

O2 (%)

0.0 0.5 1.0 3.0 20.0 40.0

8.3 8.6 11.5 9.3 9.4 9.1 14.5 12.9 12.5 12.8 13.1 12.7 13.1

Temperature (C)

Pressure drop (mm H2O)

First BF

Second BF

First BF

Second BF

153.7 146.1 155.9 135.1 139.2 136.9 155.9 152.6 130.7 137.7 148.4 142.2 124.5

140.4 144.3 122.1 150.1 149.8 148.8 122.1 141.6 140.3 143.0 144.6 138.7 127.6

100 75 231 82 94 185 225 120 135 176 119 141 124

60 60 58 33 35 33 6 15 19 20 20 99 215

1.4

60 1st BF Dioxins 2nd BF Dioxins 1st BF Dust 2nd BF Dust

1.0

50 40

0.8 0.6

30

0.4

20

Dust (mg/Nm3 )

3

Dioxins (ng-TEQ/Nm )

1.2

0.2 10 0.0 0

50

100

150

200

250

Pressure Drop (mm H2O)

Fig. 6. Dioxin and dust concentration at the outlet of bag filter with filter pressure drop.

Flue gas (N m3/h)

Dioxins (ng-TEQ/N m3)

2025 2148 2318 2057 2169 2155 2318 2153 2162 2135 2159 1994 1907

1.433 0.938 0.984 0.331 0.782 0.814 1.274 0.188 0.061 0.055 0.072 0.046 0.039

upon the particle types, which caused a differential pressure drop. Since mean diameter of the AC (10 lm) was 10 times larger than fly ash that makes fly ash and PCDDs/Fs adsorbed on the activated carbon hard to pass through the second bag filter [20,21]. The dust emission of the second bag filter was also measured with the change of pressure drop and shown in Fig. 6. The average value of dust emission was 8.4 mg/N m3 which was very low as compared to the first bag filter. Thus, pressure drop does affect the dust emission also in second bag filter but effect is insignificant (as compared to first bag filter) due to very small amount of dust emission (Fig. 5). Therefore, the removal efficiency of the second bag filter of the DBF was mainly depending on the amount of AC and pressure drop caused by the AC to a certain level. 4. Discussion

140

2

2nd BF Pressure Drop (mm H O)

160

120 100 80 60 40 20 0 0

5

10

15

20

25

30

35

40

Injected AC (kg)

Fig. 7. Pressure drop of the second bag filter with the change of injected activated carbon at the DBF pilot plant (filter area: 62 m2).

added to second bag filter. After that 40 mg of fresh AC is mixed with 800 mg of recycled AC and injected into the second bag filter (Fig. 2). The spent AC discharged was only 40 mg/N m3. Thus, more PCDDs/Fs were adsorbed on to the activated carbon and removed. It was thought that the removal efficiency of PCDDs/Fs considerably depend

The present investigation gives results of a pilot plant scale test of the removal of PCDD/F emitted from a MSWI. The removal of PCDD/F was affected by AC contents and pressure drop of bag filter at the constant temperature, filter materials, and type of AC. PCDDs/Fs were changed from gas phase to particulate phase by activated carbon adsorption, and removal efficiency is depended on the particle collection efficiency of filter bag system. The geometric mean diameter of the fly ash particle and powder AC used in this study were 0.8 lm [18] and 20 lm, respectively. Since mean diameter of the fly ash adsorbed activated carbon was larger than AC alone, the removal efficiency of the particulate phase PCDD/F adsorbed on the fly ash or activated carbon was affected by the pressure drop of the filter bag at the same conditions. In conventional single bag filter (SBF) system, PCDDs/ Fs removal efficiency increased as the feeding ratio of AC increased. However, removal efficiency decreased when more than 300 mg/N m3 of AC were used (Fig. 5). In the DBF system, dioxin removal was closely related with dust concentration at outlet (Fig. 6). Because of two kinds of particle were separated in the dual bag filter (DBF) system,

B.-H. Kim et al. / Fuel 86 (2007) 813–819

PCDDs/Fs removal pattern of two bag filters showed different results. Removal efficiency of PCDDs/Fs decreased as pressure drop increased in the first bag filter because of the increase of fine particle and fly ash load. On the other hand, removal efficiency of PCDDs/Fs increased with the increase in pressure drop of the second bag filter. Also a similar pattern of the dioxin removal existed when the SBF system was compared with the first bag filter of the DBF system. Removal efficiency of dioxins increased as pressure drop increased when the mixture of dust and the AC was present in the SBF. And the same results were obtained in the first bag filter in the DBF process. However, PCDDs/Fs removal efficiency will be decreased drastically when pressure drop of the second bag filter is more than 300 mm H2O. It was thought that high mechanical energy was forcing the activated carbon particle and adsorbed PCDDs/Fs to pass through the filter materials, and it increased thread holes of the filter bag. 5. Conclusion A 2000 N m3/h pilot scale test was conducted to develop a dual bag filter (DBF) system for the effective removal of PCDDs/Fs emitted from municipal solid waste incinerator. The following results were obtained from the comparative pilot plant test of a conventional single bag filter (SBF) system and a dual bag filter (DBF) system: • As the pressure drop increased in the SBF system, PCDDs/Fs removal efficiency increased in the beginning, but the removal efficiency decreased when AC feeding rate was more than 300 mg/N m3. • Removal efficiency of PCDDs/Fs decreased as pressure drop increased in the first bag filter of the DBF system. On the other hand, the removal efficiency of PCDDs/Fs increased with the increase in pressure drop of the second bag filter. • A similar pattern of dioxins emission with pressure drop existed between the SBF system and the first bag filter of the DBF system. In both cases, fine particle such as fly ash was susceptible to pass through the filter materials by the increased pressure drop. • Pressure drop range for the optimum operation in the first and second bag filter were 150–200 mm H2O and 170–200 mm H2O, respectively. The emission of PCDDs/Fs after removal by the DBF system was below 0.05 ng-TEQ/N m3 when pressure drop of the second bag filter was operated about 200 mm H2O. • AC consumption was also less in case of DBF (40 mg/ N m3) as compared to SBF which discharged about 100 mg/N m3. Acknowledgement Financial assistance from Technology Center for UNFC in Climate Change, Yonsei University is gratefully acknowledged.

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