Removal of Chlorinated Aromatic Organic Compounds from MWI with Catalytic Filtration

Adsorption with activated carbon and catalytic oxidation with an appropriate catalyst are two methods for the effective removal of gaseous dioxin-like congeners from flue gases. The removal efficiencies of chlorobenzene/chlorophenol (CBz/CPh) from gas streams achieved by an “activated carbon injection + bag filter” (ACI + BF) system in a municipal waste incinerator (MWI) and catalytic filter (CF) system installed in a pilot-scale module are evaluated in this study via sampling and analysis. Real flue gas obtained prior to activated carbon injection in the dust of an MWI is introduced and fed into a pilot-scale module to evaluate CBz/CPh removal efficiencies. The CBz and CPh removal efficiencies achieved with the catalytic filtration 180°C are 78.3% and 77.4%, respectively, and the efficiencies increase significantly with increasing temperature. Due to the different removal mechanisms of the ACI + BF and CF technologies, increasing removal efficiencies with more Cl-substituted CBz/CPh congeners in flue gas are found with ACI + BF technology as a result of higher boiling points of highly Cl-substituted CBz/CPh congeners. Thanks to the destruction of contaminants by a catalyst, relatively lower CBz/CPh concentrations (158 and 177 ng/g, respectively) were measured in the fly ash of the CF module compared with those in the ACI + BF system (301 and 371 ng/g of CBz and CPh, respectively). Since AC injection is not needed in the CF module, lower CBz/CPh contents and lower discharge of fly ash are observed compared with those seen with the ACI + BF technology. If the ACI + BF technology is replaced by CF technology, the discharge factors of CBz and CPh (including emission and fly ash) will be reduced significantly, from 11.9 and 15.3 mg/ton waste, respectively, to 6.13 and 6.11 mg/ton waste. Moreover, a reduced environmental impact is expected with the application of CF technology due to the lower CBz/CPh concentrations in fly ash compared with those seen with the ACI + BF technology.


INTRODUCTION
Polychlorinated dibenzo-p-dioxins, furans (PCDD/Fs) and polychlorinated biphenyls (PCBs) are commonly known as persistent organic pollutants (POPs).Seventeen 2,3,7,8substituted PCDD/Fs and twelve PCBs are considered as dioxin-like congeners due to their high toxicities, bioaccumulation and carcinogenicity for human beings and wildlife.These toxic organic pollutants are listed in Annex C of the Stockholm Convention with the goal of continuing minimization and, where feasible, ultimate elimination.
Compared with dioxin-like compounds, the toxicities of twelve chlorobenzenes (CBz) and nineteen chlorophenols (CPh) with different chlorinated levels are relatively lower.However, CBz and CPh have been proven as important precursors of PCDD/F formation in thermal processes.Previous studies indicates that significantly positive correlation is found between TEQ concentration of PCDD/Fs and mass concentrations of CPh compounds measured at flue gas of heat recovery boiler outlet, with R 2 being greater than 0.75 (Blumenstock et al., 2001).
Regarding to CBz, Lavric et al. (2005) compile the published results of literatures and also indicate that a highly positive correlation exists between PCDD/F and CBz concentrations.Moreover, R-square value (R 2 ) between PCDD/F and CBz concentrations increases with increasing chlorinated level of CBz, i.e., 0.82 for Mono-CBz vs. PCDD/Fs and 0.93 for Hexa-CBz vs. PCDD/Fs (Kaune et al., 1996;Blumenstock et al., 1999).It demonstrates that highly Cl-substituted CBz congeners may be important precursors leading to PCDD/F formation.Therefore, CBz/CPh concentrations have been regarded as an important index for formation potential of dioxin-like congeners.Furthermore, penta-and hexa-CBz are listed as POPs by Stockholm Convention (Annex A) for eventual elimination.
"Activated carbon injection follows by bag filter (ACI + BF)" technology has been generally applied in 23 largescale municipal waste incinerators (MWIs) in Taiwan for reducing the emission of dioxin-like compounds, heavy metals and particulate matter.It has been well recognized as a powerful and effective air pollutant control technology for reducing the emissions of relevant pollutants.However, gaseous pollutants are only adsorbed by activated carbon and transferred to fly ash with this technology.
Catalysts are commonly applied for reducing NO, such as selective catalytic reduction (SCR), and oxidizing organic compounds, such as catalytic combustor.The effectiveness of V 2 O 5 /TiO 2 catalyst with/without WO 3 doping in removing dioxin-like congeners from gas streams has been demonstrated (Yang et al., 2008).However, traditional SCR system is generally installed downstream of particulate matter and acid gas control devices to minimize catalyst poison, and reheating flue gas is commonly needed to provide the appropriate temperature for SCR reactions.
Catalytic filter (CF) technology has been applied in some MWIs of Japan, Europe and USA.CF technology which can simultaneously remove solid-and gas-phase PCDD/Fs from flue gases of thermal processes has been proven effective in some previous studies (Bonte et al., 2002;Šyc et al., 2006).Compared with ACI + BF system, activated carbon is not needed in CF system and hence reduces the operating cost.Furthermore, both discharge rate of BF ash and potential of PCDD/F formation are significantly reduced since activated carbon injection is not needed (Hung et al., 2011).Previous study indicates that some PAHs including naphthalene, phenanthrene, pyrene and benzo(p)pyrene can also be effectively removed via CF technology (Weber et al., 2001).However, research on CBz and CPh removal via catalytic filter technology is very limited to date.For better understanding the characteristics of CBz/CPh removal via catalytic filter technology, a pilot-scale catalytic filters module (designated as CF module) is installed in a largescale MWI located in southern Taiwan.Real flue gas after semi-dry absorber but prior to activated carbon injection is split and fed into CF module for investigating the removal and discharge characteristics of CBz and CPh congeners.

METHODS AND SAMPLE ANALYSIS
For simulating operational condition of fabric filter in real plant, a pilot-scale module with catalytic filter (designated as CF module) is constructed in this study as shown in Fig. 1(a).Part of flue gas of real MWI is introduced into CF module by ID fan.Flue gas passes through heating region, filter chamber, ID fan, and then flows back to the exhaust duct of MWI.A heating region is provided to keep a constant temperature of flue gas to evaluate the temperature effect of catalytic filter.Catalytic filter investigated in this study is composed of membrane felt layer and catalytic layer.V 2 O 5 -WO 3 /TiO 2 powder is incorporated into PTFE fibers to form the catalytic layer.In the CF module, 4 catalytic filters with the length of 80 cm and diameter of 15.5 cm are installed for removing particulate matter and organic compounds exist in flue gas.Total filtration area is 1.634 m 2 .Two sets of air pulse-jet are equipped to wash the filter cake formed on catalytic filter.Collected fly ash is stored in the dust chamber for up to 3 hours, and then discharged into the collection tank.Two temperature sensors are set at inlet and outlet of filter chamber, respectively, for making sure that operating temperature is well controlled.For evaluating removal efficiencies of pollutants, sampling points are also set at inlet and outlet of filter chamber.Pitot tube is installed at the outlet of filter chamber to monitor the flow rate and a constant air/cloth (A/C) ratio (filtration velocity) can be controlled at different operating temperatures.In this study, filtration velocity is controlled at 1 m/min for all experimental tests.
This study aims to remove CBz and CPh via catalytic filter.However, ACI + BF technology has been applied in most large-scale MWIs in Taiwan.For comparing the efficiencies achieved with catalytic filter and "ACI + BF" system, a large-scale MWI located in southern Taiwan (Fig. 1(b)) is selected because powder activated carbon (PAC) is injected between semi-dry absorber (SDA) and BF, and AC-free flue gas is available for the test and comparison.Part of the flue gas after hydrated lime injection and prior to AC injection was split and introduced into the CF module.The flue gas introduced is reheated to the designated operating temperature varying from 180°C to 210°C.Furthermore, removal efficiencies of pollutants achieved with the catalytic filter system installed and ACI + BF applied in real plant are further evaluated and discussed.
The sampling procedure of flue gas follows isokinetic sampling of USEPA Method 5. Solid-phase and gas-phase pollutants are collected with glass fiber filter and XAD-2 resin, respectively, and follows USEPA Method

Characteristics of Pollutants in Inlet Flue Gas
Concentrations of CBz and CPh in flue gas are investigated in this study.Fig. 2 shows the congener concentrations at inlet sampling point of CF module.Inlet CBz and CPh concentrations are 1,500 and 1,060 ng/Nm 3 , respectively.The concentrations of penta-and hexa-CBz are significantly higher than that of tri-and tetra-CBz.As for CPh congeners, 2,4,6-TriCPh is of the highest concentration (451 ng/Nm 3 ) while other congeners are typically lower than 200 ng/Nm 3 .The formation potential between highly and low Clsubstituted congeners is clearly different between CBz and CPh congeners.

Removal Characteristics of CBz Congeners with Catalytic Filtration and ACI + BF Technology
Three temperatures (180, 195 and 210°C) are selected for operating CF module to evaluate the temperature effect.Catalyst activity generally increases with increasing temperature.On the other hand, increasing operating temperature would increase volatility of SVOCs exist in filter cake.Hence, overall removal efficiencies may be reduced if the catalyst activity is not significantly enhanced with increasing temperature.91.6%, respectively.Therefore, catalyst activity is significantly increased for CBz destruction with increasing operating temperature, even though more CBz would vaporize due to the increase of temperature.Removal efficiencies of CBz congeners are between 71.5-85.7% at 180°C, 77.9-95.5% at 195°C and 88.0-97.6% at 210°C.
For pilot-scale module with catalytic filter, two main removal mechanisms including filtration by membrane and destruction by catalyst coexist.Commonly, removal of chemicals with low volatility achieved with filtration is relatively easier compared with that of high volatility because great part of these compounds would exist in solid phase due to the condensation on particulate matter.Boiling points of CBz congeners increase with increasing chlorination level.The boiling point of mono-CBz is 131°C and it increases to 323°C for hexachlorobenzene.Therefore, great part of highly Cl-substituted congeners would condense on particles and further removed by membrane filtration of catalytic filter.On the other hand, low Cl-substituted congeners are more easily penetrated through the filter with higher volatility.Wang and Jones (1994) indicate that half-life times of CBz congeners increase with increasing Cl-substituted level.It means chemical stability of CBz congeners may increase with increasing chlorine substitution and, therefore, destruction of low Cl-substituted CBz congeners via catalytic filter is easier than that of highly Cl-substituted compounds.It is summarized that highly and low Cl-substituted congeners can be effectively removed simultaneously via filtration and catalysis in CF technology, respectively.
Fig. 3(a) also shows the efficiencies of ACI + BF technology for CBz congeners removal in the MWI investigated.Total removal efficiency of ≥ 3 Cl-substituted CBz is 81.9%.The removal efficiencies of CBz achieved with ACI + BF system of the MWI operating at 160°C are close to that achieved with catalytic filter operating at 195°C.Regarding ACI + BF technology, removal efficiencies of CBz congeners increase generally with rising chlorine substitution.Highly Cl-substituted CBz congeners are easily adsorbed by activated carbon because they have higher boiling points.However, adsorption and filtration are two main mechanisms responsible for pollutant removal in ACI + BF technology, lower removal efficiencies are expected for low Cl-substituted congeners due to higher volatility.

Concentrations of CBz Congeners in Fly Ash Discharged from CF Module and ACI + BF System Investigated in This Study
Both CF and ACI + BF technologies are proved effective in removing CBz from gas streams.However, a significant difference of CBz contents in fly ash is found between CF module and ACI + BF system investigated in this study.Fly ash discharged from CF module and ACI + BF system are designated as CF ash and BF ash, respectively.CF ash collected is generated from CF module operating at 195°C while the operating temperature of BF in the MWI investigated is 160°C.Fig. 4(a) shows the concentrations of CBz congeners in CF and BF ashes.CBz concentrations in CF ash are generally lower than that in BF ash.Total CBz concentrations are 177 and 371 ng/g in CF and BF ashes, respectively.Interestingly, a significant difference between the concentrations of low and highly Cl-substituted congeners is found in BF ash, but not in CF ash.It is attributed to the fact that adsorption efficiency achieved with activated carbon injection is significantly enhanced with higher chlorinated level due to higher boiling points of highly Cl-substituted CBz congeners.This phenomenon is further confirmed by the fact that removal efficiencies of highly Cl-substituted CBz congeners in flue gas by ACI + BF system are higher compared with that of low Cl-substituted compounds.

Concentrations of CPh Congeners in Fly Ash Discharged from CF Module and ACI + BF System Investigated in This Study
Fig. 3(b) and Fig. 4(b) show the total removal efficiencies of ≥ 3 Cl-substituted congeners and concentration of CPh congeners in fly ash collected from both technologies discussed in this study.CPh removal efficiencies are 77.4%,85.5% and 91.5% at 180°C, 195°C and 210°C, respectively, and also increase significantly with increasing operating temperature.Removal efficiencies of CPh congeners are between 70.1-85.3% at 180°C, 79.6-91.8% at 195°C and 89.4-95.5% at 210°C.Dual mechanisms with catalytic destruction and membrane filtration can also effectively remove gas-and solid-phase CPh congeners.Furthermore, not much variations in removal efficiencies of CPh congeners are found between highly and low Cl-substituted congeners.
For ACI + BF system of the MWI investigated, total removal efficiency of ≥ 3 Cl-subtituted CPh is 77.3%.The removal efficiency of CPh achieved with ACI + BF system of the MWI operating at 160°C is close to that achieved with the CF module operating at 180°C.Fig. 3(b) shows removal efficiencies of CPh congeners increase with increasing chlorine substitution.Highly Cl-substituted CPh congeners are easily adsorbed by activated carbon because the boiling points of CPh congeners increase from 175°C of 2-chlorophenol to 309°C of pentachlorophenol.Highly Cl-substituted CPh congeners have higher potential to condense on BF ash due to adsorption and filtration of ACI + BF system.As shown in Fig. 4(b), the concentrations of CPh congeners are found to increase significantly with increasing chlorinated level in BF ash of ACI + BF system in MWI investigated.By the way, lower CPh concentration in CF ash is also found compared with that in BF ash due to different removal mechanisms between catalytic filter and ACI + BF technologies.Total CPh concentrations in CF and BF ashes are 158 and 301 ng/g, respectively.

Evaluation of Discharge Factor Compared with Catalytic Filter and ACI + BF Technologies
Adsorption is the main mechanism responsible for gaseous organic pollutant removal with ACI + BF technology.Gaseous pollutants can be effectively adsorbed by activated carbon due to extremely high specific surface area, followed by the collection of activated carbon by bag filter.However, all pollutants including solid-and gas-phase pollutants are eventually concentrated into BF ash.In contrast, gaseous CBz and CPh can be effectively destroyed by catalytic filter.Therefore, part of gaseous organic compounds was adsorbed by fly ash while passing through the filter cake, and the other was destroyed by catalysis in CF module.This study has proved that removal efficiencies of CBz and CPh from gas streams are close between catalytic filter and ACI + BF technologies, however, significantly higher CBz and CPh concentrations are measured in BF ash compared with that in CF ash.For better understanding the environmental impact caused by both technologies, discharge factors of CBz and CPh by applying CF and ACI + BF technologies are defined and calculated.
Discharge factor is the total discharge of pollutant including stack emission and fly ash and is defined as below: Discharge factor (mg/ton waste) = ((Emission rate of pollutants from stack + Discharge rate of pollutants from APCDs' residual)/Incineration rate of waste) ((mg/hr)/(ton waste/hr)) ( The discharge factors calculated in this study are based on the actual operating conditions of the MWI investigated in this study.For discharge factor via CF technology, catalytic filters replaced to original bag filters of MWI investigated are assumed and emission concentrations obtained with 195°C operating temperature of CF module are adopted for evaluating discharge factor via CF technology.The incineration rate of municipal solid waste is 12.13 ton/hr.The gas flow rate is 86,520 Nm 3 /hr and the discharge rate of fly ash generated from bag filter of the MWI is 523 kg/hr.Inlet and outlet CBz and CPh concentrations of CF module and ACI + BF system have been presented earlier, but only ≥ 3 Cl-substituted CBz and CPh compounds are measured for flue gas samples.For the same basis, we only consider the CBz and CPh congeners of ≥ 3 Cl-substituted in fly ash for the calculation of discharge factor.Total concentrations of ≥ 3 Cl-substituted CBz and CPh congeners in CF ash are 103 and 117 ng/g, respectively, while that in BF ash are 230 and 316 ng/g, respectively. Fig. 5 shows mass flow rates of CBz and CPh including system inlet and two discharging pathways including stack emission and fly ash discharge with ACI + BF and CF systems, respectively.Input rates of CBz and CPh are 130 and 91.7 mg/hr, respectively.Emission rates of CBz and CPh achieved with ACI + BF technology are 23.5 and 20.8 mg/hr, respectively, which are 18% of input CBz rate and 23% of input CPh rate, respectively.With CF technology, CBz and CPh emission rates are 20.6 and 13.0 mg/hr, respectively.The results indicate that CBz and CPh emission rates via stack achieved with both technologies are fairly close.However, CBz flow rate via fly ash discharge is 120 mg/hr achieved with ACI + BF technology and is significantly    (Chi et al., 2008).Therefore, high surface area of activated carbon has the advantage of high adsorption efficiency and disadvantage of high formation potential of organic contaminates.In contrast, CF technology can effectively destroy gaseous organic pollutants, resulting in much lower CBz and CPh contents in fly ash.Discharge factor (mg/ton waste) can be calculated by dividing the pollutant flow rates (mg/hr) by the treatment capacity of waste (ton/hr).For ACI + BF technology, total discharge factors of CBz and CPh are 11.9 and 15.3 mg/ton waste, respectively.For CF technology, total discharge factors of CBz and CPh are 6.13 and 6.11 mg/ton waste, respectively.CBz/CPh discharge via fly ash discharge is the major cause for great variation in discharge factor because emission flow rates of CBz/CPh achieved with ACI + BF and CF technologies are fairly close.CBz/CPh discharge factors achieved with CF technology are significantly lower than that achieved with ACI + BF technology due to significant formation of CBz/CPh in fly ash of ACI + BF system.The objective of CF technology developed is to replace ACI + BF technology.Therefore, activated carbon injection is not needed if CF technology is applied.AC injection rate in ACI + BF system of MWI investigated is 5.06 kg/hr.Based on the operating conditions of this study, if CF system is adopted in this MWI, discharge of fly ash is slightly decreased from 523 kg/hr to 518 kg/hr, and the discharge rates and factors of CBz/CPh from fly ash are significantly reduced.

CONCLUSIONS
This study evaluates and compares the removing capabilities of CBz/CPh via ACI + BF and CF technologies, respectively.Pilot-scale module with catalytic filters is installed in an MWI in southern Taiwan.AC-free real flue gas is introduced and fed into the pilot-scale module with catalytic filter for evaluating CBz/CPh removal efficiencies.CBz/CPh removal efficiencies achieved with pilot-scale module at 180°C and A/C ratio of 1 m/min are close to that achieved with ACI + BF system adopted in the real plant operating at 160°C.As operating temperature of the CF module is increased from 180°C to 210°C, removal efficiency of CBz is enhanced from 78.3% to 91.6% and that of CPh is also increased from 77.4% to 91.5%.Due to different removal mechanisms between ACI + BF and CF technologies, increasing removal efficiencies with higher Cl-substituted CBz/CPh congeners in flue gas are found with ACI + BF technology.CBz/CPh concentrations in fly ash are 301 and 371 ng/g with ACI + BF system, respectively, and 158 and 177 ng/g with CF module.Therefore, CBz/CPh contents in BF ash are significantly higher than that in CF ash, and this result also leads to higher discharge factors of CBz and CPh which are 11.9 and 15.3 mg/ton waste, respectively, in ACI + BF system compared with the discharge factors of CBz and CPh via CF system which are 6.13 and 6.11 mg/ton waste, respectively.Lower environmental impact is expected if CF technology is applied due to significantly lower CBz/CPh concentrations in fly ash compared with ACI + BF technology.Therefore, catalytic filter is an environment friendly air pollution control technology from the perspective of total emission control.
3542.Samples are extracted by Soxhlet extraction with dichloromethane.Isotope spiked solution of CBz and CPh (Wellington Laboratories Inc.) are added before sample extraction for quantifying the concentration of each CBz and CPh congener.The samples after extraction are analyzed by HRGC/LRMS (USEPA Method 8270).For samples of flue gas, only ≥ 3 Cl-substituted CBz and CPh congeners are analyzed due to the low adsorption efficiencies of monoand di-chlorination of CBz/CPh congeners by XAD-2 resin; for fly ashes discharged from CF module and ACI + BF system, all CBz and CPh congeners are quantified.The removal efficiency of pollutant in flue gas is calculated with the following formula.CBz/CPh removal efficiency via CF module (%) = (Concentrations at inlet of CF module -Concentrations at outlet of CF module)/Concentrations at inlet of CF module × 100% (1) CBz/CPh removal efficiency via ACI + BF system (%) = (Concentrations at inlet of CF module -Concentrations at stack of MWI)/Concentrations at inlet of CF module × 100% (2) (a) Pilot-scale module with catalytic filters.(b) Procedure of MWI and installation of CF module.

Fig. 1 .
Fig. 1.Schemes of the pilot-scale module and MWI procedure.

Fig. 3 .
Fig. 3. Removal efficiencies of CBz/CPh congeners at different operating temperatures of the catalytic filter and ACI + BF technology adopted in the MWI.

Fig. 5 .
Fig. 5. Mass flow rates of CBz and CPh with (a) the ACI+BF system and (b) the CF system, respectively.

Chlorophenol CF ACI+BF higher
than that achieved with CF technology (53.8 mg/hr).On the other hand, CPh flow rate via fly ash discharge of ACI + BF system is 165 mg/hr which is much higher than the input CPh rate (91.7 mg/hr).In contrast, CPh flow rate via fly ash discharge of CF technology is 61.2 mg/hr and is significantly less than that of ACI + BF technology.Based on the input flow rates of CBz and CPh, total output flow rates of CBz and CPh including emission of flue gas and discharge of fly ash are 111% and 203% achieved with ACI + BF technology, respectively.These results indicate that CBz and CPh may be significantly formed in fly ash, especially CPh.As for CF technology, total output flow rates of CBz and CPh are 57% and 81% of input flow rates, respectively.Interestingly, previous study indicates that high potential of PCDD/F formation is found in fly ash of ACI + BF technology due to high reaction area and additional carbon source provided by the activated carbon injected