Chemosphere 102 (2014) 31–36
Contents lists available at ScienceDirect
Chemosphere journal homepage: www.elsevier.com/locate/chemosphere
Treating PCDD/Fs by combined catalysis and activated carbon adsorption Sha-sha Ji ⇑, Yong Ren, Alfons Buekens, Tong Chen, Sheng-yong Lu, Ke-fa Cen, Xiao-Dong Li ⇑ State Key Laboratory of Clean Energy Utilization, Institute for Thermal Power Engineering, Zhejiang University, Hangzhou 310027, China
h i g h l i g h t s High-level concentrations of PCDD/Fs are hard to destroy catalytically. A mixture of catalyst and activated carbon can enhance the destruction and removal efficiency. Such a mixture performs well, even at low temperature (180, 160 °C). The AC characteristics may influence upon the adsorption and degradation ability of this mixture.
a r t i c l e
i n f o
Article history: Received 24 May 2013 Received in revised form 13 October 2013 Accepted 1 December 2013 Available online 26 December 2013 Keywords: Catalyst Activated carbon Adsorption Destruction PCDD/Fs
a b s t r a c t V2O5–WO3/TiO2 catalysts are used to destroy dioxins present in the gas phase, yet both their removal efficiency (RE) and destruction efficiency (DE) decrease with rising initial concentration (IC). Therefore, activated carbons (AC-1: based on lignite; AC-2: based on coconut shell) were mixed with the catalyst to tackle these high IC gases. A gas phase dioxin-generating system was used to supply three different stable IC-values. When the highest IC is used (20.5 ng I-TEQ N m3) without AC, at 200 °C, the RE and DE-value of PCDD/Fs reaches only 76% and 64%, respectively. At the same conditions, using a mix of catalyst and AC-2, these RE and DE-values rise to 90.1% and 82.0%, respectively. The mix catalyst/AC also shows better performance at low temperature (160 and 180 °C). The AC characteristics influence upon the adsorption and degradation abilities of the mixtures. Ó 2013 Elsevier Ltd. All rights reserved.
1. Introduction Polychlorinated dibenzo-p-dioxins (PCDDs) and dibenzofurans (PCDFs), in brief dioxins (PCDD/Fs), are formed during incineration. Governments have to be concerned about atmospheric pollution with dioxins because of the Stockholm Convention (2001). This convention was designed to reduce or eliminate emissions of persistent organic pollutants (POPs), and protects human health and the environment. Several techniques, such as adsorption on activated carbon (AC) and catalytic destruction, are used to limit PCDD/Fs emissions from waste incineration. Activated carbon is a porous material with various functional groups and a large BET surface; it performs particularly well on volatile organic compounds (VOCs), polycyclic aromatic hydrocarbons (PAHs) and PCDD/Fs (Karademir et al., 2004; Kawashima et al., 2011; Tseng et al., 2011). Therefore, activated carbon injection (ACI), followed by baghouse filtration, is a key technology to remove PCDD/Fs. Tseng et al. (2011) indicated that during 2002–2006, ACI helped to meet the toxicity equivalent emission standards (0.1 ng TEQ N m3) in Taiwan. Several factors may affect ⇑ Corresponding authors. Tel./fax: +86 571 87951294 (X.-D. Li). E-mail addresses: [email protected] (S.-s. Ji), [email protected] (X.-D. Li). 0045-6535/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.chemosphere.2013.12.008
adsorption, such as the characteristics of AC, the nature and concentration of adsorbates, and the operating conditions, including temperature and humidity (Chuang et al., 2003). Nevertheless, AC only makes dioxin transfer from vapor phase to solid phase, and used AC may be considered as hazardous waste that requires further treatment. Catalytic dioxins destruction catalyst is a promising technique. Vanadium–tungsten catalysts on titanium oxide carriers are used in selective catalytic reduction (SCR) of NOx, yet destroy organic pollutants as well (Krishnamoorthy et al., 1998; Albonetti et al., 2007; Debecker et al., 2010). Catalytic oxidation effectively reduces PCDD/Fs in flue gas to values below 0.1 ng TEQ N m3, and adsorbs and oxidizes these to harmless small molecules, such as CO2, H2O and HCl (Weber et al., 1999; Debecker et al., 2007; Wielgosin´ski, 2010). Adding tungsten or molybdenum oxides enhances the activity and makes it easier to destroy dioxins by oxidation (Wang et al., 2009). Flue gas from 19 commercial municipal solid waste incinerators (MSWI) in China showed concentrations of PCDD/Fs of 0.04– 2.46 ng TEQ N m3 with an arithmetic average of 0.42 ng TEQ N m3 (Ni et al., 2009). Those from 14 domestic-made hospital waste incinerators (HWIs) in China attained even 0.08–31.60 ng I-TEQ N m3 with average value of 4.22 ng TEQ N m3 (Gao et al.,
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S.-s. Ji et al. / Chemosphere 102 (2014) 31–36
2009). The range of dioxins in flue gas exhibits a large variation. It is questionable whether a V2O5-based catalyst can completely destroy such high concentrations in flue gas: some research indicated that the decomposition ability of catalyst is limited by the activity of the sites at the catalyst surface, or else by the conversion rate of V4+OX to higher oxides; maybe both may limit the destruction of high concentration dioxins (Bertinchamps et al., 2005). In order to evaluate the catalytic destruction of dioxins at different initial concentrations (IC), a stable dioxins-generating system is used to supply gas-phase PCDD/Fs continuously. The influence of IC on PCDD/Fs degradation is discussed, and the main aim of this study is to keep the destruction efficiency of PCDD/Fs at a high level, even if IC exceeds 10 ng I-TEQ N m3. Catalytic conversion and AC are coupled in this experiment for enhancing the oxidation activity and adsorption abilities of the catalyst/AC mix.
Off-gas was collected by XAD-2 polymeric resin and a toluene absorber bottle. After each run the reactor was thoroughly cleaned (flushing the reactor three times with toluene and blowing dry by blower); then fresh reactant and tailored filter were loaded to avoid cross contamination. PCDD/Fs contained in the reactant, the tailored filter and cleaning fluid of reactor walls (Soxhlet Extracted by toluene for 24 h) all were analyzed separately as PCDD/Fsadsorb in this paper. Every experimental condition was repeated two times, then the two collected samples were mixed together and purified; the resulting value is divided by two, taking into account that two samples were added and averaged. In this work, only 2,3,7,8-PCDD/Fs congeners are targeted. Removal efficiency and destruction efficiency is referred to as ‘‘RE’’ and ‘‘DE’’ respectively and calculated as follows:
2. Materials and methods
DEð%Þ ¼ ðPCDD=Fsinlet PCDD=Fsoff-gas PCDD=Fsadsorb Þ
REð%Þ ¼ ðPCDD=Fsinlet PCDD=Fsoff-gas Þ PCDD=Fsinlet
PCDD=Fsinlet
2.1. Characteristics of V2O5–WO3/TiO2 and AC A commercial V2O5–WO3/TiO2 catalyst (CAS-KR, China) is used. X-ray multi-crystal diffraction (XRD, PANalytical-X’Pert PRO, Holland) shows an anatase structure for the TiO2 phase. X-ray Energy Dispersive Spectroscopy (EDS, EDAX-GENESIS4000, USA) shows V, W and Ti contents of about 0.82, 6.72 and 47.12 wt.%, respectively. Other important parameters are a BET surface area of 93 m2 g1, a micropore surface area of 3.7 m2 g1 and an average pore size of 95 nm (N2-Physisorption, Quanta Chrome, USA). Two commercial ACs are also used, one based on lignite (AC-1), and the other one on coconut shell (AC-2); their specific parameters are shown in Table 1. Both catalyst and AC were sifted to 100 mesh, prior to use.
ð1Þ
ð2Þ
2.3. Analysis Dioxins were pretreated according to the US EPA method 1613 (US EPA, 1994). The recovery rate of each internal standard was established at between 72% and 113%, which conforms to the recovery standard of 40–130%. Analyses were performed by HRGC/HRMS combined with a 6890 Series gas chromatograph (Agilent, USA) and coupled to a JMS-800D mass spectrometer (JEOL, Japan). A DB-5 ms capillary column (60 m 0.25 mm inside diameter, 0.25 lm film thickness) was used to separate the congeners of PCDD/Fs. The temperature program and mass spectrometer were operated as described by Chen et al. (2008).
2.2. Experimental system 3. Results and discussion A stable dioxin generator continuously supplied dioxins in the gas-phase. The system consists of an injector/nebulizer of dioxins solution, a preheater with temperature-controller, and gas flowmeters. The solution is prepared by extracting fly ash, and then purifying and concentrating the extract. The dioxins concentration in the stock solution is adjusted by adding solvent (n-nonane) to the mother liquors. Carrier gas (N2:O2 = 9:1) is controlled at 1 L min1 by a gas mass flowmeter. Preheating is used to eliminate the solvent effect as much as possible and to volatilize dioxins more uniformly. The experiment was conducted using a vertical quartz tube (height 150 mm, inner diameter 25 mm) with heating system and heat preservation jacket, as shown in Fig. 1. The temperature controller maintained the reaction temperature at 160, 180 and 200 °C (actual temperatures of 153, 174 and 196 °C). A tailored filter cartridge placed in tube contains 1.0 g reactant (catalyst only/ AC only/the mixture of catalyst and AC) and 4.0 g SiO2 (sifted to 50 mesh). The corresponding Gas Hourly Space Velocity (GHSV) is 8000 h1. The sampling time during each run was one hour.
Table 1 Properties of AC. Properties
AC-1
AC-2
Ash (%) Moisture (%) Chloride (%) BET surface area (m2 g1) Pore volume (cm3 g1) Micropore, pore diameter 99%, and the DE-value is >95% for the lowest IC-value, of 2.05 ng I-TEQ N m3. When the IC rises to 4.57 ng I-TEQ N m3, RE and DE-values are still 95% and 85%, yet they decrease to only 76% PCDD/Fs removed and 64% PCDD/Fs destroyed for the highest IC-value, of 20.5 ng I-TEQ N m3. Chang et al. (2009) studied the catalytic decomposition of PCDD/Fs with different types of catalyst for different levels of inlet PCDD/F concentrations (1.08–3.04 ng I-TEQ N m3), and found that the PCDD/F destruction efficiencies did not change much at the same temperature for two reasons: firstly change used a higher temperature (250–320 °C), and secondly change used as highest IC-value
S.-s. Ji et al. / Chemosphere 102 (2014) 31–36
33
Fig. 1. Diagram of the experimental set-up.
Table 2 Initial concentrations and congener distribution of PCDD/Fs fed into the reactor (pg I-TEQ N m3). Toxic congeners
C1 (Average)
C2 (Average)
C3 (Average)
2378-TCDD 12378-PeCDD 123478-HxCDD 123678-HxCDD 123789-HxCDD 1234678-HpCDD OCDD PCDDs 2378-TCDF 12378-PeCDF 23478-PeCDF 123478-HxCDF 123678-HxCDF 123789-HxCDF 234678-HxCDF 1234678-HpCDF 1234789-HpCDF OCDF PCDFs Total I-TEQ
21.6 70.8 19.1 34.2 47.9 33.4 8.0 235.0 146.9 35.6 952.7 172.1 133.5 66.8 210.8 73.1 16.6 9.9 1818.3 2.1 103
44.5 205.0 52.7 111.8 69.1 102.0 24.4 609.5 269.9 88.9 1495.7 490.1 426.6 233.5 637.5 234.7 47.7 31.7 3959.3 4.6 103
261.1 1254.1 294.7 510.1 413.9 502.8 82.5 3319.2 184.1 354.9 6358.7 2233.8 2201.7 942.5 3401.1 1143.0 207.4 109.2 17136.6 20.5 103
only about one-seventh of that in this paper. Lower RE-values showed that the adsorption ability of a catalyst is also limited and higher concentration may cause part of PCDD/Fs not to be adsorbed by the catalyst. The difference between DE-values and RE-values becomes larger with rising IC, rising from 5% to 20%: part of the PCDD/Fs are adsorbed by the catalyst and temperature and oxygen supply limit the conversion between V4+OX and V5+OX (Xu et al., 2012). The RE-values for PCDDs are 0.6%, 0.2% and 6.5% higher than those for PCDFs, and the DE-values of PCDDs are 1.7%, 0.8% and 3.4% higher than those for PCDFs, when the IC changes from a low to a medium or a high value. Actually, the absolute value of the PCDF concentrations is much higher than that for PCDDs. More PCDFs need to be treated, but the catalyst capacity does not change. Therefore, the differences of RE and DE-values between PCDDs and PCDFs increase sharply when IC is at its highest level.
Fig. 2. Effect of initial concentration on the RE and DE-values for the 17 toxic PCDD and PCDF congeners (T = 200 °C, C1 = 2.05 ng I-TEQ N m3, C2 = 4.57 ng I-TEQ N m3, C3 = 20.5 ng I-TEQ N m3).
Fig. 2 shows the RE and DE-values of 17 toxic congeners at different IC-values. When IC is only 2.05 ng I-TEQ N m3, the RE-value of 6, 7 and 8 chlorinated congeners (high-chlorinated) is almost similar to that for 4 and 5 chlorinated congeners (low-chlorinated), however, the DE-values of high-chlorinated is 1.9% lower than
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those for low-chlorinated congeners. When the IC increases to 20.5 ng I-TEQ Nm3, the RE-value of high-chlorinated is 4.3% lower than for low-chlorinated, and the DE-values of high-chlorinated is 5.5% lower than those for low-chlorinated. The IC also influences upon the distribution of congeners and it is easier to adsorb high-chlorinated congeners than low. Higher chlorinated congeners have lower volatility and thus result in longer residence times on the catalyst surface (Weber et al., 1999). Chang et al. (2008) also indicated that higher chlorinated PCDD/Fs congeners are more easily adhered to the particles. 3.3. Adsorption of PCDD/Fs by AC The adsorption abilities of AC-1 and AC-2 are evaluated, for the highest IC-value, of 20.5 ng I-TEQ Nm3. Earlier experiments suggested a decrease in adsorption at higher temperature (Lu et al., 2003). However, some researchers reported a good performance of V2O5–WO3/TiO2 catalysts for treating PCDD/Fs within a range of 200–350 °C (Boos et al., 1992; Frings and Marl, 1994), and the higher the temperature, the stronger the degradation effect obtained. In order to avoid the temperature range for fast formation, experiments are conducted at 160, 180, 200 °C (Tuppurainen et al., 1998). The increasing vapor pressure makes more PCDD/Fs to be released to the vapor phase (Rordorf, 1989), so that with temperature rising from 160 °C to 200 °C the RE-value of AC-1 decreases from 88.6% to 86.1%, and the RE-value of AC-2 also decreases from 95.6% to 92.7%. Early research showed that not only the BET surface area, but also the distribution of micropores and mesopores may influence adsorption on AC, and optimal adsorption of PCDD/Fs requires an AC pore diameter of 2.3–4.1 nm (Xie, 2003). Due to the larger BET surface area and the richer micropore/mesopore structure, AC-2 has stronger adsorption abilities than AC-1. The RE-values increase with rising chlorination level for the 17 toxic PCDD/Fs congeners of AC-1; the RE-values for PCDD/Fs congeners of AC-2, however, are almost the same. It illustrates that the adsorption of PCDD/Fs congeners is related to the structure of AC and vapor pressure of PCDD/Fs congeners. If AC has a reasonably larger surface area, and a relatively larger pore network for the transport of molecules to the interior, the difference of congeners’ vapor pressure can be ignored at certain temperatures. RE-values for PCDDs of AC-1 are higher than PCDFs due to under the same chlorination level, the vapor pressures of PCDD congeners are lower than PCDF, and this result is similar to Hung et al. (2011). 3.4. Catalyst & AC degradation PCDD/Fs High-level IC reduces both the RE and DE values when using catalyst only. In order to enhance the adsorption ability at low temperature, AC is mixed with catalyst. Mechanical mixing of catalyst and AC (catalyst: AC = 1:4) is used to destroy PCDD/Fs better (Using M-1 and M-2 to represent catalyst & AC-1 and catalyst & AC-2 respectively), and the IC is also 20.5 ng I-TEQ Nm3. Without AC, only 78.6% PCDD/Fs is removed at 200 °C; in the presence of AC, the RE-values of M-1 and M-2 increase to 90.1% and 91.9%, and the RE-values of the mixture at 160 °C are still higher than catalystonly at 200 °C. Thus, adding AC to the catalyst can obviously improve the RE-values, even at high-levels of PCDD/Fs and at low temperature. The strong adsorption ability of AC prolongs the contact time between catalyst and PCDD/Fs. Fig. 3 shows that the RE-values of M-1 are always lower than those of M-2, and that the RE-values of the mix rise little at rising temperature. This result shows that (a) the larger surface and the micropores/mesopore structure of AC-2 makes that M-2 has better adsorption ability than M-1; (b) catalytic oxidation of PCDD/Fs also influences upon the RE-values of mixture. Catalysts also have pore structure, but
Fig. 3. RE and DE-values of (1) AC and (2) a mixture of catalyst and AC at different temperatures.
are lower in specific surface than AC, so that the RE-values of AC are always higher than for the mixture. The RE-value of AC-1 is lower than M-1 at 200 °C; the weaker adsorption of AC-1 and the stronger catalytic oxidation can induce this. Fig. 4 shows the RE-values for the 2,3,7,8-PCDD/Fs congeners at different conditions. The RE-values for PCDDs of the M-1 and M-2 are higher than PCDFs, influenced by catalyst and AC. The RE-values for low-chlorinated congeners of M-2 are much higher than M-1, but the RE-values for high-chlorinated congeners of M-1 and M-2 are almost the same. For M-1, the RE-values of high-chlorinated congeners are always higher than for low-chlorinated congeners, and the difference is more obvious at lower temperature; for M-2, the difference of RE-values between high and lowchlorinated congeners is not obviously (
Contents lists available at ScienceDirect
Chemosphere journal homepage: www.elsevier.com/locate/chemosphere
Treating PCDD/Fs by combined catalysis and activated carbon adsorption Sha-sha Ji ⇑, Yong Ren, Alfons Buekens, Tong Chen, Sheng-yong Lu, Ke-fa Cen, Xiao-Dong Li ⇑ State Key Laboratory of Clean Energy Utilization, Institute for Thermal Power Engineering, Zhejiang University, Hangzhou 310027, China
h i g h l i g h t s High-level concentrations of PCDD/Fs are hard to destroy catalytically. A mixture of catalyst and activated carbon can enhance the destruction and removal efficiency. Such a mixture performs well, even at low temperature (180, 160 °C). The AC characteristics may influence upon the adsorption and degradation ability of this mixture.
a r t i c l e
i n f o
Article history: Received 24 May 2013 Received in revised form 13 October 2013 Accepted 1 December 2013 Available online 26 December 2013 Keywords: Catalyst Activated carbon Adsorption Destruction PCDD/Fs
a b s t r a c t V2O5–WO3/TiO2 catalysts are used to destroy dioxins present in the gas phase, yet both their removal efficiency (RE) and destruction efficiency (DE) decrease with rising initial concentration (IC). Therefore, activated carbons (AC-1: based on lignite; AC-2: based on coconut shell) were mixed with the catalyst to tackle these high IC gases. A gas phase dioxin-generating system was used to supply three different stable IC-values. When the highest IC is used (20.5 ng I-TEQ N m3) without AC, at 200 °C, the RE and DE-value of PCDD/Fs reaches only 76% and 64%, respectively. At the same conditions, using a mix of catalyst and AC-2, these RE and DE-values rise to 90.1% and 82.0%, respectively. The mix catalyst/AC also shows better performance at low temperature (160 and 180 °C). The AC characteristics influence upon the adsorption and degradation abilities of the mixtures. Ó 2013 Elsevier Ltd. All rights reserved.
1. Introduction Polychlorinated dibenzo-p-dioxins (PCDDs) and dibenzofurans (PCDFs), in brief dioxins (PCDD/Fs), are formed during incineration. Governments have to be concerned about atmospheric pollution with dioxins because of the Stockholm Convention (2001). This convention was designed to reduce or eliminate emissions of persistent organic pollutants (POPs), and protects human health and the environment. Several techniques, such as adsorption on activated carbon (AC) and catalytic destruction, are used to limit PCDD/Fs emissions from waste incineration. Activated carbon is a porous material with various functional groups and a large BET surface; it performs particularly well on volatile organic compounds (VOCs), polycyclic aromatic hydrocarbons (PAHs) and PCDD/Fs (Karademir et al., 2004; Kawashima et al., 2011; Tseng et al., 2011). Therefore, activated carbon injection (ACI), followed by baghouse filtration, is a key technology to remove PCDD/Fs. Tseng et al. (2011) indicated that during 2002–2006, ACI helped to meet the toxicity equivalent emission standards (0.1 ng TEQ N m3) in Taiwan. Several factors may affect ⇑ Corresponding authors. Tel./fax: +86 571 87951294 (X.-D. Li). E-mail addresses: [email protected] (S.-s. Ji), [email protected] (X.-D. Li). 0045-6535/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.chemosphere.2013.12.008
adsorption, such as the characteristics of AC, the nature and concentration of adsorbates, and the operating conditions, including temperature and humidity (Chuang et al., 2003). Nevertheless, AC only makes dioxin transfer from vapor phase to solid phase, and used AC may be considered as hazardous waste that requires further treatment. Catalytic dioxins destruction catalyst is a promising technique. Vanadium–tungsten catalysts on titanium oxide carriers are used in selective catalytic reduction (SCR) of NOx, yet destroy organic pollutants as well (Krishnamoorthy et al., 1998; Albonetti et al., 2007; Debecker et al., 2010). Catalytic oxidation effectively reduces PCDD/Fs in flue gas to values below 0.1 ng TEQ N m3, and adsorbs and oxidizes these to harmless small molecules, such as CO2, H2O and HCl (Weber et al., 1999; Debecker et al., 2007; Wielgosin´ski, 2010). Adding tungsten or molybdenum oxides enhances the activity and makes it easier to destroy dioxins by oxidation (Wang et al., 2009). Flue gas from 19 commercial municipal solid waste incinerators (MSWI) in China showed concentrations of PCDD/Fs of 0.04– 2.46 ng TEQ N m3 with an arithmetic average of 0.42 ng TEQ N m3 (Ni et al., 2009). Those from 14 domestic-made hospital waste incinerators (HWIs) in China attained even 0.08–31.60 ng I-TEQ N m3 with average value of 4.22 ng TEQ N m3 (Gao et al.,
32
S.-s. Ji et al. / Chemosphere 102 (2014) 31–36
2009). The range of dioxins in flue gas exhibits a large variation. It is questionable whether a V2O5-based catalyst can completely destroy such high concentrations in flue gas: some research indicated that the decomposition ability of catalyst is limited by the activity of the sites at the catalyst surface, or else by the conversion rate of V4+OX to higher oxides; maybe both may limit the destruction of high concentration dioxins (Bertinchamps et al., 2005). In order to evaluate the catalytic destruction of dioxins at different initial concentrations (IC), a stable dioxins-generating system is used to supply gas-phase PCDD/Fs continuously. The influence of IC on PCDD/Fs degradation is discussed, and the main aim of this study is to keep the destruction efficiency of PCDD/Fs at a high level, even if IC exceeds 10 ng I-TEQ N m3. Catalytic conversion and AC are coupled in this experiment for enhancing the oxidation activity and adsorption abilities of the catalyst/AC mix.
Off-gas was collected by XAD-2 polymeric resin and a toluene absorber bottle. After each run the reactor was thoroughly cleaned (flushing the reactor three times with toluene and blowing dry by blower); then fresh reactant and tailored filter were loaded to avoid cross contamination. PCDD/Fs contained in the reactant, the tailored filter and cleaning fluid of reactor walls (Soxhlet Extracted by toluene for 24 h) all were analyzed separately as PCDD/Fsadsorb in this paper. Every experimental condition was repeated two times, then the two collected samples were mixed together and purified; the resulting value is divided by two, taking into account that two samples were added and averaged. In this work, only 2,3,7,8-PCDD/Fs congeners are targeted. Removal efficiency and destruction efficiency is referred to as ‘‘RE’’ and ‘‘DE’’ respectively and calculated as follows:
2. Materials and methods
DEð%Þ ¼ ðPCDD=Fsinlet PCDD=Fsoff-gas PCDD=Fsadsorb Þ
REð%Þ ¼ ðPCDD=Fsinlet PCDD=Fsoff-gas Þ PCDD=Fsinlet
PCDD=Fsinlet
2.1. Characteristics of V2O5–WO3/TiO2 and AC A commercial V2O5–WO3/TiO2 catalyst (CAS-KR, China) is used. X-ray multi-crystal diffraction (XRD, PANalytical-X’Pert PRO, Holland) shows an anatase structure for the TiO2 phase. X-ray Energy Dispersive Spectroscopy (EDS, EDAX-GENESIS4000, USA) shows V, W and Ti contents of about 0.82, 6.72 and 47.12 wt.%, respectively. Other important parameters are a BET surface area of 93 m2 g1, a micropore surface area of 3.7 m2 g1 and an average pore size of 95 nm (N2-Physisorption, Quanta Chrome, USA). Two commercial ACs are also used, one based on lignite (AC-1), and the other one on coconut shell (AC-2); their specific parameters are shown in Table 1. Both catalyst and AC were sifted to 100 mesh, prior to use.
ð1Þ
ð2Þ
2.3. Analysis Dioxins were pretreated according to the US EPA method 1613 (US EPA, 1994). The recovery rate of each internal standard was established at between 72% and 113%, which conforms to the recovery standard of 40–130%. Analyses were performed by HRGC/HRMS combined with a 6890 Series gas chromatograph (Agilent, USA) and coupled to a JMS-800D mass spectrometer (JEOL, Japan). A DB-5 ms capillary column (60 m 0.25 mm inside diameter, 0.25 lm film thickness) was used to separate the congeners of PCDD/Fs. The temperature program and mass spectrometer were operated as described by Chen et al. (2008).
2.2. Experimental system 3. Results and discussion A stable dioxin generator continuously supplied dioxins in the gas-phase. The system consists of an injector/nebulizer of dioxins solution, a preheater with temperature-controller, and gas flowmeters. The solution is prepared by extracting fly ash, and then purifying and concentrating the extract. The dioxins concentration in the stock solution is adjusted by adding solvent (n-nonane) to the mother liquors. Carrier gas (N2:O2 = 9:1) is controlled at 1 L min1 by a gas mass flowmeter. Preheating is used to eliminate the solvent effect as much as possible and to volatilize dioxins more uniformly. The experiment was conducted using a vertical quartz tube (height 150 mm, inner diameter 25 mm) with heating system and heat preservation jacket, as shown in Fig. 1. The temperature controller maintained the reaction temperature at 160, 180 and 200 °C (actual temperatures of 153, 174 and 196 °C). A tailored filter cartridge placed in tube contains 1.0 g reactant (catalyst only/ AC only/the mixture of catalyst and AC) and 4.0 g SiO2 (sifted to 50 mesh). The corresponding Gas Hourly Space Velocity (GHSV) is 8000 h1. The sampling time during each run was one hour.
Table 1 Properties of AC. Properties
AC-1
AC-2
Ash (%) Moisture (%) Chloride (%) BET surface area (m2 g1) Pore volume (cm3 g1) Micropore, pore diameter 99%, and the DE-value is >95% for the lowest IC-value, of 2.05 ng I-TEQ N m3. When the IC rises to 4.57 ng I-TEQ N m3, RE and DE-values are still 95% and 85%, yet they decrease to only 76% PCDD/Fs removed and 64% PCDD/Fs destroyed for the highest IC-value, of 20.5 ng I-TEQ N m3. Chang et al. (2009) studied the catalytic decomposition of PCDD/Fs with different types of catalyst for different levels of inlet PCDD/F concentrations (1.08–3.04 ng I-TEQ N m3), and found that the PCDD/F destruction efficiencies did not change much at the same temperature for two reasons: firstly change used a higher temperature (250–320 °C), and secondly change used as highest IC-value
S.-s. Ji et al. / Chemosphere 102 (2014) 31–36
33
Fig. 1. Diagram of the experimental set-up.
Table 2 Initial concentrations and congener distribution of PCDD/Fs fed into the reactor (pg I-TEQ N m3). Toxic congeners
C1 (Average)
C2 (Average)
C3 (Average)
2378-TCDD 12378-PeCDD 123478-HxCDD 123678-HxCDD 123789-HxCDD 1234678-HpCDD OCDD PCDDs 2378-TCDF 12378-PeCDF 23478-PeCDF 123478-HxCDF 123678-HxCDF 123789-HxCDF 234678-HxCDF 1234678-HpCDF 1234789-HpCDF OCDF PCDFs Total I-TEQ
21.6 70.8 19.1 34.2 47.9 33.4 8.0 235.0 146.9 35.6 952.7 172.1 133.5 66.8 210.8 73.1 16.6 9.9 1818.3 2.1 103
44.5 205.0 52.7 111.8 69.1 102.0 24.4 609.5 269.9 88.9 1495.7 490.1 426.6 233.5 637.5 234.7 47.7 31.7 3959.3 4.6 103
261.1 1254.1 294.7 510.1 413.9 502.8 82.5 3319.2 184.1 354.9 6358.7 2233.8 2201.7 942.5 3401.1 1143.0 207.4 109.2 17136.6 20.5 103
only about one-seventh of that in this paper. Lower RE-values showed that the adsorption ability of a catalyst is also limited and higher concentration may cause part of PCDD/Fs not to be adsorbed by the catalyst. The difference between DE-values and RE-values becomes larger with rising IC, rising from 5% to 20%: part of the PCDD/Fs are adsorbed by the catalyst and temperature and oxygen supply limit the conversion between V4+OX and V5+OX (Xu et al., 2012). The RE-values for PCDDs are 0.6%, 0.2% and 6.5% higher than those for PCDFs, and the DE-values of PCDDs are 1.7%, 0.8% and 3.4% higher than those for PCDFs, when the IC changes from a low to a medium or a high value. Actually, the absolute value of the PCDF concentrations is much higher than that for PCDDs. More PCDFs need to be treated, but the catalyst capacity does not change. Therefore, the differences of RE and DE-values between PCDDs and PCDFs increase sharply when IC is at its highest level.
Fig. 2. Effect of initial concentration on the RE and DE-values for the 17 toxic PCDD and PCDF congeners (T = 200 °C, C1 = 2.05 ng I-TEQ N m3, C2 = 4.57 ng I-TEQ N m3, C3 = 20.5 ng I-TEQ N m3).
Fig. 2 shows the RE and DE-values of 17 toxic congeners at different IC-values. When IC is only 2.05 ng I-TEQ N m3, the RE-value of 6, 7 and 8 chlorinated congeners (high-chlorinated) is almost similar to that for 4 and 5 chlorinated congeners (low-chlorinated), however, the DE-values of high-chlorinated is 1.9% lower than
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those for low-chlorinated congeners. When the IC increases to 20.5 ng I-TEQ Nm3, the RE-value of high-chlorinated is 4.3% lower than for low-chlorinated, and the DE-values of high-chlorinated is 5.5% lower than those for low-chlorinated. The IC also influences upon the distribution of congeners and it is easier to adsorb high-chlorinated congeners than low. Higher chlorinated congeners have lower volatility and thus result in longer residence times on the catalyst surface (Weber et al., 1999). Chang et al. (2008) also indicated that higher chlorinated PCDD/Fs congeners are more easily adhered to the particles. 3.3. Adsorption of PCDD/Fs by AC The adsorption abilities of AC-1 and AC-2 are evaluated, for the highest IC-value, of 20.5 ng I-TEQ Nm3. Earlier experiments suggested a decrease in adsorption at higher temperature (Lu et al., 2003). However, some researchers reported a good performance of V2O5–WO3/TiO2 catalysts for treating PCDD/Fs within a range of 200–350 °C (Boos et al., 1992; Frings and Marl, 1994), and the higher the temperature, the stronger the degradation effect obtained. In order to avoid the temperature range for fast formation, experiments are conducted at 160, 180, 200 °C (Tuppurainen et al., 1998). The increasing vapor pressure makes more PCDD/Fs to be released to the vapor phase (Rordorf, 1989), so that with temperature rising from 160 °C to 200 °C the RE-value of AC-1 decreases from 88.6% to 86.1%, and the RE-value of AC-2 also decreases from 95.6% to 92.7%. Early research showed that not only the BET surface area, but also the distribution of micropores and mesopores may influence adsorption on AC, and optimal adsorption of PCDD/Fs requires an AC pore diameter of 2.3–4.1 nm (Xie, 2003). Due to the larger BET surface area and the richer micropore/mesopore structure, AC-2 has stronger adsorption abilities than AC-1. The RE-values increase with rising chlorination level for the 17 toxic PCDD/Fs congeners of AC-1; the RE-values for PCDD/Fs congeners of AC-2, however, are almost the same. It illustrates that the adsorption of PCDD/Fs congeners is related to the structure of AC and vapor pressure of PCDD/Fs congeners. If AC has a reasonably larger surface area, and a relatively larger pore network for the transport of molecules to the interior, the difference of congeners’ vapor pressure can be ignored at certain temperatures. RE-values for PCDDs of AC-1 are higher than PCDFs due to under the same chlorination level, the vapor pressures of PCDD congeners are lower than PCDF, and this result is similar to Hung et al. (2011). 3.4. Catalyst & AC degradation PCDD/Fs High-level IC reduces both the RE and DE values when using catalyst only. In order to enhance the adsorption ability at low temperature, AC is mixed with catalyst. Mechanical mixing of catalyst and AC (catalyst: AC = 1:4) is used to destroy PCDD/Fs better (Using M-1 and M-2 to represent catalyst & AC-1 and catalyst & AC-2 respectively), and the IC is also 20.5 ng I-TEQ Nm3. Without AC, only 78.6% PCDD/Fs is removed at 200 °C; in the presence of AC, the RE-values of M-1 and M-2 increase to 90.1% and 91.9%, and the RE-values of the mixture at 160 °C are still higher than catalystonly at 200 °C. Thus, adding AC to the catalyst can obviously improve the RE-values, even at high-levels of PCDD/Fs and at low temperature. The strong adsorption ability of AC prolongs the contact time between catalyst and PCDD/Fs. Fig. 3 shows that the RE-values of M-1 are always lower than those of M-2, and that the RE-values of the mix rise little at rising temperature. This result shows that (a) the larger surface and the micropores/mesopore structure of AC-2 makes that M-2 has better adsorption ability than M-1; (b) catalytic oxidation of PCDD/Fs also influences upon the RE-values of mixture. Catalysts also have pore structure, but
Fig. 3. RE and DE-values of (1) AC and (2) a mixture of catalyst and AC at different temperatures.
are lower in specific surface than AC, so that the RE-values of AC are always higher than for the mixture. The RE-value of AC-1 is lower than M-1 at 200 °C; the weaker adsorption of AC-1 and the stronger catalytic oxidation can induce this. Fig. 4 shows the RE-values for the 2,3,7,8-PCDD/Fs congeners at different conditions. The RE-values for PCDDs of the M-1 and M-2 are higher than PCDFs, influenced by catalyst and AC. The RE-values for low-chlorinated congeners of M-2 are much higher than M-1, but the RE-values for high-chlorinated congeners of M-1 and M-2 are almost the same. For M-1, the RE-values of high-chlorinated congeners are always higher than for low-chlorinated congeners, and the difference is more obvious at lower temperature; for M-2, the difference of RE-values between high and lowchlorinated congeners is not obviously (
