A review of dioxin-related substances during municipal solid waste incineration.

Waste Management 36 (2015) 106–118

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Waste Management journal homepage: www.elsevier.com/locate/wasman

Review

A review of dioxin-related substances during municipal solid waste incineration Hui Zhou, Aihong Meng, Yanqiu Long, Qinghai Li, Yanguo Zhang ⇑ Key Laboratory for Thermal Science and Power Engineering of Ministry of Education, Department of Thermal Engineering, Tsinghua University, Beijing 100084, China

a r t i c l e

i n f o

Article history: Received 26 July 2014 Accepted 12 November 2014 Available online 5 December 2014 Keywords: Waste Incineration PCDD/F Precursor Indicator

a b s t r a c t Polychlorinated dibenzo-p-dioxins and polychlorinated dibenzofurans (PCDD/Fs) are among the most toxic chemicals and the main restriction on municipal solid waste incineration. To exert more effective control over the formation of dioxin homologues during municipal solid waste incineration, it is significant to investigate dioxin-related compounds. Despite the numerous studies about PCDD/Fs, a unified understanding regarding many problems has yet to be reached because the homologues of PCDD/Fs are excessive, the measurement of PCDD/Fs is difficult, and the formation mechanisms of PCDD/Fs are complicated. Firstly, this paper briefly introduces the different formation mechanisms of PCDD/Fs, including high temperature homogeneous reaction PCDD/Fs formation and low temperature heterogeneous reaction PCDD/Fs formation. Then the sources of PCDD/Fs including precursors (chlorophenols and polycyclic aromatic hydrocarbons) and residual carbon are summarized. In particular, this paper analyzes the substances that influence PCDD/Fs formation and their impact mechanisms, including different categories of chlorine (Cl2, HCl and chloride in fly ash), O2, copper, sulfur, water, and nitrogen compounds (ammonia and urea). Due to the high cost and complexity of PCDD/Fs measurement, PCDD/Fs indicators, especially chlorobenzenes and polycyclic aromatic hydrocarbons, are summarized, to find an effective surrogate for quick, convenient and real-time monitoring of PCDD/Fs. Finally, according to the results of the current study, recommendations for further research and industrial applications prospects are proposed. Ó 2014 Elsevier Ltd. All rights reserved.

Contents 1. 2.

3.

4.

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . The sources of PCDD/Fs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.1. Chlorophenol . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.2. Polycyclic aromatic hydrocarbons . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.3. Residual carbon . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . The substances that influence PCDD/Fs formation. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.1. Chlorine . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.1.1. Cl2 and HCl . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.1.2. Chloride in fly ash . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.2. Oxygen (O2) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.3. Copper . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.4. Sulfur . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.5. Water . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.6. Ammonia and urea . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . PCDD/Fs indicators . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.1. Chlorobenzene . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.2. Polycyclic aromatic hydrocarbons . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.3. Other indicators. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

⇑ Corresponding author. Tel.: +86 010 62783373; fax: +86 010 62798047 801. E-mail address: [email protected] (Y. Zhang). http://dx.doi.org/10.1016/j.wasman.2014.11.011 0956-053X/Ó 2014 Elsevier Ltd. All rights reserved.

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Conclusion and outlook . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 115 Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 115 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 115

1. Introduction Dioxins are generic terms for polychlorinated dibenzo-p-dioxins (PCDDs) and polychlorinated dibenzofurans (PCDFs). PCDDs and PCDFs consist of 75 and 135 homologues, respectively, as shown in Fig. 1. Dioxins are among the most toxic chemicals on the earth, and in particular, 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) has the highest toxicity. US Environmental Protection Agency (USEPA) has listed dioxins as a serious cancerogen which may destroy the immune system of human body and interfere with hormone regulation (Mckay, 2002). Municipal solid waste (MSW) incineration is one of the significant sources of PCDD/Fs in the environment. A considerable body of research about PCDD/Fs from MSW incineration has been carried out since the 1970s. Generally, researchers believe that PCDD/Fs are formed via two mechanisms. One is from homogeneous reactions, in the temperature range of 500–800 °C. The main process is the rearrangement reactions of chlorinated precursors in the gas, such as chlorophenol (CP) and chlorobenzenes (CBz). PCDD/Fs from this process are called homogeneous PCDD/Fs or high-temperature PCDD/Fs. PCDD/Fs can also be formed from heterogeneous reactions, in the temperature range of 200–400 °C. PCDD/Fs from this process are termed as heterogeneous PCDD/Fs or low-temperature PCDD/Fs. Heterogeneous PCDD/Fs may also come from CP or CBz (Addink and Olie, 1995a; Vermeulen et al., 2014), or from carbon in fly ash (Huang and Buekens, 1995). The main process is the surface catalytic effect of fly ash or soot, i.e. de novo process (Stanmore, 2004). It remains debated whether heterogeneous PCDD/Fs come from gaseous precursors or carbon in fly ash. The experiments of Dickson et al. (1992) show that, under the same conditions, the PCDD/Fs formation rate from precursors is 72–99,000 times higher than the formation rate from carbon in fly ash. Therefore, precursors are thought as the main source of PCDD/Fs. Luijk et al. (1994) also thought the PCDD/Fs formation from precursors was approximate 3000 times faster than the de novo process from activated carbon. In a review article, Tuppurainen et al. (1998) also argued that precursors were the main source of PCDD/Fs formation. However, the experiments of Dickson et al. and Luijk et al. had some limitations. The CP concentration was too high—much higher than the real concentration in flue gas of incineration. Moreover, the characteristics of synthetic fly ash were quite different from that of real fly ash, and the reactivity of real fly ash was higher. Due to the low concentration of precursors in flue gas, Huang and Buekens (2000) studied the proportional distribution of PCDDs and PCDFs, and concluded that the main origin of heterogeneous PCDD/Fs was the de novo process of the carbon in fly ash. Everaert and Baeyens (2002) reviewed the incinerator of MSW,

Fig. 1. The structure of PCDD/Fs.

wood and industrial waste, coal boiler, and metallurgical furnace, and obtained the same conclusion. Stanmore and Clunies-Ross (2000) also deemed that all the PCDD/Fs from MSW incinerator were related to fly ash. Their conclusions have been proved by the experiments by Hell et al. (2000) and Tame et al. (2003) and the model established by Stanmore (2002a). However, some researchers contend low chlorinated PCDD/Fs are from gas phase catalysis or non-catalytic flame reactions, while high chlorinated PCDD/Fs are mainly from de novo reactions of fly ash (Wikstrom et al., 2004a). Xhrouet et al. (2001) found that the ratio of PCDFs to PCDDs was larger than 5 in simulated de novo reactions, and the same results have been replicated in other studies (Everaert and Baeyens, 2002; Addink and Olie, 1995b; Pekarek et al., 2001; Chang and Huang, 2000). Thus it is believed that PCDFs are mainly from de novo reactions, while precursors play an important role on the formation of PCDDs. Moreover, during the heterogeneous PCDD/Fs formation from trichlorophenol (TrCP), the ratio of PCDDs to PCDFs is much larger than 1 (Hell et al., 2000; Qu et al., 2009). Given the PCDD/Fs formation mechanisms are complicated and the measurement of PCDD/Fs is difficult, continuous research efforts have been devoted to this area and a universally accepted result has yet to be obtained (Stanmore and Clunies-Ross, 2000). After the heterogeneous formation mechanisms of PCDD/ Fs were discovered, most of the research focuses on heterogeneous PCDD/Fs (Alderman et al., 2005; Vehlow, 2012). With the application of advanced industrial dust removal equipment, the formation of heterogeneous PCDD/Fs is controlled to some extent (Kim et al., 2007a; Lin et al., 2008). Accordingly, the percentage of homogeneous PCDD/Fs is increased, thus more researchers have turned to investigating the formation of homogeneous PCDD/Fs (Qu et al., 2009). PCDD/Fs may be generated from the condensation of CP and CBz, the chlorination of polycyclic aromatic hydrocarbons (PAHs), or the de novo reactions of fly ash. The present article reviews three important sources of PCDD/Fs: CP, PAHs and residual carbon, and summarizes the influences of chlorine, O2, copper, sulfur, water, and nitrogen compounds on PCDD/Fs formation. Since conventional PCDD/Fs measurement in incinerators is complicated and costly, many researchers are seeking for PCDD/Fs indicators (Pandelova et al., 2006), i.e. to find a simple method to measure PCDD/Fs emissions during incineration process. Therefore, the last part of this paper reviews the possibility of CBz and PAHs and other compounds as PCDD/Fs indicators.

2. The sources of PCDD/Fs 2.1. Chlorophenol Among many precursors, CP has the most similar structure with PCDD/Fs, and CP is the easiest to form PCDD/Fs (Xu et al., 2010; Pan et al., 2013). Both the homogeneous and heterogeneous reactions of CP are very important for PCDD/Fs formation (Tuppurainen et al., 2003). Many studies have identified the total amount and categories of PCDD/Fs using CP as an origin at elevated temperature (Hell et al., 2000), including high-temperature homogeneous PCDD/Fs formation (Kim et al., 2007b; Khachatryan et al., 2003a) and low-temperature heterogeneous catalytic PCDD/Fs formation

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(Mulholland and Ryu, 2001; Nganai et al., 2012; Hell et al., 2001a; Lomnicki and Dellinger, 2002). Adopting pentachlorophenol (PCP) as precursors, Karasek and Dickson (1987) carried out the surface catalytic reactions of the real fly ash from incinerators to test PCDD/Fs formation. The best formation temperature was 250–350 °C. The authors deemed that the metal in fly ash was the catalyst of PCDD/Fs formation. Ghorishi and Altwicker (1996) studied PCDD/Fs formation from the two different kinds of precursors, 1,2-dichlorobenzene (DCBz) and 2,4-dichlorophenol (DCP), and the results showed that levels of PCDD/Fs produced from 2,4-DCP were two orders of magnitude higher than those produced from 1,2-DCBz. Dickson et al. (1992) led PCP gas through synthetic fly ash, and the C in PCP consisted 13C atom. The results showed that 13C and 12 C were not present together in one molecular, indicating that the processes from precursors to PCDD/Fs and from de novo reactions to PCDD/Fs were entirely distinct. Under the same conditions, the PCDD/Fs formation rate from precursors was much higher than that from carbon. The conversion rate of PCP was as high as 55%. The formation of PCDD is related to the surface coverage of CP in fly ash, indicating this reaction is surface controlled. When the reaction time increased from 2 to 120 min, the surface coverage

About high-temperature homogeneous PCDD/Fs formation from CP, it has been found that the maximum conversion rate from 2,4,6-TrCP to PCDD/Fs (0.05%) occurred at 600 °C in a quartz reactor (Sidhu et al., 1995). Luijk et al. (1994) have obtained similar results. During the experiments of 2,4,6-TrCP homogeneous condensation to PCDD/Fs, when temperature increased from 300 to 600 °C, the reaction rate increased by 10 times. Khachatryan et al. (2003b) has built a kinetic model of PCDD homogeneous formation from 2,4,6-TrCP using 45 extended reactions and 12 core reactions, the calculated results highly agreed with the experimental results. Recently, a mechanistic and kinetic study of the formation of PCDD/Fs from CP precursors was carried out. Two different types of radicals from chlorophenoxy radicals, i.e., substituted phenyl radicals and phenoxyl diradicals, were proposed to serve as potential sources contributing to the formation of PCDD/Fs (Zhang et al., 2014). The typical CP concentration in MSW incinerator is 20 lg/N m3 (Jay and Stieglitz, 1995). During normal operation of the solid waste incinerator, the dioxin-related processes in furnace include high-temperature decomposition and combustion of PCDD/Fs and CP and condensation of CP to form PCDD/Fs. The condensation reaction from CP to PCDD/Fs can be illustrated as follows:

ð4Þ

ð5Þ

of CP in fly ash did not change, which meant the system formed the adsorption–desorption balance in a short time (Milligan and Altwicker, 1996). Afterward, the research of Lomnicki and Dellinger (2003) also proved the mechanisms of surface reactions. Huang and Buekens (2000) have built a catalytic reaction model from CP to PCDD/Fs:

CP þ S ! CP0

ð1Þ

CP0 ! decomposition products

ð2Þ

CP0 þ CP0 ! PCDD

ð3Þ

CP0 denotes adsorbed CP, and S denotes solid phase. This model worked well for 2,4,6-TrCP, 2,3,4,6-tetrachlorophenol (TeCP), and PCP. In high concentration, the reactions are first-order reactions; in low concentration, the reactions are second-order reactions. This model has predicted the experimental results of Milligan and Altwicker (1996) at low temperatures. About the low-temperature heterogeneous catalytic PCDD/Fs formation from CP, some conclusions can be obtained. The reaction temperature is 250–450 °C, and the best temperature is approximate 350 °C (Mulholland and Ryu, 2001). Fly ash plays an important role in this process, and the surface coverage of CP in fly ash is a key parameter.

2.2. Polycyclic aromatic hydrocarbons The chlorination of dibenzofuran (DF) at 200–400 °C may be the origin of PCDFs formation (Wikstrom and Marklund, 2000). Under the effect of CuCl2, the chlorination of DF is obvious (Ryu et al., 2003). Ryu et al. (2004) built a chlorination model of DF and dibenzo-p-dioxin (DD), and found the homologue distribution of PCDFs was consistent with the measurement results in incinerator, indicating that the chlorination of DF played an important role on the formation of PCDFs. However, the homologue distribution of PCDDs was different from the measurement results of real incinerator, indicating chlorination of DD was not important for the formation of PCDDs. Before the convective zone, the concentration of DF is 100–1000 times of the concentration of PCDFs; after the convective zone, the concentration of PCDFs increases by 10–100 times. This means the chlorination of DF is important for the PCDFs formation in some incinerators (Wikstrom and Marklund, 2000). There are two mechanism of the chlorination of DF. One is the direct chlorination of DF by Cl2:

ArH þ Cl2 ! ArCl þ HCl

ð6Þ

The other is the reaction of DF and CuCl2:

ArH þ CuCl2 ! ArHCl þ CuCl

ð7Þ


ArHCl þ CuCl2 ! ArCl þ CuCl þ HCl

ð8Þ

At 250 °C, in the presence of SiO2/MgO and CuCl2H2O, biphenyl was converted to large amounts of PCDFs, with no PCDDs. During the experimental process, 72% of the total biphenyl was converted to PCDFs, indicating that without the destroying of skeleton of the reactant, ether linkages could be formed intramolecularly. PCDFs were not derived from the condensation of CP or CBz (Wilhelm et al., 2001). PCDF can be formed directly from fluorenone, biphenyl and fluorene, and the concentration of these three PAHs can predict the concentration of PCDF (Fullana and Sidhu, 2005). Anthracene and chlorinated anthracenes can generate PCDD/Fs, and the best temperature is 300 °C (Schoonenboom and Olie, 1995). Under the same conditions, phenanthrene generates more PCDFs than activated carbon, and coronene can also generate 1,2,8,9-tetrachlorodibenzofuran (TCDF) and its derivatives (Iino et al., 1999a,b). Furthermore, Weber et al. (2001a) suggested that PCDFs, CP, polychlorinated naphthalene, and CBz all came from PAHs. Actually, due to their low vapor pressure, PAHs tend to remain in fly ash. Measurement reports that the total concentration of 16 PAHs in bag filter fly ash of mechanical grate furnace is as high as 2337 lg/kg. Among them phenanthrene, fluoranthene and pyrene have the highest concentration (Shi et al., 2009). In fact, PAHs have very similar structure with residual carbon, thus the chlorination process of PAHs can be hardly distinguished from the de novo process of residual carbon.

2.3. Residual carbon Research has found that residual carbon and soot in fly ash are one of the main sources of PCDD/Fs formation during MSW incineration (Huang and Buekens, 1995, 2000; Iino et al., 1999a; Ryan and Altwicker, 2000; Wikstrom et al., 2004b). Hell et al. (2001b) mixed 12C and 13C labeled C atoms with real fly ash, and PCDD/ Fs could be formed from the de novo process of residual carbon in fly ash, or the condensation of CP. They thought that half of the PCDDs were derived from the condensation of C6 precursors from de novo process, and the other half were directly from the de novo reactions of C12 structure. The condensation of C6 precursors was not important for the formation of PCDFs. The same conclusions have been reached by Wikstrom et al. (2004a). There are some more direct conclusions. Research has found that the generation of PCDD/Fs is approximate in proportion to the carbon content in fly ash (Wikstrom et al., 2003a). Kuzuhara et al. (2003) has obtained the same results through simulated de novo experiments, and the PCDD/Fs release rate was proportional to the carbon consumption rate. Some studies involved more extensive carbon content (from 0% to 20%) (Chang and Huang, 2000), and found that when carbon content was 0, no PCDD/Fs were formed. The best carbon content for PCDD/Fs formation was 5%. When carbon content was more than 5%, the generation of PCDD/Fs decreased significantly. The authors deemed that as the addition of more carbon, carbon participated in the competition for chlorine to form other chlorides; another reason was that the addition of carbon lowered the catalyst concentration. In fact, there might be another reason. Chang and Huang (2000) used constant oxygen concentration, and the additional carbon resulted in the competition for O2. The decreased ratio of O2 and carbon might be the reason that PCDD/Fs formation yields reduced, and this mechanism will be illustrated in detail in Section 3.2. Nevertheless, it should be noted that Chang and Huang heated the fly ash at 550 °C to remove the original carbon in fly ash. During this process, some other organic compounds might also be removed, which meant the fly ash was different from real fly ash.

109

Since the carbon content in fly ash of incinerator is generally less than 5%, in this range, the formation of PCDD/Fs is proportional to the carbon content in fly ash. Schoonenboom et al. (1995) have proposed a two-step mechanism. It is the chlorination of carbon surface, and then it is the oxidation and decomposition of chlorinated carbon that form 3,4,6,7-substituted or side by side chlorinated PCDD/Fs. Activated carbon may have catalytic effect on the heterogeneous PCDD/Fs formation of precursors. When PCP passed through the mixture of silica gel, anhydrous copper chloride and activated carbon powder, the conversion rate of PCP was as high as 55%. It should be noted that without copper catalyst, the conversion rate of PCP was still very high due to the catalytic effect of activated carbon probably (Dickson et al., 1992). Luijk et al. (1994) also believed activated carbon had catalytic effect on the condensation of CP. However, this effect has not been further studied. In conclusion, research has proved that CP and PAHs are important precursors of PCDD/Fs formation, and they are toxic substances themselves (USEPA, 1980; Luch, 2005). Residual carbon is also an important origin of PCDD/Fs. In real MSW incinerator and experiments, PCDD/Fs formation may be controlled from the control of these three substances. Therefore, the research about MSW incineration can focus on the formation of these three substances. 3. The substances that influence PCDD/Fs formation During MSW incineration, besides PCDD/Fs sources such as CP, PAHs and residual carbon, there are some other substances that affect PCDD/Fs formation. The most important substances include different forms of chlorine, O2, catalyst, sulfur, water, and nitrogen compounds (ammonia and urea). 3.1. Chlorine PCDD/Fs cannot be formed without chlorine (Hatanaka et al., 2000). The chlorine for PCDD/Fs formation may come from solid phase, such as metal chloride and chloride from incomplete combustion, or from gas phase, such as Cl2 and HCl. 3.1.1. Cl2 and HCl It is widely accepted that Cl2 and HCl have important impact on PCDD/Fs formation. Research has found that during de novo process, Cl2 can promote PCDD/Fs formation more than HCl. When HCl or Cl2 passed through real fly ash with 6.3% chlorine, the addition of Cl2 promoted reaction rate considerably, while the addition of HCl had no significant influence on PCDD/Fs formation. When HCl and O2 were added simultaneously, more PCDD/Fs were generated. The reason may be Cl2 in flue gas is increased (Wikstrom et al., 2003a, 2003b), i.e. Deacon Reaction happens, and CuCl2 is catalyst of this reaction.

4HCl þ O2 ! 2Cl2 þ 2H2 O

ð9Þ

However, Addink et al. (1995) has obtained different results. In the simulated de novo reaction, when the same concentration HCl and Cl2 was added, the PCDD/Fs yields were similar, but the ratio of PCDDs to PCDFs was different. The authors thought Deacon Reaction did not play an important role on chlorination process. It should be noted that there was 10% O2 in the reaction atmosphere of Addink et al. (1995), and there was CuCl2 in real fly ash. Therefore, it was not contradictory to the results of Wikstrom et al. (2003a, 2003b). About the influence mechanism of HCl, different researchers hold various understandings. Addink et al. (1995) believed HCl could perform chlorination effect directly, or through Cl2.


Hoffman et al. (1990) deemed HCl reacted with fly ash surface to form metal chloride, and metal chloride was the actual chlorinating agent for PCDD/Fs formation. Gullett et al. (2000a) thought Cl radical was active agent, and Cl radical was from HCl and Cl2. HCl could react with oxidizing radicals (e.g. OH) to form Cl radical. It is certain that Deacon Reaction is not the only way that HCl militates. But from the results of Wikstrom et al. (2003a,b), O2 is important for the chlorination effect of HCl. Meanwhile, it is difficult for HCl to react with fly ash directly. Therefore, it is a possible process that HCl reacts with oxidizing radicals to form Cl radicals.

3.1.2. Chloride in fly ash The chlorine concentration in MSW incinerator fly ash is high. Typical chlorine concentration is 1.8%, 4.5%, 8.65% (Stanmore, 2002b), 9.1% (Takasuga et al., 2000), 4.5% (Gullett et al., 1994), 6.0% (Addink and Altwicker, 2001), and 5.3% (Wikstrom et al., 2003a). Many studies show that chloride in fly ash is important in de novo process. In typical MSW incinerator, with the increase of chlorine content in fly ash, the homologues of PCDDs were increased (Stanmore, 2004). In the simulated de novo experiments, without HCl and Cl2, large amount of PCDD/Fs were still generated, indicating chloride in solid phase participated into the reactions (Gullett et al., 1994). When the organics in fly ash were removed, during the simulated de novo process in 10% O2, large amounts of PCDD/Fs were still formed, and the addition of NaCl had not effect on the results. When the soluble inorganic chloride had been removed, the addition of NaCl increased PCDD/Fs formation rate (Addink et al., 1998). Wikstrom et al. (2003b) further developed this conclusions, and thought the chloride in fly ash was enough for PCDD/Fs formation, and the chloride in fly ash was more important than gas phase chlorine (Cl2, HCl, and Cl radical) for PCDD/Fs formation. About the chlorine forms in fly ash, Addink and Altwicker (2001) identified 6.0 wt.% inorganic chlorine and 0.10 mg g1 organic chlorine from the fly ash of incinerator. They studied the effect of solid phase chlorine on de novo process using 37Cl isotope, and found the effective chlorine was metal chloride in fly ash and original C–Cl bond, and the chlorides of copper could exchange chlorine with chlorine in carbon. Therefore, CuCl2 was regarded as important chlorinating agent of PCDD/Fs formation. Research has found that the exchange rate of chlorine in CuCl2 and chlorine in macromolecular carbon structure of fly ash is very fast, and the reaction process is as follows:

2CuCl2 þ R-H ! 2CuCl þ R-Cl þ HCl

3.2. Oxygen (O2) Since excess O2 is usually provided during combustion process, and PCDD/Fs structure has ether linkages, O2 concentration may be an important factor for PCDD/Fs formation. Oxygen bonded in carbon basis is important oxygen origin for de novo process (Wilhelm et al., 2001), as has been illustrated in Section 2.2. The chlorination of DF structure in carbon basis is one of the sources of PCDF formation. Free oxygen plays an important role on the PCDD/Fs formation from CP. At temperature above 340 °C, CP can generate PCDD/Fs with the presence of O2, while this reaction does not happen in the atmosphere of Ar (Weber and Hagenmaier, 1999). The mechanism may be that phenoxy radicals are formed firstly, and then dimerization happens. Addink and Olie (1995b) carried out simulated de novo reactions using the mixture of activated carbon and real fly ash. Without O2, not PCDD/Fs were formed. As shown in Fig. 2, with the increase of O2 concentration from 0 to 10 mol%, the formation of PCDDs was proportional to O2 concentration, while the formation of PCDFs was proportional to the 0.5 square of O2 concentration. Addink and Olie believed that PCDDs with two oxygen atoms were more sensitive to O2 than PCDFs with only one oxygen atom. During the simulated de novo reactions of synthetic fly ash, Ryan and Altwicker have found that with carbon black as reactant, when O2 concentration was 2%, the formation rate of PCDD/Fs was the highest. Stanmore (2004) thought this might due to Deacon Reaction. The HCl conversion rate of this reaction reached plateau when O2 concentration was more than 2%. In fact, the addition of O2 may oxidize carbon to form CO2 and CO. Therefore, the effect of O2 on PCDD/Fs formation is the result of two competing reactions, and the O2 concentration for PCDD/ Fs formation has a peak. Addink and Olie have obtained different results from Ryan and Altwicker. It should be noted that the experiments of them were quite different. The former used activated carbon; the latter used carbon black; and the reactivity of activated carbon was higher. Secondly, the former used real fly ash; the latter used synthetic fly ash with FeCl2 as chlorine origin and catalyst. It has been proved that the original carbon in real fly ash is more reactive than the carbon in synthetic fly ash (Stanmore, 2004), and the CuCl2 in fly ash is more reactive than other metal compounds. Therefore, the reactants in the research of Addink and Olie’s had higher reactivity than that in the research of Ryan and Altwicker, thus could interact with higher O2 concentration. In addition, the best O2 concentration for PCDD/Fs formation is

ð10Þ

Therefore, the generation of organic chloride depends on the conversion rate from Cu(II) to Cu(I) (Weber et al., 2001b). In fact, fly ash is not only composed of chlorine, but also residue carbon and metal catalyst. Therefore, heating fly ash will produce PCDD/Fs, while when fly ash is heated together with other compounds, PCDD/Fs formation will be increased. CuCl2 in fly ash not only is catalyst of PCDD/Fs formation, but also can provide chlorine for PCDD/Fs formation. It is very difficult to solve the chlorine function mechanism, because chlorine has many forms, such as HCl, Cl2, Cl radicals, and organic chlorine and inorganic chlorine in fly ash. Research has found that the Cl concentration of most fly ash is sufficient for de novo reaction, and the plentiful inorganic chlorine and a small amount of organic chlorine have very high activity. There may be two reasons: one is the high reactivity of CuCl2 itself, and the other may be chloride in fly ash can contact with carbon and catalyst sufficiently. When Cl2 in gas phase is added, it can provide activated chlorine after CuCl2 in solid phase is exhausted, while HCl can provide chlorine only with presence of O2.

Concentration of PCDD/Fs(nmol/g of fly ash)

110

2.5

PCDDs PCDFs Square root fitting

2.0

1.5

1.0

0.5

0.0 0

2

4

6

8

10

O2 concentration (%) Fig. 2. Yields of PCDD/Fs at different O2 concentration (Addink and Olie, 1995b).


Concentration of PCDD/Fs(ng/g of fly ash)

related to the mass transfer process of reactors, and that is why different experiments obtained different optimum O2 concentration. Vogg et al. (1987) used real fly ash in simulated de novo experiments. With the increase of O2 concentration to 10%, the formation of PCDD/Fs increased continuously, as shown in Fig. 3. Similar to the research of Addink and Olie, the formation of PCDDs was proportional to O2 concentration, while the formation of PCDFs was proportional to the 0.5 square of O2 concentration. Pekarek et al. (2001) has obtained similar results with Vogg et al., using real fly ash. Only three points were experimented (0, 1%, 10%), as shown in Fig. 4. However, Vogg et al. got higher amounts of PCDDs than of PCDFs, as shown in Fig. 3, which was different from other studies (Pekarek et al., 2001; Chang and Huang, 2000). The reason might be that the fly ash used by Vogg et al. was different from that of other studies. Some experiments have studied the effect of boarder oxygen concentration (from 0% to 100%) (Chang and Huang, 2000). The results showed that the best O2 concentrations for PCDD/Fs formation in cyclone separator fly ash were 7.5% and 40%. Two different maximum indicated that O2 had complicated influences on PCDD/ Fs formation. It was also found that the ratio of PCDDs/PCDFs increased with the increase of O2 concentration. The same with 10000

PCDDs PCDFs Square root fitting

8000

6000

4000

2000

0

Addink and Olie (1995b), the authors deemed that PCDDs with two oxygen atoms were more sensitive to O2 than PCDFs with only one oxygen atom. It was also found that PCDD/Fs could be formed without O2, and the reason might be that fly ash contained dioxinlike oxygenous groups, or even solid phase PCDD/Fs (Wu et al., 2011). The influence of O2 on PCDD/Fs formation may be driven by two mechanisms. One is to transform HCl to more activated Cl2 by Deacon Reaction. Though there is no HCl added into reactions, fly ash may produce HCl itself, thus the addition of O2 will increase high chlorinated PCDD/Fs. The other mechanism may be that carbon is chlorinated by metal chloride (such as CuCl2) on the surface, and O2 increases chlorination rate through reaction with metal ions (Addink and Olie, 1995b). As shown in Section 3.1.1, the addition of O2 is important for the chlorination effect of HCl, which proves the first mechanism. However, Takaoka et al. (2005) thought O2 worked thought the second mechanism. They analyzed the catalytic process of Cu, and thought CuCl2 chlorinated carbon to form CuCl, while CuCl could be oxidized by O2 to form intermediate copper oxychloride, and then reacted with Cl2 to form original CuCl2 to finish a life cycle. Stieglitz (1998) thought the function of O2 may also lead to the oxidization and decomposition of carbon structure. The de novo process could be divided into two processes: the first process was the chlorination of carbon structure by metal chlorides to form C–Cl structure, and the second process was the oxidization of carbon structure to generate CO2 as well as PCDD/Fs. The effect of O2 on de novo process may be multiple. It may promote the Deacon Reaction, or promote the regeneration of CuCl2. Meanwhile, O2 may promote the oxidative disintegration of carbon structure and release of PCDD/Fs. In addition, the generation of PCDD/Fs has the best O2 concentration. On one hand, the addition of O2 will promote the formation of PCDD/Fs due to the above mechanisms; on the other hand, excess O2 will promote carbon combustion to form CO and CO2, thus PCDD/Fs formation will be suppressed.

3.3. Copper 0

2

4

6

8

10

O2 concentration (%) Fig. 3. Influence of oxygen on the formation of PCDD/Fs on fly ash after two hours at 300 °C (Vogg et al., 1987).

Concentration of PCDD/Fs(ng/g of fly ash)

111

14000

PCDDs PCDFs

12000 10000 8000 6000 4000 2000 0 0

2

4

6

8

10

O 2 concentration(%) Fig. 4. Formation of PCDD/Fs by de novo synthetic reaction in different oxygen concentration (Pekarek et al., 2001).

When 0.07% CuCl2H2O was added to the synthetic MSW, the formation of PCDD/Fs increased by 30% (Halonen et al., 1995), indicating that Cu might have important effect on PCDD/Fs formation. Typical Cu concentration in fly ash is 0.11 wt.% (Addink and Altwicker, 2001), 0.16 wt.% (Wikstrom et al., 2003a), and 0.39 wt.% (Bie et al., 2007). During the combustion tests of PVC, only trace amounts of PCDD/Fs were generated without Cu, and the addition of Cu promoted the formation of PCDD/Fs (Wang et al., 2002). Hatanaka et al. (2004) tested the combustion of synthetic MSW and found that PCDD/Fs formation increased with the increase of Cu concentration in MSW, indicating Cu was a catalyst of PCDD/Fs formation. In the heterogeneous PCDD/Fs formation process from precursors PCP, CuCl2 is an important catalyst (Dickson et al., 1992; Ryu et al., 2005). Through calculation, Sun et al. (2007) thought CP reacted with CuO to form HO-Cu-2,4,5-TrCP, and Cu(II) transformed into Cu(I) as it desorbed to chlorophenoxy radical. Reaction 2,4,5-TrCP + CuO ? 2,4,5-trichlorophenoxy radical + CuO was an obvious exothermic reaction and happened very easily, while CP alone decomposed to chlorophenoxy radical was difficult to happen. This demonstrated that CuO was the catalyst for the formation of polychlorinated phenoxy radical in gas phase, and the latter was PCDD/Fs precursors. When CuO catalyst was changed to CuSO4 in heterogeneous PCDD/Fs formation experiments from precursors, the generation of PCDD/Fs decreased to a great extent (Gullett et al., 1992).

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Regarding the mechanisms of catalytic effect of Cu on precursors condensation, Born et al. (1993) deemed that Cu(I) was the activated agent of nucleophilic aromatic substitutions in Ullmann Reaction; Cu catalyzed halogenated benzenes and alkali metal phenates to form biphenyl ethers. For de novo reactions, Kuzuhara et al. (2003) examined the reactivity of different chlorine compounds, and found the reactivity order was KCl < CaCl2 < FeCl3 CuCl2. Chin et al. (2012) also added 10 wt.% FeCl2, ZnCl2 and CuCl2 onto the fly ash medium, the PCDD/Fs concentration was 8.8, 25.5 and 369.5 ng/g, respectively. This indicates that the formation of PCDD/Fs was significantly increased by CuCl2. The catalyst effect of Cu and CuO is low, and the generated PCDD/Fs are below detection limits. While CuCl and CuCl2 can act as catalyst, they are chlorine sources themselves, and thus they may promote PCDD/Fs formation. Addink and Altwicker (1998) tested different types of copper compounds, such as elemental Cu, CuCl, CuCl2, Cu(NO3)22.5H2O, Cu2O, and CuSO45H2O. The results showed that only CuCl and CuCl2 increased the formation of PCDD/Fs, and with the increase of concentration of CuCl and CuCl2, PCDD/Fs formation increased. The experiments of Pekarek et al. (2007) also proved the catalytic effect of CuCl2 was strong, and the catalytic effect of CuO and CuSO4 was trivial. In fact, most research has found that Cu can act as catalyst for processes such as the oxidation of carbon, chlorination of carbon, and dechloridation. Meanwhile, CuCl2 is a very reactive chlorine source. The chlorine in NaCl is difficult to transfer out, while CuCl2 can play a positive role on chlorine transportation, and the effect of CuCl2 is stronger than iron chloride and the chlorides of other metals (Addink and Altwicker, 2001). As shown in Reaction (10), the exchange rate of chlorine in CuCl2 and chlorine in macromolecular carbon structure of fly ash is very high. About the chlorination effect of CuCl2, it is generally thought that the solid phase halide of Cu can react with PAHs to form chlorinated PAHs through the following mechanism. Ar denotes the boundary of graphitic layers or aromatic compounds.

ArH þ CuCl2 ! ArHCl þ CuCl

ð11Þ

3.4. Sulfur A significant number of studies have shown that sulfur is an inhibitor for the heterogeneous formation of PCDD/Fs (Ruokojarvi et al., 1998; Raghunathan and Gullett, 1996; Hunsinger et al., 2007; Hajizadeh et al., 2012). When (NH4)2SO4 was added to waste wood combustion at an S:Cl ratio of 6, significant reductions in the PCDD/Fs contents of the flue gas were observed (Lundin and Jansson, 2014; Lundin et al., 2013). Many researchers have attempted to unveil the mechanisms with which sulfur inhibits PCDD/Fs formation, and two mechanisms were proposed (Gullett et al., 1992, 2000a, 2000b). (a) Cl2 is regarded as the main chlorine source for PCDD/Fs formation by substitution reactions of aromatic structure. Cl2 can influence the generation of chlorinate aromatic precursors to further influence the formation of PCDD/Fs. While SO2 and Cl2 have the following reaction:

Cl2 þ SO2 þ H2 O ! 2HCl þ SO3

ð13Þ

25000

Since Cl2 is consumed by SO2, PCDD/Fs formation will be reduced. (b) Cu(II) can react with SO2 to form CuSO4, and the catalytic effect of CuSO4 is much lower than CuO. During the combustion process of natural gas in a pilot scale incinerator, SO2 and HCl were added, and when the S/Cl was 0.64, the inhibition effect of sulfur was the most obvious. The authors thought that sulfur inhibited PCDD/Fs formation through Mechanism (a) (Raghunathan and Gullett, 1996). Ogawa et al. (1996) also approved this mechanism. However, more studies supported Mechanism (b). Gullett et al. (1992) changed the catalyst of heterogeneous PCDD/Fs formation reaction from CuO to CuSO4, PCDD/Fs formation was reduced significantly. However, when SO2 was introduced into CuO, PCDD/Fs formation did not change, indicating that Reaction (13) did not happen. The authors believed that following reaction took place.

20000

CuO þ SO2 þ 1=2O2 ! CuSO4

ArHCl þ CuCl2 ! ArCl þ CuCl þ HCl

ð12Þ

Takaoka et al. (2005) identified the Cu forms in fly ash, and found that the main forms were CuCl23Cu(OH)2 and CuCl, while pure CuCl2 was not found. The CuCl23Cu(OH)2 content in fly ash had positive correlation with the formation of polychlorinated biphenyl (PCB) and CBz. Takaoka et al. (2005) analyzed the

40000 35000

PCDD/PCDFs (ng/g)

catalytic process of Cu. CuCl2 could chlorinate carbon to form organic chlorides including PCDD/Fs and CuCl2 were transformed to CuCl. CuCl could also react with carbon to form elemental Cu and PCDD/Fs, while more CuCl would be oxidized by O2 into intermediate copper oxychloride. And then the intermediate would react with Cl2 to form original CuCl2 to finish a life cycle. Takaoka et al. adopted and developed the mechanisms of Stieglitz, and the novel aspect was the consideration of regeneration of CuCl2. And there was some experimental support, which could explain the important effect of Cu on de novo process. Different forms of Cu (including CuO) may have catalytic effect on the condensation of CP, while only CuCl2 has effect on de novo process. CuCl2 is not only the catalyst of Deacon Reaction, but also chlorine source for PCDD/Fs formation, and CuCl2 has a stronger ability to provide chlorine than other substances. When CuCl2 provides chlorine, it transforms into CuCl, and can regenerate through the combined effect of O2 and Cl2/HCl.

PCDDs PCDFs

30000

15000 10000 5000 0 CuCl2

CuCl2+SO2

CuCl2+H2SO4

Fig. 5. The influence of SO2 and H2SO4 on the generation of PCDD/Fs with the catalyst of CuCl2 (Pekarek et al., 2007).

ð14Þ

However, why the addition of SO2 had no significant effect on PCDD/Fs formation has not been explained. Gullett et al. (2000a) revised their former conjecture later, and proposed SO2 transformed CuCl2 but not CuO into CuSO4. Ryan et al. (2006) also deemed that the effect of SO2 was to transform metal chloride in fly ash into sulfate. They found that without Cl2, CuO could not react with SO2. When Cl2 was introduced, oxides could be converted into chlorides. When SO2 was added, chlorides were converted into sulfates. This result negated Reaction (14). Their hypothesis about reactions was as follows.


2CuO þ Cl2 ! 2CuOCl

ð15Þ

2CuOCl þ 1=2O2 þ 2SO2 þ H2 O ! 2CuSO4 þ 2HCl

ð16Þ

Pekarek et al. (2007) carried out simulated de novo experiments using fly ash, CuCl22H2O, NaCl, and activated carbon as basis and N2 + 10%O2 as reaction gas. They found that SO2 could inhibit PCDD/Fs formation slightly, while sulfuric acid could inhibit the formation of PCDFs significantly, as shown in Fig. 5. Since the catalytic effect of CuO was low, it was CuCl2 rather than CuO that reacted with SO2, i.e.

SO2 þ CuCl2 þ H2 O þ 1=2O2 ! CuSO4 þ 2HCl

ð17Þ

Shao et al. (2010a) studied the effect of SO2 on de novo process of PCDD/Fs using a simulated system with CuCl2 and carbon. They found at 10% O2/N2 and 300 °C, SO2 could sulfate CuCl2, but could not react with CuO or CuCl2CuO. The addition of SO2 reduced PCDD/Fs formation significantly, and lowered the average chlorinated level of PCDD/Fs. Based on the above experimental results, the authors concluded that the conversion from CuCl2 and Cu2Cl2 to CuSO4 was the main mechanism of the inhibition effect of SO2. Another research found that under the atmosphere of high SO2 concentration, chlorine decreased in fly ash from 20 to 0.3 wt.%. Meanwhile, CuCl2 concentration in fly ash decreased, and PCDD/ Fs were reduced by 40% (Hunsinger et al., 2007). Recently, Wu et al. (2012) also proved that at temperatures lower than 600 °C, SO2 firstly reacts with CuCl2 to form CuSO4 through a thermodynamic equilibrium calculation. In fact, co-combustion of MSW or sewage sludge and sulfurcontaining coal or peat is a pathway of PCDD/Fs reduction (Yan et al., 2006; Zhang et al., 2013; Bajamundi et al., 2014a, 2014b, 2014c). In large incinerators, when MSW is burned blended with sulfur-containing coal, the emission of PCDD/Fs is reduced by 80% (Gullett et al., 2000b). Ogawa et al. (1996) studied the inhibition effect of sulfur sources on PCDD/Fs emission in a small-scale fluidized bed. The forms of sulfur included gas phase SO2, sulfurcontaining coal, and coal together with elemental sulfur. Under every condition, PCDD/Fs formation was reduced, but the inhibition effect of sulfur-containing coal was the most evident. In summary, the following conclusions can be reached. (1) The addition of sulfur-containing coal or SO2 into MSW fuel can reduce PCDD/Fs formation, but the inhibition effect of sulfur-containing coal is better than SO2. (2) There are two mechanisms that SO2 inhibits PCDD/Fs formation. One is SO2 reacts directly with Cl2 to form HCl, which hinders chlorination reactions. The other mechanism is that SO2 reacts with copper chloride to form low-reactivity CuSO4. The latter mechanism is dominant. 3.5. Water Since the moisture content of MSW is generally very high (Zhou et al., 2014), many researchers have studied the effect of water on PCDD/Fs formation. Briois et al. (2007) made precursor CP pass through non-carbon fly ash, and found that the addition of water inhibited PCDD/Fs formation significantly. The chlorinated level of PCDDs did not change, while the chlorinated levels of PCDFs turned to lower chlorinated levels. The influence of water may be driven by following mechanisms (Addink and Olie, 1995a): (a) To provide hydrogen source and lower the chlorinated levels of PCDD/Fs. (b) To provide oxygen source. (c) To provide –OH radicals.

113

(d) To compete with precursors for adsorption on fly ash surface. (e) To change the balance of Deacon Reaction. Wikstrom et al. (2003a) found that the addition of water promoted the oxidation of carbon, and decreased the total chlorine content and organic chlorine content in fly ash, thus PCDD/Fs shifted to low chlorinated substitutions. The authors contended that the addition of water transformed the chlorine to HCl, and that HCl was an inactive chlorine source, i.e. Mechanism (e) was preferred. With the presence of SO2, another mechanism was proposed (Liu et al., 2000):

Cl2 þ H2 O þ SO2 ! SO3 þ 2HCl

ð18Þ

H2O together with SO2 could transform Cl2 into HCl, but this mechanism was denied by the experiments of Gullett et al. (1992). Li et al. (2006a) studied the effect of waste on de novo reactions in a fixed reactor. They found water could promote the formation of PCDD/Fs, and the main pathway was to activate fly ash. Meanwhile, water could react with Cl2 to change the balance of Deacon Reaction. However, in the presence of CuCl2, water presented inhibition effect on PCDD/Fs formation, and the main reason might be that water consumed CuCl2 or inhibited the chlorination process by CuCl2. Shao et al. (2010b) found that the addition of water decreased PCDD/Fs formation by 96%, and meanwhile the addition of water increased the percentage of low-chlorinated PCDD/Fs. The authors found that the chlorine content in synthetic fly ash (CuCl2) decreased by 55% in the 10%O2/N2 atmosphere. When 10% H2O was added, the chlorine content decreased by 90%, indicating that H2O could promote the dechloridation of CuCl2 to form CuO with lower catalytic effect. The reaction process might be that:

CuCl2 þ H2 O ! CuO þ 2HCl

ð19Þ

In fact, with the presence of SO2, metal chlorides may react with SO2 and H2O, and the reactions may be (Matsuda et al., 2005):

MClx þ ðx=2ÞSO2 þ ðx=2ÞH2 O þ ðx=4ÞO2 ! ðx=2ÞM2=x SO4 þ xHCl MClx þ ðx=2ÞSO2 þ ðx=2ÞH2 O ! ðx=2ÞM2=x SO3 þ xHCl

ð20Þ ð21Þ

It should be noted that the above simulated de novo reactions are performed in real fly ash or synthetic fly ash with CuCl2. From Section 3.1.2, the CuCl2 in fly ash is a more effective chlorine source than Cl2 and HCl. Combining the experimental results of Li et al. (2006a), H2O is more likely to work with CuCl2 and transform CuCl2 to inactive CuO, or to CuSO3 and CuSO4 with the presence of SO2. 3.6. Ammonia and urea Ammonia is usually added in the post-combustion area because of the SCR or SNCR to reduce the NOx. The concentration of ammonia in real MSW incineration flue gas is between 100 and 1000 ppm (Kuzuhara et al., 2005). Also, urea is a low-cost compound that could probably be readily adapted to full-scale incinerators (Ruokojarvi et al., 2004). Ruokojarvi et al. (1998) studied the effect of NH3 on PCDD/Fs formation in the combustion of liquid fuel in a pilot-scale plant. The inhibitors were injected into the flue gas stream after the first economizer at 670 °C and just before the second economizer at 410 °C. Both PCDDs and PCDFs concentrations decreased when inhibitors were added. Ruokojarvi et al. (2004) also reported the clear reductions in PCDD/Fs by a maximum of 90% caused by urea in the same reactor. An empirical study was carried out by Kuzuhara et al. (2005) to clarify the suppression effect of ammonia and urea on the

114


formation of PCDD/Fs through the de novo synthesis. Graphite and copper chloride contained in a mixture were used as sources of carbon and chlorine, respectively. The amount of PCDD/Fs formed is reduced significantly by the addition of both ammonia and urea. Particularly, the addition of urea reduces the amount of PCDD/Fs by approximately 90%. Similar results were also obtained by Kasai et al. (2008). Recently, the influence of NH3 on PCDD/Fs generation was investigated by Hajizadeh et al. (2012) in a fixed bed reactor. When the flue gas went through the reactor with fly ash from MSW incinerator, with the injection of NH3 into the flue gas, the concentration of both PCDD and PCDF was decreased by 34–75% from the solid phase and by 21–40% from the gas phase. The total I-TEQ values of PCDD/Fs in the sum of the fly ash and exhaust flue gas were decreased by 42–75% and 24–57%, respectively. Changing the temperature from 225 to 375 °C within the tested range did not significantly affect the mechanism of PCDD/Fs inhibition by ammonia. Hajizadeh et al. also carried out a 96 h run, and there was strong evidence of the suppression of PCDD/Fs formation even under conditions suitable for their active de novo synthesis. This makes ammonia potentially suitable for use in the post-combustion zone. There are various mechanisms of the inhibition effect of NH3 on PCDD/Fs formation. Most of the mechanisms involve reducing the ability of metals to catalyze PCDD/Fs formation, just like the inhibition mechanisms of sulfur compounds. NH3 may alter the acidity of fly ash, form stable metal nitride bonds, or form stable metalammonia co-ordination compounds (Hajizadeh et al., 2012). Since nitrogen molecule with lone pair electrons can easily form stable complexes with transition metals and thus block the active sites on the particle surfaces, the mechanism of the formation of inactive complexes between the inhibitor and transition metals on the fly ash particle surface was supported by Ruokojarvi et al. (2004). Hajizadeh et al. (2012) reported that the separate analysis of solid-phase and gas-phase PCDD/Fs concentrations showed that ammonia may have affected the solid phase interactions involving the catalytic metals present in the fly ash. Some mechanisms also involve the reaction of nitrides and PCDD/Fs precursors. For example, during combustion NHi and CN radicals may form and can became attached to potential aromatic rings or may even be inserted into the rings forming aromatic amines, nitriles and pyridine-like structures (Kasai et al., 2008). Organic compounds containing amino (–NH2) or cyanide (–CN) groups and those containing nitrogen within the carbon ring frame were detected in the outlet gas in the case of urea addition by Kuzuhara et al. (2005). This suggests a possibility that hydrogen and/or chlorine combined with PCDD/Fs are also substituted by such nitrogen-containing groups, and this reduces the formation rate of their frame of carbon rings. Also, Kuzuhara et al. did not deny the interaction of nitrogen-containing compounds with copper chloride existing on the carbon surface. In brief, NH3 and urea definitely have inhibitive effect on PCDD/ Fs formation, and approximately a 90% reduction rate by urea was reported. NH3 and urea are lost-cost compounds and applied in the SCR or SNCR process, which makes them may suitable for application at post-combustion zone of real MSW incinerators. The mechanisms of the effect of NH3 and urea are still not clear, and there are two possibilities. The first one involves the reduction of the ability of metals to catalyze PCDD/Fs formation, and the second one involves the direct reaction of nitrides and PCDD/Fs precursors.

4. PCDD/Fs indicators Since the PCDD/Fs concentration in flue gas is usually very low, and there are numerous homologues, PCDD/Fs measurement is not

only complicated but also costly. An increasing number of studies are trying to develop a high-concentration and easy-to-measure PCDD/Fs indicator, thus the concentration and toxicity of PCDD/ Fs can be calculated from this indicator. These investigated indicators include CO (Yoneda et al., 2002; Weber et al., 2002), PAHs (Li et al., 2006b; Streibel et al., 2007; Yan et al., 2010), CBz (Streibel et al., 2007; Yoneda et al., 2002; Oser et al., 1998; Zimmermann et al., 1999; Kato and Urano, 2001b), CP (Yoneda et al., 2002; Tuppurainen et al., 1999; Yamada et al., 2004), low-volatile organohalogen compounds (LVOH) (Watanabe et al., 2010; Kato et al., 2000), and some specific PCDD/F (Kato and Urano, 2001a; Gullett and Wikstrom, 2000; Iino et al., 2003). According the present study, CBz and PAHs are two indicators that are positively and strongly correlated with PCDD/Fs. 4.1. Chlorobenzene Typical CBz concentration of MSW incinerator is 3 lg N m3 (Jay and Stieglitz, 1995). Since CBz is strongly correlated with international toxic equivalent quantity (I-TEQ) of PCDD/Fs, it has attracted extensive attention (Zimmermann et al., 1999; Kato and Urano, 2001a, 2001b; Kaune et al., 1998; Blumenstock et al., 2001). Manninen et al. (1996) applied partial least squares (PLS) in correlation of CBz and PCDD/Fs, and found that the accuracy of PCDD/ Fs and I-TEQ could be improved by multiple regression. The disadvantage of this method is that too many parameters called for measurement. Research found that there was a strong correlation between CBz and PCDD/Fs, and the correlation coefficient was 0.93. Especially, hexachlorobenzene (HCBz) had the following relation with PCDD/ Fs: PCDD/Fs = 0.34 H6CBz1.1, and the correlation coefficient was as high as 0.98 (Yoneda et al., 2002). Based on the analysis of 10 MSW incinerators and a large body of literature, Kato and Urano (2001a) found that I-TEQ and total PCDD/Fs concentration in flue gas had good correlation with the concentration of Cl4-6BZs and pentachlorobenzene (PeCBz). To measure I-TEQ easily or monitor the change of I-TEQ with time, PeCBz was recommended as an indicator, because PeCBz did not have isomerides and the toxicity of PeCBz was lower than that of HCBz. Blumenstock et al. (2000) analyzed the flue gas of a pilot-scale incinerator, and found that monochlorobenzene (MCBz) and DCBz are strongly correlated with I-TEQ. Later, Blumenstock et al. (2001) further studied the possibility of different kinds of CBz as I-TEQ indicator, and found that 1,2,3,4-tetrachlorobenzene (TeCBz) had the highest correlation coefficient with I-TEQ (0.91), but it was difficult to measure online. MCBz had a correlation coefficient of 0.85 with I-TEQ, which was an ideal product for online detection. The research of Pandelova et al. (2006) showed that the best indicator for the sum of PCDD/F World Health Organization toxic equivalent (WHO-TEQ) and PCB WHO-TEQ was 1,2,4,5-TeCBz, and the correlation coefficient was 0.82. In fact, CBz is proved to be a better PCDD/Fs indicator than PCB and CP (Pandelova et al., 2006; Lavric et al., 2005); the reason may be that CBz has the similar formation mechanism with PCDD/Fs. In fluidized bed reactor, the PCDD/Fs concentration and CBz concentration in fly ash presents similar pattern with the change of temperature and residence time, and the best formation temperature is 340 °C (Faengmark et al., 1994). PCDD/Fs and CBz are generated together at convective heat exchanger zone of wood combustion, indicating these two compounds have similar de novo formation mechanisms (Andersson and Marklund, 1998). This conclusion was supported by Lavric et al. (2005) later. Kuribayashi et al. developed a vacuum ultra-violet single-photon ionization ion trap time-of-flight mass spectrometer (VUV-SPI-IT-TOFMS) system for real-time monitoring of


trichlorobenzene (TrCBz) as PCDD/Fs indicator. The system had an exceeding robust performance and was able to maintain the high sensitivity in analyzing TrCBz in actual waste incineration flue gas for long months of operation (Kuribayashi et al., 2005). Gullett et al. (2012) applied gas chromatography coupled to a resonance-enhanced multiphoton ionization – time-of-flight mass spectrometry (GC-REMPI-TOFMS) system to monitor CBz online as PCDD/Fs indicators. The results showed that the correlation between 1,2,4-TrCBz and I-TEQ was 0.85 in 5 min, and the correlation between 1,2,4-TrCBz and Total measures of PCDD/Fs was 0.89. 4.2. Polycyclic aromatic hydrocarbons As shown in Section 2.2, PAHs, as incomplete combustion products, have very important relationship with PCDD/Fs formation (Iino et al., 1999a, 1999b). In addition, PAHs have the advantage of high concentration in flue gas as PCDD/Fs indicator. Measurement shows that the PAHs emission of sewage sludge combustion is 3.84–8.35 lg m3; the PAHs emission of medical waste is 8.92– 10.70 lg m3, which is 1000–10,000 times higher than PCDD/Fs concentration (Dyke et al., 2003). Research found that the tendency of total PCDFs is similar to that of total PAHs, and benzo[a]anthracene may be a reliable indicator of PCDFs (Li et al., 2006b). Yan et al. (2010) measured PCDD/ Fs and PAHs of 4 MSW incinerators of 2 typical combustion technologies. The results showed that the PAHs from grate furnace of MSW were mainly naphthalene and fluorene; while the PAHs from fluidized bed furnace of coal blended with MSW were mainly phenanthrene and fluoranthene. The unitary linear correlation coefficient between fluorene and I-TEQ was 0.62, and the ternary linear correlation coefficient of naphthalene, fluorene, and phenanthrene was 0.85. Heger et al. (1999) developed a resonance-enhanced multiphoton ionization – time-off-light mass spectrometer (REMPI-TOFMS) for online measurement of PAHs of MSW incinerators. The limit of detection for naphthalene was 10 pptv. It was demonstrated that REMPI-TOFMS enabled a real-time on-line trace analysis of PAHs of combustion flue gases or industrial process gases. 4.3. Other indicators It is commonly believed that the total concentration of PCDD/Fs and Co-PCB in the exhaust gas from incinerators decreases when its CO content is low. In a fluidized-bed furnace that enabled a relatively stable combustion, a distinct correlation of CO and total concentration of PCDD/Fs and Co-PCB was observed (Yoneda et al., 2002). The correlation of PCDD/Fs levels with the CO emissions in an industrial MSW incinerator was assessed during a four-week measurement by Weber et al. (2002). The CO concentration during the sampling time showed no significant correlation to the PCDD/Fs amount in fly ash. A correlation was found for the time integrated CO values of 3 and 4 h before sampling (R2 = 0.467 and R2 = 0.4577 respectively). This indicates a memory effect in the high temperature cooling section of several hours. The correlation between gas-phase CP and particle-phase PCDD/Fs was demonstrated by deriving predictive PLS models. The predictive ability of both individual congeners and congener classes was considerable and in some cases the cross-validated correlation coefficients were high. However, the accuracy of the PLS models was clearly poorer for the highly chlorinated congeners (Tuppurainen et al., 1999). By using fluidized-bed furnace and rotary-kiln + stoker furnace and four different kinds of industrial wastes such as waste food, coffee mill, waste oils and waste plastics, a correlation (R = 0.88) between CP and total concentration of PCDD/Fs and Co-PCB was observed (Yoneda et al., 2002).

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During the startup of two MSW incinerators, the correlation of LVOH with I-TEQ was good. The correlation of LVOH with I-TEQ involved a memory effect related to the delayed emission of less volatile compounds (Watanabe et al., 2010). A correlation of ITEQ values of PCDD/Fs and the concentration of semi- and nonvolatile organic halogen (SNVOX) was also found by Kato et al. (2000), and I-TEQ could be estimated approximately from SNVOX. It has been found that positive correlations of the I-TEQ values with the concentration of 2,3,4,7,8-P5CDF for a large number of different incineration facilities and in a wide concentration range (Kato and Urano, 2001a). Consequently, the concentration of 2,3,4,7,8-P5CDF whose measuring methods are much easier than that of obtaining the I-TEQ values, could be used as convenient substitute indices to the I-TEQ values for controlling and monitoring dioxins in stack gas from various waste incineration facilities (Kato and Urano, 2001a). An analysis of emission data from two facilities indicated that concentrations of select mono- to triCDD/Fs showed promising correlation with I-TEQ across facilities, suggesting that these compounds could act as I-TEQ indicators (Gullett and Wikstrom, 2000). 5. Conclusion and outlook PCDD/Fs, as one of the most hazardous substances, are the key factor that restricts the application MSW incineration. From the 1970s to present, studies on PCDD/Fs are numerous. A number of important conclusions have been reached, while the mechanisms of PCDD/Fs formation and the influence of many factors remain debated. The ratio of homogeneous to heterogeneous PCDD/Fs in real incinerator and the sources of heterogeneous are not clear. The conclusions from different furnace types and combustion conditions may be various, which needs to be studied and summarized by future research. As the most important source of chlorine during incineration, HCl may come from table salt (NaCl) or PVC, while the mechanisms underlying the effect of HCl on PCDD/Fs formation call for further research. Furthermore, the present article recommends future studies investigate how to control the precursors, residual carbon, chlorine, O2, Cu to destroy the conditions for PCDD/Fs formation and how to apply sulfur, water, and nitrogen compounds to remove PCDD/Fs. Finally, due to the high concentration, low detection cost, and simplicity for online monitoring, as well as strong correlation with PCDD/Fs, CBz and PAHs are worth applying in industrial incinerators as PCDD/Fs indicators. Acknowledgements The financial support from National Basic Research Program of China (973 Program, No. 2011CB201502) is gratefully acknowledged. Special thanks and love to Talia Wei for the language improvement. References Addink, R., Altwicker, E.R., 1998. Role of copper compounds in the de novo synthesis of polychlorinated dibenzo-p-dioxins/dibenzofurans. Environ. Eng. Sci. 15, 19– 27. Addink, R., Altwicker, E.R., 2001. Formation of polychlorinated dibenzo-p-dioxins/ dibenzofurans from residual carbon on municipal solid waste incinerator fly ash using (NaCl)-Cl-37. Chemosphere 44, 1361–1367. Addink, R., Olie, K., 1995a. Mechanisms of formation and destruction of polychlorinated dibenzo-p-dioxins and dibenzofurans in heterogeneous systems. Environ. Sci. Technol. 29, 1425–1435. Addink, R., Olie, K., 1995b. Role of oxygen in formation of polychlorinated dibenzop-dioxins/dibenzofurans from carbon on fly-ash. Environ. Sci. Technol. 29, 1586–1590. Addink, R., Bakker, W.C.M., Olie, K., 1995. Influence of HCI and Cl2 on the formation of polychlorinated dibenzo-p-dioxins/dibenzofurans in a carbon/fly ash mixture. Environ. Sci. Technol. 29, 2055–2058.

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