Chemosphere 123 (2015) 9–16

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PCDD/Fs’ suppression by sulfur–amine/ammonium compounds Jian-Ying Fu, Xiao-Dong Li ⇑, Tong Chen, Xiao-Qing Lin, Alfons Buekens, Sheng-Yong Lu, Jian-Hua Yan, Ke-Fa Cen State Key Laboratory of Clean Energy Utilization, Institute for Thermal Power Engineering, Zhejiang University, China

h i g h l i g h t s  Thiourea could inhibit 97.3% of PCDD/Fs with a (S + N)/Cl molar ratio of 0.47. +

 The amine functional group –NH2 is more efficient than ammonium NH4 .  The inhibition effect of PCDD/Fs by thiourea is better at 350 °C than that at 650 °C.

a r t i c l e

i n f o

Article history: Received 25 June 2014 Received in revised form 20 September 2014 Accepted 27 October 2014 Available online 4 December 2014 Handling Editor: H. Fiedler Keywords: PCDD/Fs Thiourea Aminosulfonic acid Ammonium thiosulfate Model fly ash Inhibition

a b s t r a c t Three distinct –S and –NH2 or NH+4 containing compounds, including ammonium thiosulfate, aminosulfonic acid and thiourea, were studied as polychlorinated dibenzo-p-dioxins and dibenzofurans (PCDD/Fs) inhibitors. All these three –S and –N containing compounds tested show strong suppression of PCDD/Fs formation, especially for thiourea which has not been studied before. With a (S + N)/Cl molar ratio of only 0.47, thiourea could inhibit 97.3% of PCDD/Fs and even 99.8% of I-TEQ. At an unusually high de novo test temperature (650 °C), the PCDD/Fs’ formation was still very low but also the inhibition capacity of thiourea was weak, with an efficiency of 59% for PCDD/Fs when with a (S + N)/Cl molar ratio of 1.40. The results also revealed that the inhibition capability of the combined –S/–NH2 or –S/NH+4 suppressant was strongly influenced by both the nature of the functional group of nitrogen and the value of the molar ratio (S + N)/Cl. The amine functional group –NH2 tends to be more efficient than ammonium NH+4 and within a certain range a higher (S + N)/Cl value leads to a higher inhibition efficiency. Moreover, the emission of gases was continuously monitored: the Gasmet results revealed that SO2, HCN and NH3 were the most important decomposition products of thiourea. Thiourea is non-toxic, environment-friendly and can be sprayed into the post-combustion zone in form of powder or aqueous solution. The cost of thiourea at least can be partially compensated by its high inhibition efficiency. Therefore, the application of thiourea in a full-scale incinerator system is promising and encouraging. Ó 2014 Elsevier Ltd. All rights reserved.

1. Introduction A fast developing economy is swelling significantly the amount of the municipal solid waste (MSW) stream in China. Consequently, ensuring adequate MSW disposal is becoming increasingly urgent. Incineration has been considered as a quite promising volumereduction technique (Duan et al., 2008; Mari et al., 2009). Since 2004, the share of MSW incineration in waste disposal has been steadily increasing, which reached 24.7% by 2012 (National Bureau of Statistics of China). However, considerable attention has to be paid to toxic by-products, such as polychlorinated dibenzo-p-dioxins and dibenzofurans (PCDD/Fs) (Olie et al., 1977; Liu et al., 2006; ⇑ Corresponding author. Tel.: +86 571 87952037. E-mail address: [email protected] (J.-Y. Fu). http://dx.doi.org/10.1016/j.chemosphere.2014.10.073 0045-6535/Ó 2014 Elsevier Ltd. All rights reserved.

Lee et al., 2007; Zheng et al., 2008). Specifically in China, many novel techniques are being explored to reduce these hazardous emissions. Chemical inhibition has emerged as a popular option since it can efficiently prevent PCDD/Fs’ formation at source (Hans Hunsinger, 2012). Ideally, an appropriate inhibitor which can inhibit the PCDD/Fs’ formation should be efficient, non-toxic, low-cost and environment-friendly. Inhibitors can be generally subdivided into four groups: (1) alkaline substances (e.g., CaO, KOH, NaHCO3, and Na2CO3), controlling acid gases (HCl) in the combustion gases or reducing the acidity of the fly ash reaction surface (Samaras et al., 2000; Ruokojarvi et al., 2004; Liu et al., 2005); (2) sulfur-based suppressants (e.g., SO2, SO3, Na2S, and mercaptans) (Ruokojärvi et al., 1998; Pandelova et al., 2005; Shao et al., 2010); (3) nitrogen-containing compounds (e.g., ammonia, urea, and dimethylamine) (Ruokojärvi et al., 1998; Samaras et al.,

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2001; Hajizadeh et al., 2012); and (4) compounds containing functional groups (–COOH, etc.) and compounds that easily combine with metal ions, such as ethylenediaminetetraacetic acid, nitrilotriacetic acid (Addink et al., 1996). Among all of these, inhibitors containing either sulfur or nitrogen have been most widely studied. Distinct inhibitors also affect the formation routes of PCDD/Fs differently (Ruokojarvi et al., 2004). The inhibition mechanism of –S containing compounds was generally explained by one or more of the following four steps: (1) they could react with chlorine and deplete chlorine resources (Gullett et al., 1992; Raghunathan and Gullett, 1996); (2) they could react with transition metal ions and weaken their catalytic activity (Gullett et al., 1992); (3) they could form sulfate compounds and deactivate the catalytic site (Lindbauer et al., 1994; Ryan et al., 2006); and (4) they could react with dioxin precursors and form poly-chlorodibenzothiophene (PCDT) and poly-chlorothianthrenes (PCTA) (Gullett et al., 1992). Studies on –N inhibitors are less abundant than those of –S inhibitors. PCDD/Fs’ formation is suppressed by specific functional –N containing groups like amine (–NH2), ammonium (NH+4), amide (–CONH2), nitrile (–CN), etc. NH3 (formed by thermal decomposition of compounds such as urea and (NH4)2SO4), could react with HCl/Cl2 and inhibit PCDD/Fs’ formation by depleting chlorine source (Takacs and Moilanen, 1991). Some –N inhibitors (e.g., urea) can react with dioxin precursors to form PCDD/Fs analogues, like chlorobenzonitriles, chlorobenzeneamines, and chloropyridines (Kuzuhara et al., 2004; Kasai et al., 2008). Another two influencing factors are vapor pressure (Hell et al., 2000) and the aliphatic/aromatic structure of an inhibitor. Lippert et al. (1991) studied the inhibition with employing ethanolamine. They found that –NH2 inhibit the catalytic action of copper with the only exception that using tertiary amines as the suppressant. The lone pair electrons in –NH2 make it much easier to form stable complexes with metals (Ismo et al., 1997). As a consequence, weakening the catalytic activity of transition metals is a potential mode of action of –NH2 inhibitors. However, organic compounds containing –NH2 within the carbon ring frame were detected with the addition of –N inhibitor (Kuzuhara et al., 2004). It suggested a possibility that hydrogen and/or chlorine combined with PCDD/Fs could be substituted by such nitrogen containing groups. Inhibitors containing –S or –NH2/NH+4 compounds share some common features, even though the detailed inhibition route is seriously dissimilar. The inhibition reactions of sulfur or nitrogen compounds are inclined to be synergistic rather than competitive. Pandelova et al. (2005) studied the PCDD/Fs’ suppression by 20 distinct, thermally resistant inorganic compounds with dividing them into four subgroups. The group combining sulfur and amine/ ammonium suppressants ((NH4)2SO4, (NH4)2S2O3, CO(NH2)2 + S, etc.) were proved as the most effective ones. An addition of 10 wt.% ammonium thiosulfate (ATS) of the fuels led to a 98% emission reduction of PCDD/Fs, followed by (NH4)2SO4 with 97%. The suppression effects of hydroxylamine-o-sulfonic acid (HOSA), sulfamide (SA) and aminosulfonic acid (ASA) were also investigated (Samaras et al., 2000). When 1 wt.% ASA of the fuels was added, the inhibition efficiency of PCDD/Fs was up to 96%. Yet all the researchers regarded these suppressants as sulfur containing compounds and no attempt was made to explain the inhibition effects of combining S–NH2 or S–NH+4 (Samaras et al., 2000; Pandelova et al., 2005, 2007). To this end, three compounds were picked out for tests: ammonium thiosulfate (ATS, (NH4)2S2O3), aminosulfonic acid (ASA, NH2–OSO–OH) and thiourea (TUA, CS(NH2)2). Experiments were carried out with these compounds to verify the conjecture that compounds containing both sulfur and nitrogen (herein it refers in particular to sulfur and amine or ammonium) would suppress PCDD/Fs’ formation more effectively. Both ATS and ASA were

selected on the basis of previous studies (Samaras et al., 2000; Pandelova et al., 2007). TUA was selected because of the following considerations: (1) TUA was studied here for the first time as PCDD/Fs’ suppressant; (2) the share of functional group (–NH2) and the element S seemed to be highest in CS(NH2)2, compared with all other compounds explored (Samaras et al., 2000; Ruokojarvi et al., 2004; Pandelova et al., 2007). Subsequently, a series of tests were performed with TUA under various conditions. The study on the exploration of high-effective novel inhibitors and their potential inhibition mechanism would contribute to the application of low-cost inhibitors in a full-scale incinerator system. 2. Materials and methods 2.1. Materials In most incineration and metallurgical processes PCDD/Fs are formed mainly by de novo formation with the existence of organic/inorganic chorine sources, in which fly ash is the key catalyst carrier. In this study a synthetically composed model fly ash was used to simulate PCDD/Fs’ formation during a de novo test, as such, or else suppressed by ATS (ammonium thiosulfate), ASA (aminosulfonic acid) and TUA (thiourea), with S (N) contents (in wt.%) of 33 (14), 43 (19), and 42 (37), respectively. Compared to natural fly ash, using model fly ash allows minimizing the variations in chemical composition and to improve the homogeneity of the test samples (Fangmark et al., 1991). The composition of model fly ash was based on the study of Ke et al. (2010): (in wt.%) 91.8 SiO2, 3 activated carbon, 5 NaCl and 0.2 CuCl2, corresponding to a fly ash load of ca. 3.1 wt.% Cl, 0.1 wt.% Cu, and 3 wt.% C. The model fly ash was ca. 100–150 mesh and the detailed sample preparation of model fly ash can be obtained in another reference (Ke et al., 2010). 2.2. Laboratory-scale experiments A laboratory scale system was used (Fig. 1) to try and attain reproducible and repeatable experimental results and avoid the complexity of a full-scale furnace (Samaras et al., 2000). It comprises a tubular isothermal furnace consisting of three sections a, b, and c (0.5 m for each section), featuring independent heaters and temperature controllers to maintain the desired temperature profile. The external and internal diameter of the quartz reactor tube was 45 mm and 30 mm, respectively. The inhibitors were well mixed with model fly ash (except for test M) and positioned inside the internal quartz tube using silica wool layer (cf. the partial enlarged drawing on top of Fig. 1). Then the quartz tube was placed into the furnace which had been heated up to the test temperature. The bulk residence time and the experimental reaction time were ca. 68 s and 50 min, respectively. The simulated flue gas (300 ml min1, 12 vol.% O2 in N2) flowed through the reactants in the model fly ash, triggering the de novo reactions. In each test run the amount of model fly ash was 2 g. The experimental conditions were listed in Table 1 and explained in Experimental Design (see Section 2.3). 2.3. Experimental design The test series F grouped the different de novo PCDD/Fs’ formation tests conducted without inhibitor and serving as baseline: F-1 at usual (350 °C) and F-2 at high test temperature (650 °C), respectively. The base test F-1 was conducted 5 times, with the purpose of establishing a statistically valid error analysis. In the other test series, inhibitors were added to the model fly ash. Series I explored suppression by three different –S and –N

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Fig. 1. Schematic of the reactor system: 1-mass flow-meter; 2-inner tube; 3-tube furnace; 4-glass cotton; 5-model fly ash; 6-XAD – II resin; 7-ice bath; 8-toluene; 9-outer tube; 10-temperature controller.

Table 1 Experimental test design conditions: inhibitor, temperature, (S + N)/Cl molar ratio. No.

Inhibitor

Temperature (°C)

Ratio

No.

Inhibitor

Temperature (°C)

Ratio

F-1 F-2 I-1 I-2 I-3

None None 0.06 TUA 0.06 g ASA 0.06 g ATS

350 ± 5 650 ± 5 350 ± 5 350 ± 5 350 ± 5

0 0 1.40 0.71 0.93

A-1 A-2 T Ma

0.02 g 0.10 g 0.06 g 0.06 g

350 ± 5 350 ± 5 650 ± 5 350 ± 5

0.47 2.34 1.40 1.40

TUA TUA TUA TUA

Notes: the experimental conditions of the PCDD/Fs’ formation tests F and the inhibition tests (all other tests) were: 50 min, 12 vol.% O2 in N2. a In test M, TUA was placed before MFA by the silica wool layer, and this was the only difference between test I-1 and test M.

containing compounds. Series A, T and M concentrated on TUA tested under different reaction conditions: tests A were conducted at three different (S + N)/Cl molar ratios; T tested the temperature effect at 650 °C on suppression efficiency compared to the test F-2 at the same high temperature; in test M, instead of being mixed in before the test, TUA was placed upstream of the model fly ash and separated from model fly ash by a layer of silica wool (Fig. 1). Thus the resulting inhibition may be ascribed to the emanating thermal decomposition products of TUA, since there is no direct contact between TUA and the model fly ash subjected to the de novo test. 2.4. PCDD/Fs’ analysis All 136 congeners (TCDDs to OCDF) were analyzed and the seventeen 2,3,7,8-substituted PCDD/Fs’ isomers were discussed in detail. The clean-up procedure of the PCDD/Fs’ samples was conducted according to the USEPA 1613 method (U.S EPA, 1994). The PCDD/Fs were identified and quantified by HRGC/HRMS using a 6890 Series gas chromatograph (Agilent, USA) coupled to a JMS800D mass spectrometer (JEOL, Japan). A DB-5 ms (60 m  0.25 mm I.D., 0.25 lm film thickness) capillary column was used for separating the PCDD/F-congeners (Chin et al., 2011; Li et al., 2011; Liu et al., 2013). The optimized GC temperature program was as follows: splitless injection of 1 ll at 150 °C, initial oven temperature of 150 °C for 1 min, then increased at 25 °C min1 to 190 °C, finally increased at 3 °C min1 to 280 °C and held for 20 min at the final temperature. All test experiments were conducted at least in duplicate to ensure experimental quality. 3. Results and discussions 3.1. PCDD/Fs’ inhibition using combined S–N inhibitors 3.1.1. Formation and inhibition efficiency 3.1.1.1. Tests F-1. The high average concentration (2385 ± 222 ng g1 and 39.4 ± 10.5 ng I-TEQ g1) formed from per gram model

fly ash during these tests confirmed the quality of the model fly ash as source of PCDD/Fs (Table 2). The formation level was substantial and allowed investigating a potential reduction by the inhibitor by even three orders of magnitudes (Pandelova et al., 2007). The ratio of 2,3,7,8-PCDDs to 2,3,7,8-PCDFs (ca. 40 wt.%) showed a preferred formation of PCDFs over PCDDs according with other researches (Tuppurainen et al., 1998; McKay, 2002; Stanmore, 2004). The high weight average chlorination degree (6.68 ± 0.09) of all 136 PCDD/Fs indicated strong catalytic chlorination ability of CuCl2. 2,3,4,7,8-PeCDF made the highest (28.9%) contribution to the I-TEQ value. A strong correlation between the concentration of 2,3,4,7,8-PeCDF and the I-TEQ value was also quoted in numerous studies (Fiedler et al., 2000; Kato and Urano, 2001; Iino et al., 2003; Chen et al., 2008). 3.1.1.2. Suppression tests. All the three sulfur–nitrogen inhibitors showed significant suppression of both total PCDD/Fs and I-TEQ (Table 2). PCDD/Fs’ inhibition efficiencies of TUA and ASA both exceeded 90%, while ATS was slightly less efficient with ca. 85%. All efficiencies for the suppression of I-TEQ ranged from 87.4% to 99.8%, with the same order as for PCDD/Fs: TUA > ASA > ATS. TUA, ASA and ATS would decompose when heated, and thus the sulfur contained could be converted into either sulfur or oxidation products SO2/SO3. These decomposition products are also able to inhibit PCDD/Fs’ formation, explaining at least part of the potential inhibitory role of sulfur in these inhibitors. The functional group of nitrogen is -NH2 in both TUA and ASA, while it is NH+4 in ATS. Eleven inhibitors containing ammonium- and hydroxyl-functional groups were evaluated to investigate the suppression of PCDDs’ formation in the fly ash (Dickson et al., 1989). 2-Aminoethanol and triethanolamine turned out to be the most effective. The researchers attributed this to the ability of poly-functional amines to form poly-dentate ligand complexes with metallic active sites situated on the fly ash surface, even though these poly-functional compounds might also react with several adjacent active sites simultaneously. Some other compounds consisting of organic

J.-Y. Fu et al. / Chemosphere 123 (2015) 9–16

amines such as H2N–SO4H, H2N–SO2–NH2 (SA) also effectively suppressed PCDD/Fs’ formation (Dickson et al., 1989; Samaras et al., 2000; Pandelova et al., 2007). Primary amines would convert Cu into copper nitride and thus deactivate the catalyst by the formation of nitride layers (Lippert et al., 1991). The formation of copper nitrides can besides block active sites and further contributes to the reduction of PCDD/Fs. However, the substitution of hydrogen and/or chlorine combined with PCDD/Fs by –NH2 was also a potential inhibition pathway (Kuzuhara et al., 2004). All these observations explain the effective suppressive function of –NH2 in TUA and ASA. Probably due to the –NH2 functional group ASA was more efficient than ATS, despite the higher (S + N)/Cl-ratio of ATS (0.93). The –N contained in ATS is similar to that in (NH4)2SO4: due to the presence of NH+4, NH3 would be released when ATS is heated. As shown in Table 1, the (S + N)/Cl molar ratio of TUA (1.40) is higher than that of ASA (0.71), so theoretically more transition metal can be neutralized by TUA than that by the same amount of ASA. Thus it is concluded that the type of functional group of nitrogen and the (S + N)/Cl molar ratio are the major factors in PCDD/Fs inhibition.

Notes: the weight average level of chlorination degree (Cl-PCDD, Cl-PCDF, Cl-PCDD/Fs) was analyzed on the base of all 136 PCDD/Fs (TCDD to OCDF). a

209 ± 102 604.2 ± 194 35 ± 6.1 813 ± 296 5.1 ± 1.4 65.9 ± 11.7 87.2 ± 1.8 7.84 ± 0.01 7.21 ± 0.04 7.33 ± 0.05

ng PCDD g1 model fly ash ng PCDF g1 model fly ash %, PCDDs/PCDFs ng PCDD-F g1 model fly ash ng I-TEQ g1 model fly ash %, PCDD/Fs’ %, I-TEQ Weight average level of chlorination degreea

M

0.186 ± 0.027 0.397 ± 0.012 47 ± 5.5 0.58 ± 0.03 0.021 ± 0.003 59 ± 2.6 45 ± 8.6 5.85 ± 0.09 6.15 ± 0.01 6.05 ± 0.03 0.47 ± 0.1 0.96 ± 0.1 48 1.4 ± 0.2 0.039 ± 0.014 0 0 6.40 ± 0.13 6.39 ± 0.01 6.40 ± 0.04 0.36 ± 0.12 4.21 ± 1.05 9 ± 0.3 4.6 ± 1.2 1.02 ± 0.08 99.81 ± 0.09 99.74 ± 0.1 6.20 ± 0.36 6.67 ± 0.14 6.63 ± 0.16

A-2 A-1

10.1 ± 4.2 53.7 ± 10.1 19 ± 2 64 ± 19 0.9 ± 0.7 97.32 ± 2.1 99.81 ± 0.88 7.65 ± 0.24 6.96 ± 0.17 7.03 ± 0.20 66 ± 27 283 ± 76 23.2 ± 3.3 349 ± 51 4.97 85.4 ± 4.1 87.4 ± 0.02 7.60 ± 0.12 6.74 ± 0.18 6.85 ± 0.19

I-3 I-2

30.63 ± 17.14 150 ± 62 20 ± 2.7 180.7 ± 79.6 2.35 ± 1.0 92.42 ± 3.3 94.03 ± 4.6 7.57 ± 0.01 6.86 ± 0.03 6.94 ± 0.03 0.290 ± 0.009 4.25 ± 0.32 7 ± 0.3 4.54 ± 0.33 0.07 ± 0.019 99.81 ± 0.01 99.82 ± 0.03 6.21 ± 0.32 6.91 ± 0.06 6.86 ± 0.09 687 ± 94 1698 ± 175 40 ± 3 2385 ± 222 39.4 ± 10.5 0 0 7.45 ± 0.05 6.51 ± 0.10 6.68 ± 0.09 PCDDs PCDFs PCDDs/PCDFs RPCDD/Fs’ I-TEQ PCDD/Fs’ inhibition, % I-TEQ inhibition, % Cl-PCDD Cl-PCDF Cl-PCDD/Fs’

I-1 F-1 Items

Table 2 Results regarding the inhibition of 2,3,7,8-subsitituted PCDD/Fs of various inhibitors and under various conditions with TUA.

F-2

T

Unit

12

3.1.2. Inhibition effects on the PCDDs/PCDFs ratio and the congener distribution The effect of inhibitors on the ratio of PCDDs to PCDFs and on their congener pattern varied with experimental conditions, e.g. the nature of the inhibitor, residence time and injection temperature (Ruokojarvi et al., 2004). In our study, inhibitors led to different levels of the PCDDs/PCDFs ratio and the weight average chlorination degree. Suppression of PCDDs was always stronger than that of PCDFs, in agreement with former work on sulfur-based inhibitors and NH3 (Ruokojarvi et al., 2004; Pandelova et al., 2007; Hajizadeh et al., 2012), indicating different formation pathways and inhibition mechanisms of PCDDs and PCDFs. The weight percentage distribution of the 17 toxic congeners and the weight average level of chlorination degree of all 136 PCDD/Fs are shown in Fig. 2 and Table 2, respectively. In accordance with the PCDDs homologue distribution in other studies (Samaras et al., 2000; Pandelova et al., 2005), the addition of ASA (I-2) and ATS (I-3) led to a lower proportion of low-chlorinated PCDDs and slight increase of the relative amount of OCDD, attributed both to the suppression of catalytic chlorination reactions by sulfur. The obvious rise of low-chlorinated PCDDs in test I-1 was attributed to the de-chlorination of 1,2,3,4,6,7,8-HpCDD, indicating a stronger capability of the amine group in TUA of promoting catalytic de-chlorination reactions. Similar to other research (Samaras et al., 2000; Pandelova et al., 2005), a higher chlorination of PCDF was observed; in turn this rising chlorination degree led to the higher inhibition efficiency recorded for I-TEQ than for the sum 2,3,7,8-PCDD/Fs (Table 2). In tests I-1 to I-3 the 2,3,4,7,8-PeCDF still contributed most to the I-TEQ value, with values of 29.1%, 24.9% and 26.2%, respectively. The second largest I-TEQ contributor turned from 2,3,4,6,7,8-HxCDF (F-1) to 1,2,3,4,6,7,8-HpCDF (I test series). Considered the declining distribution of OCDF, the corresponding escalation of 1,2,3,4,6,7,8-HpCDF may be caused by de-chlorination of OCDF. According to these experimental results, compounds containing S–NH2 or S–NH+4 can inhibit PCDD/Fs and I-TEQ effectively and indeed share some common suppression characteristics (e.g. the same major contributor to I-TEQ, stronger suppression of PCDDs, etc.). 3.2. TUA experiments TUA has the highest suppression efficiency of the three compounds tested in part 3.1., so that further experiments were warranted.

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PCDD Fingerprint

2378-T4CDD 12378-P5CDD 123478-HxCDD 123678-HxCDD 123789-HxCDD 1234678-HpCDD OCDD

100 90 80

Fraction / %

70 10 8 6 4 2 0

F-1

I-1

I-2

A-1

A-2

F-2

T

M

PCDF Fingerprint

70 60 50 40 30 20

Fraction / %

I-3

2378-TCDF 12378-PeCDF 23478-PeCDF 123478-HxCDF 123678-HxCDF 123789-HxCDF 234678-HxCDF 1234678-HpCDF 1234789-HpCDF O8CDF

4

2

0

F-1

I-1

I-2

I-3

A-1

A-2

F-2

T

M

Fig. 2. Effect of S–N inhibitors on the distribution of the toxic 2,3,7,8-substituted PCDD/Fs.

3.2.1. TUA experiments with different (S + N)/Cl molar ratios The amount of inhibitor added intensely influences suppression efficiency. The PCDD/Fs’ inhibition is mainly related to the functional element (–S or –N) or groups (–NH2 or NH+4) in the inhibitor. Therefore suppression is strongly influenced by the molar ratio of S/Cl (Gullett and Raghunathan, 1997; Duo and Leclerc, 2004; Shao et al., 2010) or N/Cl (Takacs and Moilanen, 1991). Ogawa et al. (1996) studied the effects of the molar ratio of S/Cl in a range of 0–4 with SO2, coal and a mixture of coal and pure sulfur and showed that a higher S/Cl led to a stronger PCDD/Fs’ suppression. However, Anthony et al. (2001) suggested that this S/Cl value, beyond its optimum value (S/Cl  1), would again increase PCDD/Fs’ formation, considering that polycyclic aromatic hydrocarbon formation also almost doubled by the addition of sulfur at an S/Cl of 2. An S/Cl molar ratio range of 0.5–3 was also studied and the most efficient result was obtained when S/Cl was 2 (Chang et al., 2006). Any variance in results may be caused by different reaction temperature, reactant and mode of sulfur addition. Since research on nitrogen inhibitors is less frequent, insufficient information is available on the influence of N/Cl. In addition, nitrogen in different inhibitors may be present as different functional groups. The suppression on PCDD/Fs’ formation is related to these specific groups, so it is meaningless to discuss the influence of N/Cl values while the nature of the functional groups are not considered.

In our tests, the molar ratios of (S + N)/Cl of test I-1 (0.47), A-1 (1.40) and A-2 (2.34) were within a range of 0–1, 1–2 and above 2, respectively. Comparing the results of test A-1, I-1 and A-2, suppression efficiency was slightly influenced by the (S + N)/Cl molar ratio. In test A-1 the inhibition efficiency of PCDD/Fs (I-TEQ) was up to ca. 97 ± 2.1% (99.81 ± 0.88%) with a 1 wt.% TUA addition of the model fly ash and a (S + N)/Cl ratio below 0.5, confirming the efficient inhibition effect of TUA further. When the (S + N)/Cl ratio augmented to 1.4 (I-1, 3% TUA), both PCDD/Fs’ and I-TEQ inhibition efficiency clearly improved. No further rise was observed when the (S + N)/Cl ratio exceeded 2 (A-2, 5% TUA). Probably because in test I-1 the inhibition efficiency already came close to 100%, an addition of 5% tended to be a surplus of TUA. So there is an optimum addition of TUA consistent with the optimum value of (S + N)/Cl (Anthony et al., 2001; Chang et al., 2006). Besides, due to different experimental conditions, the optimum value of (S + N)/Cl is specific and different (Chang et al., 2006; Wu et al., 2012). The weight average chlorination degree decreased with rising value: indeed, less catalytic copper ions were available for chlorination; this was also found in tests with urea (Ruokojarvi et al., 2004). The increasing (S + N)/Cl molar ratio also led to a surge of 1,2,3,4,6,7,8-HpCDF from 16.6% (F-1) to 42–45% (A-1, I-1, A-2), which may derive from de-chlorination of OCDF. Thus, 1,2,3,4,6,7,8-HpCDF becomes even a major contributor to I-TEQ, besides 2,3,4,7,8-PeCDF, and this differed from F-1. The results

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above indicated the ability of TUA for a suppression of chlorination or an acceleration of the de-chlorination reaction. A lower weight average chlorination degree, however, often leads to a higher I-TEQ, according to the rule of toxic equivalent factors (TEF). So the molar ratio of (S + N)/Cl is not the higher the better.

(a) 2500

3.2.2. TUA inhibition at different temperatures The temperature range of PCDD/Fs’ formation in flue gas is descending from 650 to 250 °C (Tuppurainen et al., 1998) in the post-combustion zone, with a maximum of formation at ca. 350 °C. Higher reaction temperature might lead to faster decomposition of PCDD/Fs as well as of TUA, which in turn may affect its suppression efficiency. In order to explore its applicability in terms of the temperature range, a test at a very high temperature (650 °C) was conducted. The concentration of PCDD/Fs (1.43 ± 0.19 ng g1) formed during this de novo test (F-2) at 650 °C was about three orders of magnitude lower than that in the test at 350 °C (F-1), coherent with literature: the amount of PCDD/Fs formed in the fly ash first increased and then decreased at the temperature range of the post-combustion zone. In our study, a decreasing chlorination degree (6.40 ± 0.04) was found and this may be caused by reinforced de-halogenation reactions at an increasing temperature. As seen in Fig. 2, the fractional increase of low-chlorinated PCDDs, especially for 1,2,3,7,8-PeCDD and 2,3,7,8-TeCDD, may result from de-chlorination of 1,2,3,4,6,7,8-HpCDD. As a consequence, 2,3,7,8TeCDD (26%) became one of the top contributors to I-TEQ. Addition of TUA at 650 °C further decreased the PCDD/Fs’ concentration to 0.583 ± 0.032 ng g1, with a suppression efficiency of 59% (45%) for PCDD/Fs (I-TEQ). Therefore, the application of TUA at low-temperature tends to be superior to that at a high temperature. The chlorination degree (6.05 ± 0.03) was further decreased by addition of TUA.

1500

3.3. Elementary analysis on inhibition mechanism of TUA Thiourea has the similar structure and composition with urea, except that S atom replaces O. The inhibition by urea was explained by the formation of surface nitride complexes with metal species (Tuppurainen et al., 1999) as shown by surface spectroscopy (imine, cyanides and azides). Ruokojarvi et al. (2004) concluded that in addition to urea some of its decomposition products or their combination could also cause inhibition. So, since TUA and urea share a similar chemical nature, corresponding experiments were conducted. The gases released by TUA decomposition were monitored on-line by Gasmet™ DX-4000 FT-IR Gas Analyzer. The experimental results in the gas phase were shown in Fig. 3. The discharge of carbon monoxide indirectly indicated the formation of PCDD/Fs (Fig. 3a). This result corresponded with the former findings that oxidation of carbon primarily (65–75%) led to carbon monoxide during de novo formation (Schwarz and Stieglitz, 1992). An obvious release of CO began after 5 min and lasted about 30 min (Fig. 3a). The addition of TUA led to completely different emission characteristics (Fig. 3b). CO release took a later start (15 min) yet lasted until the end of the test with a stable emission concentration of ca. 70 ppm. Decomposition of TUA included two kinds of inhibitory gases: SO2 and NH3. Gasmet tests could only provide a qualitative recognition, yet it actually also showed a small release of NH3. The SO2 discharge was much more obvious, presenting a potential explanation of suppression by the sulfur in TUA. Thus, the question arose whether the supply of decomposition products was the single source of suppression by TUA, or whether effective contact of model fly ash with TUA was also needed. Based on the first hypothesis, a test (M) in which the model fly ash was separated from TUA was conducted. The suppression efficiencies of PCDD/Fs and I-TEQ

CO SO2 NO NO2 N2O NH3 HCN

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(b) 4000 3500 3000

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time/s Fig. 3. Emission of common gaseous pollutants: (a) experiment F-1 (without any inhibitor); (b) experiment I-1 (with addition of 3% TUA).

decreased by ca. 66% and 87%, less than those in test I-1. However, especially for PCDDs, high chlorination led to a correspondingly higher inhibition of I-TEQ. The fraction of OCDD reached ca. 96% of PCDDs, while those of OCDF and 1,2,3,4,6,7,8-HpCDF rose to 69.7% and 25.7% of PCDFs, respectively. The major contributor now was 1,2,3,4,6,7,8-HpCDF, completely different from test I-1. Probably, several co-current inhibition mechanisms prevail for TUA. All above results lead to the conclusion that nitrogen in TUA is active mainly in the form of –NH2, following its reaction with Cu2+ or its replacement of hydrogen and/or chlorine combined with PCDD/Fs. 4. Conclusions Several sets of PCDD/Fs’ suppression tests were conducted in a bench-scale reactor system with the aim to investigate: (1) the inhibition efficiency attained by three distinct suppressants combining both sulfur and nitrogen and identify the most effective inhibitor; (2) the effect of these suppressants on the distribution of the 17 toxic 2,3,7,8-substituted PCDD/F congeners and the weight average chlorination degree of 136 PCDD/Fs (TCDD to OCDF); (3) the influence of a different (S + N)/Cl molar ratio and of reaction temperature on inhibition by the strongest suppressant, TUA; and (4) the potential inhibition mechanism and the efficient inhibition modes of TUA. The conditions of the post-combustion zone were mimicked by using model fly ash and simulated flue gas (12 vol.% O2) at 350 °C in de novo tests. A first test series were conducted with addition of 3 wt.% (NH4)2S2O3 (ATS, ammonium thiosulfate), NH2-OSO-OH

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(ASA, aminosulfonic acid) or CS(NH2)2 (TUA, thiourea) to the model fly ash, respectively. The inhibition efficiency relating to the sum 2,3,7,8-PCDD/Fs ranks as follows:

TUA ð99:8%Þ > ASA ð92:4%Þ > ATS ð85:4%Þ and between 87.4% and 99.8% for I-TEQ, with the same sequence as for PCDD/Fs. This strong suppression effect is attributed to the presence of functional groups of nitrogen and sulfur. The amine group (–NH2) in ASA and TUA tends to be more efficient than ammonium (NH+4): amine can either bind to metal and weaken metal’s catalytic capability or substitute the hydrogen and/or chlorine combined with PCDD/Fs, and thus makes a contribution to the reduction of PCDD/Fs. Ammonium (NH+4) contained in ATS tends to inhibit PCDD/Fs’ formation in the form of NH3. The higher (S + N)/Cl ratio (1.40) of TUA leads to a stronger suppression than that for ASA (0.93). The three inhibitors share stronger inhibition for PCDDs than for PCDFs. A second test sets were conducted with an addition of TUA to model fly ash with 3 different (S + N)/Cl molar ratios. When the (S + N)/Cl value rose from 0.47 to 1.40, the inhibition efficiency of PCDD/Fs (I-TEQ) improved from 97.3% (99.81%) to 99.81% (99.82%). No further rise was observed when the (S + N)/Cl ratio reached above 2. The suppression efficiency of TUA at 650 °C was 59% for PCDD/ Fs and 45% for I-TEQ, respectively. Considered the high inhibition efficiency during low-temperature tests, the recommended temperature for TUA utilization may be around 350 °C. The results of Gasmet test and test M indicated that the inhibitory gases released by TUA could account for the inhibition of PCDD/Fs partially. Another inhibition routes might present parallel and this is especially true for the functional group of –NH2. Studies on S–N inhibitors are helpful for the exploration and utilization of efficient and low-cost inhibitors in full-scale incinerator system, e.g. the injection of a mixture of –S and –N containing compounds at a specific proportion. Yet further exploration on its effects and mechanisms is still necessary. Acknowledgments The research was financially supported by the National High Technology Research and Development Key Program of China (No. 2012AA062803), the Major State Basic Research Development Program of China (No. 2011CB201500) and the Fundamental Research Funds for the Central Universities (No. 2012QNA4009). Moreover, we gratefully acknowledge the funds of Introducing Talents of Discipline to University (B08026). Appendix A. Supplementary material Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.chemosphere. 2014.10.073. References Addink, R., Paulus, R.H.W.L., Olie, K., 1996. Prevention of polychlorinated dibenzo-pdioxins/dibenzofurans formation on municipal waste incinerator fly ash using nitrogen and sulfur compounds. Environ. Sci. Technol. 30, 2350–2354. Anthony, E.J., Jia, L., Granatstein, D.L., 2001. Dioxin and furan formation in FBC boilers. Environ. Sci. Technol. 35, 3002–3007. Chang, M.B., Cheng, Y.C., Chi, K.H., 2006. Reducing PCDD/F formation by adding sulfur as inhibitor in waste incineration processes. Sci. Total Environ. 366, 456– 465. Chen, T., Yan, J.H., Lu, S.Y., Li, X.D., Gu, Y.L., Dai, H.F., Ni, M.J., Cen, K.F., 2008. Characteristic of polychlorinated dibenzo-p-dioxins and dibenzofurans in fly ash from incinerators in China. J. Hazard. Mater. 150, 510–514. Chin, Y.T., Lin, C., Chang-Chien, G.P., Wang, Y.M., 2011. PCDD/Fs formation catalyzed by the copper chloride in the fly ash. J. Environ. Sci. Health, Part A 46, 465–470.

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Dickson, L.C., Lenoir, D., Hutzinger, O., Naikwadi, K.P., Karasek, F.W., 1989. Inhibition of chlorinated dibenzo-p-dioxin formation on municipal incinerator fly ash by using catalyst inhibitors. Chemosphere 19, 1435–1445. Duan, H., Huang, Q., Wang, Q., Zhou, B., Li, J., 2008. Hazardous waste generation and management in China: a review. J. Hazard. Mater. 158, 221–227. Duo, W., Leclerc, D., 2004. Thermodynamic and kinetic studies of dioxin formation and emissions from power boilers burning salt-laden wood waste. Organohalogen Compd. 66, 992–1000. Fangmark, I., Marklund, S., Rappe, C., Stromberg, B., Berge, N., 1991. Use of a synthetic refuse in a pilot combustion system for optimizing dioxin emission. 2. Chemosphere 23, 1233–1243. Fiedler, H., Lau, C., Eduljee, G., 2000. Statistical analysis of patterns of PCDDs and PCDFs in stack emission samples and identification of a marker congener. Waste Manage. Res. 18, 283–292. Gullett, B.K., Raghunathan, K., 1997. Observations on the effect of process parameters on dioxin/furan yield in municipal waste and coal systems. Chemosphere 34, 1027–1032. Gullett, B.K., Bruce, K.R., Beach, L.O., 1992. Effect of sulfur-dioxide on the formation mechanism of polychlorinated dibenzodioxin and dibenzofuran in municipal waste combustors. Environ. Sci. Technol. 26, 1938–1943. Hajizadeh, Y., Onwudili, J.A., Williams, P.T., 2012. Effects of gaseous NH3 and SO2 on the concentration profiles of PCDD/F in fly ash under post-combustion zone conditions. Waste Manage. 32, 1378–1386. Hans Hunsinger, S.A., 2012. The potential of SO2 for improved power generation in wte-plants. 7th i-CIPEC, Seoul/Ilsan KINTEX, Korea, p. 8. Hell, K., Stieglitz, L., Dinjus, E., 2000. Basic compounds to dust from metallurgical plants: experimental results and discussion of inhibition mechanisms. Organohalogen Compd., 4. Iino, F., Takasuga, T., Touati, A., Gullett, B.K., 2003. Correlations between homologue concentrations of PCDD/Fs and toxic equivalency values in laboratory-, package boiler-, and field-scale incinerators. Waste Manage. 23, 729–736. Ismo, H., Kari, T., Juhani, R., 1997. Formation of aromatic chlorinated compounds catalyzed by copper and iron. Chemosphere 34, 2649–2662. Kasai, E., Kuzuhara, S., Goto, H., Murakami, T., 2008. Reduction in dioxin emissions by the addition of urea as aqueous solution to high-temperature combustion gas. ISIJ Int. 48, 1305–1310. Kato, M., Urano, K., 2001. Convenient substitute indices to toxic equivalent quantity for controlling and monitoring dioxins in stack gas from waste incineration facilities. Waste Manage. 21, 55–62. Ke, S., Jianhua, Y., Xiaodong, L., Shengyong, L., Yinglei, W., Muxing, F., 2010. Inhibition of de novo synthesis of PCDD/Fs by SO2 in a model system. Chemosphere 78, 1230–1235. Kuzuhara, S., Sato, H., Tsubouchi, N., Ohtsuka, Y., Kasai, E., 2004. Effect of nitrogencontaining compounds on polychlorinated dibenzo-p-dioxin/dibenzofuran formation through de novo synthesis. Environ. Sci. Technol. 39, 795–799. Lee, S., Choi, S., Jin, G., Oh, J., Chang, Y., Shin, S.K., 2007. Assessment of PCDD/F risk after implementation of emission reduction at a MSWI. Chemosphere 68, 856– 863. Li, H., Wang, L., Chen, C., Yang, X., Chang-Chien, G., Wu, E.M., 2011. Influence of memory effect caused by aged bag filters on the stack PCDD/F emissions. J. Hazard. Mater. 185, 1148–1155. Lindbauer, R.L.A., Wurst, F.B., Prey, T.B., 1994. PCDD/F-emission control for MSWI by SO3-addition. Organohalogen Compd. 19, 355–359. Lippert, T., Wokaun, A., Lenoir, D., 1991. Surface-reactions of brominated arenes as a model for the formation of chlorinated dibenzodioxins and dibenzofurans in incineration – inhibition by ethanolamine. Environ. Sci. Technol. 25, 1485– 1489. Liu, W., Zheng, M., Zhang, B., Qian, Y., Ma, X., Liu, W., 2005. Inhibition of PCDD/Fs formation from dioxin precursors by calcium oxide. Chemosphere 60, 785–790. Liu, H., Zhang, Q., Cai, Z., Li, A., Wang, Y., Jiang, G., 2006. Separation of polybrominated diphenyl ethers, polychlorinated biphenyls, polychlorinated dibenzo-p-dioxins and dibenzo-furans in environmental samples using silica gel and florisil fractionation chromatography. Anal. Chim. Acta 557, 314– 320. Liu, G., Zheng, M., Jiang, G., Cai, Z., Wu, Y., 2013. Dioxin analysis in China. TrAC, Trends Anal. Chem. 46, 178–188. Mari, M., Nadal, M., Schuhmacher, M., Domingo, J.L., 2009. Exposure to heavy metals and PCDD/Fs by the population living in the vicinity of a hazardous waste landfill in Catalonia, Spain: health risk assessment. Environ. Int. 35, 1034–1039. McKay, G., 2002. Dioxin characterisation, formation and minimisation during municipal solid waste (MSW) incineration: review. Chem. Eng. J. 86, 343–368. Ogawa, H., Orita, N., Horaguchi, M., Suzuki, T., Okada, M., Yasuda, S., 1996. Dioxin reduction by sulfur component addition. Chemosphere 32, 151–157. Olie, K., Vermeulen, P.L., Hutzinger, O., 1977. Chlorodibenzo-p-dioxins and chlorodibenzofurans are trace components of fly ash and flue gas of some municipal incinerators in the Netherlands. Chemosphere 6, 455–459. Pandelova, M.E., Lenoir, D., Kettrup, A., Schramm, K., 2005. Primary measures for reduction of PCDD/F in co-combustion of lignite coal and waste: effect of various inhibitors. Environ. Sci. Technol. 39, 3345–3350. Pandelova, M., Lenoir, D., Schramm, K.W., 2007. Inhibition of PCDD/F and PCB formation in co-combustion. J. Hazard. Mater. 149, 615–618. Raghunathan, K., Gullett, B.K., 1996. Role of sulfur in reducing PCDD and PCDF formation. Environ. Sci. Technol. 30, 1827–1834. Ruokojärvi, P.H., Halonen, I.A., Tuppurainen, K.A., Tarhanen, J., Ruuskanen, J., 1998. Effect of gaseous inhibitors on PCDD/F formation. Environ. Sci. Technol. 32, 3099–3103.

16

J.-Y. Fu et al. / Chemosphere 123 (2015) 9–16

Ruokojarvi, P.H., Asikainen, A.H., Tuppurainen, K.A., Ruuskanen, J., 2004. Chemical inhibition of PCDD/F formation in incineration processes. Sci. Total Environ. 325, 83–94. Ryan, S.P., Li, X., Gullett, B.K., Lee, C.W., Clayton, M., Touati, A., 2006. Experimental study on the effect of SO2 on PCDD/F emissions: determination of the importance of gas-phase versus solid-phase reactions in PCDD/F formation. Environ. Sci. Technol. 40, 7040–7047. Samaras, P., Blumenstock, M., Lenoir, D., Schramm, K., Kettrup, A., 2000. PCDD/F prevention by novel inhibitors: addition of inorganic S-and N-compounds in the fuel before combustion. Environ. Sci. Technol. 34, 5092–5096. Samaras, P., Blumenstock, M., Lenoir, D., Schramm, K.W., Kettrup, A., 2001. PCDD/F inhibition by prior addition of urea to the solid fuel in laboratory experiments and results statistical evaluation. Chemosphere 42, 737–743. Schwarz, G., Stieglitz, L., 1992. Formation of organohalogen compounds in fly-ash by metal-catalyzed oxidation of residual carbon. Chemosphere 25, 277– 282. Shao, K., Yan, J., Li, X., Lu, S., Fu, M., Wei, Y., 2010. Effects of SO2 and SO3 on the formation of polychlorinated dibenzo-p-dioxins and dibenzofurans by de novo synthesis. J. Zhejiang Univ. Sci. A 11, 363–369.

Stanmore, B.R., 2004. The formation of dioxins in combustion systems. Combust. Flame 136, 398–427. Takacs, L., Moilanen, G.L., 1991. Simultaneous control of PCDD/PCDF, HCI and NOX emissions from municipal solid waste incinerators with ammonia injection. J. Air Waste Manage. 41, 716–722. Tuppurainen, K., Halonen, I., Ruokojarvi, P., Tarhanen, J., Ruuskanen, J., 1998. Formation of PCDDs and PCDFs in municipal waste incineration and its inhibition mechanisms: a review. Chemosphere 36, 1493–1511. Tuppurainen, K., Aatamila, M., Ruokojarvi, P., Halonen, I., Ruuskanen, J., 1999. Effect of liquid inhibitors on PCDD/F formation, prediction of particle-phase PCDD/F concentrations using PLS modelling with gas-phase chlorophenol concentrations as independent variables. Chemosphere 38, 2205–2217. Wu, H., Lu, S., Li, X., Jiang, X., Yan, J., Zhou, M., Wang, H., 2012. Inhibition of PCDD/F by adding sulphur compounds to the feed of a hazardous waste incinerator. Chemosphere 86, 361–367. Zheng, G.J., Leung, A.O.W., Jiao, L.P., Wong, M.H., 2008. Polychlorinated dibenzo-pdioxins and dibenzofurans pollution in China: sources, environmental levels and potential human health impacts. Environ. Int. 34, 1050–1061.