Chemosphere 126 (2015) 60–66

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Simultaneous suppression of PCDD/F and NOx during municipal solid waste incineration Xiaoqing Lin a, Mi Yan b, Ahui Dai c, Mingxiu Zhan a, Jianying Fu a, Xiaodong Li a,⇑, Tong Chen a, Shengyong Lu a, Alfons Buekens a, Jianhua Yan a a b c

State Key Laboratory of Clean Energy Utilization, Institute for Thermal Power Engineering, Zhejiang University, Hangzhou 310027, China Institute of Energy and Power Engineering, Zhejiang University of Technology, Hangzhou 310014, China Department of Public Health, School of Medicine, Zhejiang University, Hangzhou 310058, China

h i g h l i g h t s  An S–N-inhibitor was used to suppress dioxin emissions from a full-scale MSWI.  Suppression of the total dioxins emission factor was 91.0% (81.8% for TEQ).  Simultaneously, suppression of NOx was 42.9%.  Poisoning metal catalyst was the main suppression mechanism.

a r t i c l e

i n f o

Article history: Received 17 November 2014 Received in revised form 23 January 2015 Accepted 2 February 2015 Available online 23 February 2015 Handling Editor: Gang Yu Keywords: Municipal solid waste Incineration Dioxins NOx Suppression Thiourea

a b s t r a c t Thiourea was tested as a dioxins inhibitor in a full-scale municipal solid waste incinerator with high capacity (34 t h1). The suppressant, featuring a high S- and N-content, was converted into liquor and then injected (35 kg h1) into the furnace (850 °C) through the inlets already used for Selective NonCatalytic Reduction (SNCR) of flue gas NOx. The first results show that thiourea reduces the dioxins in flue gas by 55.8 wt.%, those in fly ash by 90.3 wt.% and the total dioxins emission factor by 91.0 wt.%. The concentration of PCDD/Fs was 0.08 ng TEQ Nm3, below the national standard of 0.1 ng TEQ Nm3. The weight average chlorination degree of dioxins decreases slightly after adding the inhibitor, indicating that it suppresses both the formation and the chlorination of dioxins. Analysis of fly ash by scanning electron microscope (SEM) suggests that the particle size becomes larger after adding the inhibitor. Further analysis using an energy dispersive spectrometer (EDS) reveals that the sulphur content in fly ash rises, but the chlorine content declines when adding thiourea. These results suggest that poisoning the metal catalyst and blocking the chlorination are probably responsible for suppression. NOx reduction attains 42.6 wt.%. These tests are paving the way for further industrial application and assist in controlling the future emissions of dioxins and NOx from MSWI. Ó 2015 Published by Elsevier Ltd.

1. Introduction The generation of municipal solid waste (MSW) has sharply increased in China, from 13,650 (2002) to 17,081 (2012) million tonnes (National Bureau of Statistics of China, 2013). The volume of MSW is boosted by the rise of both economic activity and standard of living. Therefore, the problem of MSW disposal needs to be urgently addressed and solved. With growing shortage of land resources landfill is no longer possible around large cities. MSW

⇑ Corresponding author. Tel.: +86 571 8795 2628; fax: +86 571 8795 2428. E-mail address: [email protected] (X. Li). http://dx.doi.org/10.1016/j.chemosphere.2015.02.005 0045-6535/Ó 2015 Published by Elsevier Ltd.

incineration (MSWI) is technically feasible and realises waste reduction, energy recovery and resource utilisation. However, MSW incineration faces vast perception problems, mainly related to the emission of polychlorinated dibenzo-p-dioxins and dibenzofurans (PCDD/Fs) as unintentional by-products. In China the concentration of PCDD/Fs in emissions ranged from 0.042 to 2.46 ng I-TEQ Nm3, with an average value of 0.423 ng I-TEQ Nm3 (Ni et al., 2009). Only 84% of the plants comply with the old PCDD/F emission standard of 1.0 ng I-TEQ Nm3 and just 32% meet the EU emission standard of 0.1 ng I-TEQ Nm3. Recently, the national standard for dioxins emission from MSW incineration in China has been adapted to an ambitious 0.1 ng TEQ Nm3, creating considerable challenge for MSWI operation.

X. Lin et al. / Chemosphere 126 (2015) 60–66

Optimising the combustion conditions of MSWI is an effective way to control the formation of Products of Incomplete Combustion (PICs), thus reducing the formation of PCDD/F-precursors (chlorobenzenes, chlorophenols and unburned carbon) and the emission of PCDD/Fs (Weber et al., 2002). However, PCDD/Fs are easily formed within the post-combustion zone (200–400 °C) either from precursors or by de novo synthesis on the surface of fly ash particles (Karasek and Dickson, 1987; Huang and Buekens, 1995). Flue gas cleaning by activated carbon adsorption coupled with fabric filtration just transfers the gas-phase PCDD/Fs to solid-phase PCDD/Fs, without reducing the total PCDD/Fs’ output (Chang and Lin, 2001; Chang et al., 2006). Selective catalytic reduction (SCR) effectively decomposes the PCDD/Fs in the flue gas, but its high investment and operating costs limit its use in full-scale MSWI (Goemans et al., 2004; Yang et al., 2008). Using chemical inhibitors (Tuppurainen et al., 1999; Ruokojârvi et al., 2001; Chang et al., 2006; Wu et al., 2012) could overcome part of the above problems, after realising effective suppression of PCDD/Fs. Sulphur containing compounds (Pandelova et al., 2005; Chang et al., 2006; Yan et al., 2006; Chen et al., 2008; Wu et al., 2012), such as elemental sulphur (S), sodium sulphide (Na2S), sodium thiosulphate (Na2S2O3), carbon sulphide (CS2), pyrite (FeS2), sulphur trioxide (SO3), sulphur dioxide (SO2) and high-sulphur coal can all be successfully used to reduce the formation of PCDD/Fs. Several explanations have been advanced for the suppressive effects of sulphur. Already Griffin (Griffin, 1986) proposed that the high sulphur content of coal was leading to the low PCDD/Fs emissions from coal combustion. This observation was confirmed in the co-combustion of coal and waste (Gullett et al., 1992, 1998, 2000; Chen et al., 2008). Nitrogen containing compounds such as urea, ammonia, ethanolamine, ethylenediamine tetraacetic acid (EDTA), monoethanolamine (MEA), dimethylamine (DMA) and triethanolamine (TEA) are also confirmed inhibitors (Tuppurainen et al., 1999; Ruokojârvi et al., 2001). Many of these compounds feature an amine-group. Reducing the ability of catalytic metals to catalyse PCDD/F formation has been proposed as the main mechanism of inhibition by these sulphur or nitrogen containing compounds (Ruokojârvi et al., 2001). Sulphur containing compounds could convert metal chlorides into sulphates (oxidising conditions) or into sulphides (reducing conditions), thus poisoning the metal catalyst (Ryan et al., 2006; Ke et al., 2010), whereas nitrogen containing compounds undergo complex reactions with the metal catalyst, forming strongly bonded organometallic nitride complexes thus resulting in the irreversible deactivation of catalytic sites (Tuppurainen et al., 1999; Luna et al., 2000). In an alternative explanation sulphur dioxide converts chlorine (Cl2), a premium chlorinating agent, into less active HCl (Griffin, 1986; Gullett et al., 1992), whereas nitrogen containing compounds could also reduce the number of acid catalytic sites of fly ash, thus inhibiting PCDD/F formation (Ismo et al., 1997). Previous study in laboratory-scale experiments (Samaras et al., 2000; Pandelova et al., 2005) indicated that both sulphur- and nitrogen-containing compounds show high inhibition efficiency (more than 95%) based on the original PCDD/F concentration. Numerous compounds were tested, including amidosulphonic acid (ASA), sulphamide (SA), ammonium sulphate ((NH4)2SO4), thiosulphate ((NH4)S2O3) and Thiourea. However, no experiments were conducted with these compounds in a full-scale incinerator plant. The inhibition potential of PCDD/Fs in actual incineration is always affected by numerous operating parameters, including the temperature and the location of the inhibitor injection, the kind and concentration of the inhibitor, the quality of its penetration and mixing and the post-combustion conditions in the incinerator. In addition, the quality of the injection affects inhibition quite a lot; realising a homogeneous and well-spread injection, with

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sufficiently deep penetration of the reactants into the flue gas and adequate mixing at the local micro-scale is essential for attaining adequate DeNOx and DeDiox activity. Furthermore, at times too much inhibitor results in poorer combustion, causing an abnormal rise in particles- and CO-concentration and even of PCDD/Fs, in the stack gases (Samaras et al., 2000; Chang et al., 2006; Wu et al., 2012). In this study, a series of experiments to reduce the PCDD/Fs formation in a full-scale MSWI (34 t h1) was conducted by injecting thiourea, a sulphur- and amine-containing inhibitor, which has been proved an effective inhibitor in our previous laboratory-scale experiment (Fu et al., 2015). Thiourea was first dissolved in water (solubility of 150 g L1) and the resulting aqueous solutions was injected into the flue gas through the non-selective catalytic reduction (SNCR) system of the MSWI, which allowed to control the injection continuously and homogeneously, avoiding excessive injection and discontinuity of feeding. The objectives of this work were to study the ability of lessening PCDD/Fs formation by the high-temperature injection of Thiourea in a full-scale MSWI-plant, to investigate the influence of Thiourea on PCDD/F homologue patterns and distribution of congeners, to explore the inhibition mechanisms of sulphur and nitrogen containing compounds, and to study the simultaneous reduction capacity of NOx and dioxins by sulphur-amine containing compounds. 2. Materials and methods 2.1. MSWI The experiments were carried out in a full-scale (34 t h1) circulating fluidized bed (CFB) MSWI (Fig. 1) in Zhejiang province. This CFB incinerator eliminates MSW without any addition of auxiliary fuel (coal), given the high moisture content of MSW. The incinerator includes both a furnace and a secondary combustion chamber. A cyclone separates and recycles the larger solid particles to the furnace, for further burnout. The secondary combustion chamber is set at the outlet of the cyclone separator to realize a more complete burnout of unburned carbon. The flue gas is also burned out and then cooled by heat exchange in a boiler. The air pollution control devices (APCDs) include a semi-dry spray neutraliser, an activated carbon (AC) contacting chamber and finally a baghouse filter, to control both gaseous and particulate pollutants. The amounts of AC and lime injected into the flue gas are set at 150 and 3000 mg Nm3, respectively. A Selective Non-Catalytic Reduction (SNCR) system is installed at the inlet of the cyclone separator to reduce the emission of NOx (Radojevic, 1998; Baukal Jr, 2004). It is composed of 4 sets of spraying nozzles, through which aqueous ammonia solution is injected into the flue gas. The injection temperature of aqueous ammonia in the flue gas varies between 850 and 950 °C and the injection pressure of the nozzle varies between 0.15 and 0.60 MPa (Table 1). Aqueous ammonia and water were injected into the flue gas at a flow rate of 120 and 180 L h1, respectively. 2.2. Experimental design According to previous studies, the reduction efficiency of PCDD/F formation by either co-combustion with coal or addition of sulphur compounds was strongly influenced by the S/Cl molar ratio of the total input, i.e. waste feed + inhibitors (Gullett and Raghunathan, 1997; Duo and Leclerc, 2004; Chang et al., 2006; Wu et al., 2012). However, sulphur mainly remained as sulphates in the ash, with only about 16% of the sulphur converted to SO2 in the flue gas (Hunsinger et al., 2007). Therefore, our attention concentrates on the S/Cl molar ratio in the flue gas, rather than in the total input

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Fig. 1. Systemic diagram of the MSWI (1: Feeder. 2. Furnace. 3: Cyclone separator. 4: SNCR. 5: Secondary combustion chamber. 6: Boiler. 7: Semi-dry spray neutraliser. 8: AC injector. 9: Baghouse filter. 10: Stack. 11: Dissolving tank.).

Table 1 Experimental conditions of the inhibition tests.

a b

Parameters

Normal condition

Inhibition tests

Temperature (°C) Injection pressure of the nozzle (MPa) Aqueous ammonia a (L h1) Water (L h1) Aqueous thiourea b (L h1) Total mixed liquor (L h1)

850–900 0.30 120 180 0 300

850–900 0.50 120 0 350 470

The concentration of aqueous ammonia was 20%. The thiourea concentration was 10%.

to the MSWI. Similarly, we need to emphasize on the N/Cl molar ratio, especially NH3/HCl, in the flue gas. The (S + N)/Cl molar ratio ((SO2 + NH3)/HCl) in the flue gas was 0.6 under normal operating conditions and was adjusted to 1.0 by adding thiourea (increasing concentration of SO2 and NH3) during the inhibition tests. The amount of thiourea injected was controlled at 35 kg h1, accounting for 0.15% of the total input. Thiourea was first converted into aqueous solution and injected into the flue gas at a flow rate of 350 L h1 (Table 1). The PCDD/F inhibition effects by Thiourea were determined by comparing the PCDD/F concentration in the flue gas and fly ash with (inhibition tests) and without (normal condition) injection of Thiourea aqueous solutions. The experiment was conducted with the MSWI operating normally, avoiding any influences from other, uncontrolled factors. The inhibition tests were conducted after 24 h of normal operating conditions. The sampling of PCDD/Fs in flue gas was performed after 12 h of continuous injection of Thiourea aqueous solutions and lasted for 6 h (3 h for one sample). The NOx was monitored on-line from the start of the inhibition tests and lasted for 18 h. Gaseous samples of PCDD/Fs were obtained from stack, ash samples of PCDD/Fs were collected from bottom of boiler (boiler ash) and house filter (filter ash), respectively, as shown it Fig. 1.

analysis. Dioxins’ samples in the flue gas were collected by means of an isokinetic sampler (Model KNJ23, KNJ, Korea) according to US EPA method 23a, as already described in detail in previous studies (Yan et al., 2006; Chen et al., 2008). The PCDD/Fs surrogate standards were added to the XAD-2 resin for checking the PCDD/Fs sampling efficiency. After flue gas sampling, the system was rinsed with acetone and toluene, respectively, and the rinse was saved in brown glass bottle. Filter and XAD-2 resin also stored and maintained in dark below 4 on field until transferred to the laboratory. The clean-up procedures of PCDD/Fs samples followed US EPA method 1613 (US EPA, 1994). Each sample was spiked with 1 ng of 13C12-labelled internal standards and then Soxhlet extracted by 250 mL toluene for 24 h. The Soxhlet extract was concentrated to 1–2 mL by a rotary evaporator. Then the concentrated extract was washed by sulphuric acid several times until the colour of sample solution was visible. The extract was further cleaned up by mean of a multilayer silica gel column and a basic-alumina column. After cleanup procedures, the extract was concentrated to 20 lL with nitrogen and then spiked with 1 ng of 13C12-labelled recovery standards prior to analysis. All solvents were purchased from Mallinckrodt Baker Inc., USA and were pesticide residue analysis grade. Samples were analysed by means of high-resolution gas chromatography/high-resolution mass spectrometry (HRGC/HRMS) (JMS-800D, JEOL, Japan) with a DB-5MS (60 m length  0.25 mm ID  0.25 lm film) column. The mean recoveries of standards for PCDD/Fs range from 50% to 120%, which were all satisfied with the method of US EPA 1613. Details of clean-up procedure and analysis method of the PCDD/Fs can be found in our previous studies (Chen et al., 2008; Lin et al., 2013, 2014). The toxic equivalents (TEQ) were calculated using TEFWHO-05 (Van den Berg et al., 2006). All concentrations of gaseous samples were normalized to 100 kPa, 0 °C, and dry air at 11% O2.

3. Results and discussion 2.3. Sampling and analysis 3.1. SO2 and NOx SO2, NOx and other common gaseous pollutants were monitored on-line using a Gasmet detector (FTIR DX-400, Finland) in the flue gas of the stack. Additionally, the boiler and the filter ash were also sampled for further study of their chemical and physical characteristics by SEM-EDS analysis (SIRISON, FEI), as well as for PCDD/F

The mean emission values of SO2 in the flue gas before and after thiourea solution injection are 1.69 and 1.75 mg Nm3, respectively. It is due to the fact that flue gas flowed through the air pollution control devices (APCDs), especially the semi-dry spray neutraliser,

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and most SO2 were removed. During MSW incineration, NOx is mainly formed from organic nitrogen present in the fuel (FuelNOx), since the relatively low combustion temperature (826 +/ 48 °C) in the FBC-plant limits the thermal-NOx formation (Gohlke et al., 2010). The formation of NOx is also influenced by incinerator design, excess air, ratio of primary air and secondary air and occurrence of oxygen lean-zones (Baukal Jr, 2004). SNCR is an effective technique to control NOx emission at relatively low cost. The average concentration of NOx was about 140 mg Nm3 in normal conditions with aqueous NH3 injection ((Fig. 2). After adding the sulphur-amine inhibitor the concentration of NOx further diminished to 80 mg Nm3, i.e. an additional reduction efficiency of 42.9%. The sulphur-amine inhibitor thermally decomposes at 185 °C, yielding reducing amine radicals, such as NH3, NH2 and NH in the flue gas (Gohlke et al., 2010; Klippenstein et al., 2011), which could convert NOx to N2 at high temperature (Fig. 3), thus reducing NOx (Miller and Bowman, 1989; Glarborg et al., 1994). Therefore, the sulphur-amine inhibitor will not cause negative effects, such as increasing SO2 emission, on the contrary, it can reduce NOx emission. 3.2. PCDD/Fs 3.2.1. PCDD/Fs in the flue gas The concentrations of PCDD/Fs in the flue gas without and with Thiourea addition were 3.12 and 1.38 ng Nm3 and 0.15 and 0.08 ng TEQ Nm3, respectively. Thus, the inhibition efficiency of PCDD/Fs attained 55.8% (46.7% for TEQ), achieving similar suppression levels as when using sulphur (Chang et al., 2006; Wu et al., 2012) or urea (Takacs and Moilanen, 1991) as an inhibitor in MSW incineration. The PCDD/PCDF-ratio was 0.49, a very typical ratio in MSW incineration, considered characteristic of de novo synthesis (Huang and Buekens, 1995). No remarkable changes occurred for the PCDD/PCDF-ratio after adding inhibitor, indicating that the inhibition of PCDDs and PCDFs in the flue gas by Thiourea were roughly at the same level. The chlorination degree of PCDD marginally reduced from 7.30 to 7.23, that of PCDF from 6.63 to 6.54. Both SO2 and amines group may block the chlorination step during PCDD/Fs formation (Tuppurainen et al., 1999; Ryan et al., 2006; Wu et al., 2012). 3.2.2. PCDD/Fs in the ash The average concentration of PCDD/Fs augmented from 73.47 ng g1 in boiler ash to 95.49 ng g1 in filter ash, indicating only mild de novo synthesis activity during flue gas cooling. This rise might also be attributed to PCDD/Fs adsorption by AC and fly ash during filtration. The TEQ value of filter ash (2.05 ng TEQ g1)

3

NOx concentration (mg/Nm )

180

Without inhibitor

With inhibitor

160

140

140 120 80

100 80

Fig. 3. Pathways of flue gas NOx reduction.

is lower than the national standard for landfill of 3 ng TEQ g1. After injecting the Thiourea aqueous solution the concentration of PCDD/Fs in boiler ash dwindled to 1.26 ng g1 (a suppression of 98.3%), while that in filter ash diminished to 9.22 ng g1 (a suppression of 90.3%). The TEQ value in filter ash even dwindled even to 0.37 ng I-TEQ g1, almost an order of magnitude lower than the national standard for the landfill. Thus, the suppression efficiency of PCDD/Fs was higher for ash than that for flue gas. This is consistent with results reported by Tuppurainen et al. (1999), who injected liquid urea into the flue gas at 725 °C, and with the results from previous experiments (Chen et al., 2014) using sludge drying gases (containing NH3 and SO2) as inhibitor. Two possible reasons might explain this experience. One is that gaseous PCDD/Fs are mainly formed via homogeneous route rather than heterogeneous route (Qu et al., 2009). The second is that the S–N containing compounds both react with metal catalyst in the fly ash (Ryan et al., 2006; Ke et al., 2010) and/or gaseous Cl2 in the flue gas (Griffin, 1986; Gullett et al., 1992), blocking the catalytic reactions responsible for PCDD/Fs formation. The PCDD/PCDF ratio reduced only slightly after injecting the thiourea aqueous solutions, indicating that the suppression was somewhat stronger for PCDDs than for PCDFs, with suppression of 90.2% for PCDDs and 90.1% for PCDFs in the filter ash. The result was similar with those in previous studies (Chang et al., 2006; Chen et al., 2014). The difference in PCDD/PCDF-ratio between flue gas and ash also implies that the formation and inhibition of PCDD/ F requires separate consideration of flue gas and ash. The decreasing chlorination degree of PCDD/F was also established in the ash. Molecules with lone pairs of electrons (e.g., compounds containing sulphur and nitrogen) can form stable complexes with catalytic transition metals, and then block active sites for the chlorination reaction (Samaras et al., 2000; Ruokojârvi et al., 2001; Pandelova et al., 2005). Another reason is possibly that organic nitrogen compounds combined with PCDD/Fs or its chlorinated precursors, decreasing the formation of chlorinated aromatic compounds, replacing these by various nitrogen compounds (Zyaykina et al., 2003; Kuzuhara et al., 2005). Similarly, sulphonated PCDD/F- precursors, either could have prevented chlorination or helped in the formation of sulphur analogues of PCDD/Fs (Gullett et al., 1992). 3.2.3. Total emission factor of PCDD/Fs The total emission factor of PCDD/Fs was calculated as follows:

60 40 0

2

4

6

8

10

12

14

16

Time (h) Fig. 2. Concentration of NOx with and without inhibitor injection.

18

F¼

Cf  V f þ Ca  V a W

where F is the total emission factor of PCDD/Fs, C f is the PCDD/F concentration in the flue gas, V f is the volume of flue gas, C a is

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100%

3.2.4. Distribution of PCDD/Fs The congener profiles of the 2,3,7,8-substituted PCDD/Fs in the flue gas and ash are shown in Fig. 4. PCDFs contributed most to the total concentration of PCDD/Fs in the flue gas, accounting for more than 65% (Table 2) (Yan et al., 2006; Chen et al., 2008; Lin et al., 2014) and suggesting that de novo synthesis was the main way of PCDD/Fs formation. The PCDD congener profile was dominated by OCDD and 1,2,3,4,6,7,8-HpCDD, together accounting for more than 80% (Fig. 4a), while the dominant PCDF congeners were also highly chlorinated, i.e., OCDF and 1,2,3,4,6,7,8-HpCDF, representing more than 50% (Fig. 4b). Thus, the congener profiles tend towards the high-chlorinated species, suggesting the presence of chlorinating catalytic metals in the fly ash (Jay and Stieglitz, 1991). After injecting S–N inhibitor aqueous solutions, the trend towards lowchlorinated PCDD/Fs in the flue gas becomes obvious, especially for 2,3,7,8-TCDD, 2,3,7,8-TCDF and 2,3,4,7,8-PeCDF (Fig. 4), showing that chlorination is inhibited (Ke et al., 2010; Wu et al., 2012), while high-chlorinated PCDD/Fs (1,2,3,4,6,7,8-HpCDD, 1,2,3,4,6,7,8-HpCDF) strongly decline. The PCDDs in the boiler ash and filter ash are entirely dominated by OCDD and 1,2,3,4,6,7,8-HpCDD (Fig. 4a), accounting for more than 90%, while for the PCDF the dominant congener is OCDF, accounting for 50% in boiler ash and for 25% in filter ash (Fig. 4b). This profile is consistent with some previous studies (Chang et al., 2011; Pan et al., 2013). After injecting the thiourea aqueous solution into the flue gas, the PCDD congener profile stays almost the same in the boiler ash, but OCDF declined and 1,2,3,7,8PeCDF amplified for the PCDF congener profile. For fly ash, OCDD and OCDF lessened and 2,3,4,7,8-PeCDF enlarged. High-chlorinated congeners may be formed from lower chlorinated congeners by direct chlorination in the ash (Tuppurainen et al., 1999), yet this formation route was blocked by the S-N suppressants.

(a) PCDD Fingerprint

90% 80%

OCDD

70%

1234678HpCDD

60%

123789HxCDD

50%

123678HxCDD

40%

123478HxCDD

30%

12378PeCDD

20%

2378TCDD

10% 0% G1

G2

100%

BA1

BA2

FA1

FA2

(b) PCDF Fingerprint

90%

OCDF

80%

1234789HpCDF

70%

1234678HpCDF

60%

123789HxCDF

50%

234678HxCDF

40%

123678HxCDF

30%

123478HxCDF

20%

23478PeCDF

10%

12378PeCDF 2378TCDF

0% G1

G2

BA1

BA2

FA1

FA2

Fig. 4. Distribution of the 2,3,7,8-substituted PCDD/Fs: (a) PCDD fingerprint; (b) PCDF fingerprint.

the PCDD/F concentration in the fly ash (filter ash), V a is the fly ash discharge rate, and W is the MSW treatment rate. The total emission factor was calculated according to the dioxins in flue gas and in fly ash, the dioxin in the slag was not included. In our experiment, the volume of flue gas was 120,000 Nm3/h, and the fly ash discharge rate was 2.55 t h1. As calculated, the total PCDD/F emission factor was 7.173 mg t1 MSW (0.154 mg TEQ t1 MSW) during normal condition and then shrunk to 0.696 mg t1 MSW (0.028 mg TEQ t1 MSW) after injecting thiourea, with a suppression of 91.0% (81.8% for TEQ).

3.3. PCDD/F suppression mechanism SEM analysis (Fig. 5a) revealed that the particle size of the fly ash collected from the experiment with thiourea addition was larger in particle size than the test without addition of thiourea. This suggests that both physical phenomena and chemical reactions may have occurred on the surface of fly ash, such as reactions of catalytic metal chlorides (NaCl, CuCl2, FeCl3) with SO2 and of NH3 with acid gases (HCl, SO2), forming sulphates and ammonium salt fumes (Kuzuhara et al., 2005; Ryan et al., 2006; Wu et al., 2012),

Table 2 Concentrations of PCDD/Fs in the flue gas and ash with and without thiourea. Without thiourea a

RPCDDs RPCDFs RPCDD/Fs TEQ PCDD/Fs inhibition TEQ inhibition PCDDs/PCDFs Cl-PCDD Cl-PCDF Cl-PCDD/F Stand. Dev. RPCDD/Fs Stand. Dev. TEQ a

With thiourea b

G1

BA1

1.03 2.09 3.12 0.15 0 0 0.49 7.30 6.63 6.85 0.5 0.08

33.06 40.41 73.47 1.56 0 0 0.82 7.69 7.26 7.46 3.3 0.09

c

FA1

G2

BA2

FA2

42.86 51.63 95.49 2.05 0 0 0.83 7.83 6.64 7.18 3.8 0.11

0.46 0.92 1.38 0.08 55.8 46.7 0.49 7.23 6.54 6.77 0.4 0.02

0.57 0.69 1.26 0.12 98.3 92.3 0.83 7.21 6.96 7.07 0.3 0.01

4.22 5.00 9.22 0.37 90.3 82.0 0.84 7.68 6.81 7.21 1.3 0.03

ng Nm3 in flue gas; ng g1 in ash

ng WHO-TEQ % % Chlorination degree of PCDD/Fs

d

Duplicate experiments

G stands for PCDD/Fs in the flue gas. BA stands for PCDD/Fs in the boiler ash. FA stands for PCDD/Fs in the filtered ash. d Chlorination degree of PCDD/Fs was calculated as follows: P C j nj Cld ¼ ðj ¼ 4; 5; 6; 7; 8Þ. C where C j stands for the concentration of each 2, 3, 7, 8- substituted PCDD/Fs, nj stands for the amount of chlorine atom of each 2, 3, 7, 8- substituted PCDD/Fs; C stands for the total concentration of PCDD/Fs. b

c

X. Lin et al. / Chemosphere 126 (2015) 60–66

65

Fig. 5. Characteristics of filtered ash: (a) particle size; (b) element content.

condensing onto the surface of fly ash, when the flue gas cools down. Poisoning the metal catalyst and blocking chlorination thus contribute to the suppression of PCDD/F formation. Further elemental analysis of fly ash after reaction was conducted by EDS analysis (Fig. 5b). It showed that the S/Cl relative ratio in the fly ash increased from 0.20 to 0.65 after thiourea injection, confirming the aforementioned sulphate formation and condensation onto the surface of fly ash. It could be concluded that chlorination reaction on the surface of fly ash was inhibited from the raising S/Cl relative ratio. Additionally, NH3 might also react with HCl from the fly ash, forming NH4Cl, evolving from fly ash so that less chlorine was left. Further research is still needed to confirm these active catalyst-poisoning pathways. Based on our results, we suggest that the poisoning of metal catalyst and the reducing chlorine content is the main mechanism of PCDD/F-suppression.

The paper presents the first full-scale demonstration tests with this new suppressant. Further work is needed to optimize these inhibition effects and obtain even better results without increasing the suppressant’s dosage. The results are ready for industrial application to control the emissions of dioxins and other persistent organic pollutants from MSWI in China. Acknowledgments The National High Technology Research and Development Key Program of China (863 Program, 2012AA062803), the research is supported by the National Basic Research Development Program of China (973 Program, 2011CB201503) and Natural Science Foundation of China (51406182). Give our grateful acknowledgement to the funds of Introducing Talents of Discipline to University (B08026).

4. Conclusions Appendix A. Supplementary material Tests were conducted in a full-scale MSWI to demonstrate the suppression of formation of PCDD/Fs, obtained by introducing an aqueous solution of thiourea, a novel sulphur and amine containing inhibitor, through the nozzles of the non catalytic selective NOx-reduction system. This suppression activity had earlier been defined during de novo tests conducted at laboratory scale. The reduction of total dioxins from the emissions could reach more than 91.0% (81.8% for TEQ) using this new inhibitor, which accounted for an input of only 0.1 wt.% of the total waste input. Study of boiler and filter ash by SEM-EDS indicated that poisoning the metal catalyst and blocking the chlorination are probably responsible for suppression.

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