Article pubs.acs.org/est
Emission, Mass Balance, and Distribution Characteristics of PCDD/Fs and Heavy Metals during Cocombustion of Sewage Sludge and Coal in Power Plants Gang Zhang,†,‡ Jing Hai,‡ Mingzhong Ren,*,‡ Sukun Zhang,‡ Jiang Cheng,*,† and Zhuoru Yang† †
School of Chemistry and Chemical Engineering, South China University of Technology, Guangzhou 510640, China South China Institute of Environmental Sciences, Ministry of Environmental Protection, Guangzhou 510655, China
‡
S Supporting Information *
ABSTRACT: The emission, mass balance, and distribution characteristics of polychlorinated dibenzo-p-dioxins and dibenzofurans (PCDD/Fs) as well as those of heavy metals (Hg, Cd, Pb, Cr, and Cu) were investigated during the cocombustion of 5%, 10%, 15%, and 20% sewage sludge (SS) in a pulverized coal power plant. The PCDD/F emissions increased from 7.00 to 32.72 pg I-TEQ/Nm3 as the amount of SS in the mixed fuel (MF) increased. High sulfur content and relatively low chlorine levels in MF resulted in lower PCDD/F emissions. SS exhibited a remarkable difference in congener profiles compared with flue gas, bottom ash, and fly ash. The negative dioxin mass balance indicated that the cofiring of SS with coal in power plants was not a source but a sink of dioxins. The concentrations and emission factors of heavy metals in flue gas and bottom ash, as well as fly ash, all exhibited a tendency to increase with increasing input values of heavy metals in MF. The distribution characteristics of the investigated heavy metals were primarily dependent on the evaporative properties of these metals. The availability of chlorine could alter the heavy metal distribution behavior. The emitted pollutants in the power plant were below the legal limits.
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INTRODUCTION Wastewater treatment plants generate solid residue called sewage sludge (SS) that comprises high levels of water, organic matter, and pathogenic agent contents. With the rapid economic development and improved living standards in China, SS production has dramatically increased at an annual rate of 4%.1 Currently available SS disposal methods include fertilizer recycling, landfilling, sea dumping, and incineration. The application of SS as fertilizer on agricultural land has been limited by numerous setbacks because SS contains heavy metals. Limitations in recycling and landfill site capacity, as well as the planned banning of sea dumping, have raised increasing concern about the future role of SS incineration. SS incineration has several advantages, which include volume and mass reduction, detoxification, and energy recovery.2 Therefore, many countries strongly support the coincineration of SS as supplementary fuel in coal-fired power plants, cement kilns, and brick kilns. The cocombustion treatment method for SS management presents a disadvantage in that it facilitates the emission of inorganic pollutants, including heavy metals such as mercury, cadmium, and lead, as well as the emission of organic air pollutants, including polychlorinated dibenzo-p-dioxins and dibenzofurans (PCDD/Fs). These emitted pollutants have become a cause of public concern because of their high toxicity © 2013 American Chemical Society
and potential carcinogenic and mutagenic effects. The formation mechanisms of PCDD/Fs have been extensively investigated. Numerous studies3−5 revealed that PCDD/Fs are mainly formed through de novo mechanisms in the lowtemperature postcombustion zones of incinerators via heterogeneous catalytic reactions that occur in a flue gas−fly ash environment. Everaert and Baeyens6 affirmed the dominant role of de novo synthesis by analyzing the PCDD/F profiles from general large-scale thermal processes. They observed that the PCDF/PCDD ratio exceeded 1 and that the degree of chlorination indicated the presence of heptachlorodibenzo-pdioxins (HpCDD) and octachlorodibenzodioxin (OCDD) within the dioxin group as well as presence of pentachlorodibenzofuran (PeCDF), hexachlorodibenzofuran (HxCDF), and heptachlorodibenzofurans (HpCDF) within the furan group. Pulverized coal furnaces are commonly used for power generation in China. SS cocombustion in pulverized coal power plants is necessary for the disposal and utilization of SS. However, SS incineration has been identified as a significant Received: Revised: Accepted: Published: 2123
October 10, 2012 January 9, 2013 January 23, 2013 January 23, 2013 dx.doi.org/10.1021/es304127k | Environ. Sci. Technol. 2013, 47, 2123−2130
Environmental Science & Technology
Article
ashes, and fly ashes are analyzed in various sampling series. The important factors that affect PCDD/F and heavy metal emissions are also discussed in detail. Furthermore, the corresponding PCDD/F congener profiles are presented and compared.
source of PCDD/Fs and heavy metals.7 Most studies focused on PCDD/F emission and formation during SS cocombustion in laboratory-scale units1,8−11 and pilot tests.12,13 Deng et al.1 reported that PCDD/F concentrations in flue gas are lower during SS and coal cocombustion in a laboratory-scale fluidized bed incinerator compared with those during SS monocombustion. Samaras et al.8 found that the cocombustion of industrial sludge and coal in a laboratory-scale reactor decreased the PCDD/F emissions from 300 to 158 ng I-TEQ/kg fuel. Fullana et al.10 investigated PCDD/F formation and destruction of PCDD/Fs during SS incineration using a dual-chamber flow reactor system and a horizontal laboratory-scale reactor. Their results indicated that sludge ash can catalyze the oxidation and chlorination of PCDD/Fs. However, the data obtained in the laboratory experiments cannot be directly extrapolated to the actual PCDD/F emission and formation during SS cocombustion. To the best of our knowledge, intensive studies regarding PCDD/F and heavy metal emissions during SS cocombustion in a field-scale pulverized coal power plant remain insufficient. Moreover, no comparative data are available for the examination of the incremental effects of SS. A few researchers investigated heavy metal and PCDD/F emissions in a field-scale cement kiln or municipal solid waste incinerator (MSWI) when cofiring different types of wastes, such as municipal solid waste (MSW), SS, and so on. Zabaniotoua and Theofilou14 reported that metal and PCDD/F emissions are below standard levels during SS cocombustion in a cement kiln. Conesa et al.15 also found that PCDD/F and metal emissions are lower than the European Union limits when cofiring solid-recovered fuelderived MSW in cement kilns. No correlation was observed between waste feed rate and metal emission. Yan et al.16 evaluated PCDD/F emissions in fluidized bed incinerators in China by cofiring MSW with coal. They reported that cofiring MSW with coal containing a large amount of sulfur could considerably reduce PCDD/F emissions. The source identification and quantification of PCDD/Fs and heavy metals are key steps in controlling and regulating their emissions and in effectively reducing further exposure.17 The removal of pollutants from flue gas through air pollutant cleaning systems (APCS) produces solid residues. These residues include a certain amount of bottom and fly ashes that may contain significant levels of dioxins and heavy metals. The effective disposal of these residues without affecting the environment is a persistent issue. Studies on the mass balance and distribution of dioxins and heavy metals in the SS cocombustion process in power plants are significant. Information from these studies aids in the determination of how and where dioxins and heavy metals are emitted. Dioxin mass balance and distribution from MSWIs have been extensively investigated.18−20 Van de Velden et al.21 discussed the distribution behavior of eight heavy metals (Hg, As, Cd, Cu, Pb, Cr, Ni, and Zn) in different residues from a large-scale fluidized bed sludge combustor. However, few studies focused on the real mass balance and distribution behavior of PCDD/Fs and heavy metals during SS and coal cocombustion in power plants. The present study is based primarily on field measurements in a full-scale pulverized coal power plant. This study aims to characterize the emission, mass balance, and distribution of PCDD/Fs and heavy metals extensively based on the cocombustion of 5%, 10%, 15%, and 20% SS and coal. SS treated in the power plant as well as flue gas emissions, bottom
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MATERIALS AND METHODS SS and Coal Characterization. The SS used in this study was obtained from an industrial wastewater treatment plant in South China. The SS was dried using high-pressure squeezing equipment. The SS moisture content was reduced from 75% to 20%. Table 1 shows an exhaustive characterization of the SS
Table 1. SS and Coal Characterization characterization proximate analysis moisture content (wt %) ash (wt %) volatile matter (wt %) fixed carbon (wt %) low calorific value (MWh/kg) elemental analysis (wt %) C H N S Cl O heavy metals (mg/kg) Hg Cd Pb Cr Cu PCDD/Fs total ng I-TEQ/kg
sewage sludge
coal
20.00 46.88 24.98 8.14 1.512 × 10−3
5.67 36.90 7.45 49.98 9.769 × 10−3
28.50 4.32 2.56 0.21 2.98 12.30
65.40 2.14 1.24 0.75 0.17 4.28
1.48 2.80 3.80 198.13 181.50
0.21 0.50 2.00 20.70 12.90
23.43
and coal used in the experiment. The ash content was determined through calcinations at 850 °C. Calorific values, which amounted to 1.512 × 10−3 MWh/kg for SS and 9.769 × 10−3 MWh/kg for coal, were determined using a WZR-1T-CII microcomputer calorimeter. The low calorific value of SS can be attributed to its low carbon and high water content. Although sulfur content was approximately 4 times higher in coal than in SS, the chlorine content of coal was approximately 17 times lower than that of SS. Moreover, SS contained a higher heavy metal content than coal. As shown in Table 1, the SS contained high concentrations of several heavy metals such as Cr and Cu. Thus, the SS cannot be used as a fertilizer, thereby making incineration the rational method for SS disposal. Basic Information on the Pulverized Coal Power Plant. The investigation was conducted in a pulverized coal power plant in South China. Figure S1 shows the flow sheet and sampling site of the power plant. Dried SS and coal were blended using a mixer. The MF obtained from the dried SS and coal was pulverized via milling and then introduced into the combustor to achieve high combustion efficiency. The power plant produced 130 t/h of steam (3.82 MPa/450 °C) and consumed 20 t/h of fuel. The semidry scrubber (SDS) and bag filter (BG) were used as APCS to control gaseous emissions. To evaluate the emission and distribution characteristics of 2124
dx.doi.org/10.1021/es304127k | Environ. Sci. Technol. 2013, 47, 2123−2130
Environmental Science & Technology
Article
values of the different components fluctuated within a narrow range, indicating that the four sampling series were performed under stable operating conditions. PCDD/F Emission, Mass Balance, and Distribution Characteristics. Table 3 shows the I-TEQ values and emission
PCDD/Fs and heavy metals during SS and coal cocombustion, 5%, 10%, 15%, and 20% (w/w) SS were added to the coal, for a total of four series of runs. In each series, flue gas, bottom ash, and fly ash samples were collected thrice from the corresponding sampling points. The average value was determined from the three PCDD/F and heavy metal values. Sampling and Analytical Methodologies. The PCDD/F flue gas samples were sampled isokinetically from the stack sampling point with a sampling time ranging between 120 and 180 min, following the American Standard Method EPA 23A. Before flue gas sampling, 13C12-labeled sampling standards were spiked into an Amberlite XAD-2 resin. During the cocombustion of 10% SS, bottom and fly ash samples were simultaneously collected every 30 min until the samples weighed 3 kg. The methods used for the PCDD/F analysis were adopted from EPA Method 1613B. The analytical procedures were reported previously.18 Instrument analysis was conducted using a high-resolution gas chromatograph coupled with a high-resolution mass spectrometer (HRGC/ HRMS; an analysis is presented in the Supporting Information (SI)). The sampling of heavy metals required 60−120 min of isokinetic sampling at the stack sampling point. Heavy metals in the gas phase were trapped in the acidic solutions kept in impingers (except for Hg, which was trapped in acidic KMnO4 solution). For the four series, the bottom and fly ash samples were simultaneously collected every 30 min during flue gas sampling until the samples weighed 3 kg. A quantification analysis was performed using inductively coupled plasma with a mass spectroscopy detector (Elan DRC-e).
Table 3. PCDD/F Emission and Mass Balance during SS and Coal Cocombustion run series input output
flue gas
fly ash
RESULTS AND DISCUSSION Regular Parameters and Operating Conditions. The following regular parameters were continuously monitored in each sampling series: O2, NOx, SO2, and HCl. The normal cubic meter (Nm3) was calculated at 0 °C, 101.325 kPa, and at an oxygen level of 11%. Table 2 shows the average values,
5% SS 10% SS 15% SS 11.1 87 113 2.8 950 185 172 165
11.3 95 99 3.5 935 187 173 164
10.8 111 77 4.2 940 184 171 155
15% SS
20% SS
1.172
2.343
3.515
4.686
0.04
0.06
0.15
0.18
0.006
0.009
0.023
0.027
7.00
18.93
29.95
32.72
0.034
0.093
0.147
0.160
1.32
3.43
5.26
5.78
0.030
0.077
0.118
0.130
51
48
49
50
0.064
0.170
0.265
0.290
0.070
0.179
0.288
0.317
−1.101
−2.164
−3.227
−4.369
factors of the PCDD/Fs in the stack flue gases from the cocombustion of 5%, 10%, 15%, and 20% SS with coal. The total emission I-TEQ values of 5%, 10%, 15%, and 20% SS were 7.00, 18.93, 29.95, and 32.72 pg I-TEQ/Nm3, respectively. These values correspond to emission factors of 0.034, 0.093, 0.147, and 0.160 μg I-TEQ/t-MF, respectively. The emission factors for 5%, 10%, 15%, and 20% of SS, respectively, accounted for approximately 2.0%, 5.4%, 8.5%, and 9.3% of the value measured from SS monocombustion (1.732 μg I-TEQ/tSS).9 Cofiring SS with coal reduced PCDD/F emissions significantly. Significant differences were observed between the SS cocombustion proportion and that of the cocombustion and monocombustion emission factors. These differences indicate that changes in emission factors attributed to the cocombustion of different proportions of SS with coal did not result from dilution. The factors affecting PCDD/F formation are considered in the following section. The emission results indicate that all I-TEQ values of the PCDD/Fs provided emissions that were lower than the legal limit (max. 100 pg ITEQ/Nm3). To evaluate the real situation of dioxin emission in a full-scale power plant that cofires SS with coal, dioxin distribution characteristics were performed during the 5%, 10%, 15%, and 20% SS cocombustion. The dioxin input via the MF is compared with the sum of the dioxin output in flue gas, fly ash, and bottom ash. Table 3 shows that the dioxin mass balance is based on the calculation of the dioxin emission factor across the entire power plant. The fly ash samples presented dioxin levels of 1.32−5.78 ng I-TEQ/kg for 5% SS to 20% SS, which met the Japanese environmental quality standards for soil (less than 1000 ng I-TEQ/kg). However, the dioxin levels from bottom ash were 0.04−0.18 ng I-TEQ/kg, which were lower than those
Table 2. Regular Parameters and Operating Conditions during SS and Coal Cocombustion series
μg I-TEQ/ t-MF ng I-TEQ/ kg μg I-TEQ/ t-MF pg I-TEQ/ Nm3 μg I-TEQ/ t-MF ng I-TEQ/ kg μg I-TEQ/ t-MF %
removal efficiency of APCS sum of flue gas and fly μg I-TEQ/ ash t-MF total output μg I-TEQ/ t-MF mass balance μg I-TEQ/ t-MF
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O2 (%) NOx (mg/Nm3) SO2 (mg/Nm3) HCl (mg/Nm3) middle temp. of furnace (°C) exit temp. of boiler (°C) exit temp. of semidry scrubber (°C) exit temp. of bag filter (°C)
mixed fuel (MF) slag
10% SS
5% SS
20% SS 11.2 85 53 5.0 928 183 174 160
which are lower than those provided by relevant Chinese standards. HCl emission apparently increased as the amount of SS with high chlorine content increased. The SO2 reduction when the SS increased resulted from the low amount of coal introduced into the combustor, which consequently reduced the amount of sulfur in the mixture. Accordingly, the SO2 and HCl emissions in the pulverized coal power plant are strongly correlated with the sulfur and chlorine contents of MF. Considering that 90−100% of the chlorine is converted into HCl during combustion, high chlorine content linearly results in higher HCl emissions.22 The continuously monitored operating conditions are shown in Table 2. The temperature 2125
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of the fly ash. Therefore, high amounts of dioxins are transferred into fly ash in the SDS and BG process. In terms of dioxin percentage distribution, the bottom ashes constitute a minor fraction of the total dioxin output (approximately 5%− 9%), whereas the dioxin is distributed almost equally between flue gases (approximately 49%−52%) and fly ashes (approximately 41%−43%) at the four cocombustion proportions. This result differs from that reported by previous research. By investigating the dioxin mass balance in a modern MWSI, previous researchers18 found that fly ashes contributed approximately 85% to the total output, and flue gases contributed roughly 1%. These results may be attributed to the injection of activated carbon for dioxin removal from MSWI, a process that generates dioxin-enriched fly ash. Table 3 shows that the total dioxin output values are approximately 0.070−0.317 μg I-TEQ/t-MF, which are lower than the total input value (approximately 1.172−4.686 μg I-TEQ/t-MF), thus suggesting negative balance of 1.101−4.369 μg I-TEQ/t-MF during the cocombustion of 5%, 10%, 15%, and 20% SS. The dioxins present in MF are destroyed during the hightemperature process, but are then regenerated via heterogeneous catalytic reactions in off-gas treatment systems, such as waste heat recovery boiler or APCS. In the sampling campaign, the negative balance indicates that the power plant cofiring SS with coal is not a source but a sink of dioxins. Table 3 shows that PCDD/F emissions in the flue gas and fly ash tend to increase as the amount of SS increases. This increase may be related to the sulfur and chlorine contents of SS and coal. In terms of emission factors, the sum of PCDD/Fs emitted into the flue gas and fly ash has been calculated for the purpose of investigating PCDD/F formation. Figure 1 shows
combusting SS spiked with a surrogate organic mixture at a high chlorine dosage.13 Griffin24 suggested that the reaction of Cl2 + SO2 + H2O ↔ 2HCl + SO3 enhanced the conversion of Cl2 into HCl, thus inhibiting the substitution of Cl2 into aromatic structures. SO2 could still react with the Deacon process catalyst (CuO) to form copper sulfate (CuSO4), which is less active than CuO as a catalyst in the production of Cl2.25 Another possible effect of sulfur content in coal is the preferential formation of sulfur analogues into PCDD and PCDF, namely, polychlorinated thianthrene and dibenzothiophene,26 respectively. As shown in Figure 1, sulfur content decreases with increased MF content. Although the suppressant effect of sulfur on the PCDD/F formation is confirmative, the mechanism that is primarily responsible for such effect remains unclear and requires further study. The chlorine and sulfur synergies may facilitate changes in PCDD/F formation. Thus, the effect of the S/Cl molar ratios on PCDD/F formation was examined in the study. Figure 1 shows the relationship of S/Cl molar ratios with the PCDD/F sum of flue gas and fly ash. As the molar ratio increased from approximately 1.0 to 2.5, the PCDD/F formation gradually decreased. Chang et al.27 also concluded that PCDD/Fs decrease when the S/Cl molar ratio is greater than 1. Therefore, high sulfur content and low chlorine levels are unfavorable to the PCDD/F formation. It is worth noting that the same tendency can be obtained by comparing the PCDD/F emissions in flue gas and the sulfur and chlorine content. This is because the removal efficiencies achieved with APCS stay fairly stable (approximately 51%, 48%, 49%, and 50% for cocombustion proportion 5% to 20%, respectively), as shown in Table 3. It can be concluded that high sulfur content and relatively low chlorine levels result in lower PCDD/F emissions in the power plant. Several factors can affect the PCDD/F emission during SS cofiring with coal in the pulverized coal power plant. First, furnace temperature is significant. When high temperature is maintained, destruction efficiency becomes extremely high, whereas the PCDD/F emission at the furnace exit is expected to be extremely low. At temperatures above 850 °C, any dioxins/furans present in the waste will be destroyed.28 After the dioxins/furans are destroyed, the main strategies used to control the emissions are the utilization of an appropriate type of APCS and the prevention of the recombination of dioxins/ furans in the flue gas passageway. The raw gas at the boiler exit must be subjected to APCS treatment before being released into the atmosphere upon reaching stringent limits. A higher APCS removal efficiency results in lower PCDD/F emission. Finally, the flue gas temperature in the different APCS components is also an important parameter. Heterogeneous catalytic reactions, which include de novo and precursor synthesis, occur in the temperature window of 200−400 °C.29 The SDS and BG should be operated below 200 °C. Based on the results, the power plant we investigated safely emitted low dioxins during SS cofiring with coal, which may also be attributed to the high combustion temperature (>900 °C), effective off-gas treatment (SDS + BF), and low flue gas temperature in the APCS (
