Field Measurements on the Emission and Removal of PM2.5 from

Jul 14, 2016 - *E-mail: [email protected]; Telephone: +86-27-87546631; Fax: +86-27-87545526 ... a limestone-gypsum wet flue gas desulfurization (WFGD) ...
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Field Measurements on the Emission and Removal of PM2.5 from Coal-Fired Power Stations: 1. Case Study for a 1000 MW Ultrasupercritical Utility Boiler Xiaowei Liu,† Yishu Xu,† Xianpeng Zeng,† Yu Zhang,† Minghou Xu,*,† Siwei Pan,‡ Kai Zhang,‡ Li Li,‡ and Xiangpeng Gao§ †

State Key Laboratory of Coal Combustion, Huazhong University of Science and Technology, Luoyu Road 1037, Wuhan 430074, China. ‡ Electric Power Research Institute of Guangdong Power Grid Corporation, Guangzhou 510080, China § Discipline of Electrical Engineering, Energy and Physics, School of Engineering and Information Technology, Murdoch University, 90 South Street, Murdoch, WA 6150, Australia. S Supporting Information *

ABSTRACT: Enlarging the capacity of utility boiler is recognized as a good way to improve the electricity generating efficiency. An increasing number of 1000 MW ultrasupercritical (USC) utility boilers are installed in China. This contribution reports the results of systematic field measurements on PM2.5 [particulate matter (PM) with aerodynamic diameters < 2.5 μm] emitted from a 1000 MW USC utility boiler equipped with a selective catalytic reduction (SCR) denitrification (DeNOx) unit, two electrostatic precipitators (ESPs), and a limestone-gypsum wet flue gas desulfurization (WFGD) system. The PM samples were collected using a Dekati low pressure impactor (DLPI) and/or a Dekati gravimetric impactor (DGI) at multiple sampling sites. The results demonstrate that the particle size distributions (PSDs) of the PM at both inlet and outlet of the SCR unit exhibit a bimodal distribution, with a fine mode at < 0.3 μm and a coarse mode at > 0.3 μm. Passing the PM-containing flue gas through the SCR leads to the PSDs of the fine mode particles being shifted to larger size, possibly due to the formation of ammonium sulfate and/or ammonium bisulfate as well as the reduction of flue gas temperature. The removal efficiencies of the SCR for PM1 and PM2.5 are 14.3−33.6% and 13.3−30.5%, respectively, depending on the boiler load. The ESPs substantially reduce the mass concentrations of PM1 and PM2.5 from 84.6−107 mg/Nm3 and 417−440 mg/Nm3 to 0.298−1.22 mg/Nm3 and 0.812−4.41 mg/ Nm3, respectively, with overall removal efficiencies of 98.7−99.7% for PM1 and 99.0−99.8% for PM2.5. The WFGD process leads to the disappearance of the fine mode PM and an overall removal efficiencies of up to 28.7% for PM1 and 39.6% for PM2.5. Moreover, promoting the installation of 1000 MW USC utility boilers is likely to simultaneously achieve the reduction in the PM2.5 emission besides the improvement of electricity generation efficiency, particularly when advanced dust removal devices are employed. the PM2.5 emission was observed to increase from 421 mg/m3 to 552 mg/m3 as the boiler load improved from 70 to 100%. In the past few years, more and more USC boilers are implemented to improve the efficiency and reduce the pollutant emission in power stations.19,20 In China, over 60 ultrasupercritical boilers of capacity of 1000 MW are running in the power stations up to 2014, accounting for ∼8% of the total capacity.21 PM after the furnace would pass through selective catalytic reduction (SCR) denitrification (DeNOx) units, dust collectors [electrostatic precipitators (ESPs), baghouse, etc.], and wet flue gas desulfurization (WFGD) in the coal-fired utility power stations. The dust collectors can capture most of the PM, while the SCR unit and WFGD are also reported to affect the PM emission.22−25 For example, Li et al.23 reported that the SCR DeNOx process increased the contents of NH4+ and SO42− in the primary PM2.5 and finally increased the PM2.5 emission. And in our previous study on a 300 MW power

1. INTRODUCTION Particulate matter (PM) with aerodynamic diameters of < 2.5 μm (PM2.5) is a major atmospheric pollutant due to its adverse impacts on both environment and human health.1,2 In China, PM2.5 has become one of the most critical pollutions and one of the major emission sources is coal-fired power stations.3−5 To control the emission of PM2.5 from coal-fired power stations, basic information on its formation and removal characteristics is essential, which necessitates the field measurement. Extensive field studies have been carried out to characterize the PM emission characteristics of the coal-fired utility boilers, covering the boilers of various structures (tangential fired, front wall fired, opposed wall fired, etc.) and different capacities (∼215−1000 MW).6−13 PM is mainly derived from the minerals in coal and its formation progresses depend strongly on the combustion conditions, which vary distinctly with the boiler structure, capacity as well as the operating load.9,12,14−17 Studies on two different large utility boilers (615 MW and 510 MW) showed that the influence of the boiler structure on the formation of PM was even more important than the fuel property.7,18 In our previous study on a 200 MW utility boiler,9 © XXXX American Chemical Society

Received: February 22, 2016 Revised: July 3, 2016

A

DOI: 10.1021/acs.energyfuels.6b00423 Energy Fuels XXXX, XXX, XXX−XXX

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Figure 1. Schematic diagrams of (a) the power station and PM sampling sites, (b) PM sampling systems, and (c) PM sampling conditions. DLPI-D means the DLPI sampling system with a two-stage dilution module (total dilution factor: 64). DLPI-H denotes the DLPI system with all the sampling instruments and connection lines being heated to 130 °C.

Thus, this study aims to conduct systematic field measurements of the PM2.5 emitted from a 1000 MW USC utility boiler equipped with an SCR DeNOx unit, two ESPs, and a limestonegypsum WFGD system. The PM samples were collected using a Dekati low pressure impactor (DLPI) and/or a Dekati gravimetric impactor (DGI) at multiple sampling sites, including the inlets and outlets of the DeNOx unit and the WFGD system. Here, the DeNOx outlet and the WFGD inlet correspond to the inlet and outlet of the ESPs, respectively. The collected PM samples were characterized in terms of mass concentrations and particle size distributions (PSDs) and some selected PM samples were also subjected to morphology and chemical composition analyses. The influences of flue gas cleaning devices, including the SCR unit, the ESPs, and the WFGD system, on the emission behavior of PM2.5 were discussed.

generating unit, the SCR process was also observed to change the PSDs of the PM.26 As the capacity of units scale up, dimension and structure of the utility boiler as well as the flue gas cleaning devices are changed, which may affect the formation and emission of various pollutants. However, as far as we know, only one report on the PM emission of the 1000 MW USC boiler was available in public, which only reported the PM concentrations at the inlet and outlet of ESPs.22 Detailed information of the PM along the flue gas cleaning process, presenting the formation and control characteristics of the PM in the coal-fired power stations, is still not well-known. Therefore, more efforts are needed to make clear the formation and emission characteristics of the PM from the coal-fired power stations equipped with 1000 MW ultrasupercritical boilers. Abovementioned information on PM2.5 emission from the large scale utility units is also of great importance for decision-makers to optimize the structure of the coal-fired power generation industry in China in addition to the energy efficiency.

2. EXPERIMENTAL METHODS The field measurements were carried out on a 1000 MW USC coalfired power station unit, which was schematically illustrated in Figure B

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cyclone, the DLPI, and their connection lines were heated to 130 °C to avoid the condensation of the acidic gases (e.g., SO3 and HCl). This method was suitable for sampling the PM of lower concentrations. For the DLPI-D method, a two-stage dilution module with a total dilution ratio of 64 was set between the cyclone and the DLPI.29 The flue gas at the outlet of the cyclone (set at 130 °C) was first diluted by a stream of hot (130 °C) purified air with a dilution ratio of 8, and then further diluted by purified air at ambient temperature with a dilution ratio of 8. For both methods, the PM larger than 10 μm was first separated in the cyclone and that less than 10 μm (i.e., PM10) was size classified into 13 stages via the DLPI and collected on the sampling filters (either aluminum foil or polycarbonate membranes). High temperature Apiezon (Apiezon-H) grease was coated on the sampling filters to avoid particle bounce. The mass of PM10 was obtained by weighing the filters before and after an experiment using a microbalance (SARTORIUS MSA6.6S-0CE-DM). To ensure the reproducibility of the data, three parallel runs were conducted under each condition, with the average values being reported. For comparison, the PM samples collected at the inlet of the ESP (sampling site 2#) were collected using both the DLPI-H method and the DLPI-D method. The PSDs and concentrations of the PM10 obtained from the two methods are presented in Figure S1 of the Supporting Information. Clearly, the PSDs and concentrations of the PM2.5 obtained from the two methods agreed well with each other, confirming that both methods were suitable for PM sampling at the ESP inlet. To collect sufficient amount of PM samples for the compositional analysis, the DGI sampling system was employed at the inlets and outlets of the SCR and WFGD. The DGI sampling system was composed of a Dekati cyclone, a DGI, an air compressor, two mass flow controllers and a vacuum pump.13,28 Similar to that of the DLPI sampling system, ∼ 10 L/min of flue gas after the sampling probe was first directed through the cyclone to remove the particles larger than 10 μm, after which the flue gas was diluted with 60 L/min purified air before entering the DGI. In the DGI, the PM samples were sizesegregated into four size fractions (i.e., 0.2−0.5, 0.5−1, 1−2.5, and 2.5−10 μm) and collected on the polycarbonate membranes, which were dried at 45 °C for further analysis. The collected PM and total fly ash samples were tested with X-ray fluorescence probe (XRF, EAGLE III, EDAX Inc.) to obtain their chemical compositions. Selected PM samples were also subjected to morphology characterization using a field emission scanning electron microscope (FESEM, Sirion 200, FEI Inc.), with a thin layer of Pt coated onto the samples to improve the conductivity.30 Slurry in WFGD was sampled during the measurement, which was further dried at 100 °C and tested with the XRF.

1a. The power station unit consisted of an once-through tower type steam boiler (Model: SG3091/27.56-M54X), an ammonia (NH3)sprayed SCR DeNOx unit (termed as “SCR” hereafter), two threeroom four-electric-field ESPs, and a limestone-gypsum WFGD system, the detailed information of which was summarized in Table 1. The

Table 1. Key Information of the Boiler, SCR DeNOx unit, ESP, and WFGD facility

items

parameters

boiler capacity, t-steam/h dimension, m rating flue gas flow, Nm3/h, 6% O2

3091 21.48×21.48 3324959

dimension of the reactor (W×L×H), m number of reactor, # catalyst layer(s) catalyst structure pitch, mm catalyst component reducing agent NH3/NOx

14.86×27.5×12.3

SCR

1 2+1 honey comb 7 V2O5-WO3/TiO2 ammonia 0.815

ESP cathode-anode type chamber*field, # specific collection area, m2/ m3s−1 flow velocity, m/s residence time, s power supplya

wire-plate (3*4)*2 90.80

power capacity, kVA

0.93 18.4 HVHV HVPF HVPF HVHF 6 × 189 + 6 × 185 + 12 × 165

tower structure D×H, m spray layer(s) demister layer(s) sorbent liquid−gas ratio

spray tower 19.5×38.55 4 2 limestone 19

WFGD

a

HVPF: high voltage power frequency; HVHF: high voltage high frequency.

3. RESULTS AND DISCUSSION 3.1. Particle Size Distributions (PSDs) of PM at the Inlet and Outlet of the SCR Unit. Figure 2 and Figure 3 depict the particle size distributions of the PM at the inlet and outlet of the SCR as well as the fractional PM removal efficiencies of the SCR, under three boiler loads. In addition, the mass concentrations of PM1 (i.e., PM of aerodynamic diameters < 1 μm) and PM2.5 are summarized in Table 2. At least four important findings can be made from these data. First, the PSDs of the PM at both inlet and outlet of the SCR exhibit a bimodal distribution, with a fine mode at < 0.3 μm and a coarse mode at > 0.3 μm. The fine mode PM is mainly resulted from the vaporization−condensation of inorganic species,4,16,31−34 whereas char fragmentation, included minerals coalescence and excluded minerals fragmentation are major mechanisms responsible for the coarse mode PM formation4,16,35,36 PM2.5 consists of both fine mode PM and coarse mode PM, indicating that its formation is governed by the combined mechanisms aforementioned.

utility boiler burned blends of two bituminous coal and its typical fuel properties were given in Table S1 of the Supporting Information. The PM samples were collected at four sampling sites, including the inlets and outlets of the SCR and the WFGD system, under the boiler load of 100, 90, and 50%. Hereinafter, PM samples collected at the SCR outlet and the WFGD inlet correspond to those at the inlet and outlet of the ESPs, respectively. The PM samples at each sampling site were collected following the procedures described in our prior study.13,27 Briefly, a stream of flue gas (flow rate: 10 L/min) was isokinetically extracted from the center of the duct section into a high-temperature (130 °C) sampling probe and then introduced into either a Dekati low pressure impactor (DLPI) sampling system or a Dekati gravimetric impactor (DGI) sampling system for PM collection (see Figures 1b and 1c). The DLPI sampling system consisted of a Dekati cyclone (SAC-10), a DLPI, a pressure gauge (KELLER LEO 2), and a vacuum pump (SV25B).27,28 Depending on the concentration of the PM in the flue gas, either a heating method (termed as “DLPI-H”) or a dilution method (termed as “DLPI-D”) was employed. For the DLPI-H method, the C

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in the SCR. The vanadium−tungsten−titanium (V−W−Ti) SCR catalyst used, in particular the V component, is active in oxidizing SO2 in the flue gas into SO3.37,38 The formed SO3 can rapidly react with water (H2O) vapor and NH3, forming (NH4)2SO4 and/or NH4HSO4 particles.23 The formation of these particles is indirectly supported by the considerable increase in the SO3 concentration in the PM with aerodynamic diameter of 0.2−0.5 μm (PM0.2−0.5) at the outlet of the SCR, in comparison with that in PM0.2−0.5 at the inlet (see Figure 4a),

Figure 2. Particle size distributions of PM2.5 at inlet and outlet of the SCR DeNOx unit at boiler load of (a) 100%, (b) 90%, and (c) 50%.

Figure 4. Chemical composition of (a) PM0.2−0.5 and PM0.5−1 at the inlet and outlet of the SCR DeNOx unit at 90% boiler load, as well as (b) the total PM at inlet of ESP at 90% and 50% boiler load. Figure 3. Fractional PM removal efficiencies of the SCR DeNOx unit at 100%, 90%, and 50% boiler load.

without notable change in PM0.5−1. Once formed, these (NH4)2SO4 and/or NH4HSO4 particles may increase the number concentration of fine particles and thereby enhance their agglomeration and growth.23 The other reason is the reduction of the flue gas temperature from ∼350 °C at the SCR inlet to ∼135 °C at the SCR outlet (i.e., the ESP inlet). Decreasing the flue gas temperature results in enhanced relative motions among the particles and thereby the agglomeration process is enhanced. Moreover, the chemical composition of the PM passing through the SCR unit varies with the particle size, which is supposed to lead to different impacts on health and environment. As can be seen in Figure 4a,b, fine PM (e.g., PM0.2−0.5) has a higher S content than the larger PM (e.g., PM0.5−1) and total PM; while the former has lower contents of Fe, Si, and Al. Existing studies reveal that the distribution of some heavy metals, such as Hg, is correlated to the distribution of Fe and unburned carbon,39,40 implying possible different contents of the heavy metals in the fine and coarse PM. However, contents of organic components (e.g., PAHs and unburned carbon) or heavy metals (e.g., Hg) in the PM are not determined in the present study, and further exploration on this aspect is still required in the future. Third, despite the formation of (NH4) 2 SO4 and/or NH4HSO4 particles and the enhanced agglomeration of the fine particles, the concentrations of PM1 and PM2.5 at the SCR outlet are considerably lower than those of the PM at the inlet,

Table 2. Mass Concentrations of PM1 and PM2.5 at Different Sampling Site and PM Removal Efficiencies of the SCR DeNOx Unit, ESP, and WFGD 100% load SCR inlet, mg/Nm3 SCR outlet, mg/Nm3 removal efficiency of SCR, % ESP outlet, mg/Nm3 removal efficiency of ESP, % WFGD outlet, mg/ Nm3 removal efficiency of WFGD, % a

90% load

50% load

PM1

PM2.5

PM1

PM2.5

PM1

PM2.5

148 107 28.0

632 440 30.5

127 84.6 33.6

551 429 22.2

118 101 14.3

481 417 13.3

1.22 98.9

4.41 99.0

1.13 98.7

3.41 99.2

0.298 99.7

0.812 99.8

nda

nda

0.809

2.06

0.427

0.827

nda

nda

28.7

39.6

−43.4

−1.96

nd = not determined.

Second, the PSDs of the fine mode PM at the outlet of the SCR shift to larger size in comparison with their inlet counterparts, possibly due to two reasons. One is the formation of ammonium sulfate [(NH4)2SO4] and/or ammonium bisulfate [NH4HSO4] as a result of ammonia (NH3) injection D

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explains the influence of boiler load on the PM removal efficiency of the SCR. 3.2. Removal of PM2.5 via the ESPs. The PSDs of the PM before and after the ESPs are shown in Figures 2 and 5a, respectively. And the compositions of the PM in the selected size fraction (PM0.2−0.5) before and after the ESPs are presented in Figures 4a and 7, respectively. It should be noted that the SCR outlet and WFGD inlet are equivalent to the inlet and outlet of the ESP, respectively. Compared to the PSDs of the PM at the ESP inlet (see Figure 2a and c), those of the PM at the outlet remain bimodal distribution (see Figure 5a). However, the mass concentrations of PM1 and PM2.5 at the ESP outlet are reduced by 98.7−99.7% and 99.0−99.8%, respectively. Figure 5b illustrates the PM removal efficiencies of the ESPs as a function of particle size. Qualitatively consistent with that reported in the previous studies,6,13,43 the PM of sizes between 0.1 and 1 μm is not effectively removed because these particles are in the transition zone between diffusion charging and field charging and thereby more difficult to be charged.6,13,43 As for the chemical composition, after passing through the ESPs, S content in PM0.2−0.5 increases a little, which is most possibly caused by the decrease of flue gas temperature (see Figures 4a and 7). No obvious changes in the contents of other components are observed. Figure 5b also indicates that the fractional PM removal efficiency of the ESPs depends on the boiler load. Decreasing the boiler load from 100% and 90 to 50% leads to a considerable increase in the PM removal efficiency of the ESPs, possibly due to two reasons. One is the variation on the chemical composition of the PM at the ESP inlet produced at different boiler loads. As shown in Figure 4b, the concentration of SO3 in the PM at the ESP inlet produced at 50% boiler load is much higher than that in the PM produced at 90% load. A higher SO3 content leads to a lower specific resistance of the particles, which enhances the charging of the PM and thereby promotes their removal in the ESPs.13,44,45 The other reason is the reduction in the flow rate of the flue gas produced at 50% load, in comparison with that of the flue gas generated at 90% load. As a result, the ESPs’ specific collection area, defined as

with corresponding removal efficiencies of 14.3−33.6% and 13.3−30.5%, respectively. The removal of the PM in the SCR is most likely due to the deposition of these particles on the surface of the SCR catalyst.23 A closer examination suggests that the PM removal efficiency of the SCR is dependent on particle size (see Figure 3). Whereas the removal efficiencies of the SCR for the PM with aerodynamic diameters of < 0.1 μm and > 0.3 μm are generally in the range of 20−60%, those for the PM of sizes between 0.1 and 0.3 μm are generally very low or even negative. This is most likely due to both the agglomeration of ultrafine particles smaller than 0.1 μm and the formation of (NH4)2SO4 and/or NH4HSO4 particles.41,42 Fourth and last, the boiler load has effects on both the mass concentrations of PM1 and PM2.5 at the inlet and outlet of the SCR as well as its PM removal efficiency. Increasing the boiler load from 50 to 100% leads to the increases in the mass concentrations of PM1 and PM2.5 by 25.9% and 31.4% at the SCR inlet, as well as 5.78% and 5.36% at the SCR outlet. In addition, reducing the boiler load from 100 to 50% leads to a significant increase in the removal efficiency of the SCR for the PM smaller than 0.1 μm (PM0.1) but a considerable reduction in that for the PM of sizes between 0.1 and 0.3 μm (PM0.1−0.3). At a lower boiler load, less flue gas is processed in the SCR and more NH3 is injected to maintain the NOx removal efficiency (see Table 3). This results in a longer residence time and lower Table 3. Operating Conditions and Parameters of SCR DeNOx Unit. item

100% load

90% load

50% load

temperature at inlet, °C flue gas flow rate, m3/h NOx, mg/Nm3 NH3 injection rate, kg/h

386 3 155 064 186 235

377 2 998 866 196 261

338 2 403 676 376 284

temperature of the flue gas in the SCR, as well as more (NH4)2SO4 and/or NH4HSO4 particles being formed. As a result, the agglomeration of ultrafine PM (e.g., PM0.1) is enhanced and more PM0.1−0.3 is formed after the SCR. This

Figure 5. Panel (a) particle size distributions of PM2.5 collected at the ESP outlet (i.e., WFGD inlet) at 100, 90, and 50% boiler load. Panel (b) fractional PM removal efficiency of the ESP. Panel (c) particle size distributions of PM2.5 collected at the WFGD outlet at 90% and 50% boiler load. Panel (d) fractional PM removal efficiency of the WFGD. E

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Energy & Fuels the amount of collection electrode area for every cubic meter of flue gas, is substantially increased and the treating time is extended, leading to the higher PM removal efficiency of the ESP at 50% boiler load according to the Deutsch formula.44 3.3. Influence of WFGD on the Emission Behavior of PM2.5. Figure 5c presents the PSDs of the PM collected at the WFGD outlet at the boiler loads of 50% and 90%. As mentioned above, the PSDs of the PM at the WFGD inlet are equivalent to those at the inlet of the ESP. Considering those of the PM at the inlet and outlet of the WFGD together, the fractional PM removal efficiency of the WFGD is calculated and illustrated in Figure 5d. In addition, the mass concentrations of PM1 and PM2.5 at the inlet and outlet of the WFGD as well as its total PM removal efficiency are summarized in Table 2. There are at least two important findings from these data. First, whereas the PSDs of the PM at the WFGD inlet show a bimodal distribution (i.e., the ESP outlet, see Figure 5a), those of the PM at the WFGD outlet demonstrate a unimodal distribution, with the fine mode at < 0.3 μm disappeared. This can be attributed to the enhanced aggregation of these fine mode particles due to the entrainment of fine droplets from gypsum slurry.22 The enhanced aggregation of the fine mode particles is evidenced by observing the morphology of the PM with aerodynamic diameters of 0.09−0.16 μm (PM0.09−0.16) collected at the inlet and outlet of the WFGD at 90% boiler load. As shown in Figure 6, whereas PM0.09−0.16 collected at the

Figure 7. Chemical composition of PM0.2−0.5 collected at the inlet and outlet of the WFGD at 90% boiler load as well as the gypsum slurry.

indicates the formation of new particles in this size range as a result of aforementioned entrainment of gypsum slurry droplets. Overall, the removal efficiency of the WFGD for PM2.5 at 90% boiler load is ∼39.6%, whereas no obvious removal of PM2.5 via the WFGD is observed at 50% boiler load (see Table 2). 3.4. Practical Implications. From a practical viewpoint, it is important to compare the yield of PM2.5, i.e., milligrams of PM2.5 emitted per kilowatt electricity produced at the gate of a power station, from utility boilers of different capacities. Figure 8 compares the yields of PM2.5 at the inlet and outlet of the

Figure 6. Morphorology of PM0.09−0.16 collected at (a) inlet and (b) outlet of the WFGD. Figure 8. Comparison on the yield of PM2.5 at (a) inlet and (b) outlet of the PM control equipment in utility boilers of different capacities. In the figure above, reference data are taken from C. Wang et al.,13 H. Wang et al.,24 X. Liu et al.,9 H. Yi et al.,29 Z. Li et al.,23 Z. Zhao et al.,48 P. Lu et al.,49 Y. Xu,50 and some unpublished studies. Briefly, data from H. Yi et al.29 are directly reported values while other values are calculated based on the PM concentrations in the literature, assuming that O2 concentration was 5% and coal consumption rates of boilers are 328, 318, 309, 300, and 280 g/(kWh) for 100−200, 200−300, 300−600, 600−1000, and 1000 MW boilers. Unreported calorific values of coal are calculated via the Mendeleev formula.51

WFGD inlet is aggregated together in chains of small particles, that at the WFGD outlet is further aggregated in clusters, indicating that an enhanced aggregation of these fine particles takes place when the flue gas passing through the WFGD system. On the other hand, the entrainment of fine gypsum slurry droplets is supported by the variation in the chemical composition of the PM with size of 0.2−0.5 μm (PM0.2−0.5) collected at the inlet and outlet of the WFGD. As demonstrated in Figure 7, after passing through the WFGD unit, the contents of S, Ca, and Na in the PM0.2−0.5 increase notably. The three elements are major inorganic species in the gypsum slurry (see Figure 7), similar to those previously reported.46,47 Second, the fractional PM removal efficiency of the WFGD is dependent on the particle size. Generally, the PM with aerodynamic diameters of < 0.2 μm can be effectively removed with an efficiency of up to 75%, most likely due to the aforementioned enhanced aggregation of these fine particles. Furthermore, ∼65−90% of the PM with sizes larger than 2 μm is captured most likely by inertial forces. Similar finding is also reported in a prior study.8 Oppositely, the WFGD’s removal efficiency for the PM with sizes of 0.2−2 μm is relatively low, and even below zero at the 50% boiler load condition. This

dust removal equipment from the 1000 MW USC utility boilers and those of PM2.5 from boilers of lower capacities. Although the PM2.5 yields at the inlet of dust removal devices depend on many factors, such as the boiler configuration, the boiler operation, the PM removal performance of upstream flue gas cleaning equipment (e.g., DeNOx unit), and the fuel properties, statistically, the yield of PM2.5 from the 1000 MW utility boiler (1462 mg/kWh) is lower than that of PM2.5 from the majority of utility boilers with lower capacities (see Figure 8a). The yields of PM2.5 at the outlet of dust removal devices further depend on their PM removal performance. The yield of PM2.5 at the outlet of the dust removal equipment is ∼15 mg/kWh, F

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about half of the maximum yield of PM2.5 from boilers with lower capacities (see Figure 8b). Overall, the data in Figure 8 suggest that, promoting the installation of 1000 MW USC utility boilers is likely to simultaneously achieve the improvement of electricity generation efficiency and the reduction in the PM2.5 emission, particularly when advanced dust removal devices are employed.

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.energyfuels.6b00423.

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4. CONCLUSIONS With the increasing installation of 1000 MW USC power generating units, their pollutant formation and emission characteristics draw our concern. In this study, the PM2.5 emitted from a 1000 MW USC utility boiler equipped with an SCR DeNOx unit, two ESPs, and a limestone-gypsum WFGD system was systematically characterized. The PSDs of the PM at both inlet and outlet of the SCR exhibit a bimodal distribution, with a fine mode at < 0.3 μm and a coarse mode at > 0.3 μm. The SCR process leads to the PSDs of the fine mode particles being shifted to larger size, possibly due to the formation of ammonium sulfate and/or ammonium bisulfate as well as the reduction of flue gas temperature. The removal efficiencies of the SCR for PM1 and PM2.5 are 14.3−33.6% and 13.3−30.5%, respectively, depending on the boiler load. The ESPs substantially reduce the mass concentrations of PM1 and PM2.5 from 84.6−106 mg/Nm3 and 417−440 mg/Nm3 to 0.298−1.22 mg/Nm3 and 0.812−4.41 mg/Nm3, respectively, with overall removal efficiencies of 98.7−99.7% for PM1 and 99.0−99.8% for PM2.5. The WFGD process leads to the disappearance of the fine mode PM and an overall removal efficiencies of up to 28.7% for PM1 and 39.6% for PM2.5. Moreover, the increasing use of 1000 MW USC utility boilers is likely to reduce the PM2.5 emission, particularly when advanced dust removal devices are employed.

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Article

PSDs and concentration of PMs and properties of the coal fired in the power station (PDF)

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]; Telephone: +86-27-87546631; Fax: +86-27-87545526 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors acknowledge the financial support of the National Natural Science Foundation of China (51476064, 51520105008) and the National Basic Research Program of China (2013CB228501). The authors also thank the support of the Fundamental Research Funds for the Central Universities (HUST: No. CX15-018) and the Analytical and Testing Center at the Huazhong University of Science and Technology and the contributions of co-workers Wei Chen, Yang Wang, Yifan Du, and Shuang Song in the field sampling. G

DOI: 10.1021/acs.energyfuels.6b00423 Energy Fuels XXXX, XXX, XXX−XXX

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DOI: 10.1021/acs.energyfuels.6b00423 Energy Fuels XXXX, XXX, XXX−XXX