Article pubs.acs.org/EF
Particulate Matter Emission Characteristics and Removal Efficiencies of a Low-Low Temperature Electrostatic Precipitator Zhifu Qi,*,†,‡ Jingwei Li,‡ Dongli Wu,† Weiyang Xie,† Xiaodong Li,‡ and Chunhong Liu† †
Zhejiang Energy Group R&D, Hangzhou 310003, Zhejiang Province China State Key Laboratory of Clean Energy Utilization, Institute for Thermal Power Engineering of Zhejiang University, Hangzhou 310027, Zhejiang Province China
‡
ABSTRACT: The low-low temperature electrostatic precipitator (LLT-ESP) has been developed recently to improve the performance of traditional low temperature electrostatic precipitators. In this study, the particulate matter emission characteristics and removal efficiencies of were investigated on an LLT-ESP. Filterable particulate matter (FPM) was tested according to ISO standard 23210-2009, and condensable particulate matter (CPM) was tested according to U.S. EPA Method 202. The LLT-ESP showed excellent removal efficiency for FPM, with total FPM removal efficiencies of more than 99.9%. The removal efficiencies of FPM increased with the rising particulate diameter; for FPM2.5, removal efficiencies ranged from 96.5 to 98.2%. The LLT-ESP also showed remarkable removal efficiencies for CPM, with CPM removal efficiencies of more than 60.9%. The removal mechanism of CPM in the ESP was different from that of FPM. After the LLT-ESP, the quantity relationship between FPM and CPM reversed. For further reduced emission of PM for coal-fired power boiler units, more attention should be paid to the control of CPM. The load of the unit showed significant effects on CPM. CPM was generated more in lower unit loads for incomplete combustion of coal, and the organic fractions accounted for more than 65% of total CPM in the inlet flue gas of the LLT-ESP. SO42− was the main contributor of anions, and Cl− took second place. Na+ and Ca2+ were the main contributors of metal ions.
1. INTRODUCTION Particulate matter (PM), especially fine particulate matter, has been identified as one of the major air pollutants in urban areas. PM causes adverse effects to public health, and has been responsible for several respiratory diseases.1−3 Fine particulates enriched in toxic heavy metals exhibit lengthy atmospheric residence time, and deep pulmonary ingestion of fine particulates is possible. Also, PM may influence global climate change.4 To protect human health and the environment, it is necessary to control PM emission. PM can be categorized as primary particles and secondary particles by origination. Primary particles are directly emitted as liquids or solids from sources. Secondary particles are formed by gas-to-particle conversion in the atmosphere, and the formation mechanism is nucleation and condensation of gaseous precursors such as sulfur dioxide, nitrogen oxides, and volatile organic compounds.4 Also, PM can be categorized by aerodynamic diameter, such as PM2.5 with an aerodynamic diameter smaller than 2.5 μm and PM10 with an aerodynamic diameter smaller than 10 μm. Filterable particulate matter (FPM) is particles that may be physically captured on a filter during sampling. Condensable particulate matter (CPM) is particles that existed in the gas phase during sampling, and condensed to submicrometer particles after cooling. All CPM is smaller than 2.5 μm in diameter. PM can be made up of hundreds of different chemicals, including organic and inorganic components, such as carbonaceous materials, trace metals, sulfates, nitrates, and silicon dioxides.5 Sulfate and nitrate compounds are the most widely recognized forms of condensable PM emitted by combustion sources. During combustion of fossil fuels, a © XXXX American Chemical Society
small fraction of fuel-bound sulfur is converted to sulfur trioxide. Selective catalytic reduction (SCR) for denitration also catalyzes the oxidation of sulfur dioxide and generates sulfur trioxide.6,7 The oxidation of NO2 by OH and by O3 forms HNO3.8 The organic components of CPM include volatile organic compounds and semivolatile compounds. One of the important sources of particulate matter is coalfired power plants. With rapid economic growth, coal-fired power plants increased drastically in developing countries such as China and India. Recently, China imposed strict air quality rules to limit ambient particulate levels of PM2.5 and PM10. To meet the increased stringent national standards, it was necessary to control particulate emission efficiently. An electrostatic precipitator (ESP) was among the dust collection devices most utilized in coal fired power plants. The performance of the ESP was strongly affected by the electric resistivity of the dust, which depended on the dust chemical composition and flue gas conditions, especially the temperature of the flue gas.9 A traditional low temperature ESP was operated around 150 °C to avoid corrosion, while in this temperature range the electric resistivities of some kinds of coal dust were rather high. To improve the performance of the traditional ESP, a low-low temperature electrostatic precipitator (LLT-ESP) was developed. By using a heat exchanger before a traditional electrostatic precipitator, the temperature of the flue gas could be reduced to below the dew point, and the electric resistivity of the dust decreased significantly, which provided a Received: October 17, 2016 Revised: December 27, 2016 Published: January 8, 2017 A
DOI: 10.1021/acs.energyfuels.6b02692 Energy Fuels XXXX, XXX, XXX−XXX
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Energy & Fuels higher removal efficiency of dust.10,11 Besides increasing the removal of dust, LLT-ESP also removed SO3 from flue gas by cooling and condensing onto fly ash.12 CPM fractions from stationary sources are of increasing concern because studies indicated that their emissions could be significant contributors to ambient PM2.5 in some areas, while less scientific data on CPM is available for stationary combustion sources, especially for specific air pollution control devices. Although some studies were carried out on LLT-ESP or CPM, they usually focused on emission characteristics. In addition, influence factors for CPM generation and removal mechanisms of CPM were seldom studied. In this study, particulate matter emission characteristics were investigated on an LLT-ESP. Both the FPM and CPM were collected and analyzed. The effect of the load of the unit on particulate matter emission characteristics and removal efficiencies were studied. Also, the removal mechanisms of FPM and CPM were discussed.
2.2. Sampling Methods and Devices. Filterable particulate matter (FPM) and condensable particulate matter (CPM) before and
Table 2. Experimental Conditions for PM Test
2.1. Test Facilities and Fuels. This study was carried out on a 1000 MW ultrasupercritical pressure coal-fired boiler unit in Zhejiang Province, China. The unit was a state-of-the-art coal-fired unit in China. To further reduce the emission of pollutants, a unit transformation named Ultra Low Emission Reform was implemented between 2013 and 2014. After transformation, the main flue gas pollution control devices consisted of a selective catalytic reduction denitration device (SCR), a low-low temperature electrostatic precipitator (LLT-ESP), wet flue gas desulfurization (WFGD), a wet electrostatic precipitator (WESP), etc. By using a nonleakage media gas−gas heater (MGGH), the flue gas entering the LLT-ESP could be controlled accurately. The MGGH has two main parts. A heat extractor which was located after the ordinary air preheater was used to reduce the flue gas entering the LLT-ESP to below the dew point, and a reheater was used to reheat the flue gases downstream the wet electrostatic precipitator to avoid a wet stack and visible plume. Table 1 lists the properties of coal blends used to fire the boilers during the test periods. Generally, the coal blends were of medium ash and with low sulfur.
Table 1. Properties of Coal Blends of Test Unit parameter
basis
value
as received dry as received dry, ash free as received as received as received
13.5 4.5 14.51 35.93 58.77 22.24 0.63
unit load (MW)
inlet flue gas temp (°C)
outlet flue gas temp (°C)
E1 E2 E3
1000 500 500
130 126 125
100 97 93
after the LLT-ESP were collected and measured at different unit loads and inlet gas temperatures of the ESP (Figure 1). Table 2 lists experimental conditions for the PM test. FPM was tested according to the International Organization for Standardization (ISO) standard 23210-2009, and CPM was tested according to the modified U.S. EPA Method 202.13 Figure 2 shows the sketch of the particulate matter sampling system used in this study. Flue gas with particulate matter was sampled isokinetically by a sampling probe. A Dekati PM10 impactor, a three-stage cascade impactor with cut points of 10, 2.5, and 1 μm, was used for collection of FPM. The impactor fulfilled requirements of the ISO standard 23210-2009. Both the sampling probe and the PM10 impactor were heated to about 130 °C. Then flue gas entered the CPM sampling trains. The sampling trains for CPM consisted of a condenser tube, water dropout impingers in a thermostatic water bath, PM filter membranes, a moisture trap, and a silica gel trap in an ice bath. The temperature of the thermostatic water bath was maintained at 30 °C. CPM was collected by the condenser tube, water dropout impinger, backup impinger, and PM filter membranes. All glassware and connecting parts were purchased from Environmental Supply Company, Inc. USA, which met the requirements of U.S. EPA Method 202. 2.3. Particulate Matter Analysis Procedure. The PM10 impactor was utilized to collect FPM by 25 mm foil films (for PM ≥10 μm, 10−2.5 μm, and 2.5−1 μm) and 47 mm polyester filters (for PM ≤1 μm). Before sampling, the foil films and polyester filters were smeared with turpentine (dissolved in CCl4 with a mass fraction of 5%) and dried for 2 h at 130 °C. Then the foil films and polyester filters were conditioned at 25 °C in a desiccator containing allochroic silica gel for 2 h, and were weighed by an analytical balance capable of weighing to the nearest 0.01 mg. After sampling, the polyester filters and the foil films were conditioned and weighed according to the steps above successively. The CPM was analyzed according to U.S. EPA Method 202. Before sampling, all glassware and connection parts of CPM sampling trains were washed by deionized water, acetone, and hexane sequentially, and air-dried. Once the sampling of CPM was finished, an ultrahigh purity nitrogen purge was immediately conducted for 45 min on all CPM sampling trains to avoid oxidation of sulfur dioxide. After the nitrogen purge, the connecting glassware, condenser, impingers, and the front half of the CPM filter housing were rinsed twice with deionized water, acetone, and hexane, successively. The water rinses, the organic solvent rinses (including acetone and hexane), and the CPM filter were collected in clean containers separately. The condensable PM filter was extracted twice by deionized water and hexane in an ultrasonic cleaner successively, and the extracts for water and hexane were collected separately.
2. EXPERIMENTAL TEST METHOD AND FACILITY
moisture (%) moisture (%) ash (%) volatile matter (%) fixed carbon (%) heating value (MJ/kg) sulfur (%)
conditions
Figure 1. Schematic diagram of LLT-ESP. B
DOI: 10.1021/acs.energyfuels.6b02692 Energy Fuels XXXX, XXX, XXX−XXX
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Figure 2. Sketch of condensable particulate matter sampling system. The aqueous fractions and organic fractions for rinses of CPM sampling trains and extracts for CPM filters were separated by a separating funnel. Then both fractions were dried at room temperature and weighed with the analytical balance, and the total of the inorganic and organic fractions represents the CPM. Detailed procedures can be referred to in the U.S. EPA Method 202. The water-soluble ions and metal elements analyses were conducted for CPM. An ion chromatograph (ICS-2000, Dionex, USA) was used for quantitatively analyzing anion species (F−, Cl−, SO42−, NO3−). Inductively coupled plasma mass spectrometry (iCAP6300, Thermo Scientific, USA) was used for quantitatively measuring metal elements (Na+, Ca2+, Mg2+, Al3+). The correlation coefficients of the calibration lines of measured species were all above 0.995. The organic extractions were evaporated to 1 mL. An Agilent 6890N GC/5975B MSD equipped with an HP-5MS (30 m × 0.25 mm × 0.25 μm) was used for semiquantitative testing of the organic composition of condensable PM. The semiquantitative test for organic composition was performed by using scan mode, with the m/z scan ranging from 30 to 600. The inlet temperature was maintained at 250 °C. The ion source temperature was set at 230 °C, and the transmission line temperature was set at 280 °C.The temperature program was 50 °C, held for 3 min, followed by a ramp up to 300 °C at 5 °C/min, and held for 15 min. All PM (including FPM and CPM) concentrations were converted at 6% oxygen concentration, and then the removal efficiency was defined and calculated by removal efficiency =
PM inlet − PMoutlet × 100% PM inlet
Table 3. Average Mass Concentrations of PM in Flue Gas Samples (mg/m3) CPM sampling location E1, E1, E2, E2, E3, E3,
inlet outlet inlet outlet inlet outlet
FPM
org fraction
inorg fraction
FPM2.5
FPM10
FPM
111.6 21.6 236.1 10.3 287.8 21.7
47 14.7 69.2 21.5 52.9 19.1
493.6 3.8 328.7 3.8 321.6 10.1
1262 5.38 1004 4.09 916 11.74
6974 5.6 7236 4.5 7536 13.2
than that before LLT-ESP transformation (with removal efficiencies about 99.52%). The escaping FPM concentration was controlled within 15 mg/Nm3. Generally, the removal of FPM was enhanced with the decrease of the inlet flue gas temperature. The performance of an electrostatic precipitator is very sensitive to electrical resistivity. Lower temperature attenuates the thermal motion of flue gas molecules and diminishes the viscosity of the flue gas. The surface adsorbed SO3 due to condensation is very electrically conductive and effectively lowers the resistance of the bulk fly ash.14 For CPM, removal efficiencies were among 77.1−90.8%, with the escaping CPM concentration ranging from 31.8 to 40.8 mg/Nm3. Also, the concentrations of CPM after the LLT-ESP were similar for different conditions. For further reduced emission of PM for a coal-fired power boiler unit, more attention should be paid to the control of CPM after the electrostatic precipitator. Figure 3 shows the removal efficiencies for CPM, FPM2.5, FPM10, and FPM. It was found that the removal efficiencies of FPM were increased with the rising particulate diameter. For
(1)
where PMinlet is the PM concentration in flue gas upstream of the ESP (converted at 6% O2), and PMoutlet is the PM concentration in flue gas downstream of the ESP (converted at 6% O2). 2.4. Quality Assessment/Quality Control. Reagent and field blanks were analyzed during the PM test. Duplicate tests were conducted for every experimental condition. The FPM mass concentration was much larger than field and reagent blanks for ISO standard 23210-2009, and the average CPM mass concentration was 4−10 times larger than field and reagent blanks for Method 202A.
3. RESULTS AND DISCUSSION 3.1. Particulate Matter Emission Characteristics and Removal Efficiency of PM of LLT-ESP. Table 3 shows the mass concentrations of PM (FPM and CPM) in flue gas before and after the LLT-ESP under different unit loads. It was found that the quantity relationship between FPM and CPM reversed at the outlet of the LLT-ESP. The concentrations of filterable PM in flue gas before MGGH were in the range 6974−7545 mg/m3, which were much higher than those of CPM. In the flue gas downstream of the LLT-ESP, the concentrations of CPM (ranging from 31.8 to 40.8 mg/m3) were several times higher than those of FPM (ranging from 4.5 to 11.2 mg/m3). The LLT-ESP showed excellent removal capacity for FPM, with removal efficiencies of more than 99.85%, much higher
Figure 3. Removal efficiencies for PM. C
DOI: 10.1021/acs.energyfuels.6b02692 Energy Fuels XXXX, XXX, XXX−XXX
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that for E2 and E3, and the excess air coefficient for condition E1 was less than that for E2 and E3. Submicrometer particles were thought to be produced by nucleation of ash vapors, and the enrichment of these fine particles has been attributed to the vaporization of volatile constituents of the ash in the high temperature flame region of the furnace, followed by the condensation or adsorption of the vapors on the surfaces of the ash particles.15 Combustor operating conditions had significant effects on fine particles. High flame temperature and strong reducing atmosphere were beneficial for the formation of submicrometer particles.16
FPM2.5, removal efficiencies ranged from 96.5 to 98.2%. This phenomenon was in accordance with the dust removal mechanism of the electric dust collector. The performance of an electrostatic precipitator is very sensitive to the electrical resistivity and particle size distribution of the particulates. Previous results of field measurements have shown that there is a penetration window in the submicrometer size range. The reason for this window has been attributed to the decreasing charge carried by particles of smaller size and the increase of mobility with decreasing size resulting in an offsetting of the reduced charge.14 Compared with studies by Wang et al.,14 the removal efficiencies of FPM2.5 and FPM1.0 were lower, which could be attributed to the properties of coal and the operation temperatures of the LLT-ESP. The removal efficiencies of CPM were much less than those of FPM, indicating that the removal mechanisms for CPM and FPM in the LLT-ESP would be different. With the flue gas temperature decreased in the LLTESP, some organic fractions of CPM condensed and adsorbed on fly ash surfaces, and finally were removed by the electrostatic precipitator. Also, the high voltage of an electrostatic precipitator may destroy some organic fractions. For a CPM component with a low boiling point or that was difficult to ionize, the removal efficiency was rather low.
Figure 5. Component characteristics of CPM.
3.3. Effects of Unit Load on Chemical Compositions of CPM. Figure 5 shows the component characteristics of CPM. The organic fractions, which were generated mainly due to incomplete combustion, accounted for more than 65% of total condensable PM in inlet gas. Semiquantitative analysis by a gas chromatograph/mass spectrometer showed that alkanes and esters were the main components of the organic fraction for all samples. Some previous research confirmed that the inorganic fraction was dominant in CPM.6,17 There are several reasons for these differences. Low sulfur coal was burned in the test coal-fired power unit, which introduced less sulfate into the flue gas. A low NOx combustion technology by minimizing excess oxygen was utilized and fewer nitrates were generated. Meanwhile, coal may burn inadequately without enough oxygen during this low NOx combustion technology. Compared with the composition of organic fractions in the flue gas upstream of the LLT-ESP, the ratio of organic fractions decreased in the flue gas downstream of the LLT-ESP, which indicated that the removal efficiencies of organic fractions in the LLT-ESP were more than those of inorganic fractions. Figure 6 shows mass concentrations of the main anion in organic fractions of CPM, and Figure 7 shows the main metal ions. SO42− was the main contributor of anions in all inlet gas samples, and Cl− took second place. It was found that the ratio of SO42− decreased a lot in the outlet flue gas of the electrostatic precipitator. Sulfate could be treated as the indicator of sulfur trioxide; it can be concluded that the LLTESP also showed the ability to remove SO3 besides particulate matter. Small amounts of SO3 are needed for proper electrostatic precipitator performance.12,18 When the concentration of SO3 in flue gas increased, the viscosity of the particles increased while the specific resistance of the particles decreased, and the collection efficiency of the ESP increased. A lower inlet
Figure 4. Mass concentration of PM in flue gas upstream of LLT-ESP.
3.2. Effects of Unit Load on PM Concentration in Flue Gas. Figure 4 shows the PM mass concentration in the inlet flue gas of the LLT-ESP (after the ordinary air preheater). As shown in Table 1, the temperatures of the inlet flue gas were similar. No obvious difference was observed for FPM and FPM10 concentrations under different unit loads, while significant differences were observed for the mass concentration of CPM in flue gas. The concentrations of CPM for experimental conditions E2 and E3 were 305 and 341 mg/ m3, respectively, nearly 2 times that of CPM for condition E1 (159 mg/m3). It can be concluded that CPM was generated less with higher unit loads. One of the important reasons is the incomplete combustion of coal. Usually, the designs of boiler structure and operation optimization were based on 100% unit load. Although the excess air coefficient of low load was higher than that of high load, the mixing of pulverized coal and air may be inadequate, which caused the amount of products of incomplete combustion to be increased. It was also found that the mass concentration of FPM2.5 for condition E1 was more than that for E2 and E3. The flame temperature of the furnace for condition E1 was higher than D
DOI: 10.1021/acs.energyfuels.6b02692 Energy Fuels XXXX, XXX, XXX−XXX
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The unit load had significant effects on CPM generation, and CPM was generated more in lower unit loads for incomplete combustion of coal. With low NOx combustion technology utilized and low sulfur coal burned, organic fractions were the main contributor of CPM in flue gas upstream of the LLT-ESP. Alkanes and esters were the main components of the organic fraction. SO42− was the main contributor of anions, and Cl− took second place. Na+ and Ca2+ were the main contributors of metal ions.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
Zhifu Qi: 0000-0003-3043-2894 Xiaodong Li: 0000-0001-9616-7443
Figure 6. Anion mass concentrations in inorganic fractions of CPM.
Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS The research was financially supported by the Natural Science Foundation of Zhejiang Energy Group, Zhejiang Province, China.
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REFERENCES
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Figure 7. Metal ion mass concentrations in inorganic fractions of CPM.
flue gas temperature of the ESP could lead to the condensation and adhesion of SO3 to the surface of dust particulate, and the removal efficiency of SO3 in ESP increased. It was reported that the release of Cl into PM was affected by S due to the competition of cations. With the increased content of S in the coal blends, partitioning of Cl into the ultrafine PM would be reduced.19,20 The mass concentrations of NO3− were quite low, which could be attributed to the low NOx combustion and selective catalytic reduction of the denitrification catalyst. For metal elements, Na+ and Ca2+ occupied the highest concentrations in all samples, which might be caused by their high potentials of vaporization. The high concentrations of Na were consistent with former investigations.17 Generally, no obvious difference was observed for metal elements before and after ESP.
4. CONCLUSION The LLT-ESP showed excellent removal efficiency for FPM, with FPM removal efficiencies of more than 99.9%. The escaping FPM concentration was controlled within 15 mg/ Nm3. The LLT-ESP also showed remarkable removal efficiency for CPM, with CPM removal efficiencies of more than 60.9%. After LLT-ESP, the concentration of CPM was several times more than that of FPM. It was important to control CPM to further reduce the emission of PM. The mechanisms for removal of FPM and CPM were different. E
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