Physical and Chemical Characteristics of Condensable Particulate

Condensable particulate matter is the predominant contributor to the total particulate matter emissions of coal-fired power plants. In the studied ult...
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Physical and Chemical Characteristics of Condensable Particulate Matter from an Ultralow-Emission Coal-Fired Power Plant Jingwei Li,† Zhifu Qi,‡ Min Li,† Dongli Wu,‡ Chenyang Zhou,† Shengyong Lu,† Jianhua Yan,† and Xiaodong Li*,† †

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State Key Laboratory of Clean Energy Utilization, Institute for Thermal Power Engineering, Zhejiang University, Hangzhou 310027, China ‡ Zhejiang Energy Group R&D, Zhejiang Province, Hangzhou 310003, China ABSTRACT: Condensable particulate matter is the predominant contributor to the total particulate matter emissions of coalfired power plants. In the studied ultralow-emission coal-fired power plant, the emission concentrations of condensable and filterable particulate matter in the stack were 1.6 mg/Nm3 and 7.9 mg/Nm3. The organic fraction in condensable particulate matter was mainly composed of alkanes, esters, and other complex organic compounds. The organic fraction comprised 54% of the total concentrations of condensable particulate matter tested at the stack. The organic fraction in condensable particulate matter might contribute significantly to the organic carbon in atmospheric PM2.5. SO42− accounted for the highest concentrations in the inorganic fraction of condensable particulate matter. Na and Ca were predominant metal elements in the inorganic fraction. The inorganic fraction of condensable particulate matter mainly contributed to the water-soluble ions in atmospheric PM2.5. The total particulate matter elimination effect of the air pollution control devices used in the studied plant was good. The removal efficiency of the electrostatic precipitator for condensable particulate matter was much higher than those of the wet flue gas desulfurization system and the wet electrostatic precipitator. The wet flue gas desulfurization system performed well in eliminating the inorganic fraction of condensable particulate matter. Further studies should be conducted on the pollutant control effects of the wet electrostatic precipitator. It is important to study the emission characteristics, chemical compositions, and control methods for condensable particulate matter from coal-fired power plants.

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INTRODUCTION Airborne particulate matter (PM), especially fine particles (PM2.5), is considered an important adverse factor for urban air pollution. Epidemiology has confirmed that PM2.5 contains an abundance of hazardous matter, including heavy metal, organic compounds, acids, etc., and that PM2.5 has serious impact on human health.1−3 The environmental problems caused by PM have garnered the attention of the public and governments. Coal-fired power plants are thought to be one of the major stationary sources of PM emission, and many studies have been conducted on PM emissions from coal-fired power plants.4−6 PM emissions from coal-fired power plants consist of filterable PM and condensable PM. U.S. EPA Method 202 defines condensable PM as material that is in the vapor phase under stack conditions, but condenses and/or reacts upon cooling and dilution in the ambient air to form solid or liquid PM immediately after discharge from the stack. Condensable PM consists of organic and inorganic fractions. EPA Method 202 is the standard method used for condensable PM measurement of stationary source emissions after the filterable PM has been removed. Condensable PM is defined by the EPA as PM with an aerodynamic diameter less than or equal to 2.5 μm. U.S. EPA Method 202 has been a subject of controversy and recommendations in related fields, because of the positive bias cause by SO2 and NOx. The U.S. EPA made revisions to Method 202 to make the method more accurate and precise. Method 202 (dry impinger method) has been proven to substantially minimize the bias during measurement.7 Method 202 should be combined with a filterable PM test method, © 2017 American Chemical Society

which operates at high enough temperatures to cause water droplets sampled through the sampling probe to vaporize. In this study, the International Organization for Standardization (ISO) promulgated ISO 23210-2009 was used as the filterable PM test method. Previous studies have confirmed that condensable PM contributes significantly to total PM emissions from stationary sources. According to the actual Method 201/201A and Method 202 stack test results for several coal-burning boilers, condensable PM composed approximately 76% of the total PM10 stack emissions.8 Yang et al. concluded that the condensable PM concentrations were much higher than filterable PM2.5 from stationary sources, and the condensable PM fraction increased with increase in exhaust temperature.9 Pei conducted condensable PM emissions tests from 3 coalfired power plants using EPA Method 202, showing that the average condensable PM concentrations and filterable PM concentrations were (21.2 ± 3.5) mg/m3 and (20.6 ± 10.0) mg/m3.10 As shown in Table 1, most of the existing data confirmed that the inorganic fraction composed the main part of the condensable PM emissions from stationary sources, and SO42− contributed significantly to the inorganic fraction. In China, GB 13223-2011 (emission standard of air pollutants for thermal power plants) tightened the standards for pollutant emissions from coal-fired power plants. Some Received: November 5, 2016 Revised: January 8, 2017 Published: January 14, 2017 1778

DOI: 10.1021/acs.energyfuels.6b02919 Energy Fuels 2017, 31, 1778−1785

Article

Energy & Fuels

the chemical compositions of the organic fraction and the inorganic fraction of condensable PM were analyzed. The primary objectives of this work are as follows: (1) provide the distribution and emission characteristics of the condensable and filterable PM in the entire process of an ultralow-emission coalfired power plant; (2) investigate the chemical composition of condensable PM; (3) and study the control effects of different APCDs for condensable PM. The results could provide useful data on condensable PM emission and control characteristics for ultralow-emission coal-fired power plants.

Table 1. Results for the Composition of Condensable PM researcher

source

Corio8

coal-burning boilers power plant boiler brick manufacturing plant incinerator arc furnace plant steel plants coal-fired power plants

Yang9,11

Pei10

inorganic fraction (% of total CPM)

main components of inorganic fraction

average 77%

SO42−

89.0% 69.4% 72.3%

SO42− SO42− SO42−

89.8% 72.8% average 67.9% 99%

SO42− SO42− SO42−, Na+, K+, Cl−

1. MATERIALS AND METHODS 1.1. Facility and Sampling Sites. In this study, the condensable and filterable PM emission characteristics and component analysis in the whole process of an ultralow-emission coal-fired boiler unit (LH#1) were investigated. LH#1 is equipped with a 1030 MW ultra supercritical pressure one-through operation coal-fired boiler, and the main APCDs consist of a selective catalytic reduction denitration device (SCR), electrostatic precipitator (ESP), gas−gas heat exchanger (GGH), wet flue gas desulphurization (WFGD) device, and wet electrostatic precipitator (WESP). The overall process of the power plant and sampling sites are shown in Figure 1. Filterable PM and condensable PM samples were collected simultaneously from four sampling sites (Site A, Site B, Site C, Site D) at the flue sections. The sampling sites were selected on the basis of China GB 5468-91 (Measurement method of smoke and dust emission from boiler). LH#1 boiler unit was maintained at a 1030 MW condition burning the same coal while the sampling of condensable and filterable PM was proceeding. The feed coal information on LH#1 is listed in Table 2. The APCDs were kept running steadily during the test.

coal-fired power plants implemented “ultralow-emission” technology to reduce pollutants emissions, i.e., PM < 5 mg/ m3, SO2 < 35 mg/m3, and NOx < 50 mg/m3.12−14 The upgraded selective catalytic reduction denitration device (SCR), electrostatic precipitator (ESP), the wet flue gas desulfurization (WFGD) system, and the wet electrostatic precipitator (WESP) are often used in ultralow-emission technology. However, the PM emission in ultralow-emission technology only considers the filterable PM. Although the filterable PM emission concentration can be reduced substantially by ultralow-emission technology, condensable PM is not measured, because no standards require this. There are few reports regarding condensable PM emissions characteristics from ultralow-emission coal-fired power plants. Moreover, few studies have been conducted on condensable PM control technology for ultralow-emission coal-fired power plants. As more coal-fired power plants implement ultralowemission technology, it is important to investigate the real PM (filterable and condensable PM) emission amount from ultralow-emission power plants. The chemical compositions and control strategies for condensable PM in ultralow-emission coal-fired power plants should also be studied. This work conducted a comprehensive study on the distribution characteristics, chemical compositions of condensable and filterable PM in the entire process of an ultralow emission coal-fired power plant. The condensable and filterable PM were sampled simultaneously at four selected flue sections. The control effects of the air pollution control devices (APCDs) for condensable PM were investigated. Furthermore,

Table 2. Coal Information for the Studied Boiler parameter

basis

value

moisture, % moisture, % ash, % ash, % volatile matter, % volatile matter, % fixed carbon, % sulfur, % sulfur, % Qgr, MJ/kg Qnet, MJ/kg

as received dry as received dry as received dry, ash free as received as received dry as received as received

14.60 6.53 12.67 13.56 20.28 37.48 50.51 0.51 0.55 23.86 22.67

Figure 1. Schematic showing the APCDs and sampling sites: GGH(C), cooling section of gas−gas heat exchanger; GGH(H), heating section of gas−gas heat exchanger. 1779

DOI: 10.1021/acs.energyfuels.6b02919 Energy Fuels 2017, 31, 1778−1785

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Energy & Fuels The ESP of the LH#1 had five electric fields, and the fifth electric field was modified to a rotating electrode electric field. The GGH(C) was used to decrease the temperature of flue gas for the purposes of heat recovery and control of the inlet gas temperature of WFGD. Lower inlet flue gas temperature could lead to a better condensation and adhesion of SO3, as well as an enhancement in the removal efficiencies of SO3 and SO2 in the WFGD. The GGH(H) was used for increasing the exhaust temperature of flue gas to above the dew point temperature of water, in order to reduce the quantity of gypsum fine particles carried by the flue gas and eliminate the white smoke released from the stack. The studied boiler implemented joint control technology to reduce pollutant emissions by using the listed APCDs. 1.2. Sampling Equipment and Methods. The sampling method was developed with the objective of characterizing the condensable PM and filterable PM in flue gas. Filterable PM measurements were conducted according to ISO 23210-2009, and condensable PM measurements were conducted according to U.S. EPA Method 202. A Dekati PM10 Impactor was used for sampling filterable PM in this study. The Dekati PM10 Impactor is a cascade impactor that measures PM10 and PM2.5 mass concentrations simultaneously. It can divide the filterable PM in the flow into four categories: ≥10 μm, 10− 2.5 μm, 2.5−1 μm, ≤1 μm. Filterable PM was collected on the membranes fixed in the collection plates. The impactor fulfills all requirements of the ISO standard 23210, and it can be upgraded to measure PM1 mass concentration as well. The condensable PM sampling trains were purchased from Environmental Supply Company, Inc. USA. The CPM sampling trains could meet the requirements of U.S. EPA Method 202. The sampling system was a combination of Dekati PM10 impactor, condensable PM sampling train components, sampling tube, sampler, moisture meter, and recovery equipment. The sampling system could collect filterable PM and condensable PM samples simultaneously. A schematic showing the sampling system is shown in Figure 2. Sampling

1.3. Filterable PM Analysis. The impactor-specific 25 mm foil films (for PM: ≥ 10 μm, 10−2.5 μm, 2.5−1 μm) and 47 mm polyester filters (for PM: ≤1 μm) were used for filterable PM collection. Before sampling, the polyester filters and the foil films were successively smeared with the turpentine (dissolved in CCl4, m/m, 1:20), dried for 2 h at 130 °C, conditioned at 20−25 °C in a desiccator containing silica gel, and weighed using an analytical balance. The analytical balance was a Sartorius BT25s balance, accurate to at least 0.01 mg. After sampling, the polyester filters and the foil films were conditioned and weighed. 1.4. Condensable PM Analysis. The condensable PM analysis procedure was based on U.S. EPA Method 202. Condensable PM samples were collected in the condenser, connecting glassware, water dropout impinger, modified Greenburg-Smith impinger and condensable PM filter. After sampling, post-test nitrogen purge was immediately conducted according to Method 202. All sampling train components including the connecting glassware, condenser, impingers and the front half of the CPM filter housing were rinsed twice each with water, acetone, and hexane. The water rinses, the organic solvent rinses, and the condensable PM filter were collected in separate clean containers. The analytical procedures were conducted in the laboratory according to Method 202. The condensable PM filter was extracted twice by water and then by hexane in an ultrasonicator. The aqueous extracts were combined with the water rinses, and the organic extracts were combined with the organic solvent rinses. The water rinses were placed in a separatory funnel, and approximately 50 mL of hexane was added to the funnel; this was mixed well with an oscillator, and combined with the organic extraction/organic solvent rinses. The extraction procedure was repeated twice. The inorganic (water rinses) and organic (organic solvent rinses) fractions were evaporated to no less than 10 mL liquid and then dried at room temperature (20−25 °C). The inorganic and organic fractions were then removed from the desiccator, and the residues were weighed with the analytical balance. The total mass of the inorganic and organic fractions represents the CPM.8 Field blanks were also measured using Method 202. 1.5. Chemical Analysis for Condensable PM. The organic composition and inorganic components of condensable PM sampled at each sampling site were investigated in this study. The organic fractions of condensable PM samples were extracted with hexane. The inorganic fractions of condensable PM samples were extracted with deionized water, in an ultrasonicator. The organic extracts were evaporated to 1 mL. A gas chromatograph/mass spectrometer (GC/MS) (Agilent 6890N GC/5975B inert XL MSD) equipped with an HP-5MS (30 m × 0.25 mm × 0.25 μm) was used for the semiquantitative test of the organic composition of condensable PM. The injection volume was 1 μL. The inlet temperature was 250 °C, the ion source temperature was set at 230 °C, and the transmission line temperature was set at 280 °C. A temperature program was run from 50 °C (3 min hold) to 300 °C (15 min hold) at a rate of 5 °C min−1. The semiquantitative test for organic composition was performed by using scan mode, small noise peaks clipped. The m/z scan range was 30−600. The inorganic extracts were evaporated and diluted to 10 mL, half of the samples were analyzed for anion species, and the other half were analyzed for metal elements. An ion chromatograph (model, Dionex ICS-2000) was used to quantify anion species (F−, Cl−, SO42−, NO3−). The anion separation column was an IonPac Asll-HC (4 mm × 250 mm). The guard column was an IonPac AGll-HC (4 mm × 50 mm). The eluent was 10 mmol/L KOH solution. The flow velocity was 1.0 mL/min. The injection volume was 25 μL. The R2 values for the calibration curves of measured species were all above 0.995. Inductively coupled plasma mass spectrometry (ICP-MS, AAS, model XSENIES, quadrupole mass spectrometer) was used for quantitatively measuring metal elements (Na, Ca, Fe, Mg, Al). Calibration verification was performed during test. There are many published studies regarding filterable PM emissions from coal-fired power plants, including studies of mass concentration, physical and chemical properties, source analysis, etc. This study did

Figure 2. Schematic showing of the sampling system. 1, isokinetic nozzles; 2, stainless steel sampling probes; 3,6, hoses and hose connector; 4, impactor heater and heater controller; 5, Dekati PM10 impactor; 7, condenser; 8, recirculation pump; 9, water dropout impinger; 10, modified Greenburg-Smith impinger (backup impinger); 11, condensable PM filter; 12, moisture trap; 13, silica gel trap; 14, sampler; 15, water bath; 16, ice bath. probes and the Dekati PM10 Impactor were heated to 130 °C when sampling. This temperature could meet the EPA Method 5 requirement, which was higher than the temperature of flue gas. The elevated temperature could help minimize the influence of moisture. This temperature was utilized for all sampling tests in this study. A leak check was conducted before each sampling. The flue gas velocity was measured with a Pitot tube before sampling. Isokinetic sampling was achieved with the use of isokinetic nozzles. At least two samples were collected at each sampling site, and the sampling time was 1 h for each sample. Because of the high concentrations of filterable PM at Site A, we had to shorten the sampling time to 10 min. Shorter sampling time and less sampling volume might introduce bias into the analysis of condensable PM. One field train blank was recovered after the first run of the test at the sampling site. The sampling flow rate was 10 L/min. In addition, the flue gas parameters (temperature, moisture, and O2 concentration) were measured according to GB/T 16157-1996,15 China. 1780

DOI: 10.1021/acs.energyfuels.6b02919 Energy Fuels 2017, 31, 1778−1785

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Energy & Fuels not conduct physical and chemical analysis on filterable PM samples, because the filterable PM samples were used for other analyses.

vaporization of the materials in filterable PM and water droplets and would cause a positive bias for CPM testing. However, the concentrations of FPM and CPM after the ESP were relatively low in this study. Considering that 130 °C is not high for most materials in FPM, the sampling temperature was likely to have had only a very small effect on the test results. The concentrations of condensable and filterable PM at Site B, Site C, and Site D are shown in Figure 3. The filterable PM2.5

2. RESULTS AND DISCUSSION 2.1. Concentrations and Distribution Characteristics of Condensable and Filterable PM. The concentration calculation methods for condensable and filterable PM were based on ISO standard 23210 and U.S. EPA Method 202. In accordance with GB/T 16157-1996 and GB 13223-2011,16 the condensable and filterable PM concentrations were converted into standard concentrations (at 6% oxygen, dry standard, standard condition). The analysis results of the concentrations of filterable and condensable PM at each sampling site were the average values of the consecutive samples. The average concentrations of condensable and filterable PM at 4 sampling sites are shown in Table 3. This work analyzed the relative deviations of the Table 3. Concentrations of Condensable and Filterable PM (mg/Nm3)a filterable PM sampling site Site Site Site Site

A B C D

condensable PM

FPM2.5

total FPM

organic fraction

inorganic fraction

total CPM

322.1 5.6 3.1 1.1

2485.4 7.2 3.7 1.6

57.8 6.6 5.9 4.3

69.1 9.5 4.3 3.6

126.9 16.1 10.2 7.9

CPM, condensable PM; FPM, filterable PM; FPM2.5, filterable PM2.5.

Figure 3. Distributions of condensable and filterable PM. FPM, filterable PM; CPM, condensable PM; filterable PM>2.5, filterable PM except filterable PM2.5.

consecutive PM statistics at each sampling site. Most of the relative deviations for the concentrations of filterable and condensable PM were between −27.2% and 28.0%, indicating that the repeatability of the analysis procedure was good. The overall concentrations of both condensable and filterable PM decreased from Site A to Site D. The condensable PM fraction accounts for 5%, 69%, 73%, and 84% of total PM at Site A, Site B, Site C, and Site D, respectively. The relative contributions of the condensable PM fractions increased from Site A to Site D. The results agreed with previous observations, indicating that condensable PM made a significant contribution to total PM emissions for stationary sources.8−10 However, the total mass concentrations of condensable and filterable PM emitted were 1.6 mg/Nm3 and 7.9 mg/Nm3, respectively, which are quite low compared with the values reported in previous studies.8,10 In addition, the emission concentrations of total PM (condensable and filterable PM) met the requirements of the strict emission standard GB13223-2011 (PM < 20 mg/Nm3 at momentous areas in China). These emission characteristics suggest that the joint control technology implemented by the studied boiler unit could effectively control the emissions of particulate matter. In this study, the filterable PM sampling probe and Dekati PM10 were heated to 130 °C at the four sampling sites. Some concerns arose as to whether the temperature affected the results, because the flue gas temperature dropped after the ESP, and rose after the GGH(H) (Site A, 116 °C; Site B, 116 °C; Site C, 55 °C; Site D, 80 °C. According to EPA Method 5, the filter should be maintained at a temperature of 120 ± 14 °C to remove water droplets from the flue gas. When the flue gas temperature dropped, some vaporous materials probably condensed and were adsorbed in filterable PM or water droplets. Reheating the gas to 130 °C might have resulted in

comprised the main part of the total filterable PM, indicating that the ESP eliminated most of the coarse particles in flue gas. It is confirmed that the filterable PM emitted from coal-fired power plants was mainly composed of filterable PM2.5. The organic fractions accounted for 45.6%, 41.1%, 57.5% and 54.4% of total condensable PM at Site A, Site B, Site C, and Site D, respectively. In comparison, some previous studies have reported that the inorganic fraction was dominant in condensable PM.9 Corio and Sherwell investigated the organic and inorganic fractions of condensable PM emissions from four coal-burning boilers, and the limited data showed that the contribution of the average organic fraction to the condensable PM was approximately 23%. Only one boiler unit showed that the organic fraction accounted for 57% of the condensable PM.8 However, the methods of sampling condensable PM were different between Corio’s study and this study. Condensable PM was collected by deionized water in Corio’s study. The use of deionized water might lead to positive bias, because SO2 captured in the impingers could form SO42− and the purging might not be completely successful. The results of this study imply that the organic matter could account for more of the total condensable PM emissions from ultralow-emission coalfired power plants than expected. 2.2. Organic Composition of Condensable PM. Hundreds of organic substances from the organic fractions of condensable PM samples were detected with the gas chromatograph/mass spectrometer (GC−MS). Although it was impossible to quantitatively calculate the concentrations of all substances, the GC−MS generated test reports including the CAS Number, Library Research ID, retention time and area percentage of each substance. The results of the semiquantitative analysis for the organic fraction in condensable PM are shown in Figure 4.

a

1781

DOI: 10.1021/acs.energyfuels.6b02919 Energy Fuels 2017, 31, 1778−1785

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through D. However, the results provided an indication of what the organic fraction of condensable PM contained. In previous studies, the chemical characteristics of atmospheric PM2.5 were detected.17−20 SO42− emitted from coal fired power plants contributed significantly to the inorganic ions. However, the contribution of organic carbon (OC) in ambient PM2.5 might be significant. Currently, China is seriously influenced by smog. Ding21 researched the emission source of ambient PM2.5 in Nanjing and found that OC contributed approximately 23% of the mass concentrations of ambient PM2.5, as well as that it was mainly emitted from coal burning and motor vehicles. The detailed OC compositions were not investigated in Ding’s study. SO42−, NO3−, and OC were the most abundant species in atmospheric PM2.5 recently reported by Dai.22 The high proportion of organic fraction in condensable PM in this study indicated that condensable PM emitted from coal-fired power plants might be one of the main sources of OC in PM2.5 in the atmosphere. Furthermore, this study contributed some useful information on the composition of the organic fraction in condensable PM. 2.3. Inorganic Composition of Condensable PM. Previous studies reported that the inorganic fraction of condensable PM was composed mostly of SO42−, independent of source type (steel plants, power plants, incinerators) or fuel burned (coal, oil, or natural gas).11 Accordingly, the anion species (F−, Cl−, SO42−, NO3−) and the metal elements (Na, Ca, Fe, Mg, Al) in the inorganic fractions were quantitatively analyzed. Concentrations of tested species in the inorganic fractions are listed in Table 5. SO42− accounted for the highest concentrations in all samples. The results confirmed the previous research results. Cl− occupied a certain proportion in each sample as well. For metal elements, Na and Ca

Figure 4. Composition of the organic fraction.

The area percentage of alkanes was the highest in the condensable samples at Site A, Site B, Site C, and Site D. There were dozens of alkanes detected in the samples. At room temperature (