Comparison of Coal- and Oil-Fired Boilers through the Investigation of

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Comparison of Coal- and Oil-Fired Boilers through the Investigation of Filterable and Condensable PM2.5 Sample Analysis Hsi-Hsien Yang, Mahammad Arafath Shaik, Ya-Fen Wang, Jhin Yan Wu, Kuei Ting Lee, and Yueh Shu Hsieh Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.7b03541 • Publication Date (Web): 07 Feb 2018 Downloaded from http://pubs.acs.org on February 14, 2018

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Comparison of Coal- and Oil-Fired Boilers through the Investigation of Filterable and Condensable PM2.5 Sample Analysis Hsi-Hsien Yang*†, S. Md. Arafath*†, Ya-Fen Wang‡, Jhin-Yan Wu†, Kuei-Ting Lee†, Yueh-Shu Hsieh† †

Department of Environmental Engineering and Management, Chaoyang University of Technology, Taichung, Taiwan, ROC. ‡ Department of Environmental Engineering, Chung Yuan Christian University, Taoyuan, Taiwan, ROC.

ABSTRACT: This study investigated the characteristics of both filterable fine particulate matter (FPM) and condensable particulate matter (CPM) emitted from coal-fired boilers (CFBs) and oil-fired boilers (OFBs) via field sampling. FPM and CPM samples were collected using USEPA Method 201A and Method 202 respectively. Mass concentrations and chemical compositions (including water-soluble ions, metal elements and carbon contents) of collected PM2.5 samples were analyzed. The results show that PM2.5 (FPM + CPM) emission concentrations for CFBs and OFBs are 20.2 ± 10.4 and 157 ± 82.7 mg/Nm3 respectively. In terms of emission factor, emission of FPM from OFBs is 307.4 ± 50 g/kL-oil and for CFBs is 57.1 ± 13.8 g/t-coal. Significantly higher concentrations are emitted from OFBs than CFBs due to the reason that better control devices are installed in most CFBs. The average CPM fraction constitutes 58.7% and 54.8% of PM2.5 for CFBs and OFBs respectively, showing that CPM from the boilers contributes a significant fraction of PM2.5 emissions. FPM sample analysis reveals that SO42- is the primary characteristic of water-soluble ion and occupies 64.2% and 80.6% of total watersoluble ions for CFBs and OFBs respectively. SO42- is a main contributor of ions, while NO3- follows. The species in CPM are dominated by water-soluble ions, including SO42-, NO3- and NH4+. The results indicate that CPM is formed primarily by water-soluble ions. 1

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The results also show that OC concentrations are predominant for CFBs, and EC is predominant for OFBs.

1. INTRODUCTION

Industrial plants release huge amounts of particulate matter (PM) and have been considered important stationary sources in regional environment.1 The removal of PM from industrial flues is one of the most problematic environmental challenges worldwide. PM affects the climate system directly by the scattering and absorption of solar radiation, and indirectly influences cloud microphysics systems, thereby changing the hydrological cycle.2 Early studies have shown that PM2.5 accounts for 50–80% of PM emitted by coalfired power plants.3 Among the emissions caused by human activities, PM2.5 emissions resulting from the use of fossil fuel in stationary sources exceed 60%.4 The pollution attributed to PM2.5 reduces visibility significantly and causes the formation of atmospheric haze.5 PM2.5 is a mixture of diverse species and its composition is complex, owing to the inclusion of components from several sources, such as vehicle emissions, industry, biomass burning and other human activities.6 As a matter of fact, several epidemiological analyses have revealed that PM2.5 may contain a wealth of hazardous matter, including organic (soot, polycyclic aromatic hydrocarbon, spores, pollen, etc.), inorganic (metals, sulfates, nitrates, and other species) compounds. Additionally, PM2.5 potentially impacts human health through inhalation and respiratory deposition, subsequently increasing mortality and morbidity.7 Boilers play important roles in industrial manufacturing processes and are significant PM emission sources of industrial plants. Coal and oil are the major fuels used for boilers. 2


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Coal- and oil-fired boilers release injurious substances into the environment, including PM, NOx, SOx, trace metals, carbon monoxide, organic pollutants, and polycyclic aromatic hydrocarbons.8 Trace metals are emitted from the boilers in both gas phase and particulate forms during the incineration of fuel. Metal elements can act as catalysts in atmospheric transformations to form secondary pollutants, as well as contribute to corrosion of materials.9 This could contribute to economic loss. Gas emissions from boilers are from the burning of fossil fuels, which continually contribute to global warming, acid rain and smog. Most boilers are equipped with pollution control devices (e.g. cyclone, electrostatic precipitator, bag house) to reduce particulate emission. Characterization of PM2.5 from emission sources provides reliable data for identifying their contributions to ambient air and for designing corresponding control measures to follow government rules. Fundamentally, PM2.5 emissions from stationary sources are classified as “filterable” and “condensable’’ particulate matter.10 FPM is directly emitted from a source as a solid or liquid at stack, and captured on a filter media. CPM is known as a vapor phase material at stack conditions, but the material can condense and/or react upon cooling in the air to form a solid or liquid PM. FPM can be physically captured on a filter during sampling. All CPM is less than or equal to 2.5 µm in diameter, and can exist in gas phase during sampling, as well as condensed after cooling. Nitrate and sulfate complexes are the most extensively recognized forms of CPM emitted by combustion sources, whereas the volatile organic and semi-volatile compounds are in organic components of CPM.11 Many studies have been carried out to investigate FPM and CPM emissions from stationary sources at various locations worldwide.12-14 Numerous studies have confirmed that CPM contributes significantly to total PM2.5 emissions from 3


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stationary sources.10-12,15 Some studies have evaluated PM2.5 emissions from industrial coal-fired boilers.16,17 These studies illustrate characteristics of the emissions from coalfired boilers, but very few have been carried out to examine emission of oil-fired boilers,18,19 and to compare the dissimilarities of PM2.5 emissions and the emission characteristics from coal- and oil-fired boilers.20 Many studies have investigated coal-fired boilers, while very few have been done on oil-fired boilers, this is the first study to collect real world PM2.5 samples emitted from a large number of both coal- and oil-fired boilers. The numerous samples collected in this study are further classified according to the particulate control devices installed. PM2.5 (both FPM and CPM) emission factors are established and the emission levels for these two kinds of boilers with various particulate control devices are compared. In addition, chemical compositions (including carbon content, water-soluble ions and metals) for PM2.5 samples are analyzed. The results of this study provide valuable information to understand the constituents of PM2.5 emissions from industrial boilers.

2. MATERIALS AND METHODS

2.1 Sampling Plan and Boilers. In this study, both FPM and CPM samples were collected from the stacks of coal- and oil-fired boilers (abbreviated as CFBs and OFBs respectively). Overall, 26 boilers (15 CFBs and 11 OFBs) were investigated, which were divided into seven groups mainly according to particulate control devices installed in the test plants. Basic information about the test boilers is listed in Table 1. Groups C1 (n = 4), C2 (n = 5) and C3 (n = 3) are industrial CFBs which are equipped with cyclone, electrostatic precipitator (EP) and baghouse devices respectively, for particulate emission 4


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control. The kinds of industrial plants containing the test boilers include food processing, metal, leather manufacturing, paper mills, textile. In this study, PM2.5 samples were collected directly at the stacks from the test boilers. Therefore, the manufacturing processes of the test plants are unrelated and have no effect on PM2.5 emissions in this study. Group C4 (n = 3) represents coal-fired power plants, all of which use EP as a particulate emission control device. The boiler capacities of the power plants are big, and the boilers and control devices are usually more carefully operated. Thus, power plant samples are classified separately as one group. The particulate control devices were running steadily during the sampling period. Groups O1 (n = 8), O2 (n = 2) and O3 (n = 1) are OFBs which are installed with no pollution control devices, cyclone and baghouse respectively. The CFBs and OFBs use raw coal and No. 4-6 heavy oil as fuels respectively. Heavy oil is recognized as a cleaner fuel compared with coal. OFBs are not required to install particulate control devices by the government. Therefore, most OFBs do not equip with particulate control devices. Moisture, temperature, and gas compositions (CO, CO2, N2, O2) were measured before PM2.5 sampling. Gas compositions, which were measured by Orsat analyzer, are essential data for calculation of isokinetic sampling. Moisture, temperature, and gas compositions of the groups of boilers are listed in Table 1. Average OFB exhaust temperature is higher than CFBs. The high temperature results in lower water content. Thus, CFB exhaust contains higher water content than OFBs. The lowest values of moisture (2.49 ± 2.38%) and temperature (44.6 ± 5.53 oC), are found for O2 and C2 respectively. CO emissions from combustion sources depend on the oxidation efficiency of the fuel. Industrial boilers with good combustion control produce little CO.21 In this 5


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study, CO concentration was lower than the detection limit for all the test boilers. The lowest percentages of CO2 (5.35 ± 0.17%), O2 (4.83 ± 0.27%) and N2 (79.6 ± 0.35%) were found at C1, C4 and C3 respectively, whereas the highest percentages for the above parameters were determined at C2 (CO2: 14.2 ± 0.73%), O3 (O2: 11.5%) and O2 (N2: 82.2 ± 0.2%). 2.2 Sampling Method and FPM Analysis. The United States Environmental Protection Agency (USEPA) Method 201A (Determination of PM10 and PM2.5 emissions from stationary sources) and Method 202 (Dry impinger method for determining condensable particulate emissions from stationary sources) were used to measure and collect FPM and CPM samples respectively. APEX XC-5000 Automated Isokinetic Sampling Console sampling system which meets the requirement of USEPA Method 201A and Method 202 was used. Detail descriptions of the sampling system can be found in a previous study by Yang et al.15 Leakage check is conducted before each sampling. To collect sufficient samples, the sampling volume must be at least more than 2 m3. FPM samples were collected through the USEPA Method 201A which includes a front nozzle, PM2.5 cyclone, filter holder, pitot tube and stainless steel (with glass liner) sampling tube, vacuum pump, and computer control console. The sampling rate was controlled to within ± 20% isokinetically. Particulates less than 2.5 µm diameter are sucked through the cyclone and are primarily collected on a 47 mm Quartz filter. The temperature of the cyclone sampling-head was maintained within ± 10°C of the stack temperature to protect suitable sizing and prevent condensation on the walls of the cyclones. The filter temperature was kept at 20-23 oC at a relative humidity of 30-40%

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for 24 hrs before and after sampling. Afterwards, the filter was weighed according to the gravimetric technique (Sartorius balance, model Cubis 6.6S-DF). FPM filter sample was cut into four equal pieces for chemical analysis. One-fourth of the filter sample was used to measure OC and EC, with a thermal/optical carbon analyzer (DRI, model 2001, Reno, USA) using the Interagency Monitoring of Protected Visual Environments (IMPROVE) protocol. For water-soluble ion analysis, one-fourth was extracted with distilled deionized water in an ultrasonicator (Branson, model 5210) for 2 hrs. After extraction, the sample was filtered with a 0.4 µm filter and analyzed for ions (NH4+, Cl-, NO3-, SO42- Ca+, Na+, K+) by ion chromatography (IC, Dionex, model DX-120). The eluent was 20 mM methane sulfonic acid and 1.8 mM Na2CO3/1.7 mM NaHCO3 for cation and anion analysis respectively. Cation standard from AccuStandard (210125090) and anion standard from High-Purity Standards (1033506 and 1034819) were used to make calibration lines of the measured species. The R2 of the calibrations are all higher than 0.995. Blank and duplicate tests were conducted for quality control. For metal analysis, one fourth of the PM2.5 filter sample was digested with the acid mixture (HNO3: HCl = 1: 3) on a hot plate for 1 hr. The digested sample was analyzed for metal elements (Al, Ca, Fe, Mg, Mn, Si, Na, K, Pb, Zn, Ni, V, Cu, Cd, Mo, Co, Se, Sr, As, Ba, Sb, Se, Sn) by inductively coupled plasma-optical emission spectrometer (ICP-OES, Thermo Scientific, model iCAP 6000 Series). Calibration lines were made per the Merck standard (1.09492.0100). Calibration verification was performed during sample analysis. A new calibration line must be made when the bias is higher than ± 10%. 2.3 Sample Analysis of CPM. CPM samples were collected using USEPA Method 202 equipment, which includes a condenser, water dropout impinger, modified Greenburg 7


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Smith impinger and condensable PM filter. A backup filter was placed between the second and third impingers to improve collection efficiency. The CPM sample was collected by the condenser, dry impingers, pipelines and a backup Teflon filter after filterable PM was removed by a 47-mm Quartz filter. To reduce positive artifact, all CPM sampling trains were purged with Ultra-High Purity compressed nitrogen after the sample collection. The CPM train was purged at a minimum of 14 L/min for at least 1 hr. Purified water and organic solvents (n-hexane and acetone) were used twice to rinse the whole sampling pipeline, condenser, water dropout impinger, modified Greenburg Smith impinger and condensable PM filter respectively. The water rinse and organic solvent rinse samples are named as inorganic and organic fraction respectively, in Method 202. The water rinses, organic solvent rinses (including acetone and hexane), and the CPM filter were collected in clean containers separately. The aqueous (water-soluble) and organic (organic solvent soluble) fractions were dried and weighed in the laboratory. CPM is the summation of the two organic and inorganic fractions. After weighing, the dried water rinsed and organic solvent-rinsed condensable PM samples were rinsed to vials and diluted to 50 mL with distilled deionized water. A 25 mL sample was extracted following the above procedure and analyzed for ion species. Another 25 mL sample was used for metal elements analysis. Field blanks (e.g., organic solvents and water field blanks) were measured for each sampling. The average CPM mass concentration was 4−10 times larger than field and reagent blanks for Method 201A. Before commencing the sampling, all glassware and connecting parts of the CPM sampling trains were washed by deionized water, acetone, and hexane sequentially, then air-dried.

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Thermocouple calibration was performed often following the process described in Method 202. Quality checks were performed for each sampling.

3. RESULTS AND DISCUSSION 3.1. Emissions and Mass Distribution of FPM and CPM. FPM and CPM mass concentrations for the 7 groups of boilers are shown in Figure 1(a). FPM and CPM concentrations emitted from CFBs (Groups C1~C4) are in the range of 0.84 ± 0.18 to 18.7 ± 13.8 mg/Nm3 and 3.92 ± 1.09 to 22.2 ± 5.60 mg/Nm3 respectively. Low FPM emission concentrations are found at C4 and C2 groups. This is mainly due to EP which is installed along with the boilers, and has high particulate removal efficiency. Correspondingly, low CPM concentration was also measured at C2 group. In this report, only particulate control devices are discussed, because previous studies have shown that the influence of gas-air pollutant control devices on CPM emission is not as significant as particulate control devices.12 If the flue gas temperature is decreased in the EP, some organic segments of CPM could condense and adsorb onto fly ash surfaces, and are removed by EP.11 Previous studies have shown that EP can effectively reduce CPM emission.11,12 Relatively low concentration of FPM and CPM was also observed by a previous study at a coal-fired power plant with EP control devices.12 Low efficiency control device cyclone was installed at C1 group, resulting in high FPM and CPM emissions. Groups O1, O2 and O3 are OFBs which were installed without particulate control device, cyclone and baghouse respectively. Consequently, the highest concentrations of FPM (141 ± 176 mg/Nm3) and CPM (242 ± 131 mg/Nm3) are generated by O1 boilers, 9


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and the lowest FPM (2.31 mg/Nm3) and CPM (3.61 mg/Nm3) concentrations are by O3 boilers. Heavy oil has been recognized as cleaner fuel in comparison with coal. However, both FPM and CPM emission concentrations in group O1 were significantly higher than the 4 groups of CFBs due to the reason that O1 has no particulate control devices. The results show that air pollution control devices are effective and important for PM2.5 emission control. The concentrations of FPM, CPM and PM2.5 measured for CFBs and OFBs reported in the literature are listed in Table 2. Concentration of FPM and CPM was 0.84 ± 0.18 and 5.96 ± 2.21 mg/Nm3 respectively for coal-fired power plants, which are equipped with EP particle control devices. Higher CPM concentration than that of FPM for corresponding industrial boilers was obtained in this study. Numerous previous studies have shown the same trend of higher CPM concentrations for industrial boilers.12,15,22 FPM and CPM concentrations of this study was approximately equal to a previous study by 15Yang et al. which shows concentrations were 16.9 mg/Nm3 and 29.3 mg/Nm3 for FPM and CPM respectively for coal-fired boiler installed with cyclone. Previous inventories have been measured: a lower concentration of particulate matter for CFBs and higher for OFBs. The same result was observed by the current study. A slight variation was observed between previous studies and the current study concerning the PM2.5 for corresponding fuel combustion, shown in Table 2. This study indicated relatively slight variations for these measurements, which are consistent with findings in previous studies. The concentration of particulate matter emission is influenced by the corresponding fuel-combustion process, as well as control devices.29

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The emission factors of FPM, CPM and PM2.5 for the test boilers are shown in Figure 1(b). Emission factor is the measurement of a specific pollutant discharged into the environment from fuel, equipment, or source by a specific process. It can be expressed as a number of gram per ton-coal for CFBs and gram per kL-oil for OFBs. This study aggregated fuel usage of each sampling to calculate the PM2.5 emission factors. For CFBs, FPM and CPM emission factors are 57.1 ± 13.8 and 75.9 ± 19 g/t-coal respectively. For OFBs, FPM and CPM emission factors are 307 ± 50.0 and 2624 ± 1351 g/kL-oil respectively. PM2.5 emission factor for the OFBs is higher than that of the CFBs because some of OFBs were installed with no particulate control devices and others were installed with cyclone only. In the case of CFBs, the lowest emission factor of PM2.5 came from the boilers installed with EP, whereas the highest was obtained at the boilers installed with cyclones. A previous study30 revealed that in existing coal-fired power plants, EP had been one of the most efficient PM control devices, a finding that agrees with the results of this study. The emission factors of CPM for CFBs and OFBs are higher than FPM, indicating that the fuel-fired boilers make a considerable contribution to condensable PM2.5. The distribution percentages of FPM and CPM in PM2.5 for the 7 groups of boilers are shown in Figure 2. The percentages of CPM are 54.3%, 50.7%, 71.1%, 87.7%, 63.2%, 78.8% and 61.0% for group C1, C2, C3, C4, O1, O2 and O3 respectively. The results indicate that CPM contributes significantly to PM2.5 emissions for all the boilers, which is quite comparable to previous studies.15,31 The emission concentrations of CPM from industrial boilers depend significantly on the sulfur content of the fuel used.32 Consequently, CPM concentration at OFBs is high, owing to high sulfur content of fuel 11


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and resulting in large amounts of CPM.33 In measuring CPM from combustion of fuels containing sulfur, it has been shown that SO2 collected in the impingers can be oxidized to sulfate and produce a variable sulfate artifact that results in overestimation of condensable emissions.34 The high efficiency of PM2.5 removal devices can effectively reduce FPM, but does not produce the same effect on CPM. Consequently, the percentage of CPM rises after the removal device.31 The differences between CPM and FPM (CPM minus FPM) concentrations are shown in a small graph inside Figure 2. The highest difference between CPM and FPM is observed at O1, and the lowest at C2. The emission concentrations of CPM from CFBs and OFBs depend significantly on the sulfur content of the fuel used.32 Heavy oils have high sulfur content.33 Consequently, CPM concentration at O1 is high owing to high sulfur content of fuel and no control device installed, resulting in a large variation between CPM and FPM. 3.2 Distribution of Inorganic and Organic Fraction in CPM. The composition of CPM includes inorganic and organic fractions, which were separately rinsed with deionized water and organic solvent (n-hexane and acetone). Figure 3 displays the distributions of inorganic and organic fractions in CPM for the 7 groups of boilers. The average inorganic fractions are 63.4% and 35.5% for CFBs and OFBs respectively. The results show that CPM of CFBs is mainly composed of inorganic composition, and organic fraction dominates in CPM of OFBs. Chemical analysis indicated that the oilburning facilities emitted more unspecified organic compounds than the coal-burning plants.35 CPM is primarily inorganic in nature, which is emitted primarily from coal-fired boilers. A previous study also reports that the inorganic fraction is dominated in CPM 12


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emission from stationary sources.15 Another study36 reported that the contribution of the average organic faction to CPM was approximately 23% out of four coal boilers, yet one showed that organic faction accounted for 57% of the CPM. Richards et al.37 stated that the inorganic fraction dominated over organic fraction in CPM. For the current study, the two highest inorganic fractions of CPM were 86.4% and 77.1% for C4 and C1 respectively. Yang et al.15 showed that inorganic fractions accounted for 89% and 69.4% in CPM for coal-fired power plant and coal-fired boiler respectively. An evaluation of Method 202 for coal-fired boiler emissions showed that the inorganic fraction accounted for greater than 95% of the CPM and is mainly made up of SO4.34 3.3 Water-soluble Ions and Metal Elements in FPM. Average concentrations of water-soluble ions and metal elements of FPM emissions for the 7 groups of boilers are shown in Figure 4. The average concentrations of water-soluble ions are 2670 ± 1216 and 3693 ± 1035 µg/Nm3 for CFBs and OFBs respectively. The results show that a high concentration of water-soluble ions was observed at OFBs. Water-soluble ions, such as SO42-, NH4+, Na+, NO3- and Cl-, are significant emissions from the CFBs. Correspondingly, SO42-, NO3-, Na+ and K+ are the more significant ions in FPM for OFBs. The concentrations of SO42– are 3938 ± 2996, 1076 ± 425, 138 ± 130, 152 ± 41.0, 2941 ± 465, 2521 ± 1521 and 1002 µg/Nm3 for C1~C4 and O1~O3 respectively. The results confirm that SO42– accounts for the highest concentration for all the test boilers. Wang et al.38 showed that the fraction of SO42- contribution in total water-soluble ions reached 80.8% in coal-fired power plant emission. Cheng et al.39 found that SO42– is the more abundant ion in PM2.5, which accounted for 4.9-25% from OFBs emission using No. 6 fuel oil. The results of this study agree with previous reports in the literature. Whereas 13


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NH4+ (64.9 ± 1.96 µg/Nm3) and Mg2+ (66.1 ± 0.17 µg/Nm3) exhibit very low concentrations at OFBs, in the case of CFBs, the same ion concentrations account for 204 ± 148 µg/Nm3 (NH4+) and 9.58 ± 5.26 µg/Nm3 (Mg2+). Significant sulfate is emitted because of the high sulfur content in oil used for OFBs. The variation in mass concentration of water-soluble ions depends upon the type of source. Ma et al.40 also showed that SO42- is a main water-soluble ion in PM2.5 generated from CFBs. The dominant ions are SO42-, NO3-, NH4+ and Cl- accounting for more than 86% of the total water-soluble ions in FPM. F- was observed at very low concentration at both CFBs and OFBs. Wang et al.38 investigated water-soluble ions for fossil-fuel (coal, oil, and natural gas) power plants in China and they found that for coal-fired power plants, SO42- accounted for the largest proportion (81%)38. In this study, SO42- was the predominant ionic species and 45% of mass concentration was occupied in total watersoluble ions for coal-fired power plants. Cheng et al.39 reported that the mass percentage of the analyzed species for NH4+, Na+ and Ca2+ were 1%, 2.4% and 5% in PM2.5 respectively for OFBs. These three species in the current study accounted for 1%, 3.83% and 5% of PM2.5, emitted from OFBs. The emission concentrations of metal elements from the 7 groups of boilers are shown in Figure 4. The centration of metals released from the source depends on the fuel feed mechanism, composition of the fuel and combustion temperature.28 The results show that the concentration of all analyzed metals is highest for C1 (2002 µg/m3), installed with cyclone. The lowest metal emission concentration is found at C2 (466 µg/m3), which is equipped with EP. In the case of OFBs, the highest metal concentration is 1531 µg/m3 at O1, with no pollution control device installed, and the lowest value is 366 µg/m3 at O3, 14


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equipped with baghouse. The results suggest that higher metal concentration is released from CFBs than OFBs. The combustion of fossil fuel, like coal, is a primary source of trace elements in the environment. The concentration of Al, Ca, Fe, Mg and Na dominated in C3 which belongs to coal-fired boilers. For O1, O2 and O3, the dominated concentrations were Al, Ca, Fe, Na and Ni. In general, the amount of any emitted metal from CFBs and OFBs depends on the physical and chemical properties of the element itself, concentration of the metal in the fuel, the combustion conditions, type of particulate control device used, and control device removal efficiency as a function of particle size. Wang et al.38 reported that Al and Fe contributed 21.8% and 9.5% of mass concentration in total metals for coal- and oil-fired power plant respectively. In this study, the same components contributed 17.9% and 10% of mass concentration in total metal elements for CFPs and OFBs respectively. In this study the percentage of some metal elements Fe, Ni, V, Cr and accounted for 2%, 3.1%, 0.7%, 0.03, 0.02% respectively of PM2.5 emitted from OFBs. A previous study has also measured the percentage of the same elements, which were 6.6%, 2.0, 1.2%, 0.4% and < 0.1% respectively of PM2.5 emitted from OFBs.39 Therefore, the results show that many hazardous metal species are emitted from OFBs. 3.4 Water-soluble Ions and Metal Elements in CPM. 3.4.1 Inorganic fraction. The influence of inorganic element emissions from fossil fuel combustion on the environment has been recognized as a potential problem. Figure 5 illustrates the concentrations of water-soluble ions and metal elements in inorganic fraction of CPM. Water-soluble ion emissions are higher than those of metal elements for all groups of boilers. SO42- is the highest concentration ion for all the groups of boilers. If sulfur is in the fuel, it is oxidized 15


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mainly as SO4 and a minor portion is converted as SO3. It reacts in the presence of water vapor to form H2SO4 in the stack gas. The SO4 compounds are primarily made up of H2SO4. The second highest ion concentration among the analyzed water-soluble ions is NO3-. Similar results12 are not only proved by studies of water-soluble ion emissions from coal boilers, but also study41 of ambient PM2.5. The primary acid substances like SO42– and NO3– could directly affect human health and the ecosystem.38 For metal elements, concentrations of Na, Al, Ca and K are high for all groups of boilers. Na emission concentrations are 106, 120, 186, 155 and 365 µg/Nm3 for C1~C4 and O1 respectively (Figure 5). Tsukada et al.42 reported that some metal elements are in condensable phase with high concentration. According to thermodynamic equilibrium calculation, the metals, collected by Method 202, can be in gas form. The current results indicate that the high concentration of inorganic matter is a predominant fraction of CPM. Most of the large-scale industrial plants are using fossil fuels having a considerable fraction of inorganic elements.33 This result agrees with a previous study12, which reported that Na and Ca metal element of inorganic fraction are predominantly emitted from coal-fired power plant. 3.4.2 Organic fraction. Mass concentrations of water-soluble ions and metal elements in organic fraction of CPM are shown in Figure 6. Generally, organic complexes are formed by molecules composed of carbon and hydrogen, and may contain any number of other elements. For CFBs, the major chemical concentrations are SO42- (60.4 µg/Nm3) > NO3- (46.9 µg/Nm3) > Na (41 µg/Nm3) > Cl- (39.6 µg/Nm3), and the others are less than 15 µg/Nm3. In the case of OFBs, rich chemical species are SO42- (610 µg/Nm3) > NO3(438 µg/Nm3) > NH4+ (193 µg/Nm3) > Na (143 µg/Nm3) > Cl- (72.4 µg/Nm3), and the 16


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others are less than 25 µg/Nm3. Large industrial and utility boilers are generally welldesigned and well-maintained so that particulate emissions are minimized. The types of coal used in each boiler may change, which directly alters average metal emissions. The same as inorganic fraction: SO42– and NO3– are highest among the analyzed water-soluble ions. Na was the highest metal concentration in all groups of boilers, and reaming metals also occupied a portion. With the high temperature of fuel combustion, metal elements can evaporate partially or entirely and enter into the ambient air through exhaust gases after removal by various control devices. In this study, some metal elements (As, Cd, Co and Mo) were below the detection limit for both CFBs and OFBs. 3.5. OC and EC Concentrations. OC and EC compounds are mainly emitted from combustion sources and play a vital role in the chemistry of the atmosphere as a climate forcing agent. Concentrations were found to show that organic compounds established a substantial fraction of the PM and their contribution is larger than that of water-soluble ions and metal elements. Consequently, OC and EC are described distinctly in part of this study. The carbonaceous compounds (OC and EC) in emission concentrations were obtained from FPM data analysis and are represented in Figure 7(a). Differences in OC and EC (OC-EC) are shown in Figure 7(b), as well. The lowest concentrations of OC and EC are 161 ± 32.1 and 80.8 ± 11.7 µg/m3 respectively, for C2. C2 boilers were installed with EP, and the carbon content was removed effectively, along with PM. The highest values for EC (2998 ± 1182 µg/m3) and OC (2682 ± 831 µg/m3) are found at O1 which was not equipped with air pollution control devices. Average total carbon concentrations for CFBs and OFBs are 378 ± 116 and 3009 ± 1575 µg/m3 respectively. The result shows that OFBs emission concentrations are higher than that of CFBs. A similar result38 was 17


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found by previous study concerning the emissions from coal- and oil-fired power plants. From earlier study43, carbonaceous concentrations account for 17.0% ~ 40.0% of the mass concentration of PM2.5 emissions from coal-fired power plants. The results of this study elucidate that the concentration of OC and EC is based upon the fuel type and control devices. The average EC and OC concentrations for CFBs are 125 ± 39.3 and 249 ± 78 µg/m3 respectively, and are 1624 ± 833 and 1384 ± 743 µg/m3 for OFBs. The results show that OC concentrations dominate for CFBs, and EC concentrations dominate at OFBs. Particulate matter contains more EC in the emissions of OFBs, whereas OC is abundant for CFBs emissions. Primary OC and EC are emitted from the same combustion source. Nonetheless, the temperature has influence on the concentration of EC and OC.44 In general, at conditions of high temperature, EC is emitted in high quantities. Under low temperature, OC is emitted in high concentration as a result of poor combustion conditions.45 Similar results have been observed by previous studies.39,46 The ratio of OC/EC is higher for CFBs than that of OFBs (Figure 7(c)). That ratio is influenced by the boiler load and fuel type.46 The variation of OC/EC ratios in a small capacity boiler is mostly due to incomplete combustion and smoldering of coal.39 Ma et al.46 and Watson et al.47 have reported OC/EC ratios of 3.8 and 9.7 respectively, for coalfired boilers. The average values of OC-EC and OC/EC ratio are 127 ± 37 µg/m3 and 2.02 for CFBs, and are -241 ± 166 µg/m3 and 0.84 for OFBs respectively. The results also suggest that OC is predominant at CFBs, and EC is high for OFBs. Previous study48 also has reported that a concentration of EC is emitted from coal-fired power plants. Wang38 studied carbonaceous component emission from fossil-fuel power plants. In their study, the emission of carbonaceous components in PM2.5 from fossil-fuel power plants was 18


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29.6 kt in 2014. For coal-fired and oil-fired power plants, OC accounted for the largest proportions of 60.7% and 71.4% respectively.38 Cheng et al.39 studied the carbon contents from OFBs in central Taiwan and found that EC is the dominant fraction, but with high variation (value from 0.01 to 37.7%, average of 9.1%). The EC constituent measured from OFBs in this study is lower than that presented in literature from other countries.49

4. CONCLUSIONS Numerous studies have been carried out to investigate emissions from CFBs. This is the first study to intensively distinguish emissions from both CFBs and OFBs. Emission characteristics of both FPM and CPM emitted from CFBs and OFBs were investigated via field sampling. Average PM2.5 emission concentration of OFBs is higher than that of CFBs, which is caused by the fact that the test OFBs in this study have almost no particulate-control devices. The results show that the fraction of CPM is much higher than FPM fraction in PM2.5 for both CFBs and OFBs. The concentration of FPM is 18.6, 2.31 and 3.50 mg/Nm3 at the stack with cyclone, electrostatic precipitator and baghouse in CFBs respectively. In the case of OFBs, the water-soluble ions in the FPM compositions are the highest (42.7%), followed by carbon component (33.8%), with metal elements (10.2%) being lowest. For CFBs, carbon component (30.6%) and watersoluble ions (30.1%) in the FPM compositions is circa equal, and the lowest is metal (14.2%). The species in CPM are dominated by water-soluble ions, including SO42-, NO3and NH4+. The results indicate that CPM is formed primarily by water-soluble ions. For the inorganic/organic fraction of CPM, SO42-, NO3-, Na and Ca exhibit the highest concentrations in all samples. Inorganic matter occupies a significantly larger fraction of 19


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CPM than organic. Considerably elevated carbonaceous particle (OC and EC) emissions were found at CFBs and OFBs. The average fraction of OC for CFBs and EC for OFBs in total carbon is 66.8% and 54.0% respectively. The results show that OC has higher fraction for CFBs, while EC fraction is higher for OFBs. This study clearly illustrates characteristics of FPM and CPM emission from important industrial stationary sources of CFBs and OFBs. ACKNOWLEDGEMENT This research study was financially supported by the Ministry of Science and Technology (MOST) of the Republic of China.

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(27) Moretti, A. L.; Jones, C. S. Advanced emissions control technologies for coal-fired power plants. Babcock & Wilcox Power Generation Group. BR (-1886). (28) Ali, M.; Athar, M.; Ali, E. The Emissions and environmental impact of power plants operating on fossil fuel at distt. kasur, Pakistan. Ekoloji. 2007, 16 (63), 74-80. (29) Nosek, R.; HolubIík, M.; PapuIík, S. Emission controls using different temperatures of combustion air. Sci. World J. 2104, 2014, 6, ID 487549. (30) Xiong, G.; Li, S.; Chen, S.; Zhang, X.; Yao, Q. Development of advanced electrostatic precipitation technologies for reducing PM2.5 emissions from coal-fired power plants. Proc. Chinese Soc. Electr. Eng. 2015, 35(9), 2217– 2223. (31) Pei, B. Determination and emission of condensable particulate matter from coalfired power plants. J. Environ. Sci. (China). 2015, 35 (5), 1544−1549. (32) Cai, H.; Wang, M.; Elgowainy, A.; Han, J. Updated greenhouse gas and criterial air pollutant emission factors and their probability distribution functions for electric generating units; Argonne national laboratory: Energy systems division, 2012. (33) Javadli, R.; Klerk, A. Desulfurization of heavy oil. App. Pet. Res. 2012, 1 (1-4), 3– 19. (34) Corio, L. A.; Sherwell, J. In-stack condensable particulate matter measurements and issues. J. Air Waste Manage. Assoc. 2000, 50 (2), 207−218. (35) Ahlberg, M.; Berghem, L.; Nordberg, G.; Persson, S. A.; Rudling, L.; Steen, B. Chemical and biological characterization of emissions from coal- and oil-fired power plants. Environ. Health Perspect. 1983, 47, 85–102.

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Table 1. Basic Information of the Test Coal- and Oil-Fired Boilers group of boilers C1 (n = 4) C2 (n = 5) C3 (n = 3) C4 (n = 3) O1 (n = 8) O2 (n = 2) O3 (n = 1)

boiler type coal-fired boiler coal-fired boiler coal-fired boiler coal-fired power plant oil-fired boiler oil-fired boiler oil-fired boiler

PM control device Cyclone EP Baghouse EP No Cyclone Baghouse

moisture (%) 9.71 ± 2.45 8.35 ± 2.15 12.7 ± 2.21 12.2 ± 0.29 9.05 ± 0.69 2.49 ± 2.38 9.10

temp (oC)

CO2 (%)

65.3 ± 4.65 44.6 ± 5.53 83.3 ± 16.7 101 ± 2.03 148 ± 11.1 195 ± 129 125

5.35 ± 0.17 14.2 ± 0.73 12.1 ± 0.62 14.1 ± 0.09 10.2 ± 0.85 8.10 ± 2.00 6.80

27


O2 (%) 14.5 ± 0.28 5.78 ± 0.70 8.30 ± 0.26 4.83 ± 0.27 8.01 ± 1.15 9.70 ± 1.80 11.5

N2 (%) 80.2 ± 0.25 80.0 ± 0.59 79.6 ± 0.35 81.1 ± 0.35 81.8 ± 0.36 82.2 ± 0.20 81.7

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Table 2. Measurements for Coal- and Oil-Fired Boilers Comparison with Previous Studies Source Coal-fired boilers Coal-fired boilers Coal-fired boilers Coal-fired power plants Oil-fired boilers Oil-fired boilers Oil-fired boilers Coal-fired power plants Coal-fired power plants Coal-fired power plants Coal-fired boilers Coal-fired boilers Coal-fired boilers Coal-fired power plants Coal-fired power plants Oil-fired boilers Oil-fired power plant

Air pollutant control devices Cyclone EP Baghouse EP No Cyclone Baghouse EP EP EP EP EP EP EP EP EP

FPM2.5

CPM2.5

PM2.5

Reference

18.6 ± 13.7 3.83 ± 1.05 3.51 ± 3.21 0.84 ± 0.18 141 ± 76.1 22.6 ± 5.28 2.31 1.40 30.8 5.60

22.7 ± 5.61 3.92 ± 1.08 8.61 ± 4.03 5.96 ± 2.21 242 ± 131 84.2 ± 38.1 3.16 21.0 215 16.1

40.8 ± 17.4 7.72 ± 1.34 12.1 ± 4.03 6.81 ± 2.29 409 ± 131 107 5.47

Current study Current study Current study Current study Current study Current study Current study 22 Lu et al., 2010 15 Yang et al., 2014 12 Li et al., 2017 23 Li et al., 2016 24 Parve et al., 2011 25 Pechan, 2005 26 Huang et al., 2015 27 Moretti and Jones, 2012 23 Parve et al., 2011 28 Ali et al., 2007

1.90 21.3 2.40 1~10 12~36 100~120 160~328

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550

(a) FPM

450

100

400

80

350

60

300

CPM

PM2.5

CPM-FPM

CPM-FPM (mg/Nm 3)

3 FPM/CPM/PM (mg/Nm )

500

40

250 20

200 150

0 C1

C2

C3

C4

O1

O2

O3

100 50 0 C1

C2

C3

C4

O1

O2

O3

O2

O3

Group of boilers (b)

Emission factor (g/t-coal or g/kL-oil)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

6500 6000 5500 5000 4500 4000 3500 3000 2500 2000 1500 1000 500 0

FPM

CPM

PM2.5

250

200

150

100

50

0 C1

C1

C2

C2

C3

C3

C4

C4

O1

Group of boilers

Figure 1. (a) Emission concentrations of FPM / CPM / PM2.5. (b) Emission factors of FPM, CPM and PM2.5. The vertical bars indicate ± 1 standard error of the mean value for corresponding group of boilers.

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FPM

100

FPM&CPM Percentage (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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CPM

90 80 70 60 50 40 30 20 10 0 C1

C2

C3

C4

O1

O2

O3

Group of boilers

Figure 2. Percentages of FPM and CPM in PM2.5.

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Inorganic

62.4%

20.4%

13.6%

79.6% 37.6%

19.5%

20

58.9%

54.4%

77.1%

86.4%

60

40

Organic

80.5%

45.6%

80

41.1%

22.9%

100

Inorganic & Organic (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

0 C1

C2

C3

C4

O1

O2

O3

Group of boilers

Figure 3. Distribution of inorganic and organic fractions in CPM for the test boilers.

31


Energy & Fuels

FPM -

Water soluble ions (µ g/Nm3)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 32 of 35

-

F

6000

SO4 +

5000

-

Cl 2-

K

NO3 +

Na

2+

Mg

NH4 Ca

2+

4000 3000 2000 1000 0

C1

C2

C3

C4

O1

O2

O3

Group of boilers

Figure 4. Concentrations of water-soluble ions and metal elements in FPM for CFBs and OFBs. The vertical bars indicate ± 1 standard error of the mean value for corresponding group of boilers.

32


Page 33 of 35

Inorganic

14000 12000

Water soluble ions (µ g/Nm3)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

Cl-

NO-3

Na+

NH4 Ca2+

Mg2+

10000

SO24 + K

8000 6000 4000 2000 0 C1

C2

C3

C4

O1

Group of boilers

Figure 5. Chemical concentrations of water-soluble ions and metal elements in inorganic CPM for the test boilers. The vertical bars indicate ± 1 standard error of the mean value for corresponding group of boilers.

33


Energy & Fuels

Organic

1000 900 3 Water soluble ions (mg/Nm )

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

800 700

Page 34 of 35

Cl-

NO-3

Na+

NH4 Ca2+

Mg2+

SO24 + K

600 500 400 300 200 100 0 C-1

C-2

C-3

Group of boilers

C-4

O-1

Figure 6. Chemical concentrations of water-soluble ions and metal elements in organic CPM for the test boilers. The vertical bars indicate ± 1 standard error of the mean value for corresponding group of boilers.

34


Page 35 of 35

4500

(a) EC

OC

OC & EC (µ g/m3)

4000 3500 3000 2500 2000 1500 1000 500 0 C1 (b)

C2

C3 C4 O1 Group of boilers

O2

O3

200

3

OC-EC (µ g/Nm )

100 0 C1

C2

C3

C4

O1

O2

O3

-100 -200 -300 -400

Group of boilers (c)

2.0

1.5

OC/EC

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

1.0

0.5

0.0 C1

C2

C3

C4

O1

O2

O3

Group of boilers

Figure 7. (a) Concentrations of OC and EC. (b) OC-EC (c) OC/EC ratio. The vertical bars indicate ± 1 standard error of the mean value for corresponding group of boilers. 35


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