Size-Classified Variations in Carbonaceous Aerosols from Real Coal

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Size-Classified Variations in Carbonaceous Aerosols from Real CoalFired Boilers Xian Ma, Jianhui Wu,* Yufen Zhang, Xiaohui Bi, Yingming Sun, and Yinchang Feng State Environmental Protection Key Laboratory of Urban Ambient Air Particulate Matter Pollution Prevention and Control, College of Environmental Science and Engineering, Nankai University, Tianjin 300071, China ABSTRACT: The aim of this study was to characterize the carbonaceous aerosol emissions from three types of boilers: pulverized combustion technology boilers (PCBs), circulating fluidized beds (CFBs), and grate boilers (TGBs). These boilers have been used in the industrial district for heating, power, or production, and they have a maximum thermal output range of 7− 300 MW. The actual particle mass and number concentration and particle size distribution (PSD) were obtained from electrical low-pressure impactor (ELPI) measurements, and a preheated sampling system was used to manage water condensation in the stack gases. The carbonaceous composition of the particles was determined by a thermal-optical analysis. Significant differences were observed in the six boilers with regard to the contribution of carbonaceous fractions to the particle mass and number emissions. The TGBs had much higher carbon aerosol emissions with much higher levels of organic carbon (OC) and elemental carbon (EC), whereas they had higher emissions of total carbon particles, with emission rates that were approximately 15.7 and 10.2 times higher than that of the PCBs and CFBs, respectively. The OC/EC ratio was influenced by the boiler load and fuel type. In all of the boilers, the size distribution of OC and EC displayed a trimodal or bimodal distribution; however, the peaks of different particle sizes were affected by the pollution control device. The percentage of the OC1 + OC2 fraction in the OC was correlated with the temperature of the stack gases, although obvious variations were not observed with the PSD. However, the EC ratio showed variations and size differences, especially in the PM0.1. The char/soot ratio in the PM0.1 was 1.2−3.9 times that in the PM1, implying that the emission and combustion mechanisms of soot were different from those of char.

1. INTRODUCTION Carbonaceous aerosols from coal-fired boilers, including organic carbon (OC) and elemental carbon (EC), are a key type of pollutant because they are consistently associated with adverse health effects. Exposure to fine and submicrometer particles has been associated with different health effects, such as pulmonary and cardiovascular symptoms.1,2 EC has a definite effect on regional and even global climate conditions by influencing precipitation levels and enhancing greenhouse effects.3,4 Coal’s share of global primary energy consumption reached 30.1% in 2013, with a current annual increase of 2−3% per year.5 China’s coal combustion accounts for approximately onehalf of the world coal consumption.5 In China, coal is mostly used in the industrial sector and for residential heating, and combustion has the most varied uses in conversion technology.6 Because of the expected growth and high emissions apportionment by coal-fired boiler combustion, there are considerable concerns over the regional and global effects that may be caused by future coal combustion. Concern over the environmental impacts of coal-fired boilers has previously been concentrated on gaseous pollutants, such as CO, CO2, NOx, and SO2, because of their effects on climate change, acid rain, and indoor environmental pollution.7,8 However, particulate matter emitted from combustion contains toxic constituents, such as heavy metals, arsenic, mercury, silica, fluorides, organic carbon, and elemental carbon, as well as many other hazardous compounds that also have adverse effects on environmental quality and human health.9,10 In recent years, concern over carbonaceous material in the submicrometer particulate matter of ambient air has increased considerably © 2015 American Chemical Society

because of the effects of such material on air quality, climate change, and health.4,11 Globally, 1129 Tg EC and 877 Tg OC were contributed by combustion-related emissions in 1996, accounting for 14.2% and 2.6% of the total amount of EC and OC, respectively. These emissions were estimated to be responsible for 5.1−26.1% of the fine particle emission from coal-fired boilers in China, which accounts for the greatest usage of coal in the world.12 Nevertheless, our knowledge of carbon particles from pollution sources is inconsistent with the results of studies of environmental receptors. Emissions from boilers are diverse because of variations in the coal combustion processes, which include complex types of boilers, a wide range of combustion conditions, the increased development of control technologies, and variation in coal feed stocks. Although few studies have described the carbonaceous particle emissions from coal-fired boilers, the limited data show large differences among the components of particulate matter emissions from dissimilar coal burning techniques, with fly dust emissions in a FGD plant13 consisting of approximately 40% fly ash and 10% gypsum particles. The bag house collection efficiency of PM is 99.94% and 99.57% compared with that of PM 10.14 Lillieblad15 studied how a boiler’s operation load affected the emissions of particulate matter and PAHs from biofuel combustion in two traveling grate boilers with a thermal capacity of approximately 1 MW each. Relevant studies have focused on the concentration of OC and EC from coal-fired Received: August 3, 2015 Revised: November 4, 2015 Published: November 30, 2015 39

DOI: 10.1021/acs.energyfuels.5b01770 Energy Fuels 2016, 30, 39−46

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Energy & Fuels

electrical low-pressure impactor (ELPI+, Dekati Ltd., 10 l pm). ELPI+ determines the particle number concentration as a function of the aerodynamic particle size in a size range of 0.013−10.5 μm and with 11 particle size intervals.18 The relative humidity(RH) and temperature in the fuel gases were measured by the commercial instrument (TH-3012, Wuhan Tianhong Instrument Co., Ltd.). RH of the fuel gas for all boilers is in the range of 4−13%. The temperature of the fuel gas ranged from 339.7 to 433.2 K, which was much higher than the ambient air temperature. There is big difference in the water content due to the different temperature in the fuel gases. In general, these RH values were low in the air, but there was a large amount of water in the high-temperature fuel gases. Once the fuel gases cooled, a substantial amount of water had condensed. Because particulate matter is present in a high amount in the water and condensable fuel gas, high-temperature ELPI+ (HTELPI+) was used to directly measure aerosol samples at up to 70−180 °C, which effectively suppressed the vapor condensation and allowed for the collection of a high amount of water aerosols.19,20 An amount of OC may be gasified because of the heating process and fuel gases mix with ambient atmosphere. There are little studies for the loss of OC caused by the heating under the similar sampling condition. A quantitative method of losses due to elevated temperatures will be studied in our future work. The particle chemical composition was studied as a function of the particle size by analyzing the samples collected on substrates. Different substrates were selected for different research purposes. Clean and dry aluminum foil (radius of 25 mm) was used to sample the PSD. The aluminum foil was greased with a mixture of toluene and Apiezon-L vacuum grease (M & I Materials Ltd.) to suppress the particle bouncing effect.21 A quartz filter (radius of 25 mm, Pall. Ltd.) was used to analyze the carbonaceous fraction. 2.2. Analytical Methods. OC and EC collected by the quartz filter were analyzed with a DRI model 2001 Thermal/Optical Carbon Analyzer using the IMPROVE_A (Interagency Monitoring of Protected Visual Environments) protocol22 to measure the carbon fractions. The quartz filter was heated at stepwise temperatures of 120, 250, 450, and 550 °C in a pure helium environment to determine the OC1, OC2, OC3, and OC4 content, respectively. Then the environment was shifted to 2% O2/98% He, and the filter was continuously heated stepwise to 550, 700, and 850 °C to determine the EC1, EC2, and EC3 content, respectively. Pyrolyzed OC (POC) is defined as carbon that has combusted after the initial introduction of oxygen and before the laser reflectance signal achieves its original value,23 and it is assigned to the OC fraction. Therefore, OC is defined as OC1 + OC2 + OC3 + OC4 + POC, and EC is defined as EC1 + EC2 + EC3 − POC. 2.3. Plant Backgrounds. Samplings and measurements were performed in six coal-fired units in the size range of 7−300 MW. The sampled boilers are extensively used in China and include advanced and aged combustion technologies. Table 1 lists the boiler type, load capacity, and cleansing facilities for the six power units, which are mainly used as power supplements to produce, process, and supply resident heating. Two of the units were boilers based on pulverized combustion technology (PCB), two were circulating fluidized beds (CFBs) and two were traveling grate boilers (TGBs) used to produce steam for industry processes and residential heating. Three units were equipped with electrostatic precipitators (ESPs) to remove particles from the exhaust gases. In addition, three boilers were equipped with 3−4 field electrostatic precipitators (ESPs), one traveling grate boiler was equipped with a wet flue gas scrubber, and one traveling grate boiler was equipped with a gravity settling chamber. Another PCB was equipped with a bag filter. CFB2 performed wet flue gas desulfurization (WFGD) to decrease the amount of sulfur dioxide. Generally, the combustion temperatures of the pulverized coal boilers and CFB boilers were approximately 1520−1600 and 1050−1200 K, respectively. 2.4. Quality Control. The quartz-fiber filters were baked at 900 °C for 3 h before sampling to remove absorbed organic vapors. The exposed filters were stored in a refrigerator at 2 °C before chemical analysis to minimize the evaporation of volatile components.24 The aluminum foils and quartz-fiber filters were analyzed gravimetrically for

boilers; Chow’s study showed that the OC content of coal-fired power plant emissions ranged from 0.9 ± 1.2% to 62.9 ± 14.8%.16 Wierzbicka17 performed a particulate OC and EC analysis and identified 29−64% of the PM1 and mass particulates from 1% to 19% and from 1% to 56%, respectively. However, coal’s carbonaceous aerosol and fractions have rarely been reported for submicrometer particles, and their variations and internal relationships have seldom been studied. These patterns of components and PSDs can aid in the understanding and identification of the emission characteristics of the source material. In this study, particle emissions from combustion of the two most commonly used coals for heating supplies and industry, including heating, industrial processes, and power generation, were characterized. The actual particle mass and number concentration and PSD were obtained from electrical lowpressure impactor (ELPI) measurements. A preheated sampling system was used to manage the stack gases containing water droplets. On the basis of the thermal-optical method, the particle organic and elemental carbon contents were determined and ranged from PM10 to PM0.1; this result provides new insights into the composition of coal−fuel combustion particles. First, particulate matter in the condensable and high-humidity exhaust was collected and sampled by the heating ELPI+ system, and then the mass concentration of the particles and carbon fractions emitted from six boilers was quantified. Finally, the characteristics of the carbon fractions were classified according to different particle sizes, combustion methods, pollution control technologies, and stack gas temperatures.

2. EXPERIMENTAL SECTION 2.1. Sampling Methods. Figure 1 shows the setup for the particle measurements, which were performed on the stack after removing all

Figure 1. Sampling system. of the installed devices. Therefore, the direct emissions to ambient air were characterized for real-world conditions. A cyclone was placed after the probe prior to the diluter to collect coarse particles larger than 10 μm. After the cyclone, the sample flow was diluted in a diluter (Finland, Dekati Ltd.) with preheated and cleared dilution air, and the diluted sample was drawn inside the heating particle analyzer. All of the pipes were wrapped with thermal insulation material. The dilution ratio in the tube diluter was 8. The particle number concentrations in the flue gas were measured using an 40

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Energy & Fuels Table 1. Units, Temperature, and Particle Filtration Devices filtration technology boiler code

load (MW)

PCB1 PCB2 CFB1 CFB2 TGB1 TGB2 a

Ta (K)

RH (%)

300 300 105 125 7 7

7.9 13.1 5.5 4.4 7.6 12.4

dust remover b

356.3 343.7 390.4 406.9 433.2 339.7

ESP (3) ESP (4) bag filter ESP(3) gravity chamber wet scrubber

desulfuration

denitration

limestone injection WFGD no control limestone injection no control no control

SCRc SCR no control SNCRd no control no control

The temperature of exhaust. bElectrostatic precipitators. cSelective catalytic reduction. dSelective noncatalytic reduction.

their mass concentrations using an electronic microbalance with ±1 μg sensitivity (Mettler-Toledo MX5, Switzerland) after 24 h of equilibration at temperatures between 20 and 23 °C and relative humidities (RHs) between 35% and 45%. Each filter was weighed at least three times before and after sampling, and the substrate mass was obtained by subtracting the average of the presampling weights from the average of the postsampling weights. For calibration and quality control, measurements were performed with a preheated filter blank, standard sucrose solution, and replicate analysis. Field blanks were collected during sampling, and the concentration levels of OC and EC ranged from 3.84 to 4.01 μg/cm2 and from 0.83 to 0.92 μg/cm2, respectively, and these values accounted for less than 4% and 1% of the samples, respectively. The samples were corrected by the average concentrations of the blanks. The detection limit for OC and EC was 0.40 and 0.05 μg/cm2, respectively. The number of particles was dependent on the induced current related to the aerodynamic force and the Stokes diameter. A test aerosol at coal-fired boiler particle density was used.25,26 When the impactor was heated, the temperature affected the cut points of the impactor stages. The calculated cut points and flow rates were used to calculate the ELPI.

mass concentration of the six boilers. The PM mass concentrations in the CFB and PCB were much lower than the emissions in the TGB. Typically, the TGB was not equipped with a highly effective dust remover, which resulted in a high PM mass and number concentration in the emissions. Generally, a dust remover can effectively decrease the mass and number concentration of PM emissions from combustion processes.14 However, the particle numbers in the PCB2 equipped with a highly effective ESP were equal to that in CFB2 and similar to that of PCB1 (the highest). The WFGD was shown to decrease the emissions of SO2, although it can increase the particulate matter in the emissions.27,28 The mass concentrations of OC in PM10 were 0.71, 3.35, 0.73, 0.79, 31.10, and 37.96 mg/m3, which accounted for 2%, 9%, 8%, 2%, 10%, and 10% of that in the CFB1, CFB2, PCB1, PCB2, TGB1, and TGB2, respectively. The average total carbon mass concentration was 2.53, 0.96, and 64.04 mg/m3, thus contributing 3.0%, 7.5%, and 11.4% of the PM10 for the PCBs, CFBs, and TGBs, respectively, and the analogous average numbers for EC (0.50, 0.20, and 29.51 mg/m3) were 1%, 1%. and 9% in each type of furnace, respectively. A portion of the OC may be volatilized because of the heated dilution. The concentration of OC was most likely underestimated during the sampling. Studies have estimated that the value might be between 10% and 78% under ambient air sampling. In all of the sampled boilers, the carbonaceous composition was clearly enriched in the TGB emissions, indicating the important roles of combustion sources of anthropogenic origin. Figure 2 shows the number concentration distributions at the outlet of the six boilers. The number concentration of PM0.006− PM10 displays a unimodal distribution. Of the sampling units, TGB2 had the highest particulate numbers. The particulate number concentration emissions from TGB2 were lower than that of TGB1, whereas they were higher than the other boiler types. The TGBs had the highest emissions of the three types of boilers because this type is infrequently equipped with a dust collector and exhaust cleaning facilities prior to the outlet of the stack. The particulate emission characteristics in the CFBs were similar to those in the PCBs, which are generally used in large-scale commercial factories to generate power and steam. The results are different from that of earlier research on pulverized coal boilers, which was most likely because of the different combustion methods, combustion conditions, and combustion temperatures.29 Lower temperatures tend to suppress the volatilization of elements in coal, which decreases the generated concentration of submicrometer particles. 3.2. OC/EC Variations in Different Boilers. The OC/EC characteristics in the fuel gases of the boilers were classified using a linear fit. An attempt to classify the characteristics of

3. RESULTS AND DISCUSSION 3.1. OC and EC in Particulate Matter. The PM10 mass and number concentrations and the OC and EC concentrations from the six power or industrial boilers are summarized and listed in Table 2. All of the data from the boilers were corrected Table 2. PM10 Number Concentration Measured with ELPI boiler code PCB1 PCB2 CFB1 CFB2 TGB1 TGB2

particulate number (no. dN/ d log Dp)

mass (mg/m3)

OC (mg/m3)

EC (mg/m3)

× × × × × ×

9.06 33.22 29.71 35.55 297.06 369.97

0.73 0.79 0.71 3.35 31.10 37.96

0.18 0.21 0.24 0.76 24.05 34.97

7.04 1.24 3.79 1.06 2.02 2.85

104 107 106 105 107 108

under the same oxygen level. The number and mass concentrations were lower than 10 μm for the six units operating with an upper medium load as measured by ELPI+. These boilers (circulating fluidized bed (CFB), traveling grate (TGB), and pulverized coal boiler (PCB)) were classified by the type of boiler and combustion method. The PM10 number concentration ranged from 1.06 × 105 to 2.85 × 108 (dN/ d log Dp), and the TGB emissions number ranged from 2.02 × 107 to 2.85 × 108 (dN/d log Dp) and represented the highest levels in the samples for the three types of boilers. The particle number from the CFB ranged from 1.06 × 105 to 3.79 × 106 (dN/d log Dp), which was lower than that of the TGB emissions. In TGB2, the PM10 mass concentration was 369.97 mg/m3, which was approximately four times higher compared with the emission standard. PCB1 had the lowest 41

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Figure 4. Correlation between OC and EC in the TGBs.

Table 3. Comparison of PM2.5 OC/EC between Different Boiler and Fuel

Figure 2. Particle number size distributions in the flue gases after processing in a cleaning facility.

variation of the OC/EC was made, and the characteristics were compared for different boilers and fuel. The scatter plots for the OC and EC percentages on each filter are displayed in Figures 3 and 4.

boiler code

fuel

load (MW)

OC/EC

ref

PCB CFB TGB coal-fired plant coal-fired plant coal-fired plant SasDust-High SasDust-mid SasDust-low pellets-mid pellets-low

coal coal coal coal coal coal SasDust SasDust SasDust pellets pellets

300 105 7 550 600 550 1.5 1.5 1.5 1.5 1.5

3.8 4.2 1.3 23.1 35.7 9.7 27.3 4.3 0.6 0.8 0.1

this study this study this study Watson16 Watson16 Watson16 Wierzbicka17 Wierzbicka17 Wierzbicka17 Wierzbicka17 Wierzbicka17

fired boilers was approximately 2−9 times higher than that observed in this study. This result may have been caused by the preheated dilution gases decreasing the condensation of the OC on the particles, which might normally occur during direct emission to the air. Wierzbicka17 found that the concentrations of OC were strongly dependent on the operation load, regardless of the type of fuel, whereas the concentrations of EC were dependent on the load and fuel type. This result indicated that the ratio of OC and EC was strongly dependent on the boiler load and fuel type, with the ratio from coal higher than that from biofuel. Different OC/EC ratios in a small capacity boiler are most likely caused by (1) relatively poor working conditions, which provide a lower combustion temperature that may result in the incomplete combustion and smolder of coal. Also, the boilers produce a greater amount of OC and EC. The concentration of EC may be increasing more significant than OC. A similar result was reported by Wierzbicka.17 In addition, (2) the larger load boilers used to decrease emissions were equipped with a more effective dust filter and a fuel gas cleaning facility that could affect the PSD of OC and EC, thus changing the ratios in the emissions. 3.3. Size Distribution of OC and EC. The particulate OC and EC percentages for the six units and their contribution to the PM (0.01−10) mass concentrations (measured with ELPI) are shown in Figure 5. Previously studies30,31 suggested that pulverized coal fly ash particle formation is a trimodal PSD that includes a submicrometer fume region centered at a diameter of approximately 0.08 μm, a fine fragmentation region centered at

Figure 3. Correlation between OC and EC in the PCBs and CFBs.

As shown in Figure 3, a fine correlation was found between the OC and the EC in the PM10 for the two of types boilers (PCB and PCB) and the highest correlation coefficients were calculated for PCB (0.99), suggesting a common affected factor for the percentages of OC and EC. The largest OC to EC slope was estimated at 0.29 for PCB1, with the remaining slopes at 0.26, 0.20, and 0.18 for CFB2, CFB1, and PCB2, respectively. Note that the correlation of carbon species was not significant in all of the boilers. The correlation between OC and EC for the TGB is illustrated in Figure 4. The correlation factor in TGB2 was 0.16, which was lower than the value of 0.74 in TGB1. The poor correlation indicated that the affected factor for OC emissions was different from that of EC because the OC/EC characteristics from the CFB and PCB were different compared with that from the TGB. In Table 3, the OC/EC from fuel gases in the PM2.5 from other studies are listed and compared with that of similar studies. Chow16 reported that the OC/EC of PM2.5 emission sampled through dilution from coal42

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Figure 5. Size distribution of OC and EC.

a diameter of approximately a 0.7−5 μm, and a bulk or supermicrometer fragmentation region for particles at a diameter of approximately 5 μm and greater. OC was observed to increase with the particulate size variations and with the boiler. Two distinct particulate mass peaks were observed for the CFB and PCB plants. The size distribution of the TGB was different from the others. The mass size distribution of PM10 in PCB1 after ESP displayed a trimodal distribution consisting of two submicrometer modes with peaks near 1 and 0.1 μm and a fine peak near 2.4 μm. The PCB2 equipped with ESP showed a similar size distribution of OC and EC, with the size revealing two peaks near 0.37 and 6.89 μm. PCB1 and PCB2 used the same combustion method and a similar dust remover; however, these units have different gas cleaning facilities, which are based on chemical and physical principles and could affect the OC and EC size distribution. The percentage of OC and EC from a CFB stack showed two successive unimodal distributions, and both were close to 0.17− 0.99 μm. The mass percentage of OC was higher than that of EC in all of the CFB and PCB boiler unit emissions. The size distribution and mass percentage of OC and EC from the TGB were more complicated than in the other boiler types in this study. In TGB 1, the mass percentage of OC and EC was unimodal, OC accumulated close to 0.26 μm, and there was a bimodal carbon component near 0.26 and 3.97 μm. From TGB2, in the size range of 0.013−0.37 μm, the percentage of EC was higher compared with that of OC, and this trend was

inconsistent with that of the other boilers. Different OC and EC percentage distributions were observed in the two TGB sampling boilers, and the OC% was higher than the EC% in the size range of 1.61−10.15 μm, whereas the OC% was lower than the EC% in the size range of 0.003−0.64 μm. In TGB2, the sampling data were reversed, with the OC% higher than the EC % in the 0.64−3.93 μm size range and lower than the EC% in the 0.01−0.39 μm size range. These results indicated that the particle filtration device affected the carbonaceous fraction size distribution. In addition, there were gaps between the PCB and the CFB, with a lower average mass percentage of OC and EC in the CFB than in the PCB. Organic carbon contents of approximately 7.9% and 10.1% were emitted in the PM10 from PCB1 and PCB2, respectively. Approximately 2.1% and 2.7% of the OC were emitted in the PM10 from CFB1 and CFB2, respectively, and these values were much lower than those from the PCB. EC showed a similar variation according to the type of boilers, with the average EC mass at 2.7% and 0.5% in the PM10 emitted from the two CFB units. The size distribution characteristics of the boilers were affected by the coal quality, combustion conditions, and dust remover and gas cleaner presence. The boiler type and combustion temperature may be more conclusive factors for the carbonaceous aerosol. A wider set of PM size analyses is recommended to provide more specific data for evaluating the factors that produce effects attributable to coal-fired boilers. 43

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Energy & Fuels 3.4. Carbon Fraction Characteristics. The carbon fractions (OC1, OC2, OC3, OC4, EC1, EC2, EC3, and OPC) can be identified by the decomposition temperatures listed in section 2. Although the carbon fractions are operationally defined, their different evaporation/oxidation temperatures represent their unique characteristics and can help to identify the source or emission features in different boilers. OC1 and OC2 were regarded as the volatile organic compounds and semivolatile organic compounds, respectively, because of their evaporation at temperatures less than 250 °C.32 Thus, OC1 and OC2 were likely more stable at low temperatures, indicating that they do not easily pyrolyze when other factors are not considered during emission. For example, TGB1 and TGB2 are similar boiler types that present similar combustion methods, and they utilize similar quality coal and have similar load capacities. In addition, both are equipped with flue gas cleaners; however, the carbon fraction characteristics were considerably different because of gas temperature differences. In TGB2, the OC1 + OC2 fractions of the OC ranged from 65.1% to 69.6% at a gas temperature of 339.7 K. However, in TGB1, the OC1 + OC2 fractions of the OC ranged from 47.9% to 48.1% at a gas temperature of 433.2 K. In all of the samplings, the OC1 + OC2 and OC3 + OC4 fractions of the OC did not show obvious variations according to the particle size distribution. However, the ratio of soot−EC and char−EC showed variations as well as a change in particulate size. EC is not a single chemical compound; rather, it is mainly composed of two different carbon contents: combustion residues from pyrolysis and combustion emissions formed via gas-to-particle conversion.33 Char (EC1 − OPC) is defined as carbonaceous materials obtained by pyrolyzing organic substances or an impure form of graphitic carbon obtained as a residue when carbonaceous material is partially burned or heated under limited air conditions. Soot (EC2 + EC3) is defined as carbon particles that form at high temperature via gas-phase emissions.34,35 The percentage of char and soot and their ratios in the different combustion methods and PSDs have been studied and are presented in Figure 6.

the CFB, the percentage of char and soot was less than 1% and ranged from 0.24% to 0.45%. The TGB emissions of soot and char were the highest of all of the boilers. The temperature in the TGB chamber was approximately 1300 K, which was lower than that in the CFB and PCB and might have caused additional EC emissions because of incomplete combustion. Soot was more likely to be condensed in the low-temperature gases. The percentage of soot in the TGB was significantly higher than that in the CFB and PCB. When the temperature in the TGB chamber was approximately 1800 K, a large amount of soot was produced. The temperature peak for soot formation was near the range of 1800−2100 K.36 In addition, the ratio of char and soot varied with the PSD. In all of sampled boilers, the ratio was increasingly reduced as the particle sizes decreased, and this trend was particularly intense for the PM0.1; the ratio of char/soot in the PM1 was 0.4−1 times that of the PM0.1. The mechanisms that affect these carbonaceous fractions between the vapor and the particle phases are not well understood. Char mainly arises from the fragmentation of coal during combustion. The mechanisms affecting soot formation were likely different from those affecting char formation during the vapor condensation process. The condensed submicrometer carbonaceous aerosol region was centered at a diameter of approximately a 0.1 μm. The metal elements and particulate matter have similar production mechanisms,37,38 which may support our hypothesis for the condensed soot in the PM0.1. The char/soot concentrations were strongly dependent on the size distribution regardless of the temperature of the exhaust. However, whether the heated dilution sampling system could have decreased the condensation of organics on the particles, which might normally occur during direct release to the atmosphere, must still be demonstrated.

4. CONCLUSIONS In this work, the particulate matter emitted from six boilers of three different types was collected and characterized. Measurements in the stacks were obtained with an ELPI with a stage dilution sampling system. The mass concentration characteristics and size-classified carbonaceous compositions were determined using ELPI and TOR analyses. The results are as follows: (1) the particulate emissions from the sampled coalfired boilers varied between 9.06 and 369.97 mg/m3 and 7.04 × 104 and 2.85 × 108 dN/d log Dp in the PM10. The TGB was not equipped with a highly effective dust remover, which resulted in the emission of a high mass and number concentration of PM. There was a clear enrichment in the carbonaceous composition in the TGB emissions, which indicates the important role of combustion sources of anthropogenic origin. (2) The TGB with the lowest load had the highest emission levels of all of the boilers, which indicated that boiler combustion was more closely connected to the load combustion, and an increase in the thermal efficiency indirectly decreased the flue gas emissions. The most important factor for particle emissions from coal-fired boilers was the type of combustion boiler. (3) The level of particle numbers in PCB2 equipped with a highly effective ESP was 107, which is the same value observed in CFB2 and similar to that of CFB1. The particle number concentration presented much higher values compared with the other characteristics. This result verified that the WFGD can decrease the emission of SO2 and increase the particulate matter in the emissions. (4) The correlation factor (Pearson’s R) was 0.4 in the TGB, which was lower than the value (0.9) in

Figure 6. Size-classified variation of carbonaceous fraction in a different boiler.

The emission of char and soot is related to the exhaust humidity and temperature. In TGB1, the average percentage of char and soot was 5.73% and 2.63%, respectively, and their ratio in the emitted gas was 1.19 at a temperature of 433.2 K. The percentage of char and soot from the TGBs was 1.16% and 11.32%, respectively, in the emitted gas at 339.7 K. The ratio in TGB2 was 0.12, which was much lower than that in TGB1. In 44

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Energy & Fuels

mechanisms are similar to those of metal elements produced in PM0.1. In all of the sampled boilers, the char/soot ratio increased as the particulate size increased. This increasing trend was intense for the PM0.1 and had a char/soot ratio 1.2−3.9 times higher than that of the PM1. This phenomenon may support our hypothesis. (10) The char/soot concentrations were strongly dependent on the size distribution regardless of the temperature of the exhaust. However, whether the heated dilution sampling system can decrease the condensation of organics on the particles, which normally occurs during direct release to the atmosphere, must still be demonstrated. For future studies, the heating temperature of the dilution system at ambient air temperatures should be considered for OC measurements. In addition, some studies have reported that smolder and low-temperature combustion emit brown carbon, and the relationship between brown carbon and elemental carbon should receive additional attention in future studies. Different combustion conditions produce different types of particle emissions and particles with different physicochemical characteristics. Boilers with a low effective combustion produced more emissions relative to boilers with combustion under more optimal conditions, and differences in the health-related carbonaceous fractions should be determined in future studies. Moreover, additional attention should be focused on the design of combustion appliances and improvements to combustion technologies in small-scale coal-fired boilers.

the PCB and CFB. This correlation indicated that the emissions characteristics and factors in the TGB were different compared with the other boiler types. The OC and EC ratios ranged from 1.3 to 4.2 and were strongly dependent on the boiler load and fuel type, which was determined by comparing the emission of carbon with different fuels, boilers, and loads. (5) The lower OC/EC ratio in the small capacity boilers was most likely caused by the relatively poor working conditions, which provide a lower combustion temperature that may result in the incomplete combustion and smolder of coal. Also, the boilers produce a greater amount of OC and EC. The concentration of EC may be increasing more significantly than OC. The lower ratio may have been caused by the larger load boiler, which decreased emissions and was equipped with a more effective dust filter and a fuel gas cleaning facility; these factors may have affected the particle size distribution of OC and EC and changed the ratio in the emissions. (6) The OC and EC size distributions in PCB1 after ESP displayed a trimodal distribution consisting of two submicrometer modes and one fine mode. The carbonaceous species in PCB2 after ESP had two modes, with one close to the fine particle range and the other in the submicrometer range. CFB1 was equipped with a bag filter and had a similar size distribution as CFB2 (two successive unimodal distributions near the submicrometer mode), and they both presented peak distributions near the coarse size. The size distribution of OC and EC between TGB1 with TGB2 differed considerably. PCB1 and PCB2 had the same combustion method and boiler and similar dust removers, although they had different gas cleaning facilities. Although the formation of OC and EC primarily occurred during the high temperature of combustion, chemical and physical cleaning methods could affect the OC and EC size distributions. The size distribution characteristics from the boilers were affected by the coal quality, combustion conditions, and dust remover and gas cleaner presence, although the boiler type and combustion temperature may be the more conclusive factors for carbonaceous aerosol. (7) The percentage and variation according to boiler type for the OC1−4 and EC1−3 fractions in the PM10 PM2.5, PM1, and PM0.1 were studied. In TGB2, the OC1 + OC2 fraction of OC ranged from 65.1% to 69.6% in the fuel gases emitted at a temperature of 339.7 K. However, in TGB1, which had the same conditions except for temperature, the OC1 + OC2 fraction of OC ranged from 47.9% to 48.1% in the fuel gases emitted at 433.2 K. Thus, the OC1 and OC2 fractions at low temperatures were likely more stable and do not easily undergo pyrolysis during emission when other factors are not considered. In all of the samplings, the OC1 + OC2 and OC3 + OC4 fractions of the OC did not show obvious variations according to the particle size distributions. (8) The ratio of soot and char showed variations as well as a change in particulate size. In the TGB, the percentage of soot and char was 3.4% and 6.9%, respectively, and it presented the highest level for all of the boilers. The temperature in the TGB chamber was approximately 1300 K, which was lower than that in the CFB and PCB and may have caused additional EC emissions because of incomplete combustion. In addition, when the temperature in the TGB chamber was approximately 1800 K, a large amount of soot was produced. The temperature peak for soot formation ranged from approximately 1800 to 2100 K. (9) We suggest that the mechanisms of soot formation were different from those of char formation during the vapor condensation process. A greater amount of soot would have condensed at a diameter of approximately 0.1 μm, and these

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AUTHOR INFORMATION

Corresponding Author

*Phone/Fax: (086) 235-03397. E-mail: [email protected]. cn. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Financial support from the National Science Foundation in China (21207069, 21407081, and 41205089) is gratefully acknowledged.

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REFERENCES

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DOI: 10.1021/acs.energyfuels.5b01770 Energy Fuels 2016, 30, 39−46