Article Cite This: Energy Fuels 2019, 33, 5430−5437
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Distribution of Organic Compounds in Coal-Fired Power Plant Emissions Jun Liu, Tao Wang,* Jie Cheng, Yongsheng Zhang,* and Wei-Ping Pan
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Key Laboratory of Condition Monitoring and Control for Power Plant Equipment, Ministry of Education, North China Electric Power University, Beijing 102206, China ABSTRACT: The distributions of organic compounds such as methane, non-methane hydrocarbons (NMHCs), and polyaromatic hydrocarbons (PAHs) in the effluent from an ultralow-emission power plant were investigated. The methane and NMHCs in the flue gas were analyzed using a modified portable volatile organic hydrocarbon analyzer according to U.S. EPA method 25A, and a lower boiler load was found to increase the NMHC concentration. The levels of methane and NMHCs in gases evolved from the pyrolysis of solid samples were assessed, and greater amounts of both organic compounds were obtained from bituminous coal. The empirical parameter E was used to represent the amount of organic compounds in the flue gas during combustion as a percentage of the organics obtained from coal pyrolysis. The E values based on methane and NMHCs in the gas phase were below 0.2% at the boiler outlet and less than 0.1% at the stack inlet. The PAHs in solid samples were also analyzed by solvent extraction. The distributions of PAHs in solid samples for slag, selective catalytic reduction (SCR), and electrostatic precipitation were 3, 76, and 21%, respectively. In the slag location, the percentages of five- and six-ring PAHs were higher than those of PAHs with smaller rings. The percentage of three-ring PAHs was the highest in the SCR inlet fly ash in both cases. Mass balance including gas and solid sample calculations indicated that the methane and NMHC levels in the flue gas were less than 0.01 and 0.07% of the amounts obtained from coal pyrolysis, respectively. Results indicated that approximately 99.99% of the methane, 99.77% of the NMHCs, and 99.78% of the PAHs were oxidized during coal combustion in the boiler. The methane, NMHC, and PAH concentrations in fly ash were 0.01, 0.02, and 0.06%, respectively, whereas the slag contained less than 0.01% of each. Overall, less than 0.01% of the methane and 0.07% of the NMHCs were released in the stack. These results confirm the high combustion efficiency obtained from ultra-supercritical pulverized coal-fired boilers incorporating ultralow-emission air pollutant control devices that eliminate organic compounds through oxidation, condensation, and water absorption. control devices (APCDs) in conjunction with modified fly ash.6 Adding modified fly ash was found to further reduce the effluent concentration of VOCs by 10−20%, resulting in an overall VOC reduction of 40−80%.6 Assuming that all organics originate from the coal, the amount of these compounds in the pyrolysis gases under a fixed set of conditions could be set as the 100% mark so as to calculate the pollutant removal efficiency of APCDs.4 In another study, the emission of volatile organic compounds (VOCs, accounting for the largest proportion of organic compounds generated during coal combustion) from a power plant boiler, a drop tube furnace, and a thermogravimetric (TG) reactor at different heating rates were assessed to determine the effects of the heating rate on the emissions resulting from incomplete combustion.7 In this prior work, the number of organic compounds was found to decrease as the heating rate was increased, such that only several tens of organic compounds were identified in the effluent from the full-scale boiler of a power plant. A lower concentration of chlorinated hydrocarbons was observed compared with values reported by Garcia8 and Fernándeź 4 likely because coal sourced from China contains Martinez, low levels of chlorine (less than 220 ppm on average).9,10 A
1. INTRODUCTION Coal is the main energy source in the world, and pollutant emission from coal combustion has received increasing attention.1−3 Coal comprises a wide range of organic compounds, and so power plant emissions also contain numerous organic species as a result of incomplete combustion. Even low concentrations of such compounds in flue gas can be an important anthropogenic source of pollution because of the large volume of flue gas that is produced by such plants. Coal combustion involves both pyrolysis of the coal itself and burning of the evolved gases. Although pyrolysis occurs first, the two processes can take place almost simultaneously, depending on the combustion conditions such as heating rate, sample mass, and coal rank. During coal combustion in a power station, organic compounds can undergo desorption from the coal along with decomposition (that is, pyrolysis), combustion (normally incomplete), adsorption on various system components, or elimination via different effluents (gases, fly ash, or slag).4 Thus, power plant emissions are determined by both coal pyrolysis and combustion of the resulting gases, and so it is necessary to study the complete coal pyrolysis process to understand how organic compounds can be emitted in conjunction with different heating rates and ranks.5 In an earlier study, 124 organic compounds were identified in gases evolved from coal pyrolysis.5 The previous work therefore examined the removal of volatile organic compounds (VOCs) using air-pollution © 2019 American Chemical Society
Received: March 22, 2019 Revised: May 23, 2019 Published: May 27, 2019 5430
DOI: 10.1021/acs.energyfuels.9b00889 Energy Fuels 2019, 33, 5430−5437
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Energy & Fuels Table 1. Proximate and Ultimate Analysis Data for the Coal Samples Employed in this Work (Dry Basis)a proximate analysis (%)
ultimate analysis (%)
sample
V
A
FC
C
H
O
N
S
coal A coal B
32.35 29.86
11.69 16.75
55.96 53.38
63.40 66.43
4.62 4.36
19.37 10.94
0.60 0.83
0.32 0.69
Legend: V = volatile matter; A = ash; FC = fixed carbon; C = carbon; H = hydrogen; O = oxygen; N = nitrogen, and S = sulfur.
a
́ 4 studied the VOC emissions from subcritical boilers Martinez incorporating APCD systems. Methane, NMHCs, and PAHs have been used as indicators to analyze the organic compound distributions in power plant emissions. Fernandez monitored VOCs released by coal using a thermal-desorption apparatus,4,15 although only up to a temperature of 300 °C. In a prior work by our own group, VOCs were found to be released from coal during pyrolysis over the range of approximately 300−600 °C,5 and the concentrations of methane and NMHCs in the evolved gases were evidently affected by the heating rate, particle size, and other factors.7 Based on this previous research, the present study first examined the temperature range in which total amounts of methane and NMHCs evolved in gases during coal, ash, and slag pyrolysis in a fixed-bed reactor before the organic compound analysis in the solid sample.
number of the VOCs in the power plant output could not be identified because they were present at very low concentrations or did not match any standard spectra,7 although it is likely not possible or even necessary to identify every VOC during such analyses. Garcia studied the emission of VOCs by coal-fired power stations and determined that the most abundant compounds were aldehydes (including formaldehyde and acetaldehyde), aliphatic and aromatic hydrocarbons (including benzene, toluene, xylene, ethylbenzene, and benzene), and chlorinated hydrocarbons (including chlorobenzene and tetrachloroethene) in addition to aliphatics such as n-hexane ́ and n-heptane.8 Fernández-Martinez studied the VOC concentrations in gaseous emissions trapped on sorbent tubes and in solid samples (coal, fly ash, and slag) and identified 16 major components, including 1,2-dichloroethane, benzene, n-heptane, toluene, and n-octane.4 Two sampling techniques are typically used to analyze organic compounds in flue gas, gas sampling bags and sorbent tubes, although these are only suitable for monitoring a limited number of specific organic compounds, including benzene and toluene. It is estimated that such compounds account for 80− 95% of the total organic compounds in coal effluent gases. When attempting to analyze all organic compounds, EPA method 25A is preferable because the flue gas is directly sampled and sent to a gas chromatograph (GC) equipped with a flame ionization detector (FID). Using a bag-sampling technique, Yan reported that the most abundant compounds in the flue gas of a coal-fired power plant were 1-butene and ethylene, accounting for over 50% of the organics by mass.11 Because compounds greater than C5 tend to condense on the surface of the bag sampler, the concentrations of C1−C5 species such as these could be overstated. However, both methane and non-methane hydrocarbons (NMHCs) are also important emission indicators according to Chinese standards.12 Thus, a modified EPA method 25A analyzer was used to analyze the flue gas directly and to analyze the solid-sample pyrolysis with carrier gas dilution to avoid organic compound condensation. Both methane and NMHCs as the results of the analyzer are included in the total hydrocarbon (THC) concentration to indicate the total amount of organic compounds.13 Polyaromatic hydrocarbons (PAHs) are another important category of organic pollutants. These typically appear at lower concentrations compared with other organics in coal effluents but are considered to be highly toxic.14 Because of their high boiling points, the majority of PAHs tend to be contained in solid effluents, such as fly ash or slag. In the work reported herein, the concentrations of PAHs in solid samples were also investigated. Over the past 10 years, more than 100 modern ultrasupercritical pulverized coal-fired (USCPC) units have been commissioned in China, equipped with new ultralow-emission (ULE) APCDs. Thus, there is a need to better understand the synergy between USCPC facilities and ULE-APCDs with regard to lowering VOC emissions. As an example, Fernández-
2. EXPERIMENTAL SECTION 2.1. Coal Samples. Two raw bituminous coals from two boilers were used in this study, denoted as coal A and coal B, and the ultimate and proximate analysis data for these two materials are provided in Table 1. Note that each of these test results represents the average of three trials. Coal, fly ash, and slag sampling was performed according to the procedures in the ASTM D-2234 (1989) standard, with the conveyor belt running continuously during coal sampling. For each solid sample, the total sample mass was quartered and the particle size was reduced until 500 g samples were obtained. 2.2. Analytical Methods. 2.2.1. Flue Gas. VOCs were analyzed using a portable GC in conjunction with an FID (model 8807, PCF Electronica, Italy). This analytical technique was capable of continuously monitoring the THC, methane, and NMHC fractions in the effluent (calculated as the difference between the THC and methane amounts). These compounds were quantified based on the calibration of the instrument using standardized gas mixtures containing methane and propane (calibration gas, Beiwen Gas Company, China). When analyzing high-temperature (above 300 °C) flue gas, a glass-lined probe was used in the analyzer based on a modification of U.S. EPA method 5. 2.2.2. Solid Samples. The pyrolysis of solid coal samples was performed so as to assess the resulting emissions.16 In each trial, a coal specimen (approximately 20 mg, 200 mesh particle size) was placed in a ceramic crucible in a TG analyzer (STA8000, PerkinElmer) under a 30 mL·min−1 flow of high-purity nitrogen (99.999%) to ensure an inert atmosphere. The sample was subsequently heated from 50 to 1200 °C at 20 °C·min−1 and the evolved gases were swept to the sample cell of a Fourier transform infrared (FTIR) spectrometer (Frontier, PerkinElmer). The sample cell temperature was maintained at 300 °C, and spectra were acquired using the Time Base software package at a resolution of 4 cm−1 over the range of 450−4000 cm−1. The signal was the sum of absorbances at 3016 cm−1 (due to methane), 2970 cm−1 (aliphatic hydrocarbons including alkanes and olefins), and 2934 cm−1 (aromatic hydrocarbons, including benzene, toluene, and xylene). A schematic of the apparatus used to perform pyrolysis tests of the solid samples is presented in Figure 1. The specimen was placed on a sample boat in a fixed-bed reactor and heated at 20 °C·min−1 from 50 to 900 °C, and then held at that temperature for 10 min. Nitrogen carrier gas was employed to send the evolved gases to a 5 L Tedlar 5431
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according to U.S. EPA method 25A.18,19 The coal was sampled from the coal feeder prior to the boiler, and the fly ash in the SCR ash was sampled using the sampling train specified by U.S. EPA method 17. The fly ash in the ESP unit included 80% coarse (particle size greater than 10 μm) and 20% fine materials. The coarse fly ash was collected from the first stage of the unit, whereas the fine fly ash was collected from the second, third, fourth, and fifth stages. The slag was sampled from the slag bank. All specimens were obtained simultaneously under the same boiler load at the sampling points shown in Figure 2. The temperature profiles for each location are provided in Table 2.
Figure 1. Apparatus used for the analysis of methane and NMHCs released from the pyrolysis of solid samples. bag (Tedlar, HedeTech). The captured gases were analyzed using a GC to determine the amounts of methane and NMHC released. Each specimen was analyzed three times. 2.2.3. PAHs. During each PAH analysis,14,17 5−10 g of solid samples (coal, ash, or slag obtained from a power plant or fly ash taken from flue gas) were extracted with dichloromethane using the Soxhlet technique over a span of 24 h. The extracts were analyzed using thermal desorption in conjunction with gas chromatography and mass spectrometry (TD-GC/MS, PerkinElmer, Clarus SQ 8T) according to the procedure provided in the China HJ 950-2018 standard. The GC system employed a fused silica capillary column with a dimethyl polysiloxane film (Extile, 60 m × 0.25 mm × 0.25 μm film thickness). The column temperature was held at 70 °C for 2 min, then increased (heating rate 7.5 °C·min−1) to 320 °C with a holding time of 10 min. Helium was used as the carrier gas at a flow rate of 1.0 mL·min−1, the heat transfer line between the GC and MS was set at 230 °C, and mass spectra were recorded over the m/z range of 35− 500. The analytes were identified based on the NIST library and the selected ion current was used to calculate the concentration of each PAH based on calibration with standard solutions of the same compounds (O2Si smart solutions). A total of 16 PAHs having various numbers of benzene rings were quantified: two-ring (naphthalene), three-ring (acenaphthylene, acenaphthene, fluorene, phenanthrene, anthracene), four-ring (fluoranthene, pyrene, benz[a]anthracene, chrysene), five-ring (benzo[b]fluoranthene, benzo[k]fluoranthene, benzo[a]pyrene, indeno[1,2,3-cd]pyrene), and six-ring (dibenz[a,h]anthracene, benzo[g,h,i]perylene). Each analysis was repeated three times, and the standard deviations of the retention times were within ±0.1 min, whereas the relative standard deviations of the peak areas were less than 5%. 2.3. Power Plants. The experiments were conducted in 1000 MW USCPC boilers in a power plant. Each unit was equipped with a selective catalytic reduction (SCR) device for the NOx control, an air preheater (APH), first low-temperature economizer (LTE), an electrostatic precipitator (ESP), second low-temperature economizer (LTE), a flue gas desulfurization (FGD) system for the SO2 control, and a wet electrostatic precipitator (WESP) for the reduction of particular matter (PM). The effluent limits of this power plant were as follows: PM < 1 mg·m−3, SO2 < 10 mg·m−3, and NOx < 27 mg·m−3. The flue gas was sampled for hydrocarbon analysis at the inlet and outlet of the SCR unit (designated as ISCR and OSCR herein), the inlet and outlet of the ESP (IESP and OESP), and the inlet and outlet of the WESP (IWESP and OWESP). The analysis was performed
3. RESULTS AND DISCUSSION The distributions of hydrocarbons, including methane and NMHCs, in both the flue gas and the solid phase (coal, slag, and ash) are discussed first. The proportion of organic compounds released in the gas phase (which we term as the escape rate, E) was calculated based on the ratio between the compounds released during combustion and those produced by coal pyrolysis. The PAH concentrations in the solid phases were determined thereafter. The distribution of organic compounds in the power plant is proposed at the end of discussion. 3.1. Methane and NMHC Concentrations in Flue Gas. Figure 3 demonstrates that the NMHC concentration fell sharply (by approximately 30−70%) as the flue gas passed through the SCR unit, whereas the NMHC level was increased following the ESP system but decreased after the FGD and WESP units. These data also show that the methane was only approximately one-third of the NMHC concentration in the initial flue gas of the power plant and that the methane concentration was continually decreased as the effluent passed through the APCD system. The SCR unit reduced the methane and NMHC concentrations by an average of 40 and 60%, respectively. This is due to the oxidation effect in this device. After the ESP, the NMHC concentration was increased by about 25%. Sui20 indicated that the PM between 0.0134 and 0.0212 μm increased in the ESP. They pointed out that it could be from larger particles breaking down into smaller particles in the ESP system. Some VOCs adsorbed by fly ash would be expected to be released as the finer particles depart from the surfaces of larger particles. This explains the increase in the NMHC concentration at the ESP outlet. Following the FGD and WESP units, the methane and NMHC levels were decreased by approximately 4 and 17%, respectively. This is mainly due to the partial dissolution or absorption of VOCs in water. Overall, the methane and NMHC concentrations were lowered by 61 and 58%, respectively. The methane and NMHC concentrations in the two boilers at two different boiler loads are presented in Figure 4. When
Figure 2. Sampling points used to obtain flue gas and solid specimens from a power plant. 5432
DOI: 10.1021/acs.energyfuels.9b00889 Energy Fuels 2019, 33, 5430−5437
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Energy & Fuels Table 2. Temperature Distributions in the Power Plant the temperature of flue gas, °C Boiler
load, %
coal, t/h
ISCR
OSCR
IESP
OESP
IWSEP
OWESP
boiler 1
100 50 100
360 174 364
374 339 365
366 302 352
123 106 113
116 98 109
49 46 51
46 38 47
boiler 2
Figure 5. Effects of NOx concentration in the SCR unit on methane and NMHC concentrations.
Figure 3. Methane and NMHC distribution at a 100% load in two boilers.
NOx concentration would compete with the organics to oxidize the catalyst, possibly increasing the organic compound concentration. Consequently, the NMHC concentration in the flue gas after the SCR unit was increased by approximately 23% upon increasing the NOx concentration from 15 to 40 mg· m−3. However, there was no significant change in the methane level. 3.2. Methane and NMHC Concentrations in Solid Samples. Figure 6 summarizes the results obtained from the
Figure 4. Methane and NMHC distributions at different loads in the same boiler.
the boiler load was cut to 50%, the NMHC level was decreased. A lower load results in a longer flue gas residence time in the boiler, which tends to lower the emissions of organic compounds. At a 50% load, the flue gas would flow more slowly than at a 100% load, allowing for more combustion of the NMHCs. Although the methane concentration appears to have slightly increased, this may possibly have occurred because of a reduction in the flue gas volume or decomposition of the NMHCs. The final methane and NMHC concentrations after processing by the various APCDs were approximately 0.1 and 1.5 mg·m−3, respectively. These values were not significantly affected by the change in load. Figure 4 also demonstrates that oxidation in the SCR unit was the primary factor responsible for the reduction of organic compounds. The concentration of NOx in the flue gas was adjusted by varying the amount of NH3 injected into the SCR system, and the effect of this adjustment on the organic compound concentrations after the SCR unit was not significant, as shown in Figure 5. This lack of an effect is attributed to the fact that organic species are primarily reduced in conjunction with the oxidation of the catalyst in an SCR system. The effect was magnified in the flue gas after the condensation of APH and LTE. A higher (40 vs 15 mg·m−3)
Figure 6. FTIR data showing the release of organic compounds from various solid samples during pyrolysis.
pyrolysis of solid samples in the fixed-bed reactor, during which organics were generated between 400 and 900 °C. The amounts of organic compounds released from the ash and slag were very low, and it is difficult to see any obvious changed in the FTIR data with increasing temperature. The FTIR have real-time signals, but the detection limit (DL) is higher than that of the portable VOC analyzer (it has a low DL, but the analysis period is too long and is unsuitable for real-time analysis). Because organic compounds (adsorbed on the adsorbent) typically desorb below 400 °C and the unburned coal in ash and slag is equivalent to that in the original coal, it was assumed that all organic compounds would be completely 5433
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average concentration of organic compounds in flue gas, expressed in mg·m−3, and Ccoal is the organic compound concentration in the coal, expressed in mg·kg−1, as provided in Table 3. The numerical value 1 in eq 2 is used to standardize the calculations for 1 kg of coal. Finally, Vcoal represents the volume of flue gas emitted from 1 kg of coal. The average E values calculated for the two ultrasupercritical boilers are provided in Table 4. The data in the
released by 900 °C and so this was the maximum test temperature. The organic compound concentrations determined in the solid samples from both boilers at different loads are presented in Table 3. These values appear to be correlated with the Table 3. Methane and NMHC Concentrations in Solid Samples as Determined by Pyrolysis THC concentration, mg·kg−1 boiler
load, %
boiler 1
100
50
boiler 2
100
sample coal coarse ash fine ash slag coarse ash fine ash slag coal coarse ash fine ash slag
Table 4. Escape Rate (E) Values Obtained from a Power Planta
CH4
NMHC
7.11 ± 0.04 × 104 4.27 ± 0.36
2.35 ± 0.06 × 104 17.7 ± 1.2
3.85 ± 0.23 3.74 ± 0.97 3.36 ± 0.35
25.6 ± 1.5 13.9 ± 1.0 18.6 ± 1.2
boiler 1 100% load
5.17 2.76 7.01 3.68
± ± ± ±
0.28 0.17 0.12 × 104 0.79
2.32 ± 1.00 2.83 ± 0.32
22.9 20.2 1.79 34.8
± ± ± ±
a
0.9 1.4 0.05 × 104 1.8
29.6 ± 2.2 24.5 ± 0.8
(1)
where C theory = (Ccoal × 1)/Vcoal
50% load
100% load
E, %
ISCR
OWESP
ISCR
OWESP
ISCR
OWESP
CH4 NMHC
L 0.20
L 0.06
0.01 0.09
L 0.05
0.01 0.16
L 0.09
Note: L = lower than 0.005%.
ISCR column (which represent organic compounds escaping from the boiler outlet and correspond to the boiler combustion efficiency) show values below 1%, demonstrating almost complete combustion with minimal release of organics. It was shown that ultra-supercritical boilers exhibit a higher combustion efficiency compared with subcritical boilers in ́ 4 When the researches by Garcia8 and Fernández-Martinez. boiler load is increased, E also increases, meaning that a higher load results in an incomplete combustion efficiency. The data in the OWESP column represent the final stack emissions after the flue gas has been cleaned by all APCDs and correspond to the overall removal efficiency. The final E value was below 0.1%. Increasing the boiler load almost doubled the E value based on the NMHCs in the ISCR column, whereas the final value was increased no more than 0.01%. These data again demonstrate that the current APCDs worked well to remove organic compounds. 3.4. PAH Concentrations in the Power Plant. PAHs can be generated in power plants through the decomposition of coal, either by condensation or by cyclization reactions of organic compounds. Thus, PAHs can be found in the boiler at different locations such as the slag, SCR inlet fly ash, and ESP fly ash. Figure 7 summarizes the PAH concentrations identified in coal, SCR inlet fly ash, coarse fly ash, fine fly ash, and slag in this study. The PAH concentrations for coal A and coal B are approximately 62.4 and 63.1 mg·kg−1 respectively. Both coals contain more than 80% of four- and five-ring PAHs. Coal B also contains more five or six benzene-ring compounds than coal A. The percentage of three-ring PAHs is the highest in the SCR inlet fly ash in both cases (Figure 7B). In the slag location, five- and six-ring PAHs are present at a higher percentage than PAHs with smaller rings (Figure 7C). If the amount of slag and fly ash generated at three different locations are taken into consideration, the distributions of PAHs for slag, SCR, and ESP are 3, 76, and 21%, respectively. There was less than 3% of PAHs generated in the slag location. This was due to localized incomplete combustion in the boiler at such a high temperature of 1500 °C. This condition also favored the formation of five- and six-ring PAHs in the slag. When the PAHs traveled from the boiler to the inlet of the SCR unit, 70% of PAHs were collected at this location in both power plants. The majority of the PAHs are condensed and captured by the fly ash due to the cooling of the flue gas from over
volatile content of the coal as provided in Table 1. As an example, the volatile matter content of coal A from boiler 1 was higher than that of coal B from boiler 2, and the NMHCs emitted from coal B were lower than that from coal A. In addition, the methane concentrations in the coarse and fine ash from boiler 2 were lower than those from boiler 1. It is difficult to discern trends in the NMHC concentrations because the NMHC value is the sum of numerous different organic compounds and thus is affected by more factors. The boiler outlet temperature was lower in the case of boiler 2 as shown in Table 1, which would be expected to produce incomplete combustion and so increase the concentration of NMHCs in the ash and slag. The same trend was seen in the case of boiler 1 with a load of 50%, at which the NMHC concentration was higher than that at a 100% load. 3.3. Escape Rate of Organic Compounds in the Flue Gas. As noted earlier, the combustion process (including pyrolysis, oxidation of volatiles, and char combustion) is complex and it is not easy to quantify each aspect. Therefore, in this work, the empirical parameter E was used to calculate the proportion of organic compounds in the flue gas as a result of incomplete combustion.8 This value was calculated as follows E = Cgas/C theory
boiler 2
(2)
E expresses the proportion of organic compounds in the flue gas emitted from coal combustion relative to the total theoretical amount as determined by coal pyrolysis. This is a semi-quantitative parameter, and lower values of E indicate that fewer organic compounds are released by the coal combustion. Ctheory is the theoretical organic compound concentration, expressed in mg·m−3, assuming that all organics obtained by coal pyrolysis are contained in the flue gas. Cgas is the actual 5434
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temperature also dropped from 300 to 120 °C when the PAHs reached the ESP system. Around 21% of the total PAHs were collected in the ESP location. In the ESP location, the PAH concentration in the coarse fly ash was found to be higher than that in the fine ash. The coarse fly ash was collected at the first electric field and the fine fly ash was collected from the rest of the electric fields. Around 80% of the fly ash is captured in the first electric field, which has a higher unburned carbon content in the coarse fly ash to capture more PAHs. There is no specific trend for the distribution of PAHs with larger or smaller rings in this location. A general observation of the formation and distribution of PAHs indicated that condensation and adsorption are the two major mechanisms that removed PAHs from the effluent of the coal-fired power plant. 3.5. Organic Compound Distribution Throughout the Power Plant. The parameter E can be used to evaluate the relationship between methane and NMHC amounts obtained by pyrolysis and found in the flue gas from the power plant. However, these values do not provide information regarding the organic compounds in solid effluents such as fly ash and slag. For this reason, another numerical comparison was established, based on the following equation Ccoal × Mcoal = Cgas × Mgas + Cash × Mash + Cslag × Mslag + Com
(3)
where Com is the portion of organic compounds that undergoes combustion and Ccoal, Cgas, Cash, and Cslag represent the concentrations (in μg·kg−1 or mg·m−3) of organic compounds in coal, flue gas, fly ash, and slag, respectively. Considering the high boiling points of PAHs and the results of prior research,17,21,22 it was assumed that PAHs were not contained in the flue gas below 100 °C, such that the Cgas value for PAHs was zero. Mcoal, Mgas, Mash, and Mslag represent the mass or volume (in kg or m3) of coal, flue gas, fly ash, and slag, respectively, consumed or generated at a fixed full boiler load over the span of 1 h. The equation Cash × Mash = Ccoarse ‐ ash × Mcoarse ‐ ash + Cfine ‐ ash × M fine ‐ ash (4)
was also employed, where Ccoarse‑ash and Cfine‑ash represent the concentrations (in μg·kg−1) of organic compounds in the coarse and fine ash, respectively, and Mcoarse‑ash and Mfine‑ash represent the masses of coarse and fine ash generated at a 100% boiler load over 1 h (in kg). Finally, the equations
Figure 7. Concentrations of PAHs having different ring numbers in solid samples.
Mslag = Mcoal × %mineral residue × 0.1
(5)
Mash = Mcoal × %mineral residue × 0.9
(6)
Mcoarse ‐ ash = M f × 0.8
(7)
and M fine ‐ ash = M f × 0.2
1500−300 °C on passing through the duct and reaching the SCR inlet. At the same time, the five- or six-ring PAHs also generated in the boiler were oxidized and decomposed into smaller benzene rings. This is the reason that the percentage of three-ring PAHs was the highest in this location. When the PAHs went through the SCR system, the catalyst in the SCR system also promoted the oxidation of PAHs with larger rings to be decomposed to PAHs with smaller rings or other light gases such as carbon dioxide. At the same time, the
(8)
were employed. Based on the experience from operating the boiler, these equations assume that 10% of the mineral residue generated during coal combustion is converted to slag and 90% to fly ash, of which 80% is coarse and 20% fine. It was also assumed that methane, NMHCs, and PAHs were only produced from the coal, and so the proportion of each type of organic compound could be calculated. Figure 8 shows the calculated average results for the two boilers, which demonstrate that approximately 99.99% of the methane, 5435
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Figure 8. Organic compound distributions in the power plant.aNote: P = pyrolysis of solid sample; E = extraction from solid sample; L = less than 0.01%; G = gaseous sample.
water absorption processes included in APCD systems were found to reduce the levels of organics in the flue gas. The ultralow-emission technology was found to overall reduce the levels of organic species further as compared with the standard APCD system.
99.77% of the NMHCs, and 99.78% of the PAHs were burned during coal combustion in the boiler. After the flue gas was processed through the APCDs, about 0.16% of the NMHCs were removed through oxidation, condensation (that is, adsorption on the fly ash), and water absorption. The methane, NMHC, and PAH concentrations in the fly ash were less than 0.01, 0.02, and 0.06%, respectively. Overall, less than 0.01% of the methane and 0.07% of the NMHCs were released based on the APCDs shown in Figure 3. The capture efficiencies for methane and NMHCs in the SCR unit were 25 and 66.9%, respectively, whereas the capture efficiencies for the same materials in the ESP device were 9.1 and 21.6%, respectively. The ESP device also removed the greatest quantity of PAHs, primarily due to the condensation effect. This occurred because the boiling points of most PAHs are higher than 200 °C, whereas the ESP unit is held at 120 °C, such that the PAHs in the gas phase condensed on the fly ash. Figure 3 also shows that hydrocarbons were not removed to any significant extent by the FGD and WESP units. The solubility of organic compounds in water can be quite different, and this is the main factor determining the capture efficiency in the FGD and WESP units. Since methane is poorly soluble in water, its concentration was not changed significantly after the flue gas passed through the FGD and WESP locations. ́ Fernández-Martinez examined 16 organic compounds,4 representing only some of the NMHCs evaluated in this study. In the present work, a lower concentration of organic compounds (only 0.22% NMHCs) was released due to the use of a newer boiler technology compared with the levels reported ́ by Fernández-Martinez (about 3.0% VOCs).4 In addition, better control of pollutant emissions was achieved based on the ultralow-emission APCDs (approximately 0.07% of the total emissions) compared with the standard APCDs employed in ́ the study by Fernández-Martinez (about 1.12%).4 Thus, a USCPC boiler with ultralow-emission APCDs produced fewer emissions than a subcritical boiler with standard APCDs. This is due to the stronger oxidation (three layers of catalyst in the SCR system) and water absorption (the additional WESP system) capacities in an ultralow-emission power plant.
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AUTHOR INFORMATION
Corresponding Authors
*E-mail:
[email protected] (T.W.). *E-mail:
[email protected] (Y.Z.). ORCID
Yongsheng Zhang: 0000-0002-1104-5605 Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was supported by the National Key Research and Development Program of China (grant no. 2018YFB0605200), China Shenhua Research Project (SHGF-17-87), and National Natural Science Foundation of China (grant no. 51706069).
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