Energy & Fuels 2009, 23, 2031–2034
2031
Distribution of Polycyclic Aromatic Hydrocarbons in Fly Ash during Coal and Residual Char Combustion in a Pressurized Fluidized Bed Hongcang Zhou,*,† Baosheng Jin,‡ Rui Xiao,‡ Zhaoping Zhong,‡ and Yaji Huang‡ School of EnVironmental Science and Engineering, Nanjing UniVersity of Information Science and Technology, Nanjing 210044, China, and School of Energy and EnVironment, Southeast UniVersity, Nanjing 210096, China ReceiVed September 25, 2008. ReVised Manuscript ReceiVed January 27, 2009
To investigate the distribution of polycyclic aromatic hydrocarbons (PAHs) in fly ash, the combustion of coal and residual char was performed in a pressurized spouted fluidized bed. After Soxhlet extraction and Kuderna-Danish (K-D) concentration, the contents of 16 PAHs recommended by the United States Environmental Protection Agency (U.S. EPA) in coal, residual char, and fly ash were analyzed by a highperformance liquid chromatography (HPLC) coupled with fluorescence and diode array detection. The experimental results show that the combustion efficiency is lower and the carbon content in fly ash is higher during coal pressurized combustion, compared to the residual char pressurized combustion at the pressure of 0.3 MPa. Under the same pressure, the PAH amounts in fly ash produced from residual char combustion are lower than that in fly ash produced from coal combustion. The total PAHs in fly ash produced from coal and residual char combustion are dominated by three- and four-ring PAHs. The amounts of PAHs in fly ash produced from residual char combustion increase and then decrease with the increase of pressure in a fluidized bed.
1. Introduction Coal direct combustion not only has a low using efficiency but also produces a large number of pollutants, such as particulate matter, CO2, SO2, NOx, and polycyclic aromatic hydrocarbons (PAHs). Coal gasification is a clean-coal technology that presents good prospects for coal use, mainly for producing electricity with a high coal conversion efficiency and low environmental impact.1 The second-generation pressurized fluidized-bed combustion combined cycle based on coal partial gasification and residual char combustion is one of the promising clean-coal technologies in the future.2-4 To promote the power efficiency and realize the high efficiency and clean use of coal, the residual char produced in coal partial gasification combusts in the pressurized fluidized-bed combustor and fuel gas emission from the gasifier combusts to increase the entrance temperature of the gas turbine.2,3,5 There were a lot of PAHs in residual char and fuel gas during coal partial gasification in our past research.6 Considerable attention has been paid to how to oxidize and decompose PAHs in residual char and fuel gas completely in the further combustion. Formation and emission of PAHs during coal and other fuel combustion is one of the very active * To whom correspondence should be addressed. E-mail: zhouhongcang@ 163.com. † Nanjing University of Information Science and Technology. ‡ Southeast University. (1) Ocampo, A.; Arenas, E.; Chejne, F.; Londono, C.; Aguirre, J.; Perez, J. D. Fuel 2003, 82, 161–164. (2) Zhou, H. C.; Jin, B. S.; Zhong, Z. P.; Huang, Y. J.; Xiao, R. Energy Fuels 2005, 19, 1619–1623. (3) Zhou, H. C.; Jin, B. S.; Zhong, Z. P.; Huang, Y. J.; Xiao, R. Korean J. Chem. Eng. 2007, 24, 489–494. (4) Xiao, R.; Zhang, M. Y.; Jin, B. S.; Huang, Y. J.; Zhou, H. C. Energy Fuels 2006, 20, 715–720. (5) Xiao, R.; Shen, L. H.; Zhang, M. Y.; Jin, B. S.; Xiong, Y. Q.; Duan, Y. F.; Zhong, Z. P.; Zhou, H. C.; Chen, X. P.; Huang, Y. J. Korean J. Chem. Eng. 2007, 24, 175–180. (6) Zhou, H. C.; Jin, B. S.; Zhong, Z. P.; Huang, Y. J.; Xiao, R. J. EnViron. Sci. 2005, 17, 141–145.
fields in modern research of energy and environment.7-9 PAHs are harmful to the environment and the health of people because of their high degree of mutagenicity and carcinogenicity. They easily enter into the human body through breathing, eating, and drinking.10,11 It is said that about 75-90% of cancers of humans are mainly caused by PAHs.11 Therefore, the U.S. EPA has prioritized 16 PAH compounds as hazardous air pollutants. These compounds are naphthalene (NaP), acenaphthylene (AcPy), acenaphthene (AcP), fluorene (Flu), phenanthrene (PhA), anthracene (AnT), fluoranthene (FluA), pyrene (Pyr), benzo(a)anthracene (BaA), chrysene (Chr), benzo(b)fluoranthene (BbF), benzo(k)fluoranthene (BkF), benzo(a)pyrene (BaP), indeno(1,2,3-cd)pyrene [In(1,2,3-cd)P], dibenzo(a,h)anthracene (DbA), and benzo(ghi)perylene (BghiP).12 In the past, less attention had been paid on the formation, emission, and distribution of PAHs during coal partial gasification and residual char combustion in a fluidized bed, especially at elevated pressure.6 However, there has been a lot of research on the effects of the combustion variables, such as carbon content of coal, hydrogen content of coal, oxygen content of coal, sulfur content of coal, chlorine content of coal, ash content of coal, volatile matter content of coal, combustion temperature, air flow, primary/secondary air ratio, and lime/limestone feeding rate, on PAH formation and distribution in products during coal (7) Mastral, A. M.; Callen, M. EnViron. Sci. Technol. 2000, 34, 3051– 3057. (8) Liu, K. L.; Xie, W.; Zhao, Z. B.; Pan, W. P.; John, T. R. EnViron. Sci. Technol. 2000, 34, 2273–2279. (9) Domeno, C.; Nerin, C. J. Anal. Appl. Pyrolysis 2003, 67, 237–246. (10) Blasco, M.; Domeno, C.; Nerin, C. EnViron. Sci. Technol. 2006, 40, 6384–6391. (11) Zhou, H. C.; Jin, B. S.; Zhong, Z. P.; Li, F. Boiler Technol. 2003, 5, 22–26. (12) Domeno, C.; Blasco, M.; Sanchez, C.; Nerin, C. Anal. Chim. Acta 2006, 569, 103–112.
10.1021/ef8008162 CCC: $40.75 2009 American Chemical Society Published on Web 03/03/2009
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Table 1. Proximate and Ultimate Analysis of Coal and Residual Char ultimate analysis (%) Cad
Had
Oad
Nad
proximate analysis (%) Sad
Aad
Mad
Vad
Cfix
bitumite 60.36 3.72 7.24 0.92 0.44 22.90 4.42 25.24 47.44 char 47.54 0.23 0.00 0.00 0.29 47.06 4.88 1.96 46.10
combustion in a fluidized bed. Liu et al.8,13 reported that the emissions of PAHs in a fluidized bed combustion system are mainly dependent upon the excess air ratio and bed temperature. The secondary air can effectively reduce PAH emissions. The limestone injected into the fluidized bed can promote the formation of PAHs. Chlorine in the coal can lead to highmolecular-weight PAH formation during coal combustion. The total PAH emission increases with a rise of the sulfur content of coal. High-efficiency combustion results in two- or threering PAH formation. Mastral et al.14,15 investigated the effect of air flow on PAH emission in an atmospheric fluidized bed coal combustion and found that the air flow for coal fluidized bed combustion has a determinant influence on PAH emission and leads to a great increase on three- and four-ring PAH emission. The higher the excess air ratio, the lower the emission of total PAHs. Mastral et al.16 also reported that the influence of the limestone presence on PAH formation, emission, and distribution during coal atmosperic fluidized bed combustion. Limestone can not only increase PAH content in flue gas but also promote the PAH partitioning in the solid phase. The above experimental results favor the research on the formation, emission, and distribution of PAHs during coal and residual char combustion in a pressurized fluidized bed. The combustion of coal and residual char was carried out in a small bench-scale pressurized spouted fluidized bed. The 16 PAHs specified by the United States Environmental Protection Agency (U.S. EPA) in coal, residual char, and fly ash were analyzed by high-performance liquid chromatography (HPLC). To reveal the formation mechanism of PAHs during the combustion of coal and residual char, the distribution characteristics of 16 PAHs in fly ash were studied before and after the combustion of coal and residual char, which could provide a technology to control PAH emission from the coal use process. 2. Experimental Section 2.1. Experimental Materials. The analytical results of coal and residual char used in the experiment are shown in Table 1. The residual char is prepared during coal partial gasification. The apparent density and size of residual char are 650 kg/m3 and 0.3-1.5 mm, respectively. The particle size of coal is 0.3-1.0 mm. 2.2. Experimental Facility. Figure 1 shows a schematic diagram of the small bench-scale pressurized spouted fluidized bed for the combustion of coal and residual char. The whole system consists of an air compressor, a spouted fluidized-bed combustor, a coal/ char feeding section, a flue gas purification unit, and a temperature and pressure control section. The combustor is made of a stainlesssteel refractory of 80 mm inner diameter and 4.2 m in height, with a 500 mm inner diameter pressure vessel outside. The reactor has an individually controlled electric heater that supplies heat for the start-up. Two pressure taps are mounted at the bottom and outlet of the reactor to monitor the fluidization state in the reactor. Seven (13) Liu, K. L.; Han, W. J.; Pan, W. P.; Riley, J. T. J. Hazard. Mater. 2001, B84, 175–188. (14) Mastral, A. M.; Callen, M. S.; Murrilo, R.; Garcia, T. Fuel 1998, 77, 1513–1516. (15) Mastral, A. M.; Callen, M. S.; Murrilo, R.; Garcia, T.; Vinas, M. Fuel 1999, 78, 1553–1557. (16) Mastal, A. M.; Garcia, T.; Callen, M. S.; Lopez, J. M.; Murrilo, R.; Navarro, M. V. Energy Fuels 2001, 15, 1469–1474.
Figure 1. Schematic diagram of the pressurized spouted fluidized bed: (1) air compressor, (2) swing hopper, (3) feed hopper, (4) star feeder, (5) combustor, (6) slag hopper, (7) electric heater, (8) cyclone, (9) ash hopper, (10) sampling site, and (11) control valve.
thermocouples are installed across the reactor: one in the wind box, three in the dense bed, two in the freeboard, and one in the outlet of the reactor. A 60° conical distributor with 60 holes of 1 mm inner diameter perforated uniformly is mounted at the bottom of the combustor for better air distribution. A pipe with 10 mm inner diameter is used to introduce the spouting air to the reactor. About half of the air is diverted to the spouting nozzle, while the other half enters the combustor through the conical distributor. Coal/ char is fed via a variable-speed star feeder. After the flue gas leaves the reactor, a small cyclone removes particles in the flue gas, which are collected in an ash hopper. 2.3. Experimental Procedure. Each run was started with the filling of bed material up to the required height. The star feeder was turned on, and the minimum fluidizing air flow rate required to fluidize the bed material in the combustor was supplied by the air compressor. The start-up period was necessary to preheat the bed up to the required temperature before the commencement of coal feeding. When the bed temperature reached 500 °C, coal was added into the pressurized fluidized bed combustor by a star feeder. The coal feeding rate was adjusted to allow for certain excess air to achieve complete combustion of coal. The combustion of coal is shifted to the combustion of residual char by adding residual char from the feeding hopper. After the bed temperature stabilized, the components of flue gas were determined by a multicomponent flue gas analyzer at the sampling site in situ. The pressure loss and the temperature were monitored and registered at 10 min intervals. Two fly ash samples should be collected from the cyclone tank at 30 min intervals at a fixed experimental condition. The sample of coal and residual char was collected from the feeder. The coal/ char feeder was stopped once the sample collection and data recording were over. The whole fluidized bed system was shut down after the temperature dropped below the safe temperature. 2.4. Method of Analysis. Approximately 5 g of coal, residual char, and fly ash were first extracted in a Soxhlet extractor with methylene chloride for 8 h at a rate of at least 4 cycles per hour and condensed to 1 mL by a Kuderna-Danish (K-D) concentrator using a water bath at 60-65 °C, respectively. Then, the concentrated solution was made to pass through a purifying tube packed with 10 g of activated silica gel and 1 g of anhydrous sodium. The purifying tube was then eluted by a 25 mL mixture of 60:40 (v/v, %) pentane and methylene chloride to obtain PAHs in the purifying solution. Further, the purified solution was condensed to 1 mL using a K-D concentrator and a gentle stream of pure nitrogen. Finally, the samples were put into the brown vials and stored at 4 °C for HPLC analysis. The pretreatment samples were analyzed by the Waters Alliance HPLC system coupled with a Waters 2695 separations module, a
Distribution of PAHs in Fly Ash
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Table 2. Experimental Conditions and Results during Pressurized Combustion pressure bed temperature feeding rate air flow rate fluidized velocity excess air ratio bed height high heating value carbon content in fly ash combustion efficiency CO
unit
coal
MPa °C kg/h N m3/h m/s
0.3 904 2.03 12.99 1.03 1.56 300 24.81 47.21 71.26 1027
mm MJ/kg % % ppm
Table 3. Distribution of PAHs in Fly Ash during Coal Pressurized Combustion
residual char 0.2 900 1.97 8.84 1.05 1.49 300 14.80 42.93 81.49 600
0.3 894 2.49 12.47 0.99 1.43 300 14.80 26.43 85.37 375
0.45 910 3.08 17.82 0.94 1.42 300 14.80 13.56 92.26 384
Waters 2475 Multi λ fluorescence detector, and a Waters 2996 photodiode array (PDA) detector. Samples (1 µL) were injected into a C18 column (250 mm length, 4.6 mm inner diameter) containing 5 µm particles for PAHs. The mobile phase, flowing at 1.2 mL/min, was programmed to hold, held at a mixture of 60:40 (v/v, %) water and acetonitrile for 12 min, then was followed by an 11 min ramp to a composition of 100% acetonitrile, and finally, held at a mixture of 60:40 (v/v, %) water and acetonitrile for 7 min. The excitation and emission wavelengths of the fluorescence detector were 224 and 330 nm, respectively. The wavelength of the PDA detector was 254 nm. HPLC was calibrated with a diluted standard solution of 16 PAH compounds (PAH mixture-610 M from Supleco) recommended by the U.S. EPA. The recovery efficiencies and method detection limit (MDL) of 16 PAHs ranged from 60-117% and 1.97-233 pg, respectively.
3. Results and Discussion 3.1. Pressurized Combustion of Coal and Residual Char. The experimental conditions and results of coal and residual char pressurized combustion are shown in Table 2. At the invariable static bed height, bed temperature, and fluidized velocity, the feeding rate of residual char and air flow rate must change with the variable pressure during residual char pressurized combustion. From Table 2, the combustion efficiency and carbon content in fly ash during coal pressurized combustion are 71.26 and 47.21%, respectively. In comparison to the residual char pressurized combustion at the same pressure, the combustion efficiency is lower and the carbon content in fly ash is higher during coal pressurized combustion. The reason is that there are many fine particles in coal. Fine coal particles will increase the entrainment loss of coal. Thus, the combustion efficiency of coal decreases and the carbon content in fly ash increases during coal pressurized combustion. In comparison to residual char, the pore diameter, pore volume, and pore specific surface area of coal are smaller. The smaller pore diameter and pore specific surface area will not favor the adsorption of oxygen on the surface of the coal particle and the diffusion of oxygen and combustion products in the pore of the coal particle. Therefore, the oxidation velocity and combustion efficiency of coal decrease. It can also be seen that the combustion efficiencies of residual char increase from 81.49 to 92.26% and the carbon contents in fly ash decrease from 42.94 to 13.56% when the pressure increases from 0.2 to 0.45 MPa at the invariable static bed height, bed temperature, and fluidized velocity. The reason may be that the rise of pressure will reduce the combustion time of the residual char particle and increase the combustion efficiency of residual char. The increase of the oxygen content and residual char particle number in unit volume will accelerate the oxidation velocity of the residual char particle at high pressure. The high pressure can promote the combustion load of combustion facility in unit volume. However, the fluctuation of heat lose of combustion facility can be neglected. Therefore, the heat loss of unit mass
PAHs
ring total
NaP AcP AcPy Flu PhA AnT FluA Pyr Chr BaA BbF BkF BaP DbA In(123-cd)P BghiP 2 3 4 5 6
coal (mg/kg)
fly ash (mg/kg)
1.83 0.09 0.00 0.09 1.92 0.57 6.10 1.19 6.98 1.63 0.96 1.05 0.55 0.01 0.06 0.07 1.83 2.66 15.89 2.57 0.13 23.07
0.22 0.27 0.18 0.20 3.61 1.28 1.16 4.15 0.54 0.31 0.21 0.19 0.57 0.24 0.24 0.15 0.22 5.54 6.16 1.22 0.39 13.54
during coal and residual char pressurized combustion is low, and the combustion efficiency is high. 3.2. Distribution of PAHs during Coal Pressurized Combustion. The content and distribution of PAHs in fly ash during coal pressurized combustion are shown in Table 3. It can be concluded that the contents of PhA, AnT, FluA, and Pyr in fly ash during coal pressurized combustion are high. The total PAHs in fly ash are mainly dominated by three- and fourring PAHs during coal pressurized combustion. It is similar to the distribution of PAHs in coal. In comparison to coal, the percentage of two-ring PAHs in fly ash is lower than that in coal, while the percentage of six-ring PAHs in fly ash is higher than that in coal. In general, there are three pathways for PAH formation during coal pressurized combustion. The first is that the PAHs derive from the undecomposed PAHs in coal.7 The second is that the PAHs come from the pyrosynthesis of coal.17 The third is that the polymerization of free radicals at high temperature can also lead the formation of PAHs.18,19 Liu et al.8 found that the total PAHs in fly ash are three- and fourring PAHs during coal combustion in a bench-scale fluidizedbed reactor. Li et al.20 found that there were two reaction zones for PAH formation during coal combustion. One is the hightemperature combustion zone, and the other is the lowtemperature combustion zone. The dividing line of the two reaction zone is 800 °C. The PAHs derived from the pyrolysis of coal at the low-temperature zone and the synthesis of free radicals can form the PAHs at the high-temperature zone. During coal pressurized combustion, the bed temperature was 900 °C, which was between the maximum formation temperature zones (600-900 °C) of PAHs. The content of PAHs in fly ash during coal pressurized combustion was 13.54 mg/kg at 900 °C. 3.3. Distribution of PAHs during Residual Char Pressurized Combustion. The content and distribution of PAHs in fly ash during residual char pressurized combustion are shown in Table 4. It can be found that the contents of PhA, FluA, and Pyr in fly ash during residual char pressurized combustion are (17) Korenaga, T.; Liu, X. X.; Huang, Z. Y. Chemosphere 2001, 3, 117– 122. (18) Richter, H.; Howard, J. B. Prog. Energy Combust. 2000, 26, 565– 608. (19) Catallo, W. J. Chemosphere 1998, 37, 143–157. (20) Li, X. D.; Qi, M. F.; You, X. F.; Yan, J. H.; Chi, Y.; Ni, M. J.; Cen, K. F. Proc. CSEE 2002, 22, 127–132.
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Table 4. Distribution of PAHs in Fly Ash during Residual Char Pressurized Combustion fly ash (mg/kg) pressure (MPa)
PAHs
ring total
NaP AcP AcPy Flu PhA AnT FluA Pyr Chr BaA BbF BkF BaP DbA In(123-cd)P BghiP 2 3 4 5 6
char (mg/kg)
0.2
0.3
0.45
3.39 0.19 0.00 0.07 1.44 0.17 1.11 0.85 0.19 0.06 0.18 0.08 0.12 0.02 0.13 0.17 3.39 1.87 2.21 0.39 0.30 8.14
0.09 0.00 0.06 0.27 2.32 0.30 0.33 1.80 0.06 0.02 0.02 0.02 0.02 0.01 0.01 0.02 0.09 2.95 2.20 0.07 0.02 5.33
0.16 0.00 0.16 0.33 2.23 0.34 0.72 2.40 0.28 0.13 0.12 0.11 0.19 0.17 0.14 0.12 0.16 3.06 3.53 0.59 0.26 7.60
0.11 0.00 0.06 0.43 1.75 0.25 0.71 1.80 0.28 0.11 0.13 0.10 0.16 0.18 0.13 0.09 0.11 2.50 2.90 0.57 0.23 6.31
high, especially PhA and Pyr. Their percentages are all higher than 27%, and the maximum percentage reaches 43.58%. It is obvious that three- and four-ring PAHs dominate in fly ash during residual char pressurized combustion, and their percentages in fly ash are 85.52-96.69%. The distribution of PAHs in fly ash during residual char pressurized combustion is similar to the distribution of PAHs in fly ash during coal pressurized combustion, while it is not similar to the distribution of PAHs in residual char. There are many naphthalenes in residual char. The percentage of naphthalene in fly ash apparently decreases after the residual char combustion at elevated pressure. However, the percentages of three- and four-ring PAHs in fly ash during residual char pressurized combustion are higher than that in residual char. This shows that PAHs can be oxidized and decomposed into CO2, H2O, low-molecular-weight hydrocarbons, and free radicals, and they can also be regenerated via the cyclization and polymerization reaction. The high pressure favors the acceleration of the oxidation reaction velocity of PAHs, the formation of PAHs via the Dils-Alder reaction synthesis of free radicals, and the formation of heavy-molecularweight PAHs via the polymerization reaction between lowmolecular-weight PAHs. At the invariable experimental condition, the contents of PAHs in fly ash during residual char pressurized combustion increase and then decrease with the increase of pressure (shown in Table 4). The content of PAHs in fly ash during residual char pressurized combustion is 7.60 mg/kg at the pressure of 0.3 MPa, which is higher than that at the pressure of 0.2 and 0.45 MPa. The quantities of free radicals produced from the pyrolysis of residual char in unit volume increase with the pressure, which will promote the collision probability between free residuals and favor the formation of PAHs via the polymerization reaction of free radicals. According to the chemical reaction equilibrium, the high pressure favors the reaction progress toward the volume reduction of produced gas. The polymerization reaction of free radicals at high temperature to form PAHs is just a volume reduction reaction.
Therefore, the contents of PAHs in fly ash during residual char increase with the pressure. However, the high pressure can increase the combustion velocity of residual char and the oxidation velocity of PAHs, while the formation of PAHs can further polymerize into the large soot. Therefore, the contents of PAHs in fly ash during residual char pressurized combustion decrease with the further increase. Zhong and Liu21 found that the high pressure increased the quantity of PAH formation in the laminar premixed flame. In a comparison of Table 3 to Table 4, at the invariable pressure, bed temperature, and fluidized velocity, the contents of PAHs in fly ash during coal combustion are higher than that during residual char combustion at elevated pressure. The reason may be that residual char has a good pore structure, high specific surface area, and large pore volume. These characters favor heat transmission and mass transmission and promote the combustion efficiency during residual pressurized combustion. Accordingly, the carbon content in fly ash during residual char pressurized combustion is lower than that during coal combustion (shown in Table 2). The low combustion efficiency and high carbon content in fly ash favor the formation of PAHs. The content of volatile matter in coal is higher than that in residual char. The quantities of free radicals released from volatile matter that decomposed before the ignition of coal are also higher than residual char combustion, which will increase the probability of PAH formation via the polymerization reaction of free radicals at high temperature. 4. Conclusions The combustion of coal and residual char in a pressurized spouted fluidized bed was performed in this paper. The percentage of PAHs in fly ash was determinated by HPLC. The experimental results show that the combustion efficiency of residual char pressurized combustion is higher and the carbon content in fly ash from residual char pressurized combustion is lower compared to coal pressurized combustion. At the same pressure, the PAH content in fly ash from residual char pressurized combustion is lower than that in fly ash from coal pressurized combustion. The total PAHs in fly ash from coal and residual char pressurized combustion are dominated by three- and four-ring PAHs. The contents of PAHs in fly ash from residual char pressurized combustion increase and then decrease with the increase of the pressure. The shortage in this paper is the narrow operation pressure (0-0.45 MPa) during coal and residual pressurized combustion. The reason is that the adjustment of operation pressure is restricted by the structure of the fluidized bed. We plan to rebuild this fluidized bed and perform the combustion of coal and residual char in the fluidized bed at a higher pressure (0.5-1.0 MPa) in the future. Acknowledgment. This work was supported by the Key Project of State Basic Study of Ministry of Science and Technology of China (G19990221053), the National Natural Science Foundation of China (50608040), the “Qinglan Gongcheng” of Jiangsu Province, and the Foundation of Science and Research of Nanjing University of Information Science and Technology. EF8008162 (21) Zhong, B. J.; Liu, X. F. J. Eng. Thermophys. 2004, 25, 151–154.
