Energy & Fuels 2009, 23, 2018–2024
Fine Ash Formation during Pulverized Coal Combustion Tsuyoshi Teramae* and Takayuki Takarada Coal and EnVironmental Research Laboratory, Coal Business Office, Petroleum & Coal Marketing Department, Idemitsu Kosan Company, Limited, 3-1 Nakasode, Sodegaura, Chiba, Japan and Department of Chemical and EnVironmental Engineering, Faculty of Engineering, Gunma UniVersity, 1-5-1 Tenjin-cho, Kiryu, Gunma, Japan ReceiVed August 13, 2008. ReVised Manuscript ReceiVed February 10, 2009
Fine particulates that are emitted from commercial coal combustion sources can be inhaled into human respiratory systems and have been known to cause various harmful effects. Therefore, legislation has been enacted to limit the emission of fine particulate matter in many countries. A fundamental understanding of the mechanisms of fine particle formation is an important step to mitigating the environmental impacts of coal combustion. In this study, 15 pulverized coal samples were burnt in a drop-tube furnace to investigate the formation of fine particulates and the influence of coal ash properties on their emission. Coal combustion was carried out at 1673 K in air. Fine particles were collected by a cyclone and a low-pressure impactor. The elemental compositions of the collected particles were analyzed by scanning electron microscopy with energy-dispersive X-ray spectroscopy. We examined the chemical compositions of the fine particles as a function of particle diameter and examined the proportions of the elements in the parent coal samples. We determined that almost all particles less than 0.22 µm in diameter were formed by means of volatilization-condensation of SiO2 and Al2O3 in the coal. We also demonstrated that the amount of SiO2 in particle size less than 0.22 µm in diameter was related to the amount of fine included quartz and clay minerals in the parent coal. The primary components of particles greater than 0.76 µm in diameter were SiO2 and Al2O3, and as the diameter of the particles decrease, the mass fractions of iron, magnesium, calcium, and phosphorus increased. However, the particle diameter at which this tendency commenced differed depending on the element. Particles between 0.22 and 0.76 µm in diameter were thought to have been formed by the fragmentation and coalescence of particles in the coal and by the simultaneous condensation of volatilized elements onto other particles.
Introduction Possible links between fine particles and health effects were identified in the late 1980s. Since then, there has been increasing statistical evidence relating the concentration of fine particulate matter in ambient air to heart and lung problems.1 Studies have indicated that fine particle toxicology could be responsible for these adverse effects. The emission of fine particles is closely associated with the emission of toxic trace elements from coal combustion because the fine particles are often enriched with these toxic elements.2 Governments worldwide have acknowledged the adverse health effects of ambient fine particles. As a result, standards have been introduced to assist in reducing ambient fine particle concentrations. In the United States, a National Ambient Air Quality Standard (NAAQS) has been established for particulate matter with an aerodynamic diameter less than 10 µm (PM10) and less than 2.5 µm (PM2.5). In Japan, environmental regulation of suspended particulate matter with a diameter less than 10 µm has been established by the Ministry of the Environment. About 100 million tons of steaming coal is used in Japan each * To whom correspondence should be addressed. Telephone: 81-43862-9511. Fax: 81-438-60-1177. E-mail: [email protected]
(1) Sloss, L. L. The importance of PM10/2.5 emissions; IEA CCC/89; IEA Clean Coal Centre: London, U.K., 2004. (2) Lighty, J. S.; Veranth, J. M.; Sarofim, A. F. Combustion aerosols: factors governing their size and composition and implications to human health. J. Air Waste Manage. Assoc. 2000, 50, 1565–1622.
year, and the emission of fine particulate matter from coal-fired boilers must be reduced. To elucidate the mechanisms of PM10 formation, many researchers have measured the size distribution of PM10 for various coal types. In addition, they also have conducted combustion experiments in which temperature and oxygen concentration are varied. Measurements of particle size distribution are an important means of elucidating the mechanisms of fine particle formation. In recent years, there have been several reports of particle size distribution appearing as a trimodal distribution.3-5 The fine mode with an average diameter of about 0.1 µm is formed by volatilization-condensation, whereas the coarse mode of 5 µm or more is formed by the coalescence of ash particles. However, little progress has been made in discussing formation mechanisms for the central mode. Yu and co-workers6 identified and examined the origin of this central (3) Linak, W. P.; Miller, C. A.; Wendt, J. O. L. Comparison of particle size distribution and elemental partitioning from the combustion of pulverized coal and residual fuel oil. J. Air Waste Manage. Assoc. 2000, 50, 1532–1544. (4) Wendt, J. O. L. Pollutant formation in furnaces: NOx and fine particulates. IFRF Combust. J. 2003, Article No. 200301 (http://www. journal.ifrf.net/library/may2003/200301Wendt.pdf, The IFRF Electronic Combustion Journal). (5) Seames, W. S. An initial study of the fine fragmentation fly ash particle mode generated during pulverized coal combustion. Feel Process. Technol. 2003, 81, 109–125. (6) Yu, D.; Xu, M.; Yao, H.; Sui, J.; Liu, X.; Yu, Y.; Cao, Q. Use of elemental size distributions in identifying particle formation modes. Proc. Combust. Inst. 2007, 31, 1921–1928.
10.1021/ef800658w CCC: $40.75 2009 American Chemical Society Published on Web 03/23/2009
Fine Ash Formation
Energy & Fuels, Vol. 23, 2009 2019 Table 1. Properties of Coal Samples Used in This Study
E proximate 4.1 10.1 34.0 51.8
analysis (wt %) 3.3 9.1 9.7 10.6 34.4 41.4 52.6 38.9
4.9 12.7 35.6 46.8
7.4 6.6 27.4 58.6
5.8 14.4 31.8 48.0
3.7 7.2 35.6 53.5
2.8 15.0 26.3 55.9
3.3 10.0 27.7 59.0
3.1 1.9 32.7 62.3
5.2 3.2 33.3 58.3
moisture ash volatile matter fixed carbon
6.0 12.6 30.2 51.2
11.6 5.0 38.6 44.8
7.8 11.0 33.9 47.3
2.6 8.9 38.7 49.8
carbon hydrogen nitrogen sulfur oxygen (oxygen: by difference)
82.27 5.17 1.27 0.26 11.03
76.04 5.61 1.67 0.63 16.05
79.05 5.42 1.07 0.14 14.32
ultimate analysis (wt %, daf) 82.94 82.77 81.88 78.78 79.47 6.13 5.78 5.44 6.65 5.65 2.85 1.89 1.60 1.14 2.50 0.33 0.52 0.45 0.39 0.07 7.75 9.04 10.63 13.04 12.31
83.79 5.15 0.96 0.65 9.45
82.68 5.62 1.79 0.40 9.51
81.95 5.24 1.76 0.34 10.71
83.30 4.97 1.86 0.36 9.51
81.12 4.64 0.91 0.81 12.52
84.20 5.27 1.85 0.33 8.35
81.55 5.04 1.00 0.35 12.06
SiO2 Al2O3 TiO2 Fe2O3 CaO MgO Na2O K2O P2O5 MnO V2O5 SO3
52.71 29.70 1.12 3.52 7.26 0.80 0.31 1.26 0.49 0.05 0.02 1.95
44.50 21.88 0.88 12.45 6.41 1.53 0.53 1.56 0.29 0.12 0.05 5.27
61.11 17.12 0.64 3.99 9.64 1.43 0.22 0.90 0.26 0.04 0.02 3.81
elemental composition of ash (wt %) 53.59 57.78 44.57 62.06 57.88 19.98 23.41 32.58 24.29 17.79 0.81 1.29 1.61 2.10 0.72 7.44 6.11 5.29 2.60 6.93 6.32 4.06 6.69 3.14 6.04 2.72 1.40 2.38 1.09 2.40 1.24 0.39 0.42 0.85 1.59 3.00 1.01 0.53 0.33 2.82 0.26 0.38 0.39 0.03 0.38 0.05 0.04 0.08 0.07 0.07 0.03 0.06 0.05 0.06 0.02 3.03 2.73 2.90 1.73 1.89
51.63 18.63 0.73 14.11 6.14 0.89 0.51 0.49 1.21 0.07 0.02 3.26
72.71 19.03 0.90 2.10 0.80 0.57 0.30 2.25 0.04 0.00 0.02 0.49
74.57 18.36 1.16 1.61 0.82 0.53 0.33 1.98 0.05 0.00 0.03 0.46
43.90 34.02 1.60 5.50 6.74 1.31 0.48 0.80 1.16 0.04 0.03 3.50
57.94 19.01 0.65 12.89 3.15 0.78 0.28 1.50 0.30 0.24 0.02 1.44
49.76 37.30 2.97 1.43 1.09 0.72 1.33 0.51 2.01 0.08 0.40 1.99
59.75 15.06 0.47 16.71 3.53 0.82 0.53 1.41 0.35 0.11 0.00 3.80
mode based on the basis of mass-fraction size distributions of elements. They suggested that a heterogeneous condensationadsorption of vaporized species on fine residual ash particles appropriately accounts for the formation of the central mode. The primary objectives of the present study were to utilize numerous coal samples, to sample fine particles from each of these samples under the same conditions, to measure particle size distribution and elemental mass-fraction size distribution, to compare the size boundary for each mode with those obtained in previous research, and to investigate fine particle formation mechanisms. Previous studies used only a few types of coal samples, whereas in the current research, we used 15 distinct coal samples. We also focused on silicon and aluminum, which are the main constituents of fine particles in ash, and elucidated these elements’ effects on fine particle formation as well as the paths they took in becoming fine particles. Quann and coworkers7 measured SiO2 contents of 1.6-44% in the submicrometer fumes from combustion of coal in 20% O2. Buhre and co-workers8 characterized small ash particles generated from the combustion of bituminous coals in 21% and 50% O2. They showed that the main constituents of these samples are sulfur, silicon, phosphorus, and sodium and that the elemental composition and yield of submicrometer ash are greatly affected by increasing the O2 partial pressure. They also indicated that the condensation of vaporized species was responsible for the formation of ash particles smaller than 0.3 µm. In a further study by the same group, the correlation between the amounts of vaporized silica and the characteristics of five well-characterized Australian black coals was examined by combustion in 21% and 50% O2 in a drop-tube furnace (DTF). They concluded that finely dispersed siliconbearing minerals of a size less than 2 µm could substantially contribute to the amount of silica vaporization observed. (7) Quann, R. J.; Nevill, M.; Janghorbani, M.; Mims, C. A.; Sarofim, A. F. Mineral matter and trace-element vaporization in a laboratorypulverized coal combustion system. EnViron. Sci. Technol. 1982, 16, 776– 781. (8) Buhre, B. H. P.; Hinkley, J. T.; Gupta, R. P.; Wall, T. F.; Nelson, P. F. Submicron ash formation from coal combustion. Fuel 2005, 84, 1206– 1214.
However, they did not quantify the amount of particles less than 2 µm in diameter nor did they directly express these particles’ contribution to silica vaporization.9 Zhang and coworkers10 indicated that mullite (3Al2O3 · 2SiO2) generated from the decomposition of kaolinite (Al2O3 · 2SiO2 · 2H2O) contributed to the formation of fine particles with a size of 0.1-1 µm. They noted that SiO2 is also produced stoichiometrically by the decomposition of kaolinite, but the extent to which this generated SiO2 becomes incorporated into submicrometer particles is unknown. Numerous researchers have shown that the volatilization of SiO from the quartz contained in coal forms fine particles. However, the effects of clay minerals containing SiO2 and Al2O3 as sources of silicon in fine particles have hardly been discussed. In this research, we examined the role of clay minerals as a source of silicon in fine particles and the differences in the mechanisms of fine particle formation based on particle diameter. Fifteen bituminous coal samples were characterized and then burned in a DTF at 1673 K. Fine ash particles were collected by means of a cyclone and a low-pressure impactor (LPI), and the chemical composition of the particles was analyzed. Experimental Section Thirteen bituminous coals with a wide range of ash chemistries were selected for analysis. Table 1 lists the properties for these coals. Two of these coal samples (samples L and M in Table 1) were gravimetrically fractionated to obtain additional coal samples with a low ash content (samples N and O, respectively), bringing the total sample count to 15. The float fraction, which was the fraction separated by means of a liquid with a specific density of 1.3, was tested for the combustion experiments. The ash contents and chemical compositions were dissimilar among the 15 coal samples. For example, the SiO2 contents ranged from 44.5% to 74.6%, and the Al2O3 contents ranged from 17.1% to 37.3%, with SiO2/Al2O3 ratios of 1.29-4.06. (9) Buhre, B. H. P.; Hinkley, J. T.; Gupta, R. P.; Nelson, P. F.; Wall, T. F. Factors affecting the vaporization of silica during coal combustion. Fuel Process. Technol. 2007, 88, 157–164. (10) Zhang, L.; Ninomiya, Y.; Yamashita, T. Formation of submicron particles matter (PM1) during coal combustion and influence of reaction temperature. Fuel 2006, 85, 1446–1457.
2020 Energy & Fuels, Vol. 23, 2009
Teramae and Takayuki
Figure 2. Size distribution of fine particles formed from combustion of 15 samples at 1673 K in air.
Figure 1. Schematic diagram of drop tube furnace and ash collection train.
The coal samples were milled and sieved to a particle size of less than 74 µm. The coal samples were subjected to proximate and ultimate analyses (by JIS M 8812 and JIS M 8819, respectively) and analyzed by computer-controlled scanning electron microscopy (CCSEM). The chemical composition of ash generated by hightemperature ashing (1088 K, 2 h) of each sample was determined by X-ray fluorescence analysis. Ashes were also generated from the samples by low-temperature ashing (LTA), after which the particle size distribution of the ash was measured by a laser scattering particle size distribution analyzer. After characterization, the coal samples were combusted in a DTF located at Idemitsu Coal & Environmental Research Laboratory, Japan. The furnace, which features a heated tube 1200 mm in length, is shown schematically in Figure 1. The coal particles were fed by a vibrating feeder into the primary air stream. The primary air stream in turn fed the coal particles into the heated zone of the combustor at a rate of 7 g/h via a water-cooled injector probe. The secondary air was heated to 973 K and fed into the heated zone of the DTF. Combustion of the coal particles was carried out in air at 1673 K (gas temperature). Generated ash larger than 2 µm was collected by a cyclone installed at the opposite end of the DTF, and the weight of the ash was measured. Particles smaller than 2 µm were introduced into a LPI. The LPI consisted of 12 stages containing Teflon filter with aerodynamic cutoff diameters (µm) of 8.5, 5.7, 3.9, 2.5, 1.25, 0.76, 0.52, 0.33, 0.22, 0.13, 0.06, and