Occurrence of Inorganic Elements in Condensed Volatile Matter

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Energy & Fuels 2006, 20, 1482-1489

Occurrence of Inorganic Elements in Condensed Volatile Matter Emitted from Coal Pyrolysis and Their Contributions to the Formation of Ultrafine Particulates during Coal Combustion Lian Zhang,† Yoshihiko Ninomiya,*,† and Toru Yamashita‡ Department of Applied Chemistry, Chubu UniVersity, Matsumoto-cho 1200, Kasugai, Aichi, Japan, and Coal and EnVironment Research Laboratory, Industrial Energy Department, Idemitsu Kosan Co., Ltd., 3-1 Nakasode, Sodegaura, Chiba, Japan ReceiVed December 22, 2005. ReVised Manuscript ReceiVed March 21, 2006

Coal pyrolysis is the first step during coal combustion, when the injected coal particles swell to release the volatile matter (VM) at a very short residence time. Simultaneously, the organically bound fraction of inherent metals is likely emitted out too. To prove the presence of organically bound metals in coals, five bituminous coals and one anthracite coal from China were pyrolyzed in N2 in a lab-scale drop tube furnace. The gas temperature in furnace was about 900-1400 K so that almost all the inorganic elements except those containing Na hardly vaporized. The emitted VM was collected by a low-pressure impactor. The results indicate that the condensed VM (CVM) smaller than 1.0 µm has an amorphous carbon structure, which contains the inorganic elements too. Sulfur is the most prevalent, followed by sodium, silicon, chlorine, calcium, and others in the decreasing order. Apart from a portion of sodium in form of NaCl, all the inorganic elements are organically bound with CVM as determined by both TEM-EDS and XPS. These elements disperse highly in CVM; their oxidation and coagulation during VM combustion likely contribute to the majority of ultrafine particulates (PM0.1 smaller than 0.1 µm) formed during coal combustion at a relatively low temperature, 1473 K.

Introduction Emission of particulate matter is one of the most significant pollutants from coal combustion. It has been confirmed by lots of researches that the emitted fine particulates have acute adverse health effects.1,2 Specifically, the ultrafine particulates, less than 1.0 µm in diameter and termed as PM1 accordingly, are more harmful than both the fine (sizes between 1.0 and 2.5 µm) and the coarse particulates (sizes larger than 2.5 µm). This is because of their smaller sizes and the abundance of toxic volatile metals.1,3,4 PM1 can penetrate the lungs readily and cause evident respiratory distress.1,3-5 PM1 consists of two major fractions: PM0.1 for the sizes e 0.1 µm and PM0.1+ >0.1 µm.6,7 It has been clarified that these two portions are formed from different sources. PM0.1 is composed of condensed metallic vapors and their aggregates while PM0.1+ is rich in aluminum silicates.6-8 The latter fraction is mainly produced by fragmentation of inherent kaolinite during * Corresponding author. Tel: +81-568-51-9178. Fax: +81-568-51-1433. E-mail: [email protected]. † Chubu University. ‡ Idemitsu Kosan Co., Ltd. (1) Sloss, L. L. The Importance of PM10/2.5 Emissions; IEA Clean Coal Centre: London, October 2004. (2) Smith, I. M.; Sloss L. L. PM10/2.5sEmissions and Effects; IEA Coal Research: London, October 1998. (3) Lockwood, F. C.; Yousif, S. Fuel Process. Technol. 2000, 65-66, 439-457. (4) Xu, M.; Yan, R.; Zheng, C.; et al. Fuel Process. Technol. 2003, 85, 215-237. (5) Fernandez, A.; Wendt, J. O. L.; Witten, M. L. Fuel 2005, 84 (10), 1320-1327. (6) Zhang, L.; Ninomiya, Y.; Yamashita, T. Fuel 2006, 85, 1446-1457. (7) Senior, C. L.; Panagiotou, T.; et al. Prepr. Symp., DiV. Fuel Chem., Am. Chem. Soc. 2000, 45 (1). (8) Lin, W. Y.; Biswas, P. Combust. Sci. Technol. 1994, 101, 29-43.

the combustion of coal char. On the other hand, formation of PM0.1 is governed by a relatively complicated mechanism that involves several factors. Among them, the association of inherent inorganic elements is primarily important. Besides the metallic vapors generated from vaporization of inorganic compounds, the organically bound fraction of metals in raw coal is another key contributor for the formation of PM0.1.6,9 In raw coals, approximately 25% of oxygen is in the form of carboxyl acid groups. These groups act as bonding sites for various cations such as alkali, alkaline earth, etc. The organically bound fraction of metals can mostly be extracted and quantified by chemical fractionation of coal with the organic acids such as 1 M ammonium acetate.10 A relatively strong linear relationship was also found between this fraction and PM1 formed from coal combustion, implying the significant influence of organically bound metals.9 However, some inorganic elements are in the form of chelate coordination complexes with pairs of adjacent organic oxygen functional groups in raw coals, such as Si, Al, and Ca.10-13 They cannot be extracted by any weak acids including ammonium acetate; hence, the formation sources for these three elements in PM0.1 are still unknown. In principle, since the chemical bonds between C and metals are rather weak,14 the organically bound metals can decompose quickly as soon as coal particles are injected in a hot atmosphere. This reaction may occur in the first step of coal combustion, (9) Zhang, L.; Ninomiya, Y. Fuel 2006, 85, 194-203. (10) Smoot, L. D., Ed. Fundamentals of Coal Combustion for Clean and Efficient Use; Elsevier: London, 1993; pp 304-306. (11) Sakanishi, K.; Saito, L.; et al. Fuel 2002, 81, 1471-1475. (12) Sakanishi, K.; Akashi, E.; et al. Fuel 2004, 83, 739-743. (13) Wang, J.; Li, C.; et al. Fuel 2005, 84, 1487-1493. (14) Thayer, J. S. EnVironmental Chemistry of the HeaVy Elements, Hydrido and Organo Compounds; VCH Publishers: New York, 1995; pp 9-19.

10.1021/ef050429h CCC: $33.50 © 2006 American Chemical Society Published on Web 05/04/2006

Condensed Volatile Matter from Coal Pyrolysis

Energy & Fuels, Vol. 20, No. 4, 2006 1483

Figure 1. General model for pulverized coal combustion and the possible behaviors of organically bounded metals.

pyrolysis, where the injected coal particles swell quickly, the volatile matter (VM) is released, and the reducing gases including CO, CH4, etc, can be formed too.15 Simultaneously, the decomposed organic metals may emit out together with VM. The appearance of S in tar formed from the rapid hydropyrolysis of coals partially proves the above-mentioned hypotheses.16 Moreover, emission of elemental mercury is considered at a rate similar to the devolatilziation rate of coal particle, further indicating the importance of coal pyrolysis.17 The emitted VM meets oxygen and combusts to form the flame, inside it the emitted organic metals may undergo decomposition and oxidation and form the nanoscale oxides, which coagulate further in the cooling post-flame zone and cause the formation of ultrafine aggregates around 0.1∼0.2 µm eventually. This hypothesis is shown graphically in Figure 1. Given that the properties of metals in emitted VM are well-understood, it is possible to know the amount of organically bound metals in raw coal as well as to predict the formation of PM0.1 after coal combustion. Therefore, it is necessary to investigate the occurrence of inorganic elements in the VM emitted from coal pyrolysis. During coal pyrolysis at high temperature, a portion of the emitted VM changes into tar, which in part undergoes further reactions to condense into fine/ultrafine carbon particles, namely, soot.18 Up to now, a whole slew of knowledge has been obtained for the properties of soot and its precursor. Both organic carbon (OC) and elemental carbon (EC) are the most abundant.19-21 Two types of molecules, acetylene and polyaromatic hydrocarbon (PAH), are considered the most likely precursors. Once their first particle is formed, it undergoes surface growth and coagulation to form the fractal chain structures.19,22,23 Nevertheless, for metals within the fine/ultrafine carbon particles, the (15) Wu, Z. Fundamentals of PulVerized Coal Combustion; IEA Clean Coal Center: London, March 2005. (16) Xu, W.; Kumagai, M. Fuel 2003, 82 (3), 245-254. (17) Dajnak, D.; Lockwood, F. C. IFRF Combust. J. 2001, March, article no. 200103. (18) Smoot, L. D., Ed. Fundamentals of Coal Combustion for Clean and Efficient Use; Elsevier: London, 1993; pp 196-200. (19) Ma, J. L. Soot formation during coal pyrolysis. Doctoral Thesis, Brigham Young University, Salt Lake City, UT, 1996. Available at http:// www.et.byu.edu/∼tom/. (20) Solum, M. S.; Sarofim, A. F.; Pugmire, R. K.; et al. Energy Fuels 2001, 15, 961-971. (21) Jiang, Y. J.; Solum, M. S.; Pugmire, R. J.; Grant, D. M. Energy Fuels 2002, 16, 1296-1300. (22) Balthasar, M.; Frenklach, M. Combust. Flame 2005, 140, 130145. (23) Brown, A. L. Modeling soot in pulverized coal flames. Doctoral Thesis, Brigham Young University, Salt Lake City, UT, 1997. Available at http://www.et.byu.edu/∼tom/.

knowledge is still scarce except some were gotten in ref 2426, where the unburnt carbon/soot, mixed with fine ash particles e 2.5 µm, were reported to collect after the incomplete coal combustion. Through characterization by several advanced techniques including electron microscopy (EM) and extended X-ray adsorption fine spectroscopy (EXAFS), the soot was found to contain S, Fe, and Ti. The species of these elements were determined too. As to the formation mechanisms, these inorganic elements were merely considered as coated or mixed with the intrinsic carbon particles, little discussion has been conducted for interpreting the detailed reactions in the coal combustion furnace. This study first aims to elucidate the detailed occurrence of inorganic elements in emitted VM. The condensable VM (CVM) (i.e., fine/ultrafine carbon particles) is generated from pyrolysis of several coals in a lab-scale drop tube furnace (DTF). Note that the CVM defined here possibly contains both the tar generated from primary pyrolysis of coal and the soot produced from secondary reactions of its tar precursors. Behavior of carbon is not the research objective here; hence, the complex reactions for coal pyrolysis are not discussed. A low-pressure impactor (LPI) is used to segregate CVM into sizes from 12.1 to 0.03 µm. X-ray fluorescence (XRF) is adopted for quantifying the inorganic elements in the individual size of CVM; scanning electron microscopy (SEM) and transmission electron microscopy (TEM) coupled with energy-dispersive X-ray spectroscopy (EDS) are employed for observing the occurrence of inorganic elements in CVM. Additionally, X-ray photoelectron spectroscopy (XPS) is used for probing the chemical forms of inorganic elements of interest such as S, Si, Fe, and Na. Second, the amounts of inorganic elements in CVM are compared with them in PM0.1 formed after coal combustion. The contributions of the organically bound fraction of metals in raw coals to PM0.1 are discussed as well. Experimental Section Coal Properties. Five bituminous coals and one anthracite coal (JCh coal) from China were used here. These coals were ground to smaller than 125 µm and dried prior to use. Their proximate properties are listed in Table 1. The fixed carbon (FC) in these coals are in a range of 44∼58%, meanwhile their volatile matter (24) Chen, Y. Z.; Shah, N.; Braun, A.; et al. Energy Fuels 2005, 19 (4), 1644-1651. (25) Chen, Y. Z.; Shah, N.; Huggins, F. E.; Huffman, G. P. EnViron. Sci. Technol. 2005, 39, 1144-1151. (26) Chen, Y. Z.; Shah, N.; Huggins, F. E.; Huffman, G. P.; Dozier, A. J. Microsc. 2005, 217 (3), 225-234.

1484 Energy & Fuels, Vol. 20, No. 4, 2006

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Table 1. Proximate Property of Bituminous Coals Used in This Study, wt % coal

fixed carbon

volatile matter

ash

fuel ratio

JCh DT WFG ZhJ JZh Yzh

44.6 58.4 55.6 47.1 48.4 43.8

13.3 30.8 33.3 28.6 44.4 44.4

42.11 10.81 11.11 24.32 7.14 11.76

3.34 1.90 1.67 1.65 1.09 0.99

Table 2. Composition of LTA Ashes, wt %

SiO2 Al2O3 CaO Fe2O3 SO3 MgO K2O Na2O P2O5 TiO2 MnO NiO ZnO CuO BaO Cl

JCh

DT

WFG

ZhJ

JZh

YZh

45.80 31.50 5.36 6.30 4.17 0.64 2.90 0.76 0.38 1.47 0.10 0.02 0.03 0.05 0.14 0.04

33.90 19.40 4.78 17.40 18.40 0.61 1.72 0.47 1.20 1.16 0.05 0.06 0.09 0.15 0.00 0.17

37.20 25.90 3.12 9.87 14.20 0.72 1.27 0.27 0.49 4.96 0.06 0.08 0.12 0.12 0.22 0.32

45.70 30.50 6.16 8.23 3.61 0.58 0.95 0.41 0.49 2.65 0.08 0.03 0.02 0.04 0.13 0.09

24.60 14.20 10.50 21.40 24.00 1.36 1.25 0.55 0.37 1.15 0.12 0.06 0.06 0.04 0.00 0.00

14.60 10.10 14.10 25.90 32.40 0.69 0.37 0.10 0.14 0.55 0.16 0.02 0.04 0.05 0.00 0.21

Table 3. Carbon Conversion for Studied Coals, wt % on Basis of Coal in muffle furnacea JCh DT WFG ZhJ JZh Yzh a

in DTF

1088 K

1473 K

1573 K

1723 K

13.33 30.77 33.33 28.57 44.44 44.44

14.95 23.07 35.35 54.21 60.95 57.96

15.02 54.65 37.79

17.68 62.45 45.48

According to a Japan Industrial Standard (JIS) method, M8812.

(VM) ranges much widely, which is only 13% in JCh coal but as high as 44% in both JZh and YZh coals. Accordingly, the fuel ratio, weight ratio of FC to VM, ranges widely too. It is 3.34 for JCh coal and 0.99 for YZh coal, implying the distinct differences among the pyrolysis behaviors of coals studied here. The ash content in coals also varies apparently with coal type. The ash composition, as measured by XRF, is shown in Table 2. SiO2 and Al2O3 are the most prevalent except in the case of YZh coal where much SO3 is found. Contents of the other major oxides, including CaO, Fe2O3, P2O5, K2O, and Na2O, also vary greatly with coal type. Coal Pyrolysis Procedure, CVM Collection, and Characterization. Coal pyrolysis was carried out in pure N2 in a laboratoryscale drop tube furnace, whose configuration has been described in detail elsewhere.27 Temperature of the DTF wall was set at 1473, 1573, and 1723 K. For coal particles in DTF, the computational fluid dynamics (CFD) simulation estimated that their temperature is around 1000∼1400 K in the case that DTF wall temperature is 1473 K, which is about 73∼473 K lower than the set temperature. Under given conditions, all the VM in raw coals were released as shown in Table 3. CVM in this study was isokinetically collected by a sampling probe with continuous water-cooling and N2-quenching inside. The hot gas from DTF was quenched at a rate of about 5000 K/s in the sampling probe so that all the subsequent reactions were prohibited. After passing through the probe, the CVM particles were further diluted with N2 and introduced into a LPI for size segregation. LPI used here consists of 13 stages, which have the aerodynamic (27) Zhang, L.; Sato, A.; Ninomiya, Y. Fuel 2002, 81, 1499-1508.

Figure 2. Theoretical vaporization degree of inorganic metals within DT coal matrix.

diameters from 12.1 to 0.03 µm. Teflon filters were used for collection of CVM particles since they contain few contaminants except F. To avoid the particles bouncing off the filters, CVM was collected at a very short collection time, which is at most 10 min. The particle-laden Teflon filters were first analyzed by XRF (RIX 2100 of Rigaku) to quantify the contents of both C and inorganic elements in each size of CVM. Second, the presence of inorganic elements was observed by both SEM (JEM5600, JEOL) and highresolution transmission electron microscopy (HRTEM, JEM2100F, JEOL). Both of them are coupled with EDS for quantification of inorganic elements. For SEM-EDS analysis, a small piece of particle-laden filter was cut and put on the SEM observation stub, which was then observed directly without carbon coating. On the other hand, for TEM observation, half the particle-laden Telfon filter was cut and ultrasonicated in acetone. Several drops of the dissolved sample were then transferred onto copper TEM grids. After drying in a vacuum chamber, the grids were observed. Finally, the chemical forms of several inorganic elements were analyzed by XPS (ESCA-3300KM, Shimazu). For XPS analysis, small amount of the CVM particles was scraped from the filter, adhered to a piece of carbon type, and put on a cylindrical stub having a diameter of about 1.0 cm in its cross-section. The samples were analyzed under the voltage of 10 kV and the currency of 20 mA. The X-ray mode of magnesium was employed. Four major inorganic elements, S 2p, Si 2p, Fe 2p, and Na 1s were analyzed. C 1s at 284.0 eV was used for peak shift calibration. Thermodynamic Consideration on Vaporization of Inorganic Metals. At the given high temperature, a portion of inorganic minerals may vaporize. To elucidate its possibility, a theoretical calculation was conducted in the thermodynamic equilibrium viewpoint. A commercial software package, Factsage 5.2, was employed. Coal particles temperatures were set in a wide range from 900 to 1800 K. Considering the emission of reducing gases from coal pyrolysis, a reducing atmosphere of CO and N2 being balance was used as gas input. The solid input involves the elemental composition of included minerals in raw coals, which was quantified by computer-controlled SEM (CCSEM). The excluded minerals were not taken into consideration due to two reasons: (1) the inherent minerals including these excluded ones hardly vaporize in N2; (2) the excluded minerals are usually larger than 10.0 µm and difficult to vaporize due to the diffusion control and a short residence time of coal particles in DTF. The output involves three phases: ideal gas, liquid, and solid. Formation of slag was not included considering the very short residence time of samples in the furnace. Figure 2 shows the calculated vaporization of individual elements at the DTF wall temperature of 1473 K. Since coal particle is around 1000-1400 K in DTF, no elements except Na vaporize. It is noteworthy that the vaporization degree of Na as shown here was calculated from the viewpoint of thermodynamic equilibrium. The actual vaporization rate of Na should be far slower and even negligible in the short residence time.



Figure 3. Morphology of condensed VM aggregates (a) and its surface structure (b).

Figure 4. Typical EDS quantification results for the aggregate (a) and its surface (b).

Results and Discussion Formation of CVM at Wall Temperature of 1473 K and Presence of Inorganic Metals within It. At the wall temperature of 1473 K, CVM was mainly formed as aggregates having a fractal-like structure as shown in Figure 3a. The primary particle of it is around 20∼50 nm, suggestive of the condensation of liquid tar and/or its gaseous precursor. Moreover, the aggregates in CVM have a broad size distribution with two major peaks around 0.1 and 2.5 µm. Accordingly, CVM is considered to consist of two major fractions: some aggregates are less than 1.0 µm in diameter and the remainder is larger than this criteria size. The microtexture of CVM smaller than 1.0 µm was observed and illustrated in Figure 3b. Carbon was found to be amorphous and made up of randomly stacked graphic layers. The typical EDS spectra of both the aggregate (Figure 3a) and its surface (Figure 3b) exhibit a dominant carbon peak as shown in Figure 4, where the copper peaks are caused by the use of copper grid for TEM observation. There are also weak but discernible peaks for inorganic elements including sulfur, oxygen, silicon, potassium, iron, and chromium. Apparently, besides the dominance of carbon, the presence of inorganic elements in CVM cannot be overlooked. They disperse highly in the primary particles of

CVM; their formation precursors are most likely the organically bound metals in raw coals since the inorganic mineral phases seldom vaporize as explained in Figure 2. The further quantification of inorganic elements was conducted by XRF analysis and shown in Figure 5a. Consistent with the TEM-EDS analysis, S is the most prevalent, followed by Na, Si, Cl, Ca, Al, and others in the decreasing order. Not only volatile metals but also those refractory ones were found to have a relatively great amount. The CVM larger than 1.0 µm was however found to be a mixture of carbon aggregates with solid mineral particles as illustrated in Figure 6. The solid mineral particles are discernible as indicated by the arrow in this figure. Both EDS analysis of the selected particle and XRF quantification on the whole CVM showed the prevalence of silicon and aluminum (see Figure 5b). Amounts of the others are however fairly low, if not negligible. Silicon and aluminum have the molar ratio near 1.0 in this fraction, which is identical to the inherent kaolinite (Al2O3‚ 2SiO2‚H2O). Therefore, the included kaolinite in parent coals is its formation source. In other words, the larger CVM contains few metals relevant to their organically bound fraction in parent coals. Mode of Occurrence of Inorganic Elements in CVM Smaller Than 1.0 µm. The above results indicated the abundance of sulfur in CVM smaller than 1.0 µm. Its chemical forms were further analyzed by XPS and shown in Figure 7. A relatively sharp peak was formed at 163.8 eV, which is attributed to the organic sulfur. Organic sulfur is also the major form for the presence of sulfur in CVM; it should be generated from the release of a portion of the organic sulfur in parent coals. The amount of sulfur in CVM smaller than 1.0 µm was further plotted versus inherent sulfur content in parent coals and displayed in Figure 8. Except for the anthracite JCh coal (the point having sulfur content of ca. 1.0 wt %), a relatively strong linear relationship was obtained, i.e., with the content of inherent sulfur in bituminous coal increased; its amount in CVM was improved proportionally. Coal rank affects the transformation of inherent sulfur too. The chemical forms of two refractory elements, silicon and iron, were also analyzed by XPS and illustrated in Figure 9. As expected, the organically bound fraction of these two metals is prevalent. The organic Si covers a wide range, suggestive of its complex structures in the parent coals as well as in the CVM. Regarding Fe, its organic species, (Fe(C5H4COOH)2), was found at 708.4 eV. Additionally, a portion of it is in the form of oxide/ hydroxide (Fe2O3/Fe(OH)O at 711.0 eV and Fe3O4/Fe(OH)O at 725.4 eV). Since the iron-bearing inorganic species can hardly


Zhang et al.

Figure 5. Normalized composition of inorganic metals in two fractions of condensed VM (a for sizes < 1 µm and (b) for sizes g 1 µm).

Figure 8. Influence of inherent S on the occurrence of S in condensed VM < 1 µm.

Figure 6. Mixture of inherent fine aluminosilicate (around 1.0 µm) with a portion of condensed VM.

Figure 7. Mode of occurrence of S in condensed VM < 1 µm.

vaporize, presence of the above-listed oxides/hydroxides indicates the secondary reactions for organically bound metals in CVM. With the fragmentation of carboxylates in coal, a portion of the inorganic metals associated with them may be liberated and change into free radicals containing Fe. The resultant radicals are not stable and possibly subsequently react with the oxygen-containing radicals such as OH- and COO- in emitted VM to form their oxides or hydroxides. A HRTEM observation on the highly dispersed iron in carbonaceous matrix and its fast Fourier Transform (FFT) in reference26 also identified the existence of magnetite, which is consistent with the present study. Amounts of Inorganic Elements Except Sulfur in CVM Smaller Than 1.0 µm. The amounts of individual inorganic

metals in CVM smaller than 1.0 µm were quantified by XRF and shown in Figure 10, where sulfur was excluded. The unit for Y-axis was expressed as the weight percentage of individual metal on the basis of total coal ash. The results suggest the variation of metallic amounts with both coal type and metallic type. In the anthracite JCh coal, except silicon, few of the other metals were found associated with the coal matrix. On the other hand, in the studied bituminous coals, silicon is the most prevalent, ranging from 50 to 330 µg/g of coal ash. The other three refractory metals (aluminum, calcium and iron) are also detectable, having the amount around 20-50 µg/g of coal ash. No magnesium was found associated with the organic matrix. The volatile metals were found to have an amount less than 100 µg/g of ash. Among them, two halogen elements, chlorine and fluorine, are relatively prevalent. As discussed in the Introduction section, the organically bound metals, especially those in the form of chelate coordination complexes, cannot be quantified by the traditional chemical extraction method using the weak acids such as ammonium acetate. By using a polar solvent to extract two Chinese bituminous coals at 350 °C, the study in ref 11 quantified the amounts of four refractory metals associated with coal matrix. For silicon, its amount is 48 and 90 µg/g of coal ash in the extracted liquid of NT and EN coal, respectively. It has the same order of magnitude as found in the present study. Aluminum is 22∼33 µg/g of coal ash, iron is 52∼102 µg/g of ash, and calcium is 12∼102 µg/g of coal ash. All of them are comparable to the results in Figure 10. Coal type plays an important role in the amounts of these four metals in CVM, i.e., their amounts associated with the


Figure 9. XPS charts for Si (a) and Fe (b) in the condensed VM < 1 µm.

Figure 10. Amount of individual elements in condensed VM < 1 µm.

organic carbonaceous matrix in parent coals. As shown in Figure 10, the amounts of them increase in the turn of JCh < WFG < ZhJ < DT < JZh < YZh, having the reverse trend to that of fuel ratio of parent coals as listed in Table 1. This finding implies the enrichment of organically bound metals in VM of raw coals since VM generally contains more oxygen as compared to FC. Less phosphorus was found in the CVM, suggesting the lower content of organically bound fraction of this element in the studied coals. CCSEM quantification on the coal mineralogical properties indicates that phosphorus is in the forms of apatite


(Ca3(PO4)2) and a complex mixture of phosphate with aluminosilicate. Among the other highly volatile elements, the halogen elements have the highest contents. The emission of fluorine varies obviously with coal fuel ratio, indicating that this element in CVM is likely associated with the carbonaceous matrix. On the other hand, the other metals are not affected by coal type as expected. The relatively greater amounts of sodium and chlorine in CVM emitted from WFG coal imply that, besides the organically bound fraction, NaCl is likely another precursor compound deposited on CVM. As to the originality of NaCl, it is possibly present in the parent coals or formed during coal pyrolysis, i.e., chlorine may promote the vaporization of Nabearing inorganic compounds during coal pyrolysis. To prove this hypothesis, a further study was carried out by pyrolyzing three coals at elevated temperatures. The coals were selected including JCh coal having the highest fuel ratio and lowest VM, DT and WFG coals having the similar fuel ratio and medium amount of VM. Figure 11 shows the amounts of alkali elements and halogen in CVM smaller than 1.0 µm. In the case of JCh coal, no change was found for these four elements with temperature increasing, further suggesting the few amount of organically bound metals in this coal. In the case of DT coal, the amounts of sodium, potassium, and chlorine were increased somewhat with increasing reaction temperature. Fluorine was increased apparently. In the case of WFG coal, four elements behaved in the same trend, their amounts were improved exponentially with reaction temperature increasing. XPS analysis of the Na-bearing species in CVM, as shown in Figure 12, confirmed the prevalence of NaCl at the wall temperature of 1723 K. Since the vaporization of NaCl initiates at a temperature lower than 823 K,28 less of it but more organically bound Na found at 1473K proves that there is no NaCl originally existing in the parent coals. At higher temperatures, Na and Cl vaporize individually during coal pyrolysis; the resultant free radicals of Na and Cl react with each other to form NaCl depositing on the surface of CVM. This finding is also consistent with what reported elsewhere.29 Additionally, the promotion effect of chlorine is influenced by coal type too. Compared to WFG coal, DT coal contains less chlorine (see Table 2), its promotion effect on the vaporization of sodium is not obvious (see Figure 11). Contribution of Elements in CVM Smaller Than 1.0 µm to PM0.1 Formed after Coal Combustion. As stated before, the emitted VM combusts as soon as it meets oxygen in the gas atmosphere. Meanwhile the inorganic metals within it likely undergo oxidation and coagulation to form ultrafine particulates (PM0.1 having the diameter smaller than 0.1 µm) after the complete combustion of coal char. In this sense, the content of individual inorganic elements in CVM smaller than 1.0 µm is compared with them in PM0.1 and shown in Figure 13, panels a and b are for the bituminous DT and the anthracite JCh coal, respectively. For coal combustion, the DTF wall temperature was set at 1473 K. In the case of bituminous DT coal, S in CVM is reduced greatly after char combustion, since the majority of it transforms into the gaseous SO2. The remainder of it is present as sulfate in PM0.1 formed after the coal combustion.6,9,30 Essentially all (28) Tsubouchi, N.; Ohtsuka, S.; Hashimoto, H.; et al. Energy Fuels 2004, 18, 1605-1606. (29) Li, C. Z. Recent advances in the understanding of the pyrolysis and gasification behaviour of Victorian Brown coal. Proceedings of the 2005 International Conference on Coal Science and Technology, ICCS &T, Okinawa, Japan, October 9-14, 2005; CD-ROM. (30) Graham, K.; Sarofim, A. J. Air Waste Manage. Assoc. 1998, 48, 106-112.


Zhang et al.

Figure 11. Vaporization of alkali elements and halogens with increasing of temperature: (a) sodium, (b) potassium, (c) fluorine, and (d) chlorine.

Figure 13. Occurrence of individual metals in condensed VM < 1 µm and ultrafine PM0.1 formed after coal combustion (a) for DT coal and (b) for JCh coal. Figure 12. XPS charts for sodium in condensed VM (< 1 µm) emitted at the wall temperatures of 1473 K (a) and 1723 K (b).

the sulfur in raw coals vaporizes to form gaseous SO2, meanwhile a portion of it is catalyzed to form SO3, which can react with the metallic vapors to forms sulfates subsequently. For the other elements except Cl, they have nearly similar contents in both CVM and PM0.1, supporting the fact that the organically bound fraction of inorganic metals is the major source for their formation in PM0.1 emitted at low temperature.

As to the behavior of Cl, it disappears after char combustion, suggesting that almost all of it in CVM changes into HCl and vaporizes as a gaseous pollutant. In addition, it is noteworthy that the formed NaCl in CVM likely changes into the other species during char combustion. As a result, the gaseous HCl can be released too. The inorganic metals behaved differently in the case of the anthracite JCh coal. As discussed earlier, the anthracite coal has a lack of organically bound metals within it. The vaporization of inorganic phases during char combustion is therefore


the key route for formation of metallic vapors. Furthermore, since the reaction conditions are rather mild, a relatively low temperature, and a short residence time, PM0.1 and the inorganic metals within it are formed having the low contents in the case of JCh coal. Conclusions The present study leads to the following conclusions: (1) At the DTF wall temperature of 1473 K, the condensed VM from coal pyrolysis contains inorganic metals, which are mainly in form of organic species, reflecting the presence of organically bound metals in studied coals. (2) The amount of individual metal in CVM varies greatly with both coal type and metallic type. Fewer organic metals are present in the anthracite coal due to the lack of oxygencontaining functions in this coal. For the bituminous coals, sulfur is the most prevalent in CVM, which is mainly caused from the decomposition of organically bound sulfur in raw coals. Silicon is found ranging from 50 to 330 µg/g of coal ash, the others are less than 100 µg/g of coal ash. As the VM content in raw coal increases, the organically bound fraction of metals is found more.


(3) A portion of sodium is vaporized in the form of NaCl, especially at higher temperature. The inorganic species containing Na or Cl vaporize individually during coal pyrolysis. The resultant free radicals of Na and Cl likely react with each other to form NaCl depositing on CVM. (4) During the combustion of bituminous coals at the DTF wall temperature of 1473 K, the emitted PM0.1 is mainly formed from the organically metals in CVM. During the combustion of emitted VM, the organically bound metals inside likely undergo oxidation and coagulation to form PM0.1 eventually. On the other hand, for the anthracite coal, its PM0.1 amount is rather low since there are less organically bound metals within it. Acknowledgment. The authors are thankful for the Grant-inaid for Scientific Research on Priority Areas (B), 17310054, Ministry of Education, Science, Sports and Technology, Japan, and the Steel Industry Foundation for the Advancement of Environmental Protection Technology for the financial support. We are also grateful to Mr. Masunori Kawamura of the analytical center of Chubu University for his assistance in the use of TEM and EDS analysis. Lian Zhang thanks the Japan Society for Promotion of Science (JSPS) for a postdoctoral research fellowship. EF050429H

Occurrence of Inorganic Elements in Condensed Volatile Matter

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