Interactions among Inherent Minerals during Coal Combustion and

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Energy & Fuels 2007, 21, 756-765

Interactions among Inherent Minerals during Coal Combustion and Their Impacts on the Emission of PM10. 1. Emission of Micrometer-Sized Particles Qunying Wang,† Lian Zhang,†,‡ Atsushi Sato,† Yoshihiko Ninomiya,*,† and Toru Yamashita§ Department of Applied Chemistry, Chubu UniVersity, 1200, Matsumoto-cho, Kasugai, Aichi, Japan, and Coal and EnVironment Research Laboratory, Industrial Energy Department, Idemitsu Kosan Co., 3-1 Nakasode, Sodegaura, Chiba, Japan ReceiVed July 2, 2006. ReVised Manuscript ReceiVed NoVember 17, 2006

Four pulverized bituminous coals, possessing nearly identical organic properties, were burnt in a laboratoryscale drop tube furnace to investigate the formation of PM10 (particulate matter less than 10 µm in diameter) and the influence of coal mineralogical properties on its emission. Coal combustion was conducted at 1450 °C in air. A residence time of about 3 s was adopted. During combustion, PM10 was collected by the combination of a cyclone and a low-pressure impactor, and divided into two fractions: micrometer particulates g1 µm (PM1+) and submicrometer ones (PM1). These two fractions have been discussed in the present paper and the next one, respectively. Regarding the formation of PM1+, it varies with the coal mineralogical property greatly. The total amounts of four refractory elements, Al, Si, Ca, and Fe, account for more than 90 wt % in PM1+. Accordingly, PM1+ mainly consists of quartz, Al-silicate, and Ca/Fe Al-silicates. Two of the coals tested in this study, lean in Ca and Fe, release about 40 mg/g-coal of PM1+, which is about twice those emitted from the other two coals rich in Ca and Fe. This is due to the interactions between included minerals (mainly Al-silicates) and excluded ones including calcite and pyrite. In the case of coals lean in Ca and Fe, less the inherent Si and Al (regardless of their association with the carbonaceous matrix) coalesce, and hence, they transfer into PM1+ directly. Correspondingly, the amount of PM1+ formed is similar to that of inherent minerals smaller than 10 µm in raw coals. On the contrary, in the case of coals rich in Ca and Fe (mainly existing as excluded particles as found in this study), the inherent calcite and pyrite initially decompose to form finer particles, which then collide with Al-silicates released from the coal char to form the low-melting compounds and sequentially promote the coalescence of Al-silicates. As a result, less PM1+ is formed. This is further evidenced during the combustion of coal density fractions.

Introduction It is widely acknowledged that pulverized coal combustion is still a major source of electrical power generation in the foreseeable future. The pollutants formed during coal combustion are however facing stringent emission regulations due to the increasing public concern about their adverse health effects.1-3 Of these, the emission of particulate matter (PM) less than 10 µm in diameter, namely PM10, is very critical,4-6 * Corresponding author. E-mail: [email protected]. Tel.: 81568-51-9178. Fax: 81-56851-1499. † Chubu University. ‡ Present address: Energy Technology Research Institute, National Institute of Advanced Industrial Science and Technology (AIST), 16-1, Onogawa, Tsukuba, Japan. § Idemitsu Kosan Co. (1) Nalbandian, H.; Carpenter, A. M. Prospects for upgrading coal-fired power plants; IEA coal Research: London, United Kingdom, Dec 2000. (2) Soud, H. N.; Wu, Z. East Asia-air pollution control and coal-fired power generation; IEA coal research: London, United Kingdom, June 1998. (3) England, G. C.; Zielinska, B.; Loss, K.; et al. Characterizing PM2.5 emission profiles for stationary sources: comparison of traditional and dilution sampling techniques. Fuel Process. Technol. 2000, 65-66, 177188. (4) Smith, K. R.; Veranth, J. M.; Hu, A. A.; Lighty, J. S.; Aust, A. E. Interleukin-8 levels in human lung epithelial cells are increased in response to coal fly ash and vary with the bioavailability of iron, as a function of particle Size and source of coal. Chem. Res. Toxicol. 2000, 13, 118-125. (5) Sloss, L. L. The importance of PM10/2.5 emissions; IEA Clean Coal Centre: London, October 2004.

especially in most of the developing countries such as China, where coal combustion-generated PM accounts for about onethird of the annual total PM emission.1 After the complete combustion of coal, PM10 emission is merely caused by the inherent mineral matter in the parent coals. Its properties as formed are usually heterogeneous and dependent on the particulate size.7,8 The submicrometer particulates, less than 1.0 µm in diameter (PM1), are mainly formed through the vaporization-condensation pathway.9,10 Enrichment of the volatile elements is therefore the principal characteristic of this fraction. On the other hand, those formed larger than 1.0 µm (6) Smith, I. M.; Sloss, L. L. PM10/2.5-emissions and effects; IEA coal research: London, October 1998. (7) Zhang, L.; Ninomiya, Y. Emission of Particulate Matter from Coal Combustion and Its Correlation With Coal Mineral Properties. Fuel 2006, 85, 194-203. (8) Buhre, B. J. P.; Hinkley, J. T.; Gupta, R. P.; Nelson, P. F.; Wall, T. F. Fine ash formation during combustion of pulverised coal-coal prperty impacts. Fuel 2006, 85, 185-193. (9) Zhang, L.; Ninomiya, Y.; Yamashita, T. Formation of submicron particulate matter (PM1) during coal combustion and influence of reaction temperature. Fuel 2006, 85, 1446-1457. (10) Lockwood, F. C.; Yousif, S. A model for the particulate matter enrichment with toxic metals in solid fuel flames. Fuel Process. Technol. 2000, 65-66, 439-457. (11) Chen, Y.; Shah, N.; Huggins, F. E.; Huffman, G. P.; et al. Investigation of primary fine particulate matter from coal combustion by computer-controlled scanning electron microscopy. Fuel Process. Technol. 2004, 85, 743-761.

10.1021/ef0603075 CCC: $37.00 © 2007 American Chemical Society Published on Web 01/23/2007

Impact of Inherent Minerals on PM10 Emissions 1

and termed PM1+, hereafter, are mainly made up of the species containing refractory elements such as Al, Si, Ca, Fe, etc.7,11-13 Since these elements, especially the former two, are commonly the most prevalent inorganic constituents in coals,14-17 PM1+ formation is largely influenced by the transformation behaviors of inherent coal minerals at high temperature. The mechanisms governing its formation, as well as that of the above-mentioned PM1, are rather complex and the investigations on them are far from completion. The inherent Si and Al are mostly in forms of quartz (SiO2) and kaolinite (Al2Si2O5(OH)4).17-19 The latter species prefers to decompose into a spinal-like γ-aluminum and mullite (Al6Si2O13) above 827 °C. Mullite is as stable as quartz, having a melting poit higher than 1500 °C.20 Ca and Fe are mainly present as carbonate (calcite, dolomite, and siderite), sulfates, and sulfide such as pyrite.16 Decomposition is the primary behavior of these compounds. The resultant CaO and Fe2O3 can reduce the melting points of quartz and mullite via the formation of calcium Alsilicate and iron Al-silicate or the more complex compounds containing them all.18,21,22 Their interactions are greatly dependent on their association with the organic carbonaceous matrix, being included or excluded.18,23 Both the experimental and modeling studies have suggested that18,24-27 the included minerals tend to coalesce together. Their coalescence degree depends on the property of char formed after coal pyrolysis as well as the coal particle size.28,29 On the other (12) Goodarzi, F. Morphology and chemistry of fine particles emitted from a Canadian coal-fired power plant. Fuel 2006, 85, 273-280. (13) Zhang, C.; Yao, Q.; Sun, J. Characteristics of particulate mater from emissions of four typical coal-fired power plants in China. Fuel Process. Technol. 2005, 86, 757-768. (14) Vassilev, S.; Tascon, J. Methods for characterization of inorganic and mineral matter in coal: a critical overview. Energy Fuels 2003, 17, 271-281. (15) Miller, S. F.; Schobert, H. H. Effect of mineral matter particle size on ash particle size distribution during pilot-scale combustion of pulverized coal and coal-water slurry fuels. Energy Fuels 1993, 7, 532-541. (16) Shirazi, A. R.; Bortin, O.; Eklund, L.; Lindqvist, O. The impact of mineral matter in coal on its combustion, and a new approach to the determination of the calorific value of coal. Fuel 1995, 74, 247-251. (17) Ollila, H.; Daavitsainen, J.; Nuutinen, L.; et al. Mineral classification revisited: use of quasiternary diagrams in the visualization of compositional distribution of inorganic material in coal. Energy Fuels 2006, 20, 591595. (18) Zhang, L.; Wang, Q.; Ninomiya, Y.; Yamashita, T. Interactions among inherent minerals during coal combustion and their impacts on the emission of PM10. 2. Emission of submicrometer particulates e1 µm, Energy Fuels 2007, 766-777. (19) Smoot, L. D., ed. Fundamentals of coal combustion for clean and efficient use; Elsevier: London, 1993; pp 299-373. (20) Groves, S. J.; Williamson, J.; Sanyal, A. Decomposition of pyrite during pulverized coal combustion. Fuel 1987, 66, 461-466. (21) Miller, S. F.; Schobert, H. Effect of the occurrence and composition of silicate and aluminosilicate compounds on ash formation in pilot-scale combustion of pulverized coal and coal-water slurry fuels. Energy Fuels 1994, 8, 1197-1207. (22) Miller, S. F.; Schobert, H. Effect of the occurrence and modes of incorporation of alkalis, alkaline earth elements, and sulfur on ash formation in pilot-scale combustion of Beulah pulverized coal and coal-water slurry fuel. Energy Fuels 1994, 8, 1208-1216. (23) Senior, C.; Zeng, T.; Che, J.; Ames, M.; et al. Distribution of trace elements in selected pulverized coals as a function of particle size and density. Fuel Process. Technol. 2000, 63, 215-241. (24) Russell, N. V.; Me´ndez, L. B.; Wigley, F.; Williamson, J. Ash deposition of a Spanish anthracite: effects of included and excluded mineral matter. Fuel 2002, 81 (5), 657-663. (25) Yan, L.; Gupta, R. P.; Wall, T. F. A mathematical model of ash formation during pulverized coal combustion. Fuel 2002, 337-344. (26) Yamashita, T.; Tominaga, H.; Orimoto, M.; Asahiro, N. Modelling of ash formation behavior during pulverized coal combustion. IFRF combust. J. 2000, August, article no. 200008; ISSN 1562-479X. (27) Liu, Y.; Gupta, R.; Sharma, A.; Wall, T.; et al. Mineral matterorganic matter association characterization by QEMSCAN and application in coal utilization. Fuel 2005, 84 (10), 1259-1267.

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hand, the excluded minerals mainly undergo fragmentation.25-27 With respect to the interaction between CaO/Fe2O3 and Alsilicates, only a few studies had been conducted by taking the mineral association into account30,31 or merely considering it as a reaction among included minerals. For instance, more than half the included pyrite/siderite was assumed to be incorporated into iron Al-silicate glass, while the excluded merely decomposes, having no contact with either included or excluded Alsilicate.32,33 This is, however, inconsistent with the fluxing promotion effect of added calcium/iron-based compounds during gasifying coals at high temperatures,34,35 which suggests that the inherent Al-silicates are partly scavenged by the added calcium/iron-based species (which can be considered as excluded particles in the mixture). In light of these contradictive findings, there are still many incomplete and little-understood mechanisms governing the behaviors of four refractory elements, Al, Si, Ca, and Fe during coal combustion and their transformation into PM1+ as well. This paper aims at elucidating the interactions among four major refractory elements during coal combustion and their transformation into PM1+. First, four bituminous coals, possessing almost identical organic properties, were tested in order to eliminate the influence of coal organic properties. Computercontrolled scanning electron microscopy (CCSEM) was used for quantifying the mineralogical properties of raw coals, including their composition and association (included/excluded) as well. PM1+ was also characterized by CCSEM to investigate the chemical species within it and its size-dependent properties. Second, two of the selected coals, with different calcium concentrations and almost the same amounts of the other inorganic elements, were gravimetrically segregated to separate the included and excluded minerals. The different density separations were combusted, and the emitted PM1+ was collected and compared with that released from the corresponding raw coals. Finally, the interactions between calcium/iron and Al-silicate were elucidated. Note that the formation of submicrometer particulate matter (PM1) is discussed in the next paper.18 Experimental Details Coal Properties. Four bituminous coals, namely, coals A, B1, B2, and C, were tested. Coals A, B1, and B2 were mined in Australia, and the latter two were from the same coalfield. Coal C was produced in China. All of the coal samples were pulverized to less than 125 µm and dried prior to use. (28) Wu, H.; Wall, T. F.; Gupta, R. P. Ash liberation from included minerals during combustion of pulverized coal: the relationship with char structure and burnout. Fuel 1999, 13, 1197-1202. (29) Ninomiya, Y.; Zhang, L.; Sato, A.; Dong, Z. Influence of coal particle size on particulate matter emission and its chemical species produced during coal combustion. Fuel Process. Technol. 2004, 85, 1065-1088. (30) Russell, N. V.; Wigley, F.; Williamson, J. The roles of line and iron oxide on the formation of ash and deposits in PF combustion. Fuel 2002, 81, 673-681. (31) Wu, Z. Fundamentals of pulVerized coal combustion; IEA clean coal center: London, UK, March 2005. (32) Mclennan, A. R.; Bryant, G.; Bailey, C.; Stanmore, B.; et al. Index for iron-based slagging for pulverized coal firing in oxidizing and reducing conditions. Energy Fuels 2000, 14, 349-354. (33) McLennan, A. R.; Bryant, G. W.; Stanmore, B. R.; Wall, T. F. Ash formation mechanisms during pf combustion in reducing conditions. Energy Fuels 2000, 14, 150-159. (34) Jak, E.; Degterov, S.; Hayes, P.; Pelton, A. Thermodynamic modeling of the system Al2O3-SiO2-CaO-FeO-Fe2O3 to predict the flux requirements for coal ash slags. Fuel 1998, 77 (1-2), 77-84. (35) Kondratiev, A.; Jak, E. Predicting coal ash slag flow characteristics (viscosity model for the Al2O3-CaO-FeO-SiO2 system). Fuel 2001, 80, 1989-2000.

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Table 1. Properties of Coal Samples Used in This Study coal type moisture volatile fixed carbon ash carbon hydrogen nitrogen sulfur oxygena a

A

B1

B2

C

4.50 26.60 58.70 10.20

3.00 33.20 54.50 9.30

Ultimate Analysis (wt %, daf) 82.09 83.29 84.22 5.17 4.96 4.77 1.94 1.96 1.92 0.56 0.99 0.59 10.24 8.80 8.50

82.03 5.27 1.37 0.41 10.92

Proximate Analysis (wt %) 4.00 4.50 33.00 28.50 51.40 55.00 11.60 12.00

By difference.

Table 2. Elemental Composition of low-temperature ash (LTA) Ash of Four Coals in weight percent coal type

A

B1

B2

C

SiO2 Al2O3 Fe2O3 CaO MgO Na2O K2O P2O5 SO3 TiO2 MnO Cr2O3 NiO

39.1 26.8 8.7 16.8 0.53 0.01 0.28 0.8 3.6 2.7 0.07 0.05 0.1

37.7 28.6 8.6 14.2 1.0 0.20 0.49 2.9 4.4 1.6 0.10 0.02 0.01

45.3 30.8 9.0 5.2 0.95 0.22 0.59 2.4 3.6 1.6 0.06 0.02 0.01

41.9 34.5 4.0 6.7 2.8 1.1 0.49 0.74 3.4 1.6 0.08 0.0 0.07

These four coals exhibit similar properties, as shown in Table 1. On an as-received basis, the volatile matter (VM) has a content around 26.6-33.2 wt %, the fixed carbon (FC) is around 51.454.5 wt %, and ash is around 9.3-12.0 wt %. Only slight differences were found among the tested coals. Similarly, the ultimate elemental compositions of four coals are almost identical, especially in the cases of coal B1 and B2 collected from the same coalfield, further conforming the similarity of the organic structure of coals tested here. The elemental compositions of coal ashes are shown in Table 2. In term of oxide, SiO2 and Al2O3 are the most prevalent. They are mainly in the forms of kaolinite and quartz as detected by CCSEM. Fe2O3 and CaO are the next abundant compositions, and their concentrations vary with coals obviously. Fe2O3 is around 8.588.97 wt % in the ashes of coals A, B1, and B2, while it is only 3.98% in the ash of coal C. CaO is 16.8 wt % in the ash of coal A, compared to about 14.2%, 5.21%, and 6.71% in the ashes of coal B1, B2, and C, respectively. Regarding the chemical species of these two elements and their association with the carbonaceous matrix, Fe mainly partitions between pyrite and iron oxide. Their amounts vary considerably with coals. About half of the Fe is present as excluded particles in the four coals. Ca is mostly in the form of calcite. The amount of included Ca is almost the same among the four coals; meanwhile the excluded Ca varies with coals, obviously. Coals A and B1 are rich in excluded Ca, especially calcite, while coals B2 and C are lean in it. The cumulative particle size distribution (CPSD) of inherent minerals (by CCSEM) is illustrated in Figure 1. Coals A and B1 are rich in the fine mineral particles having a geometric mean diameter around 3.0 µm. The amount of particles 10, Ca < 5, Fe < 5, S < 5 Si > 10, Ca > 5, Fe < 5, S < 5 Si > 10, Ca < 5, Fe > 5, S < 5 Ca > 20, Si < 10, Al < 5, Fe < 5 Fe > 20, Si < 10, Al < 5, Ca < 5

denotes the ash particles less than 1.0 µm in diameter, which are totally collected by the LPI. Meanwhile, PM1+ represents those ranging from 1.0 to 10.0 µm. With respect to the mass balance for the major (Al, Si, Ca, Fe) and minor elements (Mg, Na, K, P, Mn, and Ni), the former elements have a deviation error less than 5%, since almost all of them were captured by cyclone and the LPI. The latter ones have a deviation error around 20% or even a little higher, which is mainly caused by the characterization instruments. PM1+ Characterization. The elemental composition of individual size in PM1+ was first quantified by X-ray florescence (XRF, RIX 2100 of Rigaku). Second, CCSEM (JEM5600, JEOL) was used for speciation. Only the cyclone-collected part was characterized. The ash pellet preparation and CCSEM characterization procedure are identical with that reported elsewhere.11,36,37 In order to quantify the distribution of four major elements, the novel mineral categories were developed and listed in Table 5. In brief, Al-Si refers to those containing more than 95 wt % Si and/or Al, which includes quartz, alumina, montmorillonite, and kaolinite/mullite. Ca-Al-Si denotes the relationship between calcium and Al-silicates, including both calcium silicate and calcium Al-silicate. So does Fe-Al-Si. Ca-rich is made up of calcite (CaCO3)/lime (CaO), dolomite (CaMg(CO3)2), and other minor species such as apatite (Ca3(PO4)2). Fe-rich includes pyrite (FeS2), pyrrhotite (FeS), and siderite (FeCO3) in raw coal, meanwhile it mainly refers to iron oxide in PM1+.

Figure 2. Particle size distribution of PM10 emitted from the combustion of four parent coals (a) for the total PM10 and (b) for PM1.

Results and Discussion PSD of PM10 and Concentrations of PM1+ and PM1. The PSDs of PM10 are first introduced and shown in Figure 2a. Regardless of coal type, the PM10 emitted from combustion is rich in the particulates larger than 1.0 µm, having a similar size range to the inherent discrete minerals. Additionally, it is obvious that coals B2 and C produce more PM1+ than the other two coals. Clearly, the influence of the coal sample is significant. PM1 has a multimode size distribution as amplified in Figure 2b. It roughly has two major fractions: e0.1 and >0.1 µm. (37) Yu, D.; Xu, M.; Zhang, L.; Yao, H.; Wang, Q.; and Ninomiya, Y. Computer-controlled scanning electron microscopy (CCSEM) investigation on the heterogeneous nature of mineral matter in six typical Chinese coals. Energy Fuels 2007, 468-476.

Figure 3. Concentration of PM1+ and PM1 emitted from the combustion of four parent coals (a) for PM1+ and (b) for PM1.

This result is identical to what was reported before.38 The coal sample affects the amount of individual sizes in PM1 too, which is much more complicated compared to that of PM1+. The concentrations of two fractions, PM1+ and PM1, are shown in Figure 3a and b. Coals A and B1 release about 20 mg/g-coal of PM1+, and coals B2 and C release about 40 mg/ (38) Zhang, L.; Ninomiya, Y.; Yamashita, T. Formation of submicron particulate matter (PM1) during coal combustion and influence of reaction temperature. Fuel 2006, 85 (10-11), 1446-1457.


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Figure 4. Concentrations of inorganic constituents in PM1+ emitted from the combustion of four parent coals (other include the elements from MgO to NiO as listed in Tables 2 and 4).

Figure 5. Mineralogical composition of PM1+ emitted from the combustion of four parent coals (a) for coal A, (b) for coal B1, (c) for coal B2, and (d) for coal C.

g-coal. Since the amount of fine minerals 10 µm to reduce the amount of PM1+. The detailed mechanism is discussed below. Figure 3b shows that the amount of PM1 accounts for 0.81.2 wt % of the inherent coal ash, which is in a range as found in the combustion of other Chinese bituminous coals.38 The influence of the coal sample on PM1 is almost opposite to that of PM1+, i.e., coals A and B1 release more of the PM1 than the other two coals. Properties of PM1+ Emitted from Raw Coal Combustion. PM1+ contains four major refractory elements, Al, Si, Ca, and Fe, as demonstrated in Figure 4. With respect to weight percentage, SiO2 is the greatest, followed by Al2O3, CaO, Fe2O3, and minor and/or trace elements. The former two elements are the most abundant in the cases of coals C and B2, constituting >90% of the total PM1+. Their amounts are also great in these two cases since more of the PM1+ is released as shown in Figure 3a. In the cases of coals A and B1, more of CaO and Fe2O3 are however found. These four elements distribute among various species in PM1+. Their contents vary with coal type too. As evidenced in Figure 5, in the cases of coals A and B1, Al-Si is the most prevalent, accounting for about half the PM1+. Ca-Al-Si and

Figure 6. Morphologies of the major species in PM1+ emitted from the combustion of two parent coals. Panels a, c, and e are for Al-Si, Ca-Al-Si, and Fe-Al-Si in the case of coal A rich in Ca and Fe, respectively. Panels b, d, and f are for the respective species in the case of coal C lean in both Ca and Fe.

Figure 7. Comparison of Al-Si in PM1+ and that in parent coals and the degree of inherent Al-Si transformed into PM1+.

Fe-Al-Si are the next, having an abundance around 30-40 wt %. The remnant species are Ca-rich and Fe-rich, having relatively large amounts too. This distribution is in quantitative agreement with the oxide composition as shown in Figure 4. Clearly, besides the relatively pure Al-Si including quartz and mullite, and Ca/Fe oxides, the associations among these species are also dominant in these two cases. On the other hand, Al-Si covers more than 75% of the PM1+ for coals B2 and C, while less of the other species were found. In other words, quartz and mullite dominate their PM1+. This is also consistent with the prevalence of Al2O3 and SiO2 in Figure 4.



Figure 8. Comparison of major mineral species before and after the combustion of three coals (a and b) for those in PM1+ and coarse ash >10 µm emitted in the case of coal A, respectively; (c and d) for those in two corresponding fractions in the case of coal B1; (e and f) for those in two fractions in the case of coal B2; and (g and h) for those in two fractions in the case of coal B2.

The Al-Si formed exhibits an irregular structure, as illustrated in Figure 6a and b. A part of Al-Si is present as a single particle with a relatively porous surface, which is likely formed by the dehydration and/or decomposition of inherent kaolinite at high temperature. Additionally, Al-Si is also observed as having a rather sintered structure as shown in panel b of Figure 6. Less of the pores but more of the molten nuclei are found on its surface. Energy dispersive spectroscopy (EDS) analysis indicates that these nuclei contain a lesser amount of volatile Na/K or of

others such as Ca and Fe, even though Si and Al account for >90 wt %. This kind of Al-Si is likely generated by the capture of volatile Na/K or other minerals such as Ca/Fe elements by kaolinite/mullite during coal combustion.39,40 The resultant Al(39) Gale, T. K.; Wendt, J. O. L. High-temperature interactions between multiple-metals and kaolinite. Combust. Flame 2002, 131, 299-307. (40) Bool, L. E., III; Peterson, T. W.; Wendt, J. O. L. The partitioning of iron during the combustion of pulverized coal. Combust. Flame 1995, 100, 262-270.


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Figure 9. Morphologies of various Ca/Fe Al-silicates as present in the coarse ashes generated from the combustion of raw coal B1.

silicates usually have a lower melting point, and hence, they can melt and change into the round shape after they move into the cooling post-flame zone. The Ca-Al-Si formed has two typical structures as illustrated in panels c and d of Figure 6. A part of the Ca-Al-Si is in the form of large agglomerates, which are aggregated by dozens of fine particles with primary round particles around 1.0 µm. The remainder is however single round particles having a diameter of about 5.0 µm. Nevertheless, both types reflect the fully melting characteristic of Ca-Al-Si under the given conditions. This can be rationalized by the lowest melting point of 1250 °C for a eutectic compound containing them.37,41 The similar structures are observed for Fe-Al-Si too, showing the promotion effect of both Ca and Fe on the melting of Al-Si. With respect to the transformation routes of these compounds into PM1+, Al-Si might be formed by the “direct” liberation of inherent Al-Si, which undergoes few phase changes. The excluded Al-Si should transform preferentially since it is not affected by the char combustion. Ca-rich and Fe-rich are most possibly formed from the fragmentation of inherent excluded calcite and pyrite. On the other hand, Ca-Al-Si and Fe-Al-Si should be initially formed through the collision between inherent Al-Si and Ca or Fe, which then leads to the formation of liquid droplets with various sizes. The resultant fine droplets may collide with each other and further grow into larger particulates, meanwhile those coarse ones possibly directly transform into PM1+ after solidification. As to which kind of Al-Si (included/ excluded) reacts with Ca/Fe, it is further discussed below. Influence of Coal Type on the Transformation of Inherent Al-Si into PM1+. The above results have revealed the importance of the behavior of inherent Al-Si on the formation of PM1+. Figure 7 shows the comparison of the inherent Al-Si amount (e10 µm) with that in PM1+. The coarse Al-Si larger than 10 µm is not taken into consideration assuming that its decomposition does not occur in the furnace. This is reasonable due to the short residence time. The inherent Al-Si is divided into two fractions: included and excluded as quantified by CCSEM. As expected, in the two cases of coals A and B1, AlSi in their PM1+ has the amount equivalent to their excluded fraction in the raw coals. Meanwhile in the cases of coals B2 (41) Levin, E. M.; Robbins, C. R.; Mcmurdie, H. F. Phase diagrams for ceramists; The American Ceramic Society: Westerville, OH, 1964; Vol. 1, p 219.

Figure 10. Cumulative particle size distribution of total ash generated from the combustion of two raw coals and their float/sink fractions (a) for coal B1 and (b) for coal B2.

and C, the amounts of Al-Si in PM1+ are larger than their excluded fraction in raw coals. Clearly, a part of the included Al-Si transformed into PM1+ too. As to their transformation degree (see the right Y-axis in Figure 7), it is rather low and nearly zero in the cases of coals A and B1 but reaches about 40% and 60% for coals B2 and C, respectively. As stated before, the interaction between Al-Si and Ca/Fe might affect the transformation of inherent Al-Si. In this viewpoint, the modes of occurrence of these four elements are compared before and after the combustion of four parent coals. Both PM1+ and the coarse ash >10 µm are investigated, and the results are shown in Figure 8. For the simplification, only the results of coals B1 and B2 are discussed here since they merely have a difference in the content of Ca, as shown in Table 2. In the case of coal B1 rich in Ca, Al-Si is the most prevalent before combustion (see panel c), whose amount is however reduced to a level near its excluded fraction. The amounts of Ca-Al-Si and Fe-Al-Si are considerably increased, while that of included Ca-rich is reduced to a level far lower than its excluded fraction. Apparently, the majority of excluded Carich, together with included Al-Si, transforms into Ca-Al-Si in the coarse ash, as shown in panel d. Similar results are also obtained in the case of coal A rich in Ca (see panels a and b). In the case of coal B2 lean in Ca, a different phenomenon is observed (see panels e and f). Al-Si in PM1+ is only a little lower than that in the raw coal, indicating that besides the excluded particles, the Al-Si included fraction is also mainly transformed into PM1+. It rarely undergoes agglomeration since Al-Si in coarse ash has the similar content before and after combustion (see panel f). This validates the preciseness of our assumption on the decomposition of little inherent coarse AlSi, as mentioned above. Small amounts of Ca-Al-Si and FeAl-Si are also formed in PM1+, which are most likely caused by the interaction between included Ca and Al-Si in the burning



Figure 11. Amounts of mineral species in both PM1+ and coarse ashes (>10 µm) generated from the combustion of raw coals and their float/sink fractions (a and b) for PM1+ and the coarse ashes in the case of coal B1, respectively; (c and d) for those in the case of coal B2.

char, since this coal has a lack of excluded Ca compared to coal B1. Furthermore, considering the lesser amount of Ca-AlSi in the coarse ash, it can be deduced that the included Ca has little influence on the promotion of Al-Si agglomeration, which might be due to its low content compared to the excluded fraction as shown in both panels c and e. As suggested by the Ca-Al-Si thermodynamic diagram, the point having a weight ratio among included Al-Si and Ca usually falls in the zone of mullite, which rarely melts, and hence, the majority of included Al-Si can be directly released out with the proceeding of char burning. The same results are also found for the coal C lean in Ca (see panels g and h). The mechanisms governing the interactions between excluded Ca and inherent Al-Si can be deduced from the SEM observation on the coarse ashes as illustrated in Figure 9. The case of coal B1 has been studied. Two typical structures are found for the coarse ashes. As shown in panel a, the large agglomeration is found having the primary particle of about 2.0 µm deposited on its surface. EDS analysis on its interior (a1) suggests the prevalence of Si and Al, while the fine particles (a2 and a3) have comparable elemental compositions among four elements. Clearly, they should be formed through the reaction between Ca/Fe and Al-Si. The former elements are mostly generated from the decomposition of excluded calcite and pyrite since their included fraction is not sufficient. The latter two are mostly generated by the direct release of their included fraction as noted already. These elements likely collide with each other in the gas atmosphere, leading to the prevalence of fully molten CaAl-Si and Fe-Al-Si droplets, which subsequently adhere to the large Al-Si surface or agglomerate with each other to form coarse ashes >10 µm.

Another possibility for their interaction is the adhesion of fine Al-Si on the surface of large Ca as illustrated in panel b. The large particle is found to be rich in Ca, which is the unreacted CaO (b1). Its surface is coated with small particles around 2.0 µm, which are rich in Al-Si and have the appreciable amounts of Ca and Fe too. These fine particles should be generated from the direct release of included Al-Si or the mentioned Ca-Al-Si. Their collision with unreacted CaO likely inhibits their transformation into PM1+. Separation of Included and Excluded Minerals To Investigate PM1+ Emission. To prove the effect of excluded Ca and Fe, further studies were conducted by separating two coals, B1 and B2, into the float and sink fractions. The excluded Ca is mostly kept in the sink fractions, meanwhile almost all the included Al-Si is kept in the coal float fractions as introduced before. Both of the fractions were combusted under the same conditions as that of raw coals. The CPSD of PM1+ as well as the coarse ashes emitted from each raw coal and its two fractions are compared. As shown in Figure 10a, in the case of coal B1, its float fraction combustion produces a large amount of PM1+, which is about 50 mg/g-raw coal and much higher than that emitted from its parent coal. Combustion of the sink fraction only produces about 5 mg/graw coal PM1+, which mainly results from the fragmentation of excluded calcite or pyrite. The CPSD of PM1+ produced from raw coal falls between those from its two fractions, proving the capability of excluded Ca and/or Fe for capturing included Al-Si. PM1+ produced from the float fraction of coal B2 exhibits a similar profile to that emitted from its parent coal, while few of which is produced from the combustion of sink fraction. The


Wang et al.

Figure 12. Physical model for the transformation of four refractory elements, Al-Si, Ca, and Fe into PM1+ and the coarse ashes.

main reason for this is that this coal is lean in Ca. Its PM1+ is therefore not affected by coal separation. Additionally, it is noteworthy that PM1+ emitted from raw coal B2 shows a similar CPSD as well as a similar amount with that produced from the float fraction of coal B1. This further proves the significance of the excluded Ca and/or Fe on reducing the PM1+ emission. The chemical species in both PM1+ and the coarse ash emitted from the combustion of coal fractions were also characterized by CCSEM, as presented in Figure 11, where the results for corresponding raw coal was also added for comparison. As expected, for the combustion of coal B1 float fraction, more of the Al-Si is found in its PM1+, meanwhile less Ca-Al-Si is found in its coarse ash. In the cases of coal B2 and its float fraction, the similar chemical compositions are found in their PM1+ as well as in the coarse ash. The main reason for this is that there is no presence of excessive excluded Ca and/or Fe. Physical Model for the Influence of Ca and Fe on PM1+ Formation. In light of this study, the transformations involved in PM1+ formation are proposed and illustrated in Figure 12. For coal lean in Ca/Fe and having Si and Al >90 wt % in its ash, e.g., coals B2 and C, the inherent Al-Si rarely (regardless of its association) undergoes agglomeration due to its high melting point. Therefore, the formed PM1+ has similar amount and PSD to the inherent minerals less than 10 µm in diameter. Al-Si is the most prevalent in PM1+, which usually has irregular structure. On the other hand, for the coal rich in Ca and Fe, which are present as excluded particles, e.g., coals A and B1, the PM1+ formation procedure likely includes (1) decomposition of excluded Ca/Fe-bearing compounds such as calcite and pyrite to produce the finer “active” CaO and Fe2O3, (2) release of the included Al-Si (such as mullite evolved from kaolinite and quartz) from the burning char, and (3) reaction of CaO and/or Fe2O3 with released Al-Si to form melting Ca-Al-Si and Fe-

Figure 13. Influence of the total amounts of excluded Ca and Fe on the emission of PM1+ from combustion of coal at 1450 °C.

Al-Si. A few of the resultant compounds are possibly solidified into round particles and transform into PM1+, meanwhile (4) the majority of molten Ca-Al-Si and Fe-Al-Si coagulate with each other or adhere to the surface of unreacted Al-Si or CaO/ Fe2O3 and promote their agglomeration into the coarse ash >10 µm. As a result of these interactions, PM1+ emission is greatly dependent on the amounts of excluded Ca and Fe in raw coal, which is generalized in Figure 13. All the raw coals and their fractions are included in the figure. The X-axis refers to the amounts of excluded Ca and Fe in each sample, and the Y-axis denotes the amount of PM1+ emitted after coal combustion at 1450 °C. A relatively reverse linear relationship is found between these two variables, i.e., with the increase in the amounts of excluded Ca and Fe in raw coals or coal fractions, the amount of PM1+ can be reduced proportionally. This finding is useful for evaluating the addition of Ca/Fe-bearing sorbents


or blending different coals to reduce the PM1+ emission. Study of both of these issues is underway by us to achieve this goal. Conclusions The present study leads to the following conclusions. (1) For four bituminous coals, burning at 1450 °C in air, their PM1+ emitted possesses different properties. Combustion of two coals lean in Ca and/or Fe leads to about 40 mg/g-coal of PM1+, accounting for about 40-60 wt % of the inherent minerals. On the other hand, during combustion of the other two coals rich in Ca and/or Fe, their PM1+ concentration is about 20 mg/gcoal, accounting for about 20 wt % of the inherent minerals, which is only half of that emitted from coals B2 and C. The emission of PM1+ is largely dependent on the interactions among minerals during combustion, especially for the case of coals with a high content of Ca and/or Fe. (2) Al-Si in PM1+ is likely generated by the direct release of both included and excluded inherent Al-Si. Less included particles coalesce into coarse particles larger than 10 µm on the condition that there is no excessive Ca or Fe in the raw coal. As a result, the PM1+ emitted has an amount similar to that of inherent Al-Si less than 10 µm in raw coals.


(3) During the combustion of coal rich in excluded Ca and/ or Fe such as calcite and pyrite, these two compounds prefer to decompose initially. The resultant finer fragments are capable of reacting with Al-Si to form the molten Ca-Al-Si and Fe-AlSi. A few of the resultant two species transform into PM1+ after their solidification, meanwhile their majority preferentially coagulate with each other or adhere to the surface of unreacted Al-Si and Ca-rich/Fe-rich to form agglomerates larger than 10 µm. As a result, a low emission of PM1+ can be achieved. (4) Separation of excluded calcium and iron from raw coal enhances the emission of PM1+ greatly, proving their significance too. Acknowledgment. The financial support from Grant-in-aid 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 is appreciated. The graduated student of our laboratory, Mr. Takayuki Minami, is also appreciated for his assistance on the combustion experiments. L.Z. also wishes to thank the Japan Society for the Promotion of Science, JSPS, for the postdoctoral research fellowship. EF0603075

Interactions among Inherent Minerals during Coal Combustion and

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