Interactions among Inherent Minerals during Coal Combustion and

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Energy & Fuels 2007, 21, 766-777

Interactions among Inherent Minerals during Coal Combustion and Their Impacts on the Emission of PM10. 2. Emission of Submicrometer-Sized Particles Lian Zhang,†,‡ Qunying Wang,† 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 bituminous coals, possessing similar organic properties, were combusted in a lab-scale drop tube furnace (DTF) to investigate the emission of particulates less than 1.0 µm in diameter (PM1). The combustion conditions are as follows: 1450 °C, air as gas atmosphere, and a residence time of about 3 s. The results indicate that PM1 is formed by two pathways: metallic vaporization and direct liberation of inherent submicrometer particles. Excluded minerals play no role in its formation. The amount of PM1 as well as the concentrations of individual elements within it vary with coal considerably. Regarding the refractory elements, the transformation of Si and Al is likely affected negatively by the amount of excluded (CaO+Fe2O3) in the parent coals. Ca and Mg in PM1 are entirely contributed from their organically bound fraction and the inherent submicrometer particles containing these two elements. A small amount of coarse particles containing Fe (>1 µm) also possibly transforms into PM1 via vaporization or fragmentation. Its extent is dependent on the mode of occurrence of Fe in raw coals. For the volatile elements in PM1, S, P, Na, and K are the most prevalent; they are mainly in forms of sulfates, phosphates, and P2O5. The amount of S is determined by the presence of alkali elements and Ca in PM1, due to their interactions. The other three are, however, affected by their original modes of occurrence. P in a complicated form containing Si, Al, Ca/Fe, and P likely vaporizes readily due to its melting propensity. The water-soluble species containing Na and K preferentially vaporizes too. The vaporized heavy metals, Mn, Ni, and Cr studied here, possibly bind with the refractory Si and Fe in PM1, which is proven by both the variation of their concentrations with particulate size and a thermodynamic equilibrium consideration.

Introduction As stated in the first paper,1 coal combustion-generated particulate matter (PM) is a severe environmental pollutant in the atmosphere. Compared to coarse particulates, those having a diameter around 1.0 µm or less, namely, PM1, are particularly troublesome, since they are enriched with the toxic heavy metals and have a high probability of escaping common air pollution control devices.2,3 Once released into air, PM1 exhibits a lengthy residence time and is capable of deep pulmonary ingestion.4-6 * Corresponding author. E-mail: [email protected]. Tel.: 81568-51-9178. Fax: 81-56-851-1499. † Chubu University. ‡ Present address: Energy Technology Research Institute, National Institute of Advanced Industrial Science and Technology (AIST), 16-1, Onogawa, Tsukuba, Ibaraki 305-8569, Japan. § Idemitsu Kosan Co. (1) Wang, Q.; Zhang, L.; Sato, A.; Ninomiya, Y.; Yamashiya, T. Interactions among inherent minerals during coal combustion and their impacts on the emission of PM10. 1. Emission of micrometer-sized particles. Energy Fuels 2007, 756-765. (2) 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. (3) Chen, Y.; Shah, N.; Huggins, F. E.; Huffman, G. P.; Dozier, A. Characterization of ultrafine coal fly ash by energy-filtered TEM. J. Microsc. 2005, 217 (Part 3), 225-234. (4) Sarofim, A.; Lightly, J.; Eddings, E. Fine particles: health effects, characterization, mechanisms of formation, and modeling. Prepr. Pap.s Am. Chem. Soc., DiV. Fuel Chem. 2002, 47, 618-621. (5) Chen, Y.; Shah, N.; Huggins, F. E.; Huffman, G. P. Transmission electron microscopy investigation of ultrafine coal fly ash particles. EnViron. Sci. Technol. 2005, 39, 1144-1151.

Typically, PM1 is contributed from two major sources generated during coal combustion, of which the primarily significant one is the metallic vapors formed by the vaporization of inorganic elements. It tends to take place in the flame-near zone, where the temperature is sufficiently higher than the boiling points of almost all the volatile elements and even some refractory ones such as Si and Fe.2,7 Additionally, direct liberation of the inherent submicrometer minerals in the coal may contribute the particulates larger than 0.1-0.2 µm.8,9 It likely occurs during the fragmentation of pyrolyzing char and is composed of refractory aluminosilicate compounds.8 With regard to the secondary reactions of these two sources in the cooling post-flame zone, the metallic vapors either nucleate to form new species due to supercritical conditions or condense onto the preexisting particles due to super saturation.9-12 Correspondingly, PM1 formed typically has two portions: (6) Limbach, L. K.; Li, Y.; Grass, R. N.; Brunner, T. J.; et al. Oxide nanoparticle uptake in human lung fibroblasts: effects of particle size, agglomeration, and diffusion at low concentration. EnViron. Sci. Technol. 2005, 39, 9370-9376. (7) Senior, C.; Panagiotou, T.; Sarofim, A. F.; Helble, J. J. Formation of ultra-fine particulate matter from pulverized coal combustion. Prepr. Pap.sAm. Chem. Soc., DiV. Fuel Chem. 2000, 45 (1), 1-6. (8) 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. (9) Lin, W. Y.; Biswas, P. Metallic particle formation and growth dynamics during incineration. Combust. Sci. Technol. 1994, 101, 29-43. (10) McNallan, M. J.; Yurek, G. J.; Elliott, J. F. The formation of inorganic particulates by homogenous nucleation in gases produced by the combustion of coal. Combust. Flame 1981, 42, 45-60.

10.1021/ef060308x CCC: $37.00 © 2007 American Chemical Society Published on Web 01/23/2007

Impact of Inherent Minerals on PM10 Emissions 2

particulates smaller than 0.1 µm, namely, PM0.1, and those larger than 0.1 µm, namely, PM0.1+.8,13 The vaporization of inorganic elements is influenced by a lot of factors. It is primarily dependent on the reaction temperature.14 It is also significantly affected by the parent coal properties and coal combustion behavior in the furnace. The initial step of coal combustion, devolatilization/pyrolysis, releases the organically bound fraction of inorganic elements.15 Subsequently, vaporization of the inorganic compounds can further take place on the surface of pyrolyzing char, where a reducing atmosphere is formed regardless of the bulk oxygen concentration, facilitating the metallic vaporization compared to an oxidizing condition.16-18 On account of these considerations, metallic vaporization should be affected by the coal pyrolysis potential and the partial pressure of a reducing gas such as CO formed on the char surface as well as the char particle properties (e.g., its temperature profile, diameter, porosity, etc.). The modes of occurrence of inorganic elements in coal can affect their vaporization as well; it is however difficult to quantify, especially for those having minor or trace amounts. Qualitatively, for a given volatile element, its organically bound fraction can vaporize readily.8,15,19,20 The inorganic species, those soluble in water such as halides, sulfates, carbonates, and nitrates, can vaporize quickly too. On the other hand, its fraction associated with alumino-silicates or pyrite vaporizes slowly because of the diffusional resistance within the mineral particle hosting it.19-21 As far as the refractory element such or Si and Fe is concerned, only their oxides were considered to be prone to vaporize via their reduction by CO.7,20,22 Little discussion had been conducted by taking the influences of the other parameters into account, for instance, the association of inherent minerals with the carbonaceous matrix, the coalescence among different species, and so on. Furthermore, it is noteworthy that all the factors as noted here may exert their impacts in a combinational manner, causing it to be more difficult to quantify (11) Jenkins, N. T.; Eager, T. W. Submicron particle chemistry: vapor condensation analogous to liquid solidification. JOM. 2003, 55 (6), 4447. (12) Buckley, S. G.; Sawyer, R. F.; Koshland C. P. Measurements of lead vapor and particulate in flames and post-flame gases. Combust. Flame 2002, 128, 435-446. (13) 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., published online August 23, http://dx.doi.org/10.1016/ j.proci.2006.07.115. (14) Zhang, Y.; Kasai, E. Effect of chlorine on the vaporization behavior of zinc and lead during high temperature treatment of dust and fly ash. ISIJ Int. 2004, 44 (9), 1457-1468. (15) Zhang, L.; Ninomiya, Y.; Yamashita T. Occurrence of inorganic elements in the condensed volatile matter emitted from coal pyrolysis and their contribution to the ultra-fine particulates formed during coal combustion. Energy Fuels 2006, 20, 1482-1489. (16) Smoot, L. D., Ed. Fundamentals of coal combustion for clean and efficient use; Elsevier: Amsterdam 1993; pp 299-373. (17) Sui, J.; Xu, M.; Qiu, J.; et al. Numerical Simulation of Ash Vaporization during Pulverized Coal Combustion in the Laboratory-Scale Single-Burner Furnace. Energy Fuels 2005, 19 (4), 1536-1541. (18) Krishnamoorthy, G.; Veranth, J. M. Computational Modeling of CO/ CO2 Ratio Inside Single Char Particles during Pulverized Coal Combustion. Energy Fuels 2003, 17, 1367-1371. (19) Senior, C. L.; Zeng, T.; Che, J.; Ames, M. R.; Sarofim, A. F.; 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. (20) Senior, C. L.; Helble, J. J.; Sarofim, A. F. Emissions of mercury, trace elements, and fine particles from stationary combustion scources. Fuel Process. Technol. 2000, 65-66, 263-288. (21) Zeng, T.; Sarofim, A. F.; Senior, C. L. Vaporization of arsenic, selenium and antimony during coal combustion. Combust. Flame 2001, 126, 1714-1724. (22) Tomeczek, J.; Palugniok, H. Kinetics of mineral matter transformation during coal combustion. Fuel 2002, 81, 1451-1258.

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the contribution of each factor, though a large amount of knowledge had been attained by studying a wide range of coals worldwide.23-27 In complement to PM1+ study as discussed in the first paper,1 the present paper focuses on the emission of PM1 from the combustion of four bituminous coals possessing the identical organic properties, i.e., equivalent volatile matter, fixed carbon, and ash contents as introduced in the first part. Specifically, two coals were collected from the identical coalfield. Therefore, for a certain element transformed into PM1, it is merely affected by the coal mineralogical properties. The influence of coal organic properties is minor and even negligible in this study. With respect to the influence of the modes of occurrence of elements, it was first investigated from the consideration of the mineral association in parent coals, i.e., included or excluded particles. Coal density fractions were combusted separately. Second, the modes of occurrence of the major elements in raw coals were determined by computer-controlled scanning eletron microscopy (CCSEM), which was used for predicting the vaporization potential of inorganic elements. This work was conducted with the viewpoint of thermodynamic equilibrium, whose results were compared with the experimental observations. Experiments PM1 Collection and Characterization. PM1 was collected by a low-pressure impactor (LPI), which segregated PM1 into several sizes ranging from 0.03 to 0.76 µm. Regarding their characterization, the filter-collected individual size of PM1 was first subjected to X-ray fluorescence (XRF, RIX 2100, Rigaku) for quantifying the elemental composition. A high-resolution transmission electron microscopy (HRTEM, JEM2100F, JEOL) coupled with an energydispersive X-ray spectroscopy (EDS) was used for morphological observation of various sizes. For TEM analysis, half the particleladen Teflon 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. Furthermore, the modes of occurrence of S and P in two typical sizes of PM1, around 0.06 and 0.52 µm, respectively, were characterized by X-ray photoelectron spectroscopy (XPS, Shimadzu), which has been proven useful for the direct identification of many elements in PM.28,29 C1s at 284.0 eV was used for peak shift calibration.

Results and Discussion Properties of PM1 Collected from Raw Coal Combustion. As introduced in the first part, PM1 emitted after combustion has amounts varying with coal type considerably. Coal B1 has (23) Linak, W. P.; Miller, C. A.; Seames, W. S.; et al. Fine and ultrafine ash particles from pulverized coal combustion. Presented at the trace elemental workshop 2002, Yokohama, Japan, July 18-19, 2002; pp 3969. (24) Quann, R. J.; Sarofim, A. F. Vaporization of refractory oxides during pulverized coal combustion. Proc. Combust. Inst. 1982, 19, 1429-1440. (25) Flagan, R. C.; Taylor, D. D. Laboratory studies of submicron particles from coal combustion. Proc. Combust. Inst. 1082, 19, 1227-1237. (26) Goodarzi, F. Morphology and chemistry of fine particles emitted from a Canadian coal-fired power plant. Fuel 2006, 85, 273-280. (27) Yu, D.; Xu, M.; Sui, J., et al. Investigation of particulate matter emission from Chinese coal combustion. Proceedings of the 2005 International Conference on Coal Science and Technology, Okinawa, Japan, Oct 9-14, 2005; CD-ROM. (28) Qi, J.; Feng, L.; Li, X.; Zhang, M. An X-ray photoelectron spectroscopy study of elements on the surface of aerosol particles. Aerosol Sci. 2006, 37, 218-227. (29) Zhang, L.; Masui, M.; Ninomiya, Y.; Koketsu, J. Properties of particulate matter emitted from a pilot-scale incineration of dehydrated sewage sludge. Fuel 2006, submitted for publication.

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Figure 1. Morphologies of two typical sizes in PM1 emitted from coal B1 combustion and their EDS analysis ((a) for the particulates smaller than 0.1 µm, (b) its EDS result, (c) for the particulates larger than 0.1 µm, and (d) its EDS spectrum).

the highest PM1 amount, which is about 1.44 mg/g-coal and accounts for 1.25% on a mass basis of the total coal ash. Coal A follows it, having a PM1 amount of 1.23 mg/g-coal and accounting for about 1% of the total coal ash. The remaining two coals have lower PM1 emission, which is about 1.12 and 1.06 mg/g-coal for coals B2 and C, respectively, accounting for less than 1% of the total coal ash. This trend is obviously different from that of PM1+, as introduced in the first part. With regard to the morphology of PM1, it is composed of two typical structures similar to those released by combustion of the Chinese bituminous coals.8 As illustrated in Figure 1, a portion of particulates, having a diameter e0.1 µm, is formed as fractal aggregates consisting of a primary particle around 20 nm (see Figure 1a). EDS analysis proves the abundance of volatile metals including S, P, and alkali elements and a small amount of Fe as well. They should be caused by the vaporization-condensation pathway. Moreover, the molten single particles around 0.1-0.2 µm were also observed (see Figure 1c), which mainly consist of refractory elements such as Si, Al, and Fe as detected by EDS (see Figure 1d). Clearly, liberation of the inherent fine minerals and their liquid droplets contributes to the formation of this kind of particles in PM1. The distribution of inorganic elements in each size of PM1 is further presented in Figure 2. As expected, the refractory elements including Si, Al, and Fe are enriched in the large sizes regardless of coal type. Conversely, the volatile S, P, and alkali elements are mostly present in the smallest sizes; their amounts are also decreased sharply with an increasing PM1 size. This is in qualitative agreement with our previous study.8,30 These four elements prefer to react with each other to form sulfates and phosphates, which subsequently nucleate and coagulate homogeneously into ultrafine particulates.10,31,32 The distribution of two alkali earth elements, especially Mg, shows a relatively uniform distribution with the particle size; their amounts are (30) Zhang, L.; Ninomiya, Y. Emission of suspended particulate matter (PM10) from laboratory-scale coal combustion and its correlation with the coal mineral properties. Fuel 2006, 85, 194-203. (31) Glarborg, P.; Marshall P. Mechanism and modeling of the formation of gaseous alkali sulfates. Combust. Flame 2005, 141, 22-39. (32) Jimenez, S.; Ballester, J. Influence of operating conditions and the role of sulfur in the formation of aerosols from biomass combustion. Combust. Flame 2005, 140, 346-358.

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also minor compared to the other refractory elements, proving the presence of their majority in the large particles >1.0 µm. What of interest is the distribution of three heavy metals, Mn, Ni, and Cr; they exhibit the enrichment in the medium size of about 0.2-0.3 µm, having a profile different from that of S and P but in portion consistent with the refractory elements, implying the possibility for their binding with each other. Coal type affects the distribution of inorganic elements and their amounts in PM1 considerably. Combining the results in Figures 2 and 3 together, it is apparent that Si and Al in the cases of coals B2 and C have relatively high concentrations in the particulates e0.1 µm too, meanwhile they are mainly present in the large sizes around 1.0 µm in the other two cases. Their amounts in PM1 (see Figure 3a), especially that of Si, also move upward with the coal type changes from coal A to coal C, having a similar trend to these in PM1+, as discussed in the first part. Conversely, the other elements in each size of PM1 emitted in the cases of coals A and B1 have a relatively high concentration compared to those in the other two cases. Accordingly, as shown in Figure 3, Fe, P, alkali elements, Mn, and Ni emitted in the case of coal B1 have the greatest amounts, followed by coals B2, C, and A in descending order. The amounts of S and Cr are however the greatest in the case of coal A. Nevertheless, all suggest the obvious dependence of the transformation of inorganic elements on the coal sample, exactly, the mineralogical properties of coal. Influence of Inherent Mineral Association. Since these four coals, particularly coals B1 and B2 collected from the same coalfield, have the similar organic structures, the PM1 emission is largely linked with their mineralogical properties, which include the mineral association with carbonaceous matrix (i.e., organically bound, included and excluded16), the chemical species (i.e., the modes of occurrence of individual elements), and the particle size. The first property is primarily important, which affects the contact between mineral particles and the burning char. To elucidate its impact, two coals, B1 and B2, were segregated into float and sink fractions. The majority of excluded minerals transferred into the sink fraction, meanwhile the included ones and those bound to the organic matrix entirely moved into the float fraction. These two fractions were combusted separately, their PM1 was collected and compared with results for the cases of raw coals. Figure 4 illustrates the cumulative particle size distributions (CPSD) of PM1 released from coal fractions and the raw coals as well. In the case of coal B1, PM1 emitted from its float fraction combustion has almost the same CPSD as in the case of raw coal. Meanwhile, only 80 µg/g-coal PM1 was released from its sink fraction combustion, compared to about 1000 µg/ g-coal PM1 emitted from itself as well as its float fraction. This phenomenon is very apparent in the case of coal B2. The amounts of individual elements in PM1 emitted from the combustion of raw coals and their sink fractions were compared too. As demonstrated in Figure 5, in the case of the coal B1 sink fraction, a small amount of Fe, S, P, and K transformed into PM1, accounting for about 6.0%, 6.6%, 1.8%, and 4.9% of the respective element emitted from the combustion of raw coal B1 (see Table 1). The other elements were however not found. Similarly, in the case of the coal B2 sink fraction, few inorganic elements except Fe evolved into PM1. Fe emitted from the sink fraction accounts for about 1.6% of the total Fe released by the raw coal (see Table 1), which is very low compared to coal B1. These results indicate the small contribution of excluded minerals to the PM1 formation. A small amount of Fe was



Figure 2. Distribution of individual elements in each size of PM1.

produced from the sink fraction combustion, whose amount is rather low, accounting for at most 6% of the total Fe released from the raw coal. This, however, can be neglected considering the about 10% relative error for the mass balance of individual elements and the possible artifacts caused during coal density segregation. Treatment of parent coal with organic solvent

possibly generates a few “artificial” included minerals by wrapping the excluded minerals with solvent, which in turn promotes their vaporization and transformation into PM1. SEM observation partially proved this speculation. With respect to the lower transformation degree of excluded minerals, it can be rationalized by three possible reasons: (1) the excluded


Figure 3. Amounts of individual elements in PM1 emitted from the combustion of four coals ((a) for the refractory elements including Si, Al, Ca, and Mg and (b) for the volatile elements including S, P, K, Na, Mn, Ni, and Cr).

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Figure 5. Amounts of individual elements in PM1 emitted from the combustion of raw coals and their sink fractions.

Figure 6. Amounts of Si and Al in raw coals and the comparison to them in combustion-generated PM1 ((a) for Si and (b) for Al). Figure 4. Cumulative particle size distributions of PM1 emitted from the combustion of float and sink fractions of coals B1 and B2.

minerals in coal usually have large sizes, whose behaviors, such as vaporization, are mostly diffusion-controlled; (2) the excluded minerals are surrounded by a large amount of bulk oxygen, which does not facilitate their vaporization compared to a reducing atmosphere formed on the char surface; and (3) the char hosting the included minerals is believed to have a

temperature higher than the surrounding gas, which favors the metallic vaporization. On the basis of these results, the following sections only focus on the behaviors of inorganic elements associated with the carbonaceous matrix of parent coals, i.e., organically bound or included mineral. Unless noted otherwise, the inorganic elements mentioned hereafter refer to these two fractions. For a given element, its organically bound fraction was determined by quantifying its amount in the volatile matter emitted from coal pyrolysis at a relatively low temper-



Figure 7. Amounts of Ca, Mg, and Fe in raw coals and the comparison to them in combustion-generated PM1 ((a) for Ca, (b) for Mg, and (c) for Fe). Table 1. Contribution of Coal Sink Fraction Combustion to Each Element in PM1 Emitted from Raw Coal Combustion in Weight Percent

Si Al Ca Fe Mg S

coal B1

coal B2

0.79 0.53 2.96 6.02 1.66 6.58

0.13 0.19 1.42 1.64 0.27 1.07

P K Na Mn Ni Cr

coal B1

coal B2

1.75 4.94 2.66 4.41 5.50 0.00

0.30 0.58 0.35 3.38 1.77 0.00

Figure 8. Influence of the mean diameter of inherent included Febearing compounds on the degree of its transformation into two fractions of PM1: e0.1 µm and >0.1 µm.

ature such as 1200 °C.15 On the other hand, its fraction as included discrete mineral was characterized by CCSEM, which had been described in the discussions for PM1+. The refractory and volatile elements are discussed separately. The former ones, including Si, Al, Ca, Fe, and Mg, undergo two routes to transform into PM1: vaporization and condensation to form PM0.1 and direct liberation to form the large PM0.1+. Meanwhile, the volatile elements in PM1, including S, P, alkali elements, and heavy metals, are mostly caused by their vaporization.

Transformation of Refractory Elements and Influences of Coal Type. The transformation behaviors of Si and Al are expressed as the comparison of their amounts in raw coals to these in PM1 and shown in Figure 6a and b, respectively. Only a portion of these two elements was transformed into PM1, whose degrees are also varied with coal type greatly. In the cases of coals A and B1, less Si and Al were found in PM1 though their amounts in coal B1 are rather high. Conversely, in the case of coal B2 having similar amounts of Si and Al to these in coal B1, the higher transformation degrees were observed. This phenomenon is more discernible in the case of coal C, where these two elements have the largest amounts in PM1 though they are rather lean in the parent coal. Clearly, the highest transformation degrees were obtained in this case. Additionally, according to the distribution of these two elements as shown in Figure 2, more of the Si and Al in coals B2 and C should undergo vaporization and transform into PM0.1. On the other hand, they mostly undergo direct liberation to evolve into PM0.1+ in the cases of coals A and B1. Assuming that reduction of oxide by CO is the sole pathway affecting the vaporization of Si and Al,20 the vaporized amounts of both of them should be in proportion to their original amounts in raw coals. Accordingly, Si and Al in PM1 would have their amounts in this order for coals B2 > B1 > C > A. Similarly, as to their amounts directly liberated into PM 0.1+, it should have the same descending order too. These postulations are however inconsistent with the empirical observation shown in Figure 6. Clearly, there are other parameters affecting the evolution of these two elements. Of these, the most important one is likely the presence of calcium and iron as discussed for the formation of PM1+. These two elements can promote the coalescence of Si and Al through reducing their melting points.33,34 The excluded fraction of Ca and Fe is possibly the most important as introduced in the first part. (33) Levin, E. M.; Robbins, C. R.; Mcmurdie, H. F. Phase diagrams for ceramists; The American ceramic society, Inc.: Columbus, OH, 1964; Vol. 1, p 219.


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Figure 9. Mode of occurrence of Fe embedded in the carbonaceous matrix of raw coals and its influence on the Fe vaporization ((a) Fe-S-Si relationship for included Fe in raw coal A, (b) Fe-S-Si relationship for included Fe in raw coal B2, (c) theoretical thermodynamic equilibrium considerations for the vaporization of Fe oxide and its sulfide, and (d) relationship between content of included pyrite and the vaporization degree of included Fe for four raw coals).

The transformation behaviors of Ca, Mg, and Fe are shown in Figure 7a, b, and c. For Ca and Mg, their amounts in PM1 are almost equivalent to these in the parent coals, suggesting that the organically bound fraction of these two elements and their amounts present as fine minerals (less than 1.0 µm in diameter) are the major sources for their formation in PM1. Less of the coarse Ca- or Mg-containing minerals in raw coals, having a diameter >1.0 µm, fragments into particulates e1.0 µm; their majorities are present in PM1+ or in the coarse ones larger than 10.0 µm. Fe behaved differently (see Figure 7c). Irrespective of coal type, more of the Fe is found in the emitted PM1, which is even higher than its original amount. One possible reason is the presence of more of the ultrafine Fe-bearing species like ferritin in the coal matrix, which however cannot be detected by CCSEM.35 Another possibility is the contribution from the (34) McLennan, A. R.; Bryant, G. W.; Bailey, C. W.; Stanmore, B. R.; Wall T. F. An experimental comparison of the ash formed from coals containing pyrite and siderite minerals in oxidizing and reducing conditions. Energy Fuels 2000, 14, 308-315. (35) Zhang, L.; Takanohashi, T.; Saito, I. unpublished data.

coarse Fe-bearing compounds, >1.0 µm, which might be via vaporization or fragmentation. The influence of the Fe-bearing compound particle size was primarily investigated as shown in Figure 8, where the X-axis refers to the geometric mean diameter of the included Fe-bearing compound in raw coals (as determined by CCSEM) and the Y-axis is the degrees of included Fe-bearing compound transformed into two fractions of PM1, which were calculated according to the following equations: Vaporization degree(%) ) M(Fe in PM0.1) - M(Fe as organically bound in raw coal) M(Fe as included discrete minerals in raw coal)

×

100 (1) Fragmentation degree(%) ) M(Fe in PM0.1+) - M(Fe as discrete fine mineral in raw coal) M(Fe as included discrete minerals in raw coal)

×

100 (2)

M represents the amount of Fe in different fractions of PM1 or in raw coal. With respect to the fragmentation of Fe into PM0.1+,



Figure 10. Mode of occurrence of S and P in two typical sizes in PM1 ((a) for S in the size of 0.52 µm, (b) for S in the size of 0.06 µm, (c) for P in the size of 0.52 µm, and (d) for P in the size of 0.06 µm).

its degree was found in reverse proportion to the mean size of Fe-bearing compounds in raw coal, indicating that the smaller Fe-bearing particles (less than 10 µm in diameter, e.g., in coals A and B1) are prone to generate the submicrometer fragments during coal combustion. Less of the larger ones undergo deep fragmentation, which might be due to a short residence time in the DTF. The vaporization degree of Fe, however, shows no relationship with its particle size. Coal B2 has a comparable amount of Fe vaporized to that of coals B1 and C, though the included Fe within it has the largest mean diameter. Clearly, the mode of occurrence of Fe is much more important, which is further elucidated by the results in Figure 9. As determined by CCSEM, Fe in raw coals has three possible chemical forms: sulfide (pyrite, pyrrhotite), oxide, and Fe aluminosilicate (FeO‚Al2O3‚SiO2); their percentages are varied with coal considerably. As illustrated by the ternary diagrams of Fe-Si-S in Figure 9a and b, most of the included Fe in coal A is enriched in the corner of Fe, suggestive of a lot of iron oxide in this coal. On the contrary, in coal B2, Fe is mostly in two forms: pyrite and Fe aluminosilicate. The vaporization potentials of different species were initially investigated with the viewpoint of thermodynamic equilibrium. A commercial calculation software package, Factsage 5.2, was used. The chemical forms of Fe-bearing compound were selected including pure pyrite and two mixtures containing pyrite and oxide at the molar ratios of 1 to 1 and 1 to 4, respectively. Fe aluminosilicate was not taken into consideration since Fe within it is fully assimilated by the aluminosilicate network; it prefers to melt and coalesce rather than vaporize. The gas atmosphere containing CO + N2 and a wide temperature range from 1000 to 2000 °C were adopted considering the locally reducing environment around the char surface. For a given Fe-bearing compound, its vaporization degree was defined as the total amounts of predicted gaseous Fe-bearing species (e.g., Fe, FeO, FeS, etc.) relative to its original amount. As shown in Figure 9c, the pure pyrite vaporizes readily. It commences at about 1450 °C. The presence of iron oxide however lessens the vaporization of Fe apparently. This can be attributed to the different melting points of two pure species. The included pyrite in char may transform to a eutectic species such as FeO-FeS,

which has a melting point around 1080 °C.34 Once melted, a portion of Fe inside may diffuse from the surface outward. S can promote the Fe vaporization too. On the other hand, a pure iron oxide such as wustite has a relatively high melting temperature around 1370 °C34 and, hence, it is difficult to vaporize. This hypothesis was experimentally evidenced by the results depicted in Figure 9d. With the weight percentage of included pyrite increasing, more of the Fe vaporizes. An asymptotic relationship, rather than a linear one, was found between these two variables, which might be due to the combinational effect of the mode of occurrence of Fe and its particle size as discussed here. Fe in coal B2 has the largest size though more than half of it is in the form of pyrite, and as a result, its vaporization degree is similar to that of Fe in coal C, in which the concentration of pyrite is lower and has the smallest size as shown in Figure 8. Transformation of Volatile Elements and Influences of Coal Type. Due to their high vaporization potential, the volatile elements, especially S and P, have comparable amounts to that of the refractory elements. As shown in Figure 10, S and P are mainly in the forms of sulfate and phosphate in the emitted PM1. Regardless of particulate size, S is bound with alkali element and calcium. Consistent with the elemental distributions in Figure 2, the alkali elements sulfates are the most abundant in the smallest sizes, which should be formed via the reactions between vaporized gaseous alkali elements with sulfur. Calcium sulfate is however the most prevalent in the large size, suggestive of the sulfur capture by submicrometer calcium. The behavior of P is a little different from that of S. Besides the alkali element phosphates, its oxide is also found, whose amount is even comparable to that of sodium phosphates in the smallest size. Clearly, once vaporized, only a portion of gaseous P reacts with the vaporized alkali elements. The remainder of P is however oxidized and mainly condenses into particulates smaller than 0.1 µm. This process most likely occurs shortly before or in the sampling probe, where the temperature is sufficiently low. Essentially, S in raw coals totally vaporizes and transforms into its gaseous oxide such as SO2, which in portion is further oxidized and reacts with gaseous metals or the submicrometer


Zhang et al.

Figure 11. Mode of occurrence of included P-bearing compounds in coal A (a) and coal C (b).

particles. The resultant sulfates transform into PM1 eventually.36 Correspondingly, the sulfur amount in emitted PM1 is dependent on the amounts of these three elements as mentioned here, Na, K, and Ca. This finding is also in agreement with our previous studies on the Chinese coals.8 On the contrary, P and its oxide have a low saturated vapor pressure, whose vaporization potential is rather low compared to S. This is consistent with studies on other coals and sewage sludge combustion,37,38 where the majority of P was found preferably coalescing into glassy aggregates, meanwhile its vaporization degree is largely dependent on the mode of occurrence of P. Little P is found organically bound within the studied coals. Thus, vaporization of inorganic P-bearing compounds is the major formation route for its presence in PM1. The mode of occurrence of P in two raw coals, A and C, is shown virtually as the ternary diagrams in Figure 11. Irrespective of coal type, P within it mainly partitions into two chemical forms: calcium phosphate (apatite, Ca3(PO4)2) and a complicated compound containing Si, Al, Ca/Fe, P, and O together; their molar ratios vary broadly as well. As suggested by a thermodynamic equilibrium calculation,37 apatite seldom vaporizes since it is stable below 1800 °C. Meanwhile, P in the complicated compound possibly vaporizes readily. It even commences at a temperature as low as 1200 °C. This is due to the coexistence of aluminosilicates with Ca and/or Fe in the complicated compound, which has a lower melting point than that of the highly pure aluminosilicates such as kaolinite.37 Consequently, a portion of P within it can be liberated from the molten aluminosilicate network and vaporize outward. On the above consideration, the maximum vaporization amount of P was further predicted for the combustion of four (36) Graham, K. A.; Sarofim, A. F. Inorganic Aerosols and Their Role in Catalyzing Sulfuric Acid Production in Furnaces. J. Air Waste Manag. Assoc. 1998, 48, 106-112. (37) Zhang, L.; Ninomiya, Y. Transformation of phosphorus during combustion of coal and sewage sludge and its contributions to PM10. Proc. Combust. Inst. 2007, 31, 2847-2854. (38) Zhang, L.; Ito, M.; Sato, A.; Ninomiya, Y.; Sakano, T., et al. Combustibility of dried sewage sludge and its mineral transformation at different oxygen content in drop tube furnace. Fuel Process. Technol. 2004, 85, 983-1011.

Figure 12. Thermodynamic equilibrium consideration for the vaporized P amount during the combustion of four coals.

coals. An averaged composition of included P-bearing compounds in raw coals, as determined by CCSEM, was used as the solid input for Factsage calculation. Once again, a reducing atmosphere containing CO and a wide temperature range from 1000 to 2000 °C were adopted. The calculation results are shown in Figure 12, which are in qualitative agreement with the experimental results as shown in Figure 3. For example, at the temperature of 1450 °C or even higher, P vaporized from coal B1 has the greatest amount, followed by coal B2 and coal A. Combustion of the latter two coals leads to the nearly same amount of vaporized P. Since the amount of P in coal B2 is far higher than that in coal A (see Table 1 in the first paper of this serial study1), the above result indicates that the majority of P is present as apatite in this coal, whereas its amount present as the complicated compound is rather low. Coal C combustion leads to the lowest emission of P, owing to a low amount of P in the raw coal as well as a small fraction of P in the complicated compound. In addition, it is noteworthy that compared to the thermodynamic equilibrium prediction, less of the P was vaporized during the combustion of the four coals. When coal B1 is taken as the example, at least 450 µg/g-coal of P could vaporize given the char temperature around 1450 °C, which is higher than the experimental finding, about 175 µg/g-coal as shown in Figure 3. Obviously, P vaporization is diffusioncontrolled.



Figure 13. Transformation of Na and K during coal combustion ((a) for Na, (b) for K, and (c) for their transformation degree).

Figure 14. Mode of occurrence of Na and K in two raw coals ((a) for Na and K in raw coal B2 and (b) for Na and K in coal C).

As shown in Figure 13, vaporized alkali elements in PM1 have amounts greatly varying with coal type too. Regarding Na, its formation source varies with coal type (see Figure 13a). In the cases of coals A and C, about half the Na in PM1 was contributed from its organically bound fraction in raw coals. On the contrary, in the other two cases, little Na is organically bound, while almost all of the Na was caused by the vaporization of inorganic species. Similarly, more than half of the K in PM1 was contributed from its organically bound fraction in the case of coal C (see Figure 13b). Such a phenomenon however did not take place in the other cases. The vaporization degrees of inorganic Na/K-bearing compounds are further demonstrated in Figure 13c. About 10% of Na was vaporized in the cases including coals A, B1, and B2,

indicating the similar properties of Na in these three coals. On the other hand, only 1% of Na in coal C was vaporized, which is far lower than the other three cases. As far as K is concerned, its vaporization degrees in coals A and B1 are the greatest, being about 20%. Coal B2 combustion caused about 10% of K to be vaporized, meanwhile in coal C, only 1.5% of K was vaporized. Clearly, the alkali elements in coal C have different modes of occurrence from these in the other coals. As illustrated by the ternary diagrams in Figure 14, Na and K in coal B2 have three possible forms: carbonate or oxide in the (Na + K) corner, chloride or sulfate near or on the line between (Na + K) and (S + Cl), and Na/K aluminosilicates in the Si corner. The former two forms are water-soluble, which readily vaporize at high temperature. The last one, having Na/K as the dilute phase


Zhang et al.

Figure 15. Possible chemical species containing three heavy metals as predicted by thermodynamic equilibrium considerations ((a, c, and e) for Mn, Ni, and Cr emitted from the combustion of coal A, respectively; (b, d, and f) for Mn, Ni, and Cr emitted from the combustion of coal C, respectively).

embedded in the aluminosilicate network, however hardly vaporizes due to its high melting point.39-41 The alkali elements in coal C are entirely bound with Si/aluminosilicate, and hence, their vaporization degrees are rather low, if not neglected. With respect to the different vaporization degree of K in the cases of coals B1 and B2, it is due to the fact that more of the K in coal B1 is water-soluble as determined by CCSEM. Finally, it is worthwhile pointing out the behaviors of heavy metals during coal combustion. The influence of coal type on their vaporization is not discussed considering the relatively poor mass balance of these elements. Moreover, their concentrations in raw coal are not high enough for CCSEM quantification on their modes of occurrence. Alternatively, the possible forms of vaporized heavy metals in PM1 are discussed on the combination (39) Murakami, T.; Naruse, I. Prediction of evolution characteristics of alkali metal compounds in coal combustion/gasification from coal properties. J. Chem. Eng. Jpn 2001, 34 (7), 899-905. (40) Zhang, L.; Ninomiya, Y. Fate of alkali elements during pyrolysis and combustion of Chinese coals. J. Chem. Eng. Jpn 2003, 36 (7), 759768. (41) Thompson, D.; Argent, B. B. The mobilization of sodium and potassium during coal combustion and gasification. Fuel 1999, 78, 16791689.

of the size-dependence of their concentrations and a thermodynamic equilibrium calculation. As shown in Figure 2, the vaporized heavy metals, including Mn, Ni, and Cr, are enriched in the medium size, implying the possible affinity of them with the refractory ones rather than S and P. A thermodynamic equilibrium calculation was further conducted to prove this hypothesis. All the inorganic elements and their amounts listed in Figure 3 were used as the solid input. The initial form for volatile elements (as listed in Figure 3b) was assumed as a gaseous metal, considering that these species are preferentially formed with the beginning of mineral vaporization. On the other hand, the refractory elements were assumed as the solid oxides since their vaporization is rather minor compared to their fragmentation. The atmosphere of air was used. Two cases, coal A and C, were simulated, and their results are displayed in Figure 15. Regardless of coal type, the vaporized Mn prefers to react with Si and/or Al to form the corresponding compounds around 1450 °C. The behaviors of Ni and Cr however vary with coal type considerably. Ni in the case of coal A likely partitions as its oxide and (NiO)(Cr2O3) equivalently. It is however entirely in the form of (NiO)(Fe2O3) in the case of coal C. With regard to Cr, the complicated compounds such as (NiO)(Cr2O3) and


K2CrO4 are favored in the case of coal A, meanwhile only FeCr2O4 is preferred in the case of coal C. Nevertheless, these results confirm the possible reactions among vaporized heavy metals with the refractory elements. As a result, the heavy metals exhibit particle size shifting to the large value compared to Na and K. Their toxicity is possibly weakened to some extent, since the above-mentioned complicated compounds are almost insoluble in water. Conclusions Four bituminous coals, possessing the similar properties, were combusted in a laboratory-scale DTF to investigate the emission of PM1. The major conclusions are drawn as follows: (1) PM1 accounts for about 1.0-1.4 mg/g-coal, varying with the coal type considerably. The refractory elements in PM1, including Si, Al, Ca, Mg, and Fe, are mainly concentrated in the large sizes. In contrast, the volatile elements, including S, P, and alkali elements are mostly enriched in the small sizes in PM1. Their distribution with particle size also shows the same tendency because of the formation of alkali sulfates and phosphates. With regard to the heavy metals such as Mn, Ni, and Cr studied here, they are however mainly enriched around the medium sizes of about 0.2-0.3 µm, partially consistent with the distribution of Si and Fe since the possible bindings exist among these four elements. (2) The association of inherent minerals with the carbonaceous matrix is primarily important for determining their


transformation into PM1. The excluded minerals barely contribute to PM1 formation. (3) The transformation behaviors of Si and Al are possibly affected by the amount of excluded (CaO + Fe2O3) in raw coals. This is similar to their transformation into PM1+. A portion of Fe-bearing large particles (>1 µm) likely transforms into PM1 too. Its vaporization is possibly affected by the chemical forms of Fe. On the other hand, the fragmentation of large Fe-bearing compounds is mainly controlled by their particle size. (4) The amount of volatile S in PM1 is determined by the total amounts of Na, K, and Ca. P in PM1 is in the forms of phosphates and its oxide, P2O5. With respect to the two alkali elements, their vaporization is influenced by their modes of occurrence. Their association with aluminosilicate rarely vaporizes. On the other hand, water-soluble species including halides and sulfates can vaporize readily. 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. We are grateful to Mr. Masunori Kawamura in the Analytical Center of Chubu University for his assistance in the use of TEM and EDS analysis. 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. EF060308X

Interactions among Inherent Minerals during Coal Combustion and

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