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A new method for identifying the modes of particulate matter from pulverized coal combustion
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Powder Technology 183 (2008) 105 – 114 www.elsevier.com/locate/powtec
A new method for identifying the modes of particulate matter from pulverized coal combustion Dunxi Yu, Minghou Xu ⁎, Hong Yao, Xiaowei Liu, Ke Zhou State Key Laboratory of Coal Combustion, Huazhong University of Science and Technology (HUST), Wuhan, 430074, China Available online 21 November 2007
Abstract Knowledge of the modality of the size distribution of particulate matter (PM) from pulverized coal combustion is of great significance from the viewpoint of exposure and risk assessment. Mass or number size distributions are usually used in modality analyses, but sometimes the central particle mode fails to be detected due to overlapping. This work provides a new method for identifying particle modes using mass fraction size distributions of the aluminum (Al). Five Chinese pulverized coals of different ranks were burnt in a laboratory drop tube furnace at 1673 K. The produced PM was size segregated by a low pressure impactor and subjected to elemental composition analysis by X-ray fluorescence (XRF). Particle mass size distributions, mass fraction size distributions of the Al and sulfur (S) were obtained for all the particle samples. The mass size distributions of four coal ash samples all show three distinct particle modes, with a central mode at approximately 2 µm, while the mass size distribution of the LPS coal ash sample only indicates two particle modes. However, the mass fraction size distributions of the Al for all ash samples, including the LPS coal ash sample, generally show three particle modes. The obscurity of the central mode in the mass size distribution of the LPS coal ash sample is expected to be a consequence of the merging of it into the coarse mode. The formation of the central mode is attributed to the more pronounced heterogeneous condensation or adsorption of vaporized species on fine residual ash particles, whose origins are still unclear at present. This is further confirmed by examination of the mass fraction size distributions of the S. These results show that mass fraction size distributions of the Al seem to be more effective in identifying particle modes and their size boundaries than particle mass size distributions. © 2007 Elsevier B.V. All rights reserved. Keywords: Coal; Particulate matter; Mode; Elemental mass fraction size distribution
1. Introduction Pulverized coal combustion is one of the major sources of particulate matter (PM) in the atmosphere, which has received much concern in recent years due to its adverse health effects [1,2]. Knowledge of the detailed characteristics of particle size distributions (PSDs), either in terms of mass or number, is very important for uncovering their formation mechanisms and dynamics both in the combustion systems and atmosphere. One such characteristic is the modality displayed by PSDs. A mode is conventionally defined as a peak in the PSD which can be described by a lognormal function for the mass size distribution of coal-derived PM. But in the context a mode sometimes ⁎ Corresponding author. Tel.: +86 27 87544779 8309; fax: +86 27 87545526. E-mail address: [email protected] (M. Xu). 0032-5910/$ - see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.powtec.2007.11.011
represents a category of particles that are formed by the same processes. Identifying the presence, location and size boundary of modes in PSDs is of great significance not only for understanding their formation mechanisms, but also for exposure and risk assessment. Furthermore, modes could serve as direct pointers to appropriate mitigation strategies to lower environmental and human effects. In most studies, two distinct modes have been observed in particle mass size distributions (MSDs) generated during pulverized coal combustion. It has been well established that these two modes are produced by different mechanisms [3–7]. The coarse mode, between 1–20 µm, is primarily produced by char fragmentation and mineral coalescence. The ultrafine mode, often centered around 0.1 µm, is mainly formed by the vaporization–condensation mechanism. However, the ultrafine mode is not always distinguishable from the coarse one. As
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indicated by Linak and Peterson [8], sometimes it was merged into the larger mode and was overlooked. An important work by McElroy et al. [3] showed that ash particles from six coal-fired utility boilers were generally bimodally distributed. But the complex structure of the larger modes suggested a superposition of more than one discrete particle mode. Regardless of that, the authors just considered the larger particle size distribution as a single particle mode there. Kang is probably the first to declare that ash particles from pulverized coal combustion may have a trimodal size distribution. In his thesis [9], Kang found that, besides a conspicuous ultrafine mode in some cases, there were always two distinct modes present in the N1 µm size range. The central mode was near 4 µm while the largest mode was centered at approximately 8 µm. Kang et al. [7] even developed a Monte Carlo model to simulate the ash coalescence and char fragmentation processes that govern the final particle size distribution. They attributed the bimodal size distribution of residual ash particles N1 µm to the competition between the particle fragmentation and the ash coalescence enhanced by the shrinking peripheral area with the progress of char reaction. Joutsensaari et al. [10] also observed a central mode in MSDs generated in a real scale power plant, in addition to a ultrafine particle mode at about 0.07 µm and a coarse one at about 3 µm. But the size of the central mode was about 0.4 µm on mass, a little different from that found by Kang [9]. Recent experimental results from Linak et al. [11], Wendt [12] and Seames [13] suggest that coal fly ash particle formation is more accurately described as a trimodal particle size distribution. Aside from the well-established ultrafine and coarse modes, a ubiquitous central mode centered at approximately 2.0 µm is highlighted in these studies. By re-examination of literature data [3,6,14,15], Linak et al. [11] found that three particle modes were in fact the rule, not the exception. Less attention to the central mode in the past work was attributed to the low resolution of particle-sampling and-sizing techniques available then. The newly found three particle modes, rather than just merely two, indicate that it is not always effective to identify particle modes by MSDs, since sometimes one mode may be merged into another [3,8,9]. Besides the number of particle modes, the mode size boundary is also an important parameter for modeling, risk assessment and development of mitigation strategies. However, it is often difficult to be defined whether in particle mass or number size distributions. Few past studies have attempted to do so, and a somewhat arbitrary value is usually given. For example, a turning point at approximately 1 µm is often assumed to be the size boundary between particles formed by the vaporization–condensation and fragmentation-coalescence mechanisms, respectively. But it is not always the case at least in two studies [16,17]. In one study, Smith et al. [16] believed that the submicron particles in the 0.1–1 µm size range were formed by the bursting of larger particles due to gas release during rapid heating, followed by coagulation and condensation of volatilized elements. In another study, Sadakata et al. [17] concluded that approximately half of the submicron fly ash larger than 0.1 µm was simply carried over from the submicron coal fragments originally contained in the pulverized raw coal, and that the remainder was newly formed through the breakup of larger coal particles in the devolatilization region.
These results show that the mechanisms contributing to the formation of submicron particles are more complex, not just the vaporization–condensation mechanism. Therefore, the identification of the size boundaries between different modes is also very important for elucidating their origins. Recently, we developed a new approach to particle mode identification by using elemental mass fraction size distributions (EMFSDs) of PM generated from combustion of a sizeclassified Chinese coal [18]. Three distinct modes were also observed in the total mass fraction size distributions of the silicon (Si) and aluminum (Al). The ultrafine and coarse modes are in the size ranges of 0.0281–0.258 and 2.36–9.8 µm, respectively. And the central mode is between 0.377 and 1.58 µm. These results, consistent with others [11–13], show the potential importance of EMFSDs in identifying particle modes. Another important feature of using EMFSDs is that, size boundaries between different modes could be more accurately defined, as indicated in the previous study [18] and will be described in more detail in the present investigation. In addition to the least volatile elements Si and Al, mass fraction size distributions of the sulfur (S) can also be used to indicate particle formation mechanisms [16]. It is reported that the S is probably the only major element that is released almost completely during coal combustion [19]. A fraction of the S is emitted as gas species and the remainder is retained in ash particles after combustion. The incorporation of the S into particles can be through homogeneous nucleation and/or heterogeneous condensation/adsorption. The S is enriched in ultrafine particles [20], because, on one hand, these fine particles contain most of the ash surface area and sulfation and condensation are directly proportional to surface area, and on the other hand, at least a portion of the ultrafine particle materials is formed by homogeneous nucleation from the gas phase such as Na2SO4, K2SO4 or H2SO4 [21]. While much less of the S heterogeneously condenses on the larger residual ash particles and forms a thin surface layer [22]. Since different processes are expected to result in different concentrations of the S in thusformed particles, the concentration variations of the S with particle size could also serve as a complementary indicator of particle formation modes. From the above considerations, the present work proposes an updated method for identifying particle modes using EMFSDs. Five Chinese coals of different ranks (from bituminous coal to lignite) were burnt in a laboratory drop tube furnace (DTF) at 1673 K. The produced PM was collected by a Dekati lowpressure impactor (DLPI) and subjected to elemental composition analysis. The mass fraction size distributions of the Al, rather than the Si and Al as used in the previous work [18], were used to characterize particle formation modes and their size boundaries. And they were supported by examination of the mass fraction size distributions of the S. The results show that EMFSDs seem to be more effective than MSDs for identifying particle modes. 2. Experimental Five Chinese pulverized coals with a particle size less than 63 µm were used in this study. The LPS, HS, PDS and PX coals
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are bituminous coals, while the XLT coal is a lignite. The proximate analysis, ultimate analysis, and composition of the low temperature ash of the coal samples are shown in Table 1. The coal samples were burnt in a laboratory DTF described in the previous study [18] at 1673 K. The DTF is 2 m long and has an inner diameter of 56 mm. It was electrically heated to the desired temperature. The atmosphere consisted of 20% oxygen with pure nitrogen being balanced. The coal particles in a Sankyo Piotech Micro Feeder (Model MFEV-10) were entrained by a primary gas stream and injected into the combustor at a feeding rate of approximately 0.2 g/min. A secondary gas stream was provided via a flow straightener on the top of the reactor to ensure that coal particles were burnt almost completely. The produced PM was routed through a watercooled sampling probe and high-purity nitrogen was supplied from the top of the probe to freeze subsequent reactions in the probe. Combustion particles were then collected by a 13-stage DLPI, before which a cyclone was used to remove particles N 10 µm. For elemental analysis, Teflon filters were used. Those particles b 10 µm (PM10) were classified into 13 size fractions (d50 0.03–10 µm). Gravimetric analysis of impactor samples was carried out by weighing the substrates carefully before and after sampling on a Sartorius M2P Microbalance (readability, 0.001 mg) in a clean, almost constant humidity laboratory room. The elemental composition of each sized PM fraction was analyzed by X-ray fluorescence (XRF). And particle morphologies were characterized by a Sirion 200 Field Emission Scanning Electron Microscope (FESEM) equipped with an Energy Dispersive Spectrometer (EDS). More detailed information on the DTF and experimental procedures can be found elsewhere [18]. Table 1 Coal properties Property
HS
PDS
PX
XLT
Proximate analysis/wt.%, a.d. Moisture 1.8 Volatile matter 21.1 Ash 27.6 Fixed carbon 49.5
2.7 15.3 50.0 32.0
1.0 36.0 18.7 44.3
1.2 19.9 40.2 38.7
15.0 48.9 24.4 11.7
Ultimate analysis/wt.%, a.d. C 57.7 H 5.0 S 3.4 N 0.7 Oa 3.8
35.4 1.4 6.2 0.5 3.8
67.0 5.3 3.8 1.1 3.1
48.7 3.1 0.8 0.9 5.1
41.0 4.8 2.9 1.2 10.7
Ash composition/wt. % SiO2 41.8 Al2O3 18.4 CaO 7.7 Fe2O3 15.2 MgO 1.0 Na2O 2.5 K2O 1.6 P2O5 0.6 9.3 SO3 Others 1.9
49.9 32.8 2.0 3.6 1.9 3.4 1.1 0.3 4.4 0.6
43.8 29.2 4.1 13.3 1.3 2.0 1.7 0.2 3.0 1.4
60.2 28.6 1.1 1.9 1.5 1.0 2.2 1.0 1.7 0.8
24.6 9.2 27.4 11.7 1.7 2.0 1.2 0.7 20.6 0.9
a
By difference.
LPS
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3. Results and discussion 3.1. Particle mass size distributions Fig. 1 depicts particle MSDs produced during combustion of five Chinese coals (see Table 1). As shown in Fig. 1a, PM from combustion of the LPS coal seems to be bimodally distributed. The ultrafine mode is near 0.1 µm and the coarse mode is beyond 0.3 µm, if assuming that the turning point at about 0.3 µm is the size boundary between the two modes. In contrast, PM from combustion of the other four coals seems to have three particle modes, as indicated in Fig. 1b, c, d, and e. Aside from the commonly observed ultrafine and coarse modes, there is a definite inflection point, if not a obvious peak, at approximately 2 µm on the shoulder of the coarse mode. It is most possibly a central mode like the one found by Linak et al. [11], Wendt [12] and Seames [13]. Some evidence for the central mode can be seen from its different particle morphology from the other two modes. As seen in Fig. 2, the coarse mode particles (about 5 µm) are primarily spherical in shape and the EDS spectra show that they are mainly aluminosilicates, indicating the production of these particles from molten minerals. Fig. 3 presents the typical ultrafine mode particle morphology (about 0.1 µm). The examined ultrafine mode particles are also primarily spherical in shape and have smooth surfaces. The EDS spectra indicate that they are relatively rich in volatile elements (alkalis and some trace elements) with respect to the coarse mode particles. The formation of the ultrafine mode can be explained by the well-established vaporization–condensation mechanism. In contrast, the particle morphologies of the central mode (about 2 µm) seem to be more complex than those of the coarse and ultrafine modes. As indicated in Fig. 4a, some central mode particles are nearly spherical in shape but often have defects. A partially melted particle can also be seen in Fig. 4a with fine irregular fragments sintered onto its surface. Agglomerates of stretched particles are common morphologies in the central mode and one typical example is shown in Fig. 4b. Such particles are expected to have higher surface-to-volume ratios than the coarse mode. Some ash cenospheres, as shown in Fig. 4c, have also been found in the central mode size range. The evolution of gas species from molten particles can account for the formation of these particles. The EDS spectra indicate that these particles are mainly Ca–Si–Al and Fe–Si–Al structures. The difference in particle morphology between the central mode and the other two suggest that, the formation of the central mode seems to be different and its existence is reasonable. As shown in Fig. 1, the HS and PDS ultrafine modes are at approximately 0.1 µm (Fig. 1b and c), while the PX and XLT ultrafine modes are a little larger, at approximately 0.2 µm (Fig. 1d and e). All the coarse modes are larger than 8 µm, if assuming that the turning point near this value is the size boundary between the central and coarse modes. Based on the same assumption, the size boundary between the ultrafine and central modes is near 0.7 µm for the HS, PDS and PX coals, but approximately 0.4 µm for the XLT coal.
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Fig. 1. Particle mass size distributions.
The ubiquitous nature of the central mode has been highlighted by Linak et al. [11] The above results for four Chinese coals of different ranks (i.e. the HS, PDS, PX and XLT coals) also show a distinct central mode. Then one would wonder if the LPS coal ash has a central mode as well, and the miss of it is simply because it is merged into the coarse mode and difficult to detect in the MSD. The answer will be given in the following subsection. 3.2. Elemental mass fraction size distributions In our previous study [18] the total mass fraction size distributions of the Si and Al were used to characterize particle
formation modes generated during combustion of a size-classified Chinese bituminous coal. Though both the Si and Al are less volatile during combustion when compared with alkali metals, the Si is actually easier to vaporize than the Al through more volatile suboxides such as SiO. For example, Kauppien et al. [20] found that the concentration of the Si in the fine particle mode from a utility boiler was 96 µg/Nm3, while that of the Al only 43 µg/Nm3, less than half of the Si concentration. Quann et al. [23] investigated the composition and size distribution of the fly ash produced by burning of a Montana lignite in a laboratory-pulverized coal combustion system at two temperatures of 2050 and 2450 K. Their results showed that SiO2 accounted for 5.11% of the submicron fume, while Al2O3
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Fig. 2. Typical morphologies of coarse mode particles (about 5 µm).
a far lower fraction of 0.97%. Calculations by Zhang et al. [24] showed that about 0.6% SiO2 and 0.3% Al2O3 vaporized for combustion of two bituminous coals at 1723 K. These results indicate that the Al is much less volatile than the Si during combustion. Its concentration variations with particle size are expected to be more easily observed than the combination of the Si and Al. Therefore, the present work focuses on mass fraction size distributions of the Al, rather than those of the Si and Al, as applied in the previous study [18]. Fig. 5 shows mass fraction size distributions of the Al for PM produced by burning the five Chinese coals. Similar to the total mass fraction size distributions of the Si and Al obtained in the previous work [18], these mass fraction size distributions of the Al can also be readily divided into three distinct size fractions based on its concentration variations with particle size. This indicates the existence and generality of three particle formation modes. As shown in this figure, except the HS coal (Fig. 5b), the corresponding particle modes for the other four coals (Fig. 5a, c, d, and e) are in the same size ranges. The ultrafine modes for the LPS, PDS, PX and XLT coals are less than 0.2 µm, the coarse modes larger than 3 µm and the central modes between 0.2– 3 µm. While the ultrafine mode for the HS coal is less than 0.3 µm, the coarse mode larger than 5 µm and the central mode
Fig. 4. Central mode particle morphology (about 2 µm).
between 0.3–5 µm. What follows is to further assess the rationality of the three modes identified in mass fraction size distributions of the Al.
Fig. 3. Typical ultrafine mode particle morphology (about 0.1 µm).
3.2.1. The ultrafine mode The ultrafine modes are seen to possess such a small, narrow size distribution (b 0.2 or 0.3 µm) that they can not be formed
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Fig. 5. Mass fraction size distributions of the Al. Error bars are standard deviations of repeated measurements.
directly from mineral particles present in the raw coal, which are usually discrete grains larger than 1 µm [25]. Additional evidence is that the ultrafine modes generally contain no more than 2.8% of the Al, far lower than the concentration of the Al in the bulk ash. Another key feature of these ultrafine modes, as shown in Fig. 5, is that the mass fraction of the Al in these particles is relatively invariant with particle size, indicating their uniform nature on a particle-to-particle basis. The independence of the concentration of the Al on particle size is the consequence of typical vaporization and subsequent condensation in the high temperature combustion region [16]. Though less attention in
the literature has been paid to the vaporization of the Al due to its inert nature during pulverized coal combustion, it does occur as shown by Kauppien et al. [20] and Quann et al. [23]. Like other refractory elements such as Si, Ca, and Mg [26], the Al usually encounters an oxygen-deficient condition near the fuel particle surface as well and is reduced by CO to more volatile species. The Al vapor species in equilibrium with a melt are Al2O and AlO, but under reducing conditions the dominant vapor species is Al2O [27]. Once formed, these species diffuse away from the parent particle and reoxidise in an oxygen-rich atmosphere. When supersaturated they will homogeneously
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nucleate to form very fine aerosols (0.01 µm) [4]. Then the particles so-formed grow to a size of 0.1–0.2 µm by condensation and coagulation. Since these ultrafine particles are formed from a well-mixed vapor phase at elevated temperatures, a nearly uniform particle composition is expected, which is consistent with the independence of the concentration of the Al on particle size. Therefore, the formation of these ultrafine modes is reasonably attributed to the well-established vaporization and condensation mechanism. 3.2.2. The coarse mode The coarse modes larger than 3 or 5 µm found in this study are clearly the consequence of the melt and coalescence of bulk mineral matter present in the original fuel particles, for the vaporization–condensation mechanism cannot possibly produce such large ash particles as suggested by the classical theory [28]. The mass fraction of the Al in these coarse modes is much higher than that in the ultrafine modes as shown in Fig. 5, but comparable to that in the bulk ash. This further reveals that these large particles are produced directly from mineral grains when the carbon matrix burns away. All of the mineral species in the pulverized coal have melting points or decomposition temperatures lower than 2100 K. But the char temperature during pulverized coal combustion in the practical boiler is very often much higher than 2100 K, up to about 2500 K [9]. Therefore, the mineral grains exposed on the char surface are usually molten and form spherical droplets due to their high surface tension [29]. As the char surface recedes, these ash droplets have a good chance to come into contact with each other and coalesce to form larger particles. If the char particle were to remain intact throughout combustion, only one fly ash particle would be produced by each coal particle. However, experimental results show that, as a consequence of char fragmentation, a much larger number of ash particles are actually formed per char particle. For example, Sarofim et al. [30] found that between 3–5 ash particles greater than 10 µm in diameter were produced per char particle, while 100–500 ash particles greater than 1 µm in diameter were generated per char particle at typical pulverized coal flame temperatures [6]. Therefore, the formation of the coarse mode is controlled by the competition between mineral coalescence and char fragmentation [9]. Since the coarse mode is derived directly from the mineral matter in coal, its composition is expected to be comparable to that of the bulk ash, as evidenced by the concentration of the Al in these coarse modes (Fig. 5). The coalescence of molten minerals tends to diminish the nonuniformity from particle to particle as well, which is shown by the relative independence of the concentration of the Al on particle size in these coarse modes (Fig. 5). 3.2.3. The central mode Similar to those found in our previous study [18], there is also a general central mode in the mass fraction size distributions of the Al in Fig. 5. These central modes are in the size range of 0.2–3 or 0.3–5 µm, qualitatively consistent with experimental results by Linak et al. [11], who highlighted the existence and generality of a central mode between approxi-
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mately 0.7–3 µm. Compared with the other two commonly observed modes, the formation mechanisms for the central mode are less clear up to now. Helble and Sarofim [6] suggested that ash particles in the 1–5 µm size range were formed by perimeter fragmentation of the char particles during combustion, but the mechanism of the shedding of ash droplets from the surface of the char was rejected. Kang [9] and Wu et al. [31] concluded that the combustion of char cenospheres with thinwalled structures gave lower chance of ash growth, hence resulted in a large number of fine ash particles. Baxter [14] attributed the same mechanism as proposed by Kang [9] to the formation of fine ash particles from combustion of bituminous coals. However, he suggested that a fundamentally different mechanism of shedding of ash droplets from the char surface should account for the formation of fine ash particles from combustion of a lignite. Obviously, this is inconsistent with the conclusions from Helble and Sarofim [6]. The above mentioned formation mechanisms for the central mode were assessed by Linak et al. [11] It was found that the central mode had irregular morphologies, little dependence on coal quality and was prevalent for both melting and non-melting coals, and no single mechanism could account for these observations. Therefore, they suggested that the formation of the central mode might be the consequence of multiple mechanisms, although they have not been elucidated in the literature. One would postulate that the central mode is a mixture of ultrafine and coarse particles. If it is the case, on one hand, the central mode should contain both ultrafine and coarse particles. But as shown in Fig. 4, the central mode has different particle morphologies from the other two (Figs. 2 and 3), and seems to lack ultrafine particles. On the other hand, the vaporized ash is often depleted of the Al (Fig. 5) and smaller than 0.5 µm [32] while the coarse particles are rich in the Al (Fig. 5) and larger than 0.5 µm [33]. Taking into account of the fact that the scavenging of fine particles by coarser ones is very small and can be neglected [34], one would expected a steep rise in the concentration of the Al around 0.5 µm, with finer particles containing little the Al but coarser ones a much larger amount of the Al, which is size-independent in both size ranges. However, it is not the case in Fig. 5. As shown in this figure, the concentration of the Al in the central mode shows a systematic increase with increasing particle size, rather than a sharp change as expected. Therefore, the combination of the ultrafine and coarse modes cannot reasonably account for the formation of the central mode. The results from Fig. 5 indicate that the composition of the central mode is more heterogeneous than the other two modes, and it is probably produced by different mechanisms. Generally, there are two main routes by which vapor species in the gas stream can be incorporated into particles. One is what has been discussed previously for the formation of ultrafine particles. When supersaturated, vapor species homogeneously nucleate to form very fine aerosols [4]. The newly-formed nuclei then grow in size by collision and coagulation. The other route is either heterogeneous condensation or adsorption of vaporized materials onto the surface of existing particles, as proposed by Davison et al. [35]. This partitioning behavior is expected to
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result in an increase of concentrations of volatile elements with decreasing particle size due to higher surface-to-volume ratios for smaller particles [16]. In other words, the concentrations of fly ash matrix materials such as the Al, are expected to increase with increasing particle size [36]. Because heterogeneous condensation or adsorption can occur on both nuclei and residual ash particles, the ultrafine particles formed in the combustion region can also have a surface layer of finite thickness composed of volatilized materials. However, since the ultrafine mode is essentially dominated by volatilized species and often undergoes coagulation, the concentration var-
iations of ash matrix materials in this mode are considered to be negligible [36], as has been evidenced by the relative independence of the concentration of the Al on particle size (Fig. 5). In contrast, the concentration of the Al in the central mode shows a systematic increase with increasing particle size, which indicates that the central mode is reasonably produced by heterogeneous condensation/adsorption of vaporized species on fine residual ash particles, rather than on ultrafine particles [18]. This mechanism is fundamentally different from those processes governing the formation of the ultrafine mode, for the cores of the particles in the ultrafine mode are formed from vaporized
Fig. 6. Mass fraction size distributions of the S. Error bars are standard deviations of repeated measurements.
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species at elevated temperatures by nucleation and coagulation, but that in the central mode from fine residual ash particles produced after the char particles burn away. According to Davison et al. [35], one would also expect a decrease in concentrations of volatile elements with increasing particle size in the coarse mode, or conversely, an increase of ash matrix materials such as the Al. But this is not obvious in Fig. 5, where the concentration of the Al seems to be nearly independent of particle size. A possible explanation for this is that much less vaporized species condense on the surface of such large particles and form a layer with a very small thickness. Therefore, the concentration variations of the Al in this mode are negligible. The above results show that heterogeneous condensation or adsorption plays a more significant role in modifying particle composition in the central mode than in the coarse mode. In this point, those mechanisms proposed by other investigators [11– 13] seem to form just the cores of the particles in the central mode size range. The heterogeneous condensation/adsorption mechanism is so pronounced in this mode that it should not be neglected. Though the origins of those fine residual ash particles are not clear at present, it is obvious that they are not formed dominantly by the coalescence of molten mineral materials, which is the controlling mechanism for the formation of the coarse mode. In support of the proposed mechanisms and assumptions, we further investigated the mass fraction size distributions of the S in PM from all the coals studied. As shown in Fig. 6, these mass fraction size distributions of the S can also be divided into three distinct size fractions, based on the relationship between the concentration of the S and particle size. It should be noted that these particle fractions from any coal are corresponding to those found in Fig. 5, indicating the same three particle formation modes. It can be seen that the concentration of the S in the ultrafine mode is generally much higher than that in the other two. The enrichment of the S in the ultrafine mode is consistent with the vaporization–condensation mechanism [20], and is complicated by homogenous nucleation and heterogeneous condensation/reaction of S-containing species [37]. It was reported that some of the SO3 in the gas stream would react with the vaporized alkali metals to form Na2SO4 and K2SO4, and some would form H2SO4. These species could easily find their way into ultrafine particles by both homogenous and heterogeneous condensation [38–40]. In addition, the sulfation of alkaline metals such as Ca and trace metals such as Zn is also an important route by which the S is partitioned into ultrafine particles [38,40]. Since homogeneous nucleation, heterogeneous condensation, and growth by coagulation are usually competitive processes, they further affect the form of size distributions of volatile elements [20]. Therefore, the concentration of the S in the ultrafine mode is somewhat scattered, as shown in Fig. 6. A sharp drop in the concentration of the S is found in the central mode. This indicates that a fundamentally different mechanism should account for the formation of these particles. It can be seen in Fig. 6 that, the concentration of the S in the central mode has a tendency of decrease with increasing particle size. This is apparently consistent with the mechanism proposed
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previously, i.e. heterogeneous condensation/adsorption of vaporized species on existing fine residual ash particles. On the other hand, the concentration of the S in the coarse mode is the lowest and seems to be nearly independent of particle size. This gives a support for the previous assumption, and accounts for the strong independence of the concentration of the Al on particle size in the coarse mode (Fig. 5). 3.2.4. Key findings of this study As discussed in the first subsection, in the particle MSD from combustion of the LPS coal only two particle modes are observed. But in the mass fraction size distribution of the Al from the same coal three distinct particle modes have been identified. They are further confirmed by exploring the mass fraction size distribution of the S. it indicates that PM from combustion of the LPS coal indeed has three distinct particle modes as well, rather than just merely two. The miss of the central mode in the MSD is quite possibly a consequence of the merging of it into the coarse mode. These results show the advantages of mass fraction size distributions of the Al over MSDs in particle mode identification. In addition to the number of particle modes, their size boundaries can be more accurately defined as well through mass fraction size distributions of the Al. This is of great significance as pointed out in the Introduction Section. The second key finding is that heterogeneous condensation/ adsorption seems to play a more significant role in modifying particle composition in the central mode than in the other two. In spite of their complicated nature, those mechanisms proposed by other investigators [11–13] are expected to form just the cores of the particles in the central mode size range. The heterogeneous condensation/adsorption of vaporized species on fine residual ash particles is suggested to more appropriately account for the formation of the central mode. 4. Conclusions PM is generated from combustion of five Chinese pulverized coals of different ranks in a laboratory DTF at 1673 K. Particle mass size distributions, mass fraction size distributions of the Al and S are investigated. The results show that, particles produced from four coals have a trimodal mass size distribution. Aside from the two commonly observed particle modes, there exists a distinct central mode at approximately 2 µm. But in the mass size distribution of the LPS coal ash sample, only two particle modes are observed. However, the mass fraction size distributions of the Al indicate that PM from all the coals, involving the LPS coal, generally has three particle modes. The miss of the central mode in the mass size distribution of the LPS coal ash sample is possibly a consequence of the merging of it into the coarse mode. The proposed formation mechanisms for the found three particle modes are further confirmed by examination of the mass fraction size distributions of the S. The heterogeneous condensation/adsorption of vaporized species on fine residual ash particles is suggested to more appropriately account for the formation of the central mode, although the origins of those fine residual ash particles are not clear at present. The
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present work shows that mass fraction size distributions of the Al seem to be more effective in identifying particle modes and their size boundaries than particle mass size distributions. Acknowledgements The financial support of the National Key Basic Research and Development Program of China (2002CB211602), and the National Natural Science Foundation of China (50325621) should be gratefully acknowledged. This work was also partially supported by the Programme of Introducing Talents of Discipline to Universities (“111” project), China and the Natural Science Foundation of Hubei Province (2006ABC002). Special thanks go to Prof. Wendt at the University of Utah for beneficial discussions. The authors would like to acknowledge the support of the Analytical and Testing Center at Huazhong University of Science & Technology. References [1] C.A. Pope, M.J. Thun, M.M. Namboodiri, D.W. Dockery, J.S. Evans, F.E. Speizer, C.W.J. Heath, American Journal Respiratory Critical Care Medicine 151 (3 Pt 1) (1995) 669–674. [2] D.W. Dockery, C.A. Pope, X. Xu, J.D. Spengler, J.H. Ware, M.E. Fay, B.G.J. Ferris, F.E. Speizer, New England Journal of Medicine 329 (24) (1993) 1753–1759. [3] M.W. McElroy, R.C. Carr, D.S. Ensor, G.R. Markowski, Science 215 (4528) (1982) 13–19. [4] M. Neville, R.J. Quann, B.S. Haynes, A.F. Sarofim, Eighteenth International Symposium on Combustion, The Combustion Institute, Pittsburgh, PA, 1981. [5] J.J. Helble, M. Neville, A.F. Sarofim, Twenty-First International Symposium on Combustion, The Combustion Institute, Pittsburgh, PA, 1986. [6] J.J. Helble, A.F. Sarofim, Combustion and Flame 76 (2) (1989) 183–196. [7] S.G. Kang, A.R. Kerstein, J.J. Helble, A.F. Sarofim, Aerosol Science and Technology 13 (4) (1990) 401–412. [8] W.P. Linak, T.W. Peterson, Aerosol Science and Technology 3 (1) (1984) 77–95. [9] S.G. Kang, Ph.D. Thesis, Department of Chemical Engineering, MIT, 1991. [10] J. Joutsensaari, E.I. Kauppinen, P. Ahonen, T.M. Lind, S.I. Ylatalo, J.K. Jokiniemi, J. Hautanen, M. Kilpelainen, Journal of Aerosol Science 23 (Suppl 1) (1992) 241–244. [11] W.P. Linak, C.A. Miller, W.S. Seames, J.O.L. Wendt, T. Ishinomori, Y. Endo, S. Miyamae, Proceedings of the Combustion Institute 29 (2002) 441–447. [12] J.O.L. Wendt, IFRF Combustion Journal 200301 (2003) Article No.
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Powder Technology 183 (2008) 105 – 114 www.elsevier.com/locate/powtec
A new method for identifying the modes of particulate matter from pulverized coal combustion Dunxi Yu, Minghou Xu ⁎, Hong Yao, Xiaowei Liu, Ke Zhou State Key Laboratory of Coal Combustion, Huazhong University of Science and Technology (HUST), Wuhan, 430074, China Available online 21 November 2007
Abstract Knowledge of the modality of the size distribution of particulate matter (PM) from pulverized coal combustion is of great significance from the viewpoint of exposure and risk assessment. Mass or number size distributions are usually used in modality analyses, but sometimes the central particle mode fails to be detected due to overlapping. This work provides a new method for identifying particle modes using mass fraction size distributions of the aluminum (Al). Five Chinese pulverized coals of different ranks were burnt in a laboratory drop tube furnace at 1673 K. The produced PM was size segregated by a low pressure impactor and subjected to elemental composition analysis by X-ray fluorescence (XRF). Particle mass size distributions, mass fraction size distributions of the Al and sulfur (S) were obtained for all the particle samples. The mass size distributions of four coal ash samples all show three distinct particle modes, with a central mode at approximately 2 µm, while the mass size distribution of the LPS coal ash sample only indicates two particle modes. However, the mass fraction size distributions of the Al for all ash samples, including the LPS coal ash sample, generally show three particle modes. The obscurity of the central mode in the mass size distribution of the LPS coal ash sample is expected to be a consequence of the merging of it into the coarse mode. The formation of the central mode is attributed to the more pronounced heterogeneous condensation or adsorption of vaporized species on fine residual ash particles, whose origins are still unclear at present. This is further confirmed by examination of the mass fraction size distributions of the S. These results show that mass fraction size distributions of the Al seem to be more effective in identifying particle modes and their size boundaries than particle mass size distributions. © 2007 Elsevier B.V. All rights reserved. Keywords: Coal; Particulate matter; Mode; Elemental mass fraction size distribution
1. Introduction Pulverized coal combustion is one of the major sources of particulate matter (PM) in the atmosphere, which has received much concern in recent years due to its adverse health effects [1,2]. Knowledge of the detailed characteristics of particle size distributions (PSDs), either in terms of mass or number, is very important for uncovering their formation mechanisms and dynamics both in the combustion systems and atmosphere. One such characteristic is the modality displayed by PSDs. A mode is conventionally defined as a peak in the PSD which can be described by a lognormal function for the mass size distribution of coal-derived PM. But in the context a mode sometimes ⁎ Corresponding author. Tel.: +86 27 87544779 8309; fax: +86 27 87545526. E-mail address: [email protected] (M. Xu). 0032-5910/$ - see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.powtec.2007.11.011
represents a category of particles that are formed by the same processes. Identifying the presence, location and size boundary of modes in PSDs is of great significance not only for understanding their formation mechanisms, but also for exposure and risk assessment. Furthermore, modes could serve as direct pointers to appropriate mitigation strategies to lower environmental and human effects. In most studies, two distinct modes have been observed in particle mass size distributions (MSDs) generated during pulverized coal combustion. It has been well established that these two modes are produced by different mechanisms [3–7]. The coarse mode, between 1–20 µm, is primarily produced by char fragmentation and mineral coalescence. The ultrafine mode, often centered around 0.1 µm, is mainly formed by the vaporization–condensation mechanism. However, the ultrafine mode is not always distinguishable from the coarse one. As
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indicated by Linak and Peterson [8], sometimes it was merged into the larger mode and was overlooked. An important work by McElroy et al. [3] showed that ash particles from six coal-fired utility boilers were generally bimodally distributed. But the complex structure of the larger modes suggested a superposition of more than one discrete particle mode. Regardless of that, the authors just considered the larger particle size distribution as a single particle mode there. Kang is probably the first to declare that ash particles from pulverized coal combustion may have a trimodal size distribution. In his thesis [9], Kang found that, besides a conspicuous ultrafine mode in some cases, there were always two distinct modes present in the N1 µm size range. The central mode was near 4 µm while the largest mode was centered at approximately 8 µm. Kang et al. [7] even developed a Monte Carlo model to simulate the ash coalescence and char fragmentation processes that govern the final particle size distribution. They attributed the bimodal size distribution of residual ash particles N1 µm to the competition between the particle fragmentation and the ash coalescence enhanced by the shrinking peripheral area with the progress of char reaction. Joutsensaari et al. [10] also observed a central mode in MSDs generated in a real scale power plant, in addition to a ultrafine particle mode at about 0.07 µm and a coarse one at about 3 µm. But the size of the central mode was about 0.4 µm on mass, a little different from that found by Kang [9]. Recent experimental results from Linak et al. [11], Wendt [12] and Seames [13] suggest that coal fly ash particle formation is more accurately described as a trimodal particle size distribution. Aside from the well-established ultrafine and coarse modes, a ubiquitous central mode centered at approximately 2.0 µm is highlighted in these studies. By re-examination of literature data [3,6,14,15], Linak et al. [11] found that three particle modes were in fact the rule, not the exception. Less attention to the central mode in the past work was attributed to the low resolution of particle-sampling and-sizing techniques available then. The newly found three particle modes, rather than just merely two, indicate that it is not always effective to identify particle modes by MSDs, since sometimes one mode may be merged into another [3,8,9]. Besides the number of particle modes, the mode size boundary is also an important parameter for modeling, risk assessment and development of mitigation strategies. However, it is often difficult to be defined whether in particle mass or number size distributions. Few past studies have attempted to do so, and a somewhat arbitrary value is usually given. For example, a turning point at approximately 1 µm is often assumed to be the size boundary between particles formed by the vaporization–condensation and fragmentation-coalescence mechanisms, respectively. But it is not always the case at least in two studies [16,17]. In one study, Smith et al. [16] believed that the submicron particles in the 0.1–1 µm size range were formed by the bursting of larger particles due to gas release during rapid heating, followed by coagulation and condensation of volatilized elements. In another study, Sadakata et al. [17] concluded that approximately half of the submicron fly ash larger than 0.1 µm was simply carried over from the submicron coal fragments originally contained in the pulverized raw coal, and that the remainder was newly formed through the breakup of larger coal particles in the devolatilization region.
These results show that the mechanisms contributing to the formation of submicron particles are more complex, not just the vaporization–condensation mechanism. Therefore, the identification of the size boundaries between different modes is also very important for elucidating their origins. Recently, we developed a new approach to particle mode identification by using elemental mass fraction size distributions (EMFSDs) of PM generated from combustion of a sizeclassified Chinese coal [18]. Three distinct modes were also observed in the total mass fraction size distributions of the silicon (Si) and aluminum (Al). The ultrafine and coarse modes are in the size ranges of 0.0281–0.258 and 2.36–9.8 µm, respectively. And the central mode is between 0.377 and 1.58 µm. These results, consistent with others [11–13], show the potential importance of EMFSDs in identifying particle modes. Another important feature of using EMFSDs is that, size boundaries between different modes could be more accurately defined, as indicated in the previous study [18] and will be described in more detail in the present investigation. In addition to the least volatile elements Si and Al, mass fraction size distributions of the sulfur (S) can also be used to indicate particle formation mechanisms [16]. It is reported that the S is probably the only major element that is released almost completely during coal combustion [19]. A fraction of the S is emitted as gas species and the remainder is retained in ash particles after combustion. The incorporation of the S into particles can be through homogeneous nucleation and/or heterogeneous condensation/adsorption. The S is enriched in ultrafine particles [20], because, on one hand, these fine particles contain most of the ash surface area and sulfation and condensation are directly proportional to surface area, and on the other hand, at least a portion of the ultrafine particle materials is formed by homogeneous nucleation from the gas phase such as Na2SO4, K2SO4 or H2SO4 [21]. While much less of the S heterogeneously condenses on the larger residual ash particles and forms a thin surface layer [22]. Since different processes are expected to result in different concentrations of the S in thusformed particles, the concentration variations of the S with particle size could also serve as a complementary indicator of particle formation modes. From the above considerations, the present work proposes an updated method for identifying particle modes using EMFSDs. Five Chinese coals of different ranks (from bituminous coal to lignite) were burnt in a laboratory drop tube furnace (DTF) at 1673 K. The produced PM was collected by a Dekati lowpressure impactor (DLPI) and subjected to elemental composition analysis. The mass fraction size distributions of the Al, rather than the Si and Al as used in the previous work [18], were used to characterize particle formation modes and their size boundaries. And they were supported by examination of the mass fraction size distributions of the S. The results show that EMFSDs seem to be more effective than MSDs for identifying particle modes. 2. Experimental Five Chinese pulverized coals with a particle size less than 63 µm were used in this study. The LPS, HS, PDS and PX coals
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are bituminous coals, while the XLT coal is a lignite. The proximate analysis, ultimate analysis, and composition of the low temperature ash of the coal samples are shown in Table 1. The coal samples were burnt in a laboratory DTF described in the previous study [18] at 1673 K. The DTF is 2 m long and has an inner diameter of 56 mm. It was electrically heated to the desired temperature. The atmosphere consisted of 20% oxygen with pure nitrogen being balanced. The coal particles in a Sankyo Piotech Micro Feeder (Model MFEV-10) were entrained by a primary gas stream and injected into the combustor at a feeding rate of approximately 0.2 g/min. A secondary gas stream was provided via a flow straightener on the top of the reactor to ensure that coal particles were burnt almost completely. The produced PM was routed through a watercooled sampling probe and high-purity nitrogen was supplied from the top of the probe to freeze subsequent reactions in the probe. Combustion particles were then collected by a 13-stage DLPI, before which a cyclone was used to remove particles N 10 µm. For elemental analysis, Teflon filters were used. Those particles b 10 µm (PM10) were classified into 13 size fractions (d50 0.03–10 µm). Gravimetric analysis of impactor samples was carried out by weighing the substrates carefully before and after sampling on a Sartorius M2P Microbalance (readability, 0.001 mg) in a clean, almost constant humidity laboratory room. The elemental composition of each sized PM fraction was analyzed by X-ray fluorescence (XRF). And particle morphologies were characterized by a Sirion 200 Field Emission Scanning Electron Microscope (FESEM) equipped with an Energy Dispersive Spectrometer (EDS). More detailed information on the DTF and experimental procedures can be found elsewhere [18]. Table 1 Coal properties Property
HS
PDS
PX
XLT
Proximate analysis/wt.%, a.d. Moisture 1.8 Volatile matter 21.1 Ash 27.6 Fixed carbon 49.5
2.7 15.3 50.0 32.0
1.0 36.0 18.7 44.3
1.2 19.9 40.2 38.7
15.0 48.9 24.4 11.7
Ultimate analysis/wt.%, a.d. C 57.7 H 5.0 S 3.4 N 0.7 Oa 3.8
35.4 1.4 6.2 0.5 3.8
67.0 5.3 3.8 1.1 3.1
48.7 3.1 0.8 0.9 5.1
41.0 4.8 2.9 1.2 10.7
Ash composition/wt. % SiO2 41.8 Al2O3 18.4 CaO 7.7 Fe2O3 15.2 MgO 1.0 Na2O 2.5 K2O 1.6 P2O5 0.6 9.3 SO3 Others 1.9
49.9 32.8 2.0 3.6 1.9 3.4 1.1 0.3 4.4 0.6
43.8 29.2 4.1 13.3 1.3 2.0 1.7 0.2 3.0 1.4
60.2 28.6 1.1 1.9 1.5 1.0 2.2 1.0 1.7 0.8
24.6 9.2 27.4 11.7 1.7 2.0 1.2 0.7 20.6 0.9
a
By difference.
LPS
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3. Results and discussion 3.1. Particle mass size distributions Fig. 1 depicts particle MSDs produced during combustion of five Chinese coals (see Table 1). As shown in Fig. 1a, PM from combustion of the LPS coal seems to be bimodally distributed. The ultrafine mode is near 0.1 µm and the coarse mode is beyond 0.3 µm, if assuming that the turning point at about 0.3 µm is the size boundary between the two modes. In contrast, PM from combustion of the other four coals seems to have three particle modes, as indicated in Fig. 1b, c, d, and e. Aside from the commonly observed ultrafine and coarse modes, there is a definite inflection point, if not a obvious peak, at approximately 2 µm on the shoulder of the coarse mode. It is most possibly a central mode like the one found by Linak et al. [11], Wendt [12] and Seames [13]. Some evidence for the central mode can be seen from its different particle morphology from the other two modes. As seen in Fig. 2, the coarse mode particles (about 5 µm) are primarily spherical in shape and the EDS spectra show that they are mainly aluminosilicates, indicating the production of these particles from molten minerals. Fig. 3 presents the typical ultrafine mode particle morphology (about 0.1 µm). The examined ultrafine mode particles are also primarily spherical in shape and have smooth surfaces. The EDS spectra indicate that they are relatively rich in volatile elements (alkalis and some trace elements) with respect to the coarse mode particles. The formation of the ultrafine mode can be explained by the well-established vaporization–condensation mechanism. In contrast, the particle morphologies of the central mode (about 2 µm) seem to be more complex than those of the coarse and ultrafine modes. As indicated in Fig. 4a, some central mode particles are nearly spherical in shape but often have defects. A partially melted particle can also be seen in Fig. 4a with fine irregular fragments sintered onto its surface. Agglomerates of stretched particles are common morphologies in the central mode and one typical example is shown in Fig. 4b. Such particles are expected to have higher surface-to-volume ratios than the coarse mode. Some ash cenospheres, as shown in Fig. 4c, have also been found in the central mode size range. The evolution of gas species from molten particles can account for the formation of these particles. The EDS spectra indicate that these particles are mainly Ca–Si–Al and Fe–Si–Al structures. The difference in particle morphology between the central mode and the other two suggest that, the formation of the central mode seems to be different and its existence is reasonable. As shown in Fig. 1, the HS and PDS ultrafine modes are at approximately 0.1 µm (Fig. 1b and c), while the PX and XLT ultrafine modes are a little larger, at approximately 0.2 µm (Fig. 1d and e). All the coarse modes are larger than 8 µm, if assuming that the turning point near this value is the size boundary between the central and coarse modes. Based on the same assumption, the size boundary between the ultrafine and central modes is near 0.7 µm for the HS, PDS and PX coals, but approximately 0.4 µm for the XLT coal.
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Fig. 1. Particle mass size distributions.
The ubiquitous nature of the central mode has been highlighted by Linak et al. [11] The above results for four Chinese coals of different ranks (i.e. the HS, PDS, PX and XLT coals) also show a distinct central mode. Then one would wonder if the LPS coal ash has a central mode as well, and the miss of it is simply because it is merged into the coarse mode and difficult to detect in the MSD. The answer will be given in the following subsection. 3.2. Elemental mass fraction size distributions In our previous study [18] the total mass fraction size distributions of the Si and Al were used to characterize particle
formation modes generated during combustion of a size-classified Chinese bituminous coal. Though both the Si and Al are less volatile during combustion when compared with alkali metals, the Si is actually easier to vaporize than the Al through more volatile suboxides such as SiO. For example, Kauppien et al. [20] found that the concentration of the Si in the fine particle mode from a utility boiler was 96 µg/Nm3, while that of the Al only 43 µg/Nm3, less than half of the Si concentration. Quann et al. [23] investigated the composition and size distribution of the fly ash produced by burning of a Montana lignite in a laboratory-pulverized coal combustion system at two temperatures of 2050 and 2450 K. Their results showed that SiO2 accounted for 5.11% of the submicron fume, while Al2O3
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Fig. 2. Typical morphologies of coarse mode particles (about 5 µm).
a far lower fraction of 0.97%. Calculations by Zhang et al. [24] showed that about 0.6% SiO2 and 0.3% Al2O3 vaporized for combustion of two bituminous coals at 1723 K. These results indicate that the Al is much less volatile than the Si during combustion. Its concentration variations with particle size are expected to be more easily observed than the combination of the Si and Al. Therefore, the present work focuses on mass fraction size distributions of the Al, rather than those of the Si and Al, as applied in the previous study [18]. Fig. 5 shows mass fraction size distributions of the Al for PM produced by burning the five Chinese coals. Similar to the total mass fraction size distributions of the Si and Al obtained in the previous work [18], these mass fraction size distributions of the Al can also be readily divided into three distinct size fractions based on its concentration variations with particle size. This indicates the existence and generality of three particle formation modes. As shown in this figure, except the HS coal (Fig. 5b), the corresponding particle modes for the other four coals (Fig. 5a, c, d, and e) are in the same size ranges. The ultrafine modes for the LPS, PDS, PX and XLT coals are less than 0.2 µm, the coarse modes larger than 3 µm and the central modes between 0.2– 3 µm. While the ultrafine mode for the HS coal is less than 0.3 µm, the coarse mode larger than 5 µm and the central mode
Fig. 4. Central mode particle morphology (about 2 µm).
between 0.3–5 µm. What follows is to further assess the rationality of the three modes identified in mass fraction size distributions of the Al.
Fig. 3. Typical ultrafine mode particle morphology (about 0.1 µm).
3.2.1. The ultrafine mode The ultrafine modes are seen to possess such a small, narrow size distribution (b 0.2 or 0.3 µm) that they can not be formed
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Fig. 5. Mass fraction size distributions of the Al. Error bars are standard deviations of repeated measurements.
directly from mineral particles present in the raw coal, which are usually discrete grains larger than 1 µm [25]. Additional evidence is that the ultrafine modes generally contain no more than 2.8% of the Al, far lower than the concentration of the Al in the bulk ash. Another key feature of these ultrafine modes, as shown in Fig. 5, is that the mass fraction of the Al in these particles is relatively invariant with particle size, indicating their uniform nature on a particle-to-particle basis. The independence of the concentration of the Al on particle size is the consequence of typical vaporization and subsequent condensation in the high temperature combustion region [16]. Though less attention in
the literature has been paid to the vaporization of the Al due to its inert nature during pulverized coal combustion, it does occur as shown by Kauppien et al. [20] and Quann et al. [23]. Like other refractory elements such as Si, Ca, and Mg [26], the Al usually encounters an oxygen-deficient condition near the fuel particle surface as well and is reduced by CO to more volatile species. The Al vapor species in equilibrium with a melt are Al2O and AlO, but under reducing conditions the dominant vapor species is Al2O [27]. Once formed, these species diffuse away from the parent particle and reoxidise in an oxygen-rich atmosphere. When supersaturated they will homogeneously
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nucleate to form very fine aerosols (0.01 µm) [4]. Then the particles so-formed grow to a size of 0.1–0.2 µm by condensation and coagulation. Since these ultrafine particles are formed from a well-mixed vapor phase at elevated temperatures, a nearly uniform particle composition is expected, which is consistent with the independence of the concentration of the Al on particle size. Therefore, the formation of these ultrafine modes is reasonably attributed to the well-established vaporization and condensation mechanism. 3.2.2. The coarse mode The coarse modes larger than 3 or 5 µm found in this study are clearly the consequence of the melt and coalescence of bulk mineral matter present in the original fuel particles, for the vaporization–condensation mechanism cannot possibly produce such large ash particles as suggested by the classical theory [28]. The mass fraction of the Al in these coarse modes is much higher than that in the ultrafine modes as shown in Fig. 5, but comparable to that in the bulk ash. This further reveals that these large particles are produced directly from mineral grains when the carbon matrix burns away. All of the mineral species in the pulverized coal have melting points or decomposition temperatures lower than 2100 K. But the char temperature during pulverized coal combustion in the practical boiler is very often much higher than 2100 K, up to about 2500 K [9]. Therefore, the mineral grains exposed on the char surface are usually molten and form spherical droplets due to their high surface tension [29]. As the char surface recedes, these ash droplets have a good chance to come into contact with each other and coalesce to form larger particles. If the char particle were to remain intact throughout combustion, only one fly ash particle would be produced by each coal particle. However, experimental results show that, as a consequence of char fragmentation, a much larger number of ash particles are actually formed per char particle. For example, Sarofim et al. [30] found that between 3–5 ash particles greater than 10 µm in diameter were produced per char particle, while 100–500 ash particles greater than 1 µm in diameter were generated per char particle at typical pulverized coal flame temperatures [6]. Therefore, the formation of the coarse mode is controlled by the competition between mineral coalescence and char fragmentation [9]. Since the coarse mode is derived directly from the mineral matter in coal, its composition is expected to be comparable to that of the bulk ash, as evidenced by the concentration of the Al in these coarse modes (Fig. 5). The coalescence of molten minerals tends to diminish the nonuniformity from particle to particle as well, which is shown by the relative independence of the concentration of the Al on particle size in these coarse modes (Fig. 5). 3.2.3. The central mode Similar to those found in our previous study [18], there is also a general central mode in the mass fraction size distributions of the Al in Fig. 5. These central modes are in the size range of 0.2–3 or 0.3–5 µm, qualitatively consistent with experimental results by Linak et al. [11], who highlighted the existence and generality of a central mode between approxi-
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mately 0.7–3 µm. Compared with the other two commonly observed modes, the formation mechanisms for the central mode are less clear up to now. Helble and Sarofim [6] suggested that ash particles in the 1–5 µm size range were formed by perimeter fragmentation of the char particles during combustion, but the mechanism of the shedding of ash droplets from the surface of the char was rejected. Kang [9] and Wu et al. [31] concluded that the combustion of char cenospheres with thinwalled structures gave lower chance of ash growth, hence resulted in a large number of fine ash particles. Baxter [14] attributed the same mechanism as proposed by Kang [9] to the formation of fine ash particles from combustion of bituminous coals. However, he suggested that a fundamentally different mechanism of shedding of ash droplets from the char surface should account for the formation of fine ash particles from combustion of a lignite. Obviously, this is inconsistent with the conclusions from Helble and Sarofim [6]. The above mentioned formation mechanisms for the central mode were assessed by Linak et al. [11] It was found that the central mode had irregular morphologies, little dependence on coal quality and was prevalent for both melting and non-melting coals, and no single mechanism could account for these observations. Therefore, they suggested that the formation of the central mode might be the consequence of multiple mechanisms, although they have not been elucidated in the literature. One would postulate that the central mode is a mixture of ultrafine and coarse particles. If it is the case, on one hand, the central mode should contain both ultrafine and coarse particles. But as shown in Fig. 4, the central mode has different particle morphologies from the other two (Figs. 2 and 3), and seems to lack ultrafine particles. On the other hand, the vaporized ash is often depleted of the Al (Fig. 5) and smaller than 0.5 µm [32] while the coarse particles are rich in the Al (Fig. 5) and larger than 0.5 µm [33]. Taking into account of the fact that the scavenging of fine particles by coarser ones is very small and can be neglected [34], one would expected a steep rise in the concentration of the Al around 0.5 µm, with finer particles containing little the Al but coarser ones a much larger amount of the Al, which is size-independent in both size ranges. However, it is not the case in Fig. 5. As shown in this figure, the concentration of the Al in the central mode shows a systematic increase with increasing particle size, rather than a sharp change as expected. Therefore, the combination of the ultrafine and coarse modes cannot reasonably account for the formation of the central mode. The results from Fig. 5 indicate that the composition of the central mode is more heterogeneous than the other two modes, and it is probably produced by different mechanisms. Generally, there are two main routes by which vapor species in the gas stream can be incorporated into particles. One is what has been discussed previously for the formation of ultrafine particles. When supersaturated, vapor species homogeneously nucleate to form very fine aerosols [4]. The newly-formed nuclei then grow in size by collision and coagulation. The other route is either heterogeneous condensation or adsorption of vaporized materials onto the surface of existing particles, as proposed by Davison et al. [35]. This partitioning behavior is expected to
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result in an increase of concentrations of volatile elements with decreasing particle size due to higher surface-to-volume ratios for smaller particles [16]. In other words, the concentrations of fly ash matrix materials such as the Al, are expected to increase with increasing particle size [36]. Because heterogeneous condensation or adsorption can occur on both nuclei and residual ash particles, the ultrafine particles formed in the combustion region can also have a surface layer of finite thickness composed of volatilized materials. However, since the ultrafine mode is essentially dominated by volatilized species and often undergoes coagulation, the concentration var-
iations of ash matrix materials in this mode are considered to be negligible [36], as has been evidenced by the relative independence of the concentration of the Al on particle size (Fig. 5). In contrast, the concentration of the Al in the central mode shows a systematic increase with increasing particle size, which indicates that the central mode is reasonably produced by heterogeneous condensation/adsorption of vaporized species on fine residual ash particles, rather than on ultrafine particles [18]. This mechanism is fundamentally different from those processes governing the formation of the ultrafine mode, for the cores of the particles in the ultrafine mode are formed from vaporized
Fig. 6. Mass fraction size distributions of the S. Error bars are standard deviations of repeated measurements.
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species at elevated temperatures by nucleation and coagulation, but that in the central mode from fine residual ash particles produced after the char particles burn away. According to Davison et al. [35], one would also expect a decrease in concentrations of volatile elements with increasing particle size in the coarse mode, or conversely, an increase of ash matrix materials such as the Al. But this is not obvious in Fig. 5, where the concentration of the Al seems to be nearly independent of particle size. A possible explanation for this is that much less vaporized species condense on the surface of such large particles and form a layer with a very small thickness. Therefore, the concentration variations of the Al in this mode are negligible. The above results show that heterogeneous condensation or adsorption plays a more significant role in modifying particle composition in the central mode than in the coarse mode. In this point, those mechanisms proposed by other investigators [11– 13] seem to form just the cores of the particles in the central mode size range. The heterogeneous condensation/adsorption mechanism is so pronounced in this mode that it should not be neglected. Though the origins of those fine residual ash particles are not clear at present, it is obvious that they are not formed dominantly by the coalescence of molten mineral materials, which is the controlling mechanism for the formation of the coarse mode. In support of the proposed mechanisms and assumptions, we further investigated the mass fraction size distributions of the S in PM from all the coals studied. As shown in Fig. 6, these mass fraction size distributions of the S can also be divided into three distinct size fractions, based on the relationship between the concentration of the S and particle size. It should be noted that these particle fractions from any coal are corresponding to those found in Fig. 5, indicating the same three particle formation modes. It can be seen that the concentration of the S in the ultrafine mode is generally much higher than that in the other two. The enrichment of the S in the ultrafine mode is consistent with the vaporization–condensation mechanism [20], and is complicated by homogenous nucleation and heterogeneous condensation/reaction of S-containing species [37]. It was reported that some of the SO3 in the gas stream would react with the vaporized alkali metals to form Na2SO4 and K2SO4, and some would form H2SO4. These species could easily find their way into ultrafine particles by both homogenous and heterogeneous condensation [38–40]. In addition, the sulfation of alkaline metals such as Ca and trace metals such as Zn is also an important route by which the S is partitioned into ultrafine particles [38,40]. Since homogeneous nucleation, heterogeneous condensation, and growth by coagulation are usually competitive processes, they further affect the form of size distributions of volatile elements [20]. Therefore, the concentration of the S in the ultrafine mode is somewhat scattered, as shown in Fig. 6. A sharp drop in the concentration of the S is found in the central mode. This indicates that a fundamentally different mechanism should account for the formation of these particles. It can be seen in Fig. 6 that, the concentration of the S in the central mode has a tendency of decrease with increasing particle size. This is apparently consistent with the mechanism proposed
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previously, i.e. heterogeneous condensation/adsorption of vaporized species on existing fine residual ash particles. On the other hand, the concentration of the S in the coarse mode is the lowest and seems to be nearly independent of particle size. This gives a support for the previous assumption, and accounts for the strong independence of the concentration of the Al on particle size in the coarse mode (Fig. 5). 3.2.4. Key findings of this study As discussed in the first subsection, in the particle MSD from combustion of the LPS coal only two particle modes are observed. But in the mass fraction size distribution of the Al from the same coal three distinct particle modes have been identified. They are further confirmed by exploring the mass fraction size distribution of the S. it indicates that PM from combustion of the LPS coal indeed has three distinct particle modes as well, rather than just merely two. The miss of the central mode in the MSD is quite possibly a consequence of the merging of it into the coarse mode. These results show the advantages of mass fraction size distributions of the Al over MSDs in particle mode identification. In addition to the number of particle modes, their size boundaries can be more accurately defined as well through mass fraction size distributions of the Al. This is of great significance as pointed out in the Introduction Section. The second key finding is that heterogeneous condensation/ adsorption seems to play a more significant role in modifying particle composition in the central mode than in the other two. In spite of their complicated nature, those mechanisms proposed by other investigators [11–13] are expected to form just the cores of the particles in the central mode size range. The heterogeneous condensation/adsorption of vaporized species on fine residual ash particles is suggested to more appropriately account for the formation of the central mode. 4. Conclusions PM is generated from combustion of five Chinese pulverized coals of different ranks in a laboratory DTF at 1673 K. Particle mass size distributions, mass fraction size distributions of the Al and S are investigated. The results show that, particles produced from four coals have a trimodal mass size distribution. Aside from the two commonly observed particle modes, there exists a distinct central mode at approximately 2 µm. But in the mass size distribution of the LPS coal ash sample, only two particle modes are observed. However, the mass fraction size distributions of the Al indicate that PM from all the coals, involving the LPS coal, generally has three particle modes. The miss of the central mode in the mass size distribution of the LPS coal ash sample is possibly a consequence of the merging of it into the coarse mode. The proposed formation mechanisms for the found three particle modes are further confirmed by examination of the mass fraction size distributions of the S. The heterogeneous condensation/adsorption of vaporized species on fine residual ash particles is suggested to more appropriately account for the formation of the central mode, although the origins of those fine residual ash particles are not clear at present. The
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