Emission of suspended PM10 from laboratory-scale coal combustion and its correlation with coal mineral properties

Emission of suspended PM10 from laboratory-scale coal combustion and its correlation with coal mineral properties

Fuel 85 (2006) 194–203 www.fuelfirst.com Emission of suspended PM10 from laboratory-scale coal combustion and its correlation with coal mineral prope...

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Fuel 85 (2006) 194–203 www.fuelfirst.com

Emission of suspended PM10 from laboratory-scale coal combustion and its correlation with coal mineral properties Lian Zhang, Yoshihiko Ninomiya * Department of Applied Chemistry, College of Engineering, Chubu University, 1200 Matsumoto-cho, Kasugai, 487-8501 Aichi, Japan Received 10 October 2004; received in revised form 29 November 2004; accepted 28 March 2005 Available online 15 September 2005

Abstract Four pulverized coals were subjected to combustion in a laboratory-scale drop tube furnace to investigate the emission of suspended particulate matter smaller than 10 mm (PM10) and to study the correlation of PM10 emission with mineral properties of the coals. Combustion conditions of 1200 8C, 2.4 s and 20% atmospheric oxygen content were used and all the carbon was consumed under given conditions. The properties of PM10 were studied including its concentration, particle size distribution and elemental composition. Two typical sizes were also subjected to Computer controlled scanning electron microscopy (CCSEM) analysis for determination of chemical species within them. To investigate the influence of coal mineral properties, the metallic elements in the raw coals were divided into three parts: organically bound, included inorganic particles and excluded ones. The results indicated that during coal combustion, about 0.5–2.5 wt% of inherent minerals changed into the suspended PM10. With an increase in the coal ash content, the concentration of PM10 increased proportionally. The resulting PM10 had a bimodal size distribution with two peaks around 2.5 and 0.06 mm, respectively. SiO2 and Al2O3 dominated the large mode around 2.5 mm, which is formed by the direct transformation of inherent minerals. On the other hand, SO3 and P2O5 were prevalent in the small mode around 0.06 mm, which is formed by vaporization of these two elements. For other metals found in PM10, the refractory metals were enriched in the large mode, with concentrations proportional to their content in the excluded minerals in the raw coal. Volatile metals were however enriched in the small mode since, they react with gaseous SO2 and P2O5 to form sulfates and phosphates in the solid phase. The study showed that experimental observations agree with thermodynamic equilibrium considerations. q 2005 Elsevier Ltd. All rights reserved. Keywords: Coal combustion; PM10; Coal mineral properties; Direct transformation; Vaporization

1. Introduction The emission of PM10, particulates less than 10.0 (m in diameter, is one of the major sources of pollution from the combustion of coal [1]. Over the past few decades, standards for control of PM10 have become progressively stringent [2]. Even so, there are still many fine particulates that elude control devices and are emitted into the atmosphere [1]. This is a major motivation for studies on the formation of coal combustionderived PM10 as well as their control. Many studies have been carried out to investigate the formation of PM10 from both fundamental and practical points of view. In principle, PM10 are formed from changes in inherent metallic elements during coal combustion. In coal, a portion of inherent metallic elements has diameters * Corresponding author. Tel.: C81 568 51 9178; fax: C81 568 51 1499. E-mail address: [email protected] (Y. Ninomiya).

0016-2361/$ - see front matter q 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.fuel.2005.03.034

smaller than 10.0 mm, which may directly transfer into PM10 without any chemical or physical changes. In addition, a portion may undergo fragmentation to form fine particulates as well, especially for refractory elements. This is termed as the solid-to-particulate pathway governing the formation of particulates larger than 1.0 mm [3–5]. On the other hand, volatile metals initially undergo vaporization in the combustion flame; the vapors produced subsequently undergo nucleation, condensation, deposition and agglomeration to form particulates smaller than 1.0 mm; this is a solid–vapor-particulate pathway [3–8]. This portion is toxic because of the prevalence of heavy metals within the particulates. In addition to investigating the formation mechanisms, parametric studies have been carried out to investigate the influence of several factors affecting PM10 formation. Coal combustion devices were thought to be paramount. The air ratio, coal rank, and particle size of coal were of concern as well. Coal combustion devices affect coal conversion greatly. The higher the carbon conversion, the lower is the

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PM10 concentration in the exhaust gas because less soot is formed [9]. With increasing oxygen pressure, more of the inherent metals directly transfer or vaporize to form PM10 [10,11]. Coal rank affects the distribution of inherent minerals within as well as carbon conversion in combustion. Accordingly, PM10 formation is affected as well [12]. Moreover, our previous study suggests that with decreasing coal particle size, more of the inherent minerals change into excluded particles, which preferentially transform into PM10 [13]. There are still several little-understood mechanisms for PM10 formation. Among them, the influence of coal properties, especially its mineralogical composition, has not been elucidated. Few studies have taken the heterogeneity of coal minerals into account. Correspondingly, there is still a lack of accurate guidelines for the selection of coal to minimize its PM10 emission in industrial plants. Furthermore, the bulk properties of PM10 have been studied until now whereas few studies have given attention to its heterogeneity. A different route, as discussed before, governs the formation of coarse particles and fine/ultrafine particles. Accordingly, the sources for the formation of different portions of PM10 must originate at different parts of raw coal. The present paper aims to link the formation of suspended PM10 with the mineral properties of coal. PM10 was segregated into several groups according to elemental composition. The coal mineral composition was analyzed by several procedures to elucidate its heterogeneity: chemical fractionation was conducted to quantify the contents of organically bound elements in raw coals. Computer controlled scanning electron microscopy (CCSEM) was used to quantitatively determine the particle size distribution and association of inorganic minerals in the coals. The association of inorganic minerals is defined as included particles existing in the carbonaceous matrix and excluded particles not associated with the carbonaceous matrix [14]. The formation of each group of PM10 was linked with different parts of inherent minerals quantified by the abovementioned analysis procedure. CCSEM speciation on two sizes of PM10 and thermodynamic equilibrium calculations were also conducted to further elucidate the relationship between coal mineral properties and PM10 formation. 2. Experimental 2.1. Coal properties Four Chinese coals, Yanzhou high-sulfur (YZHS), Yanzhou low-sulfur (YZLS), Baotou (BT) and Wangfg (WFG), were used in the investigation. The coals were ground to smaller than 125 mm and dried overnight, prior to use. Their properties are listed in Table 1. Clearly, the four coals have different compositions with the ash content ranging from 6.43 wt% in BT coal to 12.43 wt% in YZHS coal. The contents of the volatile matter and fixed carbon vary greatly with the coal type. The low-temperature ashing (LTA) ash compositions, measured by X-ray fluorescence (XRF), are shown in

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Table 1 Properties of the four coals used in this study Ultimate analysis, daf, (wt%) C H YZHS 77.85 1.75 YZLS 79.64 4.62 Baotou 77.05 4.82 Wangfg 88.52 5.47 Proximate analysis, as received, (wt%) Ash Moisture YZHS 12.43 0.40 YZLS 7.29 4.01 Baotou 6.43 1.75 Wangfg 8.23 0.71 a

N 1.60 1.05 1.25 2.22

SCOa 18.80 14.69 16.88 3.79

Volatile 41.10 34.00 35.30 28.86

Fixed carbona 46.07 54.70 56.52 62.20

By difference.

Table 2. The data further indicates the broad variation of the coals selected for this study. SiO2 and Al2O3 are the most prevalent oxides. The contents of the other major oxides, including CaO, Fe2O3, SO3, P2O5, K2O and Na2O, however vary greatly with coal type. Finally, the distributions of the inherent minerals are listed in Table 3 for the four coals. The percentage of organically bound metals was determined by extracting raw coals with a solution of 1 M ammonia acetate. Analysis of SEM images, obtained from CCSEM measurements, was used for quantifying the included and excluded inorganic minerals. It is noteworthy that the organically bound metals are salts of organic acids in the raw coals. Thus, they are too small to be detected by CCSEM due to the limitations of the CCSEM. The included inorganic metals analyzed by CCSEM are larger than 0.5 mm. The mineral distribution varies greatly with the coal type. YZHS coal has the greatest amount of organically bound metals, the majority of which is sulfur. On the other hand, WFG coal has the lowest amount of organically bound metals, but a relatively large amount of included minerals, further suggesting different particle size distributions of the minerals in these coals. Table 2 Ash composition, wt% on basis of total ash

SiO2 Al2O3 Fe2O3 CaO K2O Na2O TiO2 SO3 MgO Cl BaO P2O5 MnO ZrO2 CuO ZnO NiO

YZHS

YZLS

BT

WFG

11.98 5.77 27.50 15.63 0.43 0.17 0.80 35.91 0.36 0.40 0.38 0.14 0.22 0.13 0.08 0.06 0.04

27.91 11.14 27.11 5.83 1.80 0.29 1.80 21.29 0.46 0.30 0.00 1.21 0.29 0.26 0.08 0.09 0.14

32.88 6.71 28.95 13.72 2.23 0.68 1.51 11.11 0.81 0.30 0.00 0.29 0.44 0.00 0.13 0.10 0.14

34.07 17.24 16.43 4.90 1.76 0.71 6.71 15.73 0.36 0.48 0.00 0.47 0.21 0.29 0.24 0.23 0.15

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Table 3 Distribution of inherent minerals in the four coals

2.4. Thermodynamic equilibrium calculation

Coal

Total ash (wt%)

Organically bound (wt%)

Inorganic minerals (wt%) Included Excluded

YZHS YZLS BT WFG

12.43 7.29 6.43 8.23

4.97 2.16 3.91 1.25

0.96 2.52 0.40 2.72

To justify the formation routes of PM10 having different sizes, thermodynamic calculations were carried out using the Factsagee 5.1 software. The phase change of particulates larger than 1.0 mm, the vaporization of volatile elements, and chemical reactions among metallic vapors were investigated.

6.50 2.61 2.11 4.26

3. Results and discussion

Coal combustion was carried out in a laboratory-scale drop tube furnace, whose configuration and combustion procedure have been described in detail elsewhere [15]. The reaction temperature was maintained at 1200 8C, and the residence time of gas in the furnace was 2.4 s for all runs. Coal was fed at about 0.3 g/min into the furnace. An oxidizing atmosphere, with 20% O2 and N2 being the balance, was used. All the coals were burnt completely under these conditions. The exiting gas, containing the solid products, was initially quenched with N2 and collected by a water-cooling probe. Subsequently, coarse ash particles were collected by a cyclone. Meanwhile, the suspension of ultrafine particles was further diluted with air, and immediately directed to a Low-Pressure-Impactor (LPI) for size-segregated collection. The LPI used here is composed of 13 stages having aerodynamic cut-off diameters ranging from 0.03 to 12.1 mm. Each stage is composed of a filter above a substrate and a substrate holder. The pressure after the final stage is approximately 73.3 kPa. The cut-off sizes of both the cyclone and the first stage of LPI are noteworthy. The cyclone has a cut-off size of around 10.0 mm, which indicates that it collects the majority of coarse particles having a sizeR10.0 mm. Meanwhile, a few of the finer particles, smaller than 10.0 mm, were also collected by it. On the other hand, the first stage of LPI collected particles around its cut-off size of 12.1 mm. These collected particles were added to those collected by the cyclone to form the total coarse ash with a sizeR10.0 mm. In the present study, suspended PM10 is defined as those particles collected by the other stages of the LPI having a cut-off size smaller than 10.0 mm. The fine particles collected by the cyclone were however ignored. 2.3. PM10 characterization By the size-segregated collection using LPI, both the concentration of PM10 and its particle size distribution were obtained simultaneously. Each size of PM10 was also subjected to XRF to quantify the elemental composition. Two typical sizes, 2.5 and 0.13 mm, were also subjected to CCSEM to determine their chemical forms. The procedures for CCSEM analysis and the data interpretation have been previously explained in detail [16].

3.1. Emission of PM10 in coal combustion and influence of coal mineral properties The emission of PM10 from the combustion of the four coals is shown as a function of the particle size distribution in Fig. 1. A bimodal distribution was obtained for each case regardless of the coal type. The large mode is around 2.5 mm and the small one around 0.06 mm. A 1.0 mm size acts as the boundary line classifying PM10 into two groups: (1) a portion with size equal to and greater than 1.0 mm, termed as PM1.0C hereafter, and (2) particles smaller than 1.0 mm, termed as PM1. These findings are similar to results reported elsewhere [17]. It can be concluded that PM1C is derived from the direct transfer of inherent minerals, whereas PM1.0 is mostly generated from the vaporization of volatile metals within the coals. In addition, the amounts of these two modes vary greatly with coal type, implying their dependence on coal properties. The influence of coal properties, specifically its ash content, was initially plotted as a function of the percentage of ash transformed into PM10 as shown in Fig. 2. A fairly linear relationship was found, indicating that as the coal ash content increased, the emitted PM10 concentration also proportionally increased. In this study, it was found that about 0.5–2.5 wt% of the inherent minerals were transformed into the suspended PM10 during coal combustion, even after the collection of ashes by the cyclone installed before LPI. The distribution of individual elements is shown in Fig. 3. The concentrations of SiO2 and Al2O3 were added together as 0.7 0.6 Concentration, µg/g_ash

2.2. Coal combustion procedure and collection of suspended PM10

0.5

YZHS YZLS BT WFG

0.4 0.3 0.2 0.1 0.0 0.01

0.1

1

10

Diameter, µm Fig. 1. Particle size distribution of PM10 emitted by the combustion of the four coals.

Wt% of coalash transferred into PM10

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2.5 2.0 1.5 1.0 0.5 0.0 5

6

7 8 9 10 11 Content of ash in raw coals, wt%

12

13

Fig. 2. Influence of inherent coal ash content on the percentage of inherent ash transformed into PM10.

the aluminosilicate group; Na2O, K2O and MgO were added together as the alkali and alkaline earth compounds group; SO2 and P2O5 were added together due to the possibility that both may act as negative ions to react with the other metallic vapors.

The results shown in Fig. 3 indicate that the majority of SiO2 and Al2O3 exist in PM1C and they are the dominant elements. In the case of the other two refractory elements, CaO and Fe2O3, the former mainly exists in PM1C and its low content in PM1 implies negligible vaporization of calcium. Conversely, Fe2O3 has a bimodal distribution and the two modes (around 0.06 and 2.5 mm, respectively), have comparable content, indicating that vaporization of iron-based compounds is severe compared to that of calcium. Besides the above-mentioned elements, the existence of alkali and alkali earth elements was also found in PM1, though they are present in relatively low concentrations, implying the formation of alkali aluminosilicate salts in the larger particles. Both SO3 and P2O5 dominate PM, having contents much higher than those of alkali elements and Fe2O3. This implies the formation of sulfates and phosphates during coal combustion, which will be further discussed later. Cl has a different distribution to the above-mentioned elements. It has the largest mode around 0.2–0.5 mm, which falls between the smaller mode, 0.06 mm, and the larger one around 2.5 mm. Compared to SO2 and P2O5 in the 0.06 mm 6000

(SiO 2+Al2O 3)

Concentration, µg/g_ash

Concentration, µg/g_ash

6000 5000

197

4000 3000 2000 1000 0 0.01

0.1

1

5000 4000 3000 2000 1000 0 0.01

10

YZHS YZLS BT WFG

(CaO )

0.1

Diameter, µm

4000 3000 2000 1000 0 0.01

0.1

1

5000

10

1

10

4000 3000 2000 1000 0 0.01

10

0.1 Diameter, µm

400

400

(Na 2O+K2O)

Concentration, µg/g_ash

Concentration, µg/g_ash

1

(SO 3+P2O 5)

Diameter, µm

300 200 100 0 0.01

10

6000

(Fe 2O 3)

Concentration, µg/g_ash

Concentration, µg/g_ash

6000 5000

1 Diameter, µm

0.1

1

10

(Cl) 300 200 100 0 0.01

Diameter, µm

Fig. 3. Distribution of individual elements within PM10.

0.1 Diameter, µm

L. Zhang, Y. Ninomiya / Fuel 85 (2006) 194–203

3.2. Formation of PM1C The concentration of PM1C was plotted as a function of the content of inherent minerals, smaller than 10.0 mm, as shown in Fig. 4. No relationship was found for included minerals as a function of the PM1C concentration. The included minerals thus have little influence on the formation of the large mode

Concentration of PM10 µ g/g_ash

10000 8000 6000 4000 PM1 vs. excluded minerals PM1 vs. included minerals

2000 0 0

5 10 15 20 25 30 35 40 45 50 Content of fine minerals (<10m) in raw coal, wt%

55

Fig. 4. Relationship between the contents of PM1C with the content of minerals in the raw coals.

during combustion of these four coals. In contrast, the relatively linear relationship for excluded minerals confirmed their influence on the formation of PM1C. The relationship between the content of individual elements in raw coals (existing as excluded mineral particles smaller than 10.0 mm) and their concentrations in PM1C is shown in Fig. 5. These experimental results show that: (1) the transfer rate of all elements except Ca is proportional to their content in the raw coals; (2) the transfer rate of an individual element depends on the elemental type. The fitting lines for SiO2 and Al2O3 have the steepest slope indicating that they have the greatest transformation rates among all the elements. In addition, their similar gradient 8000

Content in PM1+, µ g/g_ash

mode, the prevalence of Cl in the medium size range implies the deposition of chloride on the medium-sized solid particles. The results described above suggest that there are three kinds of distribution of the elements in PM10. (1) SiO2, Al2O3, CaO, SO3 and P2O5 have a single mode distribution; the former three are prevalent in PM1C, whereas the latter two are abundant in PM1. The transformation of the former three metals was likely caused by the direct transformation of inherent refractory metals within the coals. A solid-to-particles pathway governs their transformation. Vaporization is however the main formation route for SO3 and P2O5 in PM, the reactions between them and the other vaporized metals allowed for the formation of ultrafine solid particles following the solid–vaporparticles pathway. (2) Fe2O3, Na2O, K2O and MgO have a bimodal distribution. The transformation of Fe2O3 should be governed by both the above-mentioned pathways, i.e. a portion of it undergo direct transformation whereas the remaining portion vaporizes and condenses into ultrafine particles. For the other three metals, their presence in the large mode is likely caused by adsorption of their vapors on the inherent minerals such as kaolinite. In other words, the presence of these three metals in PM10 likely resulted from their vaporization. (3) Cl has a single mode around 0.5 mm, suggesting the agglomeration of vaporized chloride or its deposition on the surfaces of other solid particles. As stated previously, the inherent minerals in raw coals are generally assigned to two groups, organically bound and inorganic. Inorganic minerals consist of included particles embedded within the carbonaceous matrix, and the excluded particles exist separate from the carbonaceous matrix. Coal combustion is initiated by the release of volatile matter, which is followed by char combustion. The formation of a flame during char combustion causes a higher temperature around the burning char than that of the furnace. The organically bound and a portion of the included inorganic minerals within the char then undergo vaporization leading to the formation of submicron particles. Meanwhile, a portion of the excluded minerals undergoes fragmentation in coal combustion, which may lead to the formation of large particulates (in general having size greater than 1.0 mm). The majority of included inorganic minerals, however, undergo coalescence and agglomeration to form coarse ash, having little effect on the formation of large particulates. In this respect, it is plausible that the formation of the large mode PM1C may be affected by the content of the excluded minerals having sizes smaller than 10.0 mm. Meanwhile, PM1 may be affected by the contents of both organically bound and included minerals smaller than 10.0 mm. Both of these will be discussed separately in the following sections.

SiO 2

6000

Al2 O 3

4000

2000

0 0

5

10

15

20

25

Content of excluded SiO2 and Al2O3 in coal ash, wt% 4000

Content in PM1+, µ g/g_ash

198

CaO

3000

2000

1000

Fe 2 O 3

0 0

5 10 15 Content of excluded CaO and Fe2O3 in coal ash, wt%

Fig. 5. Effect of excluded refractory metals smaller than 10.0 mm on their concentrations in PM1C.

L. Zhang, Y. Ninomiya / Fuel 85 (2006) 194–203

values suggest that these two elements have a nearly equal transfer rate, which might be due to their co-existence as aluminosilicate, e.g. kaolinite, in raw coals. The change in the inherent aluminosilicate allowed the transfer of both into PM1C. The transformation of Ca is complex. The relationship shown here, which is not very clear, implies that except for its content, the reaction of Ca with other metals may be important too. Though the extent of transformation of Fe is proportional to its content in the excluded minerals, the nearly horizontal fitting line suggests that less iron is transformed into PM1C even for higher contents. The majority of iron might be scavenged by aluminosilicate to form molten phases, which undergo agglomeration to form coarse ash particles. The chemical species in the large mode, 2.5 mm, were quantified by CCSEM and the results are shown in Fig. 6. The results shown are consistent with the elemental composition as discussed above. That is, aluminosilicate dominates and its salts, including calcium, iron and alkali elements, are relatively prevalent too. For the YZHS coal, iron oxide and calcium sulfate have minor contents. Both quartz and aluminosilicate should be formed by direct transformation of the inherent quartz and kaolinite in raw coals. Calcium sulfate should also be the inherent mineral, as shown by CCSEM analysis on raw coals reported elsewhere [15]. On the other hand, the existence of aluminosilicate salts and iron oxide indicates that the chemical reactions between different elements also play an important role in the transformation of excluded minerals during coal combustion. The formation of iron oxide is likely caused by the oxidation of pyrite. This reaction commences at 873 8C in an oxidizing atmosphere, and magnetite and hematite are regarded as the main products [4]. On the other hand, the formation of aluminosilicate salts suggests that it is plausible that aluminosilicate might react with the excluded calcium, iron, and alkali vapors via collision in the gas atmosphere. To better understand the formation mechanisms of aluminosilicate salts, thermodynamic equilibrium calculations were carried out to investigate the phases of these salts in the combustion temperature windows. Calculations were conducted by selecting the elemental composition of individual

199

particles as the input, which was obtained from CCSEM analysis. An oxidizing atmosphere was used as the input as well. The output was expressed in terms of percentage of liquid phase in each particle. An average result was finally given for each species. The calculated results are shown in Fig. 7. The Yaxis unit is expressed as the weight percentage of liquid phase in each compound. Less than 5 wt% of aluminosilicate was maintained in the liquid phase, indicating a solid state for the material. Therefore, the solid-to-particles pathway governs the transformation of inherent quartz and kaolinite. About 10–40 wt% of Ca or Fe aluminosilicate was kept in a liquid phase at 1200 8C, which indicates a semi-liquid phase, which should be due to the reaction of calcium/iron and aluminosilicate on the surface of the latter compound. In the post-flame zone, these partially molten particles condense again or agglomerate with each other to form larger clusters. Hence, a solid-semi-liquid-particles pathway should be the formation route. Furthermore, more than 60 wt% of alkali aluminosilicates was kept as liquid phase. Clearly, the reaction of vaporized alkali elements and aluminosilicate resulted in the formation of a total melt phase, and the condensation of its droplets led to formation of fine particulates in the exit gas. Therefore, a pathway of solid–liquid-particles should be adopted for the existence of alkali aluminosilicates in PM1C. 3.3. Formation of PM1 As suggested in Fig. 3, the major elements in PM are sulfur and phosphorus. The influence of these two elements was plotted as a function of the concentration of PM1, as shown in Fig. 8. The linear relationship confirmed the significant influence of these two elements. For the other volatile elements shown in Fig. 3, in principle, they may undergo physical and chemical changes after they vaporize. In this respect, the formation of PM1 is discussed in Sections 3.4 and 3.5.

100

80

Na/K Al-silicate

Ca sulfate WFG BT YZLS YZHS

60

Wt%

Iron Oxide Alkali Al-silicate

Ca/Fe Al-silicate

40

Ca/Fe Al-silicate Al-silicate

20

Al-silicate Quartz

0 700

0

20

40

60

80

100

800

900

1000 1100 1200 1300 1400 1500 1600 Reaction temperature, oC

Wt%

Fig. 6. Chemical species in the size of 2.5 mm.

Fig. 7. Predicted percentage of liquid amount in each phase in the size range of 2.5 mm.

200

L. Zhang, Y. Ninomiya / Fuel 85 (2006) 194–203

Concentration of PM1,µ g/g_ash

14000

12000

10000

8000

6000 0

1 2 3 4 Contents of S and P in raw coals, wt%

5

Fig. 8. Relationship between the formation of PM1 with the SO3 and P2O5 content in raw coals.

3.4. Vaporization of metals Fig. 9 displays the degree of vaporization for the studied elements, where the values for the Y-axis, i.e. the degree of vaporization, was calculated using the following equation: Degree of vapourization of Mð%Þ  Z Concentration of M in PM1=10 ½mg=m3 N !exit gas amount½m3 N=min

(1)

!10 =fcoal feed rate½g=min !concenration of M in coal½g=gg K4

Here, M represents the individual elements listed on the X-axis in Fig. 9. The vaporization of alkali elements was calculated on the basis of their content in PM10; meanwhile the content of other metals in PM1 was used for the vaporization calculation. This is because a portion of the alkali metals is deposited on the large mode as shown in Fig. 3. In addition, the results for each metal are shown as a bar that represents the range of the degree of vaporization for this metal. The upper end is the maximum value and the lower end the minimum. The coal type

Wt% of organic and included metals transferred into PM1

70 60 50 40 30 20 10 0 Al

Si

Ca

Mg

Fe

K

Na

Fig. 9. Weight percentage of individual metals transferred into PM1.

significantly affected the vaporization of the elements as discussed before. The degree of vaporization of elements depends on the type of element, which decreases in the order of NaOKOFeO MgOCaOSiOAl. The vaporization degree of Na ranges from 30 to 65%, the largest among the metals studied. It is followed by K, for which the degree of vaporization ranges from 3 to almost 20%. Fe has a similar range as that of K, suggesting it has a relatively high degree of vaporization among the refractory elements. 5% of Mg vaporized at most. Also, the vaporization of Ca and Si is small, if not negligible. Al hardly underwent any vaporization, accounting for its lowest value shown here. A thermodynamic equilibrium study was carried out to justify the above conclusions from a qualitative viewpoint. For simplification, a reducing gas atmosphere was adopted by assuming that the oxygen diffusing to the char surface is completely consumed. CO is the only product on the char surface, and hence, a fixed partial pressure of CO, PCO, of 0.15 atm was used as an input. The content of each element studied, M, and that of Si and Al in raw coal (both organically bound and included discrete) were also used as the input. The latter two elements are the most prevalent in char and they could react with the vaporized metals in the char. A broad temperature range from 1000 to 1800 8C was used for the calculation considering the possibility that the char has a temperature about 200–400 8C higher than that of gas. The calculation results indicate that mullite (Al6Si2O13) is the major compound formed in the char. SiO, Al and AlO are the gaseous phases formed for elements Si and Al. For the other two refractory elements, Ca and Fe, the majority of Ca is scavenged by aluminosilicate to form various salts including CaAl2Si2O8, Ca2Al2SiO7 and CaAl4O7. Meanwhile a small amount of Ca is formed as vapor. Fe does not react with aluminosilicate under the given conditions. It partitions into solid Fe in the char as well as gaseous Fe. The formation of Mg2Al4Si5O18 is also preferred. Its amount decreases gradually as the temperature increases and consequently, more gaseous Mg is formed. The formation of NaAlSi3O8(s) is only possible at temperatures lower than 1300 8C. When the temperature increases further, all the Na2O is transformed to gaseous Na. The formation of KAlSi2O6(s) is however possible at temperatures below 1600 8C and gaseous K is formed as its equilibrium phase. Fig. 10 shows the weight percent of the gas phases of individual elements as a function of the reaction temperature. Only the results for YZHS coal are shown here. Assuming that the char temperature ranges from 1400 to 1600 8C, it is likely that Na2O vaporizes completely, followed by K2O having 20% of it vaporizes at 1400 8C, which goes up to 100% at 1600 8C. SiO2 vaporizes as well to an extent ranging from 5 to 60%. MgO and Fe2O3 vaporize to a relatively small extent, under 10%. In addition, CaO and Al2O3 do not vaporize since the amounts of their gas phases are negligible. Thus, from a qualitative viewpoint, the calculated results are roughly the same as the results obtained experimentally. On the other hand, there are differences between the calculated

1.0E-01 K2O

8.0E-02

SiO2

60

6.0E-02

Fe2 O3

4.0E-02 MgO

20

Al2O3

40

CaO

Wt % transferred into gas

Na2 O

80

2.0E-02

201

Content in PM1, µ g/g_ash

100

Wt% of CaO or Al2O3 transferred into gas

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and experimental results from a quantitative viewpoint. One plausible reason for this is that the calculations simplified the elements as oxides in the char, rather than their original forms. The original form of the metals is thought to be more important for determining their vaporization [10]. Moreover, the diffusion of the vaporized metals within the pores of the char is another factor that might also lower the degree of vaporization [3]. The relationship between the content of individual elements in raw coal and their content in PM1 is shown in Fig. 11. The xaxis in the figure refers to the content of individual metals in the carbonaceous matrix of raw coals, and its unit is C.O.I. (content of organically bound and included inorganic metal (%10.0 mm)). A good linear relationship was not found for several elements including SiO2, K2O, Na2O and MgO. This suggests a complex process for the vaporization of these metals. Even so, there is an upward trend wherein increasing the C.O.I. of these elements resulted in the improvement of their vaporization. For the other elements, including Al2O3, Fe2O3 and CaO, their degree of vaporization is proportional to their C.O.I. as expected. The elemental type affected its vaporization as well. The gradient of the fitting line also varied greatly with the elemental type, further indicating their different vaporization rates during coal combustion. 3.5. Formation of chemical species in PM1 After leaving the char surface, the resultant metallic vapors initially undergo re-oxidization to form oxides. These subsequently react with the other constituents via their heterogeneous agglomeration. The chemical species for the particulate having the diameter of 0.13 mm were quantified by CCSEM and the results are shown in Fig. 12. The results show the existence of quartz, silicates, sulfates and phosphates. Quartz should be formed by the homogenous condensation of gaseous SiO. Silicates should however be formed by chemical reactions between gaseous SiO and other metallic vapors. The presence of iron oxide indicates the re-oxidation of metallic iron. Sulfates are present

Content in PM1, µ g/g_ash

Fig. 10. Predicted weight percentage of individual elements transformed into gaseous phases (for YZHS coal).

Content in PM1, µ g/g_ash

0 0.0E+00 1000 1100 1200 1300 1400 1500 1600 1700 1800 o Reaction temperature, C

Fig. 11. Relationship between the content of inherent metals in the organic matrix and their concentrations in PM.

with the greatest amounts, suggestive of sulfation of the metals, especially alkali elements. On the other hand, the amount of phosphates is relatively low compared to solid P2O5. The effect of reactions between P2O5 and other metals appears to be minor. The existence of P2O5 likely resulted from the condensation of its gaseous phase in the post-flame zone in the furnace. The existence of SO2 can be attributed to the formation of liquid sulfate droplets or ammonium sulfate, considering the existence of moisture and nitrogen in coals. The coal type affected the distribution of species as well. A number of sulfates were formed for YZHS coal due to its high sulfur content. The YZLS, WFG and BT coals follow it in the decreasing order of sulfur content in the raw coals.

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Phosphrous oxide

significant amount of K2Si4O9 (l). This is very different from the experimental results of less silicate as show in Fig. 12. Apparently, the formation of silicate is very slow and timeconsuming.

WFG BT YZLS YZHS

Na/K Phoshpate Sulfur oxide Na/K Sulfate Zn Sulfate

4. Conclusions

Ca Sulfate Iron Oxide Na/K silicate Ca/Fe silicate Quartz 0

10

20

30

40

50

Wt%

Fig. 12. Chemical forms in the size of 0.06 mm.

This further suggests the significant influence of sulfur on the formation of PM1. For a better understanding of the formation of species for particulate of 0.13 mm, the thermodynamic equilibrium calculation was conducted with the elemental composition of this size and an oxidizing atmosphere used as the input. The results expressed as a percentage of species with this size are shown in Fig. 13. Iron was not included in the results since all the iron was re-oxidized into oxide whereas some of it reacted with the other constituents as discussed above. Clearly, the formation of quartz proved the condensation of SiO into SiO2. The formation of K2SO4 (l) and the absence of Na2SO4 suggest that alkali sulfate in this size was formed by the reaction between K(g) and SO2, which allowed for the formation of ultrafine droplets, and subsequently, fine round solid particles after their condensation. In addition, the predicted variation in K2SO4 content with coal type is similar to that observed using CCSEM (as shown in Fig. 12). This suggests that this reaction reached its equilibrium under the stated conditions. The formation of alkali phosphate, Na3PO4, was also confirmed. The absence of P2O5 indicates that all the gaseous phosphorous should be used to form Na3PO4. The existence of P2O5 in Fig. 12 however implies that the formation of Na3PO4 did not reach its equilibrium. This was likely caused due to two reasons: (1) a short residence time in the furnace; (2) a low partial pressure of phosphorous vapor compared to sulfur. Finally, the calculations also show the formation of a K silicate(l) Na phosphate(s)

WFG BT YZLS YZHS

Na sulfate(s) quartz(s)

Acknowledgements

K sulfate(l) Phosphorous oxide Sulfur oxide -0

20

40

60

80

The present study leads to the following conclusions. Suspended PM10 emitted from the combustion of four different coals has a bimodal distribution with two peaks around 2.5 and 0.06 mm, respectively. When the ash content increased, the PM10 concentration also increased linearly. About 0.5–2.5 wt% of inherent minerals changed into the suspended PM10. Its amount is proportional to the ash content in raw coal as well. There are three kinds of elemental distributions in PM10. SiO2, Al2O3, CaO, SO3 and P2O5 have a single modal distribution. The former three elements dominate PM1C having a size ranging from 1.0 to 10.0 mm and a peak around 2.5 mm. They are mainly formed by the direct transformation of inherent minerals. SO3 and P2O5 are prevalent in PM1 with a size range smaller than 1.0 mm and a peak around 0.06 mm. These two elements were formed by their vaporization. Fe2O3, Na2O, K2O and MgO have a bimodal size distribution. Both direct transformation and vaporization govern the formation of Fe2O3. A portion of the vaporized Na, K and Mg was captured by the inherent aluminosilicate to form a large mode around 2.5 mm. Finally, Cl has a single distribution with a peak around 0.2–0.5 mm, which should be the result of the deposition of chlorides on other solid particles. PM1C was formed by the direct transformation of refractory elements in raw coal. Quartz and aluminosilicates within this portion were formed by transformation without phase change. Meanwhile, calcium/iron aluminosilicate in PM1C was formed by the reaction of calcium or iron with aluminosilicate, which led to a sticky surface of the latter compound. Alkali aluminosilicate was formed as melt droplets in coal combustion and it condensed into large particulates in PM1C. PM1 is rich in both sulfates and phosphates of vaporized elements. The degree of vaporization of elements is determined by their content (organically bound and included minerals smaller than 10.0 mm) in the raw coals. The elemental type also affects their vaporization greatly, which decreases in the order of NaOKOFeOMgOCaOSiOAl. This is fairly consistent with the predictions of thermodynamic equilibrium calculations. The vaporized metals reacted with gaseous SO2, P2O5 and SiO to form their compounds.

100

Wt%

Fig. 13. Predicted quantitative distribution of species in the size range of 0.13 mm.

The author would like to thank Grant-in-aid for Scientific Research on Priority Areas (B), 14380279, Ministry of Education, Science, Sports and Technology, Japan, and the Steel Industry Foundation for the Advancement of Environmental Protection Technology for financial support. The first author thanks the Japan Society for Promotion of Science, JSPS, for the postdoctoral research fellowship.

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