Influence of different distributions of Ca-mineral in coal on trimodal particulate matter formation during combustion

Influence of different distributions of Ca-mineral in coal on trimodal particulate matter formation during combustion

JOURNAL OF FUEL CHEMISTRY AND TECHNOLOGY Volume 44, Issue 3, March 2016 Online English edition of the Chinese language journal Cite this article as: J...

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JOURNAL OF FUEL CHEMISTRY AND TECHNOLOGY Volume 44, Issue 3, March 2016 Online English edition of the Chinese language journal Cite this article as: J Fuel Chem Technol, 2016, 44(3), 273278

RESEARCH PAPER

Influence of different distributions of Ca-mineral in coal on trimodal particulate matter formation during combustion ZHANG Ping-an1, YUAN Jing2, YU Dun-xi1, LUO Guang-qian1, YAO Hong1,* 1

State Key Laboratory of Coal Combustion, Huazhong University of Science and Technology, Wuhan 430074, China;

2

Middle Changjiang River Bureau of Hydrology and Water Resources Survey, Wuhan 430012, China

Abstract:

Calcium acetate was added into a bituminous coal through physically blending and impregnation to obtain the ble-Ca coal

rich in excluded Ca-mineral and imp-Ca coal rich in included Ca-mineral, respectively. The raw coal, ble-Ca coal and imp-Ca coal were burned in a drop tube furnace at 1300°C. The generated particulate matters (PMs) were collected and analyzed to study the influence of different distributions of Ca-mineral in coal on trimodal PM formation during combustion. The results showed that for the three coals, PMs with the ultrafine mode, central mode and coarse mode were all in the size range of <0.2 m, 0.2‒2.0 m and >2.0 m, respectively. The included and excluded Ca-minerals can both promote the formation of ultrafine mode PM, and the excluded one had more significant effect. The included Ca-mineral can restrain, while the excluded one can promote the formation of central mode PM. The included Ca-mineral can promote the formation of coarse mode PM, while the excluded one did not have obvious effect. Key words:

coal combustion; coal matrix; included mineral; excluded mineral; particulate matter; formation mode

So far, particulate matter (PM) is the principal air pollutant in China and the combustion of coal is an important source [1]. It is believed that the coal-derived PM has three formation modes. They are ultrafine mode, central mode and coarse mode, respectively[2‒4]. As shown in Figure 1, the ultrafine mode PM is mainly produced by vaporization-condensation mechanism[5], the central one by fragmentation mechanism[6], while the coarse one by fragmentation/coalescence mechanism[7]. Compared to the coarse mode PM, the ultrafine and central mode PMs are much more harmful to human health because of their large specific surface areas which are beneficial for toxic elements enrichment[3] and their high penetration rates in electrostatic precipitator[8]. Under the condition of complete combustion, coal-derived PM comes from the transformation of minerals in coal. According to the connections with coal matrix, the minerals can be divided into included mineral and excluded mineral. Included minerals are surrounded by coal matrix. During combustion, included minerals will experience reducing atmosphere and higher temperature than flue gas. With the consumption of coal char, the included minerals will contact and react with each other, while the excluded minerals are separated from coal matrix. During combustion, the excluded minerals will experience oxidizing atmosphere and

temperature equal to flue gas. During transformation, the excluded minerals are hard to collide or react with each other[9]. So, the distribution of minerals in coal will significantly influence the processes of mineral [10] transformation and PM formation. With the development of mineral analysis technologies, such as computer-controlled scanning electron microscopy (CCSEM)[11], the influence of mineral distribution on PM formation has attracted more and more attentions. Zhan et al[12] found that more included minerals would lead to more PM1 formation during combustion of density-classified coals. However, the previous researches almost focused on the influence of mineral distribution on PM particle size distribution, and not the formation of different modes PMs. This research aimed at the influence of mineral distribution in coal on the trimodal PM formation during combustion. First of all, the included and excluded minerals should be separated. There are three separation methods. Density separation[12,13]. Because of the density differences between coal particles and mineral particles, the low-density coal rich in the included minerals and high-density coal rich in the excluded minerals can be obtained by density separation. Synthetic char[14‒16]. The minerals added during the process of synthetic char preparation are considered as the included

Received: 15-Oct-2015; Revised: 06-Jan-2016. Foundation item: Supported by the Science and Technology Support Program of Hubei Province (2014BCB040). *Corresponding author. Tel: +86-27-87545526(O), E-mail: [email protected]. Copyright  2016, Institute of Coal Chemistry, Chinese Academy of Sciences. Published by Elsevier Limited. All rights reserved.

ZHANG Ping-an et al / Journal of Fuel Chemistry and Technology, 2016, 44(3): 273278

minerals, while the minerals added in the prepared synthetic char are considered as the excluded minerals. Addition of soluble minerals. Ninomiya et al[17] found that the soluble minerals added to coal through impregnation could be considered as the included species, while the minerals added to coal through physically blending should be considered as the excluded ones. When using the first method, density separation, the coal char characters and the mineral compositions of low-density coal and high-density coal may be quite different[18], and these differences will interfere with the experiment result. For the second method, the preparation process of synthetic char is quite complicated and the synthetic char is hard to burn out. Hence, the last method, addition of soluble minerals, was used in this study. First, a soluble mineral, calcium acetate, was added into a bituminous coal through physically blending and impregnation to obtain coals rich in the excluded Ca-mineral and included Ca-mineral, respectively. Then, the different coals were burned in a drop tube furnace and the generated PMs were collected and analyzed. Finally, the influence of different distribution of Ca-mineral in coal on the trimodal PM formation was discussed.

1

Experimental

The raw coal used was a bituminous coal and its properties are shown in Table 1. The major elements in raw coal are Si (SiO2: 57.03%) and Al (Al2O3: 32.31%), and the content of Ca is very

Fig. 1

low (CaO: 0.68%). The mineral distribution in raw coal was detected by CCSEM and is shown in Figure 2. The detailed information of CCSEM could be found in[11]. The included minerals took 15.1% of total minerals in weight and mainly were clay minerals, such as kaolinite, K Al-silicate and unknown mineral (mainly mix-aluminosilicate). The used calcium acetate was analytical reagent. Raw coal and calcium acetate were both milled to less than 200 m. The calcium acetate was added into raw coal through physically blending and impregnation to obtain the ble-Ca coal and imp-Ca coal, respectively. The mass ratio of calcium acetate to coal was 17.5/100. The imp-Ca coal was prepared as follow: first, calcium acetate was dissolved in deionized water at room temperature, and the raw coal was put into the solution and stirred for 24 h. Then, the sample was dried at 45°C under the protection of N2, and finally, the sample was milled to less than 200 m again. The combustion experiments were carried out in an electric heating drop tube furnace. An alundum tube with 2000 mm in length and 56 mm in inner diameter was used as the reactor. The temperature was set at 1300°C and the constant temperature zone was about 1100 mm in length. The feeding rates of raw coal and Ca-addition coals were 0.4 g/min and 0.47 g/min, respectively. Air atmosphere was achieved. The volume ratio of primary gas to secondary gas was 1:3. The excess air coefficient was about 3 and the particle residence time was about 2 s. A water-cooled N2-quenched sampling probe was used. The quenched N2 not only realized isokinetic sampling but also restrained secondary reaction.

Mechanisms of PM formation during coal combustion Table 1

Properties of raw coal

Proximate analysis w/%

Ultimate analysis w/%

M

A

V

FC

C

H

N

S

O*

0.68

36.57

21.54

41.21

50.58

3.14

0.82

3.58

4.64

Na2O

MgO

Al2O3

SiO2

P2O5

SO3

K2O

CaO

TiO2

Fe2O3

1

1.29

32.31

57.03

0.14

1.71

1.73

0.68

0.87

3.24

Major ash components w/%

*

: by difference

ZHANG Ping-an et al / Journal of Fuel Chemistry and Technology, 2016, 44(3): 273278

Fig. 2

Mineral distribution in raw coal (detected by CCSEM)

The particles in flue gas were collected by a cyclone and dekati low pressure impactor (DLPI). The coarse particles with aerodynamic diameter larger than 10 m were collected by the cyclone and the smaller ones were collected by DLPI onto 13 plates. The cutoff diameters of DLPI are 0.0281, 0.0565, 0.0944, 0.154, 0.258, 0.377, 0.605, 0.936, 1.58, 2.36, 3.95, 6.6 and 9.8 m, respectively. The particles collected on each DLPI plate was weighted on a Sartorius M2P electronic balance. Polycarbonate membranes were used to collect particles for the following elemental analysis through X-ray fluorescence (XRF). The schematic diagram of the drop tube furnace is shown in Figure 3 and more details can be found in[19].

2 2.1

Results and discussion Identification of PM formation modes

The different formation mechanisms lead to the different elemental compositions of different mode PM[20]. According to this characteristic, Yu et al[21] in our group developed a method by using mass fraction size distribution of Al to identify the PM formation modes, and this method was used here.

Mass fraction size distributions of Al in PMs generated by different coal combustion are shown in Figure 4. The three distribution curves all show “S” type in shape. For particles smaller than 0.2 m, the Al contents in them are quite low and nearly keep constant with increasing particle size. For particles larger than 2.0 m, the Al contents in them are relatively high and also nearly keep constant with increasing particle size. For particles between 0.2 and 2.0 m, the Al contents in them are medium and increase significantly with increasing particle size. According to Yu’s method[21], particles smaller than 0.2 m (DLPI plate 1‒4) are identified as ultrafine mode PM, those between 0.2 and 2.0 m (DLPI plate 5‒9) as central mode PM, and those larger than 2 m (DLPI plate 10‒13) as coarse mode PM. The yield of each mode PM was calculated, which represented the PM generation capability of unit coal ash. Take ultrafine mode PM as example, the yield of ultrafine mode PM = (total mass of particles collected on DLPI plate 1‒4) / (total mass of ash collected by the sampling probe) with unit of mg/g. 2.2

Formation of ultrafine mode PM

The yields of ultrafine mode PM generated by different coals combustion are shown in Figure 5. Clearly, the Ca-addition coals show higher yields than raw coal (ble-Ca coal: 7.91 mg/g, imp-Ca coal: 5.75 mg/g, raw coal: 3.68 mg/g). As mentioned before, the ultrafine mode PM was mainly produced through vaporization-condensation mechanism. The higher yields of Ca-addition coals indicated more volatile species in Ca-addition coals than in raw coal. As the additive, calcium acetate would experience decomposition at high temperature and gas phase compositions, such as acetone and CO2[22,23], would be generated. During this procedure, partial Ca steam (or fume) could enter flue gas with the generated gas and experience condensation (or coagulation) to form ultrafine mode PM[24].

Fig. 4 Fig. 3

Schematic diagram of the drop tube furnace

Mass fraction size distributions of Al in PMs generated by different coals combustion

ZHANG Ping-an et al / Journal of Fuel Chemistry and Technology, 2016, 44(3): 273278

Fig. 5

Yields of ultrafine mode PM generated by different coals

Fig. 6

combustion

The elemental analysis supported this view. The contents of Ca in ultrafine mode PM (take DLPI plate 4 as example) of different coals are shown in Figure 6. The Ca contents in Ca-addition coals are significantly higher than those in raw coal (ble-Ca coal: 8.41%, imp-coal: 8.60% and raw coal: 0.15%). The organically-bound Ca was an important source for ultrafine mode PM formation. Comparing the two Ca-addition coals, ble-Ca coal showed higher yield of ultrafine mode PM than imp-Ca coal. As mentioned before, the included Ca-minerals in the imp-Ca coal would experience reducing atmosphere and higher temperature than the excluded ones, which were beneficial for element vaporization[9]. Thus, theoretically, the volatile species generated by the imp-Ca coal should be more than the ble-Ca coal. It is opposite to the ultrafine mode PM formation as shown in Figure 5. This contradiction could be explained by the distribution of Ca-mineral in the two coals. Calcium acetate in the ble-Ca coal was excluded mineral and surrounded by flue gas. The Ca steam (or fume) could directly enter flue gas and form the ultrafine mode PM, while calcium acetate in the imp-Ca coal was included mineral and surrounded by coal matrix. The Ca steam (or fume) generated by included Ca-mineral should pass through the pores in coal to enter flue gas and might react with the included clay minerals during the passing. These interactions could scavenge the Ca steam (or fume) and restrain the formation of ultrafine mode PM. As shown in Figure 5, the ultrafine mode PM yield of the ble-Ca coal was higher than the imp-Ca coal, which indicated that under the conditions of this experiment, the restraint effect caused by mineral interaction was more significant than the promotion effect caused by reducing atmosphere and higher temperature. 2.3

Formation of central mode PM

The yields of central mode PM generated by different coal combustion are shown in Figure 7.

Contents of Ca in ultrafine mode PM (collected on DLPI plate 4) of different coals

Fig. 7

Yields of central mode PM generated by different coals combustion

The ble-Ca coal shows the highest yield (3.48 mg/g) and the imp-Ca coal indicates the lowest yield (0.80 mg/g), while the raw coal displays the medium yield (2.66 mg/g). As mentioned before, the central mode PM was mainly produced through fragmentation mechanism. Compared to raw coal, there were many fragile calcium acetate particles in ble-Ca coal and so it showed higher central mode PM yield. Figure 8 shows the contents of Ca in central mode PM (take DLPI plate 8 as example) of different coals. In the figure, it is clear that the Ca content of ble-Ca coal (9.99%) is higher than that of raw coal (2.87%), indicating the fragile Ca-mineral is contributed to the central mode PM formation. The addition amounts of calcium acetate to the ble-Ca coal and imp-Ca coal were equal, while the distributions were quite different. The excluded calcium acetate in ble-Ca coal was hard to collide or react with other minerals. The decomposition and following fragmentation of calcium acetate particles would directly produce central mode PM, while the included calcium acetate in the imp-Ca coal could easily contact and react with the included clay minerals during coal char consuming.

ZHANG Ping-an et al / Journal of Fuel Chemistry and Technology, 2016, 44(3): 273278

Fig. 8

Contents of Ca in central mode PM (collected on DLPI plate

Fig. 10

Contents of Ca in coarse mode PM (collected on DLPI

8) of different coals

The generated Ca-aluminosilicates could lead to melting generation and mineral coalescence[25], which restrained particle fragmentation and central mode PM formation. Thus, the central mode PM yield of imp-Ca coal was lower than that of ble-Ca coal, and even lower than that of raw coal. 2.4

Formation of coarse mode PM

The yields of coarse mode PM generated by different coals combustion are shown in Figure 9. The yields of raw coal, ble-Ca coal and imp-Ca coal are 4.96, 4.74 and 6.02 mg/g, respectively. The coarse mode PM is mainly produced through fragmentation/coalescence mechanism. As discussed in the previous section, the included Ca-mineral in imp-Ca coal could react with the included clay minerals, leading to melting generation and mineral coalescence. The mineral coalescence would promote the coarse mode PM formation and thus the imp-Ca coal showed the highest coarse mode PM yield. The contents of Ca in coarse mode PM (collected on DLPI plate 12) of different coals are shown in Figure 10.

plate 12) of different coals

The highest Ca content of imp-Ca coal (raw coal: 2.01%, ble-Ca coal: 10.35%, imp-Ca coal: 14.42%) indicates the most significant contribution of the included Ca-mineral to the coarse mode PM formation.

3

Conclusions

In this experiment, calcium acetate was added into a bituminous coal through physically blending and impregnation to obtain the ble-Ca coal rich in excluded Ca-mineral and the imp-Ca coal rich in included Ca-mineral, respectively. The different coals were burned in an electric heating drop tube furnace, and the generated PMs were collected and analyzed. Under the experiment conditions, the following conclusions were drawn. The ultrafine mode, central mode and coarse PMs of the three coals were all in the size range of f <0.2 m, 0.2‒2.0 m and >2.0 m, respectively. Compared to excluded Ca-mineral, the included Ca-mineral could contact and react with the included clay minerals in coal during combustion. The mineral interactions led to Ca-steam (or Ca-fume) scavenging and mineral coalescence, which restrained ultrafine and central mode PM formation, while promoted coarse mode PM formation.

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