Effect of the Devolatilization Process on PM10 Formation during Oxy

Aug 6, 2014 - ... (18–25%) of volatile elements in PM0.5–2.5 implies that the effect of vaporized ash loss during devolatilization is indecisive i...
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Effect of the Devolatilization Process on PM10 Formation during Oxyfuel Combustion of a Typical Bituminous Coal Chang Wen, Dunxi Yu,* Jianpei Wang, Jianqun Wu, Hong Yao, and Minghou Xu* State Key Laboratory of Coal Combustion, Huazhong University of Science and Technology, 1037 Luoyu Road, Wuhan, Hubei 430074, People’s Republic of China ABSTRACT: The devolatilization process has important influence on the formation of PM10 (particulate matter with an aerodynamic diameter of ≤10.0 μm) in oxy-fuel combustion of pulverized coal but has been explored little. A bituminous coal was devolatilized in either CO2 or N2 at 1573 K on a drop-tube furnace (DTF) to produce CO2-char and N2-char. Coal and its char samples were burned at 1573 K and in 29 vol % O2/71 vol % CO2. PM10 was collected and segregated into 13 size fractions, which were subjected to subsequent analysis. The results show that the particle mass size distributions of PM10 from coal and chars have similar peak and trough sizes, suggesting that the devolatilization process has insignificant influence on the major pathways of PM10 formation. Three particle modes can be identified, i.e., ultrafine mode ( 10 μm) via a matched cyclone. It should be noted that, as the flue gas contains much CO2, the actual cutoff aerodynamic sizes of cyclone and LPI are slightly different from the calibration data from Dekati Company. For each combustion case, three or four replicates were run to calculate the averages and error bars reported in this paper. The detailed procedure of char preparation and PM10 collection can be found in our earlier publications.10,14 The burnout rates of raw coal and chars were all higher than 99%. Such high burnout rates indicate that only little unburned carbon exists in PM samples, although the lack of testing techniques. The unburned carbon was reported to compose of unburned char predominantly, which formed the particles of >10 μm generally.22 On the basis of the

3. RESULTS 3.1. Properties of Raw Coal, CO2-Char, and N2-Char. Table 1 presents the properties of raw coal and its prepared chars. It shows that the content of volatile matter (VM) remaining after coal devolatilization in CO2 is slightly less than that in N2, while the content of fixed carbon (FC) in CO2-char is much lower than that in N2-char. The decreased ratios of VM and FC after the devolatilization in CO2 or N2 can be calculated on the basis of the ash trace method. The decreased ratios of VM are approximately 84.7 and 79.6% for CO2-char and N2char, respectively, while those of FC are about 45.7 and 14.7%, respectively. All of those imply that the gasification reaction of CO2 with char during CO2 devolatilization consumes the carbonaceous matter predominantly rather than the VM. The extensive gasification reaction between char and CO2 during CO2 devolatilization causes the char yield of CO2-char to be apparently lower than that of N2-char, as shown in Table 2. Note that the char yields had been presented by authors.10 The ash loss of two chars during devolatilization is calculated in Table 2. The theoretical ash mass of char is the product of assumed initial coal mass (1000 g), char yield, and ash content 5683

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(shown in Table 2). When the theoretical ash masses of chars and coal are compared, the ash loss is calculated to be 10.65 and 4.64 g of ash/kg of coal for CO2-char and N2-char, respectively. The ash loss of N2-char should derive from the release of organically bound metals, which was mentioned by Zhang et al.19 The higher ash loss of CO2-char is likely to be caused by the gasification reaction of CO2 and char during devolatilization in CO2. Kim et al. described the major heterogeneous reactions of the CO2 gasification reaction on the char particles.13

C(s) + CO2 → 2CO

(1)

As shown in eq 1, in comparison to devolatilization in N2, more carbonaceous matter would be consumed during devolatilization in CO2 (confirmed by Table 1). Accordingly, the organically bound metals associated with carbonaceous matter in CO2-char can be released more easily to cause the higher ash loss of CO2-char, and the lower content of Na in CO2-char compared to N2-char can support the speculation (see Table 1). On the other hand, eq 1 reveals that the devolatilization in CO2 will produce much CO. As shown in eq 2 reported by Quann and Sarofim, CO may react with metal oxides (MOn) to form more volatile suboxides (MOn−1) or metals (M).23 MOn(s) + CO → MOn − 1(v) + CO2

Figure 1. PSDs of PM10 formed from the oxy-fuel combustion of raw coal, CO2-char, and N2-char.

3.3. Elemental Mass Fraction Size Distributions of PM10. Several investigations have suggested that the particulate matter generated during coal combustion may be trimodally distributed.27,28 In addition to the commonly observed ultrafine and coarse modes, a central (or intermediate) particle mode exists as a region between the distinct ultrafine and coarse modes rather than merely a transition or overlapping. However, the PSD profiles in Figure 1 do not show an obvious central mode. As discussed in the previous study,26 the central particle mode cannot always be effectively identified on the particle mass size distribution profiles. Nevertheless, it can be clearly identified on the basis of the mass fraction size distribution of the elements Al + Si in particles. The total inorganic mass fractions of Al + Si in each size-segregated PM samples are calculated, and then the size distributions of inorganic mass fractions of Al + Si from different samples are presented in Figure 2. On the basis of the mass fraction size distributions of Al + Si, three particle formation modes can be readily observed and are indicated by dash lines. The ultrafine mode contains particles less than about 0.5 μm (PM0.5). The inorganic mass fraction of Al + Si in these particles is the lowest and nearly independent of the particle

(2)

One may wonder if the more release of vaporized suboxides or metal species during devolatilization in CO2 can cause the higher ash loss of CO2-char compared to N2-char. Because of the high CO2 concentration, the reduction of metal oxides shown in eq 2 might be inhibited during devolatilization in CO2. In contrast, devolatilization in N2, as generally known, can generate a high concentration of reducing gases, such as H2, CO, and CH4, which were also reported to be able to reduce the metal oxides.24,25 Therefore, there is no evidence to support the hypothesis that the devolatilization in CO2 can release more vaporized suboxides or metal species and cause higher ash loss. As a result, the release of more organically bound metals, as discussed above, is the major reason for higher ash loss of CO2char compared to N2-char. 3.2. PSDs of PM10 Produced from the Combustion of Raw Coal, CO2-Char, and N2-Char. As introduced above, the absolute mass of size-segregated PM10 samples collected by each stage of LPI can be weighed. Given this, their mass generated by an equivalent mass of ash input can be figured out for indicating the ability of ash-forming species to form PM10. Accordingly, the differential mass-based PSDs of PM10 from the combustion of coal and char samples are depicted in Figure 1. The way of data presentation is similar to other publications.10,26 It is noteworthy that the equivalent ash input content of char samples needs to be converted after considering the above-calculated minor ash loss of chars. All of the PSD profiles in Figure 1 show similar features. The ultrafine particles are centered around 0.136 μm, and the coarse particles at approximately 8.791 μm. Particles with a size of around 0.535 μm have the lowest concentration. These similarities suggest that the devolatilization process has an insignificant effect on the major pathways of PM10 formation. It is clear that the concentration of ultrafine particles decreases in the order of raw coal > CO2-char > N2-char. This result suggests that the devolatilization process has a significant effect on the formation of ultrafine particles. However, the devolatilization process seems to have little influence on the formation of coarse particles larger than about 2 μm.

Figure 2. Inorganic mass fraction size distributions of Al + Si in PM10 formed from the oxy-fuel combustion of raw coal, CO2-char, and N2char. 5684

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Figure 3. Yields of PM10 formed from the oxy-fuel combustion of raw coal, CO2-char, and N2-char: (a) PM0.5, (b) PM0.5−2.5, and (c) PM2.5−10.

size. They are believed to be primarily formed through the vaporization and condensation mechanism,2,29 which often leads to an uniform distributions of Al + Si.26 The coarse mode is roughly identified in the range of 2.5−10 μm (PM2.5−10). The mass fraction of Al + Si in this range is the highest and also seems independent of the particle size. The coalescence of coal mineral matter was reported to be the major source and tends to result in the relatively homogeneous particle composition.1,30 In addition to the two modes, there is also a clear central mode in the range of approximately 0.5−2.5 μm (PM0.5−2.5). The mass fraction of Al + Si in these particles is between that in ultrafine and coarse mode particles and apparently decreases with a decreasing particle size. The central mode particles are thought to derive mainly from heterogeneous condensation/ reaction of vapor species on fine particles, which are generated by processes such as char fragmentation.27,28,31 Considering that the formation mechanisms of central mode particles are distinct from that of the commonly observed ultrafine and coarse mode particles, the effect of the devolatilization process on the formation of central mode particles is believed to be also different from that on other particles and is worthy of being concerned. The three particle modes identified in this study are similar to those observed in previous investigations.27,32 3.4. Yields of PM0.5, PM0.5−2.5, and PM2.5−10. On the basis of the classification of PM10 into three size fractions, the yields of PM0.5, PM0.5−2.5, and PM2.5−10 from the combustion of coal and its chars can be quantified in Figure 3. As shown in Figure 3, the yields of PM0.5 and PM0.5−2.5 from char combustion are generally lower than those from coal combustion. It is more obvious for the yields of PM0.5 (Figure 3a). The PM0.5 yield from coal combustion is about 3 times that from N2-char and approximately 2 times that from CO2-char. The results suggest that the devolatilization process has important influence on the production of PM0.5 and PM0.5−2.5. The influence mechanisms will be further examined in the Discussion. In contrast, the PM2.5−10 yield from coal combustion is not evidently different from that from char combustion (Figure 3c), implying the insignificant effect of the devolatilization process. PM2.5−10 was reported to be primarily formed by the coalescence of mineral inclusions,1,30 which should be insensitive to the char combustion behavior under the investigated conditions. Figure 4 presents the composition data of major inorganic elements on different particle size fractions. The compositions of PM2.5−10 formed from raw coal and two chars are highly alike, which confirms the insignificant effect of the devolatilization process. Sarofim’s group found that not only did the char particle combustion temperature,

Figure 4. Inorganic elemental compositions of PM10 formed from the oxy-fuel combustion of raw coal, CO2-char, and N2-char.

speculated by the gas temperature, have a minimal effect on the distribution of particulate matter between approximately 5 and 10 μm16 but also the size fractions of particles from 5 to 15 μm formed from the mineral inclusions of slow pyrolysis char and rapid pyrolysis char were relatively identical.17 Similarly, Fix et al. suggested that the char combustion behavior, indicated by the oxygen/fuel stoichiometry, had little influence on the formation of coarse mode particles.32 Our results support their suggestions.

4. DISCUSSION Figures 1 and 3 suggest that the devolatilization process has an important influence on the production of PM2.5. In this paper, PM2.5 is classified into two size ranges, i.e., PM0.5 and PM0.5−2.5, because of their different formation mechanisms. How the devolatilization process affects the production of PM0.5 and PM0.5−2.5 and the difference of the effect of devolatilization in a CO2 or N2 atmosphere on PM0.5 and PM0.5−2.5 need to be clarified in this section elaborately. 4.1. Effect of the Devolatilization Process on PM0.5 Formation. Figure 3a presents that the PM0.5 yield from coal combustion is much higher than that from char combustion. As shown in Figure 4, PM0.5 mainly contains volatile elements, such as Na, K, S, and P. The total mass fraction of volatile elements Na + K + S + P in PM0.5 accounts for as high as 65%. It supports that PM0.5 is primarily formed by the vaporization of volatile elements and their condensation/coagulation upon gas cooling.2,29 As discussed in subsection 3.1, the volatile materials cause the ash loss after devolatilization in CO2 or N2. Figure 5 5685

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Figure 5. Absolute mass of major inorganic elements in (a) PM0.5 and (b) PM0.5−2.5 formed from the oxy-fuel combustion of raw coal, CO2-char, and N2-char.

presents the absolute mass of major inorganic elements in PM0.5 and PM0.5−2.5, which is normalized to equivalent mass of ash input. The masses of volatile elements Na, K, S, and P in PM0.5 from char combustion are all lower that those masses from raw coal combustion (Figure 5a), which implies that the reserved volatile materials in coal may contribute to more PM0.5 in the raw coal combustion than char combustion. On the other hand, because of the burning of volatile matter, the particle temperature during coal combustion is expected to be higher than that during char combustion.33,34 The higher particle temperature in coal combustion could enhance the vaporization of mineral matter.18,35 The mass of refractory elements shown in Figure 5a, such as Ca, Ti, Al, and Si, from the coal combustion is higher than those mass from char combustion, which positively reflects the effect of the higher particle temperature on the vaporization of refractory elements in coal combustion and also promotes the yield of PM0.5 from raw coal combustion. Therefore, the yield of PM0.5 from coal combustion is apparently higher than that from char combustion, as shown in Figure 3a. The calculations in Table 2 have shown that the ash loss during devolatilization in CO2 is higher than that in N2. One would expect that the combustion of CO2-char could produce less PM0.5 than N2-char combustion because a larger fraction of the volatile materials has been released during devolatilization in CO2. However, it is not the case. Figure 3a shows that the PM0.5 yield from CO2-char combustion is somewhat higher than that from N2-char combustion. This seeming discrepancy can be accounted for by the effect of the gasification reaction between char and CO2 during devolatilization in CO2, which could affect the char properties and char combustion behavior. Figure 6 presents the PSDs and mean volume diameters of coal and char samples. The char swelling factor is defined as the ratio of D[4,3] of char to that of coal. The value of CO2-char is 2.52, while that of N2-char is 2.54. The similarities of PSDs and swelling factor of two chars reveal that those factors in the micrometer scale are seldomly affected by the gasification reaction between char and CO2 and very likely have little effects on PM10 formation. Other relatively microfactors in the nanoscale affected by the gasification reaction between char and CO2, such as BET surface area, may be responsible for the discrepancy of the PM0.5 yield from the combustion of two chars. Table 1 shows that the char sample generated in CO2 has a much higher BET surface area than that in N2. Saastamoinen et

Figure 6. PSDs and mean volume diameters (D[4,3]) of raw coal, CO2char, and N2-char.

al.36 reported that, during coal combustion, O2 was mostly consumed on the char particle surface but abundant CO2 was able to penetrate into the interior of char particles and triggered the reaction between char and CO2 inside, therefore creating the larger BET surface area and pore volume of CO2-char. Because of this difference, the combustion rates and temperature of CO2-char particles could be higher than that of N2-char particles in oxy-fuel combustion, which was calculated in the previous work.10 Because the vaporization of inorganic materials is highly affected by the combustion temperature,18,35 CO2-char combustion would lead to more extensive vaporization of mineral matter because of their higher combustion temperature. It can be further proven by the data of volatile elements. The theoretical mass of volatile elements in chars is calculated according to their mass fractions in the theoretical ash mass of chars (shown in Table 2). The theoretical masses of Na and S in CO2-char are calculated to be 2.59 and 6.71 g of element/kg of coal, respectively, while those masses in N2-char are 5.45 and 14.62 g of element/kg of coal, respectively. In comparison to N2-char, the less content of Na and S in CO2char produces larger masses of Na and S in PM0.5, as shown in Figure 5a. Such a seemingly abnormal behavior lends clear support to the argument that CO2-char combustion can lead to more extensive vaporization of inorganic minerals. Therefore, CO2-char combustion may produce more PM0.5 than N2-char combustion because of its higher combustion temperature of 5686

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char particles, even though the ash loss during devolatilization in CO2 is slightly higher than that in N2. 4.2. Effect of the Devolatilization Process on PM0.5−2.5 Formation. Figure 3b shows that the PM0.5−2.5 yield from coal combustion is higher than that from char combustion. As illustrated in Figure 4, PM0.5−2.5 is dominated by refractory elements, such as Al and Si, whose total mass fraction accounts for 60−72%, it is consistent with the postulation that the central mode particles are primarily from char fragmentation.27,31 Besides, the sum of the mass fraction of volatile elements, including Na, K, S, and P, in PM0.5−2.5 is 18−25%. The occurrence of those elements could be attributed to the heterogeneous condensation/reaction mechanism. The loss of vaporized ash during char preparation, which was discussed above, may result in the lower mass of Na, K, and S in PM0.5−2.5 from the char combustion compared to coal combustion (Figure 5b), but the small fraction (18−25%) of volatile elements in PM0.5−2.5 implies that the effect of vaporized ash loss during devolatilization is indecisive in the production of PM0.5−2.5. The combustion of volatile matter, as mentioned above, may cause the particle temperature during coal combustion to be very likely higher than that during char combustion. This can be illustrated by comparing the morphologies of PM0.5−2.5 from coal and char combustion in Figure 7. PM0.5−2.5 from coal combustion (Figure 7a) is dominantly spherical in shape, implying the complete particle melting. In contrast, there are a number of oval-like and irregular particles in PM0.5−2.5 from the combustion of CO2-char and N2-char (panels b and c of Figure 7), suggesting the insufficient melting because of the lower burning temperature of chars. In comparison to char combustion, the higher particle burning temperature in coal combustion is expected to enhance not only the vaporization of inorganic minerals but also the fragmentation of char.15,35 The larger mass of volatile elements (Na, K, and S) in PM0.5−2.5 from raw coal combustion, as shown in Figure 5b, is in close agreement with the argument that the higher temperature of coal combustion promotes the heterogeneous condensation/ reaction of volatile elements on fine particles. Besides, Figure 5b presents that Al and Si are the most prevalent inorganic elements in PM0.5−2.5; their mass from coal combustion is higher than that from char combustion, suggesting that the char fragmentation in coal combustion is intensified to generate more fine aluminosilicates in the size range of 0.5−2.5 μm. As a result of the higher particle burning temperature, a larger amount of PM0.5−2.5 will be formed during coal combustion, as shown in Figure 3b. There is an appreciable difference in the PM0.5−2.5 yield between the combustion of CO2-char and N2-char. Subsection 4.1 refers to the gasification reaction between char and CO2 causing the higher BET surface area of CO2-char compared to N2-char and the CO2-char particles likely having a higher combustion temperature. In addition, Figure 8 further shows that, in the whole pore size range of 1.7−300 nm, the pore volume of CO2-char is much higher than that of N2-char, indicating the CO2-char possesses much more abundant pore structure compared to N2-char. All of these would result in more extensive particle fragmentation during CO2-char combustion. Further proof can be observed in Figure 5b. The mass of refractory elements Al and Si found in PM0.5−2.5 from N2-char combustion is much lower than that from CO2-char combustion, suggesting that the fine aluminosilicates in N2-char are difficult to retain their fine size and be partitioned into

Figure 7. Typical morphology of PM0.5−2.5 formed from (a) raw coal, (b) CO2-char, and (c) N2-char.

PM0.5−2.5. This phenomenon should be caused by the less char fragmentation and more mineral coalescence during N2-char combustion. As a result, the PM0.5−2.5 yield from CO2-char 5687

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Consequently, both the vaporization of inorganic materials and the fragmentation of char during CO2-char combustion are promoted to generate more PM0.5 and PM0.5−2.5 than N2-char combustion.

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AUTHOR INFORMATION

Corresponding Authors

*Telephone: +86-27-87544779. Fax: +86-27-87545526. E-mail: [email protected]. *Telephone: +86-27-87546631. Fax: +86-27-87545526. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors sincerely acknowledge the finances supported by the National Natural Science Foundation of China (Grant 51376071, U1261204), the National Key Basic Research and Development Program of China (Grant 2013CB228501), and the Program for New Century Excellent Talents in University (Grant NCET-11-0192). The XRF testing from the Analytical and Testing Center at Huazhong University of Science and Technology is also thanked.

Figure 8. Pore size and pore volume distributions of raw coal, CO2char, and N2-char.

combustion is higher than that from N2-char combustion, as shown in Figure 3b.

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5. CONCLUSION This work provides important insights into the effect of the devolatilization process on PM10 formation during advanced oxy-coal combustion. CO2-char and N2-char were prepared after devolatilization of a typical Chinese bituminous coal under a CO2 or N2 atmosphere, respectively. Despite it being difficult to acquire a char to represent an accurate state after coal devolatilization, the results from the combustion of raw coal, CO2-char, and N2-char can still help us to understand further the effect of the devolatilization process on PM10 formation. Major findings in this work include the following: (1) The PSD profiles of PM10 from the combustion of coal and chars show similar peaks and troughs, suggesting that the devolatilization process has an insignificant effect on the major pathways of PM10 formation. (2) On the basis of the mass fraction size distribution of the elements Al + Si in PM10 from coal and char combustion, three particle modes can be clearly identified. The ultrafine mode contains particles less than about 0.5 μm, and the coarse mode is roughly identified in the range of 2.5−10 μm. In addition to the two modes, there is also a clear central mode in the range of approximately 0.5−2.5 μm. (3) The devolatilization process has an important influence on the production of PM0.5 and PM0.5−2.5, especially for PM0.5. In contrast, the PM2.5−10 yield is insignificantly affected by the devolatilization process. The volatile materials are released during char preparation, which can inhibit the vaporization of inorganic elements and the formation of PM0.5 from char combustion compared to coal combustion. The burning volatile matter poses a higher particle burning temperature of coal combustion and dominantly enhances both the elemental vaporization and char fragmentation to form more PM0.5 and PM0.5−2.5 than char combustion. (4) The devolatilization in CO2 tends to produce char particles with properties that favors the formation of PM0.5 and PM0.5−2.5 compared to that in N2, but the mass of vaporized ash loss during devolatilization in CO2 is higher than that in N2. The gasification reaction between char and CO2 that occurred during devolatilization in CO2 can explain this observed behavior. It increases the pore volume and BET surface area of CO2-char apparently, leading to its higher particle burning temperature compared to N2-char.

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