Comparison of Particle Size Evolution during Pulverized Coal

Nov 4, 2013 - It may have a consequence on particle size evolution and ash particle formation during char conversion. Combustion in an O2/CO2 ...
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Comparison of Particle Size Evolution during Pulverized Coal Combustion in O2/CO2 and O2/N2 Atmospheres Yuan Chen, Guoliang Wang, and Changdong Sheng* School of Energy and Environment, Southeast University, Nanjing 210096, People’s Republic of China ABSTRACT: The present work was aimed at investigating particle size evolution during pulverized coal combustion in an O2/ CO2 atmosphere in comparison to that in an O2/N2 atmosphere. Two narrow size fractions of a bituminous coal were devolatilized in N2 and CO2 and burned in O2/N2 and O2/CO2 atmospheres in a high-temperature drop-tube furnace. The resulting chars and residual ashes as well as the parent coals were characterized with scanning-electron-microscopy-based image analysis to quantify particle size distributions, which were systematically compared. The results show that devolatilization in CO2 produces finer char particles than that in N2. It may have a consequence on particle size evolution and ash particle formation during char conversion. Combustion in an O2/CO2 atmosphere generally generates more fine ash particles than burning in an O2/N2 atmosphere at the same oxygen level. Increasing the O2 concentration decreases the amount of the fine ash particles in both atmospheres. The results also show that changing gas atmospheres has a greater influence on the evolution of the particle size distribution during the combustion of the larger coal size fraction.

1. INTRODUCTION Oxy-fuel combustion is regarded as one of the most promising technologies for pulverized-coal-fired power plants to control CO2 emissions, and it also offers additional benefits of substantially reducing NOx, SO2, and Hg emissions.1−4 Therefore, extensive research efforts have been made to cover many scientific and engineering issues on its applications.1,5,6 The technology is undergoing rapid development with a number of demonstration projects around the world, which are aimed at developing it toward commercialization.6 In an oxy-coal combustion system, oxygen with recycled flue gas (principally CO2) is used to replace air for combustion to generate flue gas with a high concentration of CO2 ready for sequestration. In comparison to conventional air combustion, there are significant changes in the oxidants and the flue gas composition in oxy-coal combustion. For instance, a higher O2 concentration, typically 30% in contrast to 21% in air, is required for achieving a similar combustion performance to that of air combustion.7,8 The resulting flue gas contains mainly CO2 and a minor amount of O2, N2, SO2, etc., while that from air combustion consists of a great quantity of N2 and CO2 and a minor amount of O2, SO2, etc. Consequently, coal combustion processes, including devolatilization and ignition,9−12 char burnout,8,13−18 and pollutant formation,19−23 are more or less different from those in air combustion. Basically, inorganic matter in coal coexists with organic matter, most of which are minerals. After pulverization, the minerals are present in coal particles as included or excluded, depending upon their associations with the coal matrix. During pulverized coal combustion, the transformation behavior of the minerals and the vaporization of inorganic matter are believed to be significantly dependent upon the temperature and gas environment.24,25 The behavior of the included minerals is to a great extent affected by the combustion of organic matter. The change from air to oxy-coal combustion was found to result in significant changes in the burning temperature8 and internal gas composition26 of char particles, which are expected to have © XXXX American Chemical Society

effects on the behavior of the included minerals. While the excluded minerals behave nearly independently of the burning of organic matter during pulverized coal combustion, the transformations of some excluded minerals can be affected by the gas environment.27,28 For example, the transformation behavior of excluded pyrite in O2/N2 and O2/CO2 combustion was shown to be considerably different.29 Previous studies indicated little difference in the elemental composition and crystalline phases between the bulk ashes of burning a coal in O2/N2 and O2/CO2 combustion.30−33 However, it was observed that O2/CO2 combustion has evident influences on the composition of mineral phases, such as iron-containing phases, in the residue ash.29,31 It was attributed to the change of coal-particle-burning temperature, which was postulated to affect the interaction behavior, i.e., the fragmentation and coalescence of the included minerals and mineral associations.33 The size distribution of residual ash from pulverized coal combustion depends upon the transformation and redistribution of the minerals during devolatilization and the competition between the coalescence of included mineral grains and the fragmentation of char/ash particles, along with the burnout of char particles. Char particles in an O2/CO2 atmosphere burn at lower temperatures than those in an O2/N2 atmosphere at the same O2 level.8 It may suggest a less extent of melting and consequently a less extent of coalescence or a more extent of fragmentation of included mineral grains within a char particle. The modeling work of Brix et al.34 indicates that, in O2/CO2 combustion, CO2 gasification can contribute to the conversion rate of char particles burning at high temperatures. In contrast to the difficulty of O2 penetration, the easiness of Special Issue: 4th (2013) Sino-Australian Symposium on Advanced Coal and Biomass Utilisation Technologies Received: August 1, 2013 Revised: November 2, 2013

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CO2 penetration into char particles may lead to relatively even burnoff of the carbon matrix, which may favor the fragmentation of char particles. On the other hand, because of the high concentration of CO2 in an O2/CO2 atmosphere, the concentration of CO within char particles is also high.26 It may decrease melting temperatures of iron-containing compounds and, therefore, favor the melting and coalescence of included mineral grains. All three aspects are expected to have influences on the sizes of ash particles formed by the included minerals. Moreover, changing atmospheres also affects the behavior of the excluded minerals, e.g., fragmentation. As a result, the sizes of ash particles from the excluded minerals can also be affected. The objective of the present work was to study particle size evolution during pulverized coal combustion in an O2/CO2 atmosphere in comparison to that in an O2/N2 atmosphere. Two narrow size fractions of a bituminous coal were devolatilized in N2 and CO2 and burned in O2/N2 and O2/ CO2 atmospheres in a high-temperature drop-tube furnace (DTF). Particle size distributions of the resulting chars and residual ashes as well as the parent coals were quantified by an image analysis technique, which were systematically compared to investigate the influence of changing atmospheres from O2/ N2 to O2/CO2 on particle size evolution and residual ash formation during pulverized coal combustion.

Table 2. Composition and Distribution of the Minerals in Two Coal Size Fractions Determined by CCSEM

50−75 μm

75−90 μm

excluded minerals

number (%)

mass (%)

number (%)

mass (%)

11.78 0.98 10.75 7.27 14.60 1.36 4.35 5.04 56.13 9.94 1.06 11.56 6.30 15.11 1.41 3.06 5.95 54.40

9.92 0.85 14.23 4.34 12.60 0.93 3.50 4.42 50.79 11.06 1.04 13.26 5.58 12.56 0.81 3.94 5.18 53.43

6.58 1.96 12.22 2.37 10.07 0.41 2.28 7.98 43.87 6.94 2.02 13.14 5.02 10.74 0.36 2.33 5.05 45.60

5.21 1.97 19.86 1.08 12.58 0.27 1.76 6.48 49.21 5.74 1.16 18.80 4.64 8.93 0.21 2.31 4.78 46.57

simulate reaction conditions that pulverized coal experienced in a practical process. The DTF and its systems are schematically shown in Figure 1. The reactor has a 1.3 m long corundum tube electrically heated by three independently controlled elements. Pulverized coal was introduced into the reactor tube at the top by a spiral coal feeder. The experimental gas, i.e., N2, O2, or the mixture of O2/N2 or O2/ CO2, was fed into the furnace through the primary and secondary gas streams. The reaction products were extracted at the bottom of the reactor using a water-cooled sampling probe, which was connected to a quartz fiber filter with a pore size of 0.3 μm to collect char or residual ash particles. 2.3. Devolatilization and Combustion Experiments. Devolatilization experiments were carried out in a N2 or a CO2 atmosphere to generate char samples, while combustion experiments were conducted to burn the coal samples in an O2/CO2 or an O2/N2 atmosphere at two O2 concentrations (21 and 30% O2) to generate residual ash particles. The furnace temperature was set at 1573 K for both devolatilization and combustion. The coal feeding rate was ∼5 g/h. The residence time of coal particles in the reactor was ∼0.5 s for devolatilization and ∼1.2 s for combustion, respectively. The flow rate of the experimental gas was 4.5−8.0 L/min. To consider the differences of gas properties, the rate for each experimental case was set different in this range. The gas flow rate, together with the settings of the sampling location, ensured the resident time to be same for N2 and CO2 devolatilization or for O2/N2 and O2/CO2 combustion. 2.4. Analyses of Char and Residual Ash Particles. The char or ash sample collected from each experimental case was examined with a FEI Sirion field-emission scanning electron microscope (SEM) to observe morphology and acquire images of the particles. The SEM images were further analyzed by the UTHSCSA Image Tool program36 to determine particle size distribution. For comparison purposes, SEM observation and image analysis were also performed on the coal size fractions. During the image analysis, each particle was delimited with its projective outline on the image. Size length L and width W were measured along the major and minor axes of the object, respectively. The equivalent spherical diameter of the particle was then calculated as d = (LW2)1/3. To obtain a statistically representative particle size distribution, tens of SEM images of each sample were analyzed, so that at least 300 large particles were counted, while all fine particles on the same images observable for image analysis were also measured and included in the reporting. It should be noted that the definition of the

2. EXPERIMENTAL SECTION 2.1. Coal Samples. A bituminous coal was used as raw material, which was ground to pass through a 200 μm screen. Its properties are presented in Table 1. The pulverized coal was further subject to wet sieving to obtain two narrow size fractions of 50−75 and 75−90 μm.

Table 1. Properties of the Coal Sample property

quartz iron oxide calcite kaolinite aluminosilicates pyrite others unknown total minerals quartz iron oxide calcite kaolinite aluminosilicates pyrite others unknown total minerals

included minerals

value

Proximate Analysis (wt %, on an Air-Dried Basis) moisture 7.79 ash 11.21 volatile matter 30.60 fixed carbon 50.40 Ultimate Analysis (wt %, on an Air-Dried Basis) carbon 64.55 hydrogen 4.94 nitrogen 0.88 sulfur 0.48 oxygen (by difference) 29.15 Ash Composition (wt %) SiO2 51.7 22.3 Al2O3 Fe2O3 5.2 CaO 13.4 MgO 1.6 TiO2 0.8 Na2O 2.0 K2O 2.8

The occurrence of the minerals in the size fractions was characterized with computer-controlled scanning electron microscopy (CCSEM). The obtained data were processed to derive the information on the included and excluded minerals based on the classification rules proposed by Zygarlicke and Steadman.35 The results are summarized in Table 2. 2.2. Experimental Facility. Devolatilization and combustion of the two samples were performed on a DTF, which was designed to B

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Figure 1. Schematic diagram of the DTF. equivalent diameter represents only a preliminary dimensional characterization of a particle. It may not give the “real” diameter particularly for a very irregular particle. Nevertheless, in the present work, particle size evolution was investigated by comparing particle size distributions of the parent coals, chars, and residual ashes, which were all determined with the definition. Therefore, it is not expected to have considerable effects on the comparisons and conclusions. In contrast to inert N2, CO2 can react with coal material at high temperatures. To evaluate its effect on particle size evolution during devolatilization, the conversions of all char samples were measured by burning the chars at 800 °C in a thermogravimetric analyzer and then calculating with the ash-tracer method. The results are presented in Table 3, together with the ash and volatile matter contents of the

The number and volume size distributions of the coal samples determined by the image analysis are presented in Figure 3. The single big and sharp peaks on the distribution profiles qualitatively indicate that both samples are fairly narrow in size. The cumulative data derived from the distributions are summarized in Tables 4 and 5 for 75−90 and 50−75 μm fractions, respectively. For both samples, although the presence of a fraction of fine particles by number implies the difficulty of completely removing very fine particles even by wet sieving, most particles either by number or volume are concentrated in the groups of the major particles (roughly defined as particles of 65−120 and 45−85 μm for the two size fractions, respectively). The narrow size distributions of the two samples allow for the observation on the behavior of particle size evolution during coal devolatilization and combustion. CCSEM analyses on the coal samples (Table 2) indicate that quartz, calcite, kaolinite, and aluminosilicates occur as major minerals, while pyrite and iron oxide (probably iron carbonates) are present with minor amounts. These are consistent with the elemental composition of the laboratory ash (Table 1). Table 2 shows that slightly more minerals (by mass) are included in both size fractions. The coarser fraction generally has more included minerals than the finer one. For individual minerals, quartz, aluminosilicates, and pyrite are preferentially included and relatively concentrate in the finer fraction, kaolinite is also more included but slightly enriched in the coarse fraction, and calcite and iron carbonates are preferentially excluded and accumulate in the finer fraction. Nevertheless, the differences in the distributions of individual species between the two size fractions are not significant. It enables the investigation to be focused on the influence of the parent coal size on the particle size evolution and residual ash formation during combustion in different atmospheres. 3.2. Evolution of the Particle Size during Coal Devolatilization. Figure 4 shows the morphology of the

Table 3. Properties of N2 and CO2 Chars Compared to Those of the Corresponding Parent Coal Size Fractions ash content (wt %, db)

a

a

coal volatile matter content or char conversion (wt %, daf)b

coal size fraction

coal

N2 char

CO2 char

coal

N2 char

CO2 char

50−75 μm 75−90 μm

12.36 11.22

22.66 20.41

26.53 23.12

37.48 38.17

51.87 50.73

60.94 57.98

db = on a dry basis. bdaf = on a dry and ash-free basis.

parent coal size fractions. Additionally, to understand the influence of the CO2 reaction on mineral redistribution during devolatilization and its consequence for ash particle formation, the chars were also examined with CCSEM to obtain the information on the distribution of included and excluded minerals.

3. RESULTS AND DISCUSSION 3.1. Characterization of Coal Samples. SEM images of the coal size fractions are shown in Figure 2. The 75−90 μm sample (Figure 2a) is fairly uniform in particle size. The 50−75 μm sample (Figure 2b) is also quite uniform, except for the presence of a very small fraction of fine particles. C

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Figure 2. SEM images of pulverized coal size fractions of (a) 75−90 μm and (b) 50−75 μm.

Figure 3. Particle size distributions of the chars compared to those of the parent coals: (a) number and (b) volume size distributions of the chars from 75−90 μm coal and (c) number and (d) volume size distributions of the chars from 50−75 μm coal.

Table 4. Size Distributions of 75−90 μm Coal and Its N2 and CO2 Chars cumulative number (%)

coal N2 char CO2 char

fine (120 μm)

number-averaged diameter (μm)

fine (120 μm)

volume-averaged diameter (μm)

89.02 51.03 17.58

5.88 9.66 2.51

92.14 74.50 39.33

0.96 2.79 9.31

79.23 68.80 62.41

19.82 28.42 28.28

106.3 110.4 106.1

char particles obtained from devolatilization in N2 and CO2 for 75−90 and 50−75 μm coal samples. Regardless of the coal

particle size and gas environment, most of the char particles are elliptical or spherical, significantly different from the morpholD

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Table 5. Size Distributions of 50−75 μm Coal and Its N2 and CO2 Chars cumulative number (%)

coal N2 char CO2 char

fine (85 μm)

number-averaged diameter (μm)

fine (85 μm)

volume-averaged diameter (μm)

75.35 40.82 28.02

4.25 4.37 2.20

58.84 41.57 32.02

1.86 3.28 5.55

83.26 75.76 76.96

14.88 20.96 17.48

74.6 74.9 71.1

Figure 4. SEM images of the char particles: (a) N2 char and (b) CO2 char from 75−90 μm coal and (c) N2 char and (d) CO2 char from 50−75 μm coal.

To quantitatively describe the differences between CO2 and N2 chars and the influence of the parent coal particle size, image analyses on all char samples were conducted. The obtained particle size distributions of the chars are shown in Figure 3 in comparison to those of the parent coals. Figure 3 illustrates that the number and volume size distributions of both CO2 and N2 chars are different from that of the corresponding parent coals. The number size distributions (panels a and c of Figure 3) exhibit much higher resolution. They show that the differences are significant, as indicated by the presence of fine size peaks centering at 10−20

ogy of the parent coal particles (Figure 2). Obviously, coal particles underwent softening and fusion during devolatilization in both N2 and CO2 environments. As seen in Figure 4, all char samples contain some small particles, which may be generated mainly by bubble rapture and particle fragmentation during devolatilization. Soot might also form during devolatilization in both environments. However, few soot particles were observed in SEM images. They might be too fine to be captured by the filter. Generally, CO2 chars contain relatively more small particles with more irregular morphology than N2 chars from the same size fraction. E

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Figure 5. SEM images of the ash particles from 75−90 μm coal burning in (a) O2/N2 with 21% O2, (b) O2/CO2 with 21% O2, (c) O2/N2 with 30% O2, and (d) O2/CO2 with 30% O2.

μm in addition to the coarse size peaks close to those of the parent coals. When the chars from the same fraction are compared, it is clear that CO2 char has a lower coarse size peak and a higher fine size peak than N2 char (panels a and c of Figure 3). The cumulative data derived from the number size distributions (Tables 4 and 5) indicate that devolatilization in CO2 produced much less major size particles and much more fine size particles than devolatilization in N2. As a result, the number average sizes of CO2 chars are much smaller than those of N2 chars. These suggest that finer char with more fine particles is formed from devolatilization in CO2 than in N2. The differences are more apparent between the CO2 and N2 chars from the coarser fraction, i.e., 75−90 μm coal (Figure 3a and Table 3). In contrast, the volume size distributions show much smaller differences (panels b and d of Figure 3). Nevertheless, they also indicate that a considerable amount of finer particles was formed after devolatilization in both N2 and CO2 and devolatilization in CO 2 generated more fine particles particularly from the coarser fraction, as indicated by the cumulative data summarized in Tables 4 and 5. The evolution of the particle size distribution during pulverized coal devolatilization is mainly attributed to two reasons. One is the result from particle shrinkage because of the release of a large quantity of organic matter as volatiles and particle swelling owing to the bubbling within the particles

blown by the volatile release.37 The other is particle fragmentation caused by the rapture of the bubbles during devolatilization. While the former is likely to yield char particles with the sizes close to those of the parent coal particles, the latter is expected to produce finer particles. Tables 4 and 5 indicate that the coarse particles of each char take the most fraction of the char volume, and the volume-averaged sizes of both N2 and CO2 chars are close to that of the corresponding parent coals. These suggest that the impacts of the shrinkage and/or swelling on the sizes of the major particles are not significant during devolatilization. Note that the fracture of the char particles during collection, storage, and SEM preparation cannot be ruled out as a source of uncertainty. However, considering the parent coals having narrow size distributions, a great number and a considerable volume of the fine particles in the resulting chars from devolatilization in both N2 and CO2 confirm significant particle fragmentation because of bubble rapture. The observation of Brix et al.34,38 also showed the evidence of the fragmentation in CO2 and N2 devolatilization, although their narrowed size coal sample contained quite a large fraction of fine particles. Furthermore, devolatilization in a CO2 atmosphere generally produced much more fine char particles and less major and coarse particles than N2 devolatilization (Tables 4 and 5). Both the number and volume size distributions (Figure 3) show these trends. One possible reason is that CO2 participates in F

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Figure 6. Particle size distributions of the ashes from O2/CO2 and O2/N2 combustion of 75−90 μm coal are compared to those of the chars from N2 and CO2 devolatilization: (a) number size distribution and (b) volume size distribution.

Figure 7. Particle size distributions of the ashes from O2/CO2 and O2/N2 combustion of 50−75 μm coal are compared to those of the chars from N2 and CO2 devolatilization: (a) number size distribution and (b) volume size distribution.

vesiculated char particles with network structure formed in CO2 than in N2. Considering that CO2 is less aggressive than O2 in air, the extent of the gasification (Table 3) and preferential formation of network char particles in CO2 imply that it is unlikely to cause such significant fragmentation, although the role cannot be excluded. 3.3. Evolution of the Particle Size during Coal Combustion. Four residual ashes were generated from the combustion experiments of each coal sample. Typical SEM images of the residual ashes from 75−90 μm coal are presented in Figure 5 as an example to show the morphology of the ash particles. The number and volume size distributions obtained from the image analysis are presented in Figures 6 and 7 for the ashes from 75−90 and 50−75 μm fractions, respectively. Considering that devolatilization is the early stage of the conversion during pulverized coal combustion, particle size distributions of the corresponding N2 and CO2 chars were replotted in Figures 6 and 7 for comparison purposes. SEM observation (Figure 5) shows that most ash particles have spherical shape, indicating that the particles experienced melting and coalescence during ash formation in the char combustion process. A fraction of small spherical particles and irregular fragments are evidently observed. It implies that coal and char particles also underwent fragmentation during combustion. For the ashes from both O2/N2 and O2/CO2 atmospheres, the ash particles generated from the combustion

cross-linking reactions and reduces the plasticity of coal organic material during devolatilization.34 It depresses the bubbling and bubble coalescence within the particles. On the other hand, the lower plasticity of coal material results in the higher pressure building up in the bubbles.39 Once the bubbles rapture, the higher internal pressure means stronger bursts, which are more likely to cause the particle fragmentation. Another possible reason is the partial gasification of char particles in a CO2 environment.15 The rough surface of some large CO2 char particles (panels b and d of Figure 4) may be evidence of CO2 gasification. The measured conversions of N2 and CO2 chars (Table 3) showed that CO2 devolatilization resulted in relatively 17 and 14% more conversion than N2 devolatilization for 50−75 and 75−90 μm fractions, respectively. It indicates the contribution of CO2 gasification to coal conversion,15 which can also cause char fragmentation. A study on char fragmentation during combustion of an Australian bituminous coal40,41 observed the fragmentation occurring around the completion of devolatilization (35.5% conversion for that coal) in N2 and air environments. It is in agreement with the present observation on N2 chars of both size fractions. The study also showed that, in air combustion, fragmentation becomes violent at burnout levels between 54 and 70%, because of the presence of a significant number of cenospherical char particles.40,41 However, the observation on the chars of a high-volatile bituminous coal14 showed that more G

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increased to 30%, both types of ashes turn to be coarser than those formed at the 21% O2 level mainly because of a higher char burning temperature enhancing mineral coalescence, as discussed above. However, the O2/CO2 ash is still much finer. Figures 6a and 7a and the related discussions above suggest that the difference in the number size distributions between the ashes from O2/CO2 and O2/N2 combustion at the same O2 level seems mainly to be the consequence of the difference between the CO2 and N2 chars. One may assume that devolatilization in CO2 and N2 represents the devolatilization processes during O2/CO2 and O2/N2 combustion. However, such an assumption may be too rough because O2 also has an impact on coal devolatilization similar to CO2.14 This impact on the in situ devolatilization actually occurs in both O2/CO2 and O2/N2 atmospheres. Considering that O2 is more aggressive than CO2, coal devolatilization and char formation behavior during O2/CO2 and O2/N2 combustion are likely to be more similar. Nevertheless, the difference in particle size distributions between the chars formed in O2/CO2 and O2/N2 atmospheres and its influence on the subsequent ash formation during char combustion may not be ruled out. The higher O2 concentration enhancing coalescence during char combustion, as indicated by the comparison between the ashes of O2/N2 combustion at two O2 levels, implies the burning temperatures of char particles playing an important role in ash formation. Char particles in O2/CO2 generally burn at the temperatures much lower than those in O2/N2 at the same O2 levels.7,8,13 Thus, less coalescence and more fragmentation of the mineral grains are expected during char combustion. It should also account for the difference between the particle size distributions of O2/CO2 and O2/N2 combustion ashes. Although nearly half of the minerals in the two coal samples are excluded (Table 2), the behavior of most included minerals is hardly influence by changing atmospheres. Only calcite and pyrite may decompose and fragment,42−44 which may be affected by the atmospheres. However, considering their percentages and the extents of the atmosphere influence, the change in their behavior is unlike to have a significant consequence for particle size evolution. In contrast, the behavior of the included minerals, i.e., coalescence and fragmentation, can be significantly affected by the combustion atmosphere and O2 concentration. Moreover, more than half of the minerals in the two fractions are included. Therefore, the behavior of the included minerals dominates ash particle size evolution, which can be affected by changing atmospheres. Figure 8 compares the percentages of the included minerals in N2 and CO2 chars to those of the parent coal size fractions.

at 30% O2 generally have more spherical particles than those from the combustion at a lower O2 level (21%). It is expected because a higher O2 concentration means higher burning temperatures of char particles,7,8,13 which enhances the melting of the minerals. Evident differences in volume size distributions can be observed between the ashes from 75−90 μm coal (Figure 6b), indicating the influences of the combustion atmosphere and O2 concentration on ash formation. In contrast, volume size distributions of the ashes from the 50−75 μm sample are quite similar, regardless of the combustion atmosphere and O2 concentration (Figure 7b), and they are also similar to those of N2 and CO2 chars as well as that of the parent coal (Figure 3d). These observations are generally consistent with those using an optical approach (volumetric method) to analyze ash particle sizes.30 Because the volume size distributions in Figures 6b and 7b has low resolution to show the differences between the ashes from the same size fraction, the following discussion is focused on the number size distributions. Figure 6a compares the number size distributions of the ashes and chars from 75−90 μm coal. It shows that the ash from O2/N2 combustion at 21% O2 has a greatly higher peak of fine particles and a much lower peak of coarse particles than N2 char. It implies that a great extend of fragmentation occurring during char combustion that consumed a large number of coarse particles and generated a great number of fine ash particles. In contrast, when the ash from O2/CO2 combustion at 21% O2 is compared to the CO2 char, the coarse particle peak shifts toward the larger sized end, which may be caused by both the coalescence and fragmentation of coarse particles during char conversion; the fine particle peak increases slightly, suggesting the considerable contribution of particle fragmentation during combustion when the burnoff of the fine char particles are also considered. When the O2 concentration is increased to 30%, the size distribution of the O2/N2 ash becomes close to that of the N2 char but the ash still contains more fine particles. It suggests that, while the coalescence of the mineral grains becomes more important in ash formation because of char particles burning at higher temperatures, the fragmentation also plays its role there. Increasing the O2 concentration to 30% in O2/CO2 combustion leads to the coarse particle peak of the ash shifting toward the small-sized end but increasing in height. These are also attributed to high O2 combustion enhancing the melting and coalescence. Meanwhile, the decrease of the fine peak with an increasing O2 concentration in O2/CO2 combustion may be mainly due to the burnoff of fine char particles containing few minerals but cannot exclude the contribution of the fragmentation. The third peak at middle sizes (Figure 6a) may be formed by large fragments. Although the number size distributions of all of the ashes and chars from the 50−75 μm coal fraction are marginally different (Figure 7a), similar trends can be observed for the evolution of particle size distributions, especially for the coarse particle peaks, when comparing the number size distributions of the O2/N2 and O2/CO2 ashes to those of the corresponding N2 and CO 2 chars. The influences of an increasing O 2 concentration on the size distribution evolution are generally similar to those observed for the coarse coal sample. Figure 6a shows that the number-size distributions of the ashes from burning 75−90 μm coal in O2/CO2 and O2/N2 at 21% O2 are slightly different and the O2/CO2 ash is generally finer than the O2/N2 ash. When the O2 concentration is

Figure 8. Mass and number percentages of the total included minerals in the chars and parent coals. H

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4. CONCLUSION Two size fractions (50−75 and 75−90 μm) of a bituminous coal were devolatilized in CO2 and N2 and burned in O2/CO2 and O2/N2 atmospheres. Particle size distributions of the resulting char and residual ash samples as well as the coal size fractions were quantified by image analysis. The influences of changing atmospheres on the particle size evolution and the related ash formation behavior during combustion were investigated through systematically comparing the size distributions of the parent coals, chars, and residual ashes. The influences of the coal particle size and O2 concentration for combustion were also studied. The main conclusions are as follows: (1) Devolatilization in CO2 leads to more particle fragmentation and the formation of more fine char particles particularly by number than devolatilization in N2. It may have a consequence for the evolution of ash particle sizes during char conversion and cause the difference in particle size distributions between the residual ashes from O 2 /CO 2 and O 2 /N 2 combustion. (2) Combustion in O2/CO2 generally generates more fine ash particles than burning in O2/N2 at the same oxygen level because of the lower char burning temperatures in an O2/CO2 atmosphere. Increasing the O2 concentration decreases the amount of the fine ash particles in both atmospheres, which indicates that a higher O2 concentration reduces char fragmentation and enhances the coalescence of included minerals. (3) Comparing the chars and residual ashes from the two size fractions indicates that changing gas atmospheres has a greater influence on the evolution of the particle size distribution during the combustion of the larger size coal.

The results show that only 2 and 3% (by mass) of total included minerals were librated from 50−75 and 75−90 μm coal devolatilized in N2; slightly more, i.e., 6 and 8%, were librated during CO2 devolatilization. These indicate that the libration of ash particles was not considerable during devolatilization, consistent with the observation of Wu et al.40 Figure 8 shows that the number percentages of total included minerals in the chars are significantly lower than those of the parent coals, implying the redistribution and coalescence of the included minerals during devolatilization. Nevertheless, the number percentages of total included minerals in the two types of the chars are nearly the same. It means that changing gas environments caused little difference in redistribution of the included minerals of both size fractions. In contrast, changing gas environments affect the char particle size distribution (Figure 3). It may have the consequence for the evolution of ash particle sizes during char combustion. Char fragmentation is dominant for ash formation at the early and middle stages, while included minerals coalescence dominates at the later stage of char conversion.40 In comparison to O2/N2 combustion, lower burning temperatures of char particles in an O2/CO2 atmosphere favor the fragmentation and do not favor the coalescence, consequently leading to the formation of finer ash particles. Increasing the O2 concentration from 21 to 30% certainly increases char burning temperatures in both atmospheres. In this case, because char combustion is diffusion-controlled, the intensive oxidation on the char surface does not favor fragmentation, while the higher burning temperatures allow for the coalescence to form relatively coarser ash particles. As shown in Figures 6a and 7a, changing the combustion atmosphere and O2 concentration has more evident influences on particle size distributions of residual ashes of the 75−90 μm fraction than those of the 50−75 μm fraction. It may be attributed to two reasons. One is the occurrence of the minerals in the parent coal. However, the similarities in the composition and distribution of the minerals between the two size fractions (Table 2) and between the corresponding chars rule out this possibility. The other reason is the particle size of the coal sample. The smaller size fraction generates more fine char particles than the larger size fraction during devolatilization in both N2 and CO2 (Figure 3). With regard to char combustion, a smaller particle generally attains a lower burning temperature than a larger particle whether burning in an O2/CO2 or an O2/ N2 atmosphere,8 resulting in less melting and coalescence of the included minerals. Moreover, a smaller particle is easier for O2 to penetrate and tends to burn more uniformly inside, which therefore favors particle fragmentation. As a result, all of the ashes formed from the fine coal fraction have a quite large fraction of fine ash particles, as shown in Figure 7. Additionally, the easy penetration of O2 also implies that the resulting ash particle size distribution is not so sensitive to the O2 concentration. In contrast, the combustion of a larger char particle is more preferentially diffusion-controlled. It implies that the combustion and consequently ash formation is sensitive to the O2 concentration. Because the penetration of O2 into the char particle is limited and that of CO2 is relatively easier, CO2 gasification may make a greater contribution in char conversion. It determines the combustion atmosphere dependence of the evolution of particle size distribution during the combustion of the large coal particles.

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AUTHOR INFORMATION

Corresponding Author

*Telephone: +86-25-83790317. Fax: +86-25-57714489. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors acknowledge financial support from the Fundamental Research Funds for the Central Universities (3203003905) and partial support from the Foundation of State Key Laboratory of Coal Combustion (FSKLCC0905) to the present work. We are also grateful to Prof. Minghou Xu of State Key Laboratory of Coal Combustion for his help on the access to use the CCSEM facility at the Analysis Center of Huazhong University of Science and Technology.

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