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Energy & Fuels 2008, 22, 3844–3851
The Formation and Emission of Particulate Matter during the Combustion of Density Separated Coal Fractions Xiaowei Liu, Minghou Xu,* Hong Yao, Dunxi Yu, Dangzhen Lv, and Ke Zhou State Key Laboratory of Coal Combustion, Huazhong UniVersity of Science and Technology, Wuhan, China, 430074 ReceiVed April 4, 2008. ReVised Manuscript ReceiVed August 8, 2008
The present study is a further effort to extend our knowledge of the included and excluded mineral characteristics responsible for the formation of particulate matter (PM). A Chinese bituminous coal was first separated into three density fractions using the float-sink method: heavy (>2.0 g/cm3), medium (1.4-2.0 g/cm3) and light (1 µm to the competition between the particle fragmentation and the ash coalescence enhanced by the shrinking peripheral area with the progress of char combustion. Helble and Sarofim17 investigated the influence of char fragmentation on the size distribution of fly ash in detail. They found that the macroporosity of the char was the important variable in determining fragmentation behavior, as evidenced by results obtained from combustion of synthetic chars. Macroporous Spherocarb doped with sodium silicate yielded 75 ash particles greater than 1 µm in diameter per char particle, whereas non-macroporous sucrose/carbon black chars doped with sodium silicate yielded only 1 ash particle per char particle. Recently, Yan and Gupta18 reported a mathematical model of ash formation during pulverized coal combustion. The model was based on the CCSEM characterization of minerals in pulverized coal. The model predicted the particle size distribution and chemical composition of ash. Buhre and Gupta4 indicated that coals showed an increase in submicron ash yield at elevated oxygen partial pressure, and char characterization and ash fusion temperature could play an important role in the minimization of the fine ash formed. Coal is a mixture of organic and mineral matter. Mineral grains can be classified as included and excluded minerals. For PM10 formation, the included minerals coalescence mechanism and the excluded minerals fragmentation mechanism determined particle size distribution together. By re-examination of literature data above and others, we found that the previous work was carried out using the raw pulverized coal. During combustion of raw coal, it is difficult to assess the relative contributions of each mechanism to the formation of particulate matter. It is difficult to separately assess the effects of the excluded mineral matter on PM formation. These have not been previously studied and are the focus of this article. To elucidate their origins, we used density-separated coal fractions. The float-sink method was primarily used for determination of major and trace element affinities in coal19-22 and char structure and burnout.23 Only a few previous studies about PM formation used a densityseparated fraction. Pulverized coal has been separated using the float-sink method into three density fractions: light (2.0 g/cm3). The reason for this was that mineral matter distribution varied with the density separation fraction. In general, a very high excluded mineral content can be achieved in the heavy density fraction. For example, in one study, Russell et al.24 showed that included and excluded mineral matter influenced ash deposition. Three density fractions were prepared. The light fraction contained little mineral matter, most of it included, whereas in the heavy fraction, most of mineral matter was excluded. The medium (16) Kang, S. G. Ph. D. Thesis, Department of Chemical Engineering, MIT, 1991. (17) Helble, J. J.; Sarofim, A. F. Combust. Flame 1989, 76, 183–196. (18) Yan, L.; Gupta, R.; Wall, T. Fuel 2002, 81, 337–344. (19) Senior, C. L.; Zeng, T.; Che, J.; Ames, M. R. Fuel Process. Technol. 2000, 63, 215–241. (20) Pusz, S.; Krzton, A.; Komraus, J. L.; Martinez-Tarazona, M. R. Coal Geology 1997, 33, 369–386. (21) Querol, X.; Klika, Z.; Weiss, Z. Fuel 2001, 80, 83–96. (22) Zhuang, X.; Querol, X.; Plana, F.; Alastuey, A.; Lopez-Soler, A.; Wang, H. Coal Geology 2003, 55, 103–115. (23) Yu, Jianglong; Lucas, J.; Strezov, V.; Wall, T. F. Energy Fuels 2003, 17, 1160–1174. (24) Russell, N. V.; Mendez, L. B. Fuel 2002, 81, 657–663.
Energy & Fuels, Vol. 22, No. 6, 2008 3845 Table 1. Weight Percent of Each Fraction coal
density (g/cm3)
weight percent (wt %)
C1 C2 C3
2.0
30.28 58.52 11.20
Table 2. The Proximate and Ultimate Analysis of Coal Sample coal
proximate analysis/wt %, dry
C1 C2 C3
Ad 4.34 28.92 79.48
Vd 27.33 21.37 16.53
FCd 68.33 49.71 3.99
ultimate analysis/wt %, dry C 81.33 56.10 6.65
H 5.41 3.19 0.49
O* 4.23 6.76 5.94
N 1.42 0.93 0.19
S 3.27 4.10 7.25
* By difference.
fraction contained more mineral matter than the light fraction, and this was largely included. From the above considerations, the goal of the work described in this paper is to demonstrate that the included mineral coalescence mechanism and the excluded mineral fragmentation mechanism have impacts on PM formation separately and to assess the relative contributions of each mechanism. The present study is a further effort to extend our knowledge of the included mineral and excluded mineral characteristics responsible for the formation of PM. A Chinese bituminous coal was first put through density separation using sink-float techniques. Three density fractions were prepared. Then, a low-temperature ashing technique was used to identify minerals from each coal fraction. Char samples were prepared in a drop tube furnace (DTF) and characterized. Char swelling behavior of each density fraction was characterized by the swelling ratio, BET surface area, and total pore volume. Each density fraction coal was burnt in a DTF, and the resulting PM was collected by a Dekati lowpressure impactor. PM characteristics including particle size distribution, emission concentration, and elemental compositions of each density fraction were investigated. In addition, in this paper, the particles with an aerodynamic diameter less than 10 µm and greater than 1.0 µm were termed as supermicron ash (PM1-10). Submicrometer ash (PM1) referred to particulates less than 1.0 µm in diameter. 2. Experimental Section 2.1. Density Separation. The pulverized coal was separated using the float-sink method into three density fractions: light (2.0 g/cm3). The selected fractions were prepared using benzene/carbon tetrachloride mixtures for density of 1.6 g/cm3 and benzene/bromoform mixtures for density of 2.0 g/cm3. The light, medium, and heavy fractions are denoted as C1, C2, and C3, respectively. The weight percent of each fraction is shown in Table 1. The properties of the coal fractions are shown in Table 2. The results suggest that with the increase of coal density, the ash content increases sharply, whereas the volatile matter content decreases. Both the carbon content and hydrogen content generally decrease with increasing coal density, whereas the sulfur content increases. 2.2. Coal Combustion. The combustion experiments of the three coal fractions were carried out in a laboratory-scale DTF. The height of the reactor tube was 200 cm, and the inner diameter was 56 mm. The reaction temperature was maintained at 1673 K. The oxygen content was 20%, balanced with pure nitrogen. The coal sample was fed at the rate of 0.2 g/min in all runs. The residence time of the particles in the tube was about 2 s. Under given conditions, all the samples burned almost completely. The exiting gas, entraining the solid products, was first quenched with N2 at the top of a water-cooled probe. Subsequently, the fly ash was collected by a cyclone having a cutoff size around 10.0 µm, and a low pressure impactor (LPI) was used for a size-segregated collection. The LPI used here was composed of 13 stages having
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Table 3. Composition of Ash ash elemental composition (wt %)
C1
C2
C3
Na Mg Al Si K Ca Ti Fe
0.72 0.97 36.43 48.09 0.87 3.52 1.62 7.78
0.27 0.79 33.09 51.66 2.30 0.42 3.10 8.37
1.47 1.09 24.23 53.69 3.10 3.32 1.32 11.78
aerodynamic cutoff diameters ranging from 9.8 to 0.03 µm. The d50 of the thirteen impaction plates were 0.0281, 0.0565, 0.0944, 0.154, 0.258, 0.377, 0.605, 0.936, 1.58, 2.36, 3.95, 6.60, and 9.80 µm, respectively. The gas flow rate was maintained at 10 L/min. 2.3. Coal Pyrolysis. Chars were also prepared in the DTF. The same temperature and coal feeding rate were selected as in the coal combustion experiments. The pyrolysis experiments were completed at 1 atm and in the N2 atmosphere with 1% (v/v) oxygen, a slightly oxidizing atmosphere that was considered necessary to avoid contamination of the char samples with soot and condensed tars. Char particles were finally collected on glass fiber filters with a pore size of 0.3 µm. 2.4. Sample Analysis. The cross section of each coal fraction was analyzed with a scanning electron microscope (SEM). Crystalline minerals presented in each coal fraction were characterized by X-ray diffraction (XRD). The LTA of each coal sample was prepared in a plasma low-temperature ash instrument at about 423 K. A Malvern particle-size analyzer was used to obtain the size distributions of the coal samples and their corresponding chars and minerals. Single-point BET nitrogen adsorption was used to determine the BET surface area and total pore volume of the char samples. By the size-segregated collection using the LPI, both the concentrations of PM10 and its particle size distributions were simultaneously obtained. The solid products, including the fly ash collected by the cyclone and PM collected by the LPI, were subjected to X-ray fluorescence (XRF) analyses to quantify their elemental composition (Table 3).
3. Results and Discussion 3.1. Mineral Distribution. To obtain the mineral distribution, the low-temperature ashing technique was used to remove the organic matter (OM) from each coal fraction with minimal disturbance and damage to mineral microstructure. The mineral particle size distribution (PSD) was analyzed using a Malvern Laser Sizer. The results are shown in Figure 1. The mineral PSDs of the coal fractions are significantly different from one another. The minerals in C1 have the smallest particles, whereas the minerals in C3 have the largest particles. To investigate whether the differences in mineral PSDs were the result of differences between coal PSDs, the coal PSDs were also subjected to particle size analyses by the same Malvern Laser
Figure 1. Particle size distribution of minerals.
Figure 2. Particle size distribution of raw coal.
Sizer. The results, illustrated in Figure 2, show that the PSDs of the three density-separated coal fractions are almost the same. This indicates that the differences in mineral PSDs are actually influenced by coal density and mineral matter content rather than the coal PSDs. Minerals in a pulverized coal are generally classified into included and excluded minerals on the basis of their associations with coal carbon matrix. The behavior of included and excluded minerals was greatly different during combustion. In this paper, the cross section of each coal fraction was analyzed under a scanning electron microscope (SEM). Examples of the backscattered electron (BSE) images from them are shown in Figure 3. In the images, darkest area denote resin. The grey area denotes carbon, and the bright areas denote mineral matter. Obviously, the light fraction contained less included minerals with the smallest mineral particle size. The proximate analysis in Table 2 showed that the light fraction had only 5% ash content and nearly 70% fixed carbon. The medium fraction contained more mineral matter than the light fraction, and this was largely included (nearly 30% ash content and 50% fixed carbon). The heavy density fraction contained mainly excluded mineral matter (nearly 80% ash content and only 4% fixed carbon). The included minerals, which were embedded within coal particles, usually undergo coalescence during combustion, to some extent depending on their physical closeness within a singe char particle. Two extreme models for coalescence behavior of included minerals have been frequently used in the past to predict ash size distribution and chemical composition, that is, no-coalescence scheme and full-coalescence scheme.18,25,26 The no-coalescence scheme assumes that one mineral grain evolves into a single ash particle regardless of the included or excluded nature of the minerals. The full-coalescence scheme, on the contrary, consideres that all included mineral grains within a single char particle coalesce to form a single ash particle after complete burnout of the char particle. A partial coalescence was more likely to occur in real situations and was expected to be somewhere intermediate between these two extreme cases.18 On the other hand, excluded minerals evolved into ash particles individually with or without fragmentation, depending on thermal behaviors of the mineral species. Among the dominant mineral species occurring in coal, silicate minerals including quartz, etc., were reported not to experience fragmentation. Calcite and pyrite indeed underwent fragmentation. Yan9 indicated that for calcite the volume-median diameter d50 decreased from 44 µm for the raw material to 34 µm for the (25) Benfell, K. E.; Bailey, J. G. Comparison of combustion and high pressure pyrolysis chars from Australian black coals. Sydney, Australia, Proceedings of AIE Eighth Australian Coal Science Conference, Australian Institute of Energy, December 7–9, 1998; pp 157-162. (26) Gupta, R. P.; Wall, T. F.; Kajigaya, I.; Miyamae, S.; Tsumita, Y. Prog. Energy Combust. Sci. 1998, 24, 523–543.
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Energy & Fuels, Vol. 22, No. 6, 2008 3847
Figure 3. The BSE images of raw coal.
residue, and for pyrite, the volume-median diameter d50 changed from 65 µm for raw pyrite to 40 µm for its residue, which indicated a certain extent of fragmentation.10 Brink27 suggested one calcite particle of 60 µm broke up into fragments approximately 10 times smaller in a laboratory facility. Although both included mineral coalescence and excluded mineral fragmentation contribute mainly to supermicron ash (PM1-10), they play a different role in the transformation extent during coal combustion. Hence, each density fraction of the parent coal produces a different weight percentage of PM. 3.2. Char Swelling. When introduced into the combustion system, coal particles became plasticized. At the same time, the particles started to release volatile gaseous species. Gas evolving through the coal matrix left pores behind. This was called the devolatilization stage. The pores generated during devolatilization assumed drastically different shapes and sizes, depending on the maceral constitution in coal. Chars of different shapes and pore structures would have different depths of (27) Ten Brink, H. M.; Eenkhorn, S.; Weeda, M. Fuel Process. Technol. 1996, 47, 233–243.
Figure 4. A comparison of PSD (by volume) of char and raw coal.
oxygen penetration and would therefore have different combustion patterns. The PSDs of the coal samples and their corresponding char particles are shown in Figure 4. It can be clearly seen that char particles produced from C1 have a much larger size distribution than the coal samples. It indicates that coal particles undergo significant swelling during pyrolysis under the used experimental condition. The swelling ratios, defined as the average particle size of the resultant char sample over that of the original coal at a certain temperature, are shown in Figure 5. C1 has the highest swelling ratio of 1.47. The swelling ratio of C2 and C3 are 1.16 and 1.02, respectively. Because C3 has only 4% fixed carbon (proximate analysis shown in Table 2) and the highest ash content (mainly excluded mineral), the char particle size distribution is almost the same as that of the coal sample. The morphology of the chars produced in the DTF was analyzed under a SEM. Char was classified in three categories: thinwalled, thick-walled, and solids. The morphological character
3848 Energy & Fuels, Vol. 22, No. 6, 2008
Liu et al. Table 4. BET Surface Area and Total Pore Volume of Char
Figure 5. Swelling ratios of chars from density fractions of coal.
Figure 6. The morphology of chars.
of chars from each coal density fraction is shown in Figure 6. C1 has mostly thin-walled. C2 largely contains thick-walled. C3 mainly consists of solids. The increase of swelling ratio with decreasing coal density was also observed by other researchers. The study by Yu et al.23 on particle swelling and char structures from density fractions of pulverized coal showed that the swelling ratio for the light density fraction ( 50 nm) pore volume (cm3/g) (10 nm < dp < 50 nm) pore volume (cm3/g) (dp < 10 nm) total pore volume (cm3/g) BET surface area (m2/g)
0.011 0.005 0.007 0.023 9.12
0.008 0.004 0.004 0.016 6.37
0.0055 0.0012 0.0003 0.007 1.06
observation was also consistent with Gilfillan et al.28 In their study, the reactivity of the density fractions was assessed using a DTF at a temperature of 1573 K, residence time of 100 ms and 1 vol % oxygen atmosphere. Char fragmentation was found to be one of the dominant mechanisms controlling the formation of ash particles. The ash particle size distribution was expected to shift toward fine ash particles. Baxter29 indicated that bituminous coals fragment more extensively than the lignite, with the extent of fragmentation exhibiting strong particle size dependence and a weaker ash loading dependence. The number of fragments produced per original bituminous char particle varies from 1 to 10 for initial char particle sizes less than 20 µm to over 100, or possibly several hundred, for initial char particle sizes greater than 80 µm. The previous studies suggested that the macropores in the shell played an important role in the char fragmentation.17 Hence, single-point BET nitrogen adsorption was used to determine the surface areas of the chars utilized in this study. The results are shown in Table 4. The BET surface area and total pore volume of chars increased with the decreasing parent coal density. During coal devolatilization, the developed char particles showed the evolution of a higher porous structure. The porosity of these particles could be as high as 80% on a volume basis, and the BET surface area of these chars was known to be at least 1 order of magnitude greater than the surface area of the original coals.30 The pore surface was recognized to form during the plastic state of coal devolatilization. Because of the different swelling ratios for each fraction, char with a larger swelling ratio had more surface area and total pore volume. Some researchers31-36 found that parent coal vitrinite content directly influenced the char mean diameter, porosity, and sphericity. The coal with the highest vitrinite content showed the highest sphericity and proportion of cenospherical chars after devolatilization. 3.3. Influence of Coal Density on PM10 Formation. The particle size distributions of PM10 are shown in Figure 7. It can be seen that PM10 from all density fractions of the coal samples were bimodally distributed, with a large mode at 4.0 µm and a small one at 0.1-0.2 µm. According to classical field and theoretical studies, the PM1-10 was believed to be formed through char fragmentation, included mineral coalescence and excluded mineral fragmentation. Char fragmentation and mineral (28) Gilfillan, A.; Lester, E.; Cloke, M.; Snape, C. Fuel 1999, 78, 1639– 1644. (29) Baxter, L. L. Combust. Flame 1992, 90, 174–184. (30) Lorenz, H.; Carrea, E.; Tamura, M.; Haas, J. Fuel 2000, 79, 1161– 1172. (31) Benfell, K. E.; Liu, G.; Roberts, D.; Harris, D. J.; Llucas, J. A.; Bailey, J. G.; Wall, T. F. P. Combust. Inst., Pittsburgh, PA 2000, 28, 1651– 1660. (32) Bailey, J. G.; Tate, A. G.; Diessel, C. F. K.; Wall, T. F. Fuel 1990, 69, 225–239. (33) Benfell, K. E.; Bailey, J. G. Eighth Australian Coal Science Conference, 1998; pp 157. (34) Yu, D.; Xu, M.; Yu, Y.; Liu, X. Energy Fuels 2005, 19, 2488– 2494. (35) Gale, T. K.; Bartholomew, C. H.; Fletcher, T. H. Combust. Flame 1995, 100, 94–100. (36) Gao, H.; Murata, S.; Nomura, M.; Ishigaki, M.; Tokuda, M. Energy Fuels 1996, 10, 1227–1234.
Particulate Matter and Coal Fractions
Figure 7. Particle size distribution of PM10.
Figure 8. Influence of coal density on weight percentage of PM1 and PM10.
coalescence were the key processes influencing the transformation of included mineral during pulverized coal combustion, and supermicron PM formation was the consequence of the competition between char fragmentation and included minerals coalescence. The excluded mineral fragmentation has been ignored in most previous work on coal combustion, although a few researches experimented with single pure minerals, and this was very different from the actual coal combustion. To explore the effect of coal density on PM1 and PM10 formation, the weight percentages of PM1 and PM10 in the total collected ash from each density fraction were calculated and are presented in Figure 8. The total ash was the sum of the mass of the fly ash collected by the cyclone (>10 µm) and by the LPI (
