Energy & Fuels 2005, 19, 2261-2267
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Determination of the Mineral Distribution in Pulverized Coal Using Densitometry and Laser Particle Sizing Hong Zhang,* Yan-xue Mo, Ming Sun, and Xian-yong Wei School of Chemical Engineering, China University of Mining and Technology, Xuzhou, 221008, Jiangsu, People’s Republic of China Received July 5, 2005. Revised Manuscript Received August 23, 2005
Coal particle size and mineral matter content have important effects on coal combustion. The mineral content of five Chinese coals was determined by a method combining densitometry and particle-size analysis. The finer particles of pulverized samples were found to contain more mineral content. Rank also had a significant influence on the particle-size ash-content distribution of pulverized coal particles. The sharpest size-ash distribution was found in pulverized anthracite samples; a broader distribution was found with bituminous coal samples, while a uniform distribution was observed in pulverized lignite samples. Ash in higher ash anthracite or lower ash bituminous coal is more evenly distributed. It is a combined effect of size distribution, yield, and proximate analysis of their density separation fractions. Mineral matter tends to distribute more evenly in finer pulverized coals. This results from a relative increase of the low-density fraction in the finer particles.
Introduction Pulverized coal burns at a rate dependent upon its properties and characteristics such as particle size, rank, maceral composition, and mineral matter content. The influence of mineral matter on the combustion of coal has attracted much interest.1-5 Usually the coals were studied as a whole. The minerals were assumed to distribute evenly in pulverized coal, which is not true.2,4 Because particle size also has a significant effect on combustion,6,7 it seems likely that a detailed understanding of how the mineral matter in coal is distributed between different sized particles will aid the accurate prediction of the kinetics of char oxidation reactions. Furthermore, when the distribution of mineral matter is known as a function of size, a creative economical process might be developed to separate the high ash part * To whom correspondence should be addressed. E-mail:
[email protected]. Telephone: (86)-516-399-5018. Fax: (86)516-399-1167. (1) Mitchell, R. E. The influence of the mineral matter content of coal on the temperatures and burning rates of char particles during pulverized coal combustion, in Proceedings of the Sixth Annual International Pittsburgh Coal Conference; The Combustion Institute: Pittsburgh, PA, 1989; pp 69-78. (2) Menendez, R.; Alvarez, D.; Fuertes, A. B. Effects of clay minerals on char texture and combustion, Energy Fuels 1994, 8, 1007-1015. (3) Mendez, L. B.; Borrego, A. G.; Martinez-Tarazona, M. R. Influence of petrographic and mineral matter composition of coal particles on their combustion reactivity, Fuel 2003, 82, 1875-1882. (4) Wigley, F.; Williamson, J.; Gibb, W. H. The distribution of mineral matter in pulverized coal particles in relation to burnout behavior, Fuel 1997, 76, 1283-1288. (5) Hurt, R. H.; Sun, J.-K.; Lunden, M. A. Kinetic model of carbon burnout in pulverized coal combustion, Combust. Flame 1998, 113, 181-197. (6) Milligan, J. B.; Thomas, K. M.; Crelling, J. C. Temperatureprogrammed combustion studies of coal and maceral group concentrates, Fuel 1997, 76, 1249-1255. (7) Field, M. A. Rates of combustion of size-graded fractions of chars from a low rank coal between 1200 K and 2000 K, Combust. Flame 1969, 13, 237.
from the rest of a pulverized coal in situ, rather than using costly and water-consuming processing in coal preparation plants. Sieving is a classic method to determine particle-size distribution. However, the sieve series is limited, and in any case, sieves are difficult to operate when the particle size is below 45 µm; therefore, it is impossible to get a detailed size distribution with this method. Optical microscopy and the newly developed computercontrolled scanning electron microscopy (CCSEM) method4,8 are ineffective because the minerals beneath the particle surface cannot be observed. In this paper, a new method combining float-sink densitometry and a laser particle sizer is described. We use the method to find an accurate distribution of mineral matter as a function of the particle size in pulverized coals. The method is applied to study the mineral matters distribution in five pulverized Chinese coals. Experimental Procedures Five Chinese coals with varied rank (from anthracite to lignite) and ash content (from 16.21 to 48.89%), designated FJ, SX, NJ, XZ, and YN herein, were selected for this study. Data of the proximate, ultimate, and petrographic analyses of the raw coals are summarized in Table 1. The samples include a medium ash anthracite, a high ash anthracite, a medium ash bituminous coal, a high ash bituminous coal, and a lignite. They were ground with a vibrating mill to a size typically used in pulverized fuel furnaces. The float-sink method has been widely adopted for the separation of coal particles with different densities in the laboratory. About 120 g of each coal were density-fractionated (8) Charon, O.; Sarofim, A. F.; Beer, J. M. Distribution of mineral matter in pulverized coal, Prog. Energy Combust. Sci. 1990, 16, 319326.
10.1021/ef050201u CCC: $30.25 © 2005 American Chemical Society Published on Web 10/04/2005
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Table 1. Proximate, Ultimate, and Petrographic Analyses of the Whole Coals proximate analysis (wt %) coal origin
Ad
Vdafa
FJ SX NJ XZ YN
23.97 34.79 22.11 48.89 16.21
4.12 11.56 38.68 41.17 56.31
a
a
ultimate analysis (wt %, daf)
petrographic analysis (vol %, mmf)
C
H
N
S
O
vitrinite
liptinite
inertinite
94.73 90.64 82.24 81.12 70.49
2.42 3.92 5.89 6.05 5.08
0.73 1.55 1.52 1.56 1.59
0.42 2.19 1.59 1.55 1.95
1.7 1.7 8.76 9.72 20.89
91.98 87.61 73.11 81.40 92.57
0.00 0.00 7.33 7.22 4.96
8.02 12.39 19.56 11.35 2.47
Ad, ash content (dry base); Vdaf, volatile matter content (dry and ash-free base). Table 2. Proximate Analyses of Density-Separated Fractions of NJ Coal
density fraction (g cm-3) yield (wt %) proximate analysis (wt %)
Ad Vd
2.0
15.23 3.10 41.30
37.61 5.97 34.82
20.65 10.48 29.34
5.76 17.05 28.18
3.53 27.45 24.19
1.03 43.30 22.71
16.18 79.43 16.62
Table 3. Relative Proportion of a Set of Size Fractions of Pulverized NJ Coal (vol %) density range (g cm-3)
100
total
7.61 15.9 8.4 5.9 1.98 0.698 1.29
11 8.7 4.1 1.8 0.02 0.002 0.01
73.4 10.5 1.2 0.1 0 0 0
100 100 100 100 100 100 100
content < 10.48%, and specific gravity > 2.0 g cm-3, with ash content > 79.43%. The laser particle-size analyzer is an effective and convenient apparatus for measuring the size distributions of particles. The size distributions of the seven fractions of NJ coal were measured, and their size graphs of cumulative percents are shown in Figure 1. The relative proportion of each size fraction can be readily obtained from the software provided with the instrument. The proportions of a set of size fractions of NJ coal are shown in Table 3. Because the fractions were preseparated according to their densities, the particle densities within each fraction vary little and can be assumed identical. The size distribution in each fraction expressed in weight percent is thus the same as when expressed in volume percent. Volume fraction is the parameter measured by laser particle sizers. The absolute amount of coal contained in any sizedensity fraction can be calculated by multiplying the yield of that density fraction in Table 2 with the proportion of the size fraction in that density fraction in Table 3. The data from the NJ coal are shown in Table 4 after being manipulated this way. Let us take density fraction 1.4-1.5 g cm-3 as an example. Its
Figure 1. Particle-size distributions of seven density fractions of NJ coal.
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Table 4. Density-Size Related Fractions of Pulverized NJ Coal (wt %) density fraction (g cm-3)
100
total
1.77 2.67 1.47 0.36 0.19 0.09 0.86 7.41
1.42 2.44 0.06 0.16 0.07 0.08 0.29 4.52
3.44 5.19 0.00 0.00 0.00 0.17 0.02 8.80
15.235 37.6 20.65 5.76 3.53 1.03 16.18 100
2.33 4.14 1.55 0.65 0.32 0.145 1.36 10.49
Table 5. Accurate Distribution of Mineral Matter within Size Classifications of Pulverized NJ Coal Particles size range (µm)
2.0 g cm-3. Thus, when considering the combustion of the ∼10-20 µm fraction, the 1.3-1.4, 1.4-1.5, and >2.0 g cm-3 fraction are important and the rest of the fractions are negligible. Influence of Rank on the Distribution of Minerals in Pulverized Coal Particles. This part describes how the mineral distributions of two other pulverized whole coals of different rank and similar ash content, namely, FJ and YN coal, were determined. Because the density of anthracites is higher than that of bituminous coals, their separation density range, 1.6-2.2 g cm-3, is also higher than that of the bituminous coals, which is ∼1.3-2.0 g cm-3. Special attention should be paid to lignite; its separation range, ∼1.3-1.6 g cm-3, is narrower than that of both anthracite and bituminous coal. The ash distributions of the three coals, which are calculated according to the method proposed above, are shown in Figure 2. Figure 2 shows that all coals tend to have more ash in the finer particles. However, coals of different rank have different ash distribution characteristics. As shown in Figure 2, the difference between maximum and minimum ash content for NJ, FJ, and YN coals is 35.5, 27.0, and 4.4%, respectively. It can be concluded that coal rank strongly influences the size-ash distribution. After studying 19 pulverized coals with CCSEM, Wigley et al. found that the organic particles with included minerals generally had smaller median and maximum sizes when compared to organic material.4 Similar phenomena with several pulverized bituminous coals were found by Palmer et al.,10 while Dick et al.11 found that ash was evenly distributed in a pulverized lignite. Coals of higher rank have a denser structure and are thus more rigid. Minerals in the coal further strengthen (10) Palmer, A. D.;Cheng, M.; Goulet, J.-C.; et al. Relation between particle sizes and properties of some bituminous coals, Fuel 1990, 69, 183. (11) Dick, E. P.; Dobrochtow, W. I.; Zalkind, I. J. Zum problem der Verschlckung bei groben dampterzeugen (russ.), Teploenergetika 1980, 3, 18-22.
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Figure 3. The size distributions of FJ- and SX-pulverized coals.
the matrix. When these harder, mineral containing particles fracture during grinding, they disintegrate into finer pieces than would softer, nonmineral containing particles. Because coal with a lower rank is more porous, particles with and without minerals then tend to fracture to the same degree. It should be noted that the pulverized coal powders in this test were made in a batch-vibrating mill. In industrial conditions, the air through the grinding mill will blow the lighter coal particles away preferably, leaving the heavier particles behind. Thus, the heavier particles are ground a longer time, which results in more severe particles segregation. Littlejohn12 found that in industrial mills the ash content in particles less than 10 µm could be twice that of the larger, remaining part. This is an interesting and commercially important phenomenon. It is believed that minerals in coals act as catalysts in low-temperature combustion while acting as inhibitors in high-temperature combustion.5 Using air separators, pulverized coal particles can be readily separated into coarse and fine parts. The former, having a high content of mineral matters, is sent to burn in low-temperature furnaces, while the latter, having a low content of mineral matters, is sent to burn in hightemperature furnaces. This technology can be applied in plants with both low- and high-temperature furnaces, as is the case in cement plants with precalciners.13 Influence of Ash Content on the Distribution of Minerals in Pulverized Coal Particles. Apparently, the mineral matter distribution of pulverized coal particles will be influenced by its ash content. In this part, two pairs of coals of similar rank but different ash content are compared. First consider SX and FJ coals. SX coal, an anthracite with an ash content of 37.89%, was ground to a similar size as FJ coal, the ash content of which is 23.97%. The size distributions of the two pulverized coals are shown in Figure 3. Their size-ash distributions calculated are shown in Figure 4. (12) Littlejohn, R. F. J. Inst. Fuel 1966, 39, 59-67. (13) Kohlhaas, B.; Labahn, O. Cement Engineers’ Handbook; Wiesbaden Bauverlag GmbH: Berlin, Germany, 1983.
Figure 4. The size-ash distributions of FJ- and SX-pulverized coals
Throughout the whole size range, the lower ash FJ coal has less ash content than an equivalent-sized, higher ash counterpart. The ash contents of the two coals in 5 µm particles. Figure 4 also shows that ash in anthracite with a higher ash content is more evenly distributed. Proximate analyses of the density fractions of the two coals shows that the high ash SX coal contains more ash than does its low ash counterpart in every specific gravity fractions except 2.0-2.2 g cm-3, as is shown in Table 6. Furthermore, although the two coals have similar size distributions overall, their composite density fractions are different. Take the 5-10 and 20-40 µm fractions as examples. Their density compositions are compared in Table 7. The higher ash, 5-10-µm-sized coal fraction has a greater proportion of low density and thus low ash particles, which results in a decrease in the ash difference between the two coals. For the 20-40-µm-sized fractions, the difference between the density proportions is decreased. The ash content of a certain density fraction in the higher ash coal is much higher than that
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Table 6. Ash Contents of Density Fractions of Pulverized FX and SX Coals (Ad, %) coal origin
2.2 g cm-3
FJ SX
1.01 10.42
2.39 14.09
5.01 14.94
10.85 16.31
17.51 20.2
34.07 26.75
63.84 76.47
Table 7. Density Compositions of 5-10 µm and 20-40 µm Fractions of FJ and SX Coals (vol %) size fraction 5-10 µm 20-40 µm
origin
2.2 g cm-3
0.03 4.37 0.23 4.68
0.87 2.69 4.17 3.45
7.60 8.28 19.81 16.15
16.07 12.32 21.42 26.3
17.35 13.77 14.34 16.09
20.46 45.40 11.44 24.26
37.64 13.19 28.58 8.89
FJ SX FJ SX
Table 8. Ash Contents of Density Fractions in Pulverized NJ and XZ Coals (Ad, %) coal origin
2.0 g cm-3
NJ XZ
3.1 6.02
5.97 9.83
10.48 15.11
17.05 21.04
27.45 35.32
43.3 43.67
79.43 66.86
Table 9. Particle-Size Distributions of SX Coals at Different Fineness (wt %) sample
100
SX1 SX2
2.28 4.84
17.23 25.27
15.56 14.11
15.41 13.85
15.13 14.46
9.97 9.90
7.32 7.83
6.20 5.43
10.89 4.31
of its lower ash counterpart, which results in a higher ash difference between the two coals. Now consider the NJ and XZ coals, two bituminous coals of different ash content. Their size-ash distributions are shown in Figure 5. Contrary to the anthracites, the ash in bituminous coal with a lower ash content is more evenly distributed. Furthermore, a greater ash difference lies in the particle range of 1.6 g cm-3, while its low ash counterpart, the NJ coal, is mainly concentrated in density fractions of
