Subscriber access provided by UNIV OF CALIFORNIA SAN DIEGO LIBRARIES
Article
Primary fragmentation characteristic of coal particles during rapid pyrolysis Tongmin Cui, Zhijie Zhou, Zhenghua Dai, Chao Li, Guangsuo Yu, and Fuchen Wang Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.5b01289 • Publication Date (Web): 24 Aug 2015 Downloaded from http://pubs.acs.org on August 28, 2015
Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a free service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are accessible to all readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.
Energy & Fuels is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.
Page 1 of 31
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Energy & Fuels
1
Primary fragmentation characteristic of coal particles during rapid pyrolysis
2
Tongmin Cui a, Zhijie Zhou a, Zhenghua Dai a, Chao Li a, Guangsuo Yu a, Fuchen Wang a,*
3
a
4
Engineering Research Center of Coal Gasification, East China University of Science and Technology, Shanghai
5
200237, PR China
6
*
7
Key Laboratory of Coal Gasification and Energy Chemical Engineering of Ministry of Education, Shanghai
Corresponding author. Email address:
[email protected] Abstract
8
Experiments were carried out to investigate the rapid pyrolysis of NM (lignite), SF (bituminous coal) and JS
9
(anthracite) with the duration time of 0 to 500 ms and the temperature of 1173 to 1773 K using a high-frequency
10
induction furnace. Interest was centered on the primary fragmental characteristic of particles including the
11
changes of mass loss, particle density and size distribution during the pyrolysis. A pair of fragmental parameters,
12
i.e. dimensionless particle diameter δ and particle distribution Sf, was proposed to characterize the fragmentation
13
during the different stages of pyrolysis. The result showed that the pyrolysis progress increase with time and
14
temperature. The fragmentation extent is also positively related to time and temperature. The progress of primary
15
fragmentation is lignite ≈ bituminous coal > anthracite. However, the particle morphology changes little during
16
the pyrolysis fragmentation. Evidence reveals NM and SF’s major fragmentations occur at the outer zone of
17
particle and the coarse fragmentation of JS is insignificant compared with the exfoliation.
18
Keywords: rapid pyrolysis, primary fragmentation, size distribution, particle morphology.
19
1. Introduction
20
Rapid pyrolysis is a complex physical and chemical process occurring as the initial stage in coal combustion
21
and gasification 1. In this process, primary fragmentation is the breakage of a raw fuel particle into smaller
22
particles 2, which has an important impact on ash formation
3-5
and conversion efficiency of combustion
1
ACS Paragon Plus Environment
6-9
. The
Energy & Fuels
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Page 2 of 31
23
occurrences of particle comminution associated with coal pyrolysis can be classified into: coarse fragmentation
24
and exfoliation 2, 10. These different categories of particle fragmentation occur in series and/or in parallel.
25
Since the fluidized-bed combustor and gasifier are particularly sensitive to the particle size, the particle
26
fragmentation plays an important role in these reactors. Therefore, the fragmentation of coal particles was firstly
27
investigated in the fluidized bed reactors
28
fluidized reactor. They observed that the coal particles fragmented mainly at a higher temperature, and the coal
29
particles with larger particle size and more volatile matter were fragile. Cui et al.
30
distribution functions to describe the fragmentation process with time. Dacombe et al.
31
the morphological changes of char particles during the fragmentation. Kim et al.
32
movements and transformations during the ignition by means of high-speed camera. Senneca et al. conducted a
33
series of excellent work on the fragmentations of coal particles during the rapid pyrolysis
34
heated strip reactor, the heating rate reached as high as 104 K/s 20. This apparatus had an advantage of not only
35
providing an extremely high heating rate, but also being capable to collect the particle samples with a very short
36
residence time and thus quantify the particle changes. They also proposed the fragmental probability Sf for
37
assessing the ratio of particles number that fragment to the initial number 10 and developed a model for predicting
38
the primary particle fragmentation process during the rapid pyrolysis 2.
39
11-15
. Zhang et al.
15
experimentally studied the coal fragmentation in
19
17
16
adopted two normal
and Liu et al. 18 studied
observed the coal particle
2, 10, 20
. By using a
In mechanism, the particle primary fragmentation is known to be resulted from temperature gradient (thermal
40
shock) and volatile release (overpressure)
41
the swelling of coal particle due to the thermal expansibility. The influences of coal swelling on the
42
fragmentations were reported in the literature 10, 21, 23-25.
2, 15, 17, 21, 22
. It is noteworthy that the mentioned factors can also lead to
43
The purpose of the present work is to investigate the fragmentation characteristics of different coals in rapid
44
pyrolysis. The variations of a pair of fragmental parameters δ and Sf with pyrolysis time or temperature are 2
ACS Paragon Plus Environment
Page 3 of 31
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Energy & Fuels
45
analyzed to characterize the fragmentation and exfoliation of coal particles. The pyrolysis time, temperature and
46
coal property were consequently found to have a great influence on primary fragmentation. In addition, the
47
morphological parameters sphericity, symmetry, B/L ratio and convexity were utilized for further analysis on
48
fragmental characteristic.
49
2. Experimental
50
2.1 Coal sample
51
Three kinds of coal were used in this study. They are Nei-meng lignite (NM), Shen-fu bituminous coal (SF)
52
and Jin-cheng anthracite (JS). The coal was ground to powder and dried at 378 K for 2 h. The properties of coal
53
are listed in Table 1 (CSN is based on British standard 26).
54 55
[Table 1] 2.2 Pyrolysis apparatus and procedure
56
Coal pyrolysis was carried out in a quartz tube reactor. The experimental apparatus is shown in Fig.1. The
57
quartz tube was heated with a high frequency induction method. A current generator was installed outside the
58
quartz tube, which generated a high frequency current of 35 kHz and formed a quickly mutative magnetic field. A
59
molybdenum cylinder was placed inside the quartz tube and was heated by the induced current in the magnetic
60
field. The quartz tube was coiled outside the wall with a hollow copper pipe through which the cooling water
61
passed to remove the heat from the coil. The height of the cylinder was adjusted to 30, 40, 60, 80, 100, 125, 150
62
mm, by which the duration time of particle through the hot zone was changed. A thermocouple bead was pointed
63
at a position of 20 mm above the bottom edge of the crucible to measure the pyrolysis temperature.
64
[Figure 1]
65
The argon gas flowed into the reactor from the top of quartz tube. The gas flow rate was controlled at 300 mL
66
(STP)/min. The coal particles dropped through a thin tube (400 mm length, 8 mm i.d.) into the reaction zone. The 3
ACS Paragon Plus Environment
Energy & Fuels
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Page 4 of 31
67
solid products were collected at the bottom of the tube. The pyrolysis temperature was controlled in the range of
68
1173~1573 K.
69
2.3 Methods for determining char particle size and residence time
70
The particle size distributions and the real densities of coal and char were measured by a Laser Granulometer
71
and a True Densitometer. The reaction time was calculated as follows. The argon flowing around the particles was
72
regarded as the climb stream because the Reynolds was estimated to be less than 1. Consequently, the particle
73
drop was treated as the free sedimentation.
74
In the light of the Stokes’ law, the terminal velocities of free sedimentation ut can be determined by
75
ut =
gd 2 ( ρ − ρ g )
eq. (1)
18µ
76
where, g is acceleration of gravity, d is the average particle diameter; ρ and ρg are the densities of particle and gas,
77
µ is the viscosity of gas. The residence times of the particles through the reaction zone at different temperatures
78
can be accordingly determined. The results are shown in Table 2.
79 80 81 82 83
[Table 2] 2.4 Mass loss of particles Mass loss is an important parameter reflecting the rate of pyrolysis. The mass loss of dry ash-free basis L0 is defined as
L0 =
w0 − w w0 (Vd + FCd )
eq. (2)
84
where, w0 and w are the masses of coal and char, respectively; Vd and FCd are the mass fractions of the volatile
85
matter and fixed carbon in the dry coal, respectively. A blank experiment was done without heating to examine
86
how much coal particle would approximately adhere to the reactor wall. The mass loss obtained by the blank
87
experiment was denoted as Lb. The mass loss is finally obtained by the following modification: 4
ACS Paragon Plus Environment
Page 5 of 31
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Energy & Fuels
L=
88
L0 -Lb 1-Lb
eq. (3)
89
Obvious difference may exist between hot experiments and cold blank runs, therefore the blank experiments
90
of different samples were conducted (Table 3). Seven kinds of size distributions were tested in the cold condition.
91
The result shows that the blank mass loss (Lb) decreases with coal particles size. It can be inferred that the loss of
92
fine particles is the major reason of mass loss in blank experiments. Based on the analysis of Table 3, the Lb of
93
samples used in blank experiment (120~180 µm) is less than 0.0003, which is negligible compared to the hot
94
experiment (up to 0.5). Therefore, the calibration as eq.(3) is credible, value of L is employed represent the
95
progress of pyrolysis.
96
[Table 3]
97
In addition, the particle size distributions of two positions (shown in Fig.1, sampling position A and B) were
98
analyzed (Fig.2) in hot experiments. The solid sampled from position A represents the particles entrained by gas
99
leaving the reactor, and that from position B represents the particles adhered on the reactor wall, both of which
100
cause the error in counting the mass loss (L0). Fig.2 indicates not only the loss particle is small compared to their
101
parent coal samples, but also large particles remain in the char gathered on the bottom of the reactor. Both Table 3
102
and Fig.2 show that it is acceptable to calculated L by eq. (3).
103 104
[Figure 2] 2.5 Fragmentation parameters
105
Particle size and particle distribution are two key parameters reflecting the changes of particle fragmentation.
106
The volume average particle diameter, which is frequently short as D[4,3], can be calculated as eq. (4).
107
Dimensionless diameter δ is defined as eq. (4), which represents the average size of the particles obtained after
108
pyrolysis relative to the initial value of the particles. 5
ACS Paragon Plus Environment
Energy & Fuels
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
109
D[4, 3] =
δ=
110
∑N D ∑N D i
i
i
i
Page 6 of 31
4
eq. (4)
3
D[4,3]char D[4, 3]coal
eq. (5)
111
where D[4,3]char and D[4,3]coal represents the volume average particle diameter of the char and the parent coal,
112
respectively.
113
Another fragmental parameter is the particle distribution function Sf, which is represented as the ratio of the
114
number of the fragmented particles to that of the initial particles. As its tiny volume, the small particle cannot be
115
completely wiped out the coal samples. As a consequence, it is hard to assess whether a small particle is derived
116
from fragmentation or originated from coal. In addition, the accurate amount of total small particles (30 µ m
eq. (6)
N in ,>30 µ m
123
where Nout,>30µm and Nin,>30µm are the number of particles with the diameter of larger than 30 µm, respectively, in
124
the char and in the original coal, either of which is calculated by
125
N >30 µ m = ∫ N ( D )dD ≈ ∑ 30
30
12 (1 − L )
ϕi ∆Di πρ ( Di 3 + Di +13 )
eq. (7)
126
where N(D) is the number of particles with the diameter D; Di and Di+1 are the diameter differential interval; φi is
127
the volume fraction of particles of Di, and φi∆Di represents the probability density function (PDF) of particles
128
between Di and Di+1. The PDFs of samples were measured by the granulation analysis. Fig.3 also shows an 6
ACS Paragon Plus Environment
Page 7 of 31
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
129
Energy & Fuels
example of the calculation of N(D).
130
[Figure 3]
131
3. Results and discussion
132
3.1 Pyrolysis progress and median diameter
133
3.1.1 Mass loss
134
Fig.4a shows the mass loss curves of NM in different temperatures. The mass loss raises approaching 0.5 in
135
100 ms at 1573 K, and then keep slowly increasing rate, which indicates a rapid coal pyrolysis at high temperature.
136
In comparison, the progress of mass loss is slower at 1173 K. Similar results are also observed in the pyrolysis of
137
SF coal (Fig.4b) and JS coal (Fig.4c), which can be inferred that pyrolysis at high temperature is intense and takes
138
short time. It is believed that high temperature and long time have positive effect on increasing of mass loss.
139
[Figure 4]
140
Table 4 presents the initial rate of the mass loss and the time of “half-pyrolysis” (the time when the mass loss
141
is half of the maximum mass loss). For lignite, the initial rate increase from 0.090 to 0.530%/ms-1 with the
142
temperature rising from 1173 K to 1573 K. The same trend is observed for the bituminous coal and anthracite. On
143
the other side, the t0.5 decrease with temperature, which also indicates the rate of pyrolysis rises with temperature.
144
Besides, the comparison of different coals shows that the order of pyrolysis rate at the same temperature is
145
lignite > bituminous coal > anthracite.
146 147 148
[Table 4] 3.1.2 True density True density of char is connected to its porosity. It is reported that the increase in porosity is expected from
149
the increasing true density
150
indicates a higher degree of porosity. Fig.5 shows true density variation in pyrolysis of three coals. The result
27
. As a consequence, the larger true density of char compared to its parent coal
7
ACS Paragon Plus Environment
Energy & Fuels
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
151
shows that both the high temperature and the long time lead to the high density. Also, the coal property affect the
152
density, lignite gets a 39.2% increasing and bituminous coal gets 44.6% increasing, however, anthracite only
153
increases 17.4%. Furthermore, the char true density rises to 2100 kg/m3 at high temperature approaching the value
154
of the graphite density of 2250 kg/m3. It seems that the rise in the true density may due to the graphitisation of the
155
carbon structure. The variation is also in accord with the reported data 27, 28.
156
[Figure 5]
157
The pyrolysis extent increases with pyrolysis time and temperature based on the variation of mass loss and
158
true density. The coal property also affects the pyrolysis, and its order of progress is lignite ≈ bituminous coal >
159
anthracite.
160
3.1.3 Median diameter
161
The overall trend of particle median diameter decreasing shows that the fragmentation occurs and influences
162
the process greatly (Fig.6). Char’s median diameter of NM (Fig.6a) and JS (Fig.6c) monotonically reduces with
163
pyrolysis time, so does the SF (Fig.6b) at lower temperature. Difference appears on SF at high temperature (1373
164
K~1573 K), that d0.5 increases with time firstly, and then decreases. This phenomenon indicates the occurrence of
165
swelling. The d0.5 of lignite changes from 172.5 µm to153.7 µm (decreases 10.9%) and bituminous coal varies
166
from 153.3 µm to138.5 µm (decreases 9.7%). The decrease of median diameter of NM and SF are more
167
significant than that of JS (decrease 5.5%).
168
[Figure 6]
169
3.2 Fragmentation progress
170
3.2.1 Effect of pyrolysis time
171
Based on eq. (5) and (6), the variations of two parameters δ and Sf were considered simultaneously. Fig.7
172
shows the variations of NM lignite. The dimensionless diameter δ appears to decrease with time, indicating that 8
ACS Paragon Plus Environment
Page 8 of 31
Page 9 of 31
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Energy & Fuels
173
the particle size becomes smaller. On the other hand, the distribution function Sf increases, which reveals that the
174
number of larger particles reduces. Fig.7a shows that these two parameters changes slowly at 1173 K. The δ
175
remains nearly constantly while the Sf increases. Fig.7b and 7c show that δ decreases and Sf increases coordinately
176
before 150 ms / 116 ms, then δ continuing decreasing with a fixed Sf. Fig .7d and 7e show that the δ decreases
177
faster than that at the low temperature, meanwhile, the turning point of Sf become earlier.
178
[Figure 7]
179
This phenomenon shows that Sf is more temperature-sensitive than δ. The δ of NM decreases monotonically
180
with the Sf increasing to its limit. During pyrolysis, coarse fragmentation and exfoliation may occur
181
simultaneously, both of which leads to a decrease of δ. However, fragmentation causes not only a smaller δ but
182
also an increase of Sf. At low temperature, coarse fragmentation is gentle and takes place with exfoliation, which
183
displays as the δ and Sf varies slowly and monotonically. As the temperature goes up, the coarse fragmentation
184
occurs more intensely because Sf sharply increases in a short period. Soon the coarse fragmentation complete and
185
leave the exfoliation only changing the δ with a constant Sf.
186
The result of Fig.8 shows the variations of SF bituminous coal, the similar trends of δ and Sf are reported. At
187
1173 K (Fig.8a) and 1273 K (Fig.8b), δ and Sf varies slowly, which indicates the occurrence of fragmentation with
188
exfoliation. At high temperature (Fig.8c~8e), fragmentation is intense and last a short period, which illustrate that
189
Sf keeps invariant after a rapid increasing. It is observed (Fig.8c~8e) that the dimensionless diameter of particles
190
rises beyond 1, before it starts to reduce. This phenomenon indicates the particles swell during the devolatilization.
191
During rapid pyrolysis, massive volatile releases to generate gas, leading the overpressure inside of the particles.
192
Radial stress may cause the increase in particle diameter, which displays as the occurrence of swelling.
193 194
[Figure 8] The situation of JS anthracite (Fig.9) seems to be similar with lignite and bituminous, δ decreases and Sf 9
ACS Paragon Plus Environment
Energy & Fuels
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
195
increases with time. However its extent of variations of δ and Sf is quite low compared with NM and JS. The
196
maximum drops in δ of NM and SF are 10.9% and 12.7%, and the rise of Sf up to 0.55 and 0.56. These two values
197
of JS are 5.1% and 0.03, even the pyrolysis temperature is 1773 K.
198 199 200
[Figure 9] Based on Fig.7~9, it can be inferred that pyrolysis time enhances the fragmentation. 3.2.2 Effect of pyrolysis temperature
201
Temperature is another dominant factor in thermal processes, so is the pyrolysis. As shown in Fig.4~6,
202
temperature has strong influence on mass loss, true density and median diameter. It is believed that temperature
203
may also affect the fragmentation. However, the impact of temperature on the process may differ with the
204
variation of duration. Therefore, the effect of temperature is discussed in two stages, initial stages (by the 30 mm
205
crucible, NM 54~61 ms, SF 77~86 ms, JS 82~88 ms) and final stage (by the 150 mm crucible, NM 271~308 ms,
206
SF 387~432 ms, JS 344~368 ms).
207
[Figure 10]
208
Fig.10a reports the variation of δ in the initial stage of pyrolysis. NM and JS particles dimensionless
209
diameters reduce with temperature, i.e. particles become smaller when temperature rises. By contrast, the drop of
210
JS’ δ is not significant as it changes from 1.002 to 0.991. In addition, the effect of temperature on SF’s δ trend is
211
opposite with the other two coals. The δ varies from 1.023 (at 1373 K) to 1.030 (at 1473 K) and finally reaches
212
1.047 (at 1573 K). The increasing trend of δ with temperature reveals that the swelling extent of SF coal is also
213
positively related to temperature. Fig.10b reports the variation of δ in the final stage of pyrolysis, three curves of
214
different coals show their clarity and consistency on the decreasing trend of δ. Compared with the result shown in
215
Fig.10a, the decrease trend is more obvious, which is reflected by all three coals.
216
Fig.10c reports the curves of Sf in short pyrolysis time. NM and SF’ Sfs increase with temperature, which 10
ACS Paragon Plus Environment
Page 10 of 31
Page 11 of 31
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Energy & Fuels
217
shows that large particle reduce when temperature rises. However, JS’ Sf remains nearly constantly, indicating that
218
the majority of large particle in JS does not change. The result (Fig.10d) shows the similar trends in last stage,
219
besides, the increasing trend of NM and SF is less obvious than that in the initial stage.
220
Fig.10 discusses the effect of temperature on fragmental parameters δ and Sf. The results show that high
221
temperature has apparently positive effect on fragmentation. It is noticed the slight difference exists in the
222
variation of δ and Sf. The impact of temperature on reducing δ is more significant in the last stage while increasing
223
Sf is more obvious in initial stage. As discussed, change of δ is not only the consequence of fragmentation, but also
224
the exfoliation. Overall, temperature enhances the fragmentation, particularly in the initial stage of pyrolysis.
225
3.2.3 Effect of coal property
226
Although time and temperature significantly affect the fragmentation, the effect of coal property cannot be
227
underestimated. As shown in Fig.4~6, mass loss, true density and median diameter have closed connected with the
228
coal properties. It is also suggested that the order of pyrolysis progress is lignite ≈ bituminous coal > anthracite. In
229
the comparison of Fig.7, 8 and 9, the changing trend of δ and Sf is rather consistent, apparently. However, the
230
range of variation of NM and SF is nearly equal which is much bigger than that of JS. For example, at 1573 K, the
231
decrease of δ of NM and SF is 2.3 and 2.7 times compared to that of JS, while the increase of Sf is 21.1 and 21.6
232
times. Fig.10 further shows that although the pyrolysis temperature of JS is higher (ranging from 1373 K to 1773
233
K), the δ decreases trend is insignificant in both initial and final stages. Meanwhile, Sf keeps constantly, and it is
234
well closed to zero. All evidences seem to indicate that the fragmentation of anthracite is far weak compared with
235
low-rank coal. It is similar that the order of fragmentation progress is lignite ≈ bituminous coal > anthracite.
236
Furthermore, Fig.8 and Fig.10a display another unique effect of coal property, i.e. the occurrence of swelling.
237
Theoretically the swelling may occurs during the devolatilization of every kinds of coal. However, its occurrence
238
depends on not only the experimental conditions but also coal property itself. The SF coal tends to swell in the 11
ACS Paragon Plus Environment
Energy & Fuels
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
239
initial stage at high temperature, which is not observed in the NM and JS.
240
3.3 Fragmental characteristic and particle morphology
241
As presented in Senneca’s 2 and Paprika’s 22 works, possible fragmentation modes can be classified into three
242
categories: exfoliation, fragmentation at outer zone and fragmentation at the center. These different modes may
243
cause diversity in the fragmental characteristics and particle morphology. As the consequence of thermal shock,
244
exfoliation describes that the out shell of coal breaks up and generates fine particles, most obvious characteristic
245
of which displays as the δ decreases with a constant Sf. The fragmentation at outer zone leads to the out zone
246
separating from mother particles, however, the main body remains. As a result，δ drops with Sf rises while the
247
morphological parameters keep nearly constantly. The fragmentation at the center is much more intense that the
248
whole structure breaks up. As consequence, the size of particle decrease shapely and giant reformation on
249
morphology. These three phenomena are listed in Table 5.
250 251
[Table 5] Four parameters
29
are utilized to describe the shape characteristic of char particle. Sphericity (Fig.11a)
252
represents the degree of which the particle (or its projection area) is similar to a circle, considering the smoothness
253
of the perimeter: Spht.=4πA/P2, where the A and P are the measured area and perimeter a particle projection.
254
Symmetry (Fig.11b) is defined as, Symm.=(1+rmin/rmax)/2, where the rmin and rmax are the minimum and maximum
255
distances from the center of area to the borders, respectively. Breadth length ratio (Fig.11c), is expressed as B/L=
256
xFe,min/xFe,max, where xFe,min and xFe,max stands for the minimum and maximum Feret diameter of the particle.
257
Convexity (Fig.11d) equals the ratio of real area and convex area of the particle projection. The result shows that
258
no significant changes in morphology have occurred in the thermal process. The NM, SF and JS maximum
259
changes of sphericity are 1.4%, 1.0% and 0.9%. The value variations of symmetry are less than 1.0%, 0.6% and
260
0.6%. The breadth length ratio of particles changes no more than 3.0%, 1.4% and 1.9%. The convexities remain 12
ACS Paragon Plus Environment
Page 12 of 31
Page 13 of 31
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Energy & Fuels
261
still and closed to 1, indicating that the entire surfaces of particles are convex. The result reveals that no
262
fragmentation occurs at the center, since the morphological parameters are constant. It is believed that
263
fragmentation occurs at outer zone during the rapid pyrolysis of NM and SF, and the fragmentation of JS is
264
insignificant.
265 266
[Figure 11]
5. Conclusion
267
A pyrolysis experiment has been carried out on fragmental characteristics of NM lignite, SF bituminous coal
268
and JS anthracite. Based on the analysis of mass loss, true density, median diameter, size distribution and particle
269
morphology, three conclusions can be made as the following points:
270
1. The pyrolysis extent is higher when residence time is longer and temperature is higher, which displays as the
271
larger value of mass loss and true density. The pyrolysis of low-rank coal is more significant than anthracite due to
272
the greater decease of solid mass and increase in density.
273
2. The fragmentation progress is also sensitive to time and temperature. High temperature and long time intensify
274
the fragmentation. At high temperature, fragmentation occurs rapidly and completes soon. Also the exfoliation
275
takes place in parallel with fragmentation. The fragmental characteristics of different coals are diverse.
276
Distinguishingly, swelling occurs in the case of SF at high temperature and short time. The overall order of
277
fragmentation progress displays as the low-rank coal is more significant than anthracite.
278
3. The morphological parameters, sphericity, symmetry, B/L ratio and convexity change little during the rapid
279
pyrolysis. Results show that no fragmentation occurs at the center of particle in the experimental condition.
280
Therefore, major fragmentation occurs at the outer zone of particle in the case of NM and SF, and the exfoliation
281
dominates the pyrolysis of JS.
282
Acknowledgements 13
ACS Paragon Plus Environment
Energy & Fuels
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
283
This work has been supported by the National Natural Science Foundation of China (21376079).
284
Reference
285 286 287 288 289 290 291 292 293 294 295 296 297 298 299 300 301 302 303 304 305 306 307 308 309 310 311 312 313 314 315 316 317 318 319 320 321 322 323
(1) Van Heek, K.; Hodek, W., Structure and pyrolysis behaviour of different coals and relevant model substances. Fuel 1994, 73, (6), 886-896. (2) Senneca, O.; Urciuolo, M.; Chirone, R., A semidetailed model of primary fragmentation of coal. Fuel 2013, 104, 253-261. (3) Helble, J.; Sarofim, A., Influence of char fragmentation on ash particle size distributions. Combustion and Flame 1989, 76, (2), 183-196. (4) Baxter, L. L., Char fragmentation and fly ash formation during pulverized-coal combustion. Combustion and Flame 1992, 90, (2), 174-184. (5) Buhre, B.; Hinkley, J.; Gupta, R.; Nelson, P.; Wall, T., Fine ash formation during combustion of pulverised coal–coal property impacts. Fuel 2006, 85, (2), 185-193. (6) Pecanha, R.; Gibbs, B. In The importance of coal fragmentation and swelling on coal burning rates in a fluidised bed combustor, Proceedings of the third international conference on fluidized combustion, 1984; 1984; pp 65-71. (7) Salatino, P.; Miccio, F.; Massimilla, L., Combustion and percolative fragmentation of carbons. Combustion and flame 1993, 95, (4), 342-350. (8) Pereira, C.; Pinho, C., Influence of particle fragmentation and non-sphericity on the determination of diffusive and kinetic fluidized bed biochar combustion data. Fuel 2014, 131, 77-88. (9) Senneca, O.; Cortese, L., Thermal annealing of coal at high temperature and high pressure. Effects on fragmentation and on rate of combustion, gasification and oxy-combustion. Fuel 2014, 116, 221-228. (10) Senneca, O.; Urciuolo, M.; Chirone, R.; Cumbo, D., An experimental study of fragmentation of coals during fast pyrolysis at high temperature and pressure. Fuel 2011, 90, (9), 2931-2938. (11) Sundback, C. A.; Beér, J. M.; Sarofim, A. F. In Fragmentation behavior of single coal particles in a fluidized bed, Symposium (International) on Combustion, 1985; Elsevier: 1985; pp 1495-1503. (12) Chirone, R.; Massimilla, L. In Primary fragmentation of a coal in fluidized bed combustion, Symposium (International) on Combustion, 1989; Elsevier: 1989; pp 267-277. (13) Arena, U.; Cammarota, A.; Chirone, R. In Primary and secondary fragmentation of coals in a circulating fluidized bed combustor, Symposium (International) on Combustion, 1994; Elsevier: 1994; pp 219-226. (14) Sasongko, D.; Stubington, J. F., Significant factors affecting devolatilization of fragmenting, non-swelling coals in fluidized bed combustion. Chemical engineering science 1996, 51, (16), 3909-3918. (15) Zhang, H.; Cen, K.; Yan, J.; Ni, M., The fragmentation of coal particles during the coal combustion in a fluidized bed. Fuel 2002, 81, (14), 1835-1840. (16) Cui, Y.; Stubington, J. F., In-bed char combustion of Australian coals in PFBC. 3. Secondary fragmentation. Fuel 2001, 80, (15), 2245-2251. (17) Dacombe, P.; Pourkashanian, M.; Williams, A.; Yap, L., Combustion-induced fragmentation behavior of isolated coal particles. Fuel 1999, 78, (15), 1847-1857. 14
ACS Paragon Plus Environment
Page 14 of 31
Page 15 of 31
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
324 325 326 327 328 329 330 331 332 333 334 335 336 337 338 339 340 341 342 343 344 345 346 347 348 349 350
Energy & Fuels
(18) Liu, G.; Wu, H.; Gupta, R.; Lucas, J.; Tate, A.; Wall, T., Modeling the fragmentation of non-uniform porous char particles during pulverized coal combustion. Fuel 2000, 79, (6), 627-633. (19) Kim, R.-G.; Li, D.; Jeon, C.-H., Experimental investigation of ignition behavior for coal rank using a flat flame burner at a high heating rate. Experimental Thermal and Fluid Science 2014, 54, 212-218. (20) Senneca, O.; Allouis, C.; Chirone, R.; Russo, S., Set up of an experimental apparatus for the study of fragmentation of solid fuels upon severe heating. Experimental Thermal and Fluid Science 2010, 34, (3), 366-372. (21) Suzuki, A.; Yamamoto, T.; Aoki, H.; Miura, T., Percolation model for simulation of coal combustion process. Proceedings of the Combustion Institute 2002, 29, (1), 459-466. (22) Paprika, M. J.; Komatina, M. S.; Dakić, D. V.; Nemoda, S. Đ., Prediction of Coal Primary Fragmentation and Char Particle Size Distribution in Fluidized Bed. Energy & Fuels 2013, 27, (9), 5488-5494. (23) Dakič, D.; van der Honing, G.; Valk, M., Fragmentation and swelling of various coals during devolatilization in a fluidized bed. Fuel 1989, 68, (7), 911-916. (24) Chirone, R.; Salatino, P.; Massimilla, L., Secondary fragmentation of char particles during combustion in a fluidized bed. Combustion and Flame 1989, 77, (1), 79-90. (25) Chern, J.-S.; Hayhurst, A. N., Does a large coal particle in a hot fluidised bed lose its volatile content according to the shrinking core model? Combustion and flame 2004, 139, (3), 208-221. (26) BS ISO 501:2012 Hard coal — Determination of the crucible swelling number. (27) Somerville, M.; Jahanshahi, S., The effect of temperature and compression during pyrolysis on the density of charcoal made from Australian eucalypt wood. Renew. Energy 2015, 80, 471-478. (28) Brown, R. A.; Kercher, A. K.; Nguyen, T. H.; Nagle, D. C.; Ball, W. P., Production and characterization of synthetic wood chars for use as surrogates for natural sorbents. Organic Geochemistry 2006, 37, (3), 321-333. (29) BS ISO 9276-6:2008, Representation of results of particle size analysis — Part 6: Descriptive and quantitative representation of particle shape and morphology.
351
15
ACS Paragon Plus Environment
Energy & Fuels
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
352
Page 16 of 31
Table 1. Characteristics of coals used. Proximate analysis/d,%
Ultimate analysis/d,%
Samples VM
FC
Ash
C
H
N
S
O
NM
42.31
41.05
16.64
56.37
3.81
0.82
0.29
22.07
SF
36.70
55.14
8.16
70.55
4.15
0.94
0.45
15.75
JS
8.86
76.71
14.42
81.39
1.98
1.00
0.67
0.54
Density
d0.1
d0.5
d0.9
kg/m3
µm
µm
µm
NM
1508.6
130.2
172.5
SF
1399.5
93.1
JS
1617.6
89.7
Samples
353
Spht.
Symm.
B/L
Conv.
CSN*
218.8
0.884
0.898
0.727
0.995
1.5
154.3
221
0.875
0.901
0.716
0.992
3
122.5
160.4
0.876
0.9
0.707
0.992
1
*CSN is short for crucible swelling number, based on BS ISO 501:2012
16
ACS Paragon Plus Environment
Page 17 of 31
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
354
Energy & Fuels
Table 2. Residence time of the particles in the molybdenum tube at different temperatures. Residence time of different mmolybdenum tube/ms
Temperature K
NM
SF
JS
30mm
40mm
60mm
80mm
100mm
125mm
150mm
1173
48.8
65.0
97.1
128.9
160.4
200.0
240.4
1273
50.3
66.8
99.1
131.7
164.7
206.0
247.9
1373
52.0
68.9
102.6
137.0
171.4
214.4
256.9
1473
54.1
71.6
106.1
141.1
176.0
219.2
262.5
1573
55.4
73.7
109.5
144.8
179.2
221.8
264.5
1173
93.0
123.8
185.7
248.0
309.9
387.3
463.3
1273
95.3
126.3
188.0
250.5
314.4
393.9
470.5
1373
93.2
123.1
185.2
248.2
311.1
389.2
465.2
1473
95.9
126.4
189.6
253.6
318.0
398.9
477.7
1573
92.1
120.4
179.9
240.2
302.1
380.7
458.3
1373
85.4
113.9
171.1
228.3
286.2
358.5
-
1573
89.6
119.5
178.1
236.4
295.2
368.5
-
1773
90.8
121.2
182.2
242.1
301.2
374.9
-
355 356
17
ACS Paragon Plus Environment
Energy & Fuels
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
357
Table 3. Mass loss of different coal samples in blank experiments. NM
Size
358 359
Page 18 of 31
SF
JS
Distribution
D[4,3]
Lb
D[4,3]
Lb
D[4,3]
Lb
µm
µm
%
µm
%
µm
%
>425
512
-
624
0.0024
499
-
180~425
221
0.0037
254
0.0050
202
0.0032
120~180*
174
0.0237
167
0.0294
125
0.0134
83~120
101
0.0184
109
0.0243
99
0.0321
60~83
71
0.0353
77
0.0204
65
0.0299
45~60
48
0.0440
48
0.0417
47
0.0564
