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The Impacts of Mild Pyrolysis and Solvent Extraction on Coking Coal Thermoplasticity Quang Anh Tran, Rohan J. Stanger, Wei Xie, Nathan D Smith, John A. Lucas, and Terry F. Wall Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.6b02018 • Publication Date (Web): 12 Oct 2016 Downloaded from http://pubs.acs.org on October 18, 2016
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Energy & Fuels
The Impacts of Mild Pyrolysis and Solvent
1
2
Extraction on Coking Coal Thermoplasticity
3
Quang Anh Tran*,a, Rohan Stangera, Wei Xiea, Nathan Smithb, John Lucasa, Terry Walla
4
a
Chemical Engineering, the University of Newcastle, Callaghan, NSW 2308, Australia
5
b
Analytical & Biomolecular Research Facility, the University of Newcastle, Callaghan, NSW 2308,
6
Australia
7
*
8
Abstract
9
The thermoplastic development of a coking coal was studied before and after treatment to study the
10
impacts on thermoswelling and volatiles release. The treatment process consisted of two consecutive
11
steps, namely mild pyrolysis and solvent extraction. The intention was to characterise the thermally
12
generated and solvent extractable material produced immediately prior to swelling onset and to study
13
the impact of its removal. The heating step removed ~5% of the coal volatiles and reduced its
14
swelling extent from 83% to 25%. Subsequent solvent extraction with tetrahydrofuran (THF) on the
15
heated coal removed up to 21% of material on a dry basis. The residue after extraction showed little
16
swelling extent and significantly altered volatiles release profile on heating, indicating the role of
17
extracted materials in thermoplastic development and tar formation. The molecular weight
18
distributions of volatile tars collected after pyrolysis shared a similar molecular weight distribution
19
spanning between 200 and 600 Da and peaking at ~347 Da. By comparison, the molecular weight
20
distribution of the THF extract peaked at ~472 Da and extended to ~3000 Da. In addition, it consisted
21
of two classes of compounds: one covered the molecular weight range of tars with repeating structures
22
every 12–14 Da up to 600 Da, and the other contained 24 Da reoccurring units at molecular weight
23
above 600 Da.
24
Keywords: Coal thermoplasticity; Solvent extraction; Metaplast
Corresponding author. Email:
[email protected] 1 ACS Paragon Plus Environment
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1
Introduction
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Coking coal when heated under an oxygen–deficient atmosphere softens, swells, and resolidifies to
28
form coke, a porous carbon residue used in the blast furnace. Such physical transformation in coking
29
coal during pyrolysis is regarded to as coal thermoplastic behaviour.1 To explain coal
30
thermoplasticity, a number of hypotheses have been proposed. Among them, the most favoured
31
theories are the γ–fraction2 and the metaplast3-5 theories. The γ–fraction (also known as the mobile
32
phase6) theory assumes that the plastic material is present in the raw coal and can be extracted by
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organic solvents (such as chloroform7 and tetralin8). The extraction of this material from coal results
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in the reduction of coal fluidity9 and a marked deterioration in the properties of the coke formed from
35
the residue10. The metaplast theory, on the other hand, assumes that the plastic material is thermally
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generated and has not escaped the coal particles either via evaporation or entrainment.5, 11 Recently, it
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has been stated that both the plastic materials that were present in the raw coal (the γ–fraction) and the
38
components that were generated during heating (the metaplast) determined coking coal fluidity.12
39
However, although the removal of the extractable materials from raw coal significantly affects its
40
plasticity, it is the metaplast that contributes the most toward coal thermoplastic behaviour.13
41
The metaplast is thought to be generated from the thermolysis of coal macromolecular structure.14, 15
42
The temperature interval over which the metaplast generation occurs coincides roughly with the
43
interval of primary devolatilization.16 Due to its transient, thermally unstable characteristics, the
44
metaplast can decompose to produce volatiles and/ or be transformed into coke.4 The amount of the
45
metaplast increases and subsequently decreases during coal pyrolysis. This thermally generated
46
component can be isolated by heating the coal to its temperature of maximum fluidity before rapidly
47
cooling and extracting it with organic solvent.3, 17 Pyridine is the most commonly used solvent for the
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structure study of coal18 and its thermally generated components.11 In fact, the molecular weight of
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pyridine extract up to 3000 Da was assumed to be the molecular weight of extractable materials in the
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functional group – depolymerisation, vaporization and cross–linking (FG–DVC) model proposed by 2 ACS Paragon Plus Environment
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Solomon et al.19. However, due to its high boiling point (115 °C), the molecular weight distribution of
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pyridine extracts are believed to reflect secondary polymerization of the extract in boiling pyridine
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solution rather than the extraction of higher coal molecular weight fractions.11 Additionally,
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significant amount of colloidal dispersions with molecular weight up to 106 Da was observed in
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pyridine extracts.20,
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viewed with caution.3 For that reason, attempts to use different solvents for the study of coal and
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metaplast structure have been made, such as carbon disulphide–pyridine mixture,22 carbon
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disulphide–NMP
59
tetrahydrofuran,26, 27 and tetralin.28 Among these solvents, tetrahydrofuran (THF, boiling temperature
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66 °C) was suggested to extract only metaplastic materials and was used for the development of the
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chemical percolation devolatilization (CPD) model.29
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This work focused on investigating the thermoplastic development of a coking coal before and after
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its metaplast material was extracted. The metaplast removal process consisted of two steps, namely
64
mild pyrolysis and solvent extraction. Previous work has shown that extraction yields are highest at
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temperatures immediately prior to softening,12 as is the molecular weight distributions of solvent
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extracts.26 The mild pyrolysis step, therefore, generated heated sample by heating coal just prior to its
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softening onset. In the solvent extraction step, the heated coal was exhaustively extracted by
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tetrahydrofuran (THF) in a Soxhlet apparatus to produce the metaplast (THF extract) and solvent
69
residue. The thermoplastic development of the raw coal, heated coal, and residue was examined via
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their thermal swelling behaviour and volatiles release profile. The swelling behaviour of samples was
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analysed by computer aided thermal analysis (CATA), whereas the volatiles release was investigated
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by thermogravimetric analysis (TGA), providing a broad overview of samples devolatilization, and
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dynamic elemental thermal analysis (DETA), differentiating tar evolution and light gases release from
74
total volatiles release. Finally, Laser desorption/ ionization time of flight mass spectrometry (LDI–
75
TOF–MS) was utilized to obtain molecular weight distribution of the metaplast and tars collected
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from pyrolysing samples.
21
As a result, pyridine extraction was suggested by some researchers to be
(1-methyl-2-pyrrolidinone)
mixture,12,
23
dichloromethane,24
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quinoline,25
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78
2
79
2.1 Samples
80
2.1.1
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Australian coals (and other Southern hemisphere coals) are well known to contain high proportions of
82
“reactive” semi-fusinite which can contribute to its thermoplasticity.30 While inertinite macerals do
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not form a fluid phase, further complexity arises in coals containing semi-fusinites as these
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metaplastic components are poorly understood. In this work, an Australian coking coal (mean
85
maximum reflectance 1.32%) wet-sieved to 100–210 µm was investigated. The results of its
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petrographic, proximate, and ultimate analyses as well as its carbonisation properties are given in
87
Table 1.
88
Experiments
Coal Sample
Table 1: Properties of the Investigated Coal Components Ash
9.8
Volatile matter
23.7
Fixed Carbon
66.5
C
89.1
Ultimate analysis
H
4.2
(wt%, daf)
N
2.0
O+Sa
4.7
RoMaxb
1.32
Vitrinite
55.6
Inertinite
44.4
Telovitrinite
50.7
Detrovitrinite
2.7
Semi-fusinite
40
Fusinite
1.3
Inertodetrinite
1.3
Proximate analysis (wt%, db)
Petrographic analysis (%, mmf)
Maceral components (%)
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Mineral matter
4
Initial softening 415 Carbonisation propertiesc
temperature (°C) Maximum fluidity
(Gieseler plastometer,
460 temperature (°C)
ASTM D2639) Resolidification 495 temperature (°C) Maximum fluidity (ddpm)
89
a
By difference
90
b
Mean maximum reflectance
91
c
Values reported on -425 µm coal particles
350
92 93 94
2.1.2
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Previous work showed that the softening onset for this coal with the particle size of 100–210 µm was
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at 433 °C.13 Given that the accuracy of sample temperature measurement was ±2 °C,31 the chosen heat
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treatment temperature was set to be 430 °C so that coal was heated just prior to its swelling onset. The
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heated sample was prepared by heating coal to this temperature using a heating rate of 5 °C/min in an
99
infrared gold image furnace (manufactured by SHINKU-RIKO Inc., Yokohama, Japan), before being
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rapidly quenched to room temperature (cooling rate of ~250 °C/min). The condensed fraction in the
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volatiles at the downstream of the tube, tar, was collected by washing the quartz tube with
102
tetrahydrofuran (THF).
103
2.1.3
104
Soxhlet extraction technique was employed in this work to extract the metaplast out of the heated coal
105
prepared at 430 °C. About 6 g heated sample was put in a quartz thimble which was then inserted into
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the Soxhlet extraction body. Tetrahydrofuran (THF) was the solvent of choice for the metaplast
107
removal study in this work.
Heated Sample
Soxhlet Residue Sample
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To conduct Soxhlet extraction experiment, 400 mL THF was filled in the boiling flask placed at the
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bottom of the Soxhlet extraction body. The heating mantle power was adjusted so that a 20 min/cycle
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reflux was applied to the extraction process. About 100 mL of fresh THF was added every 12 h to
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account for the solvent loss due to vaporization in that period. The extraction was continued until the
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condensed THF in the extraction body became colourless. At this point, the heating mantel was
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switched off so that solvent refluxing ceased to occur. In total, >400 refluxing cycles were needed to
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complete the extraction process which was equivalent to 6 days of continuous extraction. The Soxhlet
115
residue was removed from the quartz thimble, dried under vacuum for 24 h, and left to stay another 24
116
h in a desiccator prior to further investigations.
117 118
2.2 Experimental Techniques
119
2.2.1
120
The swelling behaviour of samples (including raw coal, heated coal, and Soxhlet residue) during
121
pyrolysis were analysed by computer aided thermal analysis (CATA). About 2 g of sample was
122
packed into a quartz tube (12 mm ID) at the density of ~950 kg/m3 and heated under inert atmosphere
123
generated by a 30 ml/min argon flow. During pyrolysis, the sample pellet was restrained at its front-
124
end and was allowed to expand at its back-end. The change in the length of the coal pellet was
125
measured by a linear variable differential transformer (LVDT) arranged at the back-end of the sample
126
pellet. The swelling ratio was determined by this transient change compared to the initial length of the
127
coal pellet prior to experiment.
Computer Aided Thermal Analysis
Swelling ratio =
∆L x100% Lo
(1)
128
where ∆L is the transient change measured by the LVDT, Lo is the length of the packed coal pellet
129
(~20 mm). To compare the difference in the rate of swelling of investigated samples, the swelling rate
130
was obtained as the first derivative of thermal swelling profile.
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As sample was heated at 5 °C/min rate, the pressure difference between the front-end and back-end of
132
the sample pellet was recorded and was designated as the pressure drop (∆P). The pressure drop value
133
was previously linked to the permeability of the bed pellet with high ∆P indicating lower bed
134
permeability.32 The point at which pressure drop value exceeded 2 kPa was taken as swelling onset.
135
Similarly, the temperature at which ∆P dropped below 2 kPa, indicating no resistance imposed on
136
argon flow, was also recorded. The temperature range between swelling onset (∆P > 2 kPa) and no
137
resistance on carrier gas (∆P < 2 kPa) was assigned as ∆P range and was an indication of the
138
thermoplastic range.13 The working principle of CATA technique can be found elsewhere.33
139
2.2.2
140
Thermogravimetric
141
thermogravimatric analyser Q50 (manufactured by TA instrument, Delaware, the United States).
142
After being equilibrated at room temperature (~20 °C), 10 mg sample was heated to 100 °C using a 20
143
°C/min ramp rate and was kept at this temperature for 10 min to remove excess solvent and moisture.
144
Sample was then pyrolysed to 1000 °C utilizing a heating rate of 5 °C/min under nitrogen atmosphere
145
(50 mL/min flow rate). After reaching 1000 °C, a 50 mL/min air flow was supplied into the furnace to
146
completely oxidize the remaining coke for 30 min. The ash yields of samples retrieved at the end of
147
TGA experiments were obtained and used for the calculation of volatiles release via the ash tracer
148
method.34
Thermogravimetric Analysis analysis
(TGA)
W=
experiments
were
applied
104 (A1 − A 0 ) A1 (100 − A 0 )
on
the
samples
using
(2)
149
where W is the amount of volatiles release after the coal was heat-treated (%, dry basis), A0 is the ash
150
yield of the raw coal (%, dry basis), and A1 is the ash yield of the heated coal (%, dry basis). Solvent
151
extraction yield was also calculated using Eq. (2). In this case, W is the extraction yield, A0 is the ash
152
yield of heated sample, and A1 is the ash yield of Soxhlet residue.
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2.2.3
154
Dynamic Elemental Thermal Analysis (DETA) technique consisted of two operation modes, Total
155
Volatiles and Light Gases, which corresponded to the determination of total volatiles generation and
156
light gases evolution during coal pyrolysis, respectively. In the Total Volatiles mode, sample was
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heated at 5 °C/min rate from room temperature to 800 °C under inert atmosphere generated by 100
158
mL/min argon flow. Generated total volatiles were combusted by a custom heated O2 lance placed at
159
downstream of the coal sample. The combustion products, including CO2/CO, H2O, H2, NOx, SOx,
160
and O2, were analysed after cooling and provided the elemental composition of the total volatiles.
161
In the Light Gases mode, the O2 lance was placed in a second chamber. The volatiles, generated by
162
heating samples with 5 °C/min heating rate, were supplied into an iced tar condenser instead of being
163
directly combusted by the heated lance as in the Total Volatiles mode. Light gases, the non–
164
condensable fraction in volatiles flowing out of the tar condenser, were directed to the second
165
chamber where they were oxidised by the heated O2 lance. By substracting light gases from total
166
volatiles measurement, the evolution of “tar-by-difference” as a function of temperature was
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calculated. Solid residue (coke) remaining after heating, was oxidised by supplying an additional
168
oxygen flow of 20 mL/min into the heating chamber when sample temperature reached 800 °C.
169
DETA analyses were conducted in duplicate with the deviation between runs was found to be 2 kPa
433
440
-
Swelling rate > 1%/min
450
459
-
Maximum ∆P
460
464
482
-
464
463
450
Swelling rate < 1 %/min
482
481
-
∆P < 2 kPa
539
541
-
-
675
739
-
Maximum swelling rate Swelling stop No resistance on carrier gas Contraction started
201 202
Table 3: Summary Data from CATA Experiments Maximum Maximum swelling Samples
swelling
Swelling Absolute
Maximum
Thermoplastic
contractiona
∆P
rangeb
(%)
( kPa)
(°C)
rangec
(°C)
rate
(%) (%/min) Raw coal
83
26
8
74
106
32
Heated coal
25
11
6
47
101
22
Soxhlet residue
3
0.3
1
2
-
-
203
a
The difference between maximum and final swelling
204
b
The temperature range between ∆P > 2 kPa and ∆P < 2 kPa
205
c
The temperature difference between swelling acceleration and swelling end point
206 207
The impact of mild pyrolysis on coking coal thermoplasticity was substantial. The initial heat-
208
treatment to 430 °C step reduced the coal maximum swelling from 83% to just 25% after heating. The
209
deviation in their absolute high temperature contraction values, in contrast, was less significant (only
210
~2%, Table 3). With respect to thermal swelling rate, the raw coal exhibited three peaks at 455, 464,
211
and 472 °C, while the heat-treated sample was devoid of the first and the third shoulder peak (Figure 11 ACS Paragon Plus Environment
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212
2). The heated coal swelling rate profile only consisted of one significant peak at 463 °C and with a
213
much lower magnitude (11 %/min compared to 26 %/min in raw coal, Table 3). In addition, the point
214
at which the heated sample swelling accelerated (swelling rate > 1 %/min) was at a higher
215
temperature (459 °C) compared to that of the raw coal (450 °C). In contrast, both samples stopped to
216
swell at a similar temperature (~480 °C). The shift in swelling acceleration toward higher temperature
217
while maintaining a similar swelling-stop point led to the heated sample having a narrower swelling
218
range than that in the raw coal (Table 3).
219
With respect to the resistance imposed on carrier gas during pyrolysis (the pressure drop profile), a
220
similar delay in the heated coal swelling onset temperature (defined by ∆P > 2 kPa) compared to that
221
in the raw coal was also observed. Specifically, while the heated sample started to swell at 440 °C, the
222
swelling onset of the raw coal was recorded already at 433 °C. Despite their difference in the swelling
223
onset, the ∆P profile of heated coal exhibited a similar trend as the raw coal profile, albeit with lower
224
magnitude at temperatures >450 °C. Overall, the difference in the swelling rate magnitude, the delay
225
in the swelling onset and acceleration, and the lower ∆P value led to the significant reduction in the
226
coking coal swelling extent after undergoing the mild pyrolysis step (heat-treatment to 430 °C).
227
The Soxhlet residue did not display thermoplastic swelling. The insignificant swelling rate (maximum
228
swelling rate of only 0.3 %/min) and the low ∆P value (maximum ∆P value ~2 kPa) of this sample
229
were likely the result of solid state thermal expansion. In addition, the residue still retained its
230
powdered form after pyrolysis. Therefore, the metaplast removal process, in which the heat-treated
231
sample was extracted by THF, was proved to be successful in removing the remaining
232
thermoplasticity. The THF extract, as a result, could be regarded as being a portion of the metaplast,
233
the heat-generated material that was responsible for at least 25% of the coking coal’s thermoplastic
234
development.
235
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3.2 Investigations on Volatiles Evolution
237
The volatiles evolution of the raw coal, heated coal, and Soxhlet residue was investigated by
238
thermogravimetric analysis (TGA) and dynamic elemental thermal analysis (DETA) techniques.
239
While the former technique provided a broad overview of their volatiles release, the latter offered an
240
additional insight to the pyrolysis process by separating tar evolution and light gases release from the
241
total volatiles.
242
3.2.1
243
The devolatilization profiles (DTG curve, Derivative Thermogravimetric Analysis) obtained from
244
TGA experiments was acquired to compare the volatiles release profiles of the raw coal, heated coal,
245
and Soxhlet residue. The results are illustrated in Figure 4 with summary data presented in Table 4.
Samples Volatiles Release
246 247
Figure 4: The volatiles release of raw coal, heated coal, and Soxhlet residue.
248 249
Table 4: Summary Data Obtained from TGA Experiments Samples
Volatile
Coke
Ash yield
Vaporised volatiles/
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RMaxb
TMaxc
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matter
(wt %, dry)
(wt %, dry)
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Extraction yielda
(wt %, dry)
(wt %/min)
(°C)
(wt %, dry)
Raw coal
20.4
70.4
9.2
-
0.708
479
Heated coal
17.8
72.6
9.6
5.4
0.653
482
Soxhlet residued
13.8d
74.8d
11.4d
20.6
0.396
490
250
a
Calculated via ash tracer method
251
b
Maximum rate of weight loss
252
c
Temperature at maximum rate of weight loss
253
d
Excluding
