pyrolysis

1. Journal of Analytical and Applied Pyrolysis 117 (2016) 64–71
Contents lists available at ScienceDirect
Journal of Analytical and Applied Pyrolysis
journal homepage: www.elsevier.com/locate/jaap
Effect of high-temperature pyrolysis on the structure and properties
of coal and petroleum coke
Jin Xiaoa,b
, Fachuang Lia,∗
, Qifan Zhonga
, Jindi Huanga
, Bingjie Wanga
, Yanbing Zhanga
a
School of Metallurgy and Environment, Central South University, Changsha, Hunan Province 410083, PR China
b
National Engineering Laboratory of Efficient Utilization of Refractory Nonferrous Metal Resources, Changsha 410083, PR China
a r t i c l e i n f o
Article history:
Received 1 July 2015
Received in revised form
22 November 2015
Accepted 15 December 2015
Available online 22 December 2015
Keywords:
Pyrolysis
Crystallite structure
BET surface area
Gasification reactivity
Powder resistivity
Real density
a b s t r a c t
In this study, for the purpose of substituting part of petroleum coke with low ash anthracite to prepare
carbon anode used in aluminum electrolysis, the effects of high-temperature pyrolysis on the structure
and properties of coal char and petroleum coke were studied at the temperature range of 1000 ◦
C–1600 ◦
C.
Results showed that the carbon crystallite structure of coal char and petroleum coke became more ordered
with the increase of the temperature. However, the graphitization degree of coal char was lower than
that of petroleum coke at the same pyrolysis temperature. The Brunauer-Emmett-Teller (BET) surface
area of coal char decreased with the increase of temperature, whereas the BET surface area of petroleum
coke decreased first and then increased. The increase of pyrolysis temperature generally inhibited the
gasification reactivity of coal char, whereas the gasification reactivity of petroleum coke was inhibited
first and then promoted. Moreover, the increase of pyrolysis temperature led to the rapid decrease of
powder resistivity of coal char and petroleum coke. In particular, the powder resistivity of coal char was
significantly higher than that of petroleum coke at the same pyrolysis temperature. Additionally, the
real density of coal char and petroleum coke showed a different trend with the increase of temperature.
Specifically, the real density of the former decreased, whereas that of the latter increased. The causes of
the above observations were also discussed. Overall, this study provides further understanding about the
differences between coal char and petroleum coke, which will facilitate the preparation of carbon anode
from the mixture of coal char and petroleum coke.
© 2015 Elsevier B.V. All rights reserved.
1. Introduction
Petroleum coke is the main raw material of carbon anode
used in aluminum electrolysis. Hence, petroleum coke significantly
influences the energy consumption and environmental benefits
of aluminum electrolysis. The cost of carbon anode accounts for
15–20% of the total cost of aluminum electrolysis production. With
the rapid development of the electrolytic aluminum industry in
China, the demand for high-quality petroleum coke is also growing.
However, the quality of petroleum coke is deteriorating because of
the increasing proportion of imported heavy crude oil used in China,
consequently leading to a tight supply of high-quality petroleum
coke used for aluminum electrolysis [1,2]. Thus, expanding backup
reserves of resources for carbon anode is beneficial to the sustain-
able development of the aluminum industry. Furthermore, finding
∗ Corresponding author. Fax: +86 15273113704.
E-mail address: cwd818@163.com (F. Li).
low-cost raw materials to replace petroleum coke is necessary to
reduce the cost of carbon anode.
As an important raw material for carbon products, high-
metamorphic anthracite exhibits the characteristics of high
carbon content, compact structure, high strength, good thermal
stability, and lower cost than petroleum coke [3–6]. Hence, high-
metamorphic anthracite can be an ideal raw material to replace
petroleum coke for the preparation of carbon anode. To date,
attempts have been made to produce carbon anode from coal
extracts [7–10], which were obtained by solvent extraction tech-
nology. However, this high-cost technology is complex and the
yield is low. Additionally, good physical and chemical properties
of prebaked anodes have been achieved using low-ash calcined
anthracite to partly substitute petroleum coke [1,11,12]. However,
coal and petroleum coke presented obvious differences because
of the different formation processes. Even after high-temperature
treatment, the properties of coal char and petroleum coke (such as
carbon crystalline structure, specific surface area, gasification reac-
tivity, electrical resistivity and real density) remain different, and
such difference will affect the properties and energy efficiency of
http://dx.doi.org/10.1016/j.jaap.2015.12.015
0165-2370/© 2015 Elsevier B.V. All rights reserved.

2. J. Xiao et al. / Journal of Analytical and Applied Pyrolysis 117 (2016) 64–71 65
carbon anode in aluminum electrolysis. Although, studies [1,11,12]
have demonstrated that high-quality carbon anodes can be pro-
duced by adding an appropriate amount of coal char to petroleum
coke, the effects of pyrolysis on the structure and properties of coal
and petroleum coke have not been thoroughly investigated.
High-temperature pyrolysis of coal and petroleum coke is a
primary process for their carbon products. Moreover, pyrolysis con-
ditions significantly influence the performance of calcined coke
(char), thereby affecting the performance indicators of the carbon
products. The current study systemically investigated the effects
of high-temperature treatment on the structure and properties of
coal and petroleum coke. Accordingly, the differences in pyrolysis
characteristics and performances between two kinds of calcined
cokes are determined. Overall, this study provides theoretical basis
to prepare carbon anodes by substituting coal for petroleum coke.
2. Experimental
2.1. Raw materials
Low-ash anthracite (Shenhua Group Corp., China) and
petroleum coke (Qilu Petrochemical Co., Ltd., China), which
were named A and PC, respectively, were used for the investi-
gation. Proximate analysis, ash compositions, and ash melting
properties are listed in Tables 1 and 2. The ash compositions
of anthracite and petroleum coke were determined by X-ray
fluorescence (PANalytical Axios mAX equipment).
As shown in Table 1, the fixed carbon content of coal is slightly
higher than that of petroleum coke, whereas the volatile and sul-
fur contents of coal are significantly lower than those of petroleum
coke. These features are the major advantages of coal as raw mate-
rial to prepare carbon anode for aluminum electrolysis. However,
compared with petroleum coke, the disadvantage of coal is also
obvious, that is, a higher ash content. The impurity elements in
Table 2, such as Si and Fe, will significantly affect the quality of
electrolytic aluminum, because the impurities of Si and Fe in the
carbon anode will turn into the aluminum liquid. Although the con-
tent of aluminum is also high, aluminum is considered a valuable
element. The allowed impurity content in raw carbonaceous mate-
rials for carbon anode is generally based on the standard of 2B grade
in SH/T0527-1992 [13]; this standard states that the contents of Si
and Fe should be <0.08% each. The contents of Si and Fe in coal are
relatively high, whereas those in petroleum coke are low. There-
fore, when adding an appropriate amount of coal, the content of
impurities in the carbon anode can also meet the requirements.
2.2. Preparation of char samples
The pyrolysis of coal and petroleum coke was completed in a
high-temperature furnace. The inert atmosphere was maintained
in the furnace to prevent oxidation of coal and petroleum coke. The
samples were heated to a desired pyrolysis temperature at a heat-
ing rate of 4 ◦C/min, and then held at this temperature for 120 min.
The pyrolysis temperatures were 1000 ◦C, 1150 ◦C, 1300 ◦C, 1450 ◦C,
and 1600 ◦C. When the heat preservation time was over, the sam-
ples kept in the furnace were cooled to room temperature. The
samples were then removed from the furnace, broken, and graded
in different particle sizes. Coal chars prepared under different tem-
peratures were referred to as A1000, A1150, A1300, A1450, and
A1600, whereas the petroleum cokes were referred to as PC1000,
PC1150, PC1300, and PC1450. The proximate analysis and ultimate
analysis of coal chars and petroleum cokes are listed in Table 3.
Fig.1. The schematic diagram of the testing device for gasification reactivity.
2.3. Measurements of air gasification reactivity and CO2
gasification reactivity
Air gasification reactivity and CO2 gasification reactivity are
important indicators of coke chemical activity, which significantly
affect the overconsumption of carbon materials in aluminum elec-
trolysis [14,15]. In accordance with the principle of weight loss
method, the air gasification reactivity and CO2 gasification reac-
tivity of cokes were characterized in terms of residual rate after
reaction with air and CO2 following the test standard YS/T 587.7-
2006 [16] in China.
The tests for air gasification reactivity and CO2 gasification reac-
tivity were conducted in a quartz tube reactor testing equipment;
a schematic of this testing device are shown in Fig. 1. The detailed
measurement procedure is as follows: 5 g of coke with particle
size of 1 mm to 1.4 mm was placed in the reactor; air gasification
reactivity was performed in air atmosphere at 600 ◦C for 60 min,
whereas CO2 gasification reactivity was measured in CO2 atmo-
sphere at 1000 ◦C for 100 min; and the flow rate of both gases was
50 L/h. At the end of the reaction, the cooled char was collected from
the testing equipment and then weighed. The reactivity was indi-
cated in terms of the ratio of the mass loss to the initial weight of
the sample. The gasification percentage (X) is calculated as follows:
X = 1 −
mt
m0
(1)
where m0 represents the initial weight of the sample, and mt is the
residual weight of the sample.
2.4. Analysis of carbon crystalline structure and BET surface area
of the samples
The crystallite structure of coke was characterized by X-ray
diffraction (XRD) with Cu K␣ radiation in a Rigaku D/Max 2500
diffractometer at a scan speed of 4◦ min−1.
XRD is an effective means to analyze carbon crystallite struc-
ture. The interplanar spacing d002 and the stacking height of the
carbon crystal Lc are the basic parameters for analyzing the carbon
crystallite structure. According to Eqs. (2) and (3), the parameters
d002 and Lc were calculated from the (0 0 2) peak as follows:
d002 =
␭
2sin Â002
(2)
Lc =
0.89␭
␤002cos Â002
(3)

3. 66 J. Xiao et al. / Journal of Analytical and Applied Pyrolysis 117 (2016) 64–71
Table 1
Proximate analysis and ash character temperature of raw materials.
Sample Proximate analysis(wt%) Ultimate analysis(wt%) Ash character temperature/◦
FC V A St C H N St DT ST FT
A 89.99 7.12 2.89 0.14 88.39 2.40 0.73 0.19 1400 1460 >1500
PC 88.8 11.0 0.40 3.7 92.15 1.29 1.15 3.43 1280 1300 1330
DF: initial temperature of melting; ST: softening temperature; FT: flow temperature.
Table 2
The content of the impurity elements (ppm).
Sample A Al Si Fe Ti Ca Mg Na K P V Ni
A 4960 3440 1569 160 2933 457 727 80 95 – –
PC – 66 68 12 160 – 67 – – 309 200
Table 3
Proximate analysis and ultimate analysis of coal chars and petroleum cokes.
Sample Proximate analysis(wt%,d) Ultimate analysis(wt%,d)
A V FC C H N St O*
A-1000 3.65 4.13 92.22 90.80 0.87 0.72 0.20 3.76
A-1150 3.72 1.76 94.52 94.31 0.52 0.52 0.18 0.75
A-1300 3.13 1.77 95.10 95.44 0.43 0.52 0.17 0.31
A-1450 3.14 1.69 95.27 95.55 0.41 0.46 0.16 0.28
A-1600 3.19 1.63 95.18 95.68 0.41 0.44 0.17 0.11
PC-1000 0.42 1.85 97.73 92.97 0.44 1.69 3.20 1.28
PC-1150 0.47 1.22 98.31 94.01 0.38 1.34 3.18 0.62
PC-1300 0.43 1.04 98.53 94.88 0.37 0.76 3.19 0.37
PC-1450 0.45 0.93 98.62 97.09 0.37 0.36 1.39 0.34
A:ash content; V:volatile matter content; FC:fixed carbon content; St: total sulphur content.
O* = 100–C–H–N–S–A.
where is the wavelength of the X-ray emission, Â002 is the position
of the (0 0 2) diffraction peak, and ␤002 is the angular width at half-
maximum intensity of peak (0 0 2).
N2 adsorption–desorption measurements were performed
using a Quantachrome instrument at −196 ◦C.
2.5. Powder resistivity, real density, ultimate analysis and
thermogravimetric analyses
The powder resistivity of the chars was tested using a GM-II
type resistivity meter. The testing method and basic principle are
described as follows: 3.3 g of char with a particle size of 0.315 mm
to 0.4 mm was placed into a cylindrical mold; and two conducting
plates at the end of the mold were pressed with 784 N to connect
the char and then import direct current. The powder resistivity of
coke was automatically calculated according to Eq. (4) based on
Ohm’s law:
␳ = V S/Ih (4)
where ␳ is the powder resistivity of coke, V is the voltage between
the two conducting plates, S is the inner cross-sectional area of
mold, I is the current through the sample, and h is the height of the
sample bed.
The thermal properties of cokes were studied with the SDTQ600
thermogravimetric (TG) analysis apparatus. The samples were
heated from room temperature to 1000 ◦C at a heating rate of
10 ◦C min−1 under high-purity N2 at a flow rate of 100 ml/min.
Ultimate analysis was carried out by Vario MICRO cube elemen-
tar (produced by Hanau, Germany). The analytical precision of this
instrument is <0.1% abs for C, H, N, and S.
The real density of char with particle size of <74 ␮m was
determined using the 3H-2000TD automatic real density analyzer
produced by Beishide Instrument-S&T Co., Ltd. This real den-
sity analyzer has high testing accuracy (±0.04%) and repeatability
Fig. 2. TG–DTG plots of coal and petroleum coke during pyrolysis in N2.
(±0.02%). High-purity helium gas (99.999 v/v%) was used in this
system.
3. Results and discussion
3.1. TG–DTG analysis
To understand the pyrolysis process of coal and petroleum coke,
the variation in weight of the two samples from room temperature
to 1450 ◦C were studied by TG analysis. The TG–DTG curves of the
coal and petroleum coke are displayed in Fig. 2.
Fig. 2 shows that the weight losses of coal and petroleum coke
gradually increase with the increase of pyrolysis temperature. In
low-temperature pyrolysis (below 350 ◦C), the weight losses of the
two samples are small, which are mainly caused by dehydration
and removal of a small amount of volatile matter. At 350 ◦C–800 ◦C,

4. J. Xiao et al. / Journal of Analytical and Applied Pyrolysis 117 (2016) 64–71 67
a maximum amount of gas was discharged because of depoly-
merization and decomposition reaction. The thermal instability
components, such as the side chains and functional groups around
the basic structure unit of samples, are continuously cracking.
Accordingly, the low molecular compounds are formed and evapo-
rated as volatile [17]. In high-temperature pyrolysis (above 800 ◦C),
small weight losses are still observed in the samples, which belong
to the secondary devolatilization phase [18,19]. The polyconden-
sation reactions, which mainly occur between aromatic structures
and proceed to dehydrogenation simultaneously, play a major role
in this stage.
The degrees of weight losses of coal and petroleum coke are dif-
ferent. Petroleum coke and anthracite reached the maximum rate
of weight loss at 608 ◦C and 661 ◦C, respectively; the maximum
weight loss rate of coal is higher than that of petroleum coke. Before
petroleum coke reaches the maximum weight loss rate (608 ◦C),
its weight loss is higher than that of coal. However, the weight
loss rate of coal is substantially higher than that of petroleum coke
thereafter. This finding may be attributed to the fact that coal con-
tains large amounts of stable aromatic rings, which are not easily
destroyed at low temperature, but can be decomposed at temper-
atures above 600 ◦C [18].
3.2. Effect of pyrolysis temperature on carbon crystalline
structure
Pyrolysis temperature significantly affects the structure of
carbonaceous materials, consequently determining their per-
formances. Therefore, understanding the changes in crystalline
structure of chars after high-temperature treatment is very impor-
tant. The XRD patterns of coal char and petroleum coke at different
pyrolysis temperatures are shown in Fig. 3; the corresponding
structural parameters of crystallite are displayed in Fig. 4.
As shown in Fig. 3, the intensities of the (0 0 2) peak of the two
chars gradually increased with the increase of temperature, and the
(1 0 0) diffraction peak appeared. The stacking height Lc exhibited
an increasing trend, whereas the interplanar spacing d002 exhib-
ited a declining trend with the increase of pyrolysis temperature
(Fig. 4). Thus, the crystallite structures of coal char and petroleum
coke became more ordered with the increase of pyrolysis temper-
ature. The interfacial defects between the adjacent basic structural
units (BSUs) in the carbon structure disappeared gradually, and the
aromatic nucleus of chars increased in size because of the conden-
sation reaction. However, the growth of BSUs is mainly in a vertical
direction.
At the same pyrolysis temperature, the d002 of coal char is higher
than that of petroleum coke, but the Lc of coal char is less than that
of petroleum coke. In addition, the Lc of petroleum coke increased
faster than coal with the increase of temperature. The results show
that the graphitization degree of carbon structure in petroleum
coke is higher than that in coal char after high-temperature treat-
ment. The crystallite orientation in carbonaceous materials before
heat treatment or in the early stage of carbonization significantly
influences the graphitization of carbonaceous materials. Accord-
ing to the Franklin’s structure model [20], the crystallites arranged
in order belong to easily graphitized carbon, and the crystallites
distributed randomly are regarded as non-graphitizing carbon.
Therefore, crystallite orientation in petroleum coke is more ordered
than anthracite in the initial state. These results are also consistent
with the TG analysis; the structural rearrangement of anthracite
requires higher temperatures. Several researchers have investi-
gated the influence of pyrolysis temperature on the crystallite
structure of coal and petroleum coke, and they obtained similar
results [21–23].
3.3. Effects of pyrolysis temperature on BET surface area
The effects of pyrolysis temperature on the BET surface area and
average pore sizes of coal char and petroleum coke are presented in
Table 4. The BET surface area of coal char decreased gradually from
23.88 m2 g−1 to 4.53 m2 g−1, whereas the average pore sizes exhib-
ited an increasing trend, with the increase of pyrolysis temperature
(Table 4). The pore size distribution of coal char in Fig. 5 shows that
the pore sizes of coal char concentrated in mesopore and micro-
pore, which are less than 4 nm. Moreover, the peaks of the pore
size distribution decreased. Therefore, the shrinkage degree of coal
char increases with the increase of temperature. This phenomenon
results in the reduction and disappearance of small pores (microp-
ore and mesopore), as well as the reduction of BET surface area of
the pores.
Contrary to the BET surface area of coal char, that of the
petroleum coke initially showed a decreasing trend, followed by
an increasing trend. When the pyrolysis temperature was 1000 ◦C,
the BET surface area of petroleum coke was 7.35 m2 g−1. When
the pyrolysis temperature increased to 1150 ◦C, the specific surface
area decreased to 2.64 m2 g−1. However, when the pyrolysis tem-
perature increased further, the BET surface area began to increase.
This trend is opposite to that of the coal char in this temperature
range. Wu and Gao [21] also found the similar trend of BET sur-
face area of petroleum coke at temperatures above 1000 ◦C. Thus,
the calcination temperature of petroleum coke should not exceed
1350 ◦C [24]. Similar to coal char, the average pore size of petroleum
coke increases with the increase of temperature.
The pore size distribution of petroleum coke (Fig. 5) shows that
the pores of petroleum coke are mainly mesopores and micropores
(<10 nm); this pore size distribution is different from that of coal
char. The peak of the pore size distribution decreases first and then
increases with the increase of pyrolysis temperature. This trend
is consistent with the BET surface area. Furthermore, the pores of
petroleum coke with >10 nm pore size increased significantly after
heat treatment at 1450 ◦C.
The reduction of BET surface area of coal char is partly caused
by ash melting when the temperature increased to 1450 ◦C, which
is higher than the ash melting temperature. The ash melting blocks
part of the char pores. Hence, the BET surface area decreased. The
effect of ash melting on the surface area of petroleum coke is neg-
ligible because of its low ash content.
3.4. Effects of pyrolysis temperature on air gasification reactivity
and CO2 gasification reactivity
The effects of pyrolysis temperature on air gasification reactivity
and CO2 gasification reactivity are illustrated in Fig. 6 . The gasifi-
cation rate of coal char generally shows a decreasing trend with
the increase of pyrolysis temperature. Specifically, the variation
trend of air gasification reactivity is more significant, indicating that
the increase of pyrolysis temperature can reduce the gasification
reactivity of coal char, which is very beneficial to reduce over-
consumption of carbon products. Researchers generally believe
that the carbon crystallite structure and BET surface area of car-
bonaceous materials are the two main factors affecting gasification
reactivity. Smaller BET surface area or more ordered carbon crys-
tallite structure decreases gasification reactivity [21,25–27]. XRD
and BET analyses showed that the carbon crystallite structure of
chars tends to be ordered and the surface area decreases with the
increase of temperature. They work together to inhibit the gasifi-
cation reactivity of coal char. Therefore, the gasification rate of coal
char generally decreases.
Both the air and CO2 gasification rates of petroleum coke
decreased with the increase of temperature, and the gasifica-
tion rate reached the minimum value when the temperature

5. 68 J. Xiao et al. / Journal of Analytical and Applied Pyrolysis 117 (2016) 64–71
Fig. 3. XRD patterns of coal chars and petroleum coke at different pyrolysis temperature (a)A-chars (b) PC-chars.
Fig. 4. Effects of the pyrolysis temperature on the carbon crystalline structure parameters.
Table 4
The effect of pyrolysis temperature on BET surface area and average pore size.
Samples A1000 A1150 A1300 A1450 A1600 PC1000 PC1150 PC1300 PC1450
BET/(m2
/g) 23.88 10.82 9.28 4.66 4.53 7.35 2.64 4.48 9.41
Average pore size /nm 2.62 3.73 4.60 8.37 9.45 6.13 9.83 8.89 14.37
Fig. 5. Effect of the pyrolysis temperature on the pore size distribution of coal char and petroleum coke.
is up to 1300 ◦C. The carbon structure of the petroleum coke
becomes ordered with the increase of pyrolysis temperature,
thereby decreasing the gasification rate. However, the gasifica-
tion reactivity of petroleum coke is enhanced when the pyrolysis
temperature is higher than 1300 ◦C because of the increase of
BET surface area. Accordingly, the carbon crystallite structure of
petroleum coke plays a major role in gasification reactivity at
temperatures below 1300 ◦C. The effect of BET surface area on gasi-
fication reactivity is stronger than that of the carbon structure when
the temperature exceeds 1300 ◦C.

6. J. Xiao et al. / Journal of Analytical and Applied Pyrolysis 117 (2016) 64–71 69
Fig. 6. Effects of the pyrolysis temperature on the air and CO2 gasification reactivity.
In addition, the inherent minerals exert remarkable catalytic
effects on gasification reactivity, consequently increasing the reac-
tion rate. Thus, the difference between air gasification reactivity
and CO2 gasification reactivity of petroleum coke can be explained
by the catalytic effect of impurity elements. Impurity elements
in petroleum coke, such as V, Ni, and Na, have strong catalytic
effects on carbon–air reaction. Moreover, impurity elements such
as Ca, Ni, and Na significantly influence the carbon–CO2 reaction
[14,15,28–30]. Table 2 shows that the V content in the petroleum
coke is obviously higher; hence, the air gasification reactivity is
more obvious than the CO2 gasification reactivity.
Furthermore, the CO2 gasification rate of coal char is slightly
higher than that of petroleum coke, indicating that the petroleum
coke is less reactive than coal char for CO2 gasification at this tem-
perature range. However, the air gasification rate of petroleum coke
is much higher than that of coal tar. On the one hand, coal mainly
contains impurity elements, such as Al, Fe, Si, and Ca, they inter-
act with each other to form compounds with no catalytic activity;
hence, they play an inhibiting role on air gasification reactivity
[24,31,32], and V and Ni with strong catalytic abilities are relatively
less or do not exist. On the other hand, ash melting of coal char sig-
nificantly influences the gasification reaction. The ash melts cover
the surface and block the pores of the coal char particles, thereby
hindering the gasification reaction [33,34]. Considering all these
factors, the capability of coal char to resist air gasification is more
significant than that of petroleum coke.
3.5. Effect of pyrolysis temperature on powder resistivity
The effect of pyrolysis temperature on powder resistivity is plot-
ted in Fig. 7 . Both coal char and petroleum coke show a rapid
decrease in their powder resistivity with the increase of temper-
ature. The final carbonization temperature is the most important
factor affecting the powder resistivity of coke. The rapid decrease
of powder resistivity with the increase of temperature is mainly
related to several factors [35,36]. First, the volatile substances are
continuously discharged from the material. Second, hydrogen is
released with the breaking of C H bond, consequently forming
new free electrons. Third, the conductivity of coke depends on the
formation of conjugated ␲ bond, which increases with the aroma-
tization of coke.
As shown in Fig. 7, the powder resistivity of coal char is far
higher than that of petroleum coke. According to China’s nonfer-
rous industry standard YS/T 625–2007, the powder resistivity of
calcined coke for carbon anode should be ≤610 ␮ m. However,
even after calcination at 1600 ◦C, the powder resistivity of coal char
Fig. 7. The effect of pyrolysis temperature on electrical resistivity.
remains very high; hence, improving the heat treatment tempera-
ture becomes necessary. The powder resistivity of petroleum coke
meets the requirement for carbon anode even when the pyrolysis
temperature is only higher than 1150 ◦C.
The results showed that pyrolysis temperature has significant
effect on both powder resistivity and structural parameters of
the carbon crystallites of chars. Thus, a relationship should exist
between the powder resistivity and structural parameters of crys-
tallites. The correlations between powder resistivity and structural
parameters of carbon crystallite (Lc, d002) are shown in Fig. 8. A close
exponent correlation is observed between Lc and powder resis-
tivity, and d002 demonstrates a good linear dependence relation
with powder resistivity, especially both the correlation coefficients
are >0.93. Therefore, the carbon crystalline structure plays a vital
role in determining powder resistivity. Larger Lc or smaller d002
values indicate higher ordered crystalline structure, resulting in
lower electrical resistivity [37]. Hence, powder resistivity can be
considered a structure-sensitive parameter reflecting the internal
structure of chars [22].
3.6. Effect of pyrolysis temperature on real density
Real density is related to the types of materials and heat treat-
ment conditions, as well as the porosity of materials. In the process
of aluminum electrolysis, using carbon anode with higher real den-
sity can reduce the replacement frequency; thus operational costs
are also reduced.
The effect of pyrolysis temperature on the real density of the
chars are shown in Fig. 9 . The real density of petroleum coke
increases with the increase of pyrolysis temperature, which is
attributed to the molecular structure rearrangement of petroleum
coke, while hydrogen, oxygen, nitrogen, and other impurity atoms
are continuously discharged.
Notably, the real density of coal chars declined with the increase
of temperature, and this result appears to conflict with the XRD
and BET surface area analyses, which showed that carbon crys-
tallite structure of coal char tends to be ordered, and the BET
surface area decreases. To explain the decrease of real density of
coal chars, proximate analysis and ultimate analysis of coal chars
and petroleum cokes are added in Table 3. With the increase of
temperature, the content of C increases, and the contents of N,
H, O, and S gradually decrease (Table 3). The spatial arrangement
and abundance of the elements: C, H, N, O, and S often correlate
or directly influence the coal properties, including helium density
[38]. With the increase of graphitization degree, the side chains
and functional groups on the molecules are reduced, oxygen and

7. 70 J. Xiao et al. / Journal of Analytical and Applied Pyrolysis 117 (2016) 64–71
Fig. 8. Correlation between the carbon crystalline structure and electrical resistivity.
Fig. 9. The effect of pyrolysis temperature on real density.
nitrogen elements in molecules are also rapidly reduced simultane-
ously. Although, the reduction of side chains and functional groups
is beneficial to the enhancement of density, the relative atomic
mass of oxygen and nitrogen is relatively larger than that of car-
bon. As a result, the decrease of oxygen and nitrogen contents has a
dominant effect on the real density of coal chars at 1000 ◦C–1600 ◦C,
this effect results in the decrease of the real density. For the calcined
petroleum coke, a very compact structure of aromatic compounds
was formed with the rapid increase of the order degree of the crys-
tallite structure. Accordingly, the real density of calcined petroleum
coke increased rapidly. The different trends of real density between
coal char and petroleum coke may be attributed to the fact that the
graphitization degree of coal char is lagging behind the petroleum
coke.
As a result, the real density of petroleum coke is larger than that
of coal char. For instance, after heat treatment at 1300 ◦C, the real
density of petroleum coke is 2.085 g cm−3, whereas that of coal char
is only 1.749 g cm−3. According to Chinese nonferrous metal indus-
try standard YS/T 625–2007, the real density of a carbon material
should be >2.01 g cm−3. Therefore, the real density of coal char is
relatively small, which is a disadvantage for a carbonaceous mate-
rial in preparing carbon anode.
4. Conclusion
The effects of high-temperature pyrolysis on the structure and
physicochemical properties of low-ash anthracite and petroleum
coke were investigated in this study. The differences in physical and
chemical properties were also analyzed. The results are presented
as follows:
(1) The crystallite structures of coal char and petroleum coke
became more ordered with the increase of temperature. Com-
pared with petroleum coke, structural rearrangement of coal
char requires higher temperatures. Therefore, the graphitiza-
tion degree of coal char is lower than that of petroleum coke at
the same pyrolysis temperature.
(2) With the increase of pyrolysis temperature, the BET surface area
of coal char decreased gradually, whereas the BET surface area
of the petroleum coke exhibited a decreasing trend after ini-
tial increasing trend. The average pore sizes of coal char and
petroleum coke showed an increasing trend.
(3) The carbon crystallite structure and BET surface area generally
inhibited the gasification reactivity of coal char. The gasification
reaction was first inhibited and then promoted for petroleum
coke. The air gasification reactivity of coal char is remarkably
lower than that of petroleum coke, but the CO2 gasification
reactivity of coal char is slightly stronger.
(4) The powder resistivity of coal char and petroleum coke showed
a rapid decrease with the increase of temperature, and the pow-
der resistivity of coal char is considerably higher than that of
petroleum coke. Moreover, the structural parameters of carbon
crystals (Lc and d002) are well correlated with powder resistiv-
ity.
(5) The real density of petroleum coke increased with the increase
of pyrolysis temperature. By contrast, the real density of coal
char declined.
Compared with petroleum coke, coal char presents disadvan-
tages in terms of ash content, powder resistivity and real density.
For this reason, we attempt to prepare the carbon anode by blend-
ing part of coal char with petroleum coke. In this way, the properties
of the mixed coke will be controlled within allowable ranges, and
the requirements of carbon anode will be satisfied.
Acknowledgments
This work was funded by the National Natural Science Founda-
tion of China (51374253). The authors are also grateful to Miss Wu
for the advice of this paper.
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