Energy and Power Engineering, 2013, 5, 36-40
doi:10.4236/epe.2013.54B007 Published Online July 2013 (
Study on Co-combustion Characteristics of Superfine Coal
with Conventional Size Coal in O2/CO2 Atmosphere
Yuhang Zhang, Mingyan Gu*, Biao Ma, Huaqiang Chu
Department of Metallurgy and resource, Anhui University of Technology, Ma’an shan, China
Received March, 2013
The pulverized coal combustion in O2/CO2 atmosphere is one of the promising new technologies which can reduce the
emission of carbon dioxide and NOx. In this study, the combustion behaviors of different mixing ratio of Shenhua coal
with 20 μm and 74 μm particle size in the O2/CO2 atmosphere and air atmosphere were studied by using a ther-
mal-gravimetric analyzer. The combustion characteristics such as ignition and burnout behavior were investigated in the
temperature from 20℃ to 850. The influence of mixing ratio on combustion characteristics was conduced. The re-
sults obtained showed that the ignition temperature of the two kinds of particle size in O2/CO2 atmosphere is higher than
in the air, while the activation energy in O2/CO2 atmosphere is lower. With the increasing ratio of 20 μm superfine pul-
verized coals, the ignition temperature and the activation energy decreased, while the DTG peak value increased, the
maximum burning rate position advanced. There were three trends for the ignition temperature curve with the increas-
ing of superfine coal ratio: the ignition of the mixed coal decreased rapidly, then changed less, at last reduced quickly.
Keywords: Coal Particle Size; Co-combustion; Thermo-gravimetric; Heat of Combustion
1. Introduction
Coal makes a significant contribution to energy produc-
tion and because of its availability and flexibilityit is
likely to continue to be a major part of the energy scene
for some time. However, this fuel has the disadvantage of
producing more CO2 per unit of energy produced than
the other fossil fuels which will in time require the ap-
plication of carbon capture and storage technologies.
Coal combustion in O2/CO2 is considered as a new gen-
eration of combustion technology that both direct access
to high concentrations of CO2 and comprehensively con-
trol the coal-fired pollutant emission [1-3]. During the
O2/CO2 combustion, coal is burnt in a mixture of O2 and
recycled flue gas (mainly CO2 and H2O), to yield a rich
CO2 stream. Previous results have shown that the re-
placement of N2 by CO2 can cause significant differences
in the area such as burning stability, char burnout, heat
transfer and gas temperature profiles [4-6]. The flame
propagation speed, flame stability and gas temperature in
O2/CO2 environment are lower and the unburned carbon
content is higher than those in air environment. Experi-
mental studies on coal-fired O2/CO2 combustion have
been performed in laboratory and semi-industrial scale
test facilities of different size [7-10]. The coal combus-
tion appears ignition delay and combustion instability in
O2/CO2 [11], improving the oxygen concentration [12]
and using the superfine pulverized coal combustion can
improve the combustion characteristics, and the particle
size has a great influence on the combustion characteris-
tics [13-14].
In this study, the combustion process of mixed coal
with 20 μm and 74 μm of Shenhua coal under both air
and O2/CO2 atmosphere were employed by a thermal-gra-
vimetric analyzer (TGA). The influence of superfine
pulverized coal on the combustion characteristics was
analyzed in the atmosphere of O2/CO2 and the ignition
temperature and kinetic parameters were obtained. The
results obtained provide a better understanding of coal
combustion in O2/CO2 atmosphere.
2. Experimental
A NETZSCH STA449C simultaneous thermal analyzer
was used in this work. The temperature was increased
from 20 to 850 linearly at the heating rate of 10
K·min-1. The flow rate of evolved gas was fixed at 80
ml·min-1, 7.5 mg of fuel sample with 20 μm and 74 μm
sizes were used for each test. Two kinds of different par-
ticle size with Shenhua coal and their mixture were used
in this study and their proximate analysis was listed in
Table 1. The sizes of coal are 20μm and 74μm respec-
Copyright © 2013 SciRes. EPE
3. Result and Discussion
3.1. The Coal Combustion under Different
The TG and DTG profiles for all cases were obtained to
analyze the combustion characteristic parameters. For all
TG curves, it was shown that after an initial moisture
removal, the volatile matter started to release and then a
rapid weight loss were observed. Temperature at which
the DTG curve showed peak value was denoted as the
maximum weight loss temperature. The ignition tem-
perature (Ti) was obtained by co-analyzed TG-DTG
curves, at which the DTG has its peak value and the cor-
responding slope to the intersection with respect to the
TG profile. Burnout temperature (Tb) was determined
when the mass loss reaches 99% of final weight loss at
the temperature of 850.
The TG-DTG curves for 20μm and 74μm coal in
O2/CO2 and air atmosphere were shown in Figure 1.
From these TG curves, it can be seen that, there were
three stages of weight loss during the coal burning. First
stage corresponds to the removal of inherent moisture
until the temperature exceeds 100. Following that was
the volatile matter release in the second stage; finally, the
char combustion was observed. Compared with that un-
der air atmosphere, the coal DTG curve moved to the
right and the maximum weight loss temperature in-
creased while the maximum weight loss values decreased
in same concentration of oxygen under O2/CO2 atmos-
Table 2 showed the parameters of two particle size
coal combustion characteristics in air and O2/CO2 at-
mospheres. For 20 μm coal in air atmosphere, the igni-
tion temperature increased from 375 to 384 and the
burnout temperature increased from 658 to 796.1
under O2/CO2 atmosphere. For 74 μm coal, the ignition
increased from 396 to 401 and the burnout tem-
perature increased from 648.6 to 784.5 in O2/CO2
atmosphere. The ignition temperature of coal with 20 μm
was lower than the one with 74 μm, even lower than the
ignition temperature in air atmosphere. In addition, under
O2/CO2 atmosphere, for 20μm coal, the maximum weight
loss decreased from 13.72% to 13.62%, while the maxi-
mum weight loss temperature increased from405 to
412 and the burnout rate increased from95.52% to
97.18%. For 74μm coal, the maximum weight loss de-
creased from 12.67% to 12.36%, while the maximum
weight loss temperature increased from 437 to 441
and the burnout rate increased from 99.39% to 99.96%.
Table 1. Proximate analysis (mass/%).
Fuel M A V FC
ShenHua coal(SH) 4.80 5.00 31.90 58.30
Figure 1. TG and DTG curves under O2/CO2 and air at-
Table 2. Combustion characteristic parameters of two par-
ticle size coals.
coal atmosphereTi()T
b() (dW/dt)max
Air 375 658.0 13.72 40595.52
20 μm
O2/CO2 384 796.1 13.62 41297.18
Air 396 648.6 12.67 43799.39
74 μm
O2/CO2 401 784.5 12.36 44199.96
Arrhenius kinetic parameters were obtained by ana-
lyzed the TG-DTG curves, as shown in Table 3. A sim-
ple kinetic analysis was performed which a single step
reaction process is supposed.
The kinetic parameters of two particle size coals were
listed in Table 3. It was clearly that in O2/CO2 atmos-
phere, the reaction activation energy of coal with both 20
μm and 74 μm diameters were lower than that in air en-
vironment, and the activation energy of 20 μm coals were
lower than 74 μm coal.
Copyright © 2013 SciRes. EPE
Figure 2 showed the TG and DTG curves for blending
coals with 20 μm and 74 μm Shenhua coal with different
ratio under O2/CO 2 atmosphere. From the figures, it can
be seen that, blending the 20 μm coal into 74 μm coal
moved the coal DTG curve to the left, increase the
maximum weight loss and the maximum burning rate.
Combustion characteristic parameters of blending
coals were listed in Table 4. From the figures, it can be
seen that for the 20 μm and 74 μm blending coals, with
the increasing ratio of 20 μm coal from 30% to 70%, the
Table 3. Kinetic parameters of two particle coals.
coal atmosphere
E/(kJmol-1) A/s-1 R
Air 101.46 164737.3 0.9666
O2/CO2 81.44 3246 0.9822
Air 122.94 4578628.6 0.9878
O2/CO2 91.87 17248.9 0.9788
Figure 2. TG and DTG curves for different superfine coal
Table 4 .Combustion characteristic parameters of blending
Coal Ti/°C Tb/°C (dW/dt)max
/°C η/% Cb
+70%74μm392782.612.27 438 99.697.98
+30%74μm388779.612.78 425.5 98.998.49
+20%74μm386778.113.25 415.5 97.828.89
Table 5. Kinetic parameters of blending coals.
coal E/(kJmol-1 ) A/s-1 R
30%20μm + 70%74μm 87.85 8994.2 0.9787
70%20μm + 30%74μm 82.73 3652.3 0.992
80%20μm + 20%74μm 81.79 2352.7 0.9821
maximum weight loss temperature decreased from 438
to 425.5, increasing the ratio of 20 μm coal to 80%,
the maximum weight loss rate appears at 415.5; With
the increasing ratio of superfine coal, the peak value of
DTG curves increased, and the maximum weight loss
was from 12.27% to 13.25%. Increasing the ratio of
20μm coal lower the ignition temperature from 392 to
386 and the burnout temperature decreased from
782.6 to 778.1. From Table 4, it also can be seen
that with the increasing ratio of superfine coal, the burn-
out rate decreased which may be caused by the small
coal particle size and packing density.
This paper used the flammability index to compare the
combustion characteristics of different blending coals. It
is defined as Cb= (dW/dt)max/ Ti
2 ;Cb mainly reflect the
coal combustion reaction capability which the higher the
value, the better the flammability of coal. For the blend-
ing coals, with the increasing of 20 μm coal, the flamma-
bility index of mixed coals increased from 7.98 to
Table 5 showed the kinetic parameters of blending
coals. It can be seen that increasing the ratio of 20 μm
coal decreased the reaction activation energy from 87.85
to 81.79 (kJmol-1). Higher the ratio of 20 μm coal, the
lower the activation energy value.
Figure 3 showed the ignition temperature of different
ratios of blending coals. It can be seen that with the in-
creasing of 20 μm coal, the ignition temperature tends to
a downward trend. When the ratio of 20 μm was from 0
to 20%, the ignition temperature decreased rapidly, while
the ratio was 20% to 60%, the ignition temperature
Copyright © 2013 SciRes. EPE
Figure 3. The ignition temperature of blending coals.
changed less. Increasing the ratio of 20 μm coal further,
the ignition temperature decreased rapidly again.
4. Conclusions
In this paper, the combustion characteristics of ShenHua
coals with diameters of 20 μm and 74 μm and their mix-
ture under air and O2/CO2 atmosphere had been investi-
gated using the TG analysis. The conclusions were listed
as follows.
1) The coal burning rates decreased and the ignition
temperature lowered under O2/CO2 atmosphere compared
with coal combustion in the air environment.
2) For coal burning both in O2/CO2 atmosphere and air,
the ignition temperature with superfine size coal was
lower than that of large particle size, and the maximum
weight loss rate was higher than the large particle size
3) The addition of 20 μm coal into 74 μm lowered the
ignition temperature under O2/CO2 atmosphere. With the
decrease of coal particle size, the DTG curve moves to
the left, the ignition temperature and the burnout tem-
perature decreased which indicating that the coal ignition
advanced and it's easy to reach the ignition temperature
and combustion.
4) Activation energy of blending coals combustion va-
ried with the ratio of 20 μm for blending coals. For 20
μm - 74 μm blending coals, the activation energy de-
creased with the increasing ratio of 20 μm coal.
5. Acknowledgements
The present work was supported by National key scien-
tific instrument and equipment development project
(2012YQ22011 9).
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