Journal of Power and Energy Engineering, 2013, 1, 1-7
http://dx.doi.org/10.4236/jpee.2013.12002 Published Online May 2013 (http://www.scirp.org/journal/jpee)
1
The Carbonation Behaviors of Limestone Particle in
Oxygen-Fuel Circulating Fluidized Bed O2/CO2 Flue Gas
Jianyu Shang1*, Zhongliang Liu1, Chunbo Wang2
1College of Environmental and Energy Engineering, Beijing University of Technology, Beijing, China; 2School of Energy and Power
Engineering, North China Electric Power University, Baoding, China.
Received April 12th, 2013; revised May 12th, 2013; accepted May 27th, 2013
Copyright © 2013 Jianyu Shang et al. This is an open access article distributed under the Creative Commons Attribution License,
which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.
ABSTRACT
Limestone powder is still applied as SO2 sorbent in emerging oxygen-fuel circulating fluidized bed boiler, but its car-
bonation in O2/CO2 flue gas is an unclear problem. For a better understanding of carbonation behaviors, the tube fur-
nace heating system was built for simulating circulating fluidized bed boiler flue gas by regulating the supply of O, CO2,
N2, SO2 and H2O, and Carbonation reaction was tested. Thermal gravimetric analysis and scanning electron microscopy
were used. It was found that carbonation is closely related to temperature, CO2 concentration, impurities, water vapor,
and cycle times; high temperature can promote carbonation process; high concentration of CO2 can inhibit the chemical
reaction stage speed of carbonation process, but it has little effect on the final conversion rate; water vapor can increase
the final conversion rate of carbonation; the cycle times will reduce the activity of carbonation. The presence of car-
bonation turns the traditional boiler flue gas indirect desulfurization model into indirect desulfurization mechanism
which does not have a negative impact on SO2 removal efficiency.
Keywords: Oxygen-Fuel; Circulating Fluidized Bed; Limestone Particle; Carbonation; Desulfurization; Production
Layer
1. Introduction
For many coal-fired power plants in China, limestone is
widely used in dry desulfurization technology [1-4]. In
traditional boilers, limestone (Figure 1) will be thermally
decomposed into porous CaO particles (Figure 2) in high
temperature flue gas, and then chemically react with SO2
to produce CaSO4. This type of SO2 removal is called
indirect desulfurization reaction [5-7], and the chemical
equations are
 
32
CaCOsCaO sCOg (1)
  
22 4
1
CaOsSOgO gCaSO s
2
  (2)
In fact, it may not be quite that simple for oxygen-fuel
circulating fluidized bed (CFB) flue gas, because car-
bonation will happen at that carbon-dioxide-rich atmos-
phere. CaO carbonation is actually a reversible reaction
about CaCO3 calcinations [8] as shown in Figures 3 and
4. The phenomenon that limestone is partly calcined into
Figure 1. Unreacted limestone particle.
CaO does exist at the CFB oxygen-rich combustion zone
where CO2 concentration is lower and temperature is
relatively higher, and when these CaO reach carbon-
dioxide-rich zone it will present carbonation reaction [9].
In other words, these CaO will be finally reduced to
CaCO3 again at main flue zone where CO2 concentration
is >80% [10]. The chemical equation is

 
2
CaOsCOgCaCOs

3
(3)
*Corresponding author.

The Carbonation Behaviors of Limestone Particle in Oxygen-Fuel Circulating Fluidized Bed O2/CO2 Flue Gas 2
Figure 2. Porous calcined limestone particle.
CaCO
3
CaCO
3
CaO
Figure 3. Limestone calcinations process.
CaCO
3 CaO
Figure 4. Limestone carbonation process.
So in this case without porous CaO, limestone has to
directly react with SO2, which is called direct desulfuri-
zation [11-13], and the chemical reaction equation is
  
3224 2
1
CaCOsSOgO gCaSO sCO g
2

(4)
Some discussions about carbonation can also be found
in [14-21]. In order to find out the influence on carbona-
tion from some physical parameters such as temperature,
CO2 concentration, water vapor, impurity, cycle times,
we carried out a series of experiments on an altered tube
furnace heating system. And this study also includes
carbonation impact on the desulfurization process that is
what we care about.
2. Experimental
The tube furnace heating system was rebuilt. Reactant
gas distribution is a key for this investigation, so the pip-
ing system was set as shown in Figure 5 for providing
O2, CO2, N2, SO2, and H2O. The partial pressure of every
atmosphere is based on different type of experiment. O2,
CO2 and SO2 are reactant gas, and N2 is a protective gas.
Water vapor is one innovation which was suspected to
definitely affect carbonation or desulfurization of lime-
stone in the flue gas. H2O is injected by a pump into a
heated pipe as shown in Figure 5. And Figure 6 stands
for Single channel syringe infusion pump and pipe heat-
ing system. The pump is LSP01-1A laboratorial pump
which is single channel syringe infusion mode designed
by Longer Precision Pump Co., Ltd., the acceptable sy-
ringe specification is from 10 μl to 60 ml, suitable for
high accuracy and small flow rate liquid transferring. The
thermocouple is to maintain the pipe wall above 200˚C to
heat the water into steam in the main pipeline, and the
steam flow is controlled by a solenoid valve. Powder
samples are placed in a porcelain boat and pushed into
the tube furnace, and any change of limestone on the
porcelain is surveyed by the weight monitor for investi-
gating calcinations/carbonation/desulfurization thermo-
gravity (TG) law.
3. Effect on Carbonation from Temperature
In this section, 10 mg of limestone sample was weighed
and token into the porcelain, and then it was heated up to
850˚C by 20˚C/min. For 10 minutes later, it was cooled
down to different reaction temperatures by 20˚C/min.
During above process, N2 was always protective gas in
order to avoid CaO being reacted. At different set tem-
perature, the reaction atmosphere was switched to a
mixed gas of CO2 and N2, and CO2 concentration is 80%.
The experimental results are shown in Figure 7. For
every case, chemical reaction stage is fast and lasts about
<1 minute, then the reaction rapidly approaches a slower
state which is called product layer diffusion control stage.
Figure 5. Tube furnace heating system.
Figure 6. Single channel syringe infusion pump and pipe
heating system.

The Carbonation Behaviors of Limestone Particle in Oxygen-Fuel Circulating Fluidized Bed O2/CO2 Flue Gas 3
400˚C
500˚C
600˚C
700˚C
800˚C
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0
Conversion
0 5 10 15
t/min
Figure 7. Conversion rate of CaO carbonation at different
temperatures.
As we can see, temperature directly and seriously deter-
mines the value of chemical reaction rate constant. In the
chemical control stage, the promotion of carbonation
conversion rate is 21% from 400˚C to 500˚C, 32% from
500˚C to 600˚C, or 18% from 600˚C to 700˚C. The final
conversion rate is 21% higher at 500˚C than 400˚C, 10%
at 600˚C than 500˚C, or 5% at 700˚C than 600˚C. So,
Temperature is a great influence on the conversion of
carbonation, which is because that high temperature will
exacerbate the carbonation reaction include chemical
reaction rate and final conversion rate. Otherwise, foul-
ing will more likely to occur at high-temperature heating
surface, and it should take some appropriate measures to
avoiding the unsafe operation of boiler.
4. Effect on Carbonation from CO2
Concentration
In this experiment, each reaction temperature is 700˚C,
and corresponding CO2 concentration is 16%, 70%, 80%,
and 90%. In Figure 8, the conversion rate of carbonation
include chemical reaction rate and final reaction rate is
approximately equivalent in the case of high CO2 con-
centration such as 70%, 80%, or 90%. For the case of
lower 16% CO2 concentration, the chemical reaction rate
is slower than that in high concentration CO2 flue, and it
needs 190 seconds to go into the production layer diffu-
sion control stage, but it needs only 40 seconds for the
case of 80% CO2. Their final conversion rates are almost
same, such as 73.14% for 16% CO2, 73.27% for 70%
CO2, 74.48% for 80% CO2, and 76.02% for 90% CO2. So,
CO2 concentration directly determines the carbonation
speed of chemical reaction stage, and its influence on
final reaction rate is little.
5. Effect on Carbonation from H2O
Currently it is not a lot about the investigation on any
relation with steam and carbonation. In this experimental
section, we sprayed water into the stainless steel pipe by
a pump, and then water in the tube was rapidly heated
into steam. When carbonation reaction, N2 was protective
90%
80%
70%
16%
0 5 10 15
t/min
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0
Conversion
Figure 8. Conversion rate of CaO carbonation at different
concentration of CO2 (700˚C).
gas, CO2 concentration was 80%, H2O concentration was
0% - 15%. Experimental results for four temperatures are
shown in Figure 9. When temperature is 700˚C, lime-
stone carbonation conversion rate with 5%, 10%, 15%
water is respectively 0.9%, 5.8%, 11.5% higher than that
without steam (0% water). At 600˚C, the result is almost
similar to that at 700˚C. When temperature is 400˚C,
limestone carbonation conversion rate with 5%, 10%,
15% water is respectively 3.1%, 21.5%, 28.6% higher
than that without steam. At 500˚C, the result is almost
similar to that at 400˚C. So, steam can facilitate the car-
bonation reaction of calcined limestone especially at
lower temperature conditions.
At the heating surface in the tail of CFB, partly un-
vulcanized CaO particles will have to continue the car-
bonation reaction with CO2 and cause fouling and slag-
ging phenomenon. According to above result, steam can
increase the intensity and hardness of slag which is more
difficult to be removed by soot-blowers, the convective
heat transfer coefficient of heating surface will also re-
duce.
6. Effect on Carbonation from Impurities
In fact, the calcined production of natural limestone is
not exactly CaO, also contains SiO2, Fe2O3, Al2O3, MgO
and other impurities. Pure CaCO3 and natural limestone
are our experimental samples. Figure 10 is a comparison
of two results, CO2 is 80%, and reaction temperatures are
500˚C, 600˚C, and 700˚C. At 700˚C, the carbonation
conversion rate of CaCO3 reaches 0.65 after 145 seconds,
it needs 197 seconds for limestone to reach this rate, and
both simultaneously reach 0.67 after 197 seconds. After
that time, the carbonation reaction speed of limestone
become faster than that of CaCO3, the final conversion
rate of CaCO3 is 0.71, and that of limestone is 0.73. The
case of 500˚C and 600˚C is similar to 700˚C. We can see
from above result that the carbonation reaction speed of
pure CaCO3 is faster than that of natural limestone at the
chemical reaction stage, and it is reverse at the production

The Carbonation Behaviors of Limestone Particle in Oxygen-Fuel Circulating Fluidized Bed O2/CO2 Flue Gas 4
0%
5%
10%
15%
0.9
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0
Conversion
0 5 10 15
t/min
(a)
0%
5%
10%
15%
0.9
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0
Conversion
0 5 10 15
t/min
(b)
0%
5%
10%
15%
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0
Conversion
0 5 10 15
t/min
(c)
0.9
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0
Conversion
0%
5%
10%
15%
0 5 10 15
t/min
(d)
Figure 9．Conversion rate of CaO carbonation at different
concentration of H2O. (a) 700˚C; (b) 600˚C; (c) 500˚C; (d)
400˚C.
layer diffusion stage. So, the impurities can promote the
carbonation reaction speed at the production layer diffu-
sion stage, the reason may be that impurity cause lattice
defect in production layer [22].
7. Effect on Carbonation from Cycle Times
The calcinations/carbonation experiment for 10 cycle
times was done in this section. Firstly, sample was taken
into the furnace, N2 was protective gas, calcination tem-
perature was 850˚C, insulation time was 10 minutes, the
temperature was secondly dropped down to reaction
temperature (500˚C, 600˚C, 700˚C), the reaction gas was
thirdly switched to 80% CO2, reaction time was 30 min-
utes, finally, sample was heated up to 850˚C again. This
is a calcinations/carbonation cycle, and cooling/heating
speed was 20˚C/min. The experimental result is shown in
Figure 11.
For this case, carbonation conversion rate is calculated
as
3
0CaO
0CaCO CaO
n
n
mm M
mAM M




(5)
In Figure 11, we can see that the activity of CaO
700˚C CaCO
3
600˚C CaCO
3
500˚C CaCO
3
700˚C limestone
600˚C limestone
500˚C limestone
0 5 10 15
t/min
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0
Conversion
Figure 10. Comparison of CaCO3/limestone carbonation
reaction.
700˚C
600˚C
500˚C
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0.0
Conversion
0 2 4 6 8 10
Cycle time
Figure 11. Multiple cycle calcinations/carbonation reaction.

The Carbonation Behaviors of Limestone Particle in Oxygen-Fuel Circulating Fluidized Bed O2/CO2 Flue Gas 5
reduces rapidly and the rate decreases by about 30% for
the first 5 cycles, and then its curve becomes flat for the
next several cycles especially at high temperature. This is
mainly due to serious sintering of CaO after multiple
cycles. So, in order to maintain a high efficiency, people
had to increase the amount of fresh minerals, this will
bring a series of problems, such as operating cost, in-
creased wear, pollute. Its mechanism need to be further
studied.
8. Effect on Desulfurization from
Carbonation
From the above study, we can know that there is a seri-
ous problem of carbonation in CFB oxygen-fuel atmos-
phere. In this section, two experiments were conducted to
study the influence on desulfurization from carbonation,
one was desulfurization experiment after calcinations,
and the other was desulfurization after carbonation. In
the first experiment, 10 mg limestone samples were
placed into the furnace and heated up to the calcinations
reaction temperature (750˚C, 800˚C, 850˚C) by the heat-
ing speed of 20˚C /min, N2 was the protective gas, here-
after, SO2 (3000 ppm) and O2 were switched in and the
desulfurization reaction began, the result of TG curve is
shown in Figure 12. In the second experiment, 10 mg
limestone samples were placed into the furnace, CO2
(80%) was passed into, and then the samples were heated
up to the reaction temperature (750˚C, 800˚C, 850˚C) by
the heating speed of 20˚C/min, finally, SO2 (3000 ppm)
were switched in and the desulfurization reaction began,
the result of TG curve is shown in Figure 13. As we can
see, it is a direct desulfurization reaction type in Figure
13 which is different with that indirect desulfurization in
Figure 12. Therefore, the carbonation changes the tradi-
tional mode of desulfurization.
And indirect desulfurization rate is calculated as
850˚C
800˚C
750˚C
10
5
Q
ualit
y
/m
g
0 1000 2000 3000 4000
t/s
Figure 12. Indirect desulfurization TG curve.
4
CaO
CaSOCaO 0
Mm
X
M
Mm



(6)
Direct desulfurization rate is calculated as
3
43
CaCO
CaSOCaCO 0
Mm
X
M
Mm



(7)
According to above equations, the results of desulfu-
rization rate were calculated in Figures 14 and 15. The
final conversion rate of direct desulfurization reaction is
higher than that of indirect desulfurization, especilly at
high temperatures. For example, the final rate of indirect
desulfurization is <40% at 850˚C, but the final rate of
direct desulfurization is >60% at this temperature and it
is still increasing with time. Obviously, carbonation does
not have a negative impact on desulfurization process.
Figure 16 is the micro-morphology of indirect desulfu-
rization particle by scanning electron microscopy (SEM),
there is a large number of pores formed after calcination,
and they are blocked by desulfurization production and
present concave-convex shape. Figure 17 is the micro-
850˚C
800˚C
750˚C
6.4
6.2
6.0
5.8
5.6
5.4
5.2
5.0
Qualit
y
/m
g
0 2000 4000 6000 8000 10,000 12,000 14,000
t/s
Figure 13. Direct desulfurization TG curve.
850˚C
800˚C
750˚C
0.5
0.4
0.3
0.2
0.1
0.0
Conversion
0 400 800 1200
t/s
Figure 14. Indirect desulfurization conversion rate.

The Carbonation Behaviors of Limestone Particle in Oxygen-Fuel Circulating Fluidized Bed O2/CO2 Flue Gas 6
850˚C
800˚C
750˚C
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0.0
Conversion
0 2000 4000 6000 8000 10,000 12,000
t/s
Figure 15. Direct desulfurization conversion rate.
Figure 16. Indirect desulfurization particle surface mor-
phology.
Figure 17. Direct desulfurization particle surface morphol-
ogy.
morphology of direct desulfurization particle by SEM,
the production layer surface is smooth and dense without
any pore. Direct desulfurization can achieve high effi-
ciency without significant pore, which seems to be in-
consistent with gas diffusion theory and pore model the-
ory. However, it also shows that solid state ionic diffu-
sion [22] does play a key role which needs a further in-
vestigation.
9. Conclusion
The carbonation of calcium-based sorbent is really a se-
rious problem in oxygen-fuel CFB flue gas, and it is
closely related to temperature, CO2 concentration, impu-
rities, water vapor, and cycle times. High temperature
can promote carbonation process. High concentration of
CO2 can inhibit the chemical reaction stage speed of
carbonation process, but it has little effect on the final
conversion rate. Water vapor can increase the final con-
version rate of carbonation. The cycle times will reduce
the activity of carbonation. The presence of carbonation
changes the traditional boiler flue gas desulfurization
chemical model, which is not indirect desulfurization
mechanism but direct desulfurization mechanism, and
this mechanism will not have a negative impact on SO2
removal efficiency. It should be paid attention that car-
bonation is easier to damage some flue gas equipments.
And, the microscopic mechanism about carbonation and
direct desulfurization need deep research in the future.
10. Acknowledgements
This work was supported by the National Key Technol-
ogy R&D Program of China (2012BAC24B01), the Na-
tional Natural Science Foundation of China (51276064),
the Beijing Nature Science Foundation (3132028), and
the Beijing Postdoctoral Research Foundation (2012ZZ-
19).
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