a study on the calcination and sulfation behavior of limestone during ...

A STUDY ON THE CALCINATION AND SULFATION BEHAVIOR
OF LIMESTONE DURING OXY-FUEL COMBUSTION
Juan Chen a,b , Hong Yao b , Lian Zhang a*
a : Department of Chemical Engineering, Monash University, Clayton, GPO Box 36, Vic
3800, Australia
b : State Key Laboratory of Coal Combustion, Huazhong University of Science
Technology, Wuhan, 430074, China
*: Corresponding author.
Tel.: +61-3-9905-2592; fax: +61-3-9905-5686. E-mail: lian.zhang@monash.edu
ABSTRACT
The mechanisms governing the behaviour of limestone for in-furnace desulfurisation
during oxy-fuel combustion have been investigated in this paper. Apart from the use of
pure limestone, the mixtures of limestone with a brown coal (i.e. lignite) at different
mass ratios were also examined to address the influence of the coal-bound ash-forming
metals in the behaviour of limestone. The combustion conditions tested were 1273 K
furnace temperature and different particle residence times. With respect to gas
composition, apart from pure air and a 27% O2/CO2 mixture, a SO2 content of 1000
ppmV was also doped into the two gases to examine its competition with the ashforming
metals on the interaction with limestone.
The inhibitory influence of the high CO2 partial pressure on the calcination of limestone
has been confirmed. However, upon mixing limestone with coal, an increase in the
particle temperature through radiative heat transfer from coal flame has been confirmed
to promote the calcination degree of CaCO3. The ash-forming metals in coal were also
proven promoting the calcination of limestone. It is very likely that, the Ca-Al/Si
formed on limestone surface could be partially shed away, which in turn ensured the
exposure of fresh surface of limestone to undergo decomposition and sulfation.
However, once the Ca-Al/Si compounds formed were molten and a eutectic film was
formed on the limestone particle surface, the gas diffusion resistance was increased, and
hence, both the calcination and sulfation extents of limestone were reduced. The doping
of extra SO2 through the recirculation of flue gas mainly increased the sulfation of
limestone and concurrently reduced the un-reacted limestone fraction through direct
sulfation. The interaction between limestone/lime and ash were affected little, which
still remained competitive and highly preferred for the Ca/S molar ratio of 2-4
particularly under the oxy-fuel combustion condition. To achieve an efficient in-furnace
desulfurisation through limestone injection for oxy-fuel combustion condition, a
sufficiently high Ca/S molar ratio and the low-ash coal should be considered to avoid
the ash influence.
Keywords:
Limestone, Calcination, Sulfation, Ash, Oxy-fuel combustion.
1.INTRODUCTION

Juan chen et al.
Oxy-fuel combustion is a promising technology for coal-fired power plants to mitigate
its greenhouse gas emission in the short/medium term. In this technology, the flue gas is
recirculated to mix with the air-derived pure oxygen for combustion. The resulting CO2
concentration in flue gas can reach up to 95%. Since a large portion of flue gas is
recycled, the gaseous impurities such as SO2/SO3 within it can gradually accumulate in
a furnace, which in turn potentially causes extra fouling and tube corrosion problems
through the deposition of sulfates, as well as affects the purity of the CO2 which is
eventually sequestrated. To solve these problems, an efficient capture of SO2 for flue
gas clean-up is pivotal for an oxy-fuel system.
In-furnace capture of SO2 through the use of limestone is a promisingly cost-effective
manner for flue-gas clean-up. As has been confirmed for the conventional air
combustion, the sulfur removal of limestone involves two consecutive steps: calcination
of limestone and sulfation of calcium oxide, (Borgwardt 1970) according to
Calcinatio n : CaCO3
→ CaO + CO2
(1)
Sulfation : CaO + SO2
+ O2
→ CaSO4
(2)
Direct Sulfation : CaCO + SO + 0.
5O
→ CaSO + CO
(3)
3
2
The calcination of CaCO3 by reaction (1) is affected by two factors, CO2 partial
pressure and particle temperature. The high CO2 partial pressure in an oxy-fuel furnace
is expected to play a negative role. However, it is unclear if the increase in particle
temperature through radiative heat transfer from coal flame will offset this negative role.
Regarding the sulfation reaction, apart from reaction (2), the direct sulfation of
limestone according to reaction (3) is also considerable for sulfur capture, in particular
during oxy-fuel combustion. A generalised understanding has yet to be achieved in the
literature. Liu and his co-workers (2000) considered that, as the sintering extent of
limestone in CO2 is decreased substantially, the direct sulfation of limestone exhibits a
comparable rate to that of the sulfation reaction of calcium oxide in O2/CO2. Fuertes et
al (1994) and Snow et al (1988) suggested the kinetic control and fast diffusion of gases
for reaction (3). Our previous result also confirmed the large potential for reaction (3)
in O2/CO2 under certain conditions (Chen et al., 2011). In contrast, Iisa and Hupa (1990)
hypothesized the diffusion limit for the direction sulfation of limestone, and some other
researchers also proposed the comparable significance of reaction control and
diffusivity limit (Hajaligol et al. 1988; Iisa and Hupa, 1992). Moreover, the influence of
the coal-bound inorganic metals on the behavior of limestone in O2/CO2 mixture has not
yet been studied. At the typical pulverized coal-fired temperatures, the interaction
between calcium and alumina/silicon for the formation of molten eutectics is a
significant cause inhibiting the sulfation capture efficiency in air (Zhang et al. 2002).
The present <strong>study</strong> aims to address these knowledge gaps to clarify the transformation
behavior of CaCO3 in oxy-fuel combustion. In particular, the interaction of coal-bound
metals with limestone has been elucidated in detail, through the combustion of
limestone mixed with a low-rank lignite at different mass ratios. The lignite used is
Victorian brown coal rich in organically bound metals. Its combustion at high
temperature is expected to deliver plenty of fine and even ultra-fine ash particles which
2
4
2
2

Juan chen et al.
can attack limestone through random collision. The doping of 1000 ppmV SO2 to flue
gas was also conducted to simulate the gradual accumulation of this impurity gas in an
oxy-firing furnace. Mathematical modelling through a shrinking core model was also
conducted to clarify the resistance of ash layer against the diffusion of gases during the
calcination and sulfation of limestone.
2. THEORY SECTION
The mathematical model was used based on a reversible reaction of equation (1) for the
solid-gas decomposition of limestone which preferentially occurs at the solid CaCO3
surface, and has an equilibrium decomposition pressure of CO2 as (Baker, 1962):
Pe = 1.826 × 10 7 exp (- 19680/T) (4)
The calcination rate of limestone particles is considered to be chemical-kinetic
controlled (Borgwardt, 1985). The reaction rate at both external and internal surface is
expressed as:
Rate = - ks . ACaCO3 (5)
The effect of the gaseous CO2 partial pressure on the reaction rate was incorporated into
the rate equation by modifying rate constant ks as the following expressions from
Darroudi and Searcy (1981), Hu and Scaroni (1996).
ks = ks’ for P < 10 -2 ×Pe (6)
ks = ks’ . (Pe-P)/Pe for 10 -2 ×Pe < P < Pe (7)
where
ks’ = 6.078×10 7 exp {-205000/(RT)} (8)
Which is established by Borgwardt (1985) in a calcination <strong>study</strong> on small dispersed
limestone particles. Thus, the unreacted fraction (x) of CaCO3 can be expressed by:
ln (1-x) = - ks . Sg . MCaCO3 . t (9)
Where Sg (m 2 /g) is the BET-surface area of limestone, and is estimated as 0.4 m 2 /g in
this <strong>study</strong>; MCaCO3 (g/mol) is the molecular weight of CaCO3; t (s) is the time. When a
reactor length of 1200 mm was used, the particle residence time was estimated to be
approximately 1 s including 0.2 s for the pre-heating of particles from room temperature
to about 873 K where the decomposition of limestone commences.
The sulfation of limestone is a very complicated process during coal combustion. A
number of researchers (Borgwardt 1970; Dennis and Hayhurst 1990; Borgwardt and
Bruce 1986; Borgwardt et al. 1987; Bruce et al. 1989; Ghosh-Dastidar et al. 1996) have
investigated the kinetic and diffusion of the reaction between SO2 and calcium oxide.
Moreover, many investigations have been made on the mechanism of direct reaction (3).
The shrinking un-reacted core model (Szekely et al. 1976) is the most frequently used
model for the description of the kinetics of the direct sulfation reaction. Snow et al.
(1988), Hajaligol et al. (1988), Fuertes et al. (1993), Krishnan & Sotirchos (1993), and
Liu et al. (2000) have obtained the reaction rate expressions and the corresponding rate
constants. At higher conversions, the sulfation process is controlled by diffusion in the
product layer. There are more arguments for the type of diffusion control in the product
layer. Hajaligol et al. (1988), Krishnan & Sotirchos (1993), and Liu et al. (2000)
suggested gas phase diffusion mainly based on the fact that the product layer is porous.
Iisa et al. (1992), and Fuertes et al. (1994) indicated solid-state diffusion control. There
3

Juan chen et al.
are much more things about diffusion in the product layer that need to be known. In
addition, the influence of ash layer against the gas diffusion has not been considered I
the literature. The un-reacted limestone was assumed to remain as a core on the surface
of which the lime derived from its decomposition remains a layer limiting the diffusion
of both CO2 and SO2 out of and into the particle.
The mode developed by Szekely (1976) with the consideration of both kinetic and
diffusion was used, according to
2 / 3
1/
3 ⎧Z
−
( )
[ Z + 1 − Z 1 − X ]
2 / 3 ⎫
= K ∗ 1 − 1 − X + K ∗ 3
− ( 1 − X )
(10)
Where
[ ] ( )( )
t c
d
K
ρr
⎨
⎩
Z −1
0
c = (11)
K sC0
2
0
ρr
K d = (12)
6DeC 0
molar volume of CaSO 4 Z =
= 3.
09
(13)
molar volume
of CaO
X being the fractional conversion of CaO, Ks (m/s) the chemical rate contant, ρ
(mol/m 3 ) the molar density of limestone particles, r0 (m) the effective particle radius, the
average particle size of limestone is 21 µm in this <strong>study</strong>, C0 (mol/m 3 ) the molar density
of SO2, De (m 2 /s) the effective diffusivity through the product layer.
A coefficient σ was proposed as:
K K r
= =
K 6D
d s 0
σ (14)
c
e
When σ 10, it is
diffusion controlled, and when 0.1

Juan chen et al.
temperature-ashing in muffle furnace at 873 K) accounts for 0.9 wt% and 2.24 wt%,
respectively. The inorganic metals within the brown coal are dominated by organically
bound cations and ultra-fine mineral grains embedded firmly within the closed voids.
These species are presumably highly reactive upon the collision with limestone particle.
The volatile matter accounts for more than half of the dried coal, forming a thick cloud
flame in the oxidizing atmosphere (Zhang et al. 2010). Calcination of pure limestone
and the combustion of coal mixed with limestone were carried out in a lab-scale DTF as
shown in Fig.1. The furnace temperature was fixed at 1273 K. Pure limestone or coallimestone
mixture at a feeding rate of 0.5 g/min was entrained by 1 L/min primary gas
(cold) into the furnace. The pre-heated secondary gas of 9 L/min was also fed into the
reactor. Two major bulk gases, air and 27% O2/73% CO2, were tested, considering the
fact that they match each other in terms of flame propagation speed and burning coal
particle temperature (Zhang et al. 2010). Three mass ratios of CaCO3 to coal were tested,
18%, 5% and 1%. A SO2 content of 1000 ppmV doped into mixture gas, air or 27%
O2/73% CO2, were also tested. Each run lasted for ~30 min. Two reactor lengths of
1200 mm and 600 mm were tested, considering different particle residence times. Coal
combustion ash was quenched rapidly by dry ice at approximately 5000 o C/s before
passing through a collection system including a flask installed at the bottom of the
reactor and a thimble filter with a cut-off size of 0.5µm.
The un-reacted CaCO3 fraction in a solid product was quantified through the use of a
thermogravimetric analyzer (TGA). The elemental compositions were quantified using
X-ray fluorescence (XRF, Rigaku 2100), and the crystalline structures were determined
by X-ray Diffraction Spectrometer (Rigaku RINT XRD).
4. RESULTS AND DISCUSSIONS
4.1 Calcination of limestone
The modelling results of the
unreacted CaCO3 fraction under air
and 27% O2/73% CO2 at different
temperatures are showed in fig 2,
where the experimental results appear
as discrete points for comparison.
Obviously, for the pure CaCO3, its
unreacted extent after a 1200 mm
reactor in O2/CO2 still accounts for
55.5%, in comparison to an unreacted
fraction of 18.9% in air. This is a
direct reflection of the high CO2
partial pressure in the oxy-fuel
Unreacted CaCO3, %
900 1000 1100
T, K
1200 1300
atmosphere, which inhibited the right shift of reaction (1). Interestingly, mixing CaCO3
with coal at a mass ratio as high as 18% improved the limestone calcination extent in
O2/CO2 to the similar level with that obtained in air. The fraction of the unreacted
CaCO3 remaining either in air or in O2/CO2 case, is also considerably lower than that
observed for the pure limestone. As the ash content in coal are extremely lower than
CaCO3 addition percentage, and that SO2 does not participant in reaction (1), the
percentage of calcium reacted with sulfur was only about 5% shown in fig 3 (a).
Therefore, the sole explanation for these phenomena is the formation of coal volatile
100
80
60
40
20
0
Unreacted CaCO3, %
100
80
60
40
20
0
air
oxy-fuel
air
pure CaCO3 oxy-fuel
pure CaCO3 18% CaCO3
pure CaCO3
18% CaCO3 + Coal
+ Coal
900 1000 1100 1200 1300
T, K
pure CaCO3 18% CaCO3
18% CaCO3 + Coal
+ Coal
Fig. 2 Model predictions of limestone
calcination for air and oxy-fuel
conditions at different temperatures
5

Juan chen et al.
flame during coal combustion, which substantially increased the limestone particle
temperature to a level overweighing the inhibitory effect of CO2 partial pressure. As can
be seen in fig. 2, adding 18% CaCO3 to coal combusted in DTF, increased the limestone
particle temperature from 1183 K to 1223 K in O2/CO2 and 1194 K in air.
Unreacted CaCO3, %
20
15
10
5
0
Air
Oxy-fuel (a)
18% 5% 1%
Mass percentage of CaCO3 in coal
Unreacted CaCO3, %
20
15
10
5
0
Air+SO2
Oxy-fuel+SO2 (b)
18% 5% 1%
Mass percentage of CaCO3 in coal
Fig 3Variation of the fractions of unreacted limestone with the addition of limestone
The mass ratio of limestone to ash in coal and the concentration of SO2 in flue gas were
also confirmed playing noticeable role on the extent of limestone calcination. As
demonstrated in fig. 3, in either pure gas, air versus 27% O2/73% CO2 at a reactor
length of 1200 mm in panel (a), with the decrease in the addition percentage of
limestone, i.e. increase in the mass ratio of coal ash/sulfur to limestone, the fraction of
unreacted CaCO3 was decreased drastically. If the reactions (1) and (2) shown above
are the single routes for limestone calcination and sulfation, the presence of coal ash and
sulfur in the furnace should not affect the extent of the right shift of reaction (1),
because they do not participant in this reaction. The SO2 emitted from coal-bound
sulphur is also insignificant, as the sulphur in coal only accounts for 0.9 wt%. It is thus
referrable that ash (2.24% content) plays the main role on the calcination extent of
limestone. This clearly indicates the interaction between limestone and coal ash,
according to the following reaction (16) and (17), respectively.
CaO+ Al O ⋅ SiO →CaO⋅
Al O ⋅ SiO
(16)
2
3
2
CaCO 3 + Al2O3
⋅ SiO2
→ CaO ⋅ Al2O3
⋅ SiO2
+ CO2
(17)
The Ca-Al-Si formed could be shed away from limestone surface, which in turn ensured
the exposure of fresh surface of limestone to undergo decomposition and react with SO2.
The direction reaction (17) should occur slowly in O2/CO2 with high CO2 partial
pressure when compared with the interaction (16) between calcium oxide (derived from
limestone calcination) and coal ash which is predominant in air (Zhang et al, 2002), as
indicated by the constantly higher unreacted CaCO3 fraction over the mass ratio of
limestone to coal in O2/CO2 than in air. Moreover, as suggested by the results for adding
1000 ppmV SO2 in fig. 3(b), it is clear that the un-reacted limestone fraction was
reduced quickly with the addition of SO2 compared to that without SO2 addition in
panel (a). The limestone calcination extent in O2/CO2 was even increased to the similar
level with that obtained in air. This is a clear reflection of the importance of the direct
sulfation reaction (3) at high SO2 concentration in an oxy-fuel boiler.
2
3
2
6

4.2 sulfation of limestone
Juan chen et al.
Fig. 4 shows the sulfation extent of limestone in different cases and the corresponding
modelling results for comparison.
Fraction of Ca-S, %
80
60
40
20
0
Modeling
Air
Oxy-fuel
18% 5% 1%
Mass percentage of CaCO3 in coal
18% 5% 1%
Mass percentage of CaCO3 in coal
(a)
(b)
Fig 4 Variation of the fractions of Ca-S compounds with limestone addition
Fraction of Ca-S, %
80
60
40
20
0
Modeling
Air+SO2
Oxy-fuel+SO2
As can be seen, in panel (a) for the two pure gases, air versus O2/CO2, the modelling
fractional conversions are identical, which is far higher than the experimental data. It
clearly indicates the influence of ash which reacted with limestone on the surface of
limestone/lime particles and increased the diffusion resistance of SO2. The slowest Ca-S
extent for the 18 wt% limestone addition to coal can be attributed to the insufficience of
SO2 emitted from coal-bound sulfur. Decreasing the mass percentage of limestone to
1% increased the molar ratio of ash/sulfur to calcium, thus resulting in the sulfation of
about 35% of limestone in air, in comparison to a slow increase for the fraction of Ca-S
to about 15% in O2/CO2. Combined with the limestone decomposition in fig. 3(a), the
un-reacted CaCO3 also had a slow decrease in O2/CO2 from 18% to 1% mass percentage
of limestone. This evidences that the
surface of limestone is blocked by ashcalcium
products. The Ca-Al/Si
50
compounds have low melting points and
Air
are easily molten in reducing atmosphere
40
because of the char-CO2 gasification
30
Air+SO2
reaction in oxy-fuel condition with high
Oxy-fuel
concentration of CO2 (Rathnam et al.
20
2009) , forming a eutectic film on the
Oxy-fuel+SO2
limestone surface which increase the gas
10
diffusion resistance.
The influences of ash and SO2 increase on
the Ca-S extent were also intriguingly
observed and demonstrated in the panel (b)
of fig. 4. Adding 1000 ppm SO2 into flue
gas greatly increased the sulfation extent
of calcium. Especially in O2/CO2 with 1%
limestone addition, sulfation extent got to
the similar level with that in air. 1000ppm SO2 increased the sulfur content to 3.76%,
Fraction of Ca-Si/Al, %
0
Fig 5 Variation of the fractions of Ca-
Si/Al compounds with 1000ppmV SO2
1% limestone addition in 1200 mm
reactor
7

Juan chen et al.
higher than the ash content, thus the gas-solid reaction with SO2 became the dominant
reaction, which compensated the affect of ash. As can be seen in fig. 5, the fractions of
Ca-Si/Al compounds were decreased with SO2 addition in both air and O2/CO2
conditions. The lower sulfation extent in O2/CO2 than in air was further confirmed.
Shaddix and Molina (2007) reported that O2 has a lower diffusion rate (about 20% less)
in CO2 layer than in N2. This phenomenon is explained by the lower SO2 diffusivity in
O2/CO2 than in air affecting the transport of SO2 to the surface of the limestone particle.
Fig. 6 depicts the changes to the fractions of
unreacted limestone and Ca-S with the
addition of 1000 ppmV SO2 to air and
O2/CO2 conditions. For the addition 18%
limestone to coal in O2/CO2 case, the
reduction on its unreacted fraction is nearly
equal to the increase in the percentage of
Ca-S, thus proving the sole significance of
the direction sulfation reaction (3) for a
high Ca/S molar ratio. For another two low
limestone addition percentages, in air
condition the unreacted fraction upon SO2
addition is not changed, but the Ca-S
fraction is still increased. For O2/CO2, the
reduction in the CaCO3 fraction is far lower
than the increase in the fraction of Ca-S,
particularly for the case of 1% limestone added to coal, indicative of Ca-S not only from
direct sulfation reaction (3), but also binding with sulfur from other approaches. Apart
from the increase in the S/Ca molar ratio, the interaction of coal ash with limestone via
reaction (16) and (17) may also be the cause for this phenomenon. The formed Ca-Al-Si
could be shed away from limestone surface, ensured the exposure of fresh surface of
limestone to undergo decomposition and react with SO2. Besides that, our previous
results (Zhang et al. 2002) confirmed that the added limestone reacted with
aluminosilicate in the ash and fixed into calcium aluminosilicate, rich in calcium, can
continue to react with gaseous SO2 to remove it.
5. CONCLUSIONS
18% 5% 1%
mass percentage of CaCO3 in coal
Mechanisms for calcination and sulfation of limestone added at the mass ratios of 18%,
5% and 1% to a brown coal during air and O2/CO2 mixture (27/73) in a lab-scale DTF
have been investigated. The major conclusions are drawn as follows: (1) The high CO2
partial pressure in an oxy-fuel furnace played a negative role on calcinations of
limestone. However, increase in the local temperature of limestone particles through the
radiative heat from coal flame favoured the decomposition of limestone to a level
similar with that achieved in air in a nominal gas residence time of approximately 4 s. (2)
The ash-forming metals in coal played an important role on the calcination and sulfation
of limestone during oxy-fuel combustion. The direct interaction between ash-forming
metals particularly aluminium and silicon and limestone was more favored in the CO2dominant
gas atmosphere than in air, which in turn promoted the decomposition of
limestone. The Ca-Al/Si formed could be partially shed away from limestone surface to
ensure the exposure of fresh surface to continue decomposition and sulfation. However,
Reduction/Increase in fraction, %
24
18
12
6
0
-6
-12
-18
-24
reduction in unreacted CaCO3(Air), %
increase in Ca-S(Air), %
reduction in unreacted CaCO3(O2/CO2), %
increase in Ca-S(O2/CO2), %
Fig.6 Changes to the fractions of unreacted
CaCO3 and CaSO4 upon SO2
addition to air and O2/CO2
8

Juan chen et al.
with the decrease on the limestone to ash mass ratio, the Ca-Al/Si compounds formed
are seemingly easily molten, which in turn blocked the internal pores of particles and
resulted in an resistant layer hindering the diffusion of SO2 and CO2 (derived from
limestone decomposition) into and out of limestone particle. (3) The direct sulfation of
limestone was only pronounced when the extra SO2 was introduced into flue gas and
excess Ca was present in flue gas. Otherwise, the interaction between limestone/lime
and ash will be more influential. For the highest Ca/S molar ratio of 16 tested here, the
direct sulfation accounted for approximately 6-15% of the total sulphur captured, which
eliminated the discrepancy between oxy-fuel and air combustion in which the
limestone-ash interaction is less intense and the indirect sulfation is must fast and
favored by the rapid decomposition of limestone even at a relatively low temperature.
ACKNOWLEDGEMENTS
This <strong>study</strong> is supported by the Australian Research Council (ARC) under a Future
Fellowship Grant of FT0991010. The assistance from Mr Facun Jiao and Prof
Yoshihiko Ninomiya (Chubu University, Japan) on the XRF and XRD analysis are also
gratefully acknowledged.
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NOMENCLATURE
ACaCO3 Specific BET surface area of calcium carbonate (m 2 g -1 )
Pe Equilibrium CO2 pressure for CaCO3 decomposition (atm, 0.1 MPa)
T Temperature (K)
Rate Decomposition rate (mol s -1 )
ks, ks’ Decomposition rate constants (mol m 2 s -1 )
P CO2 partial pressure (atm, 0.1 MPa)
R Molar gas constant, 8.314 JK -1 mol -1
Sg The BET-surface area of limestone (m 2 /g)
MCaCO3 The molecular weight of CaCO3 (g/mol)
t Time(s)
x The fractional conversion of CaCO3 decomposition
X The fractional conversion of CaO to CaSO4
Ks The chemical rate constant (m/s)
ρ The molar density of limestone particles (mol/m 3 )
r0 The effective particle radius (m)
C0 The molar density of SO2 (mol/m 3 )
De The effective diffusivity through the product layer (m 2 /s)
Z The ratio of CaSO4 molar volume to CaO molar volume
10