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Modelling for Flash Calcination and Surface Area Development of Dispersed Limestone Particles.

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Dev. Chem. Eng. Mineral Process., 8(3/4),pp.233-243, 2000.
Modelling for Flash Calcination and Surface
Area Development of Dispersed Limestone
Particles
2. Ying, 2. Chuguang*, L. Zhaohui and S. Xuefeng
National Laboratory of Coal Combustion, Huuzhong University of
Science and Technology, Wuhun 430074, ?? R. CHINA
~
~
~
~~
~~
A mathematical model for calcination, sintering and surface area development of
dispersed limestone parricles is presented in the papel: The model describes the
calcination by considering heat transfel; mass transfer of CO1 through a porous CaO
layer, and the chemical kinetics. Difision of CO, in a porous product layer is
discussed in detail. The extent of calcination, BET surface area, temperature and
pressure distribution in a particle were calculated. The extent of calcination and BET
surface area, obtained from the simulation, are consistent with experimental data.
Introduction
The emission of sulfur dioxide from coal fired boilers is generally viewed as a major
contributor to acid rain and as a consequence it is important to reduce the amount of
SOl released.
Dry sorbent injection into pulverized coal boilers is a simple
technology with relatively low cost, and it is especially suitable for the retrofit of
existing power plant. Dry sorbent injection into pulverized coal boilers occurs at high
temperatures (11OO-15OO K).
The sorbent initially calcinates before undergoing
sulfation, the reactions can be expressed as follows.
* Authorfor correspondence.
~
~
233
Z. Ying, Z Chuguang, L.Zhaohui and S Xuefeng
CaCO,(s) + CaO(s)+ CO,(g)
+
CaCO, MgCO, (s) + CaO MgO(s) 2C0,(g)
cuo(s)+sO,(g)++o,(g)
+ CuSO,(s)
(3)
Three potential rate-limiting processes are involved in the calcination process of
limestone. (1) Heat transfer to the surface of the particle and then through the porous
CaO product layer to the reaction interface. (2) Chemical reaction at the interface.
(3) Mass transfer of the COz released at the reaction zone away from the interface
through the product layer. Many investigators have studied experimentally the
calcination and sintering processes of dispersed limestone particles, and they have
developed some mathematical models to describe the processes.
Borgwardt [1J found that the decomposition reaction was kinetically controlled
except for the final stage of decomposition where the diffusion of C02 through the
product layer was rate limiting. Therefore, he suggested the calcination rate was
proportional to the BET surface area of unreacted CaC03.
Silcox et al. [2] developed a calcination model where the diffusion of evolved COz
was incorporated. One conclusion from this study was that the escape of C02 does
not significantly slow down the calcination reaction rate for small particles. Keener
and Kuang [3] proposed a structural pore-development model for calcination
assuming that the particle temperature was uniform and the effect of sintering could
be omitted. The model was correct at lower temperatures, however the difference
between the model predictions and experimental data increases significantly at higher
calcination temperatures.
Milne et al. [43 proposed an experimentally modified shrinking core model after
analyzing previous experimental data obtained from an entrained flow reactor. They
proposed that the decomposition process occurred in a complex reaction zone instead
of only a distinct reaction interface. A shrinking core model with a dependence on
particle size to the 0.6 power was shown to best describe the experimental data.
Hu and Scaroni [5] developed a model considering heat and mass transfer and
chemical kinetics for calcination of pulverized limestone particles under furnace
injection. It was sufficient to describe the calcination process, but the treatment of
234
Modelling for Flash Calcinationof Dispersed Limestone Particles
sintering and C02 diffusion in the product layer needed further improvement.
The objective of this study was to develop a model considering heat transfer, mass
transfer and chemical reaction in order to better simulate the calcination and sintering
processes. The modelling effort was focused on describing COz diffusion in the
product layer in detail, and surface area development during calcination and sintering.
Model Description
I Thermal Decomposition
The thermal decomposition of CaCO3 is a reversible reaction and has an equilibrium
decomposition pressure of COz, which was established from the work of Hills as:
Pe = 1.826~10~
exp(-19680/T)
(4)
The reaction rate is expressed as:
Rate = -k, .Acnco3
ACoCO3
= S g 'Ncac03 mi'4cac03
The effect on the reaction rate of the partial pressure of the C02 gaseous product
surrounding the solid is obvious, and there have been studies by different
investigators [6,7]. However, general agreement has not yet been reached. We have
used the same formula as Hu and Scaroni [5]:
where kf' is the rate equation established by Borgwardt [I] in a calcination experiment
on small dispersed limestone particles, and is as follows:
k,' = 3 . 6lo7
~ exp(-205000/ R T )
(8)
Thus, the conversion fraction (x) of CaC03 to CaO can be expressed by:
ln(1- x) = -k, .S, Mcaco3.t
(9)
235
2.Ying, 2 Chuguang, L Utaohui and S Xuefeng
I1 Heat 'kanfer
For a spherical particle, the unsteady-state heat transfer equation is:
Initial condition:
T = To
t=O
(aT/ar) = o
Boundary conditions: r = 0
r = ro
where h is the convective heat transfer coefficient (W/m2 K):
h=Nu*LId,
(11)
1
1
Nu = 2.0 + 0.69Re' PrT
(12)
where k is the thermal conductivity of a limestoneparticle. For uncalcined limestone,
k is 1.64 Wim K. For fresh calcines, k is 0.084 W/m K. For partially calcined
limestone, k is calculated using the following equation:
k = 1/((1- x)/k, + X/k2)
(13)
For fully calcined and partially sintered limestone:
k = 0.69 - 6.07X
S
(14)
I11 Mass Zhnsfer
The unsteady-state C02 diffusion equation is:
[
i a
7%
r De
$1
aP
at
= -- RTG,
t=O
Initial condition:
Boundary conditions: r = 0
p=o
aplar = 0
r = ro
- De (ap/ar) = hd (P - P b
where h,, is the convective mass transfer coefficient ( d s ) :
h, = Sh*D l d ,
(16)
1
1
Sh = 2.0+0.6Re2Sc3
236
(17)
Modelling for Flash Calcination of Dispersed Limestone Particles
where D,is the effective diffusion coefficient (m2/s), including the ordinary diffusion
given by:
coefficient (DAB)and Knudsen diffusion coefficient (DK)
Tt
D, =435.7
(18)
'-i
D,=-d
3
2RT
M C O 2
&
D,= -(l/DM + 1/D, )-'
z
-
and d is mean pore diameter:
2 = 4~/(S,p(l- &))
(21)
N Sintering Model and Su$me Area Development
German and Munir [8] developed a sintering model based on a sintering mechanism
proposed for the isothermal sintering of AI2O3,ZnO, Fez03 and TiOz. The model is:
((So -S)/Soy =k, * t
(22)
Borgwardt [9]studied experimentally the sintering process of fresh CaO under
H20 and COz atmospheres and obtained the empirical correlation of sintering rate
constant k, and the exponent "I:
In k, = 1.485+ 0.558 In pco, - 1 16601T
(23)
In r = 0.0034T + (In pco, - 1.948)/ 44.9
(24)
When the fresh CaO is obtained at t= t,which has a BET surface area of So, CaO
sinters at T for a certain shell, its BET surface area decreases to S"*at t = t, :
A fundamental assumption of the sintering model is that A&is proportional to AS:
E2
= El
- (&I /s, XSl - sz )
and S=S,p(I-E)
(26)
(27)
237
Z.Ying, Z Chuguang, L. Zhaohui and S Xuefeng
Therefore, the porosity of the shell (So) at t = t, is E""
Therefore, the surface area and porosity of the shell at t = t,, are
s a oand E a o :
k=l
(29)
Results and Discussion
A. Model Verification
Model predictions are compared with experimental data for different sorbents and
particle size fractions in Figure 1. For the different sorbents and particle sizes
calcinated, the model fits the experimental data well at all temperatures. As the
model incorporates only fundamental physical and chemical processes during heating
and calcination, it can be concluded that it can adequately describe the mechanism of
calcination under experimental conditions.
238
Modelling for Flash Calcination of Dispersed Limestone Particles
lOum 1198K
0
1198K 1Oum experimental
data(reference 1)
6 3 m 1473K
.-.-.-A
1473K 63um experimental
data (reference 5 )
- - - - -91.17m
o
0.1
0.2
0.3
0.4
0.5
0.6
1473K
1473K 91.17um t h i s study
ClS
Figure 1. Comparison between m d e l predictions and experimental data
for the calcination of dispersed limestone particles.
B. Calculation Results and LXwussion
After the particle is injected into the boiler furnace its temperature increases rapidly.
However, the particle temperature is lower than the gas temperature during
decomposition. A large inner temperature gradient in the particle exists because
calcination is an endothermic reaction and the thermal conductivity of porous CaO is
relatively poor.
Figure 2 shows particle surface temperature as a function of residence time and
particle diameter for different particle diameters injected into 1473 K nitrogen. It is
predicted that the heat transfer from entrained gas to particle surface is very rapid for
small particles, and the surface temperature is nearly constant during the course of
calcination. Actually, the temperature distribution is also uniform. So, it is clear that
the decomposition rate is controlled by chemical kinetics. However, the surface
temperature is much lower than the gas temperature during calcination for larger
particle diameters (91.17 pm, 152.33 pm).
Figure 3 shows the calculated temperature distribution as a function of residence
time for 91.17 pm particles injected into 1473 K nitrogen. It is obvious that there is a
large temperature gradient in particle. This temperature gradient will cause a large
difference in the extent of calcination within the particle because the decomposition
rate is exponentially dependent on temperature.
239
Z. Ying, Z Chuguang, L Zhaohui and S Xuefeng
1500
I
1400
t4 l300
H
\
llo0
I200
no0
loo0
1000
900
1
,
r / r O = O .4
,zr/rO,=O.6
r/rO=O.8
.
I
r/rO=l 0
0
0.1
0.2
0.3
0.4
0
0.5
0.1
t/s
Figure 2. Surface temperature as a
function of residence timefor direrent
size limestone particles injected into
1473 K nitrogen.
0.2
0.3
0.4
0.5
t/s
Figure 3. Temperature distribution within
the particle as a function of residence
time for 91.17pn limestone particles
injected into 1473 K nitrogen.
CaCO3 and MgC03 decompose rapidly during limestone calcination at high
temperatures, and a large amount of COz is released. Figure 4 shows the distribution
of COzpartial pressure within the particle as a function of residence time for 9 1. I7pn
particles injected into 1473 K nitrogen. In the initial stages of calcination, the extent
of calcination is small, and the pore structure of the product layer does not develop
completely. The porosity and C02 diffusion coefficient are thus small so that C02
cannot diffuse across the product layer into surrounding atmosphere in time. The
pressure of COz within the particle increases. Meanwhile, the maximum C02pressure
increases as the particle diameter increases. Then, the partial pressure of COz
decreases slowly because the extent of calcination increases and porosity of the
product layer increases during the calcination process.
The temperature and pressure distribution in the particle will significiantly effect
the rate of calcination of limestone, furthermore effecting the conversion rate of
CaC03 to CaO. Figure 5 shows the extent of calcination within the particle as a
function of residence time for 91.17 pm limestone particles injected into 1473 K
nitrogen. It is predicted that there exists a big difference between the particle
periphery and center in the extent of calcination. The extent of calcination at the
particle’s center is lower than that at the particle’s periphery. Therefore, a model that
omits heat and mass transfer within the particle will not predict the calcination
process of larger limestone particles adequately.
240
Modelling for Flash Calcination of Dispersed Limestone Particles
3r/rO=O.6
0.6
------c---r/ro=o. 8
r/rO=O
0.4
0.2
2
0
0
0.1
0.2
0.3
0.2
0
0.4
Figure 4. CO,partial pressure distribution
within the partkle as a function of
residence timefor 91.17 pm limestone
particles injected into 1473 K nitrogen.
tn
0.4
0.6
t/s
t/s
Figure 5. Extent of calcination as a
function of residence timefor
91.17 pm limestone particles
injected into 1473 K nitrogen.
30
13.65um
Y
0
0
0.2
0.4
t/s
0.6
Figure 6. BET surface area as
a function of residence time
for 13.65 pm limestone particle.
0
0.2
0.4
0.6
t/s
Figure 7. BET surface area as a function
of residence time for limestone particle
of different diameters (T=1373K).
In Borgwardt’s sintering experiment [9],the BET surface area of fresh CaO can be
as high as 104 m2/g. Unfortunately, the fresh CaO looses most of its surface area due
to high temperature sintering. Figure 6 shows BET surface area as a function of
residence time for 13.65 j.un limestone particles calcined at different temperatures. It
predicts that the maximum BET surface area is considerably lower than 104 m2/g, and
does not occur at the maximum extent of calcination. Figure 7 shows BET surface
area as a function of residence time for limestone particles of different diameter
calcined at 1373 K. It predicts that the maximum BET surface area decreases as the
particle diameter decreases, and it will take longer for it to reach the maximum BET
surface area. The BET surface area is clearly correlated with calcination rate.
241
Z. Ying, Z Chuguang, L Zhaohui and S Xuefeng
Conclusions
A model including heat transfer, mass transfer and chemical kinetics is developed in
this paper for limestone calcination and particle surface area development. For
calcination of dispersed limestone particles, these processes all exert a significant
influence on the calcination rate especially for larger limestone particles. There are
large temperahue and COz partial pressure gradients within the particle. Therefore
the extent of calcination in the particle is also significantly different, and a shrinking
core model and other simple models cannot adequately simulate the dispersed
limestone calcination. The BET surface area is closely correlated with the conversion
rate. It is significantly lower than the BET surface area of the fresh CaO.
Nomenclature
b C 0 3
k/?
k
hfCaC03
R
Rate
s,
sm
T
T
X
Surface area of CaC03
Mass diffksivity
D
dP
Particle diameter
Decompositionrate constants
Molecular weight of CaCO3
Gas constant
Decompositionrate
Specific BET surface area of CaC03
BET surface area
Time
Temperature
Extent of calcination
Greek letters
h
Thermal conductivity of N2
z
Tortuosity factor
&
Porosity
P*
True density
References
1.
Borgwatd~R.H. Calculation hetics and surface area of dispersed limestone particles. AIChE J.,
2.
Silcox, G.D.,Kramlich, J.C.,and Pemhing, D.W. Mathematkd model for the flash calcination of
dispersed CaCO, and Ca(Om particles. lnd. Eng. Chem. Res., 1989.28: 155-160.
1985.31: 103-111.
242
Modellingfor Flash Calcination of Dispersed Limestone Particles
3.
4.
Keener, S., and Kuang, S.J. Structural pore development model for calcination. Chem. Eng.
COIXUIIU~.. 1992,117: 279-291.
M h e , C.R., Silcox, G.D., and Pershing. D.W. Calcination and sintering models for application to
high-temperature, short-time sulfation of calcium-based sorbents. Ind. Big. Chem. Res., 1990, 29:
139-149.
5.
6.
7.
8.
Hu, N., and
Scaroni, A.W. Calcination of pulvdzed limestone particles under fumace injection
conditions. Fuel, 19%. 75: 177-186.
Wang, Y., and Thoms0n.W.J. The effect of steam and carbon dioxide on calcite decomposition using
dynamic X-raydiffraction. Chem. Eng. Sci., 1995.50: 1373-1382.
Khinast, J., Knunma, G.F., Brunner, C., and Staudinger, G. Decomposition of limestone the
influence of C& and particle size on the reaction rate. Chem. Eng. Sci., 1996.51: 623-634.
German, R.M., and Munir, Z.A. Surface area reduction during isothermal sintaing. J. Am. Ceram.
SOC.,1976.59: 379-383.
9.
Borgwardt, R.H. Calcium oxide sinering in atmospheres containing water and carbon dioxide. Ind.
Eng. Chem. Res., 1989.28: 493-500.
243
244
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