Reaction kinetics of CO2 carbonation with Mg-rich minerals

Article abstract of DOI:10.1021/jp2040899

Due to their low price, wide availability, and stability of the resulting carbonates, Mg-rich minerals are promising materials for carbonating CO ₂. Direct carbonation of CO₂ with Mg-rich minerals reported in this research for the first time could be considerably superior to conventional liquid extraction processes from an energy consumption perspective due to its avoidance of the use of a large amount of water with high specific heat capacity and latent heat of vaporization. Kinetic models of the reactions of the direct CO₂ carbonation with Mg-rich minerals and within simulated flue gas environments are important to the scale-up of reactor designs. Unfortunately, such models have not been made available thus far. This research was initiated to fill that gap. Magnesium silicate (Mg ₂SiO₄), a representative compound in Mg-rich minerals, was used to study CO₂ carbonation reaction kinetics under given simulated flue gas conditions. It was found that the chosen sorbent deactivation model fits well the experimental data collected under given conditions. A reaction order of 1 with respect to CO₂ is obtained from experimental data. The Arrhenius form of CO₂ carbonation with Mg ₂SiO₄ is established based on changes in the rate constants of the chosen deactivation model as a function of temperature.

Full text of DOI:10.1021/jp2040899

ARTICLE
pubs.acs.org/JPCA
Reaction Kinetics of CO Carbonation with Mg-Rich Minerals
2
†
,‡,†
§
†
||
Soonchul Kwon, Maohong Fan,* Herbert F. M. Dacosta, Armistead G. Russell, and Costas Tsouris
†
Georgia Institute of Technology, Atlanta, Georgia 30332, United States
‡
University of Wyoming, Laramie, Wyoming 82071, United States
§
Chem-Innovations, P.O. Box 3665, Peoria, Illinois 61612, United States
Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831, United States
ABSTRACT: Due to their low price, wide availability, and stability of the resulting
carbonates, Mg-rich minerals are promising materials for carbonating CO . Direct
2
carbonation of CO with Mg-rich minerals reported in this research for the first time
2
could be considerably superior to conventional liquid extraction processes from an
energy consumption perspective due to its avoidance of the use of a large amount of
water with high specific heat capacity and latent heat of vaporization. Kinetic models of
the reactions of the direct CO carbonation with Mg-rich minerals and within simulated flue gas environments are important to the
2
scale-up of reactor designs. Unfortunately, such models have not been made available thus far. This research was initiated to fill that
gap. Magnesium silicate (Mg SiO ), a representative compound in Mg-rich minerals, was used to study CO carbonation reaction
2
4
2
kinetics under given simulated flue gas conditions. It was found that the chosen sorbent deactivation model fits well the experimental
data collected under given conditions. A reaction order of 1 with respect to CO is obtained from experimental data. The Arrhenius
2
form of CO carbonation with Mg SiO is established based on changes in the rate constants of the chosen deactivation model as a
2
2
4
function of temperature.
’
INTRODUCTION
sorbents whose CO sorption capacities are not negatively
2
affected due to the presence of water vapor.
Because of their low price, wide availability, and the fact that
The concentration of CO in the atmosphere has been
2
continuously increasing for many years due to the emissions of
1,2
their CO adsorption capacity and rate increase in the presence
2
CO from many sources. About 40% of the total emissions of
2
of H O, magnesium-rich (Mg-rich) minerals are promising
2
CO in the U.S. are generated by power plants that use fossil fuels
2
19ꢀ21
candidates for CO separation.
Due to the fact that CO2
2
including coal, natural gas, and oil as their major energy sources.
In the recent years, various technologies have been developed to
is captured as a stable carbonate, mineral-based CO removal
2
processes can minimize the risk of CO leakage compared to
3
ꢀ13
2
capture CO2.
capture technologies are too high to be widely accepted. Without
an economically viable CO capture technology, carbon capture
The costs associated with the current CO2
other methods, thereby providing the potential for long-term
CO sequestration. Moreover, solid byproducts from the CO
2
2
2
mineralization process have many potential applications, includ-
14
21
and storage (CCS) cannot be justified. CO capture by
ing soil remediation. However, even though their associated
2
adsorption does not appear to be thermodynamically limited
reactions are thermodynamically favorable, natural Mg-rich,
and, because of the possibility of high CO selectivity and low
energy demand, adsorption appears to be one of the more
promising options among the various technologies proposed for
mineral-based carbonation processes are slow because of the
2
1
5
19,21,22
low surface area available.
Therefore, researchers have
shown increasing interest in improving the kinetics of mineral-
based, Mg-rich CO carbonation methods. One approach is to
capturing CO from power plant flue gases. Consequently, the
2
2
use an aqueous extraction process under elevated pressure and
temperature that can improve the kinetics of reaction between
Mg SiO and CO . However, because it requires either mechan-
development of economical and effective adsorbents is desirable.
Solid sorbents are an attractive choice because they are easy to
handle, are not associated with a risk of releasing solvents in the
environment, have low deployment costs, and cause negligible
corrosion problems. Further, they cause lower environmental
2
4
2
ical or chemical pretreatment of Mg-rich minerals, this process is
indirect and overly time-consuming.
By contrast, not only might the use of Mg-rich minerals for
1
6,17
impacts.
the direct carbonation of CO in flue gases overcome certain
2
Flue gases from power plants typically contain 8ꢀ12% (vol)
shortcomings of an aqueous extraction process, it might also
help to identify methods for improved rates of capture.
However, to the authors’ knowledge, studies on kinetic models
CO and 8ꢀ10% steam (H O). Due to the tendency of H O
2
2
2
molecules to also be adsorbed by the sorbent, many sorbents lose
18
their CO adsorption capacities in the presence of water vapor.
2
Accordingly, sorbents with high adsorption capacities for CO in
2
Received: May 3, 2011
the presence of water are desirable for their practical applications
in power plants. Therefore, it is important to identify solid
Revised:
Published: May 31, 2011
May 28, 2011
r 2011 American Chemical Society
7638
dx.doi.org/10.1021/jp2040899
|J. Phys. Chem. A 2011, 115, 7638–7644

The Journal of Physical Chemistry A
ARTICLE
2 4 2
Figure 1. Schematic diagram of experimental setup for Mg SiO -based CO sorption.
of the reactions between Mg-rich minerals and gaseous
Matheson Trigas FM-1050 flowmeters. An additional flowmeter
was used to determine if the flow of the inlet gas analyzer was
CO within a simulated flue gas environment, including rate
2
equation and Arrhenius form, have not yet been reported.
Thus, the development of kinetic models for the early application
normal during all CO sorption operations.
2
Adsorption of CO with Mg SiO was carried out in a
2
2
4
of Mg-rich CO carbonation in coal-fired power plants is
quartz tubular reactor with a length of 610 mm and inside
diameter of 9 mm. The fixed sorbent bed in the reactor was
formed by placing the Mg SiO particles between two pieces
2
desirable.
Pure magnesium silicate (Mg SiO ) was selected in this
2
4
2
4
research to study the kinetics of the reaction between Mg-rich
of quartz wool to serve as bed holders. The quartz reactor
was placed inside a TF55030A-1 tube furnace (Thermo Cor-
poration, Asheville, NC), with a UT150 temperature controller
minerals and CO within a simulated flue gas environment. The
2
overall reaction between Mg SiO and H O is
2
4
2
(
Yokogawa M&C Corp., Newnan, GA) to control the CO2
H2O
1
1
sorption temperature. The CO sorption unit was connected to a
steam generation unit to introduce water into the simulated
2
=2
Mg SiO4 þ CO2
s
f
MgCO þ = SiO2
ðR1Þ
2
r
s
3
2
dry flue gas stream containing CO and N . In order to avoid
2
2
Mg SiO is chosen as a model CO sorbent because it represents
2
4
2
steam condensation prior to the flow of the simulated wet
flue gas into the sorbent bed, thermotape was used to wrap the
pipelines between the vapor injection point and the inlet
gas-reactor connection, with temperature controllers employed
to control the heating rates of the thermotape. The outlet gas
from the tubular reactor was passed through a steam removal unit
before entering the ZRE gas analyzer (California Analytical
major Mg-rich minerals existing in nature (e.g., olivine and
serpentine) and because the use of a single compound simplifies
the kinetic study.
’
EXPERIMENTAL SECTION
Materials. Mg SiO was supplied by Alfa Aesar and used
2
4
without further treatment. Mg SiO4 is in the form of a
Instruments, Inc.), where the CO
2
concentration was measured.
2
white powder with 99% purity, a median particle size of 3.5
The CO concentration in the simulated dry flue gas stream was
2
3
μm, and a bulk density of 3.21 g/cm . The X-ray diffraction
also measured before each sorption test started. A data acquisi-
tion system was connected directly to the gas analyzer in order to
(
XRD) patterns of the Mg SiO4 both before and after
2
carbonation were analyzed using an X’PERT model X-ray
diffractometer (Philips), with Cu KR as the radiation resource,
and with scans performed in the 2θ range from 5° to 95° with
continuously record measured CO
Operating Procedures. The CO
Mg SiO under a given condition was determined based on
changes in measured CO concentrations after adsorption. Each
test run was conducted with fresh Mg SiO . First, 0.50 g of
Mg SiO was lightly packed into the tubular reactor, which was
2
concentrations.
adsorption capacity of
2
2
4
0
.02°/s.
2
Experimental Apparatus. The schematic of the experimental
2
4
setup for CO capture (Figure 1) consists of three parts: a flue
2
2
4
gas simulation unit, a CO sorption system, and CO analysis
preheated for 10 min to ensure constant operating conditions at
the desired reaction temperature. The tubular reactor was then
connected to the gas supply unit and the gas analyzer. At the
same time, the data acquisition unit was turned on. The
2
2
equipment. Gas cylinders containing pure N and a mixture of
2
CO and N with a flow rate of 0.5 L/s were used to conduct all
2
2
tests. Flow rates of the feed gases were manually controlled with
7
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The Journal of Physical Chemistry A
ARTICLE
Figure 3. Total and breakthrough CO
2
sorption capacities of Mg
2
SiO
4
at different temperatures [(inlet gas conditions, H
2 2
O, 4.1 mmol/L; CO ,
4
.1 mmol/L; total gas flow rate, 0.5 L/min); weight of Mg SiO , 0.5 g].
2 4
Figure 2. XRD patterns of spent Mg
.1 mmol/L; CO , 4.1 mmol/L; total gas flow rate, 0.5 L/min); weight
of Mg SiO , sorption temperature, 150 °C].
2 4 2
SiO [(inlet gas conditions, H O,
4
2
2
4
A good sorbent should, therefore, have not only a high total
sorption capacity but also a high breakthrough capacity. High
breakthrough capacity is determined by inherent characteristics
of the sorbent, including molecular structure, pore structure,
surface defect or active site, BET (Brunauer, Emmett and Teller)
surface area, and sorption operation conditions. Since it is
directly related to the reaction kinetics of (R1), temperature is
expected to be one of the more important operational factors
affecting the breakthrough capacity of Mg SiO . The total and
composition of effluent gas from the steam removal unit was
measured immediately by the ZRE gas analyzer. When the CO2
concentration in the effluent gas stream was within 1% of the
initial inlet gas-stream concentration (indicating that the
Mg SiO particle surfaces were saturated with CO ), the flow
2
4
2
of simulated flue gas into the reactor was stopped. Each reported
data point in this study represents the average value of three tests
under the same operating conditions.
2
4
breakthrough CO sorption capacities of Mg SiO at 150, 175,
2
2
4
and 200 °C (Figure 3) show that, compared to other solid
sorbents, Mg SiO has good CO sorption performance at the
2
4
2
25ꢀ31
three temperatures.
While the total sorption capacity of
’
RESULTS AND DISCUSSION
Mg SiO decreases with an increase in temperature, the break-
2 4
through capacity does not change considerably (Figure 3). Our
tests showed that the breakthrough sorption capacity achieved at
Determination of the Temperature Range for Kinetic
Study. Thermodynamic calculations have shown that the Gibbs
free energy changes of (R1) are ꢀ32.95, ꢀ20.88, and ꢀ3.88 kJ/
mol at temperatures of 25, 100, and 200 °C, respectively,
indicating that Mg SiO -based CO mineralization should occur
1
50 °C, when CO separation efficiencies were close to 100%,
2
was lower than that obtained at 200 °C. Further, the ratio of
breakthrough sorption capacity to the total sorption capacity
increased with temperature under the given experimental con-
2
4
2
in the given temperature range, which is supported by the results
Figures 2 and 3). CO adsorption occurred on the surface of
ditions (Figure 3) and was higher than those of most of the
(
31ꢀ36
2
reported sorbents.
Because its performance in the
Mg SiO at 200 °C, as demonstrated by the peaks of carbonation
2
4
1
50ꢀ200 °C range suggests that Mg SiO could react with
2
4
products (MgCO and SiO ) of (R1) (Figure 2), a finding
3
2
CO reasonably fast and with a considerable CO sorption
2
0,23,24
2
2
consistent with the observations of other researchers.
capacity, a 100ꢀ200 °C temperature range was chosen to study
The increase in the Gibbs free energy changes of (R1) with
temperature signifies that higher temperatures could have con-
the kinetics of reaction between Mg SiO and CO .
2
Sorption Kinetics. Determination of Reaction Order and Rate
4
2
siderable negative effects on the thermodynamics of CO on
37
2
Constants. Park successfully used a deactivation model to
Mg SiO . On the other hand, the rate of reaction improves at
2
4
describe the decrease in activity of Na CO during reaction of
2
3
higher temperatures. Therefore, an appropriate temperature
range should be chosen for study of the reaction kinetics of
R1). Our preliminary tests showed that the reaction rate of (R1)
NaOH with CO . Because the heterogeneous CO adsorption
2
2
process investigated in this research is very similar to that of
37
(
Park’s, his deactivation model is used to establish the rate
equation of (R1).
was quite slow in the temperature range of 0ꢀ100 °C, even
though the sorbent could achieve reasonably large CO sorption
2
According to the deactivation model, Mg CO forms gradu-
2
3
capacities over very long sorption periods. However, given that
ally during the carbonation process and covers the surface of
the volume of the emitted CO -containing flue gas is so large,
Mg SiO , thus reducing the activity or carbonation rate of the
2
2
4
slow sorption would require the construction of very bulky
adsorber units to extend considerably the time of contact
between Mg SiO and CO in order to achieve high CO capture
sorbent. Assuming that the pseudo-steady-state hypothesis is
applicable within a constant water vapor concentration environ-
ment, the deactivation equation of the Mg SiO -based CO
2
2
4
2
2
2
4
efficiencies, a prohibitively expensive approach.
carbonation process in a packed bed reactor shown in Figure 1
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The Journal of Physical Chemistry A
ARTICLE
3
7
can be expressed as
CCO2
a ¼
CCO , 0
2
2
6
4
"
!
#
3
kCH OWMg SiO
4
2
2
6
6
1 ꢀ exp
ð1 ꢀ expð ꢀ kdtÞÞ
7
7
Qg
6
7
¼
exp6
expð ꢀ kdtÞ7
1 ꢀ expð ꢀ kdtÞ
7
5
ðE1Þ
or
CCO2
2
"
!
#
3
kCH OWMg SiO
2
2
4
Figure 4. Mg SiO based CO sorption profile at 200 °C [(inlet gas
2
4
2
6
6
1 ꢀ exp
ð1 ꢀ expð ꢀ kdtÞÞ
7
7
Qg
2 2
conditions, H O, 4.1 mmol/L; CO , 4.1 mmol/L; total gas flow rate, 0.5
6
7
¼
CCO , 0 exp6
expð ꢀ kdtÞ7
2
2 4
L/min); weight of Mg SiO , 0.5 g].
6
1 ꢀ expð ꢀ kdtÞ
7
4
5
conditions, (E4) can be written as
ðE2Þ
dR
Mg SiO4
2
ꢀ
¼ k CCO2 R
ðE5Þ
d
Mg SiO4
2
where C
and CCO2 are the inlet and outlet concentrations
CO ,0
2
dt
3
(
kmol/m ) of CO in the gas stream, respectively, at any sorp-
tion time (t) and k is the initial sorption rate constant
2
If the rate-limiting reaction in the overall reaction mechanism
6
of (R1) does not change within the range of 100ꢀ200 °C,
[
m /(kmol kg min)] in the following CO isothermal conserva-
3
3
2
m
and n
should not vary either, and their values can
CO
2
Mg SiO
2
tion equation
4
therefore be obtained with the sorption data collected at any
temperature within the range. However, mMg2SiO4 = 1 and nCO2 = 1
are assumed values and were tested only under limited test
conditions. Thus, their applicability within broader ranges of test
conditions should be further supported with theoretical deriva-
tions, as discussed below.
dCCO2
mðMg SiO Þ
nðCO2Þ
4
2
ꢀ
Qg
ꢀ kCH OC
R
¼ 0
ðE3Þ
2
CO2
Mg SiO4
2
dw
Mg SiO4
2
3
Here, C
Mg SiO4
2
is the concentration of water vapor (kmol/m ),
H O
2
W
is the weight of Mg SiO (kg) in the reactor, Q is the
volumetric flow rate of the inlet gas mixture (L/min), w
consumed Mg SiO , n is the reaction order with respect to
Mg SiO4
2
defined as (W
2
4
g
Assuming (R1) proceeds through the following elementary
is
Mg SiO4
2
steps in the presence of water during the CO sorption process
2
2
4
CO2
þ
2ꢀ
CO , m
is the power of R
(the activity of Mg SiO4
CO þ H O T 2H þ CO
ðR2Þ
ðR3Þ
ðR4Þ
2
Mg SiO
2
2
2
2
3
4
ꢀ w
)/W , varying from 0 to 1),
Mg SiO
2 4
Mg SiO4
Mg SiO
2
2
4
3
and k is the deactivation rate constant [m /(kmol min)] when
1
Mg SiO4 þ 2Hþ f Mg þ H2O þ = SiO2
2þ
1
d
3
=2
2
2
nCO2 and m
are 1 in the following sorbent deactivation
2
Mg SiO4
equation
Mg2 þ CO32ꢀ f MgCO3
þ
dRMg SiO
mðMg SiO4Þ
2
4
nðCO2Þ
2
ꢀ
¼ kdC
R
ðE4Þ
(R2) is a reversible reaction that can reach its equilibrium state
quickly, leading to
CO2
Mg SiO4
2
dt
The choices for n
= 1 and m
3
= 1 are based on Park’s
2
CO2
Mg SiO
2
4
k
ðR2Þ, forwardCCO2
CH2O ꢀ kðR2Þ, reverse
C
þ
Cnet, CO32ꢀ ¼ 0
3
7
net, H
studies on the Na CO reaction system, which is very similar to
2
ðE6Þ
the one studied in this research. An iterative method was used to
determine k and k at a given temperature. The k and kd
d
or
ultimately chosen should make the CCO2 vs t relationship
predicted by (E2) under given experimental conditions match
kðR2Þ, forward CCO2 CH2O
2
C
þ
¼
k
ðE7Þ
the CCO2 vs t profiles recorded by the CO analyzer, as shown in
net, H
2
C
2ꢀ
ðR2Þ, reverse
net, CO3
Figure 4, for the results obtained at a given temperature under
different flow rates. The differences of the two CCO2 vs t
relationships, shown in Figure 4 to become larger after a period
of reaction time will be explained in one of the following
paragraphs.
where k(R2),forward and k(R2),reverse represent the forward and
reverse reaction rate constants of (R2), respectively, and CCO2
correspond to the concentrations of CO and H O,
respectively. Since H O is actually neither consumed nor gener-
and C
H O
2
2 2
2
We chose to use the sorption profile obtained at 200 °C
ated when both (R2) and (R3) are considered, C
can be
H O
2
þ
(
Figure 4) to test the deactivation model (E2), since the
treated as a constant. H O is simply an indispensible H carrier.
2
þ
slope resolution of the sorption profile at 200 °C is higher than
Cnet,H
þ
is the net concentration of H , which is the sum of
the concentrations of the H generated from the forward
reaction of (R2) and the H consumed by (R4) and the reverse
þ
that at other temperatures due to the increases of k and k with
d
þ
temperature. On the basis of the assumption that nCO2 = 1
and m
= 1 at 200 °C, and under other given reaction
reaction of (R2).
Mg SiO4
2
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The Journal of Physical Chemistry A
ARTICLE
2
Figure 5. Effect of temperature on enthalpy change of CO hydrolysis.
2ꢀ
Cnet,CO32ꢀ isthe net concentration of CO , whichisthesumof
3
2ꢀ
the concentrations of the CO32 generated from the forward
ꢀ
reaction of (R2) and the CO3 consumed by (R4) and the
reverse reaction of (R2). According to the pseudo-steady-state
3
8
2ꢀ
theory, Cnet,CO3 can be treated as a constant. Furthermore,
Figure 6. Determination of parameters of Arrhenius form of initial CO
2
3
3
2ꢀ
sorption [H O, 4.1 mmol/dm ; CO , 4.1 mmol/dm ; total gas flow rate,
Cnet,CO3 should be low for two reasons. First, since exothermal
2
2
2 4
0.5 L/min;weight of Mg SiO , 0.5 g; sorptiontemperature, 100ꢀ200 °C].
characteristics of the reaction (Figure 5) result in CO having low
2
solubility in water in the tested temperature range (particularly at
is less than 8%. Clearly, when R
continues to decrease, the
2
Mg SiO4
2
00 °C, a relatively high temperature for sorption), the forward
difference between the assumed m
(1) and the m
2
Mg SiO4
2
Mg SiO4
reaction rate of (R2) is slow. Therefore, (R2) or the interaction
(
0.5) that was postulated with the reaction mechanism leads to
between H O and CO is the rate-controlling step of the overall
2
2
an increase in (C
the later CO mineralization stage in Figure 4. Even though an
ꢀ C
), as shown in
CO ,measured
2
CO ,predicted with (E2)
2
CO sorptionprocess. Second, given thathighpressureis notused,
2
2
(
R3) and (R4) should be much quicker than the forward reaction
of (R2), leading to quick consumption of (R2)’s slowly generated
analytical solution or explicit expression of (E1) or (E2) cannot
be derived from (E3) and (E4), and numerical methods must be
2ꢀ
2ꢀ
CO3 . Thus, Cnet,CO3 can be considered to be a small constant.
On the basis of (R3), a decrease of RMg SiO with time can be
expressed as
used, it is better to use m
= 0.5 in (E3) and (E4) for
2
Mg SiO4
2
4
predicting the Mg SiO deactivation rate.
2
4
Arrhenius Form. The relationship between rate constants
dRMg SiO4
(k or k ) and reaction temperature (T) can be correlated using
2
2
0:5
d
ꢀ
¼ kðR3Þ
C
k
þ
R
38
net;H
Mg SiO
4
2
the Arrhenius form
dt
k ¼ Aeꢀ
E=RT
ðE9Þ
ðR2Þ;forward CCO2
CH2O
0:5
¼
k
ðR3Þ
R
Mg SiO
4
2
k
Cnet;CO32ꢀ
ðR2Þ;reverse
ꢀ
d
Ed=RT
k ¼ A e
ðE10Þ
d
k
k
CH2O
ðR2Þ;forward ðR3Þ
k
0:5
¼
¼
CCO R
where A and A are pre-exponential factors treated as constants
2
Mg SiO4
d
2
Cnet;CO32ꢀ
ðR2Þ;reverse
in the studied temperature range, E and E are the activation
d
energy values corresponding to k and k , and R is the ideal gas
d
0
0:5
k CCO2 R
ðE8Þ
Mg SiO
4
constant. The pre-exponential factors would also be impacted by
reactor conditions (e.g., geometry, loading), and these were kept
constant between experiments.
2
where
The values of k and k at 100, 125, 150, and 175 °C were
d
k0 ¼ k
k
CH2O
Cnet, CO32ꢀ
ðR2Þ, forward ðR3Þ
k
obtained using the same method cited previously for the k and k
d
ꢀ1
ꢀ1
ðR2Þ, reverse
values at 200 °C. The ln k vs T and ln k vs T plots of (R1)
are shown in Figure 6 and Figure 7, respectively. Regression
d
is a constant. Comparing (E8) to (E5), we find that the reaction
orders with respect to CO , i.e., n in (E5) and nCO2,
found an E value of 43.2 ( 3.7 kJ/mol and an A value of (2.3 (
2
CO ,experiment
2
6
6
0
.6) ꢁ 10 (kmol kg min), respectively. In addition, regression
3 3
d d
in (E8), obtained with experimental results and derived
with a postulated reaction mechanism, respectively, are exactly
theory
found an E value of 48.5 ( 2.4 kJ/mol and an A value of (2.5 (
5
0
.8) ꢁ 10 (L/min), respectively. Thus, the Arrhenius forms of
the same. However, m = 1 in (E5), is larger than
Mg SiO ,experiment
2
4
CO carbonation with Mg SiO within an H O environment for
2
2
4
2
m
= 0.5 in (E8). In the meantime, m
= 1
Mg SiO ,experiment
Mg SiO ,
2
4
2
4
the geometry of our experiments are
still makes (E2) fit the experimental data well at the early stage of
sorption. This is because when R
is within 0.85ꢀ1, which
6
5193=T
6
Mg SiO
2
k ¼ 2:3 ꢁ 10 e
ðkmol kg minÞ
ðE11Þ
ðE12Þ
4
3
3
is the range used in Figure 4 for comparing C
,
CO measured
and
2
C
,
CO predicted with (E2)
,
2
6
ꢀ5837=T
kd ¼ ð2:5 ꢁ 10 Þe
ðL=minÞ
ꢀ
ꢀ
ꢀ
0
:5
R
ꢀ RMg SiO
Mg SiO4
2
4
2
ꢁ
100%ꢀmaximum
The reaction order and activation energy of (R1) should not
be affected by the flow rate of the simulated flue gas mixture
0
:5
R
ꢀ
Mg SiO4
2
7
642
dx.doi.org/10.1021/jp2040899 |J. Phys. Chem. A 2011, 115, 7638–7644

The Journal of Physical Chemistry A
ARTICLE
temperature is unfavorable to (R1). Indeed, the reaction becomes
unfavorable above 200 °C.
The median particle size of Mg SiO used in this research was
2
4
3
.5 μm, which is already small for fixed-bed reactors. Further
reductions in Mg SiO particle size can create higher Mg SiO
4
2
4
2
surface areas, thereby improving CO breakthrough sorption
2
capacities within a given time period. However, this can also lead
to a considerable increase in back pressure, undesirable in fixed-
39
bed reactors because, according to Ergun’s equation
2
ð1 ꢀ εÞ
μ
1 ꢀ ε F
ΔP=L ¼ 150
2u þ 1:75
u2
ðE13Þ
3
3
ε
dp
ε
d
p
where ε is the interparticle porosity of the CO sorption bed, μ is
2
the flue gas viscosity (kg/ms), u is the interstitial velocity (m/s)
3
of the gas stream, F is the density (kg/m ) of flue gas, and ΔP/L
is the pressure drop (Pa/m) of gas in the fixed bed. As shown,
ΔP/L is significantly affected by the particle size of Mg SiO
2
4
2 4
Figure 7. Determination of parameters of Arrhenius form of Mg SiO
(
dp). Accordingly, a fluidized-bed reactor might be used to
3
3
deactivation [H O, 4.1 mmol/dm ; CO , 4.1 mmol/dm ; total gas flow
2
2
overcome the stricter particle-size constraints of a fixed bed.
The pressure of flue gas should be elevated when fluidized-bed
reactors are used. However, use of fluidized-bed reactors does
rate, 0.5 L/min; weight of Mg
2
SiO
4
, 0.5 g; sorption temperature,
100ꢀ200 °C].
not necessarily entail the increase in the overall CO capture
2
expense due to their advantages over fixed bed reactors.
Other Compounds in Mg-Rich Minerals. Mg-rich minerals in
nature often contain other metals in the form of silicates such as
Fe SiO . However, because their concentrations in the minerals
are very low, these metals should not have any significant effect
on (R1).
2
4
2
2
4
SO /NO . As with any other CO separation technologies, the
x
x
2
initial CO
2
sorption
Mg
2
SiO
4
deactivation
effects of SO /NO in flue gas on the studied CO carbonation
x
x
2
flow rate (L/min)
E (kJ/mol)
E
d
(kJ/mol)
process should be considered as well. Acids resulting from the
hydrolysis of SO /NO are much stronger than those from CO
2
0
0
1
.25
42.4 ( 5.1
43.2 ( 3.7
43.5 ( 4.6
47.9 ( 4.2
48.5 ( 2.4
47.2 ( 5.7
x
x
.5
and thus can react easily with Mg SiO . However, because their
2
4
concentrations in flue gas are only at parts per million levels (far
lower than the percentage levels of CO ), and Mg-rich minerals
2
are not intended to be used as regenerated sorbents for CO in
2
under the given CO mineralization conditions, an assumption
2
the first place, the effects of SO /NO may be negligible.
x
x
verified by conducting CO sorption tests with the same initial
2
CO concentration but with gas flow rates of 0.25 and 1.0 L/min.
2
’ AUTHOR INFORMATION
The derived values of E and E of (R1) for the three flow rates
d
Corresponding Author
*
(
0.25, 0.5, and 1.0 L/min) were consistent (see Table 1).
Application Considerations. Reactor Choice. Figure 3 shows
E-mail: mfan@uwyo.edu.
that the CO breakthrough and total sorption capacities achieved
2
with Mg SiO under the given test conditions were in the ranges
2
4
’
ACKNOWLEDGMENT
This work was supported by Caterpillar, Siemens, and the
of 50ꢀ60 and 125ꢀ175 g of CO /kg of Mg SiO , respectively,
2
2
4
which are lower than the theoretical sorption capacity of pure
Mg SiO , 630 g of CO /kg of Mg SiO . Therefore, appropriate
School of Energy Resources at the University of Wyoming.
2
4
2
2
4
measures should be taken to enhance the CO sorption capa-
2
cities of Mg SiO within given reaction conditions, since other-
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