(87b) Investigation of the Adsorbed Species of CO2 on Mg(OH)2

Introduction

Fossil
fuels supply a majority of the world's energy needs.  However, the
combustion of fossil fuels is one of the major sources of the greenhouse gas CO₂.
Integrated gasification combined cycle (IGCC) is one the most efficient power
generation systems that use coal. However, because the IGCC gas streams are at
high pressure and elevated temperatures, there is a thermal efficiency loss if
the fuel gas stream is cooled for CO₂ removal. The current
commercial technologies, such as the Selexol process,
require cooling of the gases to near ambient temperature for water and CO₂
removal. There are tremendous advantages of developing technologies for CO₂
capture at moderate/hot gas temperatures.  One advantage is that the
moderate/hot fuel gas after CO₂ removal can
be directly introduced to the turbine systems for power generation were as the Selexol process requires the fuel gas to be reheated after
low temperature capture.  IGCC systems with CO₂ removal would
be more energy efficient if the sorbents were operational at warm gas
temperatures (i.e. 150 to 350 °C).  Further, successful development of
such a CO₂ capture sorbent would enable the use of other warm gas
clean up technologies such as HCl and H₂S. 
Although warm gas clean up removal processes are expected to increase the
overall efficiency of the IGCC process significantly, only a few studies
related to regenerable sorbents with sufficient CO₂ removal capacity
at 150 to 350 °C are reported in the literature.

A
novel magnesium hydroxide-based sorbent that can capture CO₂ from
200 to 315 °C was developed and patented by NETL researchers.[1-2] The sorbent
is also regenerable at 375-400 °C.  The capture process with this novel warm
gas CO₂ removal sorbent involves a chemical reaction, as shown in
Reaction 1 below.  Mg(OH)₂ has a very high CO₂ sorption
capacity at 200?315 °C which is considerably higher than that of the commercial
Selexol process.  The capture reaction/process will be referred to as
?sorption.?

Mg(OH)₂ (s) + CO₂  (g)  →
MgCO₃ (s) + H₂O                    (1)

The carbonate formed during
the reaction will thermally decompose to release carbon dioxide and regenerate
according to Reaction 2 at 375?400 °C.

MgCO₃ (s) + H₂O  →   Mg(OH)₂
(s) + CO₂                         (2)

Reaction 2 may also occur in
two reaction steps as shown in Reactions 3 and 4:

MgCO₃ (s) → MgO (s) + CO₂
(g)                                         (3)

MgO (s) + H₂O (g) → Mg(OH)₂
(s)                                      (4)

Experimental

The
Mg(OH)₂-based sorbent contains a mixture of Mg(OH)₂,
bentonite binder, and a promoter, the detailed synthesis has been described
elsewhere.[2]  This sorbent
powder was formed into 1-2 mm pellets by combining with water followed by
drying at 150 °C for 4 hours.  The pellets were then ground into a powder and
placed in a high pressure, high temperature FTIR-DRIFTS reactor (Thermo Fisher
- Nicolet).  The reactor was purged with N₂ that flowed through a
saturator at 50 sccm.  The saturator provided approximately 4% H₂O
water to the gas flow.  After purging for 30 minutes, the pressure was set to
280 psig and the reactor was heated to 425 °C to clean the sorbent for 2 hours. 
The clean sorbent was cooled to 200 °C for 2 hours while flowing hydrated N₂
to rehydrate the sorbent per Reaction 4.  The hydrated sorbent was then exposed
to 5% in CO₂ in hydrated N₂ for 12 hours.  The FTIR was
set to series mode to scan the sorbent through the entire capture cycle.

Pellets
of 1-2 mm were placed in a bench-scale, packed-bed reactor (0.5 inch inner
diameter) to form a 6 inch bed.  The sorbent bed was then heated to 200 °C and exposed
to the capture gas consisting of 28% CO₂, 28% Ar, 10% H₂O,
and 34% N₂ gas mixture flowed downward with gas hourly space
velocity (GHSV) of 250 hr^-1.  The capture was complete when the CO₂
concentration of the outlet matched the inlet.  After the capture was complete,
the sorbent was regenerated to remove the captured CO₂. 
Regeneration occurred by flowing 10% H₂O with 90% N₂ at
500 hr^-1 GHSV through the reactor maintained at 375 °C until CO₂
was no longer present in the effluent as determined by a mass spectrometer. 
Upon finishing the regeneration, the reactor was cooled to 200 °C and a mixture
of 20% H₂O in N₂ was introduced for 2 hours to
hydroxylate the remaining MgO into Mg(OH)₂.  After hydroxylation,
the CO₂ capture gas stream was introduced and subsequent capture
cycles were conducted.

Results and Discussion

Figures 1 and 2 show FTIR spectra during CO₂
sorption over the Mg(OH)₂ sorbent.  The spectra are in absorbance
form where the single beam spectra collected immediately before the CO₂ introduction
was used as the background.  In absorbance form, species with a positive rate
of formation are shown with a positive IR band while species with negative
rates of formation are shown with negative IR bands. As Reaction 1 proceeds,
the carbonates in Figure 1 are shown increasing while the hydroxyl groups shown
in Figure 2 are decreasing.

Figure 1 shows the 1300 to 1750 cm^-1 range
which contains the carbonate formations.  The three peaks shown correlate to
two carbonate species: 1597 cm^-1 correlates to the C=O stretch of
bidentate carbonate, 1780 cm^-1 correlates to the C=O stretch from
bridged carbonate, and 1260 cm^-1 correlates to the asymmetric
stretch from bridged carbonate. [3]  After 10 minutes of CO₂ exposure the bridged carbonate reached a maximum while the bidentate carbonate concentration continued to increase.  To establish the nature of the continued bidentate carbonate growth, the hydroxyl species associated to Mg(OH)₂ are shown in Figure 2.  The hydroxyl groups show continued decreasing through the 12-hour CO₂ exposure confirming the sorbent captured CO₂ throughout the 12-hour capture period.  It is interesting to note that the rate of growth of the carbonate peaks does not correspond to the rate of disappearance of the hydroxyl groups.  This could be the result of equilibrium where as the initial hydroxyl groups are consumed, new hydroxyl groups are formed.  The
new hydroxyl groups are formed from the hydrated gas and MgO that was not hydroxylized
during the hydroxylation step.  The new hydroxyl groups replace the consumed
hydroxyl groups maintaining the surface concentration and equilibrium.

Figure 1.  FTIR spectra of the carbonate formation during CO₂
sorption.

Figure 2.  FTIR spectra of the hydroxylate consumption during CO₂
sorption.

Figure
3 shows the MS profile of CO₂ during a capture over Mg(OH)₂
sorbent.  The initial capture shows that nearly 90% of the CO₂ was
captured.  After 15 minutes of CO₂ exposure, the rate of CO₂
capture decreased to about 15% of the CO₂ flow.  Comparing this
profile to the IR data suggests the initial capture was the result of the
formation of both bridged and bidentate carbonates.  The lower rate of capture
in the bench scale reactor may correlate to the
formation of the bidentate formation. In addition, powders were used in FTIR
while pellets were used in reactor tests. Diffusional resistance through the
product layer and pores in the pellets may also contribute to the lower rates
with increasing time.

Figure 3.  The MS CO₂ profile from the packed bed
bench scale reactor during CO₂ sorption.

This
paper will discuss these results in greater detail and further investigation on
how to promote the rate of formation of bidentate carbonate.  Increasing the
rate of formation of bidentate carbonate would effectively increase the usable
capture capacity (e.g. capture capacity above 90%).

Conclusions

Two carbonate species formed during CO₂
capture over Mg(OH)₂ were identified using an FTIR.  Bridged
carbonate reached a maximum concentration after 10 minutes where as the
bidentate carbonate intensity continued to increase.  The paper is going to
investigate the correlation between the rate of formation of bidentate
carbonate and the slow secondary capture observed in the bench scale reactor. 
If a correlation exists, promoting the rate of formation of bidentate carbonate
to shift this capacity to a useable capacity will also be discussed.

References

1.Siriwardane, Ranjani V. and Robert W. Stevens Jr., Novel Regenerable Magnesium Hydroxide
Sorbents for CO2 Capture at Warm Gas Temperatures. Ind. Eng. Chem. Res.,
2009. 48: p. 2135-2141.

2.Siriwardane, R.
V., Regenerable Sorbents for CO2 Capture From Moderate and High Temperature
Gas Streams. 2008, The United States of America as represented by the
United States Department of Energy.

3.Little, L. H., Infrared
Spectra of Adsorbed Species. 1966, New York: Academic Press Inc.