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 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.

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.