Novel Regenerable Magnesium Hydroxide Sorbents for CO2 Capture

Ind. Eng. Chem. Res. 2009, 48, 2135–2141

2135

Novel Regenerable Magnesium Hydroxide Sorbents for CO2 Capture at Warm Gas Temperatures Ranjani V. Siriwardane*,† and Robert W. Stevens, Jr.†,‡ U.S. Department of Energy, National Energy Technology Laboratory, 3610 Collins Ferry Road, Morgantown, West Virginia 26507, and Parsons, P.O. Box 618, South Park, PennsylVania 15129

A novel sorbent consisting of Mg(OH)2 was developed for carbon dioxide (CO2) capture at 200-315 °C suitable for CO2 capture applications such as coal gasification systems. Thermodynamic analysis conducted with the FactSage software package indicated that the Mg(OH)2 sorbent system is highly favorable for CO2 capture up to 400 °C at 30 atm. MgCO3 formed during sorption decomposes to release CO2 at temperatures as low as 375 °C up to 20 atm. MgO rehydroxylation to form Mg(OH)2 is possible at temperatures up to 300 °C at 20 atm. The experimental data show that the sorbent is regenerable at 375 °C at high pressure and that steam does not affect the sorbent performance. A multicycle test conducted in a high-pressure fixed-bed flow reactor at 200 °C with 28% CO2 showed stable reactivity during the cyclic tests. The capture capacity also increased with increasing pressure. The sorbent is unique because it exhibits a high CO2 capture capacity of more than 3 mol/kg at 200 °C and also is regenerable at a low temperature of 375 °C and high pressure. High-pressure regeneration is advantageous because the CO2 compression costs required for sequestration can be reduced. 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 carbon dioxide (CO2). Technologies are needed that will allow for the use of fossil fuels while reducing greenhouse gas emissions. Existing commercial CO2 capture technology that exists today is very expensive and energy-intensive. Improved technologies for CO2 capture are necessary to achieve low energy penalties. The integrated gasification combined cycle (IGCC) is an efficient power generation system, and coal gasification will become an important technique for power generation in the future. The coal gas produced with an IGCC plant consists mainly of H2 and CO, which can be shifted to CO2 and H2. Removal of CO2 after the water-gas shift reactor will produce a gas stream containing a large percentage of hydrogen. Removal of impurities such as H2S, HCl, and ammonia might be necessary prior to both the water-gas shift reactor and the CO2 removal sorbent bed. Zeolites have shown promising results for separating CO2 from gas mixtures and can potentially be used in the pressure swing adsorption (PSA) process.1-12 According to a systems analysis conducted in The Netherlands,13 PSA/temperature swing adsorption (TSA) systems would be even more energy-efficient for IGCC systems if the sorbents were operational at warm gas temperatures (200-350 °C). However, only a few studies related to regenerable sorbents with sufficient CO2 removal capacities at 200-350 °C have been reported in literature. Current commercial techniques such as the Selexol process could be utilized for CO2 removal from highpressure gas streams but require gas cooling, resulting in a loss of thermal efficiency in the process. Zeolites have also been tested, but their capacities are very low at temperatures above 200 °C.6 Lithium-based sorbents that capture CO2 at 450-550 * To whom correspondence should be addressed. E-mail: [email protected]. Tel.: (304) 285-4513. Fax: (304) 2854403. † National Energy Technology Laboratory. ‡ Parsons.

°C were reported by Nakagawa and Ohashi.14 Golden and Sircar15 described a PSA process for the removal of bulk CO2 from wet high-temperature gas using alkali-promoted hydrotalcite, alkali-impregnated alumina, and double salt extrudates. Our group previously reported on the development of a sodiumbased sorbent that could absorb CO2 at 200-315 °C.16,17 The sorbent, however, required a temperature above 700 °C for regeneration. The purpose of this research work was to develop a warm-gas-temperature (200-400 °C) CO2 capture sorbent with a high CO2 sorption capacity that could be regenerated at a lower temperature. A novel magnesium hydroxide-based sorbent that can capture CO2 at 200-315 °C was developed and patented by National Energy Technology Laboratory (NETL) researchers.16 These sorbents are also regenerable at 375 °C. The capture process with this novel warm-gas CO2 removal sorbent involves the chemical reaction Mg(OH)2(s) + CO2(g) f MgCO3(s) + H2O

(1)

The novel sorbent has a very high CO2 sorption capacity at 200-315 °C, which is considerably higher than that of the commercial Selexol process (i.e., 3-4 mol of CO2/kg for the present sorbent vs 0.3 mol of CO2/kg solvent for Selexol with a CO2 partial pressure of 0.3 MPa).18 The capture reaction/ process will be hereafter referred to as “sorption”. The carbonate formed during the reaction can be thermally decomposed to release carbon dioxide and regenerate according as described in reaction 2 at 375-400 °C MgCO3(s) + H2O f Mg(OH)2(s) + CO2

(2)

Reaction 2 can also occur in two reaction steps as shown in reactions 3 and 4 MgCO3(s) f MgO(s) + CO2(g)

(3)

MgO(s) + H2O(g) f Mg(OH)2(s)

(4)

According to these reactions, Mg(OH)2 can be a potential regenerable sorbent for CO2 capture from gas streams at warm gas temperatures. Therefore, both thermodynamic analysis and

10.1021/ie8011598 CCC: $40.75  2009 American Chemical Society Published on Web 12/29/2008

2136 Ind. Eng. Chem. Res., Vol. 48, No. 4, 2009

Figure 1. Thermodynamic equilibrium analysis as a function of temperature for a system of 1 mol of CO2, 1 mol of Mg(OH)2, 1 mol of H2O, and 0.1 mol of N2 at a pressure of 1 atm.

Figure 2. Thermodynamic equilibrium analysis as a function of temperature for a system of 1 mol of CO2, 1 mol of Mg(OH)2, 1 mol of H2O, and 0.1 mol of N2 at a pressure of 30 atm.

experiments were conducted to evaluate whether the sorbent can be utilized for CO2 capture and regeneration. Thermodynamic analysis of the CO2 sorption/regeneration process with Mg(OH)2, multicycle laboratory-scale and bench-scale flow reactor studies, and XRD data for the Mg(OH)2 sorbent are presented in this article. Experimental Section Sorbents were prepared by combining solid magnesium hydroxide and a promoter, sodium orthosilicate, in a mixer. H2O was added to the mixture to pelletize the sorbent, yielding particles on the order of 2-3 mm. The pellets were dried in air at 100 °C for 1 h. The resulting sorbent pellets exhibit a low surface area, on the order of 2.4-3.0 m2/g as analyzed by the BET method. This synthesis is also described in a U.S. patent.16 A 10-cycle laboratory-scale flow reactor test was conducted with 3 g of Mg(OH)2 sorbent in a laboratory-scale reactor. The sorption was conducted at 200-250 °C and 150 psig with a gas mixture containing 28% CO2 in moist He at a gas hourly space velocity (GHSV) of 250 h-1. Regeneration was conducted at 375 °C and both 1 atm and 150 psig with He and moisture. After the regeneration, the sorbent was cooled in the presence of moisture to ambient temperature prior to the next cycle. The laboratory-scale reactor was limited to an operating pressure of 150 psig. The water content in the feed to the laboratory-scale reactor was also limited as it was delivered via a bubbler that was maintained at room temperature, yielding an upper bound on the H2O concentration of approximately 2.6%. Tests were also conducted with approximately 18 g of the sorbent in a bench-scale flow reactor to evaluate process variables (i.e., temperature, pressure, H2O feed content, etc.). Testing was conducted at 200-250 °C and 150-280 psig, with a H2O content between 10% and 30% and with 10-30% CO2. X-ray diffraction measurements were conducted with a PANalytical PW3040 X-ray diffractometer. The resulting spectra were analyzed with X’Pert HighScore Plus software, and peak assignments were determined through comparison with X-ray databases. Results and Discussion Thermodynamic Analyses. Thermodynamic analysis was conducted to understand the favorable range of temperatures and pressures for the sorption, regeneration, and rehydroxylation reactions. One mole of Mg(OH)2, 1 mol of CO2, and 1 mol of water were equilibrated at 1 atm, and the resulting products are shown in Figure 1 as a function of temperature. As shown in

Figure 3. Thermodynamic equilibrium analysis as a function of temperature for a system of 1 mol of MgCO3, 1 mol of H2O, and 0.1 mol of N2 at a pressure of 1 atm.

Figure 1, MgCO3 can be easily formed up to 300 °C at 1 atm. Results of a similar analysis shown in Figure 2 indicate that, at 30 atm, formation of MgCO3 can take place up to 400 °C. Thus, the formation of MgCO3 appears to be favored at elevated pressures. Because precombustion gas streams such as those from IGCCs are at high pressure, Mg(OH)2 is a suitable candidate for CO2 capture from precombustion gas streams. Thermodynamic analysis data for the decomposition of 1 mol of MgCO3 in the presence of 1 mol of water at 1 atm is shown in Figure 3. Decomposition initiates around 300 °C at 1 atm, and favorable products are magnesium oxide and CO2. Rehydroxylation of MgO to form Mg(OH)2 appears to be unfavorable in the presence of CO2. Decomposition of MgCO3 at 10 atm initiates at 375 °C, as shown in Figure 4. Further increasing the pressure to 20 atm during the decomposition of MgCO3 shifts the initiation temperature to 400 °C, as shown in Figure 5. The products are MgO and CO2, and Mg(OH)2 is again not formed in the presence of CO2 after the decomposition. These data indicate that the MgCO3 can be decomposed at relatively low temperatures as compared to the other metal-oxide-based CO2 capture systems. Li-based systems14 require temperatures above 800 °C for decomposition of carbonates, whereas sodiumbased systems16,17 require temperatures above 700 °C. The ability to regenerate at a lower temperature range of 375-400 °C makes the magnesium hydroxide sorbent more favorable because the regeneration energy will be lower than what is reported in the literature with other metal-salt-based systems.

Ind. Eng. Chem. Res., Vol. 48, No. 4, 2009 2137


Figure 6. Thermodynamic equilibrium analysis as a function of temperature for a system of 1 mol of MgO, 1 mol of H2O, and 0.1 mol of N2 at a pressure of 10 atm.


Figure 7. Thermodynamic equilibrium analysis as a function of temperature for a system of 1 mol of MgO, 1 mol of H2O, and 0.1 mol of N2 at a pressure of 20 atm.

After the regeneration, rehydroxylation of MgO is necessary to obtain Mg(OH)2. Therefore, thermodynamic analysis was conducted with 1 mol of MgO and 1 mol of H2O. The resulting products at 10 atm are shown in Figure 6; complete conversion of MgO to Mg(OH)2 is possible up to 275 °C at 10 atm. Thermodynamic analysis data for the rehydroxylation at 20 atm, as shown in Figure 7, reveal that the rehydroxylation is extended at the higher pressure up to 300 °C. Thus, increasing pressure increases the temperature window for the rehydroxylation process. In summary, the thermodynamic analysis showed that the Mg(OH)2 sorbent system is highly favorable for CO2 capture within a wide temperature/pressure range (up to 300 °C at 1 atm; up to 400 °C at 30 atm). MgCO3 decomposes to release CO2 at temperatures as low as 375 °C up to 20 atm. MgO rehydroxylation to form Mg(OH)2 is possible at temperatures up to 300 °C at 20 atm. Because the thermodynamic analysis showed favorable results, Mg(OH)2 was tested in a flow reactor to determine the properties of CO2 capture and regeneration. Heats of Reaction Information. The reaction of Mg(OH)2 with CO2 to form MgCO3 (reaction 1) is exothermic with a ∆H value of -19.657 kJ/mol of CO2. The ∆H values for other CO2 sorption reactions with various metal-salt-based systems were computed with HSC Chemistry software and are summarized

Table 1. Heats of Reaction for the Reaction of CO2 with Various Metal Salts at Standard Temperature and Pressure compound

reaction

∆H (kJ/mol of CO2)

Mg(OH)2 NaOH KOH Na2CO3 K2CO3 MgO CaO Ca(OH)2 Li2ZrO3 Li2SiO4

Mg(OH)2 + CO2 ) MgCO3 + H2O 2NaOH + CO2 ) Na2CO3 + H2O KOH + CO2 ) KHCO3 Na2CO3 + CO2 + H2O ) 2NaHCO3 K2CO3 + CO2 + H2O ) 2KHCO3 MgO + CO2 ) MgCO3 CaO + CO2 ) CaCO3 Ca(OH)2 + CO2 ) CaCO3 + H2O Li2ZrO3 + CO2 ) Li2CO3 + ZrO2 Li2SiO4 + CO2 ) Li2CO3 + Li2SiO3

-19.657 -127.488 -146.753 -135.521 -142.846 -100.895 -178.175 -69.021 -162.632 -141.963

in Table 1. It is important to note that the Mg(OH)2-based system has the lowest heat of sorption. This indicates that the regeneration heat required for the reverse reaction of decomposition of carbonate to form Mg(OH)2 is significantly lower than that required for the other processes. This is a significant advantage for the Mg(OH)2 system because regeneration energy is one of the critical factors for CO2 capture systems. The temperature swing requirement for the sorption and regeneration of the Mg(OH)2/CO2 system is also lower than that for most of the other systems. Thus, the low heat of regeneration and low temperature swing required for the Mg(OH)2 sorbent make this sorbent highly favorable for CO2 removal from precombustion


Figure 8. Laboratory-scale flow reactor CO2 capture data with 28% CO2/ He (moist), GHSV ) 250 h-1 at 200-250 °C and 100-150 psi; regeneration at 375 °C and 1 atm.

Figure 9. Laboratory-scale flow reactor CO2 capture data with 28% CO2/ He (moist), GHSV ) 250 h-1 at 200 °C and 150 psi; regeneration at 375 °C and 150 psig.

Table 2. Laboratory-Scale Flow Reactor Data Summary for the Mg(OH)2 Sorbent

to MgO. The MgO is converted to Mg(OH)2 through a reaction with water vapor during the cooling process. The extent of hydroxylation was not controlled during the multicycle tests. The CO2 sorption capacity during the subsequent cycles depends on the degree of rehydroxylation; the increased capacities are likely due to a greater extent of rehydroxylation. The rehydroxylation process is critical for retaining reactivity during cyclic operation and is thermodynamically more favorable at lower temperatures. The CO2 sorption data for cycles 7-10 are shown in Figure 9. The regenerations for these cycles were conducted at 150 psig and 375 °C. The CO2 removal efficiencies for these cycles remained greater than 99%, and the capture capacities were higher than those for the first six cycles when the regeneration was conducted at 1 atm. The CO2 capture capacities were in the range of 3-4 mol/kg for the sorbent as reported in Table 2. Regenerable sorbents at 200-315 °C with high CO2 capture capacities have not been reported in the literature, and these novel sorbents offer great promise for IGCC applications. The high capacities will contribute to low regeneration costs and small vessel sizes. In addition, the regeneration temperature of the sorbent is 375 °C, which contributes to a low temperature swing from absorption to regeneration. Fixed-Bed Bench-Scale Flow Reactor Tests. Bench-scale flow reactor tests were conducted with both 10% and 28% CO2 at 200 °C and 150 psi to determine the effect of the CO2 concentration on the sorbent’s performance. Tests were also conducted with 28% CO2 at 280 psi to determine the effect of pressure. The results of the first cycle of the bench-scale flow reactor tests with 10% CO2, 10% H2O, 10% Ar, and 70% N2 at a space velocity of 250 h-1 are shown in Figure 10. The sorbent was able to remove the CO2 down to part-per-million levels, yielding approximately a 99% removal efficiency. The CO2 concentration measured during the regeneration at 375 °C is shown in Figure 11. The CO2 desorption rate is high, which indicates that regeneration is feasible at 375 °C. The CO2 sorption capacities during the five-cycle test conducted after regeneration at 375 °C are reported in Table 3. A CO2 capture capacity of about 2.5 mol/kg can be obtained with 10% CO2 in the gas stream. The results of the six-cycle test with 28% CO2, 10% H2O, 28% Ar, and 34% N2 are reported in Table 4. The data for the fifth cycle are shown in Figure 12. The first four cycles were conducted at 150 psig, whereas the last two cycles were

CO2 sorption

regeneration

cycle

T (°C)

P (psig)

capacity (mol/kg)

T (°C)

P (psig)

1 2 3 4 5 6 7 8 9 10

200 250 250 200 200 200 200 200 200 200

100 100 150 150 100 150 150 150 150 150

1.47 2.11 2.13 2.84 2.40 3.46 4.09 4.14 4.27 3.22

375 375 375 375 375 375 375 375 375 n/a

0 0 0 0 0 150 150 150 150 n/a

Table 3. Sorption Capacities over Mg(OH)2 during Bench-Scale Flow Reactor Testing with a Feed Composition of 10% CO2 and 10% H2O and GHSV ) 250 h-1 CO2 sorption

regeneration

cycle

T (°C)

P (psig)

capacity (mol/kg)

T (°C)

P (psig)

1 2 3 4 5

200 200 200 200 200

150 150 150 150 150

2.72 2.53 2.40 2.25 2.41

375 375 375 375 n/a

0 0 0 0 n/a

gas streams. The reverse of reaction 1 can take place in two steps as shown in reactions 3 and 4. The decomposition of MgCO3 (reaction 3) is endothermic with a ∆H° value of 100.92 kJ/mol. The rehydroxylation (reaction 4) is exothermic with a ∆H° value of -81.35 kJ/mol. To exploit the low ∆H value for the overall reaction 2, the heat from reactions 3 and 4 have to be integrated into the process. Fixed-Bed Laboratory-Scale Flow Reactor Tests. The first six cycles of the 10-cycle laboratory-scale flow reactor test data with 28% CO2 in He at 200 °C, 100-150 psi, and 250 h-1 space velocity are shown in Figure 8. Regenerations during these six cycles were performed at 1 atm and 375 °C. The efficiency of the CO2 removal was greater than 99% during these six cycles. The breakthrough times also increased with increasing number of cycles, indicating that the sorbent’s capture capacity increased with cycle number. The CO2 capture capacities calculated from the breakthrough curves are listed in Table 2; it can be seen that there is nearly a 3-fold increase in capacity between the first cycle and the ninth cycle. During regeneration at 375 °C, both MgCO3 and unreacted Mg(OH)2 are converted


Figure 10. Bench-scale flow reactor CO2 capture data over Mg(OH)2 sorbent at 200 °C, 150 psig, GHSV ) 250 h-1 with CO2/Ar/H2O/N2 ) 10%/10%/ 10%/70%.

Figure 12. Bench-scale flow reactor CO2 capture data over Mg(OH)2 sorbent at 200 °C, 280 psig, GHSV ) 250 h-1 with CO2/Ar/H2O/N2 ) 28%/28%/ 10%/34%.

Figure 11. CO2 concentration during regeneration at 1 atm and at 375 °C with H2O/N2 after capture with CO2/Ar/H2O/N2 ) 10%/10%/10%/70%.

Figure 13. Desorption of CO2 during regeneration of the sorbent at 20 psig and 375 °C after CO2 capture with CO2/Ar/H2O/N2 ) 28%/28%/10%/34%.

Table 4. Sorption Capacities over Mg(OH)2 during Bench-Scale Flow Reactor Testing with Feed Composition of 28% CO2, 10% H2O, and GHSV ) 250 h-1 CO2 sorption

regeneration

cycle

T (°C)

P (psig)

capacity (mol/kg)

T (°C)

P (psig)

1 2 3 4 5 6

200 200 200 200 200 200

150 150 150 150 280 280

n/a 1.72 2.54 2.46 3.10 3.37

375 375 375 375 375 375

20 20 20 20 20 20

conducted at 280 psig. The increase in concentration from 10% to 28% did not impact the CO2 capture capacity, but the increase in pressure from 150 to 280 psig did have a significant effect on the capacity. Capacities close to 3 mol/kg were obtained with the sorbent at 280 psig and 200 °C. Regeneration of the sorbent at 375 °C led to desorption of all of the sorbed CO2 (Figure 13), suggesting that the sorbent is fully regenerable under the conditions employed (375 °C and 20 psig). The results also indicated that the rehydroxylation process is critical to maintain the CO2 capture capacity during cyclic processing. MgO showed a very low CO2 capture during the bench-scale flow reactor test at 200 °C, as shown in Figure 14, even though the thermodynamic analysis of the reaction of CO2 with MgO indicated that the reaction is thermodynamically

Figure 14. Bench-scale flow reactor CO2 capture data over MgO at 200 °C, 280 psig, GHSV ) 250 h-1 with CO2/Ar/H2O/N2 ) 28%/28%/10%/ 34%.

favorable. These results suggest that the kinetics of CO2 capture over MgO are much slower than those over Mg(OH)2. Therefore, the presence of Mg(OH)2 is necessary for the CO2 capture process.


Figure 15. XRD analysis of fresh Mg(OH)2 sorbent.

Figure 17. XRD analysis of Mg(OH)2 sorbent following regeneration at 400 °C and 280 psig of captured CO2.

(MgSiO4), which is believed to result from an interaction between MgO and the sodium silicate-based promoter under the conditions applied during regeneration. Conclusions

Figure 16. XRD analysis of Mg(OH)2 sorbent following exposure to CO2 at 200 °C and 280 psig.

X-ray Diffraction Analyses. X-ray diffraction studies were conducted with a fresh, unexposed sample of the Mg(OH)2 sorbent, a sorbent sample following CO2 exposure at 200 °C and 280 psig, and a sample of the CO2-exposed sorbent following regeneration at 400 °C and 280 psig. Figure 15 shows the spectrum of the fresh Mg(OH)2 sorbent sample following its preparation. Peaks representative of Mg(OH)2 dominate the spectrum, as the clay binder is amorphous. Peaks representative of the sodium silicate-based promoter are barely visible among the Mg(OH)2 peaks. The XRD analysis of the CO2-exposed sorbent is shown in Figure 16. Peaks at 2θ ) 33°, 43°, and 79° indicate the presence of crystalline MgCO3. It is interesting to note that the spectrum is still dominated by Mg(OH)2 peaks. This is likely due to bulk Mg(OH)2 being inaccessible to the gaseous CO2 during capture. Although the current CO2 capture capacities are attractive, this suggests that the sorbent’s capacity could be further improved if the sorbent’s structure were opened up to allow greater permeability of CO2. Figure 17 shows the XRD analysis of the Mg-based sorbent following regeneration at 400 °C and 280 psig. Consistent with the thermodynamic predictions, the major peaks within the spectrum are indicative of MgO, and no Mg(OH)2 peaks are present. Minor peaks representative of MgCO3 are still present at 32° and 47°, suggesting that the regeneration was not fully completed when the sample was taken. It is also interesting to note the formation of numerous small peaks assigned to forsterite

A novel sorbent consisting of Mg(OH)2 developed at NETL showed promising results for CO2 capture at 200-315 °C and high pressure. This sorbent will be suitable for CO2 capture applications such as coal gasification systems. Thermodynamic analysis indicated that the Mg(OH)2 sorbent is highly favorable for CO2 capture up to 300 °C at 1 atm and up to 400 °C at 30 atm. MgCO3 formed during sorption decomposes to release CO2 at temperatures as low as 375 °C at pressures up to 20 atm. MgO rehydroxylation to form Mg(OH)2 is possible at temperatures up to 300 °C at 20 atm. Both laboratory-scale and bench-scale flow reactor tests showed promising results with the sorbent. The sorbent demonstrated a CO2 capture capacity in the range of 3-4 mol/kg during laboratory-scale testing, with an efficiency of CO2 removal of approximately 99%. The experimental data also showed that the sorbent is regenerable at 375 °C and high pressure; the presence of steam does not adversely affect the sorbent performance. The sorbent’s capture capacity increased during high-pressure regeneration. A multicycle test conducted in a high-pressure bench-scale fixed-bed flow reactor at 200 °C with 28% CO2 showed stable, consistent performance. The capture capacity also increased with increasing pressure. The sorbent is unique because it has a high CO2 capture capacity of more than 3 mol/kg at 200 °C and it is regenerable at a low temperature of 375 °C and a high pressure of 20 psig. High-pressure regeneration is advantageous because the CO2 compression costs required for sequestration could be minimized. Acknowledgment The authors thank Mr. Craig Thomas and Mr. Frank Thomas for conducting the flow reactor tests and Mr. James Poston for his assistance in conducting the XRD analyses. Note Added after ASAP Publication: Because of a production error, Figure 12 was used in place of Figure 10 in the version of this paper that was published on the Web December 29, 2008. The corrected version of this paper was reposted to the Web January 5, 2009.


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ReceiVed for reView July 28, 2008 ReVised manuscript receiVed November 5, 2008 Accepted November 18, 2008 IE8011598

Novel Regenerable Magnesium Hydroxide Sorbents for CO2 Capture

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