Article pubs.acs.org/EF
Mg(OH)2 for CO2 Capture from High-Pressure, ModerateTemperature Gas Streams James C. Fisher, II* and Ranjani V. Siriwardane National Energy Technology Laboratory, U.S. Department of Energy, 3610 Collins Ferry Road, Morgantown, West Virginia 26507, United States ABSTRACT: Precombustion CO2 separation is considered to be the most efficient method of carbon removal for fossil fuels. Solid sorbents are promising for CO2 separation because of lower sensible heats, higher CO2 sorption capacities, and favorable adsorption/desorption temperatures. The work reported here continues an effort to develop a Mg(OH)2-based sorbent that adsorbs CO2 at IGCC fuel gas temperatures of 150−200 °C and at 280 psig. One novelty of the sorbent is that the CO2 can also be released near 300 °C at 280 psig reducing downstream compression costs of CO2 capture and storage. This work details thermodynamic equilibrium data that illustrates the optimal regeneration temperature and pressure, Fourier transform infrared data that shows the adsorbed species on the surface of the sorbents, fixed-bed performance testing, and the effect of moisture on regeneration. Additional information on potential heat integration during regeneration is also reported. These findings further demonstrate the ability of the Mg(OH)2 sorbent to capture CO2 from fuel gas streams in an IGCC plant efficiently.
1. INTRODUCTION Fossil fuels supply a majority of the world’s energy requirements. The use of fossil fuels within power plants is believed to be one of the main contributors to an increasing CO2 concentration in the atmosphere. Current CO2 removal technologies require a significant auxiliary load to the plant, which reduces its performance, resulting in a decreased net power output.1−3 Improved CO2 capture technologies are needed to minimize this impact and increase the efficiency of plants with CO2 capture. Integrated gasification combined cycle (IGCC) is a promising coal-fueled plant configuration considered for the current application. The IGCC plant effluent streams are at high pressure and elevated temperatures. Compared to standard pulverized coal power plants, the composition of the product stream in an IGCC plant also has a significantly higher concentration of CO2, which simplifies CO2 removal. A coal slurry is delivered into the gasification unit to produce a fuel gas from which contaminants (i.e., H2S, HCl, etc.) are removed immediately downstream. CO2 removal is generally carried out via a pressure swing CO2 absorption process, such as the stateof-the-art commercial Selexol process. Used in many industries, the commercial Selexol process uses liquid glycol ether to separate CO2 from high-pressure streams. The CO2-rich solvent is regenerated by reducing the pressure. Because CO2 is recovered at near 1 atm of pressure, the process results in a large CO2 compression train that requires large amounts of energy to operate, which reduces the overall efficiency of the power plant. In addition, because the Selexol CO2 removal process takes place below 50 °C, the fuel gas stream must be cooled from 200−350 °C. The Selexol process is also sensitive to moisture, requiring energy-intensive drying of the fuel gas. After CO2 separation, the fuel gas must be reheated to above 200 °C prior to introduction to the turbines; the large quantity of gas that is cooled and reheated results in a thermal loss reflected in an IGCC plant’s overall efficiency loss.4 A superior CO2 separation process would operate at 200−315 °C, thus © XXXX American Chemical Society
retaining a majority of the fuel gas’s thermal energy and contributing to improved efficiency of the IGCC plant. Additionally, if the capture media is capable of high-pressure regeneration, the energy requirement for CO2 compression may also be significantly reduced.5 Conducting the CO2 separation and contaminant removal processes at higher temperature will improve plant efficiency. Only a few studies related to regenerable sorbents with sufficient CO2 removal capacity at 150−350 °C are reported in the literature. Solid sorbents that separate CO2 at 450−550 °C have been reported; however, the regeneration temperatures and energies are very high.6 Zeolites were examined and found to have low CO2 capture capacities at temperatures above 200 °C.6−8 Nakagawa and Ohashi reported a lithium-based sorbent that captures CO2 at 450−550 °C.9 However, the regeneration temperature reported for the Li-based sorbents was near 800 °C, which requires a costly thermal swing for regeneration. Additionally, the heat of sorption and desorption with Li-based sorbents was reported to be very high, at 200 °C. Golden and Sircar reported a novel alkali-promoted hydrotalcite, alkaliimpregnated alumina, and double salt extrudates. The system used a pressure swing process to remove a majority of the CO2 from wet high-temperature gas.10 The pressure swing process recovers CO2 at low pressures requiring significant compression energy for sequestration. NETL previously reported the development of a sodium-based sorbent that could absorb CO2 at 200−315 °C.11,12 The Na-based sorbent required a temperature above 700 °C for regeneration. This large temperature swing coupled with the high heat of sorption/ desorption would require a significant amount of energy, making the process impractical for an IGCC application. We also reported a novel magnesium-based sorbent that captures Received: April 15, 2014 Revised: August 8, 2014
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CO2 between 200−315 °C and is regenerable at 400 °C, which is more compatible to an IGCC systems.13 The current report describes the development of the magnesium-based sorbent for IGCC applications and our recent findings. It builds from a previsous manuscript on material development followed by a second manuscript on process feasibility.5,13 This manuscript details the potential chemistry for an improved process. The Mg(OH)2-based sorbent possesses a high CO2 sorption capacity at 200−315 °C, which is significantly higher than the commercial Selexol process. The capture reaction proceeds as follows: Mg(OH)2 (s) + CO2 (g) → MgCO3(s) + H 2O
regeneration as well as the conditions favorable for singlestep regeneration. The intent of this study was to determine if single-step regeneration could be carried out under precombustion CO2 capture conditions. Past results have shown that regeneration at lower pressures is more favorable; however, the hydroxylation of MgO to Mg(OH)2 is favored at high pressures. To determine the dominating effect, regeneration and rehydroxylation were conducted at 0 psig and 280 psig.13 Thermodynamic simulation was carried out using 2 mol of MgO in 20 mol of H2O/N2 gas mixture with 1 mol of CO2 while varying the temperature. The simulation was then repeated for varying H2O/N2 ratios. The ratio of inert to CO2 was determined via mass spectrometer to be 20:1 in the experimental system and then matched in the thermodynamic simulation. Experimental testing also showed a maximum capacity of 5 mol/kg, which is nearly 38% of theoretical maximum of 13 mol/kg. Thermodynamic simulation accounted for this reduction by inputting 1 mol of CO2 for 2 mol of MgO. 2.2. Sorbent Preparation. The synthesis of the Mg(OH)2based sorbent containing a mixture of Mg(OH)2, bentonite binder, and a promoter has been described in detail elsewhere.11 To fabricate the sorbent, the powders were combined with water to form 1−2 mm pellets, which were dried at 150 °C as previously described.11 During bench-scale testing, a large amount of CO2 was sorbed rapidly during the initial capture followed a slow CO2 sorption for hours after the initial sorption. To understand this phenomena, a pore former (Biopolmer 110A) was added to the sorbent during synthesis. The sorbent was then dried with a heating rate of 1 °C/min and held at 600 °C for 6 h in air to remove the polymer pore former. The resulting porous sorbent pellets of 1−2 mm diameter were then characterized via SEM and tested in the fixed-bed reactor. 2.3. Fourier Transform Infrared Spectroscopy. To establish the reliability of the thermodynamic predictions, the surface concentration of the Mg(OH)2-based sorbent was monitored by Fourier transform infrared (FTIR) spectroscopy during a CO2 sorption and desorption cycle. The FTIR experiments were carried out in a Nicolet FTIR system with a Harris Praying Mantis diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) assembly, which is a highpressure and high-temperature reactor. The surface species on the sorbent were monitored continuously in situ. Additionally, a temperature-controlled water bubbler that can provided a 5 vol % H2O content to the gas stream was included in the gas inlet. The schematic of this system is shown in Figure 1. The sorbent was crushed via pestle and mortar and then sieved through a 100 μm mesh sieve to produce a fine powder for FTIR sampling. Approximately 100 mg of the powdered sorbent was placed in the reaction cell and sealed into the reactor. The reactor was then purged with N2 at 50 sccm while the sorbent was initially heated to 400 °C for 30 min to remove any adsorbed CO2 during material handling. The reactor was then cooled to 200 °C and pressurized to 280 psig. Moisture (4%) was introduced to the sample for hydroxylation for 4 h by passing the N2 through the bubbler. A gas stream of 15% CO2 in N2 was introduced to the reactor bypass for approximately 30 min. Capture was then initiated by redirecting the N2/CO2 gas flow into the FTIR reactor, which continued until the FTIR spectra came to a steady state. The CO2 and moisture flow was then replaced with a pure N2 flow, and the sorbent was regenerated by increasing the reactor temperature to 400 °C while maintaining 280 psig in flowing N2. The resulting single-
(1)
The carbonate formed during the reaction can be thermally decomposed to release CO2 and regenerate according to Reaction 2 at 375−400 °C: MgCO3(s) + H 2O → Mg(OH)2 (s) + CO2
(2)
Reaction 2 is referred to as one-step regeneration. Regeneration may also occur in a two-step reaction sequence, as shown in Reactions 3 and 4: MgCO3(s) → MgO(s) + CO2 (g)
(3)
MgO(s) + H 2O(g) → Mg(OH)2 (s)
(4)
The reaction of Mg(OH)2 with CO2 to form MgCO3 (Reaction 1) is exothermic with ΔH° value of −19.7 kJ/mol of CO2.5,13 The decomposition of MgCO3 (Reaction 3) is endothermic with ΔH° value of 100.9 kJ/mol occurring at 400 °C. The rehydroxylation (Reaction 4) is exothermic with a ΔH° value of −81 kJ/mol and occurs at 200 °C. Two-step regeneration is carried out in two separate reactor units where the endothermic reaction occurs at 400 °C and the exothermic reaction occurs at 200 °C. This temperature difference complicates integrating the heat from the reactions. Without heat integration, the decomposition of MgCO3 requires 35% of the power plant’s gross energy production, rendering the sorbent impractical. Heat integration between the hightemperature endothermic reaction and the low-temperature exothermic reaction is required to make this sorbent viable for industrial CO2 capture. The most direct and economical way to integrate the heat is to carry out the one-step regeneration shown in Reaction 2. This method requires only about 20 kJ/ mol of heat, reducing the regeneration duty from the reaction by 80%. The intent of this study is to examine the feasibility of one-step regeneration for capturing CO2 from power plants. To evaluate the feasibility of one-step regeneration, thermodynamic calculations were used to determine the conditions at which one-step regeneration could be performed. If the conditions become too extreme (high temperature, low moisture content, and/or low pressure), the sorbent will decompose to MgO, an undesirable product that will result in low heat integration and high plant energy consumption. The thermodynamic predictions were verified experimentally using Fourier transform infrared spectroscopy (FTIR) where the reaction progress can be monitored. One-step regeneration was then tested experimentally. A novel method of increasing the useable capture capacity is also presented.
2. MATERIALS AND METHODS 2.1. Thermodynamic PredictionsFactSage. FactSage 6.0 was used to determine the feasibility of single-step B
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Figure 2. Schematic of the HPR fixed-bed testing system with gas mixing and moisture into a heated cabinet to vaporize water and warm gases to the reaction temperature.
Figure 1. Schematic of the FTIR system with the DRIFTS accessory.
beam spectra were then processed into absorbance spectra for analysis. 2.4. Fixed-Bed Bench-Scale Testing. The fixed-bed reactor was used to determine the effects of the sorbent’s porosity on reactor performance. Sorbent porosity was controlled by adding a pore former during preparation. Standard sorbent pellets were placed into a fixed-bed vertical tubular reactor with a 0.5 in. diameter to form a 6 in. bed. The sorbent bed was heated in N2 to 200 °C and exposed to capture gas consisting of 28% CO2, 15% H2O, and 57% N2 in a downflow, packed bed configuration with a gas hourly space velocity (GHSV) of 500 h−1. Capture was completed when the CO2 outlet concentration matched the inlet concentration, which was monitored via mass spectrometer. After capture was completed, the sorbent was regenerated by flowing 15% H2O with 85% N2 at 500 h−1 GHSV through the reactor at 400 °C until CO2 was no longer present in the effluent as determined by a mass spectrometer CO 2 profile. Subsequent to regeneration, the reactor was cooled to 200 °C and 30% H2O in N2 was introduced for 2 h to hydroxylate the MgO to Mg(OH)2. Following hydroxylation, the CO2 capture cycle was repeated. A schematic of this system is shown in Figure 2. The empty reactor bed test was also conducted to determine the empty bed volume, and CO2 capture data were corrected for the empty bed dead volume. 2.5. Effect of Moisture Concentration on Regeneration. Single-step regeneration of the Mg(OH)2 sorbent requires a high temperature with high-pressure steam, as shown by the thermodynamic models. Experiments with a high concentration of steam were conducted in a flow reactor unit from Parr Instrument Company. This fixed-bed reactor has a 1 in. diameter and up to 6 in. bed height with 50 mL capacity. Preceding the fixed bed is a metal spiral auger that heats the incoming gas and vaporizes liquid water. The testing apparatus includes two mass flow controllers and a water pump with heattraced inlet lines for the water. The effluent was passed through a water chilled condenser maintained at 5 °C to remove the moisture. The dried effluent was monitored by mass spectrometer. The Mg(OH)2 sorbent was loaded into the reactor, resulting in a 45 mL bed weighing 63 g. The sorbent was heated to 400 °C to remove residual CO2 and H2O. Then the CO2 containing gas flow was directed to the bypass mode until the gas concentration reached steady state on the mass spectrometer. To initiate CO2 capture, the reactor was switched from bypass
to online mode, exposing the sorbent to 30 vol % H2O, 12 vol % CO2 and balance N2, and a flow rate of 1,000 h−1 GHSV at a pressure of 280 psig. The effluent concentration was monitored by mass spectrometer. The CO2 capture cycle was complete when the effluent concentration matched the reactor inlet concentration. Following CO2 capture, the single-step regeneration was carried out by heating the reactor bed while increasing the moisture content in the inlet gas stream. To maintain a dry gas flow for the mass spectrometer, the GHSV had to be increased as the moisture content increased. Initial single-step regeneration was conducted by replacing the dry N2 with 25 vol % H2O in N2 and heating the reactor to 314 °C with a GHSV of 1000 h−1. Once the CO2 concentration was below 1%, the temperature, moisture content, and GHSV were increased in three steps. The single-step regeneration was carried out over a 2 day period due to the slow rate of regeneration.
3. RESULTS AND DISCUSSION 3.1. Thermodynamic PredictionsFactSage. FactSage 6.0 was used to calculate the thermodynamic equilibrium of Mg(OH)2 in various environments. The input to the simulation was 13 mol of H2O (65 mol %), 7 mol of N2, 1 mol of CO2, and 2 mol of MgO. These results are shown in Figure 3, Mg(OH)2 decomposition to MgO initiates at 260 °C at 0 psig
Figure 3. FactSage 6.0 thermodynamic results with 65 mol % moisture at 0 and 280 psig. C
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(top) and at 280 psig (bottom). As the temperature increases, the MgCO3 reacts with the steam to form Mg(OH)2 until both species decompose into MgO. Additionally, at 65 mol % moisture concentration, only 28% of the MgO will be converted to Mg(OH)2 form while the balance will remain as MgCO3 suggesting only 28% of the sorbent will be regenerated with one-step moisture regeneration at 260 °C. With 65 mol % H2O at 260 °C, 39% MgCO3 can be regenerated. Figure 3 shows the thermodynamic simulation at 65 mol % H2O, indicating a maximum temperature of 280 °C at 0 psig; 50% of the MgCO3 can be converted to Mg(OH)2. These calculations were then repeated at 280 psig. The general trend observed was that increased amounts of moisture allowed higher temperatures for single-step regeneration to form Mg(OH)2 from MgCO3. Additionally, higher pressure increased both the decomposition temperature of Mg(OH)2 into MgO and the fraction of sorbent that could be regenerated to Mg(OH)2 via single-step regeneration. At 280 psig and 65% steam (Figure 3), the sorbent could proceed with a complete single-step regeneration temperature as high as 360−375 °C. These results indicate that experimental tests for single-step regeneration should be conducted at high pressures for complete regeneration. In addition, because CO2 is released at high pressure, compression costs for sequestration or enhanced oil recovery could be minimized. High steam concentrations could be realized in practice via a counter current regenerator. In this type of reactor the steam would flow upward while the sorbent pellets flow downward in a moving-bed configuration. As the sorbent nears the regenerator exit, the concentration of steam will increase to nearly 100%. These results suggest that one-step regeneration with 50% conversion could easily be achieved in practice, reducing the heat duty for the reaction of regenerating the sorbent from MgCO3 to MgOa significant savings. These results also provide information on temperatures and conditions at which single-step regeneration could be achieved. 3.2. FTIR. The FTIR was used to (1) qualitatively verify the thermodynamic predictions and (2) determine the type of CO2 surface species adsorbed on the sorbent. The wavenumber 1597 cm−1, which corresponds to the carbonate stretching frequency, was used to determine the rate of carbonation and to evaluate the accuracy of the thermodynamic predictions.14,15 The experiment was carried out by first exposing the sorbent at 100 °C to 100% CO2 that was humidified via a water bubbler at room temperature. When the FTIR signals reached steady state, N2 was purged through the system until the gas phase CO2 was removed. The system was then slowly heated to allow the sorbent to reach equilibrium. Thermodynamic calculations with FactSage were conducted by simulating 1 mol of MgCO3 and 50 mol of H2O in 1000 mol of N2. The extensive gas/vapor phase allows for a low partial pressure of CO2 to simulate the environment in a flowing fixed bed as found in the FTIR. The results of the thermodynamic simulation were then normalized from 0 to 1 using the formula (X − Xmin)/(Xmax− Xmin) and compared with the experimental results from the FTIR. The resulting graph in Figure 4 shows that the experimental data and thermodynamic predictions are very similar. Figures 5 and 6 show the results of reloading the sorbent into the FTIR and performing CO2 capture with 5% CO2 balance N2 with 3% moisture over a 10 h period. The single beam spectra obtained were converted to adsorbance spectra against the initial spectra just prior to introducing CO2 into the reactor. The resulting figure shows the absorbance spectra, where
Figure 4. Comparison of carbonate formation in the FTIR (green square) versus the thermodynamic prediction (solid black line).
Figure 5. FTIR absorbance spectra during CO2 adsorption.
Figure 6. FTIR absorbance spectra during CO2 adsorption.
upward trends indicate an increase in the associated species. Figure 5 shows the carbonate bands forming from the adsorption of CO2 on to the Mg(OH)2-based sorbent, whereas Figure 6 shows the disappearance of the −OH band at 3757 cm−1 as the −OH sites are consumed to form carbonates. Figure 6 shows two distinct types of carbonates that are forming, a bridged carbonate at wave numbers 1677 and 1352 cm−1 and a bidentate carbonate at 1597 cm−1.14,15 The bidentate carbonate CO stretch is associated with the 1597 cm −1; however, the 1280 and 1030 cm−1 bands associated with asymmetric and systemic stretch were either masked by the FTIR and drifts system and/or not strong enough to be observed. Interestingly, after 10 min of adsorption, the bridged carbonate achieves steady state while the bidentate carbonate continues to increase for the full 10 h. The bidentate carbonate D
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increase corresponds to the continuous decrease in the −OH band at 3757 cm−1 for 10 h. 3.3. Fixed-Bed Bench-Scale Testing. The fixed-bed bench-scale reactor was used to test the effect of adding the pore former to the sorbent as described in the experimental section. Figure 7 illustrates capture results during tests with
Figure 8. SEM images of the standard sorbent (left) and the sorbent with the pore former after high-temperature drying (right).
Initially, a typical capture cycle consisting of CO2 capture, regeneration, and hydroxylation at conditions similar to previous studies was conducted. The sorbent was then exposed to CO2, and single-step regeneration was carried out at varying steam concentrations. One-step regeneration was conducted for about 15 h. Figure 9 illustrates the mass spectrometer CO2
Figure 7. Effect of porosity of the sorbent in the fixed-bed reactor.
both the standard material without the pore former and the material with a pore former. Quantification of the CO2 released during the regeneration indicated both materials captured about 5 mol/kg but had different characteristics during the capture phase. However, examination of the CO2 profile during capture, shown in Figure 7, with the standard material indicates more than 90% of the CO2 for nearly 10 min after which the sorbent only captured about 15% of the CO2. The sorbent with the pore former showed a similar trend at first, but the initial CO2 capture phase with an efficiency greater than 90% CO2 removal continued for more than 20 min, almost doubling the useable CO2 capture capacity. This is important because the initial capacity is what will be utilized in a capture scheme, any capacity after the breakthrough (
