CO2 recycling by reaction with renewably-generated hydrogen

ly ic N International Journal of Greenhouse Gas Control 4 (2010) 44–50 ys O2 ac ca so N2 d ito . lar tio Contents lists available at ScienceDirect International Journal of G e 1. Introduction Growing concerns about global climate change have increased attention on various approaches to reduce emissions of CO2 and other GHGs. Considerable efforts are currently underway to develop practical and affordable ways to capture CO2 from major point sources, such as fossil-fueled power plants, and dispose of this CO2 bymeans of geologic sequestration. In the U.S., the electric power generation sector alone is responsible for over 40% of the country’s total CO2 emissions (U.S. DOE Energy Information Administration, 2008). As shown in Fig. 1, emissions from this sector have grown steadily over the past two decades. Several relatively small-scale carbon capture and sequestration (CCS) approaches are currently in development and demonstration stages. However, CCS still faces several technical and economic challenges. Also, CCS generally entails considerable parasitic energy loads, effectively reducing the energy output of a power plant and increasing the costs of producing electricity. Numerous other approaches are being explored to utilize the emitted CO2 either directly (as in enhanced oil recovery) or as a chemical feedstock for producing other useful products (Ritter, 2007). One product receiving considerable attention is methanol, which is commercially produced by reaction of CO2 with hydrogen (H2). However, methanol production utilizing CO2 in flue gas from a fossil-fueled power plant seems uneconomic at the present time (Cifre and Badr, 2007). Another well-known process for converting CO2 to a useful product involves reaction with H2 over a metal catalyst to produce methane. This methanation process, also called the Sabatier reaction, has been studied extensively (Naumov and Krylov, 1979a,b; Fujita et al., 1991, 1993). The same process is effective in converting carbon monoxide (CO) to methane. Methanation of both CO and CO2 are highly exothermic, as indicated by the chemical reactions shown below: CO2þ4H2 $ CH4þ2H2O ðDH298¼ �165kJ=molÞ (1) CO þ 3H2 $ CH4þH2O ðDH298¼ �206kJ=molÞ (2) Thoughmany different metals have been used to catalyze these methanation reactions, nickel and ruthenium are two of the most effective (Zhilyaeva et al., 2002; Yaccato et al., 2005). Common applications of this methanation process include scrubbing traces of CO and CO2 from H2 streams used in fuel cells (Takenaka et al., 2004; Ledjeff-Hey et al., 2000), purification of H2 used to manufacture ammonia (Du et al., 2007), removal of CO2 from confined spaces such as submarines and spacecraft, and synthesis of propellant for Martian spacecraft (Hu et al., 2007). In addition, these methanation reactions are utilized in gas chromatograph (GC) detectors that convert CO and CO2 to methane for improved sensitivity. A few researchers have discussed possible use of Sabatier methanation in reducing CO2 emissions from power plants (Inui, 1996; Hashimoto et al., 1999, 2002). However, this is only helpful for GHG mitigation if the H2 used in the reaction comes from non-fossil sources. Furthermore, this approach requires large volumes of economically produced H2. These challenges have not * Corresponding author. Tel.: +1 775 674 7065; fax: +1 775 674 7016. E-mail address: [email protected] (S.K. Hoekman). 1750-5836/$ – see front matter � 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.ijggc.2009.09.012 CO2 recycling by reaction with renewab S. Kent Hoekman *, Amber Broch, Curtis Robbins, R Desert Research Institute, Division of Atmospheric Sciences, 2215 Raggio Parkway, Reno, A R T I C L E I N F O Article history: Received 23 January 2009 Accepted 21 September 2009 Available online 17 October 2009 Keywords: CO2 recycling Greenhouse gas Methanation Sabatier reaction Renewable hydrogen A B S T R A C T A laboratory-scale reactor s reacting carbon dioxide (C according to the Sabatier re commercial methanation electrolysis of water (from custom blend of 2% CO2 in temperature, flow rates, an extent of reaction was mon CO2 occurred at 300–350 8C about 10,000 h�1with amo by increasing the H2/CO2 ra journal homepage: www. -generated hydrogen hard Purcell V 89512, USA tem was built and operated to demonstrate the feasibility of catalytically ) with renewably-generated hydrogen (H2) to produce methane (CH4) tion: CO2 + 4H2! CH4 + 2H2O. A cylindrical reaction vessel packed with a talyst (Haldor Topsøe PK-7R) was used. Renewable H2 produced by lar- and wind-generated electricity) was fed into the reactor along with a , meant to represent a synthetic exhaust mixture. Reaction conditions of gas mixing ratios were varied to determine optimum performance. The red by real-time measurement of CO2 and CH4. Maximum conversion of Approximately 60% conversion of CO2 was realized at a space velocity of ratio of H2/CO2 of 4/1. Somewhat higher total CO2 conversionwas possible , but the most efficient use of available H2 occurs at a lower H2/CO2 ratio. � 2009 Elsevier Ltd. All rights reserved. reenhouse Gas Control l sev ier .com/ locate / i jggc yet been fully overcome. However, there are numerous efforts underway to develop improvedways of producingH2 via enhanced electrolysis of water, direct photolytic water splitting, thermal- catalytic water splitting, and other means. Exciting laboratory reports about enhanced H2 production pathways appear in the issue. In small-scale applications, micro-channel reactors have been used because of their excellent thermal control (VanderWiel et al., 2000; Brooks et al., 2007). In large-scale commercial applications, some active means of temperature control would be required, with the produced heat being utilized as a valuable product of the overall process. The objective of our work was to demonstrate the feasibility of reducing CO2 in flue gas via methanation utilizing renewably- generated H2. Although not employed in these initial experiments, the ultimate goal is to capture the producedmethane and recycle it into a natural gas power plant as supplemental fuel, thereby improving overall fuel efficiency and reducing total GHG emissions from the plant. 2. Methodology A laboratory-scale reactor system was designed and built to enable control of reaction conditions andmonitoring of products. This systemwas installed and operated in a portable trailer, which provided good access and ventilation. A schematic of this system is shown in Fig. 2. Individual components of the system are described below. 2.1. Reactant gases Fig. 1. U.S. CO2 emissions by sector – 1990–2007 (million metric tons). S.K. Hoekman et al. / International Journal of Greenhouse Gas Control 4 (2010) 44–50 45 literature with increasing frequency (Kanan and Nocera, 2008). Given these developments, and the increased urgency in addres- sing GHG emissions, it seems warranted to now explore more thoroughly the application of CO2 recycling from power plant emissions via methanation processes. Depending upon specific applications and catalyst type, the Sabatier methanation reaction is typically conducted at tempera- tures of 250–400 8C. At higher temperatures, catalyst integrity can become a concern, and the reverse reaction becomes more significant. In fact, steam methane reforming (SMR) – which is the reverse of the Sabatier reaction – is commercially employed to manufacture H2 (Barelli et al., 2008). SMR is generally conducted at temperatures of 800–1000 8C. Due to the highly exothermic character of the Sabatier reaction, heat removal from the reactor becomes an important Fig. 2. Schematic of CO All H2 used in this work was produced from renewable energy sources. Electricity provided by a 1.0 kW Siemens PV array was directed to a small Proton Energy PEM electrolyzer located inside the trailer. This unit is capable of producing up to 600 cm3/h of H2 at a pressure of 200 psi. The high purity (99.999%) H2was stored on board the trailer in 4 storage tanks, having a total capacity of roughly 0.1 kg of H2 at 200 psi. A custom-blended gas mixture of 2% CO2 in N2 was used as a ‘‘synthetic exhaust’’ formethanation.While this CO2 concentration is considerably lower than typically found in gas turbine flue gases, it allowed us to explore a variety of experimental conditions without causing an excessive temperature rise within the catalytic reactor. H2 flow rates were controlled by an electronic flow controller, operated by a National Instruments compact Field Point (cFP) programmable system, using Labview 7.1 software. Flow 2 recycle system. S.K. Hoekman et al. / International Journal of Greenhouse Gas Control 4 (2010) 44–5046 rates of the synthetic exhaust were controlled manually, but monitored electronically. Mixing of theH2 and synthetic exhaustwas accomplished in the preheating section of the system. Four 200-W heaters were used to increase the temperature of the feed gases as desired. These heaters were controlled manually with their own proportional integral-derivative (PID) controller, which is a generic control loop feedback mechanism, connected to a thermocouple in the preheating section. 2.2. Methanation reactor A schematic of the methanation reactor used in this study is shown in Fig. 3. The vessel is 304 L stainless steel, with an outer diameter of 88.9 mm and a wall thickness of 4.6 mm. The total length of the reactor cylinder is about 16.5 cm (6.5 in.). The catalyst packed bed, which is supported between 30 mesh 304 stainless steel screens, occupies a length of 10.9 cm (4.3 in.), and has a volume of 0.55 L (33.3 in.3). Flow through the reactor was top to bottom, in accordance with the catalyst manufacturer’s recom- mendation. Two thermocouple ports were installed into the side of the reactor so that catalyst temperatures could be monitored throughout the experiments. The upper and lower thermocouples Fig. 3. Schematic of methanation reactor. measured temperatures 1/3 and 2/3 of the way into the catalyst bed, respectively. The reactor was heated to operating tempera- tures using two 400-W band heaters. A photograph of the reactor system installed within the portable trailer is shown in Fig. 4. 2.3. Methanation catalyst A commercially available methanation catalyst (Haldor Topsøe PK-7R) was used in all experiments. This material consists of Ni andNiO on an alumina substrate. It has a total nickel loading of 20– 25%, and a recommended operating temperature range of 190– 450 8C. This catalyst has an extruded ring shape (OD of 5 mm; ID of 2.5 mm) which decreases the pressure drop and increases the surface to volume ratio as compared to spherical or cylindrical shaped catalysts. Prior to use, the catalyst was activated within the reactor by passing 5 standard liters per minute (slpm) of 0.5% H2 in N2 through the bed at 300 8C for 1–2 h. Before eachmethanation experiment, the reactor was heated to the desired temperature (asmeasured by the upper thermocouple) with no gas flow. After initiating reactant gas flow though the reactor, the exothermic methanation reaction caused a tempera- ture rise within the catalyst bed, with the lower (downstream) thermocouple location generally being hotter than the upper location. The extent of this temperature increase is determined by the composition of the feed gas. According to themanufacturer, the following temperature increases can be expected: � 75 8C per mole % of CO converted. � 60 8C per mole % of CO2 converted. � 165 8C per mole % of O2 converted. 2.4. Gas analyzers To characterize the performance of the system, the composi- tions of the gases before and after the catalytic reactor were measured in each experiment. Two different gas analyzers were used. A DJ Electronics 5-gas analyzer with capabilities to measure O2, CO, CO2, NOx and THC was used upstream of the reactor. A DJ Electronics Methane analyzer with capabilities to measure methane, CO2, CO, O2 andNOxwas used downstreamof the reactor. The DJ 5-gas analyzer utilizes an Andros 6251A gas benchwhich has a non-dispersive infrared (NDIR) sensor for CO, CO2, and HC measurements, and electro-chemical sensors for O2 and NOx measurements.We found in these experiments that theNOx sensor also responds to H2. Since NOx measurements themselves were of no interest in these experiments, we used the NOx signal as a surrogate for H2, which was useful in synchronizing data streams from various sources. The methane analyzer used to measure the reactor exhaust stream utilizes an NDIR sensor for CO2 and CH4, and electro- chemical sensors for O2, NOx, and CO. The CO cell was configured to measure concentrations of 0–2400 ppm. However, this CO sensor also had a cross-sensitivity to H2, which resulted in saturation of the chem-cell from high concentrations of H2 in the exit gases under certain conditions. Because of this, the CO channel was turned off throughout most tests so as not to degrade the sensor. Both gas analyzers were calibrated using a variety of purchased standard gas mixtures. Separate calibrations were performed for CO2 and CH4. 2.5. System control All experiments were controlled through use of a computer control system, PID temperature controllers, and human inter- vention. Temperatures of the gasmixing chamber and the catalytic reactor were controlled by two PID controllers. The cFP computer etha S.K. Hoekman et al. / International Journal of Greenhouse Gas Control 4 (2010) 44–50 47 unit controlled the H2 flow rate and monitored the total gas flow rate into the reactor. A laptop computer monitored and logged all temperature data. The two gas analyzers continuously recorded gas concentrations in the inlet and exhaust streams. In addition, the computer unit was used for safety control by monitoring two hydrogen sensors set up to detect H2 leaks within the trailer. The computer unit also had control to shut down the system if temperatures in the reactor exceeded 450 8C, the recommended maximum temperature for the catalyst. Warning lights inside and outside the trailer were programmed to alert Fig. 4. Photograph of m users if either a leak or a temperature exceedance occurred. 3. Experimental conditions The aim of these experiments was to demonstrate the proof of concept of recycling CO2 from a synthetic combustion gas and to determine the optimum reactor parameters (temperature and flow rates) to maximize the CO2 conversion under H2-limited condi- tions. This was accomplished through a series of tests that investigated the methanation reaction under various operating conditions. In particular, reactor temperature, reactant gas mixing ratios, and total reactant feed rate were explored. All tests were conducted using a synthetic exhaust consisting of 2% CO2 in N2. This low CO2 concentration was chosen to minimize the temperature increase during the reaction but still have sufficient gas concentrations for reliable measurements. 3.1. Effect of reactant gas mixing ratios and catalyst temperature Due to the complexity and expense of producing H2 from renewable sources, a goal of any such CO2 recycling approach is to utilize the available H2 in the most efficient manner possible. Therefore, it is important to investigate the effects of reactant gas mixing ratios on CO2 conversion. To do this, a series of experiments was conducted using different mixing ratios of H2 to CO2 fed into the reactor. In these experiments, the synthetic exhaust feed rate was held constant at 81.5 slpm. At this level, a 6.5 slpm feed rate of H2 is required for a stoichiometric Sabatier reaction (H2/CO2 = 4/1). The actual H2 flow ratewas varied (using the cFP control system) from0 to 9.8 slpm in discrete steps representing 0%, 50%, 100% and 150% of the stoichiometric amount of H2 required to react all available CO2. Flow rates at each step of the test were held constant until catalyst temperature and exhaust gas concentration readings appeared relatively stable. Stabilization typically occurred in less than 5 min. The seven steps of this test sequence are illustrated in Fig. 5. The temperature of the catalytic reactor is expected to have a large effect on the CO2 methanation rate. To test the effects of nation reactor system. temperature, the entire seven-step test sequence was conducted at four reactor temperatures: 200, 250, 300, and 350 8C. In each case, the temperatures of both the pre-heater and the catalytic reactor were set to the desired point prior to introduction of the reactant gases. 3.2. Effect of space velocity Longer residence time of the feed gases in the reaction chamber increases the conversion of CO2 to CH4. Residence time ismeasured by a term called space velocity, which indicates the relationship between the volumetric flow rate and the volume of the reactor. (For our determination of space velocity, only the volume of the packed catalyst bedwas used, not the volume of the entire reaction chamber.) Space velocity defines how many reactor volumes of feed can be treated per hour; it is expressed in units of h�1. As space velocity increases, residence time decreases, and it is expected that the conversion of CO2 will decrease. To test this and determine the optimum flow rate, we conducted a series of experiments in which the total flow rates of the feed gases were varied at a constant H2/ CO2 molar ratio of 4/1. The baseline condition for this experiment was defined as the space velocity at a synthetic exhaust flow rate of 81.5 slpm, a stoichiometric amount of H2 (6.5 slpm), and a reactor temperature of 300 8C. The space velocity under this baseline condition was approximately 9000 h�1. Space velocities were then varied in steps of 25% of the baseline case by adjusting the total reactant gas feed rates while maintaining a stoichiometric mixture. S.K. Hoekman et al. / International Journal of Greenhouse Gas Control 4 (2010) 44–5048 4. Data analysis and reduction Experimental data were collected from three different sources: (1) the cFP system, (2) the inlet gas analyzer, and (3) the exhaust gas analyzer. The datawere coordinated using the time stamp from each of the three sources. Additionally, because the NOx sensor in the inlet analyzer was sensitive to H2, data from the NOx channel were used to indicate changes in the H2 flow. The total flow rate exiting the reactor was not measured, so the gas concentration data from the exhaust gas analyzer were used along with the inlet flow rates to calculate the flow rate of each exhaust gas. Due to three factors, the total gas flow rate out of the reactor was slightly different from the rate into the reactor: (1) the methanation reaction results in fewer moles of products than moles of reactants, (2) water in the product stream was dried before reaching the gas analyzers, and (3) some reactant flow was diverted to the inlet gas analyzer, and never passed through the Fig. 5. 7-Step test sequence to investigate Fig. 6. Experimental data from methanation reaction at 300 8C. Vertical s reactor. Therefore, various adjustments were necessary to calculate the correct exhaust gas flow. The experimental data were collected over finite time periods, giving a series of values for each test. For example, the test results from the seven-step test at 300 8C are shown in Fig. 6. To make comparisons between tests, a single point was chosen to represent the results at each step. This point was selected where the temperatures and gas concentrations were stabilized. As seen in Fig. 6, the lower catalyst temperature increased with each increase in H2 flow, due to the exothermic reaction, but began to stabilize after an initial jump in temperature. The range of data where the temperature was the most stable was used to determine the point values. The averages of all values occurring within this range (indicated by the shaded bands in Fig. 6) were used to define a single result at that condition. Raw data from the gas analyzers also required some corrections prior to final data analysis. The analyzers are intended for use in effects of reactant gas mixing ratio. haded bars represent regions of stable operation for data collection. nominal, target temperature. Results from the 200 8C test show low CO2 conversion at all reactant ratios. Distinctly higher conversion rateswere observed at 250 8C,withmaximumconversion occurring in the range of 300–350 8C. The slight differences in conversion rate observed between the 300 and 350 8C condition may simply represent the variability in our system, but could also indicate a small degree of the reverse Sabatier reaction occurring at higher temperatures. These tests show that 300–350 8C seems to be the optimum operating temperature range for our catalytic reaction system: it provides sufficient heat to initiate the methanation reaction, but the additional exotherm from the reaction is not so large that it reverses the reaction significantly or jeopardizes the integrity of the catalyst material. Fig. 7 also shows the variation of CO2 conversion rates as a function of H2/CO2 ratios. With a stoichiometric amount of H2 (H2/ CO2 = 4/1), approximately 60% conversion of CO2 was achieved. When less than the stoichiometric amount of H2 was added to the system (H2/CO2 = 2/1), a maximum of about 30% of the CO2 was Fig. 7. CO2 methanation results: effects of catalyst temperature and reactant gas mixing ratio. S.K. Hoekman et al. / International Journal of Greenhouse Gas Control 4 (2010) 44–50 49 automotive applications to sample tailpipe emissions. For such applications, the analyzers are designed to zero out against atmospheric conditions every time they are started-up, to correct for gases that are already present in the air. For our experiments, such zeroing was inappropriate and needed to be eliminated. Additionally, the methane analyzer that we used had significant problems with drifting and calibration that required frequent adjustment. For these reasons, all methanation results reported here are expressed on the basis of CO2 conversion, as defined below in Eq. (3). %CO2 conversion ¼ CO2 in� CO2 outCO2 in � ð100Þ (3) 5. Results and discussion 5.1. Effects of catalyst temperature and reactant gas ratios At each temperature, the entire seven-step test of various reactant gas mixing ratios was performed. To compare the results of all experiments, a single point representing the average of stable operation at each condition was used. Two points each were measured at H2/CO2 ratios of 2/1 and 4/1, since these conditions comprised two of the seven steps in each experiment. Fig. 7 shows the CO2 conversion for each H2/CO2 ratio at each temperature. Actual temperatures measured at the upper catalyst thermocouple location are shown in this figure, rather than the Fig. 8. H2 efficiency in CO2 methanation: effects of catalyst temperature and reactant gas mixing ratio. converted. Adding excess H2 into the system (H2/CO2 = 6/1) increased the conversion of CO2 to a maximum around 80%. In large-scale applications, H2 is expected to be the limiting reactant. Therefore, it is important to determine the amount of CO2 conversion compared to the amount of H2 available, as shown in Fig. 8. In the Sabatier reaction, only 1/2 of the H2 reactant is converted to CH4; the other 1/2 is converted to water. Additionally, 2 moles of H2 are required to produce one mole of CH4. Therefore, themaximum theoretical CO2 conversion that could be achieved is only 25% per mole of H2 used. Fig. 8 shows that the efficiency of H2 utilization is best at lower H2/CO2 ratios. A value of 16% CO2 conversion per mole of H2 was achieved at a H2/CO2 ratio of 2/1. The conversion efficiency dropped to about 14% and 12% for H2/CO2 ratios of 4/1 and 6/1, respectively. 5.2. Effect of space velocity Tests to investigate the effect of space velocity on CO2 methanation were conducted at an upper catalyst temperature of 300 8C and a stoichiometric ratio of H2/CO2 = 4/1. Both the synthetic exhaust flow and H2 flow were adjusted in step to maintain this stoichiometric ratio while changing the total flow rate. In this manner, the space velocity was varied from 50% to 200% of the baseline condition of 81.5 slpm. For data analysis, the same method of selecting a single point for comparison was used as described above. The CO2 conversion results at seven different space velocities are plotted in Fig. 9. At lower space velocities, the gases have a longer residence time in the reactor, which results in higher Fig. 9. Effect of space velocity on CO2 methanation. Catalyst temperature of 300 8C; stoichiometric H2/CO2 molar ratio of 4/1. amounts of CO2 conversion. As the flow rate increases, the amount of CO2 conversion decreases. A linear trend line through the data illustrates the overall decrease in CO2 conversion from 70% at 4000 h�1 to 55% at 20,000 h�1. 6. Conclusions and recommendations A series of experiments was conducted to test the concept of recycling CO2 in combustion exhaust tomethane, and to determine the most effective operating conditions for this process. The experiments included testing the effects of the reactant gas mixing ratios of H2 to CO2, the catalyst temperature, and the space velocity. The results confirm the proof of concept and offer guidelines for the most effective operating conditions. A methanation catalyst temperature in the range of 300–350 8C was shown to provide maximum conversion of CO2 to CH4. Operating temperatures above 350 8C should be avoided to fabricating the reactor system is acknowledged, as well as the expert help of Ms. Vicki Hall in preparation of this manuscript. References Barelli, L., Bidini, G., Gallorini, F., Servili, S., 2008. Hydrogen production through sorption-enhanced steam methane reforming and membrane technology: a review. Energy 33, 554–570. 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Hoekman et al. / International Journal of Greenhouse Gas Control 4 (2010) 44–5050 minimize reversal of themethanation reaction andpreserve catalyst lifetime. Approximately 60% conversion of CO2was observed in this temperature rangeusingastoichiometric ratioofH2/CO2of4/1anda space velocity of approximately 10,000 h�1. Although more CO2 is converted with higher concentrations of H2, there is decreasing conversion efficiency for each additional mole of H2 added. Because H2 is likely tobe thefinancially limiting reactant in the system,using excessive amounts of H2 should be avoided. Therefore, a stoichio- metric ratio of 4:1 H2:CO2 (or less) is recommended. The space velocity tests showed that increasing flow rates through the catalytic reactor reduced CO2 conversion efficiency. However, the impact was not large; only a 15% reduction in CO2 conversion occurred (from 70% to 55%) over a five-fold increase in space velocity (from 4000 to 20,000 h�1). 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