Ind. Eng. Chem. Res. 2010, 49, 11073–11078
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Assessment of Methanol Synthesis Utilizing Exhaust CO2 for Chemical Storage of Electrical Energy Liisa K. Rihko-Struckmann,*,† Andreas Peschel,† Richard Hanke-Rauschenbach,† and Kai Sundmacher†,‡ Max Planck Institute for Dynamics of Complex Technical Systems, and Otto-Von-Guericke UniVersity Magdeburg, D-39106 Magdeburg, Germany
The thermodynamic and operational boundaries to store electrical energy chemically are evaluated in this contribution. Methanol is considered as a candidate for chemical energy storage. The production of methanol from exhaust CO2 could be one way to recyle CO2 and lower the global CO2 emissions. Energetic analysis reveals that exergy losses are most severe in the parts of the system when electrical energy is converted to chemical (electrolysis) and when chemical energy is converted to electrical (power generation). In methanol production, the exergetic efficiency is 83.1%, when the chemical exergy of hydrogen and methanol, the exergy of the power input and the released heat are taken into consideration. The exergetic efficiency of the overall energy conversion-storage system including methanol as storage medium was evaluated to be between 16.2 and 20.0% depending on the applied conversion technology. Methanol is suitable not only as stationary energy storage, but it could also be used as fuel for transportation. The energy storage system with hydrogen as storage medium shows higher exergetic efficiency than the methanol route. However, the storage of hydrogen is clearly more complex and cost-intensive. 1. Introduction The geological sequestration of CO2 has been regarded as one promising solution to reduce the ever increasing atmospheric CO2 emissions caused by burning fossil fuels in power plants.1,2 Sequestration is principally an available and technologically feasible way to reduce the CO2 emission into the atmosphere, although there are still obvious ecological, environmental, and safety aspects to clarify before it can be implemented in large scale.3 However, the sequestration technologies handle the concentrated CO2 stream mostly as a waste to dispose. The highly complex removal process by aqueous amine absorption and recovery procedure does not produce added value for the power plant, but causes significant additional costs and a remarkable efficiency deficit. The utilization of CO2 as carbon source for chemical synthesis and the production of fuels has got a great deal of attention because it has been emphasized that the reutilization and recycle of the exhaust CO2 could contribute positively to the global climate change.4-7 CO2 is used industrially as feed stock, for example, in the production of urea, inorganic carbonates, pigments, or salicylic acid. As a new application, a commercial methanol production plant using recycled CO2 will be launched in 2010 in Iceland.8 Furthermore, CO2 is cofed to CO stream in the industrial production of methanol. However, the existing production of various chemicals from CO2 can have a positive but only insignificant impact on the global carbon balance as the total amount of CO2 used in these processes is negligible compared to the global CO2 emissions. The utilization of CO2 in the fuel production or as a chemical storage of energy could, in turn, make a significantly larger impact as only 16.8% of the world oil consumption was used in 2007 for nonenergy purposes.9 * To whom the correspondence should be addressed. E-mail:
[email protected]. Tel: +49-391-6110318. † Max Planck Institute for Dynamics of Complex Technical Systems. ‡ Otto-von-Guericke University Magdeburg.
Solar and wind energy are highly desired as renewable energy sources because the renewable energy can be directly converted to electricity by wind turbines and solar cells. However, the generation of electricity by these renewable sources suffers from intermittent and fluctuating character. It has not only daily fluctuations but also the seasonally varying meteorological conditions that directly influence the quantity of the produced electricity. Furthermore, the general demand and the supply of solar and wind electricity do not match with each other, which necessitates a storage of electricity. Most common storage systems for electricity, for example, water reservoirs, pumpedstorage power stations, flywheels, NaS, vanadium redox flow and Li ion batteries,10 suffer either from high costs or from limited system capacity and unfavorable discharge/loading characteristics. Hydrogen can be seen as a chemical storage for renewable electricity because it can be produced by electrolyzing water to hydrogen and oxygen by applying electrical power. Hydrogen is a clean fuel, its burning causes no harmful emissions, and it has a gravimetric heating value three times higher than typical hydrocarbon fuels. On the other hand, hydrogen, as a very small molecule, is a light gas at ambient conditions, and is extremely cost-intensive to store. Basically, five methods can be applied to store hydrogen: compression, liquefaction, physisorption, and the use of metallic and complex hydrides.11 The low gravimetric or volumetric hydrogen storage density of all these methods is the main difficulty in the storage and utilization. However, hydrogen is likely the key component also in the CO2 reutilization, as discussed below. Our objective is to evaluate the existing thermodynamic limitations, the energetical and exergetical efficiency of the reuse of CO2 in systems storing electrical energy available in the future from fluctuating renewable energy sources. A system with liquid storage medium, applying CO2 as a carbon source in the production, is compared to a system where hydrogen is used directly as the storage medium.
10.1021/ie100508w 2010 American Chemical Society Published on Web 09/17/2010
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Figure 1. Optional routes for the use of CO2 to store electrical energy chemically on large scale. MTBE, methyl-tert-butyl ether; DMC, dimethyl carbonate; DME, dimethyl ether. Processes: MTG, methanol to gasoline; F-T, Fischer-Tropsch; R-WGS, reverse water gas shift; DR, dry reforming.
2. Evaluated Process Routes Figure 1 illustrates the main routes where CO2 could be involved to store energy chemically. Methanol (CH3OH), methyl-tert-butyl ether (MTBE), dimethyl carbonate (DMC), and dimethyl ether (DME) are candidates that could be used to store energy in chemical compounds. Furthermore, these chemicals could be used also as traffic fuel, where they are applicable easily with minor modifications with the existing infrastructure and vehicle fleet. The production of MTBE, DMC, and DME requires methanol as feed, and therefore, it is likely that the energetic efficiency is lower than that of methanol. Therefore, in the present evaluation we concentrated on the methanol synthesis. Aside to the production of the above-mentioned chemicals, one route for CO2 activation could be the dry reforming of methane.12 This process produces synthesis gas with a ratio of H2 and CO suitable for Fischer-Tropsch synthesis to produce hydrocarbon fuels. The route: “Dry reforming of methane f Fischer-Tropsch f Hydrocracking of the F-T waxes” would provide one complete process chain from CO2 to high quality liquid fuels to be directly applicable in the existing vehicle fleet. There are, however, still severe technological limitations especially in the dry reforming process, e.g. the coke formation and sintering of the Ni metal particles leading to catalyst deactivation. The methanation of CO2 with hydrogen to produce CH4, the well-known Sabatier reaction, is one further, highly interesting and technologically feasible option to store renewable energy chemically.13 The handling and storage of methane is a mature technology, and closely similar to processing natural gas. The further possible reactions are illustrated in Figure 1. In the present study, we carried out an energetic evaluation in order to assess the overall efficiency of various energy conversion-storage systems for the electrical energy. Two systems for storing energy in chemical compounds were compared in detail: methanol-based and the hydrogen-based systems. Both energy conversion-storage chains in the present study are illustrated schematically in Figure 2. In the former route (Figure 2a), the hydrogen is used as cofeed component in the conversion of electrical energy to methanol by utilization of CO2. The inputs to the system include the input of electrical energy and water to produce hydrogen, the chemical stream of cleaned and concentrated CO2 from the power plant to the methanol synthesis. The output of the system consists of electricity generated in the last step (MCFC or CC-PP) and the excess heat generated in the methanol process. The main benefit
Figure 2. (a) Simplified process scheme for the energy storage-conversion system with methanol as storage medium. MCFC: Molten carbonate electrolyte fuel cell. CC-PP: Combined cycle power plant. (b) Simplified process scheme for the energy storage-conversion system with hydrogen as storage medium. PEMFC: Polymer electrolyte fuel cell. CC-PP: Combined cycle power plant.
of the methanol system route is undoubtedly the simple storing of the storage medium because methanol is in liquid phase at ambient conditions. The technology to carry out the CO2 capture is still under development and, therefore, reliable energetic and exergetic data of these processes and the quality specifications of the recycled CO2 are scarce. Therefore, the expected efficiency loss due to the CO2 recovery in the power plant is excluded in this study but should be considered in the analysis when the overall energy chain is considered. In the latter route (Figure 2b) hydrogen is directly used as energy storage. The thermodynamic limitations of each step were included in the analysis. The system input here is the water and power input for the electrolysis, and the output is the power output aside the exhaust gas water. 2.1. Hydrogen Production via Water Electrolysis. It is obvious that hydrogen, which is needed for methanol synthesis, has to be produced in a sustainable way without CO2 emissions, if the goal of the activity is the CO2 recycle and a possible reduction of green house gas emissions. There are numerous alternatives to produce hydrogen, for example, reforming of natural gas by steam, plasma, or aqueous phase or by partial oxidation of methane.14 The conventional and the most cost efficient production of hydrogen is done by steam reforming of
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Figure 3. Scheme of simulated low-pressure methanol production process with CO2 and H2 streams with ratio nH2/nCO2 ) 3:1 as feed gases, pressure 0.1 MPa. Each reactor inlet temperature Tin ) 493 K, pressure p ) 5.0 MPa.
natural gas. However, the steam reforming of methane causes CO2 emission in a molar H2/CO2 ratio of 4:1, and therefore, it is not here as reasonable alternative. Water electrolysis provides a CO2-free production for hydrogen. It is a well-known, old electrochemical process typically carried out in alkaline electrolyzers,15 although the recent progress in applying solid polymer16 or solid oxid17 electrolytes or even biocatalyzed electrolysis18 might open new ways to produce hydrogen with higher efficiency. The reversible cell voltage U0 of 1.18 V is obtained from thermodynamic data by the equation U0 ) ∆Rg(T)/(z · F)at the assumed conditions T ) 353 K and p ) 0.1 MPa, taking into consideration the temperature dependence of ∆Rg(T), ∆Rh(T), and ∆Rs(T). In the present analysis, a real operational voltage of U ) 1.96 V was assumed for the electrolyzer. Due to the system irreversibilities, the dissipated energy, qirr, was 150.5 kJ/mol, which is partly consumed by the electrolysis reaction to supply heat to the system, and partly released as heat loss to the surroundings. Under these real conditions, the molar energy consumption for hydrogen production is 378.9 kJ/mol (16.9 MJ/m3). If no irreversibilities exist, that is, qirr ) 0, the molar energy consumption will be only 227.7 kJ/mol. 2.2. Methanol Synthesis. The second part of the investigated system is methanol synthesis. CO2 reacts with hydrogen to methanol according to eq 119 CO2 + 3H2 T CH3OH + H2O ∆Rh(300K) ) -49.16 kJ/mol (1) Considering a reaction mixture consisting CO2 and H2, the reverse water gas shift reaction (eq 2) producing CO is taken into account CO2 + H2 T CO + H2O
∆Rh(300K) ) 41.16 kJ/mol (2)
In the presence of CO, the direct reaction of CO with H2 to methanol occurs as well (eq 3). This is the main reaction in the presence of Cu/ZnO2/Al2O3 catalysts in the large scale production of methanol.20 CO + 2H2 T CH3OH
∆Rh(300K) ) -90.77 kJ/mol (3)
The methanol production process was simulated under the following conditions: Four adiabatic reactor units in cascade were considered assuming equilibrium conversion with the defined stoichiometric reactions (1-3). The molar feed nH2/nCO2
ratio was 3:1 in the simulation. The input temperature Tin was 493 K in each adiabatic reactor, and intermediate cooling between the reactors was simulated by locating heat exchangers between the reactor units. The total reactor pressure was 5 MPa, and a pressure drop ∆p ) 25 kPa was assumed in each reactor and heat exchanger. The selected pressure and temperature ranges correspond to typical industrial conditions for low pressure methanol synthesis. After the reactor section, the pressure was released down to 1.2 MPa, and the unreacted feed compounds CO, CO2, and H2 were separated in the main flash unit, recycled and pressurized back in two-step to the reactor pressure 5 MPa. The fresh H2 and CO2 feed was supplied into the system at ambient pressure (101.325 kPa) and the three step compression (from atmospheric pressure to 0.4 MPa, from 0.4 to 1.2 MPa, and from 1.2 to 3.0 MPa) with intermediate cooling of the feed stream to 3 MPa was included in the analysis. The nonidealities of the reaction mixtures were accounted by the use of the predictive Soave-Redlich-Kwong method (PSRK).21 The compressors were assumed to work either isentropic or polytropic with a compression efficiency of 0.8 and work efficiency of 0.9. A scheme of the simulated methanol process is shown in Figure 3. The recycle ratio (molar flow ratio of the reactor input to the fresh feed) of 4.2 was established in the system to reach high conversion of CO2 (96.8%). The remaining CO2 and traces of H2 solved in the production stream containing methanol and water was removed in a flash unit at 323 K prior to the final methanol distillation to avoid too low temperatures in the distillation column condenser. The simulations were executed with the Aspen Plus 6.5 Flowsheet Simulator. Because of the exothermic methanol formation reactions (eqs 1-3) heat was generated in the reactors, and the temperature of the product stream increased in each reactor unit. The heat was removed after the reactor units in the heat exchangers after the reactors (Hx-2 to Hx-5, refer to Figure 3). Furthermore, heat exchangers were used to cool down the compressed feed gas (including in Comp-Feed unit in Figure 3). As a summary, heat was released in seven heat exchangers, in the main flash unit and in the distillation column condenser. Heat supply was necessary in three units: in the distillation column reboiler, in the heat exchangers to heat the reactor input stream (Hx-1 in Figure 3), and in the flash before the distillation unit. Mechanical work, wMeOH,in was applied for the compression of the feed and the recycle gas to the reaction pressure 5.0 MPa. The units containing the compressors and also intermediate cooling are illustrated as Comp-Feed and Comp-1 in Figure 3, respectively.
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Table 1. Heat Duty and Compressor Power Demand in the Methanol Synthesisa Heat duty, Q
[kW]
Hx-1a Hx-1b Hx-2 Hx-3 Hx-4 Hx-5 Hx-Feed1 Hx-Feed2 Main Flash Unit Distillation Flash Unit Distillation Reboiler Condenser
-38.0 32.5 -21.3 -14.3 -9.4 -38.6 -17.5 -17.8 -116.2 3.2
Table 2. Material and Chemical Exergy Values of the Streams Using Methanol as Chemical Storagea
Compressor work, wMeOH,in
[kW]
methanol route
Comp-Feed1 Comp-Feed2 Comp-Feed3 Comp-1 Comp-2
20.2 17.8 13.9 34.1 43.0
55.5 -52.4
a
Values related to input 9.502 kmolH2/h, corresponding to 3600 MJ/h (1 MW) power input into the electrolyzer.
Because of the high exothermicity of the reactions and the heat generation during several compression steps, it was obvious that the excess heat has to be removed from the system, that is, cooling is necessary. However, on the other hand, heat supply was needed in the system to warm up the reactor input stream and the reboiler. To use the available heat (hot streams) in the process energetically in a most efficient way for internal heating (cold streams), a heat integration by the pinch analysis22 was included in this contribution. The pinch analysis is a highly suitable method to include the heat integration in the exergy estimation. A rigorous optimization of a heat exchanger super structure could possible yield even better results, but it leads to a much more complex model formulation and computational effort.23 To evaluate the heat duty of the system, temperature - heat duty illustration was generated according to the heat integration principles applied in the pinch analysis. The values of the heat duties were taken from the Aspen simulation results. A summary of the process work and heat duties is given in Table 1. Figure 4 illustrates temperature-heat duty dependence of the methanol synthesis. As clearly seen in Figure 4, the amount of heat released in the methanol production process is clearly higher than the heat demand. A minimal necessary temperature difference, ∆Tmin, between the hot and cold streams was taken to be 10 K, and this condition was limiting at the hot stream pinch temperature 493 K, as illustrated in Figure 4. As seen in Figure 4, the internal heat utilization range exists between the temperatures 388 and 503 K (hot stream). It was assumed that the heat released in this range was used completely to supply the heat demand of
exergy flow [MJ/h]
9.502
3600 2230b
electrical energy input hydrogen output methanol synthesis methanol output compressor work excess heat electrical energy output CC-PP (ηel ) 0.57c) MCFC (ηel ) 0.50c)
3.030
2153b -467 87 1101 966
a CC-PP: Combined cycle power plant. MCFC: Molten carbonate electrolyte fuel cell. b Chemical exergy. c Efficiency estimation based on the fuel lower heating value.
the system. However, the investigated methanol production unit released heat below and above the internal heat utilization range, and this excess heat was included in the exergy output of the methanol synthesis. The heat duty varied as a function of the temperature and the exergy of the heat was calculated according to eq 4. Equation 4 describes the exergy of the heat released below the internal heat utilization range eheat )
Figure 4. Temperature as a function of the process heat duty in the methanol synthesis process. [s] Heat released by the system (hot stream). [s] Heat required by the system (cold stream). Pinch point at 493 K to fulfill the criterion ∆Tmin ) 10 K.
molar flow [kmol/h]
∫
Tint
T0
dQ (1 - Tref /T)dT dT
(4)
where T0 was the lowest temperature where heat was released in the system (303 K), Tint the lower limit of the internal heat utilization range, and Tref the reference temperature in the environment. The exergy of the heat released the temperature range above the internal heat utilization range was calculated accordingly. The excess heat (198 kW) released below the internal heat utilization range (T < 388 K) had a low exergetic value (10.8 kW) as the temperature of the released heat was close to the Tref (293 K), as seen in Figure 4. The heat (31.5 kW) obtained above the internal heat utilization range (T > 503 K) was exergetically clearly more valuable (13.3 kW). 2.3. Conversion of Chemical Energy to Electricity. Last part of the investigated energy conversion chain was the conversion of methanol to electricity, which was calculated applying either a molten carbonate fuel cell (MCFC) or a combined cycle power plant (CC-PP). The industrially proven electric system efficiency, ηel, of the MCFC fuel cell fed with natural gas is close to 50%,24 which was taken here the calculation basis also for the exergy efficiency for such systems.25 For the combined cycle power plant an electric efficiency of 57% calculated on the lower heating value of the fuel was assumed. For the comparative hydrogen route, the conversion in polymer electrolyte fuel cells (PEFC) and CCPP was calculated as high electric system efficiency of such systems could be expected. For PEFC the electric efficiency ηel ) 56% and for the hydrogen fueled CC-PP ηel ) 57% were assumed. 3. Analysis of the Energy Conversion-Storage Systems The molar material flows and the chemical exergy values of the investigated system are available numerically in Table 2 (methanol storage) and Table 3 (hydrogen storage). The electrical energy input in the electrolyzer and in the methanol production process and the exergy of heat released in the methanol production process are also included in the table. The overall exergy consumption in the energy conversion-storage system over methanol (system corresponding to Figure 2a) is illustrated in Figure 5. As a simplification in this study, the exergy of the fuel streams are calculated from the chemical
Ind. Eng. Chem. Res., Vol. 49, No. 21, 2010 Table 3. Material and Chemical Exergy Values for the Streams Using Hydrogen As Chemical Storagea hydrogen route electrical energy input hydrogen output electrical energy output CC-PP (ηel ) 0.57c) PEFC (ηel ) 0.56c)
molar flow [kmol/h]
exergy flow [MJ/h]
9.502
3600 2230b 1310 1287
a CC-PP: Combined cycle power plant. PEFC: Polymer electrolyte fuel cell. b Chemical exergy. c Efficiency estimation based on the fuel lower heating value.
exergy values of these compound,26 and the contribution of physical exergy27 is not taken into consideration. The exergy available as the heat that was generated during the exothermic methanol formation reaction and removed in the heat exchangers made only a minor contribution (3.9%) of the overall exergy output stream in the methanol synthesis. As can be seen in Figure 5 and numerically in Table 2, in the methanol process, the exergy of the methanol product stream is only slightly lower than that of the hydrogen input feed stream. The ratio of chemical exergies of the methanol to hydrogen streams in the simulation is 0.967. This is to be expected as the stoichiometric molar ratio of the produced methanol to consumed hydrogen is nCH3OH/nH2 ) 3.0 (eq 1), and the ratio of the chemical exergy values of methanol and hydrogen is ECH3OH/EH2 ) 3.027. However, in the analysis of the methanol process, the compression work of the fresh feed and the recycle streams (see Table 1) and, at the same time, the available exergy of the released heat have to be taken into consideration. If we consider the methanol process including the chemical compound streams, the compressor work and the released heat, the exergy efficiency of methanol synthesis is 83.1%. However, one should note that the methanol process was simulated here under conventional low pressure process conditions, but in the same time under the assumption that the equilibria of the reactions (eqs 1-3) are attained in each reactor unit. Of course, this is only possible at infinite amount of catalyst. If reaction equilibria were not reached, the recycle stream volume would increase and so the energy demand for the compression. Similarly, if the process was operated under-stoichiometric in terms of hydrogen, the recycle stream increases strongly, and so the energy demand of the recycle stream compression. However, the losses in the methanol process are not the main contribution for the exergy losses in the energy conversion-storage system with methanol.
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The process steps, where either electrical energy is converted to chemical (electrolysis) or chemical energy is converted back to electrical (MCFC and CC-PP), contributes significantly more for the exergy loss of the overall system. In the electrolyzer, the exergy loss is 38.1%. This is slightly lower than other published values.28 In the conversion of methanol to electrical energy, we obtain exergy efficiencies of 51.1% and 44.9% if we compare directly the exergy of the obtained electrical energy to the chemical exergy of the methanol feed. A potential heat recovery of the high temperature units (MCFC and CC-PP) could possible still improve the exergetic efficiency. As overall efficiency, if we consider only the ratio of the initial electric power to the finally available electric power, we obtain for the system containing methanol synthesis 13.8% and 17.6% with MCFC and CC-PP, respectively. The exergetic efficiency of these processes is improved to some extent if the exergy of the heat released in the methanol synthesis is included to the calculation (16.2 and 20.0%, respectively). With the compared system (Figure 2b) utilizing hydrogen as storage medium, overall efficiencies of 35.7% and 36.4% in PEM-FC and CC-PP can be estimated (see Figure 6). Comparing these values with the former ones, we might conclude that both systems have considerable exergy losses. The methanol-based storage system is energetically more unfavorable. However, the main and very significant advantage of the methanol system would be a simple and cost-efficient storage and, furthermore, the possibility to use the storage medium as transportation fuel in the conventional fleet. 4. Conclusions In this study, the idea to use captured CO2 from power plants for the production of energy storage medium has been evaluated. The reuse of exhaust CO2 as carbon source for methanol production could be seen as one potential way to store electrical energy in chemical storage and simultaneously to reuse CO2. The energetical and exergetical efficiencies of two energy conversion-storage systems for storing electrical energy have been compared here. The energy and exergy flows of the system containing methanol as storage medium were analyzed including the chemical synthesis of methanol from hydrogen and CO2. The electrical energy demand and the heat release of the methanol synthesis and the exergetic flows were estimated by a flowsheet simulation of the process. To the best of our knowledge, a detailed consideration of the methanol production
Figure 5. Simplified energy diagram for the energy storage-conversion chain using methanol as energy storage medium with flows of electrical energy (dark gray symbol), heat (dark gray symbol), and chemical exergy (material streams, pale gray symbol). MCFC: Molten carbonate electrolyte fuel cell. CC-PP: Combined cycle power plant.
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Figure 6. Simplified energy diagram for the energy storage-conversion chain using hydrogen as energy storage medium with flows of electrical energy (dark gray symbol) and chemical exergy (material streams, pale gray symbol). PEFC: Polymer electrolyte fuel cell. CC-PP: Combined cycle power plant.
process including process heat generation, pressure losses in the units and the pinch point analysis for the heat integration were accounted in an exergetic analysis for CO2 utilization for the first time. The exergetic efficiency of the overall energy conversion-storage system including methanol as storage medium was between 13.8 and 17.6% depending on the applied conversion technology. Including the exergy of the heat released in the methanol synthesis the values improved to 16.2 and 20.0%. The exergetic efficiency of the system using hydrogen as storage medium was higher, namely, close to 36%. However, the main and very significant advantage of using methanol as chemical storage medium would be the unproblematic storage of the liquid methanol. Literature Cited (1) Bachu, S. Sequestration of CO2 in geological media in response to climate change: Road map for site selection using the transform of the geological space into the CO2 phase space. Energy ConVers. Manage. 2002, 43, 87. (2) RECCS Report. Comparison of renewable energy technologies (RE) with carbon capture and storage (CCS) regarding structural, economic and ecological aspects. http:/www.bmu.de/erneuerbare_energien/downloads/doc/ 38826.php. (3) Costas, D. A. Separation of CO2 from flue gas: A review. Sep. Sci. Technol. 2005, 40, 321. (4) Halmann, M. M.; Steinberg, M. Greenhouse Gas Carbon Dioxide MitigationsScience and Technology; CRC Press: Boca Raton, FL, 1999. (5) Song, C. Global challenges and strategies for control, conversion and utilization of CO2 for sustainable development involving energy, catalysis, adsorption and chemical processing. Catal. Today 2006, 115, 2. (6) Arakawa, H.; Aresta, M.; Armor, J. N.; Barteau, M. A.; Beckman, E. J.; Bell, A. T.; Bercaw, J. E.; Creutz, C.; Dinjus, E.; Dixon, D. A.; Domen, K.; DuBois, D. L.; Eckert, J.; Fujita, E.; Gibson, D. H.; Goddard, W. A.; Goodman, D. W.; Keller, J.; Kubas, G. J.; Kung, H. H.; Lyons, J. E.; Manzer, L. E.; Marks, T.; Morokuma, K.; Nicholas, K. M.; Periana, R.; Que, L.;
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ReceiVed for reView March 6, 2010 ReVised manuscript receiVed August 18, 2010 Accepted August 25, 2010 IE100508W
