Evaluation of Formic-Acid-Based Hydrogen Storage Technologies

Sep 22, 2014 - the storage of hydrogen energy has been evaluated. The efficiency is highly dependent upon the way formic acid is produced. Options bas...
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Evaluation of Formic-Acid-Based Hydrogen Storage Technologies Irma Schmidt, Karsten Müller,* and Wolfgang Arlt Institute of Separation Science and Technology, Friedrich-Alexander-Universität Erlangen-Nürnberg (FAU), Egerlandstraße 3, 91058 Erlangen, Germany S Supporting Information *

ABSTRACT: To make hydrogen a feasible energy carrier, its transformation into another chemical is advisible. Formic acid may constitute an attractive option to store hydrogen in a dense and safe form. The efficiency of formic-acid-based process chains for the storage of hydrogen energy has been evaluated. The efficiency is highly dependent upon the way formic acid is produced. Options based on reactions of hydrogen with carbon dioxide show a rather poor energetic performance, while routes using renewable production of intermediate carbon monoxide can reach competitive efficiencies. For the storage of electrical energy, an overall efficiency of about 12% is possible using the current state of the art technology, but alternative processes show potential for higher efficiencies.

1. INTRODUCTION Because of fluctuations in the power production by renewable energies, the development of new storage technologies for energy is necessary. Hydrogen is a very interesting option to overcome the drawbacks of other storage options. However, because the density of elemental hydrogen is very low at ambient conditions, storage requires high pressures or very low temperatures, causing new issues. The conversion of hydrogen into another form is therefore reasonable. There are two possibilities to do this: reversible and irreversible conversion of hydrogen. Reversible conversion is realized by hydrogenating an unsaturated organic compound and, subsequently, dehydrogenating it when energy is needed. These so-called liquid organic hydrogen carriers (LOHCs) are a subclass of energycarrying compounds.1,2 In the case of irreversible conversion, hydrogen reacts with a carbon oxide, mostly CO2, to a fuel. Potential products of these reactions are mainly methane,3 methanol,4,5 or long-chain alkanes.6 Because the energy content of the products is below the energy demand of the hydrogen used, the overall efficiency is rather low. Nevertheless, these fuels can be stored in a rather safe and easy manner. In this contribution, the focus is set on the storage of hydrogen in the form of formic acid (FA), whereas the hydrogenation of carbon dioxide will be the main part of this contribution. In the literature, a number of studies dealing with the decomposition of FA to gain hydrogen are already available.8,12 Hence, this process step will not be discussed in detail. In recent years, a number of research groups7−11 have performed intensive research on FA as an alternative for hydrogen storage. These works mainly focus on the catalytic release of hydrogen from FA and its production from CO2 and hydrogen. Complex systems of ruthenium- and rhodium-based catalysts are normally used for these reaction steps.12,13 FA is a strong, corrosive, but stable acid. Its formal H2 content is 4.4 wt %. The storage and transportation conditions are wellestablished.7 Therefore, the initial conditions to use FA as a hydrogen storage material are quite good. Beller and co-workers7 as well as Laurenczy and co-workers8 suggested the catalytic decomposition of FA as a practical © 2014 American Chemical Society

method for hydrogen release. The reaction can be performed at mild conditions (100 °C and 1 bar), and CO2 as the co-product of the decomposition could be rehydrogenated into FA. The catalyst in this reaction is a Ru-based catalyst system, which allows for hydrogenation of CO2 as well as dehydrogenation of FA. Its stability was tested over 1 year without any loss in activity.13 FA could be considered as a sustainable storage material for hydrogen if it could be produced by direct hydrogenation of CO2. However, thermodynamics limit the extent of this reaction to very low yields. Elevated pressures could increase the maximum conversion, but it would still be far too low for a technical process. When bases or alcohols are applied as co-reactant, the equilibrium can be shifted toward higher conversions, because FA is removed from the reactive system by a subsequent reaction. Nevertheless, a more demanding recovery of the free acid from the product mixture must be performed for higher conversions. Nowadays, FA is produced from carbon monoxide and methanol in a two-step reaction. CO, which is produced by reforming of fossil energy carriers, and methanol are converted into methyl formate. This reacts in the presence of water and a tertiary amine in a subsequent step into FA.10 Because this is the common way of generating FA, one can consider this process as the benchmark. Several research groups are currently investigating different processes for the generation of FA from hydrogen and carbon dioxide, but until now, no alternative process is yet commercialized.

2. MODELING In this contribution, the efficiencies achievable by the individual process options proposed in the literature9,10,14−16 have been determined and the results have been compared to a renewable modification of the benchmark process. The focus is set on the production of FA for energy storage purposes based on different processes. In total, four process options will be discussed: (1) reaction Received: August 12, 2014 Revised: September 19, 2014 Published: September 22, 2014 6540

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Figure 1. Process flow sheet of the FA production using amine and diol as additional reactants. of CO2 and H2 only, (2) reaction of CO2, H2, and an amine,14 (3) reaction of CO2, H2, and an amine using a diol as a supporting compound,9 and (4) renewably modified benchmark process.10 The processes under consideration have been modeled using the software Aspen Plus, version 8.0. In every process, CO2 and H2 are the reactants. Both undergo a direct chemical reaction to form FA. The reaction conditions in the individual processes differ. Therefore, every model in Aspen Plus was adjusted to the operation conditions, which will be discussed in the following. The evaluation of all processes is based on the efficiency for hydrogen storage. This efficiency can be defined as

ηH = 2

In the simulation of this approach, the reaction conditions are assumed as proposed by Beller’s group.14 They suggested a temperature of 100 °C and a pressure of 60 bar, whereas CO2 and H2 are added in a stoichiometric ratio of 1:1. For the process model in Aspen Plus, pressurization of the CO2 stream to 60 bar was assumed to be performed in a two-step compression with intermediate cooling. H2 at a pressure of 30 bar is obtained directly from electrolysis and is further compressed to 60 bar. Both compounds are passing a heat exchanger to preheat the stream before it is fed into the reactor. After the reaction, the separation of FA from the reactants is performed using flash distillation. Because the reaction of CO 2 and H 2 to FA is a thermodynamically uphill reaction, the obtained yield of FA using only these two components is far below 1%. Taking into account all results from the simulation to calculate the electrical efficiency according to eq 2, the electrical efficiency ηel will be below 1%. Also, the efficiency for hydrogen storage, ηH2, stays far below 1%. The energy demand of this process is about 9.4 GJ/molFA.. 3.2. In Situ Product Removal Using an Amine. The second process alternative studied is the formation of FA in the presence of an amine to form a FA amine adducts. When a base is added, the reaction equilibrium (eq 3) is shifted to the product side, because the protonation of the base provides an additional driving force to the reaction. The Gibbs free energy of reaction therefore becomes negative.9 Possible bases for this task are amines, such as triethylamine13 or trihexylamine.9 The model used for the first process alternative was extended by an amine stream, and the influence of the increased conversion on the efficiency of FA production was studied. Again, 100 °C and 60 bar have been assumed as reaction conditions. The ratio of CO2, H2, and amine was 1:1:0.4. The separation of FA from the educts was realized using flash distillation. A total yield of 2% for FA has been assumed as reported by ref 14. Applying a solvent, such as dimethylformamide, could enhance the reaction, but the additional separation effort does render this unreasonable. When this modification is applied, the electrical efficiency ηel increases to 6% and the efficiency of hydrogen storage ηH2 goes up to 13%. This is still a rather low value, but efficiency could be increased significantly compared to the

nFA LHVH2 − Q Decomp (n in − nrecyc)LHVH2 + EKomp

(1)

where LHVH2 is the lower heating value of hydrogen stored in 1 mol of FA (LHVH2 = 241.9 kJ/mol). nFA is the molar amount of FA; nin is the amount of hydrogen that enters the hydrogenation reactor; and nrecyc is the amount of unconverted hydrogen, which is recycled. QDecomp is the heat needed for the decomposition of FA to release hydrogen, and EKomp is the mechanical work that is used for the hydrogenation of CO2. To account for the whole process of storing electrical energy, efficiencies of electrolysis and fuel cell (ηelectrolysis ≈ 0.6, and ηFC ≈ 0.5) have to be taken into account as well. Therefore, the electrical efficiency ηel can be obtained ηel =

nFA LHVH2ηFC − Q Decomp LHVH2

(n in − nrecyc) η

electrolysis

+ E Komp

(2)

3. RESULTS 3.1. Direct Reaction of CO2 and H2. The first approach analyzed was the direct reaction of CO2 and H2 to FA without any further reaction steps. Equation 3 shows the reaction equation and the corresponding thermodynamic properties CO2 + H 2 ⇋ HCOOH ΔR g = +32.9 kJ mol−1,

(3)

ΔR h = −31.2 kJ mol−1

As seen, the reaction is exothermal but still thermodynamically unfavored because of a high positive Gibbs free energy of reaction. The thermodynamic equilibrium is therefore far on the side of the reactants. 6541

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Figure 2. Process scheme of the modified benchmark process.

First, CO2 and H2 are fed into a reversed water−gas shift (RWSG) reactor, where they are converted into CO and H2O. The RWGS reaction is performed at 950 °C and 30 bar. A stoichiometric starting mixture of carbon dioxide and hydrogen is assumed. The equilibrium conversion is 72%. The subsequent conversion of CO with methanol to methyl formate is then performed in a second reactor at 80 °C and 40 bar. Equation 4 shows the chemical reaction equation.

previous process option. The energy demand for the production of FA is only 1.5 MJ/molFA. 3.3. In Situ Product Removal Using Liquid−Liquid Separation with an Amine and a Diol. The third process under consideration has been proposed by the group of Schaub.9 The process scheme is shown as an Aspen flow sheet in Figure 1. As shown above, adding amine as a supporting agent to the hydrogenation of CO2 influences the reaction equilibrium positively. FA amine adducts are poorly miscible with the free amine phase; hence, a diol phase, e.g., 2-methyl-1,3-propanediol, is added as an additional compound, in which these adducts will be solved. Thus, a liquid−liquid two-phase system with the free amine as the second phase is formed. The hydrogenation of CO2 is then performed in this system. Lipophilic ruthenium catalysts are used for this reaction. These catalysts dissolve preferably in the amine phase. After the reaction, the two phases are separated. The amine phase containing a high amount of catalyst will be recycled to the hydrogenation step. The small amount of catalyst, which remains in the product phase, will be extracted in a separate column. Afterward, the catalyst is removed totally from the diol and amine phases, and FA can be obtained from these phases by distillation. In our simulation, we assumed a stoichiometric mixture of H2 and CO2. The reaction conditions are different from those in the processes described above. The temperature of the reactor is only 50 °C at a pressure of 60 bar. The obtained yield of FA was set to 2% as reported by ref 14. The distillation column for purification of FA is run at 150 mbar with an inlet temperature of 150 °C.8 On the basis of the results from this simulation, an electrical efficiency, ηel, of approximately 2% is obtained for this process. When hydrogen is looked at as the system boundary, the efficiency ηH2 is 5%. The reason for this low efficiency is the energetic effort one has to spend for the purification of FA. In total, the energy demand for this process is 4.3 MJ/molFA. A total of 2.3 MJ/ molFA of this demand is caused alone by the distillation step. 3.4. Modified Benchmark Process. The last process option studied is the modification of the benchmark process run on renewable hydrogen. This process is composed of two process steps. In Figure 2, the ASPEN flow sheet for the process simulation is shown.

CO + CH3OH ⇋ HCOOCH3

(4)

The obtained intermediate, HCOOCH3, is forwarded to the second process step. Together with water and 2,6-dimethylpyridine, methyl formate is provided to a hydrolysis reactor in a ratio of HCOOCH3/H2O/AMINE = 1:1:0.9.9 The conditions in the reactor are assumed to be 120 °C and 12 bar. The reaction equation is shown in eq 5. tert ‐N + CHOOCH3 + H 2O ⇋ [tert ‐NH]+ [HCOO]− + CH3OH [tert ‐NH]+ [HCOO]− ⇋ HCOOH + tert ‐N

(5)

The tertiary amine (tert-N) 2,6-dimethylpyridine is used as supporting media to provide the hydrolysis of methyl formate. In the reactor, the formate formed is then converted into FA. Afterward, the reaction mixture is separated into FA and the tertiary amine by thermal processing in a distillation column. The modified benchmark process is composed of a high number of technical equipment. Nevertheless, for the whole process, an electrical efficiency of 12% is achieved. The efficiency of the hydrogen storage ηH2 reaches a value of 30%. The energy demand of the process is only 561 kJ/molFA, which is far below the energy demand of the process options described above. A more detailed description of all process models is presented as Supporting Information.

4. DISCUSSION To obtain an overview of all process options, Table 1 gives a summary of the efficiencies for electric energy and hydrogen as the system boundary for the individual process options discussed. It should be obvious that all of the studied processes are contemplated ideally and do not consider “real world effects” that occur when operating a technical plant. Catalyst deactivation as well as non-ideal purification steps would have 6542

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to about 25% for the process using amine and diol as additional streams would be required. In the case of amine as supporting media, a yield of only 7% must be reached to obtain the same efficiency as the modified benchmark process. It can be concluded that the new process option of the direct reaction of hydrogen with carbon dioxide in the presence of an amine might be an alternative to the modified benchmark process. The efficiency obtained could be slightly higher. However, namable increases in yield are required to achieve this. Applying elevated pressures and/or an optimized reaction temperature program, the yield could be raised, whereas the influence of the temperature is dominant over the pressure. Further, in real processes, a suitable solvent that favors thermodynamics as well as an increased amount of catalyst would lead to higher yields. Furthermore, the energy demand of this process option decreases with a higher yield of FA, because most of the energy is used for the compression and heating of the reactive compounds before entering the reactor. Achieving higher yields, the energy demand per mole of FA reduces to 42 kJ/molFA. One big challenge of this process option is the economic separation of FA from the FA amine adducts. Lowboiling amines and the adducts form azeotropic mixtures from which pure FA is only difficult to obtain by distillation.9 One proposal is an exchange of the base of the FA amine adduct in the hydrogenation using a high boiling base, such as imidazole.9 This leads to an additional operation step, which consumes further energy, so that the efficiency would decrease. The disadvantage of the modified benchmark process is the high number of apparatuses needed and, hence, rather high investment costs. However, because of the better energy balance, this process seems to be the best choice. To see which process step carries the highest risk because of uncertainties concerning the practical implementation, a pareto plot has been created for the modified benchmark as the best process option (Figure 4). Herein, the relative decrease of

Table 1. Summary of the Resulting Electrical Efficiency and the Efficiency for Hydrogen Storage of the Individual Process Options 1, 2, 3, 4,

only CO2 and H2 CO2 and H2 with amine CO2 and H2 with amine and diol modified benchmark process

efficiency ηel (%)

efficiency ηH2 (%)

1.20 × 10−3 6 2 12

2.5 × 10−3 13 5 30

negative impact on the yield, and therefore, the efficiency will be lower. It can be seen that the electrical efficiency only reaches values about one-half of the efficiency only considering hydrogen storage (not taking the electrochemical steps into account). For processes 2 and 3, the overall yield in process modeling was assumed to be 2%, as reported in the literature. Drawing a comparison of these two processes, the second process option shows the best efficiency and the lowest demand of energy. In the modified benchmark process, a yield of 72% is achieved and, therefore, high efficiencies in both criteria are reached. To determine the dependency of efficiency upon the yield for FA for the process options listed above, especially for processes 2 and 3, a sensitivity analysis has been performed. A comparison to the modified benchmark process is also shown. In Figure 3, the yield of FA was varied between zero and full conversion and the resulting electrical efficiency was determined.

Figure 3. Electrical efficiency as a function of the yield of FA. The efficiencies for the conversions taken from the literature and actually assumed in this work are marked.

One can see that, by increasing the yield of FA, the electrical efficiency is also rising. The maximum electrical efficiency that is achieved at total conversion of FA is 28% for the process using only amine as a supporting agent. In the process with amine and diol as supporting media, the electrical efficiency is leveling off to a saturation value of about 22% at full conversion. The efficiency ηH2 with hydrogen as the system boundary shows a similar behavior. The maximum value for this parameter is 86% for process 2 and 64% for process 3. The electrical efficiency of the modified benchmark process at its conversion of 72% assumes a value of 12%. This value is below the optimum efficiency of the second and third process options if total conversion is assumed for them. With an efficiency of nearly 28 and 22% in the case of total conversion, these process options could exceed the conventional process. However, to break even with the conventional process, an increase of yield

Figure 4. Potential negative influence of uncertainties concerning the implementation of different process steps on the overall efficiency for the modified benchmark process.

overall efficiency because of potential drawbacks in the realization of different elements of the process is listed. The uncertainties are arranged in descending order concerning influence on the overall process performance. The highest risk concerning overall electrical efficiency is the actual efficiency of the electrochemical steps. When the efficiency of the fuel cell is reduced to 44% and the efficiency 6543

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Table 2. Comparison of Electrical Efficiencies for the Conversion of Hydrogen into FA and Other Alternative Fuels and Storage Options electrical efficiency (%)

FA

methane3

Fischer−Tropsch products13

compressed hydrogen14

liquefied hydrogen14

12−28

21−28

17

27−29

17−24

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of the electrolysis is reduced to 47.6%, a deficit in efficiency of 13 and 10%, respectively, must be registered. These two values are the lowest efficiencies that are listed in the literature for the electrochemical process steps and are applied to cover the worst case. For the elements of the process chain that belong to the FA production itself, the release of FA from the ester (reactor 3) is the most critical element. If the conversion of the reactants would be 20% lower compared to the values assumed in the process simulation above, the loss in efficiency would be about 9%. The sensitivity of the overall efficiency on the yield in the other reactions is far lower (only 3% if the conversion in reactor 2 is lowered by 20%). More important is the energy demand of the subsequent purification step. An increase of this energy demand by 20% would result in an efficiency loss by about 6%. The compression of the reactants CO2 and H2 has only a minor influence. Here, the deficit in efficiency would be only about 1% (even if the isentropic efficiency of the compressor would be lowered by 50%). For the comparison to other approaches for the storage of hydrogen, an overview is given in Table 2. Conversion of hydrogen into FA (on the basis of the modified benchmark process) shows the potential to reach a higher efficiency than, e.g., conversion into Fischer−Tropsch products, such as gasoline or diesel, if the yield in the new process alternatives can be increased. The best case efficiency for methanization is similar to the best case efficiency for FA (if a combined cycle power plant is assumed for power generation from methane). Thus, conversion into FA shows the potential to compete with methane as a potential storage option. However, using the current state-of-the-art technology, it has to be stated that FA is not competitive with other storage options. Nevertheless, the most efficient way of storing hydrogen remains compression, but the low storage density (which is the main reason for the intensive research on hydrogen storage in the last few years) renders compression not suitable for mobile as well as largescale applications. Liquefaction is a way to reach higher storage densities than in the case of compression, but in terms of energy balance, it does not seem competitive to other options (especially because the boil-off losses during storage add to the energy demand for liquefaction).

ASSOCIATED CONTENT

* Supporting Information S

More detailed description of the processes. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*Telephone: +49-9131-8527455. E-mail: karsten.mueller@cbi. uni-erlangen.de. Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS This work was performed within the framework of the Energie Campus Nürnberg and was funded by the state of Bavaria. The authors also thank Prof. Beller and Dr. Junge from the Leibniz Institute for Catalysis at the University of Rostock for valuable discussions.

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REFERENCES

(1) Teichmann, D.; Arlt, W.; Wasserscheid, P.; Freymann, R. Energy Environ. Sci. 2011, 4, 2767. (2) Pez, G. P.; Scott, A. R.; Cooper, A. C.; Cheng, H.; Wilhelm, F. C.; Abdourazak, A. H. U.S. Patent 7,351,395 B1, 2008. (3) Müller, B.; Müller, K.; Teichmann, D.; Arlt, W. Chem. Ing. Tech. 2011, 83, 2002. (4) Olah, G. A. Angew. Chem. 2005, 117, 2692; Angew. Chem., Int. Ed. 2005, 44, 2636. (5) Asinger, F. MethanolChemie- und Energierohstoff; Springer Verlag, Berlin, Germany, 2011. (6) Schüth, F. Chem. Ing. Tech. 2011, 83, 1984. (7) Boddien, A.; Gärtner, F.; Federsel, C.; Sponholz, P.; Mellmann, D.; Jackstell, R.; Junge, H.; Beller, M. Angew. Chem. 2011, 123, 6535− 6538. (8) Fellay, C.; Dyson, P. J.; Laurenczy, G. Angew. Chem. 2008, 120, 4030−4032. (9) Schaub, T.; Paciello, R. Angew. Chem. 2011, 123, 7416−7420. (10) Karl, J.; Hauk, A. European Patent 1,784,375 B1, 2007. (11) Moret, S.; Dyson, P. J.; Laurenzy, G. Nat. Commun. 2014, 5, 4017. (12) Loges, B.; Boddien, A.; Junge, H.; Beller, M. Angew. Chem. 2008, 120, 4026−4029. (13) Joó, F. ChemSusChem 2008, 1, 805−808. (14) Boddien, A.; Federsel, C.; Sponholz, P.; Mellmann, D.; Jackstell, R.; Junge, H.; Laurenzy, G.; Beller, M. Energy Environ. Sci. 2012, 5, 8907−8911. (15) Müller, K.; Geng, J.; Arlt, W. Energy Technol. 2013, 1, 42−47. (16) Müller, K.; Arlt, W. Energy Technol. 2013, 1, 501−511.

5. CONCLUSION Generally, FA is a potential energy carrier, because the efficiency is in the range of other commercially used processes for hydrogen storage. However, FA-based energy storage is likely to remain a niche application, because there are other processes, such as methanization, showing higher efficiencies. The highest efficiencies are not achieved using direct reaction of hydrogen with carbon dioxide but by conversion of carbon dioxide into carbon monoxide by RWGS and, subsequently, producing FA in the conventional way. Nevertheless, optimizing the process scheme using amine as a co-reactant may constitute an attractive alternative in the future. 6544

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