Without reliable storage technology, neither the political goals of the German government nor the EU's “Green Deal” will be able to be implemented in practice. Throughout living nature, the three elements C, H and O are the basis of energy storage. Their present reaction products water (H2O) and carbon dioxide (CO2) are ideally sufficient to produce synthetic fuels such as hydrogen (H2), as well as hydrocarbons (CnHm) especially methane (CH4) and ethene (C2H4), generally hydrocarbon compounds, from fluctuating regenerative electricity with the help of “power-to-gas” technologies and to use them as regenerative storage substance, but also as sustainable industrial raw material. For this purpose, however, it is imperative to increase energy efficiency in the technical implementation. As described in (M. Thema, F. Bauer, M. Sterner: Power-to-gas: electrolysis and methanation status review. Renewable and Sustainable Energy Reviews 112 (2019) p 775-787 Table 2), the average efficiency for H2 electrolysis is 77% and 41% for methanation of H2 with CO or CO2, which is essentially based on the Sabatier reaction, although lower values are certainly cited in the literature. Since H2 is an essential feedstock for the implementation of the Sabatier reaction, all the processes mentioned above are also linked to the efficient supply of H2.
The task of the invention is to significantly improve the above-mentioned efficiencies for “power-to-gas” technologies with the aid of an integrated overall concept, to minimize investment costs by means of new circuits and their constructive design, and thus to contribute to sustainable system integration that guarantees a secure and stable power supply despite fluctuating feed-in and contributes to a sustainable circular economy through CO2 recycling.
Problem Analysis, Solution Approaches and Task Definition
A problem analysis carried out with the help of the methodology of reversible process structures described in (Wolfgang Georg Winkler: Sustainable product development based on second law of thermodynamics. September 2011. Applied Energy 88(9): 3248-3256 DOI: 10.1016/j.apenergy.2011.03.020), showed that mainly two effects with their impact on further process integration lead to these relatively low efficiencies. These are the heat requirement for the evaporation of the supplied liquid H2O and the separation of the O2 produced during the thermal methanation by an oxidation with H2 to be additionally produced for this purpose and the subsequent condensation of the H2O produced. In addition, there are various deficiencies in system integration that are easily avoidable with the reversible structure as a design basis.
In electrolytic H2 generation, H2O is supplied in a liquid state and the necessary evaporation heat is generated electrically. This reduces the theoretically possible efficiency of H2O electrolysis to 83% (Wolfgang Winkler, Ai Suzuki, Akira Miyamoto, Harumi Yokokawa, Mark C. Williams: Performance Envelope for Electrolyser Systems. ECS Trans. 2015 65(1): 253-262). From the consideration of reversible process control of electricity storage using H2 according to (Wolfgang Georg Winkler: Sustainable product development based on second law of thermodynamics. September 2011. Applied Energy 88(9): 3248-3256 DOI: the necessary improvement options can be derived. For this purpose, the starting point for the overall electrolysis processes considered here are the reaction equations:
H2O=H2½O2 (1)
and
CO2=CO+½O2. (2)
Whereby the last equation is only important for the integration of the electrolysis processes for CH4 production. Both processes are thermodynamically similar, however, Eq. (2) for the representation of carbon monoxide (CO) is relevant here only for processes in the gas phase and thus, initially, for the analysis of the optimal system integration of the evaporation process, only the reversible process control based on Eq. (1) is relevant.
Electrolysis processes today operate at either high or low temperatures, so the thermodynamic differences between these two process options need to be explained. The reason for the additional power supply, to cover the heat requirement for evaporation in electrolysis is that H2O is required for the reaction in gaseous form and not in liquid form as it is supplied. Recovery of the heat of condensation of the combustion exhaust gases given off to the environment is impracticable, and the issue is generally addressed formally in technical publications by the “thermoneutral voltage” defined with the higher heating value (compare, for example, “Electrochemical Thermodynamics,” Karl Winnacker Institute DECHEMA, https://dechema.de/kwi_media/Downloads/ec/5++Elektrochemisch+Thermodynamic s-p-976.pdf), which thus simply includes the evaporation heat in the voltage calculation. Not discussed is the possibility of using a heat pump to thermodynamically upgrade low-value heat with it and then use it for evaporation, thus reducing power requirements.
Also significant for the thermal system design is the temperature at which the electrolyzer must be operated in order to convert as much excess renewable electricity as possible into the chemical potential of the H2 when used as an electricity storage system. To explain this is the task of FIG. 19, where the temperature-dependent distribution of the reversible work ΔRG and reversible heat T·ΔRS to be supplied during electrolysis is plotted above the temperature T, starting with the ambient temperature T0, in order to obtain the reaction enthalpy ΔRH required for the reaction. It follows directly from FIG. 19 that the higher the temperature T, the less reversible work ΔRG can be converted to chemical potential. Thus, if the reversible work is available as fluctuating electric work, less electric work can be stored by means of a high-temperature electrolysis, operated at temperature T2, for the same hydrogen production than by a low-temperature electrolysis, operated at temperature T1. The heat requirement for evaporation, assuming the same pressure, depends only on the amount of H2 produced. Therefore, a high temperature electrolysis requires more heat of evaporation, relative to the electrical work supplied, than a low temperature electrolysis. This shows that low-temperature electrolysis best fulfills the task of converting excess electricity and non-usable waste heat into usable chemical potential.
In contrast, high-temperature electrolysis is the more suitable process when sufficient high-temperature heat is available and little electrical power is available. With sufficient external heat supply, the required evaporation heat can also be provided without difficulty and without generating additional electricity consumption. However, it must be taken into account that the use of high-temperature heat for evaporation leads just as much to considerable exergy losses and thus does not solve the thermodynamic problem. The methanation processes in use today use H2 and CO2 as reactants. Optimization of electrolysis is therefore a basic prerequisite for further optimization of these processes.
FIG. 1 shows the process flow diagram of a reversible H2 electricity storage process, in which the thermodynamic variables required for balancing are entered for easier orientation. With this approach, the entire product cycle of the H2 is considered from the extraction of the H2O in the liquid state via its evaporation, the extraction of the gaseous products H2 and O2 by means of electrolysis with the supplied electrical work, the storage of the products, the subsequent conversion in the fuel cell with the output of the generated electrical work and condensation of the gaseous reaction product H2O up to the discharge of the liquid H2O. Since reversibility of all processes is assumed, occurring losses can only be caused by faulty system structures, which are thus easily identifiable. It is then the task of the technical implementation to come as close as possible to these theoretical structures with technically and economically reasonable solutions. However, their influence on system efficiency always remains directly identifiable by comparison with the ideal solution.
The main components of this isothermal system are the electrolyzer (1) and the fuel cell (2), which are interconnected via the two gas reservoirs (3a) and (4a) and the associated lines (3) and (4) and are operated at temperatures T above the associated saturated steam temperature, practically above 100° C. The conduit system (3) contains H2 in the case of an H+-conducting electrolyte and O2 in the case of an O2−-conducting electrolyte, and accordingly the conduit system (4) is filled with O2 in the case of H+-conducting electrolytes and with H2 in the case of O2−-conducting electrolytes. However, this is irrelevant for thermodynamic considerations as long as H2 and O2 remain separate in systems 3 and 4. The gases H2 and O2 stored separately in the gas storage tanks (3a) and (4a) are fed to the fuel cell (2) when required (lack of current) and are converted back to H2O there, and the free enthalpy of reaction −ΔRG is released to the outside as reversible work (here and in the following, the resulting signs are prefixed for better understanding and the quantities ΔRG, ΔRS and Δsv are then to be understood consistently as absolute values). H2O is condensed in the condenser (5) and fed to the H2O tank (6) and the condensation heat −T·Δsv is transferred to the heat accumulator (7), where Δsv stands for the entropy change due to the phase change. In parallel, the reversible waste heat −T·ΔRS generated by the fuel cell (2) due to the reaction entropy ΔRS must be supplied to the heat accumulator (8). The liquid H2O taken from the H2O tank (6) in case of excess of current is fed to the evaporator (9), where it is evaporated by means of the supply of the required heat of evaporation +T·Δsv from the heat accumulator (7) and fed to the electrolyzer (1) via the H2O line (10). The electrolyzer (1) is supplied with the free enthalpy of reaction +ΔRG as reversible work from outside and the heat supply +T·ΔRS required because of the reaction entropy ΔRS is supplied from the heat accumulator (8). This closes the circuit and describes the functioning of its components. If we now summarize the energy supplied to and removed from the system, the following applies to the supplied energy Ezu:
E
zu=ΔRG+T·ΔRS+T·Δsv (3)
and for the energy to be removed Eab:
E
ab=−ΔRG−T·ΔRS−T·Δsv (4)
This process structure is exactly loss-free and real occurring losses in such plants are only due to the imperfection of the practical implementation with real lossy components. If we consider only the production of hydrogen which is spent elsewhere, the possibilities of recuperation via heat accumulators are omitted and even with fully reversible components, the heats T·ΔRS and T·Δsv have to be supplied from outside. It follows immediately that the main loss of H2 production is that H2O is supplied in the liquid state in most electrolyzers in use, and no heat stores exist to allow recuperation of heat given off elsewhere during oxidation of the H2 at other times. However, the contribution of reaction entropy to heat demand is comparatively small in this case.
An important approach of the “power-to-gas” technology, however, refers precisely to the use of the synthetic gases at a spatial distance from the place where they are produced, which means that the direct spatial connection between fuel cell and electrolysis no longer exists and thus the internal heat exchange necessary for high efficiency is no longer possible. In theory, however, this dilemma can initially be remedied by overcoming the spatial separation of the connection between reversible electrolysis and reversible fuel cell by shifting the system border of the storage process as shown in FIG. 1 in such a way that the environment simultaneously becomes a heat sink for the fuel cell and a heat source for the electrolyzer. This succeeds according to (Wolfgang Georg Winkler: Sustainable product development based on second law of thermodynamics. September 2011. applied energy 88(9): 3248-3256 DOI: 10.1016/j.apenergy.2011.03.020) using a reversible heat-power process that reversibly cools the H2O exiting the fuel cell at the condenser (5) to the ambient state now prevailing in the H2O tank (6), and using a reversible heat pump that preheats the liquid H2O of the H2O tank (6) needed to operate the electrolyzer and evaporates it in the evaporator (9). Since the work generated by the reversible heat engine, is equal to the work required by the reversible heat pump, the reversibility of the overall system is maintained. However, this initially purely theoretical consideration provides some guidance as to how, in the case of isolated synthetic gas generation without the inclusion of the gas consumers themselves, an improvement in efficiency can be achieved with the aid of reducing the losses to provide the evaporation heat. Practicable ways to exploit this principle include using solar heat or geothermal sources to vaporize the H2O needed for electrolysis, or waste heat from any process, reheated by heat pumps if necessary. This is the first step towards improving the efficiency of the generation of H2 and thus of CH4, as well as for the “power-to-gas” technology as a whole.
This will be explained in more detail in the following. As mentioned above, electrolyzer and fuel cell are usually operated separately, which means that the direct spatial connection between fuel cell and electrolysis no longer exists and thus the internal heat exchange necessary for high efficiency is no longer possible. The extension to this case is therefore necessary in order to understand and technically implement the design principles of electrolysis, which is essential for any “power-to-gas” technology.
The closed reversible cycle of an energy storage process based on H2 described in FIG. 1 does not describe the more detailed design of the H2O tank (6), the heat recovery via the condenser (5), the heat accumulators (7) and (8), and the evaporator (9), but only their ideal functioning. Thus, the system boundary within which these functions must be fulfilled is also freely selectable. It must only be possible to deliver H2O, H2 and O2 reversibly to the respective reservoirs and to take them out again reversibly, as well as to exchange the reversible heats T·ΔRS and T·Δsv reversibly between fuel cell and electrolyzer. Assuming the possibility of reversible transport of H2 from the electrolyzer to the fuel cell via, say, ideal conduits, the environment can serve as a reservoir of H2O, and O2. Then only the securing of the necessary reversible heat exchange between fuel cell and electrolyzer remains to be solved in order to be able to describe a reversible process control with the environment as storage.
According to (Wolfgang Georg Winkler: Sustainable product development based on second law of thermodynamics. September 2011. Applied Energy 88(9): 3248-3256 DOI: 10.1016/j.apenergy.2011.03. 020), such a process control can be represented by a combination of reversible heat engine and reversible heat pump, as shown in FIG. 20. Between the temperature levels T and T0, the heat (T0·ΔRS+T0·Δsv) coming from the fuel cell (2) (negative sign) is thus supplied to the environment (100) and from there to the electrolyzer (1) (positive sign) with the aid of Carnot processes, once operating as a heat engine (2a) and once as a heat pump (1a). For this purpose, the work WC must be dissipated once (negative sign) and supplied once (positive sign). The same principle is also applied to the outgoing and incoming substance flows from the fuel cell and electrolyzer (Wolfgang Georg Winkler: Sustainable product development based on second law of thermodynamics. September 2011. Applied Energy 88(9): 3248-3256 DOI: 10.1016/j.apenergy.2011.03.020), where the reversible work input or output corresponds to the associated exergy change of the respective substance flow. This overall system, which is independent of the distance between fuel cell and electrolyzer, is formed with the help of the system boundary (101), within which only reversible processes take place. The environment thereby forms a reversible storage for all heat and substances exchanged with it, while at the same time a reversible (electrical) energy network is tacitly assumed to be a reversible storage of the reversible work supplied and discharged. Furthermore, the choice of the environment as the storage eliminates the need to include the refilling of the storage in the considerations, because the reversible heat and substance extraction by the electrolysis system does not change the thermodynamic state of the environment. Similarly, the withdrawal of the required reversible Carnot work for the heat pump does not affect the state of a global reversible power supply system of ambient character. Thus, obviously, the electrolysis system for electricity storage is the most thermodynamically advantageous, which draws the required heat input from the environment with the least possible effort, converting the largest possible amount of electric work into thermodynamic potential of the formed H2. In contrast, high-temperature electrolysis may be the more advantageous solution when high-temperature heat is sufficiently available and electric power is scarce or too expensive. Whereby it must not be neglected that high temperature heat can be used for electricity generation as is well known and thus its thermodynamic value is significantly higher than that of low temperature waste heat.
This also indicates the design principle of how, in principle, the losses to provide the required evaporation heat can be minimized in any isolated synthetic gas generation with H2 as reactant. With the aid of a heat pump, the required evaporation evaporation heat can always be provided in a more energy-efficient manner in low-temperature electrolyzers than is the case with the dissipation of electrical work that is common today. The energy requirement of the heat pump can be further reduced if any waste heat can be used as its heat source instead of ambient heat. The heat pump can be dispensed with if the temperature of the waste heat is above the required evaporation temperature, usually 100° C. In the sense of the reference process, waste heat can be interpreted as any heat which, if not used for evaporation, would dissipate in the environment and thus contribute to its entropy increase.
The second main influence follows from the substance separation in thermal processes for CH4 generation according to Sabatier. Here, too, all possible process paths are analyzed accordingly (Wolfgang Georg Winkler: Sustainable product development based on second law of thermodynamics. September 2011. Applied Energy 88(9): 3248-3256 DOI: 10.1016/j.apenergy.2011.03.020). A reversible CH4 generation in principle would be the reversal of CH4 oxidation according to:
2H2O+CO2=CH4+2O2 (5)
In contrast to H2 production by electrolysis, there are currently no operational processes besides the thermal Sabatier process for CH4 production to electrochemically remove O2 from the reactor via an electrolyte. However, since O2−-conducting electrolyte materials are available as essential components for such processes and are constantly being further developed, and since catalyst development is also currently making interesting progress, the results of a corresponding thermodynamic analysis are included here. However, partial O2 removal for product separation is currently only possible via upstream electrochemical H2 or CO generation. In the first case, the reaction equation Eq. (5) must then be replaced by:
4H2+CO2=CH4+2H2O, (6)
so that the reaction products can be separated by condensation of the H2O. The disadvantage here is that the number of moles of H2O supplied to the system (electrolyzer and thermal methanation reactor), and thus the required evaporation heat, must be doubled compared to Eq. (5). Therefore, the processes used to remove O2 from the methanation reactor and thus in the preceding process steps have a decisive influence on the losses. A schematic diagram as shown in FIG. 2 serves to analyze the possible process steps. There, the starting materials and the thermodynamically possible variants of reactions are shown in 3 columns, with arrows indicating the corresponding links. H2O and CO2 enter the system as starting materials. The first step before thermal methanation is upstream electrolysis. For methanation, reduction to at least 2 mol of H2 per mol of CH4 to be produced is mandatory. To implement Eq. (6), H2 generation is sufficient as the only electrochemical process because CO2 is a reactant in Eq. (6). However, the reduction of CO2 to CO can also be represented electrochemically and has the advantage that the reaction proceeds at ambient conditions without phase change. Thus, in addition to the methanation reaction Eq. (6), which requires the provision of 4 mol of H2 per mol of CH4, the thermal methanation reaction Eq. (7) is also an option, requiring only 3 mol of H2 per mol of CH4. This reduces the evaporation loss by 25% compared to the reaction Eq. (6). However, two different electrolysis processes are then required simultaneously:
3H2+CO=CH4+H2O (7)
In the third column of FIG. 2, combinations are given only for those processes which are possible with an O2−-conducting electrolyte via which O2 can be discharged electrochemically. The combination of 2 upstream electrolysis processes for the production of CO and H2 fulfills this requirement, combined with the removal of ½ mol O2 in the methanation reactor according to the reaction Eq. (8):
In an adaptation of Eq. (8), if electrolysis is omitted to produce CO, the reaction equation (9) follows, in which CO2 is fed directly into the methanation reactor, in which 1 mol of O2 must then be removed:
2H2+CO2=CH4+O2 (9)
Accordingly, it is also possible to adapt Eq. (7) in such a way that H2 production is omitted and 2 H2O is fed in. The electrolysis then produces only CO and in the methanation reactor 3/2 mol O2 must then be removed according to Eq. (10):
For the energetic evaluation it is still important, which reversible work is required for the reactions according to Eqs. (6) to (10), or what work, if any, they could deliver. This can be determined with the help of FIG. 3. There, above the reaction temperature, the reversible methanation work of the methanation reactions dealt with here, eqs. (5) to (10), with the amount of O2 discharged as a parameter and with reference to the associated reaction equation. It becomes clear that the O2 removal in the methanation reactor is very well suited together with the reactor temperature to describe the energetic reactor state. With the exception of a narrow operating range in which thermal reactions are possible, in the range of reactions described by Eqs. (5) to (10), work must always be supplied. The thermal reaction Eq. (6) is reversible only at 862 K and reaction Eq. (7) only at 892 K, where in each case the reversible work vanishes (ΔRG=0). At lower temperatures, ΔRG<0 holds and the methanation reactor could supply work, but due to the existing thermal reactor design, this work must be dissipated and removed as heat. At higher temperatures, work would have to be supplied to the methanation reactor because of ΔRG>0. The conversion of the thermally driven processes according to Eqs. (6) and (7) into electrochemical processes with H+-conducting electrolytes, however, allows in principle to recover electric work during the methanation reaction. All of the above reversible methanation reactions Eqs. (6) to (10) always give off heat to cover exactly the demand of the upstream electrolysers.
The combination of H2O or CO2 electrolysis with the reactions described by Eqs. (6) to (10) also requires a more detailed analysis of the heat fluxes in the linked reactions. Analogous to FIG. 3, FIG. 4 plots the heat release (−) from the methanation step and the total heat absorption (+) from the electrolyzers versus temperature. As the temperature increases, the amount of heat given off and the amount of heat absorbed increases. This indicates that therefore lower temperatures in the processes lead to lower investment costs. The working ranges of the electrolysis and methanation processes are also plotted, as is the amount of O2 discharged as a parameter, and the assignment to the electrolysis and methanation processes can be seen from the equation numbers noted. Since the temperature of the methanation is usually higher than that of the associated electrolysis, more heat is given off than is needed for the electrolysis. This results in a surplus in heat output that can be used to evaporate the water for electrolysis. This effect occurs because electrolysis operating at a lower temperature requires more electrical power than at a higher temperature, where more energy is supplied by heat. These different temperatures are process-related because of the operating temperature of the catalysts, and the excess (irreversible) heat thus supplied can be used as evaporation heat in the system.
All processes described herein for methanation according to Eqs. (6) to (10) have three different temperature levels as a common feature of the process control. These are, with increasing temperature, the ambient state, the common temperature level of electrolyzer and evaporator, and the temperature level of the methanation stage. As discussed in (Wolfgang Georg Winkler: Sustainable product development based on second law of thermodynamics. September 2011. Applied Energy 88(9): 3248-3256 DOI: 10.1016/j.apenergy.2011.03.020), the ideal process can be approximated very well by a consistent heat exchange between the heating and cooling substance flows between the temperature levels. FIG. 5 serves to explain these circuits using the example of process control according to Eq. (6). Here, an O2−-conducting electrolyte in the electrolyzer (1) is taken as a basis and, for clarification, reference is made to the resulting assignment to O2 and H2. The methanation reactor (11) is supplied with H2 via line (3) and with CO2 via line (12). Of the heat released there, to cover the heat balance, the T·ΔRS portion is supplied to the electrolyzer (1) and the T·Δsv portion to the evaporator (9). The CH4—H2O mixture is discharged from the methanation reactor (11) via the line (13) according to Eq. (6). The CH4—H2O mixture is then passed through the heat exchangers (16) to the H2 preheater and the second stage CO2 preheater (17) finally to the condenser (15) for separation of CH4 and H2O. The condensed H2O is stored in the vessel (6) and the CH4 is discharged separately (14). To produce 1 mol of CH4, 4 mol of H2O must be supplied to the electrolyzer, half of which is thus provided by recirculation and the other from outside. The required H2O is supplied to the evaporator (9) via the condenser (15) after preheating and to the electrolyzer (1) as steam via the line (10). The O2 leaving the electrolyzer is fed via the line (4) to the heat exchanger (17) as the first preheating stage for preheating the incoming CO2 and is cooled there.
With this preliminary work, the tasks set for improving the efficiencies of the synthetic production of H2 and CH4 and other hydrocarbons, generally hydrocarbon compounds, which behave according to the characteristic diagrams of FIGS. 3 and 4, and for their technical implementation can be specified in the following points. Depending on the availability of suitable catalysts and electrodes, the use of O2−-conducting electrolytes can also enable direct methanation according to Eq. (5). This means that for each mol of CH4 produced, at least 2 mol of H2O must be supplied. Naturally, the corresponding evaporation heat must be supplied to the system at the same time. Although the reaction entropy is vanishingly small for reactions according to (5), the reversible electrochemical methanation reactor in the comparative process must again be supplied reversibly from the outside with the evaporation heat (T0·ΔRS+T0·Δsv) according to FIG. 20.
TASK OF THE INVENTION
- 1. to use the principles of the reversible comparison process according to FIG. 1 to improve the energy efficiency of “power-to-gas” technologies based on H2 and synthetic hydrocarbons, generally hydrocarbon compounds, as elements of large-scale electricity storage and to use CO2 as a raw material in closed cycles without releasing CO2 to the environment;
- 2. improving the energy efficiency of H2 generation using electrolysis through devices to provide H2O in gaseous state upstream of the electrochemical electrolysis cell, largely independent of the operating temperature;
- 3. improvement of thermal integration of upstream electrolysis for H2 generation and Sabatier exothermic methanation reaction, and utilization of the working potential of this reaction for recovery, thus improving efficiency.
- 4. improvement of thermal integration of upstream electrolysis for H2 generation and exothermic methanation in potential new electrochemical processes.
- 5. system integration of the newly developed above gas generators into an electricity storage system and with integrated recycling for CO2 as a sustainable industrial feedstock.
- 6. transfer of the process logic and device to other related process designs.
Solution of the Task
The elaborated reversible process control of electricity storage with the aid of H2, as indicated in FIG. 1, can be directly implemented with minor adaptations into a technically realizable device for electricity storage for grid stabilization with high efficiency, the design of which leads directly to the sought-after devices for energy-efficient generation of H2 and advantageously CH4 as well as other synthetic hydrocarbons, generally hydrocarbon compounds. Crucial for the transfer of the principles to a device is that the electrolyzer is always provided with the required H2O only in the gas phase by a suitable heat recovery or use of waste heat, in order to avoid high heat losses due to the necessary H2O evaporation.
The technical solution of the electricity storage device, as shown in FIG. 6, differs from this reversible basic structure only in that real occurring temperature and pressure differences are taken into account. For this purpose, the fuel cell (2) is supplied with H2 and O2 from the gas accumulators (3a) and (4a) when electricity is required and is operated at a temperature so much higher than the electrolyzer (1) that reliable heat exchange via the heat accumulator (8) is ensured. The pressure of the exhaust steam of the fuel cell (2) is appropriately increased by a steam compressor (18) so that the heat accumulator (7) can be supplied by the condenser (5) with waste heat at a sufficiently high temperature during the condensation of the exhaust steam of the fuel cell, and the condensate is supplied to the H2O tank (6). When there is a power surplus, the electrolyzer (1) is supplied with steam from the H2O tank (6) via the feed pump (19), the evaporator (9) and the H2O line (10), and then refills the two gas reservoirs (3) and (4) with H2 and O2. The heat required for this is taken from the heat accumulator (7).
The amount of heat stored in the heat accumulator (8), which follows from the release of the reaction entropy of the fuel cell, is relatively small at low operating temperatures of the electrolyzer. It is therefore advisable to check here whether the operating conditions of the electrolyzer permit an economical additional storage installation, or whether it is more sensible to compensate for the heat loss by electrical heating or to seek other solutions.
According to the invention, the evaporation heat from the heat source (22, 27) is waste heat from processes or waste heat obtained from the environment. Waste heat from (industrial) processes is in particular industrial waste heat, preferably with a temperature of maximum 400° C., further preferably maximum 300° C., still further preferably maximum 200° C. Waste heat from processes is advantageously external waste heat, i.e. it is supplied to the device according to the invention from outside and originates, for example, from an external industrial process and not from processes within the device according to the invention, in particular not from the waste heat of a fuel cell in the device according to the invention, unless this heat would otherwise be discharged into the environment as intended. A heat pump can be omitted if the temperature of the waste heat is above the required evaporation temperature.
A simplified device shown in FIG. 7 with the same mode of operation differs from the basic solution discussed above in that the system of condenser (5), H2O container (6), heat accumulator (7) and evaporator (9) is replaced by a steam accumulator (20) which supplies the electrolyzer (1) with steam in the event of a power surplus and is recharged by the exhaust steam from the fuel cell via the steam compressor (18) when power is required. To increase the security of supply, the steam accumulator (20) can be equipped with electrical heating for pressure maintenance or a connection to a steam network, if available, or further connected steam accumulators or other heat accumulators and thus be integrated into an industrial sector coupling with a large storage volume.
The device described can also be used with regenerative fuel cells (1/2), which can also be operated as electrolyzers, as FIG. 8 shows. For this purpose, the steam accumulator (20) is connected to the regenerative fuel cell (1/2) and the steam compressor (18) simultaneously via a three-way valve (21). In fuel cell mode, the three-way valve (21) connects the regenerative fuel cell (1/2) to the steam compressor (18) and the steam accumulator (20) is charged with the vaporous H2O formed. In electrolysis mode, the three-way valve connects the steam accumulator (20) to the regenerative fuel cell (1/2) which is in electrolysis mode. To increase operational reliability and storage volume, the steam storage tank (20) can also be connected to other steam storage tanks in a steam network and integrated into an industrial sector coupling.
As already explained on the basis of the extension of the balance boundary of the electrolyzer-fuel cell system as shown in FIG. 20, the possibilities of process improvement shown in FIG. 1 can therefore also be used for open cycles for H2 production and consequently for CH4 production and, under certain conditions, also for the production of further hydrocarbons (CnHm) or hydrocarbon compounds. Thermodynamically, the reversible process described above can be approximated if the evaporation heat released during H2 oxidation—whether in a fuel cell or in a combustion reactor—is released to the environment during its condensation due to lack of recuperation capability, if conversely the evaporation heat of the H2O required for electrolysis can be recovered from the environment. This can be achieved, or at least approximated, as part of a system integration of the electrolysis (1) using a circuit as indicated in FIG. 9. H2O is fed from the tank (6) via the feed pump (19) to the evaporator (23), where it is supplied with sufficient heat of evaporation from the heat source (22), and the resulting steam is then fed to the electrolyzer (1). In its structural design, the evaporator (23) can integrate the steam accumulator (20) into the evaporator (9). If, for example, the heat source (22) is supplied with solar heat or geothermal heat with evaporation heat, the requirement for a reversible process would be well approximated, since the environment would have (at least approximately) resupplied the evaporator with the entropy given off during H2 oxidation by this type of heat transfer. Although this ideal case is not always achievable, a setup according to FIG. 9 can always be used to utilize waste heat from various processes, if necessary within the limits of economic viability, with the aid of heat pumps, as already shown in the comparative process according to FIG. 20, to evaporate the H2O supplied to the electrolyzer in order to significantly reduce the exergy losses of 17% during H2 production. The advantage of this approach is that in industrial plants steam with low exergy can be used for this purpose in order to save the dissipation of electrical work.
The device shown in FIG. 10 shows an advantageous installation of an electrolyzer (1) in an integrated evaporator (26), which also serves as a steam accumulator (20) or for steam storage, similar to a shell boiler. Heating can be carried out in various ways using a heat source (27). For example, various heat-emitting fluids can be passed through tubes, or exothermic reactions that need to be cooled can take place there. The heat sources (27) are to be arranged in parallel in such a way that an orderly evaporation process and water circulation in the evaporator section (9) can be ensured. The electrolyzer (1) consists of parallel arranged cell groups (24), which can be plate- or tube-shaped but also micro-process modules, which are surrounded by a cladding tube (25). The steam is directed via the steam dome (28), the steam line (29) to the distributors (30) and then to the cell groups (24) located in the cladding tube (25). The parallel flow distributors (30) connect the cell groups (24) and the associated cladding tubes (25) to the parallel headers (31) and (32). The gases of systems (3) and (4) then exit from headers (31) and (32). If an H+-conducting electrolyte is used for the cell groups (24), H2 exits as a gas in (3) and O2 exits as a gas in (4). When an O2−-conducting electrolyte is used, H2 exits as a gas in (4) and O2 exits as a gas in (3). H2O is supplied via the inlet connection (33), whereby the entering water should already be preheated to close to the saturated steam temperature. When using the electrolyzer in industrial plants with steam networks, the inlet connection (33) can also be used to feed external steam to maintain the temperature of the system during shutdowns and also, depending on the possibilities of system integration, for steam heating instead of the heat source (27). For start-up and as an emergency supply, electrical heating of the evaporator is also possible. In the case of an integrated H2 electricity storage system in accordance with the structures discussed in FIG. 7, the individual fuel cell groups (2) are surrounded analogously to the electrolysis cells (24) with or without cladding tubes corresponding to (25) and, while retaining the circuitry indicated in FIG. 7, are simultaneously used as a heat source (27). It should be noted that, in comparison with the design of the electrolyzer (1), the gases involved, H2 and O2, enter the fuel cell via two separate flow distributors in the fuel cell process in accordance with the process-related reversal of the flow direction, and the product H2O is fed to the steam compressor (18) via an outlet collector and is then fed to the integrated evaporator (26) via the inlet (33). One option is for the steam compressor (18) to inject the higher pressure steam not only into the evaporator section (9), but also into the steam chamber (35a). Instead of electrical heating, gas accumulators (3a) and (4a) can also be used to supply heat to maintain the pressure of the integrated evaporator, and thus integrated H2 burners supplied with O2 that feed their exhaust gas H2O directly into the evaporator section (9). In the case of integrated H2 electricity storage systems corresponding to FIG. 8, the integration of the regenerative fuel cell (1/2) shown here in place of the electrolyzer (1) into the integrated evaporator (26) alone is sufficient, and the latter takes over the tasks of the steam accumulator (20) and the heat accumulator (8). The H2O exiting the fuel cell (1/2) is returned to the integrated evaporator (26) via the steam compressor (18).
FIG. 11 shows a variation of the device shown in FIG. 10, in which the flow distributors (30) and the collectors (31) and (32) are replaced by chambers. The flow distributor (30) is replaced by the steam chamber (34), which is separated from the actual evaporator section (9) by a tube sheet (35). Similarly, the two headers (31) and (32) are replaced by the two gas chambers (36) and (37) formed with the tube sheets (35). The flow routing and gas distribution is analogous to what was said for FIG. 10. In the case of an integrated H2 electricity storage unit according to the variants presented in FIG. 7 or 8, what has been said above in the description of FIG. 10 applies.
A further simplification of the construction of the device is shown in FIG. 12. The electrolyzer is constructed as in FIGS. 10 and 11 with the difference that the cladding tube (25) is omitted and the cells are arranged directly in the evaporator section (9). This reduces the number of tube sheets (35) to only one and the discharge of the generated gases O2 and H2 takes place via the associated outlet nozzles of lines (3) and (4). It is possible that steam is also entrained with the outgoing product gases O2 or H2. This effect can be largely reduced with cyclones and, if necessary, by subsequent condensation. Which gases escape at (3) and (4) depends on the electrolyte selected and corresponds to what has already been said above. In the case of an integrated H2 electricity storage system according to the structures discussed in FIG. 7, the same applies as in the description of FIG. 10.
As FIGS. 3 and 4 show, the device shown in FIGS. 10 to 12 also fulfills very well the requirements for optimizing devices for CH4 generation and its thermal integration with the upstream electrolysis. For this purpose, FIG. 13 shows the heating (11) of the integrated evaporator using the example of the variant shown in FIG. 12 by means of an exothermic reaction based on the methanation reaction Eq. (6) instead of or as a supplement to the general heat source (27). At the same time, this also shows the principle structure of the associated further necessary system integration for an integrated CH4 generator. In principle, all of the systems integration steps given in Eqs. (6) to (10) are exothermic and therefore in principle suitable for heating the evaporator, as already explained for FIG. 4. The basic principles required to design integrated CH4 generators combining electrolytic H2 generation with both thermal and possible electrochemical CH4 methanation reactors have already been presented in FIG. 5. For further clarification of the design principles of a highly integrated CH4 generator, the reaction control according to Eq. (6) already selected and explained in FIG. 5 is therefore also further used in FIG. 13. As in FIG. 12, the assignment of system (4) to O2 and of system (3) to H2 is immediately clear here, because only H2 and not O2 is allowed to flow into the methanation reactor (11). The fluid in the piping system (12) is CO2 and flows to the methanation reactor (11) without upstream electrolysis. The system design corresponds to the formulated rules described in (Wolfgang Georg Winkler: Sustainable product development based on second law of thermodynamics. September 2011. Applied Energy 88(9): 3248-3256 DOI: 10.1016/j.apenergy.2011.03. 020). According to these rules, reactants and products must first be used for heat recovery, taking into account the individual temperature levels of the reactions involved, before additional heat may be added. In the first stage, the products H2 and O2 are obtained from the reactant H2O. In the second reaction stage, CH4 and H2O are produced from the reactants H2 and CO2 according to Eq. (6). This results in three different temperature levels: ambient, evaporator/electrolyzer and methanation, between which the heat recovery shown here takes place. The CH4—H2O mixture is discharged from the methanation reactor (11) via the line (13) according to equation (6). The CH4—H2O mixture is then passed through the heat exchangers (16) to the H2 preheater and the second stage CO2 preheater (17) and finally to the condenser (15) for separation of CH4 and H2O. The condensed H2O is stored in the tank (6) and the CH4 is discharged separately (14).
From the electrolyzer, 4 moles of H2O are required to produce 1 mole of CH4, half of which is thus provided by the recirculation and the other from outside. These 4 mol H2O/mol CH4 are supplied to the evaporator section (9) via the condenser (15) after preheating via the line (33) and are supplied internally to the electrolyzer (1) as steam. The O2 leaving the electrolyzer is used via the heat exchanger (17) as the first preheating stage to preheat the incoming CO2. In this process, the condenser (15) can also serve as a waste heat source for the heat pump.
Since the electrolytic H2 generation must also be combined together with the electrolytic generation of CO, FIG. 14 shows a corresponding addition to the system setup of the device shown in FIG. 13. However, the changes in the system only affect the area of the device that comprises the supply line of the CO2 (12) and the components associated with it. In particular, these relate to the change in preheating associated with the supply and discharge of the process gases CO2 and CO. The second preheating stage of CO2 is replaced by a preheating of CO by the CH4—H2O mixture in the heat exchanger (42) corresponding to the temperature level. The design options for integrating the CO2 electrolyzer (39) can be taken from FIGS. 10 and 11 using the example of integrating the H2O electrolyzer. The solution shown in FIG. 14 is based on FIG. 10. When simultaneously integrating the electrolyzers for the generation of H2 and CO, special care must be taken to avoid short circuits in the integrated evaporator (26). The CO2 is fed via the line (12) to the distributor (38) of the CO2 electrolyzer (39), where it is converted into CO while releasing ½ O2 into the evaporator section (9) in the same way as for H2O electrolysis. This is led via the collector (40) and the line (41) to the preheater (42) and from there to the methanation reactor (11). The conception of the treatment of the CH4—H2O mixture leaving the methanation reactor (11) remains unchanged compared to FIG. 13.
Analogous to FIG. 11, this device can be further simplified in terms of design, as shown in FIG. 15. There, the distributor (38) is replaced by a CO2 inlet chamber (43), which serves to supply gas to the electrolysis cell (39). The CO formed in the electrolysis cell (39) is then fed into the H2 outlet chamber (36) and the H2—CO mixture is fed to the methanation reactor (11) via the line (3) after preheating in the heat exchanger (16), as in FIG. 13.
Another possibility for improving the efficiency of methanation according to Eqs. (6) and (7) results from utilizing the unused potential indicated in FIG. 3 for the recovery of electrical work in these thermal processes. FIG. 16 serves to explain the practical implementation. H2 and CO2 flow into the methanation reactor (11) according to Eq. (6) or CO according to Eq. (7). In both cases, additional H2 alone is needed compared to the CH4 generation requirement, in order to be able to separate O2 after oxidation with H2 by condensation of CH4. The available potential to supply electric work can be utilized by adding H2 using an H+-conducting fuel cell. H+ ions then emerge from the outer surface of the fuel cell and react with CO2 and CO, respectively, to form CH4 and H2O. In all highly integrated systems according to FIGS. 10 to 16, special care must be taken to ensure that no short circuits can occur between the integrated current-carrying components and that they are excluded by design.
In the case of direct electrochemical generation of CH4 using O2−-conducting electrolytes, already discussed above, the design principles derived above can be adapted to the appropriate devices. FIG. 21, which arises from the design according to FIG. 11, shows as an exemplary example a device for a methanation reaction according to Eq. (5). The electrolysis cells (24) are replaced by methanation reactors (11), the walls of which are formed from O2−-conducting electrolytes, whereby the channel (37a) is formed with the cladding tube (25a), which serves to discharge the O2 produced during the reaction. Steam is supplied to the steam chamber (34) via the steam line (29). CO2 is supplied there through the line (12) after its preheating in the heat exchanger (17), and the mixture of H2O CO2 is then fed into the methanation reactor (11). According to the design of the separate gas inlet by means of distributors or chambers according to FIG. 10 or FIG. 11, a separate feed of CO2 and vaporous H2O into the methanation reactor can also take place. Depending on the catalysts used, different operating temperatures are possible for such methanation reactors, therefore, due to the possible different operating conditions, reference is made to the already quoted design rules for heat recovery in (Wolfgang Georg Winkler: Sustainable product development based on second law of thermodynamics. September 2011. Applied Energy 88(9): 3248-3256 DOI: and to the presented embodiments in FIGS. 10 to 15.
Using combinations with the devices for generating H2 and CH4 according to Eqs. (6) to (10), the inlet gas concentrations of the methanation reactors (11) can be optimized for different catalysts. For this purpose, the device according to FIG. 21 only has to be supplemented by the installation of H2O and/or CO2 electrolysis cells according to the embodiments of FIGS. 10 to 15. If, for example, an admixture of H2 is desired, the device according to FIG. 21 can be combined with H2O electrolysis, as FIG. 22 shows using the example of Eq. (9). For this purpose, only the chambers (34, 36, 37) at the inlet and outlet must be divided with a partition (35a) and/or supplemented or replaced with the design of flow distributors (30) and collectors (31), (32). The corresponding division at the outlet results in a chamber (36) for CH4 and a chamber (36a) for H2. The construction of the electrolysis section corresponds to FIG. 11 with an O2−-conducting electrolyte. The H2 formed is admixed to the CO2 line (12) via the line (3). Alternatively, the H2 can also be added on the steam side in the line (29) or in the inlet chamber (34); in this case, an optional H2 withdrawal is also possible via the branch (3a). A control device (29a) is used to control the amount of steam supplied to the electrolysis (1, 24) and thus the H2 production. In the same way, the supply of CO and CO2 to the methanation reactors (11) can be controlled in the required ratio. If only a smaller addition of H2 is required for methanation, the cladding tube (25) can be dispensed with, following FIG. 12, if an H+-conducting electrolyte is used, the steam and the H2 formed being fed to the methanation reactor (11) via the evaporator section (9) and the line (29). The O2 exiting the electrolyzer (1, 24) is then discharged via the gas compartment (37), and the partition (35a) and gas compartment (36a) are omitted. The installation of a CO2 electrolyzer (39) is analogous to that of an H2O electrolyzer (1) with O2−-conducting electrolytes and accordingly a cladding tube analogous to (25) and with the associated gas chambers (43) and or flow distributors (38) and collectors (40). The pipe (41) conducts the formed CO to the methanation reactor (11). The geometrical arrangement of the individual integrated methanation reactors (11) and the electrolysers (1) and (39) is decisively determined by their influence on the heat and substance concentrations as well as the heat and substance transport and, if necessary, optimized with internals for flow control.
The further integration of the device of an integrated CH4 generator (44) designed according to the above mentioned design principles into a sustainable energy system is essential for the sustainability and CO2 freedom of its operation. Again, the reversible comparative process shown in FIG. 1 represents the theoretical basis of the process control. Accordingly, the CH4 produced is stored in existing gas storage tanks (45) and used in fuel cells (2) to generate electricity and heat. The resulting flue gas contains only CO2 and H2O, and residual heat utilization in a flue gas condenser (46) allows CO2 and H2O to be separated. After compression in the CO2 compressor (47), the resulting CO2 is fed via a CO2 piping system (12) to the CO2 storage (48), which in turn feeds the CH4 generator (44) again. To improve the sustainability of the energy system and the raw material economy, it is expedient to cover the CnHm demand, generally demand for hydrocarbon compounds, of industry and commerce (49) from the gas storage facilities (45). However, since CH4 synthesis requires CO2 as a feedstock, this supply relationship of regenerative CH4 supply inevitably results in the unavoidable need to also recycle CO2 from plastic waste and return it to the CH4 generator through the pipeline (12). Since the volumetric energy density of CH4 is about four times higher than that of H2, CH4 can be used to hold significantly more energy to cover seasonal longer severe supply shortages of renewable power than would be possible with H2. Therefore, it is appropriate to conceptually provide for the possibility of H2 generation from CH4 (50) to secure H2 supply even in the event of prolonged generation shortfalls of renewable electricity generation, thus significantly increasing H2 supply security. Conversely, hydrogen supply (4) to industry from ongoing hydrogen production (51) is also a useful addition. The H2O system (10), which is also shown, is intended to illustrate the H2O demand of the processes described, but in practice this is probably only of interest at highly integrated industrial sites.
The process control of the devices derived here from the example of CH4 production and the technical solutions shown here for their plant engineering implementation can also be used, as already indicated several times, for the production of C2H4 and other hydrocarbons CnHm or hydrocarbon compounds. The prerequisite for this is that the thermodynamics of their process control correspond to the characteristic diagrams of reaction work, reaction heat and O2 removal in the electrolyzers and in the methanation reactor shown in FIGS. 3 and 4, so that these structures can be used. FIG. 18 shows an example of a compilation of comparable reaction equations for CH4 and C2H4 and the associated methods of O2 removal.
Patent Application
20230392265
