OPERATING METHOD FOR A SOLID OXIDE CELL SYSTEM

Abstract

A method of operating a solid oxide cell system comprises generating an electrochemical conversion from one of: (i) water steam H.sub.2O(g); and (ii) a mixture comprising water steam H.sub.2O(g) and carbon dioxide CO.sub.2. A quantity of at least one other substance is added into the one of the water steam H.sub.2O(g) and the mixture comprising water steam H.sub.2O(g) and carbon dioxide CO.sub.2. The at least one other substance comprises a hydrocarbon C.sub.mH.sub.n. The quantity of the at least one other substance is converted into a syngas CO+H.sub.2. An endothermic reforming of the mixed-in hydrocarbons occurs by coupling-in waste heat from the electrochemical conversion. The additional quantity of the at least one substance is added compensate for effects of a degradation of the solid oxide cells of the solid oxide cell system. A total quantity of the hydrogen H.sub.2 generated by the solid oxide cell system is kept constant.

Claims

1-7. (canceled)

8. A method of operating a solid oxide cell system, comprising: generating an electrochemical conversion from one of: (i) water steam H.sub.2O(g); and (ii) a mixture comprising water steam H.sub.2O(g) and carbon dioxide CO.sub.2; adding a quantity of at least one other substance into the one of the water steam H.sub.2O(g) and the mixture comprising water steam H.sub.2O(g) and carbon dioxide CO.sub.2, wherein the at least one other substance comprises a hydrocarbon C.sub.mH.sub.n; and converting the quantity of the at least one other substance into a syngas CO+H.sub.2, wherein an endothermic reforming of the at least one other hydrocarbon occurs by coupling-in waste heat from the electrochemical conversion, wherein the additional quantity of the at least one substance compensates for an effect of a degradation of the solid oxide cells of the solid oxide cell system, and wherein an additional quantity of hydrogen H.sub.2 is produced and a total quantity of the hydrogen H.sub.2 generated by the solid oxide cell system is kept constant over a time.

9. The method according to claim 8, wherein the total quantity of the hydrogen H.sub.2 generated by the solid oxide cell system is kept constant relative to an operation under rated load.

10. The method according to claim 8, wherein at least one of: (i) a quantity of hydrogen produced; and (ii) a quantity of syngas produced, is determined using Faraday's Law relying on a measured current intensity.

11. The method according to claim 8, further comprising providing a ramp configured for a standard volumentric flow of the at least one other substance.

12. The method according to claim 8, wherein the solid oxide cells of the solid oxide cell system are configured to be cooled by the endothermic reforming, such that they are configured to be operated thermoneutral over their entire service life.

13. The method according to claim 8, wherein an internal reforming of the at least one substance occurs within the solid oxide cells of the solid oxide cell system.

14. The method according to claim 8, wherein an external reforming of the at least one substance occurs by means of a reformer arranged upstream of the solid oxide cell system, and wherein heat is led away from the solid oxide cell system to the reformer to cool the solid oxide cells of the solid oxide cell system.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0044] In the drawings:

[0045] FIG. 1 schematically illustrates an embodiment of an operating method for a solid oxide cell system;

[0046] FIG. 2a graphically illustrates an embodiment of a curve of hydrogen output or of the efficiency over time t, for methods according to the prior art in which no compensation for the degradation occurs;

[0047] FIG. 2b graphically illustrates an embodiment of the operating method for a solid oxide cell system in which hydrocarbons CmHn are used to compensate for the degradation; and

[0048] FIG. 3 schematically illustrates an of an SOEC system that is suitable for an embodiment of the disclosed operating method for a solid oxide cell system.

DETAILED DESCRIPTION OF THE INVENTION

[0049] FIG. 1 shows the basic functional principle of the operating method for a solid oxide cell system according to the invention. The solid oxide cell system depicted as an example comprises a stack 1 of SOEC or respectively rSOC that can be operated in an electrolysis mode. With regard to the functional principle, the stack 1 can also be understood as a single cell or as a stack module. The stack 1 comprises an oxygen electrode 2 (anode) and a hydrogen electrode 3 (cathode). Air 8 is fed to the stack 1 as a flushing medium on the anode side, and a gaseous reformate of CH4, H2O, CO2, CO and H2 24 is fed to the stack 1 on the cathode side, and via the electrochemical reaction, preferably using renewable electricity, syngas (H2, CO) is generated as a product gas 31, while on the anode side flushing medium 9 enriched with oxygen O2 is released.

[0050] An additional quantity of hydrogen can be generated for balancing out a degradation of the cells or for covering an extra demand short-term or at short-notice, if hydrocarbons CmHn are fed to the solid oxide cell system. The hydrocarbons can be pre-reformed (only partially converted) or fully reformed in an external reformer 23, together with the water steam or respectively the mixture comprising water steam and carbon dioxide, which is provided in the conventional manner in high-temperature co-electrolysis. For this purpose, heat Q.sub.R can be coupled in from the stack 1. Alternatively or additionally, an internal reforming is possible in which the hydrocarbons, here preferably methane, are converted into H.sub.2 and CO directly at the catalytically active hydrogen electrode. In this context, heat Q.sub.E is led away from the electrochemical conversion through the endothermic reforming. Alternatively, the work can be done without the external reformer 23. The educt gas together with the hydrocarbons C.sub.mH.sub.n is led directly to the stack 1 and reformed in the cells of the stack 1.

[0051] The method depicted can always be applied when hydrogen/syngas is to be provided and waste heat due to the ohmic losses in the stack is available. This is the case when the stack is degraded and the ohmic resistance therefore increases.

[0052] The method can also be applied when the solid oxide cell system is connected to a synthesis, e.g. Fischer-Tropsch synthesis. Therein a hydrocarbon-rich residual/circulation gas arises, which is converted into H2 and CO in the external reformer 23 or directly in the stack, using heat from an exothermically operated electrolysis. By this heat decoupling, the electrolysis can be operated at a power density at which the stack would otherwise either overheat, or the system is cooled by air and this reduces the efficiency. The heat feed-in to the external reformer 23 can occur in various manners known in the prior art.

[0053] In FIG. 2a and FIG. 2b, the relative curve of the hydrogen output V or rather of the overall efficiency ETA over time for the method according to the prior art and the method according to the invention, wherein hydrocarbons CmHn are used to compensate the degradation are depicted. L2 and L6 depict the curve over time of the hydrogen output or rather of the efficiency for the method according to the prior art in which no compensation occurs for the degradation of the cells by mixing-in hydrocarbons. The SOEC system is operated increasingly exothermically over time, by which although a virtually constant output of hydrogen can be ensured, the efficiency falls with the increasingly exothermic process management. L3 and L4 depict the curve over time of the hydrogen output or rather of the efficiency for the method according to the prior art in which no compensation occurs for the degradation of the cells by mixing-in hydrocarbons. Although the SOEC system continues to be operated at its thermoneutral point, by which the efficiency can be kept constant over time, the quantity of the hydrogen donated falls over time, however, relative to the rated value of the hydrogen output. If hydrocarbons are then fed to the system for compensating the degradation, this has the effect that additional hydrogen (and additional syngas) is generated, on the one hand, and the stack is cooled by the endothermic reforming and therefore can be operated at its thermoneutral point, on the other hand. Thus both the H2 output as well as the efficiency can be kept constant in a very simple manner (cf. L1 and L5).

[0054] FIG. 3 shows a possible embodiment of an SOEC system that can be operated with the method according to the invention. It consists of an SOEC stack or stack module 1, which in turn comprises an oxygen electrode 2, a hydrogen electrode 3, and an oxygen-conducting electrolyte (not shown). Temperature-controlled air or another temperature-controlled flushing medium 8, such as nitrogen, CO2, water steam, or other gases that are inert with respect to oxygen are fed to the oxygen electrode 2 of the stack 1. The cold flushing medium 4 is first recuperatively heated by a heat exchanger 5. The preheated flushing medium 6 is additionally adjusted to the desired stack inlet temperature by means of an electric heater 7 before it is fed to the oxygen electrode 2. The oxygen-enriched flushing medium 9 leaving the stack via the oxygen electrode 2 is cooled by means of a heat exchanger 10, wherein the heat exchangers 5 and 10 preferably form one component. The cooled flushing medium 11 can be fed to an additional heat exchanger 12, to decouple usable heat again before it leaves the system as cold circulation medium 13.

[0055] Water or rather water steam 20 is fed to the system on the hydrogen electrode side. When fluid water is used, it is first transformed into the gaseous state in the evaporator 21. The evaporator 21 can be implemented as a heat store in which excess waste heat is stored, to generate steam with low energy expenditure. Other than heat/waste heat, the energy feed-in can also be electric. The water steam 22, with process gases 27, e. g. hydrocarbons C.sub.mH.sub.n, particularly methane CH.sub.4, or return gases from a connected synthesis system (not shown), are fed to a catalytic reactor/reformer 23. In this context, the catalytic reactor/reformer 23 can be implemented such that heat from the stack module 1 can be coupled into it. In this context, the catalytic reactor/reformer 23 can be structured directly as a heat exchanger, or also implemented as a series circuit of at least one heat exchanger and reactor stage. The catalytic reactor/reformer 23 generates a reformate 24 from the process gases 27, said reformate 24 containing CH.sub.4, H.sub.2O, CO.sub.2, CO and H.sub.2. If necessary, the reformate 24 can be further heated by means of a recuperator 24a before it is fed to the hydrogen electrode 3. Alternatively or additionally, an electric heater (not shown) can also be used for this. Hydrocarbon C.sub.mH.sub.n, CO.sub.2, water steam and/or return gas from a synthesis (hydrocarbons, CO, CO.sub.2, H.sub.2, H.sub.2O) 25 is fed to the catalytic reactor 23 as a medium to be reformed. The educts 25 are heated in a recuperator or electric heater 26, and fed to the reactor 23 as overheated process gas 27. The reactor 23 is operated such that hydrocarbons can be reformed at a high rate with the external pre-reforming and the internal reforming in the SOEC stack 1, and the stack 1 can be effectively cooled. For this purpose, the reactor 23 can also be operated as a methanation reactor.

[0056] The reformate 24, which is fed to the hydrogen electrode 3, should only still contain methane (CH4) as a hydrocarbon, which is catalytically reformed very effectively by nickel in the SOEC stack before the educt is electrochemically converted. In this context, the internal reforming consumes heat from the ohmic losses of the electrochemical reaction. The product gas or rather residual gas 31 led away from the hydrogen electrode 3, said gas 31 containing H2, H2O, CO and CO2, is first cooled in a recuperator 32 to lead away heat. The recuperator 24a and the recuperator 32 can be implemented as a common unit.

[0057] The cooled residual gas 33 is further cooled in a shift stage 34. CO and H2O are converted into CO2+H2 in a hydrogen shift reaction in the shift stage 34. A CO fine-purification wherein the residual quantities of CO are converted occurs if desirable. The purified residual gas 35 is then cleaned up or rather conditioned. A purification stage 36 can serve for removing H2O and/or CO2, or other impurities. Pressure swing adsorption (PSA), temperature swing adsorption (TSA), amine scrubbing, membrane methods, and/or cryogenic separation methods can serve for purification. A simple condenser or the previously mentioned methods can be used to remove water or rather water steam. The purified hydrogen/the syngas 37 is then led out of the system, and can be fed to a downstream system, e.g. for Fischer-Tropsch synthesis. Normally the separated residual gases 38, such as e.g. H2O and CO2 are reused, e.g. to save water to be supplied or to increase the CO2 conversion. For this purpose, the CO2 in gaseous form is recirculated and fed to the educts 25. The standard volumentric flows of the fed-in process gases 4, 20, 25 can be monitored and adjusted by means of suitable actuators, e.g. mass flow controllers (MFC) 70. Equally the quantity of the standard volumentric flow of the purified hydrogen/of the syngas 37 can be monitored by means of a mass flow controller 70. These are only indicated as an example in FIG. 3, in the interest of clarity. At least one temperature sensor 80 is provided at the SOEC stack 1, to monitor the outlet temperature of the product gas 31 and therefore the operation of the stack 1 at the thermoneutral point, and to be able to adjust the operation of the stack. Alternatively, the sensor 80 can also be integrated into the stack 1 at a suitable location.

[0058] The SOEC stack 1 is supplied with electrical power P by means of an AC/DC converter 40. The AC/DC converter 40 is configured to consume power from an electrical supply grid 41, and to provide it as direct current, with a suitable voltage and current intensity, to the electrodes 2, 3 of the SOEC stack for the electrochemical reaction. Alternatively or supplementarily, a DC/DC converter (not shown) can be provided, to be able to feed direct current from a DC current source (photovoltaic system) and/or a battery store (also not shown, respectively) to the SOEC stack 1. The DC/DC converter can be structurally unified with the AC/DC converter 40. A depiction of individual phases of the electrical lines between assemblies 1, 40 and 41 is dispensed with in FIG. 3 in the interests of improved clarity.

[0059] In the method according to the invention a control apparatus 50 is provided for controlling the electrolysis process, and particularly the compensation of the degradation of the SOEC stack 1. The control apparatus 50 receives measurement signals 72 from the mass flow controllers 70, and provides signals for the control 71 of the electrolysis process to the mass flow controller 70. Measurement signals 72 can be the current standard volumentric flows of the individual process gases or rather gas mixtures, for example. The composition of the gases can also be monitored by means of suitable sensors. Furthermore the control apparatus 50 receives measurement signals 82 from the AC/DC converter 40, and provides signals thereto for the control 71 of the electrolysis process. Measurement signals 82 are the current stack voltage and the current intensity, for example. Alternatively to the current intensity, the current density, which can be calculated from the current intensity relative to the active area of the cells, can be used as a measurement signal as a variable derived from a measured variable. The control apparatus 50 also receives measurement signals 72 from the temperature sensor 80, particularly the temperature sensor provides measurement signals for the stack temperature as a measurement signal 72. Target values for the stack temperature and standard volumentric flows are provided in the control apparatus 50, and parameters and look-up tables required for the control are stored in said control apparatus 50. The control apparatus 50 is connected to a master control apparatus 60, and can receive therefrom target value specifications for the control of the electrolysis process, e.g. the currently required hydrogen quantity, and can provide information about the available hydrogen quantity and other status information. The control apparatus 60 can be part of a master SCADA system.

[0060] For exchanging measurement signals and control commands as well as further information, if needed, the control apparatus 50 is connected communicatively to the AC/DC converter 40, the master control apparatus 60, as well as the mass flow controllers 70 and the temperature sensor 80, via corresponding signal lines (not shown in the interest of clarity).

[0061] The control apparatus 50 can be selected as a programmable logic controller (PLC) or industrial computer, and comprises at least one processor that is configured to process information, including the information from the mass flow controllers 70 and the temperature sensor 80. The control apparatus 50 comprises a transitory memory and a non-transitory memory for storing and providing information. The method according to the invention can be held as software or firmware in the non-transitory memory. The control apparatus 50 can further comprise at least one input/output apparatus that can comprise any apparatus known in the prior art for providing input data 72, 82 for the control system 50 and/or providing output signals 71, 81 to the mass flow controllers 70 and the AC/DC converter 40. The embodiment of command sequences is not restricted to a certain combination of hardware circuits and software commands, regardless of whether they are described and/or depicted here. The control apparatus 50 can also contain at least one interface that enables the control apparatus 50 to communicate with the mass flow controllers 70, the temperature sensor 80, and the AC/DC converter 40. The sensor interface can be or contain e.g. one or more analog/digital converters that convert analog signals into digital signals that can be used by the processor. The sensor interface can also be configured to process information by means of various data transmission protocols.

[0062] LIST OF REFERENCE SIGNS

[0063] 1 SOEC stack/cell/stack module

[0064] 2 oxygen electrode

[0065] 3 hydrogen electrode

[0066] 4 flushing medium

[0067] 5 heat exchanger

[0068] 6 pre-warmed flushing medium

[0069] 7 electric heater

[0070] 8 air/flushing medium

[0071] 9 oxygen-enriched air/flushing medium

[0072] 10 heat exchanger

[0073] 11 cooled flushing medium

[0074] 12 heat exchanger

[0075] 13 cold flushing medium

[0076] 20 water steam/H.sub.2O

[0077] 21 evaporator

[0078] 22 water steam/H.sub.2O

[0079] 23 catalytic reactor/reformer

[0080] 24 reformate/educt gas

[0081] 24a recuperator/elec. heater

[0082] 25 C.sub.mH.sub.n, CO.sub.2, CO, H.sub.2, H.sub.2O

[0083] 26 recuperator/electric heater

[0084] 27 overheated process gas

[0085] 31 residual gas

[0086] 32 recuperator

[0087] 33 cool residual gas

[0088] 34 shift stage

[0089] 35 purified residual gas

[0090] 36 cleaning-up stage

[0091] 37 purified hydrogen H.sub.2/purified syngas H.sub.2+CO

[0092] 38 separated residual gases

[0093] 40 AC/DC converter

[0094] 41 electrical supply grid

[0095] 50 control apparatus

[0096] 60 master control apparatus

[0097] 70 mass flow controller

[0098] 80 temperature sensor

[0099] 71, 81 control signals

[0100] 72, 82 measurement signals

[0101] L1 curve over time of the hydrogen output according to the method according to the invention

[0102] L5 curve over time of the efficiency according to the method according to the invention

[0103] L2, L3 curve over time of the hydrogen output according to the prior art

[0104] L4, L6 curve over time of the efficiency according to the prior art

[0105] Q.sub.ec electric heat feed-in

[0106] Q thermal heat feed-in

[0107] Q.sub.R heat led away for the external reforming

[0108] Q.sub.E heat consumed in the internal reforming