METHOD AND SYSTEM FOR PRODUCING HYDROGEN, ELECTRICITY AND CO-PRODUCTION

Abstract

A combined hydrogen and electricity supply system for producing hydrogen, electrical Power (P) and co-production, the system including a variable electrical load for varying the amount of impedance on the system, a pre-reformer connected to a stream of carbonaceous fuel, a stream of steam and connected to a heating source. The pre-reformer produces a first reformate gas having at least hydrogen, carbon monoxide and unconverted carbonaceous fuel. The pre-reformer is responsive to the amount of heat provided by the heating source, a solid oxide fuel cell stack coupled to the variable electrical load and coupled to the first reformate gas. The ratio between electrical power (P) and amount of hydrogen produced depends at least on the variable electrical load and the heat provided by the heating source.

Claims

1. A method for producing purified hydrogen and electrical power (P) in a combined hydrogen and electricity supply system whereby the ratio between purified hydrogen and electrical power (P) can be adjusted, the method comprising the steps of: introducing a carbonaceous fuel and steam into a pre-reformer, and in the pre-reformer reforming part of the carbonaceous fuel by steam reforming into a first reformate gas (S1) comprising hydrogen and carbon monoxide so that unconverted carbonaceous fuel remains; introducing the unconverted carbonaceous fuel and the first reformate gas (Si) into an anode side of a solid oxide fuel cell stack; in the solid oxide fuel cell stack reforming at least part of the unconverted carbonaceous fuel by internal steam reforming into a second reformate gas (S2) comprising mainly hydrogen and carbon monoxide, introducing an oxygen containing gas into a cathode side of the solid oxide fuel cell stack, in the solid oxide fuel cell stack converting oxygen of the oxygen containing gas as well as hydrogen and carbon monoxide of the first and second reformate gas (Si, S2) into electrical power (P) and an anode off-gas; introducing the anode off-gas into a hydrogen separation unit, and converting in the hydrogen separation unit the anode off-gas into purified hydrogen and an off-gas, whereby the reforming in the pre-reformer is performed as external reforming, wherein the pre-reformer being thermally separated from the solid oxide fuel cell stack to allow independent thermal control of the pre-reformer and the solid oxide fuel cell stack to separately control external reforming and internal reforming, that a controllable heating source is thermally coupled to the pre-reformer to provide the pre-reformer with controlled heat to control the reforming rate of the pre-reformer, that the electrical power (P) production is controlled to provide heat for internal reforming and to control internal reforming, and that the amount of purified hydrogen as well as the amount of electrical power (P) produced is modulated by a combined control of external reforming, internal reforming and a fuel utilization rate (FU) of the solid oxide fuel cell stack.

2. The method of claim 1, further comprising the step of heating the solid oxide fuel cell stack through external electrical energy, to provide heat to the solid oxide fuel cell stack for internal reforming.

3. The method of claim 1, wherein the purified hydrogen is not recirculated into the solid oxide fuel cell stack.

4. The method of claim 1, further comprising the step of controlling the fuel utilization rate (FU) by varying an external electrical load connected to the solid oxide fuel cell stack.

5. The method of claim 1, further comprising the step of controlling the fuel feed flow of the carbonaceous fuel.

6. The method of claim 1, further comprising the step of controlling the fuel utilization rate (FU) by varying the fuel flow of the carbonaceous fuel.

7. The method of claim 1, further comprising the step of controlling the reforming rate of the pre-reformer by controlling the pre-reformer outlet temperature (T2).

8. The method of claim 6, further comprising the step of controlling the heat provided to the pre-reformer, limiting the rate of the external reforming by keeping the pre-reformer outlet temperature (T2), which means the outlet temperature of the first reformate gas (Si) and the remaining carbonaceous fuel, below 450 C., so that reforming of up to 90% takes place in the solid oxide fuel cell stack by internal reforming, to allow a high electrical power (P) production.

9. The method of claim 6, further comprising the step of controlling the heat provided to the pre-reformer, keeping the pre-reformer outlet temperature (T2), which means the outlet temperature of the first reformate gas (Si) and the remaining carbonaceous fuel, between 450 C. and 850 C., and varying the hydrogen production by controlling the external electrical load.

10. The method of claim 1, further comprising the step of burning the off-gas and/or a make-up gas- to thereby provide heat to the pre-reformer and/or a steam generator.

11. The method of claim 1, further comprising the step of electrically heating at least one of the pre-reformer, the steam generator, the SOFC stack, a fluid flowing such a carbonaceous feed or oxidant flow.

12. The method of claim 1, further comprising the steps of: splitting a stream of carbonaceous fuel and steam in a first part and a second part, feeding the first part into the pre-reformer, bypassing the pre-reformer with the second part, combining the first and second part after the pre-reformer to a combined stream, and controlling the amount of the first and second part to thereby control the reforming rate of the combined stream.

13. The method of claim 1, further comprising the step of controlling the reforming rate of the solid oxide fuel cell stack by measuring a temperature of the solid oxide fuel cell stack, in particular the outlet temperature (T1) of the cathode outlet, and based on the measured temperature of the solid oxide fuel cell stack cooling the solid oxide fuel cell stack by controlling the amount of the oxygen containing gas introduced in the cathode side.

14. The method of claim 1, further comprising the step of managing the heat balance between solid oxide fuel cell stack and pre-reformer by allowing internal reforming of up to 90% to take place in the solid oxide fuel cell. Stack.

15. A combined hydrogen and electricity supply system for producing hydrogen, electrical Power (P) and co-production, the system comprising: a solid oxide fuel cell stack, a pre-reformer, an electrical load consuming the electrical Power (P), a hydrogen separation unit, a control unit, and a carbonaceous fuel source, the solid oxide fuel cell stack being coupled to the electrical load and being coupled to the hydrogen separation unit wherein the pre-reformer being thermally separated from the solid oxide fuel cell stack to allow independent thermal control of the pre-reformer and the solid oxide fuel cell stack, that a steam source provides a stream of steam, that the pre-reformer being connected to a stream of the carbonaceous fuel source and the stream of the steam source, wherein said pre-reformer produces a first reformate gas (Si) comprising at least hydrogen, carbon monoxide and unconverted carbonaceous fuel, that the solid oxide fuel cell stack being coupled to the pre-reformer to receive the first reformate gas (Si) and the unconverted carbonaceous fuel (20a); that the electrical load is a controllable, variable electrical load, that the pre-reformer is thermally coupled to a controllable heating source, and that the control unit is adapted to at least control the variable electrical load and the heat provided by the heating source (9) to independently control internal and external reforming, to thereby control the ratio between electrical power (P) and amount of hydrogen being produced.

16. The system according to claim 15, characterized in means for providing external heat to the solid oxide fuel cell stack.

17. The system according to claim 15, wherein there is no recirculation of purified hydrogen back to the solid oxide fuel cell stack.

18. The system according to claim 15, wherein a controllable valve is fluidly connected with the carbonaceous fuel source, and that the control unit is adapted to control by controllable valve the flow of the stream of carbonaceous fuel to the pre-reformer, to thereby control the amount of H2 produced by unit time and the electrical power P produced by unit time.

19. The system according to claim 15, wherein an electrical heating is adapted to provide heat to at least one of the pre-reformer, the steam generator and the SOFC stack.

20. Use of a combined hydrogen and electricity supply system comprising a solid oxide fuel cell stack and an external pre-reformer according to claim 15, to convert a carbonaceous fuel to hydrogen by only using electrical power (P) as a heat source for external reforming, to store hydrogen, and to convert stored hydrogen to electrical power (P).

21. Use of the system according to claim 15 in an electrical grid to balance between production and consumption of electrical energy.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0049] FIG. 1 shows a schematic representation of a combined hydrogen and electricity supply system of a first embodiment of the invention;

[0050] FIG. 2 shows a combined hydrogen and electricity supply system of a second embodiment of the invention comprising a control unit and a heat exchanger network for the heat management in the system;

[0051] FIG. 3 shows a control strategy of the combined hydrogen and electricity supply system according to FIG. 2;

[0052] FIG. 4 shows a relation between the H2/(H2+P) ratio, pre-reforming rate and Fuel Utilisation (FU) of the combined hydrogen and electricity supply system according to FIG. 2 to fulfil heat balance;

[0053] FIG. 5 shows a thermodynamic equilibrium calculation of natural gas (CH4) steam reforming as a function of temperature;

[0054] FIG. 6 shows a further example of a relation between pre-reforming rate and fuel utilization in a combined hydrogen and electricity supply system to fulfil heat balance;

[0055] FIG. 7 shows the relation according to FIG. 6 and in addition the effect of varying the amount of fuel;

[0056] FIG. 8 shows a further embodiment of a combined hydrogen and electricity supply system;

[0057] FIG. 9 shows a further embodiment of a combined hydrogen and electricity supply system;

[0058] FIG. 10 shows an option for controlling the degree of pre-reforming;

[0059] FIG. 11 shows a second option for controlling the degree of pre-reforming.

DESCRIPTION OF PREFERRED EMBODIMENTS

[0060] The present invention provides hydrogen, electricity and co-production system. The invention will be described below relative to illustrative embodiments. Those skilled in the art will appreciate that the present invention may be implemented in a number of different applications and embodiments and is not specifically limited in its application to the particular embodiment depicted herein.

[0061] FIG. 1 shows a schematic embodiment of a combined hydrogen and electricity supply system 1 suitable for producing as the output hydrogen 80, electrical power P or a combination of both hydrogen 80 and electrical power P. The system 1 comprises a blower 101, a solid oxide fuel cell stack 2, a pre-reformer 3, an electrical load 6, a hydrogen separation unit 8 and a burner 9. The pre-reformer 3 is separated from the SOFC stack 2, and is not in direct physical contact with the body of the SOFC stack 2, so that there is no direct heat transfer between the pre-reformer 3 and the SOFC stack 2 through physical contact, thus allowing independent thermal control of the pre-reformer 3 and the SOFC stack 2. The solid oxide fuel cell stack 2 comprises a cathode side 21, an electrolyte 22 and an anode side 23. The solid oxide fuel cell stack 2 is coupled to a variable electrical load 6 that consumes electrical power P. The solid oxide fuel cell stack 2 is adapted to receive input reactants, such as a carbonaceous fuel 20, most preferably natural gas, and an oxidant 100, most preferably air. The air 100 is fed to the blower 101 and through conduit 102 to the cathode side 21 of the solid oxide fuel cell stack 2. The fuel 20 and water 40 is fed to a steam generator 11 and then to the pre-reformer 3 to produce by external reforming a first reformate gas S1 comprising hydrogen and carbon monoxide so that unconverted carbonaceous fuel 20a remains. The first reformate gas S1, unconverted carbonaceous fuel 20a and steam is fed by conduit 205 to the anode side 23 of the solid oxide fuel cell 2. In the solid oxide fuel cell stack 2 at least part of the unconverted carbonaceous fuel 20a and steam is reformed by internal reforming into a second reformate gas S2 comprising mainly hydrogen and carbon monoxide. In an exceptional method step, the whole carbonaceous fuel 20 may be converted in the pre-reformer 3 by external reforming, thus no unconverted carbonaceous fuel 20a remains that could be fed from the pre-reformer 3 to the solid oxide fuel cell stack 2. Such an external reforming may be achieved by providing the pre-reformer 3 with sufficient heat, for example by electrically heating the pre-reformer 3, preferably using surplus electricity, such as electricity produced by wind energy or solar energy.

[0062] An oxygen depleted air stream 105 is fed from the cathode side 21 to the burner 9. An anode off gas 208 is fed from the anode side 23 to the hydrogen separation unit 8. The hydrogen separation unit 8 is adapted to separate at least a portion of hydrogen from the anode off gas 208 and is adapted to generate purified hydrogen 80 and an off gas 215 that is fed to the burner 9 where it is burnt. In an advantageous embodiment, disclosed in FIG. 2, also a make-up gas 19 may be introduced to the burner 9. The make-up gas 19 may be controlled by controller 17 through command line 17f and valve 18. In a preferred embodiment heat 9b, generated in the burner 9 by burning off gas 215 and/or make-up gas 19, is transferred to the pre-reformer 3, for example through heat exchangers 9a and 3a, which may be connected, as disclosed in FIG. 2, or for example through heat exchanger 3a only, as disclosed in FIG. 9. In a preferred embodiment the heat, for example the heat generated in the burner 9 or the heat T3 measured at the exit of heat exchanger 9a, may be controlled by controlling at least one of the off gas 215 and the make-up gas 19. The heat of the off gas 215 and/or the make-up gas 19 provided to the pre-reformer 3 may be controlled by controller 17 to provide heat management and to control the pre-reforming rate in the pre-reformer 3.

[0063] Suitable techniques for the hydrogen separation unit 8 may be adsorbtion based, for example pressure-swing adsorption, or membrane base, for example palladium-based, or protonic, or electrochemical base, for example electrochemical pumps based on protonic conductors.

[0064] FIG. 1 also shows a schematic representation of the process. Water 40 enters the evaporator 11, also referred to as steam generator. Steam is generated in the evaporator 11 and mixed to the carbonaceous fuel 40, most preferably natural gas NG, before entering the pre-reformer 3. In the pre-reformer 3, natural gas NG is partially reformed to hydrogen H.sub.2 and carbon monoxide CO, the remaining being methane, steam and carbon dioxide CO.sub.2. The pre-reforming rate is determined by the outlet temperature T2 of the prereformed gas leaving the pre-reformer 3. Preferably the outlet temperature is kept as a fixed value. The prereformed gas enters the anode side 23 of the SOFC stack 2 where the remaining methane is reformed. In a preferred method, the resulting syngas is in the SOFC stack 2 partially converted to water H.sub.2O and CO.sub.2 through electrical power P production in the SOFC stack 2. The anode off-gas 208 contains H.sub.2, CO, CO.sub.2 and H.sub.2O. The anode off-gas 208 is fed to the hydrogen separation unit 8, where at least part of the H.sub.2 of the anode off-gas 208 is separated to produce a hydrogen stream 80. The off-gas 215 or tail-gas 215 of the hydrogen separation unit 8 is then burnt in burner 9 with the hot depleted air 105 exiting the cathode side 21 of the SOFC stack 2. The heat 9b generated by the burner 9 is most advantageously used for the steam generation and pre-reforming reaction.

[0065] The input fuel reactant 20 can be any suitable hydrocarbon fuel known to those of ordinary skill in the art. The input oxidant reactant 100 can comprise any suitable oxygen-containing fluid.

[0066] The system 1 can be operated in a number of select modes to produce and modulate electrical power P, purified hydrogen 80 and a combination thereof. The main purpose of the method and system according to the invention is the production of electrical power P, hydrogen in the form of purified hydrogen 80 and a combination thereof from a carbonaceous fuel feed. The present description distinguishes between hydrogen produced within the system, and the output of the system, which are electrical power P and/or hydrogen in the form of purified hydrogen 80.

[0067] The illustrated system 1 is a multi-function system. In addition to the traditional function of electrical generation, the SOFC stack 2 can be utilized to perform reforming for hydrogen production or the co-production of hydrogen and electricity. In a traditional mode of operation, the SOFC stack 2 generates electricity by electrochemically reacting the input fuel reactant with the input oxidant reactant to generate power, waste heat and exhaust, which includes carbon dioxide and water. In an alternative mode of reformer operation, the SOFC stack 2 reforms the input fuel reactant to generate a hydrogen exhaust without simultaneously generating electricity. Additionally reactant by-products that can be included in the exhaust include carbon monoxide, carbon dioxide, and water. In a combined or co-production mode of operation, the SOFC stack 2 simultaneously generates both a hydrogen exhaust and electricity. The exhaust can include additional reaction species such as carbon monoxide, carbon dioxide and water respectively steam.

[0068] As used herein, the term reforming and the like refers to a chemical process performed by the pre-reformer 3 or the SOFC stack 2 that reacts hydrocarbon fuels, at an elevated temperature, such as above 250 C., and preferably between about 400 C. and about 1000 C., in the presence of steam and without oxygen to generate a reformate. In the present invention, the pre-reformer 3 and the SOFC stack 2 reforms hydrocarbon fuels to produce hydrogen by reacting the hydrocarbon fuel with water.

[0069] The combined hydrogen and electricity supply system 1 disclosed in FIG. 1, allows, based on an input feed of carbonaceous fuel 200, producing as output hydrogen in the form of purified hydrogen 80 and electrical power P. The system 1 can switch among the different operational modes to generate as output either electricity, which means electrical power P, or hydrogen or vary the ratio of hydrogen to electricity produced in the combined mode, by controlling the fuel utilization rate of the SOFC stack 2 by the amount of load by way of the variable load 6 that is applied to the SOFC stack 2 through the electrical connector 22a, and preferably by simultaneously controlling the pre-reforming rate in the pre-reformer 3.

[0070] FIG. 2 shows a second embodiment of a combined hydrogen and electricity supply system 1 in more detail. FIG. 2 shows a system 1 comprising fluid regulating devices 14, 18, 101, 404 and a controlling device 17 providing control signals 17a-17f for controlling at least one or more of the fuel 20 being fed to the reformer 3, of the oxidant reactants 100 being introduced to the solid oxide fuel cell 2, of the water 40 respectively steam 40a being introduced to the reformer 3, and of a make-up gas 19 introduced to the burner 9. In addition means such as heat exchangers 103, 203, 206, 209, 212, 3a, 9a, 11a, sensors and connecting lines, not shown in detail, are provided for controlling the heat exchange within the solid oxide fuel cell system 1.

[0071] The combined hydrogen and electricity supply system 1 disclosed in FIG. 2 allows producing purified hydrogen 80 and electrical power P from a carbonaceous fuel feed 200 converted into a reformed process gas feed 205, and in particular allows to control the amount of purified hydrogen 80 produced by unit time and the electrical power output P produced. The system 1 has the advantage that the amount of purified hydrogen 80 produced by unit time and the electrical power output P produced may be varied in a wide range according to the demand.

[0072] The modulation between electrical power P and output hydrogen production is managed by a control unit 17 as shown in FIG. 2. The control unit 17 can act for example on the natural gas valve 14, make-up gas valve 18, air blower 101, water recirculation pump 404, hydrogen separation unit 8 the SOFC stack 2, and the electrical load 6. The control unit 17 monitors the pre-reformer outlet temperature T2, the SOFC stack outlet temperature T1 at the air side and the burner temperature T3. To change the operating point respectively the H2/electrical power P ratio, the amount of H2 available in the anode off-gas 208 for separation will be controlled by providing the pre-reformer with heat and by controlling the outlet temperature T2, so that the pre-reforming rate in the pre-reformer 3 is adapted. In addition, to change the operating point, the fuel utilisation FU in the SOFC stack will be adjusted by the electrical load 6 and accordingly as disclosed in FIG. 4. Preferably also the air flow, controlled by air blower 101, will be used for fine tuning of the SOFC stack outlet temperature T1. For each point of operation (e.g. fixed H2/(H2+P)), the production can in addition be scaled by varying fuel 20 and water 40 feed concurrently so as to keep the same steam to carbon ratio.

[0073] The fuel utilisation (FU) relates to the total electrical current (I) and the fuel flow per cell (f) through the relation:

FU=I/(nF*f)

[0074] Where n is the number of electrons involved in the oxidation of 1 molecule of fuel (e.g. 8 for CH4), F is the Faraday constant (=96485 C/mol). It can therefore be varied either by changing the total current in the SOFC by varying the external electrical load 6, or by changing the fuel flow.

[0075] A hydrocarbon fuel 20, typically biogas or natural gas, is fed to a controllable valve 14 and optionally to a fuel pretreatment unit 13, typically a desulphurization unit, to get a poison-free carbonaceous fuel. The carbonaceous fuel feed 200 is mixed with steam 40a, preheated in a heat exchanger 203, and fed through conduit 204 into a pre-reformer 3 to produce the reformed process gas feed 205. Steam 40a is generated in a steam generator 11. The pre-reformer 3 is provided with heat by heat exchanger 3a. The reaction in the pre-reformer 3 preferably takes place in the presence of a reforming catalyst in a temperature range of 500 to 800 C. The reformed process gas 205 is heated in heat exchanger 206 and is fed to the anode side 23 of the SOFC stack 2. The anode off-gas 208 leaving the SOFC stack 2 is cooled in heat exchanger 209 to for example about 300 C., and is first fed into a water gas shift reactor 4, is then cooled in a heat exchanger 212, and is fed to a water separator 15, a condenser, which at least separates water 40 from the gas stream 213, so that a water depleted stream 214 results. Water 40 is stored in a water tank 402, and may then be fed through conduits 403 and 405 and a water pump 404 to the steam generator 11. The water depleted stream 214 is fed to the hydrogen separation unit 8 to generate purified hydrogen 80 and a carbon dioxide rich gas stream 215 comprising also unseparated H2 and some CO, which is fed to the burner 9.

[0076] The SOFC stack 2 also comprises a cathode side 21 as well as an electrolyte 22. The SOFC stack 2 keeps the oxidant stream respectively air stream 100 and the reformed process gas 205 separated, so that they do not mix. No further details of the solid oxide fuel stack 2 are shown. Air 100 is slightly compressed in blower 101 to compressed cold air 102, is heated in heat exchanger 103 to pre-heated air 104 and is then fed to the cathode side 21 of the solid oxide fuel stack 2. The small overpressure of the compressed cold air 102, for example about 50 mbar, is required to overcome the pressure drops in the heat exchangers and the SOFC stack 2. A hot depleted air stream 105 leaving the cathode side 21 of the SOFC stack 2 is fed to the burner 9. Electricity produced by the SOFC stack 2 is converted from DC to AC and is forwarded to a variable electrical load 6 not shown in detail. The electrical load 6 consumes the electrical power P.

[0077] Control unit 17 preferably controls through control lines 17a to 17g at least one of the blower 101, the H2 separation unit 8, the SOFC stack 2, the valve 14, the valve 18, the water pump 404 and the electrical load 6. In addition, in a preferred embodiment, the exit temperature T1 of the depleted air 105 leaving the SOFC stack 2, the exit temperature T2 of the reformate 205 leaving the pre-reformer 3 and the exit temperature T3 of the heat leaving the heat exchanger 9a is measured by control unit 17.

[0078] The control of the exit temperature T1 has the advantage that at any operating conditions defined by the pre-reforming rate and the FU, the temperature of the SOFC stack 2 may be monitored using sensor T1, which measures the temperature of the hot depleted air stream 105 exiting the SOFC. The temperature T1 can be adjusted or can be fine-tuned by varying the air flow via the blower 101. Increasing the air feed flow will increase the SOFC stack cooling and thereby lower the SOFC stack temperature.

[0079] Depending on the required amount of heat 9b, make-up gas 19 may be fed to the burner 9 to provide heat 9b to heat exchanger 9a, which provides heat to heat exchanger 3a.

[0080] FIG. 3 shows one aspect of a control strategy of the system 1 disclosed in FIG. 2. FIG. 3 shows a set of possible operation conditions that can be achieved with the system 1. The system 1 can be operated at specific conditions defined by a hydrogen yield, i.e. the amount of purified H.sub.2 80 produced by unit time, and an electrical power P output. The H2 yield and the electrical power P are shown in arbitrary units [a.u.]. The system 1 can be controlled in such a way that the system 1 delivers a given, predetermined amount of H.sub.2 by unit time and a given electrical power P. Depending on external demand, the required amount of H.sub.2 and electrical power P may change during operation of the system 1. The control unit 17 is able to control the system 1 such that the required amount of H.sub.2 and electrical power P is achieved. This is achieved by simultaneously change the pre-reforming rate of the pre-reformer 3 and the fuel utilisation FU of the SOFC stack 2.

[0081] In addition to the pre-reforming rate and the fuel utilisation FU, also the fuel feed flow may be controlled. The different lines LD1, LD2, LD3, LD4, MD1, MD2, MD3, MD4 in FIG. 3 correspond to different amounts of fuel feed flow, LD1 and MD1 to 100% fuel feed, LD2 and MD2 to 75% fuel feed, LD3 and MD3 to 50% fuel feed and LD4 and MD4 to 25% fuel feed. The system 1 may be operated at different operation points, whereby each operation point is defined by a H2 yield and an electrical power P, or is defined by the ratio H2/(H2+P) and a fuel feed flow.

[0082] In the ratio H2/(H2+P) H2 refers to the H2 produced by the system 1 by unit time, which means the purified hydrogen 80 by unit time. P refers to the produced electrical Power, which means electrical energy per unit time. The figures show H2 and P in arbitrary units. The unit of electrical power is Watt. For H2 the equivalent power based on the LHV (low heating value) may be used, which means H2=molar flow (mol/s)*LHV(J/mol)=Watt.

[0083] In the example disclosed in FIG. 3, showing arbitrary units of H2 and P, three operating points L1, L2, L3 are represented corresponding to the same H2 yield (0.33) but with different electrical power outputs. The electrical power is 0.3 for point L1, 0.17 for point L2 and 0 for point L3. Point L1 can be achieved at full fuel feed (100%) and at a H2/(H2+P) ratio of 0.52. Point L2 can be achieved at partial fuel feed (75%) and H2/(H2+P)=0.68. Point L3 can be achieved at 50% fuel feed and H2/(H2+P)=1.

[0084] Any possible specific operating points L1, L2, L3, . . . . L100 in FIG. 3, defined by the fuel feed flow and the H2/(H2+P) ratio, can be achieved by the following three measures:

1) controlling the fuel feed flow with valve 14 and the water flow with water pump 404 concurrently in order to maintain the correct steam-to-carbon ratio in the feed stream 204 and to maintain the correct fuel feed stream;
2) adjusting the pre-reforming rate in the pre-reformer 3 by controlling the heat provided to the pre-reformer 3, and
3) adjusting the fuel utilisation FU in the solid oxide fuel cell 2, as disclosed in FIG. 4, by controlling the produced electrical power P.

[0085] Of particular interest is an operation of system 1 with the ratio H2/(H2+P)=0, which means that no purified H2 is produced, but only electrical Power P is produced. Of particular interest is also an operation of the system 1 with the ratio H2/(H2+P)=1, which means that only purified H2 is produced, but no electrical Power P is produced. As indicated in FIG. 3 by lines LD1, LD2, LD3, LD4, MD1, MD2, MD3, MD4, the amount of electrical Power P, with no purified H2 produced, respectively the amount of purified H2 with no electrical Power P produced, may be controlled based on the fuel feed flow.

[0086] FIG. 4 discloses the relation between the H2/(H2+P) ratio and the pre-reforming rate R and fuel utilisation FU for the system 1 disclosed in FIG. 2. For instance, in the example disclosed in FIG. 3, operating point L1 has a H2/(H2+P) ratio of 0.52. Operating point L1 can be achieved by setting the fuel feed flow at full scale (100%), and, as disclosed in FIG. 4, setting the pre-reforming rate at 0.37 and the fuel utilisation FU at 0.57. Similarly, according to FIG. 3, operating point L2 has a H2/(H2+P) ratio of 0.68. According to FIG. 3, operating point L2 can be achieved by reducing the fuel feed flow to 75%, while, according to FIG. 4, changing the pre-reforming rate R to 0.52 and the fuel utilisation FU to 0.45.

[0087] FIG. 5 discloses that the pre-reforming rate R of an equilibrated pre-reformer 3 is related to the pre-reformer outlet temperature T2. This relation is shown in FIG. 5 in the case of steam reforming of methane with a steam to carbon ratio of 2. In order to achieve a pre-reforming rate of 0.37, the outlet temperature T2 of the pre-reformer 3 should be equilibrated at 510 C. Similarly a pre-reforming rate R of 0.52 corresponds to an equilibrium temperature of 565 C. The outlet temperature T2 can be controlled, for example, through the amount of heat transferred from the burner 9 by heat exchanger 9a to the pre-reformer 3 by heat exchanger 3a, by burning the hydrogen separation off-gas 215, and if required, in addition make-up gas 19, and/or by heat provided by electrical power, for example by the use of a resistive heating element arranged in or at the pre-reformer 3.

[0088] FIG. 6 shows an example of a relation between pre-reforming rate R, fuel utilization FU, H2 yield, electrical power P and the H2/(H2+P) ratio in a SOFC system 1 to fulfil heat balance. FIG. 7 shows, besides the graphs of FIG. 6, in addition the different lines LD1, LD2, LD3, LD4, MD1, MD2, MD3, MD4, which correspond to different amounts of fuel feed flow, LD1 and MD1 to 100% fuel feed, LD2 and MD2 to 75% fuel feed, LD3 and MD3 to 50% fuel feed and LD4 and MD4 to 25% fuel feed.

[0089] FIG. 6 in particular shows the relation between pre-reforming rate R and fuel utilization FU in the SOFC stack 2 for heat management and the resulting electrical power P and H.sub.2 production for each point of operation.

[0090] The modulation strategy is as follows. The modulation between the production of hydrogen and electrical power is achieved by varying the degree of pre-reforming and adjusting the fuel utilization FU in the SOFC stack 2 according to the relation given in FIG. 6. The degree of pre-reforming may be achieved by controlling the pre-reforming outlet temperature T2. The pre-reformer outlet temperature T2 is measured for example with a thermocouple and adjusted by varying the amount of heat provided to the pre-reformer 3 for the reforming reaction. This heat is for example generated by burning the hydrogen separation unit off-gas 215 and/or makeup-gas 19. It can also be produced by an electrically heated device in certain circumstances. In a further control option the outlet temperature T2 of the pre-reformer is controlled and adjusted as described in order to remain constant, and in addition, the feed stream comprising carbonaceous fuel and steam is split in two streams according to the desired degree of pre-reforming using for example regulating valves 3b, 3c, as disclosed in FIG. 11.

[0091] The pre-reforming rate R is the key control parameter in combination with FU. FIGS. 6 and 7 disclose an operation map for the combined hydrogen and electricity supply system 1. The starting point is the choice of a H2/(H2+P) operating point according to the demand of electrical power P and hydrogen H.sub.2. Thereby the fuel feed flow is also adjusted. The pre-reforming rate R and the FU are then adjusted simultaneously according to FIG. 7. For instance, if it has been decided to operate the system 1 at H2/(H2+P)=0.7, corresponding to the production of 200 kW equivalent H.sub.2 (LHV=Lower Heating Value) and 80 kW electrical power P. According to FIG. 6 or 7, the pre-reforming rate should be adjusted to 0.55 and the FU to 0.45.

[0092] In a first example the pre-reforming rate R can be adjusted by changing the pre-reformer outlet temperature T2 to 570 C., controlling the heat provided to the pre-reformer 3 by burning the hydrogen separation unit off-gas and/or additional make-up gas 19. In a second example the pre-reforming rate R can be adjusted by adjusting a by-pass as disclosed in FIG. 11. The fuel utilization FU can be adjusted for a fixed feed 20 flow by changing the electrical load P on the solid oxide fuel cell stack 2, or by changing the feed flow at fixed electrical load P.

[0093] By way of example, the operating point shall now be changed to 50 kW equivalent H.sub.2 keeping the electrical power P at 80 kW, which means H2/(H2+P) becomes 0.385. As disclosed in FIG. 6 or 7, the corresponding pre-reforming rate R would be 0.25 and the fuel utilization FU is 0.66. As the electrical power output P remains unchanged, the latter would be achieved by lowering the feed flow 20 and 40 concurrently from 100% to 50%, as can be seen in FIG. 7, while lowering the pre-reformer outlet temperature T2 to 460 C. or increasing the by-pass as disclosed in FIG. 11.

[0094] FIG. 8 shows a further combined hydrogen and electricity supply system 1, which distinguishes over the combined hydrogen and electricity supply system 1 disclosed in FIG. 2 in particular insofar as at least one of the pre-reformer 3 and the steam generator 11 is heated using electricity 500. FIG. 7 shows this operation mode in more detail. In FIG. 7, the two operation points indicated by EL-SRM refer to electrical heated steam methane reforming and correspond to the case where excess electrical power is available, preferably at low cost, for example from the electrical grid or from solar panels, making its use economically favorable for the production of hydrogen. In this case the upper EL-SRM point refers to a hydrogen yield of 100%, and the lower EL-SRM point refers to electrical power P. The electrical power is negative (0.55) as it is consumed in the process. The electrical power is used to produce heat, in particular through resistive heating elements, for the generation and the super-heating of steam in the evaporator 11, the reforming reaction in the pre-reformer 3, and the heating of the reformate gas 205. An advantageous embodiment is depicted in FIG. 8. In a further preferred embodiment for example also the SOFC stack 2 may be heated with electricity 500. One advantage of such an embodiment is that hydrogen may be produced in the SOFC stack 2 by using only or by using most of the heat produced by electricity. A further advantage is, that when the SOFC stack 2 is electrically heated, most of the carbonaceous fuel is reformed in the SOFC stack 2, which allows reducing the size of the pre-reformer 3.

[0095] In addition any other part of the system 1 that need heat, for example fluids flowing, such as carbonaceous feed, oxidant flow, evaporator 11, steam superheating, make-up-gas may be electrically heated.

[0096] In a further advantageous embodiment the hydrogen 80 may be stored in a hydrogen storage container 81. In a further advantageous embodiment the hydrogen 80 stored in the container 81 may be fed to the SOFC stack 2, to produce electrical power. The system according to the invention may therefore be used to withdraw electricity from the electrical grid, and to later on supply the electrical grid with electricity. In an advantageous embodiment the system according to the invention may be used for grid control, to control supply and demand of electrical energy.

[0097] FIG. 9 shows a further embodiment of a combined hydrogen and electricity supply system 1. The controlling device 17 for controlling various fluid regulating devices 14, 18, 101, 404 and for measuring various state variables such as temperatures or feed flows is not disclosed in detail. They are similar as disclosed in FIG. 2.

[0098] The controlling device 17 providing control signals 17a-17g for controlling at least one or more of the fuel 20 being fed to the reformer 3, of the oxidant reactants 100 being introduced to the solid oxide fuel cell 2, of the water 40 respectively steam 40a being introduced to the reformer 3, of a make-up gas 19 introduced to the burner 9, and the electrical load 6 being controlled. In addition means such as heat exchangers 103, 203, 206, 209, 212, 3a, 11a, sensors and connecting lines, not shown in detail, are provided for controlling the heat exchange within the system 1.

[0099] The system 1 disclosed in FIG. 9 allows producing purified hydrogen 80 and electrical power P from a reformed process gas feed 205, and in particular allows to control the amount of purified hydrogen 80 produced by unit time and the electrical power P produced.

[0100] The control unit 17 can act on the fuel valve 14, air blower 101, water recirculation pump 404, hydrogen separation unit 8, the electrical load 6 and the SOFC cell 2. It monitors the pre-reformer outlet temperature T2, the SOFC outlet temperature T1 at the air side and the burner temperature T3. To change the operating point respectively the H2/power ratio, the H.sub.2 separation rate will be changed so as to reach the desired pre-reformer outlet temperature T2. The fuel utilisation (FU) in the SOFC stack 2 will be adjusted accordingly to FIG. 6 and the air flow will be used for fine tuning of the SOFC stack outlet temperature T1. For each point of operation (e.g. fixed H2/(H2+P)), the production can in addition be scaled by varying fuel and water feed concurrently.

[0101] A hydrocarbon fuel 20, typically biogas or natural gas, is fed to a controllable valve 14 and to a fuel pretreatment unit 13 to get a poison-free carbonaceous fuel. The carbonaceous fuel feed 200 is mixed with steam 40a and fed through conduit 204 into a pre-reformer 3 to produce the reformed process gas feed 205. Steam 40a is generated in a steam generator 11. The pre-reformer 3 is provided with heat 9a by heat exchanger 3a. The reformed process gas 205 is heated in heat exchanger 206 and is fed to the anode side 23 of the solid oxide fuel cell stack 2. The anode off-gas 208 leaving the solid oxide fuel cell stack 2 is cooled in heat exchanger 206, and is first fed into the steam generator 11 and then into a water gas shift reactor 4, is then cooled in a heat exchanger 212, and is fed to a water separator 15, a condenser, which at least separates water 40 from the gas stream 213, so that a water depleted stream 214 results. Water 40 is stored in a water tank 402, and may then be fed through conduits 403 and 405 and a water pump 404 to the steam generator 11. The water depleted stream 214 is fed to the hydrogen separation unit 8 to generate purified hydrogen 80 and a carbon dioxide rich gas stream 215, which is fed to the burner 9.

[0102] The solid oxide fuel cell stack 2 also comprises a cathode side 21 as well as an electrolyte 22. The solid oxide fuel cell stack 2 keeps the air stream 100 and the reformed process gas 205 separated, so that they do not mix. Air 100 is slightly compressed in blower 101 to compressed cold air 102, is heated in heat exchanger 103 to pre-heated air 104 and is then fed to the cathode side 21 of the solid oxide fuel cell 2. A hot depleted air stream 105 leaving the cathode side 21 of the solid oxide fuel cell stack 2 is fed the heat exchanger 103 and then to the burner 9. Electricity produced by the solid oxide fuel cell stack 2 is converted from DC to AC and is forwarded to a variable electrical load 6. The electrical load 6 consumes the electrical power P.

[0103] Control unit 17 preferably controls through control lines 17a to 17g at least one of the blower 101, the H2 separation unit 8, the solid oxide fuel cell stack 2, the valve 14, the valve 18 and the water pump 404. In addition, in a preferred embodiment, the exit temperature T1 of the depleted air 105 leaving the solid oxide fuel cell stack 2, the exit temperature T2 of the reformate 205 leaving the pre-reformer 3 and the exit temperature T3 of the heat leaving the heat exchanger 9a is measured by control unit 17.

[0104] Depending on the required heat 9b, make-up gas 19 may be fed to the burner 9.

[0105] The combined hydrogen and electricity supply system 1 according to the invention has also the advantage that heat may be provided for external use. For example in FIG. 9, excess heat can be recovered from heat exchanger 212. One advantage is that the heat can be valorised, which increases the overall system efficiency. The temperature levels and heat amount vary depending on the operating point. Heat could be available for example between 400 C. and 250 C. It could be used to produce steam or hot water, for example on the level of 35 to 55 C. for example for domestic or sanitary water, for car cleaning, or even cooling, using an adsorption chiller.

[0106] FIG. 10 shows a first embodiment for controlling the degree of pre-reforming in the pre-reformer 3, namely by control of the outlet temperature T2. The pre-reformer 3 may be heated by an electrical power source 500 and a heat exchanger 3a or by any other heat source. In FIGS. 10 and 11, the element 3a is a heating element. However in FIG. 9 the element 3a is a heat exchanger. The control unit 17 controls the exit temperature T2 and controls the electrical power source 500 so that the exit temperature T2 corresponds to a predetermined temperature to control the pre-reforming rate.

[0107] FIG. 11 shows a second embodiment for controlling the degree of pre-reforming. The pre-reformer 3 is heated such by the electrical power source 500 or any other heat source that the outlet temperature T2 is kept constant. The pre-reforming rate is then adjusted by varying the flows 3d and 3e through the control valves 3b and 3c, so that part of the fuel feed/steam stream 204 bypasses the pre-reformer 3.