The invention relates to a method and to a device for generating electrical energy, especially for generating electrical energy for an aircraft.

DE 10 2004 026 226 A1 discloses an aircraft having an integrated electrochemical supply system. A first electrochemical reactor is provided here, which splits wastewater that arises within the aircraft, after purification or desalination, into the constituents hydrogen and oxygen. The hydrogen and the oxygen can be fed to a second reactor which is, for example, a fuel cell. By means of the fuel cell, electrical energy and water are produced from the hydrogen and the oxygen and hydrocarbonaceous energy carriers, for example kerosene. The first and second reactor are surrounded by a gastight protective housing filled with an inert gas.

DE 10 2005 053 692 B3 discloses a fuel cell for use in an aircraft. For fire protection, the fuel cell is accommodated in a protective housing filled with an inert gas. The inert gas used is nitrogen-enriched cathode offgas from the fuel cell. The inert gas formed from the cathode offgas disadvantageously has a relatively high content of residual oxygen.

US 2009/0155642 A1 discloses a reformate-operated fuel cell system. In this case, hydrogen is produced from a liquid fuel, for example methanol, in a first reactor in the form of a reformer, and is then supplied to a downstream fuel cell for generation of electrical energy. The first reactor may comprise a purification stage with which a proportion of extraneous gases in the hydrogen is produced.

DE 10 2012 023 531 A1 discloses a method of producing an operating medium on board an aircraft. In this case, a fuel cell unit having at least one fuel cell is accommodated in a first trolley. A second trolley accommodates a storage unit for uptake of hydrogen for operation of the fuel cell. The first trolley and the second trolley may be coupled to one another such that the fuel cell can be supplied with fuel. The fuel cell is accommodated in the first trolley in a protective housing filled with an inert gas. The inert gas is produced using cathode offgas and/or anode offgas.

Cathode offgas contains a relatively high content of residual oxygen of about 9%. Anode offgas can also be used as inert gas depending on the composition. As a result, using cathode offgas and/or anode offgas as inert gas, it is not possible to entirely rule out a risk of explosion in the event of escape of hydrogen from the fuel cell.

It is an object of the invention to eliminate the disadvantages according to the prior art. More particularly, a method improved in terms of safety for generation of electrical energy by means of a fuel cell system operated with reformate gas is to be specified. In a further aim of the invention, the efficiency of the method is to be improved. It is a further object of the present invention to specify a fuel cell system improved from a safety point of view. The fuel cell system is also to have improved efficiency with regard to the generation of electrical energy.

This object is achieved by the features of claims1and17. Appropriate configurations of the invention are apparent from the features of claims2to16and18to26.

According to the invention, a method of generating electrical energy by means of a fuel cell system operated with a reformate gas is proposed, having the following steps:

providing a fuel cell system having a first reactor for conversion of a fuel to a first fuel gas and a second reactor connected downstream of the first reactor, where the second reactor forms a fuel cell for generation of electrical energy with formation of a cathode offgas and an anode offgas, and where at least the second reactor is surrounded by a protective housing,
providing a gas separation unit with which at least one of the gases CO2, N2and/or a partial gas mixture comprising CO2and N2is separable from the first fuel gas or a further gas mixture formed therefrom,
feeding at least a portion of the first fuel gas or of the further gas mixture formed therefrom to the gas separation unit,
separating the gases N2or CO2or the partial gas mixture from the first fuel gas or the further gas mixture,
feeding the gases N2or CO2separated or the partial gas mixture separated as inert gas into the protective housing and
feeding an H2-enriched tail gas formed in the gas separation unit as second fuel gas for operation of the fuel cell or a further fuel cell.

In the context of the present invention, a “fuel” is any hydrocarbon of the general formula CxHyOzor a mixture of different hydrocarbons. The hydrocarbons may, for example, be kerosene, natural gas, alcohol, especially methanol, or the like.

A “reactor” in the context of the present invention is understood to mean a unit with which substances supplied, for example hydrocarbons, are converted to a further substance under the action of temperature and/or pressure and/or auxiliaries, such as catalysts. The substance may be a substance mixture, especially a liquid or gas mixture. For conversion of the substance in the reactor it is typically necessary to expend energy.

A “fuel gas” is understood to mean a gas or gas mixture suitable for operation of a fuel cell.

A “further gas mixture” is understood to mean a further fuel gas suitable for operation of a fuel cell, a cathode or anode offgas formed by the fuel cell or an offgas which is formed by an offgas cleaning stage downstream of the fuel cell.

The method of the invention relates to a fuel cell system operated with a reformate gas. This involves supplying a fuel to a first reactor in which the fuel is converted to a first fuel gas. For this purpose, the first reactor may comprise a conventional reformer. In the fuel cell system, following a main flow direction of the first fuel gas or of further gas mixtures formed therefrom, a second reactor is arranged downstream, which comprises a fuel cell surrounding by a protective housing. The second reactor may also comprise multiple fuel cells. Also provided is a gas separation unit with which especially the gases CO2, N2and/or a partial gas mixture comprising CO2and N2are separable from the first fuel gas or a further gas mixture formed therefrom. In the method of the invention, at least a portion of the first fuel gas or of the further gas mixture formed therefrom is fed to the gas separation unit. The separately removed gases N2or CO2or the separately removed partial gas mixture are then used as inert gas for filling the protective housing. In particular CO2, but also N2, is of excellent suitability as inert gas. They feature improved inertizing action especially compared to conventionally used oxygen-depleted air (ODA).

It is thus particularly effectively possible to lower the risk of explosion in the event of a leak in the fuel cell. The removal of the gases N2, CO2can have a positive effect on a subsequent reaction and/or reduce the gas flow in terms of mass. As a result, reactors provided downstream can have smaller dimensions or the pressure drops in downstream reactors can be reduced. As a result, it is possible to reduce energy to be expended for gas transport.

The remainder, or a portion of the remainder, of the fuel gas or further gas mixture supplied to the gas separation unit that remains in the gas separation unit, after removal of one of the gases CO2, N2and/or a partial gas mixture containing CO2and N2, is referred to as tail gas.

The tail gas constitutes a second combustion gas which is supplied to the fuel cell or a further fuel cell for operation. The tail gas or second fuel gas differs in its composition from the first fuel gas. The second fuel gas is enriched with H2or is formed by H2.

In the gas separation unit, a tail gas having a reduced content of N2and/or CO2or enriched with H2or formed from H2is thus formed.

A “gas separation unit” is appropriately understood to mean a unit with which a gas mixture is separable without supply of thermal energy and/or an increase in pressure. It is possible to separate off N2, for example, by the Linde method and to separate off H2by the use of palladium platelets. Also known is a separation of H2, N2or CO2by means of films or by absorption. In addition, CO2can be separated off by adsorption, i.e. by binding to a solid. In mobile applications, for performance of the separation of the aforementioned gases, membrane methods in particular are used. In this case, a permeable membrane separates the gas stream into the constituents.

The term “main flow direction” is understood to mean the direct route, i.e. without branches and/or diversions, of the gas stream through the series-connected reactors.

In the method proposed, it is possible to feed merely a portion of the first fuel gas to the gas separation unit. In this case, the other portion of the first fuel gas that remains in the main flow is unchanged in its composition. Alternatively, it is possible to feed the first fuel gas entirely to the gas separation unit. In this case, by means of the gas separation unit, a further or second fuel gas that differs in its composition from the first fuel gas is produced. The further or second fuel gas especially has a reduced content of N2and/or CO2. A reduction, especially in the CO2content in the main gas stream, can have a positive effect on reaction equilibria in downstream reactions, for example in the water-gas shift reactor.

The gas separation unit can also separate off H2separately as gas and it can be used as second fuel gas for operation of the fuel cell or a further fuel cell. This increases the efficiency of the method.

The H2, in the case of splitting into two H2partial gas streams, can also be used to operate different fuel cell types. This enables a dissimilar redundant mode of operation which is especially advantageous in the case of use in aircraft. For example, in redundant operation, it is possible to operate a low-temperature polymer electrolyte membrane fuel cell (LTPEM) and a solid oxide fuel cell (SOFC).

The separation of H2from the main gas stream can also bring about a positive shift in the reaction equilibrium in the water-gas shift reaction. This can improve the efficiency of the method and the efficacy of generation of electrical energy.

In an advantageous configuration of the method, a proportion of CO in the first fuel gas is reduced by means of a third reactor designed as a water-gas shift stage and connected downstream of the first reactor and/or of a fourth reactor configured as a PrOx stage. The abbreviation “PrOx” stands for “preferential oxidation”. This is a preferred oxidation of a usually gaseous substance over a catalyst. In the context of the present invention, this is especially the oxidation of CO over a catalyst to give CO2. The use of the third and/or fourth reactor proposed is especially advantageous when fuel cell types having an efficiency reduced by a high content of CO in the flue gas are used. The provision of the third and/or fourth reactor is advantageous especially when the fuel cell used is a low-temperature polymer electrolyte membrane fuel cell (LTPEM) or a high-temperature polymer electrolyte membrane fuel cell (HTPEM).

In a further configuration of the process, it is also possible to feed the cathode offgas and/or anode offgas at least partly as further gas mixture to the gas separation unit. It is thus possible to separate a residue of H2from the anode offgas and reuse it as second fuel gas for operation of the fuel cell or a further fuel cell. In addition, especially the gases N2and/or CO2or a partial gas mixture containing CO2and N2can be obtained from the cathode and/or anode offgas and used as inert gas. Such an inert gas is particularly low in residual oxygen.

In a further configuration of the invention, a fifth reactor for conversion of the cathode offgas and/or anode offgas is connected downstream of the second reactor, in which case an offgas formed in the fifth reactor can be fed at least partly as further gas mixture to the gas separation unit. It is also especially possible to separate CO2as gas from the offgas and use it as inert gas.

The first fuel gas or the first gas mixture may contain gaseous H2O. The gaseous H2O can be separated off by means of the gas separation unit and likewise used as inert gas or for production of moist, oxygen-depleted air (ODA). Inert gas admixed with gaseous H2O counteracts the spread of flames. Moist, oxygen-depleted air can be used, for example, in cargo holds of aircraft for inertization.

In a further configuration of the process, the gaseous H2O can be converted to liquid H2O and discharged for external use. “Discharge for external use” is understood to mean that a corresponding substance is conducted out of the method and used elsewhere. For example, the water produced can be sent to a water reservoir in an aircraft.

The first fuel gas or the first gas mixture may contain gaseous H2O. The gaseous H2O can be converted by means of a condenser to liquid H2O and discharged for external use and/or sent to an internal use. The liquid H2O can also be separated off upstream of the gas separation unit.

Heat released in the conversion of gaseous H2O to liquid H2O can be discharged for external use. For example, the heat can be used via a heat exchanger for heating of drinking water or cabin air in an aircraft.

The first fuel gas and/or the further fuel gas are separated into the individual gases in the gas separation unit, preferably using membranes. Suitable membranes for separation of CO2are especially those made of polyethylene glycol, polyamide, polyimide and the like. Hollow fiber membranes are suitable for separation of N2. H2can be removed, for example, by means of membranes produced from metal foils. Metal foils of this kind may have been produced from palladium, palladium alloys, refractory metals such as vanadium, niobium or tantalum, or alloys thereof.

The fuel used in the method of the invention may especially be hydrocarbon of the general formula CxHyOzor a mixture of different hydrocarbons, especially kerosene, natural gas, alcohol, preferably methanol. It is of course also possible to use either liquid or gaseous hydrocarbons.

In an advantageous configuration of the method, CO2and/or N2separated off by means of the gas separation unit can be discharged for external use, especially for inertization of a cargo hold and/or a tank hold in an aircraft. In addition, it is possible to use CO2and/or N2separated off by means of the gas separation unit, mixed with gaseous H2O, as extinguishant for a cargo hold in an aircraft.

In a further configuration, it is possible to branch off a portion of the first fuel gas and discharge it for external use, especially for operation of a further fuel cell. It is also possible to discharge the anode offgas to external use for use as a further fuel gas.

In an advantageous configuration, not just the second reactor but multiple reactors, preferably all reactors, are surrounded by the protective housing. In addition, it is also possible for the gas separation unit, one or more conduits that connect the reactors to one another, feeds connected to the gas separation unit and/or control valves inserted into the conduits and feeds to be surrounded by the protective housing.

In a further provision of the invention, a fuel cell system is proposed, having

a first reactor for conversion of the fuel to a first fuel gas,

a second reactor connected downstream of the first reactor, wherein the second reactor forms a fuel cell for generation of electrical energy using a cathode offgas and an anode offgas, and wherein at least the second reactor is surrounded by a protective housing,
a gas separation unit with which at least one of the gases CO2, N2and/or a partial gas mixture containing CO2and N2is separable from the first fuel gas or a further gas mixture formed therefrom,
a first feed unit for feeding at least a portion of the first fuel gas or the further gas mixture formed therefrom to the gas separation unit,
a second feed unit for feeding the gases N2or CO2separated off by means of the gas separation unit or the partial gas mixture separated off to the protective housing and
a third feed unit for feeding an H2-enriched tail gas as second fuel gas to the second reactor or to a sixth reactor comprising a further fuel cell.

In the context of the invention, a “feed unit” is understood to mean a conduit connected to the gas separation unit. The feed unit may comprise one or more valves by which flow through the conduit can be enabled, shut off or controlled in terms of amount. The feed unit may also comprise at least one of the following sensors: temperature sensor, pressure sensor, mass flow sensor and the like. By means of parameters detected by the sensors, it is possible to control an amount of the first fuel gas or further gas mixture supplied to the gas separation unit through the conduit. Equally, it is possible to control an amount of separated gases produced by the gas separation unit of a partial gas mixture by open- or closed-loop control.

In the context of the invention, multiple first feed units may be provided to supply the first fuel gas or the further gas mixture formed therefrom to the gas separation unit. The first feed units here each branch off at different points in the main gas stream. For example, it is possible to provide a first feed unit downstream of each reactor. In a similar manner, it is also possible for multiple third feed units to be provided in order to enrich the main gas stream with hydrogen upstream of defined reactors. It is possible to provide a controller with which the feed devices are controllable in such a way that an inert gas of minimum oxygen content is produced and supplied to the protective housing. The control is appropriately configured so as also to improve the efficiency of the production of electrical energy by means of the at least one fuel cell.

In an advantageous configuration, the gas separation unit is also set up such that it can also be used to separate H2separately from the first fuel gas or the further gas mixture formed therefrom. H2can be supplied to the fuel cell and/or to a further fuel cell as fuel.

A third reactor in the form of a water-gas shift stage and/or a fourth reactor in the form of a PrOx stage may be connected downstream of the first reactor. In a further configuration, a fourth feed unit is provided for feeding at least a portion of the cathode offgas and/or anode offgas as further gas mixture to the gas separation unit. In addition, a fifth reactor for conversion of the cathode offgas and/or the anode offgas may be connected downstream of the second reactor. The provision of the fourth feed unit ensures that the gas separation unit is supplied at any time with a sufficient amount of gas mixtures, such that a sufficient amount of inert gas can be produced and supplied to the protective housing. This can also achieve the effect that hydrogen unutilized in the method is used as the second fuel and hence particularly efficient generation of electrical energy can be achieved.

In addition, a unit for conversion of gaseous H2O separated off by means of the gas separation unit to liquid H2O may be provided. This may be a condenser. The liquid H2O may be discharged for external use. For this purpose, a separate conduit may be provided.

In a further configuration, a condenser for separating off gaseous H2O present in the first fuel gas or in the further gas mixture is provided. Such a condenser may be connected upstream or else downstream of the gas separation unit.

In addition, a unit may be provided for discharge of heat released in the conversion of gaseous H2O to liquid H2O. This may be a heat exchanger for example.

The gas separation unit is preferably a gas separation unit provided with specific membranes, with which the respective gases are separable from the first fuel gas or from the further gas mixture.

Advantageously, not just the second reactor but multiple reactors, preferably all reactors, are surrounded by the protective housing. In addition, the gas separation unit, one or more conduits that connect the reactors to one another, feeds connected to the gas separation unit and/or control valves inserted into the conduits and feeds may also be surrounded by the protective housing.

