WO2010022732A1

WO2010022732A1 - Fuel cell system and method of operating such fuel cell system

Info

Publication number: WO2010022732A1
Application number: PCT/DK2009/050212
Authority: WO
Inventors: Rasmus Høyrup REFSHAUGE
Original assignee: Dantherm Power A/S
Priority date: 2008-08-25
Filing date: 2009-08-24
Publication date: 2010-03-04

Abstract

The fuel cell system (1) comprises a fuel cell stack (9), a methanol reformer (6) and an evaporator (4). The fuel cell stack (9) is composed by a number of PEM fuel cells and having an anode inlet (8), an anode outlet (10), a cathode inlet (13) for ambient air, and a cathode outlet (16). The PEM fuel cells have an operating temperature of at least 120º C. The methanol reformer (6) is a single-step steam reformer that is directly coupled to the anode inlet (8) of the fuel cell stack (9) without any intermediate CO cleanup system. Furthermore, a method of operating a fuel cell system (1) is disclosed.

Description

Fuel cell system and method of operating such fuel cell system

The present invention relates to a fuel cell system comprising a fuel cell stack, a methanol reformer for converting evaporated methanol and water into hydrogen, and an evaporator for feeding the methanol reformer with evaporated methanol and water, the fuel cell stack being composed by a number of PEM fuel cells and having an anode inlet for hydrogen containing gas, an anode outlet for anode waste gas, a cathode inlet for ambient air, and a cathode outlet for cathode exhaust gas, and the PEM fuel cells having an operating temperature of at least 12O⁰C.

US 2006/0099467 A1 discloses a fuel cell system including a fuel cell stack that includes PEM fuel cells. Each fuel cell has an operating temperature of at least 120 ⁰C. The fuel cell stack has a cathode inlet to receive a flow of ambient air and a cathode outlet to provide a cathode exhaust flow. The fuel cell system includes a fuel processing reactor that has an inlet and an outlet. The inlet and outlet are in fluid communication with a catalyst that is suitable for converting a hydrocarbon into a gas that contains hydrogen and carbon monoxide. The outlet is in fluid communication with an anode chamber of the fuel cell, and the outlet of the fuel processing reactor is in fluid communication with the cathode outlet. However, in such systems it is typically necessary to reduce CO levels to less than 100 ppm to avoid damaging the fuel cell catalyst. For this reason the fuel processor may employ additional reactions and processes to reduce the CO that is produced. For instance, a Hb permeable membrane of palladium-silver alloy may be utilized to reduce CO levels. To this end, it is necessary to provide the hydrogen containing gas at high pressure, thereby reducing the efficiency of the fuel cell system. The object of the present invention is to provide a fuel cell system of simplified structure.

In view of this object, the fuel cell system is characterized in that the methanol reformer is a single-step steam reformer that is directly coupled to the anode inlet of the fuel cell stack without any intermediate CO cleanup sys- tern. By coupling the single-step methanol steam reformer directly to the fuel cell stack, a simplified fuel cell system is achieved in that any intermediate CO cleanup system is avoided.

In an embodiment, the methanol reformer comprises a fuel processing chamber in which the evaporated methanol and water is brought into contact with a catalyst, and the fuel processing chamber is in direct communication with the anode inlet of the fuel cell stack. Thereby, a fuel cell system of a further simplified structure may be achieved.

In an embodiment, the methanol reformer is heated by means of ex- haust gas from a catalytic burner in which the anode waste gas from the anode outlet of the fuel cell stack is combusted. As the fuell cell does not consume the entire content of hydrogen contained in the gas from the methanol reformer, this remaining fuel may advantageously be combusted in the catalytic burner for heating the methanol reformer, whereby a better total effi- ciency of the fuel cell system may be achieved. Furthermore, remaining methanol and carbon monoxide as well as other pollutants in the anode waste gas may be burned in the catalytic burner, thereby minimizing emissions from the system.

In an embodiment, the cathode exhaust gas from the cathode outlet of the fuel cell stack is fed partly into the catalytic burner, and preferably between 1/2 and 3/4, and more preferred approximately 2/3 of the cathode exhaust gas is fed into the catalytic burner. Thereby, a suitable gas flow rate and temperature in the catalytic burner may be obtained, and the overall efficiency of the fuel cell system may be improved, as thermal energy of the heated cathode exhaust gas from the cathode outlet of the fuel cell stack may be utilized.

In an embodiment, the cathode exhaust gas from the cathode outlet of the fuel cell stack is fed partly into the evaporator, preferably between 1/4 and 1/2, and more preferred approximately 1/3 of the cathode exhaust gas is fed into the evaporator. Thereby, the overall efficiency of the fuel cell system may be improved even further, as thermal energy of the heated cathode exhaust gas from the cathode outlet of the fuel cell stack may be utilized. In an embodiment, the exhaust gas from the catalytic burner is fed into the evaporator. Thereby, the overall efficiency of the fuel cell system may be improved even further, as thermal energy remaining in the exhaust gas from the catalytic burner, after that the methanol reformer has been heated by means of said exhaust gas, may be utilized.

In a structurally particularly advantageous embodiment, the system is arranged within a housing so that the PEM fuel cells of the fuel cell stack are stacked in a longitudinal direction of the housing and so that the catalytic burner, the methanol reformer and the evaporator are arranged within an in- sulating enclosure in continuation of each other in said longitudinal direction of the housing.

In a structurally particularly advantageous embodiment, the cathode inlet for ambient air is arranged along a side of the fuel cell stack opposing a first side wall of the housing, the cathode outlet for cathode exhaust gas is arranged along a side of the fuel cell stack opposing the insulating enclosure, an inlet for cathode exhaust gas for the catalytic burner is formed as a first opening in a front wall of the insulating enclosure, and an inlet for cathode exhaust gas for the evaporator is formed as a second opening in the front wall of the insulating enclosure. In this embodiment, the cathode exhaust gas from the cathode outlet of the fuel cell stack may be fed partly into the catalytic burner through the first opening in the front wall of the insulating enclosure and partly into the evaporator through the second opening in the front wall of the insulating enclosure. By suitably dimensioning and positioning of the first and the second openings in the front wall of the insulating enclosure, a suit- able relation between the quantity of cathode exhaust gas fed into the catalytic burner and the quantity of cathode exhaust gas fed into the evaporator may be achieved.

In an embodiment, the fuel processing chamber of the methanol reformer communicates with the anode inlet of the fuel cell stack through a pipe extending in the longitudinal direction of the housing, between the cathode outlet for cathode exhaust gas of the fuel cell stack and the insulating enclosure, and the pipe preferably extends along at least 1/2 and more preferred 2/3 of the length of the fuel cell stack. Thereby, the hydrogen containing gas produced by the methanol reformer may be cooled appropriately by means of the cathode exhaust gas of the fuel cell stack, so that a temperature of said gas suitable for the anode of the fuel cells may be obtained.

In a structurally advantageous embodiment, the catalytic burner has the form of a perforated tube surrounded by a catalytic element, and the catalytic element preferably is in the form of a cylindrically extending wire mesh coated with a catalyst.

In a structurally particularly advantageous embodiment, a body of the evaporator is composed by a first block and a second block, each block hav- ing an internal contact face and an outer heat exchanging surface provided with ribs, the internal contact faces of the first block and the second block, respectively, are mounted in close contact with each other, the internal contact face of the first block is provided with a zigzagging evaporator channel for a mixture of methanol and water to be evaporated, and the internal contact face of the second block is provided with an inlet opening for the mixture of methanol and water to be evaporated and a vapour outlet opening for a mixture of evaporated methanol and water, said inlet opening communicating with a first end of the zigzagging evaporator channel and said vapour outlet opening communicating with a second end of the zigzagging evaporator channel.

In a structurally particularly advantageous embodiment, the fuel processing chamber of the methanol reformer has a first end at which the catalytic burner is located and a second end at which the evaporator is located, an inlet manifold for a mixture of evaporated methanol and water is located at the first end of the fuel processing chamber and an outlet manifold for hydrogen containing gas is located at the second end of the fuel processing chamber, and a reformer inlet tube for the mixture of evaporated methanol and water connects the vapour outlet opening of the evaporator and the inlet manifold of the fuel processing chamber, the reformer inlet tube extending inside the fuel processing chamber of the methanol reformer. Thereby, excessive temperature of the hydrogen containing gas exiting the methanol reformer may be avoided, as the hot exhaust gas from the catalytic burner may flow along heat exchanging ribs of the methanol reformer, substantially in parallel with the flow of the mixture of evaporated methanol and water being processed in the fuel processing chamber of the methanol reformer.

In a structurally advantageous embodiment, a blower is arranged in an insulated casing arranged in the housing between a first end wall of the hous- ing and the insulating enclosure, a blower inlet opens out through the first end wall of the housing, and a blower outlet channel extends from the blower, across said first end wall, and opens out at the first side wall of the housing, thereby communicating with the cathode inlet for ambient air of the fuel cell.

In an advantageous embodiment, the methanol reformer is a packed catalytic bed reformer.

The present invention further relates to a method of operating a fuel cell system, whereby a mixture of methanol and water is evaporated and brought into contact with a catalyst, thereby reforming said mixture into a hydrogen containing gas, whereby hydrogen containing gas is fed to the anode of a fuel cell stack being composed by a number of PEM fuel cells, and whereby the PEM fuel cells are operated at a temperature of at least 12O⁰C.

The method according to the invention is characterized by that the hydrogen containing gas is steam reformed in a single-step and flows directly from the catalyst to the anode of the fuel cell stack without any intermediate CO cleanup. Thereby, the above-discussed effects may be achieved.

In an embodiment, the evaporated methanol and water is heated during reformation by catalytic combustion of anode waste gas exiting from the anode of the fuel cell stack. Thereby, the above-discussed effects may be achieved. In an embodiment, a part, preferably between 1/2 and 3/4, and more preferred approximately 2/3 of the cathode exhaust gas from the cathode of the fuel cell stack is combusted catalytically. Thereby, the above-discussed effects may be achieved.

In an embodiment, the mixture of methanol and water is heated to evaporate by means of a part, preferably between 1/4 and 1/2, and more preferred approximately 1/3, of the cathode exhaust gas flowing from the cathode of the fuel cell stack. Thereby, the above-discussed effects may be achieved. The invention will now be explained in more detail below by means of examples of embodiments with reference to the schematic drawing, in which

Fig. 1 is a schematic diagram showing a fuel cell system according to the present invention, Fig. 2 is a top view of an embodiment of the fuel cell system shown in

Fig. 1 , whereby the top cover has been removed,

Fig. 3 is a side view of the embodiment of the fuel cell system shown in Fig. 2, whereby the fuel cell stack and a side wall have been removed,

Fig. 4 is a cross-section along the line IV-IV through the embodiment of the fuel cell system shown in Fig. 2,

Fig. 5 is a cross-section along the line V-V through the embodiment of the fuel cell system shown in Fig. 2,

Fig. 6 shows a longitudinal cross-section through evaporator and reformer of the embodiment of the fuel cell system shown in Fig. 2, Fig. 7 shows a plan view of an internal contact face of a first block of the evaporator of the embodiment of the fuel cell system shown in Fig. 2,

Fig. 8 shows a perspective view of a second block of the evaporator of the embodiment of the fuel cell system shown in Fig. 2,

Fig. 9 shows a perspective view of the evaporator and reformer of the embodiment of the fuel cell system shown in Fig. 2,

Fig. 10 shows a perspective view of the internal of the reformer of the embodiment of the fuel cell system shown in Fig. 2, and

Fig. 11 shows a perspective view of the blower end of the embodiment of the fuel cell system shown in Fig. 2. Fig. 1 shows diagrammatically a fuel cell system 1 according to the present invention, whereby Fig. 2 shows a top view of an embodiment of such a fuel cell system 1 being arranged within a housing 44. A pump 2 is arranged outside the housing 44 to supply the fuel cell system 1 with a mixture of methanol and water through a supply line 3. The mixture of methanol and wa- ter is fed into an evaporator 4, in which it is heated to evaporation. The evaporated mixture of methanol and water is subsequently through a line 5 fed into a methanol reformer 6 for converting the evaporated mixture of methanol and water into a hydrogen containing gas. The hydrogen containing gas then flows through a line 7 to an anode inlet 8 of a the fuel cell stack 9 being composed by a number of PEM fuel cells. The fuel cell stack 9 has an electric connection 47. The fuel cell stack 9 has an anode outlet 10 for anode waste gas connected through a line 11 with a catalytic burner 12. The fuel cell stack 9 further has a cathode inlet 13 that is fed via a line 14 and a manifold 45 with ambient air by means of a blower 15. In addition, the fuel cell stack 9 has a cathode outlet 16 that via a branching point 17 feeds the catalytic burner 12 and the evaporator 4 with cathode exhaust gas. The combustion gas from the catalytic burner 12 is passed through heat exchange ribs 18 of the methanol reformer 6, whereby the methanol reformer 6 is heated, and subsequently passed through heat exchange ribs 19 of the evaporator 4, whereby the evaporator 4 is heated. The ambient air supplies the cathode of the fuel cells with oxygen as reactant but also cools the fuel cell stack 9.

As an example, suitable approximate operating temperatures may be 16O⁰C for the fuel cell stack 9, 18O⁰C for the cathode exhaust gas entering the evaporator 4, 12O⁰C for the evaporated mixture of methanol and water entering the methanol reformer 6, 200⁰C for the hydrogen containing gas leaving the methanol reformer 6, 32O⁰C for the combustion gas leaving the catalytic burner 12, and 22O⁰C for the combustion gas leaving the methanol reformer 6. It should be noted that the hydrogen containing gas leaving the methanol reformer 6 is cooled down to a suitable temperature lower than 200⁰C before entering the fuel cell stack 9. Said cooling of the hydrogen containing gas takes place in that the gas passes through line 7 that is located in the flow channel forming the branching point 17 for cathode exhaust gas from the cathode outlet 16, see Figs. 2 to 4. The membrane of the fuel cells may be based on Polybenzimidazole (PBI), whereby a temperature up to 220⁰C may be reached without using water management.

Fig. 6 shows a longitudinal cross-section through the unit comprising evaporator 4 and methanol reformer 6 of the embodiment of the fuel cell sys- tern 1 shown in Fig. 2. The evaporator 4 is formed by a body composed by a first block 20 and a second block 21 , whereby each block 20, 21 has an internal contact face 22 and an outer heat exchanging surface provided with ribs 19, see Figs. 7 to 9. The internal contact faces 22 of the first block 20 and the second block 21 , respectively, are mounted in close contact with each other. The internal contact face 22 of the first block 20 is provided with a zigzagging evaporator channel 23 for the mixture of methanol and water to be evaporated. The internal contact face 22 of the second block 21 is provided with an inlet opening 24 for the mixture of methanol and water to be evaporated and a vapour outlet opening 25 for a mixture of evaporated methanol and water. In the assembled state of the first block 20 and the second block 21 of the body of the evaporator, the inlet opening 24 communicates with a first end 26 of the zigzagging evaporator channel 23 and the vapour outlet opening 25 commu- nicates with a second end 27 of the zigzagging evaporator channel 23. The contact faces 22 of the first block 20 and the second block 21 , respectively, are, in the assembled state, pressed against each other by means of clamping bolts 28. The inlet opening 24 is fed with the mixture of methanol and water from the pump 2 via the line 3. The vapour outlet opening 25 of the evaporator 4 is via the line 5 communicating with an inlet manifold 29 of the methanol reformer 6, whereby the evaporated mixture of methanol and water is fed into a fuel processing chamber 30 of the methanol reformer 6 comprising a packed bed of catalytic material. The processed mixture of methanol and water exits the fuel process- ing chamber 30 through an outlet manifold 31 . The outlet manifold 31 has outlet opening 32 communicating with the anode inlet 8 of the fuel cell stack 9 via the line 7. Fig. 10 shows a perspective view of the internal of the fuel processing chamber 30 of the methanol reformer 6; however, the packed bed of catalytic material is not shown in the figure. The evaporator 4 and the methanol reformer 6 are clamped together in continuation of each other by means of a clamping bolt 33 extending in the longitudinal direction of the evaporator 4 and the methanol reformer 6. The catalytic burner 12, the evaporator 4, and the methanol reformer 6 are enclosed by means of an insulating enclosure 34 having a front wall 35. An inlet 36 for cathode exhaust gas for the catalytic burner 12 is formed as a first opening in the front wall 35 of the insulating enclosure 34, and an inlet 37 for cathode exhaust gas for the evaporator 4 is formed as a second opening in the front wall 35 of the insulating enclosure 34. The anode outlet 10 for anode waste gas of the fuel cell stack 9 is connected through the line 11 with the catalytic burner 12. The line 11 enters through the bottom part of the first opening forming the inlet 36 for cathode exhaust gas in the front wall 35 of the insulating enclosure 34, see Fig. 3. The catalytic burner 12 has the form of a vertically extending perforated tube 38 surrounded by a not shown catalytic element in the form of a cylindrically extending wire mesh coated with a catalyst. The not shown wire mesh is made from tantalum wire coated with ceramics, whereby the ceramic coating is coated with platinum; however, any suitable catalytic element may be used. The catalytic burner 12 is provided with oxygen containing air through the inlet 36 for cathode exhaust gas in the front wall 35 of the insulating enclosure 34. The exhaust gas from the catalytic burner 12 passes through the heat exchanging ribs 18 of the methanol reformer 6, subsequently through the heat exchanging ribs 19 of the evaporator 4, and eventually the exhaust gas exits through an exhaust exit opening 39 in an end wall 43 of the housing 44 of the fuel cell system 1 , see Fig. 2.

The blower 15 is arranged within an insulating enclosure 46 arranged next to the insulating enclosure 34 inside the housing 44 of the fuel cell system 1. The blower 15 draws ambient air in through an inlet opening 40 of an end wall 42 of the housing 44, see Fig. 11. The ambient air passes from the blower 15 through a cut-out 41 in an internal part of the end wall 42 to the manifold 45 extending along the fuel cell stack 9. The ambient air then enters the cathode inlet 13 of the fuel cell stack 9, passes through the cathode of the fuel cells and exits from the cathode outlet 16. Subsequently, the cathode exhaust gas is fed to the catalytic burner 12 and to the evaporator 4 via the branching point 17, as explained above.

Claims

1. A fuel cell system (1 ) comprising a fuel cell stack (9), a methanol reformer (6) for converting evaporated methanol and water into hydrogen, and an evaporator (4) for feeding the methanol reformer (6) with evaporated methanol and water, the fuel cell stack (9) being composed by a number of PEM fuel cells and having an anode inlet (8) for hydrogen containing gas, an anode outlet (10) for anode waste gas, a cathode inlet (13) for ambient air, and a cathode outlet (16) for cathode exhaust gas, and the PEM fuel cells having an operating temperature of at least 12O⁰C, characterized in that the methanol reformer (6) is a single-step steam reformer that is directly coupled to the anode inlet (8) of the fuel cell stack (9) without any intermediate CO cleanup system.

2. A fuel cell system according to claim 1 , wherein the methanol reformer (6) comprises a fuel processing chamber (30) in which the evaporated methanol and water is brought into contact with a catalyst, and wherein the fuel processing chamber (30) is in direct communication with the anode inlet (8) of the fuel cell stack (9).

3. A fuel cell system according to claim 1 or 2, wherein the methanol reformer (6) is heated by means of exhaust gas from a catalytic burner (12) in which the anode waste gas from the anode outlet (10) of the fuel cell stack (9) is combusted.

4. A fuel cell system according to any one of the preceding claims, wherein the cathode exhaust gas from the cathode outlet (16) of the fuel cell stack (9) is fed partly into the catalytic burner (12), and wherein preferably between 1/2 and 3/4, and more preferred approximately 2/3 of the cathode exhaust gas is fed into the catalytic burner.

5. A fuel cell system according to any one of the preceding claims, wherein the cathode exhaust gas from the cathode outlet (16) of the fuel cell stack (9) is fed partly into the evaporator (4), wherein preferably between 1/4 and 1/2, and more preferred approximately 1/3 of the cathode exhaust gas is fed into the evaporator (4).

6. A fuel cell system according to any one of the claims 3-5, wherein the exhaust gas from the catalytic burner (12) is fed into the evaporator (4).

7. A fuel cell system according to any one of the preceding claims, wherein the system (1 ) is arranged within a housing (44) so that the PEM fuel cells of the fuel cell stack (9) are stacked in a longitudinal direction of the housing (44) and so that the catalytic burner (12), the methanol reformer (6) and the evaporator (4) are arranged within an insulating enclosure (34) in continuation of each other in said longitudinal direction of the housing (44).

8. A fuel cell system according to claim 7, wherein the cathode inlet (13) for ambient air is arranged along a side of the fuel cell stack (9) opposing a first side wall of the housing (44), wherein the cathode outlet (16) for cath- ode exhaust gas is arranged along a side of the fuel cell stack opposing the insulating enclosure (34), wherein an inlet (36) for cathode exhaust gas for the catalytic burner (12) is formed as a first opening in a front wall (35) of the insulating enclosure (34), and wherein an inlet (37) for cathode exhaust gas for the evaporator (4) is formed as a second opening in the front wall (35) of the insulating enclosure (34).

9. A fuel cell system according to claim 7 or 8, wherein the fuel processing chamber (30) of the methanol reformer (6) communicates with the anode inlet (8) of the fuel cell stack (9) through a pipe (7) extending in the longitudinal direction of the housing (44), between the cathode outlet (16) for cath- ode exhaust gas of the fuel cell stack (9) and the insulating enclosure (34), and wherein the pipe (7) preferably extends along at least 1/2 and more preferred 2/3 of the length of the fuel cell stack.

10. A fuel cell system according to any one of the preceding claims, wherein the catalytic burner (12) has the form of a perforated tube (38) sur- rounded by a catalytic element, and wherein the catalytic element preferably is in the form of a cylindrically extending wire mesh coated with a catalyst.

11. A fuel cell system according to any one of the preceding claims, wherein a body of the evaporator (4) is composed by a first block (20) and a second block (21 ), each block having an internal contact face (22) and an outer heat exchanging surface provided with ribs (19), wherein the internal contact faces of the first block and the second block, respectively, are mounted in close contact with each other, wherein the internal contact face (22) of the first block (20) is provided with a zigzagging evaporator channel (23) for a mixture of methanol and water to be evaporated, and wherein the internal contact face (22) of the second block (21 ) is provided with an inlet opening (24) for the mixture of methanol and water to be evaporated and a vapour outlet opening (25) for a mixture of evaporated methanol and water, said inlet opening (24) communicating with a first end (26) of the zigzagging evaporator channel (23) and said vapour outlet opening (25) communicating with a second end (27) of the zigzagging evaporator channel (23).

12. A fuel cell system according to any one of the claims 2 to 11 , wherein the fuel processing chamber (30) of the methanol reformer (6) has a first end at which the catalytic burner (12) is located and a second end at which the evaporator (4) is located, wherein an inlet manifold (29) for a mixture of evaporated methanol and water is located at the first end of the fuel processing chamber (30) and an outlet manifold (31) for hydrogen containing gas is located at the second end of the fuel processing chamber (30), and wherein a reformer inlet tube (5) for the mixture of evaporated methanol and water connects the vapour outlet opening (25) of the evaporator (4) and the inlet manifold (29) of the fuel processing chamber (30), the reformer inlet tube (5) extending inside the fuel processing chamber (30) of the methanol reformer (6).

13. A fuel cell system according to any one of the claims 8 to 12, wherein a blower (15) is arranged in an insulated casing (46) arranged in the housing (44) between a first end wall (42) of the housing and the insulating enclosure (34), wherein a blower inlet (40) opens out through the first end wall (42) of the housing (44), and wherein a blower outlet channel (14) ex- tends from the blower (15), across said first end wall (42), and opens out at the first side wall of the housing (44), thereby communicating with the cathode inlet (13) for ambient air of the fuel cell.

14. A fuel cell system according to any one of the preceding claims, wherein the methanol reformer (6) is a packed catalytic bed reformer.

15. A method of operating a fuel cell system (1 ), whereby a mixture of methanol and water is evaporated and brought into contact with a catalyst, thereby reforming said mixture into a hydrogen containing gas, whereby hydrogen containing gas is fed to the anode of a fuel cell stack (9) being com- posed by a number of PEM fuel cells, and whereby the PEM fuel cells are operated at a temperature of at least 12O⁰C, characterized by that the hydrogen containing gas is steam reformed in a single-step and flows directly from the catalyst to the anode of the fuel cell stack (9) without any intermediate CO cleanup.

16. A method of operating a fuel cell system according to claim 15, whereby the evaporated methanol and water is heated during reformation by catalytic combustion of anode waste gas exiting from the anode of the fuel cell stack (9).

17. A method of operating a fuel cell system according to claim 15 or

16, whereby a part, preferably between 1/2 and 3/4, and more preferred approximately 2/3 of the cathode exhaust gas from the cathode of the fuel cell stack (9) is combusted catalytically.

18. A method of operating a fuel cell system according to any one of the claims 15 to 17, whereby the mixture of methanol and water is heated to evaporate by means of a part, preferably between 1/4 and 1/2, and more preferred approximately 1/3, of the cathode exhaust gas flowing from the cathode of the fuel cell stack (9).