A fuel cell system comprising - a fuel tank and a reformer-fuel evaporator for evaporating liquid fuel from the fuel tank and providing the evaporated fuel to a reformer system, - a reformer system with an extruded reformer chamber that has a constant cross section along a longitudinal axis with an inner reformer volume enclosed by reformer walls, the inner reformer volume containing a catalyst for catalytic conversion of the evaporated fuel into syngas when the fuel in evaporated form flows through the inner reformer volume; - a fuel cell fluid-flow connected to the reformer chamber for receiving the syngas from the reformer chamber and configured for converting the syngas into electricity, - a burner for providing hot burner gas to the reformer chamber, wherein the reformer chamber comprises a plurality of fins on the outer side of the reformer walls, the fins being integral extruded with the reformer walls, the fins being provided in gas-flow connection to the burner for guiding the hot burner gas form the burner along the fins for taking up heat from the hot burner gas and conducting the heat through the reformer wall into the inner reformer volume, wherein the reformer chamber comprises a plurality of ribs integral extruded with the reformer walls and extending from an inner side of the reformer walls towards a central region of the inner reformer volume for heat transfer from the ribs to the evaporated fuel in the inner reformer volume when the evaporated fuel is flowing through the inner reformer volume, characterized in that a plurality of guide plates (4a, 4b) are provided inside the inner reformer volume, the plurality of guide plates (4a, 4b) comprising a first type of guide plates and a second type of guide plates, the first type and the second type of guide plates being arranged in alternating configuration; wherein the first type of guide plates comprises a central aperture and obstruction blades arranged between the ribs for blocking flow of evaporated fuel through spaces between the ribs and for forcing flow of evaporated fuel through the central aperture; wherein the second type of guide plates comprises a central blocking plate and partially obstructing blades extending from the blocking plate partially into the spaces between the ribs for blocking flow of evaporated fuel through the central region of the inner reformer volume and for forcing flow of evaporated fuel through the spaces between the ribs; wherein the alternating configuration is configured for forcing the evaporated fuel in the inner reformer volume from the central region towards the reformer walls and into the spaces between the ribs and back into the central region for radial mixing of the evaporated fuel and for improved heat transfer between the reformer walls and the evaporated fuel. - a fuel tank and a reformer-fuel evaporator for evaporating liquid fuel from the fuel tank and providing the evaporated fuel to a reformer system, - a reformer system with an extruded reformer chamber that has a constant cross section along a longitudinal axis with an inner reformer volume enclosed by reformer walls, the inner reformer volume containing a catalyst for catalytic conversion of the evaporated fuel into syngas when the fuel in evaporated form flows through the inner reformer volume; - a fuel cell fluid-flow connected to the reformer chamber for receiving the syngas from the reformer chamber and configured for converting the syngas into electricity, - a burner for providing hot burner gas to the reformer chamber,