There is disclosed a fuel cell assembly comprising: at least one horizontally arranged fuel cell stack that has numerous fuel cells, each comprising an anode, a cathode and an electrolyte situated between the anode and the cathode; combustible gas supply means for supplying combustible gas to the anodes of the fuel cells; anode gas withdrawal means for withdrawing the anode exhaust gas from the anodes; cathode gas supply means for supplying cathode gas to the cathodes of the fuel cells; cathode gas withdrawal means for withdrawing the cathode exhaust gas from the fuel cells; and recirculation means for recirculating at least one part of the anode exhaust gas and/or the cathode exhaust gas to cathodes of the fuel cells. The fuel cell assembly according to the invention is characterised in that the assembly consists of modular sub-assemblies that are independent of one another and that communicate with one another via standardised interfaces. A first sub-assembly comprises the fuel cell stack, the combustible gas supply means and the anode gas withdrawal means, a second sub-assembly comprises the cathode gas supply means together with a cathode gas distributor, a start-up heater and a heat exchanger, a third sub-assembly comprises a cathode exhaust gas collector, a cathode exhaust gas line, a gas mixer for mixing fresh air, the cathode exhaust gas and the anode exhaust gases, a catalytic burner and a collection line of the recirculation means, and a fourth sub-assembly comprises a delivery unit of the recirculation means.