Fuel Cell Systems Explained - from and for SET students

thus leads to higher efficiency. Heat can also be used to preheat fuel and oxidant.Ensure high electrical and thermal efficiency and reduce the exergy losses are the key aspects indesigning such a system.7.2 Common features7.2.1 An introduction to fuel reformingThe production of hydrogen from hydrocorabons usually involves steam reforming.The steam reforming reaction with natural gas is an endothermic reaction, that means heat needs tobe suuplied for a suficient reaction rate. In most cases fuel reforming takes place at about 500°Cand can be used from the exhaust gases of the fuel cell itself. The PAFC requires additional burningof fuel to achieve that temperature, what reduces the effcicncy to 40-45%. For the MCFC and theSOFC efficiencies over 50% can be achieved.In a PAFC as in a PEM must the reformed gas further processed with the gas shift reaction, due toreducing the CO content in the gas. An aspect considering all fuel cells using hydrocarbonat,asnatural gas, is the desulphurisation of the fuel .Even small amounts of sulfur deactivate theelectrodes. This proces also needs heating up the fuel over 350°C. The higher operationtemperatures of theMCFC and the SOFC allows the basic steam reaction taken place in the fuel cellstack itselfs. Furthermore the steam neede is already present in the stack. The exhaust steam at theanode is used for the fuel reforming. That simplifies the design of MCFC and SOFC.7.2.2 Fuel utilisationFuel utilisation has always to be taken into account when the hydrogen is fed in a mixture to thestack. As the gas mixture passes through the cell, the hydrogen is utilised, the other componentssimply pass through unconverted, and so the hydrogen concentration falls. At the end of the cell thepartial pressure of hydrogen will be very low and thus lower the cell voltage.From the Nernst equation we can get the voltage change in respect of a change in hydrogenpressure.If the partial pressure of hydrogen falls from P1 to P2, the voltage change will always be negative.The RT term means that this drop will be greater for higher temperatures. The same occur for theoxygen utilisation from the air. Figure 7.1 shows the effect of it in four cases.1. Standard hydrogen fuel cell operating at 1bar with pure hydrogen and oxygen.2. Using air at the cathode and a mixtue of 4 parts hydrogen and 1 part carbon dioxide at theanode.3. Nernst exit voltage for 80% fuel utilisation and 50% air utilisation.4. Nernst exit voltage for 90% fuel utilisation and 50% air utilisation.