Cogeneration of Energy and Chemicals: Fuel Cells pps

Cogeneration of Energy and Chemicals: Fuel Cells P-L Cabot, F Alcaide, and E Brillas, Universitat de Barcelona, Barcelona, Spain & 2009 Elsevier B.V. All rights reserved. Introduction Fuel cells (FCs) are electrochemical systems that con- tinuously produce electric energy and heat, where the reactants (fuel and oxidant) are fed to the electrodes and the reaction products are removed from the cell. The chemical energy of the reactants is directly converted into electricity, reaction products, and heat without in- volving combustion processes. The efficiencies of the FCs are about twice those of the heat engines because the latter are affected by the limitations imposed by Carnot’s theorem. Electricity is normally the main product of FCs, the chemicals and heat generated be ing the waste prod- ucts of the first (or primary) cycle. In this case, the re- action products should be environmentally friendly and the heat produced could be used to obtain additional energy in a secondary cycle. The reaction product is water when the fuel is pure hydrogen and the oxidant pure oxygen. This case is the most advantageous to avoid polluti on of the environ- ment in electricity-generating FCs. However, different reactants lead to other reaction products that could be valuable chemicals for particular applications. One then refer s to chemical cogeneration or electrogenerative processes when the main cycle i s the formation of such valuable chemicals. The current delivered and the heat produced during the electrochemical reaction can be used in other secondary cycles. The FC can be suc- cessfully transformed into an electrolytic reactor when the only object is the production of a given chemical. In this case, the consumption of external electric power allows increasing the generation rate of the cor res- ponding product. The important point here is the economic study to decide the adequate operation mode. Fuel cells are thus electrochemical power sources in which different combined-cycle processes can be per- formed. The primary cycle is the generation of the main product and the secondary cycles result from the appli- cation of the waste by-products. The primary and sec- ondary cycles depend on their mode of operation. Fuel cells operate at low and high temperatures. Aqueous FCs (such as alkaline fuel cells (AFCs)), proton- exchange membrane fuel cells (PEMFCs), and phos- phoric acid fuel cells (PAFCs) operate at low tempera- tures. The molten carbonate fuel cells (MCFCs) and solid oxide fuel cells (SOFCs) operate at high tempera- tures (from 500 1C). The electrolytes can be aqueous (used in low-temperature FCs), molten (used in intermediate- and high-temperature FCs), and solid (used in intermediate- and high-temperature FCs). In this article, the combined-cycle processes in which these FCs are involved will be examined from the sci- entific, technological, and economical points of view. At the end, combined-cycle processes resulting in the pro- duction of electricity and chemicals, not electrochemical in origin, in which the products can be used in electro- chemical power sources, will also be briefly examined. Cogeneration of Chemicals and Electricity Chemical Cogeneration as Electrosynthesis Electrosynthesis of organic and inorganic compounds by electrolysis of particular reactants actually employs the FC technology by introducing gas diffusion electrodes (GDEs) in which the gas consumption/evolution re- actions take place. A GDE provides a large specific area for the electrode reaction and greatly favors diffusion of gases. This has allowed a significant saving of energy in important industrial processes such as hydrodimerization of acetonitrile and in chlorine/alkali cells. A proper choice of half-reactions in porous electrodes leads to FCs in which the spontaneous reactions produce useful chemicals and electricity (see the scheme of Figure 1). The important difference is that electricity is consumed in the electrolytic cell, whereas it is produced in the FC. This attractive difference has led to the study of many cogeneration processes that have been thought to be interesting from the economical and/or the en- vironmental point of view. It is worth to note in this regard that the use of FCs can allow simplifying a complicated chemical industrial process in a one-step production and developing alternative process when the demand for a final product decays. The first systematic works were performed in the middle of the twentieth century, mainly devoted to the study of the oxidation of hydrocarbons and petroleum fuels. Further works have described several tens of cogeneration processes involving chemical products with interesting industrial applications. The main cogenerated chemicals reported in the literature are some inorganic and organic compounds obtained through reactions such as hydrogenations, dehydrogenations, and oxidations, involving hydrocarbons, benzene, alcohols, ketones, and their derivatives, with increasing complexity. 146 . proton- exchange membrane fuel cells (PEMFCs), and phos- phoric acid fuel cells (PAFCs) operate at low tempera- tures. The molten carbonate fuel cells (MCFCs) and solid oxide fuel cells (SOFCs) operate at. Cogeneration of Energy and Chemicals: Fuel Cells P-L Cabot, F Alcaide, and E Brillas, Universitat de Barcelona, Barcelona, Spain & 2009 Elsevier B.V. All rights reserved. Introduction Fuel. appli- cation of the waste by-products. The primary and sec- ondary cycles depend on their mode of operation. Fuel cells operate at low and high temperatures. Aqueous FCs (such as alkaline fuel cells