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 reacti
Trang 1Cogeneration 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 being 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 pollution 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
refers to chemical cogeneration or electrogenerative
processes when the main cycle is 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
corres-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
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