Cogeneration of Energy and Chemicals: Fuel Cells pps
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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.
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