Elettrochimica

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FUEL CELLS Considerazioni aggiuntive
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Allaumentare della corrente di scarica (a cui
corrisponde un aumento di potenza), diminuisce la
tensione della cella (a causa dellaumento delle
sovratensioni) e quindi diminuisce la sua energia
specifica.
Conseguentemente viene scelto un valore di
compromesso che consenta di operare con potenze
accettabili senza abbassare troppo il valore
dellenergia.
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Direct Methanol Proton Exchange Fuel Cell The
large potential market for fuel cell vehicle
applications has generated a strong interest in a
fuel cell that can run directly on methanol.
Operation on liquid fuel would assist in rapid
introduction of fuel cell technology into
commercial markets, because it would greatly
simplify the on-board system as well as reduce
the infrastructure needed to supply fuel to
passenger cars and commercial fleets. Performance
levels achieved with a direct methanol PEFC using
air are now in the range of 180 mA/cm2 to 250
mA/cm2.
Problems with methanol crossover and high
overpotentials still inhibit performance.
Research has focused on finding more advanced
electrolyte materials to combat fuel crossover
and more active anode catalysts to promote
methanol oxidation. Significant progress has been
made over the past few years in both of these key
areas.
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SISTEMI Nella maggior parte delle applicazioni è
necessario considerare il sistema complessivo,
di cui la fuel cell è solo uno dei componenti.
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Fuel Processing Technology Descriptions Fuel
Conversion Description The generic term
generally applied to the process of converting
liquid or gaseous light hydrocarbon fuels to
hydrogen and carbon monoxide is reforming.
There are a number of methods to reform fuel. The
three most commercially developed and popular
methods are 1) steam reforming 2)
partial-oxidation reforming 3) autothermal
reforming Each of these methods can be used to
produce a fuel suitable for the fuel cell.
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Steam reforming (SR) provides the highest
concentration of hydrogen and can obtain a high
fuel processing conversion efficiency. Partial
oxidation (POX) is a fast process, good for
starting, fast response, and a small reactor
size. Non-catalytic POX operates at temperatures
of approximately 1,400 C, but adding a catalyst
(catalytic POX or CPOX) can reduce this
temperature as low as 870 C. Combining steam
reforming closely with CPOX is termed autothermal
reforming (ATR).
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Relevant reactions for the steam reformer are
presented below CH4 H2O ? 3H2 CO
(Steam Reforming Reaction) CO H2O ? CO2 H2
(Water Gas Shift Reaction) A third
relevant reaction is also presented below.
However, this reaction is simply a combination of
the other two. Of the three reactions, any two
can be utilized as an independent set of
reactions for analysis, and should be chosen for
the user's convenience. Here we have chosen the
steam reforming and the shift reactions. CH4
2H2O ? 4H2 CO2
(Composite Steam Reforming Reaction)
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The CH4 in the natural gas is usually converted
to H2 and CO in a SR reactor. Steam reforming
reactors yield the highest percentage of hydrogen
of any reformer type. The basic SR reactions for
methane and a generic hydrocarbon are CH4 H2O
? CO 3H2 CnHm nH2O ? nCO (m/2 n) H2 CO
H2O ? CO2 H2 In addition to natural gas, steam
reformers can be used on light hydrocarbons such
as butane and propane and on naphtha with a
special catalyst. Steam reforming reactions are
highly endothermic and need a significant heat
source. Often the residual fuel exiting the fuel
cell is burned to supply this requirement. Fuels
are typically reformed at temperatures of 760 to
980 C (1,400 to 1,800 F).
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