Re: GAS-L: Fuel Cells -- ethanol -- reforming natural gas

[Peter Singfield <snkm@btl.net>]

Just a little regarding Fuel cell technology -- kind of a Readers digest
version.

Get the big picture at:

http://www.benwiens.com/energy4.html

A very complete explaination of tham any various fuel cell configurations.

Following are short blurbs on just two types.

The "Solid Oxide Fuel Cell" which can use synthesis gas directly -- both
the H2 and the CO

The "DIRECT ALCOHOL FUEL CELL" -- which uses alcohols -- directly.

There are numerous other versions -- efficiencies are quite variable.


Peter Singfield / Belize



    The Solid Oxide Fuel Cell is considered to be the most desirable fuel
cell for generating electricity from hydrocarbon fuels. This is because it
is simple, highly efficient, tolerant to impurities, and can at least
partially internally reform hydrocarbon fuels.
    The SOFC runs at a red-hot temperature of 700-1000°C. Westinghouse has
worked at developing a tubular style of SOFC for many years which operates
at 1000°C. These long tubes have high electrical resistance but are simple
to seal. Many companies such as Global Thermoelectric are now working on a
planar SOFC composed of thin ceramic sheets which operate at 800°C or even
less. Thin sheets have low electrical resistance and possible high
efficiencies. Cheaper materials can be used at these lower temperatures.
Experts previously predicted that the SOFC was a long way to becoming
commercial reality. Many now believe that these lower temperatures may lead
to a quicker solution to these problems. 
    One of the big advantages of the SOFC over the MCFC is that the
electrolyte is a solid. This means that no pumps are required to circulate
hot electrolyte. Small planar SOFC of 1 kw could be constructed with very
thin sheets and result in a very compact package. 
    A big advantage of the SOFC is that both hydrogen and carbon monoxide
are used in the cell [3]. In the PEFC the carbon monoxide is a poison,
while in the SOFC it is a fuel. This means that the SOFC can readily and
safely use many common hydrocarbons fuels such as natural gas, diesel,
gasoline, alcohol and coal gas. In the PEFC an external reformer is
required to produce hydrogen gas while the SOFC can reform these fuels into
hydrogen and carbon monoxide inside the cell. This results in some of the
high temperature waste thermal-energy being recycled back into the fuel.
    Because the chemical reactions in the SOFC are good at the high
operating temperatures, air compression is not required. Especially on
smaller systems this results in a simpler system, quiet operation and high
efficiencies. Exotic catalysts are not required either.
    Many fuel cells such as the PEFC require an expensive liquid cooling
system but the SOFC requires none. In fact insulation must be used to
maintain the cell temperature on small systems. The cell is cooled
internally by the reforming action of the fuel and by the cool outside air
that is drawn into the fuel cell. 
    Because the SOFC does not produce any power below 650°C, a few minutes
of fuel burning is required to reach operating temperature. While the SOFC
is also being proposed as an automotive powerplant, this time delay is
considered to be a disadvantage. Because electric powerplants run
continuously, this time delay is not a problem. Still you may be interested
to know that the manager for bus development at Ballard, who develop PEFC,
is now working for Global, who develop SOFC. The SOFC may well be suited to
at least certain vehicles which run more continuously.
    Because of the high temperatures of the SOFC, they may not be practical
for sizes much below 1,000 watts or when small to midsize portable
applications are involved.
    Small SOFC will be about 50% efficient [4] from about 15%-100% power.
To achieve even greater efficiency, medium sized and larger SOFC are
generally combined with gas turbines. The fuel cells are pressurized and
the gas turbine produces electricity from the extra waste thermal-energy
produced by the fuel cell. The resulting efficiency of the medium SOFC
could be 60% and large one's up to 70%. 
    A SOFC suitable for producing 1-30 kW and using natural gas as it's
fuel is shown in Fig 5. On the anode side, natural gas is first ejected
into a reforming chamber where it draws waste thermal-energy from the stack
and is converted into hydrogen and carbon monoxide. It then flows into the
anode manifold where most of the hydrogen and carbon monoxide is oxidized
into water and carbon dioxide. This gas stream is then partly recycled to
the reforming chamber where the water is used in the reforming chamber. On
the cathode side, air is first blown into a heat exchanger where it reaches
nearly operating temperature. The air is brought up to the operating
temperature of 800°C by combustion of the remaining hydrogen and carbon
monoxide gas from the anode. The oxygen in the cathode manifold is
converted into an oxygen ion which travels back to the anode. 

9. DIRECT ALCOHOL FUEL CELL (DAFC)
    Several companies around the world are presently working on DAFC. Even
in 1999 there has been a marked shift away from developing the PEFC in
favor of the DAFC [5]. ]. In this type of fuel cell, either methyl DMFC or
ethyl DEFC alcohol is not reformed into hydrogen gas but is used directly
in a very simple type of fuel cell. Its operating temperature of 50-100°C
is low and so is ideal for tiny to midsize applications. It's electrolyte
is a polymer or a liquid alkaline. This type of fuel cell was largely
overlooked in the early 1990s because its efficiency was below 25%. Most
companies rather pursued the PEFC because of its higher efficiency and
power density. There has been tremendous progress made in the last 6 years.
Efficiencies of the DMFC are much higher and predicted efficiencies in the
future may be as high as 40% [6] for a DC automobile powerplant. Power
densities are over 20 times as high now as in the early 1990s. It is
expected that the DMFC will be more efficient than the PEFC for automobiles
that use methanol as fuel. Presently the power density of the DEFC is only
50% of the DMFC but hopefully this can be improved in the future.
    Fuel crossing over from the anode to the cathode without producing
electricity is one problem that has restricted this technology from its
inception. One company, Energy Ventures Inc claimed in Dec1999 that it has
completely solved this cross-over problem. Another problem however is that
there are often chemical compounds formed during operation that poison the
catalyst.
    There are already working DMFC prototypes used by the military for
powering electronic equipment in the field. 


Fig 6 A small simple 30 kw Direct Methanol Fuel Cell 

    Figure 6 illustrates a type of DMFC that could be used in a 30 kw
system. Even smaller ones for use as battery replacements do away with the
air blower and the separate methanol water tank and pump. Such fuel cells
are not much different than batteries in construction.
    Recently there has been much concern about the poisonous aspects of
methanol--methyl alcohol. As of 2001 methanol is "out" and ethanol is "in".
Already several companies are now working on DEFC. Presently the power
density is only 50% of the DMFC but hopefully this can be improved. 


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