Page 378 - Boiler_Operators_Handbook,_Second_Edition

Plants and Equipment 363

two electrodes in water (the electrolyte), pass a direct edly does neither unless it’s fueled with pure hydrogen
current through them and the water, then watch gas which has to be a very expensive fuel. Most of the fuel
bubbles form at each electrode and rise into an inverted cells do operate at temperatures low enough that they
test tube. One contained oxygen, the other contained don’t produce any NO and there’s no way for particu-
x
hydrogen; the process broke the water down into its two late matter to get through the liquid electrolyte.
basic parts. A fuel cell does the opposite, using reaction As for carbon monoxide and volatile organics we’ll
of hydrogen gas and oxygen to produce direct current have to assume they can’t get through the electrolyte ei-
electricity and water. ther although a reformer could dump them out with the
Fuel cells became the mainstay of electric power carbon dioxide under upset conditions. I would be a lot
in the space program because they generated a lot of happier about the future of fuel cells if someone would
power with very little weight and produced water that admit that they could go wrong and produce most of the
could be consumed by the crew or jettisoned without other criterial pollutants except possibly NOx and sulfur
degrading the environment. Their relatively low op- oxides.
erating temperatures and lots of careful development Sulfur oxides aren’t considered because the sulfur
produced a highly reliable electricity generator. When would poison most of the electrolytes used in fuel cells
used in earthbound applications the direct current pro- to stop their operation in short order. Fuel cell applica-
duced has to be inverted to alternating current. They’re tions require special fuel pretreatment to remove any
used principally in plants where highly reliable backup sulfur. It’s also highly probable that a fuel cell will re-
electric power is required. The important thing to note quire good air filters and some air pretreatment to limit
is that they are designed for, and work well with, pure the effects of the normal allotment of particulate and
hydrogen. nasty gases that can be in the air around industrial sites.
Since there are no hydrogen pipelines or storage How much air cleaning and fuel preparation will be de-
tanks out there a conventional hydrogen—oxygen fuel termined to extend the operating life of the fuel cell as
cell is not the sort of thing that someone is ready to in- we gain experience with them.
vest in. There’s considerable hype around the develop- As I understand it right now a fuel cell has to be
ment of fuel cells for automobiles as clean burning and dismantled and rebuilt on at least a five-year schedule.
that may result in some domestic supply of hydrogen It’s the sort of operation the manufacturer insists on do-
but not enough to power any large systems. The typical ing, probably to retain secrecy regarding their methods
earth based fuel cell installations currently burn a com- of construction and other details. The schedule may be
mon hydrocarbon fuel such as natural gas with some based on experience with the degradation of the electro-
modification. lyte from the problems we’re already aware of, contami-
The modification of a fuel cell to burn hydrocarbons nated fuel, particulate and stray gases in the air, etc. so
incorporates a ‘reformer’ which modifies the fuel to pro- programs of life extension based on chemical treatment
duce pure hydrogen. As I understand the cryptic descrip- or reconditioning of the electrolyte may be able to ex-
tions available, the reformer produces heat, generating tend that operating period in the future.
steam. The steam is then exposed to the fuel in a catalyst The actual operating temperature of a fuel cell
where the hydrogen in the water is released as the carbon depends on which electrolyte is used and can vary from
combines with the oxygen to form carbon dioxide. That very low (about 350°F) to high (about 1200°F) so the
way some of the energy produced by the carbon is used temperature of the exhaust can vary considerably and
to create more hydrogen. The source of the oxygen is air. the possible uses of the exhaust heat will vary as well. If
That means that the exhaust of a fuel cell contains carbon fuel cells reach the potential that many people try to give
dioxide and water just like a normal boiler. them the exhaust will only be good for heating service
So, when you see those articles and advertisements water.
that say a fuel cell produces less carbon dioxide than a Currently fuel cell applications are limited to sites
boiler you should treat everything the author says as where reliable electricity supply is all important and, by
a potential lie because there’s no alternative. Any hy- installing several fuel cells, an owner can be reasonably
drocarbon has to produce carbon dioxide and water; to certain the power will never be interrupted. You may
claim it doesn’t is blowing smoke. The only alternatives be called on to monitor fuel cell operation and, once
are to produce carbon monoxide, something we don’t again, the important thing to do is read that instruction
want to do, or leave pure carbon. A fuel cell suppos- manual.