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07/01/2004
Hydrogen Not Guilty
A four-letter word beginning with “f”, uttered by a high level
member of the administration, garnered considerable attention
this past week. Last week I wrote about fugu. There’s another
four-letter f-word that concerns many more people – fuel. I
understand from my golfing buddy Bob that on Cape Cod the
price of gasoline ranged from $2.13 to $2.33 a gallon. Rising gas
prices invariably spur talk about alternate energy sources. Last
year President Bush announced a nearly $2 billion program to
develop fuel cell-powered vehicles.
In a fuel cell, hydrogen is fed to one electrode while oxygen (in
our air) is fed to the other electrode. The hydrogen and oxygen
react to form water – no carbon dioxide, no carbon monoxide or
other pollutants to foul our air and contribute to global warming.
The vision of fuel cell-powered vehicles leads to the concept of a
“hydrogen economy”, in which you drive up to your former
“gas” station, fill up with hydrogen and be on your way.
I had planned to write about whether or not this utopian view of a
hydrogen economy is realistic. However, in researching the
subject, I was led to the Department of Energy’s Web site, which
has a plethora of information on hydrogen and fuel cells. One
concern you might have is the safety of driving around with a
tank of hydrogen in your vehicle. I worked with hydrogen on
occasion at Bell Labs and admit to never having been totally
comfortable with it. I imagine we’ve all seen those horrifying
film clips of the hydrogen-filled Hindenburg dirigible going up
in flames, right here in New Jersey. The Department of Energy’s
Web site addresses the safety issue and has a link to the
Deutscher Wasserstoff Verband Web site and an article by
Addison Bain of NASA and Ulrich Schmidtchen of the Federal
Institute for Materials research and Testing in Berlin.
Their article “Afterglow of a Myth. Why and how the
‘Hindenburg’ burnt” puts forth a convincing case that, contrary
to the prevailing view, the dramatic demise of the Hindenburg
was not due to the hydrogen. Furthermore, they claim that the
damage and loss of life would have been virtually the same had
the Hindenburg been filled with helium.
The Hindenburg was a humongous airship, measuring 804 feet in
length and 135 feet in its maximum diameter. It was built at the
Zeppelin works in Germany following the success of its
predecessor, the LZ 127 Graf Zeppelin. The LZ 127 was buoyed
up by hydrogen but, following a fatal accident involving a
hydrogen-filled British airship, the decision was made to make a
new airship, the LZ 129 in which both helium and hydrogen
would be used. The plan was to vent the lighter hydrogen gas
when the crew wanted to make the airship less buoyant, as when
landing. However, the U. S. was the only country making
helium and a law prohibited its export. As a result, the LZ 129,
later dubbed the “Hindenburg”, ended up using only hydrogen.
The Hindenburg was completed in 1936 and by May of 1937 had
logged over 60 trips and had carried over 2800 passengers. Her
longest trip was a 7,000-mile voyage from Frankfurt am Main to
Rio de Janeiro. The accommodations for the 50-72 passengers
were “small and expedient” but served by five cooks and nine
stewards. A physician was on board and there was even a piano,
built of aluminum to conserve weight! In spite of the hydrogen
close by, there was a smoking room, watched over at all times by
a crew member.
On May 3,1937, the LZ 129 took off from Frankfurt headed for
Lakehurst, New Jersey carrying 61 crew members, 36
passengers, a couple of dogs and assorted mail and luggage. It
arrived over New York at about 2 PM on May 6 and had to fly
up and down the coast until a line of squalls and thunderstorms
cleared the Lakehurst area. At about 6 PM it arrived over
Lakehurst. In Germany, it would drive itself down to ground
level before being secured. However, the U. S. Navy specialized
in another approach and the LZ 129 dropped to 200 feet above
the ground, at which point the crew dropped landing ropes so the
craft could be pulled down and secured.
A few minutes after dropping the ropes, a fire started on the back
of the LZ 129 close to the fin near the top of the airship and
spread rapidly. Significantly the fire burned downwards as well.
The cells containing the hydrogen burst, the venting hydrogen
thrusting the ship forward and causing two water containers to
drop. This resulted in the airship becoming tail-heavy, tilting the
ship tail down. Until that point, the LZ 129 remained level.
With the tail down, the fire spread rapidly upwards and those in
the front of the ship were in greater danger. In only about half a
minute, the ship hit the ground tail first.
The Hindenburg and Navy crews worked valiantly to rescue
people from the fire but the death toll included 22 crew
members, 13 passengers and 1 ground crew member. Although
the event was popularly described as an explosion at the time,
Bain and Schmidtchen emphasize that it was not an explosion
but a fire. The hydrogen burned in less than a minute, while the
diesel fuel used to power the craft’s engines burned for hours.
The official consensus that hydrogen was the cause of the
disaster, most likely through a hydrogen leak of some kind,
prevailed for the rest of the 20th century. Since there was no
explosion, the hydrogen burned. The light given off when
hydrogen burns is a faint bluish light, barely visible in daylight.
However, the fire was a bright fire, described as like a bonfire.
The fire also burned downwards at the beginning. Hydrogen
being lighter than air, we would expect the fire to burn upwards.
How do Bain and Schmidtchen explain the fire? Remember that,
prior to landing, the LZ 129 was killing time in the stormy
weather around Lakehurst. In that storm, the airship would
become “charged” to the same electrical potential as the
surrounding atmosphere. (It may be a stretch but think of it this
way. Sitting at my desk, I’m at the same potential as my
surroundings. But suppose someone scuffs his feet across the
carpeting and builds up a charge. When he touches me there’s a
spark. Static electricity.) Now the ship comes in and the ropes,
wet from the rain, are tossed to the ground. The wet ropes are
conductors and charge is exchanged between the airship and the
ground. The airship is grounded.
Or is it? The frame of the ship to which the ropes were attached
would be grounded but not necessarily the outer cover, or
shroud, of the ship. The LZ 127 Graf Zeppelin had flown
through thunderstorms and never had a problem. But there was
difference. A new paint formulation had been used in the
Hindenburg. The new paint was a poor electrical conductor,
which would make it more difficult to even out the charge
differences. Even more significant, the paint on the outer cover
of the LZ 129 was flammable, its composition resembling that of
the solid fuel used to boost the Space Shuttle off the ground!
The paint consisted of a layer containing iron oxide, then layers
of a mixture of a compound known as cellulose butyrate acetate
containing aluminum powder.
Put aluminum powder, iron and oxygen together under the right
circumstances and you can get a pretty violent reaction going.
The proposal is that the fire started when a difference in potential
caused a discharge (think of that static electricity spark) either
between different parts of the shroud at different potentials or
between the shroud and the frame through the dry cords
connecting them. In the latter case the cords could have caught
fire. Also, high voltages typically arise at protruding parts of an
object and indeed the fire arose near the fins of the LZ 129.
Remember that this accident occurred in the 1930s when Hitler
was in power. The German reports and investigations of the
incident were incomplete and seemed to be slanted to avoid any
implications of errors in design. This was the Third Reich.
Searches through old records revealed that experiments were
done in the lab trying to mimic the disaster conditions. Max
Dieckmann got original samples of the outer covers of both the
Hindenburg and the Graf Zeppelin. He set up experiments with
wet cloth and electrodes in hydrogen-air mixtures. Official
reports cited his experiments with the Hindenburg cloths in
which the hydrogen-air mixtures ignited.
But Bain and Schmidtchen found that Dieckmann had not found
ignition of the hydrogen mixtures when the experiments were
done with the Graf Zeppelin cover material. This fact was not
recorded in Dieckmann’s notes but in a note by a translator of a
Dieckmann paper into English. The translator apparently learned
this from someone who had visited Dieckmann during the
experiments.
Bain dug into the archives of museums, especially the Zeppelin
Museum at Friedrichshafen in Germany. The archives revealed
the smoking gun. Letters written by Otto Beyersdorff, an
engineer involved in the investigation, attributed the actual cause
of the fire to “the extremely easy inflammability of the covering
material brought about by the discharges of an electrical nature.”
Furthermore, after the accident engineers working on the LZ 130,
then under construction, made a number of changes. The
composition of the paint was changed with the addition of a
flame retardant compound and the aluminum was replaced with
bronze powder. In addition, graphite was incorporated in the
ropes connecting the shroud to the frame, making the ropes more
conductive.
Now that hydrogen has been proved not guilty, we can return to
fuel cells next week.
Allen F. Bortrum
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