HYDROGEN:
THE FUEL OF THE FUTURE
IN CONJUNCTION WITH FUEL CELL SYSTEMS,
HYDROGEN HAS THE POTENTIAL TO TRANSFORM
THE WORLD'S ENERGY LANDSCAPE.
Hydrogen offers tremendous advantages as a clean energy
carrier. In the coming decades, hydrogen and fuel cells
will assume a greater role in meeting the world's energy
needs. Hydrogen, like electricity, is
an energy carrier, and when derived
from renewable energy sources,
hydrogen has the potential to provide
an inexhaustible supply of energy to fuel
our cars, homes and industry without
generating pollution or greenhouse
gases of any kind. It can also be
economical and have a relatively
high margin of safety when properly
produced, stored and dispensed.
PRODUCTION AND USE OF HYDROGEN
Hydrogen is an important chemical commodity. It has been
mass-produced for more than 50 years, and in the U.S.
alone, more than eight million tons are produced annually,
mostly by steam reforming of natural gas. NASA was the
first to use fuel cells powered by hydrogen as a dependable
source of both electric power and potable water on manned
missions, and today, hydrogen is routinely transported
safely both as a cryogenic liquid and as a compressed gas
by rail, barge, truck and pipeline for use in the aerospace,
food, petrochemical and semiconductor industries. These
industries have an excellent safety record with hydrogen
because they understand the risks and how to manage them.
Hydrogen is most commonly used as a gas, and is generally
piped directly from production to consumption. When hydrogen
is stored, it is stored in either a compressed state under
high pressure (typically 2,400 pounds
per square inch in cylinders or tube
trailers) or as a cryogenic liquid at
temperatures less than minus 253
degrees Celsius (20 degrees above
absolute zero).
HYDROGEN PROPERTIES AND CHARACTERISTICS
· Hydrogen is an energy carrier like electricity or gasoline,
not an energy source. The distinction is that as an energy
carrier, hydrogen is a way of transporting useful energy to
users. Energy sources like oil and natural gas are found in
nature as developable resources.
· Hydrogen is the lightest of all elements. With a specific
gravity of 0.0696, about 1/14th that of air, it has a very
high diffusion rate. This causes it to be buoyant and to
rapidly disperse when released in air-two great safety
assets in open environments, because a leak or release is
quickly diluted and rendered harmless. Hydrogen diffuses
four times faster than methane, and ten times faster than
gasoline vapors.
· Hydrogen is colorless, odorless and has no taste.
· Hydrogen burns with a pale blue flame that is virtually
invisible in daylight.
· A hydrogen fire radiates very little heat compared to a
petroleum fire. Injuries rarely occur unless someone actually
comes in contact with the flame.
· Hydrogen has a wide flammability range: 4% to 75% by
volume in air (the range for gasoline is 1.4% to 7.4% by
volume in air). Actual limits vary with pressure, temperature,
ignition energy and water vapor content.
· For a flammable mixture to exist, a three times higher
concentration is required than with gasoline (1.3% versus
4%), yet hydrogen dissipates ten times faster than gasoline
vapor. Similar comparisons are true of methane and propane
versus hydrogen.
For a leak from a pipe or vessel
containing a 100% concentration
of a fuel gas under pressure, the
plume of escaping gas will only
be at 100% at its origin and at
progressively lower concentrations
as it mixes with entrained air. From
a practical standpoint, the risk is
characterized by the volume of
the plume measured at the lower
flammability limit (LFL). The upper
flammability limit is of little consequence.
In fact, hydrogen's lower
flammability limit of 4% and
propensity to dissipate rapidly will
tend to reduce the risk relative to
other fuels.
· Hydrogen has a low ignition energy-as little as 0.017
millijoules at 30% concentration, in contrast to 0.25
millijoules for other hydrocarbon fuels. However, at their
lower flammability limits, methane and hydrogen have
very similar ignition energies of about 10 millijoules.
The wide range of flammability of hydrogen-air mixtures compared to
other
combustibles is in principle a disadvantage with respect to potential
risks.
A hydrogen vapor cloud could potentially have a greater volume within
the
flammable range than a methane cloud formed under similar release
conditions. On the other hand, there are only minor differences between
the
lower flammability limits (LFLs) of hydrogen and methane, and those of
propane and gasoline are even lower. In many accidental situations, the
LFL
is of particular importance as ignition sources of sufficient energy
are often
present to ignite a fuel-air mixture once a flammable concentration has
been
reached. In some circumstances (eg. Low momentum releases) the
dispersion
characteristics of hydrogen may make it less likely that a flammable
mixture
will form than for the other fuels. In addition the 4 vol.% LFL for
hydrogen
only applies to upward propagating flames. For downward propagating
flames,
experiments have shown that between 9 and 10 vol.% hydrogen is
required.
For methane, the difference between LFLs for upward and downward
propagating flames is less-5.3 versus 5.6 vol.%.
In practical release situations, the
lower ignition energy of hydrogen
may not be as significant a
differentiation between the fuels
as it first seems. The minimum
ignition energy tends to be for
mixtures at around stoichiometric
composition (29 vol.% for hydrogen).
At the LFL, the ignition energy
for hydrogen is similar to that
of methane. In addition, many
so-called weak ignition sources,
such as electrical equipment
sparks, electrostatic sparks and
sparks from striking objects involve
more energy than is required to
ignite methane, propane and other
fuels. A weak electrostatic spark
from the human body releases
about 10 millijoules.1
· Hydrogen has a small molecular size, allowing it to leak
more easily through porous materials, cracks or bad joints
then other common gases at equivalent pressures.
· Hydrogen is non-toxic and non-poisonous. In contrast, the
combustion products of diesel and other conventional
fuels, along with methanol, are toxic if inhaled.
· As with any gas, asphyxiation is possible if hydrogen
displaces oxygen.
· There are few significant environmental hazards associated
with the accidental discharge of hydrogen. In contrast,
spills or leaks of gasoline or diesel fuel can poison water
supplies or pose a serious risk of fire or explosion if
concentrated in a waterway, drainage system or sewer
system. Estimates of remediation of ground and surface
water contamination resulting from oil and gas leakage
are in the billions of dollars annually.
SETTING THE RECORD STRAIGHT
· Today, most people take for granted their use of
hydrocarbon fuels such as gasoline, propane and natural
gas-yet an 1875 report by the U.S. Congressional
Horseless Carriage Committee warned that stores of
gasoline hurtling down the street at 15 miles per hour
would constitute a fire and explosive hazard of the first
rank.2 Tens of millions of people now pump gasoline into
their cars each day.
· Natural gas and propane are commonly piped into homes
for use in appliances such as water heaters, stoves, and
furnaces. The risks associated with the use of these fuels
are accepted because systems that use them achieve
sufficient levels of safety.
· Hydrogen as a fuel is a new idea for most people, but its
use dates back to the 19th century. In the United States,
"town gas" was used to light city streets and was piped
into homes to fuel lamps, cooking stoves and gas heaters.
Town gas was produced from coal and contained about
50% hydrogen along with methane, carbon dioxide and
carbon monoxide. Town gas is still used safely today in
parts of Japan, China and other Asian countries.
Many people unfamiliar with hydrogen believe that it is
dangerous due to its common association with the
Hindenburg incident. In fact, the attributes of hydrogen
actually helped save 62 lives in the 1937 disaster. An
investigation by NASA scientist Dr. Addison Bain found that
the 35% who died in the accident were killed by jumping
Flammability Limits
Properties of Hydrogen
Property Hydrogen Methane Propane Gasoline
Lower Flammability Limit (%) 4 5.3 1.7 1.3
Lower Detonation Limit (%) 18.3 6.3 3.1 1.1
Upper Detonation Limit (%) 59 13.5 9.2 3.3
Upper Flammability Limit (%) 75 17 10.9 6.0
Auto Ignition Temperature 585 C 537 C 450 C 215 C
Minimum Ignition Energy 0.017 mL 0.274 mJ 0.240 mJ 0.240 mJ
out of the airship or by the burning diesel oil, canopy and
debris. The hydrogen-fueled flames swirled harmlessly
overhead. It is highly likely that no one on board was killed
by a hydrogen fire, and that the incident would have
occurred even if the airship had utilized non-flammable
helium instead of hydrogen. The root cause was determined
to be local thunderstorm activity which ignited the fabric
skin-which was made of cotton treated with aluminized
cellulose acetate butyrate dopant, a component of rocket fuel.3
GENERAL SAFETY CONSIDERATIONS
All fuels are energetic materials and pose fire and explosion
risks, so they must be treated with care. A potentially
flammable mixture is formed whenever hydrogen is exposed
to air in a closed environment. As with other gaseous fuels,
if burning hydrogen gas is confined, the resulting pressure
and temperature rise can trigger an explosion. A 1997
Directed Technologies Inc. Hydrogen Vehicle Safety Report
concluded, "Hydrogen has a greater tendency to detonate
than gasoline, but this probably is so small in unconfined
spaces that it can be ignored." 4
When storing and dispensing fuels, there is an increased
chance of a leak resulting from a structural failure to a tank
or connector. Systems storing liquid hydrogen and gaseous
hydrogen at high pressure are robustly designed to handle
the pressure and temperature extremes without leaking.
Because hydrogen is colorless and odorless, it is more
difficult to detect than other fuels. NASA and industry
employ sensors to detect leakage in hydrogen containment
systems, and both have excellent safety records.
Well-engineered hydrogen powered vehicles and associated
hydrogen dispensing systems can be expected to be at least
as safe as gasoline, natural gas and propane vehicle
systems. In some cases, such as a collision in an open
space, a hydrogen-powered vehicle poses less of a hazard
than a traditional gasoline-powered vehicle because of the
reduced risk of a flammable fuel leak.
COMPARATIVE SAFETY OF HYDROGEN AND OTHER FUELS
Dr. Michael Swain, a researcher from the University of
Miami, has been studying the comparative safety of hydrogen
and other fuels for more than a decade. Dr. Swain found
that one characteristic of burning hydrogen that gives it a
safety advantage over hydrocarbon fuels is its relatively low
heat radiation. When hydrocarbon fuels such as gasoline and
diesel burn in air, they produce high temperature carbon
soot that radiates heat. Because hydrogen produces virtually
no soot during combustion, this greatly reduces the radiant
energy from the hydrogen flame.
In the tank leak test performed in 2001, Dr. Swain
simulated two car fires, one created by a 1/16th inch
puncture in a gasoline fuel line, and the other by a leaking
hydrogen connector. He videotaped the experiment to
document what would happen if the leaks ignited. As the
photo below, taken at one minute after ignition, illustrates,
the effects of the gasoline fire
were much more severe. While the
gasoline-fed fire eventually consumed
the second test vehicle, leaving it a
smoldering heap of charred steel and
melted glass, the hydrogen fire was
over in less than two minutes and
left the hydrogen-tank equipped test
car virtually undamaged. Temperature
sensors placed inside the car and
around the outside barely registered
above normal even when the high
pressure flame was no more than a
foot away.5
While every situation is different, experiments suggest that
when used in properly designed systems, hydrogen is not
more dangerous than fuels like gasoline, propane and
natural gas, and in some ways, hydrogen may be safer than
the hydrocarbon fuels used for nearly a century. This was
the conclusion of a 1997 Ford study sponsored by the DOE
as well as a 2002 Norwegian study.6
ADDITIONAL RESOURCES
For more information, visit http://www.hydrogenus.com ,
http://www.eere.energy.gov , and http://www.rmi.org .
1 Compilation of Existing Safety Data on Hydrogen and Comparative
Fuels,
July 2001.
2 The summary of the Report of the Congressional Horseless Carriage
Committee, 1875.
3 http://www.dwv-info.de/pm/hindbg/hbe.pdf .
4 DOE/CE/50389-502 May 97.
5 http://www.eere.energy.gov/hydrogenandfuelcells/pdfs/30535be.pdf .
6 Directed Technologies and http://www.bellona.no/en .
All other information from "Companion Guide to Hydrogen: The Matter
of
Safety," Hydrogen 2000 and Plug Power Inc.
Tank Leak Test performed
by Dr. Michael Swain,
University of Miami, 2001.
Hydrogen-powered vehicle
on left, Gasoline-powered
vehicle on right.
Time: 1 minuite, 0 seconds
after ignition.
HEADQUARTERS
968 Albany-Shaker Road
Latham, New York 12110
Phone: (518) 782.7700
Fax: (518) 782.9060
WASHINGTON, D.C.
499 South Capitol Street, SW
Suite 606
Washington, D.C. 20003
Phone: (202) 484.5300
Fax: (202) 554.2896
EUROPE
P.O. Box 880
7301 BC Apeldoorn
The Netherlands
Phone: 31 55 53 81 000
Fax: 31 55 53 81 099
www.plugpower.com
A proud member of:
U.S. Fuel Cell Council
National Hydrogen Association
Business Council for Sustainable Energy
Sustainable Energy Coalition
CPRINTED ON RECYCLED PAPER
XML site map