Hydrogen fuel is a zero-emission fuel burned
with oxygen. It can be used in fuel cells or internal combustion engines. It
has begun to be used in commercial fuel cell vehicles such as passenger cars,
and has been used in fuel cell buses for many years. It is also used as a fuel
for spacecraft propulsion.
]As of 2018,
the majority of hydrogen (∼95%) is produced from fossil
fuels by steam reforming or partial oxidation of methane and coal gasification
with only a small quantity by other routes such as biomass gasification or electrolysis
of water.
Hydrogen is
found in the first group and first period in the periodic table, i.e. it is the
first element on the periodic table, making it the lightest element. Since the
density of hydrogen is less than air, it rises in the atmosphere and is
therefore rarely found in its pure form, H2. In a flame of pure
hydrogen gas, burning in air, the hydrogen (H2) reacts with oxygen
(O2) to form water (H2O) and releases energy.
2H2
(g) + O2 (g) → 2H2O (g) + energy
If carried
out in atmospheric air instead of pure oxygen, as is usually the case, hydrogen
combustion may yield small amounts of nitrogen oxides, along with the water
vapor.
The energy
released enables hydrogen to act as a fuel. In an electrochemical cell, that
energy can be used with relatively high efficiency. If it is used simply for
heat, the usual thermodynamics limits on the thermal efficiency apply.
Hydrogen is
usually considered an energy carrier, like electricity, as it must be produced
from a primary energy source such as solar energy, biomass, electricity (e.g.
in the form of solar PV or via wind turbines), or hydrocarbons such as natural
gas or coal. Conventional hydrogen
production using natural gas induces significant environmental impacts; as with
the use of any hydrocarbon, carbon dioxide is emitted.
Production and Storage
Because pure hydrogen does not occur
naturally on Earth in large quantities, it usually requires a primary energy
input to produce on an industrial scale. Hydrogen fuel can be produced from
methane or by electrolysis of water. As of 2018, the majority of hydrogen (∼95%)
is produced from fossil fuels by steam reforming or partial oxidation of methane
and coal gasification with only a small quantity by other routes such as
biomass gasification or electrolysis of water.
Steam-methane reforming, the current
leading technology for producing hydrogen in large quantities, extracts
hydrogen from methane. However, this reaction releases fossil carbon dioxide
and carbon monoxide into the atmosphere which are greenhouse gases exogenous to
the natural carbon cycle, and thus contribute to climate change. In electrolysis,
electricity is run through water to separate the hydrogen and oxygen atoms.
This method can use wind, solar, geothermal, hydro, fossil fuels, biomass,
nuclear, and many other energy sources. Obtaining hydrogen from this process is
being studied as a viable way to produce it domestically at a low cost.
Hydrogen fuel is hazardous because of
the low ignition energy and high combustion energy of hydrogen, and because it
tends to leak easily from tanks. Explosions at hydrogen filling stations have
been reported. Hydrogen fueling stations generally receive deliveries of
hydrogen by truck from hydrogen suppliers. An interruption at a hydrogen supply
facility can shut down multiple hydrogen fueling stations.
Hydrogen as Energy
Hydrogen is locked up in enormous
quantities in water, hydrocarbons, and other organic matter. One of the
challenges of using hydrogen as a fuel comes from being able to efficiently
extract hydrogen from these compounds. Now, steam reforming, which combines
high-temperature steam with natural gas, accounts for the majority of the
hydrogen produced. This method of hydrogen production occurs at temperatures
between 700-1100°C, and has a resultant efficiency of between 60-75%. Hydrogen
can also be produced from water through electrolysis, which is less carbon
intensive if the electricity used to drive the reaction does not come from
fossil-fuel power plants but rather renewable or nuclear energy instead. The
efficiency of water electrolysis is between about 70-80%, with a goal set to
reach 82-86% efficiency by 2030 using proton exchange membrane (PEM)
electrolyzers. Once produced, hydrogen can be used in much the same way as
natural gas - it can be delivered to fuel cells to generate electricity and
heat, used in a combined cycle gas turbine to produce larger quantities of
centrally produced electricity or burned to run a combustion engine; all
methods producing no carbon or methane emissions. In each case hydrogen is
combined with oxygen to form water. The heat in a hydrogen flame is a radiant
emission from the newly formed water molecules. The water molecules are in an
excited state on initial formation and then transition to a ground state; the
transition releasing thermal radiation. When burning in air, the temperature is
roughly 2000 °C (the same as natural gas). Historically, carbon has been the
most practical carrier of energy, as hydrogen and carbon combined are more
volumetrically dense, although hydrogen itself has three times the energy
density per weight as methane or gasoline. Although hydrogen is the smallest
element and thus has a slightly higher propensity to leak from venerable
natural gas pipes such as those made from iron, leakage from plastic
(polyethylene PE100) pipes is expected to be very low at about 0.001%.
The reason steam methane reforming has
traditionally been favoured over electrolysis is because whereas methane
reforming directly uses natural gas, electrolysis requires electricity. As the
cost of producing electricity (via wind turbines and solar PV) falls below the
cost of natural gas, electrolysis becomes cheaper than SMR.
Uses of Hydrogen Fuel
Hydrogen fuel can provide motive power
for liquid-propellant rockets, cars, trains, boats and airplanes, portable fuel
cell applications or stationary fuel cell applications, which can power an
electric motor. The problems of using hydrogen fuel in cars arise from the fact
that hydrogen is difficult to store in either a high pressure tank or a
cryogenic tank.
Internal combustion engine conversions
to hydrogen
Combustion engines in commercial
vehicles have been converted to run on a hydrogen-diesel mix in the UK, where
up to 70% of emissions have been reduced during normal driving conditions. This
eliminates range anxiety as the vehicles can fill up on diesel. Minor
modifications are needed to the engines, as well as the addition of hydrogen
tanks at a compression of 350 bars. Trials are now underway to test the
efficiency of the 100% conversion of a Volvo FH16 heavy-duty truck to use only
hydrogen. The range is expected to be 300km/17kg; which means an efficiency
better than a standard diesel engine (where the embodied energy of 1 gallon of
gasoline is equal to 1 kilogram of hydrogen).
At a low cost price for hydrogen
(€5/kg), significant fuel savings could be made via such a conversion in Europe
or the UK. A lower price would be needed to compete with gasoline in the US, as
gasoline is not exposed to high taxes at the pump.
Fuel cells
Using a fuel cell to power an electric
motor is two to three times more efficient than using a combustion engine. This
means that much greater fuel economy is available using hydrogen in a fuel
cell.
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