Hydrogen is the energy market’s new player on the block, attracting attention for its exciting potential, variable supply sources and cleanliness as a fuel. Hydrogen has incredible energy density by weight at 120MJ/kg, with the potential to revolutionise energy storage and supply to turn one of the key global polluters into an emissions-free industry.
The
various “types” of hydrogen, classed according to their production method –
green, blue, brown and gray – all present different potential upsides and
downsides to the energy market, but in the end the produced H2 gas is
still a product that needs to be transported.
As a gas, hydrogen can be compressed to improve the efficiency of
transport. Compressed hydrogen can be transported in several ways; in gas
cylinders at 200-500bar and atmospheric temperature, as compressed cryogenic
gas cooled to -196°C and pressurised, or fully liquid.
Compressed gas transportation is used at
the smaller scale, and truckloads of compressed hydrogen cylinders, known as
tube trailers, are typically limited to 250bar, transporting up to 900kg of H2
per trailer. A liquefied hydrogen road tanker can contain up to 49,200L, or
nearly 3.5t, of hydrogen, but presents larger losses as boil-off gas.
Pipeline transport is expected to become a
widespread transport method, as gaseous hydrogen can be put into pipelines and
pumped using turbines and compressors in much the same way as natural gas.
Unfortunately for gas investors, hydrogen gas is incompatible with current
natural gas pipeline design, even leaving aside pressure and density design considerations.
Hydrogen causes significant embrittlement
in steel as it penetrates the metal’s crystalline structure, leading to
increased risk of cracking, and can reduce the metal’s strength by a factor of
ten. This embrittlement causes significant leak risk in pipelines and means
that current natural gas pipelines would be unsuitable for hydrogen without a
significant retrofit to protect the containers. New pipeline investment or
significant investment into refits would be needed.
One prospective use of hydrogen is
blending up to 15% H2 into a natural gas pipeline to reduce the carbon
footprint of the natural gas. In this case, pipelines may only require very
minimal modifications to their structures to reduce risk.
The natural gas strategy of liquefaction is
a strong contender for the transportation of hydrogen. Hydrogen’s compression
factor is even greater than that of methane at 1/800th volume, compared to
natural gas’s 1/600th. Hydrogen is, however, more energy intensive to liquefy –
while natural gas liquefies at -163°C, hydrogen gas needs to be cooled all the
way down to -253°C, or most of the way down to absolute zero.
While the strategy is still viable, the
cooling process is significantly more energy-intensive. LNG plants already often lose up to 15%
of their gas creating the energy needed to chill the product gas, making a
potential hydrogen chilling process expensive and potentially uneconomic.

Australia recently shipped the world’s
first liquid hydrogen cargo on the Suiso Frontier out of the Port of Hastings.
The 1,250m3, 88.7t cargo will be delivered to Kobe, Japan, after roughly two
weeks at sea. As a maiden voyage, the ship is roughly the same size (116m) as the
1959 maiden LNG voyager, the Methane Pioneer (103m), but it has only a quarter
of the Pioneer’s 5,000m3 storage.
Much of this discrepancy can be
attributed to superior design techniques, safety guidelines and restrictions
implemented since the late 1950s, such as double shell structure and vacuum
insulation, but concern remains that the hydrogen shipping process will be less
space-efficient in design
than that of the LNG industry.
Unfortunately, hydrogen’s unrivalled
energy density by weight may fall prey to its poor energy density by volume.
Even at liquefaction point -253°C, hydrogen only reaches an energy density of 2350
kWh/m3. Its closest substitute, ammonia, has energy density of 3730kWh/m3, 59%
higher, even without including the energy required to keep both substances
liquid.
Conversion to ammonia for transportation via the Haber Bosch process is likely to become
a preferred strategy, but the conversion from hydrogen to ammonia and back to
hydrogen may reduce efficiency to an unacceptable level.
The most efficient commercial Haber Bosch
plants currently operate at roughly 50% energy efficiency – a ton of ammonia
contains roughly 5MWh of energy as hydrogen, but takes 10MWh of energy to
produce. Estimates suggest ammonia will have the highest efficiency at around
34-37%, followed by liquid hydrogen at 30-33%, and trailed by methylcyclohexane
(MCH) at around 25% energy efficiency.
Ammonia is a well-studied, long understood
chemical widely used as a key fertiliser ingredient, so if efficiencies can be
maintained, this may be a straightforward and popular conversion process. Ammonia’s
boiling point is a mere -33.5°C, making it a much more transportable material.
Once on site, ammonia can be
catalytically decomposed to reform hydrogen, or else used as a fuel directly at
the cost of contamination with nitrous oxides. The conversion of ammonia back to hydrogen is an
endothermic (cooling) reaction with a variety of potential catalysts. The
theoretical efficiency for the catalytic cracking reaction is roughly 85% of
the energy, lower heating value (LHV), of the released hydrogen.
Anhydrous ammonia is usually transported as
a pressurised liquefied gas, via railways in tank cars, via road in tanker
trucks and via pipelines in populated areas. It is a well understood chemical,
but presents a risk in its transportable form. The chemical is liquefied either
by chilling to -34°C or by being pressurised to approximately 870kPa at room temperature, usually a mixture
of the two methods.
Pipelines have been regularly used to
transport ammonia and are typically pressurised to 1720kPa (~17 atmospheres). While
liquid ammonia is non-corrosive, making internal corrosion is a minimal issue,
it can cause stress corrosion
cracking, usually when contaminated with air, and proven steps need to be taken
to protect the pipelines.
Ultimately, hydrogen represents a significant
step in revolutionising the energy industry as a way to solve the largest
problem with renewable power, that of transmission and storage. Now that the
seal has been broken on transporting hydrogen fuel, a variety of potential
products and methods lie before the industry, and selecting which is the best fit
for purpose will be the next challenge.
