February 2022
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.