Decarbonisation Technology - May 2023 Issue

Hydrogen property

Gaseous (compressed) hydrogen

Density

Release rate

Being one-eighth of the density of methane, in equivalent conditions the volumetric flow rate of hydrogen is 2.8 times that of methane; conversely, the mass flow of methane is 2.8 times that of hydrogen. Isolated hydrogen pressure systems will depressurise faster than for methane, but larger flammable clouds may result. The higher energy density per unit mass of hydrogen means the energy flow (like for like) is similar. Hydrogen is more buoyant than methane and will have a strong tendency to move upwards, an aspect that can be used to minimise the potential for hazardous concentrations to develop. The minimum spark energy required to ignite a hydrogen-air mixture is less than a tenth of that required for methane or natural gas. However, this does not significantly increase the chance of ignition. Testing by DNV has shown that many potential ignition sources either ignite both hydrogen and natural gas mixtures or neither. Only a small proportion will ignite hydrogen but not natural gas. Additionally, equipment approved for use in hydrogen systems is readily available. Concentrations of hydrogen in air between 4% and 75% are flammable, which is a much wider range than for natural gas (5-15%). This will increase the likelihood of ignition. Released compressed hydrogen gas will burn as a jet fire. Flame lengths correlate well with the energy flow rate, and as this is similar for hydrogen and methane, in like-for-like conditions, the jet fire hazards are similar. The explosion potential for hydrogen is much greater compared to methane as at higher concentrations in air (>20%), the speed of the flame is much more than for methane. In addition, hydrogen-air mixtures can undergo transition to detonation in realistic conditions, which would not occur with methane.

Dispersion and gas

build-up

Ignitability

Ignition energy

Flammability

Combustion

Fire

Explosion

Liquid hydrogen (additional to compressed gas hazards) Temperature Liquefaction Liquid hydrogen (additional to compressed gas hazards)

In many ways, liquid hydrogen is a cryogenic liquid like liquefied natural gas (LNG). But due to the lower temperature, spillages can liquefy and solidify air from the atmosphere. The resulting mix of liquid hydrogen and liquid/solid air has exploded in small-scale field experiments. This does not occur with LNG.

Density

Buoyancy and dispersion As liquid hydrogen vapourises and mixes with air, it cools the air, increasing its density. Consequently, a hydrogen-air cloud produced from a liquid hydrogen release will not be as strongly buoyant as in a gaseous hydrogen case. This also occurs with LNG, but in this case the LNG-air mixture will be denser than air.

Table 1 Comparison of hydrogen and natural gas/methane properties and hazardous outcome

Safety represents a significant business risk to investors and developers. There have already been examples where incidents at hydrogen refuelling stations have halted hydrogen use in vehicles for significant periods.

The industry has tried-and-tested methods for managing the safety of flammable gases that have been used for decades and these come with some very important, hard-won lessons:  Safety must be based on an understanding of