Background
The fast-developing energy transition, with a target of net-zero greenhouse gas emissions, will include a significant expansion in the use of hydrogen. The roles for hydrogen being considered include energy transportation and storage, land transport, maritime propulsion, domestic heating and ‘hard to de-carbonise’ industry. Even if this is only partially realised, there will be a considerable growth in hydrogen production facilities.
Hydrogen can be stored in gaseous and liquid form. In gaseous form, due to its low density, it tends to be stored at high pressure, often measured in 100s of bar, though the pressure range will depend on the application. As with hydrocarbon fuels, there is the potential for jet fires to occur on hydrogen facilities and there will undoubtedly be the need to protect critical structures and equipment using PFP.
DNV and ThorntonTomasetti have developed a technical memo considering hydrogen and PFP for PFPNet. In the memo they compare hydrogen jet fires with those produced by the release of high-pressure methane or natural gas.
Observations
By referencing experimental studies and jet fire modelling the team has shown that, for like-for-like releases (same pressure and hole size), the external flame characteristics (flame length, thermal radiation) are not significantly different, with hydrogen flames being about 10% shorter than like-for-like methane/natural gas flames.
They also identify that a transient release from isolated, like-for-like, inventories of hydrogen and methane/natural gas, produces a resulting hydrogen jet fire that will have a duration of about one third of that of a methane/natural gas jet fire due to the higher volumetric outflow in the case of hydrogen.
The understanding of the internal characteristics of the flame are less certain. Hydrogen jet fire flame temperatures are greater than those in methane/natural gas jet fires. Modelling suggests the temperature difference is at least 150°C, but it could be more. The higher temperature suggests that the convective thermal flux to an object in the flame may be higher in the case of hydrogen compared to methane/natural gas, but there is no confirmatory experimental evidence available at this time.
Modelling also indicates that the gas velocities and densities in the bulk of the flame are not appreciably different between hydrogen and methane/natural gas, suggesting that erosive forces will be similar.
Why is this important?
Understanding these properties, particularly flame temperature and convective and total heat flux is important. The understanding will enable industry to assess whether this affects the performance of certain PFP materials and whether the current ISO22899-1 and HHF tests, developed for fires generated by ignited methane/natural gas releases, are sufficient to indicate likely PFP performance when subjected to hydrogen jet fires .
And alongside this there is a need to define representative hydrogen jet fire scenarios for the various hydrogen applications to understand both the likely size and duration of such fires. It is highly likely that in some instances, the ignited hydrogen releases will have characteristics that mean that PFP is not required for mitigation.
With hydrogen of increasing interest to the membership of PFPNet, a working group is being established by PFPNet to consider the optimal route to obtaining the required data and incorporating this into recommendations regarding testing and PFP specification for hydrogen facilities.