This section provides recommendations for hydrogen injection into the existing gas grid in terms of materials, devices, and practices. It also sets out some key safety elements and provides recommendations on design aspects and other safety measures.
Risk-based Regulatory Design for the Safe Use of Hydrogen
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22. Domestic use: safety of hydrogen in buildings with focus on hydrogen use in cooking stoves and boilers
Abstract
Domestic use of hydrogen
The use of the existing infrastructure of natural gas pipelines to deliver hydrogen inside buildings will contribute to a faster energy transition. Several projects have investigated the effect of hydrogen blending on the infrastructure integrity and safety of existing pipelines, with the aim to eventually develop a 100% pure hydrogen network for heating and/or cooking purposes in houses and other residential and commercial buildings. Pilot and demonstrations projects worldwide (e.g. in the UK, Germany, France and China) have been launched, to assess the use of hydrogen up to certain composition in existing gas network. None of the pilots have reported any accidents.1
The infrastructure typically used for hydrogen use in residential areas consists of:2
local hydrogen production via electrolyser (pressure at 10 – 40 barg) and/or local hydrogen storage in tube trailers (typical pressure at 200 barg – 300 barg);
low distribution pressure systems, i.e. 4-8 bar, with a reduced to 100 mbar in pipework into houses for heating purposes;
fiscal metering in gas cabinets, tubing, and H2 heaters/boilers in domestic premises.
While this report focuses mainly on the safety concerns of the domestic use of hydrogen, it also provides recommendations regarding the use of the existing gas network for hydrogen blends and/or pure hydrogen.
Integrity issues of existing gas network
A primary concern with hydrogen in domestic use is the capability of the current gas distribution network to manage hydrogen/natural gas mixtures or pure hydrogen, as hydrogen can damage pipelines through embrittlement.3 Another concern is the efficiency and appropriateness of the existing appliances that use natural gas. However, this is beyond the scope of this report.
Recommendations for hydrogen injection into the existing gas grid
Materials
Use existing carbon steel transmission pipelines in medium to high pressure systems, as they can tolerate pressures between 55 to 210 bar and can withstand hydrogen concentrations up to 15% v/v without any significant impact (Capelle et al., 2008[1]), (Meng et al., 2017[2]) (Elaoud, Abdulhay and Hadj-Taie, 2014[3]), (Witkowski et al., 2018[4]).
Use plastic pipelines, which are commonly used in low-pressure systems, as they are generally unaffected by hydrogen injection up to 20 v/vand pose no danger in embrittlement. Generally speaking, up to 20% v/v blend of hydrogen with natural gas is still compatible with the existing infrastructure and heating appliances.
A phased transition to 100% polyethylene network is recommended, since most observed flammable gas leaks are caused by metallic network components. However, even with 100 % polyethylene pipelines for a 100 % hydrogen network additional mitigation measures should be implemented downstream of the gas meter to achieve fatality risk lower than the current network and as safe as the natural gas network.
Devices
The existing domestic pressure regulators can be safely used with hydrogen, and it is therefore unnecessary to replace the regulators as part of the conversion to hydrogen.
Practices
The seal tightness specifications in current pipelines should be stricter, ensuring that the maximum permissible leakage rate for hydrogen as 74% of that of natural gas.
Mechanical crimp fittings should be used in pipework instead of soldered joints, which are more prone to leakages. It can be considered safe to use the same materials and fittings for internal pipework for hydrogen as is currently used for methane, at least in the short term, in the context of a community trial.
Key safety/failure elements
A major concern when using hydrogen/natural gas blends or pure hydrogen either for heating or in cooking stoves in buildings relates to safety. Blending of hydrogen with natural gas up to 20 vol% concentration results in only a modest increase of developed overpressures and thus in a small increase in event severity of a leakage (a factor of about 1.2 greater overpressure for 20% hydrogen blend) compared to pure natural gas (Lowesmith et al., 2011[5]).
Experiments (Crewe, Johnson and Allason, 2020[6]) that assessed the potential for household electrical items (including white goods in new and used condition, plugs and switches, light fittings and extractor fans) to ignite hydrogen or methane mixtures with air showed that in 20 out of 43 tests, no ignition occurred with hydrogen. In two tests, ignition occurred with both hydrogen and methane. Very few domestic appliances caused hydrogen to ignite, but not methane. These included hair dryers, toasters, vacuum cleaners, tumble dryers and irons. Nearly all of these appliances can only be used with a human operator present, who would most likely be able to smell a gas release (provided that odorants are added as per the following recommendations, see below).
Based on relevant research even with pure hydrogen, for short-term, low-rate hydrogen releases inside properties flammable concentrations are unlikely to be formed even in scenarios with low air permeability rates.
Existing technical norms
Domestic use of hydrogen is still an application under development. Pilot and demonstration projects have been reported in several countries including Germany, France, the United Kingdom, the Netherlands and China, which inject hydrogen in the natural gas grid (up to 20% v/v) to supply houses in selected neighbourhoods for heating and/or cooking purposes.
As the domestic use of hydrogen is currently at the stage of piloting, there is no international standards that apply to this application and there is usually no or only limited regulation regarding the distribution and domestic use of hydrogen. However, China has published a group of standards for natural gas and hydrogen mixing stations and in Japan and South Korea the domestic use of hydrogen involves fuel cell systems, which are subject to regulations that apply to fuel cells in general.
Recommendations on key safety elements to ensure acceptable level of risk for domestic use of hydrogen
Design4
The gas metre should be installed outside of the property, where possible, and comply with current best practice and BS6400-1:2016.
Provide sufficient ventilation and venting in any cavity should be mandatory, as specified by Building Regulations (i.e. an exemption should not be granted for hydrogen appliances). Such mitigation measures can reduce the maximum concentration of hydrogen and the risk of explosion.
Fit wall vents (non-closable) at the upper part of the room (no more than 50 cm from the ceiling) in all rooms with gas appliances or installed hydrogen-carrying pipes.
Fit vents to all the cupboards and other compartments (e.g. boilers) where hydrogen appliances are present should have vents.
Simple vent geometry, like rectangular vent area, should be promoted.
High aspect ratio (height/length) of the vent is also recommended as it provides more efficient ventilation.
Open ventilation grids can reduce to half the maximum concentration and up to 2 vol% (half of the LFL of 4 vol% hydrogen in air) for rates typical of leak through the piping connected to the gas meter. At such low concentration, ignition is unlikely to take place.
The use of airbricks in basements can be helpful, but current research studies have not reached conclusive results.
Safety devices
Fit leak detection and alarm systems in the upper part of the rooms and inside cavities inside buildings, as hydrogen tends to accumulate in the ceiling and might be trapped inside cavities. The alarm should be activated as soon as hydrogen is detected at concentration above some fraction of the LFL.
Fit excess flow valves (EFV) to stop the flow of hydrogen in the service pipes when it reaches a certain level and emergency control valves (ECV) should be deployed to safely isolate a customer’s pipe from the network. The first EFV should be placed in the service pipe or immediately after the emergency control valve and the second one should be integrated in the hydrogen gas metre or added upstream of the metre.
Install flame failure devices (FFDs) to all hydrogen appliances.
Practices
Odorise hydrogen supply gas for the early detection of hydrogen gas leaks. Odorant NB, which is a blend of 78% t-butyl mercaptan and 22% dimethyl sulphide and THT have also been tested and are found to be effective and compatible with network components and hydrogen appliances.
Provide a stronger flexible pipe at the rear of cookers to limit the likelihood of damage when the cooker is moved. Additionally, the cooker should be fixed to the wall using a chain and Rawl bolts to limit the loading on the flexible cooker connection.
Controls
Inspection and maintenance in all equipment should be performed at a regular base by specialised personnel.
A more general recommendation for the domestic use of hydrogen is to aim at a smooth conversion of the system. In the short-term, a 20 vol% blend of hydrogen with natural gas for heating can be preferred, which would still be compatible with the existing infrastructure and household heating appliances and will not increase the risk (Khalil, 2019[7]).
References
[1] Capelle, J. et al. (2008), “Sensitivity of pipelines with steel API X52 to hydrogen embrittlement”, International Journal of Hydrogen Energy, Vol. 33/24, pp. 7630-7641.
[6] Crewe, R., M. Johnson and D. Allason (2020), Ignition Potential Testing with Hydrogen and Methane, DNV GL.
[3] Elaoud, S., B. Abdulhay and E. Hadj-Taie (2014), “Effect of hydrogen injection into natural gas on the mechanical strength of natural gas pipelines during transportation”, Arch. Mech., Vol. 66/4, pp. 269-286.
[7] Khalil, Y. (2019), Hydrogen Production Workshop.
[5] Lowesmith, B. et al. (2011), “Vented confined explosions involving methane/hydrogen mixtures”, Int. J. Hydrogen Energy, Vol. 36, pp. 2337–2343.
[2] Meng, B. et al. (2017), “Hydrogen effects on X80 pipeline steel in high-pressure natural gas/hydrogen mixtures”, International Journal of Hydrogen Energy, Vol. 42/11, pp. 7404-7412.
[4] Witkowski, A. et al. (2018), “Analysis of compression and transport of the methane/hydrogen mixture in existing natural gas pipelines”, International Journal of Pressure Vessels and Piping, Vol. 166, pp. 24-34.
Notes
← 1. In this OECD report, see Part III – Review of international experience with hydrogen pilot projects.
← 2. Shared data by the Dutch counterparts in mail communications.
← 3. Embrittlement is a significant decrease of ductility of a material, which makes the material brittle.
← 4. The recommendations are mainly based on the conditions that apply in the United Kingdom, because most of the projects for domestic use of hydrogen reported in the OECD Output-Literature review, from which this report has been taken input, have been carried out in the United Kingdom. However, they can provide guidelines for other countries as well.