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How Safe is Hydrogen?

Several disasters have been attributed to the use of hydrogen. However, this doesn't necessarily mean that hydrogen is completely unsafe. It is a gas with different properties that need to be understood.

Properties and Characteristics

Hydrogen is a colourless, odourless, and tasteless gas that burns with a pale blue flame that is virtually invisible in daylight. It is also a small molecule and, thus, has a high propensity to leak. Hydrogen would leak nearly 3 times faster from a leak of given size than Natural Gas and over 5 times faster than propane[13].

Hydrogen is the lightest of all elements; it is 14 times lighter than air. This causes it to be buoyant and rapidly disperse when released in air. This can be an important safety asset in the event of a leak where hydrogen would quickly diffuse through air and hence from buildings compared to other fuel gases[14].

Hydrogen also has a high propensity to ignite; the range of hydrogen/air mixtures that will explode is between 4% v/v hydrogen (Lower Explosive Limit) up to 75% v/v (Upper Explosive Limit) at standard atmospheric conditions. This wide flammability range is a disadvantage however the LEL, which is considered to be of greater significance from a safety perspective, is only slightly different to that of methane (5.3%) or propane (2.1%)[15].

For explosions, the energy needed to start a hydrogen/air explosion is diminutive; small sparks such as those produced from dropping a plastic-cased pen is capable of igniting hydrogen/air mixtures.

The maximum burning velocity of a hydrogen/air mixture is also high; it is about 8 times greater than that of Natural Gas mixtures. This makes it difficult to confine hydrogen flames and explosions, particularly in closed environments.

However, on a positive note, this rapid rate of deflagration means that hydrogen fires convey less heat to the surroundings than other gaseous fuel fires, reducing the risk of creating secondary fires in neighbouring materials[16].

Where are the gaps in terms of safety standards?

Standards and regulations are the key to solving the 'chicken and egg' dilemma, which asks the question of what to roll out first - hydrogen technologies and equipment (chicken) or the infrastructure to distribute it (egg). The International Standardisation Organisation (ISO) currently have a technical committee responsible for developing standards on systems and devices for the production, storage, transport, measurement and use of hydrogen - ISO/TC 197[17].

In the UK, the British Standards Institution (BSI) acts as a national mirror body and information hub for the activities going on within the ISO community. The UK Hydrogen and Fuel Cell Association (UKHFCA) also facilitate industry involvement in document reviews to prepare UK positions and feedback to ISO draft standards.

Gastec at CRE (part of KIWA Ltd) has provided CE marking services to a wide range of hydrogen appliance manufacturers.

However, for local installations there is a lack of guidance for both the installers and permitting authorities, both of whom may not be experts in hydrogen technologies.

References and Links

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