Hydrogen revolution

WHO REMEMBERS unleaded petrol? The days of choosing between “Four-Star” and unleaded (does my engine need to be converted) seem far behind us. Progress. It happens. The rumblings of another energy revolution are afoot, to cite the WEF “The green hydrogen revolution has started, and it won't be stopped”. 

It’s the year of COP26, and hydrogen projects and innovations abound: hydrogen buses, trains, shipping and planes, hydrogen fuel cells, electrolysers of all sorts and scales, hydrogen refuelling stations, hydrogen in gas networks, hydrogen CHP plants. One would be hard pressed not to notice. All to be celebrated, and somehow satisfying that the oldest element in the universe would hold so much promise, so much potential to solve one of humankind’s newest and most pressing problems: the climate crisis. The allure of ‘zero emission’ green hydrogen is understandable.

Hydrogen ready

For those embracing the move to hydrogen, a question: is your gas detection hydrogen ready?

Although it’s the most abundant element in the universe, most of the hydrogen on our planet exists in water (good old H₂O). In its gaseous state, hydrogen consists of two hydrogen atoms, bonded together (H₂), each containing just one proton and one electron. The simple structure of hydrogen accounts for its properties: hydrogen is odourless, much lighter than air and relatively easy to ignite.

Hydrogen is not toxic, but can be an asphyxiant when oxygen is displaced, however the principle and most well recognised risk from hydrogen gas is its flammability. Hydrogen is easily ignited and has a very wide flammable range, with a lower explosive limit (LEL) of 4% Volume¹, and an upper explosive limit of 77% Volume in air. Compare this to methane, the main component of natural gas and biomethane, which has an explosive range of 4.4% to only 17% Volume in air. Outside of the flammable range the gas mixture is either too weak or too rich to burn. Within its flammable range, the mixture can be ignited by a spark or by contact with a surface exceeding its ignition temperature. A build-up of hydrogen can therefore present a flammable or even explosive risk, and over a wider concentration range than methane.

Hydrogen has a much lower density than air (relative density 0.07 which means it’s about 14 times lighter than air) so leaked hydrogen will tend to accumulate in overhead spaces within an enclosed environment. 

Detection for leaks of flammable vapours and gases, is usually concerned with monitoring for levels below the LEL, so as to provide an early warning of leaks before they become flammable. For this reason, flammable gas detectors usually have a range of 0-100% LEL, with alarm levels typically deployed at 10, 20 or 40 % LEL.

In some applications, ppm (part per million) level detection of hydrogen can be appropriate.

Methane detection systems

One area which seems set to move towards hydrogen fuel is CHP, Combined Heat & Power. We’ve all become accustomed to CHP plants running on natural gas or biomethane, and most facilities will have some sort of gas detection in place. On the face of it, methane and hydrogen would appear to present similar risks – methane is after all also lighter than air, and easily ignited. However legacy systems, which have originally been selected for methane detection may not be suitable for hydrogen. Alarmingly (or not, no pun intended) it might not respond to hydrogen at all! If in any doubt, please check with your gas detection supplier.

So why might your methane detection system not be suitable? Consider first the sensor technology: a number of historic sensor technologies are used by gas detection equipment manufacturers, and not all of these are suitable for hydrogen detection. 

Pellistor, or catalytic bead technology, has been used for flammable gas and vapour detection since the 1960s. Counter-intuitively these devices contain platinum wire, embedded in ceramic beads, which are electrically heated to around 500°C (don’t worry, it’s all quite safe as it takes place in a controlled environment, behind a sinter). The sensing bead, being coated or impregnated with a catalyst, will increase in temperature as the sample gas is oxidised, changing its electrical resistance. The reference bead, without the catalyst, will not react to the sample gas, and it’s temperature and resistance are unaffected. The differential resistance can be used to measure the concentration of flammable gas present. The pellistor response will not be the same for all flammable gases; manufacturers often publish data on relative response. It’s important to realise that a pellistor based detector calibrated for 0-100% LEL methane, will not give a true %LEL response to Hydrogen; it could over or under respond; and an under-response is potentially dangerous.

If your existing gas detectors use Infra-Red technology, they won’t respond to hydrogen at all. Infra-Red (IR) sensor technology relies on the principle of IR energy, of a particular wavelength, being absorbed by chemical bonds in the target gas. For methane detectors, the wavelength is tuned for absorption by the hydro-carbon bonds, which are not present in hydrogen, hence hydrogen is invisible to these detectors.

Then there are the ATEX requirements to consider. The funny numbers on the equipment label might not mean much to the lay person, but they are there to pronounce the equipment’s suitability for use in potentially flammable atmospheres. Suitably rated and installed equipment will not be an ignition source. Equipment installed for a methane application might not be suitable for a hydrogen application: 

Hydrogen is in Equipment Group IIC, Methane is in Gas Group IIA. The groups are differentiated by the properties of the potential gases that may be present. The C rating is the safest, and many equipment manufacturers will ensure that their equipment meets the IIC requirements, but if your detectors are labelled as IIA, be aware that they’re not approved for use in a potential hydrogen atmosphere. 

And we’ve not even mentioned flame detection. Some gas detection systems will have integrated flame detection. Hydrogen burns with a near invisible flame: not something you’d want to walk into. If you already have IR (infra-red) open path flame detection installed for methane of fossil fuels, this will need reviewing for hydrogen applications: hydrogen flames emit very little IR energy, so specialised multi-spectrum IR, or ultra-violet (UV) detectors are required.

As you can see, there are many considerations. Embrace the hydrogen revolution, but please don’t overlook your humble gas detectors, they’re there to protect you after all.

References

1. Depending on the standard, and test method used. LEL of 4% according to ISO/IEC 80079-20-1: 2017

Jackie Marsh is technical specification engineer at Crowcon Gas Detection Instruments. For more information, visit www.crowcon.com