Hydrogen purity: Why do we comply with 99.999% if the standard says 99.97%?
One of the key points when selecting an electrolyzer is the purity of the gas. Many projects propose to sell hydrogen for mobility, where there are standards that establish the minimum purity of the element, as well as limits for its impurities.
So, if you are going to supply hydrogen for automotive, this is important.
When we start reading the specifications of a hydrogen project we usually find (sooner or later) somewhere the following figure: 99.999%, referring to the purity of the hydrogen product in that project (or, in other words: 5.0, which means five nines followed by a zero) but if we go into the standards: SAE J2719, ISO 14687 or ISO 17124, we see a slightly different number: 99.97%.
All these standards share a table that starts with hydrogen purity and continues with impurities. The first surprising thing is to see that 99.97% minimum purity (seen from the other point, 3.7, three nines followed by a seven).
At this point, our customers often call us to ask us what is going on and then, we have to explain it to them as we are going to do now with you.
The famous table has many impurities: sulfur, ammonia, hydrocarbons, water, nitrogen, oxygen, etc. If you notice, not all of them should affect you in an electrolytic production. The standards themselves indicate which impurities must be taken into account. In the case of electrolysis, only water and oxygen must be considered, which have a limit, by standard, of 5 ppmV, five parts per million volumetric.
One part per million equals 0.0001% so, 5 ppmV is 0.0005%, as there are two impurities, water and oxygen, this adds up to 0.001%. If we subtract: 100 – 0.001 = 99.999 do you understand?
Basically, the obligation to meet impurity limits is more restrictive than the obligation to meet the minimum purity requirements.
So, what’s the point of a 99.97 value? It simply reflects the diversity of hydrogen sources, as hydrogen is not only produced through electrolysis. It can also be produced by natural gas reforming, biomass (or coal) gasification, or petroleum cracking, among others. If you’re interested in this topic, we recommend this book (link to Naturgy’s book).
If you’ve made it this far, hang tight because there’s more to learn. We want you to know that at Lean Hydrogen, we can help you with your hydrogen project. We are a consulting and engineering firm specialized in hydrogen production projects through electrolysis, injection into natural gas (blending), and mobility (hydrogen refueling stations).
Visit our blog “Lean Hydrogen” for more interesting content about this gas: https://leanhydrogen.whdev.es/blog/.
