Defining The Hydrogen Economy From A to Z: P is for Purity
Continuing in our Defining the Hydrogen Economy from A to Z series and revisiting the alphabet letter P, today we discuss Purity and its importance to the liquid hydrogen value chain.
When we think of the word or term Purity, the first thing that comes to mind might not necessarily be its relationship to Hydrogen. However, as we think about the general definition of “the freedom from contamination,” then it sets the introduction to the following discussion, especially when transitioning specifically to liquid hydrogen.
Purity in chemistry and materials is important and products of a chemical nature contain specific substance characteristics. Following standard requirements and analytical testing of purity composition address safety, compatibility, and performance.
Generally, Hydrogen gas comes in different purity levels or into three grades, pure hydrogen (hydrogen purity ≥ 99.99%), high pure hydrogen (hydrogen purity ≥ 99.999%), and ultrapure hydrogen (hydrogen purity ≥ 99.9999%). Impurities in hydrogen can interfere with the proper functioning or performance of equipment that stores, distributes, or uses the hydrogen. The impact of impurities varies with the specific equipment using the hydrogen and the nature of the impurity. For example, hydrogen combustion boilers can tolerate a higher percentage of impurities than a vehicle using a fuel cell. Inert impurities such as nitrogen are usually less harmful than other reactive species that might be present. Since the specific impurity matters, standards such as ISO 14687:2019 have been developed that provide more detailed requirements on hydrogen purity for specific applications.
The presence of impurities in hydrogen gas depends on the production process. Hydrogen produced from natural gas or methane reforming contain carbon dioxide and carbon monoxide by-products which must be separated out along with other impurities, while hydrogen produced by electrolysis and renewables routinely include trace oxygen and water, which usually must be removed prior to use. Purity analysis is critical to the rigorous specification demands. Sampling of hydrogen is challenging, and caution must be taken to ensure that impurities are not introduced through the sampling process to the hydrogen supply chain.
Now that we understand more about the importance of the purity of hydrogen produced, meeting standard requirements, and the challenges of producing and testing/sampling for verifying hydrogen purity level, the process of liquefaction is a critical step in the hydrogen value chain. Liquefaction and GenH2’s approach combined with controlled storage inherently produces ultrapure hydrogen, since the conversion of hydrogen gas to liquid hydrogen freezes out the impurities and will be left behind. Ultrapure hydrogen is critical to the operations of fuel cell vehicles and the next generation of hydrogen aircraft. Therefore, hydrogen gas of different purities can be further purified and densified to liquid hydrogen as an energy carrier to meet the time savings needs and purity requirements in moving forward to a global cleaner energy economy.
Please look for next week’s blog as we discuss the alphabet letter Q, the symbol for Heat, Heat Flow and Heat Flow Rate.
Blog written by GenH2 team member: Martha Williams, Ph.D.