Physics:Hydrogen purity

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Hydrogen purity or hydrogen quality describes the presence of impurities in hydrogen when used as a fuel gas. Impurities in hydrogen can interfere with the proper functioning of equipment that stores, distributes, or uses hydrogen fuel.

Hydrogen Purity Requirements

The impact of impurities varies with the specific equipment used and on the physio-chemical nature of the impurity. For example, hydrogen boilers that combust hydrogen will generally tolerate higher concentrations of impurities than a vehicle using a polymer electrolyte membrane fuel cell (PEMFC)[1] and inert impurities such as nitrogen are usually less harmful than reactive species such as hydrogen sulphide.[2]

As the specific impurity matters it is not sufficient to rely on normal metrics of gas purity, often reported using nines (e.g. >99.9990% or 5.0N),[3] as this does not provide adequate information about which impurities may be present at trace levels. Instead, standards have been developed that provide more detailed requirements on fuel purity for specific applications. The international standard ISO 14687:2019 [2] specifies maximum permissible concentrations for many key impurities depending on use. This standard is being adopted into legislation in many jurisdictions. For example, in Europe the Directive 2014/94/EU[4] on the deployment of alternative fuels infrastructure states that the hydrogen purity dispensed by hydrogen refuelling points shall comply with the technical specifications included in ISO 14687-2.

Fuel Cell Electric Vehicles

Fuel cell electric vehicles commonly use polymer electrolyte membrane fuel cells (PEMFC) which are susceptible to a range of impurities. Impurities impact PEMFC using a range of mechanisms, these may include poisoning the anode hydrogen oxidation reaction catalysts, reducing the ionic conductivity of the ionomer and membrane, altering wetting behaviour of components or blocking porosity in diffusion media. The impact of some impurities like carbon monoxide, formic acid, or formaldehyde is reversible with PEMFC performance recovering once the supply of impurity is removed. Other impurities, for example sulphurous compounds, may cause irreversible degradation.[5] The permissible limits of hydrogen impurities are shown below.

Fuel Quality Specification For Gasseous Hydrogen Supplied to PEMFC Road Vehicles [6]
Maximum Permissible Concentration / μmol mol−1
Total non-hydrogen gasses 300
Water 5
Total Hydrocarbons Except Methane [Carbon atom basis] 2
Methane 100
Oxygen 5
Helium 300
Nitrogen 300
Argon 300
Carbon Dioxide 2
Carbon Monoxide 0.2
Total Sulphur Compounds [Sulphur atom basis] 0.004
Formaldehyde 0.2
Formic Acid 0.2
Ammonia 0.1
Halogenated Compounds [Halogen ion basis] 0.05
Maximum Particulate Concentration 1 mg kg−1

Efforts to assess the compliance of hydrogen supplied by hydrogen refuelling stations against the ISO-14687 standard have been performed.[7][8][9] While the hydrogen was generally found to be 'good'[7] violations of the standard have been reported, most frequently for nitrogen, water and oxygen.

Combustion Engines and Appliances

Combustion applications are generally more tolerant of hydrogen impurities than PEFMC, as such the ISO-14687 standard for permissible impurities is less strict.[10] This standard has itself been criticised with revisions proposed to make it more lenient and therefore suitable for hydrogen distributed through a repurposed gas network.[1]

Fuel Quality Specification For Gaseous Hydrogen Supplied to Combustion Engines and Appliances [11]
Impurity Maximum Permissible Concentration / μmol mol−1
Total non-hydrogen gasses 20 000
Water Non-condensing
Total Hydrocarbons [Carbon atom basis] 100
Carbon Monoxide 1
Sulphur [Sulphur atom basis] 2
Combined water, oxygen, nitrogen, argon 19 000
Permanent Particulates Shall not contain an amount sufficient to cause damage.

Sources of Hydrogen Impurities

The presence of impurities in hydrogen depends on the feedstock and the production process. Hydrogen produced by electrolysis of water may routinely include trace oxygen and water, which must be usually be removed prior to use. Hydrogen produced by reforming of hydrocarbons is produced as a mixture with a stoichiometric mixture with carbon dioxide and carbon monoxide which must be separated, additionally trace impurities from the feedstock such as sulphur compounds may be present in the final hydrogen supply. Impurities may also be introduced during storage, distribution, dispensing or as a result of equipment malfunction. Examples of this include distribution of hydrogen through repurposed gas networks which may be contaminated with a range of impurities or malfunctioning of equipment at refuelling stations.[1] Some impurities may be added deliberately, for example odorants to aid detection of gas leaks.[12]

Methods for Hydrogen Purity Analysis

As the permissible concentrations for many impurities are very low this sets stringent demands on the sensitivity of the analytical methods. Moreover, the high reactivity of some impurities requires use of a properly passivated sampling and analytical systems.[13] Sampling of hydrogen of is challenging and care must be taken to ensure that impurities are not introduced to the sample and that impurities do not absorb on or react within the sampling equipment, there are currently different methods for sampling but rely on filling a gas cylinder from the refuelling nozzle of a refuelling station.[14] Efforts are underway to standardise and compare sampling strategies.[15][16] A combination of different instruments is needed to assess hydrogen samples for all of the components listed in ISO 14687-2.[17] Techniques suitable for individual impurities are indicated in the table below.

Example Analytical Methods for Asessing The Concentration of Impurities in Hydrogen[18][19]
Impurity Possible Analytical Methods Detection Limits
Total non-hydrogen gasses
Water Quartz crystal microbalance

or CRDS

1.3 or 0.030
Total Hydrocarbons Except Methane [Carbon atom basis] GC-Methaniser-FID 0.1
Methane GC-Methaniser-FID, GC-EPD 0.1
Oxygen GC-PDHID, GC-EPD 0.3
Helium GC-TCD 10
Nitrogen GC-PDHID, GC-EPD 1
Argon GC-PDHID, GC-EPD 0.3
Carbon Dioxide GC-Methaniser-FID, GC-EPD 0.02
Carbon Monoxide GC-Methaniser-FID, GC-EPD 0.02
Total Sulphur Compounds [Sulphur atom basis] GC-SCD, GC-EPD 0.001
Formaldehyde GC-Methaniser-FID 0.1
Formic Acid FTIR 0.2
Ammonia GC-MS or UV-visible spectroscopy or FTIR 1 or 0.03 or 0.1
Halogenated Compounds (Halogen Ion Equivalent) TD-GC-MS 0.016

In addition to rigorous laboratory analysis analytical methods that can be operated in the field continuously assessing hydrogen for impurities are being developed. These include techniques such as electrochemical sensors [20][21] and mass spectrometry.[22]

Methods for Purifying Hydrogen

See also: Hydrogen purifier

Purification of hydrogen is an important aspect of hydrogen distribution and there are a range of technologies available depending on the impurities present and process conditions.[1]

See also

References

  1. ↑ 1.0 1.1 1.2 1.3 "WP2 Report Hydrogen Purity" (in en-US). https://www.hy4heat.info/reports. 
  2. ↑ 2.0 2.1 "ISO 14687:2019" (in en). https://www.iso.org/cms/render/live/en/sites/isoorg/contents/data/standard/06/95/69539.html. 
  3. ↑ "Purity, Grades and Concentration" (in en). https://www.boconline.co.uk/en/contact-and-support/technical-advice/speciality-products-advice/purity-grades-concentration/purity-grades.html. 
  4. ↑ "Directive 2014/94/EU on the deployment of alternative fuels structure". https://eur-lex.europa.eu/legal-content/EN/TXT/PDF/?uri=CELEX:32014L0094&from=EN. 
  5. ↑ X. Cheng, Z. Shi, N. Glass, L. Zhang, J. Zhang, D. Song, Z.-S. Liu, H. Wang and J. Shen (2007). "A review of PEM hydrogen fuel cell contamination:Impacts, mechanisms, and mitigation". Journal of Power Sources 165 (2): 739–756. doi:10.1016/j.jpowsour.2006.12.012. Bibcode: 2007JPS...165..739C. 
  6. ↑ "ISO 14687:2019" (in en). https://www.iso.org/cms/render/live/en/sites/isoorg/contents/data/standard/06/95/69539.html. 
  7. ↑ 7.0 7.1 Aarhaug, Thor Anders; Kjos, Ole; Bacquart, Thomas; Valter, Vladimir; Optenhostert, Thomas (2021-08-18). "Assessment of hydrogen quality dispensed for hydrogen refuelling stations in Europe" (in en). International Journal of Hydrogen Energy. HYDROGEN ENERGY SYSTEMS 46 (57): 29501–29511. doi:10.1016/j.ijhydene.2020.11.163. ISSN 0360-3199. 
  8. ↑ Aarhaug, Thor A.; Kjos, Ole S.; Ferber, Alain; Hsu, Jong Pyong; Bacquart, Thomas (2020). "Mapping of Hydrogen Fuel Quality in Europe". Frontiers in Energy Research 8: 307. doi:10.3389/fenrg.2020.585334. ISSN 2296-598X. 
  9. ↑ "HYDRAITE public report D3.1 | HYDRAITE" (in en-US). https://hydraite.eu/public-reports/. 
  10. ↑ "ISO 14687:2019" (in en). https://www.iso.org/cms/render/live/en/sites/isoorg/contents/data/standard/06/95/69539.html. 
  11. ↑ "ISO 14687:2019" (in en). https://www.iso.org/cms/render/live/en/sites/isoorg/contents/data/standard/06/95/69539.html. 
  12. ↑ "Hydrogen Odorant and Leak Detection Project Closure Report". https://sgn.co.uk/sites/default/files/media-entities/documents/2020-09/Hydrogen_Odorant_and_Leak_Detection_Project_Closure_Report_SGN.pdf. 
  13. ↑ Bacquart, Thomas; Moore, Niamh; Hart, Nick; Morris, Abigail; Aarhaug, Thor A.; Kjos, Ole; Aupretre, Fabien; Colas, Thibault et al. (2020-02-14). "Hydrogen quality sampling at the hydrogen refuelling station – lessons learnt on sampling at the production and at the nozzle" (in en). International Journal of Hydrogen Energy. 22nd World Hydrogen Energy Conference 45 (8): 5565–5576. doi:10.1016/j.ijhydene.2019.10.178. ISSN 0360-3199. https://www.sciencedirect.com/science/article/pii/S0360319919340510. 
  14. ↑ Arrhenius, Karine; Aarhaug, Thor; Bacquart, Thomas; Morris, Abigail; Bartlett, Sam; Wagner, Lisa; Blondeel, Claire; Gozlan, Bruno et al. (2021-10-11). "Strategies for the sampling of hydrogen at refuelling stations for purity assessment" (in en). International Journal of Hydrogen Energy 46 (70): 34839–34853. doi:10.1016/j.ijhydene.2021.08.043. ISSN 0360-3199. 
  15. ↑ Practice for Sampling of High Pressure Hydrogen and Related Fuel Cell Feed Gases, ASTM International, doi:10.1520/d7606-17, http://dx.doi.org/10.1520/d7606-17, retrieved 2021-11-01 
  16. ↑ DIN ISO/TS 22002-3:2017-09, https://www.iso.org/obp/ui/#iso:std:iso:19880:-1:ed-1:v1:en, retrieved 2021-11-01 
  17. ↑ Murugan, Arul; Brown, Andrew S. (2015-03-22). "Review of purity analysis methods for performing quality assurance of fuel cell hydrogen" (in en). International Journal of Hydrogen Energy 40 (11): 4219–4233. doi:10.1016/j.ijhydene.2015.01.041. ISSN 0360-3199. https://www.sciencedirect.com/science/article/pii/S0360319915000804. 
  18. ↑ "Hydrogen purity" (in en). https://www.npl.co.uk/products-services/gas/hydrogen-purity. 
  19. ↑ Bacquart, Thomas; Arrhenius, Karine; Persijn, Stefan; Rojo, Andrés; Auprêtre, Fabien; Gozlan, Bruno; Moore, Niamh; Morris, Abigail et al. (2019-12-31). "Hydrogen fuel quality from two main production processes: Steam methane reforming and proton exchange membrane water electrolysis" (in en). Journal of Power Sources 444: 227170. doi:10.1016/j.jpowsour.2019.227170. ISSN 0378-7753. Bibcode: 2019JPS...44427170B. 
  20. ↑ Mukundan, Rangachary (2020). "Development of an Electrochemical Hydrogen Contaminant Detector" (in en). Journal of the Electrochemical Society 167 (14): 147507. doi:10.1149/1945-7111/abc43a. Bibcode: 2020JElS..167n7507M. 
  21. ↑ Noda, Z.; Hirata, K.; Hayashi, A.; Takahashi, T.; Nakazato, N.; Saigusa, K.; Seo, A.; Suzuki, K. et al. (2017-02-02). "Hydrogen pump-type impurity sensors for hydrogen fuels" (in en). International Journal of Hydrogen Energy 42 (5): 3281–3293. doi:10.1016/j.ijhydene.2016.12.066. ISSN 0360-3199. https://www.sciencedirect.com/science/article/pii/S0360319916336138. 
  22. ↑ "HydrogenSense" (in en). https://www.vandf.com/en/products/analyzers/hydrogensense/,%20https://www.vandf.com/en/products/analyzers/hydrogensense/. 



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