From One Gas to Another

In many areas of the world there are well-established natural gas distribution infrastructures providing natural gas to numerous industrial, commercial, and residential customers. The United States alone has about 3 million miles of natural gas pipeline, which delivered approximately 27.7 trillion cubic feet of natural gas to around 77.3 million customers in 2020.1 This massive natural gas infrastructure includes gas storage (underground and above ground), compressor stations, instrumentation, and piping components.

As the world explores hydrogen as a potential carbon emission free fuel source, utilizing the existing infrastructure could provide significant cost and time savings as opposed to developing a new hydrogen infrastructure from the ground up. Aside from the cost involved, items such as permitting, environmental regulations, land rite of ways, etc. could all be significant roadblocks preventing the development of a new hydrogen infrastructure.

The question thus arises, can we use or repurpose the existing gas infrastructure for use with a blend of hydrogen or, ultimately, 100% hydrogen? The short answer is yes, but there are many factors to consider, and some modifications most likely required.

Hydrogen Embrittlement

Hydrogen embrittlement, or hydrogen-induced cracking, occurs when hydrogen diffuses into a material. Hydrogen diffusion can reduce the material’s strength and tensile ductility, which can increase the pipe’s susceptibility to blistering or cracking.

Although hydrogen embrittlement is a concern for all steel pipe, in general high-strength steels (>100 ksi yield strength) are more susceptible. Low-strength steels are typically only subjected to loss in tensile ductility.2 Hydrogen concentration, internal pressure and internal pressure fluctuations are critical factors that impact the likelihood of hydrogen embrittlement.

What does this mean?

• All sections of pipelines will need to be evaluated and inspected on a case-by-case basis for material, design and operating conditions, age, and overall pipe condition.
• Most of the existing natural gas infrastructure piping is of lower strength steel and plastic pipe that are not at significant risk to hydrogen embrittlement. Reduction in the service life of pipelines due to hydrogen diffusion does not seem very likely.3
• Generally, crack-like defects need to already exist in the pipeline for hydrogen-induced cracking to occur. Crack-like defects are uncommon, and pipeline inspections can determine there are no defects before hydrogen is introduced.3
• Maintenance and inspection frequency and rigor of inspections likely need to increase to ensure safe pipeline transport of hydrogen.
• There is no magic hydrogen blend percentage that pipelines can tolerate without modifications.
• For some pipelines to accept 100% hydrogen, pipeline pressures and flowrates may need to be de-rated to ensure safety.

Hydrogen leakage

Hydrogen is a smaller molecule than methane, meaning the potential for leakage through piping joints and through pipe walls increases. Leakage also occurs with methane, but within pipes typically used in the natural gas infrastructure, the permeation rate of hydrogen is four to five times faster than methane.2

Leakage in steel pipes is more significantly found through threads, mechanical joints, and piping components such as valves and instrumentation, but permeation through the pipe walls can occur as well. Leakage in plastic pipes due to high permeation rates is more significantly found through the walls of the pipe as compared to the pipe joints (seals).

What does this mean?

• The suitability of fittings, seals, valves, control valves and their propensity for leakage must be evaluated and potentially replaced as hydrogen levels increase.
• More leak detection monitoring and sensing systems will likely be required.
• After about 20% hydrogen blend, packing materials of valves and sealing gaskets and instrumentation will likely need to be replaced.
• Adding 20% hydrogen to natural gas in plastic pipe approximately doubles the total gas loss via permeation through the pipe walls. Although this rate of leakage increases, actual total natural gas leakage is lower, resulting in a net reduction in the greenhouse gas impact.2

Pipeline size, equipment, components, and end user compatibility:

On a BTU/lb basis, hydrogen has about 2.5 times the energy density of natural gas. On a volumetric basis, however, you need approximately three times the volume of hydrogen as compared to natural gas to get the same amount of energy. The flame speed of hydrogen is almost ten times that of natural gas, and it burns hotter. All these factors have the potential to impact the pipeline sizes required for equivalent energy density and the compatibility of hydrogen with equipment, components, and end user compatibility.

What does this mean?

• End users such as gas turbines, HVAC, and cooking equipment, etc. will all be able to tolerate different percentages of hydrogen before modifications or ultimate replacements will be necessary.
• Due to the heating value characteristics and volumetric flow characteristics of hydrogen, the existing pipe sizes of the gas infrastructure should be sufficient for blended hydrogen flow with only a slight reduction in energy density.
• To match energy flow, the required pipeline gas compressor’s driving power will increase as hydrogen blend percentages increase. For 100% hydrogen, compressors driver power will increase by up to three times the driver power for natural gas. Compressors will therefore need to be modified or replaced due to increased driver power requirements as hydrogen blend percentages increase.3
  — Up to 10% hydrogen blend – No changes required
  — Up to 40% hydrogen blend – Internals (impellers, feedback stages, etc.) replaced or adjusted
  — Beyond 40% hydrogen blend – Complete compressor replacement with additional modifications or compressor replacements as blend increases to 100% hydrogen.
• Beyond material concerns and leakage, flow control valves, pressure regulators etc., will need to be adjusted or replaced as hydrogen percentages increase.

Conclusion

With a well-established natural gas infrastructure within the Unites States and other developed areas of the world, it could be very cost-effective and advantageous to use this infrastructure for hydrogen transport—and it’s certainly possible. But as with almost any major retrofitting project, there are many factors at play. The material, size and function of individual pipelines and components will all need to be evaluated on a case-by-case basis to determine the overall safety and functionality of transporting hydrogen.

References:

1. U.S. Energy Information Administration. (2021, November 5). Natural Gas Explained: Natural Gas Pipelines. Retrieved February 7, 2022, from https://www.eia.gov/energyexplained/natural-gas/natural-gas-pipelines.php.
2. Melaina, M. W., Antonia, O., & Penev, M. (2013, March). Blending Hydrogen into Natural Gas Pipeline . Retrieved February 7, 2022, from https://www.nrel.gov/docs/fy13osti/51995.pdf.
3. Siemens Energy. (2021). Hydrogen Infrastructure – The Pillar of Energy Transition: The Practical Conversion of Long-Distance Gas Networks to Hydrogen Operation. Retrieved February 7, 2022, from https://assets.siemens-energy.com/siemens/assets/api/uuid:3d4339dc-434e-4692-81a0-a55adbcaa92e/200915-whitepaper-h2-infrastructure-en.pdf.
4. St. John, Jeff. (2020, November 30). Green Hydrogen in Natural Gas Pipelines: Decarbonization or Pipe Dream? Retrieved February 7, 2022, from https://www.greentechmedia.com/articles/read/green-hydrogen-in-natural-gas-pipelines-decarbonization-solution-or-pipe-dream.

Article originally published on the POWER Engineers website.