Blending hydrogen into the gas network could affect the molecular structure of plastic gas pipes and has the potential to nearly double the volume of leakage, a literature review from the US government scientists has indicated.
Researchers at the National Renewable Energy Laboratory (NREL) in Colorado found that there are large gaps in data around the effects of hydrogen blends in gas infrastructure such as underground storage and pipelines — even when using polyethylene plastic pipes, which have long been touted by the gas industry as a safe way to transport hydrogen or blends of up to 20% hydrogen and 80% fossil gas.
The US government-backed NREL’s report, Hydrogen Blending as a Pathway Toward US Decarbonisation, found some data to suggest that the physical properties of polyethylene — both its crystalline structure and its density — are changed by exposure to H2, especially in lower-density plastics.
More research is needed to identify exactly how these changes affect the structural integrity of polyethylene pipes carrying blends of hydrogen and gas, the NREL said. This suggests that carrying hydrogen in pipes made with these materials comes with unknown risks.
And there are also gaps in the body of scientific understanding of how the molecule interacts with different grades of polyethylene under different pressures, in varying temperatures or in the presence of contaminants — or with different types of polyethylene resins formed at the pipes’ joins when they are welded together with heat.
The matter is especially pertinent for distribution networks — the smaller pipes which take gas from the main transmission lines and deliver them at low pressure to end users — which are typically made from steel, copper or polyethylene.
Large, high-pressure transmission lines that ship gas long-distance are almost always made from steel, which is prone to embrittlement when hydrogen is added to the mix.
This is because the tiny hydrogen molecule easily permeates metal, where it is absorbed and diffused throughout the structure, making the pipe more prone to cracking.
For this reason, polyethylene pipes are usually cited by the gas distributors as the safest way to distribute hydrogen or hydrogen blends. As much of the distribution networks in gas-consuming countries are already made up of a mix of plastic pipes alongside steel, hydrogen proponents often claim that these networks can be economically converted to carry hydrogen, either in its pure form or as a blend.
In the UK, the pipeline operators trade body, the Energy Networks Association (ENA), says that the network will be ready to begin carrying gas blends of 20% hydrogen (to 80% methane) as soon as this year.
But leakage is a significant concern for low-pressure distribution networks, due to the high surface area of pipe walls compared to a transmission pipe and H2’s low density and high diffusivity.
Hydrogen is even prone to leak out of the walls of polyethylene pipes. In fact, the rate at which hydrogen permeates through the walls of the pipe is around five times that of methane.
And there are conflicting conclusions in the literature on how blending hydrogen into the gas network affects the overall leak rate in plastic pipes, with one study finding that a 20% blend actually reduced leakage and another finding that the same blend ratio almost doubled it.
Both studies concluded that the angle of the pipeline played a significant role in leak rate, with pipes on a 15 degree incline leaking more — and concluded that any leakage was negligible from an economic perspective.
However, any leaks have the potential to contribute to global heating. Hydrogen is a potent indirect greenhouse gas, with more than 11 times the warming potential of carbon dioxide over 100 years, or 33 times more powerful over 20 years, according to data from the UK government.
The NREL also noted that the data showed that hydrogen would leak freely through many “elastomeric” (rubber-like) materials used in gas infrastructure, such as gaskets and diaphragms.
“The concept of blending hydrogen into natural gas pipelines is not new and has been around for decades, but there remain large knowledge gaps,” said Mark Chung, NREL's hydrogen systems analysis lead. “We hope this study serves as a useful reference for those considering blending demonstrations or future areas of related research.”
The study also flagged significant unknowns in using hydrogen blends in underground gas storage — including a lack of data on how H2 would interact with residual hydrocarbons in depleted reservoirs or how the presence of microbes might affect the product — and gas turbines.
As a result of these knowledge gaps, each blending project should be considered on an individual basis until they are filled, the NREL warned.
“The tolerance of natural gas pipeline systems to the introduction of hydrogen must be considered on a case-by-case basis while accounting for network structure, gas composition, flow rates, existing facilities, and end-use constraints,” it said.
Many gas companies say that existing networks can safely handle blends of up to 20% green hydrogen without any changes required to pipes or appliances, making it an easy way of reducing greenhouse gas emissions. But according to the Fraunhofer Institute (IEE) in Germany, doing so would increase energy costs by 43% while delivering emissions cuts of only 6-7%.
And a report last year by the International Renewable Energy Agency found that 20% green hydrogen blending would equate to emissions abatement costs of $500 per tonne of carbon dioxide, placing it among the expensive methods in the world of reducing CO2.