Climate risk of hydrogen leakage

When hydrogen is produced, transported, stored, and used, some fraction of the gas will leak to the atmosphere. In the existing value chain, there is very little data on the magnitude of these leakages and how this will evolve in a future growing hydrogen economy.

The majority of hydrogen sensors available in the commercial market are designed to detect elevated concentrations in the parts per million range, indicative of an imminent safety hazard. Given this lack of adequate monitoring equipment, minuscule leaks accumulating throughout the value chain go undetected, thus posing a challenge in quantifying the amount of hydrogen escaping into the atmosphere through empirical data. Current estimates span from 0.2% to 20%!

In Part V of our newsletter, we break down where we stand in comprehending the likely worldwide atmospheric consequences if there's a surge in hydrogen use in the future, in Europe alone...

Hydrogen’s climate impact

Hydrogen has been somewhat overlooked and underexplored by the atmospheric research community. This lack of attention is mainly due to the fact that, from the moment it's released into the atmosphere until it's taken out, hydrogen doesn't cause any immediate harm to human health or the targeted ecosystems. As a result, it isn't classified as an atmospheric pollutant.

However, hydrogen is involved in atmospheric chemical reactions that affect the lifetime and abundances of other gases that have an impact on the climate. Therefore, hydrogen is an indirect greenhouse gas (GHG).

Four main climate impacts are associated with increased hydrogen levels:

1. A longer methane (CH4) lifetime and hence increased methane abundances.
2. An enhanced production of tropospheric ozone (O3) and changes in stratospheric O3.
3. An increased stratospheric water vapor (H2O) production.
4. Changes in the production of certain aerosols.

The most important reaction driving these impacts is the destruction of hydrogen by OH producing water vapour (H2O):

H2 + OH --> H2O + H

H2O can act as GHG due to its extensive infrared absorption spectrum, featuring more and wider absorption bands compared to CO2. Its presence in the atmosphere is constrained by air temperature, causing radiative forcing by water vapor to escalate with global warming, creating a positive feedback loop.

How to quantify climate impact?

The concept of Global Warming Potential (GWP) was introduced by the IPCC in 1990 to aid policymakers in evaluating the climate consequences of various trace gases' future emissions. GWP is expressed as the time-integrated commitment to climate forcing relative to carbon dioxide. It considers the immediate emission of a trace gas, its decline over time, and potential direct and indirect greenhouse warming effects. For CO2 itself, the GWP was set at one.

Meantime, global chemistry-transport models have evolved, allowing for more accurate estimations of direct and indirect GWPs. In 2001, the IPCC highlighted the potential climate impact of future hydrogen emissions in a fuel-cell economy. At that time, a GWP of 5.8 was calculated for hydrogen considering its impact on methane and ozone only. In 2023, a study published in Nature by climate scientists from four countries and across six different institutions led by Norway’s Centre for International Climate Research (CICERO) arrived at a GWP of 11.6 +/- 2.8 for hydrogen taking account of the effective radiative forcing contributions by methane, ozone, and stratospheric water vapor.

In practical terms, the GWP of atmospheric hydrogen may be between 8.8 and 14.4 depending on a range of factors, including how much hydrogen is absorbed into soil at ground level and how effectively it interacts with other airborne molecules that cause global warming.

Hydrogen blending in gas pipelines hindered by leaks

The European Hydrogen Backbone will make use mainly of existing infrastructure, which will be properly converted. Such ambitious undertaking is poised to extend its impact far beyond the realm of end-user energy supply, reaching into uncharted territories with potentially unintended consequences.

Researchers with the Argonne National Laboratory found that blending 30% hydrogen by volume into gas pipelines yielded a relatively modest 6% decrease in lifecycle GHG emissions. A major factor in Argonne's estimate was its finding that hydrogen blending at that level can double leakage from pipelines.

Emissions from gas transmission pipelines remain relatively unchanged when an operator introduces a 30% hydrogen blend without changing the pipeline's flow rate. However, since hydrogen has one-third of the energy density of methane, pipeline operators must replace a standard cubic meter of gas with three standard cubic meters of hydrogen to deliver the same amount of energy. To that end, the pipeline flow rate and pressure must be increased by about 30% and 70%, resp., and the compression power must be doubled. If the flow rate is increased to deliver the same amount of energy that a pure gas system can deliver, a 30% hydrogen blend increases transmission leakage by 100% according to Argonne.

Proposed fixes for hydrogen blending

Blending hydrogen will require replacement or modification of certain parts along transmission systems to address mismatches between maximum allowable operating pressure in pipe segments carrying gas and hydrogen. The US DOE National Renewable Energy Laboratory has suggested 3 approaches to overcoming a key constraint to hydrogen blending:

1. Replacing those segments with pipes of the same diameter but appropriate material grade and wall thickness.
2. Pipeline looping, meaning installing pipes that operate parallel to existing segments.
3. Adding compressor stations between segments that do not match the standard.

The second and third suggested approaches would reduce system design pressure to a level consistent with the industry standard for hydrogen piping, while increasing volumetric flow rate to meet the same end-use demand. Both involve additional right-of-way costs.

Possible consequences of a future growing hydrogen economy

The environmental impact of a prospective hydrogen economy hinges on the widespread adoption of hydrogen energy across various sectors and the chosen pathways for hydrogen production.

Today, the prevalent method of hydrogen production involves reforming natural gas into hydrogen and CO2. This process results in "grey hydrogen" if the CO2 is released into the atmosphere and "blue hydrogen" if the CO2 is captured and stored permanently. When renewable sources power water electrolysis for hydrogen production, the term "green hydrogen" is used. Leakage rates from production of the different types of hydrogen vary, with estimates ranging from 0.5% to 1.0% for grey hydrogen produced from unabated natural gas, 0.0% to 1.5% for blue hydrogen, and 0.03% to 9.2% for green hydrogen.

At present, grey hydrogen is the most commonly used type of hydrogen, accounting for around 95% of global hydrogen production. As evident from the preceding paragraph, factoring in a combination of blue and green hydrogen leads the CO2 equivalent emissions from hydrogen production to increase.

And there's more to consider!

In a combined blue & green hydrogen economy, a 1% leakage rate would lead to an increase in the CO2 equivalent emissions spanning from 20% (taking the hydrogen high heating value, HHV) to 24% (taking the hydrogen low heating value, LHV), while a 10% leakage rate would result in an increase between 64% with the HHV and 76% with the LHV for a 100?year time-horizon.

Even in the case of a green hydrogen economy with the low leakage rate of 1%, the hydrogen CO2 equivalent emissions would increase from 1.2% with the HHV to 1.4% with the LHV based on the GWP100, and from 13% with the HHV to 15% with the LHV for a 100?year time-horizon with the high leakage rate of 10%.

The intricacies of these scenarios, along with the fact that the authors of the above mentioned Nature paper came up with a 100-year GWP of 12.8?±?5.2 and a 20-year GWP of 40.1?±?24.1 for hydrogen using state-of-the-art methodology, highlight the challenges and nuances involved in navigating the path toward a sustainable hydrogen future.

And remember, at least in this context, what happens in Europe does not stay in Europe!??