Publication of the paper ‘How green is blue hydrogen?’ in the journal Energy Science and Engineering in mid-August caused controversy in the global hydrogen industry. In the paper, authors Robert Howarth and Mark Jacobson argue blue hydrogen has no role to play in a carbon-free future.

“We undertake the first effort in a peer-reviewed paper to examine the lifecycle greenhouse gas emissions of blue hydrogen, accounting for emissions of both carbon dioxide and unburned fugitive methane,” the academics say. “Far from being low carbon, greenhouse gas emissions from the production of blue hydrogen are quite high, particularly due to the release of fugitive methane.”

A number of critics questioned the findings, on Twitter and elsewhere, including David Joffe, head of carbon budgets at the UK Climate Change Committee (CCC).

“The paper’s calculations potentially represent a case where blue H₂ is done really badly and without any sensible regulations,” he wrote in a 13 August tweet. “And then cherry-picked a climate metric to make it look as bad as possible.”

A summary of the Howarth and Jacobson paper and a critique based on a Hydrogen Economist interview with David Joffe follows.

The case against blue hydrogen

To make their case, Howarth and Jacobson provided a base-case scenario, as well as sensitivity analyses around their three core assumptions and a best-case scenario for blue hydrogen, to support the validity of their initial findings.

“The paper’s calculations potentially represent a case where blue hydrogen is done really badly and without any sensible regulations” Joffe, CCC

Their base-case scenario is based on what the academics refer to as their ‘default’ assumptions: an 86 times global warming potential (GWP) for methane emissions compared with CO₂, assuming a 20-year timeframe, 3.5pc lifecycle methane emissions for natural gas relative to consumption, and a carbon-capture rate of 85pc from steam methane reforming (SMR) for blue hydrogen projects.

The rationale Howarth and Jacobson give for assuming a 20-year timeframe for GWP—rather than 100 years, which is standard for the UN’s Intergovernmental Panel on Climate Change and the IEA—is as follows: “We prefer the use of 20-year GWP, since it better captures the role of methane as a driver of climate change over the time period of the next several decades, and the 100-year timeframe discounts the importance of methane over these shorter timeframes.”

Their 3.5pc default assumption for methane emissions from grey hydrogen, blue hydrogen and natural gas is based on top-down estimates for emissions from 20 different studies in ten different gas fields plus a top-down estimate for emissions from gas transport and storage.

In terms of the 85pc capture rate from SMR, the academics hint that this assumption is a best guess given a lack of blue hydrogen projects to date.

Based on their three default assumptions, Howarth and Jacobson find: “Total carbon dioxide equivalent emissions for blue hydrogen are only 9-12pc less than for grey hydrogen. While carbon dioxide emissions are lower, fugitive methane emissions for blue hydrogen are higher than for grey hydrogen because of an increased use of natural gas to power the carbon capture. Perhaps surprisingly, the greenhouse gas footprint of blue hydrogen is more than 20pc greater than burning natural gas or coal for heat and some 60pc greater than burning diesel oil for heat, again with our default assumptions.”

Sensitivity analysis

The alternate values Howarth and Jacobson chose for the three core assumptions, compared with their default values, are as follows: for GWP of methane emissions, 86 and 105 for a 20-year timeframe, and 34 for a 100-year period. For lifecycle methane emissions, 4.3pc, 2.54pc and 1.54pc compared with the default value of 3.5pc.  For carbon-capture rates from the SMR process—ignoring capture from relatively inconsequential flue exhaust for the sake of brevity—78.8pc and 90pc in addition to the 85pc default value.

Based on these alternative values, the academics conclude: “These sensitivity analyses show that our overall conclusion is robust: the greenhouse gas footprint of blue hydrogen, even with capture of carbon dioxide from exhaust flue gases, is as large as or larger than that of natural gas.”

Best-case scenario

Under their best-case scenario for producing blue hydrogen, where Howarth and Jacobson assume renewable energy is used to power carbon capture rather than natural gas, they acknowledge that greenhouse gas emissions see a “substantial reduction”. But they go on to say it is “not a low-emissions strategy” since emissions are still 47pc of natural gas based on default values for their core assumptions —and a third of those for grey hydrogen.

The academics conclude: “This best-case scenario for producing blue hydrogen… suggests to us that there really is no role for blue hydrogen in a carbon-free future.”

The paper finds that the total emissions of blue hydrogen are 139gCO₂e/MJ of hydrogen without flue-gas capture and 135gCO₂e/MJ hydrogen with flue-gas capture, assuming SMR is used.  This compares with 21.4gCO₂e/MJ hydrogen assumed in the UK’s national hydrogen strategy for SMR processes and 16gCO₂e/MJ hydrogen assuming autothermal reforming (ATR) processes are used.

Critique

In his interview with Hydrogen Economist, Joffe highlights three major issues with the approach taken by Howarth and Jacobson: their focus on a 20-year timeframe for GWP in the bulk of their paper, a significant overestimation of methane emissions for natural gas on a lifecycle basis and a significant underestimation of carbon-capture rates.

21.4gCO₂e/MJ hydrogen – UK hydrogen strategy estimate of blue hydrogen emissions

A 20-year timeframe distorts the role of methane emissions in global warming compared with CO₂ emissions, in turn penalising natural gas—and therefore blue hydrogen—according to Joffe.

“The impact of methane emissions decline quite quickly, from 86 times in 20 years to just 34 times in 100,” he says. “In contrast, CO₂ emissions are basically permanently there. That is why 100 years is the international standard.”

Rearview mirror estimates

Joffe acknowledges the paper’s default assumptions for lifecycle methane emissions and carbon-capture from SMR are likely fair based on past practice but says “data for blue hydrogen projects presently underway in Europe suggest methane emissions less than 1pc are likely, well below the range provided by the authors in their sensitivity analysis. And future projects will be using ATR to capture CO₂ emissions, rather than SMR, with capture rates of at least 95pc, according to large engineering companies such as Equinor—well above the paper’s sensitivity range.”

Norway’s Equinor declined to comment for this article, but in a letter to UK newspaper The Times shortly after the paper was published, Al Cook, an executive vice president with the company, stated that methane emissions associated with Zero Carbon Humber—set to be the UK’s largest blue hydrogen project—“is in fact less than one hundredth” the amount assumed by Howarth and Jacobson.

Not a best-case scenario

Joffe concludes that the paper’s best-case scenario for blue hydrogen is no such thing as it is not based on the use of the best technologies.

“Based on methane emissions of less than 1pc and carbon emissions reduction of 95pc, the CCC finds that blue hydrogen could save up to 85pc of emissions on a lifecycle basis, compared to the unabated use of fossil gas over a hundred year period,” he says.

A report from the CCC shows a combination of blue and green hydrogen is consistent with the UK reaching net-zero emissions by 2050, and the CCC has recommended that the government use blue hydrogen as a ‘bridge’ to green.

A paper from industry association the UK Hydrogen and Fuel Cell Association (HFCA), published last month, said blue hydrogen was ‘essential’ to enabling the UK to meet its net-zero targets.

The paper does acknowledge the use of best-practice technologies is vital in reducing lifecycle emissions of blue hydrogen.

“Using the highest efficiency and lowest-carbon footprint technologies, which consume the least natural gas per unit of hydrogen produced, and sources of natural gas with lower emissions profiles, can lead to blue hydrogen production with lifecycle emissions at very low levels when compared with grey hydrogen, and if some of the feedstock were biomethane the production could be zero or negative emissions,” it says.

HFCA chair Chris Jackson resigned late last month over the organisation’s support for blue hydrogen.

---

Author: Vincent Lauerman