Part 4: NOx and Greenhouse Warming Potential

Series 1 previously published on LinkedIn, January 10, 2023 (with edits and additions)

Jun 12, 2023

NASA Goddard Space Flight Center from Greenbelt, MD, USA, CC BY 2.0, via Wikimedia Commons

Preface to the Republished Article

This eleven-part series was published on January 10, 2023 in response to an eruption of anti-hydrogen rhetoric on LinkedIn. Part 3 set the scene for the remaining nine parts dedicated to the deconstruction of an opinion piece1 that appeared in at least two online periodicals.

The article was presented as an expert view on the ARUP+ study2 into the safety of domestic hydrogen. Yet it demonstrated scant appreciation of the document under review, and the findings were more consistent with keyword searching rather than reading. Part 4 is dedicated to exploding an untruth that continues to be shared about NOx.

Original article with minor edits follows.

Greenhouse Warming Potential

Here, in preparation for what follows, I need to briefly define ‘Greenhouse Warming Potential (GWP)’. GWP is a scale to compare the relative impacts of various gases on radiative forcing, or, the mechanism for climate warming. The reference gas on that scale is CO2 with a GWP of 1. The duration of these impacts are governed by the longevity of the gas in the atmosphere so there is a dimension of time too.

It is remarkably easy to get this wrong, viz.

Another downside to hydrogen, however, is it produces more NOx

‘Is it safe to burn hydrogen in the home? Let’s look at the evidence’ Hydrogen Insight, Tom Baxter

When we see a phrase like ‘more NOx’, the immediate question should be, ’under what conditions?’ I return to this in Part 5 when I briefly describe some of the mechanisms of NOx formation in combustion processes and how they differ between fossil fuels and hydrogen. The reality is that there are fewer mechanisms for producing NOx with hydrogen combustion than with any fossil fuel.

This is foundational to understanding the conversation about NOx, but apparently not for starting one, so it is with a little second-hand chagrin, I have this to address.

Hydrogen Howler: NOx is not N2O

[NOx is]— a greenhouse gas 265 times more damaging to the planet than CO2 over a 100-year period — than natural gas when combusted. (sic)

‘Is it safe to burn hydrogen in the home? Let’s look at the evidence’ Hydrogen Insight, Tom Baxter

So by ‘more damaging’ Mr Baxter is referring to the GWP and he incorrectly attributes the GWP of 265 to NOx. This is due to a misreading of the IPCC AR5 Global Warming Potential (GWP) reference which he links. It actually refers to N2O or nitrous oxide (now officially called dinitrogen oxide by the International Union of Pure and Applied Chemistry (IUPAC)). This is one of the errors that does not appear in the moderated version published by The Chemical Engineer which does their review process some credit.3

NOx is classed as having only indirect effect on radiative forcing so figures for GWP are not supplied by any of the major climate bodies, including the IPCC, UNFCCC and the EPA. The role of NOx as a greenhouse gas can be said to be ambiguous, viz.

Nitrogen oxides (NOx) act as indirect greenhouse gases by producing the tropospheric greenhouse gas 'ozone' via photochemical reactions in the atmosphere. The impact of NOx gases on global warming is not all bad though. Like tropospheric ozone, NOx gases also affect the global greenhouse gas budget through their effect on the atmospheric abundance of hydroxyl (OH) radicals.
The breakdown of NOx gases gives rise to increased OH abundance and so helps to reduce the lifetimes of greenhouse gases like methane.

Other Greenhouse gases - NOx, ghgonline.org

So while NOx contributes to the evolution of certain greenhouse gases it reduces the atmospheric residency of others; the net effect of these confounding dynamics are beginning to be understood and I provide more evidence for this in Part 5. As you will see there, calling NOx 'ambiguous' is a massive understatement on my part.

Hydrogen Howler: Nitrous Oxide (N2O) is not a by-product of Hydrogen

The main thing to recognise for now is that the species of NOx does not include nitrous oxide. This is what Tom Baxter gets completely wrong and Hydrogen Insight have so far, failed to correct. Once again, it seems that The Chemical Engineer identified this, since it does not appear in the version they published.

The fact this happened is surprising since the author is a founding member of the Hydrogen Science Coalition (HSC). Presumably his colleagues at HSC haven’t read this article, because to believe otherwise would invite the unwelcome thought that they don’t know the difference between NOx and N2O either. What sort of science they are coalescing around is anyone's guess. The problem is that this type of misinformation just gets rolled into anti-hydrogen folklore.

Although it’s true that nitrous oxide (N20 i.e. not NOx) is, ‘a greenhouse gas with 265 times’ the GWP of CO2 this is formed almost exclusively from hydrogen cyanide (HCN). Hydrogen cyanide is a product of burning hydrocarbons not hydrogen. Incidentally, the EPA quote a factor of 273 and the UNFCCC use 310, but what they are absolutely agreed upon, is they are definitely not talking about NOx.

We need to reflect upon the impact of getting such basic information wrong and then putting it into the public domain. A good example is the recent article in Hydrogen Insight, ironically titled, 'A campaign built on dishonesty | Hydrogen heating trial residents accuse gas distributor Cadent of withholding key safety data', where amongst other claims I am disputing in this series, those about hydrogen and NOx are repeated.

In the next part I will discuss some of the mechanisms of NOx formation and also what the latest research tells us about its impact in the atmosphere, before returning to the main critique of Mr Baxter's Hydrogen Insight article.

Thank you for reading The Vigne Intervention. This post is public so feel free to share it.

Next - Part 5: NOx and the Availability of Nitrogen

‘Is it safe to burn hydrogen in the home? Let’s look at the evidence’ Hydrogen Insight, Tom Baxter

Safety Assessment Conclusions Report incorporating Quantitative Risk Assessment (QRA)

Mr Baxter, denying responsibility for this misinformation, said that this was not a mistake picked up by Chemical Engineer, rather it was a paragraph added by the Hydrogen Insight editor (Leigh Collins), viz.

My republished LinkedIn post on this can be found here. Note that the article has not been corrected as of June 12, 2023, 6 months after I identified the issue. Mr Baxter is right when he says this is not a mistake.