I recently received a query from one of my LinkedIn followers about hydrogen and greenhouse gas emissions. The person wondered “why no one has an issue with water vapour being released into the atmosphere when burning hydrogen. Water vapour is a far more potent greenhouse gas than CO2 yet no one seems to be concerned about it when burning hydrogen.”
This piqued my interest and, without wishing to tread on the toes of Tom Baxter – whose well-argued articles and posts on the subject of the unseemly rush towards hydrogen as a supposed replacement for fossil fuels on LinkedIn I recommend highly – I thought I would take a look at the current thinking more closely.
The questioner is right that very few people are concerned about water vapour as a greenhouse gas, other than a few who we might most kindly call contrarians. There are a number of reasons for this:
- It is not just disputed whether water vapour is a more potent greenhouse gas than carbon dioxide: it is disputed whether the global warming potential of water vapour can even be calculated. Unlike the more commonly recognised greenhouse gases (carbon dioxide, CFCs, methane, nitrogen oxides and ozone), water is condensable, and there are large bodies of it all around Earth. Higher temperatures consequently make for more atmospheric water vapour, which brings us to point 2:
- It is therefore commonly held that either the amount of water vapour in the atmosphere is controlled by the temperature, rather than vice versa, or that these two things are related in a complex way.
- Whilst maybe 50% of global warming is due to water vapour in the atmosphere, the overwhelming majority of that water vapour is nothing to do with human activity.
Water vapour is most commonly held to be an example of positive feedback from the environment on rising temperatures, amplifying the effect of other, more persistent greenhouse gases. However, this does not mean that burning hydrogen in air is emission free, even if hydrogen combustion only has one direct product – water vapour. This is because burning pretty much anything in air produces nitrogen oxides (NOx). Although there are quibbles and spin about the magnitude of the effect, it is clear that burning hydrogen in air generates a lot more NOx than burning methane in air per unit energy output.
NOx are already major urban pollutants, despite the mandatory NOx abatement on all new internal combustion-based vehicles. They have significant effects on human health, and produce a wide range of secondary pollutants including ozone (a greenhouse gas high up in the atmosphere, and a health hazard low down) and particulate matter. Hydrogen combustion is not inherently or inevitably emission free, even if it is “net zero”.
NOx emissions can be avoided by burning hydrogen in pure oxygen, using fuel cells, or requiring NOx abatement technology. All of these are rather too expensive, novel and/or rather complex to roll out into every home and car in the country, and are rarely included in consideration of the costs of doing so. Sure, you could do it, but look at how much trouble people are having just with domestic heat pumps!
Of course, hydrogen combustion can only be “net zero” if the hydrogen being burned is so-called “green” hydrogen, produced without carbon emissions. Hydrogen is not a fuel at all, just an energy carrier. As of now, green hydrogen represents less than 1% of production. There is a long way to go before hydrogen is even genuinely free of greenhouse gas emissions.
So, burning hydrogen in air only produces water vapour directly, which causes no direct global warming. But it also produces NOx, which indirectly cause global warming, and directly impact human health. Making the hydrogen to be burned almost certainly caused significant greenhouse gas emissions. Clearly a great opportunity for greenwash! Like so many things in engineering, the barriers to a zero-emissions hydrogen economy are mostly not technical – they are political. But that does not mean that they are trivial. On the contrary.
