Scientist warns of NOx urban pollution from hydrogen boilers

Ahead of the rumoured publication of the government’s Hydrogen Strategy next week, a leading atmospheric chemist has said that switching to hydrogen boilers could widen inequality in exposure to air pollution.

In a letter to the leading scientific journal Nature, professor Ally Lewis, chair of the DEFRA’s Air Quality Expert Group and a director of the National Centre for Atmospheric Science, while acknowledging hydrogen’s promise “as a clean, low carbon fuel”, warned that it would be “mostly burnt in engines and boilers rather than being used in fuel cells. The burning of hydrogen generates toxic nitrogen oxides (NOx) as well as steam. This pollution could disproportionately impact the urban poor.”

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The formation of NOx from hydrogen combustion results from a reaction with the nitrogen that makes up 78% of the atmosphere. But the issue has been neglected in ongoing discussions about the costs, benefits and practicalities of using it for home heating, transport and other energy needs.

A report from the industry group the Energy and Utilities Alliance (EUA), backing hydrogen boilers, ignored the issue. But so did a letter from think tank E3G to energy secretary Kwasi Kwarteng opposing the use of blue hydrogen – produced from natural gas – rather than green hydrogen, produced using renewable power. It has also never been raised in parliament, despite hydrogen being an increasing topic of debate in recent years and months.

“There is no question that very low NOx hydrogen boilers could be made, but the question is whether they will be — in the absence of additional regulation. NOx emissions are traded off against cost and energy efficiency. A likely outcome under the status quo would be hydrogen boilers that met current air quality standards but didn’t improve on them. Given these are locked in for up to 20-30 years, it leaves a substantial legacy,” Lewis told ENDS.

“By the 2040s, hydrogen combustion for domestic heating could be the last major source of NOx in cities, as ever more road vehicles switch to electric power. Nitrogen-oxide emissions from hydrogen boilers will be concentrated in areas of high-density housing, often associated with low-income households. A widening of inequality in exposure to nitrogen dioxide could be an unwelcome side effect of this net-zero policy without new regulation and innovation in after-treatment technologies,” his letter to Nature explained.

According to the latest leaks from Whitehall, the forthcoming Heat and Buildings Strategy will stop the installation of natural gas boilers from 2040, seven years after the Climate Change Committee’s recommended date. But this would not stop hydrogen-ready boilers being fitted – assuming that hydrogen to feed them can be produced at sufficient scale.

In an accompanying paper in the journal Environmental Science: Atmospheres, Lewis emphasised that the use of hydrogen is not “without side effects and the widely claimed benefit that only water is released as a by-product is only accurate when it is used in fuel cells”, an electrochemical rather than combustion process.

“New more demanding hydrogen-specific NOx emissions standards are required for a range of appliance sectors to ensure that low carbon infrastructure associated with the adoption of hydrogen also delivers a step-change in air quality. Placing hydrogen power within existing air quality regulatory frameworks (for example the Ecodesign Directive or Euro vehicle standards) may see NOx emissions, efficiency and cost optimised in a way that leads to hydrogen appliances matching current fossil fuel emissions performance, but potentially not improving on them. This would be a major missed opportunity to further reduce NOx emissions and improve air quality as a co-benefit of net zero commitments and low carbon investment,” it adds.

“As we take the necessary steps to move away from fossil fuels, it is essential we select solutions which are cleaner and fairer. Today’s warning shows that a shift to hydrogen could repeat the problems of the past,” said E3G senior policy advisor Juliet Phillips.

“Swapping one polluting technology for another is not the solution. Action and innovation is needed to drive down NOx emissions before hydrogen-ready boilers are installed in homes. Rather than delay the transition away from fossil gas, the government can focus on promoting readily available clean heat solutions, such as heat pumps and energy efficiency measures, which are already supporting access to low carbon heat across Europe,” she added.

Joshua Emden, a research fellow at think tank IPPR, agreed. “We cannot have a two-tier system where polluting hydrogen boilers are installed in lower-income households, while other houses have cleaner low-carbon heating options available. This inequality risks being hugely disruptive to people’s confidence at a key moment where we need to be scaling up home decarbonisation,” he told ENDS.

“Our research and focus groups consistently show that previous government policy failures like the recent Green Homes Grant and the diesel emissions scandal have a negative effect on people’s trust, so tackling this issue is crucial to delivery,” Emden added.

But the EUA dismissed such concerns. “Using zero carbon hydrogen to heat homes not only improves the air quality from already historically low levels from appliances, it also totally eliminates carbon monoxide, the silent killer, that can be emitted from faulty and poorly maintained products,” said a spokesperson.

“Testing on hydrogen boilers shows them halving NOx levels from the current EU standard requirements for a natural gas boiler. That has to be good news,” he added.