Optimising air quality co-benefits in a hydrogen economy: a case for hydrogen-specific standards for NOx emissions

A global transition to hydrogen fuel offers major opportunities to decarbonise a range of different energy-intensive sectors from large-scale electricity generation through to heating in homes. Hydrogen can be deployed as an energy source in two distinct ways, in electrochemical fuel cells and via combustion. Combustion seems likely to be a major pathway given that it requires only incremental technological change. The use of hydrogen is not however 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. The burning of hydrogen can lead to the thermal formation of nitrogen oxides (NOx – the sum of NO + NO2) via a mechanism that also applies to the combustion of fossil fuels. NO2 is a key air pollutant that is harmful in its own right and is a precursor to other pollutants of concern such as fine particulate matter and ozone. Minimising NOx as a by-product from hydrogen boilers and engines is possible through control of combustion conditions, but this can lead to reduced power output and performance. After-treatment and removal of NOx is possible, but this increases cost and complexity in appliances. Combustion applications therefore require optimisation and potentially lower hydrogen-specific emissions standards if the greatest air quality benefits are to derive from a growth in hydrogen use.