Understanding hydrogen combustion and NOx emissions

With fossil fuels being phased out due to their finite nature, and impact upon the environment and global climate, economies are looking to more sustainable ways of meeting their energy demand in accordance with moves towards Net Zero carbon emissions.

In the near future, alongside electrification, hydrogen as a fuel will be an important part of our energy mix. This is because hydrogen:

• can be produced with low carbon emissions and does not emit carbon when combusted;
• offers a low carbon solution to difficult-to-electrify aspects of the economy which are currently powered by fossil fuels (for example industrial heat and transportation);
• can be blended with natural gas in National Gas grids to reduce carbon emissions while economies transition away from maintaining national gas networks; and
• offers a means of energy storage for renewable electricity during times when electricity supply outstrips demand.

As a fuel source, hydrogen is energy-dense and has a wide flammability range making it reliable and efficient.

When combusted, it does not produce many common pollutants associated with fossil fuel combustion which are harmful to human health and the environment such as carbon monoxide, carbon dioxide, sulphur dioxide or hydrocarbons. However, combustion of hydrogen is not entirely emission-free. Combustion of hydrogen produces oxides of nitrogen (NOx) as a minor waste product.

NOx

Thermal NOx is formed by the reaction of atmospheric nitrogen (N2) and oxygen (O2) molecules in combustion flue gas at elevated temperatures.

NOx reacts in air to form nitrogen dioxide (NO2) - a globally regulated air pollutant, harmful to human and environmental health, that contributes to formation of photochemical ozone, fine particulates such as PM2.5, nitric acid (acid rain) and eutrophication. Given the potentially harmful nature of NOx emissions, in the UK they are subject to regulations such as the Environmental Permitting (England and Wales) Regulations and Air Quality Standards Regulations.

With the electrification of combustion sources, the overall mass of NOx emissions to atmosphere and associated impacts is predicted to decline, however it is still important to ensure that any residual emissions of NOx are kept as low as reasonably practicable.

This will include using established methods to minimise emissions of NOx from combustion sources, such as:

• use of lean burn;
• managing combustion temperature;
• decreasing flue gas residence time in the high temperature zone;
• reducing the excess O2 in the fuel to air mixture; and
• use of secondary abatement, for example catalytic converters or selective catalytic reduction (SCR).

With the predicted growth in use of hydrogen as a fuel, hydrogen-specific NOx emission standards and guidance on applicability of NOx reduction methods on an international, national, and industry-specific scale, are necessary. In the UK, the EA are in the process of drafting Emission Limit Values (ELVs) for hydrogen combustion plants, in order to manage NOx emissions as technologies develop on an industrial scale.

If you have any questions regarding combustion of hydrogen and its regulation, SLR will be happy to help. Some examples of where we can help include:

1. Prepare environmental permit applications for hydrogen production and consumption as a fuel source;
2. Undertake detailed dispersion modelling of hydrogen combustion emissions;
3. Assess the options and viability of switching from fossil fuel sources to renewable energy sources.
4. Advise on applicability of NOx mitigation measures;
5. Guide on emissions standard in emerging guidance;
6. Undertake safety and environmental risk assessment for the production, storage, and use of hydrogen; and,
7. Act as Owner’s Engineer on hydrogen projects.

Please get in touch to discuss your project.

- Written by Greg Altria (Technical Director, Environmental Management, Permitting & Compliance) and Hannah Jacobs (Senior Consultant, Air Quality)