Emissions of NOx from blending of hydrogen and natural gas in space heating boilers

Alastair C. Lewis*, Madeleine L. Wright

*Corresponding author for this work

Research output: Contribution to journalArticlepeer-review


As part of climate change commitments, the United Kingdom is considering an incremental transition from natural gas to hydrogen for domestic heating, blending up to 20% of hydrogen (by volume) into the national gas network. We consider the possible impacts of this policy on nitrogen oxides (NOx) emissions, a minor waste by-product from combustion. A meta-analysis of changes in NOx emissions from hydrogen/natural gas blends used in gas burners is undertaken, with focus on mixtures between 5% and 20% v/v. Literature reports are highly variable: for a 5% hydrogen blend, changes in NOx emissions, when compared to burning pure natural gas, vary over the range –12% to þ39%, with a mean change across 14 studies of þ8%. These estimates required an important assumption to be made that, when not explicitly described, all literature data on changes in NOx emissions and/or concentrations were suitably corrected for the reduced energy density and heat output arising once hydrogen is added. A NOx increase can be rationalized through the increased adiabatic flame temperature generated from hydrogen combustion. The associated range of plausible damage costs of a 5% hydrogen blend is estimated to fall within the range –117 million GBP to þ362 million GBP per year; 20% hydrogen (the maximum that could be accommodated with existing infrastructure) would lead to a change in emissions in the range –50 to þ154% with a change in damage costs of between –467 million GBP and þ1,146 million GBP per year. The mean change is estimated at 292 million GBP per year. For existing poor performing boilers, an economic case can be made for scrappage and replacement based primarily on NOx damage costs avoided. The response of older boilers to added hydrogen is a critical evidence gap that needs filling before further decisions on hydrogen as a heating fuel are made.

Original languageEnglish
Article number00114
Number of pages15
Issue number1
Publication statusPublished - 31 May 2022

Bibliographical note

© 2022 The Author(s).

Funding Information:
ACL acknowledges financial support from the National Centre for Atmospheric Science National Capability underpinning program of Natural Environment Research Council.

Funding Information:
This study has been supported by the ment Research Council.


  • Domestic combustion
  • Hydrogen
  • Natural gas
  • Net zero
  • Nitrogen oxides
  • Space heating

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