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

Abstract

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 +39%, with a mean change across 14 studies of +8%. 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 +362 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 +154% with a change in damage costs of between –467 million GBP and +1,146 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.