30% of the natural gas fueling UK homes and businesses could be replaced by hydrogen without requiring any changes to the nation’s boilers and ovens, a study by Swansea University researchers has shown. Over time the move could cut UK carbon dioxide emissions by up to 18%. The open-access study is published in the RSC journal Sustainable Energy & Fuels.
The difference between the two percentages—30% hydrogen enrichment and a potential 18% drop in carbon emissions—arises from the fact that hydrogen-enrichment lowers the calorific content of the fuel, necessitating a higher volume of gas for a given energy output.
Domestic gas usage accounts for 9% of UK emissions. In an effort to reduce annual carbon emissions, there is presently a concerted effort from researchers worldwide to offset the usage of natural gas. Enriching natural gas with hydrogen (HENG) is one option. Experiments have shown that modern-day gas appliances work safely and reliably with hydrogen-enriched natural gas as the fuel. It is already used in parts of Germany and the Netherlands. A £600-million (US$805-million) government-backed trial is taking place in the UK this year.
Natural gas naturally contains a small quantity of hydrogen, although current UK legislation restricts the allowed proportion to 0.1%. The question the Swansea team investigated was how far they could increase the percentage of hydrogen in natural gas, before it became unsuitable as a fuel, for example because the flames became unstable.
European trials have already been conducted to investigate the practical implementation of hydrogen-enriched natural gas (HENG) within a mains gas supply. In this work, the limitations of such a strategy are evaluated based on a novel meta-analysis of experimental studies within the literature, with a focus on the constraints imposed by the phenomena of flash-back and blow-off.
Through consideration of the Wobbe Index, we discuss the relationship between molar hydrogen percentage and annual carbon dioxide output, as well as the predicted effect of hydrogen-enrichment on fuel costs. It is further shown that in addition to suppressing both blow-off and yellow-tipping, hydrogen-enrichment of natural gas does not significantly increase the risk of flash-back on ignition for realistic burner setups, while flash-back at extinction is avoided for circular port diameters of less than 3.5 mm unless the proportion of hydrogen exceeds 34.7 mol%. It is thus proposed that up to 30 mol% of the natural gas supply may be replaced in the UK with guaranteed safety and reliability for the domestic end-user, without any modification of the appliance infrastructure.—Jones et al.
The team—Dr. Charles Dunnill and Dr. Daniel Jones at the University’s Energy Safety Research Institute (ESRI)—found:
An enrichment of around 30% is possible, when various instability phenomena are taken into account.
Higher percentages make the fuel incompatible with domestic appliances, due to hydrogen’s relatively low energy content, its low density, and a high burning velocity.
30% enrichment by hydrogen nevertheless equates to a potential reduction of up to 18% in domestic carbon dioxide emissions.
The research was supported by the Welsh Government Sêr Cymru Program, FLEXIS, which is partially funded by the European Regional Development Fund, and the King Saud University as well as by the Welsh Government through the Sêr Cymru Chair for Low Carbon Energy and Environment. Co-author is Waheed Al-Masry an assistant professor in the Department of Chemical Engineering at King Saud University, Riyadh, Saudi Arabia.
Daniel R. Jones, Waheed A. Al-Masry and Charles W. Dunnill (2018) “Hydrogen-enriched natural gas as a domestic fuel: an analysis based on flash-back and blow-off limits for domestic natural gas appliances within the UK” Sustainable Energy & Fuels 2, 710-723 doi: 10.1039/C7SE00598A