The technology comprises a gas boiler fueled by 100% hydrogen gas (H2). It is analogous to the widely used natural gas condensing boiler. However, compared to a natural gas boiler, it would need adjustments in different components of its design (e.g. the burner, flame detection device) to be able to burn hydrogen (in a safe and efficient manner) (Frazer-Nash, 2018a). To be considered renewable, the hydrogen has to be produced from a renewable source, such as electrolysis using renewable electricity (i.e. green hydrogen). In this factsheet only the boiler itself is considered; not the hydrogen supply chain.
In the boiler hydrogen gas is burnt in a combustion section. The heat is transferred to water through water-cooled walls and through a water heat exchanger after the combustion section. In a condensing boiler, a second heat exchanger is placed before the flue gas exit to collect the latent heat contained in the flue gases. The hot water from the gas boiler is circulated in the radiators of the building (DAE, 2016).
As a consequence of its diffusive and buoyant properties hydrogen is more prone to leakage through joints, valves and the materials in some types of pipework compared to natural gas. A new odorant may need to be developed for hydrogen. Hydrogen will rise and dilute with ambient air and the odorant will therefore need to be detectable in low concentrations (Frazer-Nash, 2018b).
Due to a greater flammability range than natural gas (4-75% compared to 5-15%), there is an increased risk of unintentional combustion, but research so far has shown that the diffusive and buoyant properties cause the hydrogen to disperse before critical concentrations were reached (Frazer-Nash, 2018b).
For hydrogen boilers a new burner will need to be developed. A new Flame Detection Device (e.g. IR or UV) will be required for hydrogen as ionisation sensors currently used with natural gas are not appropriate for hydrogen. For visibility a colourant could either be added to the unburnt gas prior to combustion or added to the combustion section (Frazer-Nash, 2018b). It will also be necessary to select and test materials that are suitable for higher temperature combustion. Adapting natural gas boilers to hydrogen is, in theory, possible although given the lack of space inside the appliances this will be practically challenging. (Frazer-Nash, 2018a).