NIA 344: Ignition Consequences, Final Report

The UK government has passed into law its ambition to reach net zero greenhouse gas (GHG) emissions by 2050. The scale of this task is such that policy makers are required to make decisions now in order to enable the necessary reduction in GHG emissions in all aspects of life. One of the largest contributions of GHG in the UK is from heating homes, which predominantly uses the natural gas transported via transmission and distribution networks directly to homes where it is burned in boilers to heat water and, to a lesser extent, to cook with or heat spaces in gas fires. One alternative for reducing GHGs from home heating being examined by the government is to transition the UK gas network to the supply of 100% hydrogen. A key element of any policy decision is to have an informed position on the relative safety of such a change.
Flammable gases such as natural gas and hydrogen have the potential to cause an explosion if an accidental release occurs within a building. Although such explosions are very rare with natural gas, it is important to understand how the differing properties of the two gases (natural gas and hydrogen) might influence the explosion behaviour, as this will assist in both understanding the relative risks and the benefit provided by additional mitigations that could be introduced at the time of transition.
Gas explosions in buildings are complex as they involve an interaction between the explosion process and structural failure, and even subtle changes in phenomena can significantly alter the severity of an explosion. To better understand the effect of changing from natural gas to hydrogen, the NIA344: Ignition Consequence Project has been undertaken. The project had the objective of performing experimental research into the various phenomena contributing to the severity of explosions in domestic structures and had three phases. The experiments have mostly been performed in a purpose-built explosion chamber where different configurations inside the chamber and at the venting (front) face were set up.
Measurements of explosion overpressure inside and around the chamber allowed assessments of the comparative severity and consequence of the explosions to be assessed and compared between hydrogen and methane fuel : air explosions (methane representing natural gas, of which it is the primary constituent). A general arrangement figure and a collage of an experiment in progress are given in Figure 1 and Figure 2 respectively.
In this programme a total of 95 experiments have been conducted to study the effect on explosion severity of five different phenomena, individually and in combination:
♦ The effects of gas concentration and accumulation patterns (established in other programmes) on pressure development in an explosion (Build-up effects).
♦ The vent face was covered with a polythene sheet only.
♦ The effects of weak structures on explosion severity (Confinement).
♦ The effects of furniture / obstacles on pressure development (Congestion).
♦ The effects of interconnecting rooms (Multi-room).
Detonation in hydrogen-air mixtures either through deflagration to detonation transition (DDT) or direct initiation (Detonation Initiation);
One important factor affecting explosion severity is the laminar burning velocity, which is a fundamental property of flammable gases in well-defined conditions. The laminar burning velocity of hydrogen : air mixture at 15 %vol is similar to the maximum for methane : air mixtures (observed at around the stochiometric concentration of 9.5 %vol) and falls well below this as the hydrogen concentration reduces further. Based on laminar velocity alone, hydrogen explosions at a concentration of 15 %vol should be no worse than the worst case with methane (or natural gas) and considerably less at lower hydrogen concentrations. However, real explosions in domestic settings are likely to feature multiple explosion phenomena together. Given that proposed mitigations for end users have the objective of limiting the potential for higher hydrogen concentrations, it is important to understand if explosion severity is characterised by laminar burning velocity alone or whether other factors play a part.
This report was submitted to HSE for their assessment of the safety evidence for 100% hydrogen heating, which can be found at Hydrogen heating: HSE assessment of the safety evidence - GOV.UK.
Queries should be directed to DESNZ: https://www.gov.uk/guidance/contact-desnz.