Gas Build-up in a Domestic Property Following Releases of Methane/Hydrogen Mixtures

1. Gas Build-up in a Domestic Property Following Releases of Methane/Hydrogen Mixtures Barbara Lowesmith, Geoff Hankinson, Catalina Spataru and Michael Stobbart

2. Naturalhy Project • Aim is to assess the possibility of using the existing natural gas pipeline network for conveying natural gas/hydrogen mixtures • Provides ‘greener’ fuel if burned directly • H2 can be separated out for use in fuel cells • Provides a means of introducing and developing the Hydrogen economy at an earlier stage

3. Naturalhy Project • Naturalhy Project studying the impact of introducing Hydrogen into Natural Gas pipelines • Effect on durability and integrity of pipeline network • Effect on safety of the public • Separating the Hydrogen from the mixture • Using the mixture in existing appliances • Socio-economic aspects

4. Objective of Safety Work • To assess the change in risk to the public caused by adding hydrogen to the gas network • In particular, for Domestic customers – does the risk in the home increase?

5. Domestic Customers • To answer this question we are: • Assessing gas accumulation behaviour by performing large scale experiments and developing mathematical models • Determining minimum ignition energies and ignition probabilities by laboratory experiments • Assessing explosion severity by models and experiments

6. Gas Build Up Experiments • ‘Room’ - 3m x 3m x 2.3m • Door in front wall • Release in centre of back wall • Vent openings in two opposing walls – upward crossflow ventilation by wind

7. Gas Build-up Experiments • Methane, or methane:hydrogen mixtures (80:20 and 50:50) • At 20-30mbar from 5 or 10mm diameter hole close to floor or 1.1m above floor • Measured gas concentration throughout enclosure with time using oxygen cells

8. Typical Results – Release at 1.1m above floor

9. Typical Results – Layer Formation

10. Mathematical Model - Objectives • To predict with time the position of the interface and the concentration in the upper layer • To validate against experimental data • To apply model to study effect of different release conditions

11. upper buoyant layer wind air entrained into jet Qout Qj Hl upper vent opening H jet Qs lower vent opening interface h hs Qin lower layer horizontal cross-sectional area, S Mathematical Model

12. Mathematical Model • Equations describing: • Volume of layer formed and concentration within it • Air flow as a combination of wind and buoyancy induced flow • Buoyancy induced flow as a function density (and hence concentration) of layer • Air entrained into jet of gas released

13. Equations: Layer • Volume of Layer: • Concentration within Layer:

14. Equations: Ventilation • Combination of wind driven and buoyancy induced: • Wind driven: • Buoyancy Induced: where:

15. Equations: Jet • Air entrainment proportional to local mean jet velocity • Entrainment constant of 0.05 used

16. Validation of Model- Transient Prediction

17. Steady State Comparison

18. Application of Model-Effect of H2 Content

19. Conclusions • Adding H2 to Natural Gas results in increased volume flow when a leak occurs • This leads to increased gas concentration in an accumulation formed • However, the buoyancy forces are greater for the lighter gas resulting in increased ventilation • This mitigates the gas concentrations to some extent

20. Conclusions • However, the concentrations are higher and the flammability limits are wider • This means that a flammable inventory persists for longer • Hence the risk to the domestic customer may increase

21. Further Work • Extend capabilities of model • Quantify the ignitability of accumulations following methane/hydrogen releases • Assess the change in risk to the public in the home environment as a result of introducing a given level of hydrogen to the natural gas