Decarbonisation Technology - May 2023 Issue

If concentrations are kept below about 15% hydrogen in air, it is no worse than methane at similar concentrations. The implication is that a key element of managing hydrogen safety is the control of gas dispersion and build-up to prevent the concentration of hydrogen in air from exceeding 15% as far as is practicable. This is a particular challenge where dispersal space is constrained – for example, on board ships. Gas detection and rapid isolation of hydrogen inventories will be key measures. Consideration of ventilation rates and ventilation patterns is also critical. Importantly, current simulation methods can model gas dispersion and build-up with reasonable confidence. In summary, although hydrogen’s high explosion reactivity is justifiably concerning, by being aware of this issue and designing to avoid high hydrogen concentrations in the atmosphere, it is reasonable to expect we can engineer facilities that are as safe or better than widely accepted natural gas facilities. If based on a sound technical understanding and addressed in early design, the cost implications of such engineering solutions may not be significant. Hydrogen derivatives Arguably, the most important hydrogen derivative in relation to hazard management is ammonia. Ammonia is flammable, but it is relatively difficult to ignite, and as its burning velocity is well below that of methane, the explosion risk is small. The key hazard with ammonia is its toxicity; it is harmful to personnel at concentrations well below its lower flammability limit of 15% in air. For example, UK HSE indicates a concentration of 0.36% could cause 1% fatalities given 30 minutes of exposure. Concentrations of 5.5% could cause 50% fatalities following five minutes of exposure. While ammonia has been widely manufactured for more than 100 years and is used in considerable amounts in the manufacture of fertilisers, its potential hazards need now to be understood in the context of new energy transition applications, as is the case with hydrogen. A very relevant example is the likely use of ammonia as a fuel in the maritime sector. An ammonia release within the hull of a ship has the possibility to develop

hydrogen flames can burn about an order of magnitude faster than natural gas and much faster than most commonly used hydrocarbons. To add to this, when a flame travels very fast, going supersonic, the explosion can transition to a detonation. A detonation is a self-sustaining explosion process with a leading shock of 20 bar that compresses the gas to the point of autoignition. The subsequent combustion provides the energy to maintain the shockwave. Detonability varies from fuel to fuel, and detonations would not occur in any realistic situation with natural gas but are entirely credible for hydrogen. It is also notable that current explosion simulation methods used by industry are not able to model the transition to detonation but only indicate when it might occur, though there is still considerable uncertainty in this area. This sounds like bad news for hydrogen facilities, yet we know that these properties depend on the concentration of the fuel in air. Figure 2 DNV’s HyStreet Facility sits at the end of the most complete onshore ‘beach to burner’ demonstration of hydrogen use anywhere in the world. DNV’s HyStreet supplies the domestic end-use with 100% hydrogen boilers providing heating, Northern Gas Network’s H21 project demonstrates distribution in the below 7 barg regime, and National Gas’s currently under- construction FutureGrid facility will demonstrate transmission in large-diameter, high-pressure systems (up to 70 barg) Credit: DNV


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