Your questions about hydrogen blending trial Nearly 700 Gas Safe registered engineers from the Gateshead and Newcastle areas joined a webinar recently to find out more about the trial that will introduce blended hydrogen into the natural gas supply in Winlaton in early 2021. During and after the webinar, organised by Northern Gas Networks, attendees asked a number of questions, some of which are answered here by Tommy Isaac, principal engineer at Progressive Energy. Appliance operation Which appliances have been tested with the hydrogen blend? Extensive appliance research has taken place at HSE and Kiwa Gastec laboratories, along with expertise from Blue Flame Associates and appliance manufacturers. Safety and performance testing has been carried out on boilers, cookers, ovens, grills, fires and heaters to assess heat outputs, temperature profiles, combustion efficiencies and flue gas analysis, as well as safety devices such flame detectors. The overall conclusion of the appliance research has been to demonstrate that current domestic appliances operate safely and within the bounds of normal operation with a blend of hydrogen up to 20 vol%. Will the heat output of appliances change? The heat output of an appliance is determined by the Wobbe number of the fuel being supplied. The Wobbe number of a gaseous fuel is the amount of heat that will be provided for a given pressure drop over a gas valve. The gas characteristic takes account of both the calorific value of the fuel as well as the fluid properties that dictate its flowrate through a valve – namely its density. Therefore, gases with the same Wobbe number will provide the same heat output within an appliance. A 20 vol% hydrogen blend remains within the current non-emergency natural gas Wobbe limits, as stipulated by the GS(M)R legislation, which is 47.2-51.4 MJ/m3. Therefore, the heat output of appliances operating in the UK won’t be 30 February 2021
changed because the range of fuel quality provided to appliances won’t be impacted. Will I need a new flue gas analyser? Flue emissions are of key concern with any change in gas quality, and a reduction in CO2 emissions is the principal motivation behind the adoption of hydrogen in the gas network. The appliance research has demonstrated that both CO
manufacturers Kane and Crowcon (manufacturers of the Anton Sprint), using a bespoke research facility constructed through the HyDeploy programme at Keele University. The research confirmed that current flue gas detectors (both direct and indirect CO2 measurement) can continue to be used with a hydrogen blend, without the need to know if the blend is present or not. This
“Much like spraying deodorant in the air, hydrogen and methane will mix by themselves and stay mixed.” and CO2 production reduce when hydrogen blends are introduced: therefore, from a toxicity perspective, the combustion results in a safer emission profile. There is also a general reduction in NOx emissions. This effect is created because of a slight cooling of the flame, which in turn is the result of a slight increase in excess air, caused by the lower volumetric air requirement of hydrogen to fully combust. Flue gas analysis research was conducted in collaboration with Worcester Bosch, Vaillant, Baxi and Ideal as well as gas detection
The Boiler Research Facility at Keele University
research was published by Gas Safe Register in Technical Bulletin 157, published in May 2020. Gas characteristics Could hydrogen and methane separate in the pipework? The notion of separation within pipework is a common misconception about hydrogen blends. A great deal of research, both desk-based and experimental, has taken place in the field of gas characteristics to understand the fundamental physics of a hydrogen blend so as to inform operational procedures and technical assessments. Hydrogen and methane are two of the most ideal gases available: therefore, they mix extremely effectively. Much like spraying deodorant in the air, the two gases will mix by themselves and stay mixed. The natural tendency of a system to maximise its entropy results in a uniform mixture of gases that stay mixed throughout the pipework.
Will a hydrogen blend leak more readily than natural gas? Some people have pointed out that hydrogen is a very small molecule and so it might find leak paths through which natural gas can’t travel. This is a misconception: a hydrogen molecule is smaller than a methane molecule (its diameter is 84 per cent of a methane molecule), but both molecules are infinitesimally small relative to a leak path. If a leak path was a mountain tunnel, methane would be a bee and hydrogen would be a fly. Leak rates are determined by flow regimes. Small leaks, such as those identified through drop testing, operate in the laminar flow regime: for these potential leaks there is a negligible change in leak rates of less than 1 per cent. For larger leaks, such as a full-bore rupture, the leak flow will be turbulent. In this flow regime, the leak rate could increase by up to 10 per cent: however, because the blended gas is more buoyant, the resulting ventilation rate within the room also increases. During a large gas leak, the gas will rise and escape. This draws air into the room and, over time, an equilibrium concentration of gas occurs. Even though a large leak would increase by up to 10 per cent, the resulting ventilation caused by the escaping gas will also increase. This results in no change to the ultimate concentration of fuel in the room – it is a self-correcting system because both the leak rate and resulting ventilation rate are governed by the gas’s density.
