Flashback, burning velocities and hydrogen admixture: Domestic appliance approval, gas regulation and appliance development

摘要

Introducing natural gas/hydrogen mixtures to an installed population of domestic natural gas appliances necessarily implies considering the risk of flashback. Previously, we quantified this risk via an interchangeability analysis using calculated laminar burning velocities. With an increasing contribution of renewable energy, still higher hydrogen fractions will become of interest to improve the economic viability in power-to-gas chains. To extend the possibilities for hydrogen admixture beyond the limits given by extant ranges of Wobbe Index and burning velocity, appliance approval standards and gas regulations must be examined to assess the degree to which higher hydrogen fractions are, or can be, justified. However, the current standards and regulations do not consider the risk of flashback in terms of the laminar burning velocity explicitly, leaving the justification of higher hydrogen fractions to empirical observations followed when the approval standards were codified. Here, we reframe the approval and regulation standards in terms of the calculated laminar burning velocity, which quantifies the notion of a 'safety margin' to safeguard appliance performance with respect to flashback, for a group of natural gases that is commonplace in the European Union (EU) but representative for many international situations. The method presented can be applied for any local regulatory area. In plots of burning velocity vs. equivalence ratio, ranges of regulated gas qualities are represented as a curve for natural gases, while for natural gas/hydrogen mixtures they appear as areas indicating the variations in hydrogen fraction for different gas compositions that do not increase the risk of flashback. To quantify the safety margin, the approval gas used in the EU for flashback (G222) is taken as an example, because of the many decades of experience in using this gas to safeguard appliance performance. Using the assumed range of gas quality and approval gas as an example, for appliances whose primary equivalence ratio is fuel rich (at greatest risk for flashback), a safety margin of 11.5 cm/s is determined and used in analyses for determining the composition of flashback limit gases in approval standards for a situation in which higher hydrogen fractions are desired. Situations considering both variable and constant fractions of hydrogen in natural gas are examined. The end-use demand for a minimum degree of thermal comfort, by having a minimum Wobbe Index in the regulated range of gas quality, automatically complicates grid management schemes for hydrogen addition: the maximum hydrogen admixture is necessarily coupled to the composition of the natural gas to which it is added. The only solution for having a constant hydrogen fraction without regard to the gas composition is by relaxing this demand on thermal comfort; in the example used here, 20% hydrogen admixture reduces the thermal comfort in the worst case by 4.7%. Fuel suppliers, grid operators and end users must agree to this loss of fitness for purpose to maximize the decarbonization of the gas supply by hydrogen admixture.