Flameback identification and air intrusion prevention in small flow hydrogen flare stack emissions

Abstract

In some hydrogen production, storage and applications, the flare stack is used as an indispensable protection system for the emission and ignition of surplus hydrogen. However, there is a risk of flameback when emissions at small flow rate. Thus, a series of studies were carried out to explore the flameback identification and air intrusion prevention in small flow emissions. In this study, a flare stack with a molecular sealing, which was 12 m in height and 100 mm in diameter, was selected as the physical object. And a 3D numerical model was established. (1) The small flow hydrogen flare stack emissions were performed when the nitrogen purge was completed, and in this study the purge was finished in 80 s (2) The flameback case of the numerical model in this paper was used along with the US Bureau of Mines experimental to verify the accuracy of the flameback limit flow theoretical model. Furthermore, the lower limit of the theoretical curve was extended from 100 mm to 1 mm by theoretical calculation. (3) In order to comprehensively consider the combined influence of turbulence and buoyancy at the flare stack outlet, we proposed a new dimensionless parameter Fd to represent the flow state by the Reynolds and Richardson numbers. Based on this, a general flameback identification criterion was proposed. (4) When the nitrogen/hydrogen ratio was within the range of 40–56 vol%, nitrogen auxiliary pressurization effectively inhibited air intrusion and the expansion of the flammable range.