Study on the combustion characteristics of NH3 and the inhibition characteristics of R134a on NH3

Abstract

NH3 has been considered as an alternative to hydrofluorocarbon (HFC) refrigerants due to its environmentally friendly properties, while it is flammable. Therefore, it is necessary to investigate the flammability of NH3 (R717) and find its matching flame retardant to broaden its application. In this paper, the combustion behaviors of R717 and the inhibition characteristics of flame retardant R134a on R717 were studied experimentally and theoretically. Firstly, the flame propagation pattern, flammability limit, laminar burning velocity, and Markstein length of pure R717 were measured by a constant volume combustion bomb from 300 K to 400 K. Then the flammability characteristics of R134a/R717 mixture were further tested. The results indicated that R134a had a distinct flame retardancy on the flammability limits of R717. Further, the flame retarding mechanism of R134a on R717 was analyzed by density functional theory (DFT) on the 6–311++G(d,p) level. The flame retarding effects of R134a were classified to physical and chemical mechanisms. The physical force mainly depended on the dilution effect of additional R134a. The chemical mechanism was contributed to the newly flame retarding reactions introduced by R134a, which reflected in the dehydrogenation of R717 and the consumption of OH and H active radicals by fluorine-contained radicals (CF3, CF2, and F).