(256c) Bench Scale Saes St 909 Tests for Methane and Carbon Dioxide Removal from Helium, Hydrogen, and Nitrogen Gas Streams

SAES St 909, a Zr-Mn-Fe alloy, was developed for decomposing, or ?cracking?, tritiated water in fusion reactor effluents. St 909 has also been shown to crack methane in helium, but its ability to purify hydrogen streams was uncertain, especially with nitrogen impurities. To better characterize St 909 impurity removal performance, bench scale (six gram) screening tests were performed using ten whole St 909 pellets. Cold (non-radioactive) tests were conducted to determine the ability of St 909 to crack methane and carbon dioxide. The tests used up to 5 vol% carbon dioxide plus methane concentrations to examine the impact of methane decomposition at 700°C and 800°C at 101mPa (760 torr). Carrier gases were helium, helium with 2 vol% ammonia, hydrogen, and nitrogen at a total flow of 10 sccm. Methane cracking was most efficient in helium and less efficient in hydrogen, due to the reduced driving force for the methane decomposition reaction, and in nitrogen, due to nitrogen absorption by the St909. A reduced methane cracking rate in the presence of ammonia was attributed to nitrogen absorption and not the hydrogen released from ammonia cracking. Carbon dioxide decomposition was most efficient in hydrogen, followed by helium, then nitrogen. High carbon dioxide decomposition rates in hydrogen were due to not only carbon and oxygen gettering by the St909, but also by methane, then later, water formation reactions. Carbon dioxide decomposition in nitrogen was reduced by nitrogen absorption by the St909 interfering with the decomposition and diffusion processes. Methane decomposition rates were reduced when carbon dioxide was present. In a helium and a nitrogen carrier, carbon dioxide can effectively stop methane cracking. In hydrogen, carbon dioxide can be reformed into methane to increase the exit methane concentration above its feed concentration. The tests and proposed mechanisms showed carbon dioxide decomposing to carbon monoxide. The carbon monoxide produced was still preferentially decomposed over methane thus suppressing the over-all methane cracking rate.