Claus plants process acid gas removed from the raw natural gas. The acid gas first passes through a thermal stage in which H2S is partially burned to SO2 which reacts with H2S to form elemental sulfur and water. In subsequent catalytic stages, H2S and SO2 are converted to sulfur at lower temperatures. The oxygen enriched air supply of the Claus furnace allows a higher throughput of acid gas with lower energy consumption and less hydrogen demand in tail gas treatment. A robust and reliable oxygen analyzer controls the oxygen levels in the combustion air after O2 injection.