Effects of CO2 impurity in the range of 0 - 50 mol% in acid gas feed to a Claus once through process have been studied for air and pure oxygen combustion media. The process comprising a reaction furnace, three sulfur condensers and two converters is simulated. A computer program is developed. Results showed the effects on adiabatic flame temperature, heat recovery, sulfur recovery and equipment sizes using air and pure oxygen in combustion. Correlations were obtained for these effects. Adiabatic flame temperature could exceed the maximum allowable limit in furnace design when mol% CO2 is lower than 44% pure oxygen is used. In this case, SURE double combustion technique can be applied. Net heat recovery increases by about 3%, whereas overall sulfur recovery is practically not changed. S-x and COS in tail gas exceed ambient air allowable limit whereas CS2 is within the allowable limits, so further tail gas treatment is necessary. An increase in equipment size in the range of CO2 studied reaches up to 34% when air is used in combustion, whereas it reaches up to 98% when oxygen is used. By replacing air with oxygen for a specified CO2 concentration, a decrease up to 65% in equipment size is predicted.