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Chapter 44.4.7 Increasing the O 2 /SO 2 ratio of the inlet gasDescriptionThe thermodynamic equilibrium limits the conversion and, hence, impacts on the residual SO 2emission level. This equilibrium very much depends on the oxygen and sulphur dioxide contentof the inlet gas, and also the O 2 /SO 2 ratio. Modern sulphur burning plants operate with a feedgas of 11.8 % v/v SO 2 and a residual 8.9 % v/v O 2 , resulting in an O 2 /SO 2 ratio of 0.75. Thisenables the plant to match emission levels below 2 kg SO 2 per tonne of H 2 SO 4 produced(equivalent to a conversion rate of about 99.7 %).Plants frequently supply high strength SO 2 gas, particularly those using oxygen-enriched air ordilution air to adjust the O 2 /SO 2 ratio.Achieved environmental benefitsA reduction of the SO 2 emissions can be achieved. The example plant achieved an emissionreduction of approximately 30 %.Cross-media effects• lower energy output• consumption of oxygen or oxygen-enriched air.Operational dataNo specific information provided.ApplicabilityIn particular this measure is applicable for existing sulphuric acid plants, if the actual productionis lower than the production capacity. At new plants, a higher dimensioning of the plant wouldhave to be considered.EconomicsTable 4.14 gives a cost estimate for increasing the O 2 /SO 2 ratio of the inlet gas.Driving force for implementationReduction of SO 2 emissions.References to literature and example plants[57, Austrian UBA, 2001, 67, Daum, 2000], Donauchemie GmbHLarge Volume Inorganic Chemicals – Ammonia, Acids and Fertilisers 183