Previous
Table of Contents
Next
At the high temperatures and oxygen concentrations typical of combustion, sulphur combines with carbon, hydrogen and oxygen to form SO2, SO3, SO, CS, CH, COS, H2S, S and S2 (Hunter (1982)). Under such circumstances almost all of the sulphur is in the +4 oxidation state (Harris (1990)), hence sulphur dioxide (SO2) is the predominant sulphur compound formed in combustion. Even with 20 % air deficiency, 90 % of the sulphur is in the form of SO2 and as little as 0.1 % is as SO3; SO accounts for the remainder of the sulphur.
At lower oxygen concentration (40 % deficiency) H2S, S2 and SH are also present in significant proportions, while SO3 becomes negligible. During combustion these species are in superequilibrium concentrations. As the gases cool their rates of consumption decrease and equilibrium may be "frozen" before the products reach room temperature (Cullis and Mulcahy (1972)).
In oxygen-rich flames, ie normal operation in combustion facilities, SO and SO3 are present, as well as H2SO4 as a result of the combination of SO3 and H2O. Sulphuric acid is responsible for corrosion in combustion equipment. This is a major reason for controlling sulphur combustion.
The European Union has set the limits of SO2 emissions to be dependent on the power output of the plant (EEC: Council Directive of 24 November 1988). Thus, for less than 300 MW, the emissions of sulphur dioxide allowed are 1,700 mg SO2/nm3. For power output greater than 500 MW, only 400 mg SO2/nm3 are allowed.
Previous | Table of Contents | Next |