The product composition and structure of sulfur compounds

When elemental sulfur or a sulfur-bearing compound is present in any combustion system, the predominant product is sulfur dioxide. The concentration of sulfur trioxide found in combustion systems is most interesting. Even under very lean conditions, the amount of sulfur trioxide formed is only a few percent of that of sulfur dioxide. Generally, however, the sulfur trioxide concentration is higher than its equilibrium value, as would be expected from the relation 

These higher-than-equilibrium concentrations may be attributable to the fact that the homogeneous reactions that would reduce the S03 to S02 and 02 are slow. This point will be discussed later in this section. 

It is well known that S03 has a great affinity for water and that at low temperatures it appears as sulfuric acid H2S04. Above 500°C, sulfuric acid dissociates almost completely into sulfur trioxide and water. 

Under fuel-rich combustion conditions, in addition to sulfur dioxide, the stable sulfur products are found to be hydrogen sulfide, carbonyl sulfide, and elemental sulfur. 

FIGURE 8.10 Equilibrium distribution of sulfur-containing species in propane-air flames with unburned gases initially containing 1% S02 (from Johnson et al. [40]). 

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