Corrosive properties of combustion gases produced from liquid fuels

The mechanism of formation of sulphur trioxide in combustion gases has been investigated using liquid fuel firing combustor with laminar pre-mixed flame. The combustion chamber was constructed from a stainless steel tube for minimizing any affect of catalysis. A temperature controlled evaporating chamber was designed where liquid fuel was evaporated and mixed with total combustion air prior to entering the combustion zone. The vapour mixture then passed through a silica sintered disc and a flat flame was produced. Six sampling ports at 0.304m (1 ft) intervals along the combustion tube were used for sampling the gases for analysis and for the measurement of sulphur trioxide with respect to residence time. Diesel oil, Kerosine, Cyclohexane, n-hexane, and n-Pentane were used in the investigation: Sulphur content of these fuels were raised to 3.4% (wt.) by addition of appropriate amount of carbon disulphide (CS2) in each fuel. It was found that SO3 was only formed when there was an excess oxygen in the combustion gases and under sub-stoichiometric conditions, no SO3 could be detected. An increase in the quantity of combustion air in excess of stoichiometric requirement lead to an increase in the concentration of SO3 in the combustion gases. However, the level of SO3 content in the flue gases reached a maximum at about 4% excess oxygen concentration in the flue gases. It was established that ignition properties of fuels have an effect on the oxidation of S02. Ignition delay v temperature curves for the fuels employed in the research programme were determined. This was done with the help of Ignition-Delay apparatus (PART II). It was found that fuels having shorter ignition delay times at. temperatures. prevailing near flame zone produced less SO3 under identical combustion conditions. The results of the effect of residence time of combustion gases in the high temperature zone showed that in the first instance amount of SO3 formed was in excess of those predicted from thermodynamic considerations involving molecular oxygen. Sulphur trioxide thus formed began to dissociate back into sulphur dioxide and oxygen as the gases continued to pass along the combustion chamber. It was thought that oxygen atoms, produced in the flame, being a reactive oxidising species are responsible for the oxidation of SO2. The theory proposed to explain the experimental results appear to be that of a consecutive reaction [see full text for details].