Sulphur oxides (SOx)
Unlike the complex formation of NOx nitrogen oxides, SOx sulphur oxides formation takes place during combustion processes in diesel engines only by the oxidation of the sulphur components contained in the fuel. In general, the quantity of sulphur eventually absorbed by the lubricant or emitted as particulate matter is so small that it is possible to consider the SOx emissions directly proportional to the fuel sulphur content and fuel consumption.
The S compounds, both organic and inorganic, contained in the fuel are rapidly (at the combustion temperatures in the cylinder) oxidized to SO2; only a small amount of SO2 becomes SO3 (5%), since the oxidation reaction that leads to the formation of sulphuric anhydride is very slow, through O or O2 radicals:
Fuel S + O2 → SO2
SO2 + O → SO3
2SO2 + O2 → 2 SO3
The SO3 fraction grows with the increasing of combustion temperature and of excess air, but it cannot exist free if there are traces of water vapour because in this case, with a rapid reaction, it creates a cloud of sulphuric acid, even at low temperatures, immediately outside the funnel in the air. The reaction is:
SO3 + H2O → H2SO4
Inside the engine cylinder, part of the sulphuric acid reacts with the basic components of the fuel, with the production of neutral sulphates. On certain surfaces or particles, the sulphuric acid can condense depending on the content of SO3 and on the humidity of the gas. Furthermore, there is a given temperature (called acid dew point, about 110-160°C) below which the gases should not be cooled, because the drops of this condensate and the acid particles are very corrosive.