What happens to sulphur oxides in an exhaust gas cleaning system using seawater?
Seawater scrubbing requires the exhaust gases to be intimately mixed with seawater in order to dissolve the sulphur oxides. Manufacturers use various techniques to achieve mixing without unduly obstructing the passage of exhaust gas, as this could result in a “back pressure” outside of the engine builder’s limits and adversely affect engine operation. The sulphur oxides of shipping exhausts typically consist of ~95% sulphur dioxide, SO2 and the remaining ~5% of sulphur trioxide, SO3. When dissolved, a reaction occurs whereby the sulphur dioxide is ionised to bisulphite and sulphite, which is then readily oxidized to sulphate in seawater containing oxygen [1].
Relevant Chemistry
Sulphur Dioxide
SO2 + H2O <> “H2SO3” (sulphurous acid) <> H+ + HSO3– (bisulphite)
Note: there is no evidence that sulphurous acid exists in solution, but the molecule has been detected in the gas phase
HSO3– (bisulphite) <> H+ + SO3 2- (sulphite)
SO32- (sulphite) + ½ O2 <> SO42- (sulphate)
Sulphur Trioxide
SO3 + H2O > H2SO4 (sulphuric acid)
H2SO4 + H2O > HSO4– (hydrogen sulphate) + H3O+ (hydronium)
HSO4– (hydrogen sulphate) + H2O > SO42- (sulphate) + H3O+ (hydronium)
The ionisation to bisulphite and sulphite produces excess hydrogen (H+) ions i.e. acidity, as does the sulphuric acid formed from sulphuric trioxide. This will be initially neutralized by the seawater’s buffering capacity or alkalinity, which is mainly imparted by its natural bicarbonate content. However once the initial buffering capacity is consumed and the pH reduces to approximately 3 the ionisation of sulphur to sulphite is negligible [1] and exhaust gas cleaning becomes limited.
The wash water flow of exhaust gas cleaning systems is optimised, so that sulphur dioxide can dissolve and an appropriate amount of buffering capacity is available to enable emissions to be reduced to the required level. Too little effective wash water flow, mixing or alkalinity and the required reduction in SO2 is not achieved, however too much wash water is inefficient in terms of pumping power and component size and weight. A system designer will also consider the temperature of the water available for scrubbing as the lower temperature the greater the SO2 solubility.
