How wet is the exhaust gas after scrubbing?

A significant amount of water is produced by the combustion of hydrocarbon fuel oils and we have already seen in the summary of typical emissions from a slow speed 2-stroke diesel engine that the exhaust can contain over 5% water.

So why after an exhaust gas cleaning unit, which would seemingly add water to the exhaust do we not see condensation and a plume above the funnel?

Temperature is a key parameter in determining the mass of water a given quantity of exhaust gas can contain within the exhaust duct i.e. the higher the temperature, the greater the mass of water that can be held before saturation is reached.  Between the engine and an EGC unit the exhaust temperature can be approximately 300°C, but after passage through the EGC unit the temperature is reduced very significantly – perhaps by 85% and water in the gas phase is condensed out through contact with the relatively cold washwater. This means the mass of water in a given quantity of exhaust gas can actually be less at exit from an EGC unit when compared with that at entry i.e. water is not necessarily added to the exhaust gas by scrubbing.

Whilst this depends upon the washwater temperature and the actual mass may be reduced, the exhaust gas will be fully saturated when it exits the EGC unit.  Typically a demister removes any entrained liquid droplets, and then a re-heater [16], [20], [21] is used to raise the temperature so the exhaust is no longer fully saturated. This prevents water in the gas phase forming water vapour and condensing onto cold surfaces. On exit from the funnel into the atmosphere the exhaust gas is immediately diluted, dropping saturation levels. This again prevents any water condensation so a plume cannot form.

Is there a risk of acidic liquid carry-over with the exhaust gas after an EGC unit?

When sulphur dioxide is dissolved, a reaction occurs whereby it is ionised to bisulphite and sulphite, which is then readily oxidized to sulphate in water containing oxygen [1].

The ionisation to bisulphite and sulphite produces excess hydrogen (H+) ions i.e. acidity in the washwater, as does the sulphuric acid (H₂SO₄) formed from the 3-5% sulphuric trioxide (SO₃) in the exhaust gas.

Exhaust pipes on unscrubbed engines are typically maintained at around 180°C as this is above the dew point for sulphuric and sulphurous acids preventing condensation onto metal surfaces and therefore corrosion [3]. Scrubbing significantly reduces the exhaust gas temperature and any sulphur-based acids are condensed within the EGC unit. Sulphuric acid is highly soluble in water and sulphurous acid, which has been detected in the gas phase but not in solution, ionises to bisulphite and sulphite producing excess hydrogen ions i.e. acidity as shown above. In order to prevent acidic liquid carry-over with the exhaust gas, a demister at the EGC unit exit is used to remove any entrained liquid droplets.

In addition a re-heater [16], [20], [21] is used to raise the exhaust gas temperature so that it is no longer fully saturated with water. This prevents water in the gas phase forming vapour and condensing onto cooler exhaust pipe surfaces. On exit from the funnel into the atmosphere the volume of exhaust gas is immediately diluted, which reduces saturation levels, which again prevents water vapour from forming.

Without the formation of water vapour any gaseous SO₂ that remains unscrubbed (typically < 2%) cannot be dissolved and the risk of subsequent acidification is mitigated.