Scrubbers have become the ultimate solution in the post IMO2020 scenario but seldom do we know how the different types of scrubber work, especially when the open-loop scrubbers are facing heat all over the world. Today we are going to highlight how a seawater scrubber work.
What is an Open-type Seawater Scrubber?
In a seawater or “open” type scrubber, seawater is used as wash water for
scrubbing, and the resulting wastewater is treated and discharged back to sea. The natural
alkalinity of the seawater is used to neutralize the acidity that results from SOx removal
Typically, seawater systems use 45 m3 of wash water per megawatt-hour3
(MWh) for scrubbing (MEPC, 2008). Seawater scrubbing of the exhaust from a representative 10 MWh engine would generate 450 m3/hr or 2.8 million gallons per day (MGD) of washwater effluent.
How does it work?
- The volume of washwater required for a given scrubbing efficiency increases with elevated alkalinity and with higher water temperature (DNV, 2009).
- The energy consumption of a seawater scrubber is 2 to 3 percent of the engine power output (Filancia, 2009).
- Seawater scrubbing requires the exhaust gases to be mixed with seawater to dissolve the sulfur oxides.
- Manufacturers use various techniques to achieve mixing without unduly obstructing the passage of exhaust gas, which could result in a “back pressure” outside of the engine builder’s limits and adversely affect engine operation.
- The sulfur oxides of engine exhausts typically consist of approximately 95 percent SO2 and 5 percent SO3.
- When dissolved, a reaction occurs whereby the sulfur dioxide is ionized to bisulfite and sulfite, which is then readily oxidized to sulfate in seawater containing oxygen (Hassellöv and Turner,2007).
- The ionization to bisulfite and sulfite and the sulfuric acid formed from sulfur trioxide
produces acidity (Karle and Turner, 2007).
- The acidity is neutralized initially by the alkalinity of the seawater, due to its natural bicarbonate content.
- After the initial buffering capacity is consumed, the pH of washwater is reduced to approximately 3.
- At low pH, the ionization of sulfur to sulfite is negligible and exhaust gas cleaning is limited.
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