Open-loop vs. closed-loop scrubbers

An exhaust gas cleaning system, also referred to as a scrubber, can take three forms; open-loop scrubber, closed-loop scrubber, and hybrid scrubber. Today there is adiposity of open-loop scrubbers installed in vessels due to low installation and operation costs. This, despite the closed-loop scrubbers are the best solution to comply with the tightening environmental regulations. The scrubbers have significant differences and their impact on our ecosystems, wildlife, and human health. Let us dive more into this in the following.

An open-loop scrubber utilizes seawater to scrub the exhaust gases. Thus, this method exploits the natural chemical composition of seawater. The seawater is pumped into and fed to the exhaust gas cleaning systems, and afterward, it is discharged. Still, the discharged water must abide by the IMO’s scrubber wash water discharge limits. Learn more about the scrubber water discharge limits here. 

To obtain this water quality, huge quantities of seawater are pumped into the system. This ultimately delivers low turbidity, low pH, low temperature, as well as a low concentration of polycyclic aromatic hydrocarbons (PAHs). As this process demands large quantities of seawater, much wash water is generated, which is both more acidic and turbid than natural seawater. This is discharged overboard without any wash water treatment, which harms ecosystems and wildlife as the water can contain gaseous and particulate emissions, particulate matter (PM), PAHs, heavy metals, nitrate, and unwanted particulates such as soot and ash. Moreover, as this solution requires much water, it requires a large pumping capacity.

Simply put, open-loop scrubbers move the pollution from air to sea, causing ocean acidification. Several countries have adopted a prohibition on open-loop scrubbers to combat ocean acidification. See the complete list here. If a vessel operating an open-loop scrubber wants to enter these countries or the ECA, it must convert to a hybrid scrubber or a closed-loop scrubber. Alternatively, it can also switch to a more expensive compliant fuel containing a lower amount of sulfur or an alternative fuel with no sulfur. To accommodate this, an increasing number of open-loop scrubbers are designed as hybrid-ready, making it easy for shipowners to upgrade to a hybrid scrubber.

Furthermore, a paper conducted by Ship & Offshore on Open-loop scrubber discharge bans identifies that open-loop scrubbers are ineffective if salinity levels are low. If salinity is low, more water must be pumped into the system. Both freshwater, brackish water, and seawater are not adequate for an open-loop scrubber at high ambient temperature. Therefore, an open-loop scrubber is highly dependent on its operating conditions. This makes it an inadequate technology for areas where the salinity levels are low.

Closed-loop scrubbers typically utilize alkaline-dosed seawater to scrub the exhaust gas. A closed-loop scrubber only requires half the amount of water to clean the exhaust gas streams compared to the open-loop scrubber. Thereby, the closed-loop scrubber is more energy-efficient.

Seawater is pumped into a process tank, where the pH is adjusted by adding alkalis such as sodium hydroxide (NaOH), also called caustic soda or magnesium hydroxide (MgOH). Dosage can also be inline before the scrubber. Besides this alkaline, no other chemicals are needed to neutralize the scrubber wash water. From the process tank, the water is pumped through the scrubber tower and back to the process tank.

The bleed flow from the scrubber’s process tank is continuously filtered by a water treatment unit (WTU) to remove particulate matter and PAHs. Moreover, the water quality is reached by adjusting the pH value and maintaining a low temperature by cooling the scrubber water with seawater in a heat exchanger. Thus, the discharge water always abides by IMO’s scrubber water discharge limits.

Closed-loop scrubbers retain the scrubber sludge for safe onshore disposal. Therefore, it is essential to have adequate sludge handling facilities onboard a ship. The sludge can be dewatered with a filter press for further solid and liquid separation before onshore disposal to lower the handling fees. The wash water and sludge can be handled correctly and sustainably by utilizing a closed-loop scrubber, and the treated water can be discharged without damaging our ecosystems and wildlife.

As the name implies, a hybrid scrubber combines the open-loop scrubber and the closed-loop scrubber, operating with seawater and freshwater. Hybrid scrubbers typically operate in open-loop mode at sea and in a closed-loop mode in regulated waters, such as in the ECA and other areas, which have prohibited the use of pen-loop scrubbers and wash water discharge. This means you can discharge your scrubber wash water at sea where it is allowed while treating it in the seas and oceans where discharge is prohibited. Thus, this solution secures compliance with global and local restrictions. Still, most ships with a hybrid scrubber operate in open-loop mode whenever possible to avoid scrubber sludge discharge fees. Although this method delivers low OPEX, sulfuric wash water discharged into the oceans harms the environment and aquatic species by causing ocean acidification.