To Scrub, or Not to Scrub -That Is the Question

By Nicholas Confuorto

On January 1, 2015, the IMO Annex VI, ECA zone requirements came into effect.  Ship owners and operators are now faced with having to decide between switching to a lower sulfur fuel or embracing alternate solutions such as exhaust gas cleaning systems (Scrubbers) and LNG.  Worldwide, about 300 scrubbing systems have already been sold to date for marine applications.  Many of these have been sold for vessels operating in the North European ECA and many for the global cruise/ferry industry (both in the US and Europe).  The U.S. based non-cruise segments have taken a wait and see attitude with only a few opting for alternate technologies (primarily in the Great Lakes).  Since the general belief is that the cost differential between low and high sulfur fuels will most likely increase as the low sulfur fuel demand increases, the question begs: why is everyone waiting?  One possible reason is that the general shipping industry knowledge of scrubbers is minimal and therefore contributing to the slow implementation of this option.  However, with each day costing the industry millions of dollars in additional fuel costs, is waiting a wise decision?  In a shipping industry where small incremental savings could mean the difference between being competitive or losing money, the scrubber alternative could make a big difference.   
This article aims to provide up-to-date information about the regulations and about the scrubber technology option.  Demystifying this option should help ship owners in their evaluation of the available options.


On January 1, 2015, the IMO Annex VI, ECA zone requirements came into effect.  Ship owners operating in these areas are now faced with having to decide between switching to a 0.1% Sulfur fuel or adopt alternate solutions such as exhaust gas cleaning systems (also known as “Scrubbers”) or LNG.  Annex VI also set January 2020 as the start of a global 0.5% Sulfur cap for all areas other than those covered by the 0.1% Sulfur ECAs  (this 2020 sulfur cap is subject to a review and confirmation in 2018).  Additional emission restrictions are being considered for Mexico, the Mediterranean, the Norwegian Sea, Singapore and Hong Kong.   Although LNG is considered a promising alternative to low sulfur fuel, it’s high initial cost, global availability, and present bunkering capabilities have not allowed it to grow as fast as the scrubber alternative.

The Scrubber Alternative
The cruise and ferry industries account for the majority of the reported 300 scrubber installations, as the industry has seemingly embraced scrubbing technology as a solution to the ECA requirement.  Other shipping segments have also installed scrubbers but not to the same extent.  Many seem to be taking a “wait and see” attitude.  While the recent reduction in fuel costs has given the industry a financial break, the net price differential between low sulfur and high sulfur fuels has not decreased. 
Forecasting the shipping industry’s need for scrubbing systems, several marine equipment companies developed their scrubber design and entered that market.  Other companies, such as CR Ocean Engineering (CROE), modified its proven designs to incorporate the requirements of the maritime industry. Most scrubber companies provide full guarantees and warranties allowing ship owners to feel confident that they can continue using the lower cost high sulfur fuel oil even in the Environmental Controlled Areas (ECA).   
A properly designed marine scrubbing system includes certain features.  It has to be light weight, relatively small in both diameter and height, highly efficient and it must be cost effective.  It also has to have low backpressure and all metal construction.  It shall be used as a silencer when wet or dry and it shall not require a bypass.  The system shall also include the required wash-water treatment equipment and all required monitoring and control equipment.  An up-flow configuration seems to be preferred by many ship owners because it requires less space than the side-entry designs.
Scrubbing Systems are available in Open Loop (a once trough design using seawater to neutralize the collected sulfur emissions), Closed Loop (using a freshwater solution with an alkaline solution to neutralize the collected sulfur compounds) or Hybrid configurations (deigned to be both Open Loop and Closed loop and able to switch from one configuration to the other on demand).  Specific selection is based on ship routing or owner’s preference.

The Open Loop Design
An Open Loop system is one where the scrubber uses seawater in and out on a once-through basis, a design that uses less equipment and fewer controls.  However, because seawater is not as good a neutralizing agent as caustic (NaOH), it will require larger piping and larger pumps.  For the same reason, it is constructed using higher grade alloys to allow for the lower pH environment.  The pH in the system is adjusted by the amount of seawater used. The pumped seawater is distributed within the scrubber vessel by strategically located nozzles. 
Each nozzle is designed to optimize the droplet size, distribution and droplet residence time to allow for the maximum contact between seawater and flue gas SO2.  The SO2 is absorbed into the droplets.  The absorbed SO2 reacts with water to form sulfurous acid.  Sulfurous acid dissociates into bisulfite. The bisulfite further dissociates to sulfite and sulfates.  The sweater pH is then used to neutralize the acidity of this discharge water in order to meet the pH requirements set up by IMO.  CROE and various other scrubber suppliers have provided several of these open loop systems to ship-owners globally. 

The Closed Loop Design
For ship-owners who travel in fresh or very low alkalinity waters, or wish to keep the scrubber water discharge on board (in a tank) for a period of time, the Closed Loop scrubber design could be the preferred configuration. 
The scrubber features used in a closed loop operation are the same as those used in open loop or hybrid.  However, the pH in the system is controlled by the amount of caustic added to the circulating loop.  A Closed Loop system uses a solution of fresh water and caustic as the reagent to remove SO2 from the exhaust gas stream. This design earns its name from the closed loop used to circulate the caustic solution between tank and scrubber.  Because the caustic solution is much more efficient in removing the SO2 than seawater, much less water is circulated a closed loop system than in an open loop system.  This means that the piping, valves and the pumps used for a closed loop system are smaller and require less power. 
Unfortunately the circulating solution in a closed loop system cannot circulate untouched for a long time because the sulfates/sulfites/bisulfites and sludge content in the circulating solution continue to build up (due to continued capture of SO2 and particulates).  Should the concentration of these collected pollutants exceed a predetermined maximum level, the scrubbing efficiency in the scrubber will decrease significantly and the system will eventually stop scrubbing.  To avoid that excessive buildup, a closed loop system incorporates a small continuous bleed (a slip stream) from the circulation loop.  That stream is continuously replaced by fresh water to keep the system in balance.  Additional fresh water is also added to replace the water that is evaporated by contact with the hot exhaust gas.   Since the untreated discharge water from a closed loop system has much higher pollutant content due to the normal build-up that occurs in the closed circulation, additional water cleaning equipment is required prior to discharge at sea.  The typical water cleaning equipment used in closed loop operation is a centrifuge or some type of positive filtration.  As with the open loop system, the closed loop system will also include all monitoring and control equipment. Additionally, it will require caustic storage and feed system as well as purge water storage tank if desired.  One major advantage of a closed loop system is that is can be constructed of typical stainless materials because the pH levels are easily controlled by caustic.  Of course chloride levels may affect the ability to use stainless steel materials and therefore a proper review of the water and caustic used is required to make sure that the maximum chloride levels are not exceeded in the circulating solution.

The Hybrid Design
A hybrid design is a combination of an open lop and a closed loop system. As such a hybrid scrubber will be the most expensive but the most flexible solution.  A hybrid system will have all the additional equipment associate with a closed loop system but will also circulate more water and be constructed of higher grade alloys as the open loop system (because of the use of seawater in open loop operation). 
The advantage of a hybrid system is that it can be switched from open loop to closed loop operation on command.  One can operate in closed loop mode when in port of fresh water and then switch to open loop when in open seas.  This will minimize the amount of caustic used and in long run save operating money.   Each ship with a hybrid scrubber installation will be able to optimize their operation based on route and local port state requirements.
Stack Monitoring Equipment
The stack emissions are monitored by a Continuous Emission Monitoring system (CEM). The CEM sensor is located in the stack above the scrubber unit.  The CEM unit will measure SO2 and CO2. The emission data collected by this unit is displayed on the CEM control panel screen. The exhaust gas emission data is also archived in the CEM panel along with the wash-water monitoring data (PAH, pH and turbidity) and the ship GPS.

Water Monitoring Equipment
Per the IMO guideline for exhaust gas cleaning systems, the effluent water from the scrubber shall be monitored for pH, turbidity, and PAH.  The data collected by these sensors is displayed on the wash-water monitoring control panel screen and is also archived on the CEM control panel.  Since IMO has set the turbidity and PAH as increased levels from the normal surrounding waters, it may be a good precaution to install turbidity and PAH monitors on the water intake as well as the water discharge points to better define the differential between inlet and outlet.  This may be very useful in proving compliance even when the normal water intake levels of turbidity and PAH exceed the required IMO established values.

Scrubbers work.  Many scrubbers are now in operation and many others in design and installation phases.  Because of this initial problem some suppliers, the Exhaust Gas Cleaning Systems Association (EGCSA) was established and all members vowed to a very strict code of ethics.  EGCSA member companies work very hard at maintaining a high standard of design and reputable sales and marketing approach.  Additionally they are supporting the shipping industry in working with IMO in clarifying and simplifying some of IMO’s requirements.  
All EGCSA member companies have presently provided scrubbers to the shipping industry and have had significant successes.  As an example, CR Ocean Engineering (CROE) has several on board units in successful operation and many others in the design and fabrication stage for some very reputable shipping/cruising companies.  Having had more than 60 years of successful scrubbing systems design under its belt, CROE’s success in the marine industry started with it’s first installations on a Great Lakes bulk carrier. 
The moral of the story is that scrubbing systems can save significant money if a ship operates in an ECA for more than about 40 % of the time.  Risks can be managed by contracting with a known and reputable company with proven scrubbing system design experience (using EGCSA member companies is one way to minimize risk).  In the end all parties want a successful system operation for many decades and all can benefit by this IMO established equivalency. 

The Author

Nicholas Confuorto is President and COO of CR Ocean Engineering LLC ( Confuorto is also Chairman of the London-based Exhaust Gas Cleaning Systems Association (EGCSA). 
e: [email protected]
t: +1 (908) 209-9701

(As published in the September 2015 edition of Maritime Reporter & Engineering News -

Maritime Reporter Magazine, page 20,  Sep 2015

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