SCRUBBER INSTALLATION - TECHNOLOGY AND COMPLIANCE

There is a general notion that the only commercial technology solution currently available to meet compliance with MARPOL Annex VI Regulation 14 is an exhaust gas cleaning system, namely scrubbing. Remember Regulation 14 requires ships to use 0.10%S fuel in ECAs and 0.50%S fuel globally from 2020. There are also regional ECAs with slight variations. Examples include the 0.50%S in some rivers and coastal areas in China.

Here we briefly discuss on various type of scrubbers and their working method, the cost consideration is not a part of this article.

Scrubber types
Four different scrubber types are available today:

1. Seawater scrubbers (open loop) utilize untreated seawater, using the
   natural alkalinity of the seawater to neutralize the sulphur from exhaust gases. The negative characteristic of an open loop system is its greater energy consumption compared to a close loop system, but there is no need for chemical additives like caustic soda in a closed loop system.
   
   
   
   
2.  Freshwater scrubbers (closed loop) are not dependent on the type of the water the vessel is operating in, because the exhaust gases are neutralized with caustic soda, which is added to freshwater in a closed system. Circulating water is processed after the scrubber and dosed with caustic soda in order to restore the alkalinity of wash water. The amount of the water which is needed in a closed loop process is about half of the flow in an open loop system.
   
   
   
3.  Hybrid scrubbers give the possibility to either use closed loop or open loop technology. Hybrid scrubbers are generally used as an open loop system when the vessel is operating in the open sea and as a closed loop system when operating in harbour or estuaries, where water discharge is prohibited. Among the different types of scrubbers a hybrid scrubber is becoming increasingly common because of its flexibility and restrictions.
   
   
   
   
4. Dry scrubbers do not use any liquids in process but exhaust gases are cleaned with hydrated lime-treated granulates. There is not any discharge to the sea from the system. As a result of the process a gypsum, which is used to manufacture wallboard, is generated. An advantage of a dry scrubber is its lower energy consumption compared to a wet scrubber.
   
   (The Technology is still developing even though commonly used on shore companies)

Scrubber working principles

A scrubber generally consists of:

•  The exhaust gas cleaning unit serves as a contact chamber that enables the exhaust gas stream from an engine or boiler to be intimately mixed with water, either seawater, freshwater, or both. In the contact chamber, SOx is converted to sulphuric acid. Due to space and access limitations, the exhaust gas cleaning units tend to be high up in the ship, in or around the funnel area.

•  The wash water treatment plant differs by scrubber type and design. Generally, physical separation techniques are used to capture suspended solids, if captured. The treatment process typically includes a multicyclone, or a cyclonic separator similar to that used to separate water from residual fuel prior to delivery to the engine. Heavier particles may also be trapped in a settling or sludge tank for disposal.

•  Sludge handling to retain sludge removed by the wash water treatment process for disposal shoreside.

Wet scrubbers are not supposed to be operated without wash water flowing. Therefore, a separate bypass is generally installed in case the scrubber is non- operational for any reason

Seawater scrubber

0. The most important chemical reactions that take place during the  process are the following:
   SO2 (gas)+H2O+1⁄2O2 →SO42- +2H+HCO3- +H+ →CO2 +H2O
   SO2 absorbed in seawater reacts with oxygen to form sulphate ions and hydrogen ions. Increased concentration of hydrogen ions means increased acidity and decreased pH. Bicarbonate ions (HCO3-) from the seawater react with hydrogen ions and thereby neutralise the acidity and raise the pH again.
   In addition to the capture of SO2 some NOx may be removed from the exhaust gas in the scrubber. It is mainly the NO2 fraction that is captured, which will be emitted with the wash water as nitrates.
   
   
   
   Open loop scrubbers have larger water flow rates than closed loop scrubbers because there is less control over water alkalinity  and more water is needed to make the scrubbing process effective when lower alkalinity water is used. The Baltic Sea is known as having a low alkalinity, especially the northern part.
   
   
   
   
   

   

   Sea Surface Salinity 
   
   

   (In seawater carbon dioxide becomes carbonic acid, which in turn forms bicarbonate and carbonate ions. The levels of these are important because they control the pH of seawater. This is a measure of how acid or alkaline the water is. In fact, seawaterhas a relatively stable pH of about 8.3.)
   
   The efficiency of the scrubber process increases with higher alkalinity. Lower alkalinity implies a higher need for wash water and results in higher energy consumption.
   
   In a marine seawater scrubber the flue gas either passes through a spray of seawater or is bubbled through seawater. System manufacturers have their own techniques for how the scrubber mixes the exhaust gas and the water. Wash water residues can be collected for deposit on land, but not necessarily.

Freshwater scrubber

In freshwater scrubbers, SO2 combines with a salt and consequently does not react with the natural bicarbonate of seawater. The following reactions occur:

2NaOH + SO2 → Na2SO3 + H2O (Sodium Sulfite);

Na2SO3 +SO2 +H2O → 2NaHSO3 (Sodium Hydrogen Sulfite); SO2 (gas) + H2O + 1⁄2O2 → SO42- + 2H+;

NaOH + H2SO4 → NaHSO4 + H2O (Sodium Hydrogen Sulfate); 2NaOH + H2SO4 → Na2SO4 + 2H2O (Sodium Sulfate).

A freshwater scrubber discharges typically 250 times less water than a seawater scrubber. For fresh water systems, the bleed off is considerably smaller (0.1-0.3 m3/MWh) and as a consequence, the pollutant concentration is higher, easing the wash water cleaning.
In a closed loop-type system, the scrubber’s wash water is always collected in a process or circulating tank. A limited quantity of wash water from the bottom of the process tank, where the residuals have collected, is extracted using a low suction, and it goes to a hydrocyclone or separator, where the residuals are removed.

Dry scrubber

Dry scrubbers use granulates with caustic lime (Ca(OH)2) which reacts with sulfur dioxide .
(SO2) to form calcium sulfite: SO2 + Ca(OH)2 → CaSO3 + H2O.

Calcium sulfite is then air-oxidized to form calcium sulfate dehydrate or gypsum:
 CaSO3 + 1⁄2O2 → CaSO4.

Reaction with sulphur trioxide (SO3) is: SO3 + Ca(OH)2 → CaSO4 + H2O. 
Which with water forms: CaSO4 • 2H2O (Gypsum).

A dry scrubber works by feeding dry pellets of hydrated lime treated granulates through a packed bed absorber. The hydrated lime reacts with the hot exhaust gas and absorbs the SOx components to form pellets of gypsum. Dry SOx scrubbers claim there is a modest market for the used granules in building materials. Testing has been carried out so far only with two vessels with a medium-speed main propulsion engine.

Water Discharge Criteria.

1. The discharge wash-water should have a pH of no less than 6.5 measured at the ship’s overboard discharge with the exception that during manoeuvring and transit, the maximum difference between inlet and outlet of 2 pH units is allowed measured at the ship’s inlet and overboard discharge.

2. The Max limit of PolyCyclic Aromatic Hydrocarbons should not be greater than than 50 μg/L PAHphe (phenanthrene equivalence) above the inlet water PAH concentration
(or a 15-minute period in any 12-hour period, the continuous PAHphe concentration limit may exceed the limit described above by up to 100%. This would allow for an abnormal start up of the EGC unit.)

3. The maximum continuous turbidity in washwater should not be greater than 25 FNU (formazin nephlometric units) or 25 NTU (nephlometric turbidity units) or equivalent units, above the inlet water turbidity.
(For a 15-minute period in any 12-hour period, the continuous turbidity discharge limit may be exceeded by 20%.)

4. The wash-water treatment system should prevent the discharge of nitrates beyond that associated with a 12% removal of NOx from the exhaust, or beyond 60 mg/l normalised for wash-water discharge rate of 45 tons/MWh whichever is greater.