Scrubber in ship or Exhaust Gas Control Systems (EGCS) are used to remove harmful components such as particles and sulfur oxides (SOx) and nitrogen oxides (NOx) from exhaust gases produced by the combustion process of marine engines to control environmental pollution. These scrubber systems are designed and used to treat exhaust gases from land-based and marine engines, auxiliary engines, and boilers to prevent people and the environment from being harmed by toxic chemicals.
Emissions of sulfur from ships into the atmosphere will be limited by new and updated international regulations under the MARPOL Convention that will come into force on 1 January 2020. International Maritime Organization (IMO) regulations require that the sulfur content in fuels transported by commercial ships be limited to 0. 50% worldwide and within the ECAs (Emission Control Areas, Baltic Sea Area, North Sea Area, United States, Canada and the Caribbean), while the United States) must be limited to 0.10% month-on-month.
Working Principle of Exhaust Gas Scrubber
The exhaust gas stream is directed to a scrubber on a ship in the presence of alkaline scrubbing material to neutralize the exhaust gas’s acidic properties and remove all particles. The used cleaning agent is collected together with the rinse water and can be stored or immediately disposed of as wastewater. The purified exhaust gases are released from the system into the atmosphere. The cleaning agent is selected so that appropriate chemical reactions can remove specific pollutants such as SOx and NOx.
Shipboard scrubbers use lime or caustic soda for desulphurization, so sulfur-containing salts are formed after treatment. However, they pose no threat to the environment and can be easily disposed of. Since they are alkaline, scrubbers can use seawater, freshwater with added calcium/sodium sorbents, or hydrated lime pellets as the cleaning medium.
Classification of Marine Scrubbers
Marine scrubbers can be classified as wet and dry scrubbers, depending on how they work. Dry scrubbers use solid lime as an alkaline detergent to remove sulfur dioxide from the exhaust gases. Wet scrubbers use water sprayed into the exhaust for the same purpose.
Wet scrubbers are further classified as closed circuit and open circuit scrubbers. Closed circuit scrubbers can use fresh water or seawater as the cleaning fluid. When closed cycle scrubbers use fresh water, the water quality around the vessel has no effect on the scrubber in ship performance or wastewater discharge. Open scrubbers use seawater in the scrubbing process.
Wet Scrubbers
In wet scrubbers, the cleaning fluid used can be seawater or freshwater with chemical additives. The most commonly used additives are caustic soda (NaOH) and limestone (CaCO3). The cleaning fluid is sprayed through nozzles into the exhaust stream for effective dispersion. Most scrubbers are designed with the cleaning fluid flowing downstream, but some have the cleaning fluid flowing upstream.
The exhaust inlet of the scrubber in ship may be in the form of a venturi, where the gas enters from the top and water is sprayed in the form of a spray into the throat or into the high exhaust velocity area above the throat. The exhaust inlet is at the side or bottom of the tower. This design ensures that the sulfur oxides present in the exhaust gas pass through the scrubbing fluid and react with it to produce sulfuric acid. The naturally corrosive sulfuric acid can be neutralized when diluted with alkaline seawater.
Open Scrubber System
This system uses at seawater as the scrubbing and an neutralizing medium, and no additional chemicals are required to desulfurize the gas. The exhaust gas stream from the engine or boiler enters the exhaust gas scrubber in ship and is treated with alkaline seawater only, the amount of which depends on the size and power of the engine.
This system is very efficient, but the amount of seawater required is very large, so large pumping power is required. If the seawater used for scrubbing is sufficiently alkaline, the open circuit system works perfectly satisfactorily. However, it is ineffective and cannot be used with seawater at high ambient temperatures, freshwater, or even brackish water. For these reasons, the open scrubber in ship cycle is not considered a suitable technology for areas with low salinity, such as the Baltic Sea.
Reactions Involved:
SO2 (gas) + H2O + 1/2O2 → SO4 2- + 2H+ (sulfate ions + hydrogen ions)HCO3- + H+ → CO2 + H2O (carbon dioxide + water)
Advantages:
- Few moving parts, it is a simple design, and it is easy to mount on a board.
- Little maintenance of the system is required apart from debris removal and operational checks.
- This system does not require waste storage.
Disadvantages:
- Cooling of exhaust gases is an issue with wet scrubber in ship systems.
- The operation of the system depends on the alkalinity of the available water,
- so it is not suitable for use in all conditions.
- Large amounts of seawater are required for efficient scrubbing, which consumes a lot of electricity.
- Expensive fuel has to be consumed in the ECA zone and at port.
Closed Scrubber System
Works on a similar principle to the open system. Instead of seawater, it uses fresh water treated with a chemical (usually sodium hydroxide) as the scrubbing medium. SOx from the exhaust gas stream is converted to harmless sodium sulfate. The wash water passes through a process tank where it is cleaned before returning to the circuit from the closed wash system.
The process tank is also necessary for the operation of the circulation pump and prevents the pump suction pressure from dropping too low.
The vessel can either carry fresh water in the tank or produce the required water from an onboard freshwater generator.
Small amounts of wash water are periodically transferred to a storage tank where fresh water can be added to avoid the formation of sodium sulfate in the system.
A closed system requires almost half the amount of wash water compared to an open version but requires more tanks. These include process and buffer tanks, recovery tanks where discharge into the sea is prohibited, and storage tanks where sodium hydroxide, usually used as 50% water, can be thermostated between 20 and 50°C. Solution.
Dry sodium hydroxide also requires a lot of storage space. Hybrid systems combine both wet systems and can operate as an open loop system if water conditions and effluent regulations allow, and as a closed loop system otherwise. Thus, hybrid systems prove to be the most popular, as they can handle a wide variety of conditions.
The reaction involving
2NaOH + SO2 → Na2SO3 + H2O (sodium sulfite).
Na2SO3 +SO2 +H2O → 2NaHSO3 (sodium bisulfite); SO2 (gas) + H2O + 1/2O2 → SO42- + 2H+; NaOH + H2SO4 → NaHSO4 + H2O (sodium bisulfate); 2NaOH + H2SO4 → Na2SO4 + 2H2O (sodium sulfate).
Advantages:
- Little maintenance is required.
- Independent of the vessel’s operating environment.
- With wet scrubbers, cooling of the exhaust gases is an issue.
Disadvantages
- Storage space (buffer tank) is required to store the effluent until it can be discharged.
- The selective catalytic reduction system must be operated before the wet scrubber.
- Assembling the system can be very complicated, especially for dual-fuel engines.
Hybrid Scrubber Systems
These systems offer an simple solution for retrofitting ships with scrubbers capable of operating in both open and closed circuits. These systems operate in open circuit at sea and closed circuit in ECA zones or ports, and can easily switch between their uses. The systems can operate for long periods of time around the world using low-cost fuel, overcoming high initial costs and economically meeting international regulations.
Advantages
- 1 Suitable for long and short voyages around the world
- 2 Vessels equipped with hybrid cleaning systems can spend more time in ECA zones and ports than vessels equipped with open circuit systems
- Cheap heavy fuel oil (HFO) can be used at all times.
Disadvantages
- Further structural modifications are required to use this system.
- A lot of storage space is required to store the chemicals and additives.
- The installation of the system is time-consuming and expensive.
Dry Scrubber
This type of scrubber does not use water as a cleaning material but uses hydrated lime pellets to remove the sulfur. These scrubbers have the advantage of burning off all soot and oily residues in the system. Because they run at higher temperatures than wet scrubbers. The calcium in the caustic lime granules reacts with the sulfur dioxide in the flue gas to form calcium sulfite. The calcium sulfite is oxidized in the air to form calcium sulfate dihydrate, which reacts with water to form gypsum. The spent pellets are stored on board and discharged at ports. However, the gypsum formed is not considered waste as it can be used as fertilizer and building material. Dry scrubber systems do not require a circulation pump and, therefore, consume less power than wet systems. However, they weigh as much more than wet systems.
Reactions Involved
SO2 + Ca(OH)2 → CaSO3 + H2O (calcium sulfite)CaSO3 + 1/2O2 → CaSO4 (calcium sulfate)SO3 + Ca(OH)2 → CaSO4 + H2O (gypsum)
Advantages
- Efficiently removes nitrogen oxides and sulfur oxides
- This type of system does not produce liquid wastewater that must be disposed of overboard.
- Gypsum after flue gas cleaning treatment can be sold for various industrial applications.
Disadvantages
- A significant amount of onboard storage capacity is required to handle the dry bulk reactants and products of the process.
- The reactants must be available at all times.
- The reactants used are expensive, especially urea for NOx reduction and calcium hydroxide for SOx reduction.
Selection of Exhaust Gas Scrubbers
When selecting the best flue gas scrubber to install on board, shipping companies must consider many factors. These include, but are not limited to, the installation space available on board, the vessel’s operating area and charter schedule, the power and output of the onboard engines and boilers, and the availability of fresh water and power available on board to operate the system under various conditions.
