As discussed in our previous article (NOx & Tier), one of the major global environmental concerns today is the air pollution from maritime transportation. The main elements of pollution are NOx and SOx. Sulfur oxides emission is mainly due to the presence and burning of sulfur compound in the fuel.
Causes and Effects of SOx Emission
The sulfur oxides emission is due to the presence of sulfur compound in the marine fuels used in marine engines on board vessel. Better the grade, lower will be the sulfur content as it is removed by refining of the fuel.
The smoke containing sulfur oxides emitted by the combustion of marine fuel will form sulfuric acid which is a major cause of acid rain.
It has also been recognized that the emission of SOx contributes in formation of secondary inorganic aerosol gases, fine particles which are harmful to humans.
When the ship is within emission controlled area
- The sulfur content of any fuel oil used on board ships must not exceed 0.1% m/m (mass/mass).
How to limit the SOx Emission
Use of Low sulfur fuel oil: It is expensive but most commonly used method to comply with Annex VI of MARPOL while entering emission controlled Area or ECA.
Due to the nature of crude oil and refinery operations meeting the 0.10% sulfur limit is not expected to be widely available.
So it is anticipated that low-sulfur distillate oil (LSDO) will generally be used to comply. It is also the simplest way to comply.
LSDO will normally consist of marine diesel oil (MDO) or marine gas oil (MGO); the terms MGO and MDO have no precise definition other than that both are distillates and therefore do not require heating before injection. The ISO standard 8216:2010 categorises MGO and MDO as distillate marine (DM) grades.
- MGO refers to the ISO 8217:2012 DMA and DMZ grades
- MDO corresponds to the ISO 8217:2012 DMB grade.
Cylinder Lubrication: Good quality cylinder lubrication along with efficient control systems such as Pulse or Alpha lubrication systems can neutralize the sulfur in the fuel and reduce SOx emissions from the engine.
But The main purpose of Alpha lubrication is avoid the loss, cold corrosion and saving the money.
What is Alpha lubricator?
■ High pressure injection directly into the piston ring pack
■ Precise Injection Timing ( Electronically )
■ Optimal Utilization of the Oil with minimum of loss
■ Easy and Precise control of the feed rate
Back to the SOx emission control:
In order to neutralize the acid, the cylinder lube oil contains alkaline components
– usually calcium salts. Normally, the Base Number (BN or TBN) is a measure of the cylinder lube oil’s ability to neutralize acid. The higher the BN, the more acid can be neutralized.
Low BN oils should be chosen for low- sulfur fuels, and high-BN oils for high-sulfur fuels.
Note: The minimum feed rate for proper oil distribution and oil film thickness has so far been set at down to 0.6 g/kWh, which at the Lub.oi BN70 that sulfur neutralization ability can be reached at 3% sulphur.
As an example, an engine using 1% sulphur fuel at a dosage of 0.6 g/kWh would therefore be over-additivated (CaCo3) – If The engine operate more than one week.
Exhaust Gas Scrubber Technology: The exhaust gas from the engine is passed through the scrubber tower where a liquid is showered over it. Fresh water blended with caustic soda (NaOH) is used as a scrubbing liquid which reduces the SOx to 95%. The scrubbing water is then sent to a water treatment effluent emulsion breaking plant after which it can be discharged overboard – open loop.
- Washwater discharge to sea
The IMO MEPC.184(59) Guidelines place emission limits on washwater discharge ( pH limit 6.5 ), but do not contain any geographical restrictions; providing the emission limit values are satisfied, they allow for exhaust gas cleaning systems to operate while discharging washwater to sea with no further restrictions. They do not reference open or closed loop modes.
In the US, the Environmental Protection Agency (EPA) has put in place Vessel General Permit (VGP) requirements for discharges incidental to the normal operation of ships. Ships with exhaust gas cleaning systems must have a VGP in order to discharge SOx scrubber washwater.
- Compatibility with waste heat recovery units and SCR systems
All wet SOx scrubbers significantly cool the exhaust gas. Therefore, they are not suitable for installation before a waste heat recovery unit. For the same reason, it would not be possible to install a wet SOx scrubber before an SCR system unless a reheater was fitted after the wet scrubber to raise the exhaust gas temperature back up to around 300oC see the NOx control solution.
- Using EGR with wet scrubbing
If burning high sulfure fuel oil, a closed loop wet scrubber is needed to clean and cool the re-circulated gas. The cleaning action prevents the acidic and corrosive gas from causing accelerated engine wear. The cooling action prevents excessive cylinder temperatures.
If EGR is applied to engines restricted to LSDO or other clean fuels, it may be possible to eliminate the scrubber, making the system less complex. ( But the Money !!!! 🙂 )
However, although EGR scrubbers have been found to remove up to 80% of SOx in the recirculated gas, this does not mean they will achieve compliance with ECA-SOx requirements.