VIEW OF LATEST ECA REQUIREMENT, CHALLENGE FOR C/E FOR PURIFICATION, STORAGE AND TREATMENT OF FUEL OIL
Latest emission control requirement caps sulphur content below 0.1% in fuel oil in ECA areas. In ECA areas, distillate fuel oil below 0.1% sulphur is used, however new fuel types usually referred to as ultra low sulphur fuel oil (ULSFO), which are usually not traditional distillates, but blended products are also now being used. Diesel engine manufacturers claim that these fuels are well suited for 2-S as well as for 4-S engines. General characteristics of ULSFO fuels are:-
- They might have higher viscosity than distillate grades
- They might contain catfines
- They might have higher pour points
- There could be compatibility issues when blending with other fuels.
All above factors pose serious challenge for ship staff in general and chief engineer in particular regarding its storage, purification and treatment.
A) With traditional fuel oil purification system existing on ships
Traditional fuel oil separators are typically equipped with gravity discs, the purpose of which is to control the interface between water and fuel in the separator bowl. The gravity disc has to be selected according to the prevailing parameters, including its density. When ULSFO has to be used, new gravity disc will be required as the density of ULSFO is less than HSFO. The density can be obtained fro BDN. For better separation oil feed in purifier should be at 98 deg C. However the viscosity of this ULSFO at 98 deg can be less than ideal range of 10-15 cst. So. it will need to be cooled down to bring it to that ideal range, The ideal temperature of injection can be found out fro lab result. To ensure optimum cleaning of fuel oil, a second separator can be used in the series operation, for example a purifier followed by a clarifier.
Otherwise for effective removal of catfines, both separators can be used as purifiers in parallel with minimum feed rate and optimum teperature.
B) Advanced Computer based fuel cleaning system
In advanced computer based fuel cleaning system like ALCAP, which operates as a clarifier, does not use gravity disc. Clean oil is continuously discharged from the clean oil outlet. Separated sludge and water accumulate at the periphery of the bowl. Water and sludge is discharged at a preset time. If separated water approaches the disc stack, before the discharge interval, some water droplets start to escape with the clean oil. A water transducer is installed at the clean oil outlet which detects water in oil and signals the control unit. The control unit measures the quantity of water, and when it reaches the preset limit, control unit initiates an automatic discharge of water through water drain valve.
Second issue which remains, is the catfines. New system designed for the monitoring of catfines (at outlet of purifier) have been developed and are being installed now a days. GEA Westfalia Separator CatfineMaster is one such example.
C) Fuel Storage and stability issue
Whenever two different fuels are mixed, there is a risk of incompatibility, which may cause clogging of filters and separators and sticking of fuel injection pumps. Compatibility problems are related to the fuel's stability reserve. HFOs are rather aromatic and do contain asphaltenes. If the stability reserve of such fuel is low, it can not stand mixing more paraffinic LSFO into it, and as a consequence asphaltenes will precipitate out of the blend.
Possible mitigation are:-
- As much as possible avoid mixing two different oil. Do not even mix them in settling tank even if only one settling tank is provided. If mixing becomes unavoidable, try to maintain 80-20 % ratio or below that if possible.
- Perform a compatibility test on board before mixing the fuels in question.
- If tie permits send the fuel samples to lab to get the compatibility test.
Pour point of ULSFO could be higher than the distillate grades. They should be stored above their pour point, Thumb rule is to always store them 10 deg above their pour point.
D) FO Additives and treatment
More stringent emission regulations and the lowering of sulphur fuel limits are resulting in fuel related problems such as sludge, low lubricity, bacterial contamination, corrosion and waxing.
Modern marine engines are designed for fuel efficiency and reduced emissions. For an engine to operate at peak level, it needs to burn fuel completely throughout the entire combustion cycle. Unburned or partially burnt fuel wastes energy and increase emissions. The right combination of fuel additives can deliver enhanced combustion, protect against wear and prevent deposit build up.
Low sulphur fuel do reduce emissions, but they also reduce the lubricating property of the fuel oil. This loss of lubricity results in reduced wear protection and potential equipment failure. To improve lubricity, lubricity improver additives are added in the fuel. ULSFO together with recent fuel injection equipment design changes, has been identified as a potential reason for an increase in internal diesel injector deposits(IDID), that can cause injectors to seize and stop performing. This problem can be alleviated with the use of certain injector cleanliness additives.