We have fuel performance issues when we blend low sulphur marine fuels from different sources. How can we resolve this?Apr-2021
Simon Calverley, KBC (A Yokogawa Company), Simon.Calverley@KBC.global
A known problem with fuel oils, including low sulphur fuel oils, is instability. In an unstable fuel oil, asphaltenes precipitate so the fuel oil will have solids present. Asphaltenes are a heavy (high molecular weight) component so are found in the heaviest hydrocarbons on a refinery; lighter streams (cutters) are used to blend with the heavy streams so that the resultant fuel oil meets the specification. Some hydrocarbon species have a tendency to precipitate asphaltenes whereas others will keep them in solution; generally speaking, the more paraffinic cutters are the more likely they are to precipitate asphaltenes, whereas the more aromatic they are the more likely that asphaltenes will stay in solution. Refiners will blend fuel oils such that they are stable, thus avoiding asphaltene deposition. However when fuel oils are mixed they can become unstable; the fuel oils are said to be incompatible.
Low sulphur fuel oils may have a tendency to be incompatible with each other as they may have more light material in them than higher sulphur specification marine fuel oils. This is because sulphur is more likely to be a constraining property than it is in higher sulphur fuel oils. Thus it becomes necessary to blend for sulphur as well as other parameters such as viscosity. A higher mass or volume of cutters may be used to achieve the sulphur specification than is the case in a higher sulphur fuel oil. These lighter materials may have a relatively high paraffin content (this is not always the case, there are some aromatic cutters such as FCC LCO) and thus be more likely to precipitate asphaltenes.
There are numerous tests that can be performed and algorithms that can be used to predict whether a fuel oil blend, or mix of blended fuel oils are likely to produce a stable or unstable fuel oil.
Marcello Ferrara, ITW Technologies, email@example.com
The difficulty for marine engines to keep up with the times has been severely tested by the new IMO 2020 regulations which require the use of low sulphur fuels which may create deposits and fouling inside the combustion chamber and consequently cause severe damage to the pistons, liners and pistons rings, and in the worst cases lead to a sudden stop of the engine. Additionally, all of the system upstream of the engine will suffer from fouling issues.
While lower sulphur in fuels will mean fewer harmful emissions, the loss of lubricity that sulphur brings can also make engine operations more challenging.
Distillate ageing also contributes to increasing issues. The percentage of distillate blend components in VLSFO often come from complex secondary refinery streams where much of the natural stability has been removed or weakened.
Distillate ageing is a chemical process that produces sludge in the presence of elevated temperature, oxygen, or by a catalyst present in the fuel (for instance, a metal). These processes can be prevented by the use of proprietary additives.
Unstable distillates oxidise (most commonly under elevated temperatures and pressures), forming sand-like sediments which block injector nozzles, while gums become impregnated with inorganics (catalyst fines, metals, sediments), creating a grinding paste on small tolerance contact surfaces (fuel pumps, injectors, and so on).
Long chain paraffins present in many fractions of VLSFO are also responsible for wax formation in fuels. Injector blockages and separation failures indicate aged distillate material.
The major problem when blending VLSFO comes however with asphaltene stability. It is well known in the industry that, by blending different stocks, asphaltene stability will be impacted and finally asphaltene will associate into larger molecules and precipitate. Additionally, the paraffinic matrix of the VLSFO will make asphaltene naturally unstable as it will dilute the resins that keep asphaltene in solution. These larger asphaltenic molecules require more time and oxygen than is available within one combustion cycle to burn. These unburnt residuals deposit on liners and pistons in the combustion chamber; or burn later and contribute to poor ignition and airborne emissions.
Adding proprietary chemistries for chemically rebalancing the fuel and restoring stability is king for improving fuel performance and for avoiding all of the operating issues, as asphaltenes can be kept in solution and sustain inappropriate blending.
Both ship owners and fuel suppliers have no chance of controlling the blending in that a ship can bunker at any place in the world and the fuel makers receive their feeds from any place in the world.
The problem can therefore be addressed only by enhancing asphaltene stability in situ, either while bunkering or at the fuel supplier tank.
ITW has a long track record in asphaltene stabilisation, ranging from blending incompatible stocks to getting out-of-range values of sediments back on-spec by hot filtration, as well as dissolving precipitated asphaltene and coke-like materials.
Additionally, ITW has a long track record in reducing airborne emissions in engines and in power stations firing any type of fuel oil, including the ones with very high asphaltene content.