Low Sulphur Fuel

• Ships use of low sulphur fuel, typically with a sulphur content of 1.5%, is the primary abatement approach considered by the current international and regional legislation
• Fuel with a sulphur content of 1.5% or lower is currently only 1% of all the fuel used by ships around the world
• The availability and price of low sulphur fuel is a source of much debate and discussion in both the shipping and oil industries ahead of the implementation of legislation
• For more information about estimated availability and cost of low sulphur fuel for ships, see (library documents)
• For a better understanding of the chemistry and production of low sulphur fuel, see INDEPTH

INDEPTH
Virtually all fossil fuels include sulphur in what can be extracted from the planet’s endowment of such fuels. It is not a desirable addition. Indeed, the word for elemental sulphur – brimstone – has overtones of Hades and hellish origins. Oil, the fuel of choice for ships, is no exception. In refineries, the crude oil is separated into various useful components, such as petrol, diesel and kerosene. The process is largely one of distillation, so the products can be described as “distillates” and “residuals”. Technological development and investment in refineries has increased the proportion of the more valuable distillates and reduced the volumes of residuals. Marine fuels are generally blends of residuals. They are difficult to use, and so cheaper, but have slightly lower energy densities. The content of sulphur also reduces the energy as combustion of sulphur produces less energy than the combustion of carbon.

The sulphur that remains in distillates can be separated – “desulphurised” – down to very low levels (10 ppm is now considered to be equivalent to “zero sulphur”). There are several ways to do this, all of which consume energy. The sulphur extracted may become elemental sulphur (a yellow crystal, or, more usually, a hot yellow liquid) or sulphuric acid (H2SO4) – an important industrial chemical. It has a role as a fertiliser, for “pickling” or cleaning surfaces, and in vulcanising rubber. The bulk of this sulphur, after use, finds its way harmlessly into our rivers and other run-off into the oceans and seas.

The use of low sulphur distillates clearly avoids the emissions of sulphur compounds when it is used. However, it greatest importance arises because exhaust gas treatments for other emissions – NOx and hydrocarbon particulates – can be more efficient and cheaper, as many catalysts are poisoned by sulphur. These benefits may offset the extra energy costs incurred in the refinery.

Desulphurisation of residuals is harder and rarely done. Residuals tend to be long chain molecules with the sulphur deeply incorporated. So to remove the sulphur, the molecules need to be broken open, and this is an energy intensive process, invariably involving the addition of hydrogen. The output tends to be lighter products, which are suitable for blending with the higher value distillates rather than for use as heavy fuels.

An alternative and cheaper process is “coking”, by which higher value distillates are extracted from the residuals to leave a carbon dust – petroleum coke. If pure, there is a limited market for graphite made from the coke, but most is used as a low value additive to coal in plants with flue gas desulphurisation.

In the USA all refineries are fitted with Cokers. This results in complete residue conversion and the import of bunker fuels for local supply from abroad. However US refineries do from time to time supply residue fuel for the bunker markets.

This arises when;

• Refineries undergo maintenance and the cokers may not be working
• Upset conditions in the refinery with certain processes shutdown
• Exceedances of regulatory permits
• Refinery equipment breakdown

So, in general, desulphurising marine fuels will result in a different, more expensive product, whose formation needs more energy (and so creates greater emissions). The benefits may justify these costs, but only if no cheaper ways are possible.