Low-sulphur fuels explained
Why is there a drive towards the use of low-sulphur fuels?
Whilst shipping, with relatively low CO2 emissions compared with other forms of transport, is regarded as the most energy-efficient means of mass transportation, the global trend towards reducing emissions from all industry sectors has led to recent calls to improve energy efficiency and control emissions in international maritime transport. Shipping is becoming recognised as one of the most significant sources of localised air pollution and acidification in the European Union. A significant element of the pollution from shipboard sources has been identified as being from the flue gas emissions containing sulphur dioxide (SOx).
Some major players in the shipping industry have already taken the initiative of switching to low-sulphur fuel and investing in finding alternative sources of “green” fuel.
But why is sulphur such a villain?
Sulphur (brimstone) is a naturally occurring element and is found freely within the earth’s crust and is essential to life, forming a constituent part of body fats and bone. It is from here that sulphur finds its way, through the passage of millions of years, into the earth’s supply of crude oil. Most of the oil supply is found between layers of sedimentary rock, this being formed by the layering of sea bed deposits and free organic material settling. The industrious work of anaerobic bacteria consumes the free organic matter converting it into simple hydrocarbons with the leaching of sulphur from the surrounding rock, resulting in crude oils having varying degrees of sulphur content.
There are several methods for extracting sulphur. The cheapest and most widespread is hydrotreatment, a process that involves treating the product with hydrogen. The removed sulphur is often in the form of H2S and must be converted into either elemental sulphur or sulphuric acid, both of which are saleable commodities. The recovery process is highly efficient, with a typical recovery rate of 99 per cent.
Not all crude oils extracted from the ground are identical.
Not all crude oils extracted from the ground are identical. Crude oil is made up of many different hydrocarbon components, ranging from light ends (aromatic hydrocarbons) to the heavier ends (non-aromatic – oily). There are many forms and within those forms there are many different compositions. In the traditional crudes there is a large difference in viscosity and sulphur content. The more viscous oils tend to contain significant amounts of heavier ends, having a lower API1 gravity and higher sulphur content, whereas the less viscous oils are predominantly made up of the lighter ends and have a lower sulphur content. The heavier the crude, the greater the expense to extract usable components. It is possible with some very light sweet crudes to burn them directly in diesel engines without any significant processing. Crude oil extracted from the ground is classified as sweet or sour, depending on the level of sulphur naturally occurring in the crude. High concentrations of sulphur dictate sour crude, while low levels are indicative of sweet crude. Commercially, sweet light crude has been preferred and therefore these resources have been consumed at a far greater rate than the less commercial heavier sour crudes. However, in the future, refineries will need to use the heavier crudes as supplies of the light crudes diminish. As the supply of light sweet crude has progressively declined since 2000, the demand has increased and this has been matched by the increased requirement for low-sulphur fuels.
Why is sulphur undesirable in fuel?
Sulphur has several undesirable properties when combined with the internal combustion engine. It is acidic in nature, which can result in the corrosion of the constituent metal parts, and is known to poison catalytic converters,2 i.e., to reduce catalytic activity, thereby reducing the effectiveness of exhaust systems.
Extraction of sulphur through hydrotreatment involves treating the sulphur with hydrogen. Hydrogen is highly reactive and can reduce the lubrication properties of diesel, a significant problem for rotary injector pumps which use the diesel for lubrication. Varying levels of hydrotreatment result in the lubrication properties of diesel varying, depending on the location of the refinery or the source of their crude oil.
Sulphur, when burnt in air, converts into sulphur dioxide (SO2), which, when released into the atmosphere, can form an acidic solution, dissolving in rain to form acid rain. This causes widespread damage to the environment, affecting lakes and forests, and has been blamed for erosion damage to buildings and structures of historic importance.
Although acid rain is so weak as not to cause immediate danger, a prolonged build-up does have significant effects on forests, causing the nutrients in the soils to dissolve and be washed away. The release of aluminium into the soil, which blocks the trees’ ability to absorb nutrients, and the washing of the tree leaves’ natural waxy deposits, restrict their ability to perform photosynthesis. This combined attack can leave trees open to disease and damage by insects and weather. Minor changes in the PH levels of rivers and lakes can affect some species while not affecting others, causing very intense plankton blooms, which go on to form very deadly toxins.
The relationship between acid rain, pollution and fossil fuels was first discovered in 1852 by Scottish chemist Robert Angus Smith, but it was not until the middle of the 20th century that the first significant steps to control air pollution were taken, gradually progressing into the current focus on shipping emissions.
1 American Petroleum Institute.
2 A catalytic converter is a vehicle emissions control device which converts toxic by-products of combustion in the exhaust of an internal combustion engine to less toxic substances by way of catalyzed chemical reactions.
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