Photo: Illustration; Image courtesy: Wärtsilä Open Loop Scrubber System

Study: Scrubbers have lower well-to-wake CO2 footprint than low-sulphur fuels

Wartsila scrubbers
Illustration; Image courtesy: Wärtsilä Open Loop Scrubber System

Using exhaust gas cleaning systems (scrubbers) or low-sulphur fuel oils to meet the IMO 2020 Sulphur Cap both result in an increase of well-to-wake CO2 emissions, according to a recent study carried out by research and consultancy firm CE Delft.

A scrubber requires energy which is generated by engines running on fuel oil and thus generate CO2, while desulphurisation in a refinery requires hydrogen which is generally produced from methane, emitting CO2 in the process, as well as energy.

The study found that CO2 emissions associated with using a scrubber vary between 1.5% and 3% for a number of representative ships.

The CO2 footprint of using an EGCS depends on the sulphur content of the fuel and the amount of fuel a ship uses in an ECA. The higher the difference between the sulphur content of the fuel and the allowed emissions of sulphur oxides, the more energy a scrubber requires.

CO2 emissions associated with producing and installing a scrubber are small compared to the operational emissions, the findings show.

In contrast, by discharging acidic washwater into the ocean from a scrubber results in CO2 emissions in the ocean, which are of a similar order of magnitude as the CO2 emissions from operating the EGCS. In total, CO2 emissions increase typically by 1.5-3%.

The study has used multiple reference ships and data about the EGCSs received from manufacturers, which they would have installed on the selected reference ships.

These include:

  • cruise ship (100,000 GT);
  • small container ship (4,000 TEU);
  • large container ship (18,000 TEU);
  • bulk carrier (80,000 dwt);
  • oil tanker (200,000 dwt).

On the other hand, various processes can be used to produce low-sulphur fuels, and the choice will depend on the refinery design and the crude oil slate the refinery uses.

The CO2 footprint of desulphurising fuel oil in the refinery depends on the crude oil used and the layout of the refinery.

Using a generic refinery model, the study analysed the CO2 impact of two options: hydrotreatment of residual fuel and hydrocracking in combination with hydrotreatment.

In both cases, the fuel quality inevitably improves as unsaturated bonds and aromatics are saturated and the fuel becomes more paraffinic.

Many low-sulphur fuels have better qualities in terms of viscosity and aromatics content than required by the applicable standards.

“Whether or not the removal of sulphur on board or removal of sulphur in the refinery generates lower CO2 emissions depends on whether or not the inevitable fuel quality improvements are taken into account,” the consultancy said.

The study added that the mere removal of sulphur generates less CO2 emissions than the use of an EGCS, whereas sulphur removal plus fuel quality improvement has more CO2 emissions than using an EGCS.

“The increase of emissions associated with desulphurisation in a refinery is higher than 1% and in many cases multiple times higher, depending on the quality improvement of the fuel, the refinery layout, and the crude used,” CE Delft said.

The study ‘Comparison of CO2 emissions of MARPOL Annex VI compliance options in 2020’ has been commissioned by 3 major EGCS suppliers, Alfa Laval, Yara Marine and Wärtsilä.