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Capturing the CO2 in ships' emissions is a technical challenge

SCRUBBING OUT CO2

Robert Allen of the UK’s University of Plymouth provides an expert opinion on whether scrubbing could be a viable option to reduce carbon dioxide emissions

The burning of one tonne of a 3% sulphur heavy fuel oil (HFO) will typically release 60 kg of sulphur dioxide (SOx), 60 kg of nitrogen oxides (NOx) and 3,200 kg of carbon dioxide. Proven technologies exist to reduce both SOx and NOx but shipborne technologies to reduce CO2 are yet to be fully developed.

October 25, 2021

The burning of one tonne of a 3% sulphur heavy fuel oil (HFO) will typically release 60 kg of sulphur dioxide (SOx), 60 kg of nitrogen oxides (NOx) and 3,200 kg of carbon dioxide. Proven technologies exist to reduce both SOx and NOx but shipborne technologies to reduce CO2 are yet to be fully developed.

 

Overall emissions have been reduced by the implementation of efficiency indexes and IMO has stated that ‘’by 2025, all new ships will be 30% more energy efficient than those built in 2014’’ which means that with less fuel consumed lower emissions are achieved. On this basis, to reach a 50% reduction by 2050 a further 20% must be achieved.

 

Onshore technologies do exist to capture CO2 from the atmosphere and the technology employs scrubbing the air with a solution of potassium hydroxide (KOH).

 

It is the hydroxide ions (OH-) which react with the CO2 to form a carbonate and water. The equation is: CO2 + (OH-) + (OH-) = (CO3)– + H2O, and for a theoretical 100% capture, 44 kg of CO2 requires 34 kg of hydroxide ions.

 

Sodium hydroxide (NaOH) is used in fresh water closed loop ship scrubbers for SOx reduction, but if applied for the removal of CO2 excessive amounts would be needed. A simple calculation indicates that to scrub all the CO2 produced by the burning of one tonne of HFO would require some 5.8 tonnes of NaOH and even a 20% reduction in CO2 emissions would require approximately one tonne of NaOH in a solution.

 

This an oversimplification and does not consider the efficiency of a scrubber or that the washwater may need further treatment to achieve the IMO minimum pH level of 7.5. The conclusion is that the use of alkali metal hydroxides in solution, to reduce CO2 emissions on a large scale, is likely to be impractical.

Alkaline water containing OH ions can be obtained from the electrolysis of water used to produce hydrogen. However, based on information on the energy to produce hydrogen and thus a solution containing OH ions, and if the electrical energy for the electrolysis is from a diesel generator, then it is estimated only 20% of the emitted CO2 could be captured using the hydroxide ions produced. In short on-board electrolysis is a net emitter of CO2 and this is a non-viable solution for ships. It is only viable if the electrical energy is from a renewable land-based resource.

 

A ship trial undertaken by a scrubber manufacturer used alkaline water of pH 9.5 (produced by electrolysis) in a two-stage scrubber. This trial demonstrated significant reductions in the levels of all three acidic gas emissions. A presentation about this trial performance was presented to the IMO, but exact details of the trial have never been published, nor was the trial work continued, so an accurate assessment of its viability cannot be made.

 

Other methods of producing alkaline water are under investigation. One method was identified using known non-chemical water purification technology and a small-scale tests rig was built and evaluated. A scaling up exercise indicated it would be possible to produce 20 m3/hr of pH 9.5 water on a ship for a production cost of less than US$10 per hour. Work is yet to be undertaken on how efficiently CO2 is captured in a simulated scrubber using alkaline water, including the quality of the resultant wash water.

 

The road to zero/low GHG emissions will unlikely be achieved using a single technology and any system adopted will need to be low cost, robust and perform efficiently in service. Alkaline water scrubbing has been proven in land-based CO2 capture and the goal is now to develop this technology for ships.

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