Marine and shipbuilding

Nasiri Ltd. provides functional design, industrial automation, and instrumentation services for the Scrubber and SCR. To find out more about these services please check our service page.

Broadly, ships are categorized as cargo and passenger. Cargo ships include container, roll-on/roll-off, bulk, multi-purpose, refrigerated, and tanker ships which are used for transportation of goods and fluids. Passenger ships include cruise and ocean liners that are used to transport people. Shipping is one of the most energy-efficient and economical methods for mass transport of cargo and it is and will remain to be the dominant method in world trade. The shipping industry emits approximately 3.1% of annual CO2, and about 15% and 13% of human-made NOx and SOx respectively, and to a large extent fuel type affects these emissions; additionally, fuel cost accounts for about 50%-60% of the total operational cost. NOx and SOx are harmful components and responsible for ozone-depleting and acid rains.

For maritime safety, the International Maritime Organization (IMO) contains rules about ships in the MARPOL (International Convention for the Prevention of Pollution from Ships). MARPOL sets the limits on NOx and SOx emissions. In 2016, on the 70th session of the Marine Environment Protection Committee (MEPS) meeting, IMO has required ships to cut their emissions of SOx by 80% (reduced from 3.5 wt% to 0.5 wt%) within the Emission Control Areas (ECAs) from 1st of 2020 onward (also referred as IMO 2020).

MARPOL (regulation 13 of Annex VI) sets the emission limits on NOx to be applied on marine diesel engines with an output power of more than 130 kW. The limits depend on the maximum operating speed (n, rpm) which is shown below:

With only combustion process optimization, the Tier II can be reached, but Tier III requires NOx emission control technologies. As these regulations are getting tighter, ship owners need to consider lighter fuels like LNG or to look for new standard solutions. The Scrubber system is the best method to remove SOx and it has open-loop and closed-loop modes. In both modes, a scrubber tower is placed in the way of engine exhaust gas to wash out the SOx (and NOx). Unfortunately, in the open-loop mode, the exhaust gas is washed with seawater and this acidic solution is directly dumped back into the sea.

SO₂ + H₂O → H₂SO₄ ←→ H+ + HSO₄ ←→ 2H+ + SO_4²

NO₂ + H₂O → HNO₃ ←→ H+ + NO₃

However, in the closed-loop mode, the exhaust gas is washed with NaOH solution following the equation below, and the resulting solution is not discharged to the sea directly but is sent to a water treatment unit where the effluent emulsion is treated to be reused or being discharged overboard to the sea. This method can reduce the SOx by 95%.

SO2 (aq) + 2 NaOH (aq) → Na2SO3 (aq) + H2O (l)

The reaction of NaOH with SO2 results in sodium sulfite (Na2SO3) that, according to the material safety data sheet (MSDS), causes little oxygen depletion in aquatic systems if dumped into the sea. To remove the NOx, Selective catalytic reduction (SCR) is the most efficient way to convert NOx into N2 and H2O by using a catalyst. In this method, exhaust gas is blended by urea solution and passed through the catalytic reactor which reduces NOx emission up to 90-95%.