Oxywash: an alternative to reducing NOx emissions

Compliance with IMO Tier III and EPA IV regulations in terms of nitrogen oxide (NOx) emissions is a major challenge for the superyacht building sector industry. The key issue is that diesel-propulsion engines cannot be modified to deliver the necessary reduction in emissions. 

In order to ensure compliance, builders and prospective owners have had only one option: bulky and impractical selective catalytic reduction (SCR) systems for exhaust gas treatment. However, Feadship has recently developed new Oxywash technology that provides a more practical and efficient solution.

To understand the benefits of Oxywash, we must first reflect on the main drawback of SCR for treating exhaust gases. That is, SCR requires carrying a considerable volume of diesel exhaust fluid (DEF). On a 60m yacht, the DEF tank can easily have a volume of 10m3 – weighing a tonne when empty and 12 tonnes when full.

Feadship’s engineers and designers recognised that fitting such a bulky system was taking up a great deal of premium floating real estate. Especially so when you consider that SCR systems are off line – with NOx being emitted directly into the atmosphere – up to 90 per cent of the time. SCR technology only works when the exhaust temperature around the catalyst is at a certain level. Unfortunately, this only happens when sailing at high speed.

Based on an idea by Roderick de Vries, technical director at Feadship Koninklijke De Vries Scheepsbouw, a better solution has been devised. Partnered with the gas treatment department of the Netherlands Organisation for Applied Scientific Research (TNO), Feadship engineers have developed the new Oxywash technology.

Because they are smaller and divided into modules (see above), Oxywash systems are much easier to fit inside a yacht than standard SCR solutions Moreover, Oxywash systems remove NOx irrespective of the engine temperature and work in any situation, making them more environmentally friendly than standard SCR solutions (see below).

The working principle behind Oxywash is simple: NOx is first made water-soluble and then immersed in seawater. When the exhaust gas temperature is high, the reaction takes place via a catalyst. If the temperature is lower, ozone is instead injected into the gas to make it water-soluble. The ozone required is generated on board from the air, removing the need to place bulky chemicals on board. And the water-soluble NOx is transferred directly into seawater without ever being emitted into the atmosphere.

When water-soluble NOx is immersed in seawater, it is transformed into nitrite and nitrate. These substances are naturally present in seawater and are a normal part of the nitrogen cycle. Nitrites and nitrates are also called fixed nitrogen forms and are essential for the growth and reproduction of marine plants and animals. Therfore, Oxywash avoids NOx emissions in the atmosphere by directly transferring fixed nitrogen to where it naturally belongs.

The Oxywash technology has been tested, and the main features of the technology demonstrated in TNO’s laboratory in Delft. A large-scale prototype test programme is currently ongoing and will explore additional features of the technology, such as its capacity to remove particulate material. The actual systems will be ready for fitting on the first yachts soon.

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