Hard-wearing sensor defying soot and heat

Measuring the content of nitrogen oxides (NO_x) in the exhaust gases from a ship is, in itself, not ground-breaking. But doing it online day after day, without the instrument sooting up, is quite a feat—now made possible by a Danish innovation project.

NO_x are hazardous substances produced in varying degrees in all types of combustion. The emissions from ships of a number of substances are regulated by the international maritime organization, IMO. The limits are gradually increased, and in 2016, new rules will enter into force that will restrict emissions of NO_x in specific areas. In addition, several ports worldwide are now requesting shipping companies to restrict their NO_x emissions.

"With a new sensor, shipping companies can document their compliance with the requirements. Also, the information about the NO_x contents in the exhaust gases is used to adjust engine operation," explains Poul K. Sørensen, Executive Technical Advisor in the Danish company Green Instruments.

More customers making green requirements
A prototype of the sensor has just passed a practical test where it has been in operation for more than four months on board one of Maersk Line's container vessels. The shipping company participates in an innovation project which was initiated in 2011.

The objective of the project was to develop a reliable method which matches the NO_x requirements that the IMO will introduce in 2016. That will enable shipping companies at the forefront in the environmental area to gain competitive advantages. 

Shipping is responsible for much of the world's NOx emissions. It thus makes good sense to develop a sensor which both measures emissions and enables reducing them.  Photo: Maersk

First with direct measurements
"Right from the start, it became clear that infrared sensors would be able to perform the required measurements. The big challenge was that the sensors would be required to continue to deliver reliable measurements over very long periods in an aggressive environment," says Mogens Havsteen Jakobsen, Associate Professor at DTU Nanotech. He has been coordinating DTU's participation in the project.

The problem is especially the high temperatures and the level of soot and various aggressive substances that the sensors are exposed in the exhaust gases from a marine engine during day-to-day operation. There are already solutions on the market, which involve pre-processing the gases. In this way, the temperature is reduced and some of the aggressive substances can be removed before the measurements are performed. But the processing also creates the risk of new sources of error.

"Our ambition was always to measure the process gases directly as close to the source as possible. However, the challenges were, of course, immense," says Mogens Havsteen Jakobsen.

The solution was a combination of a nanostructured surface and a coating, which in combination makes the sensor glass dirt-repellent. In other words, the glass in the instrument is not in itself 'nano'—it is about a centimetre in diameter, but the structure of the glass surface is. The details are so far secret due to patenting.

The way forward
The shipping company A. P.  Møller-Mærsk has tested the prototype in practice. Marine Engineer Per Hother Rasmussen says:

"The on-board test showed that the prototype with the coated glass from DTU and Green Instruments has a good reliability. It is to be expected that the finished product will live up to the requirements in the shipping industry."

Following successful testing at sea, the sensor will now be put through a series of tests at Green Instruments which has instruments that can measure the contents of a number of inert gases—i.e. gases that typically are not chemically reactive—in exhaust gases. The company will also be responsible for producing and marketing the new sensor.

Edited article from DYNAMO no. 41, DTU's quarterly magazine in Danish.