A possibility of reducing NOx and CO2 emissions
Exhaust Gas Recirculation (EGR) has firmly shown its ability to comply with Tier III NOx limits, which will enter into force in January 2016. Over 1000 hours of EGR operation have now been achieved aboard the Maersk Cardiff, a container newbuild from A.P. Moller-Maersk with a two-stroke MAN B&W 6S80ME-C9 engine.
Developed by MAN Diesel & Turbo, the EGR process already relies on Alfa Laval’s PureNOx, which provides the vital cleaning of the scrubber water. Now Alfa Laval is cooperating with MAN Diesel & Turbo to implement an EGR boiler, which could potentially add major energy benefits to the secure emission compliance.
Having demonstrated reliable compliance with Tier III NOx legislation, as well as the potential for fuel savings in Tier II operation, the vessel’s EGR installation will soon be expanded with another remarkable technology: a pressurized EGR boiler from Alfa Laval.
“We have shown that EGR works smoothly and that it complies fully with the NOx limits appearing in 2016,” says Michael Witt, Head of Retrofit Development R&D at MAN Diesel & Turbo. “Together with Alfa Laval and A.P. Moller-Maersk, we are now looking beyond compliance to optimize the system and to utilize opportunities for energy savings.“
The reasons for developing an EGR boiler are straightforward. In the EGR process, around 30% of the exhaust gas is directed back into the engine, which reduces the combustion temperature and thereby the production of NOx. This also reduces waste heat recovery by 30%, since only 70% of the gas reaches the traditional exhaust gas boiler after the turbocharger. The remaining heat energy is dissipated in the EGR scrubbing process.
“The idea is to place an in-line boiler ahead of the EGR wet scrubber, before the scrubber jets cool the exhaust gas,” says Nielsen. “In this location, the boiler would have access to much higher temperatures and pressures than traditional exhaust gas boilers. That would allow a lighter and more compact boiler, which would be self-cleaning due to the high speed of flow.“
Fuel savings through even slower steaming
Given the higher output, the increased efficiency in waste heat recovery would be interesting for any vessel like the Maersk Cardiff. Yet the Aalborg XW-HPX could prove its worth in still other ways. Most importantly, using it in an integrated boiler system would allow waste heat recovery to occur at lower main engine loads than today – creating the possibility for even slower steaming.
“The Maersk Cardiff has around 20 sister vessels, all with traditional waste heat recovery systems to compare with,” says Nielsen. “With the Aalborg XW-HPX, we should be able to both increase the efficiency and extend the operating hours of waste heat recovery, which would enable slower steaming and a massive fuel savings. That would be reflected not only in a lower fuel bill, but also in a CO2 reduction on top of the NOx abatement.”
“This is perhaps the most exciting prospect of Alfa Laval’s EGR boiler,” says Witt. “If the Aalborg XW-HPX can shift the operating balance for waste heat recovery by even a small percentage, the potential savings for ship owners and operators is enormous.“
Potential savings within the EGR circuit
Finally, employing the Aalborg XW-HPX within the EGR circuit could create synergies within the EGR system itself. By removing some of the exhaust gas heat prior to the wet scrubber, it might allow the already compact PureNOx water cleaning unit to be downsized.
“When you take exhaust gas at 450C and spray water through it to cool it to 50C, you need a lot of water and a high-capacity water cleaning unit,” explains Nielsen. “If the EGR boiler can first make use of most of that heat, there will be less heat left to enter the scrubber. Potentially, that could mean less water needed and an even smaller PureNOx.“
Ready for testing at sea
The theory behind the EGR boiler is solid, and land-based concept tests have already shown that a smaller installation can withstand the operating extremes. What remains to determine, however, is the boiler’s effect on the NOx abatement process. This is the reason for adding an Aalborg XW-HPX to the existing EGR installation on the Maersk Cardiff.
“Up to now, we have wanted to run the EGR system aboard the Maersk Cardiff without the influence of additional equipment,” says Witt. “Based on the good results seen there, however, we are confident of EGR’s ability to meet the NOx limits appearing in 2016. This means we can take the next step, installing the Aalborg XW-HPX as a part of energy-optimizing the NOx abatement technology.“
“With an EGR boiler as part of the complete EGR system, it should be possible not only to reduce NOx by 80%, but also to reduce the emission of CO2, since there is the potential of substantially lower fuel consumption,” Nielsen says. “Nothing is proven yet, but this is absolutely what we expect.“
Source and Image Credit: Alfa Laval
