Boiler adds energy saving benefits to EGR
Alfa Laval and MAN Diesel & Turbo have been jointly developing a system that adds waste heat recovery to exhaust gas recirculation, effectively permitting fuel saving and TierIII NOx compliance at the same time – previously, reducing NOx meant a fuel consumption penalty.
For the forthcoming Tier III NOx limits, MAN Diesel offers two options for its two-stroke engine portfolio, the optimum choice depending basically on the proportion of time the ship spends in an ECA. The two options are selective catalytic reduction (SCR) and exhaust gas recirculation (EGR).
Either solution can be activated for sailing in the ECA (i.e. Tier III mode) and bypassed for use on the open sea (i.e. Tier II mode), but EGR has the additional benefit that, tuned correctly, it can actually save fuel when running in Tier II mode.
According to Alfa Laval, even further savings are possible with EGR if the installation is expanded to include a waste heat recovery system centred on a pressurised EGR boiler, a concept that is currently being developed jointly by Alfa Laval and MAN Diesel & Turbo.
The MAN Diesel & Turbo EGR process relies on another Alfa Laval system, PureNOx. The EGR needs effective cleaning of the wash water in the wet scrubber, which operates in a closed loop. Harmful combustion products are effectively removed by the PureNOx water treatment system based on high-speed centrifugal separation. Centrifugal separation is said to be an efficient and stable system, while offering compact size and low operating costs.
“PureNOx keeps soot and other compounds from the exhaust gas from accumulating in the scrubber,” Kristina Effler, Alfa Laval business manager, water treatment exhaust gas emissions explains. “It safeguards efficiency, reduces maintenance and prolongs the service life of engine components by protecting them against corrosion. In addition, PureNOx enables the discharge of excess water in accordance with IMO criteria.”
The first MAN EGR installation was on the 1,092teu Maersk container ship Alexander Maersk. This prototype system has logged over 2,200 hours, leading to the first production installation, on another Maersk container ship, the 4,500teu newbuild Maersk Cardiff. This has now achieved over 1,000 hours, demonstrating the ability for Tier III compliance from its EGR-fitted 6S80ME-C9 engine, as well as the potential for fuel savings in Tier II operation. This ship will be used for trials of the Alfa Laval Aalborg pressurised EGR boiler.
“We have shown that EGR works smoothly and that it complies fully with the NOx limits appearing in 2016,” said Michael Witt, head of retrofit development R&D at MAN Diesel & Turbo. “Together with Alfa Laval and AP Moller-Maersk, we are now looking beyond compliance to optimise the system and to utilise opportunities for energy savings.”
According to Anders Busk Nielsen, Alfa Laval boiler project manager, one such opportunity is waste heat recovery. “EGR is a fantastic technology that is already well served by Alfa Laval’s separation know-how. Now we are adding our expertise in waste heat recovery, by placing a pressurised exhaust gas boiler in the EGR circuit. If successful, it could add substantial energy savings to NOx emission compliance.”
The company explains that 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 Mr 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.”
The heat would be recovered from the EGR circuit, and the availability of service steam would be unchanged from current practice. Moreover, as part of an integrated waste heat recovery system, the EGR boiler would enable a major increase in efficiency.
According to Mr Nielsen, the theory is simple, but constructing an EGR boiler is a challenge. In current waste heat recovery systems the temperatures involved are around 270°C, but before the EGR scrubber this figure can be doubled. In addition, the potential gas pressure in the EGR circuit is 2-4bar(g), which necessitates a pressurised boiler casing.
The new Aalborg XW-HPX pressurised EGR boiler is designed to be lighter and more compact than any other boiler of equivalent output. Tentatively planned for market launch in 2015, the boiler is scheduled for test installation aboard the Maersk Cardiff late this summer.
“As a front-runner in the pursuit of green technologies, we are interested in testing the new EGR boiler aboard the Maersk Cardiff in order to hopefully demonstrate its promising potential for energy optimisation in connection with the existing EGR installation,” says Ole Christensen, senior machinery specialist at AP Moller-Maersk.
The XW-HPX is expected to provide the same saturated steam as a traditional exhaust gas boiler, though it would be more cost-efficient and easier to install. However, the company believes the maximum potential lies in integrating the XW-HPX with existing waste heat recovery after the turbocharger, so that it shares a steam drum with the traditional exhaust gas boiler. With the output of the traditional boiler feeding into this drum, the XW-HPX would be capable of producing high-quality steam at a temperature of 400°C, bringing the waste heat recovery system to a much higher level of efficiency.
“Producing steam of this quality would greatly increase the performance of the steam turbine,” says Mr Nielsen. “To give an indication, we’ve made calculations together with a leading steam turbine supplier. Those calculations put the power generation increase at 7.5-10%, depending on installation factors.”
On Maersk Cardiff, for example, the additional generated power could be used to cool containers, thereby limiting the fuel consumed by the genset. Alternatively, it could be fed to a shaft generator, to increase the efficiency of the propulsion system, with a considerable fuel saving. Such 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 Mr 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.”
Mr Witt sees this as potentially the most exciting prospect: “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.”
As a further benefit, because the boiler will remove some of the exhaust gas heat in the EGR circuit prior to the wet scrubber, it could allow a smaller PureNOx water cleaning unit to be used. Mr Nielsen explains: “When you take exhaust gas at 450°C and spray water through it to cool it to 50°C, you need a lot of water and a high-capacity water cleaning unit. 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.”
The main question mark hanging over the process is the boiler’s effect on NOx abatement, and this will be discovered in the Maersk Cardiff sea trials “Up to now, we have wanted to run the EGR system aboard the Maersk Cardiff without the influence of additional equipment,” says Mr 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.”
According to Mr Nielsen, “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. Nothing is proven yet, but this is absolutely what we expect.”
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