Focus on Propulsion: NOx rules put an end to decades of complaceny

Shipowners are confronted with emissions technology choices for newbuilds that will force them to consider propulsion designs more carefully than ever before.

By Wendy Laursen

Gone are the days when the choice of propulsion was largely a matter of taking the safe option offered by the shipyard. A fundamental design shift is underway, says John Bradshaw, Lead Technical Specialist for Machinery at Lloyd’s Register: “Over the last 30 years, ships have tended to follow the same design concept – a slow-speed diesel engine with three or four diesel generators or, for vessels such as ROROs and cruise ships, medium-speed diesel engines. Because of the changes in emissions requirements, we are going to see major diversions from this and a much more bespoke approach to design and construction because there isn’t a ‘one size fits all’ solution. Ship operators are going to have to tailor the available technologies.”

LNG Is Leading the Race
The choice of solution for meeting future NOx and SOx emissions requirements is intimately tied to vessel construction date and trading pattern, and one of the three main technologies able to meet the strictest requirements is the use of LNG (liquefied natural gas) as bunker fuel. LNG can deliver dramatic reductions in NOx emissions along with significant reductions in SOx, particulate matter and CO2, without the need for after-treatment of engine exhausts.

Gas-fuelled engines, either dual-fuel or lean burn mono-fuel, are being developed by a range of suppliers including Wärtsilä, Rolls-Royce, MAN Diesel, Kawasaki Heavy Industries and Mitsubishi Heavy Industries. Wärtsilä claims an unmatched track record with over three million running hours in both land-based and marine applications for its dual-fuel engines. The current order book for vessels burning LNG as bunker stands at around 20 vessels and includes ROROs, passenger vessels, liquid bulk tankers, special cargo ships and high-speed ferries. LNG is currently around 50 per cent cheaper than low-sulphur diesel fuels, so despite the increased construction cost the payback time can be short.

However, LNG is yet to be readily available at bunker stations globally, and the equipment installation and storage tanks can take up to seven times as much space as that of traditional fuels. Safety and equipment maintenance requirements can be more technically challenging too for shipowners not used to the technology.

There is little competition for LNG as a fuel solution. According to analysis undertaken by DNV, LPG (liquefied petroleum gas) is more expensive and in limited supply. It also carries safety concerns as it has different flammability and dispersion properties than LNG. Another alternative, CNG (compressed natural gas) requires twice the volume to achieve the same sailing distance. Biofuel blends and alternative energy sources such as wind power, fuel cells and nuclear power still face significant technical challenges and require implementation timetables longer than LNG.

After-Treatments Still on the Test Bench
Selective Catalytic Reduction (SCR) is an after-treatment technology able to meet the strictest NOx emissions requirements, and systems are available from manufacturers such as Wärtsilä, MAN Diesel, ABB, Canopus Marine Solutions, Wilhelmsen Technical Solutions and Niigata Power Systems. SCR can be used for both low-speed and medium-speed engines. But because the exhaust temperature is lower for low-speed engines, the SCR unit must be placed before the turbocharger, while for a medium-speed engine it can be placed after the turbocharger. SCRs have to be hot to operate, so a vessel like a short sea ferry that is constantly in and out of port and only at sea for a few hours at a time may have problems.

SCRs work significantly better with-low sulphur exhaust gas. “With high-sulphur fuels you are delivering metals that can spoil the catalyst and sulphates that can foul the catalyst pores, so it becomes very challenging to keep SCRs functioning,” explains Kevin Reynolds, Senior Associate of Glosten Associates in the U.S. “If high-sulphur fuel and a scrubber are used, consideration must be given to the sequencing. Dry scrubbers are not an issue because they don’t cool the exhaust gas, but if a wet scrubber is used the SCR must operate before it.”

Dual-fuel engines will have added complexity if they are combined with SCRs as the SCR won’t need to be operated when the engine is running on LNG. “Would you have two stacks,” asks Reynolds, “one with SCR and one that goes straight through? If you can get your keel laid in the next five years, however, dual-fuel engines are a great option, especially for vessels such as breakbulk carriers, which do not have regular trade routes.”

Exhaust gas recirculation (EGR) is another after-treatment technology expected to meet the strictest NOx requirements but, says Reynolds, it comes at a significant reduction in overall fuel economy. EGR can be applied to both low- and medium-speed engines, but to keep the installation compact only low-sulphur fuels should be used, making them an expensive option for high-power, low-speed engines.

MAN Diesel has already introduced SCRs for its medium-speed engine range, and the company is currently investigating EGR as it has the advantage over SCR of not requiring media such as urea. Although EGR is generally considered less technically mature in marine applications, MAN has test results demonstrating the technology’s ability to meet the strictest NOx standards. To investigate long-term effects on engine components, an EGR system has been installed onboard A.P. Moeller Maersk’s 1,100-TEU container vessel Alexander Maersk.

Small Engines Get a Free Ride
For small commercial engines, the EPA recognized that space is likely to make after-treatment systems such as SCR and EGR impractical. For John Deere, the EPA requirements can be met through engine technology; and the transition from EPA Tier 2 to Tier 3, effective in 2013 or 2014 depending on engine size, has seen Deere move to 4-valve cylinder heads and high-pressure, common rail systems for most engines. Options for third-party after-treatment systems are also available.

Caterpillar Marine Power Systems will enter full production of EPA Tier 3-compliant Cat 3500C marine engines in 2012. Compliance will be achieved through fuel and air system modifications without the need for after-treatment. One of the engines has already been installed on Ocean Titan, a Z-drive tug owned by Western Towboats.

MTU is also able to meet most current emission regulations with internal engine measures. Marine engines of the Series 60, Series 2000, Series 4000 and Series 8000 variety comply with all key emission regulations worldwide, including IMO Tier II and EPA Tier 2. For specific projects, Series 396 and 1163 engines complying with IMO Tier II are also supplied. For a further reduction of emission levels, MTU offers product and project-specific after-treatment systems for all its engines.

More Options on the Drawing Board
High-pressure turbochargers and turbocharging systems are being developed for large diesel engines by a number of manufacturers including ABB Turbocharging, whose Power2 two-stage, high-pressure turbocharging system has completed type testing and been introduced commercially on two gas engines. The 2-stage technologies push charge air pressure up so that inlet valves can be closed earlier without loss of charge air mass when employing Miller Cycles on 4-stroke engines. The early valve closure allows the intake air to continue to expand, and the associated cooling reduces combustion temperature. This reduces NOx emissions by a large margin. However, the technology may not completely meet the strictest emissions requirements without being combined with other solutions.

Also on the horizon is a hybrid emissions solution from Ecospec of Singapore. The CSNOx system has a first stage similar to an open loop wet scrubber. A second stage scrubber uses seawater that has been specially conditioned by ultra-low-frequency waves to absorb the remaining SOx as well as NOx and CO2. The CSNOx system is currently undergoing verification testing on the Tanker Pacific vessel White Sea.

The risks associated with installing technologies with limited maritime operational time mean that their adoption ahead of mandates is not widespread, especially if the vessel’s anticipated trading pattern does not involve a high percentage of time in Emission Control Areas (ECAs). Scrubbers to remove SOx are still viewed with skepticism by some shipowners. However, in a report prepared for the Ship Operations Cooperative Program in the U.S., Glosten Associates showed an analysis of three ship types operating at least partially within an ECA that predicts the cost of installing a scrubber would reduce expenses so significantly that some ship operators may find their installation a competitive necessity.

Not everyone is convinced. The first of four 8,500 DWT multipurpose dry cargo vessels was launched by Jiangsu Yangzijiang Shipbuilding in China in September and, although the vessel featured the innovative Groot Cross-Bow developed by the Dutch design and engineering company Groot Ship Design, it did not incorporate LNG engines or after-treatment technologies. The owner, Caribrooke Shipping of the UK, decided not to install additional equipment because it was not yet decided where the vessels will sail and the economics of exhaust gas-cleaning equipment were not working out. Instead, space was left and preparations made for future installation.

Energy Efficiency Doesn’t Count
Technologies that improve fuel efficiency are not able to meet SOx and NOx regulations because the IMO and EPA emissions requirements relate to instantaneous end-of-pipe measurements rather than overall propulsion efficiency. Despite this, energy efficiency will feature strongly in future designs as a way of reducing operational costs and CO2 emissions.

For most vessels, electric propulsion gives the best overall efficiency because energy production and consumption are split, says Hallvard Slettevoll, Director of Stadt, an electric propulsion system manufacturer in Norway. Electric propulsion also has another advantage: “Combustion engines running on LNG have a much weaker torque performance compared to diesel engines,” states Slettevoll, “and they perform quite poorly at low RPMs and low power. Such engines will benefit greatly from a fully electric solution, typically with four equal engines running dynamically in parallel, thus avoiding low-load operation. The electric motor produces the needed torque for the propeller much better than the LNG engine.”

Optimized propellers are another way of significantly improving energy efficiency, and Voith Schneider Propeller claims efficiency advantages of almost 40 percent in certain operating environments. The Voith Schneider Propeller can be a superior propulsion solution, says Product Manager Lee Erdman, particularly for vessels requiring precision maneuvering, zero-speed roll stabilization and precise dynamic positioning.

Dates Unlikely to Slip
Shipowners will soon be forced to make emissions-based propulsion decisions for their newbuilds. An IMO correspondence group is being established to evaluate if next-generation NOx emissions abatement technologies are advanced enough to meet the 2016 deadline. The answer, expected to be announced in autumn 2012, is likely to be yes, and the EPA is confident enough about its requirements not to have even scheduled a review.

“It is going to be an additional expense and this equipment will have a space requirement,” says Bradshaw of Lloyd’s Register. “The flipside is the alternative perspective that land-based industries have been subject to increasingly rigorous emissions controls for several decades. The marine industry has had an opt-out from such requirements until now.” – MarEx

Wendy Laursen is based in Australia. This is her first appearance in the magazine.