Advances in Clean Marine Propulsion
Can LNG-fueled vessels save the planet?
By Richard Carranza
Mention propulsion to someone and they generally think of jet engines or a 12-cylinder Ferrari. And while propulsion does mean power, it also means pollution, particularly when it comes to powering ships. Burning diesel and HFO (Heavy Fuel Oil) creates greenhouse gases, mainly SOx, NOx and CO2, that are emitted into the atmosphere. Propulsion systems, because they require seals and bearings to reduce friction, can also leak lube oil into the ocean when seals weaken due to harsh conditions and varying loads. These are among the issues being addressed by the International Maritime Organization (IMO) in its continuing efforts to reduce maritime pollution worldwide. The IMO’s Tier III regulations, scheduled to take effect in 2016, would require dramatic reductions in greenhouse gas emissions from ships, including a massive 80 percent in NOx when transiting designated Emission Control Areas.
The Natural Gas Solution
Faced with looming deadlines and high compliance costs, natural gas as an alternative energy source is looking more and more viable. For one thing, methane (the main ingredient in natural gas) has the potential to deliver large amounts of energy. During complete combustion, fuel plus oxygen are converted to carbon dioxide and water. The heat derived from combustion propels the ship. Compare the heats of combustion (kJ/g) for the following fuels (approximate values): methane, 55; propane, 50; gasoline, 47; diesel, 45; and ethanol, 30. Methane delivers more energy than any other fuel.
Moreover, the combustion of natural gas is much cleaner. During complete combustion, the only byproducts are carbon dioxide (a greenhouse gas) and water, both colorless. Natural gas is, in fact, a “no sulfur fuel” and can reduce NOx emissions by up to 90 percent and CO2 by up to 20 percent with little or no smoke. In the case of incomplete combustion, as occurs with other fuels, side reactions and other compounds (including carbon monoxide and elemental carbon) are produced and give off black smoke. Diesel has a tendency to produce smoke.
Liquefied natural gas (LNG) can be used as a fuel in ships because it occupies one-six hundredth the volume of natural gas. In the 1970s LNG cargo ships were propelled primarily by steam turbines, and the fuel used to generate the steam for propulsion was the LNG cargo itself. In the 1990s there was a paradigm shift in the industry, and LNG ships employed both diesel propulsion and dual fuel propulsion (engines with the capability to use either natural gas or diesel). Since the 1990s LNG has supplied the energy source for the dual fuel electric propulsion system. Despite the fact that the LNG cargo is insulated, heat transfer does occur, thereby resulting in vaporization – methane gas is generated. The vapors are either liquefied via cryogenic refrigeration and returned to the storage system or collected and used to propel the ship.
Wartsila Ship Power Solutions
Wartsila, a Finnish company, is presently the market leader in large industrial maritime propulsion due to its ingenious dual fuel electric propulsion system that utilizes both natural gas and diesel. The system primarily works with natural gas but can be switched to diesel when necessary. The fuel is sent first to electric generators. The electricity is then used to drive electric motors, which are linked to the ship's propulsion shaft. Although the energy transfer undergoes one additional step by using the dual fuel electric propulsion model, the dual fuel system is preferred because the Wartsila engines are highly efficient. Variable frequency drives help keep the system operating at its optimum point as load conditions change, which is not possible under traditional fixed-speed motor installations. In other words, the dual fuel system actually provides higher efficiency. Moreover, the excess electricity is portable to other ship services, like cargo pumps.
Tony Teo of the ship classification society DNV in Houston believes that duel fuel electric propulsion utilizing LNG should be applied to smaller vessels as well as large LNG carriers: “The time is ripe for LNG to be used by ferries, offshore support vessels, tugs, and the Coast Guard. Growing in popularity on the Baltic and North Seas, today there are 21 vessels operating with LNG fuel and another 25 or so on order. These are all on short sea trading or coastal routes. Using LNG as fuel has big cost benefits and even bigger emissions benefits with the potential to reduce SOx completely, NOx by 80-85 percent, and CO2 by 20-25 percent.”
While acknowledging that small ship owners are concerned about the availability of LNG, Teo stated that “The supply can be secured with many of the LNG terminals located along the U.S. coast. There is a huge potential market for new, small-scale LNG distribution systems in the U.S. We have spoken to several terminals and they are eager to sell their stock as LNG. They prefer not to compete with the pipeline gas market.”
Thordon’s Seawater-Lubricated Stern Tube Bearings
On the high seas a ship's propeller system is often put in peril. Fishing lines and fishing nets can get entangled in the propellers. If the ship is an ice breaker in the Arctic, then impact with ice can also place stress on the system. The net effect is that bending moments are exerted on the shaft that cause the stern tube seals and bearings to fail. Upon failure, lubricating oil is spilled into the sea. Governments around the world are concerned about oil releases originating from stern tubes. Oil spills can result in civil and criminal indictments, fines, and even imprisonment. Furthermore, unscheduled stern tube repairs are costly, both in terms of capital costs and lost time.
In an effort of circumvent unscheduled stern tube repairs, Thordon Bearings of Canada offers a seawater-lubricated stern tube bearing system instead of the traditional oil-lubricated system. The bearings are made of elastomeric polymer instead of metal. The seawater is drawn from the ocean, pumped through the bearings and back into the sea. No oil is required. It should be noted that seawater-lubricated bearings may be more expensive than oil-lubricated. Since salt water is highly corrosive, bronze liners are required as well as corrosion protection for the shaft. However, since there is no lubricating oil present, the initial investment costs are recouped by lower maintenance costs related to seal maintenance, not to mention stern tube oil costs and the costs related to oil pollution.
“Offering bearing packages with reliability equal to or better than an oil and white metal system has made switching to pollution-free bearings an easy decision for many ship owners,” stated Ryan Edmonds, Thordon’s Marketing Coordinator. “Our in-house engineering team and distributor network in over 100 countries offer worldwide availability and technical support. Many ship owners are taking advantage of converting their ships with an existing oil-lubricated system to seawater-lubricated Thordon COMPAC bearings that have a predictable wear life and a 15-year guarantee for newbuilds.”
Caterpillar Marine Diesel Electric Propulsion Engines
Although natural gas engines have a tremendous advantage when we look at emissions, new electric propulsion systems are allowing diesel fuel to maintain a strong position as well. Caterpillar is making significant advancements in the field of diesel electric propulsion and increasing the power efficiency relative to traditional direct drive diesel systems. In traditional diesel systems, the engine is connected directly to the propulsion line. In the electric propulsion systems, the engine is connected to an electric generator. The power is then relayed through a switchboard, transformer, frequency converter, electric motor and, ultimately, the propeller. The additional hardware adds about 10 percent energy consumption, but Caterpillar’s advanced technology reduces overall fuel consumption, increases efficiency and lowers CO2 emissions.
Caterpillar does this by utilizing an electronic unit injector fuel system (EUI) in conjunction with an ADEM?™ III governor (controller). EUI does not rely on the mechanical synchronization of the injectors; rather, it uses electric solenoids to control fuel delivery. ADEM III controls the solenoids so that the timing and the length of fuel injection are matched perfectly with the speed and load requirements of the moment. ADEM III also has programmable features such as fuel/air ratio control. EUI, together with ADEM III, allows higher combustion efficiency, lower fuel consumption and reduced emissions.
Ships do not always run at full load or optimum efficiency level. Diesel electric propulsion gives the user the ability to run appropriately sized generators based on the current demand, as opposed to simply having a large engine drive a propeller. Considering that diesel electric propulsion vessels use multiple engines of different sizes, the end user can exploit this combination and allow the vessel to operate at its highest efficiency. In addition, the ability to put generators on and off line leads to a double cost benefit: less fuel consumption and less maintenance (since the engine is periodically rested).
Maneuvering and dynamic positioning are essential aspects of the offshore business. In some traditional diesel systems, the engine is full-on at low thrust; low speeds are controlled with propeller pitch. Contrarily, electric propulsion systems apply variable frequency motors. In other words, electric propulsion provides zero RPMs when there is little or no thrust requirement. Energy savings can be as high as 15 percent.
Diesel electric propulsion is suitable for a very large variety of applications: workboats, cruise ships, ferries, tankers, ice breakers and warships. Furthermore, special environmental certification from organizations like DNV and Lloyd's Register is available, and this can sometimes lead to lower harbor fees at some international ports. Luiz Pustiglione of Caterpillar Marine Power Systems added, “When we analyze marine construction trends over the last three years, we see that investment in diesel electric propulsion vessels has doubled while the construction of purely mechanically propelled vessels has slowed.”
Toward a Greener Planet
The future of the maritime industry is sure to see more inspections and regulations as the U.N., IMO and local governments unite to make the air and water cleaner. Oil leakage from ship machinery and air emissions from propulsion systems will be significantly reduced by implementing advanced technologies including increased use of LNG-fueled engines. Seawater-lubricated bearings reduce shaft seal maintenance, and electric propulsion reduces engine maintenance and fuel consumption – all of which save money. Aside from saving the planet, it appears that tougher environmental regulations can also save the bottom line. – MarEx
Richard Carranza is a chemical engineer and MarEx contributor.