Royal Belgian Institute of Marine Engineers Marine Fuels and Engines
By Chris Townsend, E.I. Naval Architect, Guido Perla & Associates, Inc.
Marine propulsion starts at the prime A combination of these factors can Heavy Fuel Oil (HFO) mover and ends at the propeller. In this, also be cost-effective over the life of -Historically the least expensive of in the second article in our series of marine the engine.One of the flaws in choosing use marine fuels, heavy fuel oil is used propulsion systems, we will discuss the an engine that burns the cheapest fuel extensively in the low-speed recipro- most common types of marine prime available is the unpredictable nature of cating engines that power many of the movers and the different fuels that they both global and local marine fuel mar- world’s largest commercial vessels. use. Many factors go into deciding which kets. Prices could go up, prices could go Though the cost of the fuel is low, the prime mover best fits a given vessel. down, or regulations could limit or pro- use of heavy fuel oil for marine propul- These factors include the intended serv- hibit the use of certain types of fuel. sion requires a cleaning system to re- ice of the vessel, as well as the weight, move excess impurities, as well as a size, regulatory compliance, and cost of heating system to lower the viscosity the prime mover. Cost considerations Fuels for efficient combustion. The energy re- typically include the initial cost of the quired to deliver the fuel to the engine engine, lifetime maintenance expenses, The selection of a primary fuel can be ready for combustion is quite high. The and fuel cost. influenced by other factors, such as space and weight required for the heat- Optimization of fuel costs can be storage arrangements and auxiliary sys- ing and treatment systems associated achieved one of two ways: choosing tems associated with the chosen fuel with HFO make using this type of fuel an engine that burns the cheapest fuel type. Consider briefly the major types impractical for smaller vessels. possible, or choosing an engine that is of marine fuel in use today: Ever growing emissions standards are particularly fuel-efficient. also making the primary use of HFO
SeaNG Rendering One of three Compressed Natural Gas (CNG) carrier versions designed by Guido Perla & Associates, Inc. This 118-meter vessel carries 50 mmsCF of CNG in 16 coselles and features a dual-fuel mechanical drive propulsion system. more expensive and less practical. Current and future regulation will further provides, without being unduly bur- New regulations allow for the creation restrict the allowable levels of sulphur in dened by the size and weight associated of Sulfur Emission Control Areas (SECA), distillate fuels, reducing pollution but al- with the nuclear reactor. The high power in which levels of sulfur emissions must most certainly increasing costs as well. density and high endurance capability of be below those achievable when burn- a nuclear propulsion system also makes ing currently available HFO. Natural Gas it an attractive choice for icebreaking This requires that a vessel be rerouted First commonly used in marine propul- operations. However, there are certain around the SECA, install a scrubber sys- sion on LNG vessels, natural gas is now drawbacks to nuclear propulsion that tem to remove sulfur from emissions, or becoming a more popular marine fuel, generally limit its use to military ves- switch to a more expensive low-sulfur primarily due to its clean burning char- sels. distillate fuel while transiting the SECA. ac-teristics, producing signi-ficantly less These constraints include the necessity Reports from refiners indicate that there carbon dioxide than other fossil fuels for specialized (and expensive) manning, is little interest in developing the tech- for the same amount of heat generated. high initial cost, the complexities of han- nologies necessary to dramatically re- LNG additionally produces less NOx, dling and disposing of nuclear materi- duce the sulfur content of HFO. SOx, and particulate matter than tradi- als, and extremely stringent worldwide Instead, the refiners argue money should tional liquid fuels. regulation. be spent on increasing the overall refin- Natural gas must be stored as a liquid ing process, yielding more distillates under pressure, and then warmed to a Biodiesel from residual wastes. gas before combustion, creating some Biodiesel is a clean burning alternative challenges in storage and piping ar- fuel that is gaining traction in the marine Marine Diesel Fuel/Marine Gas Oil rangements. industry. As its name suggests, biodiesel These fuels are blends of distilled fuel is a fuel similar to diesel, but contains no combined with small amounts of HFO. Nuclear petroleum products. Biodiesel is created They have lower levels of sulfur content Nuclear fuel for marine propulsion, per- through a chemical process called trans than HFO and are cleaner burning. haps the most controversial of all pro- esterification, which separates glycerin Though these fuels are more expensive, pulsion fuels, is typically limited to large from fat (such as soybean oil). When elimination of heating and extensive capital military vessels. These vessels done in the presence of alcohol, this treatment systems help justify the higher take advantage of the essentially unlim- process leaves behind methyl esters, costs. ited cruising range nuclear propulsion more commonly called biodiesel. This type of fuel can be used in a typical com-pression type reciprocating engine with little or no modification. Biodiesel burns cleaner than conven- tional diesel distillates, in addition to being made from renewable sources - these factors make biodiesel very envi- ronmentally friendly.
Engines
The generation of power, whether to send down a shaft or an electrical cable, has been at the core of marine propulsion since the application of steam power to ships in the late 19th century. Years of refinement and fine-tuning have resulted in a large array of possible prime movers suited to many different ships in a wide variety of services.
Internal Combustion Reciprocating The most common type of marine pro- pulsion prime mover is a reciprocating internal combustion engine (such as a diesel engine). This type of engine is characterized by the intermittent rapid burning of air and fuel inside a com- bustion chamber, releasing energy that MAN Diesel Engine Cross Section moves a piston, yielding linear motion A cross sectional view of the vee configuration version of the MAN Diesel 51/60DF dual- that is converted into rotational motion. fuel gas engine. This versatile prime mover is being installed in LNG carriers, as well as Reciprocating internal combustion en- car and passenger ferries. (Image courtesy of MAN Diesel) gines typically fall into one of three cat- engine speeds. Natural gas can be used engine eases maintenance and down- egories describing the speed at which in a slightly modified diesel engine in a time - the small size makes it possible the engine rotates. Low-speed engines dual-fuel arrangement. Diesel is injected to quickly swap out turbines instead of typically rotate between 100 and 500 as pilot fuel to ignite the natural gas, spending time at the dock doing in-place RPM. which will not detonate under extreme maintenance or repair. The relative sim- Medium-speed engines rotate at speeds compression alone. Alternatively, natu- plicity of the engine is another second- of 700-1200 RPM. High-speed engines ral gas can be used as a sole fuel source ary advantage - there are fewer moving generally rotate at 1800-4000 RPM. The in a spark-ignited reciprocating internal parts, none that reciprocate. particular advantages and disadvantages combustion engine. As mentioned previ- Some of the disadvantages of gas tur- of each of these types of engines must be ously, diesel engines can also be made bines are the strict inlet and exhaust understood to properly match the engine to run on biodiesel with little or no modi- pressure requirements, the need for to the application. Low-speed engines fication necessary. large, heavy gearboxes - particularly if are large, heavy, and expensive, but pay Diesel engines are well suited for a wide used in a direct drive configuration, high dividends on lower fuel, operating, and variety of marine applications - in fact die- exhaust gas temperatures, higher initial maintenance costs when compared to a sel engines can be found in nearly every cost, and lower relative fuel economy high-speed engine of the same output. type of commercial marine vessel, from - particularly at idle. A gas turbine can Installation of a low-speed engine can passenger vessels to tug boats. Diesel burn nearly any grade of refined petro- also eliminate the need for a gearbox, engines have recently gained traction in leum such as marine diesel fuel and saving further cost and weight. LNG tanker propulsion, marking a shift kerosene, as well as natural gas. Though Perhaps one of the primary advantages away from burning boil-off gas. Though these fuels are expensive, the gas tur- of a recipro-cating internal combustion the installation of a reliquefaction plant bine’s fuel flexibility makes it attractive engine is the widespread acceptance adds complexity and cost to the ves- for volatile fuel markets. and use of this type of engine in the ma- sel, rising market prices of LNG cargo A gas turbine works well in high-speed rine industry. The high production num- and the greater achievable efficiency of applications, where weight is always a bers drive down cost and promote reli- dual-fuel and diesel-electric propulsion concern. Additionally, there is a growing ability, while allowing for easy access to systems has made this option attractive market for gas turbines in vessels that parts and trained mechanics. and financially viable. cannot afford excessive downtime, such Reciprocating internal com-bustion en- as shuttle tankers gines are not without fault. At the larger Internal Combustion Rotational end of the spectrum, they do not have Rotational internal com-bustion engines External Combustion Rotational good power-to-weight ratios, though (such as a gas turbine) are also utilized Though internal com-bustion engines this is generally offset by greater vessel for marine propulsion. This type of engine (parti-cularly diesel internal combus- displacement. is charac-terized by continuous combus- tion engines) are the dominant source Reciprocating internal com-bustion en- tion of air and fuel and conversion of this of power in the marine industry, a ro- gines are capable of burning a variety of energy directly into rotational motion. tational external combustion propulsion different fuels. Use of heavy fuel oil is The principle advantage of installing a plant (such as a nuclear-powered steam traditionally limited to use in low-speed gas turbine engine is the large power- plant) still plays an important role in cer- engines, but marine diesel fuel and natu- to-weight ratio this type of prime mover tain services. An external combustion ral gas are suited for a wider range of can achieve. The compactness of the engine heats the working fluid (typically
MaK Diesel The MaK 9 M 43C propulsion engine shown here is a four- stroke medium-speed diesel engine, well suited to a variety of common applications on intermediate-sized vessels. (Image Copyright Caterpillar Marine Power Systems 2008) steam) outside the mechanism (turbine), upon which the working fluid expands, producing rotational motion. Like their gas turbine counterparts, a steam turbine’s advan- tages include the simplicity of fewer moving parts that rotate, a good power-to-weight ratio, and compact size. The steam boilers, however, which are neither small nor light, are a nec- essary compromise in this type of system. A steam system also requires constant upkeep and mainte- nance. Further disadvantages include a slower response time compared to internal combustion engines, both from a cold start and while underway. In most modern steam applications, steam is generated one of three ways - burning marine diesel, burning natural gas, or using nuclear fuel. Large vessels are the most practical platform for a traditional steam power plant, regardless of the heat source. The nature of their size requires a substantial installation of power, but also allows for the installation of a large, heavy propulsion system without an excessive penalty to the vessel’s perform- ance.
Conclusion The real change in marine propulsion in recent times hasn’t been the types of engines installed, but the fuels that power them. Regulatory and environ-mental changes are shifting the industry away from the exclusive use of HFO, towards cleaner burning distillates. LNG is undergoing a radical shift as it becomes less popular as a fuel for LNG tankers but more common in environmentally friendly ferry, tug, and PSV operations. Teamwork alone is not Even nuclear power for commercial ships is an old idea being resurrected. In this day and age, prime mover selection must enough – you also have to be a balance of modern environmental consciousness and old world economic savvy. be in the right boat
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SOURCE: MARITIME REPORTER & ENGINEERING NEWS NOVEMBER 2008 WWW.MARINELINK.COM