The members of the shipping industry bear in their mind that the liquefied natural gas is a safe product either as a shipped cargo or as a fuel for the engines of the vessels. An excellent safety record concerning LNG operations stands for the highest award for them. Both the operation of LNG tankers for more of 40 years and the use of the natural gas as a marine fuel since 2001 is enclosed in the hinderland of the previous statement (Basdani and Lignou 2011; DNV 2010).
Some 20 vessels are composing the world fleet of LNG - powered vessels at the present (GL - Nonstop 2012). Most of them can be categorized as small ferries sailing along the coastline of Norway where there is the infrastructure for supplying the ships with gas fuels. Worth citing that the largest vessel which runs on LNG is a chemical tanker named "Bit Viking" (see Photo III - Appendix A).
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The safety issues of the existing fleet are secured by the appliance of the existing regulations. More specifically, in the region of Baltic Sea a set of rules and regulations in accordance with the EU regulations will be applicable both for LNG fuel operations and equipment. Additionally, the IMO IGC Code and SIGGTO Guidelines provide instructions and should be applied in the near future so that to come into force. Regardless of the fact that national and port regulations are capable of being applied, there are no common standards or operational proceedings in all the LNG bunkering operations and bunker stations, too. As long as gas spills are sufficiently avoided, the fuel in the tanker will be in a safe condition and no boil - off or flashing can harm the receiving vessel by using the appropriate means (GL - Nonstop 2012). The requirements and the procedures for a safe ship - to - ship transfer and LNG bunkering operations may be secured more through a joint industry plan which is called the Gothenburg STS Project. Until the fulfillment of this goal, a stricter case - by - case risk analysis will cover the demanding levels of safety and will secure a successful prospective for the LNG - fuelled vessels (Basdani and Lignou 2011; DNV 2010).
A regulatory structure for natural fuelled ships
The mitigation of environmental problems imposes the implementation of measures for the maritime transportations which can be classified as one of the key sources of pollution in a worldwide scale (Pruzan - Jorgensen and Farrag 2010). Penetrating into this field, the progressing courses of actions tend to apply a green legislation controlling the emissions allowed for carriers (Basdani and Lignou 2011; DNV 2010).
Towards this direction, the International Maritime Organization (IMO) has already established plenty of areas such as the Baltic Sea, the North Sea and the North - American coastlines as Emission Control Areas (ECAs) in order to control pollution (see Photo II - Appendix A) through the "International Convention on the Prevention of Pollution from Ships", widely known as MARPOL 73/78. Furthermore, the MARPOL Convention has been amended by Annex VI which is titled "Regulations for the Prevention of Air Pollution from ships". The abovementioned annex puts into force limits on NOx and SOx emissions from vessels exhausts and forbids deliberate discharges of ozone depleting substances. The acceptable emission standards by IMO are incorporated in references as Tier I, II and III. The requirements of these regulations are divided into two basic groups: the global requirements and the more severe ones which are applicable in Emission Control Areas where the three types of emissions such as SOx, suspended particulate matter (or PM) and NOx are explicitly designated. In brief, the new international legislation related with the maritime engine emissions for the period 2010 - 2020 (see Appendix B) mainly includes (SMM News 2011; DNV 2010; OECD 2011):
IMO requires diminution of fuel sulphur content so that the SOx emissions in ECAs to be adequately reduced. The fuel sulphur content must be below 1% from 1st July 2010 and it is planned further reduction below 0.1% from 1st January 2015.
Accordingly to the European Union (EU) directive 2005/33/EC which is already valid from 1st January 2010, there are limitations about the fuel sulphur content to very low percentages of 0.1% while the vessels are berthed in EU harbors and inland waterways. Based on this requirement, the shipowners are forced to comply with the current regulation by changing the used fuel quality or type or even cleaning the exhaust gas to the acceptable levels.
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IMO attempts not only to specify but also to shorten present and future NOx emission limits globally for marine fuel engines through Tier I and II. In particular, the limit for NOx emissions referred in Tier II already postulates from 1st January 2011 the considerable reduction of 20% in NOx emissions for newly built vessels compared to the established standards of Tier I. Additionally, Tier III presupposes another 75% NOx reduction for vessels built after 1st January 2016 especially for ECAs.
IMO Resolution MSC.285 (26) - "Interim Guidelines on Safety for Natural Gas-fuelled Engine Installations in Ships" was adopted on 1 June 2009 but it is non-mandatory regulation (LaRoche 2012)
"Code of Safety for Ships Using Gas or Other Low - flash Point Fuels with Properties Similar to Liquefied Natural Gas" (IGF Code) is under development at IMO and is expected to come into force in the period of 2015-2017
IACS Unified Requirements M59: Control and Safety Systems for Dual Fuel Diesel Engines
The liquefied natural gas industry must also comply with the current and the emerging regulations which will cover the entire supply chain from the liquefaction facilities, LNG carriers, the gasification vessels and the terminal stations (Basdani and Lignou 2011; Honeywell 2012). Pruzan - Jorgensen and Farrag note (2010) that over the coming years, it is expected a continued patchwork of local regulations and various voluntary schemes, additional US ECA zones and initiatives from Asian states with local health concerns to step up on emissions.
Future options for using LNG fuels
Viewing the three alternative concepts for covering the ECA requirements, LNG offers the best economic and environmental solution to ship-owners operating in the Baltic Sea. During the past few years the technical drawbacks to application of this solution have been addressed and overtaken; LNG as fuel has already been operated and tested on 20 ships currently sailing in Norwegian waters with no significant problems. Moreover, LNG operations have been proven safe based on the record from LNG carriers through the past 40 years without any major safety incident recorded.
Best economic performance:
DNV has established economic models for the abovementioned three solutions for covering the ECA conditions. These models have been created for comparing the
three different ECA abatement measures, thus only differential costs between low sulphur fuel, scrubbers and LNG fuel are included.it must be mentioned however that revenue is not included in these models. Below is presented an example of cost calculation for a typical Baltic Sea cargo ship of approximately 2,700 gross tonnes, 3,300 Kw main engine and 5,250 yearly sailing hours. Usually, new LNG powers vessel ships require a 10-20% extra investment cost compared to the traditional powered ships. This additional cost occurs chiefly due to the sophisticated LNG storage tanks, the complex fuel piping system and in some cases due to modifications that demand the construction of a slightly larger ship. Assessing the references from currently operating ships as well as under construction ships, the additional capital investment has been estimated to USD 3.6 million. Although there is little experience and limited available information on scrubber installation costs, it is believed that the installation and operation cost is higher that the estimated cost mentioned above making the investment less profitable.
Dual fuel engines
Due to the fact that LNG has a high auto ignition temperature, an additional ignition source is required to ignite in combustion engines. Nowadays engines can be divided in two major categories; duel fuel engines and single fuel engines.
Additionally dual fuel engines can be subdivided in two main categories; two stroke diesel engines and four stroke Otto engine which had been produced firstly by Wärtsila corporation.(@)
The two stroke diesel engines can run on fuel oil only or on a mixture of gas supplemented with fuel oil. These engines which are mainly used on low speed large commercial vessels cannot meet the upcoming Tier III NOx requirements, but Wärtsila has currently developed a two stroke low speed engine that can meet even these stringiest limits.
However, the operation of the four stroke Otto engines can be more flexible. These engines can operate on the same fuel oils as a conventional marine diesel engine or on natural gas. When they operate on lean natural gas, their working principle is based on the Otto cycle; when they operate on fuel oils their working principle is based on the diesel cycle (DNV, 2011). These engines however, can overtake even the forthcoming regulations concerning the emissions without additional modifications.
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Other major players on the marine engine sector such as Rolls-Royce and MAN delivers single fuel gas engines of the Otto/Miller type with spark ignition achieving high efficiency and low emission at the same time. This solution has the disadvantage of losing the flexibility of operating fuel oil, a very important factor due to the fact that very few ports currently offer LNG refueling facilities.
Dual fuel engines can be fueled with either marine grade diesel, heavy fuel oil, or liquefied natural gas (LNG). Having multiple fuel options gives vessel the advantage to transit without relying on one type of fuel. Studies have shown that LNG is the most effective among the fuels even though the limited access to LNG fueling stations holds back the production and development of such engines. Vessels currently providing services in the LNG industry have been powered by dual-fuel engines which have been proved to be very reliable. Some of the benefits that dual-fuel engines have are: fuel and operational flexibility, high efficiency, low emissions, longer maintenance periods, and operational cost advantages. Liquefied natural gas engines have been proved to be an environmentally friendly alternative for the marine transportation industry to provide power to vessels. In 2010 STX Finland and Viking Line signed an agreement of building the largest environmentally friendly cruise ferry so far. Construction of NB 1376 will be accomplished in 2013. According to Viking Line, vessel NB 1376 will principally use liquefied natural gas which will be changed over to fuel oil only in extreme cases. The cruise ferry is estimated to have an emission reduction of almost 90% compared to conversional fuelled engines. Constructors of project Vessel NB 1376 calculate approximately that SOx emissions will be at least 80% below the International Maritime Organization's (IMO) standards and NOx emissions will be almost zero. Rising greenhouse gases connected with global warming is one of the most important world problems that also significant affect the maritime industry. Marine propulsion technology improvements that create a more environmentally friendly profile are necessary, thus the use of LNG fuel is one step toward achieving a healthier globe. Company profits from tax reduction benefits and operational cost advantages have led to the gradual growth of LNG fuel use in engines.
DUAL FUEL MEDIUM SPEED MARINE DIESEL ENGINES
A very common dual fuel medium speed marine diesel that is widely preferred by ship-owners is Wärtsilä DF. The DF is a four stroke, turbocharged and intercooled dual fuel engine which uses direct injection of liquid fuel in combination with indirect injection of gas fuel. This provides the flexibility to the operators to run it either in gas mode or in diesel mode when gas is not available Photo No1: Dual-Fuel Wärtsilä engines "Wartsila 50DF" -Wärtsilä Ship Power Technology June, 2010.
Method of Gas Injection: The gas is injected and mixed with air before the inlet valve, but without the need of a spark plug; instead, a diesel pilot flame is used for the desired ignition. When operating on gas only mode the combustion chamber delivers a low emission of NOx and other emission components. These emissions are a little higher compared to the spark ignited Otto engines because of the diesel fuel that has to be blended with the gas. At lower loads the ratio of energy delivered by the diesel flame increases due to the increased demand for fuel in order to sustain the desired burn. Consequently, the relative emission of NOx and other emissions components deriving from the diesel flame increases at lower working loads. Incomplete combustion and residues of unburned methane can arise both in dual fuel engines as in lean burn engines. When operating at lower loads, a dual fuel engine will switch over to diesel only running mode. However, depending on when this shift occurs, many of the problems with the high methane emission at low loads will be dissolved.
DUAL FUEL SLOW SPEED MARINE DIESEL ENGINES
Another major player on the marine engine construction is MAN B&W who has designed a slow-speed ME-GI engine exclusively for the LNG carrier market. This newest development can additionally provide land based services when refueling facilities are available. The knowhow has been gained from the existing on service MC-GI engines combined also with the electronically controlled ME engines.
The supply of the required gas has been achieved with the use of a reciprocating compressor reaching high pressure injection in the intake of The Slow Speed ME-GI while ignition is ensured by diesel fuel injection (2010).
The ME engine function is based on hydraulic controlled mechanical system for activation of the fuel injection and exhaust valves (technology which has already been proved reliable). The actuators are electronically controlled by an integrated engine control system. Finally MAN has specifically developed both the hardware and the software, supplying to the customers an integrated and simplified Engine Control System.
Photo No 2. Shows a a dual fuel slow speed engine. Retrieved from "MAN B&W ME-GI Engine Selection Guide," by MAN Diesel and Turbo, 2010.
DUAL FUEL MARINE GAS TURBINES
Among the most important characteristics of modern marine gas turbines are that they are compact and efficient prime movers that have good application in the marine environment as long as they operate with good quality fuel. The component is often supplied with low quality fuels which contain metal vanadium and that is an issue that concerns particularly the gas turbine designers and operators. The presence of metal vanadium in the gas turbine fuel results in high temperature corrosion issues. Vaporized LNG is metal vanadium free and that makes it an optimum economical and efficient fuel solution for gas turbines.
An actual benefit from the use of gas turbines as the prime-mover for commercial vessels is that the power plant can be constructed in a way that will permit extra cargo space for the vessel. This is important because in some vessels and trades, the increased cargo capability often moderates the lower efficiency of the gas turbine comparing to conventional engines. Figure 16 shows the relative efficiency of various marine prime-movers.
Converting Diesel Engines to Dual Fuel (Pros and Cons)
IMPORTANCE OF DUAL FUEL ENGINES AVAILABILITY
Conventional diesel engines can be modified to dual fuel natural gas engines quite easily for the reason that major engine characteristics like the engine compression ratio, cylinder head, or basic operation remain the same in both cases. Another benefit is the flexibility the dual fuels system offers as it can be easily removed, to bring the engine back to its initial condition. These modifications are easy to apply and easy to operate. This option gives to customers the advantage of having a new technology dual fuel engine at a very low cost.
Advantages of Spark Ignited Engines
They are specifically designed with optimized cam timing and a piston compression ratio providing the best efficiency and power in conjunction with the lower emissions possible.
Diesel fuel storage is not mandatory.
Catalytic converters can be easily installed in the exhaust gas piping in order to achieve lower emission levels.
There is only one fuel system for maintenance
Disadvantages of Spark Ignited Engines
They deliver a lower power density. In order to achieve equal power they will need a bigger engine which means a larger price tag.
They have a different cam timing than diesels which consequently creates higher exhaust gas temperatures. The combination of high temperatures with the dry gas fuel results in considerably higher valve seat wear rates.
They require additional maintenance in spark plugs which vary between 600, 1500, 2500 hours and maybe longer when conditions permit it and special plugs are used.
They have limits to fuel content, temperatures and relatively lower power capacity.
Combustion knock set the limits for the operation of a spark ignited engine even though timing adjustment can reduce this affect.
Advantages of Dual Fuel Engines
Possibility of modification of the initial diesel engine to an environmental friendly dual fuel engine.
Fuel flexibility, if gas supply is interrupted then back up diesel operation is immediately available in order not to stop power generation.
Full original power capacity.
Diesel cam timing results in higher power density and longer valve life due to lower exhaust gases that are generated when operating in diesel.
Higher compression ratio, which leads to higher engine output compared to spark gas engines.
Longer service internals on the ignition system and consequently lower maintenance cost due to pilot fuel provides lubrication to valves and rings, when combined with clean gas maintenance service
Engine warranty will be sustained in most cases due to the modification of a diesel engine to dual fuel engine does not affect the reliability of the engine
Spark ignition system, ensures reduced misfire, higher efficiency, higher power density
Exhaust emissions, specifically Nitrogen oxides, CO2 and particulates are significantly reduced.
Disadvantages of Dual Fuel engines
Diesel in dual fuel engines is mandatory, without the presence of it engine cannot be work.
Greater emissions of CO compared to traditional diesel engines but similar emissions of CO to spark gas that are not fitted with catalytic converter.
More complex double fuel supply system that has to comply with strictest regulations regarding safety making it difficult to operate.
Requires the support of additional supplier, not just the OEM (original engine manufacturer).
Engine warranty maybe no longer valid if engine has been modified but no installed in it has been manufactured.
Excess wear in some components of the engine due to smaller quantities of oil contaminates carried on the fuel compared to a diesel oil engine that uses it as self-lubrication for some engine components.