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.
Other ways to copy with ship emissions
1) Diligent fuel switching
High quality fuel ultra-low in sulphur percentage can be an alternative solution to reduce the air emissions but long term viewing this can be proved too expensive and not financially viable solution. It can be mentioned that for vessel trading only in SECA areas no capital cost is requiring in the vessels fuel supply system as long as it switches all its tanks in low sulphur fuel. On the other hand Vessels that trade worldwide must make arrangements so they can store various grades of fuel onboard and switch to the low-sulfur fuel only when within the SECA boundaries. This can demonstrated to be hard for vessel due to engine operational problems must be overtaken but also special amendments should be carried out on vessels fuel oil storage and supply system in order to meet the new requirements.
2) Use of Scrubbers
Exhaust Gas Scrubber is the after treatment method that applied with the intention of
remove sulphur residues and particulate matter from exhaust. Great advantage of this method is that no fuel switching is required but it has to operate in combination with Selective Catalytic Reduction (SCR) technology to minimize NOx emissions. Additionally basic working principle of SCR is the direct injection of a neutralizer compound of urea or ammonia into the hot exhaust just after the catalysts. This has as a result the convention of harmful NOx into harmless nitrogen and oxygen. Nowadays EGS and SCR technology is still consider expensive and not sufficient tested.
3) Cold ironing
Vessel at ports continue to need power in order to operate the auxiliary systems such as loading or unloading equipment heating or refrigerating devices for cargoes or even ballast pumps as well as to keep vessel stable and within allowable stresses.
This power is supplied from ship's auxiliary generators that ship need when cruising at sea .However, such engines are consider great pollutants at the ports where they berth, and these are often close to large centers of population. A possible solution to this problem can be a high voltage shore power provided to vessels instead of using own vessels systems .However at present, this has some major disadvantages such as: restricted available ports, different voltage and frequency requirements and not internationally standardized cable connectors, issues that doesn't permit enlargement in use of this method.
A) Primary methods Engine modifications
Injection timing retard
This method is well known among engine manufactures and operators and has been recognized as one of the most effective ways for controlling engine NOx emissions. By retarding fuel injection inside the cylinder we achieve reduced peak temperature on it without any additional modification. According to MAN B & W shows that 1 o CA retarding brings nitrogen oxides down by 4 to 5% (Zhou, Thorp, 1997) California Air Resource Board reports reduction in the range 5-30% (State of California Air Resources Board, 1997). Moreover another major engine manufacturer like Wartsila Diesel claims that retarded injection can bring nitrogen oxides down to meet the 600 ppm limitation with no more than 5 g/kWh SFOC increase (typical 2-3%). It has been found that the rise of fuel consumption is unacceptable when more than 20% of nitrogen oxides is reduced purely by retarded injection.8888888888. On the other hand except of increased consumption, it has been estimated that:1 °CA retarding increases the SFOC (specific fuel oil consumption) approximately by 1%.Futrhermore by using this way has also and main drawbacks like excessive thermal loads on turbochargers, exhaust valves but also increased particulate residues.
2) Lowering maximum gas temperature in the cylinder
A significant reduction of NOx emissions up to 80% can be reached by reducing the gas temperature in the cylinder lower than the ordinary but this can find application only in high temperature and performance two stroke diesel engines.
3)Exhaust gas recirculation
Working principle of this method is a partially introduction of the exhaust gases back into the combustion chamber aiming to reduce the max combustion temperature. The heat capacity that recirculated gases contain due to the presence of CO2 on them is about 25% higher than the atmospheric air that other engines use for combustion, this results in lower combustion temperature rise. Moreover due to lower O2 concentration of the recirculated exhaust gases a reduction of combustion temperature is achieved which results in additional temperature decrease. According to the already tested marine EGRs (not very common solution yet) a 50% reduction of NOx has been stated. Major concern about its use is the high concentrations of sulphur and abrasive particles that with combination with lower combustion temperatures lead in excessive engine wear and corrosion. This problem can be condensed in combination with the use of fuel oil emulsion (see below) with even greater percentage of NOx emission but on a slightly higher consumption.
4)Non catalytic reduction
This technique has the same working principle with the selective catalytic reduction that mentioned before but with the difference that injection of ammonia (NH3) is now conducted direct into the combustion chamber of the engine. At approximately 1000 C ammonia reacts with the nitrogen oxides. These conditions can be reached only when: (i) ammonia is injected after the completion of the main combustion process as NH3 is also flammable ;( ii) it requires homogenously mixture with the air-fuel mixture so as to burn without residues. To obtain a 50% reduction
in nitrogen oxides , 4 times the stoichiometric NH3 is required. This means that only
10-12% of ammonia is actively reacted with nitrogen oxides and the rest probably
burned off.88888888. the increase consumption of ammonia(prized almost same as heavy fuel oil) and the special conditions that are required didn't allowed further development of this application.
5)Water injection
There are two ways that water injection can be carried out, the direct water injection from a water injector inside combustion chamber or at the intake manifold before intake valves. Both of these ways aim to create humidification of the intake air and finely atomized water droplets that will absorb heat and will decrease the peak combustion temperature as the previous methods mentioned before. Water injection can be consider a good way to lower the Nitrogen Oxide emissions and especially by direct injection (which is more efficient) these can be reduced up to 60%.
6)Water/Diesel mixture (Emulsified fuel)
Emulsified fuel has been an alternative way to mitigate the emission and increase the thermal efficiency of the diesel engines with almost same working principle as the direct water injection. An average decrease of 24% to 50% can be reached but this varies depending on the type of engine, the working load and the water content in oil. Water mixture proved to have better effects on medium speed engines regarding NOx emissions in addition to the high speed engines that had more effects in CO emission and visible smoke reduction. According experiments it has been found that the greater the percentage of water in fuel the lower the emissions to the environment but available fuel injection systems on engines set the limit on this. Furthermore it has been except the lower emissions emulsified fuel benefits the clearness of the combustion chamber of the engines reducing the wear rate and increasing the periods between overhauling. As disadvantages of this method we can mention that we can achieve significant results in emission reduction only in a restricted range of engines working load but also a slightly increase in vessel fuel oil consumption.
7)New ship design & modification
Except the engine based modifications shipping industry has developed a numerous other techniques that can be utilized separate or in combination aiming to reduce the share of maritime industry as global air pollutant .Some of them can be the speed reduction due to fleet increase or to port efficiency, the regular hull cleaning and special coating paintings, measures that can reduce up to 30% the vessel's emission and consumption due to lower friction. Other sources of energy like kite towing, fixed sails wind generators or solar panel installation have shown the future in the battle for a cleaner and more environmental friendly marine sector. Latest naval construction technology has also been a great assistant on this way providing solution such as: optimized hull design, air cavity lubrication. Lastly operators themselves can attain reduction of air emission by more efficient operation of energy devices like: optimized trim, weather rooting, ballast optimization and reduction of time at port.
B) Secondary methods
1) Selective catalytic reduction
Selective catalytic reduction function is based on the convention of nitrogen oxide into diatomic nitrogen (N2) and water (H2O). These are considering totally harmless and safe when released to atmosphere. This actually achieved by a combination of NOx with a reductant such as ammonia (NH3), which with the presence of the catalyst a reaction that separates the NOx into the N2 and H20 takes place. By using SCR NOx e missions can be reduced by 70 to 95% depending the type of engine it has been fitted.
2) Scrubber - seawater washing system
Exhaust gas scrubber is a device that is installed on the funnel space of the vessels inside the exhaust pipe and threw special nozzles that spray alkaline water direct to the hot exhaust gases they can remove up to 99% of SOx 40% of NOx and 85% of particulates when operated on a 3.5% S HFO 380 fuel which is very common nowadays. Water under pressure is capable of filtering liquid hydrocarbons together solid particles which then stored in a special tank through a water separator. Even if is consider among the most efficient ways of minimizing air emissions the high installation and operation cost (extra feed water pumps and piping) together with the challenging modification on the existing vessels doesn't allow its massive growth. Likewise the solid waste that remained form the scrubbed water treatment have to be collected and send to special shore facilities where applicable at a significant high cost.
The perspectives concerning the green fuel of the future
Having recognized that LNG fuel is an economic choice for the shipping companies and especially for ships sailing in ECAs, it is very likely that the natural gas will become one of the most common fuel in particular environmental systems (e.g. in the region of the Baltic Sea) and more acceptable in the international sea trade routes (Basdani and Lignou 2011; SMM News 2011). The use of LNG fuels in carriers is quite limited in spite of their superior qualities compared to the conventional ones. Therefore, what surely needs to be done is the building of efficient infrastructure for LNG fuels in the forthcoming years. The members of the local governments have to to move first and adopt additional ecological policies which will promote the role of natural gas and through decrees to require at least this green fuel for publicity operated vessels (GL - Nonstop 2012; DNV 2010).
Because of the poor existing fuel infrastructure for LNG - fuelled ships, several options have been developed so that one of them to be selected as the best one in the close future. Namely these are:
i) Conveying of natural gas through pipelines and small scale of liquefaction in order to be distributed to the local bunker stations. This option is already available to the majority of the LNG - fuelled vessels operating in the Norwegian Seas. However, there are serious concerns about whether it is a viable solution for a long term economic point of view (because a liquefaction of gas is closely dependent on economies of scale and consequently, small quantities of LNG for liquefaction means high cost per unit)
ii) Chartering large size LNG carriers (approximately 145,000m³) from the international LNG market which could anchor in a safe berth for further distribution of the cargo to the associated bunker stations seems to be a competitive solution (long term contracts for large quantities of LNG are a good start for low pricing policies)
iii) Chartering also large size LNG carriers from the international market in order to deliver fuel to the local LNG import terminal. In this case the local terminal will have the role of a hub for further distribution of fuels to the bunker stations. This can be done by small LNG carriers (e.g. 20,000 m³) and also represents a competitive solution.
iv) Bunkering with LNG fuels directly from a regional LNG import terminal is not a realistic solution because such terminals could not supply with fuels a considerable number of vessels.
There are projects which stress the imperative need of establishing a sufficient number of LNG import terminals in the area of the Baltic Sea. They are planned to start up in the period of 2012 - 2014 and the proposed locations are Estonia, Lithuania, Poland, Sweden and Germany. Besides, the Netherlands is implementing four different projects along the region of Rhine in order to establish LNG refueling stations for the inland vessels. The planned facilities will be in full operation in a period of 18 months (GL - Nonstop 2012; DNV 2010).
The interdependent requisites of a widespread use of natural gas as a common fuel are both a well - organized supply infrastructure and a significant tonnage of LNG - fuelled vessels. On the one hand, the shipowners will decide to invest in building a LNG -fuelled carrier when the LNG fuel supply infrastructure is in position to fully service this investment and on the other hand, the LNG fuel suppliers are hesitant to commit capitals in infrastructure which will serve only a small fleet of LNG - fuelled vessels. The real challenge is a close cooperation and coordination between the representatives of the local governments and the investors or entrepreneurs who are activating in the private sector (Ashworth 2012; DNV 2010).
Würsing supports (2012 cited in GL - Nonstop 2012) that the natural gas as an alternative fuel may soon drive notable changes in the seaborne trade owing to LNG heralds a new period in ships propulsion. It is expected that more LNG - powered carriers will be entering service after 2015 as their operation depends on the availability of an adequate number of refueling stations in the major ports. Although, MGO and LNG will compete in the market for few years, the advantages are without doubt on the side of the green fuels (Ashworth 2012; SMM News 2011).
Conclusions
The proportion of vessels using LNG as a fuel is low, but increasing. It is by no means a perfect fuel from an environmental perspective. Some emissions still occur. But compared to conventional shipping fuels there are many advantages. LNG is a fuel that can be used in an economically sustainable way with existing technology. It is therefore likely that LNG will in future play an increasingly important role in the shipping industry's efforts to reduce their environmental impact.
The potential of LNG to be a 'fuel of the future' for the shipping industry is a subject we hear about almost daily. There is no doubt that LNG ticks the box in terms of emissions reductions compared to conventional fossil fuels. But the expansion of LNG as a fuel into the mainstream shipping industry throws up a combination of technological and regulatory challenges that the industry must address.
Expanding the application to new vessels types in diverse configurations has created a need for construction and arrangement requirements as well as standards to maintain existing levels of safety in the shipping segments using the new fuel
Regarding the fuel itself, owners also need to understand both its properties and handling. LNG is a mixture, not a homogenous product. It has different compositions, which result in variable properties. The energy in each cubic meter and the methane number can impact the volume of fuel required and the way the fuel is handled as well as engine performance.
Other items to consider include the power profile of the vessel and to what degree it operates below or at maximum power, an issue engine manufacturers already are addressing.
For ships operating mainly in the IMO's ECAs, LNG could be a very attractive option, and here we are talking about as soon as 2015 and 2020 or 2025, depending on how the IMO judges low-sulphur fuel availability. That in turn could lead to a ramping up of bunkering infrastructure, further strengthening the argument in favour of LNG.
Of course, this too will have its consequences. Training - and who will train the crew who will be handling this bunker fuel - is an important issue which needs to be addressed. The ISO bunkering standard mentioned above will help, and ABS is also developing training resources around this issue, but this is something the industry should take note of.
The success of the LNG industry in training crews to handle the product safely and in increasing ship size as demand has grown shows these challenges can be overcome.
How LNG as a fuel works in this new environment is a somewhat different question. With many more gas-powered vessels in operation, the level of risk potentially is increased, so these risks have to be addressed and mitigated.
This does not mean the technology issues cannot be overcome. Instead, using LNG as a fuel becomes an issue of design and suitability on a project-by-project basis rather than a one-size-fits-all solution.
As the LNG fuel is the supreme solution, the governments have to encourage the establishment of LNG bunkering infrastructure in the main ports and especially the fuelling of ferries, military and coast guard vessels. This green solution should be speeded up as it is largely based on the excellent environmental impact of LNG fuels in the marine systems and on its efficiency to cut down the costs. An immediately conversion of all the vessels to take advantage of the benefits deriving from the use of natural gas is infeasible for numerous reasons (additional costs, aged vessels, no availability of space for gas tanks etc.). However, there is hope that the deliveries of new built vessels in the near future or public - owned fleet will adopt this ecological fuel for the engines of their tonnage. It should be also mentioned that the future additional requirements (2015 - 2016) for ECAs can be considered as an ideal opportunity for the governments in the region of the Baltic Sea not only to promote the role of an environmentally friendly way of shipping in the local seawaters but also to develop green innovative solutions which could be globally practicable (Pruzan - Jorgensen and Farrag 2010; DNV 2010).
References
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EXHAUST EMISSIONS FROM SHIP ENGINES - SIGNIFICANCE, REGULATIONS, CONTROL TECHNOLOGIES Laurie Goldsworthyâˆ-
Infrastructural challenges for LNG fueled vessels in the SECA
LNG AS A MARINE FUEL - POSSIBILITIES AND PROBLEMS - Jerzy Herdzik
Gdynia Maritime University - Marine Power Plant Department- e-mail: [email protected]
THE LNG USE AS A MARITIME FUEL: ENVIRONMENTAL CHALLENGES AND PERSPECTIVES- BASDANI E.I., Mrs, C.Eng, - LIGNOU M.N., Mrs, Mech.Engineer,
Natural Gas: A Viable Marine Fuel in the United States (EM680) by
Edward J. Eastlack United States Merchant Marine Academy, Kings Point, NY
2011 Submitted to the Department of Marine Engineering in Partial Fulfillment of the Requirements for the Degree of Masters of Science in Marine Engineering at the United States Merchant Marine Academy August 2011
Converting Diesel Engines to Dual Fuel -The Pros and Cons of Common Gas Engine Types - Scott Jensen - Energy Conversions Inc.- 1/12/06
http://www.cormetech.com/selective-catalytic-reduction.htm
Anthony Fournier *
Donald Bren School of Environmental Science and Management University of California Santa Barbara February 2006 *Work supervised by Professor Charles D. Kolstad ([email protected]) and supported by the UC Institute for Global Conflict and Cooperation
Appendix A
The role of the attached photos in this section is auxiliary and it aims at the comprehension of the objectives of the current study.
Photo No1.Dual-Fuel Wärtsilä engine "Wartsila 50DF
Photo No 2. Shows a a dual fuel slow speed engine. Source by "MAN B&W ME-GI Engine Selection Guide," by MAN Diesel and Turbo, 2010.
Photo I: A LNG - fuelled vessel
Source: DNV (2010)
Photo II: Marpol Annex VI - ECAs mid 2012
Source: Ashworth (2012)
Photo III: The chemical tanker LNG - powered "Bit Viking"
Source: GL - Nonstop (2012)
Photo IV: The procedure of bunkering
Source: GL - Nonstop (2012)
Appendix B
Namely the new international legislation related with the maritime engine emissions for the period 2010 - 2020 is included in the Table 1, as follows:
Table 1: New international legislation on maritime engine emissions for the period 2010 - 2020
Enforcement
Reference
