Technologies | Primary energy convertors combustion and non-combustion

Relevant types of combustion prime movers are subject of this work package. The state of the art of current technical development is described in brief, including DF and Gas applications. The work package will also cover fuels cells and gas turbines. A special focus is also given to alternative fuels related to these prime movers.

For the shipbuilders this is the base to determine the possible developments with regards to technical development, reduction of lifecycle cost and further reduction of fuel consumption and emissions.

Regarding the types of engines, certain types are better suited for certain applications i.e. certain transport tasks and thus certain ship types.

LNG as a fuel with a high potential for clean emission ships and efficient conversion of chemical into kinetic energy shall be described as well as the application of different types of fuel cells.

This document contains also an evaluation of the current fuels as well as the fuels which have a large potential for future applications such as Bio Fuels.

Total investment costs must also be subject to the evaluation in JOULES. This report lays the base for such an evaluation.

Specific technology applied to the 2-Stroke engine

Trends in the 2-Stroke Diesel engine development have aimed at separating the different processes in the combustion cycle and trying to develop them each for a broader range of operation, load and number of revolutions. This applies to the fresh air supply using e.g. turbocharger characteristics, which are independent from the current load of the engine.

The lasts development is to integrate Common Rail technology for large, slow turning 2-Stroke engines. The latest generation of 2-Stroke Diesel Engines make use of electronically-controlled common-rail technology. This is usually combined with an extra-long stroke, which makes the engines extremely efficient in terms of both fuel consumption and emissions, with fuel consumption reductions of up to ten per cent compared to engines without that technology. Their lower CO₂ emission levels make it easier for shipyards to achieve a better Energy Efficiency Design Index (EEDI). Emissions of SOₓ are thereby also much lower (mainly affected by the increase in efficiency).

The state of the art report refers to several subsystems and improvements on the 2 stroke engines which have become available such as : improvement of the layout field, solenoid actuated common rail, voyage profile adaption, electronic monitoring and control, tribologic optimisation, injection valve technology and compliance with Tier II and Tier III IMO regulations.

2- Stroke engines are traditionally used in applications for large vessels. In order to utilize the benefits of the 2-Stroke engine also for smaller vessels, engine developers set the small size vessels up for the use of the new generation engines with bores less than 400mm. These engines can now be used for a wide variety of ship types, such as small bulk carriers and product tankers, general cargo vessels, reefers, feeder containerships, and small LPG carriers.

The so called “Mid Size” 2-Stroke engines with a bore less than 750mm are designed specifically for vessels such as Panamax bulk carriers, Capesize bulk carriers, Aframax tankers, Suezmax tankers and also feeder and Panamax container ships (1600-4500 TEU).

Above that bore range new “large” engines have been extended to the highest power outputs especially in the container vessel sector.

Natural Gas as Fuel for 2-Stroke Engines

The operating cost of a vessel is an important part of the operational cost, and is still expected to continue rising. Currently bunker fuel represents up to 70% of those costs. With US and European Emission Control Areas (ECAs) limits of 0.1% sulphur coming into force in 2015, vessels choosing to operate on distillate fuels within those areas could see costs dramatically increase; with a current 45% premium for distillates compared to conventional HFO with x% average sulphur content world with heavy fuel oil (HFO). Moreover, from 2020 or 2025 according to IMO plans, all vessels will be required to burn 0.5% sulphur fuel. With this in mind, owners and operators are taking decisions now on how they will meet these financial and regulatory challenges, weighing up the benefits of the only two viable alternatives to costly distillates: exhaust gas cleaning systems or LNG (liquefied natural gas) as a fuel.

Natural gas is a straightforward fuel for complying with upcoming ECA regulation through its inherent SOₓ emission reduction of 99%, and a NOₓ emission reduction potential - compared to liquid fuels - of more than 85%, ensuring that Tier III NOₓ regulation can be met from 2016. LNG could also provide ongoing compliance for a range of potential future legislation, as - compared to HFO - it emits 99% less harmful particulate emissions and provides a 20% reduction in greenhouse gases including the effect of methane slip. To date the use of LNG as a fuel has been largely focused on medium-speed 4-Stroke engines. Engine makers however aim to extend the benefits and experience from the 4-Stroke business across the industry by applying gas engine (Otto engines, spark ignited) and dual-fuel technology to its low-speed two stroke engines.

The main feature of the DF technology is lean-burn Otto-cycle combustion resulting in superior net efficiency and low engine emissions which can all be reached with a low pressure gas supply system. Ignition of the gas fuel is triggered by the combustion of a pilot quantity of marine diesel oil (MDO) injected into pre-combustion chambers by designated fuel injection valves. At Wärtsilä the first pilot installation is planned to be operational in 2014. At MAN the development has targeted high pressure Dual Fuel engines which have been shown to meet Tier III requirements in combination with exhaust gas recirculation.

'Going for gas' is a viable option for the shipping industry in the future as an increasing number of installations will have a need for flexibility, fuel efficiency and compliance with more strict emission legislations.

General considerations for 4-Stroke Engine

In a 4-Stroke engine the piston completes four separate strokes - intake, compression, power, and exhaust - during two separate revolutions of the engine's crankshaft. As the intake and exhaust cycles are happening during different strokes of the engine, a much cleaner gas exchange and more time for the complete combustion of the fuel is possible. This results in the fact, that the segment of slow turning large 2-Stroke engines with a low power to weight ratio and number of revolutions below 200 are usually used in large vessels, whereas medium speed engines, usually ranging between 500 and 1000 number of revolutions, are a major force in cargo and passenger ship propulsion. Engines with higher number of revolutions are usually referred to as High Speed Engines. Most of the power generation in a Diesel-Electric application in ships is covered by medium speed constant number of revolutions engines. Four-stroke engines of this type offer a power to weight ratio that makes them ideal for cruise vessels, ferries and large multi-purpose freighters. High Speed Engines are used as auxiliaries or as prime movers in light and fast ships, or vessels of a smaller size.

Several new techniques have been introduced to improve the performance and reduce emissions: common rail technology, flexible turbo charging, engine flexibility, flexibility in adjusting the control systems during service operations.

Natural Gas in 4-Stroke Engines

The application of natural gas in 4-Stroke engines has started 10 to 15 years ago with application derived from onshore power plants. Recent trends show, that this has proven to be a true success story. In marine applications LNG as a fuel is largely focused on medium-speed engines. In excess of 2000 gas engines were sold by one single engine supplier, engines which collectively have exceeded seven million operational running hours in both land-based and marine applications. Drawbacks for the technology so far were the relatively high price and inconvenient logistics of the fuel. This situation is expected to change rapidly, when the strict emission control measures of IMO come into force, which require enormous investments for exhaust gas after treatment when HFO is used Or the use of expensive Low Sulphur Diesel Oil / Marine Gas Oil.

Same as the described 2-Stroke technology, the main feature of the DF technology is lean-burn Otto-cycle combustion resulting in superior net efficiency and low engine emissions. Ignition of the gas fuel is triggered by the combustion of a pilot quantity of marine diesel oil (MDO) injected into pre-combustion chambers by designated fuel injection valves.

Dual-fuel engines can be run on natural gas, marine diesel oil (MDO) or heavy fuel oil (HFO). The engine can be switched from fuel oil to gas operation and vice versa smoothly during engine operation. During switchover, the fuel oil is gradually substituted by gas. This operation flexibility is a real advantage with the dual-fuel system.

The latest generation Dual Fuel engines provide complete engine safety systems and local monitoring. The completely integrated automation significantly improves operational efforts and safety issues.

4-Stroke Dual fuel engines are well suited for constant speed generating sets as well as variable speed mechanical drives for main engine applications. Modular design of the engines gives sufficient space saving even in smaller applications, such as small cargo vessels, ferries or tug boat installations. The multi-fuel operation offers new opportunities especially in complete multi-fuel engine room concepts with combined auxiliaries and prime movers. Typical installation examples are RoPax or LNG carriers.

Fuel Cell and reformers for maritime applications

The description of fuel cells and reformers for this document will focus on the state of the art in fuel cell and fuel processing technology for maritime applications. The fuel of choice for fuel cell applications in the maritime sector is liquefied natural gas. The described fuel consists mainly of methane with possibly small amounts of higher hydrocarbons like ethane, propane and butane. In the low temperature (60-200°C) fuel cell technologies this fuel needs to be processed first in a fuel processor (reformer) to be converted into a hydrogen rich gas mixture. The high temperature (700-1000°C) fuel cell application can be fed with the primary fuel in many cases.

The choice of technology in both the fuel processing and fuel cell technology should be made based on the system requirements of the application. Parameters like peak electrical load, number of start/stops over its lifetime, system size and weight and others determine the technology selection.

Fuel cells and hydrogen get more attention of the maritime sector as shown by initiatives of the EU/ FCH-JU who organized a workshop: Fuel Cells and Hydrogen for Maritime and Harbour Applications in June 2013 in Venice (I).

There are several types of fuel cells in the market available. There are more fuel cell technologies in the research stage. These will not be covered in this document. The three most advanced, commercially available technologies are:

  • Proton Exchange Membrane fuel cells (PEM)
  • Solid oxide fuel cells (SOFC)
  • Molten carbonate fuel cells (MCFC)

Alternative fuels can be used for fuel processing to produce hydrogen for fuel cells. Most of the existing technologies run on natural gas of which the main constituent is methane, like in LNG. Hydrogen production based on LPG/propane, (bio) diesel, kerosene and other hydrocarbon fuels are subject of study in a variety of applications including reforming for the maritime industry. Each fuel needs to be evaluated in terms of impurity (mostly sulphur) content and reforming conditions like steam to carbon ratio and oxygen to carbon ratio. For liquid fuels the temperature of evaporation is important as the reforming takes place in the vapour phase mixed with hot steam.

Gas Turbines

Gas turbines are found in many maritime applications. Key requirement for the application of gas turbines are compact size, high peak loads and fast availability. The efficiency of gas turbines is generally lower than the efficiency of other primary converters, and for a commonly available aero-derivative gas turbine engine, the thermal efficiency quoted is >40%. The values can be enhanced using technologies such as inter cooling and recuperation which offer additional fuel savings and have quoted efficiency values of >44%, and using a gas turbine in combined cycle with a steam turbine driven from the waste heat have achieved combined cycle efficiencies of 45-54%.

High shaft speeds mean that Gas turbines can be used as a mechanical drive via a gearbox, or used to drive a 50 or 60Hz alternator directly. A free power turbine can be also used which is not connected mechanically to the gas generator shaft but still extracts the energy available from the engine exit.

Alternative Fuels

In the preceding sections, standard fuels have been looked at. This Chapter will also evaluate, if the future bares opportunities for alternative fuels such as H2 in Combustion engines, or different kinds of Bio Fuels. Alternative fuels considered are: methanol, H2, Bio Fuels, Bio Diesel and DME (dimethyl ether). Some of these fuels are liquid; some are gas mixtures and require proper handling. Not clear at all is the availability of these fuels. Fuel flexibility will therefore remain an important aspect for future developments.