Review of propulsion systems on LNG carriers

1364-0321/ © 2016 Elsevier Ltd. All rights reserved.

Renewable and Sustainable Energy Reviews 67 (2017) 1395–1411

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Review of propulsion systems on LNG carriers a,⁎

b

b

Ignacio Arias Fernández , Manuel Romero Gómez , Javier Romero Gómez , A. Álvaro Baaliña Insuab

crossmark

a

Energy Engineering Research Group, University Institute of Maritime Studies, ETSNM University of A Coruña, Paseo de Ronda 51, A Coruña 15011, Spain Energy Engineering Research Group, University Institute of Maritime Studies, Department of Energy and Marine Propulsion, ETSNM University of A Coruña, Paseo de Ronda 51, A Coruña 15011, Spain b

A R T I C L E I N F O

A BS T RAC T

Acronyms: 2STwo-Stroke 4SFour-Stroke AFRAir/Fuel Ratio BLRBoiler BOGBoil-Off Gas COGESCombined Gas turbine Electric & Steam system DFDual Fuel DFDEDual-Fuel (medium speed) Diesel Electric propulsion) DFDM(HP)Dual-Fuel (low speed) Diesel Mechanical propulsion (high pressure) DFDM (LP)Dual-Fuel (low speed) Diesel Mechanical propulsion (low pressure) DFGEDual-Fuel Gas turbine Electric propulsion DFSMDual-Fuel Steam turbine Mechanical propulsion DWIDirect Water Injection GCUGas Combustion Unit GTGas Turbines HAMHumid Air Motor HFOHeavy Fuel Oil HPHigh Pressure IPIntermediate Pressure LDLow Duty LPLow Pressure LNGLiquefied Natural Gas NGNatural Gas BDCBottom Dead Centre TDCTop Dead Centre RHReheater SCRSelective Catalytic Reduction SFDM+RSingle-Fuel (low speed) Diesel Mechanical propulsion with Reliquefaction STSteam Turbine USTUltra Steam Turbine

Vessel ozone depleting emission regulations are regulated in Annex VI of the MARPOL Convention, wherein the maximum levels of NOx, SOx and suspended particles are established. These increasingly strict regulations, together with the increase in natural gas consumption and its price, have conditioned propulsion systems implemented on board vessels. This article reviews the different propulsion systems used on board vessels for the transport of Liquefied Natural Gas (LNG). The study describes the main characteristics of the propulsion systems, and the advantages and drawbacks that come along with these, from its very beginnings up to the systems installed to date. The described propulsion systems include both gas and steam turbines, combined cycles, 2 and 4 stroke internal combustion engines, as well as reliquefaction plants, while encompassing mechanical, electric and Dual Fuel (DF) technology systems. The propulsion systems implemented have undergone continual alteration in order to adjust to market needs, which were always governed by both efficiency and the possibility of consuming boil-off gas (BOG), always in compliance with the strict antipollution regulations in force. The current direction of LNG vessel propulsion systems is the installation of 2-stroke DF low pressure engines due to their high efficiency and their possibility of installing a BOG reliquefaction plant. Another great advantage of this propulsion system is its compliance with the IMO TIER III emission regulations, without the need to install any supplementary gas treatment system.

Keywords: Boil-off gas Dual fuel Efficiency

⁎

Corresponding author. E-mail addresses: [email protected] (I.A. Fernández), [email protected] (M.R. Gómez), [email protected] (J.R. Gómez), [email protected] (Á.B. Insua).

http://dx.doi.org/10.1016/j.rser.2016.09.095 Received 15 October 2015; Received in revised form 21 December 2015; Accepted 22 September 2016

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1. Introduction

Table 2 Thermo-physical properties of medium LNG.

The Liquefied Natural Gas (LNG) commerce is under constant growth owing to its vast demand worldwide [1–4]. Such demand is provided for mainly by maritime transport, which is the mainstay of bulk material transportation. To meet current demand, the number of LNG vessels has increased considerably in recent years, both on international markets as well as short-haul sea shipping [1,2,4]. The design of LNG vessels is determined by the characteristics of the load, since it is transported in a liquefied state at cryogenic conditions of −163 °C, and with a pressure slightly above atmospheric [5–7]. The classification of this vessel type is carried out according to its design, with the integration of the reliquefaction plant being the main characteristic, given that the availability of boil-off to be burned in the systems of power generation and propulsion depends on this [8]. Technological developments implemented on LNG vessel propulsion systems is conditioned by factors that are both economic as well as environmental, interlinked by the MARPOL Convention, since the restriction of emissions implies the use of higher quality fuels and hence, an increase in costs [7,9]. To date there is no standard propulsion system for LNG vessels [10]. After an exhaustive review of works related to LNG vessel propulsion systems, an extensive variety of systems installed on board has been found, ranging from turbines to internal combustion engines with endless variants. There is, however, no work that carries out a comparison of all systems installed. Therefore, the purpose of this review is a study of LNG vessel propulsion systems, taking into account the latest technological developments in this field. The paper is organised as follows: Section 2 presents the thermophysical properties of LNG as well as its composition, and the boil-off gas concept is defined. Section 3 studies the different LNG carrier propulsion systems, delineating the advantages and disadvantages of each system, followed by the final section in which the conclusions of the study are presented.

Heavy LNG

Methane (CH4) Ethane (C2H6) Propane (C3H8) Butane (C4H10) Pentane (C5H12) Nitrogen (N2) Density (kg/m3) (−162 °C/ 1.3 bar) LHV (kJ/kg)

98.60 1.18 0.10 0.02 – 0.10 427.58

92.30 5.00 1.50 0.60 0.10 0.50 451.58

85.87 8.40 3.00 1.20 0.23 1.30 474.87

49,935

49,557

48,984

1.3 bar −159.16 °C 19.396 g/mol 447.56 kg/m3 49.557 MJ/m3

Heat transfer to the LNG from the environment through insulated spaces and holding tanks results in the boiling of the load, with the consequent formation of steam, referred to as boil-off gas (BOG) [5,16]. The greatest production of BOG is generated during cargo transportation. The main reasons for this are: – Heat transfer due to the difference in temperature between the ambient and the cargo being carried [5,17]. – The cooling of tanks by spraying the LNG over the cargo itself while navigating in ballast to maintain the ideal inside temperature [5]. – A consequence of the energy dissipated from the slogging between the walls and the fluid, caused by the load being shaken by the movement of the vessel. It is on journeys in bad weather conditions that the BOG produced is increased considerably [5]. The BOG produced in the tanks must be removed in order to maintain the design pressure within the tanks [15]. This is given by means different utilities on the vessel: – In vessels without a reliquefaction plant it is used as a fuel in the propulsion system and the excess, depending on the system used, is burned in the gas combustion unit (GCU) or in the boilers, without exploiting any of the energy to control the pressure level in the tanks [18]. – On the contrary, on those vessels that do have a reliquefaction plant, the BOG produced is reliquefied, returning it to the inside of the cargo spaces in a liquid state. Such option requires a high consumption of energy from the reliquefaction plant [5,8,19].

Table 1 LNG classification based on density and composition [6]. Medium LNG

Pressure Temperature Molecular weight Density Higher heat value

2.1. Boil-off

LNG vessels are designated to the transportation of natural gas (NG) from producing countries to consumers, with the principal characteristic being that the load is transported in a liquid state at no less than −160 °C and at atmospheric pressure [5–7]. NG is a mixture of light hydrocarbons, whose main component is methane (CH4) with a ratio of between 85–96% in volume, with minor proportions of ethane (C2H6), propane (C3H8), butane (C4H10), pentane (C5H12), and nitrogen (N2) as inert component [11]. LNG can be classified into three groups, according to its density: heavy, medium or light. Their composition is depicted in Table 1 [5,12–14].

Light LNG

Value

In order to transport LNG, the load temperature must be preserved below its boiling point, at the corresponding pressure inside the tank [5]. Table 2 displays the thermo-physical properties of medium LNG for an average pressure inside the tank of 1.3 bar, calculated using commercial software REFPROP, whose library uses the fundamental Helmholtz equation to determine the fluid properties with the utmost precision [15].

2. Liquefied natural gas

Molar Composition (%)

Parameter

The following sections discuss different methods of BOG treatment, depending on the purpose for which it is intended as well as on the vessel´s propulsion system. 3. Propulsion systems The propulsion system for LNG vessels is closely related with the generation and consumption of the cargo boil-off [5,19]. One way of classifying LNG vessel propulsion systems is according to the purpose appointed for the BOG produced in the cargo spaces, as illustrated in Fig. 1. 1396

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Fig. 1. Classification of propulsion systems depending on the purpose of the Boil-off.

These engines, capable of consuming different fuel types, are known by the acronym DF (Dual Fuel). The DF engine adopted the lean-burn concept from the Otto-cycle, and a small amount of diesel as the pilot fuel, approximately 1 to 8%, which is used for ignition in the combustion chamber in its operation with gas as fuel (gas mode) [21,22]. DF engines developed around the year 2003 are 4-stroke (4S). At present, however, owing to technological advances which enable the use of NG in 2-stroke engines (2S), a new change in propulsion systems to be implemented on LNG vessels is occurring [21]. To follow, a description of the main LNG vessel propulsion systems is detailed, highlighting their main advantages and disadvantages on board.

Both the fuels used as well as the emissions regulations are factors that influence the direction of LNG vessel propulsion systems [7,9]. Another propulsion system classification is based on the fuel to be used, as illustrated in Fig. 2 [20], allowing the possibility of selecting the propulsion system based on future lines of development. Steam turbine (ST) based propulsion has been the main system implemented on LNG vessels since 1960, as this system allows the simultaneous burning in boilers of heavy fuel-oil together with the BOG generated during transportation, which in turn feed the propulsion turbines and electric turbo generators [20,21]. Since 2003, LNG vessel propulsion systems have been at a turning point. STs are being replaced by internal combustion engines due to improvements in the efficiency of the latter and because, as abovementioned, these permit the burning of both heavy fuel oil as well as BOG from the cargo [22]. This shift is reflected in the ordering of 159 methane tankers from mentioned date to be constructed with engines as the propulsion system [21].

3.1. Steam turbines (ST) A propulsion plant with turbines usually comprises two boilers with a generating capacity of 80–90 t/h of superheated steam at a pressure of 60–70 bar at 520 °C [23], with the purpose of feeding the high and

Fig. 2. Propulsion systems based on fuel used [20].

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– Process 7-1: Transmission of heat at constant pressure in the condenser, from the steam to the refrigeration circuit, with the working fluid reaching subcooled liquid state. – -Process 1-2: Feedwater compression, up to the high pressure by means of a turbo pump powered by the very steam generated in the boilers. At this stage, the process of removing non-condensables from the system is also performed. 3.1.2. Advantages and drawbacks The lack of competition in the design of propulsion systems that allow the consumption of several fuel types such as “heavy fuel oil” (HFO) and the BOG from the cargo, along with its ease of use, intrinsic reliability and reduced maintenance costs [10,21], have resulted in STs being the most popular system on board LNG vessels up to the beginnings of the new millennium [21]. It is also worth highlighting the other advantages of this plant type, such as the easy control over the use of BOG, low vibrations and reduced consumption of lubricating oil [20]. The main drawback, owing to which new on board propulsion systems are being sought and implemented, is their poor efficiency, approximately 35% at full cargo, as well as their excessive emissions of CO2, and a large engine room when compared with other systems [23].

Fig. 3. Configuration of a basic propulsion system through an ST.

low pressure turbines. The arrangement of the turbines is typically cross-compound [24], with a net power of between 35MW and 45 MW [23,25]. The steam, once expanded in both turbines, is condensed in the main condenser and sent back to the boiler by means of pumps, after passing through a number of heaters which, by taking advantage of the residual heat, increase the thermal efficiency of the cycle. Once in the boiler, the corresponding change of state occurs again through the input of heat, returning again to steam phase, thus closing the cycle [10,19,20,26]. The generation of electric energy on board is provided by two turbo generators which are fed by the steam itself generated in both boilers, as illustrated in Fig. 3 [20]. Each generator has an average power of 10 MW which, together with a diesel generator of around 3 MW, would be sufficient to meet the power demand of the vessel in any given situation [23]. The boilers are designed to simultaneously consume different fuel types such as fuel-oil and BOG, giving versatility to the system to be highlighted [19]. The supply of gas from the cargo tanks to the boilers is performed through single stage centrifugal compressors denominated LD (Low Duty). Such compressors have variable pitch blades, carried by a variable speed electric engine, regulating the gas supply to the boilers depending on the demand at any given moment [21]. The excess of BOG generated while the vessel is at port or at anchor, a situation in which the propulsion system is out of service, is also burned in the boilers, producing steam without any kind of energy exploitation. This steam is sent directly to the condenser after passing through a laminating and tempering process known as “dumping”. The purpose of this process is to provide the capacity to stabilise the pressure in the tanks.

3.1.3. Current improvements in ST plants ST manufacturers are at a point at which they must improve their performance if they wish to continue as a propulsion system to be considered on LNG vessels. The continuous rise in fuel prices [7,9] together with stricter emissions regulations [1] makes them an unattractive propulsion system. With the aim of enhancing the efficiency of ST-based propulsion plants, a system referred to as Ultra Steam Turbine (UST) has been developed [21]. When compared with existing systems, the key difference resides in the existence of a reheating stage to improve thermodynamic efficiency, as well as the installation of an intermediate pressure turbine (IP). With such developments implemented in the system the efficiency increases around 15%, but still remains lower than or equal to that achieved by internal combustion engines [21,25]. As shown in Table 3, both the temperature as well as the steam pressure in the BLR of a UST system are greater than that of a conventional system, reaching 560 °C at 10 MPa compared with the 515 °C and 6 MPa of a conventional system. The UST system comprises an intermediate pressure turbine (IP) where the steam expands after passing through a reheating stage, in which the temperature of the steam rises from 365 °C to 560 °C to a pressure of 2 MPa, as is shown in Fig. 5. The main advantages offered by a UST system when compared with a conventional ST system are as follows [25]: – The space taken up in the engine room is not increased despite the increased number of elements of which it is composed. – Increase in performance of around 15%. – Highly reliable, comparable to the conventional system. – Low emissions, reduced by around 15% of NOx, SOx and CO2.

3.1.1. Thermodynamic cycle ST plants are run under a representative thermodynamic cycle called the Rankine Cycle [27,28]. Fig. 4 depict the different process of a Rankine cycle, based on a conventional propulsion system using a ST on a merchant ship. The steam plant shown in Fig. 4(A) and the TS diagram shown in Fig. 4(B) are integrally interconnected, passing through the different processes described below:

3.2. Gas turbine The gas turbine (GT) was a technological innovation introduced on LNG vessels because of their ability to consume diesel and BOG without any limitations [19], their high reliability derived from the aeronautical industry and a very high power/weight ratio, meaning a reduced size of the system [21,23]. The first vessels to install a GT as a main propulsion system were those belonging to the navy and also passenger ships. These combined the GT with ST or diesel generators to produce electric power. On the contrary, LNG vessels with GT propulsion are not combined with any

– Process 2-5: Heat transfer to the working fluid at constant pressure in economizer and the boiler. – Process 5-7: Steam expansion in the HP and LP turbines from the boiler pressure to the condenser pressure. 1398

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Fig. 4. Configuration of the steam plant with turbines in a cross-compound arrangement and T-S diagram for the steam cycle of the plant.

the GT enables the recovery of waste heat for the implementation of a combined cycle, thereby increasing plant efficiency to 40%. There is also the possibility of using the BOG as fuel, which would be an option to consider installing as a propulsion system on LNG vessels [21,32]. There are different combined cycle-based system configurations, which can be subdivided into two groups:

Table 3 Comparison of an ST conventional system and a UST [25]. Conventional plant

UST

Steam conditions in the boiler

6 MPa at 515 °C

Arrangement of the system elements

BLR-HP-LP

HP: 10 MPa at 560 °C BLR-HP-RH-IP-LP

– Power driven combined cycle [20]. – Combined Gas turbine Electric & Steam system (COGES) [10,19– 21,23,33] 3.2.1. Power driven combined cycle The power driven combined cycle is an unusual layout on LNG vessels, because all the advantages of the flexibility provided by the DFGE system are dismissed with the installation of an auxiliary power generator. Fig. 7 illustrates a power driven combined cycle arrangement. The system comprises a GT of around 36 MW [20,33] which is responsible for supplying the required torque through a reducer, to rotate the ship’s propeller. The exhaust gases generated in the GT are sent to the recovery boiler where they provide the heat input required to generate steam that is sent to a turbine of around 10 MW [20,33], coupled to a generator that supplies power to the vessel during navigation. The plant also includes three auxiliary generators with a combined capacity of between 6 and 12 MW [20] used for power generation at port, when both turbines are stopped. 3.2.2. Combined gas turbine electric & steam system (COGES) The COGES are electric propelled combined cycles. These systems are composed of elements similar to those that form a power driven combined cycle system, but with a difference in the layout of its components and with the main propulsion being electric [21]. Two arrangements can be distinguished within the COGES, each associated with their manufacturers, these being Rolls-Royce and General Electric [21,23]. Fig. 8 depicts the arrangement of a COGES designed by the RollsRoyce manufacturer. The plant has two GTs with different powers, one

Fig. 5. Configuration of an Ultra Steam Turbine plant.

other generation system, because all the BOG is used as fuel, thus coping the energy demand of the vessel [21]. GTs are combined with electric propulsion, called the DFGE system (dual-fuel gas turbine electric propulsion) [10], shown in Fig. 6. The high specific consumption along with the need to use costly clean fuels [20] so as to comply with ISO-F DMA regulations [29–31], make the turbines a less attractive option to be used on ships. However,

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Fig. 6. DFGE (dual-fuel gas turbine electric propulsion).

not burn different fuels simultaneously [19,34]. This, however, has changed with the development of technology. In 2003, DF engines that stand out because of their ability to burn both liquid fuel as well as gas, become the new tendency in LNG vessel propulsion systems, reaching 159 units built by the end of 2006 [21]. 4S electric DF engines were the first to open up the market, given that they had already been in use on shore since the early 1990s for generating electric power [35] and having to wait until mid-2008 when DF engines were aimed towards 2S [22].

of 36 MW and another of 5 MW, designed in such a way that the exhaust gases of the GT with greatest power are exploited in a heat recovery steam generator [20,21]. The steam generated is used to power a 10 MW ST, which, together with the more powerful GT, provide the electric power and propulsion demand during ship sailing. On the other hand, the purpose of the less powerful turbine (5 MW) is to generate power at port, hence avoiding high fuel consumption caused by the main GT [33]. In Fig. 9 the COGES plant is designed by the manufacturer General Electric [21,23]. The plant has two gas turbines, each of 20 MW. The reliability of this type of system increases because, in the event that there is a fault in a GT, the system could guarantee 50% of the electric power supply to continue with the voyage. The disadvantage, however, is its high consumption while at port as it does not have a low power auxiliary generator as in the case of the Rolls-Royce design. This system´s ST is more powerful because, as a result of having two 20 MW gas turbines, gas generation increases in comparison with the previous system, allowing the possibility of installing a 15 MW turbine [20,21,23,33].

3.3.1. Two stroke slow speed diesel engine with reliquefaction plant Two strokes slow speed engines were used as the main propulsion system in merchant shipping because of its low maintenance costs, high efficiency and the option of burning low-quality fuels [23]. This system is used on LNG carriers of over 200,000 m3 on long distance crossings [21] but with the peculiarity of integrating a reliquefaction plant and a GCU [19], as shown in Fig. 10. The reliquefaction plant has the task of reliquefying the BOG generated in cargo tanks and returning it to into a liquid state inside, avoiding any wastage of the LNG being transported [8,23]. On the other hand, the GCU is designed to burn the BOG generated which, if there were a breakdown in the reliquefaction plant, would be impossible to treat, avoiding the pressure increase in the tanks and could cause great damage [19]. Reliquefaction plants installed onboard are governed by the reverse Rankine cycle with a three stage cascade configuration [8] in which N2 is considered the most widely used refrigerant [36]. The BOG extracted from the tanks passes through various stages in the reliquefaction plant [37]. First the BOG is extracted from the tanks and is subjected to a process in which all the drops that it may contain are separated to avoid damage to the compressors in the compression stage. Compression is performed in two stages to then conduct the exchange with the cooling fluid, in this case N2. Once through the heat exchangers, the BOG reaches the tank return temperature, prior passing through a stage of separating N2, which is released into the

3.2.3. Advantages and drawbacks GT plants provide high system reliability, of around 99%. The auxiliary systems, however, must be taken into account since the overall reliability is hampered as a result of these and so it is necessary to carefully evaluate all those systems that comprise a GT plant [21,23]. One of the main advantages provided by the installation of a GT or COGES system is the considerable decrease in engine room size, but the main drawback is a high consumption of gas and diesel [20]. 3.3. Internal combustion engines Internal combustion engines have become the predominant propulsion system in all sectors of marine transport, encompassing 80% of the fleet, with the exclusion of LNG vessels [10,20] since these could 1400

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Gas turbine Gear box Steam Electricity

Exhaust gas

Heat recovery steam generator

G

Cargo pumps G Load Steam turbine Bow thruster

G Aux. Engine

Gas fuel compressors G Aux. Engine G Aux. Engine

Fig. 7. Power driven combined cycle.

on the power demand of the plant, is composed of either 3 or 4 power generators [21]. Take, for the sake of an example, a 149,000 m3 LNG vessel. The reliquefaction plant has a consumption of 3.5–7 MW depending on the BOG generated in the cargo tanks. The plant consumption represents 20% of the energy available in the recovered BOG or of approximately 20 t of fuel per day [20,23].

atmosphere. This process is represented in Fig. 11, where the basic diagram of a reliquefaction plant is illustrated, both of the NG part as well as the N2 refrigerant. Two-stroke engines are highly efficient, but when considering the overall performance of the system, the reliquefaction plant must be taken into account. The operation of a reliquefaction implies a high power consumption supplied by auxiliary generators which, depending

Fig. 8. COGES designed by Rolls-Royce.

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Fig. 9. COGES designed by General Electric.

3.3.2. Advantages and drawbacks The 2S engines used on vessels have a high efficiency, of approximately 50% which, together with the use of a reliquefaction plant, has the advantage that there is no wastage in the LNG cargo transported. On the contrary, the system has a number of disadvantages when compared with an ST plant, namely its high maintenance costs, the high consumption of the reliquefaction plant, and elevated emissions of NOx and SOx [20]. Merchant ship emissions are regulated by the MARPOL Convention, Annex VI “Regulations for the Prevention of Air Pollution from Ships”. These regulations establish the maximum emission limits of both NOx y SOx [38,39]. There are several methods to reduce NOx emissions, as reflected in Table 4 [40]. By contrast, the removal of SOx can only be achieved through the use of fuel with low sulphur content or a technology to clean the exhaust gases with a maximum permissible limit of 6.0 g SOx/kWh [41].

3.4. Four stroke medium speed diesel engine (Diesel electric)

Fig. 10. Configuration of a propulsion system with 2-stroke diesel engines and reliquefaction plant.

Since early 2003, the number of newly built LNG vessels with a propulsion system using dual-fuel diesel-electric engines (DFDE) has increased considerably, reaching 159 units [21]. This fact demonstrates that the propulsion system tendency on LNG vessels is geared towards the use of DF engines, which are able to consume both gas and liquid fuels [19–21]. The typical configuration of a diesel-electric propulsion system through DF engines is shown in Fig. 14. This configuration has four DF engines coupled to electrical generators that supply energy to the entire ship including propulsion, which is done by means of two electric engines [10,20]. The main variations in configuration through DF engines are found in the propulsion, as shown in Fig. 15, which displays two similar configurations with regard to power generation with four DF engines. The difference, however, lies in the arrangement of the propulsion system: in Fig. 15(A) it is formed by two electric engines and two DF

The vessel´s net auxiliary power is increased due to the reliquefaction plant, between 14 and 16 MW when compared with a system of 4stroke DF engines without a reliquefaction plant [23]. The propulsion system through 2S engines with a reliquefaction plant has two arrangements, mainly depending on the number of main engines that it includes [10,20,21,23]. The first option, as illustrated in Fig. 12, consists of a single main engine that drives a fixed pitch propeller with power generation being provided by three 4S diesel generators also fed by heavy fuel [21,23]. As the LNG trade and vessel classification societies set high standards with regard to maintenance and equipment redundancy, the simplest 2S diesel engine plant on board a methane vessel is in greater demand, and includes two 2S engines, each coupled to a single axis line [23], as shown in Fig. 13. 1402

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Fig. 11. Basic schematic of reliquefaction plant.

Fig. 12. Propulsion system with 2S main engine and reliquefaction plant.

State C (Combustion): When the cylinder is located before the TDC, pilot fuel injection is performed, which is of approximately 1% of the consumption, which releases the energy required to start combustion and thereby moving the cylinder up to the BDC (bottom dead centre). State D (Exhaust): After the working stroke, the cylinder begins again to move towards the TDC where, with the exhaust valve open, all flue gases are released. On the contrary, if diesel or heavy fuels are used, the DF engine works under a diesel cycle (diesel mode). Fig. 17 depicts the 4 work stages of a DF engine in diesel mode. To follow is a description of each: State A (Intake): In the intake stroke only air is introduced into the cylinder because the gas valve remains closed. State B (Compression): The air is compressed by the cylinder on its ascent until it reaches the TDC. State C (Combustion): When the cylinder is located before the TDC, both the main and pilot fuel injection is performed, releasing the energy required to start combustion and moving the cylinder up to the BDC.

engines coupled to independent reducers, while Fig. 15(B) is based solely on electric propulsion [42]. Other authors propose a different configuration, which is to couple an organic Rankine cycle at propulsion system for recovering waste heat from the engines [43]. DF engines have different operation modes depending on the fuel to be used. When gas is burned as fuel (Gas Mode), they adopt the concept of the lean Otto cycle. Fig. 16 illustrates the different work stages in gas mode [10,20,21,23]. A brief description of the various work stages of a DF engine in gas mode is given below: State A (Intake): the gas is supplied to each cylinder individually through a valve at the inlet, where it is mixed with air before entry to the combustion chamber [34]. The gas intake is performed at low pressure, at approximately 5 bar [21], considerably reducing the risks that the use of methane at high pressure in the engine room would entail. State B (Compression): The mixture is compressed by the cylinder on its ascent to the TDC (top dead centre). 1403

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Fig. 13. Propulsion system with two 2S main engines and reliquefaction plant.

State D (Exhaust): After the working stroke, the cylinder begins again its move towards the TDC where, with the exhaust valve open, all flue gases are released into the atmosphere. The combustion control system is one of the main characteristics that must be taken into account in DF engines because, depending on the operating mode, the cycle to be performed by the engine varies between Otto and Diesel. When the engine runs in Diesel mode, combustion regulating is done by the controlling of exhaust temperatures in order to ensure process optimisation [34,42]. By contrast, regulating the combustion in gas mode is much more complex because, depending on the engine load, the control is adapted to different variables [42]. When the engine load is below 65%, regulation is performed using the exhaust temperatures as in the Diesel cycle but, once surpassing this load value, combustion is controlled by Knocking sensors [20,34,42].

Table 4 Methods for reducing NOx [40]. Technique

Reduction of NOx

Alternative Fuels Emulsified Fuel - Water Addition Basic IEM - Slide Fuel Valves Injection timing retardation Compression Ratio Modification Injection System Modification Scavenge/Charge Air Cooling Scavenge/Charge Air Pressure Increase Direct Water Injection (DWI) Humid Air Motor (HAM) Exhaust Gas Recirculation (EGR) Selective Catalytic Reduction (SCR)

50–60% 50–60% 20% 30% 10–30% 30% 14% 10–40% 40–60% 70–80% 80–98% 80–99%

Fig. 14. Configuration of diesel-electric propulsion using DF engines (4S).

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Fig. 15. Configurations of diesel-electric propulsion using Dual-Fuel engines (4S).

Fig. 16. Dual-fuel engine work stages (4S) in gas mode.

hydrocarbons that make up the NG in a system called Oil Mist Separator [21,44]. Before using the methane in the engines, it must be pre-treated in a plant similar to that shown in Fig. 19, where it can be seen that once the methane has been separated in the mist separator from the other components of which the NG is composed, it is drawn in by the Lowduty compressors (LD) [21,22,45], raising it to a pressure of 5–6 bar at which the engines will be fed [19,21,35]. Once the pressure is stabilised, the temperature of the gas is then stabilised through an exchanger fed with sea water [42], after passing through the engines and the GCU.

DF engines have a Knocking sensor per cylinder as combustion is controlled and monitored individually. As the engine load is increased, and with it the mean effective pressure, the parameters must be adjusted more accurately, as illustrated in Fig. 18 [20], to remain within the area known as the operating window. Conditioning processes of the BOG generated in the cargo tanks, or forced if necessary, are of paramount importance in DF engines, which is why, before being burned as fuel, it must be treated. DF engines are designed to use methane as gas fuel. It is therefore essential to separate the other NG components to ensure correct combustion, avoiding Knocking [21,35]. This requires separating the methane from the other

Fig. 17. Work stages of a DF engine (4S) in Diesel mode.

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Fig. 18. Graph of a Dual-Fuel engine operation (4S) [21].

The GCU is used to burn the excess BOG generated that cannot be consumed in the engine or if, for some reason, pressure exceeds the limits established in the tanks [19]. Vent valves are used as a final safety measure of the overpressure in the tanks. 3.4.1. Advantages and drawbacks The propulsion system by means of DF engines presents a number of advantages such as high efficiency when compared with ST systems, high redundancy due to the configuration of the propulsion system and reduced SOx emissions as the BOG consumed has no sulphur [20,35,39], complying with the IMO TIER III gas emission regulations when operating in gas mode [42]. The drawback of this propulsion system is the increased amount of equipment of which it is composed, hence implying high installation and maintenance costs [20].

Fig. 20. Ratio of Vessel speed and amount of BOG reliquefied [20].

around 0.15% per day [8,37], and the speed of the ship, which, depending on the needs of each vessel, could be an attractive option to consider. 3.5. Two stroke slow speed diesel engine (Diesel electric) Propulsion through DF engines is an LNG vessel-based technology, with the first installed on board 4S in 2003 due to the existence of similar units in industry on land from the 80s [21,35]. On the contrary, the evolution of 2S DF engines was slower, with no 2S engine with gas injection being developed until 1994 [42], and having to wait until the beginning of the XXI century for the first ships to be installed with such technology [35,42]. Two models can be distinguished in 2S DF engines according to the gas injection pressure, those of high pressure (MAN) and those of low pressure (Wärtsilä) [46].

3.4.2. Four stroke medium speed diesel engine (Diesel electric) with reliquefaction plant Installing a reliquefaction plant on a vessel with a 4S DF engine propulsion system provides high flexibility to the system. But two important factors must be taken into account, such as the ensuing high costs the plant along with the amount of BOG consumed in the engines and that generated in the tanks [20]. As shown in Fig. 20, the feasibility of installing a reliquefaction plant on vessels with a DF system is conditioned by the BOG generated, of

Fig. 19. Gas management system in a DF engine system (4 S).

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Fig. 21. Gas management in 2S DF engines.

Fig. 22. Installation of 2 S DF engines.

Fig. 24. Gas injection system components in a low pressure 2S DF engine cylinder.

3.5.1. High pressure The manufacturer MAN was the first to develop 2S DF engines to be installed on LNG vessels, following the tendency of using high pressures in the gas injection of its industrial engines. The main difference from 4S DF engines is the injection of gas, since it is

Fig. 23. Gas injection valve.

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Fig. 25. Gas supply system of a low pressure 2S DF engines, in which eight parts are highlighted [35].

Fig. 26. Propulsion system configuration of 2S low pressure DF engines.

performed directly in the combustion chamber at high pressures, 250– 300 bar [10,19,42]. Two gas management systems from within the 2S DF engines developed by MAN can be highlighted (Fig. 21), in both options the gas supply pressure being of around 300 bar. To achieve this high pressure displacement compressors must be installed with the capacity to deal with the total demand of the engines individually, as reflected in Fig. 22, wherein the arrangement of the components of a plant with several propulsion engines is highlighted [42]. Fuel injection is performed in the cylinder head through gas valves (Fig. 23) and pilot fuel [39]. The intake of gas towards the injection valve takes place through holes in the cylinder head, preventing any gas leakage through the use of double-wall conduits with detection sensors [19,21]. The gas pressure at around 300 bar is exerted permanently over the shaft, using oil at a pressure of between 25 and 50 bar above that of gas for a correct sealing and to prevent any leakage towards the control system [21,42].

Table 5 Fuel sulphur contents: global and ECA limits [7]. Date

Global limit (% mass)

Date

ECA limit (% mass)

Prior 1/1/2010

4.5%

1.5%

After 1/1/2012

3.5%

After 1/1/2020

0.5%

Prior 1/7/ 2010 After 1/7/ 2010 After 1/1/ 2025

1.0% 0.1%

Table 6 NOX emission reduction programme [7]. Tier

Tier I Tier II Tier III

Date

2000 2011 2016

NOX limit (g/kWh) n < 130

130 < n < 2000

n > 2000

17 14.4 3.4

45×n−0.2 44×n−0.23 9×n−0.2

9.8 7.7 1.96

3.5.2. Low pressure In the 1980s, Wärtsilä released high pressure DF industrial engines on the market but, owing to the complex treatment of the gas, the expensive safety systems to be installed and negligible improvement in

n: engine speed (rpm)

Fig. 27. Propulsion systems technology and anti pollution regulations chronology.

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Table 7 Propulsion systems features. Propulsion system ST

UST

GT

COGES

2S + Reliq.

DFDE 4S

DFDE 4S + Reliq.

DFDM 2S(HP) DFDM 2S (LP)

Elements

Advantages

Disadvantages

Fuel used

– – – –

– Allow the consumption of several fuel types. – Intrinsic reliability – Reduced maintenance costs. – Low vibrations. – Reduced consumption of lubricating oil. – Increase in performance of around 15%. – Highly reliable. – Low emissions, reduced by around 15% of NOx, SOx and CO2.

– Poor efficiency, approximately 35% at full cargo. – Excessive emissions of CO2. – A large engine room when compared with other systems.

HFO or Gas

– Increased number of elements of which it is composed.

– High system reliability, of around 99%.

Turbines HP & LP Boiler Pump Condenser

– Turbines HP, IP & LP – Boiler – Reheater – Pump – Condenser – GT – Aux. Engine – 2 Electric motor – 2 GT – Steam turbine. – Heat recovery steam generator – 3 Aux.engine – 2S Engine – 3 Aux. engine – BOG reliquefaction – GCU – DFDE – Electric motor – GCU

– – – – – – – –

4 DFDE Electric motor GCU BOG reliquefaction DFDM (HP) 3 Aux. engine 1 DFDM (LP) 3 Aux. engine

Emissions(g/kWh) SOX NOX CO2 1.00 11.00 930

Part. 2.50

HFO and/ or Gas

0.75

8.25

697

1.87

– High consumption of gas and diesel

GAS or MGO

0

2.50

590

0.01

– High system reliability

– High consumption of gas and diesel

GAS or MGO

0

14.00

590

0.01

– The 2S engines have a high efficiency, of approximately 50%.

– High maintenance costs. – The high consumption of the reliquefaction plant. – Elevated emissions of NOx and SOx. – The increased amount of equipment of which it is composed, imply high installation and maintenance costs.

HFO

17.00

12.90

550

0.50

HFO, Gas or MDO

12.00

13.60

612

0.40

– High costs the plant.

HFO, Gas or MDO

12.00

13.60

612

0.40

– Use of an exhaust gas recirculation system to reduce emissions – Only when operating in gas mode, pass IMO TIER III gas emission regulations.

HFO and/ or Gas HFO and/ or Gas

0.85

10.12

469

0.31

0.17

2.68

412

0.01

– High efficiency. – High redundancy due to the configuration of the propulsion system. – Reduced SOx emissions as the BOG consumed has no sulphur. – High flexibility system.

– The high efficiency of 2S engines. – The high efficiency of 2S engines. – Reduced NOx emissions. – Simple, reliable and most economical low-pressure gas supply system, with the fewest components. – Stable operation on gas over the entire load for port-to-port operation and manoeuvring.

Emissions are a factor to consider for those engines designed at present due to the limitations of the pollution regulations [7,9]. For this reason, the reduced NOx emissions of the Wärtsilä 2S DF low pressure engines should be emphasised, as they require no device in order to comply with the OMI Tier III regulation [7,35]. On the contrary, MAN DF high pressure engines, which have higher NOx emissions, require the use of an exhaust gas recirculation system to reduce emissions and so comply with current regulations [19,42].

emissions, these engines were ruled out. In the late 90s gas injection low pressure 4S DF engines were introduced onto the market. Low pressure 2S DF engines were then developed based on this engine, which are those installed from 2013 [35]. Gas injection is performed at a pressure below 16 bar when the cylinder is in mid-stroke, mixing with the dry air blast, as shown in Fig. 24 [8]. Once the mixture is compressed, combustion is started by injecting the pilot fuel, this being 1% of the amount injected at full load. In order to ensure stable ignition in all conditions, the pilot fuel is injected into pre-combustion chambers (Fig. 23) [35]. Fig. 25 depicts the gas supply system of a low pressure 2S DF engines, in which eight parts are highlighted [35]. The configuration of a propulsion system for LNG vessels that implement low pressure 2S DF engines coupled directly to two independent axes is shown in Fig. 26. In such a configuration, each engine features an individual gas treatment system to achieve greater safety in the provision of gas [35]. To generate electric power, they are equipped with three 4S DF engines coupled to their corresponding generators.

3.6. Development of propulsion systems and environmental legislation The development of propulsion systems has always been conditioned by strict anti-pollution regulations. Annex VI of the MARPOL 73/78 Convention, which deals with the prevention of air pollution by vessels, includes, among its amendments, sulphur oxide (SOx) and nitrogen oxide (NOx) emission limits [7.9]. SOx limits are established by global limits and ECA limits, as shown in Table 5, while those of NOx are established by Tier limits, depicted in Table 6 [7]. The close link between the development of propulsion systems and anti-pollution regulations can be observed in Fig. 27, in which a chronological analysis of both is carried out from 1960 to 2020. ST propulsion is established as the main propulsion system [20,21] from

3.5.3. Advantages and drawbacks The high efficiency of 2S engines, both high and low pressure, is the most prominent advantage [21,22]. 1409

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1960, until a turning point took place in 2003 and the internal combustion engines took over the market up to the present day [22]. Antipollution regulations are becoming more stringent over time, as shown in Fig. 27, where the movement is for emissions of both NOx and SOx in the years 2026/2020 to be reduced considerably. By way of summary, Table 7 depicts the advantages and disadvantages of each propulsion system, as well as their main equipment and the major polluting emission values. 4. Conclusion – Propulsion systems on LNG vessels ships are under constant development in order to adapt to pollution regulations as well as economic market tendencies. In light of the above analysis, a conclusion is reached of the advantages and disadvantages delivered by each propulsion system. – The STs possess high capabilities of burning both gas and liquid fuels as well as low maintenance costs and high reliability, but the disadvantages regarding low efficiency, of approximately 35% at full load, along with excessive CO2 emissions, discard it as a system to be used at present. However, this performance can be increased by 15% with improved as the UST cycles. – GTs exhibit high reliability and a reduced size. Its manufacturing costs and high consumption, however, result in it being an unattractive option. – 2S engines with a reliquefaction plant has always been an attractive option thanks to its increased efficiency, low maintenance costs and that there were no losses in the cargo to be transported. Notwithstanding, high emissions of both NOx y SOx, along with the high consumption of the reliquefaction plant has displaced it as an option to consider. – DF engines, both 4S and 2S, are the propulsion systems currently installed on LNG carriers due to their high efficiency, high redundancy, because of the propulsion system configuration and the reduced emissions of SOx, complying with IMO TIER III gas emission regulations when operating in gas mode, with the exception of MAN 2S DF engines. On the downside, this propulsion system comprises an increased amount of equipment, implying a high installation and maintenance costs. DF technology is that which predominates today thanks to the versatility provides the system with. Regulations on pollution, however, are increasingly stringent, so the use of clean fuels such as hydrogen is not to be discarded, as these can be easily obtained from the NG, which is the cargo to be transported. Likewise, the use of engines that consume hydrogen is a technology based on the industrial market, hence adaptation is viable on board ships. These are the future lines of research to be followed by authors in attempts to reduce atmospheric emissions on board vessels. References [1] Maxwell D, Zhu Z. Natural gas prices, LNG transport costs, and the dynamics of LNG imports. Energy Econ 2011;33(2):217–26. [2] U.S. Energy Information Administration, Annual Energy Outlook; 2014. [3] Kumar S, Kwon H-, Choi K-, Hyun Cho J, Lim W, Moon I. Current status and future projections of LNG demand and supplies: a global prospective. Energy Policy 2011;39(7):4097–104. [4] Aguilera RF, Aguilera R. World natural gas endowment as a bridge towards zero carbon emissions. Technol Forecast Soc Change 2012;79(3):579–86. [5] Dobrota Dorđe, Lalić Branko, Komar Ivan. Problem of boil – off in LNG Supply Chain. Trans Marit Sci 2013;02:91–100. [6] Querol E, Gonzalez-Regueral B, García-Torrent J, García-Martínez MJ. Boil off gas (BOG) management in spanish liquid natural gas (LNG) terminals. Appl Energy 2010;87(11):3384–92. [7] Burel F, Taccani R, Zuliani N. Improving sustainability of maritime transport through utilization of liquefied natural gas (LNG) for propulsion. Energy 2013;57:412–20. [8] Romero Gómez J, Romero Gómez M, Lopez Bernal J, Baaliña Insua A. Analysis and efficiency enhancement of a boil-off gas reliquefaction system with cascade cycle on

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