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GAS COMBUSTION UNITS: HIGH PERFORMANCE TECHNOLOGIES FOR SAFE DISPOSAL OF EXCESS

BOIL OFF GAS ON THE NEW GENERATION OF LNG CARRIERS

Damien Feger Space Engines Division

Snecma Forêt de Vernon

27208 Vernon France

ABSTRACT

The new generation of LNG carriers use Dual Fuel Diesel Electric or Slow speed Diesel combined with an on board re liquefaction plant. Compared to previous Steam turbines LNG carriers propulsions systems, where excess boil off gas coming from the cargo tanks could be burned in the boiler and the corresponding excess steam dumped in the condenser, these new types of propulsion systems require either in normal operation or as a back up, a capability to dispose of the excess boil off gas, which cannot be used as fuel or treated by the re liquefaction plant, in a safe and environmentally friendly way.

This is provided by specific equipment, the Gas Combustion Unit (GCU).

This paper presents a review of the corresponding requirement’s, the main design features and operational performances of the GCUs proposed by Snecma based on North American Stordy burner technology and already on order for three other vessels being built in Far East shipyards, as well as the first operational feed back of the GCU provided for the “Provalys” LNGC delivered by Aker Yards to Gaz de France.

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1. INTRODUCTION

1.1 PRESENTATION

Liquefied Natural Gas (LNG) carriers are part of the LNG chain, which is based on three links:

• the liquefaction terminal, in the producing country, which purifies, liquefies and stores (under ambient pressure and cryogenic temperature) the natural gas prior to its loading into the LNG carrier,

• the LNG carriers, which ship the LNG from the loading terminal to the off-loading one,

• the regasification terminal, in the gas consuming country, which stores, pressurises and regasifies the LNG prior to injecting it into the gas pipe, which distributes it to the gas consumers.

In LNG carriers, the liquefied gas is stored in a boiling state, at cryogenic temperature (- 160°C) slightly above the atmospheric pressure in insulated tanks. Due to the heat leaks getting though this insulation into the liquefied gas, a part of the cargo is boiling off the tanks (typically 0,1 to 0,3 % per day).

To avoid wastage of this boil off vapours, the thermal performance of the insulation is usually optimised so that the boil off vapours flow can be used to provide part of the ship's propulsion needs when it is on its way. For this purpose the propulsion system is of a dual type, compatible with the use as fuel of either the heavy oil either, when available, the natural gas boil off vapours coming from the cargo tanks.

When the ship propulsion requirements are reduced, during harbour manoeuvres or at anchor for example, the boil off vapours exceeds the propulsion needs, although the cargo tank pressure has to be kept within acceptable limits. To dispose of this excess boil off and avoid a pressure rise in the cargo tanks, several strategies can be considered :

• implement an on board re liquefaction plant which re liquefies the vapours and send back to the cargo tanks the boil off vapours in a liquefied state.

• dispose of this excess boil off by burning it in an on board thermal oxidiser complying with safety and environmental regulations which do not allow direct release of natural gas into the atmosphere for both safety and environmental concerns (green house gas effect of methane which is very significantly higher than the one of carbon dioxide).

The standard approach : steam turbine propulsion

Up to now, most LNG carriers strategy has been to use for this reason a steam turbine propulsion system as it allows to use either heavy oil or boil off vapours for fuel, the steam boiler being equipped with heavy oil and natural gas burners. This propulsion system had the further advantage that the excess boil vapours could be disposed of directly in the steam boiler, the corresponding excess steam being sent to the sea water cooled condenser rather than to the propulsion turbine, without requiring any specific equipment, other than a bypass valve towards the condenser to fulfil this additional boil off disposal function.

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Although it has been considered attractive for LNG carriers for decades, this steam propulsion approach is very much challenged nowadays as it has the following drawbacks :

• compared with other propulsion modes, such as Diesel or Gas turbine, it is bulkier, and therefore leads, for the same hull size, to a lower shipping capability,

• its fuel efficiency is 30 % instead of 45 %,

• although it is very reliable, technological improvements, maintenance and crew recruitment is getting more and more problematic as these are the last remaining cargo ships using steam propulsion…

Two sets of alternative technologies and propulsion system architectures are presently proposed by the industry:

• - Dual fuel Diesel Electric

• - Slow speed Heavy fuel Oil (HFO) Diesel combined with an on board reliquefier.

1.2 TWO NEW PROPULSION SCHEMES, BOTH REQUIRING A GCU

1.2 (a) First new approach: Dual Fuel Diesel Electric

This approach uses:

• specifically developed dual fuel Diesel engines

• electric propulsion system similar to the one developed for cruise ships and oil tankers.

Figure 1 gives an example of such Dual Fuel Diesel electric propulsion system. Its main features are the following :

• the boil off gas is sent to diesel driven power generators which provide power for the ship propulsion systems and the "hotel" load;

• the fixed pitched propeller is connected to two redundant high speed electrical motors, though a gear box;

• in the case the flow of gas used by the power generators ( ship at anchor or at sslow speed…) exceeds the ship 's energy needs, a GCU is used to burn and dispose off this excess boil off gas.

GCU

G

DUAL FUEL DIESEL GENSET

G

DUAL FUEL DIESEL GENSET

G

DUAL FUEL DIESEL GENSET

M/G

M

M

FPP

BOIL OFF

HOTEL LOAD

Figure 1: Dual Fuel Diesel Electric propulsion architecture

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GCU

Reject/BOG

RELIQ. PLANT

LNG RETURN

BOG

SLOW SPEED DIESEL

CARGO TANKHEAVY FUEL OIL

Because high conversion rate of both dual fuel diesel engine and electric propulsion of the power, the system have a significant improved efficiency of about 43 % from boil off to propeller shaft power, instead of 30 % for a steam propulsion system.

This represents on a typical 150 000 m3 LNG carrier a saving of 10 000 tons of LNG per year. Taking into account a LNG price of 110 € per ton, this represents a saving of 1,1 M€ per year.

1.2 (b) Second new approach: slow speed HFO Diesel combined with on board re liquefier

Figure 2: Slow speed Diesel + on board reliq propulsion architecture

This approach is based on the use of heavy oil instead of the boil off gas for the ship propulsion system. This is cost effective when the price difference between the heavy fuel oil is lower than LNG : in this case, ship owners will prefer to use this less expensive fuel and implement an on board liquefier to re cool and liquefy the boil off vapours and send them back into the cargo tanks, see Figure 2 an example of such configuration.

Reliquefaction systems have been proposed and considered at regular intervals ever since building of LNG carriers started in earnest around. It is presently baseline for all the 200 to 250 000 m3 LNGCs being built to ship gas from Qatar to United States and Europe. With this system :

• The total quantity of Btu’s loaded can now be delivered to the buyer.

• The LNG nitrogen content is reduced during the voyage

• Standard, high efficiency, slow speed Diesel can be used for propulsion

Economical merits:

• Increased cargo quantity delivered..

• Reduced heel required on ballast voyage

• Large savings in total fuel consumption

• Allows more economical freight contracts

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1.2 (c) New need for a Gas Combustion Unit within the boil off management strategy With regard to boil off, the main difference, between steam and these two other

propulsion approaches, is that the propulsion system cannot be used anymore to dispose all or part of it in an environmentally friendly and safe way : a specific equipment, is required either to re liquefy or to burn it.

In the case of Dual fuel Diesel electric propulsion, the approach presently proposed by shipyards is to use a GCU to burn the excess boil off which is not used to full fill the ship energy needs.

In the case of slow speed Diesel, the boil off vapours are not used as fuel and a re liquefier is used to re liquefy them.

GCU is required for both type of Diesel propulsion as :

- in the case of Dual Fuel engine, the excess boil off not used by the ship propulsion system has to be burnt,

- in the other case, class societies require a back up to deal with BOG if the on board reliquefier fails.

Although similar, the requirements on the thermal oxidiser will be therefore different for the two types of propulsion systems :

• The Dual Fuel Diesel Electric Configuration :

For the Dual Fuel Diesel Electric configuration, the GCU is, operating as a pressure regulation system, spending most of its time in standby mode when all the boil off is used to provide the ship's energy needs, getting in active mode at short notice (a few seconds) to avoid a pressure overshoot in the Diesel engine feed line due, for example, to a sudden slow down of the Diesel engines.

For this configuration, one main objective is to reduce the power consumption in stand by mode and to have a high transient capability.

• The slow speed Diesel + on board liquefier Configuration :

For the slow speed Diesel + on board liquefier configuration, the GCU acts both as a back up system as required by classification society to manage the excess boil off and as a solution to dispose off properly of the high nitrogen content part of the boil off gas, as explained below.

One feature of reliquefaction plant operations is that the nitrogen in the boil-off that can be as high as 20 % is rejected in a separator together with a very small quantity of methane. This reject gas cannot be released to atmosphere but must be disposed of by burning or otherwise. For LNG compositions with lower nitrogen content, the complete boil-off will be reliquefied and returned to the cargo tanks. The flow rate of reject gas can typically be 400-600 kg/h of nitrogen and 50-100 kg/h of methane and the GCU has to dispose of this gas with the minimum power consumption as the re liquefaction plant is operating nearly all year round.

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• The disposal of natural gas / inert gas mixture:

Apart from this main role of disposing of excess boil off during normal operations, the GCU has to burn as well the gas coming from the tanks during their inerting operations prior to inspection. During this operation, the tanks are warmed up and the remaining natural gas vapours are replaced by inert gas generated by the ship's inert gas generator. This replacement is performed by injecting the heavier inert gas at the bottom of the tanks and by removing the lighter natural gas vapours at the top of the tanks

During these operations, the corresponding mixture of natural gas vapours and inert gas collected at the top of the tanks has to be disposed by the GCU in order to avoid any safety and environmental hazards.

This means that for these operations, the GCU has to be able to operate compatible with low Power of Combustion Index (PCI) fuel gas.

1.3 GCU CLASS REQUIREMENTS

From class point of view, the first main safety requirement is to dispose safely of the excess Boil Off Gas by avoiding the creation of a hot spot which could act as an ignition point if a natural gas cloud is released at the exhaust of the GCU.

Minimum ignition temperature for methane has been investigated thoroughly and varies , typically between 540°C and 705°C. The USCG, which were the first to issue a GCU maximum temperature requirement, proposed, to have some margins, 535°C. This temperature threshold has been followed by most major classes since.

The second main requirement for the GCU is on the GCU exhaust plume to avoid damage to aerials or injury to the crew. GCU outlet is therefore required to be positioned away from the bridge.

Depending on class redundancy strategy , which depends on the type of vessel considered, the redundancy requirements in case of one fan failure is specified on a case by case basis.

Other safety requirements are usually covered with the relevant application of the International Gas Code or IMO requirements, fans noise limitations being one of the most critical specification.

2 INNOVATIVES TECHNOLOGIES AND DESIGN SOLUTIONS

2.1 MAIN TECHNOLOGIES USED

Figure 3 shows the overall architecture of a GCU. Its main components can be identified :

• a burner and its igniter which are to offer a wide operating range in terms of BOG flow and nitrogen content;

• a set of motor fans which are to provide air for both combustion of the BOG, but , as well, the cooling by dilution of the corresponding hot gases;

• a combustion chamber which is used to contain the flame and dilute the combustion gases with the fresh air;

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• a gas supply system which is used to control the flow of BOG to be disposed off in the GCU.

For each of this components, Snecma has searched for the best available technology

with regard to safety , capital and operation expenses, in order to meet class, shipyards , ship owners and charterers combined objectives.

Figure 3: GCU main components

First attention was focused on the key burner technology and Snecma , has reviewed several candidates technologies before selecting the Versaflame technology developed by North American Stordy. The main features of this burner technology are the following :

• primarily developed for the disposal of low Power of Combustion Index (PCI) gasses coming from organic wastes in land fields applications, it was optimised for the burning of methane mixed with high content of CO2 inert gas, and had already several success full operating references demonstrating this capability;

• it requires a very low inlet pressure ( a few millibar) , offering a very wide operating range, without requiring any active control and control of the fuel /air mixture;

• its vertical/cylindrical geometry is easy to integrate inside a cylindrical combustion chamber;

• it is fully static and passive equipment (typically like a filter), bringing a very high reliability and no maintenance.

Once the Versaflame technology has been selected, Snecma and North American Stordy have set a joint agreement to develop a burner specifically optimised for GCU requirements. The main feature of this development was the building of a full size test bench (see Figure 4) at Snecma facilities in order to map the burner operating domain and demonstrate margins with regard especially to :

• maximum/minimum BOG disposal capability;

• high nitrogen content flow;

• compliance with safety and emissions requirements.

Apart from the burner, attention was given to the selection of the igniter technology. To perform the main burner ignition several options were identified :

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• an gas pilot ignited either by a spark igniter or an oil pilot;

• an oil pilot ignited by a spark igniter

• direct ignition of the main burner by a spark igniter.

Driven by a KISS (Keep IT Safe and Simple) development approach, and the fact that most on shore gas burner applications do use direct ignition vby spark igniters, the objective has been to try to not to use an oil pilot which requires a specific oil supply system and brings the additional hazards of an oil fire in the GCU room. In the same way, next action was to see if a gas pilot was really necessary, as it brings additional complexity to the gas supply system, and a specific test program was performed and demonstrated that a direct ignition of the main burner was possible. Thanks to the Normandy weather, ignition in adverse rain conditions was has well demonstrated, free of charge!

The technologies for the combustion and dilution air supply were reviewed in parallel. One of the main questions was to assess whether it makes sense to have different type of fans for combustion and dilution. Very quickly, use of the same type of fans for both combustion and dilution air was preferred as :

• this reduces the overall number of fans driving down installation and maintenance costs;

• this simplifies the combustion chamber geometry.

The centrifugal fans technology was selected against the helicoidal one as :

• its allows to meet pressure head requirements with margins;

• it is less noisy, so the additional cost , pressure drops and volume of noise dampers is not to be considered.

The number and sizing of the fans being project dependant, the approach has been to

feed the combustion chamber through an “air room located below the combustion chamber, to which the optimum number of fans can be attached. This approach brings a lot of integration flexibility for the shipyard, suppress the need for air ducts, and allows, latter on, the ship owner to optimise fans operations according to the GCU real air requirements which depend, among others on the actual BOG rate and composition, and on the outside air temperature…

The choice of the technologies for the combustion chamber where focused on :

• the thermal insulation strategy;

• the ease of integration on board;

• an efficient mixing of the hot gas with the fresh air.

Several strategies can be considered to protect the GCU room from the combustion chamber temperatures, knowing that class requirements is to avoid touch temperature above 60 °C :

• internal refractory insulation inside an outer combustion chamber shell;

• external insulation outside a refractory steel inner shell;

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• air cooled heat shield inside a non refractory outer shell.

Due to the high thermal transient (typically from idle to full capacity in less a minute in case of a gas driven engine trip on a DF/DE LNGCs) and its sensitivity to adverse weather and vibration, refractory insulation was rejected. Outside insulation with a refractory steel shell was considered but it use of a high temperature steel is a major cost driver, so attention was focused on the implementation of an heat shield surrounding the flame and cooled on its periphery by a fraction of the dilution air flow. This way, use of refractory steel was kept to the minimum, and, as a bonus, holes in the thermal shields were implemented to drive in, in an optimal way, dilution air in the flame, improving the mixing with the hot gases…

Once this combustion chamber technology was selected, it was implemented for the full size test bench which was use to provide a thermal mapping of the combustion chamber walls to check material compatibility as well as the temperature distribution in the gas flow in order to optimise the mixing of the fresh air with the hot gases and check compliance with the required maximum exhaust outlet temperature…

For the gas supply system, no specific technologies or architecture were considered, as this equipment use standard approved gas valves and control and command devices and its architecture is basically driven by the corresponding applicable International Gas Code and class rules.

Figure 4: Full size GCU test bench at Snecma facilities

Figure 5 : Burner operating at full capacity

3 IMPLEMENTATION ON BOARD

Location of the GCU is driven by the following requirements :

• it has to be close to the BOG line going to the DF/DE engine room or to the reliquefaction plant;

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• it has to be as far as possible away from the bridge, and accommodation quarters to avoid exposure to the hot exhaust gas plume as well as to the noise coming both from the combustion itself and the motor fans;

• easy fresh, gas free, air supply from the outside.

Based on this, GCU preferred location is at the aft of the vessel, above the main engine room, within an extension of the engine exhaust pipes casing. This way :

• the length of the GCU exhaust pipe to reach the top of the funnel is reduced as much as possible, driving down corresponding cost and pressure drops;

• the GCU gas line can be directly derived from the gas line feeding either the DF/DE engine room or there liquefaction plan;

• there is ample room to implement large air louvers to supply of the combustion and dilution air with low pressure drops;

• noise towards the navigation bridge and the crew accommodation is kept to a minimum (especially, if, as illustrated in figure 5, the GCU is located at the aft of exhaust pipe casing , with air inlets facing the aft of the vessel and the main engine exhaust pipe casing acting as a noise shield between the GCU and the forward part of the vessel);

• the GCU hot gas exhaust plume is located away from the navigation bridge and the aerials, reducing the risk of hot gas exposure.

With this latter aspect, specific modelling (see figure 7) and reduced scale smoke test

may need to be performed by the shipyard to demonstrate to class that this hazard is controlled with margins and to check for possible interference between the main engines and GCU exhaust plumes.

Thanks to the compactness of its design, Snecma GCU can be implemented within 4 decks only , with a very reduced footprint :

• one fan deck for the fans, the “air room”;

• one burner deck for the burner, the combustion chamber, the gas supply and control command systems;

• two decks for the exhaust pipe leading to the top of the funnel casing.

For the shipyards, this compactness brings the opportunity to use the available spaces below the GCU to accommodate and within the same funnel casing, to accommodate equipment’s such as the emergency power generator, the fire fighting foam generator, the inert gas or nitrogen generators with the corresponding savings in steel structure.

Thanks to its burner wide operating range in terms of flow, gas composition and inlet pressure, Snecma GCU provides large operating margins to match the pressure relief requirements of DF/DE engines or reliquefaction plant, simplifying operations of this equipments as well as the one of the compressor room and reducing trouble shooting operations during gas trials….

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Figure 6: Location of the twin combustion chamber s GCU provided by Snecma for the two Gaz de France NYK 156 000 m3 DF/DE LNGCs

being built at Saint Nazaire shipyards

Figure 7: Example of exhaust plume model used to assess exhaust plume hazards

4 FIRST OPERATIONS

Snecma provided the GCUs for the “Provalys” (see figure 8) , the first DF/DE LNGCs in operations which has been delivered in the second half of 2006 by Aker Yards ( Former Alstom Marine) to Gaz de France. For this vessel, ship owner and class required to have a high level of redundancy on the GCU, so Snecma provided a GCU with two fully independent combustion chambers, hence the two large diameter exhaust pipes at the rear of the funnel casing. GCU were first successfully tested with the main engines at quay but their real challenge were the full size gas trials. These, are for this equipment certainly the most demanding part of their operational life for many reasons. First, it is during these test that the regulation laws and full operational domain of the different components ( boil off gas compressors, heat exchangers, fuel pumps, forced vaporisers, dual fuel engines , etc…) of the complete natural and forced boil of boil gas management system have to be explored and secured. This leads inevitably to extreme variation in the flow, temperature and transients of the gas to be disposed safely through the GCU. Therefore to have a flexible, reliable and safe GCU during all these operations is a key for the success full completion of these tests. Second, these tests can be expected to be not only performed close or even (in the Provalys case, above!) the design flow of the GCU, but with an “aged” LNG coming from a receiving terminal which has therefore a

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low nitrogen content and, therefore, a high calorific value…Apart from the ability of meeting, with margins, these extremely demanding operating conditions, these tests were, as well, the opportunity to check other key GCU requirements such as noise or safe power black out transients..

Following “Provalys” and her sister ship for Nippon Yusen Kaisha , Snecma has received order for two vessels from Mistubishi Heavy Industry for Malaysian International Shipping Company and from Samsung Heavy Industries for Petronet confirming that the technologies proposed are definitely “fit for purpose”.

Figure 8: View of Provalys, built forGaz De France by Aker Yardsand equipped by Snecma ‘s GCU : the first Diesel Electric LNG carrier in operation, note the two

large diameter exhaust pipes at the rear of the casing… (picture credit : Channel Photography)

5 CONCLUSIONS

Snecma and NA Stordy have jointly developed optimised Gas Combustion Units for the new propulsion systems use for LNGCs.

This GCU offer superiors benefits in terms of :

- high flexibility of operations;

- maintenance free combustion chamber;

- compactness and lightweight…

The corresponding technologies have been fully qualified on a full size test bench with regard to the safety and operational requirements specified by classification societies, shipyards, ship owners and charterers.

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6 AUTHOR’S BIOGRAPHY

After two decades of engineering in space cryogenics ranging from liquefied hydrogen and oxygen tanks for the space launcher Ariane , cryocooler for satellite sensors to freezer for the International Space Station, the author has joined Snecma to develop activities in the LNG industry, derived from the know how Snecma has built up during the development and production of liquefied hydrogen and oxygen rocket engines. As a result, Snecma has delivered the Gas Combustion Units for the world first 156 000 m3 Dual Fuel Diesel Electric LNGCs for Gaz de France and Nippon Yusen Kaisha, and has been ordered two others from Mitsubishi Heavy Industries for Malaysian International Shipping Company and one from Samsung Heavy Industries fro Petronet.. Snecma is currently proposing as well other innovative equipment’s for LNG carriers such as cargo valves and pumps….