2016_1 Boil-Off Gas handling onboard LNG fuelled ships master

Boil-Off Gas handling onboard LNG fuelled ships

Clean burning natural gas has emerged as an important fuel for ships as the marine industry seeks ways of complying with increasingly stringent environmental regulations. These restrictions limit emissions of sulphur oxides (SOx), nitrogen oxides (NOx) and particulates. The options for compliance are to employ after-treatment systems when using conventional marine fuels, or to use cleaner fuel having fewer harmful emissions, such as natural gas.

Text: BJÖRN NYGÅRD Photo:

One drawback of natural gas is that it has very low energy density compared to traditional fuels. In order to serve as a convenient energy source, the density needs to be increased. This is done by cooling the gas to cryogenic temperatures, creating liquefied natural gas (LNG). The liquefied gas can be stored in insulated tanks, keeping it in a liquid state for longer periods. However, heat flux from the surroundings will increase the temperature inside the tank, thus causing the liquid to evaporate. The generated gas from this is known as boil-off gas (BOG). 

The larger volume of gaseous natural gas created by this BOG will increase the tank pressure. To manage this, pressure vessels are utilised to contain the pressure. For longer storage periods, however, the pressure increases might be too high, which will require alternative solutions to handle the gas pressure.

Wärtsilä, a leading developer of gas and dual-fuel marine engine technologies, has extensively studied the handling of BOG onboard LNG fuelled ships. This article is based on these studies.

2016_1 Boil-Off Gas handling onboard LNG fuelled ships 2
Fig. 1 - Simplified system layout for BOG handling, when using two-stroke main engines and four-stroke auxiliary engines. Ideally consumption should match BOR, resulting in a BOG balance of zero.
2016_1 Boil-Off Gas handling onboard LNG fuelled ships 3
Fig. 2 - Illustration of an isochoric temperature increase of methane from -162°C at atmospheric pressure to 45°C (upper design temperature for worldwide service). The result is a pressure of around 60 bar.

BOG handling requirements

There are various means of handling the pressure build-up in LNG tanks. One is to contain the pressure for the ambient temperature of the fuel. Other methods include reliquefaction, thermal oxidation, and pressure accumulation. The IGF code – the international safety code for ships using gases or other low flashpoint fuels – also accepts cooling of the fuel in a liquid state. For reliquefaction, a direct system, where the evaporated gas is compressed and condensed before being returned to the tank, is one solution. The other alternative is an indirect system, where the gas is condensed or cooled with an external refrigerant, without being compressed.

Apart from handling the maximal BOR in the tank, the selected method also needs to cope with zero or low BOR’s. In the case of failure, the system must provide a redundant system that can maintain the tank pressure.  Venting gas to the atmosphere is not an alternative for pressure control, and is only allowed in emergency situations. 

Reliquefaction

Liquefaction is the process where, using a refrigerant cycle, warm gas is cooled and condensed into a liquid. Reliquefaction indicates the process whereby evaporated LNG is cooled and reverted to a liquid state.

ts, a very high thermodynamic efficiency can be achieved. Conversely the process is complicated and requires a large number of components, meaning the size requirement is large and the capital cost is high. The high efficiency and high investment cost makes it suitable for large land based liquefaction plants. 

Mixed refrigerant

Mixed refrigerant liquefaction is also based on the Rankine cycle. However, contrary to cascade cycles, a blend of refrigerants is used to obtain a close following of the natural gas cooling curve. By mixing refrigerants a temperature glide can be attained, which means the temperature at phase change will not be constant. This is because the components in the mixture evaporate at different temperatures, causing a change of concentration, which can be adapted to the process gas cooling curve. In reality, the mixed refrigerant will cause a curved temperature profile, which will lower the thermodynamic efficiency, compared to the cascade cycle. The mixed refrigerant process is suitable for small-scale liquefaction plants where the low equipment count and simplicity can be a substitute for high efficiency. 

Expander cycle

The expander cycle differs from the other liquefaction cycles by using an expander instead of a J-T valve. The expander is connected to the compressor, and extracts useful power from the compressed gas. The refrigerant used is a pure gas, and is only in gaseous phase, making it insensitive to motion. This also eliminates issues relating to the distribution of liquid refrigerants in the heat exchangers, thereby allowing rapid start-up. A gaseous phase refrigerant, however, has a limited enthalpy difference, and requires a higher refrigerant flow than two-phase refrigerants, which limits the capacity. The process does not follow the cooling curve of the process gas very well, which results in lower efficiency than with other technologies. This, on the other hand, makes the process more forgiving to variations in the gas composition.

Most expander processes utilise the reversed Brayton cycle, either closed or open loop, to generate cooling. This is done either in a single or dual stage or with pre-cooling. By using an open-loop expander cycle, a fraction of the process gas is utilised as a refrigerant. This eliminates the need for excess refrigerants.

The reversed Stirling cycle is another type of expander process used for liquefaction. The Stirling cycle is a modified Carnot-cycle, where heat from the compression stage is utilised in the expansion stage, making it a regenerative cycle. 

2016_1 Boil-Off Gas handling onboard LNG fuelled ships 4
Fig. 3 - The three-stage cooling curve of natural gas (15–-161°C), with pre-cooling, liquefaction and sub-cooling.
2016_1 Boil-Off Gas handling onboard LNG fuelled ships 5
Fig. 4 - The Wärtsilä LNGPac system allows the safe and convenient utilisation of gas fuel.

Offshore reliquefaction

The selection of liquefaction technology for offshore applications differs from the onshore equivalents. Space on marine vessels is limited, which increases the need for a compact solution.

The use of hazardous hydrocarbons has to be limited for safety reasons.

Small-scale offshore reliquefaction is, from a capacity perspective, quite similar to onshore peak-shaving plants. The expander cycle is a proven technology for these small-scale plants and is a viable choice for offshore reliquefaction. 

Thermal oxidation

Another method for handling BOG is by thermal oxidation, i.e. combustion. This is primarily done by feeding the excess gas to the consumers, i.e. the ship’s engines. Two- and four-stroke internal combustion engines are normally used for propulsion and power generation, while two-stroke engines usually have a high power output and are used for direct propulsion. Four-stroke engines can be used both as main and auxiliary engines, the latter being used while in port as well as when at sea. Additionally, auxiliary boilers can be used to produce steam or hot water.  If the amount of BOG does not correspond to the rate of consumption, the gas can be fed to a gas combustion unit (GCU). The GCU is a burner which combusts the BOG in a controlled manner without the risk of releasing unburned natural gas to the atmosphere. Although a possible solution for BOG handling, no useful energy can be recovered from a GCU, which is why it should primarily be recovered by other means.

Compression

Feeding gas to the engines is one way of handling BOG in the tanks. Four-stroke engines usually have a suitable fuel pressure need for type C tanks and can consume the gas at tank pressure. Two-stroke engines, however, demand a higher pressure. Therefore, in order to consume the BOG, the pressure must be increased to that required by the engines. 

When choosing the compressor type, pressure ratio and gas flow are the most important aspects that need to be evaluated. For safety reasons when using LNG as feed gas, contamination from lubricants and the risk of gas leaks need to be considered. Either a piston or a rotary screw compressor should be used for gas flows below 1000 m3/h. Piston compressors have a compression ratio suitable for high pressure engines. Screw compressors, with their lower compression ratio, are suitable for low pressure engines. 

In oil-free piston compressors, non-contact seals are usually used between the piston and cylinder. To minimise leakage, the contact surfaces are lined with sharp edges, called labyrinth seals. Rotary screw compressors can also be designed to operate without lubricants. These compressors are driven by synchronised gears, making small clearances possible without rotor contact.  

Fuel sharing

In order to match BOG generation with engine consumption for a desired load, fuel sharing can be utilised. Dual-fuel engines are capable of running on both diesel and gas, which can be used to even out variations in gas supply or quality. With normal gas operation, around 1—5% of the pilot fuel is needed to ignite the gas. With fuel sharing, the amount of gas can be varied between around 15% and 85%, with the rest being diesel. 

Summary

There are basically three suitable BOG handling methods which should be considered, namely boilers, auxiliary engines or reliquefaction units. Auxiliary engines are more suited to gas consumption than main engines. Additionally, the power generated is usually needed - even in port. Reliquefaction units using an expander cycle have rather low efficiency, which means they should be avoided for large BOR. For such cases, thermal oxidation in either a boiler or an auxiliary engine is a better solution.

By comparing the different BOG handling methods, it is clear that there is no universal solution that works for all systems. On the contrary the solution is rather sensitive to tank size and consumer types. This means that the BOG handling solution has to be evaluated on a case by case basis.

 

Leave a comment

Load more comments

Related articles