A new reliquefaction technology could offer environmental benefits coupled with lower capital costs
Environmental concerns, such as the widening of Emission Control Areas (ECAs), IMO’s 0.5% sulphur cap regulations, and the Energy Efficiency Design Index (EEDI) to reduce CO2 emissions by 30%, are all driving demand for LNG as a bunker fuel.
As a result, there is a steadily increasing fleet of LNG bunker vessels coming into service around the world. The issue of boil-off gas (BOG) as a result of liquid vaporising due to heat ingress into the cargo containment system is of particular relevance for these vessels.
In a paper titled An Innovative Approach to Re-Liquefy BOG Through Cold Energy Recovery presented at Gastech by Keppel Gas Technology Development’s R&D product manager Ravindu Atapattu, process engineer William Kim, project manager Samuel Chong and executive director Dr Foo Kok Seng, the drawbacks of existing methods of tackling BOG and the potential benefits of cold-energy recovery were outlined, with a particular focus on bunker vessels and small-scale LNG carriers.
Simply burning off the BOG in a gas combustion unit (GCU) is one of the most straightforward and lowest-cost methods. But given that the process of thermal oxidation releases greenhouse gasses, this rather defeats the environmental argument for using LNG as a means of lessening greenhouse gases (GHGs).
The authors of the paper cite the example of a 7,500 m3 LNG bunker vessel (LBV), which would produce approximately nine tonnes of BOG per day. When in idle mode, such as when awaiting a receiving vessel, the LBV’s fuel gas consumption is reduced by half in comparison to normal navigational mode, meaning that 50% of the BOG generated will need to be oxidised. In addition, if the bunkering takes place with vapour return, the additional BOG from the receiving vessel will also need to be handled by the LBV. The report’s authors note that burning one tonne of LNG generates 2.75 tonnes of CO2 emissions.
Another option involves reliquefaction based on the Brayton cycle, where the BOG is converted into liquid, which brings down the gas pressure of the tank gradually and allows the liquid to be maintained at an equilibrium saturated temperature.
Alternatively, the LNG can be directly cooled to a temperature a few degrees below the LNG saturation temperature, and the resulting relatively cold liquid can be sprayed from the top of the tank top to recondense BOG.
Reliquefaction or direct cooling can be achieved using gas expansion-based refrigeration systems. This option has been in consideration since the early 1970s but has been economically unfeasible until relatively recently. In recent years however, nitrogen expander-based BOG reliquefaction plants have become a popular option for larger LNG fleet operators.
Still, gas expansion liquefiers are operationally complex, require a lot of pieces of equipment – including multiple compressors and expanders – have a correspondingly higher footprint and also a higher capital cost. There are also safety concerns surrounding storage of flammable refrigerants. The report’s authors also question whether this method is suitable for small LNG carriers or LBVs, with a capacity range of 7,500 m3-30,000 m3.
The Stirling cycle is another method of reliquefying or direct cooling LNG. It involves compression and expansion of helium in a closed cycle to produce cryogenic energy to reliquefy or cool down LNG. The main drawback of this method is per-unit capacity limitations and the need for multiple units to meet required capacity. The report’s authors note that to achieve a 12 tonne per day reliquefication capacity, 10 units of cryogenerators would be required.
To address these deficiencies, a technology based around the reliquefaction of BOG through cold-energy recovery has been developed, primarily aimed at LNG bunker vessels and smaller LNG carriers. This technology uses cold energy recovered from vaporising LNG to liquefy all BOG generated from the cargo containment system. ABS has awarded Approval in Principal to the design.
In a design comparison study based around a 7,500 m3 bunker vessel, this new technology was found to consume more than 50% less power than any of the existing alternative methods, with lower capital expenditure than any method aside from gas GCU, and a considerably smaller total package footprint than other reliquefaction technologies.
Dr Foo told LNG World Shipping: “We feel that our solution is lower in cost and footprint compared to [current] BOG reliquefaction solutions, as we leverage existing assets and maximise BOG cold energy.
“For opex it should be comparable with current technology; however, in a carbon-tax environment, our solution will [result in savings] for customers.”
Keppel has finished simulations of the technology and is ready to start exploring the solution with potential customers, Dr Foo added.