As more operators in the marine industry consider using liquefied natural gas (LNG) to power their vessels, dealing with boil-off gas (BOG) is the critical step in shifting to the fuel source.
LNG is on the upswing in the marine market in part because of concerns over emissions and the fuel’s relative price advantage over low-sulfur fuels.
“Customers are attracted to LNG propulsion for a variety of reasons, and it’s being considered for many vessel types besides LNG carriers,” said Markus Tauriainen, Manager Exhaust & Combustion System Sales at Alfa Laval. Alfa Laval technologies and expertise – especially in the area of boilers – can help smooth the transition.
Cost and environmental considerations
Tauriainen said the interest in LNG really stems from cost and environmental considerations—usually a combination of the two. LNG propulsion can offer a fuel price advantage, and the increased availability of LNG today makes that advantage more accessible, he said.
How the factors of cost and emissions weigh against each other varies from customer to customer, but Alfa Laval usually enters the discussion after customer has already decided that LNG is the best option, Tauriainen said. At that point, the discussion with Alfa Laval focuses on how to enable the fuel choice that’s already been made.
Tauriainen said a simplified decision chain for a ship owner might be as follows:
- Evaluation of fuel types for propulsion (LS fuel oil, HFO with scrubber or LNG)
- Decision based on fuel availability and analysis of CAPEX vs OPEX, as well as compliance factors and perceived green advantages
- Following a decision to go with LNG, determination of how best to create a safe and easily operated system (GCU or boilers) within the vessel’s constraints
In order to use LNG as fuel, vessels must be able to manage boil-off gas (BOG), the evaporated gas that can increase tank pressure. Alfa Laval is well known for BOG management on LNG carriers, where the Alfa Laval Gas Combustion Unit (GCU) is designed to safely burns BOG in compliance with the International code for Gas as Cargo (IGC). When it comes to using LNG for propulsion under the International code for Gas as Fuel (IGF), Alfa Laval is also managing BOG with dual-fuel Alfa Laval Aalborg boilers – and is tipping the balance in LNG’s favor for a far larger number of vessels.
When an LNG fuel tank is incorporated into a vessel, the pressure created by BOG must be kept at a level that avoids ventilation to atmosphere via the safety valve. Reliquefaction of the gas is possible, but doing this on board can add too much complexity and cost for most vessels. Likewise, burning the gas in auxiliary engines or gensets requires pressurization, which demands a redundant expensive compression train.
“On some vessels you can use a Type C tank to allow pressure to accumulate,” says Tauriainen. “But what most vessels need is a safe solution for burning off the gas at low pressure. That can be a GCU on LNG carriers, but the best and most economical solution for vessels using LNG as fuel is usually a dual-fuel boiler.”
A boiler, either for producing steam or for heating thermal oil or water, is equipment that vessels need anyway. Since an Aalborg dual-fuel boiler can be fired with LNG, it can take on the additional safety function of BOG management.
“Boilers are a compact, simple and highly affordable way to handle boil-off gas,” Tauriainen said. “As strange as it may sound, our Aalborg dual-fuel boilers have been the enabler for LNG propulsion in many, many projects.”
Aalborg boilers, like the GCU, have yet another advantage when it comes to BOG management, he said. Every fifth year, when a vessel enters dry dock for inspection, the tank environment must be made non-hazardous by replacing the LNG with inert gas. During this process, Aalborg dual-fuel boilers are capable of burning the mixture of inert gas and LNG.
“Inert gas itself doesn’t burn, so the mixture resulting from the inerting process can’t be fed to an engine or genset,” Tauriainen said. “It can be fed to an Aalborg dual-fuel boiler, however. So the boiler offers a time-saving and economical solution to this recurring issue, which otherwise has to be solved by visiting an onshore disposal terminal before and after dry docking.”
Retrofit solutions are also possible, and Alfa Laval expects retrofits to pick up in the coming years, Tauriainen said. However, the retrofit market may not expand as much or as quickly as the newbuild market has.
Compared to building in LNG from the start, conversions often present much larger cost or disruption obstacles. The decision to perform a conversion is driven by the same criteria as for newbuilds—cost and environmental compliance—but factors like vessel age weigh heavily. Whether or not a vessel was designed/supplied as ‘gas-ready’ can make a huge difference with regard to a possible conversion, Tauriainen said.
Alfa Laval has supplied gas-ready boiler systems, although doing so doesn’t completely eliminate conversion costs and disruption time. For example, the gas valve unit still needs to be procured, engineered and installed. But it does ensure that the original system is supplied with sufficient space and other allowances to avoid having to rearrange or re-engineer the whole engine room for a boiler conversion. If supplied gas-ready, LNG-related equipment can be slotted in with relative convenience as part of a conversion project.
Aalborg’s dual-fuel solutions are designed with boil-off gas in mind and are influenced by our experience with the GCU, Tauriainen said.
“The boiler uses LNG efficiently for heating, but by managing the boil-off gas and making LNG propulsion possible, it also enables far greater savings,” he said. “GCU or boiler, steam or thermal fluid—we have a solution to make LNG propulsion happen, on any size or type of vessel.”
Since every aspect of BOG management is linked to safety, Alfa Laval’s organizational strengths also come into play, he said. As a full-scope marine supplier, Alfa Laval provides not only in-house engineering and manufacturing, but also peace of mind through commissioning and a well-established global service network.
“Responsiveness is crucial in safety applications,” says Tauriainen. “We’ve spent decades building up our boiler services, and we can meet vessels at their next port in the vast majority of cases. In an emergency, we also have field service engineers who can be dispatched to sea.”
In fact, Alfa Laval service expertise is closer than ever, thanks to a growing range of digital services. Alfa Laval BOG solutions all make use of Alfa Laval Touch Control, a next-generation control solution with an easy-to-use touchscreen interface. As well as providing crews with common controls for all BOG-related equipment, Alfa Laval Touch Control offers a platform for data-driven maintenance and optimization.
“Customers today want immediate solutions for converting to LNG propulsion,” says Tauriainen. “They’ll find them at Alfa Laval. But we’ll also support customers as time goes on, so that they can achieve greater savings and peace of mind over their solution’s lifetime.”