Greater design efficiency and the application of an array of proven technologies will provide a pathway to longer term emissions reductions, writes Rene Sejer Laursen, director of fuels and technology, ABS.
Like many specialist vessels, LNG carriers are built for 25 years or more of operation, so how easily will LNG carrier owners find compliance with tightening IMO regulation on carbon emissions and intensity and ultimately move towards net zero carbon? The answer may lie in fully separating the vessel’s cargo containment and propulsion systems, employing renewable fuel in the engine and reliquefying the boil off gas.
At present, LNG carriers have the option to re-use gas that has boiled off from the cargo in a dual fuel main engine, though at the scale most LNG carriers carry gas, this is most likely to be produced from fossil fuels. This provides reductions in SOx, NOx, PM and CO2 emissions in operation but does not account for the gas’ lifecycle carbon footprint or methane slip.
Faced with decisions over which propulsion systems to use for newbuildings, the options appear numerous, but in reality are closer to three.
These include high pressure two-stroke gas injection engines which have high efficiency – generally beneficial from an EEXI perspective – while minimising methane slip. The first gas engines system in service are equipped with cylinder cut-out in gas mode intended to maximize gas use. This can be important in meeting regulations requiring lower CO2 emissions in daily operation. The IMO’s decision on whether to include other greenhouse gases alongside CO2, the high pressure engine gives further some degree of future-proofing.
High pressure gas injection engines can also be equipped with Selective Catalytic Reduction or Exhaust Gas Recirculation system of the owner’s choice to enable compliance with NOx Tier III standards
The next well-established choice is low pressure two-stroke gas injection principle. In their latest iterations, these are increasingly fitted with an Exhaust Gas Recirculation type of system to create a combined solution to reduce methane slip and increase engine efficiency when using a low-pressure engine. This engine solution offers a capex competitive solution when looking at the total cost including the cost for the fuel gas supply system.
The third option is four-stroke low pressure diesel-electric, examples of which have been ordered recently for LNG carriers operating in ice conditions. These days this solution is considered less efficient compared to gas-injection two-stroke engines, when direct propulsion is mostly needed. Diesel-electric’s suitability lies in the power that it can generate – in cases of very high electricity power demand such as for some typical LNG carrier design for ice conditions, high propulsive power and electricity can be produced at high efficiency.
Both of the two-stroke gas engine solutions can be equipped with a shaft generator and clutch to produce electricity to minimise generator engine use in port and during normal operation. A clutch with a power take-off system to the main engine can count as a generator set, potentially providing some additional redundancy from a class perspective so potentially fewer generator engines need to be installed.
However adding a PTO and clutch is a function of how much electricity is needed versus how much propulsion power, so the benefit is lower fuel consumption (and thus lower CO2 emissions) and lower maintenance costs.
A battery might also be needed to manage the power from a two-stroke engine as this may not be as stable as a four-stroke at high rpm. Capacitors can also be used in place of batteries, providing a cheaper option but one with much shorter term storage.
As this suggests, increasing efficiency has often been a process of applying additional technology, such as a PTO or a waste heat recovery system, though previously such decisions were related more to fuel prices than regulation. Today we expect this type of system will likely be more common as a means to reduce emissions and future proof the system.
The need to increase efficiency on LNG carriers in future suggests a bigger leap could be considered, separating the established relationship between the containment system from the propulsion.
Choice of containment system is very much the result of which shipyard is selected since each has its preference and has knock-on implications for the gas handling system and ultimately on the choice of engine.
The efficiency of gas containment systems is such that boil off rates of 0.05/7% per day are not likely to fall much further. The issue is that to capture and use this gas in the engine adds to the CO2 budget and limits reduction potential since it is derived from fossil sources.
By applying full reliquefaction to boil off gas or sub-cooling the cargo to remove it completely, the owner/operator could use bioLNG as fuel and radically reduce their carbon emissions on a well to wake basis. Both high and low pressure injection engines can use biofuel right away with only minimal changes required to the engines. It should be remembered that not all biofuels provide carbon neutrality, though biofuel produced from biowaste has the biggest potential for the lowest carbon footprint.
This could provide a way forward for newbuildings in the LNG carrier market, with better control of methane slip in the engine and a renewable fuel source, compliance with 2030 and even 2050 targets could become easier to achieve.