- Supporters of LNG as a marine fuel contend, it offers immediate environmental benefits and a clear path to decarbonisation.
- On the other hand, critics focus on the problem of methane slip.
- LNG as a marine fuel is considered the cleanest burning fossil fuel, virtually eliminating or significantly reducing NOx, SOx and particulate matter.
- Carbon dioxide (CO2) emissions, too, are about 25 to 28% lower as compared with liquid fossil fuel counterparts, according to a study by consultancy Thinkstep.
- The study was commissioned by industry organisations SEA/LNG and The Society of Gas as a Marine Fuel (SGMF).
A recently published article in Riviera by John Snyder reveals the use of LNG and its benefits besides it also talks about the problem of methane slip.
A growing segment of shipowners invested in LNG
While over 90% of the world’s oceangoing fleet opted for very low sulphur fuel oil to comply with IMO’s 2020 0.50% global sulphur cap, a growing segment of shipowners invested in LNG-fuelled tonnage. LNG-fuelled newbuild orders surged in 2019, with shipowners contracting for 100 vessels in 2019, treble the annual average ordered between 2016 and 2018, according to class society DNV GL. As a result, the LNG-fuelled vessel orderbook grew to 205, with deliveries stretching to 2027. The current fleet stands at 182, with cruise ships, ferries, container ships, tankers, car carriers and offshore vessels comprising major segments.
“We see LNG an alternative that provides the benefits of global health and complies with IMO 2020, and it moves us forward in the right direction,” SEA/LNG chairman Peter Keller tells LNG Shipping & Terminals.
He added, “There is almost no sulphur or particulate matter, and NOx emissions are reduced 90%,” he says. “In terms of carbon, in the studies we performed with Thinkstep, there are reductions of up to 28%, depending on the technology you use.”
Bio methane
With IMO decarbonisation targets set for 2030 and 2050, SEA/LNG has commissioned Dutch research consultancy CE Delft to study the future availability and costs of liquefied bio methane (LBM) and liquefied synthetic methane (LSM). The two biofuels could serve as net-zero carbon ‘drop-in’ fuels for LNG.
“In combination, the studies we have commissioned prove that, through LBM and LSM, LNG offers a clear pathway to net zero-carbon emissions from shipping while also future-proofing ship owners’ investments,” says Mr Keller. “By investing in LNG-fuelled vessels now, ship owners can realise immediate GHG benefits – up to 21% on a well-to-wake basis and 28%, tank-to-wake, including the impact of methane emissions.”
BioLNG underway
Mr Keller notes that there are several major ventures underway to produce bioLNG. In one of the largest renewable natural gas joint ventures in the US, utility company Dominion Energy and pork processor Smithfield Farms are investing US$500M in Align Renewable Natural Gas. Dominion and Smithfield plan to capture methane from hog farms in North Carolina, Virginia and Utah and convert it into renewable energy to heat homes, power businesses and fuel vehicles. The Virginia-based companies are aiming to produce enough renewable natural gas or bioLNG to supply electricity to 70,000 homes by 2029.
CMA CGM bets on LNG
One operator betting on LNG to lower its GHG emissions is French shipping company CMA CGM, which is building nine 23,000-teu, LNG-powered containerships in China. It signed a 10-year contract in 2017 with French oil and gas giant Total to supply 300,000 tonnes of LNG per year starting in 2020. In December, CMA CGM announced it had reached another deal with Total to provide 270,000 tonnes of LNG over a 10-year period for a series of 15,000-teu containerships that will operate between Asia and the Mediterranean. LNG will be supplied at the Port of Marseille-Fos.
The problem of methane slip
The Achilles’ heel for LNG in marine propulsion is methane slip, the unwanted release of methane during the supply of LNG, bunkering, or during the combustion process. As a result of methane slip, critics of LNG say any environmental benefits of the fuel quickly evaporate when it escapes into the atmosphere. This is because methane has a global warming potential 28 to 34 times that of CO2 according to the respected Intergovernmental Panel on Climate Change (IPCC) Assessment Report 5.
Various issues that impact the amount of methane slip
The amount of methane slip for gas-fuelled engines is affected by various issues and OEMs account for these during the optimisation of the engine design. The way the engine is operated, including engine speed, load and transient operations, are all factors.
Any amount of methane slip from an internal combustion engine will depend on engine cycle, whether it is a two-stroke, four-stroke, gas turbine or boiler, engine speed, fuel (single or dual-fuel) and the thermodynamic combustion cycle (Otto or Diesel).
The lifecycle analysis on the use of LNG as marine fuel report by Thinkstep contains average data for two- and four-stroke engines and is based on data from test-bed engines.
SGMF and SEA/LNG support initiatives to further reduce methane slip and welcome regulatory measures to drive this development. At the Intersessional Meeting of the Working Group (ISWG) on the Reduction of GHG Emissions from Ships held by IMO’s Marine Environment Protection Committee (MEPC) 72 in February, SGMF submitted a proposal to add methane slip to CO2 emissions to form a CO2-equivalent to be used in EEDI calculations and for other CO2 reductions and guidelines.
IMO regulations for GHG emissions
With IMO developing regulations around GHG emissions, including methane slip, engine designer MAN Energy Solutions is offering guaranteed methane slip value performance for its two-stroke, gas-burning ME-GI engines in newbuilding contracts. The methane slip guarantees are in line with heat rate and gas consumption guarantees. MAN Energy Solutions expects that continuous optimisation of the injection system will lead to further reductions in methane slip. The engine designer has set methane slip values across the entire load range and incorporated them into its Computerised Engine Application System (CEAS) calculation tool.
Wärtsilä’s take on methane slip
Meanwhile, Wärtsilä reports that methane slip in the combustion process from its dual-fuel engines has been slashed by 75% over the past 25 years and anticipates that it will make further advancements over the next three years, based on its recent successes with land-based power generation applications.
“Engine advances and emission abatement improvements in the fuel production and supply chain mean that all our gas engines will soon offer a decisive emissions advantage over diesel,” says the company.
Wärtsilä also disputes the results of a recent study released by the International Council for Clean Transport (ICCT) which contends that gas engines are worse GHG emitters than those burning conventional diesel.
Among the flaws Wärtsilä sees in the ICCT report is that it uses methane emission levels that do not reflect the latest four-stroke gas engine technology.
The ICCT study uses an average methane slip for four-stroke, low-pressure, dual-fuel gas engines (of 5.5g/kWh) that is higher than that of all but one of its engines, according to the engine designer. It says a more appropriate four-stroke engine for the analysis is its Wärtsilä 46DF. Using this engine, methane slip measured under similar conditions as those in the ICCT and Thinkstep studies would be 2.8g/kWh.
“Using the assumptions from the Thinkstep report, this level of methane slip would lead to engine emissions that are 14% lower and total well-to-wake emissions that are marginally lower than marine gas oil over a 100-year timeframe,” says Wärtsilä . It continues: “Modern dual-fuel, four-stroke engines are already better for the environment over the most widely used timeframe.”
Wärtsilä’s R&D efforts
In order to mitigate methane slip, Wärtsilä is also pouring its R&D efforts into designing a combustion chamber that minimises cooler areas and eliminates any crevices where unmixed methane can escape.
Other areas of research are focusing on the timing of gas admission and valve overlap duration. The overlap is the time that inlet and exhaust valves are open at the same time. This is often used to allow partial cooling of engine components between combustion cycles – to reduce NOx formation – but exacerbates methane slip, says Wärtsilä. Reducing overlap time, both through the engine control system and the valve train, will decrease methane slip.
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Source: Riviera
