The need to achieve a 50% cut in greenhouse gas emissions by 2050 has the shipping industry in a spin. LNG’s credentials as a transitional fuel are the greenest of the currently viable alternatives
The alleged environmental advantages of LNG as one of the three mainstream propulsion fuel options available to shipowners have been called into question of late.
University Maritime Advisory Services (UMAS), in a study published in June 2018, concluded that the European Union’s (EU) projected spending on LNG bunkering infrastructure would have no significant climate benefits. Furthermore, UMAS, a University College London consultancy division, has noted such investments could potentially increase greenhouse gas (GHG) emissions from shipping as a result of methane slip in the LNG supply chain.
The EU has spent US$250M to date on projects promoting the use of LNG as transport fuel for ships, inland waterway vessels and vehicles. These monies have accounted for up to 50% of the total cost of individual schemes, usually matching the private sector’s contribution.
If this level of subsidy was to be maintained over the long term, and if one of the high LNG use scenarios considered by UMAS comes to pass, the EU could be forking out up to US$22Bn through 2050. Such a high-use case might occur if LNG prices remain comparatively low and alternative fuels like hydrogen are unavailable.
The UMAS consultants concluded that such a financial commitment would deliver, at best, only a 6% reduction in GHG emissions by 2050 compared to the replaced diesel fuel. Such a level is well below IMO’s objectives of at least a 50% cut by 2050 and the goal of zero emissions sometime during the following half century.
Although it may be the cleanest-burning of the fossil fuels, LNG has some inherent drawbacks, according to UMAS. For a start, LNG only provides a 20-25% reduction in CO2 emissions compared to diesel.
Secondly, the LNG supply chain, from natural gas production at the wellhead and LNG liquefaction to LNG handling, regasification and gas consumption, is prone to some degree of methane slip. Natural gas and the LNG processed for carriage by sea and use as bunker fuel is, typically, composed of 95% methane.
Methane is a potent GHG due to its ability to trap radiation and hence promote global warming. A recent US study on methane slip by the University of Delaware and the Rochester Institute of Technology for the US Maritime Administration (MARAD) found that on a volume basis, CO2 accounts for about 82% of all GHGs from human activities in that country, while methane is responsible for 9%. However, in a tonne-for-tonne comparison, methane has an impact on climate change that is 25 times more adverse than CO2.
Losses of methane throughout the supply chain have been put at less than 2.5%. However, although methane emissions volumes are relatively small, the escape of natural gas has a disproportionately high global warming potential (GWP) quotient.
Methane slip in the context of LNG-powered ships occurs as a result of gas leaks during bunker transfers and also when a small proportion of the natural gas introduced into engine combustion chambers fails to burn and escapes through the exhaust system to the atmosphere.
The methane that escapes the combustion chamber in a gas-fuelled engine as an unburned hydrocarbon stems from the poor combustion of methane under very lean methane/air mixtures; variations in flame propagation dynamics; and “blow-by” of unburned methane during cylinder valve operations.
“When it comes to environment-friendly alternatives to oil, LNG is the only scalable and economic fuel currently available for the vast majority of deepsea ocean shipping”
The three most popular engines utilised by LNG-powered ships are: lean-burn, spark-ignited engines operating on the Otto cycle; diesel dual-fuel (DDF) compression-ignited engines, operating like a lean-burn engine on the Otto cycle, but with Diesel cycle injection to ignite the methane/air mixture; and diesel-injected, compression-ignited engines operating with natural gas on the Diesel cycle.
The first type, the lean-burn gas engines of the type manufactured by Rolls-Royce and Mitsubishi, can achieve lower downstream CO2 emissions levels than DDF engines at similar air-fuel ratios. Such engines can also operate on a much leaner fuel-air mixture and at higher compression ratios using advanced spark timing. However, such engines are more prone to methane slip than compression ignition engines
DDF engines typically use a port-injected, air/methane mixture which is ignited by Diesel cycle injection and burns with the same flame propagation as in Otto cycle combustion. Such engines, including Wärtsilä’s four-stroke and Winterthur Gas & Diesel’s two-stroke units, offer lower levels of methane slip than lean-burn gas engines.
In high-pressure, gas-injection Diesel cycle engines, such as MAN’s ME-GI units, the combustion process utilises pilot fuel ignition and is diffusion-controlled, as in conventional diesel engines. Such two-stroke engines provide high levels of reliability, thermal efficiency and fuel flexibility, as well as low levels of methane slip, to the point of being negligible.
However, high-pressure gas-injection engines do not result in NOx reductions sufficient to meet IMO’s upcoming Tier III requirements. Such units have to be provided with an exhaust gas recirculation (EGR) system and/or a selective catalytic reduction system to achieve the required NOx reductions.
As a general rule, methane slip occurs only in the Otto cycle mode, including in dual-fuel engines, but not in the Diesel cycle mode. Methane slip tends to be greater at lower engine loads and is also dependent on the composition of the natural gas and the engine speed.
All manufacturers of gas-burning engines continue to work to minimise methane slip. This is done by ongoing development of their combustion chamber technologies to improve the combustion process; using catalysts to oxidise unburned methane; and optimising turbocharging arrangements. After-treatment systems utilising catalysts for methane oxidation are acknowledged to be a technology requiring further development.
In praise of LNG
The LNG community promotes the use of gas in its liquefied form to power ships as a clean-burning alternative to the low-sulphur marine diesel oil and heavy fuel oil plus exhaust gas scrubber options. It is the only one of these three currently viable options that fully meets the requirements of IMO’s 0.5% global sulphur cap and 0.1% sulphur emission control areas (SECAs) restrictions, without the need for ancillary equipment.
CO2 is not the only component of ship atmospheric emissions; there are also sulphur oxides (SOx), nitrogen oxides (NOx) and particulate matter (PM). LNG does well when it comes to these other components; burning natural gas results in 100% reductions in SOx and PM emissions and a cut in NOx escapes of over 90%. NOx emissions vary with the gas-burning engine type.
Notwithstanding the immediate advantages offered by LNG, and the possibility of additional reductions in methane slip, Transport & Environment (T&E), the non-governmental organisation (NGO) that commissioned the UMAS report, believes that the shipping industry should reject the LNG option from the outset. “It would simply be throwing good money after bad,” declared the group, “and LNG assets could end up stranded.”
T&E called on the European Commission to amend its 2014 Directive on Alternative Fuels, which contains provisions mandating LNG refuelling and bunkering facilities. Furthermore, pointed out the group, the EU should “instead back future-proof technologies that will deliver the much greater ship emissions reductions that will be needed, including liquefied hydrogen infrastructure and jettyside charging of battery-powered vessels”.
However, considerable advances in chemistry and technology will be needed if batteries of the size capable of powering large, oceangoing ships are to be provided. A global recharging infrastructure would be required, with access to electricity from renewable energy, along with more frequent port calls to permit recharging.
Cold boxes and bunker tanks on LNG-powered Viking Grace: the roro passenger ferry was provided with a rotor sail in April 2018 in the ongoing drive for reduced greenhouse gas emissions
Future fuel challenges
SEA\LNG, the broad industry consortium that promotes the commercial case for LNG as marine fuel, was among the first to point out a flaw in the conclusions reached by the UMAS researchers. SEA\LNG states that, at the moment, alternative fuels such as hydrogen and ammonia are not economic, not available at the scale needed and unproven for shipping operations.
These alternatives are justifiably called future fuels, as the technologies on which they rely are yet to be commercialised. Of course, the shipping industry and governments need to work at developing the necessary future emissions-free fuel technologies, but huge investments over decades will be required. For the time being, when it comes to environment-friendly alternatives to oil, LNG is the only scalable and economic fuel available for the vast majority of deepsea ocean shipping.
The use of hydrogen, both with fuel cells and as a power source in its own right, has been touted. The fact that it offers zero emissions operations is a powerful draw. However, providing hydrogen in the quantities required to fuel ships, especially large vessels, poses considerable challenges.
Hydrogen can be produced by electrolysing water or from reforming hydrocarbon fuels and, of these fuels, natural gas is the most appropriate feedstock. A further technology under development which bodes well for the future is power-to-gas, whereby hydrogen gas is produced from surplus renewable electricity.
Production of liquefied hydrogen (LH2) in the volumes required to power ships entails high costs, due to the work needed to free it from other elements, as does the provision of the necessary supply infrastructure.
LH2 is a low-density substance with a boiling point of -253˚C. To store the fuel in its cryogenic state onboard ship requires large, expensive, well-insulated tanks constructed of special materials.
Hydrogen also has a wide flammability range; it burns in air concentrations in the range 4 to 75%. The provision of sophisticated ventilation, alarm and fire protection systems would be required to minimise the flammability risk stemming from a hydrogen leak on an LH2-powered ship.
Another option is to compress hydrogen rather than liquefy it. However, the energy concentration would not be so great and it would probably be impractical to utilise compressed gaseous hydrogen on larger ships engaged in longer voyages.
The provision of hydrogen for use with fuel cells also has its challenges. For large ships, fuel cells would be called upon to provide upwards of 500 kW of power, necessitating high investment costs and consideration of not only their expected lifetime, but also how the dimensions and weight of the required cells could be accommodated on the ship.
MAN has stated that the adaptation of its ME-GI engine to run on hydrogen would be relatively straightforward but believes the cost of the onboard storage arrangements and ancillary systems required for LH2 would be too high. The propulsion system manufacturer has been weighing up a solution involving the conversion of hydrogen to methanol for consumption in its existing ME-LGI engines, a variant of the ME-GI unit.
While methanol is a clean-burning, biodegradable liquid fuel, it is currently costlier than diesel and less efficient to burn. It is also toxic and contains about 67% of the energy of gasoline on a per litre basis.
LNG in the intermediate mix
Advocates of LNG as marine fuel have faith in the ability of gas-burning engines to ensure a smooth transition to an emissions-free future over an extended period. They also believe that investments in LNG bunkering over the next couple of decades will not be misplaced but, rather, will reap rewards.
Quite aside from the immediate environmental benefits resulting from a switch to natural gas from conventional oil fuels, the shipping industry is also working to further reduce atmospheric pollution caused by gas-burning engines. Greater use of biomethane from renewable and carbon-neutral biogas sources is one option.
On a grander scale, the maritime industry’s commitment to IMO’s Energy Efficiency Design Index (EEDI) has already helped reduce shipping emissions from the peak levels recorded in 2008. The EEDI regime continues to be tightened and consideration is now being given to additional conditions which might be applied to new vessels delivered after Phase 3 has been implemented in 2025.
As IMO, industry and maritime administrations work towards adopting acceptable and workable emissions reduction measures in the years ahead, LNG as marine fuel has a key role to play as a transitional power source.