LNG as a marine fuel – its hazards and benefits
While it is usually from a fossil source, natural gas can also be produced from the decomposition of waste products. Natural gas is predominantly methane and is only referred to as LNG when it has been liquified which can be done either by refrigeration or pressurisation. Usually refrigeration is the chosen method for ease and safety of non-pressurised storage or transport.
Liquifying the gas is necessary because of the fact that it reduces the volume to around 1/600th of that of natural gas in its gaseous state. If this was not done, the fuel tanks needed for a vessel would be so large as to preclude almost all cargo space. An alternative to liquifying natural gas for use as fuel is for it to be compressed. Compressed Natural Gas (CNG) occupies about 2.4 times the volume of LNG at 250bar pressure for the same amount of energy density. The very high pressure under which CNG is stored is a safety risk and under the IGF Code, CNG tanks must not be housed under deck on a ship.
Even with the volume reduction that liquifying the natural gas allows, LNG still cannot match the energy density of oil fuels. Typically, oil will occupy just 60% of the space that is needed for LNG for the same energy quantity. In practice, the special construction need for LNG storage tanks makes the actual space requirement nearer a factor of two to two and a half. This is one of the main reasons why LNG has been consistently disregarded as economic by operators of long haul ships.
LNG is often talked about as though it is a product with a universal formula that will not vary wherever it is taken on as bunkers. This is very far from the truth. Natural gas in its raw state is in fact a mixture of various hydro carbon gases and its composition can vary quite considerably as shown in the table below.
Typical Composition of Natural Gas as extracted
Methane CH4 - 70-90%
Ethane C2H6 )
Propane C3H8 ) 0-20%
Butane C4H10 )
Carbon Dioxide CO2 - 8%
Oxygen O2 - 0-0.2%
Nitrogen N2 - 0-5%
Hydrogen sulphide H2S - 0-5%
Rare gases A, He, Ne, Xe - trace
Raw natural gas is processed to remove most of the non-methane gases and contaminants before it is liquified and made available for use but the resultant LNG will still vary in composition. After treatment, LNG typically contains more than 90% methane. It also still contains small amounts of ethane, propane, butane, some heavier alkanes, and nitrogen. The purification process can be designed to give almost 100 percent methane but if it does become a commonly used marine fuel, it is very likely that there will be quite a divergence in composition of LNG from various sources.
The use of LNG as a marine fuel has been promoted almost entirely on the claimed environmental advantages when compared to residual oil fuels. Being almost pure methane, LNG produces less of the three main pollutants attributed to shipping.
SOx is almost entirely eliminated since it is mostly absent from the fuel. This is the reason why LNG is touted as the easiest method for ships to meet the 2020 global sulphur cap – although that presupposes that the ships have dual-fuel or gas burning engines.
The chemical composition of methane means that there is less carbon and bonded nitrogen than in oil fuels. This means that there is a 25% reduction in CO2 and an 80% reduction in NOx. However, the picture is not all rosy and LNG can have a downside.
A study made in December 2013 on-board a cruise ferry running on LNG in the Baltic Sea. The ship was equipped with lean-burn dual fuel engines, using MGO as pilot fuel. Emissions were measured under different engine loads and both LNG and MGO were used for propulsion. When using LNG for propulsion a small amount of MGO (1-5% of total energy) was injected to ignite the LNG.
As expected, the measurements revealed that emissions of particulates (both number and mass), NOx and CO2 were all considerably lower for LNG compared to MGO and other marine fuel oils. However, emissions of carbon monoxide and total hydrocarbons were higher. Analysis of the exhaust gases showed that around 85% of hydrocarbon emissions from LNG were methane.
Emissions of unburnt methane known as the ‘methane slip’ were around 7g per kg LNG at higher engine loads, rising to 23–36g at lower loads. This increase could be due to slow combustion at lower temperatures, which allows small quantities of gas to avoid the combustion process. These escaped emissions are significant, as methane has a global warming potential which is 28 times higher than that for CO2 over a 100-year perspective, and 84 times higher over 20 years. Although overall particulate emissions were lower from LNG than MGO, LNG particulate emissions were dominated by very small (ultrafine) and volatile particles, while combustion with MGO resulted in a smaller fraction of these particle types.
LNG’s ‘clean’ characteristics are likely to make it a significant marine fuel in the future, but its operational issues will likely have a major impact on ship design and construction. A leak of LNG to the outside of the ship will not be the polluting event that it would be in the case of oil fuels. Under such circumstances, the gas would rapidly dissipate.
However, the cryogenic temperatures and low flash point could have very different consequences if the leak is inside the ship. The very many risks are covered by the IGF Code as are the protection measures needed to address them. Unlike oil fuels which, although they may deteriorate over long term storage, do not pose particular problems, LNG will slowly evaporate when stored so a means to deal with boil off gas is required.