“LPG”, Liquefied Petroleum Gas, is a mixture of light hydrocarbons which can be liquefied under moderate pressure at normal temperature but are gaseous under normal atmospheric conditions. It consists of propane, butane, propylene and other light hydrocarbons . Since LPG can be liquefied at low pressures at atmospheric temperature, its storage and transportation is easier than LNG. Propane is a gas under ambient conditions, but it has a boiling point of -42°C and hence by applying a moderate pressure it can be handled as a liquid at room temperature. At pressures above 8.4 bar at 20°C, propane is a liquid. LNG, when discharged into the environment is a greenhouse gas whereas LPG is not classified as such. LPG is not labeled as a greenhouse gas. LNG is due to its methane content.
Establishment of LPG as a marine fuel is a forward leap towards green shipping and cleaner air. The cost to install LPG as fuel equipment including storage is lower than that of LNG. Availability is similar to that of LNG and increased due to the evolution of shale gas. The fundamental difference is that the infrastructure for bunkering LPG is more mature and already in place at storage facilities, export terminals, coastal refineries around the world. Modern propulsion engines controlled electronically can be easily converted into LPG burning. The tsunami of new-building orders comes also with a tsunami of environmental regulations and the mandate to go to cleaner fuels and propulsion technology. The use of LPG as an engine fuel has not so far received the attention of its cousin the LNG and has not made a dent in the maritime world, being almost absent even in smaller commercial and recreational vessels. The advent of the global sulphur cap in 2020 has started to draw attention to various alternative fuels, among them the LPG. This is due to the fact that LNG is more difficult and expensive to implement. LPG availability has a healthy surplus ranging from 15 to 27 million tonnes per year. LPG prices are dropping driven by the shale gas mining. Similarly to LNG it has low emissions with no particulate matter. Emission factor of 3 for propane and 3.03 are not as low as the 2.75 one for methane but definitely better than the ones of HFO and MGO. Furthermore LPG fuelled engines do not suffer from the detrimental “methane slip”.
The development of LNG dual fuel engines like the MAN ME-GI paves the way to convert to high power density LPG fuelled engines as a natural evolution like the ME-LGI engine. Conversion to LPG is also more attractive in smaller engines. Engines burning LPG using diesel oil as pilot fuel represent the most appropriate candidates for marine propulsion. It is noted that the density of LPG is higher than the one of air, thus introducing more challenges than LNG regarding safety. At the same time smaller-cheaper piping can be used. Adequate training on LPG handling is a must for the crews in LPG fuelled vessels. LPG appears also to be more appropriate for small tankers, container vessels and ro-ro ships operating in coastal areas and on inland waterways with supply infrastructure in proximity.
LPG was not considered to be a commercially viable alternative due to its historically higher price compared to fuel oil. As a result no engine manufacture has yet offered a commercially available concept in that field. There is lack of technological development regarding LPG burning engines compared to LNG. There are three main engine-families in the market that can run on LPG: the MAN ME-LGI series in a Diesel, two-stroke cycle; the Wartsila 34SG series in an Otto lean-burn, four stroke cycle and the GE LM2500 series gas turbine. The ME-LGI engine has well-proven electronic controls and safety concepts developed for the dual-fuel ME-GI engine for natural gas operation. The ME-LGI engine concept includes the booster fuel injection valve. The valve depicted in Figure 1, was developed for low-flash-point liquid fuels, so that a low-pressure fuel gas supply system can be employed, reducing costs and enhancing reliability.
LNG-ready vessels can easily fit LPG as fuel in the near future. Short and set sea routers and ships with low fuel consumption are also prime targets for LPG fueling. Suitable vessels are also the ones operating mostly within ECAs and/or trading close to existing LPG bunkering infrastructure. LPG has higher energy density compared to LNG and cleaner combustion. It has high calorific value compared to LNG with a yield of 93.2MJ/m3 vs. 38.7MJ/m3. Thus, less LPG is required to produce the same amount of heat. It also has high-octane number with an octane rating of over 106. Its high-octane number makes it suitable for spark ignition engines (SI), while its low cetane number makes it less favorable for use in compression ignition engines. Wartsila has introduced a Gas Reformer to increase the methane number of LPG.
LPG fuelled ships must comply with the IGF Code, International Code of Safety for Ships using Gases or other Low-flashpoint fuels.
The main differences between the ME-GI and ME-LGI series are in some components and auxiliary systems necessary to address the different properties of liquid fuels. ME-GI burns gas fuel, whereas ME-LGI burns liquid fuel. Fuel injection takes place via a so-called fuel booster injection valve, which uses hydraulic power to raise the fuel pressure and thus eliminates the need for high-pressure fuel lines. The fuel supply system is shown in Figure 2. Both fuel oil and LPG injectors are mounted on the cylinder cover. The fuel oil injector is used to inject pilot oil when operating on LPG. An additional feature of the ME-LGI engine series is the sealing oil system integrated into the engine (Figure 1). This is necessary to provide the fuel injection components with the required lubrication and sealing that prevents LPG contamination of the system oil. The fuel valve train connects the fuel supply system with the engine through a master fuel valve. For purging purposes, the valve train is also connected to a nitrogen source. Typically, the valve train will be placed outside the engine room above the weather deck to improve safety. From the valve train, the fuel is fed to the engine in a double -walled ventilated pipe through the engine room. The system is monitored by hydrocarbon sensors (sniffers). If LPG vapor is detected inside the double- walled pipe, the safety system will switch to fuel oil operation smoothly and without any loss of power. ME-LGI engine uses liquid gas for injection all the way from tank to engine with non-cryogenic pumps i.e. LPG auxiliary systems are less expensive.
LPG Needs More Oxygen. For proper combustion, LPG requires an air to gas ratio of approximately 25:1 whilst natural gas requires a 10:1 ratio. The clean burning properties of LPG give reduced exhaust emissions namely in particulate matter and polycyclic aromatic hydrocarbons, extended lubricant and engine life. Exhaust fumes and smoke are practically eliminated and noxious gases are nearly eliminated as a result of the improved combustion of LPG. LPG does not contain or require any additives to improve combustion or increase the octane rating of the fuel such as lead or lead substitutes. The absence of petrol acids and carbon deposits in LPG combustion mean less wear and tear on the engine as the engine oil is not diluted and polluted with un-burnt petrol and carbon particles. Lubricating oil retains its protective power much longer and needs to be changed less frequently.
A more useful comparison between the two fuels is not price per unit, but rather BTU. BTU is a measure of heating efficiency – in the case of propane vs. natural gas, propane has more than double the BTU rating (1030 to 2490). Natural gas contains approximately 1,030 BTU per cubic foot and propane contains 2,490 BTU per cubic foot. This means that, even if you are paying more for propane on a per-unit basis, the fuel you’re buying will burn cleaner and provide thermal energy with greater efficiency.
In the selection of the fuel for a new vessel, there is no “silver bullet” solution. Drop in the price of LPG has made it as attractive as LNG. The drop of oil prices since 2014 has affected the price of LPG. Nowadays, LPG is priced equally or lower than LNG in the USA. LPG is also cheaper than HFO. A decoupling of LPG and oil prices is due to the increased production of propane from shale gas. The main advantage of LPG is the price. A liter of LPG costs less than half the price of a liter of petrol. Historically LNG has been a forerunner in gas applications but it is likely that the price and availability of LPG will attract the marine fuel market and that it will develop toward also using LPG as fuel.