Marine LNG Engine

A marine LNG engine is a dual fuel engine that uses natural gas and bunker fuel to convert chemical energy in to mechanical energy. Due to natural gas’ cleaner burning properties, the use of natural gas in merchant ship propulsion plants is becoming an option for companies in order to comply with IMO and MARPOL environmental regulations. The natural gas is stored in liquid state (LNG) and the boil-off gas is routed to and burned in dual fuel engines.[1] Shipping companies have been cautious when choosing a propulsion system for their fleets. The steam turbine system has been the main choice as the prime mover on LNG carriers over the last several decades. The decades old system on steam propelled LNG carriers uses BOG (boil-off gas). LNG carriers are heavily insulated to keep the LNG at around -160 °C – to keep it liquefied. What happens is that even with all the insulation, the LNG containment area is penetrated by heat which allows for naturally generated boil-off gas (BOG).[2]

Boil-off gas

The natural gas that fuels dual fuel engines is carried on ships as a boiling liquid, and transported at slightly higher than atmospheric pressure. When tank insulation is penetrated by any influx in heat, it will cause the temperature of the liquefied natural gas to rise, which allows for vaporization from liquid to gas. When heat penetrates the tank, the tank’s pressure increases due to boil-off. The insulation of the tanks is designed with the most advanced technology. Even still, the insulation of the tanks is penetrated by heat. The boil-off occurs during the ships voyage. During a storm, the LNG cargo moves and sloshes around in the tanks. The boil-off gas represents 0.1% - 0.25% of the ships capacity per day. Tanks need to be maintained at a steady pressure. If the pressure in tanks is not controlled relief or safety valves are forced to open, venting the boil-off into the atmosphere until pressure is relieved. At this point, it has been proven that onboard LNG reliquefaction is uneconomical for most ships. Instead, the gas produced by this boil-off effect is routed to the ship’s propulsion system and used as fuel for power plants such as steam boilers and dual fuel marine diesel engines. This reduces the use of bunker fuel, reducing fuel costs and equipment maintenance costs.[3]

Propulsion systems

Most propulsion systems in LNG carriers use the BOG and liquid fuels. In a steam plant, the BOG is used to fire the boilers and produce steam. The steam drives the turbines and propels the ship. The advantage to this type is that when the LNG cargo tank pressure is elevated the excessive BOG is burned simultaneously with liquid fuel. If there isn’t enough BOG, liquid fuel (heavy fuel oil or HFO) is used to keep the plant operating.[2] An alternative to the steam turbine engine is the dual-fuel marine diesel engine. Commercial ship propulsion system manufacturers such as Finland’s Wartsila and Germany’s MAN Diesel SE are producing large bore dual-fuel diesel engines. The MAN B&W ME-GI Engines have extremely flexible fuel modes that range from 95% natural gas to 100% HFO and anywhere in between. A minimum of 5% HFO for pilot oil is required as these are compression ignition engines and natural gas is not self-combustible.[4]

Cost benefits

Recent research has been focused on using LNG for fuel on ships other than LNG tankers. These studies show that LNG stands out in terms of emissions reduction and reduced operational costs.[1] Some economic incentives have been shown to be advantageous to running an LNG propulsion system. When certain systems such as waste heat recovery (using waste heat to do work rather than dissipate) are added to the power plant, significant savings can be observed. One study shows that an LNG engine with a WHR system saves money compared to a diesel engine with WHR. There is a higher initial investment cost but it is a cost efficient method and environmentally sound one.[5]

Environmental issues

Natural gas does have challenges. For example, there is an issue called methane slip. Methane slip is when gas leaks unburned through the engine. Methane has a GWP100 (100-year global warming potential), which is 25x higher than CO2. If the methane slip isn’t controlled, environmental benefits to using natural gas are reduced. Another challenge is hazards associated with the LNG being stored at very low temperatures. Insulation of the tank is critical, and there’s possibilities of structural brittleness and personnel frostbite injuries.[1] Essentially, since it is established that LNG for ship propulsion reduces CO2 and other pollutants compared to common heavy fuel oils, LNG implementation depends on these key factors: Gas availability, demand for ships, emission limits (emission controlled areas), LNG tank installation, and safety requirements.[1] Challenges related to the use of LNG should be taken into consideration. Challenges such as the lack of infrastructure in the majority of commercial ports, crew’s limited experience running engines with gas fuels, the future price of gas, and the required safety measures all are critical points to be considered.[5] U.S.-based TOTE, Inc., one of America’s largest shipping companies, announced recently that its two Washington-to-Alaska Orca-class vessels are going to be outfitted with new LNG-capable engines.[6] For companies like TOTE, installation costs of gas propulsion systems are taken into account. It can be estimated that initial investment in these systems cost more than traditional diesel solutions. However, taking into account fuel costs, economic costs, maintenance costs, and installation costs, it is possible to create cost analysis figures for the life span of the ship and an initial investment payback period.

References

  1. 1 2 3 4 Burel, Fabio; Taccani, Rodolfo; Zuliani, Nicola (2013). "Improving sustainability of maritime transport through utilization of Liquefied Natural Gas (LNG) for propulsion". Energy. 57 (1): 412–420. doi:10.1016/j.energy.2013.05.002.
  2. 1 2 Chang, Daejun; Rhee, Taejin; Nam, Kiil; Chang, Kwangpil; Lee, Donghun; Jeong, Samheon (2008). "A study on availability and safety of new propulsion systems for LNG carriers". Reliability Engineering & System Safety. 93 (12): 1877–1885. doi:10.1016/j.ress.2008.03.013.
  3. ↑ Tusiani, M. D., & Shearer, G. (2007). LNG: A nontechnical guide. Tulsa, Okla: PennWell.
  4. ↑ MAN Diesel and Turbo. (2013, Feb 28). Flexible Dual Future - MAN B&W ME-GI Engine [Video file]. Retrieved from https://www.youtube.com/watch?v=V0MVdIQYonM
  5. 1 2 Livanos, George A.; Theotokatos, Gerasimos; Pagonis, Dimitrios-Nikolaos (2014). "Techno-economic investigation of alternative propulsion plants for Ferries and RoRo ships". Energy Conversion and Management. 79: 640–651. doi:10.1016/j.enconman.2013.12.050.
  6. ↑ Newcomb, T. (2012, December 17). Natural gas–burning megaships soon to leave port. Popular Mechanics. Retrieved January 26, 2014, from http://www.popularmechanics.com/technology/engineering/extreme-machines/natural-gas-burning-megaships-soon-to-leave-port-14858070
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