The use of Liquefied Natural Gas (LNG) in transport is a suitable option to power, large long distance trucks in areas where gas is transported as LNG because there are indigenous gas supplies and no gas network. The use of LNG in passenger cars is far less viable because on average passenger cars stand idle more often, which would give rise to high evaporative losses. The use of LNG requires storage facilities for the cold (-162 0C) liquid natural gas at the roadside refueling stations and special fuelling equipment which can handle cryogenic temperatures. In addition, the trucks must be equipped with special dual fuel engines to be able to use LNG. Moreover, the fuel tank on board of the truck needs to be adapted for LNG usage. These requirements make the use of LNG relatively expensive. Nevertheless, the use of LNG in the transport sector can still have substantial environmental benefits. It is reported that a truck powered by a dual fuel LNG-diesel engine can emit up to 75% lower NOx emissions and about 13% lower Well-To-Wheel CO2 emissions compared to diesel powered trucks. Overall, the technology to use LNG as a transport fuel is well developed, but is expected to remain a niche market.
Although natural gas is a fossil fuel, it is the cleanest burning fuel available today. The increasing world wide demand for cleaner energy gives leads to an increasing trade in liquefied natural gas. The natural gas is liquefied by cooling it to approximately −162 °C. Liquefied natural gas occupies a much smaller volume than compressed natural gas which makes it much more cost-efficient to transport over long distances to places without a local source for natural gas, and where pipelines do not lead to. Where moving natural gas by pipelines is not possible or economical, LNG can be transported by ships or by trucks equipped with special cryogenic containers. At its destination the LNG is re-gasified and distributed as pipeline natural gas, but using LNG in heavy duty trucks can be a viable option as well.
Special LNG trailers can deliver the liquid fuel from the storage tanks to LNG fueling stations. At the site of the fueling station the LNG has to be stored in very well insulated tanks. The insulation only, however, will not keep the temperature of LNG low enough. LNG is stored as a so called "boiling cryogen", a very cold liquid at its boiling point which stays at the low temperature by evaporative cooling. (McMullen et al, 2002) As long as the LNG vapor is allowed to leave the storage tank, the temperature will remain constant. At the refueling site this boil off natural gas vapor can be compressed to CNG (Compressed Natural Gas) and be sold to passenger cars. This fuel type is called “LCNG”, Liquefied Compressed Natural Gas and is preferable for passenger cars.
The use of LNG in vehicles is a tradeoff between the duty cycle (i.e. the time in operation) of the vehicle and the evaporation rate (boil off rate). In general the annual mileage of passenger cars is too low to compensate for the boil off losses. However, the direct use of LNG can be an attractive alternative for heavy duty vehicles, which travel high mileages in one go.
LNG is essentially a fuel for the niche market (McPherson,1999). Liquefying and shipping natural gas is expensive, making the LNG route only attractive for areas where there is a shortage of indigenous gas supplies and where competition from pipeline gas is limited. In addition, the use of LNG requires large investments in terminal and fueling infrastructure. In large quantities LNG can only be transported by sea, so its large-scale use is confined to locations which are accessible via a port. There are only a few LNG projects worldwide, and most of them supply East Asia, which lacks local resources.
Boil off (evaporation) losses in the fuel tank of the vehicle require a high mileage of the vehicle and make the direct use of LNG only economically feasible for Heavy Duty trucks. LNG has a much higher storage density than compressed natural gas, making it more suitable as an alternative to diesel fuel than compressed natural gas (California Energy Commission, 2006). However, the heavy duty trucks need to be equipped with a special natural gas diesel dual-fuel engines (Frailey, 1998). Moreover, sufficient special refueling stations are needed with a storage tank for the cold liquid natural gas. The boil off losses at the site of the fueling station can be compressed to CNG and be used in passenger cars.
Although the technology is mature, it is a niche technology to be used for trucks in areas where there is access only to liquefied natural gas. There are a few pilot projects in the world which supply LNG to trucks such as the Clean Energy California LNG Plant (Clean Energy, 2008). The new Clean Energy California LNG Plant in Boron, California began startup operations in November 11, 2008. It was built to produce up to 160,000 gallons of LNG per day and has a 1.5-million-gallon LNG storage tank for reserves to meet unexpected LNG demand from customers and to accommodate plant maintenance procedures. The plant is expected to be the major supplier for the Clean Truck Program of the ports of Los Angeles and Long Beach. Within this program, it is envisioned to add up to 8,000 LNG trucks to serve the need for moving goods at the ports.
Although natural gas is a fossil fuel it is the cleanest burning fuel available today. It can be used in the form of liquefied compressed natural gas (LCNG) to fuel cars or liquefied natural gas (LNG) to fuel trucks, or as compressed natural gas. LNG is a much cleaner burning fuel than diesel. Nitrogen oxide (NOx) emissions reductions of over 75% (Dual Fuel technology) compared to the diesel vehicles being replaced have been reported by the manufacturers (Frailey, 1998). Therefore LNG and LCNG can be used to improve the local air quality in urban environments. LNG can for example be used to power refuse collection trucks, in urban settings, because of their high duty cycle.
It has been shown that the Well to Wheel (WTW) CO2 emission of LNG can be about 13% lower than the WTW CO2 emission of a diesel powered truck (Kroon, 2009). However, this percentage may differ depending on the origin of the LNG and hence the energy it takes to transport the LNG to its destination. The LNG transport ship can lose up to 6% of the natural gas due to evaporation. Furthermore it is assumed that the efficiency of a LNG truck engine is equivalent to the efficiency of a diesel truck engine. This is only the case for so called natural gas diesel dual fuel engines (Frailey, 1998). The natural gas diesel dual-fuel engine combines the advantages of natural gas with the diesel engine’s high efficiency rating, which is about 20 percent superior to that of the compressed natural gas engine. However, few truck manufacturers are building these natural gas diesel dual-fuel engine. It is estimated that the he overall greenhouse gas emission saving of LNG powered trucks compared to conventional diesel trucks is about 10%. (Kroon, 2009)
Most LNG in the world is transported via ships equipped with special cryogenic tanks. A 2006 study estimates that natural gas can be economically produced and delivered as LNG in a price range of about $2.5-$4.6 per Giga Joule, depending largely on terms established by producing countries for exploration and production investment and shipping distance and cost (Centre for Energy Economics, 2006). In most cases the LNG is re-gasified in the port and transported further inland via pipelines. However, when the LNG is to be used directly in trucks, it has to be transported via special LNG trailers to the inland refueling sites. A 2006 case study in Sweden reported the following investment costs for infrastructure with a capacity of 2,4 TWh/year (Petterson, 2006) (Fig 1)
Clean Energy (2008): The Clean Energy California LNG Plant. Available at http://www.cleanenergyfuels.com/pdf/CE-OS.Boron.pdf
California Energy Commission (2006): Liquefied Natural Gas (LNG) as a transportation fuel. Available at http://www.consumerenergycenter.org/transportation/afvs/lng.html
Centre for Energy Economics (2006). How much does LNG cost. Available at http://www.beg.utexas.edu/energyecon/lng/LNG_introduction_09.php
Frailey (1998): Development of LNG-powered Heavy Duty trucks in Commercial Hauling. M. Frailey, NREL/SR-540-25154, December 1998
Kroon, P. (2009): Ketenemissies van nieuwe transportbrandstoffen: Broeikasgasemissies van winning tot verbruik (Concept rapport). Unpublished
McMullen et al, (2002): New technologies and alternative fuels: Working paper on Alternative propulsion and fuel technology review. John J. McMullen Associates Inc with Booz Allen Hamilton
McPherson, C. (1999): Natural Gas, private sector participation and market development,
Petterson (2006): LCNG Study - possibilities with LNG supporting supply of methane as a vehicle fuel in Sweden. Vattenfall 2006-11-21: A. Petterson, S. Liljemark and M. Losciale.
Roeterdink, W.G., Uyterlinde, M.A., Kroon P. and Hanschke, C.B. (2010): Groen Tanken: Inpassing van alternatieve brandstoffen in de tank- en distributie infrastructuur. ECN report E—09082