Liquefied natural gas (LNG) is the liquefied form of natural gas, produced by cooling natural gas to temperatures below -260° F. It is both cleaner burning and more economical than traditional petroleum fuels. The energy content of a given amount of natural gas remains the same regardless of whether it is in the liquid (LNG) or gaseous (CNG) state. However, LNG has higher energy density than CNG and thus offers significant potential in NGV market segments where long vehicle ranges are required. Because LNG must be stored at extremely low temperatures, large insulated tanks are required to maintain these temperatures on vehicles. This makes LNG most appropriate for heavy-duty vehicles, which can accommodate the volume needed for LNG storage. LNG also requires fairly consistent vehicle use as the fuel slowly heats from the tank’s warmer surroundings, which can lead to tank venting. Typical LNG fuel tank hold times are about one week if the vehicle is not driven, but venting will not occur if the vehicle is driven every few days. Given that heavy-duty vehicles are the primary users of LNG, the development of LNG infrastructure must accommodate these vehicles, including driving ranges, duty cycles, and fueling logistics.
Basic elements of the LNG vehicle fuel cycle
- feedgas extraction
- use in vehicles
Feedgas for LNG may come from the natural gas wellhead, from pipelines, or from sources of renewable natural gas (landfills or anaerobic digestors), and liquefaction may be performed at one of seven types of facilities (see table below). Distribution of LNG is primarily performed by tanker trucks that deliver the fuel from the liquefaction facility to the station, where the fuel is dispensed into the vehicles that will use it.
LNG as a vehicle fuel has the potential to be successful in select vehicle market segments based upon favorable economics. Success for LNG is an integrated network of public access stations and LNG infrastructure across the country that can support significant penetration of LNG natural gas vehicles for long distance, cross-country travel. LNG is unlike most other transportation fuels, and an effective LNG infrastructure business model requires an integrated effort by LNG providers, station owners and operators, and prospective LNG vehicle owners. Successful LNG infrastructure implementation seeks to minimize one or more of the three main cost components of the LNG supply chain: feedgas cost, liquefaction and upgrade cost, and transportation cost.
With the exception of onsite liquefaction, LNG fueling station requirements and design are independent of the LNG infrastructure pathway. Profitable and sustainable LNG infrastructure development requires careful selection of station locations and capacities and maximum use of standardized designs, in addition to targeting specific market segments for LNG penetration. LNG stations that dispense LCNG (CNG produced from LNG) have the benefit of supporting both natural gas fuel types. With strategic expansion of an LNG infrastructure network in specific regions, successful capture of the LNG Class 8 tractor market promises attractive economics and large market potential.
LNG Station Design
LNG fueling stations generally receive their LNG supply from a liquefaction plant via tanker truck specially designed to distribute cryogenic fuels. At the fueling site, LNG is offloaded into the facilety’s storage system. In most LNG stations, the fuel passes through a pump to an ambient air vaporizer that serves as a heat exchanger. In this vaporizer, the temperature of the LNG is increased. “Green” LNG is saturated at -200°F, and “blue” LNG is saturated at approximately -220°F.. The pressure increases at these temperatures, but the fuel remains a liquid. This process is called conditioning. After conditioning, LNG is stored in large cryogenic vessels that can be configured horizontally or vertically, and are typically found in capacities of 15,000 or 30,000 gallons. When needed, LNG is dispensed as a liquid into cryogenic tanks onboard the vehicle.
Unlike conventional fueling stations, LNG stations must address various unique design and functionality requirements, including tank truck off loading, fuel conditioning, cryogenic fluid storage and processing, vapor management and venting minimization, codes and standards compliance, and special metering and dispensing needs. New technology is in development for producing LNG at warmer temperatures, which introduces upcoming opportunities to lower LNG infrastructure costs.
LNG station designers, some of whom are also cryogenic equipment manufacturers, have developed standardized or “cookie cutter” station designs. However, most stations installed to date have been custom designs in order to accommodate the particular site requirements described above. Further progress toward installing LNG stations at truck stops and building more “greenfield” stations (as opposed to trying to fit them within existing return-to-base truck and bus terminals) will enable increased use of more economical standardized designs.
LCNG Station Design
A variation of the LNG station is the LCNG station, which uses LNG to make CNG. Some LCNG stations can dispense both CNG and LNG, while others can only dispense CNG and are typically built due to inadequate natural gas pipeline access. LCNG stations use a separate pump to pump LNG to an ambient air vaporizer, where the LNG is warmed to approximately 40°F and becomes a gas. The gas is then odorized and goes through a priority fill system, fuel storage vessels, a sequential system, temperature compensation system, and dispensed into the vehicle.
LCNG stations receive and store truck-delivered LNG, which is pumped to high pressures and vaporized to fuel CNG vehicles. LCNG capability is an inexpensive addition to LNG stations. LNG and LCNG station costs act as a function of LNG storage capacity. Station costs also depend on many other factors, such as site requirements, number of dispensers, and maximum dispensing rate.
To learn more about LNG safety, please review our LNG Safety Q&A Reference document.
*Permission was granted to use material in this section from American Natural Gas Alliance (ANGA). Content is derived from ANGA-commissioned report published by TIAX – “U.S. & Canadian Natural Gas Vehicle Market Analysis: Liquefied Natural Gas Infrastructure.”