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frankzappa

Differences between LNG and CNG fuels

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Though LNG and CNG are both considered NGVs, the technologies are vastly different. Refueling equipment, fuel cost, pumps, tanks, hazards, capital costs are all different.

One thing they share is that due to engines made for gasoline, computer controlled valves to control fuel mixtures are required for both of them, often being proprietary and specific to the manufacturer. The on-engine technology for fuel metering is the same for LNG and CNG.

CNG as an auto fuel
CNG, or compressed natural gas, is stored at high pressure, 3,000 to 3,600 pounds per square inch (21 to 25 MPa). The required tank is more massive and costly than a conventional fuel tank. Commercial on-demand refueling stations are more expensive to operate than LNG stations because of the energy required for compression, the compressor requires 100 times more electrical power, however, slow-fill (many hours) can be cost-effective with LNG stations [the initial liquefaction of natural gas by cooling requires more energy than gas compression]. Time to fill a CNG tank varies greatly depending on the station. Home refuelers typically fill at about 0.4 GGE/hr. "Fast-fill" stations may be able to refill a 10 GGE tank in 5–10 minutes. Also, because of the lower energy density, the range on CNG is limited by comparison to LNG. Gas composition and throughput allowing, it should be feasible to connect commercial CNG fueling stations to city gas networks, or enable home fueling of CNG vehicles directly using a gas compressor. Similar to a car battery, the CNG tank of a car could double as a home energy storage device and the compressor could be powered at times when there is excess/ free renewable electrical energy.

LNG as an auto fuel
LNG, or liquified natural gas, is natural gas that has been cooled to a point that it is a cryogenic liquid. In its liquid state, it is still more than 2 times as dense as CNG. LNG is usually dispensed from bulk storage tanks at LNG fuel stations at rates exceeding 20 DGE/min. Sometimes LNG is made locally from utility pipe. Because of its cryogenic nature, it is stored in specially designed insulated tanks. Generally speaking, these tanks operate at fairly low pressures (about 70-150 psi) when compared to CNG. A vaporizer is mounted in the fuel system that turns the LNG into a gas (which may simply be considered low pressure CNG). When comparing building a commercial LNG station with a CNG station, utility infrastructure, capital cost, and electricity heavily favor LNG over CNG. There are existing LCNG stations (both CNG and LNG), where fuel is stored as LNG, then vaporized to CNG on-demand. LCNG stations require less capital cost than fast-fill CNG stations alone, but more than LNG stations.

wikipedia.org

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