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If you are confused about the difference between Compressed Natural Gas (CNG) and Liquefied Natural Gas (LNG), and which is the best for a transportation application, join the club. Speaking to a number of transportation and fleet managers this month, I've found that there are many questions about the proper application.
Let’s simplify. It is a matter of storage capacity, weight, and refueling time.
It does not matter which you use, CNG or LNG; the engine technology is the same. There is no such thing as an LNG engine or a CNG engine. The engine always uses natural gas in the form of a gas. The real difference is in the fuel storage equipment and the length of time that the truck can be parked unused.
It is density that drives the difference. Natural gas is not as energy dense as diesel, as measured by volume. Liquids are measured in gallons, while gas is measured in cubic feet. The space of one US gallon of liquid is .133681 cubic feet, so 7.48 gallons of liquid fit into a cubic foot of space. Diesel fuel is one of the densest fossil fuels, carrying more power than gasoline or natural gas. Even when natural gas is liquefied, 1.7 gallons of LNG equal just one gallon of diesel. Compressed to 2,300 psig it takes 3.8 gallons of CNG to equal the power of one gallon of diesel.
At 7.15 pounds per gallon, a truck with twin 150-gallon tanks carries 1,072.5 pounds of diesel in each tank — a total of 2,145 pounds of fuel, not including the tank. It takes 510 gallons of LNG to produce the same energy as 300 gallons of diesel. LNG tanks and related fuel systems weigh much more than the tanks for diesels, so accounting for the additional weight of the liquid fuel and the additional weight of the tanks, the equivalent LNG fuel and tanks weigh almost 3,500 pounds, 62 percent more.
Diesel fuel systems are more than just the fuel and tank. Because of EPA regulations, diesel trucks must use emission fluids to reduce emissions. Spark-ignited natural gas engines don’t use DEF, so the combined 550 pounds dedicated to the SCR equipment, including the DEF and tank, are unneeded. As the chart above shows, some LNG tank systems weigh less than the energy-capacity equal diesel systems.
CNG fares much worse. It is not practical to get 300 gallons of diesel equivalent in CNG onboard a truck — the combined weight of the gas and the system is over 10,000. If you work the weight of the fuel, 300 gallons of diesel = 1,140 gallons of CNG, which weighs 1.81 pounds per gallon, for a total of 2,072 pounds. Add another 1,800 pounds for the CNG tanks, and about 1,300 pounds for the racks and protective plates, and the fully loaded CNG system weighs in at over 5,172 pounds, 141 percent heavier than the full 300-gallon diesel tanks.
That is a load. Either of the alternatives increases the weight on the tractor, reducing the cargo load the tractor can haul.
The typical diesel pump can dispense 15 – 30 gallons of fuel per minute, with some reporting speeds up to 60 gpm. Even at 15 gallons per minute, a 150 gallon tank takes about 10 minutes to fill, so refueling stops don’t take excessive time away from the productive driving day.
What about natural gas? Again, there is a clear difference between CNG and LNG.
CNG is simply compressed natural gas. The first systems tapped into the existing gas distribution systems provided by the local gas utilities, compressed the gas to 2,000 psig, and then dispensed the gas. When Honda introduced the first CNG Accords, it sold an optional home-based refueling system, called Phill, that tapped into the supply lines to the owner's home. The owner parked their car for the night, hooked up Phill, and the small compressor unit pumped the gas into the car’s onboard CNG pressure vessel.
The natural gas fuel industry calls that process complete fill. The vehicle is slowly refilled overnight, and the gas is compressed at a speed that does not heat it up, making it harder to compress. Complete fill takes longer to perform — only about 4 – 6 gallons per minute. With compressed gas, the compressor does not need to be as large or power hungry. Complete fill systems are more compact, small enough to tuck into garages or sheds, like the Phill system. Many of the CNG filling station options use this small footprint, but these options are not truck friendly. The 4 – 6 gpm fill rate is fine for cars, because a 20-gallon tank takes 4 – 6 minutes to fill, but it is not productive for vocational trucks or busses. The larger 80-gallon tanks can take as long as 20 minutes to fill — a big chunk of productive time.
Tapping into the local distribution system sounds like an easy solution, but a combination of filtering, supply pressures, and volume creates challenges. Any CNG operation tapping into local utility gas lines must include a filter and separator system. The engines need natural gas free of moisture and other compounds. Utilities add a chemical odor to the gas for safety, to make it possible for people to detect gas leaks, and they add other liquid chemicals to help prevent corrosion in the lines and keep rubber seals supple. The gas also picks up moisture that naturally occurs in the gas lines from leaking connections under the water table level in the distribution system. The local gas lines must provide gas at sufficient pressure and volume to support the compressor operation. The CNG station takes the gas at a local pressure of 50 – 100 psig and compresses it to 2,300 psig, reducing the volume of the gas by as much as 20 times, so supply lines operating at 100 psig must provide 220 gallons of gas to create 10 gallons of CNG @ 2,300 psig. If the supply line pressure or volume is insufficient, it takes longer for the compressor to fill the tanks.
Complete fill is used by most domiciled CNG-burning truck fleets, like refuse collection companies. The return-to-base option, where the driver hooks the truck up to the filling system when parking the truck for the night, works well. Once all of the trucks are parked, the system automatically fills the fleets in a cascade process. The complete fill method allows for the tanks to get a complete fill at a cool tempatuture, allowing for the densest fill. (See the video at the end of this article for great demonstrations.)
There is a fast-fill method, by which larger supply lines feed larger compressors to fill the tanks. The fast-fill methods come at the price of a rather steep investment and operating cost. Because of the speed and power behind a fast-fill process, the compressor creates more heat in the gas, lowering the density of the fuel by as much as 20 percent The space required for a fast-fill station increases to at least ½ acre, increasing the acquisition costs. The minimum supply pressure into a fast-fill station is 100 psig, with a volume of at least 5,000 cf of gas per minute, so the station must be close to a large main supply line. The compressors grow in size, as do the electric motors that drive the compressor. It takes about two megawatts of power to drive a 2,500 hp electric motor, so the power consumption is large. Industry data indicates that a fast-fill CNG refueling station is about twice the investment of an equivalent LNG station.
The investment in the refueling system is part of the equation, as the payments for the investment must roll into the cost of the fuel. As the chart above indicates, the additional investment in a fast-fuel station — one that is able to dispense 7 – 10 gallons of CNG per minute — adds about $1 to the cost of each gallon of CNG. The rate of fill plays a significant part in the economic model of a CNG operation.
Here is a video about CNG refueling processes.