Natural gas vehicles are hitting the road like never before. There are about 175,000 vehicles in the United States fueled by natural gas and more than 23 million worldwide, according to NGVAmerica. Most of these vehicles run on compressed natural gas (CNG). What exactly is CNG and how do they make it?
Putting the pressure on
Pipeline gas becomes CNG when its pressure is increased to at least 3,600 pounds per square inch (psi), reducing its volume to less than 1% of its original. Before it's compressed, the gas must be dried and cleaned of contaminants.
Air-cooled reciprocating piston compressors are typically used for CNG refueling stations. With this type of compressor, the intake gas enters the suction manifold and flows into the compression cylinder. The intake and discharge valves then close. In the cylinder, multiple pistons move in a reciprocating motion.
During compression, the volume within the cylinder is reduced as the pistons advance, increasing the pressure and temperature of the gas. At a certain pressure, the discharge valves open. The pressure remains fixed while the volume decreases for the rest of the advancing stroke. The pistons stop briefly before reversing direction. As the pistons reverse, the discharge valve closes and more gas is injected into the cylinder and the cycle continues.
Setting the stage
Most CNG compressors use several stages of compression to reach a target pressure of 4,500 pounds per square inch gauge (psig). A multiple-stage compressor consists of several cylinders of decreasing size. The smaller the diameter, the higher the pressure.
Another type of CNG compressor uses a non-compressible hydraulic fluid to drive a free floating piston up the cylinder. The second stage completes compression to 3,600 psi. The natural gas and hydraulic fluid never mix because they're always at an equal pressure at the piston ring. Potential advantages include less maintenance, a smaller footprint and lower costs than conventional reciprocating technology.
Going with the flow
CNG compressors handle flows ranging from 28 to 1,250 standard cubic feet per minute (scfm) and require inputs ranging from 40 brake horsepower (bhp) to more than 1,200 bhp. Horizontally opposed designs, in which the pistons move in opposite directions horizontally instead of vertically, are popular because they produce less vibration, are easy to service and can be used in very high horsepower applications.
Because there are 132 standard cubic feet of natural gas per diesel gallon equivalent (DGE), a mid-size compressor can easily supply 250 DGE per hour (at 550 scfm). Some fueling stations store CNG on site, which can reduce fill-up time significantly.
The benefits of CNG
CNG has a number of advantages for fleet managers and drivers. Storage and distribution costs are typically much lower than with diesel or gasoline. As a cleaner burning fuel, CNG also reduces emissions and engine wear, resulting in cleaner air and extending vehicle life.
Pensacola Energy operates three CNG stations, two of which are open to the public with a gasoline gallon equivalent price of $2.05. Open 24/7 and well lit, the stations make for an excellent waypoint when traveling across the Florida panhandle. Our stations offer full redundancy reliability.
To learn more about Pensacola Energy's Natural Gas Fueling, click here.