How Do They Do That? Compressing Natural Gas for Vehicle Fuel
- Compressed natural gas has a volume less than 1 percent of pipeline gas.
- Reciprocating piston compressors are used for increasing the gas pressure.
- Several stages of compression are used to help lower discharge temperatures.
Natural gas vehicles are hitting the road like never before. There are about 150,000 vehicles in the United States fueled by natural gas and more than 15 million worldwide, according to the U.S. Department of Energy. 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 percent 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 following occurs:
- As the pistons advance, the volume within the cylinder is reduced, 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, 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.
One manufacturer uses tandem cylinder sets for up to four compression stages. This helps to lower discharge temperatures, extending component life. At higher inlet pressures, fewer stages and less horsepower are required to produce the same standard cubic feet per minute (scfm) of flow.
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 and hydraulic fluid never mix since 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 scfm and produce outputs 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 diesel or gasoline. As a cleaner burning fuel, CNG also reduces engine wear, extending vehicle life.