From a fueling infrastructure view, the process of constructing a new compressed natural gas (CNG) station begins at the gas utility connection to the site of the station. The gas is metered at this connection, and there are several steps typically required to make the gas ready for use in vehicles.
The municipal “inlet” gas connection will require sufficient flow rate and pressure for the designed application. Many CNG infrastructure applications can use the standard low pressure available in municipal gas lines, but it is good to know the pressure available at the line and if the envisioned application will require a larger line or more pressure. It is recommended that potential station owners and operators check with the local utility or gas supplier to determine the “guaranteed” minimum inlet pressure available at your selected location.
Gas Quality—Drying and Filtering
The quality of inlet gas primarily concerns moisture content and also scale or other foreign matter that may be contained in the inlet line. Moisture content in natural gas is measured in millions of parts per cubic foot. Inlet gas with high moisture content will require “drying” in order to make it serviceable for fueling vehicles, and dryers are standard equipment in most fueling applications. Further, a filter may occasionally be necessary if there is a quantity of pipe scale or foreign matter in the gas line. Filters come standard on many compressor models.
Dried and filtered inlet gas is compressed by one or more compressors and often stored in tanks, or delivered directly to a fuel dispenser. This pressurized gas is now CNG that is ready for vehicle fueling.
Moving the CNG from the compressor to storage tanks or directly to the vehicle requires directed control, and this function is supplied by a computerized priority panel. Priority panels direct the flow of CNG from the comp ressor to on-site storage tanks. Sequential panels direct the flow of CNG from the compressor or tanks to fuel dispenser units or vehicles. Based on the pressure measured in the vehicle tank, the priority panel switches between the low, medium, and high pressure tanks to ensure a complete fill.
Fast-fill CNG applications will require pressurized gas to be stored in high pressure tanks to accommodate more vehicles fueling faster. CNG storage tanks often come in cascades of up to three tanks in a bank of cylinders or in spheres. Cascade banks are most often maintained at three different pressure levels (high, medium, low) to accommodate faster vehicle refueling, and ensure a proper fill. Natural gas storage tanks are required by law to be installed above ground.
CNG dispensers come in many different sizes, shapes, and varieties. However, they all conform to either a fast-fill or a time-fill configuration and are available in different hose configurations and with different flow rates and methods of metering. Time-fill units typically dispense fuel through a fixed pressure regulator. When the fuel flow reaches a minimum rate, the fuel flow is shut off. Fast-fill units measure the pressure in the tank, then a small amount of precisely measured fuel is dispensed into the tank and the pressure rise is measured. From these figures, the volume of the tank is calculated and the tank is filled rapidly to this level. When the tank is full the flow is shut off. Many dispensers come with temperature compensators that ensure a complete fill in cold environments.
A number of vendors are developing and offering mobile fueling options for CNG, LNG, and sometimes both within the same unit. Depending on the application, time and space requirements, and other factors, a mobile fueling option may be useful. Mobile fueling typically involves equipment such as the CNG dryer, compressor, control panels and dispensers being installed on a mobile skid or flat-bed vehicle trailer. These systems are most often fast-fill and are appropriate for light, medium, and heavy-duty applications. They save on infrastructure costs and can be installed anywhere there is a gas line.
Don’t have space or a gas line available? Some companies offer scheduled mobile fueling where the fueling truck arrives at regular intervals to refuel vehicles.
Single Unit or Home Refueling
There are single unit time-fill systems available for the individual or small fleet owner who wants the ability to refuel a small number of vehicles. These systems are primarily made for light-duty CNG vehicles and refuel a single vehicle on a time-fill using an existing gas connection.