CNG Stations

Natural gas is a cleaner alternative to petroleum products, having lesser environmental impact energy. It can be used for industrial or residential heating, power generation, or at compressed natural gas (CNG) refuelling stations for vehicular applications. As far as CNG supply is concerned, the stations receive gas either directly from natural gas pipelines or through mobile cascades, which are essentially a bunch of cylinders mounted on trucks.

Types of CNG Stations

For transportation of natural gas within a city, a pipeline is laid from the city gate station where natural gas is pushed at a pressure of 15 bars or even lower. In order to dispense gas to CNG vehicles, there are CNG stations at various locations, where the gas is compressed up to 250 bars initially and then dispensed to vehicles at a pressure of 200 bars. The following are the types of CNG stations:

Online station

• Natural gas is transporter from CGS to Online stations via. steel pipelines.
• At the online station, natural gas is received at a pressure of 19 bars and then compressed up to a pressure of 250 bars with the help of a reciprocating compressor
• It is used to compress gas directly from the pipeline and the discharge can be used to fill the CNG vehicles at CNG retail outlet.
• The capacity of such a compressor is selected based on the vehicle likely to be filled in that station and with the provision for expected future expansion. It may be equipped with a 1200 SCMH capacity compressor for compressing the natural gas from 49 bar to 225 bar. All categories of vehicles are refueled i.e. Bus, Car, LMV, LCV, and autos.
• Cascade filling facilities will not be available at the online station as it is in Mother station. The investment of an online station is midway between daughter station and mother station.

Typically, a CNG online station consists of the following equipment:

• CNG compressor
• CNG dispenser
• Storage cascade
  
  
  

Mother station

• A mother station is similar to an online station. It has the facility to refuel mobile cascades in addition to the local demand of vehicles. 
• A mother station is connected to natural gas pipelines & has preferably more than one compressor.
• The mother station compressor is of capacity of around 1200 SCMH & it compresses natural gas from a pressure of 15-19 bar to 200-250 bar. The compressor is of high capacity.
• The mother CNG station fills mobile cascades for daughter stations and also fills stationary cascades for dispensing CNG into vehicles. All categories of vehicles can be refueled at a mother station, i.e. bus, car, light motor vehicle, light commercial vehicle & autos.
• The mother station has a facility to feed two to three daughter stations. Mother station are connected to steel grid, through which gas is transported from CGS to mother station.
• Gas from the distribution pipeline is compressed using a multistage compressor into a cascade storage system. This system is maintained at a pressure higher than that in the vehicles on-board storage, so that gas flows to the vehicle under differential pressure. Typically the cascade storage operates in the range of 205 bar to 250 bar, while the vehicles maximum onboard cylinder pressure is 200 bar. In order to make the utilization of compressor and buffer storage, more efficient fast fill CNG station usually operates using a three stage cascade storage system.
• From the natural gas tank, gas goes through refueling connector and a non-return valve. After that it passes through a solenoid valve, pressure regulator, engine management & injection system & is finally filled to the vehicle.

Daughter station

• Mother stations feed gas to daughter stations . CNG is dispensed using mobile cascades. Mobile cascades at daughter stations are replaced when pressure falls. The pressure depleted mobile cascade is refilled at mother station.
• In this case the investment is least among all stations. In each successive filling, there is a decrease in storage pressure. It is to be noted that once storage pressure drops, refueling time increases and quantity of CNG dispensed to vehicle also decreases.
• The sizing of mobile cascades and trucks is decided based on the distance between mother stations to daughter station. Number of LCV/HCV, to and fro frequency is decided based on the load.
• Daughter station has a facility to feed small vehicle alone.
• It is easy to set up daughter station and it can be set up at small towns, junctions & villages (where steel pipeline network does not exist). This facility has proved to be boon for villages, which do not have direct access to natural gas. This type of station may be converted to booster or online as per demand.

Daughter booster station

CNG is dispensed to vehicles based on the principle of pressure equilibrium. Once the pressure of a mobile cascade drops below 200 bars at daughter stations, vehicles get less amount of gas, which is below 200 bars. With each fill thereafter, the amount of gas dispensed to vehicles starts decreasing and the filling time starts increasing, thereby leaving the customer dissatisfied.

Daughter booster stations address the issue. These are similar to daughter stations. However, in order to cater to customers with regard to the amount of gas dispensed as well as filling time, a booster compressor (hydraulic type with variable suction pressure) is installed in between the mobile storage and the CNG dispenser. The booster compressor increases the pressure above 200 bars once the pressure of the mobile cascade falls below 200 bars. Thus, the maximum amount of gas stored in the mobile cascade is dispensed to the daughter booster station.