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) refueling 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 stations are directly connected with the natural gas pipeline. 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 in order to enhance the onboard storage capacity. Thereafter, it is dispensed to vehicles locally through CNG dispensers at a pressure of 200 bars. Typically, a CNG online station consists of the following equipment:
- CNG compressor
- CNG dispenser
- Storage cascade
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. These mobile cascades are then despatched to stations that do not have pipeline connectivity. Mother Station-CNG Station connected with Steel pipeline and having CNG Dispensing and mobile Cascade filling facility
Daughter stations are not connected to natural gas pipelines. At these stations, CNG is transported through mobile cascades at a pressure of 240-250 bars and then dispensed to vehicles through CNG dispensers. In fact, if the CNG compressor stops functioning, a mother/online station acts as a daughter station and dispenses gas from local storage cascades.
Online Station-CNG Station connected with Steel pipeline and having CNG Dispensing only
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.Daughter Booster Station-Gas is transporting to station on mobile cascade with boosting pressure to dispense CNG
Daughter Station-Gas is reaching to station on mobile cascade and dispense as CNG without boosting of pressure