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
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 on-board 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
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. These mobile cascades are then despatched to stations which do not have pipeline connectivity.
Daughter station
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.
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.
Mother-daughter refuelling system
A mother-daughter refuelling system helps supply natural gas to remote stations that are not connected to the local gas grid without having to incur the expense and effort of installing the necessary infrastructure. The mother-daughter refuelling system represents a virtual pipeline.
The mother station is connected to the gas grid. It supplies CNG to the mobile storage system via integrated refuelling equipment. The mobile storage system is transported to the remote offline (daughter) station. Thereafter, the storage system is emptied to supply natural gas to the daughter station. Both the mother station and the daughter station are equipped with fill posts so that vehicles can be refuelled. All the series in the mother-daughter refuelling system are modular by design. They can be expanded relatively easily and at reasonable cost and have been designed for high capacity performance at various pressure stages.
The advantages of the mother-daughter refuelling system are as follows:
- Provides temporary supply of natural gas to pipelines undergoing scheduled or unscheduled maintenance.
- Service customers not connected to the existing distribution systems.
- Offers emergency distribution service in case there is a line break.
Costing of a daughter booster station
Unlike the mother/online station, the daughter booster station does not require much infrastructure – existing petrol pumps and other facilities can be used for setting up the required infrastructure. Therefore, the land price does not constitute a major component in the costing of such stations. Transportation cost, which includes the cost of trailer packages, trucks and fuel, and driver’s salary, has a significant impact on the total cost. Other factors on which the cost depends are civil works, safety equipment and the dispensing mechanism.
Recent developments
The city gas distribution system in the country has provided a shot in the arm with the Petroleum and Natural Gas Regulatory Board (PNGRB) all set to distribute natural gas through cascades and CNG daughter booster stations across the country. Recently, Sanwariya Gas approached the PNGRB for setting up a daughter booster station on the Delhi-Mathura highway, for which the regulator has decided to provide permission. Meanwhile, entities willing to distribute natural gas through cascades to specific customers having a requirement of less than 50,000 million standard cubic metres per day or set up CNG daughter booster stations in an area are required to apply to the PNGRB. The regulator will then examine all such applications on a case-to-case basis and may grant permission for setting up the network.
In a separate development, Rajasthan State Gas Limited (RSGL) has commissioned a CNG station at Neemrana and a daughter booster centre at Kukas to deploy a green corridor. The Rajasthan government had, in its budget 2014-15, announced the setting up of the CNG project between Neemrana and Kukas. RSGL was appointed as the nodal agency to set up retail gas infrastructure in the state. The company is also planning to set up CNG corridors on the Kota-Jaipur and Kota-Baran-Indore-Mumbai stretches.