CNG & PNG

The classification of the Gas type in CGD,

·         Compressed Natural Gas (CNG).

·         Piped Natural Gas (PNG)

 

Compressed Natural Gas.

 

The compressed natural gas is used mainly for the Vehicles for light and heavy motor vehicles. The vehicles using this compressed natural gas are, cars, busses, autos, etc. The pressure of CNG inside the vehicles is normally 250barg. The supply to the CNG stations is from the main line which is in the pressure of 26barg. These are then compressed in the CNG station using high power staged compressors to attain the pressure of 250barg. The CNG stations are of two types,

·         Mother stations.

·         Daughter stations.

Mother Stations 

Compressed  natural  gas  is  commonly  delivered  from  production  fields  or  storage  locations  to  utility companies and/or end users via pipeline.  The degree to which the presence of pipeline infrastructure coincides with the need for natural gas refueling facilities in a given geographic area will determine the type of refueling facilities required.  Refueling facilities with access to a  continuous  supply  of  natural  gas  from  a  pipeline  are  often  referred  to  as  pipeline  or  mother  stations.  These stations always have the fuel available to refill vehicles.  Mother stations usually have the following major equipment components:

·         Compressors

·         Dryers

·         Storage

·         Controls

·         Dispensers

·         Valves & fittings

Daughter Stations 

 

Mother or pipeline stations can also act as fuel suppliers to other stations not connected to the  pipeline.  These other stations, often called daughter stations, are sited in a natural gas vehicle  territory to provide sufficient fuel to supply the entire vehicle population.  Periodic delivery of  fuel from the pipeline or mother station to the daughter station is required to ensure that  the fuel  demand of the daughter station is met.  The daughter station will contain sufficient gas storage  tankage delivered from the mother station to supply its requirements for refueling CNG vehicles.   When  a  trailer  truck  delivers  CNG  to  a  daughter  station,  the  truck  will  connect  to  a  daughter  station’s  booster  compressor  which  in  turn  connects  to  the  daughter  station’s  cascade  storage  tanks.    The  booster  compressor  and  the  cascade  storage  tanks  serve  as  a  buffer  to  refuel  the vehicles.    The  cascade  storage  tanks  will  maintain  pressure  high  enough  to  fill  the  vehicles.   When the pressure in the cascade storage tanks drops to a certain level, the booster compressor  will  be  activated  and  more  CNG  will  be  drawn  from  the  truck,  compressed  by  the  booster  compressor, and stored in the cascade storage tanks for refueling.

The cascades are divided in to three stages,

High pressure

Medium pressure

Low pressure

 

Piped Natural Gas(PNG).

The second category of the gas in the CGD is the PNG. The major difference between the CNG & PNG is that, the PNG which is supplied through the MDPE from DRS, the pressure is 2-4 bar. The pressure at which the PNG reaches the domestic, industrial and commercial customers. The various applications of PNG are as follows,

Cooking purpose

Heating/ furnace

Air conditioning

Gas fire places

Hotels, restaurants, hospitals.

Industrial

The PNG is called the convenient fuel due to the following reasons,

Continuous supply of gas.

Necessity of changing the cylinders are not required.

Useful during the emergency.

No problem of space occupancy as cylinders.

The payment is after consumption based on how much consumed.

It is totally combustible containing 94% of combustible material and does not leave any residues.

It does not darken the vessels.

It contributes to a cleaner society.

Comparing the PNG & LPG.

PNG is comparatively cheaper than LPG.

The billing is done based on the consumption by the customer.

PNG is safe as,

The property of natural gas is that it catches fire when it forms a mixture of 5-15% mixture with air, whereas LPG is combustible when at its 2% mixture with the air.

Since natural gas is lighter than air, in cases of leak, it just rises up and disperses in to the air. Comparing LPG, it being denser than air settles down in case of leakage, which is highly hazardous. The technical factor involved in this is the flammability limit of the gas.

Large quantity of LPG i.e. around 14.2 kg of LPG is compressed into the LPG cylinders. Whereas, the PNG installation inside the premises is very less and is only at pressure range of 21mbar.