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Frequently asked Questions on Natural Gas

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What is Natural Gas, PNG, CNG & LNG?

Natural gas as its name indicates is a Gaseous fuel, consisting of 87-92% of Methane with a small percentage of other higher hydrocarbons.

PNG, CNG & LNG are different forms of Natural gas under varied temperature & pressure.

Piped Natural Gas(PNG) is generally under atmospheric temperatures but the pressure can up to as high as 90 Kg/cm2 in transmission lines and limited to less than 24 Kg/cm2 in City Gas grids due to risk involved in movement of high pressure gas inside crowded cities/towns .

Compressed Natural Gas (CNG) means natural gas used as fuel for vehicles, typically compressed to the pressure ranging from 200 to 250 kg/cm2 in the gaseous state

It is used pre-dominantly for automotive purpose as the energy requirement for running a car (for 150-200 km) can be packed in a container of size, which can fit in to the luggage compartment or boot space when pressurized gas volume reduces by nearly 300 times. There are few instances of CNG being used for industrial purposes for shipping desired quantities through cylinder cascades .However in such cases pressure reduction at the end use is inevitable.

(Refer OISD-179 for Safety Requirements for Compression, storage & refueling stations for CNG in Automotive sector)

LNG (Liquefied Natural Gas)is liquid form of Natural gas, generally intended for transportation of large volumes through ships, where cross country pipelines are not feasible. There will be high degree of economy in transportation logistics due to large scale of operations involved.

How LNG is stored and handled?

Natural gas can be reduced to liquid state by cooling to 160 C so that it will occupy 1/600th of its gaseous volume and it is called as Liquefied Natural Gas which eliminates more space for gas transportation. In fact during the process of liquefaction of NG to LNG lot of impurities are eliminated.

Hence LNG is the purest form of Natural Gas.

LNG is stored in large insulated tanks that are specially designed for this purpose. The tanks are so-called "full containment" tanks. Each tank consists of a special nickel-steel inner tank and a (roughly 80-cm thick) concrete outer tank. The space between the walls of the two tanks is filled with a thick layer of special insulating material that ensures that virtually no heat can get into the inner tank. However, even with the most efficient insulation, some ambient heat will always reach the inner tank. This slight quantity of heat will cause a very small amount of the LNG (approximately 0.065% a day) to turn back into a gas. All of this so-called evaporation loss in the tank is collected, so that none of it is released into the atmosphere. It is again condensed into LNG and added to the tank as LNG liquid. It may also be used at the terminal as a fuel to heat up the LNG.

At receiving terminals (like Dahej, Hazira), LNG is re-gasified and transmitted for further usage in NG form. These receiving terminals are also known as Re-Gasification terminals

LNG vessels (Ships) are also there to carry 2,50,000 m3 of Gas at a time

In India we have LNG import-cum-re-gasification terminals and we do not have liquefaction plants, as India is not a gas-exporting nation.

What is Rich gas and Lean gas?

Rich gas contains 5 or 6 gallons or more of recoverable liquid hydrocarbons per thousand cubic feet. Whereas Lean gas contains less than 1 gallon of recoverable liquid hydrocarbons per thousand cubic feet.

Why is natural gas processed?

Natural gas is processed in LPG fractionator plant to separate it into different fractions as it is required by different applications. The following shows the different applications

  • Methane -fuel and feedstock for urea plants and fuel for power plants
  • Ethane -Production of petrochemicals
  • Propane -LPG and industrial fuel
  • Butane -Production of LPG

C5 and heavier -Production of solvents and pentane

What is associated and non-associated gas?

Based on the type of source Natural gas is classified as associated and non-associated gas and there is of no significance when end use is concerned.

  • Associated gas is natural gas found in crude oil reservoirs either dissolved or in conjunction with crude oil deposits and it is also called as Oil well gas.
  • Non-associated gas is Natural gas found in reservoirs separate from crude oil wells and it is also called as Dry gas.

What is sweet & sour gas?

Based on the sulphur content the natural gas is classified as sweet & sour gas. Sour gas contains higher level of sulphur content in its composition and sweet gas has very little or nil sulphur.

What is PNGRB and what is its role in awarding CNG projects?

Petroleum and Natural Gas Regulatory Board External Website that opens in a new window (www.pngrb.gov.in) (PNGRB) was formed on 1st Oct-2007 to

  • Protect the interest of consumers by fostering fair trade and competition amongst the entities
  • Regularize and authorizing entities involved in the business

CGD projects will be awarded to entities based on bidding process wherein weightages for different qualifying parameters will be given for entities participating in the bid.

Where the LNG terminals are located in India ?

As of Nov. 2008, India has

  1. Petronet LNG Limited (PLL), Dahej (Gujarat)
  2. Shell Hazira, Hazira (Gujarat)
  3. Dhabol , Ratnagiri for Captive usage of Power plant at Ratnagiri

Can Natural gas be stored?

Generally natural gas is stored only in CNG outlets and in LNG terminals. Apart from that natural gas will not be stored and it is in continuous flow through pipelines.

There are few Natural Gas underground caverns in some parts of the world.

How natural gas is generally traded?

Gas trade is based on $/MMBTU (Million British Thermal Units)

What is the composition of natural Gas:

Composition of Natural gas varies widely from source to source.

Natural gas is a combustible mixture of hydrocarbon gases. While natural gas is formed primarily of methane, it can also include ethane, propane, butane and pentane. Depending on the application the buyer/user can seek pure form of Methane (as high as 95%)

The thermal efficiency varies from 9000 k.cal to 13500 K.cal per Kg based on the composition of Gas.

1 MMBTU is 252000 K.cal

1 MMBTU (Million British thermal unit) = 19 to 23 kg or 23 to 30 SCM approx. based on the composition of the gas.

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