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LNG is Liquefied Natural gas and CNG is Compressed Natural Gas. Both are Natural gas in two physical forms.
Natural gas produces the least emissions of all the fossil fuels. Gas turbines use the gas in its natural form for power generation with only minimum treatment. The drawback of natural gas is its availability is restricted to certain regions or countries. In its natural form it is an inconvenient fuel source for portability or shipping.
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To overcome this drawback of availability, natural gas shipping takes place from available regions to other countries in liquid form. This is Liquefied Natural Gas (LNG). LNG has volume that is 1/600th of its original volume. This reduces transportation costs considerably. Liquefaction and transport require require special treatments and are highly capital intensive.
- Liquefaction and transport of natural gas is at -162 Deg C. This requires specially insulated tanks in the ships that carry the LNG .
- Removal of all impurities like sulphur, water and other elements is necessary for liquefaction which requires an elaborate pretreatment plant.
- The receiving side also should have an LNG terminal for re-vapourising the LNG to its gaseous form.
- A very high level of investment is required to set up an LNG plant. A typical plant would cost in the range of 2 to 4 billion US Dollars. In addition, investment is required to build receiving terminals and LNG carriers.
- As the backbone of the project, the investor should have long-term supply contracts.
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Transportation of LNG is much less risky than transporting oil. LNG transportation is at atmospheric pressure so any leakage of the liquid will vaporise to the atmosphere. This avoids any costly oil spills. Since all the impurities like sulphur are already removed this is the purest form of Natural gas.
Qatar, Indonesia, and Malaysia are the leading exporters of LNG. Japan, South Korea, Spain, and the United States are the main importers.
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CNG is Compressed Natural Gas and another physical form of natural gas to make it convenient to handle. Natural gas is compressed to 200 bar pressure so that the volume is almost 1% of the original. This makes it much more convenient to carry along.
Being a much cleaner fuel source, the increased portability and reduced cost has made CNG very popular as a transportation fuel. Because of the heavier and voluminous tank, it is yet to be popular in small vehicles.
However, there is an increase in the use of CNG for public transportation. Cities like New Delhi have made it mandatory to use CNG for public transportation. This has resulted in a considerable pollution reduction.
Even though there is an element of risk because of the pressurised cylinder, the benefits far outweigh these. Unlike LNG, the plant, machinery, and investment required for CNG production is considerably less.
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LPG is Liquefied Petroleum Gas.
LPG production happens during the refining of crude oil. The composition is predominantly propane, butane, or a mix of these and other gases. In addition, extraction of LPG takes place directly from some of the oil wells. The calorific value is higher than the Natural gas in the range of 95 MJ /kg.
LPG can be in liquid form at a relatively low pressure of 2 to 5 bar at atmospheric temperature. This makes it convenient to handle in small cylinders. This coupled with the high heating value has made it the most popular household cooking gas in most of the third world countries. Governments subsidize LPG for domestic use, which makes it costly for other uses.
Because of its potential to vaporize immediately, any leaks can be hazardous. Basic knowledge of safety is necessary for using LPG. The addition of odorizers helps make it easy to identify any leakage.
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Volumetric Energy Density.
Ultimately, it is the Volumetric Energy Density that decides the convenience of use.
Comparing the volumetric Energy Density of Diesel, which is 34 MJ/L, Natural gas, has 0.364 MJ/L, LNG 21 MJ/L, and CNG 9 MJ/L.
LPG has a volumetric Energy Density of 25 MJ/L.