Difference Between CNG and LPG


Introduction

From our young age we have been listening to our parents saying that our gas cylinders were filled with LPG and these were distributed to the customers in cooking gas cylinders. This was the cleanest fuel for cooking rather than the previously used kerosene stove or coal burning in “Chulha” which emitted copious amount of smoke. Most recently, we again learnt that clean fuel vehicles will replace all the vehicles which ran on petrol and diesel after these are suitably modified. This “clean fuel gas” is known as CNG it produces low hydrocarbon emissions and vapours that are not ozone producing. Almost all trucks, buses and cars were immediately converted to CNG from diesel and petrol to tackle the increased level of pollution in the atmosphere. So now the most common questions that arises in our mind is “what are CNG and LPG”, and how are they different from each other? 


  • What is CNG?

CNG stands for compressed natural gas which is actually compressed pure methane gas (CH4) that is used to create a clean fuel which is non-pollutant and almost clean to the atmosphere. So, CNG is also known as “clean fuel”. CNG is made by compressing natural gas, (which is mainly composed of methane, CH4), to less than 1% of the volume it occupies at standard atmospheric pressure. It is stored and distributed in hard containers at a pressure of 20–25 MPa (2,900–3,600 psi), usually in cylindrical or spherical shapes. CNG is much cheaper than high quality petrol and so this is an incentive for car owners to switch to this clean fuel from petrol or diesel. 


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  • What is LPG?

LPG stands for liquefied petroleum gas and it is a mixture of light gaseous hydrocarbons, such as propane, butane and other light petroleum products converted to liquid by applying very high pressure. LPG is produced during oil refining or is extracted during the production of Natural Gas. LPG is filled in pressurized cylinders for further transportation or storage. A slight quantity of mercaptans, a fowl smelling substance, is introduced into these pressurized cylinders so that any leakage from these pressurized cylinders can be immediately detected from the foul smell. 

 

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Difference between CNG and LPG

  • The main constituent of CNG is methane gas, whereas the main constituents of LPG are propane, butane and other lighter hydrocarbon gases.

  • The sources of CNG is “coal bed methane wells” and “natural gas wells”. In case of LPG, the main sources are natural gas fields and crude oil refineries of petroleum.

  • CNG is lighter than air, whereas LPG is heavier than air. So, in case of any leakage, CNG quickly disperses in air while LPG spreads to the lower ground/floor areas. Thus, LPG creates more risk in case of spillage compared to CNG mainly because it does not diffuse in the atmosphere like CNG. The thermal efficiency of CNG is higher than LPG, as its octane number rating is high. 

  • CNG is a gas and remains a gas under pressure(compression) whereas LPG is a gas at normal temperature and pressure but changes to liquid form under high pressure. So, both CNG and LPG have to be packed in strong containers.

  • CNG is used as a clean fuel substituting gasoline. Whereas, LPG has mostly been used in cooking, as an industrial fuel and also in refrigeration.


Safety for the environment:

  • CNG constituting methane releases very less quantity of greenhouse gases (CO2 and CO) but LPG produces carbon dioxide (a greenhouse gas) but it is much cleaner compared to gasoline. Thus, CNG is cleaner than LPG and also much cheaper. To foster a cleaner, healthier and efficient world the World LPG Association (WLPGA) that is the authoritative voice of the global LPG industry promotes the use of LPG.            


CNG vs LPG

Calorific values of LPG and CNG

LPG has high calorific value of 90 to 95 MJ/m3 compared to CNG which has calorific value of as low as 35 to 40 MJ/m3. Thus, LPG is more efficient fuel than biogas, petrol, diesel, kerosene and CNG.


Cost of CNG vs LPG:

CNG is cost effective and convenient compared to LPG gas.


Advantages of both CNG and LPG

Compared to gasoline-powered vehicle, CNG powered vehicles are safer and more efficient. CNG is cheapest, cleanest and so less impacting on the environment. As CNG is lighter than air it distributes faster and proportionately in air. 


Both the fuels, CNG and LPG, are clean in nature with less amount of residue. Therefore, as the deposition of residues on the machines and vehicle machinery is less in amount, the maintenance cost of the machines and vehicles almost decreases. Due to this reason the life of the vehicles and machines also increase.   


LPG has replaced chlorofluorocarbons within refrigerators, thus freeing the Earth from chlorofluorocarbons that is one of the greenhouse gases. These gases deplete the ozone layer of our atmosphere’s outermost layer that blocks the entry of Ultraviolet rays of the sun. It has long been used in hospitals, industries, agriculture sectors, construction, etc. It is used in a cost-effective way to illuminate homes, heating water, cooking, etc especially in those places where electricity is not yet available. Decentralized electricity can be generated from LPG in rural areas by using combining heat and power technologies (CHP). LPG has been efficiently used as an aerosol propellant or fuel for rockets. As both CNG and LPG are gases, they pose less risk of pollution to both the Earth’s surface and its atmosphere.    


Disadvantages of both CNG and LPG

Both CNG and LPG, because of presence of methane gas that burns readily in air and having low ignition temperature, have the risk of catching fire when the nozzle of the cylinder is left open or in case of any kind of leakage. The storage space required for the cylinders that are kept in isolation for their safety in both the types of gases is more compared to liquid fuels. In both the gases, the consumption is more as compared to liquid fuels like petrol and diesel. Moreover, extra modification of vehicle is required for installation of CNG cylinders in the vehicles.     


Conclusion

LPG was thought to be an only clean fuel till the arrival of CNG. Nowadays CNG has replaced most of the petrol and diesel run vehicles. Also, CNG has been accepted as a clean fuel but due to its volatility in nature they have not been used as cooking gas, since it has high auto-ignition temperature (540°C) and a narrow range of flammability of 5 to 15%.


LPG still remains a unique product for lighting homes and cooking gases. Nowadays most of the households have LPG pipelines as the liquid petroleum product can be easily and safely transported through the pipelines. Thus, CNG and LPG are both very much useful fuels, even if both have few unavoidable drawbacks.