CNG is Compressed Natural Gas, which is mainly methane compressed at a pressure of 200 to 248 bars. LPG is Liquefied Petroleum Gas, a mixture of propane and butane liquefied at 15 °C and a pressure of 1.7 - 7.5 bar. Some variants of LPG are primarily propane so LPG is often colloquially called propane. CNG is cheaper and cleaner, but LPG has a higher calorific value. Distribution is easier for natural gas over long distances via pipelines.
Natural gas is created over hundreds of years deep within the earth. It has to be extracted (pumped to the surface) and cleaned to remove impurities including water to be marketable. The byproducts of natural gas processing are ethane, propane, butane, pentane and higher molecular weight hydrocarbons.
LPG is manufactured during the refining of petroleum (crude oil), or extracted from petroleum or natural gas streams as they emerge from the ground.
CNG is generally cheaper than LPG. However, propane produces over twice the amount of energy than natural gas. So LPG could be more cost effective than natural gas depending upon local prices (which tend to fluctuate).
Here's an NBC news story on how cost effective and convenient CNG is over gasoline:
There is an extensive distribution network for both CNG and LPG. But the retail availability of LPG is more compared to that of CNG. CNG distribution is mainly via pipeline networks that run from natural gas production wells to various distribution centers and finally into residential neighborhoods and commercial areas.
LPG, vaporized and at atmospheric pressure, has a higher calorific value (94 MJ/m3 or 26.1kWh/m3) than natural gas (38 MJ/m3 or 10.6 kWh/m3). This LPG produces more energy than the equivalent amount of natural gas.
Safety and Environmental Impact
Although CNG does produce greenhouse gases upon combustion, it is a more environmentally clean alternative to other fossil fuels like gasoline or LPG. CNG is also safer than other fuels in the event of a spill because natural gas, being lighter than air, disperses quickly when released.
Substituting LPG for CNG
Because LPG produces more energy than CNG, it cannot simply be substituted for natural gas. Only gases that have the same Wobbe index are interchangeable. For LPG to have the same Wobbe index as CNG, it has to be mixed with air. A 60:40 ratio for LPG:air mixing is common but it depends upon the composition of LPG because LPG itself is a mixture of propane and butane. This mixture of LPG and air to substitute natural gas is called synthetic natural gas (SNG). Use of SNG is common before the CNG distribution network is fully operational.
Ethanethiol or ethyl mercaptan is commonly added to both LPG and CNG. Mercaptan has a foul, rotten-egg smell that makes it easy to detect gas leaks.
- Automobiles: Any vehicle running on gasoline can be converted to a bi-fuel vehicle (gasoline/CNG). It involves installing a CNG cylinder in the trunk, installing the plumbing system, installing CNG injunction system and the electronics. In the US, CNG vehicles have been introduced in a wide variety of commercial vehicles, from light duty trucks and sedans like taxi cabs to medium duty trucks like UPS delivery vans and postal vehicles to heavy duty vehicles like transit buses, street sweepers and school buses. In California, transit agency buses run on CNG. Coco Cola Enterprises delivered its beverages to London Olympics Games 2012 in bio methane fueled lorries.
- Locomotives: Napa Wine Valley Train has been converted to be run with CNG. FerroCarril Central Andino in Peru has been running a freight line on CNG run locomotive since 2005. Iran, Pakistan, Argentina, Brazil and India have the highest number of CNG vehicles run.
- Heating: LPG is used an alternative to heating oil and electricity in places where there is no natural gas pipe line.
- Cooking: LPG is the most common cooking fuel in India and urban Brazil.
- Refrigeration: Gas absorption refrigerators and air conditioning systems use LPG. But its use in motor vehicles for air conditioning has been discouraged due to the risk of fire.
- Automobiles: A large number of vehicles, light, medium and heavy duty, around the world are fueled by LPG. There about 1,600 bi-fuel Ford F-150 pickup trucks in the California state fleet. Propane fueled forklifts are used both inside and outside warehouses and at construction sites.
The following video shows how to convert a car to use LPG along with gasoline.
Advantages of using CNG and LPG in vehicles
- Lead or benzene is not used. Hence lead fouling of spark plugs is eliminated.
- Natural gas vehicles have lower maintenance costs when compared with other fossil fuel-powered vehicles.
- The fuel systems are sealed. This prevents any spill or evaporation losses.
- CNG does not contaminate or dilute the crank case oil thus increasing the life of lubricating oil.
- The emission of greenhouse gases is reduced 80% when compared to gasoline vehicles.
- Natural gas disperses easily in air and is not flammable. Hence CNG vehicles are safer than gasoline vehicles.
- CNG is cheaper than gasoline or diesel.
- LPG is cheaper than diesel or gasoline.
- LPG vehicles have lower maintenance costs.
- It has lower emission than gasoline or diesel.
- It is non-toxic and non-corrosive.
- It requires lesser space than CNG for storage.
Disadvantages of using CNG and LPG in vehicles
CNG occupies more space than gasoline. Additional space in the trunk of a car or the bed of a pickup truck is used to store the CNG tanks. This problem has been mitigated in factory-built CNG vehicles by installing the tanks under the vehicle body, e.g. Fiat Multipla, Fiat Panda, Volkswagen Touran Ecofuel, Volkswagen Caddy Ecofuel, and Chevvy Taxi.
- LPG has lower energy density than gasoline or diesel and hence the equivalent fuel consumption is more.
- LPG provides less upper cylinder valve lubrication. If a LPG-fueled engine is not suitable modified, it will lead to valve wear.
- It does not disperse easily and is readily inflammable.