This post is a part of BioBiz’s Bio-CNG Perspectives.
BioBiz, a division of EAI, is a leading market intelligence & strategic consulting firm for the Indian bio-based sectors.
This blog post uses the terms bio-CNG and renewable natural gas (RNG) interchangeably.
Bio-CNG or bio-compressed natural gas, also known as sustainable natural gas or biomethane, is a biogas which has been upgraded to a quality similar to fossil natural gas and having a methane concentration of 90% or greater. As the gas is derived from natural and renewable sources, it is also termed renewable natural gas (RNG).
Introduction
Renewable natural gas has properties similar to conventional natural gas and thus has the potential to be a replacement for CNG and PNG. It also has some properties such as calorific value similar to conventional fuels such as LPG. While RNG’s sustainability properties and calorific value is well known, it would be good to have a comparison of the fuels LPG and natural gas/RNG. This blog post provides the key differences between the different fuels.
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Comparison of LPG and natural gas/RNG
| Parameters | Notes | LPG | Natural gas/RNG |
| Boiling temperature | Denotes the temperature at which a fuel becomes a gas vapour | -42°C | -161.5°C |
| Flame temperature | Adiabatic flame temperature is the temperature attained after complete combustion of fuel in stoichiometric air without heat loss to the surroundings. | 1967°C | 1950°C |
| Gas volume | Volume of gas per kg of the fuel | 0.540 m3/kg | 1.499 m3/kg |
| Specific gravity | The specific gravity of a gas, γ, is the ratio of the density of the gas at standard pressure and temperature to the density of air at the same standard pressure and temperature | 1.5219 | 0.5537 |
| Density @15ºC | Defined as the ratio of the mass of the fuel to the volume of the fuel at a reference temperature of 15°C | 1.899 kg/m3, heavier than air, does not disperse in air easily | 0.668 kg/m3, lighter than air, hence disperses easily |
| Operating pressures | Refers to the pressure at which appliances operate | LPG appliances operate at 2.75 kPa | Natural gas appliances operate at 1.1 kPa |
| Calorific value | The calorific value or the heat value of a gaseous fuel is defined as the number of heat units developed by the complete combustion of unit mass or unit normal volume of a given fuel. | 93.2 MJ/m³ | 38.7 MJ/m³ |
| Auto ignition temperature | The auto ignition temperature is defined as the limit temperature above which a fuel/air mixture ignites if given enough time. | 470°C | 564°C |
| Flammability limits | Flammable limits apply generally to vapors and are defined as the concentration range in which a flammable substance can produce a fire or explosion when an ignition source (such as a spark or open flame) is present. | 2.15-9.6% (volume % in air) | 4-15% (volume % in air) |
| Maximum flame velocity | Flame velocity is generally used for a fuel, a related term is explosive velocity, which is the same relationship measured for an explosive. | 0.575 m/s | 0.36 m/s |
| Freezing point | The fuel freezing point is the temperature at which wax crystals, which form in the fuel as it cools, completely disappear when the fuel is rewarmed. | -188°C | 0°C |
| Flash point | The flash point of a volatile material is the lowest temperature needed to evaporate enough fluid to form a combustible concentration of gas. | -104°C | 221°C |
| Air to gas volume ratio | Air–fuel ratio (AFR) is the mass ratio of air gaseous fuel present in a combustion process. The air-fuel ratio determines whether a mixture is combustible at all, how much energy is being released, and how much unwanted pollutants are produced in the reaction. | 25:1 | 10:1 |
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