LPG and CNG

Liquefied Petroleum Gas (LPG) and Compressed Natural Gas (CNG) are two of the most widely used gaseous hydrocarbon fuels in the modern world. While both serve as cleaner alternatives to traditional solid and liquid fossil fuels (like coal, petrol, and diesel), they differ significantly in their chemical composition, physical properties, source of origin, and storage mechanisms.

Liquefied Petroleum Gas (LPG)

Chemical Composition

LPG is a flammable mixture of hydrocarbon gases, primarily belonging to the alkane series.

• Primary Components: Propane (C₃H₈) and Butane (C₄H₁₀).
• Minor Constituents: Propylene (C₃H₆), Butylene (C₄H₈), and Ethane (C₂H₆).
• Proportion Variability: The ratio of propane to butane varies by region and season. In colder climates, LPG contains more propane because it has a lower boiling point, preventing the fuel from freezing or failing to vaporize inside cylinders.

Physical State and Storage

LPG is gaseous at atmospheric pressure and ambient temperature. However, it is liquefied under moderate pressure (typically 7–10 bar) for ease of storage, transport, and handling.

• Liquid-to-Gas Expansion: When liquefied, its volume is reduced to about 1/250 of its gaseous state, allowing large quantities of energy to be stored in compact steel cylinders.

Odorization for Safety

Pure LPG is naturally colorless and completely odorless. Because it is highly flammable and denser than air (causing it to settle in low-lying areas or floors during a leak), it poses a severe explosion hazard.

• Ethyl Mercaptan (C₂H₅SH): To ensure immediate leak detection, a trace amount of a foul-smelling sulfur compound called Ethyl Mercaptan (or ethanethiol) is intentionally added. It imparts a distinct “rotten egg” smell.

Compressed Natural Gas (CNG)

Chemical Composition

CNG is an eco-friendly fuel derived directly from natural gas deposits found deep within the earth’s crust.

• Primary Component: Methane (CH₄), which makes up roughly 75% to 95% of the total volume.
• Minor Constituents: Ethane (C₂H₆), Propane (C₃H₈), and trace amounts of nitrogen, carbon dioxide, and water vapor.

Physical State and Storage

Unlike LPG, methane cannot be easily liquefied at room temperature by simply increasing pressure, because its critical temperature is very low (-82.6°C). Therefore, natural gas is compressed to less than 1% of its standard atmospheric volume.

• Storage Pressure: Stored in high-strength, thick-walled cylindrical tanks at a pressure of 200 to 250 bar.
• Density Characteristics: CNG is significantly lighter than air (it has a lower vapor density). In the event of a leak, CNG disperses rapidly upward into the atmosphere, reducing the localized risk of a ground-level fire compared to LPG.

Comparative Chemical and Physical Analysis

Environmental Footprint and Combustion Chemistry

Complete Combustion Equations

Both fuels burn cleanly when supplied with adequate oxygen, producing significantly lower particulate matter (PM2.5 and PM10) than coal, diesel, or petrol.

• Methane (CNG) Combustion:
  CH₄ + 2O₂ → CO₂ + 2H₂O + Heat
• Butane (LPG) Combustion:
  2C₄H₁₀ + 13O₂ → 8CO₂ + 10H₂O + Heat

Green Credentials of CNG over Liquid Fossil Fuels

CNG is considered one of the cleanest running fuels for the transportation sector due to its low carbon-to-hydrogen ratio.

• Carbon Monoxide (CO) Reduction: Emissions are reduced by up to 80% compared to petrol vehicles.
• Carbon Dioxide (CO₂) Mitigation: Produces roughly 20–30% less greenhouse gas emissions per unit of energy generated than petrol or diesel.
• Hydrocarbon Emissions: Emits fewer reactive hydrocarbons, leading to less ground-level ozone and photochemical smog formation.

Policy Frameworks and Indian Initiatives

Pradhan Mantri Ujjwala Yojana (PMUY)

Launched by the Ministry of Petroleum and Natural Gas, this flagship social welfare scheme aims to safeguard the health of women and children living below the poverty line (BPL) by providing clean cooking fuel (LPG). It targets replacing hazardous, polluting solid biomass fuels (wood, coal, dung cakes) that cause severe indoor air pollution and chronic respiratory diseases.

City Gas Distribution (CGD) Networks

The Petroleum and Natural Gas Regulatory Board (PNGRB) authorizes entities to develop CGD networks across designated Geographical Areas (GAs) in India. The network focuses on delivering two forms of gas:

• Piped Natural Gas (PNG): Natural gas supplied directly to households, commercial entities, and industries via a network of pipelines. It removes the logistical need for heavy LPG storage cylinders.
• CNG Stations: Fueling stations established to supply high-pressure natural gas for public transport networks and private vehicles.

Key Facts and Trivia for Prelims

• Dr. Walter Snelling: Discovered in 1910 that volatile gases in petrol could be converted into liquids and bottled, effectively inventing the LPG industry.
• The “Lighter than Air” Safety Shield: Because CNG’s auto-ignition temperature (540°C) is much higher than petrol (246°C) and diesel (210°C), it is significantly less likely to ignite accidentally on hot engine surfaces.
• Equivalent Heat Footprint: One kilogram of LPG produces an amount of heat equivalent to roughly 2.5 kg of coal, 1.3 liters of petrol, or 4.3 kg of seasoned firewood.
• The Cryogenic Distinction: Do not confuse CNG with LNG (Liquefied Natural Gas). While both are primarily methane, CNG is compressed gas at room temperature, whereas LNG is natural gas cooled to a cryogenic liquid state (-162°C) for long-distance oceanic shipment.