Liquefied Natural Gas (LNG) is natural gas that has been cooled to -162°C (-260°F), transforming it from a gaseous state into a clear, colorless, non-toxic liquid. This phase change reduces the volume of natural gas by approximately 600 times, making intercontinental transportation economically viable.
What is LNG?
LNG is not a different substance from natural gas—it is simply natural gas in its liquid phase. The transformation occurs through cryogenic cooling at atmospheric pressure. When warmed back to ambient temperature, LNG instantly reverts to its gaseous state (a process called regasification).
Key Definition
Liquefied Natural Gas (LNG): Natural gas cooled to approximately -162°C (-260°F) at near-atmospheric pressure, reducing its volume by 600:1 for efficient storage and transport.
Chemical Composition
Natural gas is primarily composed of methane (CH4), but also contains heavier hydrocarbons and impurities that must be accounted for or removed:
| Component | Chemical Formula | Typical Range (%) | Boiling Point (°C) |
|---|---|---|---|
| Methane | CH4 | 85-95% | -161.5°C |
| Ethane | C2H6 | 4-9% | -88.6°C |
| Propane | C3H8 | 1-3% | -42.1°C |
| Butane | C4H10 | 0-1% | -0.5°C |
| Nitrogen | N2 | 0-1% | -195.8°C |
Impurities Removed During Pre-Treatment
- Carbon Dioxide (CO2): Reduced to <50 ppm to prevent freezing and corrosion
- Hydrogen Sulfide (H2S): Removed to <4 ppm for safety and to prevent equipment damage
- Water (H2O): Reduced to <0.1 ppm to prevent ice formation in cryogenic equipment
- Mercury (Hg): Removed to prevent liquid metal embrittlement of aluminum heat exchangers
Physical Properties of LNG
| Property | Value | Comparison |
|---|---|---|
| Temperature | -162°C (-260°F) | Boiling point at 1 atm |
| Density | 420-450 kg/m³ | ~45% of water density |
| Specific Gravity | 0.42-0.45 | Floats on water |
| Energy Density | 22.2 MJ/L | ~60% of gasoline |
| Volume Reduction | ~600:1 | vs. gas at STP |
| Heat of Vaporization | 510 kJ/kg | High cooling effect |
| Flammability (liquid state) | Non-flammable | Must vaporize to burn |
The 600:1 Volume Reduction Explained
The dramatic volume reduction is LNG's defining advantage:
- 1 cubic meter of LNG = ~600 cubic meters of natural gas (at 15°C, 1 atm)
- 1 tonne of LNG = ~1,380 cubic meters of natural gas
- Energy content: 1 tonne LNG ≈ 52 MMBtu (million British thermal units)
This means a single Q-Max LNG carrier with 266,000 m³ capacity can transport approximately 159 billion cubic meters of natural gas energy equivalent—enough to power a city of 1 million people for several weeks.
The Thermodynamics of Liquefaction
The liquefaction process exploits the Joule-Thomson effect and multi-stage refrigeration cycles to cool natural gas below its critical temperature.
Phase Diagram of Methane
- Critical Temperature (Tc): -82.6°C — Above this, methane cannot be liquefied by pressure alone
- Critical Pressure (Pc): 45.99 bar
- Boiling Point at 1 atm: -161.5°C (-260°F)
- Triple Point: -182.5°C at 0.117 bar
Since methane's critical temperature is -82.6°C, it cannot be liquefied at ambient temperature regardless of pressure (unlike propane or butane). This is why cryogenic cooling is mandatory, not just compression.
Why -162°C Specifically?
The target liquefaction temperature of approximately -162°C is chosen because:
- It is near methane's boiling point at atmospheric pressure (minimal pressurization needed)
- Atmospheric pressure storage reduces tank complexity and weight (critical for shipping)
- Lower temperatures would require exotic materials and higher energy input
LNG vs. Pipeline Gas vs. CNG
Natural gas can be transported in three primary forms, each with distinct trade-offs:
| Transport Method | State | Temperature | Pressure | Volume Reduction | Best For |
|---|---|---|---|---|---|
| LNG | Liquid | -162°C | ~1 bar | 600:1 | Intercontinental shipping |
| Pipeline Gas | Gas | Ambient | 50-100 bar | ~50-100:1 | Regional distribution |
| CNG | Gas | Ambient | 200-250 bar | ~200:1 | Vehicle fuel, off-grid |
Economic Comparison
- Pipeline: Lowest cost per unit distance, but requires continuous infrastructure and fixed route
- LNG: High upfront capital (liquefaction/regasification plants, specialized ships) but flexible routing and no infrastructure between endpoints
- CNG: Limited by tank weight/volume; only economical for short distances or niche applications
Break-even distance: LNG typically becomes cost-competitive with pipelines for distances exceeding ~3,000-4,000 km over water, or when pipeline routes are geopolitically complex (e.g., crossing multiple nations).
Boil-Off Gas (BOG)
Even with excellent insulation, LNG tanks absorb heat from the environment, causing a small percentage of the liquid to evaporate. This phenomenon is called Boil-Off Gas (BOG).
BOG Rates
- Modern LNG carriers: 0.1-0.15% per day
- Older carriers: 0.2-0.3% per day
- Land-based storage: 0.03-0.05% per day (better insulation)
Managing BOG
On LNG carriers, BOG is typically:
- Used as fuel for the ship's engines (dual-fuel or steam turbine propulsion)
- Re-liquefied using onboard refrigeration systems (on newer vessels)
- Burned in a GCU (Gas Combustion Unit) if excess (avoided when possible)
Modern membrane-type carriers with reliquefaction systems can achieve near-zero net BOG, critical for long-duration voyages.
Energy Content & Heating Value
The energy content of LNG varies slightly based on composition (methane vs. heavier hydrocarbons):
| Metric | Value (Typical) | Notes |
|---|---|---|
| Higher Heating Value (HHV) | ~55 MJ/kg | Includes condensation heat |
| Lower Heating Value (LHV) | ~50 MJ/kg | Excludes water vapor heat |
| Energy per tonne (HHV) | ~52 MMBtu | Industry standard unit |
| Energy per cubic meter | ~22.2 MJ/L | Volume-based density |
Conversion Factor: 1 million tonnes LNG per annum (MTPA) ≈ 48 billion cubic meters of natural gas (bcm) ≈ 1.3 billion cubic feet per day (Bcf/d)
Why LNG? The Strategic Rationale
LNG exists to solve a geographic mismatch: the world's largest natural gas reserves (Russia, Qatar, Iran, USA) are often far from demand centers (Asia, Europe). LNG enables:
- Global Arbitrage: Price differentials between Henry Hub (USA), TTF (Europe), and JKM (Asia) create trading opportunities
- Energy Security: Countries without domestic gas can diversify supply (e.g., Japan post-Fukushima, Europe post-Ukraine invasion)
- Stranded Gas Monetization: Remote fields (e.g., offshore Australia, Arctic Russia) become economically viable
- Flexible Routing: Unlike pipelines, LNG cargoes can be redirected mid-voyage based on price signals
Case Study: In 2022-2023, European LNG imports surged from 80 MTPA to over 120 MTPA to replace Russian pipeline gas, demonstrating LNG's role as a "swing supply" mechanism in global energy markets.
Key Takeaways
- LNG is natural gas cooled to -162°C, reducing volume by 600:1
- Composition: 85-95% methane (CH4) plus ethane, propane, nitrogen
- Density: ~450 kg/m³ (less than half of water, will float if spilled)
- LNG is non-toxic, non-corrosive, and non-flammable in liquid state
- Requires cryogenic cooling (not just compression) because methane's critical temperature is -82.6°C
- Boil-Off Gas (BOG) is managed via ship propulsion, reliquefaction, or combustion
- 1 tonne LNG ≈ 52 MMBtu energy content