Liquefied Natural Gas (LNG)

Liquefied natural gas (LNG) enables long-distance natural gas transportation and provides strategic storage capacity for utilities and industrial users. The liquefaction process reduces natural gas volume by approximately 600:1, transforming it into a cryogenic liquid at -260°F (-162°C) for efficient storage and shipping. Understanding LNG systems helps HVAC professionals recognize supply chain dynamics, anticipate composition variations during peak events, and evaluate distributed LNG as an alternative fuel source.

LNG Liquefaction Process

Natural gas liquefaction employs cascaded refrigeration cycles to progressively cool gas from ambient temperature to -260°F while removing impurities that would freeze during the process. Pre-treatment removes water, carbon dioxide, hydrogen sulfide, and heavy hydrocarbons (C5+) that solidify at cryogenic temperatures or compromise storage tank metallurgy.

The liquefaction sequence follows these thermodynamic stages:

1. Pre-cooling: Propane refrigeration reduces gas temperature to approximately -40°F
2. Main cooling: Ethylene or mixed refrigerant cycles cool gas to -150°F
3. Final liquefaction: Methane refrigeration completes cooling to -260°F

Each stage operates at progressively lower temperatures and pressures, following the fundamental refrigeration cycle with compression, condensation, expansion, and evaporation. Total liquefaction energy consumption ranges from 8-12% of the natural gas energy content, representing substantial parasitic load.

Modern liquefaction trains employ multiple refrigeration technologies:

• Cascade cycles: Separate refrigerants (propane, ethylene, methane) optimized for specific temperature ranges
• Mixed refrigerant: Single refrigerant mixture with multiple boiling points spanning the cooling range
• Expander cycles: Gas turbine expansion providing refrigeration effect

Large baseload liquefaction facilities achieve economies of scale with trains processing 4-8 million tons per year. Smaller peak shaving plants operate at 1-5 tons per hour for utility load management.

LNG Storage in Cryogenic Tanks

LNG storage requires specialized cryogenic containment systems maintaining -260°F while preventing heat influx and managing boil-off gas. Storage tank designs fall into three categories:

Above-ground double-wall tanks: Inner container fabricated from 9% nickel steel or aluminum alloy tolerates cryogenic temperatures. Outer carbon steel shell contains insulation (perlite, polyurethane foam) maintaining thermal barrier. Tank sizes range from 40,000 gallons (small peak shaving) to 600,000 barrels (import terminals).

In-ground storage: Reinforced concrete walls with cryogenic liner and insulation below grade, offering protection from external hazards and reduced visual profile. Ground freezing requires foundation insulation and heating systems.

Membrane tanks: Used for marine transport, featuring thin stainless steel or Invar membrane supported by insulation within the ship hull structure.

Heat leakage through insulation, penetrations, and supports causes LNG evaporation (boil-off) at 0.05-0.20% of tank volume per day for large storage. Smaller tanks exhibit higher boil-off rates due to unfavorable surface-to-volume ratios. Boil-off gas serves as fuel for plant operations or supplements pipeline gas, preventing waste and pressure buildup.

Tank design pressure remains low (0.5-2 psig) due to minimal pressure resistance required for liquid storage. Pressure relief valves, vacuum breakers, and rollover prevention systems ensure safe operation.

LNG Regasification Terminals

Regasification facilities convert LNG back to gaseous form for pipeline injection. The process involves:

1. LNG transfer: Specialized cryogenic pumps move LNG from storage to vaporizers
2. Pressure buildup: High-pressure cryogenic pumps raise pressure to 600-1,400 psi
3. Vaporization: Heat exchange converts liquid to gas at pipeline pressure
4. Odorization: Mercaptan injection for leak detection (if required)
5. Metering and pressure control: Final conditioning for pipeline delivery

Vaporization methods include:

Open-rack vaporizers (ORV): Seawater or freshwater flows over aluminum panels while LNG passes through internal passages, providing heat transfer. ORV systems offer high reliability and minimal energy consumption but require adequate water supply and create thermal discharge.

Submerged combustion vaporizers (SCV): Burners heat water bath surrounding LNG coils, independent of ambient conditions. SCVs provide rapid startup, compact footprint, and operation flexibility but consume fuel and emit combustion products.

Ambient air vaporizers: Natural convection or forced air flow over finned coils, requiring no fuel or water but occupying large area and experiencing capacity reduction in cold weather.

Intermediate fluid vaporizers: Closed-loop glycol or propane circuit transfers heat from seawater or air to LNG, preventing seawater freezing and simplifying maintenance.

Terminal sendout capacity ranges from 100 MMscfd (small utilities) to 4 Bcfd (major import terminals). Regasification energy consumption remains minimal (0.5-2% of gas energy content), primarily for pumping and auxiliary systems.

LNG Transport Ships

Marine LNG transportation employs specialized vessels maintaining cryogenic cargo throughout voyages spanning 10-30 days. Ship capacities range from 30,000 m³ (small coastal) to 270,000 m³ (Q-Max vessels), with modern vessels typically 150,000-175,000 m³.

Containment systems include:

Moss spheres: Independent spherical aluminum alloy tanks protruding above deck, offering proven reliability and easy inspection

Membrane systems: Corrugated stainless steel or Invar membrane supported by plywood and insulation within ship hull, maximizing cargo capacity for given ship dimensions

SPB (Self-supporting Prismatic type B): Angular tanks spanning hull width, balancing capacity and construction efficiency

Boil-off gas during transport (0.10-0.15% per day) provides fuel for ship propulsion, reducing or eliminating the need for conventional bunker fuel. Modern vessels employ reliquefaction plants recondensing boil-off when gas exceeds propulsion requirements, preventing venting and preserving cargo.

Specialized loading and unloading arms transfer LNG at rates up to 12,000 m³/hr while maintaining cryogenic integrity and preventing leaks. Double-walled, vacuum-insulated transfer lines minimize heat influx during operations.

Volume Reduction: 600-to-1 Ratio

LNG’s fundamental advantage lies in dramatic density increase compared to gaseous natural gas. At standard conditions (60°F, 14.7 psia), natural gas occupies 600 times more volume than the equivalent mass in liquid form at -260°F.

This volume reduction follows from density relationships:

• Gaseous natural gas: 0.045 lb/ft³ (60°F, 14.7 psia)
• LNG: 26.4 lb/ft³ (-260°F, 14.7 psia)
• Density ratio: 26.4 / 0.045 ≈ 587 (rounded to 600:1)

The 600:1 ratio makes ocean transport economically viable and provides compact energy storage. A 20,000-gallon truck transports LNG equivalent to 12 million cubic feet of gaseous natural gas, enabling distribution to areas without pipeline access.

Storage economics similarly benefit from volume reduction. Underground natural gas storage in depleted reservoirs requires massive geological formations, while equivalent LNG storage occupies small tank volumes at atmospheric pressure.

LNG Safety Considerations

LNG safety encompasses cryogenic hazards, flammability risks, and rapid phase transition potential. Proper understanding prevents incidents and guides facility design.

Cryogenic hazards: Direct LNG contact causes immediate frostbite and embrittlement of ordinary carbon steel. Materials selection requires aluminum alloys, 9% nickel steel, stainless steel, or other cryogenic-rated metals. Personnel protective equipment and emergency procedures address potential exposure.

Flammability: LNG vapor mixes with air at concentrations of 5-15% create flammable atmosphere. Vapor is initially heavier than air due to cold temperature, accumulating in low areas until warming reduces density. Adequate ventilation, gas detection, and ignition source control prevent vapor accumulation and ignition.

Rollover: Stratified LNG layers of different density may undergo sudden mixing, rapidly generating boil-off and potential tank overpressure. Proper filling procedures, density monitoring, and rollover prevention systems mitigate this risk.

Rapid phase transition (RPT): LNG spilled on water may undergo explosive vaporization if superheating occurs at the interface. LNG terminals incorporate spill containment, controlled drainage, and emergency response procedures.

Dispersion modeling: Vapor cloud modeling predicts hazard zones for facility siting, determining safe separation distances from populated areas and critical infrastructure.

LNG facilities incorporate multiple safety layers: isolation valves, emergency shutdown systems, fire detection, water deluge protection, and comprehensive emergency response planning.

LNG Peak Shaving Plants

Utility peak shaving facilities liquefy natural gas during low-demand periods (summer) and store it for vaporization during high-demand events (extreme winter cold). This strategy reduces pipeline capacity requirements and provides supply security during demand surges.

Peak shaving plant components include:

• Small-scale liquefaction (1-10 tons/hr)
• Cryogenic storage tanks (100,000 to several million gallons)
• Vaporization equipment sized for peak sendout
• Backup power generation for critical systems

Economic justification balances capital cost against avoided pipeline capacity charges and supply curtailment risks. Plants typically operate 10-30 days per year during extreme weather, making them standby facilities requiring high reliability.

For HVAC professionals, peak shaving operations may introduce composition variations as LNG preferentially contains lighter hydrocarbons (methane, ethane) compared to pipeline gas. Equipment calibrated for pipeline gas typically accommodates these variations without adjustment due to similar Wobbe Index, though minor performance changes may occur.