Liquified Natural Gas: Properties, Uses, Origin, Composition, LNG Process (With PDF)

Liquefied Natural Gas or LNG is natural gas with the primary element as methane. The Liquefied Natural Gas is converted to liquid form for ease of transport and storage. While in liquid form, Liquified Natural Gas takes up around 1/600^th of the volume of its gaseous form. So, LNG can easily be transported in liquid form in locations where natural gas transportation through pipelines is not feasible. Special tankers carry this liquefied natural gas to the terminals where the LNG is returned to the gaseous phase and distributed through pipelines.

Characteristics of Liquefied Natural Gas

Liquefied natural gas or LNG is colorless, odorless, non-toxic, and non-corrosive. The main characteristics of liquefied natural gas are

It is a cryogenic liquid so must be handled using special materials and technologies.

LNG is stored in special containers.
It is a fossil fuel created by organic deposited materials.
The boiling point of LNG is typically -162 Deg. C

The density of liquefied natural gas varies between 430 Kg/m³ to 470 Kg/m³.
At ambient conditions, LNG will convert to vapor form.
LNG is non-flammable.

Liquefied Natural Gas has a very hot flame temperature means it rapidly burns and creates huge heat because its heat of combustion is 50.2 MJ/kg.
It is hazardous if not contained properly.
LNG is a very good source of energy.

Uses of Liquefied Natural Gas

LNG or liquefied natural gas is used widely for the following applications.

To generate electricity or power.
Used as fuel for industrial and commercial boilers.

for heating water and buildings, to cook in residential applications.
For Road transport as LNG vehicles
For sea transport in ships, ferries, etc

Used as fuels for furnaces, fluid bed dryers
As marine fuel

Origin of Natural Gas

Natural gas exists in nature under pressure in rock reservoirs in the Earth’s crust, either in conjunction with and dissolved in heavier hydrocarbons and water or by itself. It is produced from the reservoir similarly to or in conjunction with crude oil.

Natural gas has been formed by the degradation of organic matter accumulated in the past millions of years. Two main mechanisms (biogenic and thermogenic) are responsible for this degradation. Natural gas produced from geological formations comes in a wide array of compositions. The varieties of gas compositions can be broadly categorized into three distinct groups:

Non-associated gas – it occurs in conventional gas fields
Associated gas – it occurs in conventional oil fields, and

Unconventional natural gas.

Unconventional gas

It occurs outside of the former two. The most common types of unconventional gas are:

Tight gas – natural gas produced from reservoir rocks with such low permeability that massive hydraulic fracturing is necessary to produce the well at economic rates;

Coalbed methane – methane adsorbed into the solid matrix of the coal;
Natural gas from geo-pressurized aquifers;
Gas hydrates – methane clathrate is a solid clathrate compound in which a large amount of methane is trapped within a crystal structure of water, forming a solid similar to ice;

Deep gas

Composition of Natural Gas

Natural gas is a complex mixture of hydrocarbon and non-hydrocarbon constituents and exists as a gas under atmospheric conditions.

Raw natural gas typically consists primarily of methane (CH4), the shortest and lightest hydrocarbon molecule. It also contains varying amounts of:

Heavier gaseous hydrocarbons: ethane (C2H6), propane (C3H8), normal butane (n-C4H10), iso-butane (i-C4H10), pentanes, and even higher molecular weight hydrocarbons.
Acid gases: carbon dioxide (CO2), hydrogen sulfide (H2S), and mercaptans such as methanethiol (CH3SH) and ethanethiol (C2H5SH).
Other gases: nitrogen (N2) and helium(He).

Water: water vapor and liquid.
Liquid hydrocarbons: crude oil and/or gas condensates.
Mercury: only trace amounts.

Refer to the table in Figure 1 for a typical composition of Natural gas.

Table showing typical composition of natural gas — **Fig. 1: Table showing the typical composition of natural gas**

LNG Process

Naturally, liquefaction is advantageous as it can be transported or stored in a greater quantity. The LNG Process is the process of liquefaction. The process of cooling the gaseous LNG to -259°F or -162°C for transforming it into liquid is known as the LNG Process. The process is actually a chain of methods, hence popularly known as LNG Process Chain.

Natural Gas – Exploration to End-User

Fig. 2 below shows the flow chart for Liquefied natural gas exploration.

Flow chart showing exploration of natural gas — **Fig. 2: Flow chart showing the exploration of natural gas**

LNG Plant

An LNG plant refines the crude natural gas received from deep within the earth and condenses it into a pure, concentrated, efficient, liquid form of energy. Three basic processing steps are performed in the LNG plant. These are:

Purification of the extracted natural gas by removing dust, acid gases (CO2), helium, water, and heavy hydrocarbons.
Liquefaction by condensing and cooling it to approximately −162 °C.

Transportation of the liquefied natural gas to the consumer by sea or road transport.

Typical processes of a 2-train LNG plant are shown in Fig. 3.

A typical 2 train LNG plant — **Fig. 3: A typical 2-train LNG plant**

Liquefaction Temperatures of LNG

**Fig. 4: Image showing liquefaction temperature**

LNG Process Flow

Fig. 5 shows a Schematic of a Simple Refrigeration Cycle (LNG Process Flow)

Natural Gas Liquefaction Techniques

Different LNG Process liquefaction techniques include:

Single Refrigeration cycle
Multiple Refrigeration cycles

Self Refrigerated cycles
Cascade Processes
The cooling of natural gas involves the use of refrigerants which could either be pure component refrigerants or mixed component refrigerants.

**Fig. 5: Schematic of a Simple Refrigeration Cycle**

LNG Process Liquefaction Technologies

LNG process liquefaction is performed using various technologies mentioned below:

CASCADE PROCESS by ConocoPhillips
C3MR or AP-X by Air Products

DMR by Shell
Mixed Fluid Cascade – MFC by Linde
Liquefin by Axens / Air Liquide

Liquefied Natural Gas by CASCADE Process

Most Straight Forward of All Processes
Kenai Plant Continuous Operation 1969
CoP License, Plant Build by Bechtel.

The raw gas is first treated to remove typical contaminants.
Next, the treated gas is chilled, cooled, and condensed to -162 ˚C in succession using propane, ethylene, and methane.
The last stage is pumping LNG to storage tanks and awaiting shipment.

**Fig. 6: Schematic of Cascade process**

Pure component Refrigerants
Specific operating ranges for each component
Mixed Refrigerants

Modified to meet specific cooling demands.
Helps improve the process efficiency
Mixed refrigerants are mainly composed of hydrocarbons ranging from methane to pentane, Nitrogen, and CO2. Typically, Methane – 25-30%, Ethane – 45-55%, Propane – 15-20%, Nitrogen – 1-5%, and Butane – 1-2%.

Liquefied Natural Gas by Single MR Process

Significant improvement from Cascade Process
The use of Coil wound Heat Exchangers & MR refrigerant simplified the process.
Mixed Refrigerant offered a way to provide the required refrigeration over the temperature range required.

C3MR process of Liquefaction of LNG Process

Introduction of Propane as Pre-cooling to liquefication
Improved Efficiency, increased single train capacity
Reduction in MR refrigerant volumetric flow due to pre-cooling by Propane

Train size continued to grow with larger drivers & larger compressors
Liquefication capacity up to 5 MMTPA.

Schematic of C3MR process — **Fig. 7: Schematic of the C3MR process**

Liquefied Natural Gas by AP-X Hybrid LNG Process

Improved C3-MR process – pre-cooling by Propane, liquefaction using MR, and sub-cooling using Nitrogen Cycle.

Nitrogen Cycle has a simple & efficient expander loop.
Increased capacity by a reduction in volume flow of MR (40%of C3MR) & Propane (20% of C3MR).
Liquefication capacity up to 8.0 MMTPA.

Nitrogen Cycle is a simplified version of the cycle employed by Air Products in Air Separation plants.

Why Nitrogen:

Higher vapor pressure at the required liquefication temperature of Natural Gas
The relatively smaller volumetric flow rate in low-pressure Nitrogen circuits.

Improved efficiency by reducing pressure losses

DMR LNG Liquefaction Process

DMR – Dual Mixed Refrigerant is very similar to C3MR
The difference is in the utilization of a second pre-cooling refrigerant component.

The use of two mixed refrigerant cycles allows full utilization of power in a design with two mechanically driven compressors.
It allows keeping the compressors at their best efficiency point over a very wide range of ambient temperature variations and changes in feed gas composition.
The natural gas stream is cooled via two stages. The first stage cools natural gas to -50°C while the second column cools natural gas to LNG at -160°C.

Liquefied Natural Gas using Liquefin by Axens (Air Liquide)

Developed by IFPEN and AXENS, now owned by Air Liquide.
a highly efficient process and provides the most cost-competitive LNG product per ton.
is optimized best with the Brazed Aluminium Heat Exchanger, leading to further cost reductions and scalable output.

Compact and modular design
Balanced refrigeration power allows for identical refrigerant compressor drivers
Very cost-effective solution

Codes and Standards for Liquefied Natural Gas

Stringent code and standard guidelines are followed at every step of the LNG process to ensure safety. The primary LNG codes and standards are

NFPA 59A
EN1473

EN 1160
EN 14620
EN 1474

EN 1532
EN 13645
33 CFR Part 127

API 620
JGA-107-RPIS
JGA-108-RPAS

JGA-102
JGA-103
OISD 194

NFPA 30.

LNG pricing

The pricing of Liquefied Natural Gas is not straightforward. In the current LNG contracts, three major pricing systems are prevalent as mentioned below:

Oil-indexed contract. Primary user countries are Japan, Korea, Taiwan, and China.

Oil, oil products, and other energy carriers indexed contracts. Specifically used in Continental Europe; and
Market indexed contracts. Used mostly in the US and the UK.;
The equation used for the calculation of an indexed price is as follows:

CP = BP + β X

Here,

BP: constant part or base price
β: gradient

X: indexation

The above-mentioned formula finds its wide use in Asian LNG SPAs. The base price is represented by various non-oil factors but is usually a constant determined by negotiation at a level that can prevent LNG prices from falling below a certain level. It thus varies regardless of oil price fluctuation.

Quality of Liquefied Natural Gas

In the LNG Business, the quality of LNG is one of the most important issues. During trading, any natural gas that does not meet the agreed specifications is termed as “off-specification” or “off-quality” LNG. That’s why the LNG Quality must be regulated. Such regulations serve the following purposes:

Ensures the distributed gas is non-corrosive and non-toxic.
Guards against liquid or hydrate formation in the networks.
Allow interchangeability of the distributed gases by limiting the parameter variation ranges. Such parameters are the content of inert gases, calorific value, Incomplete Combustion Factor, Wobbe index, Soot Index, Yellow Tip Index, etc.

The quality of liquefied natural gas is measured at the delivery point by instruments like gas chromatograph.

Amount of the sulfur and mercury content and the calorific value are the most important gas quality concerns. To ensure the lowest concentration of sulfur and mercury in LNG, the liquefaction process must be accurately refined and tested.
The other concern for LNG is the heating value. In terms of heating value, the natural gas markets can be grouped into three markets as follows:

Asia (Japan, Korea, Taiwan) with a gross calorific value (GCV) higher than 43 MJ/m3(n), i.e. 1,090 Btu/SCF, known as rich gas distribution.

the UK and the US, with a GCV usually lower than 42 MJ/m3(n), i.e. 1,065 Btu/SCF, known as lean gas distribution
Continental Europe with the acceptable GCV range is quite wide: approx. 39 to 46 MJ/m3(n), i.e. 990 to 1,160 Btu/scf.

Sometimes to increase the heating value of liquefied natural gas, propane and butane are injected. In general, the price of lean LNG in terms of energy value is lower as compared to the rich LNG.

LNG Safety

Natural gas is the most environmentally friendly fossil fuel with the lowest CO₂ emissions per unit of energy. But, Natural gas, being fuel and a combustible substance, must be handled with care. For the design, construction, and operation of liquefied natural gas facilities, proper measures must be taken to ensure safe and reliable operation.

However, LNG in its liquid form can not ignite and is not explosive. For LNG to burn, it must vaporize first and mix with air in the proper proportions. During leakage, LNG rapidly vaporizes and turns into a gas that easily mixes with the air. In such a case, there is a risk of ignition causing fire and thermal radiation hazards.