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 1/600th 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 the 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.
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.
Naturally, liquefaction is advantageous as it can be transported or stored to a greater quantity. 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 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.
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.
Liquefaction Temperatures of LNG
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.
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.
- 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%, 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.
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.
- Higher vapor pressure at the required liquefication temperature of Natural Gas
- Relatively smaller volumetric flowrate in low-pressure Nitrogen circuit.
- 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 allowing 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
- EN 1160
- EN 14620
- EN 1474
- EN 1532
- EN 13645
- 33 CFR Part 127
- API 620
- OISD 194
- NFPA 30.
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
- 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 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 of 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 to be 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 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 delivery point by instruments like gas chromatograph.
Amount of the sulphur 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 increasing 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.
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.