Liquified Natural Gas - LNG

Characteristics of LNG

LNG – Liquefied Natural Gas – is a clear, odorless, colorless, non-corrosive and non-toxic, cryogenic fuel which is composed primarily of methane and be stored below methane’s boiling point, -161ºC, at near atmospheric pressure. At this temperature and pressure the methane exists as a liquid. Depending on the composition of the feed gas, the volume ratio between the gaseous state and the liquid state is about 600:1. This makes LNG suitable for storage and transportation in large quantities. The heating value of LNG is estimated to a lower heating value of about 21000 MJ/m³. LNG is very efficient as energy carrier and so there are some advantages to use it.

• Environmentally friendly - methane, a dangerous greenhouse gas, is captured and processed instead of vented to the atmosphere.

• Cost-effective fuel source - processed gas powers onsite equipment or can be sold to nearby gas pipelines.

• Potential for carbon credit trading as a qualified emissions reduction.

Processes of coal mine methane (CMM) upgrade and liquefication

The properties of CMM depend on the coal bed source and the method of its recovery. They may vary over time and depend on the conditions of its extraction. The typical components of CMM include methane, carbon dioxide and nitrogen. Substances as hydrocarbons of longer chains, water vapor, oxygen, hydrogen, carbon monoxide, helium, hydrogen sulfide, chlorine hydrogen, fluoride hydrogen, ammonium and mercury may occur in smaller or trace amounts. Similarly as in the case of other mine gases e.g. natural gas, CMM requires purification before any application or processing including liquefaction in order to remove the undesired substances.

CMM purification

Removal of dust and stray oils

All CMM-to-LNG plants must have inlet separators to remove dust, compressor oil and any entrained water or hydrocarbons. Liquids and solids are removed by gravity separation-coalesced on filter media or mesh pad. Compounds which can freeze in LNG must be reduced.

Dewatering

Gas extracted from coal beds usually contains water vapor. Its content in CMM may pose serious problems in low-temperature (cryogenic) gas processing installations because of the condensation and formation of ice traps and crystalline hydrocarbon hydrates which may block the installation. Therefore gas must be dried before further processing. In the case of high-methane content gas liquefaction, the drying must be very deep to the level <1 ppm.

H₂S and CO₂ removal

Hydrogen sulfide as well as other sulfur compounds e.g. mercaptans must be eliminated from CMM due to the following reasons:

• The toxic properties of hydrogen sulfide disqualify the gas with H₂S content for household use.

• Hydrogen sulfide causes intensive metal corrosion. Therefore its presence in the gas results in damages of the fittings and the gas processing installation.

• The purify specifications for gas application for chemical syntheses regarding hydrogen sulfide are very high as it is toxic for the catalysts. Moreover, the presence of H₂S in the raw material may affect the quality of the obtained chemical product.

The above mentioned reasons indicate that hydrogen sulfide removal from high-methane gas is a technological necessity. If contained in the gas, carbon dioxide is removed together with hydrogen sulfide. In the case of low temperature gas processing, there is a risk of CO₂ solidification which may cause stoppage in the fittings

Degasolination

C3+ hydrocarbons are eliminated from gas by degasolination. The outputs: liquid gas and light benzine (called gasoline) are valuable fuels and materials for chemical syntheses.

Denitrification

Gases with high nitrogen content represent much lower calorific value then low- nitrogen gases. Therefore nitrogen as gas component should be reduces as far as possible.

Oxygen management - thermal oxidation

This type of oxygen removal system is based on catalytic combustion of oxygen. The process operates below the auto-ignition temperature of methane at process pressure. The reactor contains the catalyst and is designed to have a sufficient volume of catalyst necessary to burn the amount of oxygen required. When the oxygen content in the feed gas stream is high, the oxygen in the feed gas must be diluted to prevent overheating and damage to the catalyst due to the exothermic reaction. The catalytic reaction oxidises methane and produces water and carbon dioxide. Following the gas flows through two heat exchangers. Condensed water is removed in a separator and the gas exits the oxygen removal unit.

Liquefaction of the mine gas

The choice of the gas liquefaction technology depends on the desired efficiency performance of the installation, gas composition (content of CO₂, H₂S, N₂, heavier hydrocarbons) and its pressure. There are three hydrocarbon gas liquefaction technologies presented in the literature sources:

• classical cascade cycle;

• auto refrigerant cascade cycle (using mixed refrigerant of hydrocarbons extracted from the liquefied gas);

• decompression cycle with a turbo-expander unit.

Classical cascade cycle

The task of the classical cascade process is cooling the natural gas in three refrigeration cycles. Propane, ethane and methane are used as refrigerants. Feed gas stream purified from water and CO₂ content flows under the pressure of 3-4 MPa into a train of cryogenic exchangers. It is cooled in three successively lower refrigeration levels forming a cascade train. After decompression each of the refrigerant streams (propane, ethane, methane) undergoes a successive, several-stage compression to increase the energy savings of the process. Low energy consumption is the primary advantage of the classic cascade method. About 0,5 kWh is used to liquefy 1 m³ of gas. All these requirements result in high operational costs of the installation.

Auto refrigerant cascade cycle

This method is a classical cascade technology. However, only one compressor and one refrigerant are applied. The refrigerant is a mixture of hydrocarbons extracted from the C₂⁺ fraction condensed from its methane fraction. A mixed refrigerant cascade train with propane cycle pre-cooling consumes about 0,6 kWh/kg LNG. It is by several per cent more than the classical cascade process. Another important advantage of this system is the production of the circulating refrigerant directly from the liquefied natural gas.

Decompression cycle

The decompression cycle based installations for hydrocarbon gas liquefaction perform according to the principle similar to the classical Joule-Thompson natural gas liquefaction method and installations for liquid oxygen and nitrogen production by cryogenic air fractionation. Gas liquefaction by decompression cycle is characterized by low efficiency; however, it is simple and requires relatively low investment outlays. The key element of the process is a turboexpander, in which 85% of the gas is decompressed and cooled to cryogenic temperature. Decompression cycle trains are usually built in locations where the energy needed for the process is cheep as the energy demand of these installations is much higher than in cascade cycles.

Examples of Use

•Snøhvit, Norway: The project is an example for offshore LNG utilization and starts in the fourth quarter of 2007. The capacity of the LNG venture is 4.3 million tonnes per annum.

•Krupinski Coal Mine near Katowice, Poland: Planned for conversion of10000 Gallons of LNG per day.

•Kwinana LNG Plant, Western Australia: Since August 2009 the plan was started up by Wesfarmers Energy which liquefied 175 t LNG per day.