№7 July - August 2011Table of contents Issue Archive
№7 July - August 2011Table of contents Issue Archive
№ 4 (April 2008)
Liquefied natural gas (LNG) production is based on significant (approximately 600 times) reduction of volume of this gas
By A. Golubov, I. Katorgin
It results in considerable reduction of expenses on storage and transportation which can be realized by special-purpose vessels, containers, motor and rail transport, etc., which enables LNG to become a serious competitor of the traditional pipeline gas. Moreover, it is rather cheap to store LNG; a huge “energy buffer storage” is formed, access to which is ensured within few seconds.
Technologies for natural gas liquefaction were first applied in the 1960s. They found an application in the countries obtaining huge reserves of natural gas and situated far from the main gas market, such as Algeria and Middle East countries. Productivity of large plants for natural gas liquefaction exceeds 2 million tons a year or 6,000 tons of LNG a day. Depending on the plant productivity, technologies used for gas treatment and liquefaction, chemical composition of the supplied compressed gas, the cost of LNG plants can vary from dozens of millions to several billions of dollars. Immense investment is often compensated by relatively low power consumption.
In the last years, new technologies were developed, which are actively used at LNG plants of medium productivity (about 1.5 million tons of LNG a year). Mixed refrigerant cycle (MRC) is usually used at these plants. Plants of this type consume more energy, but this is compensated by lower capital costs. A plant with daily output of 200 tons a day (0.07 million tons a year) should be defined as the LNG mini-plant. The total area taken by such a plant is 10,000 sq. m (of which 5,000 sq. m falls on the systems of gas preliminary treatment and liquefaction; 5, 000 sq. m – on the auxillary systems, LNG storage, and the LNG trailers filling system). The maximum height of the utilized equipment does not exceed 10 m (the LNG storage tanks make an exception).
Technologies Used for Natural Gas Liquefaction
In general, there are two technological solutions that may be used for LNG plants, and these are:
the “open cycle” technology;
the “nitrogen expansion cycle” technology.
The “Open Cycle” Liquefaction Technology
In this case, pressure of supplied gas is used as a source of energy for its cooling. High-pressure gas passes through turbines, gas expansion results in its cooling, and in the result, liquid is formed at the outlet of the system. Application of this technology results in liquefaction of only 15 percent of gas, and the remaining 85 percent leave the system in the form of low-pressure gas. The problem of utilization of the remaining 85 percent of gas can be solved if there is a consumer close by. In case of absence of consumers in the region, certain energy will be required for gas re-compression, so that gas can be supplied to the gas pipeline.
The following can be listed as advantages of this technology:
Low power consumption: only 0.01 kW/h of electric power is used for every ncu. m/h of the plant output;
Low operating costs.
Disadvantages of the system include the following:
Relatively high capital expenses. As only 15 percent of the supplied gas is liquefied, a cooling chamber and a system for preliminary gas treatment are necessary for the process. For a system having capacity of 6,000 ncu. m/h (100 tons/day), it is necessary to treat and supply gas to the cooling chamber at the rate of approximately 60,000 ncu. m/h.
Gas turbines are also necessary for this system, with rather high safety requirements.
The “Nitrogen Expander Cycle” Technology
This technology is based on nitrogen circulation in the closed loop system including compressors and turbines. Nitrogen is cooled and supplied to heat exchangers, in which LNG is formed. Then nitrogen is heated again, supplied to the compressor, turbine, cooled, etc.
Advantages of the system:
Low capital expenditure, 100 percent liquefaction of supplied gas and, in consequence, the use of relatively small cooling chamber and preliminary treatment system.
Safety. In fact, nitrogen only compresses and expands. That is why the gas processing cycle includes nitrogen circulation through a simple heat exchanger. Safety requirements are minimal.
Simplicity of the technology.
Ease of operation.
Possibility to quickly bring into operation.
Minimal requirements to space occupied by the plant.
High level of power consumption: depending on the plant size, it varies from 0.42 to 0.5 kW/h per each ncu. m/h of the plant output.
LNG is stored in tanks designed for cryogenic fluids, which ensure high degree of heat insulation. If even small amount of heat is received from the surrounding environment, then the liquid (LNG) partially passes to gaseous phase, which results in gas leakage. Solutions offered by Promtechnokom, Ltd. will reduce this leakage to the minimum possible level.
Tanks for LNG storing are designed as “vessel inside a vessel”. The inner vessel is made of stainless steel and can withstand temperature to minus 196 Celsius, and the simpler in design outer vessel is made of carbon steel. The space between the vessels is filled with special heat-insulating material, which generally consists of pearlite and vacuum. Vacuum acts as the major heat insulator.
The plant-manufacturer can manufacture LNG storing vessels virtually of any capacity. To reduce costs, including saving on the delivery cost and installation at the operation site, vessels are often manufactured at the place of operation. Basic differences between vessels manufactured at the plant and those manufactured at the operation site are as follows:
The higher pressure could be maintained in the first ones.
The latter have no vacuum between the inner and outer vessels. Only pearlite ensures heat insulation.
Among significant problems of larger capacity vessels manufactured at the plant one should mention the cost of their transportation to the operation site. Transportation costs increase significantly in cases when the vessel capacity exceeds 75 cu. m. For example, expenses on delivery of a vessel with capacity of 300 cu. m from Europe to Tyumen region amount to nearly 120,000 euros. Construction of vessels at the site can take up to several months. For example, it takes six months to manufacture a vessel with
1 200 cu. m capacity. Moreover, a lot of preparatory work should be done to fix a place to install such a vessel (which takes nearly three months). Operators should consider this type of vessels only if the total volume of the storage exceeds 600-1 000 cu. m. Many factors need to be taken into consideration in the process of decision-making regarding selection of the vessel type. For example, if the LPG storage is located far from the main motor and railway roads, and its delivery by motor transport seems problematic, the option of vessel fabrication at the place of operation will be more appropriate.
LNG Storing: Dealing with the “Venting” Problem
The temperature of LNG stored in the vessels can increase causing methane transition from liquid to gaseous state. Technologies of Promtechnocom which offers the Russian customers turn-key fabrication of LNG mini-plants make it possible to produce supercooled LNG. This means that product losses due to evaporation will begin only two weeks after the moment of production and will amount to 0.25 percent of the total volume of stored LNG per day. Therefore, no systems for gas return are provided for at the plant. In case if the losses (which are really insignificant due to the small size of the storage) occur anyway, gas is released to the atmosphere. Certainly, it is a “lost product”, but if this is not done, it will be necessary to shut down the plant, down-time of which will cost much more.