Liquefied natural gas

Liquefied natural gas (LNG) is a state-of-the-art type of fuel produced by liquefaction of natural gas using cryogenic technologies. Liquefied natural gas in its operating, economic and ecological properties compares favorably with other types of fuel. Firstly, LNG possesses all advantage of natural gas. Secondly, energy concentration in LNG volume unit is almost 600 times more than in gaseous state. In addition storage and use of liquefied natural gas is considerably more safe that storage of compressed natural gas or liquefied petroleum gas.

In spite of all advantages and the largest natural gas reserves in the world there is no a unified LNG production and consumption system in Russia. Currently five LNG production plants operate in Russia. The main of them have been built by Cryogaz CJSC and located in the Leningrad region.

The Natural Gas Liquefaction Plants are installed at the following sites in the gas industry:

- Automatic Gas Filling Compressor Stations (AGFCS);

- Gas Distribution Stations (GDS) of cross-country gas pipelines.

These plants are distinguished by low capital investments as they use standard compressor equipment (at AGFCS) or the existing gas pressure drop (at GDS).

A variant of modernization of standard Natural Gas Liquefaction Plant at AGFCS is described in this article.

Standard low-capacity LNG Production Plant at AGFCS

A standard Liquefaction Plant for ACSGF built in 1997 operates in high-pressure circuit with pre-cooling by freon refrigerating machine. The refrigerating machine operates on R22 freon. Its refrigeration capacity is 131 kW in rated mode. The structural scheme of the Liquefaction Plant is shown in Fig.1. ACSGF standard compressor equipment is used for natural gas compression up to 20 MPa. Compressed natural gas in the amount of 3600 nm3/hour is fed from recuperative heat exchanger (Т1). Compressed natural gas is cooled down to ~267К (-6 ºС) in the heat exchanger by means of heat exchange with low-pressure return flow. Then the gas is fed to evaporator exchanger where it is cooled down to ~235К (-38 ºС) by means of heat exchange with freon boiling in intertubular space. Cooled compressed natural gas is fed to recuperative heat exchanger (Т2) where it is cooled down to ~183К (-90 ºС). After heat exchanger (Т2) the flow is throttled on throttle orifice (DR) to vapor-liquid space up to pressure 0.56 MPa (abs.). The vapor-liquid mixture is fed to LNG storage tank (Е) where the liquid phase is condensed and vapor phase is returned to the Liquefaction Plant as return flow. The performance of this plant is approximately 1000 kg of liquefied natural gas per hour. Liquefaction factor at throttle is -41 %.

Fig.1 Structural scheme of Liquefaction Plant.

Energy costs of liquefied natural gas production for this Plant are provided in Table 1.

Table 1. Main electric equipment of standard Natural Gas Liquefaction Plant

Electric consumer	Power, kW
AGFCS methane compressors
Compressor motor (2 pcs.)	640
Oil pump	2.2
Cooling water pump	8
Fan motor (2 pcs.)	7.4
Total	657.6
К-127 refrigerating machine
Compressor and oil pump motors	171
Cooling air fan motors	22
Total	193

Total power consumption for LNG production by a standard Plant is 850.6 kW.

Therefore at operation in design conditions specific energy consumption for production of 1 kg of LNG is 0.85 kW/kg.

Energy costs are one of the main components of liquefied natural gas production cost.

Proposal for Liquefaction Plant modernization

Throttling process used in the standard Plant is inconvertible. Therefore it is expedient to use an expansion machine (expander). Parallel connection of an expander-generator unit will allow:

- increasing the Liquefaction Plant capacity;

- reducing specific energy consumption and production cost of liquefied natural gas;

- conversing compressed natural gas energy into electric energy;

- increasing the Liquefaction Plant reliability.

The structural scheme of the Liquefaction Plant based on an expander-generator unit is shown in Fig.2.

Fig.2 Structural scheme of modernized Liquefaction Plant. The modernized Liquefaction Plant differs from initial version by an expander-generator unit connected in parallel.

The new Plant can work in three modes:

- expander mode;

- refrigerating machine mode;

- expander and refrigerating machine mode;

When working in expander mode the high pressure gas (3600 nm3/h) after compressors is fed to tubular space of heat exchanger Т1. In heat exchanger Т1 gas is cooled down to temperature 250К (-23 °С) by means of heat exchange with return flow taking place in intertubular space. Then gas bypassing disconnected evaporator exchanger EA is divided in two flows; the first flow (2270 nm3/h) goes for expansion in expander and the second flow (1330 nm3/h) is fed to heat exchanger Т2 where it is cooled down by vapors from storage systems and by gas return flow. After heat exchanger Т2 high pressure cooled gas is throttled on throttle valve DR to pressure 0.5 MPa. The liquid phase from formed vapor-liquid mixture is condensed as LNG and vapor phase is fed as return flow to heat exchanger T2. The cryogenic cycle described in T-S diagram is shown in Fig.3.The modernized plant capacity at operation without refrigerating machine is 815 kg of LNG per hour. In addition the throttle liquefaction factor is 95 % and electric power generated by the generator is 100 kW.

As a result total electric power consumed for production is 557.6 kW and specific energy consumption is 0.68 kW/kg.

The main indicators of the modernized Liquefaction Plant operation are given in Table 2.

Table 2 Main indicators of modernized Liquefaction Plant operation

Parameter	Unit of measure	Operation mode
		Refrigerating machine mode	Expander mode
Capacity by LNG	kg/h	1000	815
Throttle liquefaction factor	%	41	95
Power consumption	kW	850.6	557.6
Specific energy consumption	kW/kg	0.85	0.68

Expander operation mode is preferred as the main one because it allows producing LNG with lower production cost at high output.

Refrigerating machine operation mode is designated for attaining maximum output.

Joint operation of expander and refrigerating machine can be performed at high environment temperature and insufficient gas cooling after compressors.

Specific energy consumption of the modernized Liquefaction Plant is comparable to the one of large-capacity 100 % natural gas liquefaction systems (0.63 kW/kg). Therefore the cycle can be also applied to high consumptions and capacities. In contrast with 100 % liquefaction systems with nitrogen cycle the proposed Plant is significantly cheaper; it does not require use of Nitrogen Intake and Storage System; it requires less space and it is easier-to-work.

The modernized Natural Gas Liquefaction Plant can be built on the base of an Automatic Gas Filling Compressor Station and become the basis for development of LNG production and consumption system in any region of Russia.

Kutsak M. Y.