The main component is methane, is recognized as the cleanest energy on earth. Colorless, odorless, non-toxic and non-corrosive, its volume is about 1/600 of the volume of the same amount of gaseous natural gas, liquefied natural gas weighs only about 45% of the same volume of water. The manufacturing process is to first purify and treat the natural gas produced in the gas field, and after a series of ultra-low temperature liquefaction, it is transported by LNG ships. Combustion of air pollution is very small, and put out a large amount of heat, so the liquefied natural gas is good.
It is natural gas compressed, cooled and liquefied at -160 degrees. Its main component is methane, transported by special ships or tanker trucks, and re-gasified when in use. since the 1970s, the world's liquefied natural gas production and trade has increased rapidly, and in 2005, the international trade volume of LNG amounted to 188.81 billion cubic meters, and the largest exporter is Indonesia, which exported 31.46 billion cubic meters; and the largest importer is Japan, which exported 76.32 billion cubic meters.
Second, the domestic and international overview and development trend
In 1941, the world's first industrial-scale LNG plant was built in Cleveland, USA, with a liquefaction capacity of 8500 m3 /d. The LNG industry has been in the process of developing into an industrial-scale LNG plant since the 1960s. From the 60's, the LNG industry has developed rapidly, the scale is getting bigger and bigger, the basic load type liquefaction capacity is 2.5 × 104 m3 /d. According to [3], there are more than 160 LNG units in operation in various countries, and the total amount of LNG exported is more than 46.18 × 106 t/a.
The main component of natural gas is methane, which has an atmospheric boiling point of -16 1 ℃, a critical temperature of -84 ℃, and a critical pressure of 4.1MPa. LNG is short for liquefied natural gas, which is formed when natural gas is purified (dewatered, dehydrocarbonized, and deacidified) [4], and methane is turned into a liquid using a process of throttling, expansion, and refrigeration from an external cold source [5].
2.1 The current status of foreign research
The liquefaction device abroad is large, complex process, equipment, high investment, basically using step refrigeration and mixed refrigerant refrigeration process, the two types of devices are currently in operation, the new commissioning design of the main mixed refrigerant refrigeration process, the study's main purpose is to reduce the energy consumption of liquefaction. Refrigeration process from the order type refrigeration improvement to mixed refrigerant refrigeration cycle, currently reported and C Ⅱ -2 new process [6], the process has the advantages of pure component cycle, such as simplicity, no phase separation and easy to control, but also have the advantages of mixed refrigerant refrigeration cycle, such as natural gas and refrigerant refrigeration temperature and level with the better, high efficacy, less equipment, etc. Advantages.
Axens France, in cooperation with the French Petroleum Institute (IFP), *** with the development of a new advanced natural gas liquefaction process - Liquefin first industrialized, the process laid the foundation for the LNG market. The Liquefin process has a 15%-20% higher production capacity and a 25% lower production cost than common methods. With the Liquefin method, production rates of more than 600 × 104 t/y per liquefaction unit can be achieved. The cost of LNG production using the Liquefin process can be reduced by 25% per ton [7]. The main advantage of the process is the use of finned heat exchangers and a thermodynamically optimized process that allows the construction of very large liquefaction units. Axens has proposed the process to several major regions in the US, Europe, and Asia, and is in the process of conducting pre-design and feasibility studies. The safety, environmental friendliness, practicality and innovative features of the Liquefin process developed by IFP and Axens have recently been recognized worldwide, with the process receiving the Institution of Chemical Engineers Award for Engineering Excellence [8].
The University of Texas Engineering Laboratory has developed a new patent-pending technology for natural gas liquefaction, GTL. This technology is more suitable for small-scale installations than the currently developed GTL technology and can process 30.5 × 104 m3 /d of natural gas. GTL at the pilot plant has been licensed to Synfuels. The company has set up a GTL pilot plant near the A & M University campus and is currently conducting economic simulations. The new process is much simpler than existing technologies, does not require syngas, and does not require the use of oxygen except for power generation. It is different from the normal Fischer-Tropsch GTL process in terms of economics, scale and production. The first industrial unit is likely to be built in the first half of 2004 [9].
2.2 Current status of domestic research
As early as in 1960s, the State Science and Technology Commission formulated the development plan of LNG, and the industrial test was completed in the mid-1960s. The Weiyuan Chemical Plant of Sichuan Petroleum Management Bureau owns the earliest industrial production unit of deep cooling separation and liquefaction of natural gas in China, which produces LNG in addition to He. In 1991, the plant supplied 30tLNG to the Ministry of Spaceflight as rocket test fuel. Unlike the foreign situation, the domestic natural gas liquefaction research is aimed at small-scale liquefaction process, the literature on this published more [10], the following on the existing domestic natural gas liquefaction plant process is briefly introduced.
2.2.1 Sichuan LNG plant
The 300l/h natural gas liquefaction plant developed by Beijing Koyang Gas Liquefaction Technology Joint Company of the Chinese Academy of Sciences in cooperation with Sichuan Janyang City Koyang Cryogenic Equipment Company is a demonstration project of using LNG as industrial and civil gas peak shifting and replacing oil with gas. The plant was completed in 1992 to provide LNG for LNG automobile research.
The device makes full use of the pressure of natural gas itself, using gas turbine expander refrigeration to make natural gas liquefaction, used for civilian natural gas peaking or production of LNG, the process is reasonable, the use of gas turbine expander, technology is more advanced. The device basically does not consume water, electricity, is an energy-saving project, but the liquefaction rate is very low, about 10%, which is consistent with its design principles.
2.2.2 Jilin oilfield liquefied natural gas device
Jilin oilfield, China National Petroleum Corporation and the Chinese Academy of Sciences cryogenic center jointly developed and developed 500l / h skid mounted industrial test device in December 1996 the overall test successfully, the device uses nitrogen as a refrigerant expander cycle process, the entire device consists of 10 skid block, all equipment Localization [11].
The device adopts gas bearing turbine expander; domestic molecular sieve depth removal of water and CO2 in natural gas, the process is simple, using skid structure, in line with the characteristics of small devices. Adopting pure nitrogen as the refrigeration medium, the power consumption is higher than that of the expander cycle using refrigerant. The pressure of natural gas itself is not fully utilized, and the natural gas is liquefied under medium pressure (around 5.0MPa) (liquefaction under higher pressure can increase the refrigeration temperature of nitrogen and reduce the refrigeration load), so the power consumption of the device is large.
2.2.3 Shaanxi North Gas Field LNG
The 2 × 104 m3 /d "Shaanxi North Gas Field LNG Demonstration Project", which was put into operation in January 1999, is a pilot project for the development of China's LNG industry, and also China's first small-scale LNG industrialized device. The plant adopts natural gas expansion and refrigeration cycle, low-temperature methanol washing and molecular sieve drying for raw gas purification, gas wave refrigerator and turbine expander for low-temperature refrigeration, gas engine as the power source of the compressor cycle, and the exhaust gas of the gas engine as the heat source of heating molecular sieve regeneration. The device equipment all localization. The successful commissioning of the device for China's remote oil and gas fields on the use of natural gas to produce LNG provides experience [12].
2.2.4 Zhongyuan Oilfield LNG Plant
Zhongyuan Oilfield once built China's largest LNG plant, with a feedstock gas scale of 26.6 5 × 104 m3 /d, a liquefaction capacity of 10 × 104 m3 /d, a storage capacity of 1200 m3, and a liquefaction rate of 37.5% [13]. At present, on the basis of fully absorbing foreign advanced technology, combining domestic and foreign equipment, and mainly focusing on the characteristics of its own gas source, the LNG process technology program has been researched [14]. The process uses the commonly used molecular sieve adsorption method of dehydration, liquefaction process selection of propane pre-cooling + ethylene pre-cooling + throttling.
The plant achieved a high yield of 51.4% at a feed gas capacity of 30× 104 m3 /d, with an energy consumption of 0.13 Kwh/Nm3. The advantage is that each refrigeration system is relatively independent, reliable and flexible. However, the process is relatively complex, requiring two kinds of refrigeration media and circulation, and high investment in equipment. Since the plant makes full use of the pressure energy of natural gas from oilfield gas wells, the liquefaction cost is low.
2.2.5 Small-scale liquefied natural gas (LNG) plant of Tianjin University
Compared with large-scale plant, small-scale LNG plant not only has the advantages of raw materials, market advantages and low investment, relocatable and flexible [15]. LNG plant is mainly used in several steps such as pre-treatment of natural gas with amine-based solvent system to remove impurities such as CO2; molecular sieve dehydration; and liquefaction. The plant utilizes a single-stage hybrid refrigeration system; a closed-loop refrigeration cycle compresses the refrigerant with a compressor. The single-stage mixed refrigerant process is easy to operate, highly efficient, and suitable for small LNG plants.
The compressor drive can be a gas turbine or an electric motor. In areas with low electricity prices, electric motors are preferred (low cost and simple to maintain). In areas with low fuel gas prices, a gas turbine would be a better option. The results of the economic evaluation show that the investment cost of a liquefaction unit with a gas turbine drive unit is $2 million to $4 million higher than the option of an electric motor. According to the cost estimation of a 15 × 106ft 3 /d liquefaction plant, a peak LNG project would have a storage volume of 100,000 m3 , while an LNG project for automotive fuels would require only 700 m3 of storage, resulting in a final cost of US$2.03 to US$2.11 per 1000 ft3 of peak LNG, while the cost of automotive LNG would be only US$0.98 to US$0.99 per 1000 ft3 . ft3.
2.2.6 Southwest Petroleum Institute Liquefaction Process
The process handles 3.0 × 104 m3 of natural gas per day, and consists of the systems of raw gas (CH4: 95.28%, CO2: 2.9%) CO2 removal, dehydration, propane pre-cooling, air-wave refrigerator refrigeration, and recirculation compression. The natural gas liquefaction process software was developed using the SRK equation of state as the base model. The natural gas compressor is powered by a natural gas engine, and the small-load electrical equipment is powered by a natural gas generator set, which solves the problem of no electricity or tight electricity in remote areas. Since there is no gathering pipeline in remote areas, the natural gas that cannot be liquefied is compressed in a cycle to improve the natural gas liquefaction rate of the whole unit.
The unit utilizes the monoethanolamine method (MK-4) to remove CO2. Due to the small capacity, the absorption tower and regeneration tower for CO2 removal should be a high-efficiency packed tower [16]. Due to the mixed refrigerant, there is no mature domestic technology and design, operation and management experience, the instrumentation and control system is more complex. At the same time, considering the high methane content in the raw gas, there is pressure energy can be utilized. Therefore, natural gas direct expansion refrigeration is used as the natural gas liquefaction cycle process [17]. Gas wave refrigeration belongs to the isentropic expansion process, and the gas wave refrigeration machine is developed on the basis of the thermal separator, using the theory of gas wave motion. Some of the advantages of the thermal separator are absorbed in the structure, while the microwave absorption chamber is added as a key device, which is significantly different from the thermal separator in principle, and the pressure of the gas is more effectively utilized to improve the refrigeration efficiency.
2.2.7 Harbin Gas Engineering Design and Research Institute and Harbin Institute of Technology
The LNG system mainly includes natural gas pretreatment, low-temperature liquefaction of natural gas, low-temperature storage of natural gas, and natural gas gasification and output [18]. The treated natural gas is liquefied through a multi-stage single-mix condensation process and the refrigeration compressor is driven by a natural gas engine. The LNG storage tank is a bimetallic-walled insulated tank with inner and outer tanks made of nickel and carbon steel, respectively [19].
Recirculating gas compressors are typically driven by natural gas, providing a quick payback in terms of operating cost savings. The compressors are generally of non-lubricated special design to avoid contamination of natural gas with lubricating oil [20]. Turbines with electronic speed control systems are used, and new turbines with final stage blades made of diamond alloy improve mechanical operation. The new clutches mounted on the turbocompressors are flexure, they are more reliable and can also be adjusted for clearance.