1. Liquefied Natural Gas (LNG)
The main component is methane, which is recognized as the cleanest energy on earth. It is colorless, odorless, non-toxic and non-corrosive. Its volume is about 1/600 of the same volume of gaseous natural gas. The weight of liquefied natural gas is only about 45% of the same volume of water. The manufacturing process is to first purify the natural gas produced in the gas field, and then transport it using liquefied natural gas ships after a series of ultra-low temperature liquefaction. After burning, it causes very little air pollution and releases a lot of heat, so liquefied natural gas is good.
It is formed by compressing, cooling and liquefying natural gas at -160 degrees. Its main component is methane, which is transported by special ships or tankers and re-gasified when used. Since the 1970s, world liquefied natural gas production and trade volume have increased rapidly. In 2005, the international trade volume of LNG reached 188.81 billion cubic meters. The largest exporter was Indonesia, with exports of 31.46 billion cubic meters; the largest importer was Japan, with 76.32 billion cubic meters.
2. Overview and development trends at home and abroad
In 1941, the world's first industrial-scale LNG unit was built in Cleveland, USA, with a liquefaction capacity of 8,500 m3/d. Since the 1960s, the LNG industry has developed rapidly and its scale has become larger and larger. The basic load liquefaction capacity is 2. 5 × 104 m3 /d. According to information [3], there are currently more than 160 LNG units in operation in various countries, and the total LNG export volume has exceeded 46.1 8 × 106 t/a.
The main component of natural gas is methane. The normal pressure boiling point of methane is -16 1 ℃, the critical temperature is -84 ℃, and the critical pressure is 4.1MPa. LNG is the abbreviation of liquefied natural gas. It is formed by converting methane into liquid through the process of throttling, expansion and external cold source refrigeration after natural gas has been purified (dehydration, dehydrocarbon, and acidic gas removal)[4][5] .
2.1 Current status of foreign research
Foreign liquefaction plants have large scales, complex processes, many equipments, and high investments. They basically use cascade refrigeration and mixed refrigerant refrigeration processes. Currently, the two All types of devices are in operation, and the newly put into production design is mainly a mixed refrigerant refrigeration process. The main purpose of the research is to reduce liquefaction energy consumption. The refrigeration process has been improved from cascade refrigeration to mixed refrigerant refrigeration cycle. Currently, there are reports of a new CⅡ-2 process [6]. This process has the advantages of pure component cycle, such as simplicity, no phase separation and easy control. It also has the advantages of a mixed refrigerant refrigeration cycle, such as better refrigeration temperature coordination between natural gas and refrigerant, high efficiency, and less equipment.
Liquefin, an advanced new natural gas liquefaction process jointly developed by the French company Axens and the French Petroleum Institute (IFP), was industrialized for the first time. This process laid the foundation for the LNG market. Its production capacity is 15%-20% higher than the general method, and the production cost is 25% lower. After using the Liquefin method, the output of each unit of liquefaction unit can reach more than 600 × 104 t/y. The cost of producing LNG using the Liquefin process can be reduced by 25% per ton [7] . The main advantage of this process is the use of fin heat exchangers and a thermodynamically optimized process, which allows the construction of ultra-large-capacity liquefaction devices. Axens has made proposals for the use of this process to several major regions such as the United States, Europe, and Asia, and is conducting preliminary design and feasibility studies. The safety, environmental protection, practicality and innovative features of the Liquefin process developed by IFP and Axens have recently been recognized around the world, with the process receiving the Engineering Excellence Award from the Institution of Chemical Engineers [8] .
The University of Texas Engineering Experiment Station in the United States has developed a new natural gas liquefaction technology - GTL technology and has applied for a patent. 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. The experimental station's GTL is licensed to Synfuels.
The company has built a GTL pilot plant near the A&M University campus and is currently conducting economic simulation analyses. The new process is much simpler than existing technologies and does not require syngas, nor does it require the use of oxygen in addition to generating electricity. Its economics, scale and production differ from ordinary Fischer-Tropsch GTL processes. The first industrial unit may be completed in the first half of 2004 [9].
2.2 Domestic research status
As early as the 1960s, the State Science and Technology Commission formulated an LNG development plan. In the mid-1960s, industrial tests were completed. Weiyuan Chemical Plant of Sichuan Petroleum Administration Bureau It has the earliest industrial production equipment for natural gas cryogenic separation and liquefaction in China. In addition to producing He, it also produces LNG. In 1991, the plant provided 30tLNG as rocket test fuel to the Ministry of Aerospace. Different from foreign situations, domestic natural gas liquefaction research is all aimed at small-scale liquefaction processes. There are many publications in this area [10]. The following is a brief introduction to the existing domestic natural gas liquefaction device processes.
2.2.1 Sichuan Liquefied Natural Gas Unit
A 300l/h natural gas liquefaction unit jointly developed by Beijing Keyang Gas Liquefaction Technology Co., Ltd. of the Chinese Academy of Sciences and Sichuan Jianyang Keyang Cryogenic Equipment Co., Ltd. , is a demonstration project for using LNG as peak shaving for industrial and civil gas and replacing oil with gas. The unit was built in 1992 to provide LNG for LNG automotive research.
This device makes full use of the natural gas's own pressure and uses a gas turbine expander to refrigeration to liquefy the natural gas. It is used for civil natural gas peak shaving or LNG production. The process is reasonable, using a gas turbine expander, and the technology is relatively advanced. advanced. This device consumes basically no water or electricity and is an energy-saving project. However, the liquefaction rate is very low, about 10%, which is consistent with its design principles.
2.2.2 Jilin Oilfield Liquefied Natural Gas Unit
The 500l/h skid-mounted industrial test device jointly developed by Jilin Oilfield, China National Petroleum Corporation and the Low Temperature Center of the Chinese Academy of Sciences was launched in 1996. The overall test run was successful in December 2016. The device adopts an expander circulation process using nitrogen as refrigerant. The entire device consists of 10 skids, and all equipment is domestically produced [11].
The device uses a gas bearing turbine expander; the domestic molecular sieve deeply removes water and CO2 from natural gas. The process is simple and it adopts a skid-mounted structure, which is in line with the characteristics of a small device. Using pure nitrogen as the refrigerant fluid has higher power consumption than an expander cycle using refrigerant. The natural gas's own pressure is not fully utilized and the natural gas is liquefied under medium pressure (about 5.0MPa) (liquefaction under higher pressure can not only increase the refrigeration temperature of nitrogen but also reduce the refrigeration load), so the power consumption of the device is high.
2.2.3 Northern Shaanxi Gas Field LNG
The 2 × 104 m3/d "North Shaanxi Gas Field LNG Demonstration Project" completed and put into operation in January 1999 is an important step in the development of my country's LNG industry. The pilot project is also the first small-scale LNG industrialized device in my country. The device uses a natural gas expansion refrigeration cycle, low-temperature methanol washing and molecular sieve drying to purify the raw gas. A gas wave refrigerator and a turbine expander are combined to perform low-temperature refrigeration. The gas engine is used as the power source of the cycle compressor, and the exhaust gas of the gas engine is used as the power source. Heat source for heating molecular sieve regeneration gas. All the equipment is domestically produced. The successful operation of the device provides my country with experience in using natural gas to produce LNG in remote oil and gas fields [12].
2.2.4 Zhongyuan Field LNG Unit
Zhongyuan Field once built my country’s largest LNG unit, with a raw gas capacity of 26.6 5 × 104 m3/d and a liquefaction capacity of 1 0 × 104 m3 /d, storage capacity is 1200 m3, and liquefaction rate is 37.5%[13]. At present, on the basis of fully absorbing foreign advanced process technology, combined with the situation of domestic and foreign related equipment, and mainly focusing on the characteristics of its own gas source, LNG process technology solutions have been developed [14]. The process uses the commonly used molecular sieve adsorption method for dehydration, and the liquefaction process uses propane precooling + ethylene precooling + throttling.
When the feed gas volume of the device is 30×104 m3/d, the yield is as high as 51.4%, and the energy consumption is 0.13 Kwh/Nm3. The advantage is that each refrigeration system is relatively independent and has good reliability and flexibility. However, the process is relatively complex, requiring two refrigerant media and circulation, and the equipment investment is high. Since the plant fully utilizes the pressure energy of natural gas from oil field gas wells, the liquefaction cost is low.
2.2.5 Tianjin University’s small-scale liquefied natural gas (LNG) unit
Compared with large-scale units, small-scale LNG units not only have raw material advantages and market advantages, but also have low investment, relocation, and Great flexibility [15]. The LNG unit mainly uses an amine-based solvent system to pretreat natural gas to remove impurities such as CO2; molecular sieve dehydration; and liquefaction. The device uses a single-stage hybrid refrigeration system; a closed-loop refrigeration cycle uses a compressor to compress the refrigerant. The single-stage mixed refrigerant process is easy to operate and highly efficient, making it suitable for small LNG installations.
The compressor driver can be a gas turbine or an electric motor. Areas with low electricity prices may give priority to electric motors (low cost and easy maintenance). In areas where fuel gas prices are low, gas turbines will be a better choice. Economic evaluation results show that the investment cost of a liquefaction plant using a gas turbine driver is $2 million to $4 million higher than that of an electric motor. According to the cost estimate for a set of 15 × 106ft 3 /d liquefaction unit, the storage tank capacity of the LNG project for peak shaving is 100,000 m3, while the LNG project for vehicle fuel only requires a 700m3 storage tank, resulting in the final peak shaving The cost of LNG used is 2.03 ~ 2.11 US dollars/1000 ft3, while the cost of vehicle LNG is only 0.98 ~ 0.99 US dollars/1000 ft3.
2.2.6 Southwest Petroleum Institute’s new liquefaction process
This process can process 3.0 × 104 m3 of natural gas per day, mainly removing CO2 from raw gas (CH4: 95.28%, CO2: 2.9%) , dehydration, propane pre-cooling, gas wave refrigerator refrigeration and cycle compression systems. Using the SRK state equation as the basic model, a natural gas liquefaction process software was developed. 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 power shortage in remote areas. Since there are no gathering and transportation pipelines available in remote areas, the natural gas that cannot be liquefied is cyclically compressed to increase the natural gas liquefaction rate of the entire device.
The device uses the monoethanolamine method (MK-4) to remove CO2. Due to the small processing capacity, the absorption tower and regeneration tower for carbon dioxide removal should use high-efficiency packed towers [16]. Due to the mixed refrigerant, there is no mature technology, design, operation and management experience in China, and the instrument control system is relatively complex. At the same time, considering the high methane content in the raw gas, there is pressure energy that can be utilized. Therefore, natural gas direct expansion refrigeration is used as the natural gas liquefaction cycle process [17]. Gas wave refrigeration is an isentropic expansion process. The gas wave refrigerator is developed based on the thermal separator and using the theory of gas wave motion. The structure absorbs some advantages of the thermal separator, and at the same time adds a key device, the microwave absorption cavity, which is significantly different from the thermal separator in principle. It can more effectively utilize the pressure of the gas and improve the refrigeration efficiency.
2.2.7 Harbin Gas Engineering Design 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 gasification and output etc.[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 an insulated tank with double metal walls. The inner tank and the outer tank are made of nickel steel and carbon steel respectively [19].
Circulating gas compressors are generally driven by natural gas, which can save operating costs and quickly recover investment. Compressors generally adopt non-lubricated special designs to avoid contamination of natural gas by lubricating oil [20]. A turbine equipped with an electronic speed control system is used, and the last few blades of the new turbine are made of diamond alloy, which improves mechanical operation.
The new clutches installed on turbine compressors are flexible, they are more reliable and the clearance can be adjusted.