The main component is methane, which is recognized as the cleanest energy source on the earth. Colorless, odorless, non-toxic and non-corrosive, its volume is about 1/600 of that of gaseous natural gas with the same volume, and the weight of liquefied natural gas is only about 45% of that of water with the same volume. Its manufacturing process is to purify the natural gas produced in the gas field, liquefy it at a series of ultra-low temperatures, and then transport it by LNG carrier. After combustion, the air pollution is small and the heat released is large, so LNG is good.
It is made of natural gas after compression, cooling and liquefaction at-160℃. Its main component is methane, which is transported by special ships or tankers and gasified when used. Since 1970s, the output and trade volume of world LNG have increased rapidly. In 2005, the international trade volume of LNG reached 18885438+0 billion cubic meters, among which Indonesia was the largest exporter, with an export volume of 3/kloc-0.40 billion cubic meters. The largest importer is Japan, with 76.32 billion cubic meters.
Second, the general situation and development trend at home and abroad
194 1 year, the world's first industrial-scale LNG plant was built in Cleveland, USA, with a liquefaction capacity of 8500m3/d. Since the 1960s, the LNG industry has developed rapidly and its scale has become larger and larger. The liquefaction capacity of basic load is 2. 5×104m3/d. According to the data [3], at present, more than 160 LNG plants have been put into operation in various countries, and the total LNG export has exceeded 46.1.8×1.06t/a. ..
The main component of natural gas is methane. The atmospheric 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, which is formed by purifying natural gas (dehydration, hydrocarbon removal and acid gas removal) [4] and turning methane into liquid [5] through throttling, expansion and additional cold source refrigeration.
2. 1 research status abroad
Foreign liquefaction plants have large scale, complex technology, many equipments and high investment. They basically adopt cascade refrigeration and mixed refrigerant refrigeration technology. At present, both types of factories are in operation, and the newly put into production mainly adopts mixed refrigerant refrigeration technology. The main purpose of the study is to reduce the energy consumption of liquefaction. The refrigeration process developed from step-by-step refrigeration to mixed refrigerant refrigeration cycle. At present, there are reports about the new process of Cⅱ-2 [6]. This process not only has the advantages of simple pure component cycle, no phase separation and easy control, but also has the advantages of mixed refrigerant refrigeration cycle, such as good temperature and temperature coordination between natural gas and refrigerant, high efficiency and few equipment.
Axens Company of France cooperated with IFP to develop an advanced natural gas liquefaction process-Liquidin for the first time, which laid the foundation for LNG market. Its production capacity is 15%-20% higher than that of ordinary methods, and its production cost is 25% lower. After adopting liquefaction method, the output of each liquefaction unit can reach more than 600× 104 t/y, and the cost of LNG production by liquefaction method can be reduced by 25% per ton [7]. The main advantage of this process is that it can build a super-large capacity liquefaction plant by using fin heat exchanger and thermodynamic optimization process. Axens has proposed to use this process in several major regions such as the United States, Europe and Asia, and is conducting preliminary design and feasibility study. The safety, environmental protection, practicality and innovation of the Liquefin process developed by IFP and Axens have recently been recognized by the world, and this process has won the "Engineering Excellence Award" issued by the Institute of Chemical Engineers [8].
——GTL technology, a new technology for natural gas liquefaction, has been developed by the Engineering Experimental Station of the University of Texas, USA, and has been patented. This technology is more suitable for small devices than the GTL technology currently developed, and can handle 30.5 × 104 m3 /d natural gas. The GTL of the experimental station has been authorized to the synthetic fuel company. The company has established a GTL pilot plant near the campus of A&M University, and is currently conducting economic simulation analysis. Compared with the prior art, the new process is much simpler, and does not need synthetic gas, and oxygen is not needed except for power generation. Its economy, scale and output are different from ordinary Fischer-Tropsch GTL process. The first set of industrial devices 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 the development plan of LNG, and completed the industrial test in the mid-1960s. Weiyuan Chemical Plant of Sichuan Petroleum Administration Bureau has the earliest industrial production device for cryogenic separation and liquefaction of natural gas in China, which produces LNG except he. 199 1 year, the factory provided 30 tons of rocket test fuel for the space department. Different from the situation abroad, the research of natural gas liquefaction in China is aimed at small-scale liquefaction process, and many documents have been published in this regard [10]. The following is a brief introduction to the process of existing natural gas liquefaction plants in China.
Sichuan liquefied natural gas plant
The 300l/h natural gas liquefaction plant jointly developed by Beijing Keyang Gas Liquefaction Technology Co., Ltd. of Chinese Academy of Sciences and Sichuan jianyang city Keyang Cryogenic Equipment Co., Ltd. is a demonstration project of using LNG as industrial and civil gas for peak shaving and replacing oil with gas. The device was built in 1992 to provide LNG for LNG vehicle research.
The device makes full use of the pressure of natural gas itself, and uses the gas turbine expander to refrigerate liquefied natural gas for peak shaving of civil natural gas or LNG production. The technological process is reasonable, and the gas turbine expander with advanced technology is adopted. The device basically does not consume water and electricity, which is an energy-saving project, but the liquefaction rate is very low, about 10%, which is consistent with its design principle.
2.2.2 LNG Plant in Jilin Oilfield
The 500l/h skid-mounted industrial test device jointly developed by Jilin Oilfield, China Petroleum and Natural Gas Corporation and Cryogenic Center of Chinese Academy of Sciences was successfully put into trial operation on June 5438+0996+February 65438. The device adopts expander cycle technology with nitrogen as refrigerant. The whole device consists of 10 skid blocks, and all the equipment is localized [1 1].
The device adopts a gas bearing turboexpander; Domestic molecular sieve deeply removes water and CO2 from natural gas, with simple process flow and skid-mounted structure, which accords with the characteristics of small-scale devices. Using pure nitrogen as refrigerant, the power consumption is higher than that of expander cycle using refrigerant. Natural gas is liquefied at medium pressure (about 5.0MPa), and its own pressure is not fully utilized (liquefaction at higher pressure can not only improve the refrigeration temperature of nitrogen, but also reduce the refrigeration load), so the power consumption of the device is high.
2.2.3 liquefied natural gas in northern Shaanxi gas field
12 × 104 m3 /d "North Shaanxi Gas Field LNG Demonstration Project" completed and put into operation in June, 1999 is a pilot project to develop China's LNG industry and the first set of small-scale LNG industrialization devices in China. The device adopts natural gas expansion refrigeration cycle, low-temperature methanol washing and molecular sieve drying to purify raw gas, gas wave refrigerator and turboexpander for low-temperature refrigeration, gas engine as the power source of circulating compressor, and gas engine tail gas as the heat source for heating molecular sieve regeneration gas. All devices of the device are localized. The successful operation of this device provides experience for China to produce LNG from natural gas in remote oil and gas fields [12].
2.2.4 LNG Plant in Zhongyuan Oilfield
Zhongyuan Oilfield once built the largest LNG plant in China, with the scale of feed gas of 26.6 5 × 104 m3 /d, liquefaction capacity/kloc-0 ×104m3/d, storage capacity 1200 m3 and liquefaction rate of 37.5% [1d]. At present, on the basis of fully absorbing foreign advanced technology, combining with the relevant equipment at home and abroad, and aiming at the characteristics of its own gas source, the LNG technical scheme has been formulated [14]. The process adopts the commonly used molecular sieve adsorption method for dehydration, and the liquefaction process adopts propane precooling+ethylene precooling+throttling.
When the feed gas volume is 30× 104 m3 /d, the output is as high as 5 1.4% and the energy consumption is 0. 13 Kwh/Nm3. Its advantage is that each refrigeration system is relatively independent and has good reliability and flexibility. However, the process is relatively complex, requiring two kinds of refrigeration media and circulation, and the equipment investment is high. Because the plant makes full use of the pressure energy of natural gas in oil field gas wells, the liquefaction cost is low.
2.2.5 Small LNG Plant of Tianjin University
Compared with large-scale plants, small LNG plants not only have raw material advantages and market advantages, but also have low investment, convenient relocation and great flexibility [15]. LNG plant mainly uses amino solvent system to pretreat natural gas to remove impurities such as CO2. Molecular sieve dehydration; Several steps of liquefaction. The device adopts a single-stage mixed refrigeration system; A closed-loop refrigeration cycle uses a compressor to compress refrigerant. The single-stage mixed refrigerant process is simple and efficient, which is suitable for small LNG plants.
The compressor can be driven by a gas turbine or an electric motor. In areas with low electricity price (low cost and simple maintenance), electric motors can be given priority. In areas where gas prices are low, gas turbines will be a better choice. The economic evaluation results show that the investment cost of gas turbine-driven liquefaction plant is 2 million to 4 million dollars higher than that of motor-driven liquefaction plant. According to the cost estimation of a set of 15 × 106ft 3 /d liquefaction plant, the storage tank capacity of LNG project for peak shaving is 65438+ 1 100 million m3, while that of LNG project for vehicle fuel only needs 700m3. The final LNG cost for peak shaving is 2.03 ~ 2. 1 1 USD/1000ft3, while the vehicle LNG cost is only 0.98 ~ 0.99 USD/1000 ft3.
2.2.6 New Liquefaction Technology of Southwest Petroleum Institute
The daily processing capacity of this process is 3.0 × 104 m3 natural gas, which is mainly composed of feed gas (CH4: 95.28%, CO2 :2.9%) and is used for decarbonization, dehydration, propane precooling, gas wave refrigerator refrigeration and cyclic compression. Based on SRK equation of state, the software of natural gas liquefaction process is developed. The natural gas compressor is powered by the natural gas engine, and the small-load electrical equipment is powered by the natural gas generator set, which solves the problem of no electricity or lack of electricity in remote areas. Because there is no gathering and transportation pipeline in remote areas, LNG will be recovered and compressed to improve the liquefaction rate of the whole plant.
This unit adopts monoethanolamine process (MK-4) to remove CO2. Because of the small capacity, the decarbonization absorption tower and regeneration tower should adopt high-efficiency packed tower [16]. Due to the use of mixed refrigerant, there is no mature technology and experience in design and operation management in China, and the instrument control system is more complicated. At the same time, considering the high methane content in feed gas, pressure energy can be used. Therefore, the natural gas liquefaction cycle process adopts natural gas direct expansion refrigeration [17]. Gas wave refrigeration belongs to isentropic expansion process, and gas wave refrigerator is developed on the basis of thermal separator and using gas wave theory. In structure, it absorbs some advantages of thermal separator, and at the same time, it adds microwave absorption cavity, a key device, which is obviously different from thermal separator in principle, making more effective use of gas pressure and improving refrigeration efficiency.
2.2.7 Harbin Gas Engineering Design and Research Institute and Harbin Institute of Technology
LNG system mainly includes natural gas pretreatment, low-temperature liquefaction, low-temperature storage, gasification and natural gas export [18]. The treated natural gas is liquefied by multi-stage single mixed condensation process, and the refrigeration compressor is driven by natural gas engine. The LNG storage tank is a bimetallic insulated tank, and the inner tank and the outer tank are made of nickel steel and carbon steel respectively [19].
Generally, the circulating gas compressor is driven by natural gas, which can save operating cost and recover investment quickly. Compressors generally adopt special design without lubrication to avoid natural gas being polluted by lubricating oil [20]. The turbine equipped with electronic speed regulation system is adopted, and the last blade of the new turbine is made of diamond alloy, which improves the mechanical operability. The new clutches installed on turbocompressors are flexible, their reliability is relatively high, and the clearance can be adjusted.