Utilization Approaches of LNG Cold Energy
Natural gas, as a clean and efficient energy source, plays a significant role in controlling atmospheric pollution in China. In recent years, the consumption of natural gas in China has been growing rapidly. Liquefied Natural Gas (LNG), as the liquid form of natural gas, is formed when natural gas is purified and cooled to -162°C, reducing its volume to 1/600 of its original size. The presence of LNG increases the flexibility of natural gas storage, transportation, and utilization, expanding the range of natural gas applications. The facility to produce one ton of LNG consumes approximately 850 kWh of electricity. During the vaporization of LNG, a significant amount of cold energy is released, approximately 830-860 kJ/kg, theoretically providing about 230 kWh of usable cooling energy per ton of LNG through heat exchange vaporization. However, under normal circumstances, this cold energy is often wasted in LNG vaporizers, resulting in significant energy waste and environmental pollution. Recovering this cold energy not only effectively utilizes energy but also reduces the substantial electricity consumption of mechanical refrigeration, leading to considerable economic and social benefits. Therefore, the utilization of LNG cold energy has attracted widespread attention from scholars both domestically and internationally.
1. Utilization Applications of LNG Cold Energy
1.1 Primary Utilization Methods of LNG Cold Energy
The utilization of LNG cold energy generally includes two main approaches: direct utilization and indirect utilization. Direct utilization primarily focuses on low-temperature power generation, air separation, dry ice manufacturing, light hydrocarbon separation, ultra-low temperature refrigeration, seawater desalination, automobile air conditioning, low-temperature breeding, cultivation, etc. Indirect utilization involves using LNG cold energy to produce liquid nitrogen or liquid oxygen, which are then used for various processes such as low-temperature grinding, low-temperature biotechnology, and sewage treatment.
1.2 Prospects of LNG Cold Energy Utilization
With the increasing demand for natural gas consumption and the escalating trend of natural gas imports in China, LNG imports occupy a considerable proportion. It is anticipated that by 2020, the supply-demand gap in the Chinese natural gas market will reach 141.5 million tons. To bridge this gap, it is foreseeable that China's LNG imports will further increase, creating a bright future for LNG cold energy utilization technologies. Currently, LNG cold energy recovery technology has received widespread attention from governments and enterprises worldwide, with the number of large LNG receiving terminals increasing globally. Japan leads the world in LNG cold energy utilization technology, with its low-temperature power generation, air separation, liquefied carbon dioxide, dry ice manufacturing, and low-temperature cold storage technologies reaching international advanced levels, achieving an LNG cold energy utilization rate of approximately 20%-30%. China's LNG cold energy utilization technology started relatively late, and its development is still immature, with overall utilization rates not being high. However, companies like China National Offshore Oil Corporation (CNOOC) have made significant progress in various technical areas related to LNG cold energy utilization, demonstrating competitiveness among international counterparts. China National Petroleum Corporation (CNPC) and China Petroleum & Chemical Corporation (Sinopec) are also intensifying their research and development efforts on LNG cold energy utilization technology, enjoying certain advantages in catching up. In the coming decades, the development of LNG cold energy utilization technology will be of great significance for China's comprehensive energy utilization.
1.3 Comparison of Cold Energy Utilization Methods
Table 3 summarizes the main utilization methods of LNG cold energy and analyzes their respective advantages, disadvantages, and cold energy demands, providing assistance in selecting suitable cold energy utilization methods based on local conditions.
2. Research Progress on Domestic and International LNG Cold Energy Utilization
Whether it is direct or indirect utilization of LNG cold energy, it involves the recovery and utilization of LNG cold energy through a single approach, which, from a thermodynamic perspective, cannot fully utilize LNG cold energy, resulting in significant losses. Currently, many industry experts suggest integrating multiple recovery methods to improve the utilization efficiency of LNG cold energy.
2.1 LNG Cold Energy Utilized in Ice Storage Air Conditioning
Chen Qiuxiong et al. developed a technology that combines LNG cold energy with ice storage air conditioning systems. This technology stores the cold energy released during LNG vaporization in the form of ice storage, which is then used to provide cooling through heat exchange to the air conditioning circulating water, effectively reducing peak electricity demand and balancing power loads, thus significantly saving user electricity costs. Lin Yuan proposed a two-stage refrigerant heat transfer process for using LNG cold energy in ice storage air conditioning. R404a and a 30% ethylene glycol solution are used as first and second-stage refrigerants, respectively, with efficiencies of 30.88% and 43.86% for the two heat exchangers, respectively. Through analysis, it is evident that the main reason for the loss is the high temperature difference during heat exchange. The authors further optimized the process from the perspective of reducing heat exchange temperature differences.
2.2 LNG Cold Energy Utilized in Air Separation
Xia Hongyan et al. proposed using LNG cold energy for air separation equipment, mainly by using the cold energy of liquefied natural gas to replace the refrigeration cycle of the expansion mechanism. The low-temperature cold energy of LNG is used to liquefy high-pressure nitrogen gas, while the ambient temperature cold energy is provided to the ethylene glycol cooling water system. This LNG cold energy air separation equipment saves 50% more energy than conventional air separation equipment and has obvious energy-saving and water-saving effects on the supporting circulating cooling water system. Wei Linrui et al. proposed a scheme using liquid nitrogen as a refrigerant to cool down the separation unit, maintaining continuous operation of the LNG air separation unit, thus solving the problem of frequent shutdowns due to fluctuating gas demand at different times and seasons, and comparing and analyzing the economic advantages of using liquid nitrogen for continuous operation maintenance compared to direct shutdowns followed by restarts.
2.3 LNG Cold Energy Utilized in Cold Storage
Yang Chun et al. proposed a liquefied natural gas (LNG) cold energy utilization device for cold storage and refrigeration water sleds, comprising three systems: LNG vaporization system, refrigerant circulation system, and cold water production system. LNG transfers its cold energy to the refrigerant through the vaporization system, and the refrigerant then uses the cold energy for cold storage and supplies it to cold water through the refrigeration water system. The refrigerant addresses the asynchronous nature of LNG gasification and application in terms of time and space. Xiao Fang et al. improved the process of LNG cold energy utilization for cold storage refrigeration technology, addressing the problem of insufficient LNG cold energy supply and insufficient refrigerant cooling capacity when natural gas user demand is low. They compared LNG cold energy refrigeration technology with traditional cold storage electric compression refrigeration technology, concluding that the former is more cost-effective, has higher process efficiency, shorter investment payback period, and lower operating costs. La Rocca studied an industrial facility that uses LNG cold energy to deep freeze agricultural products in supermarkets and can adjust air, providing a concept design, thermodynamic analysis, and economic analysis of its feasibility, applicability, and profitability, providing a new approach for efficient utilization of LNG cold energy.
2.4 LNG Cold Energy Utilized in Fuel Ships
Du Lingguang applied LNG cold energy to ocean freight refrigeration, combining cold storage technology with frozen cargo refrigeration technology, reducing the initial investment of ship refrigeration systems, recovering LNG cold energy, and reducing the
cost of ocean freight refrigeration. Tian Kun et al. designed and developed a comprehensive utilization scheme for ship LNG cold energy based on the principle of "temperature matching, cascading utilization," using LNG cold energy for cold storage and cold water air conditioning, and selecting a condensation method or a direct output method for BOG treatment based on the opening status of generators and internal combustion engines. This scheme fully utilizes the cold energy generated by LNG gasification and reduces the fuel required for refrigeration on ships, significantly reducing the electrical energy consumption required for compression refrigeration equipment to operate.
2.5 LNG Cold Energy Utilized in Power Generation
Chen Liqiong et al. summarized six cold energy power generation technologies that have been applied, including direct expansion, secondary media, combined, mixed media power generation, Brayton cycle, and gas turbine utilization. They pointed out that the Brayton cycle power generation has the highest efficiency, reaching 55%, but it requires cooling temperature requirements. He Lei et al. proposed a process combining LNG cold energy Rankine cycle power generation with air conditioning refrigeration, using high-grade cold energy for power generation and low-grade cold energy for air conditioning refrigeration through segmented recovery, achieving cascading utilization of LNG cold energy and effectively improving cold energy utilization efficiency.
2.6 LNG Cold Energy Utilized in Seawater Desalination
Huang Meibin et al. combined LNG cold energy with freezing seawater desalination and proposed two process options for whether the refrigerant undergoes phase change: non-phase change and phase change. The results showed that the non-phase change process is simpler and easier to control but has a larger mass flow rate of refrigerant. The phase-change process has a smaller mass flow rate of refrigerant but has more complex processes, equipment, and controls, with larger gas phase flow rates requiring larger gas pipeline diameters and correspondingly larger heat exchanger sizes. Jiang Kezhong et al. comprehensively analyzed three main seawater desalination technologies currently: membrane, distillation, and freezing. They proposed using a hybrid desalination process, namely, combining LNG cold energy freezing with low-temperature distillation membranes or other membrane processes, as the new direction for LNG cold energy utilization in seawater desalination. CAO studied the process of indirect contact freezing seawater desalination using LNG cold energy, selected suitable intermediate cooling agents to transfer heat, and provided the most suitable seawater crystallization temperature. The results showed that 1 kg of LNG cold energy could produce 2 kg of melted ice, with almost no energy consumption during the mixing process of LNG and seawater.
2.7 LNG Cold Energy Utilized in Butyl Rubber
Han Junshi analyzed the feasibility of using LNG cold energy in the butyl rubber industry, proposing two schemes: secondary refrigeration using LNG and propylene and direct refrigeration using LNG. After comparing and analyzing the schemes with the conventional use of ethylene and propylene combined refrigeration, it was found that using LNG in conjunction with propylene refrigeration could save 1100 kWh of electricity per ton of rubber, reducing investment costs by 10%-20%; using LNG direct refrigeration is more energy-saving, saving up to 2000 kWh of electricity per ton of rubber. Chen Maochun et al. compared and analyzed two schemes of LNG, ethylene, and propylene combined refrigeration and LNG and propylene combined refrigeration for a conventional butyl rubber plant with an annual output of 50,000 tons, concluding that the latter has advantages such as low LNG vaporization pressure, low design pressure for refrigeration equipment, and a small footprint for the plant. Compared with traditional refrigeration processes, it greatly simplifies the refrigeration process, with fewer process equipment and lower investment, reducing the consumption of public engineering and other advantages.
2.8 LNG Cold Energy Utilized in Heavy Truck Air Conditioning
Wang Fang et al. designed a combined device for utilizing LNG cold energy in heavy truck air conditioning systems, including LNG cylinders, vaporizers, air conditioners, refrigerators, engines, and controllers. LNG undergoes counter-flow heat exchange with the refrigerant in the vaporizer, releasing cold energy, then becomes ambient temperature fuel vaporized into the engine for heavy truck use. After the refrigerant receives the cold energy released by LNG, its temperature decreases, accumulating cold energy. Low-temperature refrigerant exiting the vaporizer is pressurized by a pump and enters the air conditioning evaporator, where the cold energy is transferred to the cabin through a fan to adjust the room temperature. During this process, the refrigerant undergoes only temperature changes without phase changes. This utility model device saves fuel for driving the compressor, recovers the cold energy released during LNG vaporization, saves energy and reduces consumption, and has a simple process, making it easy to promote and use.
3. Outlook
Improving the utilization rate of LNG cold energy is crucial for comprehensive energy utilization, alleviating the pressure of energy shortage, responding to the call for energy conservation and emission reduction, and increasing economic and social benefits. However, currently, the utilization efficiency of LNG cold energy is generally low, with a single utilization method, slow project progress, and serious implementation lag. Therefore, the following suggestions are proposed:
(1) Reasonably select cold energy utilization projects based on the size of LNG receiving stations, local economic conditions, and market demand.
(2) Develop specific process technologies for cascading utilization of LNG cold energy to improve LNG cold energy utilization rates from both single efficient utilization and comprehensive cascading utilization perspectives.
(3) Develop devices for accumulating and storing cold energy to separate the cold energy recovery and utilization processes using refrigerants, and supply cold energy to various cold energy users through refrigerant pipelines to practice the principle of "temperature matching, cascading utilization."
(4) Increase research and development of cold storage media. Currently, there are few refrigerants on the market that do not undergo phase changes during heat exchange with LNG. Therefore, accelerating the research and development of non-phase change refrigerants is of utmost importance.
In addition to the above, actively exploring new cold energy utilization methods is still a direction that requires effort. In summary, in the process of LNG cold energy utilization, the concept of circular economy should be implemented, actively exploring LNG cold energy utilization technologies, realizing the full utilization of LNG cold energy, and forming a healthy industrial network system.
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