BOG (boil-off gas) has different treatment methods due to different sources and scenarios. The following is a detailed introduction to common BOG treatment methods:
Recondensation process
Principle: The recondensation process is to introduce BOG into the recondenser to fully contact and exchange heat with the cryogenic liquid (such as LNG) extracted from the storage tank. The gas molecules in the BOG release heat, gradually cool and liquefy, return to the liquid state, and then are transported back to the storage tank for storage.
Application scenario: This process is widely used in LNG receiving stations. At the LNG receiving station, a large amount of LNG is transported to the station by ship, and a large amount of BOG will be generated during the unloading and storage process. The recondensation process can effectively reliquefy and recycle these BOG, reducing the impact of BOG emissions on the environment and energy efficiency.
Advantages and Disadvantages
Advantages: High recovery efficiency, can efficiently reliquefy BOG, so that it can be stored and used; can reduce the impact of BOG emissions into the atmosphere on the environment, in line with environmental protection requirements.
Disadvantages: high energy consumption, the recondensation process requires a large amount of cryogenic liquid as a cold source to achieve the recondensation of BOG, which increases energy consumption and operating costs to a certain extent; the equipment investment cost is high, and special recondensers, cryogenic liquid delivery pumps and other equipment are required, and the investment cost of these equipment is relatively high.
Use as fuel
Principle: BOG has a certain calorific value and can be used as a fuel in a variety of equipment. Taking the gas turbine as an example, after BOG is mixed with air, it burns in the combustion chamber to produce high-temperature and high-pressure gas, which drives the impeller of the gas turbine to rotate, and then drives the generator to generate electricity. In the boiler, the heat generated by the combustion of BOG as a fuel is used to heat water or other media, generate steam or hot water, and provide heat energy for production and life.
Application scenario: On LNG carriers, BOG is usually introduced into the ship's gas turbines or boilers and other equipment as fuel for combustion, providing electricity and heat energy for the navigation of the ship and the operation of various equipment. In addition, in some industrial production processes fueled by natural gas, BOG can also be used as a supplementary fuel to meet the energy needs of the production process.
Advantages and Disadvantages
Advantages: Reasonable energy utilization, can effectively utilize the energy of BOG and reduce energy waste; reduce dependence on external fuel supply, improve the energy self-sufficiency of the system, which is of great significance for some remote areas or places where it is difficult to obtain external fuel supply.
Disadvantages: Unstable combustion performance. The composition and calorific value of BOG will fluctuate with the changes in factors such as the source of LNG, production process and storage conditions. This leads to unstable combustion performance and energy output when BOG is used as fuel, which may affect the normal operation and efficiency of the equipment; the equipment modification and maintenance costs are high. Using BOG as fuel requires special design and modification of the equipment to adapt to the combustion characteristics and supply conditions of BOG. The design and modification of these equipment requires a lot of capital and technical strength, and strict maintenance and management are required during operation to ensure the safe and reliable operation of the equipment and the effective use of BOG.
Compression storage
Principle: BOG is compressed by a compressor to increase its pressure and reduce its volume, which is convenient for storage. During the compression process, the temperature of BOG will rise, so it is necessary to equip corresponding cooling equipment, such as a cooler, to cool the compressed BOG and reduce its temperature to a suitable storage temperature range. After compression and cooling, BOG is stored in a special storage container, such as a high-pressure storage tank.
Application scenario: In some small LNG filling stations or industrial production sites, since the amount of BOG produced is relatively small, the compressed storage method can conveniently store BOG for subsequent processing or utilization. In addition, in some places where the flexibility of BOG processing and utilization is required, the compressed storage method can also meet their needs, because the stored BOG can be processed and utilized at different times and occasions according to actual conditions.
Advantages and Disadvantages
Advantages: The operation is relatively simple. Compared with some other complex BOG processing methods, such as the recondensation process, the operation process of the compressed storage method is relatively simple, and does not involve complex heat exchange and liquefaction processes. Therefore, the technical requirements for operators are relatively low; storage and utilization are flexible. The compressed and stored BOG can be flexibly processed and utilized according to actual needs. For example, it can be used as fuel in subsequent production processes, or it can be transported to other places for further processing or utilization.
Disadvantages: The storage cost is high. In order to ensure that the compressed BOG can be stored safely and stably, special high-pressure storage containers and corresponding safety facilities are required. The investment cost of these equipment is large, and regular maintenance and inspection are required during the storage process to ensure the normal operation of the equipment and the safety of storage, which also increases the storage cost; there are safety risks. Since BOG is a flammable and explosive gas, during the compression and storage process, if the equipment is not tightly sealed, improperly operated, or the safety facilities are not perfect, BOG may leak. Once it encounters a fire source, it will cause safety accidents such as fire or explosion, posing a serious threat to the safety of personnel and property.
Adsorption separation
Principle: Adsorption separation technology uses the difference in the adsorption capacity of adsorbents for different components in BOG to achieve the separation and purification of BOG. Adsorbents usually have a large specific surface area and rich pore structure, which can effectively adsorb impurity gases in BOG, such as carbon dioxide, hydrogen sulfide, water, etc., while retaining the main component of BOG, methane. During the adsorption process, BOG first removes solid particles and part of the water in it through a pretreatment system, and then enters the adsorption tower to contact with the adsorbent. The adsorbent selectively adsorbs the impurity gas in BOG, so that the content of impurity gas in BOG after adsorption treatment is significantly reduced, and the purity of methane is improved. When the adsorbent reaches the saturated adsorption state, the adsorbent needs to be regenerated to restore its adsorption capacity. The regeneration of the adsorbent usually adopts the method of desorption by pressure reduction or desorption by temperature increase to release the impurity gas adsorbed on the surface of the adsorbent so that the adsorbent can be regenerated. The regenerated adsorbent can be put back into the adsorption tower for BOG adsorption treatment to achieve the recycling of the adsorbent.
Application scenario: In some industrial production processes with high requirements for BOG purity, such as natural gas chemical production, it is necessary to remove the impurity gas in BOG to improve the purity of methane and meet the requirements of the production process. Adsorption separation technology can effectively separate and purify BOG and improve the utilization value of BOG. In addition, in some places with high environmental protection requirements, such as LNG receiving stations, the use of adsorption separation technology can further reduce the content of impurity gas in BOG and reduce the impact of BOG emissions on the environment.
Advantages and Disadvantages
Advantages: Good separation effect, can effectively remove impurity gases in BOG, significantly improve the purity of methane, and meet the requirements of BOG purity in different industrial production processes; relatively low energy consumption, compared with some BOG treatment technologies that require complex heat exchange and phase change processes, such as recondensation process, the energy consumption of adsorption separation technology is relatively low, because the adsorption process is mainly based on the physical adsorption of impurity gases by the adsorbent, and does not require a large amount of heating or cooling operations; the equipment occupies a small area, and the adsorption separation equipment is usually composed of an adsorption tower, a pretreatment system, a regeneration system, etc. The structure of these equipment is relatively compact and occupies a small area, which is suitable for use in some industrial production sites or LNG receiving stations with limited space.
Disadvantages: The cost of adsorbent is high. High-performance adsorbents usually require special materials and preparation processes, which leads to high cost of adsorbents and increases the investment cost of BOG treatment. The regeneration process of adsorbents is relatively complicated. The regeneration of adsorbents requires specific methods, such as desorption by pressure reduction or desorption by temperature increase. In addition, the temperature, pressure, gas flow rate and other parameters need to be precisely controlled during the regeneration process to ensure the regeneration effect and service life of the adsorbent. This makes the regeneration process of adsorbents more complicated and requires high technical requirements for operators. The processing capacity is limited. The processing capacity of adsorption separation technology is limited by factors such as the adsorption capacity of the adsorbent, the size and number of adsorption towers. For large-scale BOG treatment needs, it may be necessary to increase the number of adsorption towers or use larger adsorption towers, which will lead to an increase in equipment investment costs and floor space.
Membrane separation
Principle: Membrane separation technology uses the difference in the permeation rate of different components in BOG by special membrane materials to achieve the separation and purification of BOG. The membrane material has the characteristics of selective permeation, which allows certain components in BOG to preferentially pass through the membrane while other components are retained. During the membrane separation process, BOG first passes through the pretreatment system to remove solid particles and part of the water in it, and then enters the membrane separation device to contact the membrane material. Different components in BOG pass through the membrane at different permeation rates under the pressure difference on both sides of the membrane. For example, light hydrocarbon gases such as methane usually have a higher permeation rate and can preferentially pass through the membrane, while impurity gases such as carbon dioxide, hydrogen sulfide, and water have a lower permeation rate and are intercepted. In this way, the separation of different components in BOG is achieved, and the purity of the target components in BOG is improved.
Application scenario: In a natural gas purification plant, BOG may contain a variety of impurity gases, such as carbon dioxide, hydrogen sulfide, etc. The presence of these impurity gases will affect the quality of natural gas and subsequent processing and utilization. Membrane separation technology can effectively remove impurity gases in BOG, improve the purity of natural gas, and meet the production requirements of natural gas purification plants. In addition, in some small LNG production units or LNG filling stations, membrane separation technology can also be used to treat BOG, remove impurity gases, improve the utilization value of BOG, and reduce the impact of BOG emissions on the environment.
Advantages and Disadvantages
Advantages: High efficiency and energy saving. The membrane separation process is mainly based on the difference in permeation rates of different components under the pressure difference on both sides of the membrane to achieve the separation of BOG. It does not require complex heat exchange and phase change processes, so the energy consumption is low and it has the advantages of high efficiency and energy saving. It is simple to operate and easy to control. The structure of the membrane separation device is relatively simple, and the operation process is relatively clear. By adjusting the pressure, temperature, gas flow and other parameters on both sides of the membrane, the membrane separation process can be accurately controlled to meet different production needs, and the technical requirements for operators are relatively low, which is easy to promote and apply. The separation accuracy is high. The membrane material has the characteristics of highly selective permeation, which can efficiently separate different components in BOG and achieve high-purity extraction of the target component, meeting the needs of some industrial production processes with extremely high requirements for product purity.
Disadvantages: The membrane cost is high. The research and development and production of membrane materials require a lot of money and technical strength, so the cost of membrane materials is relatively high, which not only increases the initial investment cost of the membrane separation device, but also affects the economy of membrane separation technology in large-scale applications; the membrane life is limited. In actual application, the membrane material will be affected by the chemical erosion and physical wear of various components in BOG, as well as operating conditions such as temperature and pressure, which will cause the performance of the membrane to gradually decline. The service life of the membrane is limited, and the membrane material needs to be replaced regularly, which not only increases the production cost, but also affects the continuity and stability of the production process; the pretreatment requirements are high. BOG usually contains solid particles, moisture, and some impurity gases that may cause pollution or damage to the membrane material. In order to ensure the normal operation of the membrane separation device and the service life of the membrane, the BOG needs to be strictly pretreated to remove the solid particles, moisture and impurity gases, which increases the investment cost and operation management difficulty of the pretreatment system.
