BOG, abbreviated as BOG, is commonly used in low-temperature or high-pressure storage and transportation scenarios such as liquefied natural gas (LNG) and liquefied petroleum gas (LPG)
Production principle
Taking LNG as an example, LNG is usually stored in specially insulated storage tanks at atmospheric pressure and a low temperature of about -162 ℃. However, despite the good insulation measures of the storage tank, the heat from the external environment will inevitably enter the tank. These incoming heat will cause some LNG to absorb energy, transforming from liquid to gas, thereby producing BOG.
For LPG, its storage pressure is relatively high. When the pressure of the storage container fluctuates due to external temperature changes and other factors, some LPG will evaporate from liquid to gas, forming BOG.
Handling method
Recondensation process: In some large LNG receiving stations, BOG is collected and re liquefied through a recondensation system. The specific process is that BOG is first introduced into the recondenser and fully contacts and exchanges heat with the low-temperature LNG liquid extracted from the LNG storage tank. During this process, the gas molecules in BOG will release heat, gradually cool and liquefy, returning to the LNG liquid state, and then be transported back to the LNG storage tank for storage.
As fuel: BOG has a certain calorific value and can be used as fuel in various equipment. For example, on LNG transport ships, BOG is usually introduced into equipment such as gas turbines or boilers on board as fuel for combustion. In a gas turbine, BOG is mixed with air and burned 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, providing power support for the navigation of ships and the operation of various equipment.
Impact and significance
Safety impact: If the generation of BOG is not handled properly, it can pose serious safety hazards. Due to the fact that BOG is usually heavier than air, in the event of a leak, it can accumulate on the ground or in low-lying areas. When the concentration of BOG in a local area reaches a certain range (i.e. the explosive limit of combustible gas), once it encounters a fire source (such as open flame, electrostatic spark, electric arc generated by electrical equipment, etc.), it will cause severe combustion or even explosion, causing serious damage to personnel safety and facility equipment.
Economic significance: Effective recycling and utilization of BOG has significant economic value. On the one hand, by condensing BOG back into LNG or using it as fuel in related equipment, energy waste caused by direct BOG emissions can be avoided, thereby reducing production costs for enterprises. On the other hand, with the increasing demand for clean energy and the increasingly strict requirements for energy conservation and emission reduction, efficient BOG processing technology and utilization solutions can help enterprises enhance their market competitiveness and sustainable development capabilities, which is in line with the overall trend and long-term interests of industry development.
The characteristics of BOG generation in different fields
1. Liquefied natural gas (LNG) industry chain
During the LNG production process, natural gas undergoes a series of purification, cooling, and liquefaction treatments before being stored in large low-temperature storage tanks. Due to the fact that the production environment is not absolutely adiabatic, the continuous invasion of external heat can cause some LNG to evaporate into BOG. According to statistics, in a typical LNG production plant, the daily amount of BOG generated due to external heat input can reach thousands of cubic meters.
The LNG transportation process also generates a large amount of BOG. Although LNG ships use efficient insulation materials and structural designs in their cargo tanks during transportation, they still cannot completely prevent the entry of external heat. With the extension of transportation time, LNG in the cargo tank continuously absorbs heat from the outside, thus continuing to evaporate and produce BOG. The rate of BOG generation during transportation varies among LNG ships of different types and sizes. Generally speaking, large LNG carriers can generate tens of thousands of tons of BOG during a single transoceanic transportation voyage.
At the LNG receiving station, when LNG is unloaded from the transport ship into the storage tanks at the receiving station, due to differences in pressure, temperature, and other conditions between the tanks and the liquid cargo tanks on the transport ship, some of the LNG quickly evaporates and produces a large amount of BOG. In addition, the storage tanks at the receiving station are also affected by external environmental heat during the storage of LNG, resulting in continuous generation of BOG. To cope with these BOGs, LNG receiving stations are typically equipped with a series of specialized BOG processing facilities, such as recondensers, compressors, vaporizers, etc., to achieve effective recovery, treatment, and utilization of BOGs.
2. Storage and transportation of liquefied petroleum gas (LPG)
The pressure and temperature changes inside the storage container of LPG during storage are the main causes of BOG generation. For example, in the high temperature environment of summer, storage containers are affected by solar radiation and the high temperature of the surrounding environment, and the temperature of LPG inside the container will significantly increase. Due to the increase in vapor pressure of LPG with temperature, when the pressure inside the container exceeds the saturated vapor pressure of LPG, some LPG will quickly evaporate and transform into gas, thereby producing a large amount of BOG. In addition, if the insulation performance of the storage container is poor, or if a large amount of external heat is transferred into the container during loading, unloading, transportation, and other operations, it will also promote the evaporation of LPG and produce BOG.
During the transportation of LPG, whether by road tank trucks, railway tank trucks, or sea transport vessels, LPG will produce BOG due to various factors such as vibration, temperature changes, and pressure fluctuations during transportation. For example, during the operation of a road tanker, it is constantly subjected to road bumps and vibrations, which can cause the LPG liquid inside the tanker to shake and rub, resulting in an increase in the temperature of the LPG liquid and the evaporation of some LPG to produce BOG. In addition, during transportation, when there is a significant change in the external environmental temperature, such as driving from a cold area to a hot area, the temperature of LPG in the transport container will also increase, which will promote the evaporation of LPG and produce more BOG. In order to reduce the amount of BOG generated during the transportation of LPG, a series of insulation, heat preservation, and vibration reduction measures are usually taken for the transportation containers. At the same time, real-time monitoring and control of temperature, pressure, and other parameters during transportation are also carried out.
3. Other low-temperature liquid storage scenarios (such as liquid nitrogen, liquid oxygen, etc.)
Low temperature liquids such as liquid nitrogen and liquid oxygen are widely used in industrial production and scientific research experiments. These low-temperature liquids are usually stored in specially designed low-temperature storage tanks, which are designed to minimize the transfer of external heat as much as possible to maintain the liquid storage state of the low-temperature liquids. However, even with efficient insulation materials and advanced insulation structure design, the heat from the external environment will still slowly enter the storage tank in various ways. For example, through the conduction of the outer shell of the storage tank, heat transfer at the pipeline connection, and a small amount of gas exchange at the sealing of the storage tank, external heat will gradually accumulate inside the storage tank, leading to an increase in the temperature of low-temperature liquids. When the temperature of the low-temperature liquid rises to a certain degree, some of the low-temperature liquid will absorb enough energy to evaporate and transform into a gaseous state, thereby producing an evaporative gas similar to BOG.
The evaporation rate of low-temperature liquids such as liquid nitrogen and liquid oxygen during storage is closely related to various factors. Firstly, the insulation performance of the storage tank is one of the key factors affecting the rate of evaporation gas production. Efficient insulation materials and well-designed insulation structures can significantly reduce the rate of external heat transfer, thereby reducing the evaporation of low-temperature liquids and decreasing the rate of evaporation of gases. For example, low-temperature storage tanks with multi-layer vacuum insulation structures have much better insulation performance than ordinary single-layer insulation tanks. Under the same storage conditions, the evaporation of low-temperature liquids in tanks with multi-layer vacuum insulation structures will be significantly reduced, and the rate of gas evaporation will also decrease accordingly. Secondly, factors such as temperature and humidity in the storage environment can also affect the rate of gas evaporation. In high temperature and high humidity environments, external heat is more likely to enter the storage tank. At the same time, air with higher humidity may form condensed water on the surface of the storage tank, further promoting heat transfer and accelerating the evaporation rate of low-temperature liquids, resulting in an increase in the generation rate of evaporated gases. In addition, factors such as the service life, maintenance status, and internal structural integrity of the storage tank can also affect the evaporation of low-temperature liquids and the rate of gas generation. For example, if the insulation layer of a storage tank is damaged, aged, or dampened, it will lead to a decrease in the insulation performance of the tank, an increase in the rate of external heat transfer, and an increase in the evaporation rate of low-temperature liquids and the generation rate of evaporated gases. Therefore, in order to effectively control the evaporation gas generation of low-temperature liquids such as liquid nitrogen and liquid oxygen during storage, a series of measures need to be taken from multiple aspects, such as selecting storage tanks with good insulation performance, optimizing storage environmental conditions, strengthening daily maintenance of storage tanks, and regularly testing and evaluating the insulation performance and internal structure of storage tanks, to ensure the storage safety and stability of low-temperature liquids and reduce the potential impact of evaporation gas emissions on the environment and production processes.
Development and Innovation of BOG Processing Technology
1. The principle and application limitations of traditional BOG processing technology
Recondensation process: Recondensation process is a commonly used method in traditional BOG treatment technology. The basic principle is to introduce the generated BOG into the recondenser, and fully contact and exchange heat with the low-temperature LNG liquid extracted from the LNG storage tank. During this process, the gas molecules in BOG will release heat, gradually cool and liquefy, returning to the LNG liquid state, and then be transported back to the LNG storage tank for storage. The recondensation process has been widely used in LNG receiving stations and other places, which can effectively liquefy and recover BOG, reducing the impact of BOG emissions on the environment and energy utilization efficiency. However, the recondensation process also has some application limitations. Firstly, this process requires the consumption of a large amount of low-temperature LNG liquid as a cold source to achieve the recondensation of BOG, which to some extent increases energy consumption and operating costs. Secondly, the recondensation process requires high equipment requirements, such as specialized recondensers and low-temperature liquid transfer pumps. These devices have high investment costs and require strict maintenance and management during operation to ensure the normal operation of the equipment and the stability of the process. In addition, the processing capacity of the recondensation process for BOG is limited by equipment scale and cold source supply. If the amount of BOG produced exceeds the processing capacity of the recondensation equipment, it may result in some BOG not being processed in a timely manner, thereby affecting the operational efficiency and safety of the entire system.
As fuel: Using BOG as fuel is another traditional way of processing BOG. BOG has a certain calorific value and can be used as fuel in various devices. For example, on LNG transport ships, BOG is usually introduced into equipment such as gas turbines or boilers on board as fuel for combustion. In a gas turbine, BOG is mixed with air and burned 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, providing power support for the navigation of ships and the operation of various equipment. In boilers, the heat generated by the combustion of BOG as fuel is used to heat water or other media, producing steam or hot water to provide thermal energy for the ship's living facilities, power systems, etc. Using BOG as fuel has certain advantages. It can effectively utilize the energy of BOG, reduce energy waste, and also reduce dependence on external fuel supply, improving the energy self-sufficiency of the system. However, this processing method also has some application limitations. Firstly, the composition and calorific value of BOG will fluctuate with factors such as the source, production process, and storage conditions of LNG. This leads to unstable combustion performance and energy output of BOG when used as fuel, which may affect the normal operation and efficiency of equipment. For example, if the methane content in BOG is low and the content of other impurity gases is high, it will lead to a decrease in the calorific value of BOG, resulting in insufficient heat generated during combustion, thereby affecting the output power and operating efficiency of gas turbines or boilers. Secondly, using BOG as fuel requires specialized design and modification of equipment to adapt to the combustion characteristics and supply conditions of BOG. For example, gas turbines need to be equipped with specialized BOG intake systems, burners, and control systems to ensure that BOG can mix evenly with air, burn stably in the combustion chamber, and automatically adjust the operating conditions of the gas turbine based on changes in BOG flow rate, pressure, and calorific value parameters. The design and renovation of these devices require significant investment of funds and technical expertise, as well as strict maintenance and management during operation to ensure safe and reliable operation of the equipment and effective utilization of BOG. In addition, the use of BOG as fuel also needs to consider environmental factors. Although BOG combustion produces relatively fewer pollutants, it still emits a certain amount of pollutants such as carbon dioxide, nitrogen oxides, carbon monoxide, and particulate matter, which have a certain impact on the environment. In order to reduce the impact of BOG combustion on the environment, a series of environmental protection measures need to be taken, such as optimizing the design and operating parameters of the burner, improving the combustion efficiency of BOG, and reducing the emissions of incomplete combustion products; Adopting technologies such as flue gas denitrification, desulfurization, and dust removal to purify the flue gas generated by BOG combustion and reduce the emission concentration of pollutants; Strengthen monitoring and management of BOG combustion process, real-time grasp of pollutant emissions, and adjust environmental protection measures and equipment operating parameters in a timely manner based on monitoring results to ensure that pollutant emissions comply with national and local environmental standards.
2. Research and application cases of new BOG processing technology
Adsorption separation technology: Adsorption separation technology is a new type of BOG treatment technology that utilizes the differences in adsorption capacity of adsorbents for different components in BOG to achieve separation and purification of BOG. Adsorbents usually have a large specific surface area and rich pore structure, which can effectively adsorb impurity gases such as carbon dioxide, hydrogen sulfide, water, etc. in BOG, while retaining the main component methane in BOG. During the adsorption process, BOG first removes solid particles and some moisture through a pretreatment system, and then enters the adsorption tower to come into contact with the adsorbent. The adsorbent selectively adsorbs impurity gases in BOG, significantly reducing the content of impurity gases in the treated BOG and improving the purity of methane. When the adsorbent reaches a saturated adsorption state, it needs to be regenerated to restore its adsorption capacity. The regeneration of adsorbents usually adopts the methods of depressurization desorption or temperature rise desorption, which releases the impurity gases adsorbed on the surface of the adsorbent and regenerates the adsorbent. The regenerated adsorbent can be reintroduced into the adsorption tower for BOG adsorption treatment, achieving the recycling of the adsorbent. Adsorption separation technology has broad application prospects in the field of BOG treatment. It has the advantages of high treatment efficiency, good separation effect, low energy consumption, and small equipment footprint, which can effectively improve the utilization value of BOG and reduce the impact of BOG emissions on the environment. At present, adsorption separation technology has been applied in some LNG receiving stations, LNG refueling stations, and industrial waste gas treatment projects, and has achieved good results. For example, in a certain LNG receiving station, adsorption separation technology is used to treat BOG. After adsorption treatment, the purity of methane in BOG reaches over 99%, and the content of impurity gases is significantly reduced, effectively improving the utilization value of BOG. At the same time, the application of adsorption separation technology also reduces the impact of BOG emissions on the environment, with good economic and environmental benefits.
Membrane separation technology: Membrane separation technology is another new type of BOG treatment technology that utilizes special membrane materials to separate and purify BOG based on the differences in permeation rates of different components in BOG. Membrane materials typically have the characteristic of selective permeation, allowing certain components in BOG to preferentially pass through the membrane while other components are intercepted, thereby achieving separation of BOG. In the membrane separation process, BOG first removes solid particles and some moisture through a pretreatment system, and then enters the membrane separation device to come into contact with the membrane material. The different components in BOG permeate through the membrane at different permeation rates under the pressure difference on both sides of the membrane.
