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• A high cooling capacity Stirling cryocooler is developed to liquefy boil-off gas. • The relative Carnot efficiency of the cryocooler reaches 33.28% at 77 K. • The cryocooler is installed in the skid-mounted BOG liquefaction system. • The system produces 27.3 L/h LNG and shows a great application potential. Natural gas is a cost-effective energy supply for the development of low-carbon economy and environmental protection worldwide. Despite the environmentally friendly properties of natural gas, methane is a potent greenhouse gas. Therefore, efficiently recovering the boil-off methane gas (BOG) generated from liquefied natural gas (LNG) devices is becoming a pressing problem involving both the economy and the environment. The Stirling cryocooler is a promising technology in dealing with BOG. This technology has superior characteristics, including large cooling capacity, high efficiency, compact configuration, and flexible operating characteristics. In the study, a high cooling capacity Stirling cryocooler was developed. The influences of the regenerator on the cryocooler were studied. The refrigeration performance and the operating characteristics were systematically analyzed. A cooling power of 1050 W at 77 K with a relative Carnot efficiency of 33.28 % was achieved. Accordingly, the cryocooler outperforms the reported Stirling-type cryocoolers in terms of overall performance. A BOG liquefaction system based on the cryocooler was installed in an LNG refueling station. On-site test results demonstrated that the system can produce 27.3 L/h LNG with a power consumption of 14.5 kW. The present research lays a good foundation for future optimizations and applications of the cryocooler in liquefying BOG. [ABSTRACT FROM AUTHOR]

In this study, cargo boil-off gas (BOG) re-liquefaction systems for ammonia-fueled liquefied carbon dioxide (LCO2) carriers were analyzed. These systems use cold energy from ammonia to reliquefy the CO2 BOG. In this study, a system that can completely reliquefy the CO2 BOG at all engine loads using only one heat exchanger is proposed, instead of the existing cascade system that requires multiple components. R744, which has a low global warming potential, was used as the working fluid for the refrigeration cycle in the CO2 BOG re-liquefaction system. The organic Rankine cycle (ORC) was used to reduce the net power consumption of the system. The existing and proposed systems were classified into Case 1 (existing system), Case 2 (our proposed system), and Case 3 (Case 2 combined with an ORC). Thermodynamic and economic analyses were conducted. Case 2 is a system with a simpler configuration than Case 1, but it has a similar thermodynamic performance. Case 3 has a higher exergy destruction rate than Cases 1 and 2, owing to the ORC, but it can significantly reduce the net power consumption. The economic analysis shows that Cases 2 and 3 reduce the total annual costs by 17.4% and 20.1%, respectively, compared to Case 1. The proposed systems are significantly more advantageous for long-term operation than existing systems. [ABSTRACT FROM AUTHOR]

The importance of the safe transport of liquefied carbon dioxide (LCO2) is increasing owing to environmental issues. When transporting a low-temperature liquid, boil-off gas generation and self-pressurization occur due to heat ingress, affecting the holding time of a low-temperature liquid tank. This study developed and compared three thermodynamic self-pressurization models to estimate the holding time of LCO2: Thermal homogeneous model (THM), Thermal two-zone model (TTZM), and Thermal multi-zone model (TMZM). Thermodynamic differential equations were solved for THM, and software was used for TTZM. For TMZM, the parameters were optimized using experimental data to determine the heat ratio parameter f and heat transfer parameters K1 and K2. THM and TTZM estimated an unreasonably long holding time, approximately 42 days. The TMZM, however, showed a satisfactory holding time of 12–13 days. These results can help predict the self-pressurization in the storage tanks of LCO2 and be applied to actual LCO2 carrier cargo handling systems, with the modeling results indicating that the 12–13 days of LCO2 self-pressurization based on the TMZM appears to be the most suitable.

The boiling point of liquid hydrogen is very low, at −253 °C under atmospheric pressure, which causes boil-off gas (BOG) to occur during storage and transport due to heat penetration. Because the BOG must be removed through processes such as re-liquefaction, venting to the atmosphere, or incineration, related studies are required to estimate the heat transfer of storage and transport devices and to improve insulation to reduce BOG generation. In this study, a vaporization analysis was performed on a vacuum-jacketed valve used in liquid hydrogen storage and transport devices to calculate the amount of BOG generation considering the flow characteristics at the vena contracta and the saturation temperature. At a pressure of 1 bar in the liquid hydrogen storage tank, the maximum fluid flow velocity and minimum static pressure occurred at the vena contracta, with values of 62.9 m/s and −0.4 bar, respectively, and the BOG generation rate was estimated as 0.132 m3/h, where the saturation temperature was minimized at 19.3 K. Furthermore, through case studies, when the pressure in the liquid hydrogen storage tank increased to 1.5 and 2 bar, the static pressure and saturation temperature decreased, and the BOG generation rate increased to 0.221 and 0.283 m3/h, respectively. [ABSTRACT FROM AUTHOR]

In this study, cargo boil-off gas (BOG) re-liquefaction systems for ammonia-fueled liquefied carbon dioxide (LCO2) carriers were analyzed. These systems use cold energy from ammonia to reliquefy the CO2 BOG. In this study, a system that can completely reliquefy the CO2 BOG at all engine loads using only one heat exchanger is proposed, instead of the existing cascade system that requires multiple components. R744, which has a low global warming potential, was used as the working fluid for the refrigeration cycle in the CO2 BOG re-liquefaction system. The organic Rankine cycle (ORC) was used to reduce the net power consumption of the system. The existing and proposed systems were classified into Case 1 (existing system), Case 2 (our proposed system), and Case 3 (Case 2 combined with an ORC). Thermodynamic and economic analyses were conducted. Case 2 is a system with a simpler configuration than Case 1, but it has a similar thermodynamic performance. Case 3 has a higher exergy destruction rate than Cases 1 and 2, owing to the ORC, but it can significantly reduce the net power consumption. The economic analysis shows that Cases 2 and 3 reduce the total annual costs by 17.4% and 20.1%, respectively, compared to Case 1. The proposed systems are significantly more advantageous for long-term operation than existing systems.

In liquid hydrogen (LH 2) storage tanks, the temperature difference between LH 2 and the environment leads to the inevitable heat ingress into the storage tanks. Understanding the details of the thermal performance in LH 2 storage tanks is necessary to minimize or avoid boil-off gas losses. This paper presents a thermodynamic non-equilibrium model to study the thermal differences between cylindrical and spherical LH 2 tanks with the same volumetric capacity. The accuracy of the model is validated with experimental data obtained from cylindrical and spherical tanks. The numerical results show that, in self-pressurized tanks, a spherical geometry results in the lowest pressure increase rate. The pressure increase in horizontal, cylindrical tanks is slower than the corresponding increase in cylindrical tanks oriented vertically. The difference can be related to more effective heat transfer between the vapor and liquid phases in the horizontal cylindrical tanks. A higher initial liquid level increases the thermal mass of the liquid, which suppresses the evaporation rates due to larger heat requirements to warm and evaporate the liquid hydrogen. For tanks under atmospheric pressure, the boil-off gas rate is lowest in the spherical tank while the boil-off gas rates in the horizontal and vertical cylindrical tanks are almost the same. Under these conditions, and quasi-steady state, the total heat absorbed by the tanks is used to evaporate LH 2. The spherical tank has the smallest surface area and thus the lowest evaporation rate and the lowest heat transfer from the environment. The evaporation rate is not affected by initial liquid levels. • A non-equilibrium model is developed to predict the thermal performance of liquid hydrogen (LH 2) tanks. • Cylindrical (horizontal and vertical) and spherical geometries are considered. • The thermal performance is analyzed at moderate (atmospheric) and higher pressures. • A parametric study is conducted to analyze evaporation processes. [ABSTRACT FROM AUTHOR]

Growing demand for cleaner energy resources has led to the increased global production of liquefied natural gas (LNG). At the same time, there are some economic and environmental impacts from LNG evaporation forming the boil-off gas (BOG) during production, storage, loading, transportation, and unloading. This study presents the technical and economic aspects of BOG utilization to a value-added hydrogen product (H 2) at LNG exporting terminals using a steam reforming production process. The steady-state process simulation using Aspen HYSYS software achieves an overall C 1 + process conversion of 100% with an H 2 gas product purity of 99.99%. Heat integration between the recycle stream and the final purification section presents 12% energy savings. Additionally, the process revealed production costs as low as $0.23/kg of H 2 produced, which is competitive with production from resources such as coal and natural gas. The low cost enabled the investigation of three monetization pathways: direct H 2 sales to domestic markets, liquefied H 2 sales to international markets, and ammonia sales to international markets. The production profitability of H 2 sales as the liquefied H 2 product shows the highest NPV of $66.2 billion for a project with a lifetime of 25 years and an interest rate of 10%. These approaches empower decision-makers to diversify selling portfolios in the LNG industry and support the transition to H 2 using existing infrastructure. • Boil-off gas (BOG) produced by LNG production plants is burned as waste energy. • LNG management plants must be improved to enhance efficiency and increase profit. • BOG from LNG facilities is used for H 2 production by steam reforming process (SRP). • SRP's H 2 production costs can be as low as $0.23/kg competing with other fossil fuels. • The NPV of H 2 production from BOG was $66.2 billion over a 25-year project lifetime. [ABSTRACT FROM AUTHOR]

Bridge technologies incorporating integrated energy production systems are used to adapt to the growing global energy demand while reducing greenhouse gas emissions. This study aims to propose a system to produce energy in the form of turquoise hydrogen, ammonia, and urea from natural gas. In this study, natural gas is fed to a fluidized bed reactor where high temperatures crack the light-end hydrocarbons into hydrogen and solid carbon. The hydrogen is split between the stored final product and the remainder is further processed as an intermediate product to produce ammonia and urea. This integrated process uses a zero-emission thermochemical cracking unit utilizing solar power as the primary heat source. This work analyzes the feasibility of transporting the produced energy carrier fuels; turquoise hydrogen, ammonia, and urea, overseas for fuel cell power production. The processes are modelled using Aspen Plus© and Engineering Equation Solver software. The main system feed is 4969 kg/h of natural gas, 3733 kg/h nitrogen, and 1991 kg/h of oxygen producing 397 kg/h of hydrogen, 2136 kg/h of ammonia and 4752 kg/h of urea. An integrated Rankine cycle generates 957 kW of power in a steam turbine from waste heat. The supply chain's boil-off gas rates are 1% and 0.02% for hydrogen and ammonia, respectively. The total electrical energy output of the energy carriers used in the fuel cells is 185,900 MWh from hydrogen, 362,100 MWh from ammonia and 458,700 MWh from the urea fuel. [Display omitted] • Thermocracking of natural gas is an alternative to meet the growing hydrogen demand. • Concentrated solar power system with thermal storage used for thermocracking. • Hydrogen and solid carbon from natural gas are used to produce ammonia and urea. • Optimized fuel type and storage conditions allow for effective transportation of energy carriers. • Fuel cells produce clean electrical energy from hydrogen, ammonia and urea energy carriers. [ABSTRACT FROM AUTHOR]

This study was carried out to assess effect of boil-off gas (BOG) on the safety and economic effectiveness of LPG storage tank. It includes analysis of the thermodynamic properties of LPG; heat absorbed from ambient air by the storage tank that leaks into the LPG, and consequently generates boil-off gas in the supply chain, by utilizing appropriate thermodynamic and heat transfer equations. Analyzing the heat leakage required the estimations of the convective heat transfer coefficient of the ambient air in the supply location and the LPG supply chain, which amounted to 3335.9W/m2K and 21.058W/m2K, respectively, in the system under study. Analyzing the thermodynamic properties such as specific volume, entropy and enthalpy of the LPG, show that the entropy of LPG in the storage tank is negative, which suggested an endothermic process, validating that heat is added to the system from the surroundings. The heat absorbed in the LPG from the ambient air by the storage tank amounted to 1.785kW. The boil-off generation rate due to the storage tank heat leakage was 0.0049kg/s, which translates to a cost equivalent loss of 0.0069$/s at LPG selling price of 1.42$/kg. It was recommended that maintenance of insulation and other external factors such as wind speed, solar radiation, ambient temperature and thermal conductivity of the storage tank material are key factors in minimizing the heat leaks into LPG; hence BOG generation, which is of utmost importance in ensuring safety and economic loss in the LPG supply chain. [ABSTRACT FROM AUTHOR]

In a natural gas liquefaction plant, boil-off gas (BOG) is generated in both liquefaction and loading processes. During the holding mode (no loading operation), the fuel gas supply, including the BOG generated in the liquefaction process, is less than the fuel gas demand. Therefore, make-up gas is added to the fuel gas system. BOG recovery systems are designed to absorb the maximum amount of BOG during liquefied natural gas (LNG) loading to ships (loading mode). Additional BOG is generated during the loading process (loading mode). At this stage, the fuel gas supply is higher than the demand. To avoid fuel gas flaring, BOG from the liquefaction process is reduced by cooling the LNG from the main heat exchanger (MHE) to a lower temperature, which consequently reduces LNG production by more than 1%. This study aimed to optimize liquefaction, storage, loading, fuel gas and BOG recovery processes to increase LNG production. In this study, a natural gas liquefaction plant producing 8 mtpa (million tonnes/ year), where LNG production was limited by the capacity of the BOG recovery system and fuel gas demand, was modelled using UniSim software. In the model, the optimization of BOG recovery during the holding mode increased LNG production by 46,850 tonnes/ year. The volumetric flow rate of BOG generated during LNG loading was reduced by decreasing the temperature of the ship’s cargo tanks, decreasing the loading pumps’ discharge pressure, reducing the discharge pressure of the ship’s compressors, and reducing the loading rate. This study shows that BOG generated during loading can be significantly reduced by around 50% from the initial rate, which increases LNG production by around 36,576 tonnes/ year. In total, the potential production increase from BOG recovery and fuel gas optimization was around 90,260 tonnes/ year or equivalent to 1.4 cargo of LNG per year.

At least 24 liquefied natural gas (LNG) rollover incidents have been reported since 1960. During rollover, because of the heat ingress through the tank walls, a stratified LNG may be suddenly homogenized while releasing massive amounts of vapor. It can result in an overpressure in the tank and significant amounts of potentially explosive LNG vapor being vented out. Both of these factors represent considerable hazards. Rollover is a physical mixing process in a single tank with two or more different cells of LNG of different compositions, temperatures, and densities that can manifest in large boil-off rates. It can exceed venting equipment capacities, and vapor pressure in tank increases rapidly and in extreme cases can lead to tank damage. This paper presents numerical approach for determination of time of rollover occurrence in storage tank. The presented model is based on the energy balance of the stratified cryogenic liquid and the gas phase as separate three thermodynamic systems in the storage tank. As a result of proposed model, for the adopted assumptions and cylindrical tank volume of 78,500 m3, the approximate time of the rollover occurrence was determined for two cases. In the first case, for heavier LNG, the rollover phenomenon will occur 193.25 h after the start of the calculations from the assumed initial conditions. In the second case, for light LNG with a higher initial liquid level in the tank, the rollover will occur after 150.25 h. [ABSTRACT FROM AUTHOR]

Accurate evaluation of thermo-fluid dynamic characteristics in tanks is critically important for designing liquid hydrogen tanks for small-scale hydrogen liquefiers to minimize heat leakage into the liquid and ullage. Due to the high costs, most future liquid hydrogen storage tank designs will have to rely on predictive computational models for minimizing pressurization and heat leakage. Therefore, in this study, to improve the storage efficiency of a small-scale hydrogen liquefier, a three-dimensional CFD model that can predict the boil-off rate and the thermo-fluid characteristics due to heat penetration has been developed. The prediction performance and accuracy of the CFD model was validated based on comparisons between its results and previous experimental data, and a good agreement was obtained. To evaluate the insulation performance of polyurethane foam with three different insulation thicknesses, the pressure changes and thermo-fluid characteristics in a partially liquid hydrogen tank, subject to fixed ambient temperature and wind velocity, were investigated numerically. It was confirmed that the numerical simulation results well describe not only the temporal variations in the thermal gradient due to coupling between the buoyance and convection, but also the buoyancy-driven turbulent flow characteristics inside liquid hydrogen storage tanks with different insulation thicknesses. In the future, the numerical model developed in this study will be used for optimizing the insulation systems of storage tanks for small-scale hydrogen liquefiers, which is a cost-effective and highly efficient approach. [ABSTRACT FROM AUTHOR]

The boiling point of liquid hydrogen is very low, at −253 °C under atmospheric pressure, which causes boil-off gas (BOG) to occur during storage and transport due to heat penetration. Because the BOG must be removed through processes such as re-liquefaction, venting to the atmosphere, or incineration, related studies are required to estimate the heat transfer of storage and transport devices and to improve insulation to reduce BOG generation. In this study, a vaporization analysis was performed on a vacuum-jacketed valve used in liquid hydrogen storage and transport devices to calculate the amount of BOG generation considering the flow characteristics at the vena contracta and the saturation temperature. At a pressure of 1 bar in the liquid hydrogen storage tank, the maximum fluid flow velocity and minimum static pressure occurred at the vena contracta, with values of 62.9 m/s and −0.4 bar, respectively, and the BOG generation rate was estimated as 0.132 m3/h, where the saturation temperature was minimized at 19.3 K. Furthermore, through case studies, when the pressure in the liquid hydrogen storage tank increased to 1.5 and 2 bar, the static pressure and saturation temperature decreased, and the BOG generation rate increased to 0.221 and 0.283 m3/h, respectively.

Reduction of energy expenditures required for various technological processes is a pressing issue in today's economy. One of the ways to solve this issue in regard to liquefied natural gas (LNG) storage is the recovery of its vapours from LNG tanks using an ejector system. In that respect, studies on the outflow of the real gas through the nozzle, the main element of the ejector, and identifying differences from the ideal gas outflow, are of high relevance. Particularly, this concerns the determination of the discharge coefficient µ as the ratio of the actual flowrate to the ideal one, taking into account the energy losses at gas outflow through the nozzle. The discharge coefficient values determined to date for various nozzle geometries are, as a rule, evaluated empirically and contradictory in some cases. The authors suggest determining the discharge coefficient by means of an experiment. This paper includes µ values determined using this method for the critical outflow of air to atmosphere through constrictor nozzles with different outlet diameters (0.003 m; 0.004 m; 0.005 m) in the pressure range at the nozzle inlet of 0.5–0.9 MPa. The obtained results may be used for the design of an ejector system for the recovery of the boil-off gas from LNG tanks, as well as in other fields of industry, for the design of technical and experimental devices with nozzles for various applications. [ABSTRACT FROM AUTHOR]

The price of constructing hydrogen generation units is very high and sometimes it is not possible to build them in the desired location, so the transfer of hydrogen from the hydrogen generation system to the units that need it is justified. Since the storage of hydrogen gas needs a large volume and its transportation is very complex, so if hydrogen is stored in liquid form, this problem can be resolved. In transporting liquid hydrogen (LH2) over long distances, owing to heat transfer to the environment, the LH2 vaporizes, forming boil-off gas (BOG). Herein, in lieu of only reliquifying the BOG, this study proposes and assesses a system employing the BOG partially as feed for a novel liquefaction process, and also the remaining utilized in a proton exchange membrane fuel cell (PEMFC) to generate power. Using the cold energy of the onsite liquid oxygen utility of the LH2 cargo vessel, the mixed refrigerant liquefaction cycle is further cooled down. In this regard, by using 130 kg/h BOG as input, 60.37 kg/h of liquid hydrogen is produced and the rest enters PEMFC with 552.7 kg/h oxygen to produce 1592 kW of power. The total thermal efficiency of the integrated system and electrical efficiency of the PEMFC is 83.18% and 68.76%, respectively. Regarding the liquefaction cycle, its specific power consumption (SPC) and coefficient of performance (COP) were achieved at 3.203 kWh/kgLH 2 and 0.1876, respectively. The results of exergy analysis show that the exergy destruction of the whole system is 937.4 kW and also its exergy efficiency is calculated to be 58.38%. Exergoeconomic and Markov analyses have also been applied to the integrated system. Also, by changing the important parameters of PEMFC, its optimal performance has been extracted.

Due to tighter environmental regulations, newly built liquefied natural gas (LNG) carriers are equipped with a re-liquefaction system to minimize combustion of surplus boil-off-gas (BOG). Thus, this paper comparatively analyzes the re-liquefaction system for a low-pressure gas injection engine according to the refrigerant (no external refrigerant or single mixed refrigerant) with three key performance indicators: energy, economic, and environmental aspects. For an energy efficiency analysis, we proposed several process alternatives and optimized them to minimize the specific power consumption required to liquefy BOG. In economic analysis, minimizing total annualized cost is the objective. For an environmental analysis, CO2 emissions at each optimal point is calculated and comparatively analyzed. The results show that the process without external refrigerant has 10% better performance in terms of economy, while the single mixed refrigerant process is suitable in terms of energy efficiency (6%) and environmental (15%) impact.

There exists no single optimal way for transporting hydrogen and other hydrogen carriers from one port to the other globally. Its delivery depends on several factors such as the quantity, distance, economics, and the availability of the required infrastructure for its transportation. Europe has a strategy to invest in the production of green hydrogen in Africa to meet its needs. This study assessed the economic viability of shipping liquefied hydrogen (LH2) and hydrogen carriers to Germany from six African countries that have been identified as countries with great potential in the production of hydrogen. The results obtained suggest that the shipping of LH2 to Europe (Germany) will cost between 0.47 and 1.55 USD/kg H2 depending on the distance of travel for the ship. Similarly, the transportation of hydrogen carriers could range from 0.19 to 0.55 USD/kg H2 for ammonia, 0.25 to 0.77 USD/kg H2 for LNG, 0.24 to 0.73 USD/kg H2 for methanol, and 0.43 to 1.28 USD/kg H2 for liquid organic hydrogen carriers (LOHCs). Ammonia was found to be the ideal hydrogen carrier since it recorded the least transportation cost. A sensitivity analysis conducted indicates that an increase in the economic life by 5 years could averagely decrease the cost of LNG by some 13.9%, NH3 by 13.2%, methanol by 7.9%, LOHC by 8.03%, and LH2 by 12.41% under a constant distance of 6470 nautical miles. The study concludes with a suggestion that if both foreign and local participation in the development of the hydrogen market is increased in Africa, the continent could supply LH2 and other hydrogen carriers to Europe at a cheaper price using clean fuel. [ABSTRACT FROM AUTHOR]

Liquefied natural gas (LNG)-fueled ships have the effect of reducing most pollutants, which is advantageous for responding to strict regulations. Because boil-off gas (BOG) is generated in the LNG storage tank of an LNG-fueled ship, a BOG re-liquefaction system is required. The representative systems for LNG-fueled ships were proposed by Kwak and Shen, but their exergy efficiencies were only 19.6% and 24.9%, respectively. To improve the system, this paper proposes novel BOG re-liquefaction systems combined with the fuel gas supply system. The systems utilize LNG cold energy in the BOG stream and N2 reverse Brayton cycle, respectively. The proposed systems were simulated using a commercial program and were optimized using a genetic algorithm. The results of energy, exergy, and economic (3E) analyses performed for comprehensive evaluation of the proposed system show that the system in which LNG cold energy is applied to the BOG stream has the best performance. Specific energy consumption, exergy efficiency, and total annual costs of this system were improved by up to 78.6%, 69.2%, and 68.2%, respectively, compared to those of the existing systems. The overwhelmingly superior system is expected to greatly contribute to the improvement of the BOG re-liquefaction system for LNG-fueled ships. [ABSTRACT FROM AUTHOR]

Copyright of Journal of Engineering for Thermal Energy & Power / Reneng Dongli Gongcheng is the property of Journal of Engineering for Thermal Energy & Power and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

In this research, a novel liquefied natural gas (LNG) reproduction process is developed to re-liquefy generated boil-off gas from the LNG tank through an ejector refrigeration unit driven by an organic Rankine cycle, an auto-cascade refrigeration, and an ethylene compression refrigeration cycle. The developed system is evaluated through exergy efficiency, exergy destruction, coefficient of performance, and specific energy consumption indexes. Exergy destruction evaluation shows that heat exchangers and compressors are the most destructive equipment in the developed layout. The presented cycle obtains an exergy efficiency of 0.5936. It has been shown that increasing the adiabatic efficiency of the compressor leads to increasing the coefficient of performance while it decreases the required specific energy consumption. In addition, it has been monitored that increasing the boil-off gas re-liquefaction pressure yields an increase in the specific energy consumption. Besides, it has been illustrated that turbine inlet temperature is a crucial factor in the performance of the system, and achieving higher temperature has a positive factor on the whole system. [ABSTRACT FROM AUTHOR]

• Explored sloshing in cryogenic hydrogen tanks for trains. • Simulations conducted at 4 vibration cases. • Used transient volume-of-fluid phase change model. • Increased frequency led to increased boil-off gas generation. • Highlighted temperature and pressure changes with vibrations. This study examines the effects of hydrogen sloshing on internal pressure, temperature, and fluid behavior liquefied-hydrogen storage tanks designed for train usage by applying the natural frequency and frequency conditions from train vibration test standards. Notably, it investigates the impact on BOG generation using a transient volume-of-fluid phase change model. Here, simulations were conducted at vibrations of 0, 0.53, 1.53, and 3 Hz, which were established using sine wave acceleration. The results demonstrated that sloshing increased with higher frequencies, thereby resulting in a more intense heat transfer between the wall of the tanks and free surface of hydrogen and an increase in the BOG generation. Compared to the 0 Hz baseline, BOG generation increased by 13, 44, and 66 % at 0.53, 1.53, and 3 Hz, respectively. [ABSTRACT FROM AUTHOR]

Re-liquefication of boil-off gas (BOG) from liquefied natural gas is essential for maintaining LNG tank safety and facilitating methane recovery. This study aims to reduce the energy consumption of the BOG re-liquefaction process based on the reverse Brayton cycle (RBC) and eliminates the need for a BOG compressor. The classical Kapitsa cycle is improved by making the expander backpressure lower than the pressure of liquid nitrogen after throttling and employed in the BOG re-liquefaction process without a BOG compressor. A comprehensive analysis is conducted, including the sensitivity of independent operating parameters, global optimization using a genetic algorithm, and subsequent thermodynamic, economic, and environmental assessments based on the optimization results. Furthermore, the impact of BOG flowrate and temperature variations on system performance is investigated. Results showed that the proposed process is more efficient than the conventional BOG indirect processes employing the classical Kapitsa cycle or RBC. A minimum specific energy consumption (SEC) of 0.82 kWh·kg−1 is achieved, and the figure of merit is up to 30.9 %. The payback period of total capital investment is estimated to be merely one month. Carbon dioxide emissions are decreased by 137462 kg·h−1 compared to direct BOG emissions. SEC could reach 0.82 kWh·kg−1 across varying BOG flowrates by adjusting the refrigerant flowrate. • The calssical Kapitza cycle is improved and acts as a nitrogen liquefier. • BOG is liquefied using the liquid nitrogen and the BOG compressor is eliminated. • Global optimization is performed using genetic algorithm. • The process is assessed in terms of thermodynamics, economy, and environment. • Minimum SEC of 0.82 kWh·kg−1 and payback period of about 1 month are achieved. [ABSTRACT FROM AUTHOR]

At least 24 liquefied natural gas (LNG) rollover incidents have been reported since 1960. During rollover, because of the heat ingress through the tank walls, a stratified LNG may be suddenly homogenized while releasing massive amounts of vapor. It can result in an overpressure in the tank and significant amounts of potentially explosive LNG vapor being vented out. Both of these factors represent considerable hazards. Rollover is a physical mixing process in a single tank with two or more different cells of LNG of different compositions, temperatures, and densities that can manifest in large boil-off rates. It can exceed venting equipment capacities, and vapor pressure in tank increases rapidly and in extreme cases can lead to tank damage. This paper presents numerical approach for determination of time of rollover occurrence in storage tank. The presented model is based on the energy balance of the stratified cryogenic liquid and the gas phase as separate three thermodynamic systems in the storage tank. As a result of proposed model, for the adopted assumptions and cylindrical tank volume of 78,500 m3, the approximate time of the rollover occurrence was determined for two cases. In the first case, for heavier LNG, the rollover phenomenon will occur 193.25 h after the start of the calculations from the assumed initial conditions. In the second case, for light LNG with a higher initial liquid level in the tank, the rollover will occur after 150.25 h.

The present study is an analysis of the processes in the components of the LPG (propane/butane) reliquefaction plant under the conditions of co-mingling in tanks when transporting by sea. For the analysis, the monitoring data of an LPG cargo operation have been used. An energy analysis of the mixture-based reliquefaction plant has been performed. The characteristics of the mixture in the tanks, the operating conditions of the reliquefaction plant, and the performance of the system have been considered. The method of equivalence has been applied for thermodynamic analysis. The result of the substitution of actual processes with equivalent ones allows for the accomplishment of the parameters control of each working fluid within the mixture as a pure working fluid. It is shown that the low-boiling component determines the operating parameters of the entire reliquefaction plant. The method of equivalence and visualization of the processes within the LPG as a mixture using the thermodynamic diagrams of pure working fluids is recommended to shorten the path to set up the appropriate reliquefaction plant management strategy. The energy analysis performed using the method of equivalent cycles has been validated with the existing reliquefaction plant characteristics. The inaccuracies are in the limit of 4%. [ABSTRACT FROM AUTHOR]

To cope with climate change, emerging fuels- hydrogen, ammonia and methanol- have been proposed as promising energy carriers that will replace part of the liquefied natural gas (LNG) in future maritime scenarios. Energy efficiency is an important indicator for evaluating the system, but the maritime supply system for emerging fuels has yet to be revealed. In this study, the energy efficiency of the maritime supply chain of hydrogen, ammonia, methanol, and natural gas is investigated, considering processes including production, storage, loading, transport and unloading. A sensitivity analysis of parameters such as ambient temperature, storage time, pipeline length and sailing time is also carried out. The results show that hydrogen (2.366%) has the highest daily boil-off gas (BOG) rate and wastes more energy than LNG (0.413%), with ammonia and methanol both being lower than LNG. The recycling of BOG is of great importance to the hydrogen supply chain. When produced from renewable energy sources, methanol (98.02%) is the most energy efficient, followed by ammonia, with hydrogen being the least (89.10%). This assessment shows from an energy efficiency perspective that ammonia and methanol have the potential to replace LNG as the energy carrier of the future and that hydrogen requires efficient BOG handling systems to increase competitiveness. This study provides some inspirations for the design of global maritime supply systems for emerging fuels.

Accurate evaluation of thermo-fluid dynamic characteristics in tanks is critically important for designing liquid hydrogen tanks for small-scale hydrogen liquefiers to minimize heat leakage into the liquid and ullage. Due to the high costs, most future liquid hydrogen storage tank designs will have to rely on predictive computational models for minimizing pressurization and heat leakage. Therefore, in this study, to improve the storage efficiency of a small-scale hydrogen liquefier, a three-dimensional CFD model that can predict the boil-off rate and the thermo-fluid characteristics due to heat penetration has been developed. The prediction performance and accuracy of the CFD model was validated based on comparisons between its results and previous experimental data, and a good agreement was obtained. To evaluate the insulation performance of polyurethane foam with three different insulation thicknesses, the pressure changes and thermo-fluid characteristics in a partially liquid hydrogen tank, subject to fixed ambient temperature and wind velocity, were investigated numerically. It was confirmed that the numerical simulation results well describe not only the temporal variations in the thermal gradient due to coupling between the buoyance and convection, but also the buoyancy-driven turbulent flow characteristics inside liquid hydrogen storage tanks with different insulation thicknesses. In the future, the numerical model developed in this study will be used for optimizing the insulation systems of storage tanks for small-scale hydrogen liquefiers, which is a cost-effective and highly efficient approach.

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Reduction of energy expenditures required for various technological processes is a pressing issue in today’s economy. One of the ways to solve this issue in regard to liquefied natural gas (LNG) storage is the recovery of its vapours from LNG tanks using an ejector system. In that respect, studies on the outflow of the real gas through the nozzle, the main element of the ejector, and identifying differences from the ideal gas outflow, are of high relevance. Particularly, this concerns the determination of the discharge coefficient µ as the ratio of the actual flowrate to the ideal one, taking into account the energy losses at gas outflow through the nozzle. The discharge coefficient values determined to date for various nozzle geometries are, as a rule, evaluated empirically and contradictory in some cases. The authors suggest determining the discharge coefficient by means of an experiment. This paper includes µ values determined using this method for the critical outflow of air to atmosphere through constrictor nozzles with different outlet diameters (0.003 m; 0.004 m; 0.005 m) in the pressure range at the nozzle inlet of 0.5–0.9 MPa. The obtained results may be used for the design of an ejector system for the recovery of the boil-off gas from LNG tanks, as well as in other fields of industry, for the design of technical and experimental devices with nozzles for various applications.

Liquefied natural gas (LNG)-fueled ships have the effect of reducing most pollutants, which is advantageous for responding to strict regulations. Because boil-off gas (BOG) is generated in the LNG storage tank of an LNG-fueled ship, a BOG re-liquefaction system is required. The representative systems for LNG-fueled ships were proposed by Kwak and Shen, but their exergy efficiencies were only 19.6% and 24.9%, respectively. To improve the system, this paper proposes novel BOG re-liquefaction systems combined with the fuel gas supply system. The systems utilize LNG cold energy in the BOG stream and N2 reverse Brayton cycle, respectively. The proposed systems were simulated using a commercial program and were optimized using a genetic algorithm. The results of energy, exergy, and economic (3E) analyses performed for comprehensive evaluation of the proposed system show that the system in which LNG cold energy is applied to the BOG stream has the best performance. Specific energy consumption, exergy efficiency, and total annual costs of this system were improved by up to 78.6%, 69.2%, and 68.2%, respectively, compared to those of the existing systems. The overwhelmingly superior system is expected to greatly contribute to the improvement of the BOG re-liquefaction system for LNG-fueled ships.

The production, transportation, and storage of liquefied natural gas (LNG) is a promising area in the gas industry due to a number of the fuel's advantages, such as its high energy intensity indicators, its reduced storage volume compared to natural gas in the gas-air state, and it ecological efficiency. However, LNG storage systems feature a number of disadvantages, among which is the boil-off gas (BOG) recovery from an LNG tank by flaring it or discharging it to the atmosphere. Previous attempts to boil-off gas recovery using compressors, in turn, feature such disadvantages as large capital investments and operating costs, as well as low reliability rates. The authors of this article suggest a technical solution to this problem that consists in using a gas ejector for boil-off gas recovery. Natural gas from a high-pressure gas pipeline is proposed as a working fluid entraining the boil-off gas. The implementation of this method was carried out according to the developed algorithm. The proposed technical solution reduced capital costs (by approximately 170 times), metal consumption (by approximately 100 times), and power consumption (by approximately 55 kW), and improved the reliability of the system compared to a compressor unit. The sample calculation of a gas ejector for the boil-off gas recovery from an LNG tank with a capacity of 300 m3 shows that the ejector makes it possible to increase the boil-off gas pressure in the system by up to 1.13 MPa, which makes it possible to not use the first-stage compressor unit for the compression of excess vapours. [ABSTRACT FROM AUTHOR]

Growing interest in liquefied natural gas (LNG) as a rocket fuel demands reliable prediction and an improved understanding of the changes in its composition arising from the preferential boil-off of lighter components during long duration storage. Unfortunately, current methods of predicting boil-off gas (BOG) evolution from cryogenic liquids are based on limited experimental data. This work reports a series of new experiments which measure the temporal change in BOG production, composition, and pressure at industrially relevant conditions for both ternary mixtures of methane, ethane, and nitrogen and an LNG mixture used as rocket fuel. Faster pressure build-up rates are consistently observed with decreasing initial liquid volume fraction, whilst a decline of 8% in the LNG higher heating value was observed after thirty-three days of weathering. The data is compared with a robust and efficient superheated vapor (SHV) model, implemented in the software package BoilFAST, which allows for reliable calculations of self-pressurisation and boil-off losses for different tank geometries and thermal insulation systems. The model exhibits good agreement with the experimental data across all conditions explored. Finally, the potential effect of LNG composition and heating value changes on rocket engine performance was assessed by examining changes in the adiabatic flame temperature and burnt gas volume ratio. While our data suggest that the rocket engine performance would improve as a result of weathering, the effectiveness of the weathered LNG as a coolant in a regeneratively cooled rocket engine decreases. Two unique apparatuses providing comprehensive sets of data for LNG fuel boil-off Boil-off-gas (BOG) measurements of LNG mixtures conducted for various conditions In-depth analysis of pressure build-up, composition change and BOG rates A robust model to predict the dynamic weathering of LNG fuel Potential effect of LNG composition changes on rocket engine performance [ABSTRACT FROM AUTHOR]

At present, LNG vessels without reliquefaction plants consume the BOG (boil-off gas) in their engines and the excess is burned in the gas combustion unit without recovering any of its energy content. Excess BOG energy could be captured to produce H2, a fuel with high energy density and zero emissions, through the installation of a reforming plant. Such H2 production would, in turn, require on-board storage for its subsequent consumption in the propulsion plant when navigating in areas with stringent anti-pollution regulations, thus reducing CO2 and SOX emissions. This paper presents a review of the different H2 storage systems and the methods of burning it in propulsion engines, to demonstrate the energetic viability thereof on board LNG vessels. Following the analysis, it is identified that a pressurised and cooled H2 storage system is the best suited to an LNG vessel due to its simplicity and the fact that it does not pose a safety hazard. There are a number of methods for consuming the H2 generated in the DF engines that comprise the propulsion plant, but the use of a mixture of 70% CH4-30% H2 is the most suitable as it does not require any modifications to the injection system. Installation of an on-board reforming plant and H2 storage system generates sufficient H2 to allow for almost 3 days’ autonomy with a mixture of 70%CH4-30%H2. This reduces the engine consumption of CH4 by 11.38%, thus demonstrating that the system is not only energy-efficient, but lends greater versatility to the vessel.

In a Boil-off Gas (BOG) compressor, there could be a carryover phenomenon of lubricants oil which flows with a solvent gas. To protect process system downstream of the compressor, it is necessary to select low solubility lubricants at supercritical state. In this study, the experiment of solubility measurement was conducted at 300 bar of methane with six different types of lubricating oils using supercritical fluid extracting devices operated at different temperature conditions. The solubility measurement results were analyzed according to the density of methane in the supercritical states and the vapor pressure of the lubricants. This indicates that there is a possibility of developing the criteria to choose the proper lubricant oil used by the equipment operating in the supercritical state.

Re-liquefying boil-off gas (BOG) of LNG is of great significance for energy conservation and environmental protection. BOG is usually compressed before being liquefied with nitrogen reverse Brayton cycle, which makes the system's capital investment and energy consumption high. A novel compression-free re-liquefication process of BOG using liquid nitrogen (LN 2) is proposed. BOG is liquefied with latent heat of LN 2 , reducing the heat-transfer temperature difference in BOG condenser and the required amount of refrigerant. LN 2 is produced using LNG pre-cooled Joule-Thomson cycle. Overall optimization is performed using genetic algorithm. Thermodynamic, economic, and environmental assessments are conducted based on the optimized results. Sensitivity analysis of the principal design parameters is performed. It is found that the proposed process is significantly profitable because it fully utilizes the cold energy of LNG and LN 2 and eliminates the required cost for BOG compressor, turbine expander, and LNG regasification process. The minimum specific energy consumption of 0.74 kWh·kg−1 is achieved, and the figure of merit is up to 34.95%. The payback period of total capital investment is only one month. Carbon dioxide emission is reduced by 36242.66 kg·h−1 compared to the direct emission of BOG. Pinch temperature difference of 3∼5 °C is recommended for cryogenic heat exchangers. •A compression-free BOG liquefaction process using LN 2 is proposed. • LN 2 is produced with LNG cold energy pre-cooled J-T cycle. •Global optimization is performed using genetic algorithm. •The process is evaluated in terms of thermodynamics, economy, and environment. •Minimum SEC of 0.74 kWh·kg−1 and payback period of 1 month are achieved. [ABSTRACT FROM AUTHOR]

In this study, a novel boil-off-gas handling process was proposed to optimize refrigerant mix and flow rate considering different boil-off gas rates under different weather conditions. The proposed design utilizes a nitrogen expander and mixed refrigerant cycle to generate subcooled liquified natural gas to cool the tank, preventing boil-off gas generation. The optimized design in terms of exergy destruction minimization is assessed in a multiple steady-state simulation based on seasonal boil-off gas and heat ingress rates which are predicted by a machine learning algorithm. Furthermore, an economic analysis of the proposed reliquefaction processes was conducted. The result shows that the conventional process has 46% and 8% lower exergy destruction compared to the proposed one for the nitrogen expander and mixed refrigerant cycle respectively. However, the proposed process outperforms the conventional process in terms of investment costs. The proposed process has 36% and 27% lower investment costs for the nitrogen and mixed refrigerant cycle compared to the conventional one. This leads to higher profit for the novel process and performs better in the face of uncertainty of liquified natural gas price. [Display omitted] • This study proposed a novel BOG handling process for minimizing capital costs. • This study offers BOG handling using subcooled LNG refrigerated by MR and N 2 cycle. • Utilizes ML for predicting BOG and heat ingress rates in varying weather conditions. • The study lowers capital costs by 36% and 27% compared to the traditional approach. [ABSTRACT FROM AUTHOR]

The effect of sloshing motion on the changes in the boil-off gas (BOG) and thermodynamic characteristics in a cryogenic liquid fuel tank were studied. Multiphase thermal flow simulations were performed by implementing a phase-change model with a pressure-dependent boiling-point model. Experiments were conducted on a vertically oriented C-type fuel tank with an insulation system using liquid nitrogen (LN 2) to validate the simulation results. Data of the temperature inside the tank and the BOG mass flow rate for different filling ratios (FRs) and periods of sloshing motion were collected. A forced roll motion with an absolute magnitude of 30° was applied for the sloshing motion. The BOG amount, as measured by the mass flow rate at the outlet, increased significantly compared to that in the static condition and tended to increase with the FR. When the sloshing period was the shortest among the experimental conditions (3 s), the BOG amount increased rapidly, and the tendency of BOG to change with FR differed from that of other longer-period conditions. The maximum relative error in the BOG amount predicted by the present simulation was approximately 7% compared with the experimental results under the same conditions, confirming the accuracy of the numerical method used. Simulations for shorter sloshing periods, which could not be performed because of the limitations of the experimental facility, showed that as the sloshing period approached the natural frequency of the tank, the sloshing resonance became stronger, and the BOG amount, temperature, and pressure inside the tank increased significantly. Visualization and analysis of the simulated physical quantities and flows were used to explain the causes of these phenomena. • Investigated effect of sloshing motion on boil-off gas in cryogenic liquid fuel tank. • Performed multiphase-thermal flow simulations and experiments in dynamic state. • Implemented phase-change model with pressure-dependent boiling point model. • Simulation results show good agreement with experimental results. • Sloshing effect is important when designing and optimizing cryogenic liquid tanks. [ABSTRACT FROM AUTHOR]

Transporting energy in liquefied forms results in reduction in volume, which enables energy to be transported economically over long overseas distances. In this study, liquefied natural gas, liquid ammonia and methanol are proposed to transport the energy of natural gas in different forms to overseas. Due to temperature difference between the energy storage medium and the ambient, a portion of liquefied energy carriers mass is lost as boil-off gas (BOG). Therefore, a technical assessment based on energy and exergy analyses is conducted in this work to assess the total required energy and losses due to BOG for each energy carrier. To make a fair comparison among the energy carriers, the ship volume capacity is the fixed factor. The results show that the total daily energetic BOGs for LNG, ammonia, and methanol are calculated as 0.610%, 0.098%, 0.034% while the exergetic BOGs are 0.491%, 0.068%, 0.032%, respectively. Ammonia and methanol generate significantly less daily BOG, respectively, compared to LNG during the full supply chain, which make them alternative for efficient energy carrier transport. Image 1 • Boil-off gas determination of potential energy carriers for overseas transportation. • Energy required for production of LNG: 6.73 MJ/kg and ammonia: 6.73 MJ/kg. • Daily sea shipping BOG rates are LNG: 0.12%, ammonia: 0.024%, methanol: 0.005%. • Comparison of LNG, ammonia, and methanol using energy losses within supply chain. [ABSTRACT FROM AUTHOR]

A model has been developed and implemented in the software package BoilFAST that allows for reliable calculations of the self-pressurization and boil-off losses for liquid hydrogen in different tank geometries and thermal insulation systems. The model accounts for the heat transfer from the vapor to the liquid phase, incorporates realistic heat transfer mechanisms, and uses reference equations of state to calculate thermodynamic properties. The model is validated by testing against a variety of scenarios using multiple sets of industrially relevant data for liquid hydrogen (LH2), including self-pressurization and densification data obtained from an LH2 storage tank at NASA’s Kennedy Space Centre. The model exhibits excellent agreement with experimental and industrial data across a range of simulated conditions, including zero boil-off in microgravity environments, self-pressurization of a stored mass of LH2, and boil-off from a previously pressurized tank as it is being relieved of vapor.

The production, transportation, and storage of liquefied natural gas (LNG) is a promising area in the gas industry due to a number of the fuel’s advantages, such as its high energy intensity indicators, its reduced storage volume compared to natural gas in the gas-air state, and it ecological efficiency. However, LNG storage systems feature a number of disadvantages, among which is the boil-off gas (BOG) recovery from an LNG tank by flaring it or discharging it to the atmosphere. Previous attempts to boil-off gas recovery using compressors, in turn, feature such disadvantages as large capital investments and operating costs, as well as low reliability rates. The authors of this article suggest a technical solution to this problem that consists in using a gas ejector for boil-off gas recovery. Natural gas from a high-pressure gas pipeline is proposed as a working fluid entraining the boil-off gas. The implementation of this method was carried out according to the developed algorithm. The proposed technical solution reduced capital costs (by approximately 170 times), metal consumption (by approximately 100 times), and power consumption (by approximately 55 kW), and improved the reliability of the system compared to a compressor unit. The sample calculation of a gas ejector for the boil-off gas recovery from an LNG tank with a capacity of 300 m3 shows that the ejector makes it possible to increase the boil-off gas pressure in the system by up to 1.13 MPa, which makes it possible to not use the first-stage compressor unit for the compression of excess vapours.

With the tightening of environmental regulations (i.e., IMO Tier III), natural gas (NG) has been spotlighted as an eco-friendly fuel with few air pollutants other than nitrogen oxides (NOx) and sulfur oxides (SOx). For reasons of economic efficiency, it is mainly stored and transported in a liquid state at -163 °C, which is a cryogenic temperature, using a liquefied gas storage tank. Accordingly, it is necessary to reduce the boil-off gas (BOG) occurrence due to the heat flow according to the temperature difference between the inside and outside of the storage tank. Therefore, in this study, a BOG measurement test on an independent-type storage tank made up of SUS304L was carried out. The test results showed the tendency for BOG occurrence according to the temperature under different filling ratios.

Liquefied natural gas has attracted attention through an explosive increase in demands and environmental requirements. During this period, the Energy Efficiency Design Index (EEDI), which was adopted by the International Maritime Organization, expecting to significantly reduce CO2 from ships, has become an important key. It has triggered a change in use from steam turbine systems and dual fuel diesel electrics to high-efficiency main engines such as ME-GI engines to meet the EEDI requirements. However, since the ME-GI engines use 300 bar of fuel gas pressure, it is necessary to resolve problems of the pressure controllability and to prevent the reductions of the re-liquefaction amount caused by clogging of the lubricant mixed with the fuel gas during the compression. The purpose of this study is to propose a novel control strategy with a newly developed configuration for controlling the pressure so as not to trip the BOG compressors when the ME-GI engines are tripped, and for preventing a reduction on re-liquefaction amount. Unlike the typical configuration used in the current vessels, this proposal separately provides the fuel gas at 150 bar without lubricants to the re-liquefaction. In addition, three control strategies are proposed, depending upon the application of multi-controllers and the location of the pressure transmitters. A simulation was conducted to verify the efficacy of the proposed method, focusing on the controllability of the pressure and the re-liquefaction amount, in comparison with the typical configuration. As results of the simulation, the proposal showed excellent controllability without trips of the BOG compressors even in abnormal conditions and confirmed the great re-liquefaction performance.

Liquefied natural gas (LNG) is attracting increasing attention as an alternative fuel in the maritime sector, as it can reduce harmful emissions for compliance with stricter environmental regulations. Owing to this environmental advantage, the number of ships using LNG as a fuel is increasing; thus, the demand for ship-to-ship LNG bunkering is increasing. One of the challenges of ship-to-ship LNG bunkering is boil-off gas (BOG) management, as it is more difficult than normal BOG management. This study analyzed the influences of the parameters on vapor generation, including the temperature difference between the bunker tank and receiving tank, bunkering flow rate, insulation performance, and compositions. A model based on a typical bunkering system was established, and a dynamic simulation was conducted using a commercial process simulator, Aspen HYSYS. The results indicated that as the initial temperature of the receiving tank increased, the amount of vapor return increased proportionally. In addition, increasing the bunkering flow rate decreased the amount of heat entering through the pipes and tanks; however, the heat dissipated by the pump shaft power increased. Different LNG compositions in the bunker tank led to changes in the initial pressure of the bunker tank, influencing the vapor return and vapor generation in the receiving tank. Through a parametric study, it was found that the pressure of the tank is the most important factor in terms of vapor return and vapor generation. As such, a pressure control method was proposed for the tank, so as to reduce vapor generation and vapor return. With pressure control, the total amount of vapor return to the bunker tanks is reduced from 7392 to 3317 kg. The net vapor generation in the receiving tank is reduced by up to 4047 kg and the net vapor generation in the overall system is reduced by 16.2%. [ABSTRACT FROM AUTHOR]

At present, LNG vessels without reliquefaction plants consume the BOG (boil-off gas) in their engines and the excess is burned in the gas combustion unit without recovering any of its energy content. Excess BOG energy could be captured to produce H2, a fuel with high energy density and zero emissions, through the installation of a reforming plant. Such H2 production would, in turn, require on-board storage for its subsequent consumption in the propulsion plant when navigating in areas with stringent anti-pollution regulations, thus reducing CO2 and SOX emissions. This paper presents a review of the different H2 storage systems and the methods of burning it in propulsion engines, to demonstrate the energetic viability thereof on board LNG vessels. Following the analysis, it is identified that a pressurised and cooled H2 storage system is the best suited to an LNG vessel due to its simplicity and the fact that it does not pose a safety hazard. There are a number of methods for consuming the H2 generated in the DF engines that comprise the propulsion plant, but the use of a mixture of 70% CH4-30% H2 is the most suitable as it does not require any modifications to the injection system. Installation of an on-board reforming plant and H2 storage system generates sufficient H2 to allow for almost 3 days' autonomy with a mixture of 70%CH4-30%H2. This reduces the engine consumption of CH4 by 11.38%, thus demonstrating that the system is not only energy-efficient, but lends greater versatility to the vessel. [ABSTRACT FROM AUTHOR]

Enlarging the fleet of gas carriers would make it possible to respond to the growing demand for hydrocarbon gases, but it will increase carbon dioxide emissions. The International Maritime Organization (IMO) has developed the energy efficiency design index (EEDI) with the objective of carbon emission reduction for new ships. In this paper, thirty gas carriers transporting liquefied natural gas (LNG) and liquefied petroleum gas (LPG) and equipped with various types of main engines are considered. As shown by the calculation of the attained EEDI, 2 of the 13 LPG carriers and 6 of the 17 LNG carriers under study do not comply with the EEDI requirements. To meet the stringent EEDI requirements, applying thermochemical regenerators (TCRs) fed by main engine exhaust gases is suggested. Mathematical modeling is applied to analyze the characteristics of the combined gas-turbine-electric and diesel-electric power plant with thermochemical recuperation of the exhaust gas heat. Utilizing TCR on gas carriers with engines fueled by syngas produced from boil-off gas (BOG) reduces the carbon content by 35% and provides the energy efficiency required by IMO without the use of other technologies. [ABSTRACT FROM AUTHOR]

A new CFD model relevant to non-isobaric cryogen evaporation has been developed. It considers the heat influx, from the surroundings to the liquid and vapour phases, and the heat transfer between the phases. The phase change is modelled considering two contributions: an interfacial energy balance far from the tank wall, and by direct wall-to-liquid conduction near the wall. The model provides well resolved liquid and vapour temperature and velocity profiles, as well as pressure build-up and evaporation rates. It shows that below the vapour-liquid interface, liquid natural convection is dampened by thermal stratification, which is driven by the increase of the interfacial temperature due to increase in pressure. The pressure build-up is mainly governed by the heating of the vapour, particularly at the beginning of the evaporation. The model was used to analyse liquid velocity profiles for two different sets of experimental data for evaporation of liquid nitrogen by optimizing empirical parameters, namely the wall heat partitioning fraction and the overall heat transfer coefficients. The results show that a large fraction of the vapour heat ingress is not transferred to the vapour phase bulk. Instead, it is transported through the walls and transferred to the liquid at the liquid-vapour-wall contact point, driving phase change. Heat conduction from the walls to the interface has a significant effect, even for low heat fluxes, and cannot be neglected. • A CFD model for the non-isobaric evaporation of stored cryogens is developed. • Interfacial evaporation, bulk phase change and wall evaporation are considered. • Pressure, vapour and liquid temperatures and liquid velocity profiles are provided. • Most of the heat ingress from the surroundings is transferred directly to the liquid. • Thermal stratification dampens natural convection below the vapour-liquid interface. [ABSTRACT FROM AUTHOR]

As stricter national standards for volatile organic compounds (VOCs) emissions are implemented, the emission of VOCs is a growing concern. There is an urgent necessary to optimize the existing condensate recovery system structure to meet national emission standards for boil-off gas. In existing VOCs recovery methods, the drawbacks include the frequent regeneration of adsorbent materials, limitations in the condensation recovery of isomers, and the lack of self-sufficiency in power generation. Therefore, a novel boil-off gas condensate recovery system by liquid nitrogen (LN) was proposed in this study. Based on the first and second laws of thermodynamics, a numerical optimization model of the system was established and the system performance of thermodynamic, economic and environmental was analyzed. The results show that the system achieves condensation and recovery of boil-off gas. The LN pump has an optimal outlet pressure. When the pressure is 1.95 MPa, the system achieves maximum net power output, and when the pressure is 0.85 MPa, the system achieves maximum LN savings. Additionally, the higher the ambient temperature, the greater the net power output. At lower ambient temperatures, the system supplies a greater cooling capacity. The system demonstrates good economic and environmental performance under higher ambient temperatures and outlet pressure of LN pump. When the pressure is 1.95 MPa, the system achieves the maximum net present value, standard coal savings and carbon dioxide emission reduction. The system has a maximum payback period of only 0.35 years. The system could be used in engineering applications to improve the overall performance of the liquefied natural gas plant. • A novel boil-off gas condensate recovery system by liquid nitrogen is proposed. • The electric power generated by nitrogen is a byproduct for boil-off gas recovery. • The system greatly decreases the consumption of liquid nitrogen. • The system displays a satisfactory economic and environmental performances. [ABSTRACT FROM AUTHOR]

Port towns are strategic places from the point of view of transport systems. They form integration junctions for various transport branches , apart from the traditional - road and railway ones , also for water( sea) transport which is active there. Moreover, air transport comes also into consideration , whose efficient functioning must be connected with good accessibility, that concerns sea transport as well. Efficient and safe servicing the ports is crucial for their functioning. Problems associated with the overloading of lorries, which leads to degradation of road surface structure , observed in Gdynia, are discussed as an example in this paper. Problems of road traffic safety (RTS) are presented in this paper on the example of Gdańsk. The two issues: the road traffic safety and road surface degradation constitute only some transport problems of port towns , but they are very important, from the point of view of their specificity, for integration junctions of all the transport branches for people and goods. However, in discussing selected aspects of transport in port towns it is necessary to refer to the managing of integrated transport system with taking into account its traffic safety aspects.

Recently, a new type of LNG tank named “KC-1 membrane LNG tank” has been developed by Korean Gas Corporation (KOGAS), and Samsung Heavy Industries (SHI) is currently building KC-1 membrane type LNG carriers. Unlike other LNG tanks, the KC-1 membrane LNG tank has a single-insulation structure rather than a double-insulation structure. For a given tank’s boundary condition, heat transfer analysis is performed from the external to the internal environment of the LNG tank by numerical simulation for three tanks. In each tank, the main thermally resistant layer of insulation is assembled with a High density rigid Polyurethane Foam (H-PUF), which is blown with one of three different types of hydrofluorocarbons-namely-HFC-365mfc, 245fa, and 245fa-e (enhanced). Advantage of such blowing agents is that it has a lower Ozone Depletion Potential (ODP) than HCFC-141b or carbon dioxide (CO2) that has been used in the past as well as having low thermal conductivity. A Reduced Order Model is utilized to a 3-dimensional section of the insulation to calculate equivalent thermal conductivity. The equivalent thermal conductivity of the insulation is then applied to the rest of LNG tank, reducing the size of tank simulation domain as well as computation time. Tank’s two external and internal boundary conditions used are those defined by the International Code for the Construction and Equipment of Ships Carrying Liquefied Gases in Bulk (IGC) and the United States Coast Guard (USCG) conditions. Boil-off Rate (BOR) of the tank that has the insulation with H-PUF blown with HFC-245fa resulted in 0.0927 %/day and 0.0745 %/day for IGC and USCG conditions, respectively.