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The objective of this study is to propose an optimal plant design for blue hydrogen production aboard a liquefied natural gas (LNG) carrier. This investigation focuses on integrating two distinct processes—steam methane reforming (SMR) and ship-based carbon capture (SBCC). The first refers to the common practice used to obtain hydrogen from methane (often derived from natural gas), where steam reacts with methane to produce hydrogen and carbon dioxide (CO2). The second refers to capturing the CO2 generated during the SMR process on board ships. By capturing and storing the carbon emissions, the process significantly reduces its environmental impact, making the hydrogen production "blue," as opposed to "grey" (which involves CO2 emissions without capture). For the SMR process, the analysis reveals that increasing the reformer temperature enhances both the process performance and CO2 emissions. Conversely, a higher steam-to-carbon (s/c) ratio reduces hydrogen yield, thereby decreasing thermal efficiency. The study also shows that preheating the air and boil-off gas (BOG) before they enter the combustion chamber boosts overall efficiency and curtails CO2 emissions. In the SBCC process, pure monoethanolamine (MEA) is employed to capture the CO2 generated by the exhaust gases from the SMR process. The results indicate that with a 90% CO2 capture rate, the associated heat consumption amounts to 4.6 MJ per kilogram of CO2 captured. This combined approach offers a viable pathway to produce blue hydrogen on LNG carriers while significantly reducing the carbon footprint. [ABSTRACT FROM AUTHOR]

During LNG storage and transportation by ship, a fraction of the LNG in the cryogenic tanks evaporates due to heat ingress through the insulation, resulting in boil-off gas (BOG) production and a change in LNG composition, a phenomenon known as LNG ageing. Common practice is to assume that BOG composition and related density are identical to the initial LNG or pure methane, resulting in inaccuracy in BOG mass flow measurements. This is particularly important regarding LNG shipping economics and the utilization of BOG as a fuel for ship propulsion. This work investigates the influence of LNG ageing on the produced BOG thermodynamic properties relevant to the mentioned inaccuracies' estimation and correction. An established, simplified, dynamic boil-off model is utilized for the simulation of LNG and BOG properties' changes during the voyage. Four cases represented by limiting the minimum and maximum values of methane and nitrogen content are used to estimate the general influence of the compositional variability over the whole range of practically possible LNG source mixtures. Research results provide an insight into the relevant BOG properties' variability and confirm that BOG flow measurements should be corrected with dynamic model simulations results due to significant differences between the total BOG mixture and forced BOG mixture corresponding to the LNG composition. [ABSTRACT FROM AUTHOR]

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During LNG storage and transportation by ship, a fraction of the LNG in the cryogenic tanks evaporates due to heat ingress through the insulation, resulting in boil-off gas (BOG) production and a change in LNG composition, a phenomenon known as LNG ageing. Common practice is to assume that BOG composition and related density are identical to the initial LNG or pure methane, resulting in inaccuracy in BOG mass flow measurements. This is particularly important regarding LNG shipping economics and the utilization of BOG as a fuel for ship propulsion. This work investigates the influence of LNG ageing on the produced BOG thermodynamic properties relevant to the mentioned inaccuracies’ estimation and correction. An established, simplified, dynamic boil-off model is utilized for the simulation of LNG and BOG properties’ changes during the voyage. Four cases represented by limiting the minimum and maximum values of methane and nitrogen content are used to estimate the general influence of the compositional variability over the whole range of practically possible LNG source mixtures. Research results provide an insight into the relevant BOG properties’ variability and confirm that BOG flow measurements should be corrected with dynamic model simulations results due to significant differences between the total BOG mixture and forced BOG mixture corresponding to the LNG composition.

The use of liquefied natural gas (LNG) in the transportation sector is increasing, and boil-off gas (BOG) management systems are considered viable options to increase economic efficiency and reduce greenhouse gas emissions at LNG refueling stations. The present study proposed an economically optimized method by investigating four refueling station scenarios, including different BOG management systems. Among the four scenarios, the scenario in which compressed natural gas was produced from BOG had the lowest minimum selling price (MSP) and was the most economical. Sensitivity and uncertainty analyses were conducted for the economically optimal scenario, which identified the factors with the most influential impact and their uncertainties on the MSP. Finally, to determine the feasibility of the business through profitability analysis, the net present value, discounted payback period, and present value ratio due to changes in the discount rate were presented, and the discounted cash flow rate of return was found to be 13.22%. As a result of this study, a BOG management system can contribute to improving the economic feasibility for LNG refueling stations by reliquefying BOG and re-selling it (the most efficient way is scenario 4) and will provide an economical guide for countries with much demand for LNG in the transport sector. [ABSTRACT FROM AUTHOR]

As the boil-off gas (BOG) rate is tested to evaluate the thermal insulation performance of cryogenic containers at a 90 % liquid level in standard. This study aims to predict the BOG rate of cryogenic containers at different liquid levels based on a single test. If the BOGs at a 90 % liquid level can be evaluated based on the BOG test at a low liquid level, the large amount of working medium can be saved to reduce the test cost and the equilibration time can be shortened to improve the test efficiency. In this study, the QHALW (heat absorbed by the liquid from its corresponding outer wall) and QHTVL (heat transferred from the corresponding wall of the vapor to the liquid) are analysed to establish the energy equation of the liquid in the BOG test, indicating latent heat of BOG in cryogenic containers includes the QHALW and QHALW. Therefore, the equation between the latent heat of BOG and the two parts of heat isbuilt to predict BOGs at different liquid levels with the tested BOG. The maximum error of the experimental results is not more than 7.0 %, which shows that the BOG is successfully predicted at different liquid levels with a single-tested BOG. [ABSTRACT FROM AUTHOR]

This paper proposes the optimal shape of an LNG fuel tank with a lattice pressure vessel (LPV) design for a tugboat. The LPV is a Type C tank with a design philosophy of “design by analysis,” which facilitates greater variability of shape compared with other traditional Type C tanks. Further, compared with conventional cylindrical fuel tanks, the LPV provides better volumetric efficiency. Considering the shape of a fuel tank room, a trapezoidal shape of the LPV is concluded as the most optimal design. This study performs two major analyses of the LPV: structural and heat transfer analyses. First, a design procedure of the LPV based on structural analyses is elaborated. The finite element method is used for the analyses. Furthermore, the results guarantee that the maximum stresses by applied loads do not exceed an allowable stress limitation. Second, the heat transfer analysis of the LPV is conducted. LNG boil-off gas generation is analyzed based on various insulation materials and the degree of acuum.

Increasing renewable energy is posing a challenge to the stability of electricity grid, and the importance of an energy storage system (ESS) is becoming clearer. A Liquid Air Energy Storage System (LAES) is proposed as one of the promising ESS technologies. LAES converts electricity to liquid air efficiently and economically. However, since the storage temperature of liquid air is −196 °C, loss of liquid air is inevitable due to evaporation, i.e., boil-off gas (BOG). A loss of liquid air has negative effect on the round-trip efficiency of LAES; therefore, the BOG rate must be minimized. In this research, the insulation performance of liquid air tank is evaluated. To calculate the heat ingress and BOG rate, a partial equilibrium model is developed. The insulation performance is evaluated with varying insulation thickness, emissivity, aspect ratio and materials. The results show that the BOG rate of vacuum insulation can reach 1.8 %/day. For foam glass insulation, the BOG rate can become 2.25 %/day. With optimal geometry, 0.7 %/day for vacuum insulation and 1 %/day for foam glass insulation can be achieved. The vacuum insulation generally shows better insulation performance, however, there is area where the foam glass insulation shows better performance depending on the insulation thickness and emissivity. • A partial-equilibrium thermo-fluid model for liquid air tank is developed. • The model predicts the heat ingress and boil-off gas rate of liquid air tank. • The effects of insulation thickness, emissivity, and aspect ratio are investigated. • Vacuum insulation generally shows better insulation performance than foam glass. • The change of oxygen concentration should be carefully monitored. [ABSTRACT FROM AUTHOR]

• BOG cold exergy and PTORC are employed to recover waste heat of a PEM fuel cell. • The PEM fuel cell is simulated mathematically and results are validated. • Thermodynamic and exergoeconomic analyses are performed for the system. • A parametric study is considered to illustrate effects of important parameters. • Proposed system generates 1353 kW power and overall total cost rate is 154.66 $/h. A heat recovery system is employed for power generation from a PEM fuel cell's waste heat. The proposed system involves the parallel organic Rankine cycle for utilizing the provided heat and hydrogen boil-off gas (BOG) stream as a heat sink for the cycle. Also, this stream generates power by using an expander and prepares some chilled water that enhances system performance. The PEM fuel cell as the major component of the system is modeled and then the parallel two-stage ORC (PTORC) and BOG streams are analyzed in points of the energy and exergy view. Exergoeconomic concept is applied to evaluate the economic view of the system besides energy and exergy analyses. Also, a sensitivity analysis is performed by system key parameters which provided some valuable information to comprehend the performance of the overall system. The results show that the proposed system can generate 1353 kW power. Exergoeconomic factor, energy efficiency and exergy efficiency of the overall system are computed to be 26.21%, 58.15%, and 36.64%, respectively. Also, it is concluded that the higher current density and lower operating temperature, will raise the total cost rate. [ABSTRACT FROM AUTHOR]

The increase demand in liquefied natural gas (LNG) leads to an increase in the import and export of LNG cargo using LNG carriers with composite insulation and double-hull structures for membrane-type storage tanks. Therefore, the transient characteristics of cryogenic heat transfer in LNG cargo containment systems have become important for understanding the physics when LNG vaporizes into the gas phase to minimize the cargo loss of boil-off gas (BOG). However, public papers on fundamental research on long-term computational fluid dynamics (CFD) simulations are still lacking. In the present study, the transient flow behaviors of LNG fluids and the heat transfer characteristics were numerically studied by applying the simplified single-layer insulation in an LNG cargo containment system, based on an initial temperature of T LNG,0 = 110.5 K and saturation temperature of T LNG,sat = 111.5 K in the boundary condition of IGC code. The proposed analysis procedure can serve as a reference for relative comparison of predicted BOG difference in LNG cargo holds. Consequently, the heat transfer mode between the two-phase methane and the solid insulation could be typically classified into two different regimes: (1) a subcooled regime before reaching the saturation temperature from the initial temperature, and (2) a boiling regime after reaching the saturation temperature. In the subcooled regime, the subcooled liquid exhibited bulk flow motion induced by natural convection. The loss rate of LNG mass per time interval was extracted from the best-fit correlations as −0.0448 kg/s at 1 d (105 s) to cool down the insulation system and −0.0072 kg/s at 5 d (5 × 105 s) to reach the LNG saturation temperature. When the predicted value of the loss rate of LNG mass by the CFD simulation was regarded as the generation rate of boil-off gas, it was smaller than the designed value of the boil-off gas of 0.464 kg/s corresponded to boil-off rate (BOR) about 0.2 %/day. In the boiling regime and under sufficiently cooled conditions, the loss rate of LNG mass per time interval did not consistently exceed the BOR value. The liquid flow exhibited a turbulent-like motion despite the relatively low velocity at 10 d (106 s). Especially, the increase in the effective thermal conductivity was a good indicator for distinguishing between the boiling and subcooled regimes. • A novel boil-off gas analysis procedure using long-term simulation • A full-scale membrane-type LNG storage tank was applied. • Differences in boil-off gas and heat flux characteristics between 1-D and 3-D results. • Boil-off gas generation characteristics distinguished by two heat transfer modes. • Thermofluid behaviors inside the LNG cargo were investigated. [ABSTRACT FROM AUTHOR]

Because environmentally-friendly fuels such as natural gas and hydrogen are primarily stored in the form of cryogenic liquids to enable efficient transportation, the demand for cryogenic fuel (LNG, LH) ships has been increasing as the primary carriers of environmentally-friendly fuels. In such ships, insulation systems must be used to prevent heat inflow to the tank to suppress the generation of boil-off gas (BOG). The presence of BOG can lead to an increased internal pressure, and thus, its control and prediction are key aspects in the design of fuel tanks. In this regard, although the thermal analysis of the phase change through a finite element analysis requires less computational time than that implemented through computational fluid dynamics, the former is relatively more error-prone. Therefore, in this study, a cryogenic fuel tank to be incorporated in ships was established, and the boil-off rate (BOR), measured considering liquid nitrogen, was compared with that obtained using the finite element method. Insulation material with a cubic structure was applied to the cylindrical tank to increase the insulation performance and space efficiency. To predict the BOR through finite element analysis, the effective thermal conductivity was calculated through an empirical correlation and applied to the designed fuel tank. The calculation was predicted to within 1% of the minimum error, and the internal fluid behavior was evaluated by analyzing the vertical temperature profile according to the filling ratio.

This paper proposes an LNG fuel gas supply system in which the compressors are used for multiple purposes. Referred to as the compressor-assisted fuel gas supply system (CA-FGSS), the proposed system uses its compressors to both manage the boil-off gas (BOG) in the LNG fuel tank and generate a pressure differential for drawing LNG from the tank. The feasibility of the defined operation sequence of the CA-FGSS was verified by dynamic process simulations in which the system was applied to two-stroke dual-fuel engines that required medium and high fuel gas injection pressures (16 and 300 bar), respectively. An availability analysis revealed 99% system availability of the CA-FGSS, which has a lower number of redundant equipment compared to a conventional pump-based fuel gas supply system (PB-FGSS). This is because the CA-FGSS does not use components with high failure rates and long maintenance times, such as in-tank pumps in the LNG fuel tank. These findings demonstrate the economic feasibility of the CA-FGSS, which also has a 10% lower capital expenditure compared to a PB-FGSS. A sensitivity analysis of the entire propulsion system of the CA-FGSS further showed that it was operationally more economical than a PB-FGSS for the same gas supply pressure. Moreover, a high-pressure propulsion system that utilizes the CA-FGSS was found to be more economical than a medium-pressure propulsion system for a high LNG price. [ABSTRACT FROM AUTHOR]

Liquefied natural gas (LNG) is recognized as a preferable alternative fuel for ship owners, since it can substantially reduce harmful emissions to comply with stricter environmental regulations. The increasing number of LNG-fueled vessels has driven up the number of LNG bunkering vessels (LNGBVs) as well. A key issue of LNGBVs is boil-off gas (BOG) generation, especially the huge amount of BOG that is generated during loading and unloading (bunkering) processes. This study proposes a hybrid system that combines conventional onboard LNG-fueled generators with an energy storage system (ESS) to solve the BOG issue of LNGBVs. This hybrid system is targeted at an LNGBV with the cargo capacity of 5000 m3. The amount of BOG generation is calculated based on assumed operation modes, and the economic study and the environmental analysis are performed based on the results. By comparing the conventional system to the proposed ones, some benefits can be verified: about 46.2% BOG reduction, 66.0% fuel saving, a 7.6-year payback period, and 4.8 tons of greenhouse gas (GHG) reduction for one voyage in the best case, with some assumptions. This proposed hybrid system using the ESS could be an attractive green solution to LNGBV owners.

This paper covers the experimental and numerical investigation of the changes in boil-off gas and thermodynamic characteristics due to filling ratio (FR) in a cryogenic liquid fuel tank. An experiment on a vertically oriented C -type fuel tank with an insulation system was carried out using LN2 to allow validation of the simulation results. The data for temperature inside the tank and mass flow rate of boil-off gas were acquired with respect to the different FRs. It was observed that the amount of BOG tends to increase in the form of a quadratic function based on the trend line, as FR increases. The simulation seemed to well express the complex physical phenomenon associated with heat transfer, heat flow, and vaporization occurring in the entire system of the tank. Especially the flow visualization of the simulated physical quantities helped to understand physical phenomena more easily, such as a mechanism of BOG caused by vaporization, which are not easy to approach in experiments. On the other hand, the BOR according to the FR showed a very slight difference of less than 3% on average compared to the experiment, which made it possible to confirm the validity of the multi-physics simulation technique. • Experiments are conducted according to filling ratio in a cryogenic liquid fuel tank. • BOG and temperature are measured to analyze thermodynamic characteristics in the tank. • Multiphase-thermal flow simulations are used to validate with experimental data. • The influence of phase change model is shown in cryogenic fuel tank with vaporization. • The structure of thermal flow is understood in cryogenic fluid through visualization. [ABSTRACT FROM AUTHOR]

Abstract: The development of new LNG offshore facilities addresses new technical challenges that need to be deeply investigated to overcome the new constraints raised from the offshore operability. One of the key issues concerns the management of LNG storages. As future offshore LNG projects could be implemented in areas with harsh environmental conditions, the motions of the floating structure combined to the design and operating parameters should widely affect the behavior of the LNG inside partially filled tanks. Therefore, the knowledge available from onshore LNG storages has to be completed taking into account the offshore environment. The first concern deals with the production of Boil-Off Gas (BOG) during LNG’s unloading and storage operations under offshore conditions, where specific offshore parameters would contribute in increasing the production of BOG. For that purpose, a new optimized LNG storage process based on an increased operating pressure (P = 500 mbarg) in the TRITON FSRU’s tanks has been investigated during LNG’s unloading and storage operations with LNG emission. It mainly showed that by operating at 500 mbarg, the production of BOG is nearly 4 times reduced during unloading operations compared to LNG carrier standard operating pressure (100 mbarg) and nearly suppressed during the storage phase. The second concern deals with managing the rollover risk under offshore conditions. In this context, GDF SUEZ performed a pioneer qualitative assessment to highlight the impact of the liquid motion on the rollover mechanisms in the TRITON FSRU’s tanks. The liquid motion completely modifies the mixing mechanisms in LNG stratified layers. The mixing is no longer governed by the molecular diffusion induced by the thermal effects but by the dynamic flow effects induced by the fluid wave movement into the layers. Moreover, this assessment showed that the time needed for a stratification to evolve up to a complete mix is severely reduced compared to the onshore LNG storage while the assessment of the induced BOG peak depends as usual on the energy stored in the stratification when being mixed and could be of the same order of magnitude as for onshore rollover. [Copyright &y& Elsevier]

• A high-fidelity model was developed to describe BOG generation during LNG loading. • A computationally efficient model was derived for efficient use of the high-fidelity model. • An MPC system was designed to regulate the pressure and temperature of an LNG tank. • In the MPC, optimal vapor outlet flow and amount of LNG spray were manipulated. • The designed MPC was able to achieve a safe operation of the cool down process. During liquefied natural gas (LNG) loading, excessive boil-off gas (BOG) is generated due to the temperature difference between the LNG and a tank at room temperature. While one way to deal with this challenge is to gradually reduce the tank temperature via an LNG, this cool-down process may also produce excessive BOG, if not properly handled, leading to a sudden pressure change inside the tank. To this end, a model predictive control (MPC) system was formulated in this work to simultaneously regulate the pressure and temperature of the tank by manipulating the vapor outlet flow rate and the amount of LNG spray injected during the cool-down process. Specifically, a high-fidelity model of cool-down process was developed and simplified to a reduced-order model (ROM). Then, the ROM was incorporated into the developed MPC scheme which successfully computed an input profile to drive the pressure and tank temperature to target values. [ABSTRACT FROM AUTHOR]

Because environmentally-friendly fuels such as natural gas and hydrogen are primarily stored in the form of cryogenic liquids to enable efficient transportation, the demand for cryogenic fuel (LNG, LH) ships has been increasing as the primary carriers of environmentally-friendly fuels. In such ships, insulation systems must be used to prevent heat inflow to the tank to suppress the generation of boil-off gas (BOG). The presence of BOG can lead to an increased internal pressure, and thus, its control and prediction are key aspects in the design of fuel tanks. In this regard, although the thermal analysis of the phase change through a finite element analysis requires less computational time than that implemented through computational fluid dynamics, the former is relatively more error-prone. Therefore, in this study, a cryogenic fuel tank to be incorporated in ships was established, and the boil-off rate (BOR), measured considering liquid nitrogen, was compared with that obtained using the finite element method. Insulation material with a cubic structure was applied to the cylindrical tank to increase the insulation performance and space efficiency. To predict the BOR through finite element analysis, the effective thermal conductivity was calculated through an empirical correlation and applied to the designed fuel tank. The calculation was predicted to within 1% of the minimum error, and the internal fluid behavior was evaluated by analyzing the vertical temperature profile according to the filling ratio. [ABSTRACT FROM AUTHOR]

Liquefied natural gas (LNG) is recognized as a preferable alternative fuel for ship owners, since it can substantially reduce harmful emissions to comply with stricter environmental regulations. The increasing number of LNG-fueled vessels has driven up the number of LNG bunkering vessels (LNGBVs) as well. A key issue of LNGBVs is boil-off gas (BOG) generation, especially the huge amount of BOG that is generated during loading and unloading (bunkering) processes. This study proposes a hybrid system that combines conventional onboard LNG-fueled generators with an energy storage system (ESS) to solve the BOG issue of LNGBVs. This hybrid system is targeted at an LNGBV with the cargo capacity of 5000 m3. The amount of BOG generation is calculated based on assumed operation modes, and the economic study and the environmental analysis are performed based on the results. By comparing the conventional system to the proposed ones, some benefits can be verified: about 46.2% BOG reduction, 66.0% fuel saving, a 7.6-year payback period, and 4.8 tons of greenhouse gas (GHG) reduction for one voyage in the best case, with some assumptions. This proposed hybrid system using the ESS could be an attractive green solution to LNGBV owners. [ABSTRACT FROM AUTHOR]