Solubility of hydrogen in ice Ih at pressures up to 8 MPa

Experimental studies of the solubility of hydrogen in ice Ih (usual low-pressure ice) at temperature −1 to −2 °C and pressures up to 8 MPa were carried out. At a pressure equal to 1.90 and 8.04 MPa, hydrogen solubility in the ice was found to be 0.15 and 1.32 cm³/g, respectively (hydrogen volume was reduced to the normal conditions).     

Simulation of hydrogen leak and explosion for the safety design of hydrogen fueling station in Korea

Hydrogen has been used as chemicals and fuels in industries for last decades. Recently, it has become attractive as one of promising green energy candidates in the era of facing with two critical energy issues such as accelerating deterioration of global environment (e.g. carbon dioxide emissions) as well as concerns on the depletion of limited fossil sources. A number of hydrogen fueling stations are under construction to fuel hydrogen-driven vehicles. It would be indispensable to ensure the safety of hydrogen station equipment and operating procedure in order to prevent any leak and explosions of hydrogen: safe design of facilities at hydrogen fueling stations e.g. pressurized hydrogen leak from storage tanks. Several researches have centered on the behaviors of hydrogen ejecting out of a set of holes of pressurized storage tanks or pipes. This work focuses on the 3D simulation of hydrogen leak scenario cases at a hydrogen fueling station, given conditions of a set of pressures, 100, 200, 300, 400 bar and a set of hydrogen ejecting hole sizes, 0.5, 0.7, 1.0 mm, using a commercial computational fluid dynamics (CFD) tool, FLACS. The simulation is based on real 3D geometrical configuration of a hydrogen fueling station that is being commercially operated in Korea. The simulation results are validated with hydrogen jet experimental data to examine the diffusion behavior of leak hydrogen jet stream. Finally, a set of marginal safe configurations of fueling facility system are presented, together with an analysis of distribution characteristics of blast pressure, directionality of explosion. This work can contribute to marginal hydrogen safety design for hydrogen fueling stations and a foundation on establishing a safety distance standard required to protect from hydrogen explosion in Korea being in the absence of such an official requirement.     

Numerical simulation of hydrogen leakage diffusion in seaport hydrogen refueling station

Ningbo's seaport hydrogen refueling station was used as the research object. The effects of different leakage angles, wind direction, roof shape, leakage hole diameters, temperature, and humidity on the diffusion of hydrogen leakage were studied by numerical simulation. The influence of leakage angle on hydrogen leakage is mainly reflected in the presence or absence of obstacles. The volume of the flammable hydrogen cloud was reduced by 31.16%, and the volume of the hazardous hydrogen cloud was reduced by 63.22% when there was no obstacle. The wind direction can significantly impact hydrogen leakage, with downwind and sidewind accelerating hydrogen discharge and reducing the risk. At the same time, headwind significantly increases the volume of the flammable hydrogen cloud. Compared with no wind, the volume of the flammable hydrogen cloud increased by 71.73% when headwind, but the volume of the hazardous hydrogen cloud decreased by 24.00%. If hydrogen shows signs of accumulation under the roof, the sloping roof can effectively reduce the hydrogen concentration under the roof and accelerate the hydrogen discharge. When the leakage angle θ = 90°, the sloping roof reduced the volume of the flammable hydrogen cloud by 11.74%. The leakage process was similar for different leak hole diameters in the no wind condition. The inverse of the molar fraction of hydrogen on the jet centerline was linearly related to the dimensionless axial distance of the jet in different cases. Using a least squares fit, the decay rate was obtained as 0.0039. In contrast, temperature and humidity have almost no effect on hydrogen diffusion. Hydrogen tends to accumulate on the lower surface of the roof, near the roof pillars and the hydrogen dispenser. In this paper, a set of hydrogen detector layout schemes was developed, and the alarm success rate was verified to be 83.33%.     

Estimation of temperature change in practical hydrogen pressure tanks being filled at high pressures of 35 and 70 MPa

Some complete experimental data sets, not only on the hydrogen temperature within the tank during filling, but also on the supplied temperature and pressure from the station have been opened for analysis of the temperature change with time. The data were independently obtained for 6 different conditions and have been analyzed and checked to validate the Monde et al. model. It is found that the measured temperatures are well predicted using the software based on the model and the heat loss during filling with hydrogen is also well predicted, if a suitable heat transfer coefficient is adopted.     

Effect of hydrogen ratio on turbulent flame structure of oxyfuel syngas at high pressure up to 1.0 MPa

The CO/H₂/CO₂/O₂, CO/H₂/CO₂/air turbulent premixed flames as the model of syngas oxyfuel and syngas/air combustion were studied experimentally and compared to that of CH₄/air mixtures at high pressures up to 1.0 MPa. Hydrogen ratio in syngas was set to be 35%, 50% and 65% in volumetric fraction. Four perforated plates are used to generate wide range of turbulence intensity and scales. The instantaneous flame structure was measured with OH-PLIF technique and then statistic flame structure parameters and turbulent burning velocity were derived to interpret the multi scale turbulence-flame interaction. Results show that the flame structure of syngas is wrinkled and convex cusps to the unburned mixtures are sharper and deeper comparing to that of CH₄ flames. Pressure has a dominating effect on flame wrinkling other than mixtures composition at high pressure of 1.0 MPa. The flame surface density, Σ of syngas is larger than that of CH₄. The Σ of syngas flames is almost independent on pressure and hydrogen ratio especially when hydrogen ratio is over 50% which is a significant feature of syngas combustion. Larger flame surface density for syngas flames mainly comes from the finer structure with smaller wrinkles which is the result of more intensive flame intrinsic instability. The S_T/S_L of syngas is larger than CH₄ and it slightly increases with the pressure rise. The S_T/S_L of syngas oxyfuel is similar to that of syngas/air flames in the present study. The S_T/S_L increases with the increase of hydrogen ratio and keeps almost constant when hydrogen ratio is over 50%.     

Turbulent flame topology and the wrinkled structure characteristics of high pressure syngas flames up to 1.0 MPa

The turbulent flame topology characteristics of the model syngas with two different hydrogen ratios were statistically investigated, namely CO/H₂ ratio at 65/35 and 80/20, at equivalence ratio of 0.7. The combustion pressure was kept at 0.5 MPa and 1.0 MPa, to simulate the engine-like condition. The model syngas was diluted with CO₂ with a mole fraction of 0.3 which mimics the flue gas recycle in the turbulent combustion. CH₄/air flame with equivalence ratio of 1.0 was also tested for comparison. The flame was anchored on a premixed type Bunsen burner, which can generate a controllable turbulent flow. Flame front, which is represented by the sharp increased interface of the OH radical distribution, was measured with OH-PLIF technique. Flame front parameters were obtained through image processing to interpret the flame topology characteristics. Results showed that the turbulent flames possess a wrinkled character with smaller scale concave/convex structure superimposed on a larger scale convex structure under high pressure. The wrinkled structure of syngas flame is much finer and more corrugated than hydrocarbon fuel flames. The main reason is that scale of wrinkled structure is smaller for syngas flame, resulting from the unstable physics. Hydrogen in syngas can increase the intensity of the finer structure. Moreover, the model syngas flames have larger flame surface density than CH₄/air flame, and hydrogen ratio in syngas can increase flame surface density. This would be mainly attributed to the fact that the syngas flames have smaller flame intrinsic instability scale l_i than CH₄/air flame. S_T/S_L of the model syngas tested in this study is higher than CH₄/air flames for both pressures, due to the high diffusivity and fast burning property of H₂. This is mainly due to smaller L_M and l_i. V_f of the two model syngas is much smaller than CH₄/air flames, which suggests that syngas flame would lead to a larger possibility to occur combustion oscillation.     

Study on vapour dispersion and explosion from compressed hydrogen spill: Risk assessment on a hydrogen plant

Hydrogen produced from renewable resources is one of the cleanest fuels and could be used to store intermittent solar, wind and other energies. The main concern about using hydrogen is its hazards, such as high storage pressure, wide-range flammability, low mass density, and high diffusion. This study investigated the hazards of compressed hydrogen storage by developing a CFD model to understand the gas dispersion behaviour. The model was validated using the past experimental data and showed a good agreement, which could demonstrate the diffusion characteristics and gas stratification of a buoyant gas. A case study of an accidental release of compressed hydrogen from a storage tank was investigated to evaluate the risk of a hydrogen plant. A mathematical model of the jet spill was used to account for the choking effect from a high-pressure release to ensure the input velocity in CFD simulation is suitable for modelling gas dispersion using verified spatial and temporal scales, then the simulation results were used as inputs of vapour cloud explosions (VCEs) to investigate the potential overpressure effect. It was found the CFD model could predict a more reasonable flammable gas amount in cloud than using the bulk hydrogen release rate. The safety distance based on the overpressure prediction was reduced by 35%. The method proposed in this study can provide more validity for the consequence analysis as part of risk assessment.     

Experimental study on small-scale hydrogen liquefaction of 0.5 L/h

A small-scale hydrogen liquefaction device based on two-stage G-M refrigerator was designed and manufactured. Many practical operation processes on the liquefaction device were conducted in the open-air test base. The experimental results shown that 1) The direct liquefaction scheme of micro-positive pressure with normal temperature hydrogen realized by two-stage pressure reducing valve was feasible and effective; 2) Design of four-stage heat exchanger for G-M refrigerator cold head was reasonable and reliable; 3) The liquefaction rate in pure hydrogen was 0.47 L/h, and liquefaction pressure can maintain the range of about 120 kPa～160 kPa; 4) After venting hydrogen-helium mixture, the liquefaction rate of hydrogen was 0.439 L/h again. In other words, the previously filled helium in the liquid hydrogen Dewar could be discharged through multiple venting method. The residue helium had little effect on the hydrogen liquefaction rate; 5) The scheme of simultaneous liquefaction and transmission was proved to be feasible; 6) Operation process experience and safety precautions on the hydrogen liquefaction were summarized. The testing results provided a technical support for design and operation of small-scale hydrogen liquefactions.     

Development of a Sievert apparatus for characterization of high pressure hydrogen sorption materials

A volumetric gas absorption (Sievert) apparatus has been developed to measure hydrogen absorption and desorption at pressures up to 700 bar and temperatures between 240 K and 320 K. The apparatus is designed to reduce uncertainty for high pressure measurements while maintaining proper temperature control in the sample. Pressure-composition isotherms (PCI) and kinetics measurements of a well-studied material, LaNi₅ have been obtained for validation of the apparatus. Measurements of both absorption and desorption PCI curves as well as full absorption kinetics data have been obtained for TiCrMn to examine the performance at high pressures, as well as to examine the thermodynamic hysteresis effect in TiCrMn for applications in metal hydride system design. Due to this hysteresis, the thermodynamics of the absorption reaction differ significantly from those of the desorption reaction, which must be accounted for when considering thermal design of a metal hydride reactor and the suitability of the metal hydride for energy storage applications.     

Investigation of hydrogen production using wood pellets gasification with steam at high temperature over 800 °C to 1435 °C

A fixed-bed gasifier was developed to study the effects of steam flow rate and temperature on the hydrogen production during biomass gasification at high temperature over 800 °C to 1435 °C. An optimum steam flow rate for peak of hydrogen yield was found. As temperature increases, amount of hydrogen increases first, subsequently decreases and then increases again with a maximum peak of hydrogen yield at 917 °C. In the temperatures of 1018 °C through 1435 °C post the peak hydrogen production increases with temperature. The maximum volume fraction of hydrogen and hydrogen production ratio are 60% and 76%, respectively. Chemical equilibrium calculation was also done using ASPEN software, which demonstrates that the more the steam flow rate, the lower the temperature for maximum hydrogen yield; the higher the temperature, the lower the effect of steam flow rate. The results are expected to develop high temperature gasification technology.     

Para-H2 to ortho-H2 conversion in a full-scale automotive cryogenic pressurized hydrogen storage up to 345 bar

Hydrogen vehicles offer the potential to improve energy independence and lower emissions but suffer from reduced driving range. Cryogenic pressure vessel storage (also known as cryo-compressed storage) offers the advantage of higher densities than room temperature compressed although it has the disadvantage of cryogenic operating temperatures which results in boil-off when the temperature of the gas increases. In order to understand and optimize the time prior to boil-off, we have examined heat absorption from the transition between the two quantum states of the hydrogen molecule (para–ortho) in a full-scale (151 L internal volume) automotive cryogenic pressure vessel at pressures and temperatures up to 345 bar and 300 K, and densities between 14 and 67 g/L (2.1–10.1 kg H₂). The relative concentration of the two species was measured using rotational Raman scattering and verified by calorimetry. In fifteen experiments spanning a full year, we repeatedly filled the vessel with saturated LH₂ at near ambient pressure (2–3 bar), very low temperatures (20.3–25 K), varying densities, and very high para-H₂ fraction (99.7%). We subsequently monitored vessel pressure and temperature while performing periodic ortho-H₂ concentration measurements with rotational Raman scattering as the vessel warmed up and pressurized due to environmental heat entry. Experiments show that para–ortho H₂ conversion typically becomes active after 10–15 days of dormancy (“initiation” stage), when H₂ temperature reaches 70–80 K. Para–ortho H₂ conversion then approaches completion (equilibrium) in 25–30 days, when the vessel reaches 100–120 K at ∼50 g/L density. Warmer temperatures are necessary for conversion at lower densities, but the number of days remains unchanged. Vessel dormancy (time that the vessel can absorb heat from the environment before having to vent fuel to avoid exceeding vessel rating) increased between 3 and 7 days depending on hydrogen density, therefore indicating a potentially large benefit for reduced fuel venting in cryogenic pressurized hydrogen storage.     

Comparative study of turbulence models performance for refueling of compressed hydrogen tanks

Compressed hydrogen gas is a popular mode of fuel storage for hydrogen powered vehicles. When hydrogen gas is filled at high pressure, the gas temperature increases. The maximum gas temperature should be within acceptable safety standards. Numerical studies can help optimize the filling process. There is a high level of turbulence in the flow as the high velocity inlet jet is penetrating the nearly stagnant gas in the tank. Selection of a suitable turbulence model is important for accurate simulation of flow and heat transfer during filling of hydrogen tanks. In the present work, a comparative study is performed to identify suitable turbulence model for compressed hydrogen tank filling problem. Numerical results obtained with different turbulence models are compared with available experimental data. Considering accuracy, convergence and the computational expenses, it is observed that the realizable k-ε model is the most suitable turbulence model for hydrogen tank filling problem.     

A sieverts apparatus for measuring high-pressure hydrogen isotherms on porous materials

A high-pressure Sieverts apparatus, specifically designed to investigate carbon and other low-density materials for hydrogen storage, has been constructed and used to investigate potential storage materials for which the volume of the sample is uncertain or difficult to define. The apparatus can be managed from a computer via a graphical interface and used to measure gas sorption isotherms from 77 K to 873 K, utilising a computer driven piston pressure booster to compress the gas to 340 bar. Based on measurements of activated carbons and graphene-like samples, this article demonstrates the low sensitivity of the apparatus to uncertainty in the sample volume and the quality of data obtained for H₂ adsorption isotherms measured at 296 K and 77 K at pressures up to 300 bar.     

Effects of hydrogen addition on the combustion characteristics of diesel fuel jets under ultra-high injection pressures

Many applications use hydrogen addition and high-pressure fuel injection technology to improve combustion performance. In this study, spray atomization and combustion characteristics of a diesel fuel jet, under the injection pressure of 350 MPa, injecting into a constant volume combustion vessel filled with air-hydrogen mixture at the diesel engine relevant condition are investigated by simulation method. A simplified mechanism of the n-heptane (C₇H₁₆) oxidation chemistry mechanism consisting of 26 reactions and 25 species integrated with the Kéromnès-2013 hydrogen combustion mechanism and EDC combustion model are utilized to predict the diesel fuel spray auto-ignition and combustion. The ambient gas is the mixture of air and hydrogen range in volume fraction from 0% to 10%. The ambient temperature and pressure is set to 1000 K and 3.5 MPa, respectively. The results indicate that as the hydrogen volume fraction is 2%, the minimum overall droplet SMD (Sauter Mean Diameter) is approximately 0.95 μm, which is obviously smaller than that of the case with the conventional high injection pressure. In cases that H₂ v/v% larger than 4%, the maximum gaseous temperature increased significantly up to 2700 K. There are two peaks in the temperature growth rate curves as the hydrogen fraction of 8% and 10%. The high temperature at the outer edge of the spray is clearly seen due to its high value when the hydrogen fraction is larger than 4%. The hot reaction layer is the main location of CO formation. The H, OH radicals are formed at the edge of the spray where the temperature is high. The hydrogen species obviously promotes the oxidation and combustion of diesel fuel.     

Numerical investigations on the fast filling of hydrogen tanks

High-pressure storage of hydrogen in tanks is a promising option to provide the necessary fuel for transportation purposes. The fill process of a high-pressure tank should be reasonably short but must be designed to avoid too high temperatures in the tank. The shorter the fill should be the higher the maximum temperature in the tank climbs. For safety reasons an upper temperature limit is included in the requirements for refillable hydrogen tanks (ISO 15869) which sets the limit for any fill optimization. It is crucial to understand the phenomena during a tank fill to stay within the safety margins.The paper describes the fast filling process of hydrogen tanks by simulations based on the Computational Fluid Dynamics (CFD) code CFX. The major result of the simulations is the local temperature distribution in the tank depending on the materials of liner and outer thermal insulation. Different material combinations (type III and IV) are investigated.Some measurements from literature are available and are used to validate the approach followed in CFX to simulate the fast filling of tanks. Validation has to be continued in the future to further improve the predictability of the calculations for arbitrary geometries and material combinations.     

Comparison of two-layer model for hydrogen and helium jets with notional nozzle model predictions and experimental data for pressures up to 35 MPa

A two-layer, reduced order model of high pressure hydrogen jets was developed which includes partitioning of the flow between the central core jet region leading to the Mach disk and the supersonic slip region around the core. The flow after the Mach disk is subsonic while the flow around the Mach disk is supersonic with a significant amount of entrained air. This flow structure significantly affects the hydrogen concentration profiles downstream. The predictions of this model are compared to previous experimental data for high pressure hydrogen jets up to 20 MPa and to notional nozzle models and CFD models for pressures up to 35 MPa using ideal gas properties. The results show that this reduced order model gives better predictions of the mole fraction distributions than previous models for highly underexpanded jets. The predicted locations of the 4% lower flammability limit also show that the two-layer model much more accurately predicts the measured locations than the notional nozzle models. The comparisons also show that the CFD model always underpredicts the measured mole fraction concentrations.     

Fast filling strategy of type III on-board hydrogen tank based on time-delayed method

In order to study the fast filling problem of the type III on-board hydrogen tank, a 3D computational fluid dynamics (CFD) simulation model is proposed. Several simulation calculations are completed to simulate the fast filling process under different initial conditions. In order to control the temperature rise during the fast filling process, the effects of different mass flow rates are studied. Based on the control of mass flow rate, various time-delayed filling strategies for different conditions are proposed to meet the requirement of shortening the filling time as much as possible without exceeding the maximum temperature limit. It is found that if the delay duration is determined, how the filling time is allocated has little effect on the final temperature rise. The proposed strategy can complete the filling within 155s in a general environment, which saves 62% of the time compared with the filling with constant mass flow rate. This research provides the theoretical basis and technical support for mass flow control strategies of fast filling in the hydrogen refueling stations and has guiding significance for the actual filling process of large-capacity hydrogen tanks.     

Performance assessment of 700-bar compressed hydrogen storage for light duty fuel cell vehicles

Type 4 700-bar compressed hydrogen storage tanks were modeled using ABAQUS. The finite element model was first calibrated against data for 35-L subscale test tanks to obtain the composite translation efficiency, and then applied to full sized tanks. Two variations of the baseline T700/epoxy composite were considered in which the epoxy was replaced with a low cost vinyl ester resin and low cost resin with an alternate sizing. The results showed that the reduction in composite weight was attributed primarily to the lower density of the resin and higher fiber volume fraction in the composite due to increased squeeze-out with the lower viscosity vinyl ester resin. The system gravimetric and volumetric capacities for the onboard storage system that holds 5.6 kg H₂ are 4.2 wt% (1.40 kWh/kg) and 24.4 g-H₂/L (0.81 kWh/L), respectively. The system capacities increase and carbon fiber requirement decreases if the in-tank amount of unrecoverable hydrogen is reduced by lowering the tank “empty” pressure. Models of an alternate tank design showed potential 4–7% saving in composite usage for tanks with a length-to-diameter (L/D) ratio of 2.8–3.0 but no saving for L/D of 1.7. A boss with smaller opening and longer flange does not appear to reduce the amount of helical windings.     

Theoretical study and performance evaluation of hydrogen production by 200 W solid oxide electrolyzer stack

High temperature steam electrolyzers, taking advantage of high temperature heat, can produce more hydrogen by using less electrical energy than low temperature electrolyzers. This paper presents an experimental study on hydrogen production by using a 200 W solid oxide stack working in reverse mode. A thermodynamic study of the process was performed by measuring the heat and mass balance of stack at different operating conditions. Different definitions of efficiency were used to highlight the limit and potential of the process. The I–V curve, the flow rate measurements and the GC analysis on outlet flows were used to calculate the hydrogen and oxygen productions. In addition, the influence of steam dilution, water utilization and operating temperature on conversion efficiency and stack's thermal balance was evaluated. With this aim, the tests were performed at three operating temperature (700 °C, 750 °C and 800 °C) over a range of steam inlet concentration from 50% to 90% and water utilization up to 70%. The hydrogen and oxygen flows produced by electrolysis, at different loads, were directly measured after water condensation: net flows up to 2.4 ml/(min cm²) of hydrogen and 1.2 ml/(min cm²) of oxygen were measured and compared to the theoretical ones, showing a good agreement.     

Study on leakage and explosion consequence for hydrogen blended natural gas in urban distribution networks

It appears to be the most economical means of transporting large quantities of hydrogen over great distances by the existing natural gas pipeline network. However, the leakage and diffusion behavior of urban hydrogen blended natural gas and the evolution law of explosion characteristics are still unclear. In this work, a Computational Fluid Dynamics three-dimensional simulation model of semi-confined space in urban streets is developed to study the diffusion process and explosion characteristics of hydrogen-blended natural gas. The influence mechanism of hydrogen blending ratio and ambient wind speed on the consequences of explosion accident is analyzed. And the dangerous area with different environmental wind effects is determined through comparative analysis based on the most dangerous scenarios. Results indicate that the traffic flow changes the diffusion path of the jet, the flammable gas cloud forms a complex profile in many obstacles, high congestion level lead to more serious explosion accidents. Wind effect keeps the flammable gas cloud near the vehicle flow, the narrow gaps between the vehicles aggravate the expansion of the flammable gas cloud. When the wind direction is consistent with the leakage direction, hydrogen blended natural gas is gathered in the recirculation zone due to the vortex effect, which results in more serious accident consequences. With the increase in hydrogen blending ratio, the higher content of H and OH in the gas mixture significantly increases the premixed burning rate, the maximum overpressure rises rapidly when the hydrogen blend level increases beyond 40%. The results can provide a basis for construction safety design, risk assessment of leakage and explosion hazards, and emergency response in hydrogen blended natural gas distribution systems.