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.     

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%.     

Effect of wind condition on unintended hydrogen release in a hydrogen refueling station

Ambient condition, especially the wind condition, is an important factor to determine the behavior of hydrogen diffusion during hydrogen release. However, only few studies aim at the quantitative study of the hydrogen diffusion in a wind-exist condition. And very little researches aiming at the variable wind condition have been done. In this paper, the hydrogen diffusion in different wind condition which including the constant wind velocity and the variable wind velocity is investigated numerically. When considering the variable wind velocity, the UDF (user defined function) is compiled. Characteristics of the FGC (flammable gas cloud) and the HMF (hydrogen mass fraction) are analyzed in different wind condition and comparisons are made with the no-wind condition. Results indicate that the constant wind velocity and the variable wind velocity have totally different effect for the determination of hydrogen diffusion. Comparisons between the constant wind velocity and the variable wind velocity indicate that the variable wind velocity may cause a more dangerous situation since there has a larger FGC volume. More importantly, the wind condition has a non-negligible effect when considering the HMF along the radial direction. As the wind velocity increases, the distribution of the HMF along the radial direction is not Gaussian anymore when the distance between the release hole and the observation line exceeds to a critical value. This work can be a supplement of the research on the hydrogen release and diffusion and a valuable reference for the researchers.     

Modeling the diffusion of flammable hydrogen cloud under different liquid hydrogen leakage conditions in a hydrogen refueling station

Constructing hydrogen refueling stations will be popular for hydrogen energy use in the future, and investigating the diffusion characteristics of hydrogen in a leakage incident is quite significant. The instantaneous evolution of flammable hydrogen clouds arising from liquid hydrogen leakage in a hydrogen refueling station is predicted using Ansys Fluent, and parametric analyses are conducted to reveal the effects of storage pressure, source height, and leakage direction on the distributions of the flammable regions. In addition, the feasibilities of heating the ceiling or the ground of the station after the leakage of liquid hydrogen to accelerate the hydrogen dilution are examined. The results show that the flammable region is stabilized at 90 s, the corresponding flammable hydrogen cloud volume is about 333 m³, and the extensions of downwind and vertical directions reach 10 m and 9.3 m. Storage pressure has a finite effect on the downwind diffusion distance of the flammable cloud. A lower source height tends to format the high-concentration hydrogen cloud near the ground while a higher source height helps separate the flammable clouds from the ground. The upward leakage direction leads to the maximum downwind diffusion distance of about 10.2 m while the downward leakage direction makes the high hydrogen concentration region confined below the ceiling. Just maintaining the ceiling at the initial temperature of 300 K is effective for accelerating the hydrogen dilution in the upward leakage. The maximum hydrogen concentration and the flammable volume can be reduced at rates of 0.35 vol % and 8% for every 50 K increase in heating temperature. For the downward leakage, keeping the ground at the initial temperature just works for the first 40 s in reducing the maximum hydrogen concentration, while increasing the heating temperature receives a gradually declined effect on reducing the flammable volume.     

Dispersion modeling and assessment of natural gas containing hydrogen released from a damaged gas transmission pipeline

This paper performs a simulation and assessment of dispersion of natural gas containing hydrogen released from transmission pipeline using a Computational Fluid Dynamics (CFD) approach. A 3D CFD model is established to evaluate the dispersion behavior of hydrogen-enriched natural gas in the hydrogen-natural gas mixing station. The simulations include a matrix of scenarios for hydrogen doping ratios, gas release rates, wind speeds and wind directions. The development process of flammable gas cloud is predicted, and the dangerous area generated in the hydrogen-natural gas mixing station is assessed. Additionally, the effects of some critical factors on flammable gas dispersion behavior are analyzed. The simulations produce some useful outcomes including the parameters of flammable gas cloud and the dangerous area in the station, which are useful for conducting a prior risk assessment and contingency planning.     

Safety analysis of leakage in a nuclear hydrogen production system

Due to its unique advantages, such as clean and pollution-free, hydrogen energy has gradually improved its energy transition position. Constructing nuclear hydrogen production systems is a necessary means to achieve large-scale hydrogen production, and the study of hydrogen leakage and diffusion behavior is critical to commercializing hydrogen production systems. In engineering practice, the distance between the hydrogen storage device and the nuclear power plant is an important indicator to measure the safety of nuclear hydrogen production. To study the influence of gas storage tank's own conditions and external environmental conditions on leakage diffusion, influencing factors such as wind speed, leakage direction, leakage diameter, leakage height, and leakage angle are discussed in the present study. By calculating severe working conditions combined with the above multiple factors, the longest distance of hydrogen diffusion is determined. Finally, peak overpressure impact generated by hydrogen explosion was evaluated, and the minimum separation distance required to avoid safety risks was predicted. The results demonstrate that when the wind direction is consistent with the leakage direction, and the leakage angle is 0°, the higher the wind speed, the larger the leakage diameter and the lower the leakage height, resulting in a longer diffusion distance. Under more extreme and severe working conditions, the diffusion distance of combustible hydrogen cloud can reach as far as 237 m. Once hydrogen diffusion explodes, the minimum separation distance required is about 338 m. This research provides an effective method for safety risk assessment of a nuclear hydrogen production system.     

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.     

Modeling the development of hydrogen vapor cloud considering the presence of air humidity

A three-dimensional CFD model for large-scale liquid hydrogen spills is developed and validated by the experiments carried out by NASA. The effect of humidity on the development of hydrogen vapor cloud is emphasized, with the modified expressions of Lee model accounting for the phase changes of water and hydrogen. The results show that the numerical prediction is more consistent with the experiment considering the presence of air humidity. The condensation of water in the atmosphere increases the buoyancy of the vapor cloud, and promotes the diffusion of the cloud in vertical direction. The dimension of the cloud in streamwise direction changes little under different humidity, due to the balance between the height-dependent wind speed and the induced buoyancy. The scope of visible cloud indicated by the condensed water vapor expands with the increasing air humidity, and still lies within the flammable domain when the relative humidity approaching to 75%. Water vapor condensation induces the cloud temperature rise under the same concentration, and the leeward part is more influenced compared with the upwind part.     

Simulation and risk assessment of hydrogen leakage in hydrogen production container

In this paper, the hydrogen leakage and diffusion characteristics analysis and risk assessment are carried out on the container where a 2 Nm³/h alkaline hydrogen production device is located. Firstly, the transient and steady process of hydrogen leakage from hydrogen production container is analyzed. Secondly, the dynamic balance of combustible hydrogen cloud is analyzed, the concept of critical ventilation flow is put forward. It was found that in order to reduce the flammable volume by 85%, the installed ventilation can only cover the leakage flow of 1.0 Nm³/min. Finally, TNT equivalent method is used to evaluate the hazard degree of hydrogen leakage. It is found that the existed hydrogen production container ventilation device can only exhaust the hydrogen-air cloud with small flow leakage, while the accumulation of gas cloud still exists in large flow leakage. Under the critical ventilation flow, the minor injury radius can be reduced from 4.8 m to 2.78 m. The effect of critical ventilation flow was verified.     

Numerical investigation of flammable cloud on liquid hydrogen spill under various weather conditions

The investigations of hydrogen leakage during the latest decades, have developed our knowledge level. However, few studies concerned the whole dispersion history of the flammable cloud (from generating to disappearing in the atmosphere). A Non-Homogeneous Equilibrium Model (NHEM) was used, and validated by a large scale LH₂ spill experiment. The predicted data displayed good agreement with the experiment. Moreover, the experiment was further investigated on the colourless flammable cloud. Three primary questions of the hydrogen dispersion process were concerned: the maximum spreading range, the minimum distance above the ground, and the duration time of the flammable cloud in the atmosphere. Three major influence factors were selected to simulate various weather conditions, including ambient temperature (coupled with ground temperature), wind speed and atmospheric pressure. The hydrogen dispersion can be excited with the increased wind speed, and be impeded with the increased atmospheric pressure. The hydrogen dispersion process in four seasons of a year appears a different trend.     

Numerical simulation of leakage and diffusion distribution of natural gas and hydrogen mixtures in a closed container

The transportation and utilization of hydrogen blended natural gas have received extensive attention. The dangerous characteristics of hydrogen such as high diffusivity and wide flammability/explosion limit also increase the leakage risk of hydrogen blended natural gas. In this paper, a numerical model is established for the leakage and diffusion of hydrogen blended natural gas in a closed container. The evolution of the distribution, diffusion law and flammable area of different proportions of hydrogen blended natural gas after leaking into a closed container is investigated. The results show that the flammable area with low hydrogen ratios (20% and below) will disappear within 2.7 s–11.1 s after the leakage, which is relatively safer, while the high hydrogen ratio (80% and above) reaches 3875 s–4555 s with a significant increase in risk duration. After the 50% hydrogen ratio leakage, the thickness of the flammable area is higher than 15.67% for the 80% hydrogen ratio and 30.25% higher than pure hydrogen at 120 s after leakage, and the risk is higher in a short time. Due to the difference in the diffusion rates between methane and hydrogen, hydrogen diffuses to the middle and lower part of the enclosed container faster, and the risk in the middle and lower part also deserves attention.     

Investigation of the hazardous area in a liquid hydrogen release with or without fence

Constructing a fence for potential liquid hydrogen leakage is a common protective measure to prevent the unrestricted spread, and critically evaluating the effects of a fence is essential for the safe use of hydrogen energy. Based on NASA's large-scale experiment of liquid hydrogen release, a 3D numerical model considering the existence of fence is proposed to study the dynamic spread behaviors of hydrogen flammable cloud and liquid hydrogen pool. The results show that the liquid hydrogen pool keeps a downwind radius of about 1.75 m most of the time during the release, and the hydrogen flammable cloud has a limit spread distance. Three cases are set to comparatively evaluate the effect of the fence, which are full fence, downwind half fence and no fence. Under the three cases, liquid hydrogen pools all evaporate completely within 5 s. Full fence scenario has the modest spread scale of hydrogen flammable cloud in the atmosphere but the longest duration for 86 s, and the upwind fence causes an easily overlooked hazardous region due to the vortex here, thus constructing a full fence is not an ideal strategy when the spread direction is constant. Just constructing a downwind half fence leads to the greatest spread distance of the flammable hydrogen cloud, which are 33.6 m and 68 m in vertical and downwind directions. No fence case corresponds to the smallest diffusion scale of flammable hydrogen cloud in atmosphere but the longest near-ground spread distance for 43.35 m. Four stages can be divided to summarize the entire life period of the flammable cloud for the cases with and without a fence, while three stages for the case with a downwind half fence.     

Dilution of flammable vapor cloud formed by liquid hydrogen spill

The flammable vapor cloud formed by liquid hydrogen spill can severely threaten the safety of life and property, which is one of the primary concerns during handling liquid hydrogen. Mixture model and Realizable k-ε model are adopted in ANSYS Fluent to predict the two-phase flow of liquid hydrogen spill. Dilution of the hydrogen vapor cloud formed by liquid hydrogen spill is analyzed, including the turbulent disturbance during cloud dispersion, and the cloud dilution characteristics especially its behavior under different wind speeds. The results show that the turbulent disturbance in the upper part of flammable vapor cloud can be 7.21 times of that in the background atmosphere, due to the mixing of the vapor cloud with the incoming wind and the upper air. The dilution time, needed for the vapor cloud to be diluted below the lower flammable limit since the end of the spill, increases firstly and then turns to decrease with the increasing wind speed. The atmospheric turbulence, the turbulent disturbance induced by the upward movement of vapor cloud, and the cloud frontal area all affect the dilution behavior.     

Modeling and analysis of hydrogen diffusion in an enclosed fuel cell vehicle with obstacles

Hydrogen has been utilized in FCV and leakage can cause safety issues. In this study, the vehicle is simplified as a cuboid enclosure with obstacles. The influence of obstacle locations on the hydrogen diffusion behavior is investigated with the iso-surfaces of 1% and 4% volume fraction of hydrogen. The time of the iso-surface of 4% volume fraction to reach the ceiling and the sidewalls without any obstacles is 1.42 and 1.25 times of that with an obstacle, respectively. Both height and the width of the flammable zone in the enclosure with an obstacle are greater than that without obstacles. The distance between the obstacle and the leakage affects significantly the hydrogen diffusion and the influence of the obstacle on the hydrogen diffusion strengthens with the decrease of the distance. Yet as the distance keeps same, the obstacle position relative to the leakage has no significant effect on hydrogen diffusion, and it makes little effect difference whether the obstacle is located in front of leakage or side of leakage.     

Explosion-prevention strategies of airflow controlling and closed-inerting for hydrogen dilution in utility tunnel

Under the large-scale application of hydrogen energy and the acceleration of utility tunnel construction in China, the hydrogen pipeline would enter the utility tunnel soon. The explosion-prevention strategy is of important to hydrogen transportation in utility tunnel. Thus, a series of comparative studies were carried out in the present study, to explore the high efficiency dilution methods for unexpected hydrogen leak scenarios. (1) Effects of source type and domain extension were further validated based on our previous numerical model. The small-hole model was found to be simple and feasible for pipe release simulation, and has the same high accuracy like the notional nozzle model. The influence of unexpanded domain is acceptable. (2) Investigation of the explosion-prevention strategies was performed based on the finding of the worst leakage case, and hydrogen leaks at the middle of the upwind tunnel with upward leak direction was found to be the worst leakage case for the normal utility tunnel. (3) Compared with the layouts of lateral vents and the single horizontal air distributor, the middle exhaust vent was found to be the optimal airflow controlling strategy. (4) Furtherly, compared to the optimal airflow controlling strategy, closed-inerting was validated to be the most efficient dilution method. In the closed-inerting cases, the peak value of the flammable gas volume which is only ∼40% of the unimproved case appears. After that, the flammable cloud volume decays gradually, while the value increases steadily in the initial unimproved case. No obvious difference was found between the N₂ and CO₂ injection cases.     

A CFD simulation of hydrogen dispersion for the hydrogen leakage from a fuel cell vehicle in an underground parking garage

In the present study, the dispersion process of hydrogen leaking from an FCV (Fuel Cell Vehicle) in an underground parking garage is analyzed with numerical simulations in order to assess hazards and associated risks of a leakage accident. The temporal and spatial evolution of the hydrogen concentration as well as the flammable region in the parking garage was predicted numerically. The volume of the flammable region shows a non-linear growth in time with a latency period. The effects of the leakage flow rate and an additional ventilation fan were investigated to evaluate the ventilation performance to relieve accumulation of the hydrogen gas. It is found that expansion of the flammable region is delayed by the fan via enhanced mixing near the boundary of the flammable region. The present numerical results can be useful to analyze safety issues in automotive applications of hydrogen.     

Numerical study of hydrogen dispersion in a fuel cell vehicle under the effect of ambient wind

In the rescue of hydrogen-fueled vehicle accidents, once accidental leakage occurs and hydrogen enters the cabin, the relatively closed environment of the vehicle is prone to hydrogen accumulation. Excessive hydrogen concentration inside the vehicle cabin may cause suffocation death of injured passengers and rescue crews, or explosion risk. Based on hydrogen fuel cell vehicle (HFCV) with hydrogen storage pressure 70 MPa, four different scenarios (i. with opened sunroof, ii. opened door windows, iii. opened sunroof and door windows and iv. opened sunroof, door windows and rear windshield) under the condition of accidental leakage were simulated using computational fluid dynamics (CFD) tools. The hydrogen concentration inside the vehicle and the distribution of flammable area (>4% hydrogen mole fraction) were analyzed, considering the effect of ambient wind. The results show that in the case of convection between interior and exterior of the vehicle via the sunroof, door windows or rear windshield, the distribution of hydrogen inside the vehicle is strongly affected by the ambient wind speed. In the least risk case, ambient wind can reduce the hydrogen mole fraction in the front of the vehicle to less than 4%, however the rear of the vehicle is always within flammable risk.     

The simulation and analysis of leakage and explosion at a renewable hydrogen refuelling station

The number of hydrogen refuelling stations (HRSs) is steadily growing worldwide. In China, the first renewable hydrogen refuelling station has been built in Dalian for nearly 3 years. FLACS software based on computational fluid dynamics approach is used in this paper for simulation and analysis on the leakage and explosion of hydrogen storage system in this renewable hydrogen refuelling station. The effects of wind speed, leakage direction and wind direction on the consequences of the accident are analyzed. The harmful area, lethal area, the farthest harmful distance and the longest lethal distance in explosion accident of different accident scenarios are calculated. Harmful areas after explosion of different equipments in hydrogen storage system are compared. The results show that leakage accident of the 90 MPa hydrogen storage tank cause the greatest harm in hydrogen explosion. The farthest harmful distance caused by explosion is 35.7 m and the farthest lethal distance is 18.8 m in case of the same direction of wind and leakage. Moreover, it is recommended that the hydrogen tube trailer should not be parked in the hydrogen refuelling station when the amount of hydrogen is sufficient.     

Surface effects on flammable extent of hydrogen and methane jets

Studies on the effect of surfaces on the extent of the flammable cloud of high-pressure horizontal and vertical jets of hydrogen and methane are performed using CFD numerical simulations. For the horizontal jets, two scenarios pertaining to the location of the surface are studied: horizontal surface (the ground), and vertical surface (side wall). For a constant flow rate release, the extent of the flammable cloud is determined as a function of time. Effects of the proximity of the surface on the flammable extent along the axis of the jet and its impact on the maximum extent of the flammable cloud is explored and compared for both hydrogen and methane. The results are also compared to the predictions of the Birch correlations for flammable extents. It is found that the presence of a surface and its proximity to the jet centerline result in a pronounced increase in the extent of the flammable cloud compared to a free jet.     

Leakage and diffusion behavior of a buried pipeline of hydrogen-blended natural gas

Buried pipelines are one method of conservation transfer for widely used gases such as natural gas and hydrogen. The safety of these pipelines is of great importance because of the potential leakage risks posed by the flammable gas and the special properties of the hydrogen mixture. Estimating the leakage behavior and quantifying the diffusion range outside the pipeline are important but challenging goals due to the hydrogen mixture and presence of soil. This study provides essential information about the diffusion behavior and concentration distribution of underground hydrogen and natural gas mixture leakages. Therefore, a large-scale experimental system was developed to simulate high-pressure leaks of hydrogen mixture natural gas from small holes in three different directions from a pipeline buried in soil. The diffusion of hydrogen-doped natural gas in soil was experimentally measured under different conditions, such as different hydrogen mixture ratios, release pressures, and leakage directions. The experimental results verified the applicability of the gas leakage mass flow model, with an error of 6.85%. When a larger proportion of a single component was present in the hydrogen-doped natural gas, the leakage pressure showed a greater diffusion range. In addition, the diffusion range of hydrogen-doped natural gas in the leakage direction was larger at 3 o'clock than that at 12 o'clock. The hydrogen blend carried methane and diffused, which shortened the methane saturation time. Moreover, a quantitative relationship between the concentration of hydrogen-doped natural gas and the diffusion distance over which the hydrogen-doped natural gas reached the lower limit of the explosion was obtained by quantitative analysis of the experimental data.