Consequence modeling of a compressed hydrogen tube trailer explosion – theoretical study

. In the present context, with the increasing focus on decarbonization and the transition toward sustainable energy systems, due to the imminent climate crisis, hydrogen has emerged as one of the key players in the quest for clean and renewable energy solutions. If this alternative fuel will be widely distributed, it can be used as an energy source for industry, households, and transportation, thereby promoting a carbon-free society. However, hydrogen's safe handling and transportation present significant challenges due to its unique properties and potential hazards. Given the high level of interest in the future of hydrogen applications, assumed by European Commission in the European Hydrogen Strategy and transposed in national strategies and action plans for the member states, special safety measures must be taken to avoid accidents. The study aims to provide an overview of the physical and chemical properties of compressed hydrogen and the specific characteristics of tube trailers used for its transportation and to examine the potential scenarios that may lead to a tube trailer explosion, such as ignition sources, leakages, or accidents during transit. Consequence modeling techniques are employed to evaluate the potential hazards and impacts of accidental hydrogen release from a tube trailer. This includes dispersion modeling to assess the extent of the hydrogen cloud and its potential for ignition, which can lead to jet fire or rising fireballs. Process Hazard Analysis Software Tool (PHAST) was utilized to evaluate the hazardous extent of the occurrence area and to estimate the effects of the theoretical explosion scenarios presented.


Introduction
Increasing global energy demand and the urgent need to address the alarming consequences of greenhouse gas emissions, have necessitated a paradigm shift toward cleaner and more sustainable energy sources.As traditional fossil fuels, utilized for decades due to their energy density and ease of availability, have become increasingly incompatible with future environmental goals, the search for innovative alternatives that can provide reliable and low-carbon energy has become critical in this setting.Among many sustainable energy choices available, hydrogen has emerged as a particularly promising candidate.As an energy carrier, hydrogen has numerous significant advantages, including high energy density, abundance, and the potential for zero-emission operations.Furthermore, hydrogen's capacity to be generated from a variety of renewable resources, such as solar, wind, and biomass, makes it a crucial component of a sustainable energy mix.If it becomes a globally adopted clean energy source, hydrogen may have significant applications in industry, households, and transport, supporting a carbon-free society, often referred to as the hydrogen economy [1].
Utilizing renewable hydrogen supports the goals of the European Green Pact, including achieving climate neutrality by 2050 and reducing greenhouse gas emissions by at least 55 percent by 2030 relative to 1990 levels, as well as economic growth, job creation, and postpandemic recovery.The European Commission has already made substantial investments in clean technologies and value chains for sustainable growth, identifying hydrogen as one of the essential elements of the energy transition [2].
However, hydrogen poses certain hazards and risks, especially regarding its flammability and potential for explosions.It is a highly flammable gas that can ignite easily in the presence of an ignition source.It has a wide flammability range in the air (between 4 to 75.6% by volume) and can easily form explosive atmospheres when mixed with air.Hydrogen flames have a high propagation rate due to their low ignition energy, making them challenging to control or extinguish.Hydrogen fires are hard to detect, due to their flames which burn with a pale color, increasing the risk of accidental exposure or delayed response.Hydrogen molecules are small and can easily escape through tiny gaps or leaks, rapidly dispersing and accumulating in poorly ventilated areas, increasing the risk of a flammable or explosive atmosphere.Proper safety measures must be implemented when handling, storing, and transporting hydrogen, to mitigate these hazards.
The recently launched European Hydrogen Strategy [2,3] recognizes the critical role that hydrogen transportation will play in achieving renewable hydrogen penetration in Europe.Several member states have released national hydrogen strategies, with some of them considering hydrogen imports.Access to significant volumes of renewable hydrogen at low cost will be critical in the next years, in order to accomplish the European Strategy.The final cost of hydrogen resources is dictated by the costs related to its production and delivery.The cost of producing renewable hydrogen depends on the nature and capacity factor of the available renewable energy sources.The cost of delivery depends on the quantity of hydrogen transported, the transport distance, the chosen transport mode, and the state in which hydrogen is transported (the 'packaging' mode).The term 'packing' refers to the process of compressing or liquifying hydrogen, as well as transforming it into a chemical carrier.The opposite term, 'unpacking', refers to the process of reversing the previous step to obtain hydrogen that is pure, gaseous, at a defined pressure and purity level at the usage site.Speaking of hydrogen, the main packaging modes are compressed hydrogen, liquefied hydrogen, and chemical hydrogen carriers (ammonia -NH3 and liquid organic hydrogen carriers -LOHC).
The main options for transporting hydrogen to a site are ships and -in the case of compressed hydrogen-pipelines.Trains and trucks are also considered in the delivery scenarios involving a distributed network of hydrogen demand locations (e.g., automotive hydrogen fueling stations).In terms of transporting hydrogen by road, trucks are suitable for compressed hydrogen, over short distances, in ranges of approx.200 km, after which this means of transport becomes inefficient in terms of costs and carbon footprint.Rail transport is considered efficient for transporting hydrogen over distances of 100-800 km, considering the existence of a rail network and loading/unloading infrastructure.The form of transport is LOHC or ammonia, due to the low volumetric density per unit of energy specific to hydrogen gas.For storage, transport, and distribution there is the possibility of providing multi-functional refueling stations (stations where hydrogen is produced, stored, and distributed).A multi-purpose refueling station could consist of several stations for different types of transport, including vehicles (motor vehicles, commercial vehicles, etc.).In such a situation, the operating costs would be lower, because transport from hydrogen production stations to refueling stations will be avoided.The benefit of a multi-purpose refueling station would be to diversify the fleet of vehicles that can benefit from hydrogen refueling.Ship transport is anticipated to be used for intercontinental transport, in the form of ammonia or LOHC.
Small to medium quantities of hydrogen gas can be transported by truck, in compressed gas containers.Several pressurized gas cylinders or tubes are bundled together on so-called CGH2 tube trailers, for transporting larger volumes.Typically, the cylinders are made of steel and have a substantial net weight, which leads to mass-related transport restrictions.The most recent pressurized storage systems use composite storage containers, for transport by trailers.
The low density of hydrogen also affects its transport.Under standard conditions (1.013 bar and 0°C), hydrogen has a density of 0.0899 kg/m 3 , also known as normal cubic meter (NM3).Under typical conditions, compressing hydrogen to 200 bars, the pressure raises its density to 15.6 kg hydrogen per cubic meter; compressing it to 500 bars, its density would reach 33 kg/m 3 .Hydrogen, on the other hand, may be delivered in the form of liquid in tanker trucks and other vehicles.Since the density of liquid hydrogen is greater than that of gaseous hydrogen, an LH2 trailer can transport a greater quantity of hydrogen than a pressure gas tank.Since the density of liquid hydrogen is only about 800 kg/m 3 , only minor amounts of hydrogen are transported even in this circumstance.With a loading volume of 50 m 3 , you may transport 3,500 kg of liquid hydrogen, or about 40,000 Nm3 at a density of 70.8 kg/m 3 .When traveling greater distances, it is typically more cost-effective to transport hydrogen in liquid form due to the larger capacity of a liquid hydrogen tank compared to a pressurized gas tank.Hydrogen is placed into cryogenic tanks for transit as a liquid.The average range for an LH2 trailer is around 4,000 kilometers.As the cryogenic hydrogen inside the container warms up throughout the trip, the internal pressure increases [4].
Due to its flammable nature, a comprehensive risk analysis is crucial to assess and mitigate potential hazards associated with hydrogen transport and storage.Therefore, many risk assessment studies were conducted on the safe design of Hydrogen Refueling Stations (HRS).Risk assessment is the overall process of hazard identification, risk analysis, and risk evaluation [5,6].
This paper uses PHAST software to simulate the hydrogen dispersion distance and jet fire radiation, in the case of hydrogen road transport, using compressed gas hydrogen tube trailers (CGH2), involved in traffic accidents.A hypothetical transport of hydrogen in compressed form, using a trailer with different configurations, involved in a road accident in an urban area, is analyzed to determine the impact on the environment and the objectives in the area considered.

Consequence modeling in PHAST software
PHAST (Process Hazard Analysis Software Tool) is a widely used software package developed by DNV (Det Norske Veritas) for conducting process hazard analysis in various industries.PHAST is designed to assess and analyze the potential hazards associated with industrial processes, including chemical, petrochemical, oil and gas, and pharmaceutical facilities.The key features of the software include hazard identification, consequence analysis, using a range of models and algorithms to assess the consequences of the events (fire, explosion, toxic release, jet and pool fires, vapor cloud explosions, and hazardous materials' dispersion), and risk assessment, by combining identified hazards with data on the probability or frequency of occurrence [7].Also, PHAST is a useful tool in the development of mitigation and safety measures, its validation being based on the incorporation of the current industry standards and regulatory guidelines, enabling users to assess their compliance with specific regulations and codes such as those from regulatory bodies like the Occupational Safety and Health Administration (OSHA) and the European Union's Seveso Directive.
Many company engineers, competent authorities, and researchers use PHAST for predicting the dispersion of hazardous materials or simulating catastrophic scenarios, making it one of the most widely utilized software programs for safety engineering applications [8,9,10,11,12].
The jet fire consequence results obtained by PHAST were extracted for three different thermal radiation levels namely, 37.5 kW/m 2 , 12.5 kW/m 2 , and 4 kW/m 2 , that correspond to 99%, 50%, and 1% lethality levels (with 20 seconds exposure time).

Case study
Within the theoretical study, a hypothetical traffic collision that occurred at a junction in an urban area was considered, involving a compressed hydrogen tube trailer.The risk analyses are those associated with unintentional releases of hydrogen.
A typical hydrogen steel tube trailer contains several high-pressure cylinders for hydrogen storage, as shown in Figure 1.Four different configurations of the CGH2 tube trailer were considered, differing in the number of installed tubes: 1, 4, 9, and 15.Relevant input parameters corresponding to the mentioned scenarios are presented in Table 1 below.
According to other risk assessments from the literature, the high accident rate is related to transportation, handling, and loading.In case of vehicle accidents or collisions, these incidents can potentially result in cylinder damage, leaks, or ruptures, exacerbating the flammability and explosion risks associated with hydrogen [13,14,15].The weight, size, and characteristics of the trailers and cylinders necessitate careful handling, secure anchoring, and adequate loading and unloading techniques to minimize the risk of injury or property damage.Ensuring the integrity of hydrogen cylinders and preventing leaks is also crucial.However, due to the small molecular size of hydrogen, it can diffuse through materials more easily compared to other gases.Any leaks from the cylinder valves, fittings, or connections can result in the release of hydrogen gas, leading to fire hazards or asphyxiation risks in confined spaces.Even small sparks or sources of heat can ignite hydrogen, resulting in fire or explosion incidents.
A catastrophic rupture of a hydrogen tube trailer refers to a severe failure of the trailer's cylinders, resulting in the sudden and uncontrolled release of hydrogen gas.This type of incident can have significant consequences, including safety hazards, property damage, and potential environmental impacts.This is the case taken into consideration for the studied scenarios.To simplify the case, the accident is considered to produce a catastrophic rupture, the entire volume of hydrogen being considered as a whole (i.e.all tubes are equally affected).4 Result and discussion

Dispersion
When hydrogen is released into the atmosphere, it tends to rapidly disperse due to its low molecular weight and buoyancy.The accurate modeling and prediction of hydrogen dispersion are crucial for assessing potential risks and designing safety measures.
To simulate the dispersion of hydrogen in the atmosphere, PHAST uses mathematical models.These simulations consider variables including the release rate, wind direction and speed, atmospheric stability, and local topography.These variables are considered by PHAST to forecast the spatial distribution and concentration of hydrogen throughout time.In the studied scenarios, the input parameters for weather conditions are presented in Table 1 above.The dispersion charts for the simulated cases are shown in Figure 2, Figure 3, Figure 4, and Figure 5.Given the high quantity of hydrogen released in the case of catastrophic rupture, due to vehicle collision, the ignition of hydrogen cloud by an ignition source (open flames, sparks, or electrical equipment) can lead to the formation of a fireball, which is a rapidly expanding and intensely burning mass of flames.The fireball emits intense heat radiation, which can cause thermal hazards to nearby structures, equipment, or personnel.A catastrophic rupture followed by ignition can generate blast effects.These effects include overpressure waves, fragmentation of the trailer, and potential projectiles, which can cause severe damage to nearby structures and pose risks to persons in the vicinity.
The simulation results are shown in Figures 6 -9, for the four cases taken into account.

Conclusions
Using PHAST software, this paper aims to model the hydrogen dispersion distance and fireball radiation in case of a catastrophic rupture produced to a CGH2 tube trailer, involved in a hypothetic road accident, in an urban area.For this study, four scenarios were considered, for four different numbers of cylindrical tubes loaded on the tube trailer.The study found that after a major rupture, the sudden and uncontrolled release of hydrogen gas can have significant consequences, hydrogen being able to form highly explosive mixtures with air.If the released hydrogen gas or the mixture cloud encounters an ignition source, it can ignite and result in a fire or explosion, depending on the amount of hydrogen transported.In the case of large volumes of compressed hydrogen, the study showed the formation of a fireball, a rapidly expanding and intensely burning mass of flames.
By understanding hydrogen dispersion patterns and concentrations, safety measures can be implemented, such as adequate ventilation, proper storage and handling procedures, and the placement of ignition sources at safe distances.Effective emergency response plans can also be developed to mitigate the consequences of hydrogen releases, should they occur.In this context, consequence modeling carried out using PHAST software has proved very good applicability in the studied scenarios.

Table 1 .
Input parameters for selected scenarios.