Vented hydrogen deflagrations in weak enclosures: Experimental results and implications for industrial practice

It is common practice in industry to install equipment for hydrogen energy applications in containers and smaller enclosures. Fires and explosions represent a significant hazard for such installations (Skjold et al., 2018), and specific measures are generally required for reducing the risk to a tolerable level (Skjold et al., 2017). Explosion venting is a frequently used measure for mitigating the consequences of hydrogen deflagrations in confined systems. Whereas most enclosures used for hydrogen applications in industry are inherently congested, most of the experiments that have been used for validating the empirical or semi-empirical engineering models in international standards, such as EN 14994 (2007) and NFPA 68 (2018), were performed with empty vessels. This paper reviews selected results from the experimental investigation of vented hydrogen deflagrations performed in 20-foot ISO containers as part of the EU-funded HySEA project (Skjold 2018ab; Skjold et al., 2019a). The parameters investigated in the experiments include hydrogen concentration, vent area, type of venting device, homogeneous and inhomogeneous mixtures, initial turbulence and level of congestion inside the enclosures. The measurements included maximum reduced explosion pressure, external blast waves and the structural response of the container walls. The results demonstrate the strong effect of congestion and mixture stratification on the maximum reduced explosion pressure, neither of which is accounted for in the current version of the European standard for gas explosion venting protective systems (EN 14994, 2007). The discussion elaborates on the implications of the results for industrial applications.publishedVersio

Computational fluid dynamics simulations of hydrogen releases and vented deflagrations in large enclosures

This paper presents model predictions obtained with the CFD tool FLACS for hydrogen releases and vented deflagrations in containers and larger enclosures. The paper consists of two parts. The first part compares experimental results and model predictions for two test cases: experiments performed by Gexcon in 20-foot ISO containers (volume 33 m3) as part of the HySEA project and experiments conducted by SRI International and Sandia National Laboratories in a scaled warehouse geometry (volume 45.4 m3). The second part explores the use of the model system validated in the first part to accidental releases of hydrogen from forklift trucks inside a full-scale warehouse geometry (32 400 m3). The results demonstrate the importance of using realistic and reasonably accurate geometry models of the systems under consideration when performing CFD-based risk assessment studies. The discussion highlights the significant inherent uncertainty associated with quantitative risk assessments for vented hydrogen deflagrations in complex geometries. The suggestions for further work include a pragmatic approach for developing empirical correlations for pressure loads from vented hydrogen deflagrations in industrial warehouses with hydrogen-powered forklift trucks.publishedVersio

Modelling breakdown of industrial thermal insulation during fire exposure

The aging of many of the installations in the oil and gas industry may increase the likelihood of loss of containment of flammable substances, which could lead to major accidents. Flame temperatures in a typical hydrocarbon fire may reach 1100–1200 °C, which are associated with heat flux levels between 250 and 350 kW/m2. To limit or delay the escalation of an initial fire, passive fire protection (PFP) can be an effective barrier. Additionally, both equipment and piping may require thermal insulation for heat or cold conservation. Previous studies have investigated whether thermal insulation alone may protect the equipment for a required time period, e.g., until adequate depressurization is achieved. The present study entails the development of a numerical model for predicting the heat transport through a multi-layer wall of a distillation column exposed to fire. The outer surface is covered by stainless-steel weather protective cladding, followed by PFP, thermal insulation, and finally an inner column of carbon steel of variable thicknesses. The model for the breakdown of thermal insulation is based on observed dimensional changes and independent measurements of the thermal conductivity of the insulation after heat treatment. The calculated temperature profiles of thermally insulated carbon steel during fire exposure are compared to fire test results for carbon steel with thicknesses of 16, 12, 6 and 3 mm. The model's predictions agree reasonably well with the experiments. The degradation of the thermal insulation at temperatures above 1100 °C limits its applicability as fire protection, especially for low carbon-steel thickness. However, the model predicts that adding a 10-mm layer of more heat-resistant insulation (PFP) inside the fire-exposed cladding may considerably extend the time to breakdown of the thermal insulation.publishedVersio

Simulating vented hydrogen deflagrations: Improved modelling in the CFD tool FLACS-hydrogen

Under embargo until: 2021-10-06This paper describes validation of the computational fluid dynamics tool FLACS-Hydrogen. The validation study focuses on concentration and pressure data from vented deflagration experiments performed in 20-foot shipping containers as part of the project Improving hydrogen safety for energy applications through pre-normative research on vented deflagrations (HySEA), funded by the Fuel Cells and Hydrogen 2 Joint Undertaking (FCH 2 JU). The paper presents results for tests involving inhomogeneous hydrogen-air clouds generated from realistic releases performed during the HySEA project. For both experiments and simulations, the peak overpressures obtained for the stratified mixtures are higher than those measured for lean homogeneous mixtures with the same amount of hydrogen. Using an in-house version of FLACS-Hydrogen with the numerical solver Flacs3 and improved physics models results in significantly improved predictions of the peak overpressures, compared to the predictions by the standard Flacs2 solver. The paper includes suggestions for further improvements to the model system.acceptedVersio

CFD Analysis of Explosions with Hydrogen-Methane-Air Mixtures in Congested Geometries

Hydrogen is an enabler for de-carbonising the energy system in Europe by 2050. In the UK, several projects are investigating the feasibility of gradually blending hydrogen into the natural gas pipelines with the aim to reach 100% hydrogen in the gas network. However, the safe use of hydrogen as a fuel presents different challenges than conventional hydrocarbon-based fuels Advanced consequence models are powerful tools that can be used to support the design process and estimate the consequences of potential accidents. This paper analyses the predictive capabilities of two combustion models for explosion for hydrogen, methane and hydrogen-methane blends. The analysis involves the default combustion model in the commercial version (FLACS-CFD v21.2), and a new combustion model implemented in an in-house development version where the model for premixed turbulent combustion incorporates Markstein number effects (FLACS-CFD v21.2 IH). Experiments performed by Shell in unconfined pipe-racks, some of which were part of the EU funded project NaturalHy, are considered. The simulation results from both versions of FLACS-CFD are within a factor of 2 of the overpressures observed in the experiments. However, FLACS-CFD v21.2 IH appears to give an improved representation of the overpressure trend with variations in the hydrogen equivalence ratio observed in the experiments.publishedVersio

The Effect of Solid Inhibitors on Hydrogen-air Combustion

The use of hydrogen as an energy carrier is a promising solution for enabling the transition towards increased use of renewable energy sources in the global energy mix. However, hydrogen-air mixtures are highly reactive, and conventional technologies for explosion protection have limited applicability for hydrogen systems. As such, it is not straightforward to achieve the same level of safety for hydrogen energy systems, compared to systems based on conventional hydrocarbon fuels. The last decades have seen the development of innovative solutions for chemical inhibition of vapour cloud explosions with solid inhibitors, such as sodium bicarbonate and potassium carbonate (Roosendans and Hoorelbeke, 2019). Both substances are non-toxic, non-flammable, lowcost and relatively harmless to the environment, compared to for example halons. Although solid suppressants can be highly effective for hydrocarbons (Babushok and Tsang, 2000), experiments indicate that the same compounds are not very effective for the inhibition of hydrogen-air mixtures. The absence of carbon implies that hydrogen combustion is inherently different from hydrocarbons, however, the combustion of hydrocarbons includes the elementary reactions involved in combustion of hydrogen-air mixtures. These elementary reactions change when exposed to solid inhibitors like sodium or potassium compounds (Roosendans, 2018). Simulations of chemical kinetics based on these elementary reactions show that potassium compounds should yield a significant reduction of flame velocity. The same simulations show a significantly higher generation of radicals for hydrogen combustion compared to hydrocarbon combustion. Thus, more inhibitor is needed for effective inhibition of premixed hydrogen-air flames. For a solid inhibitor to be effective, the compound must evaporate in the flame zone, and this process appears to be the main hurdle for effective inhibition of hydrogen explosions. This paper presents results from dedicated experiments and simulations with chemical kinetics software that elaborate on previous findings and improve the understanding of the underlying mechanics of solid inhibitors in hydrogen-air combustion.publishedVersio

In the green? Perceptions of hydrogen production methods among the Norwegian public

This article presents findings from a representative survey, fielded through the Norwegian Citizen Panel, examining public perceptions of hydrogen fuel and its different production methods. Although several countries, including Norway, have strategies to increase the production of hydrogen fuel, our results indicate that hydrogen as an energy carrier, and its different production methods, are still unknown to a large part of the public. A common misunderstanding seems to be confusing ‘hydrogen fuel’ in general with environmentally friendly ‘green hydrogen’. Results from a survey experiment (N = 1906) show that production method is important for public acceptance. On a five-point acceptance scale, respondents score on average 3.9 for ‘green’ hydrogen, which is produced from renewable energy sources. The level of acceptance is significantly lower for ‘blue’ (3.2) and ‘grey’ (2.3) hydrogen when respondents are informed that these are produced from coal, oil, or natural gas. Public support for hydrogen fuel in general, as well as the different production methods, is also related to their level of worry about climate change, gender, and political affiliation. Widespread misunderstandings regarding ‘green’ hydrogen production could potentially fuel public resistance as new ‘blue’ or ‘grey’ projects develop. Our results indicate a need for clearer communication from the government and developers regarding production methods to avoid distrust and potential public backfire.publishedVersio

Simplified modelling of explosion propagation by dust lifting in coal mines

Dispersion of accumulated layers of combustible dust by turbulent flow or shock waves ahead of the propagating flame may sustain explosion propagation in coal mine galleries and other industrial facilities. The mechanisms involved in transforming dust layers into dust suspensions are rather complex, and detailed numerical modelling of this process is therefore practically impossible, at least on industrial scales. In the computational fluid dynamics code DESC (Dust Explosion Simulation Code), a simplified empirical relation describes the dust-lifting phenomenon. The relation originates from experimental work in a laboratory-scale shock tube, and a small wind tunnel, at Warsaw University of Technology. The present paper describes the modelling of dust lifting in the current version of DESC, and illustrates the performance of the code by simulating some large-scale dust explosion experiments conducted in a 100-m surface gallery at the Experimental Mine Barbara in Katowice, Poland. Although there are significant uncertainties associated with this type of calculations, the results suggest that a simplified approach to dust lifting may become a useful tool for risk assessments in the future.publishedVersio

Simulating the Effect of Release of Pressure and Dust Lifting on Coal Dust Explosions

Dispersion of accumulated layers of combustible dust by turbulent flow or shock waves ahead of a propagating flame can play an important role for explosion propagation in coal mines. In 1924, Greenwald and Wheeler investigated this process in a series of large-scale dust explosion experiments, performed in a 750 feet long experimental gallery. In the present paper, these experiments are simulated with the computational fluid dynamics code DESC (Dust Explosion Simulation Code), using a simple empirical model to describe the process of dust lifting

Flame Propagation in Dust Clouds: Challenges for Model Validation

Modelling of industrial dust explosions poses a formidable challenge to researchers and safety engineers. Whereas current best practice with respect to modelling the consequences of vapour cloud explosions in the petroleum industry involves the use of computational fluid dynamics and a probabilistic approach to risk assessment, current safety practice for powder handling plants still rely on empirical guidelines. Although there has been significant progress in the predictive capabilities of phenomenological tools and CFD codes in recent years, such methods still require further development and verification. The first part of the paper contains a brief review of medium to large-scale dust explosion experiments suitable for validating consequence models for dust explosions. The second part demonstrates typical modelling challenges by simulating selected experiments from a study of vented explosions in a connected vessel system with the CFD code DESC. The simulation results are evaluated based on comparison with experimental results, and with respect to the observed response to moderate variations in the input parameters. Finally, the discussion highlights main knowledge gaps with respect to available experimental data and current modelling capabilities