Publications

As a part of a broader project on hydrogen production by reforming of methane in a membrane catalytic reactor this paper outlines the research activity performed at the University of Pisa Department of Chemical Engineering aimed at developing and testing composite panels that can operate as thermal protective shields against hydrogen jet fires. The shield design criterion that appears to give a more practical and convenient solution for the type of installation to be protected is the one that suggest to realize composite panels. Composite material are made of two elements fiber and matrix. In this study composite panels will be realized with basalt fabric as fiber and epoxy-phenolic resins as matrix. Therefore following the indications given by norms as UNI 9174 and ASTM E 1321-93 a test method has been studied to obtain temperature data from a specimen impinged by an hydrogen flame. Thanks to thermocouples applied on backside of the sample and an infrared video camera to realize thermal images of specimen surface impinged by flame this type of test try to characterize the behaviour of composite materials under the action of hydrogen flame simulating in a simple way the action of hydrogen jet fires.

The current work is devoted to problems connected with application of CFD tools for safety analysis of large-scale industrial structures. With the aim to preserve conservatism of overall process of multistage procedure of such analysis special efforts are required. A strategy which has to lead to obtaining of reliable results in CFD analysis is discussed. Different aspects of proposed strategy including: adequate choice of physical and numerical models procedure of validation simulations and problem of ‘under-resolved’ simulations are considered. For physical phenomena which could cause significant uncertainties in the course of scenario simulation an approach which complements CFD simulations by application of auxiliary criteria is presented. Physical basis and applicability of strong flame acceleration and detonation-to-deflagration transition criteria are discussed. In concluding part two examples of application of presented approach for nuclear power plant and workshop cell for hydrogen driven vehicles are presented.

The present work proposes a novel safe method for obtaining metallic hydrides. The method is called SHS (Self-Propagating High temperature synthesis). A novel high pressure gas reactor governed by an electromechanical control device has been designed and built up in order to synthesise metallic hydrides. This system is provided with a control system that allows calculating the amount of gas coming into the reaction vessel at every stage of the process. The main feature of this method is that metallic hydrides can be safely synthesised using low gas reaction pressures. In order to validate the assessing system the main kinetic regularities of SHS in Ti-H2 system were studied. In addition phase analysis (by means of X ray diffraction) as well as chemical analysis have been performed.

Experimental studies were carried out to examine the lift-off and blow-out stability of H2/CO2 H2/Ar H2/C3H8 and H2/CH4 jet flames. The experiments were carried out using a burner with a 2mm inner diameter. The flame structures were recorded by direct filming and also by a schlieren apparatus. The experiments showed that the four gases affected the lift-off and blow-out stability of the hydrogen differently. The experiments showed that propane addition to an initially attached flame always produced lifted flame and the flame was blown out at higher jet velocity. The blow-out velocity decreased as the increasing in propane concentration. Direct blow-off of hydrogen/propane was never observed. Methane addition resulted in a relatively stable flame comparing with the carbon dioxide and propane addition. Comparisons of the stability of H2/C3H8 H2/CH4 and H2/CO2 flames showed that H2/C3H8 produced the highest lift-off height. Propane is much more effective in lift-off and blow out hydrogen flames. The study carried out a chemical kinetic analysis of H2/CO2 H2/Ar H2/C3H8 and H2/CH4 flames for a comparison of effect of chemical kinetics on flame stability.

When hydrogen gas is used or stored within a building as with a hydrogen-powered vehicle parked in a residential garage any leakage of unignited H2 will mix with indoor air and may form a flammable mixture. One approach to safety engineering relies on buoyancy-driven passive ventilation of H2 from the building through vents to the outside. To discover relationships between design variables we combine two types of analysis: (1) a simplified 1-D steady-state analysis of buoyancy-driven ventilation and (2) CFD modelling using FLUENT 6.3. The simplified model yields a closed-form expression relating the H2 concentration to vent area height and discharge coefficient; leakage rate; and a stratification factor. The CFD modelling includes 3-D geometry; H2 cloud formation; diffusion momentum convection and thermal effects; and transient response. We modelled a typical residential two-car garage with 5 kg of H2 stored in a fuel tank; leakage rates of 5.9 to 82 L/min (tank discharge times of 12 hours to 1 week); a variety of vent sizes and heights; and both isothermal and nonisothermal conditions. This modelling indicates a range of the stratification factor needed to apply the simplified model for vent sizing as well as a more complete understanding of the dynamics of H2 movement within the building. A significant thermal effect occurs when outdoor temperature is higher than indoor temperature so that thermocirculation opposes the buoyancy-driven ventilation of H2. This circumstance leads to higher concentrations of H2 in the building relative to an isothermal case. In an unconditioned space such as a residential garage this effect depends on the thermal coupling of indoor air to outdoor air the ground (under a concrete slab floor) and an adjacent conditioned space in addition to temperatures. We use CFD modelling to explore the magnitude of this effect under rather extreme conditions.

Design of hydrogen fueling components is critical for safety and reliability. Intensive usage of such components in urban public environment is expected in the near future. Any leakage of gas or failure of equipment will create potential hazards. Materials for such category of equipment must have specific mechanical characteristics including hardness (influence on the durability of the equipment and on the resistance to hydrogen) and be easy to machine. Air Liquide has developed a test program for qualifying equipment representing the present state of the art. Studies on the susceptibility of various steels to hydrogen embrittlement have been done. Test specimens were exposed to static and cyclic loads with hydrogen and an inert gas the inert gas representing a reference. Various tests are described here. As a result the importance of further development in the design and selection of appropriate materials for critical hydrogen components is required. Various options are presented and discussed.

Safety is a high priority for a hydrogen refueling station. Here we propose a method to safely refuel a vehicle at optimised speed of filling with minimum information about it. Actually we identify two major risks during a vehicle refuelling: over filling and overheating. These two risks depend on the temperature increase in the tank during refuelling. But the inside temperature is a difficult information to get from the station point of view. It assumes a temperature sensor in a representative place of the tank and an additional connection between the vehicle and the station for data exchange. The refuelling control may not depend on this parameter only. Therefore out objective was to effectively control the filling particularly to avoid the two identified risks independently of optional and safety redundant information from the vehicle. For that purpose we defined a maximum filling pressure which corresponds to the most severe following conditions: if the maximum temperature is reached in the tank or if the maximum capacity is reached in the tank. This maximum pressure depends on a few filling parameters which are easily available. The method and its practical applications are depicted.

This paper presents a compilation and discussion of the results supplied by HySafe partners participating in the Standard Benchmark Exercise Problem (SBEP) V1 which is based on an experiment on hydrogen release mixing and distribution inside a vessel. Each partner has his own point of view of the problem and uses a different approach to the solution. The main characteristics of the models employed for the calculations are compared. The comparison between results together with the experimental data when available is made. Relative deviations of each model from the experimental values are also included. Explanations and interpretations of the results are presented together with some useful conclusions for future work.

Microgrid with hydrogen storage is an effective way to integrate renewable energy and reduce carbon emissions. This paper proposes an optimal operation method for a microgrid with hydrogen storage. The electrolyzer efficiency characteristic model is established based on the linear interpolation method. The optimal operation model of microgrid is incorporated with the electrolyzer efficiency characteristic model. The sequential decision-making problem of the optimal operation of microgrid is solved by a deep deterministic policy gradient algorithm. Simulation results show that the proposed method can reduce about 5% of the operation cost of the microgrid compared with traditional algorithms and has a certain generalization capability.

A method for determination of hazardous zones for hydrogen installations has been studied. This work has been carried out within the NoE HySafe. The method is based on the Italian Method outlined in Guide 31-30(2004) Guide 31–35(2001) Guide 31-35/A(2001) and Guide 31-35/A; V1(2003). Hazardous zones for a “generic hydrogen refuelling station”(HRS) are assessed based on this method. The method is consistent with the EU directive 1999/92/EC “Safety and Health Protection of Workers potentially at risk from explosive atmospheres” which is the basis for determination of hazardous zones in Europe. This regulation is focused on protection of workers and is relevant for hydrogen installations such as hydrogen refuelling stations repair shops and other stationary installations where some type of work operations will be involved. The method is also based on the IEC standard and European norm IEC/EN60079-10 “Electrical apparatus for explosive gas atmospheres. Part 10 Classification of hazardous areas”. This is a widely acknowledged international standard/norm and it is accepted/approved by Fire and Safety Authorities in Europe and also internationally. Results from the HySafe work and other studies relevant for hydrogen and hydrogen installations have been included in the case study. Sensitivity studies have been carried out to examine the effect of varying equipment failure frequencies and leak sizes as well as environmental condition (ventilation obstacles etc.). The discharge and gas dispersion calculations in the Italian Method are based on simple mathematical formulas. However in this work also CFD (Computational Fluid Dynamics) and other simpler numerical tools have been used to quantitatively estimate the effect of ventilation and of different release locations on the size of the flammable gas cloud. Concentration limits for hydrogen to be used as basis for the extent of the hazardous zones in different situations are discussed.