ANALYSIS METHODOLOGY FOR HYDROGEN BEHAVIOUR IN ACCIDENT SCENARIOS

Hydrogen is not more dangerous than current fossil energy carriers, but it behaves differently. Therefore hydrogen specific analyses and countermeasures will be needed to support the development of safe hydrogen technologies. A systematic step-by-step procedure for the mechanistic analysis of hydrogen behaviour and mitigation in accidents is presented. The procedure can be subdivided into four main parts: 1. ) 3D modelling of the H2-air mixture generation, 2. ) hazard evaluation for this mixture based on specifically developed criteria for flammability, flame acceleration and detonation on-set, 3. ) numerical simulation of the appropriate combustion regime using verified 3D-CFD codes, and 4. ) consequence analysis based on the calculated pressure and temperature loads. 1.0 INTRODUCTION Hydrogen and fuel cells are seen as a sustainable energy system that could meet the demands for energy supply security, cost-competitiveness and CO2-reduction in a post-fossil era [1]. Such a future hydrogen based economy will bring many new hydrogen-fueled applications into the now existing work and living environment and will lead to widely decentralized use of hydrogen by a large population. For the acceptance of hydrogen as an energy carrier its safety level must be comparable to that of current fossil fuels, for which safe procedures have been developed. The question therefore arises: are there differences in the safety of hydrogen and hydrocarbons? Fig.1 compares safety relevant properties of hydrogen with those of methane (the major component of natural gas), propane and gasoline vapor. The properties are grouped in six blocks. Each of these blocks corresponds to an accident phase which potentially can be encountered in a hydrogen accident scenario: (1) hydrogen release, (2) distribution and mixing with air, (3) ignition, (4) initial slow laminar deflagration, (5) fast turbulent deflagration, and (6) detonation. All hydrocarbon properties in Fig.1 are normalized to the corresponding hydrogen value. The resulting property profiles of the three hydrocarbons are quite similar, especially in the reactive part of the accident. However, the property differences to hydrogen reach almost a factor of 10 in each of the six accident phases. Since he outcome of a gas release depends on many parameters and complex physics, hydrogen specific safety investigations will be needed for the development of safe hydrogen technologies . 2.0 ANALYSIS METHODOLOGY The Research Center Karlsruhe (FZK) is developing numerical codes and methods aiming at the consistent analysis of hydrogen behaviour in accident scenarios and at the scientific foundation of suitable countermeasures. The present paper outlines a general analysis procedure and gives some examples from recent investigations for safe use of hydrogen. The emphasis is put on a step-by-step description of the deterministic methodology. The general approach is based on 3D computational fluid dynamics (CFD) simulations. References covering the important issue of code validation are given throughout the text. The complete and self-consistent modeling of hydrogen behavior in accidents requires information for four main steps which are outlined in Fig.2. For each of these steps specific input information is needed.