Fire & Explosion - General
DIN No Open Government Status TD5_048 Fully Open Issue Date Review Date 18 December 2002 18 December 2004
Fuel Cells - hazards and risk management by Gordon Newsholme, Technology Division
Introduction Since the start of the new millennium the interest and activity in hydrogen and fuel cell technology has been accelerating rapidly. The purpose of this note is to provide an introduction to the principal hazards that are likely to be present in fuel cell installations. Basic Principles of fuel cell operation In general terms a fuel cell is an electrochemical device that oxidises a hydrogen rich fuel and delivers a significant amount of the energy released as electricity. In virtually all fuel cells the oxidising agent employed is oxygen from the surrounding air and the most commonly used fuel at present is hydrogen, which may come from: • • • • •
cylinders of compressed gas cryogenic liquid storage metal hydride storage systems hydrocarbons e.g. LPG or methane methanol
The hydrogen produced from hydrocarbons and methanol is obtained using a high temperature catalytic reformer. This feeds a hydrogen stream directly into the fuel cell. In addition, cells are now being developed that can operate directly on methanol without the need for a reformer. The use of a liquid fuel simplifies storage and the elimination of a high temperature component is a significant safety advantage. An extensive review of the main fuel cell types and typical uses is available in DIN TD5/045. Hazards associated with fuel cell installations All the fuels listed above present a significant fire and explosion hazard. In order to reduce the risks when storing and using these materials appropriate measures need to be taken. Hydrogen is, currently, the most common fuel for cells and in some ways is the most hazardous. The measures necessary to reduce the risks from hydrogen are discussed in detail below. The different or additional hazards arising from the other materials that may also be present in fuel cell installations are discussed separately later. Fire and explosion hazards of hydrogen Hydrogen is a flammable gas and readily forms an explosive mixture with air. The range of air/hydrogen concentrations that will explode is extremely wide. Mixtures containing from as little as 4% v/v hydrogen up to as much as 75% v/v will readily explode3. For the bulk of this range (18-69% v/v) there is a significant risk that a confined hydrogen/air mixture will detonate. The likelihood of an explosion occurring is further increased as a result of the very low ignition energy necessary to initiate a hydrogen/air explosion. The ignition energy for hydrogen/air mixtures is so low, 0.02 mJ, that the absence of ignition sources should not be relied upon as a basis of safety.
Hydrogen is very buoyant relative to air. Consequently, any leak of hydrogen will rapidly dissipate upwards. If the leak occurs in an open or well-ventilated area these properties will help to reduce the likelihood of a flammable atmosphere being formed. Conversely, there is a serious risk of explosion when hydrogen leaks occur within enclosed areas containing electrical equipment or other sources of ignition. The risk is particularly high when the source of ignition is close to a ceiling or other impervious high-level barrier. Storage Hydrogen will usually be supplied from high pressure compressed storage in cylinders and then fed into the fuel cell stack under very modest pressure (< 1 barg). Other modes of storage likely to be encountered include cryogenic storage of liquid hydrogen and, in the near future, absorption of hydrogen in metal hydrides. When high pressure storage is used the cylinders should be located in secure outside storage in the open air. Indoor storage of hydrogen cylinders, although not recommended, is permissible provided that extensive safety measures; including effective ventilation, noncombustible construction and explosion relief,...
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