Safety of Hydrogen as an Energy Carrier
Contract No SES6-CT-2004-502630

Deliverable D113
Guidance for using hydrogen in confined spaces

Results from InsHyde
Proposed table of content

EXECUTIVE SUMMARY
Lead: INERIS

1 INTRODUCTION

Lead: INERIS

1.1 Referenced documents

2 RISK CONTROL MEASURES WHEN USING HYDROGEN INDOOR

Lead: INERIS

2.1 Fuel supply and storage arrangement

Lead: INERIS

2.2 Detection

Lead: JRC; Contributors: BAM, INERIS, StatoilHydro

2.3 Ventilation and exhaust

Lead: INERIS; Contributors: BRE, UNIPI

2.4 Fire and explosion safety

Lead: BRE, HSL; Contributors: HSL, UU, TNO, FZK, UNIPI

2.4.1 Hazard segregation principle

2.4.2 Fire proof walls / segregation of hazards

2.4.3 Storage limitation

2.4.4 Fire detectors

2.4.5 Escape routes

2.4.6 Review of existing best practices

2.4.7 Review of ignition sources

Hydrogen can burn in various combustion modes: as a laminar or turbulent jet fire, anchored at a particular location, as a laminar or turbulent deflagration with a flame zone propagating through space, and, as a detonation wave. For this to occur, two additional requirements have to be fulfilled: the presence of oxygen (air contains 21% oxygen) and an ignition source. Since air is omnipresent, risk reduction should be concentrated on the identification and avoidance of possible ignition sources: open flames, hot surfaces, electric sparks, mechanical sparks, static electric sparks, friction.

2.4.7.1 Open flames

Combustible hydrogen-air mixtures within the flammability limits will inevitably ignite upon contact with open flames.

2.4.7.2 Hot surfaces

Ignition by a hot surface occurs as a result of local heating of the hydrogen-oxidant mixture to the point where a sufficiently large volume reaches the autoignition temperature and the combustion reaction is initiated. For this to occur generally requires the surface to be at a temperature well above the autoignition temperature (see [14]), although the actual temperature depends on a number of factors in addition to the usual mixture concentration, ambient temperature etc. These additional factors determine the hot surface ignition behaviour of flammable gases and not just hydrogen, and include the size and shape of the hot surface, the degree of confinement around the surface, the strength of the convection currents across the surface (see Laurendau [21]) and the material of the surface.

For a particular hot surface, ignition is characterised by an ignition delay, which under ideal circumstances multiplied by the power for ignition, gives a linear relationship between the product (energy) and ignition delay (Carleton et al. [23]). The offset on the y-axis in this plot is the minimum power for ignition for that arrangement.

The temperatures required to cause ignition of mixtures of hydrogen with air and oxygen (see the review by Buckle & Chandra [24] and Carleton et al. [23]) range from 640°C to 930°C, the spread of temperatures being explained the size, geometry effects etc described in the first paragraph. While the temperatures quoted are above the auto ignition temperature, the increase is much less than seen for hydrocarbon-fuel air mixtures, as illustrated by the IIA curve in Figure Figure 3.3. In terms of simple modelling of hot surface ignition, Laurendau [21] presents a simple model in terms of a one step reaction chemical kinetics model.

Interestingly, the most easily ignited mixture of hydrogen with air lies lean of stoichiometric (See Calcote and Gregory [25]) while work using very small hotsurfaces (Carleton et al. [23]) suggests that mixtures as low as 10 to 15% are the most easily ignited. For hydrogen-oxygen mixtures, the work of Buckle & Chandra [24] indicates a fairly flat H2 concentration depenedence (slight positive slope with increasing hydrogen concentration) between roughly 20 and 90% hydrogen in oxygen. Catalytic surfaces (e.g plantinium) have a dramatic effect on the ignition temperature required (Cho & Law [26]), ignitions reported at temperatures as low as 70°C.

2.4.7.3 Autoignition

Generally auto-ignition results from either the exothermic or the chain branching character of the oxidation reactions which, at certain conditions, self-accelerate to reach high conversion and heat release rates. Auto-ignition limits can be established experimentally or theoretically for a homogeneous mixture of volume V filling a vessel whose walls have a temperature Tw. Once the heat release rate in the volume due to reactions exceeds the heat lost to the walls, or, if the reaction rates in the vessel exceed the reaction quenching (termination) rates by the walls or in the gas a thermal or branched chain (isothermal) auto-ignition occurs. Typically, as almost all combustion reactions are exothermic, chain auto-ignitions also cause self heating and are accelerated by both factors. Obviously autoignition limits are not only a feature of the mixture composition and parameters (pressure, temperature), but also of the vessel size, wall properties and internal flow conditions.

This is illustrated in Figure 3.4 showing the auto-ignition limits often called also explosion limits for a stoichiometric mixture of hydrogen and oxygen providing the important parameters of the test vessel. We may note the logarithmic scale of pressure and linear scale of temperature showing that pressure effects on reaction rates are weaker than temperature effects as one would expect by the consideration of Arrhenius chemistry.

The first and second limits, although interesting from the fundamental point of view, correspond to very low pressures (up to about 0.3 bar) and are thus of little practical interest. The third limit follows the trend that one would expect from simple density considerations. As the pressure increases, the initial densities of the reactants increase and a lower temperature is necessary for the reactions to reach a critical reaction rate for explosion. For safety considerations explosions in large volumes where wall effects can be neglected at atmospheric conditions and for most violently reacting, i.e. stoichiometric mixtures are considered. Thus, in typical safety manuals a temperature of 585 °C is given as the autoignition temperature for hydrogen air systems. The initial reaction rate in auto-ignition is very small thus a certain time must pass before the reaction has reached a defined rate. This time interval is called ignition delay. Ignition delays are particularly important for operation of engines as they provide the engine speed limits where operation is possible due to auto-ignition (compression ignition engines) or where auto-ignition can be avoided when detrimental (knock in spark ignition engines).

Most accurate ignition delay measurements can be performed in shock tubes in wall reflected shocks where the heating of the mixture is practically instantaneous. A research issue is then prediction of the ignition delays using available kinetic data. The state of the art in this field is far from satisfactory as illustrated in Figure 3.5 (after [27]) where a comparison of measured and calculated ignition delay times using different chemical reaction mechanism, available in the literature is provided.

2.4.7.4 Electric sparks

Electrical sparks are discontinuous electrical discharges across a gap, between at least two electrodes, that occur when the potential difference exceeds the breakdown voltage.

2.4.7.5 Mechanical friction and impact

2.4.7.5.1 Mechanical rubbing

2.4.7.5.1 Mechanical impact

2.4.7.5.1 Mechanical sparks

2.4.8 Preventing and limiting ignition sources

2.4.8.1 Prevention

2.4.8.2 Segregation

2.4.8.3 Containment

2.4.9 Zoning and choice of appropriate equipment (ATEX)

2.4.10 Fire permit

2.4.11 Maximum tolerable hydrogen mass that can be released (FZK tests)

2.4.12 Explosion venting

2.4.13 Building design

(no accumulation of h2 at highest point, e.g. avoid double ceiling, building strength and inclusion of weak points)

2.5 Commissioning, inspections, training and worker protection

Lead: INERIS

3 HYDROGEN BEHAVIOUR IN ACCIDENTAL SITUATIONS

Lead: NCSRD

3.1 Hydrogen release and dispersion

Lead: NCSRD; Contributions: FZK, CEA, INERIS, HSL

3.2 Hydrogen ignition

Lead: HSL, UU

3.3 Hydrogen explosion

Lead: FZK; Contributions: WUT, TNO, CEA, HSL

3.4 Hydrogen fire

Lead: BRE; Contributions: HSL, UU

4 RISK ASSESSMENT RECOMMENDATIONS

Lead: DNV

4.1 Risk assessment methodology

Lead: DNV, RISOE, UNIPI, TNO

4.2 Consequence assessment

Lead: NCSRD

4.2.1 Engineering approach that can be used

Lead: TNO; Contributions: CEA

4.2.2 Using CFD

Lead: GEXCON; Contributions: NCSRD, UPM, UU, HSL, CEA, DNV

4.2.3 Performing Experiments

Lead: FZJ; Contributions: FZK, CEA, INERIS

5 REFERENCES

ANNEX-1: Experiences from HYSAFE members

Lead: FZJ