Three things are needed for a fire or explosion to occur: a fuel, oxygen (mixed with the fuel in appropriate quantities) and a source of ignition. Hydrogen, as a flammable fuel, mixes with oxygen whenever air is allowed to enter a hydrogen vessel, or when hydrogen leaks from any vessel into the air. Ignition sources take the form of sparks, flames, or high heat.

A.3.2.1 Flashpoint

All fuels burn only in a gaseous or vapour state. Fuels like hydrogen and methane are already gases at atmospheric conditions, whereas other fuels like gasoline or diesel that are liquids must be converted to vapour before they burn. The characteristic that describes how easily these fuels can be converted to vapour is the flashpoint. The flashpoint is defined as the temperature at which the fuel produces enough vapour to form an ignitable mixture with air at its surface. If the temperature of the fuel is below its flashpoint, it can not produce enough vapour to burn since its evaporation rate is too slow. Whenever a fuel is at or above its flashpoint, vapour is present. The flashpoint is not the temperature at which the fuel bursts into flames; that is the autoignition temperature.

_Table A-3 Chemical properties of Hydrogen and comparative Fuels [36, 46, 17, 227]






Higher Heating Value (kJ/g)





Lower Heating Value (kJ/g)





Flammability limit (vol%)

4 - 75

5.3 - 15

2.1 - 9.5


Detonability limit (vol%)

18.3 - 59

6.3 - 13.5

2.6 - 7.4


Ignition energy (mJ)





Autoignition Temperature (°C)




230 - 480

Flame temperature (°C)





Burning speed (m/s)

2.65 - 3.25

0.37 - 0.45

0.43 - 0.52

0.37 - 0.43

Quenching gap (mm)





Flash point (°C)





A.3.2.2 Flammability Range

The flammability range of a gas is defined in terms of its lower flammability limit (LFL) and its upper flammability limit (UFL). The LFL of a gas is the lowest gas concentration that will support a self-propagating flame when mixed with air and ignited. Below the LFL, there is not enough fuel present to support combustion; the fuel/air mixture is too lean. The UFL of a gas is the highest gas concentration that will support a self-propagating flame when mixed with air and ignited. Above the UFL, there is not enough oxygen present to support combustion; the fuel/air mixture is too rich. Between the two limits is the flammable range in which the gas and air are in the right proportions to burn when ignited.

A stoichiometric mixture occurs when oxygen and hydrogen molecules are present in the exact ratio needed to complete the combustion reaction. If more hydrogen is available than oxygen, the mixture is rich so that some of the fuel will remain un-reacted although all of the oxygen will be consumed. If less hydrogen is available than oxygen, the mixture is lean so that all the fuel will be consumed but some oxygen will remain. Practical internal combustion and fuel cell systems typically operate lean since this situation promotes the complete reaction of all available fuel. One consequence of the UFL is that stored hydrogen (whether gaseous or liquid) is not flammable while stored due to the absence of oxygen in the cylinders. The fuel only becomes flammable in the peripheral areas of a leak where the fuel mixes with the air in sufficient proportions.

Hydrogen is flammable over a very wide range of concentrations in air (4 - 75%) and it is explosive (detonate) over a wide range of concentrations (15 - 59%) at standard atmospheric temperature. The flammability limits increase with temperature as illustrated in Figure A.2. As a result, even small leaks of hydrogen have the potential to burn or explode. Leaked hydrogen can concentrate in an enclosed environment, thereby increasing the risk of combustion and explosion. The flammability limits of comparative fuels are shown in Table A-3.

A.3.2.3 Autoignition Temperature

The autoignition temperature is the minimum temperature required to initiate self-sustained combustion in a combustible fuel mixture in the absence of a source of ignition. In other words, the fuel is heated until it bursts into flame. Each fuel has a unique ignition temperature. For hydrogen, the autoignition temperature is relatively high at 585 °C. This makes it difficult to ignite a hydrogen/air mixture on the basis of heat alone without some additional ignition source. The autoignition temperatures of comparative fuels are indicated in Table A-3.

600 K

Hydrogen Flammability

Percent Hydrogen in Air

600 K

Percent Hydrogen in Air

Figure A.1. Variation of Hydrogen Flammability Limits with Temperature [46] A.3.2.4 Ignition Energy

Ignition energy is the amount of external energy that must be applied in order to ignite a combustible fuel mixture. Energy from an external source must be higher than the autoignition temperature and be of sufficient duration to heat the fuel vapour to its ignition temperature. Common ignition sources are flames and sparks.

Although hydrogen has a higher autoignition temperature than methane, propane or gasoline, its ignition energy at 0.02 mJ (Table A-3) is about an order of magnitude lower and it is therefore more easily ignitable. Even an invisible spark or static electricity discharge from a human body (in dry conditions) may have enough energy to cause ignition. Nonetheless, it is important to realize that the ignition energy for all of these fuels is very low so that conditions that will ignite one fuel will generally ignite any of the others.

Hydrogen has the added property of low electro-conductivity so that the flow or agitation of hydrogen gas or liquid may generate electrostatic charges that result in sparks. For this reason, all hydrogen conveying equipment must be thoroughly grounded.

A.3.2.5 Burning Speed

Burning speed is the speed at which a flame travels through a combustible gas mixture. It is different from flame speed. The burning speed indicates the severity of an explosion since high burning velocities have a greater tendency to support the transition from deflagration to detonation in long tunnels or pipes. Flame speed is the sum of burning speed and displacement velocity of the unburned gas mixture. Burning speed varies with gas concentration and drops off at both ends of the flammability range. Below the LFL and above the UFL the burning speed is zero. The burning speed of hydrogen at 2.65-3.25 m/s (Table A3) is nearly an order of magnitude higher than that of methane or gasoline (at stoichiometric conditions). Thus hydrogen fires burn quickly and, as a result, tends to be relatively shortlived.

A.3.2.6 Quenching Gap

The quenching gap (or quenching distance) describes the flame extinguishing properties of a fuel when used in an internal combustion engine. Specifically, the quenching gap relates to the distance from the cylinder wall that the flame extinguishes due to heat losses. The quenching gap has no specific relevance for use with fuel cells. The quenching gap of hydrogen (at 0.064 cm) is approximately 3 times less than that of other fuels, such as gasoline (Table A-3). Thus, hydrogen flames travel closer to the cylinder wall before they are extinguished making them more difficult to quench than gasoline flames. This smaller quenching distance can also increase the tendency for backfiring since the flame from a hydrogen-air mixture can more readily get past a nearly closed intake valve than the flame from a hydrocarbon-air mixture.

Flammable Hydrogen Methane Propane
Figure A.2. Flammability Ranges of Comparative Fuels at Atmospheric Temperature [46] _Table A-4 Combustion properties of hydrogen in air at 1 Atm and 25°C[36]_
Combustion properties_Deflagration Detonation Units

Heat of reaction (high)




Lower lammability limit in air






kg/m3 of air

Upper flammability limit in air






kg/m3 of air

Optimum detonation mixture ratio in air




Detonation maximum overpressure in air




Auto-ignition temperature




Minimum ignition




Maximum flame temperature in air



°C 3

Explosion energy



kg TNT/m3 of NTP gas

laminar burning velocity in air




Detonation velocity in air




maximum overpressure ratio



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  • bell brown
    What percentage of of biogas must be methane in order for the biogas to be flammable?
    7 years ago
  • uwe
    What ppercentage of biogas must be methane in order for it to be flammable?
    4 years ago
  • Feaven
    What is the flash point of BIOGAS?
    2 years ago
  • stefano lombardo
    How to find flash point of biogas?
    1 year ago
  • eliisa
    How to increase flamability of biogas slideshare?
    3 months ago
  • jasmine
    What is flash point of fermentation?
    2 months ago

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