Chemical Engineering December 2020 - 21

Facts At Your Fingertips
Hydrogen flame hazards and leak detection
Department Editor: Scott Jenkins
T
he use of molecular hydrogen
is common across the chemical
process industries (CPI). Annually,
70 million metric tons of H2 are
produced worldwide. Most industrial
H2 production currently occurs via
steam reforming of methane, but production
via water electrolysis is growing.
This one-page reference provides
information on H2 flame hazards and
leak detection. Tables 1 and 2 outline
major industrial uses of H2 and its key
properties, respectively.
Hydrogen flammability
Although H2 is nontoxic, it is highly
flammable and explosive. The National
Fire Protection Association (NFPA;
Quincy, Mass.; www.nfpa.org) rates
H2 as a " 4 " on the flammability scale
(the highest rating), because H2 is
flammable when mixed with air, even
in small amounts, and the minimum
ignition energy (MIE) is small (0.019
mJ for a gas-air mixture). Hydrogen
can also self-ignite without energy
from an external source when it is
leaking from a pipe at high pressure.
Hydrocarbon flames differ from H2
flames. A H2 flame emits low levels
of infrared radiation and visible light,
so it will not give off intense heat and
light. Therefore, it cannot be easily
detected by human senses. It is difficult
to see a H2 flame even up close.
Plant workers may see a shimmering,
mirage-like area or possibly sparks,
Chemical formula
Molecular weight
Boiling point at atmospheric pressure
Density of gas at boiling point
Melting point
U.S. Units
H2
-423°F
4.23 lb/ft3
-434°F
Latent heat of vaporization (at boiling) 191.7 Btu/lb
Specific gravity of H2 at 1 atm (air = 1)
0.0696
Health = 0
NFPA ratings
Flammable limits in air
Autoignition temperature
Department of Transportation Hazardous
classification scheme label code
Flammability = 4
Instability = 0
4% to 75%*
752°F
2.1
(flammable gas)
*Flammable limits are the lower volume limit concentration of a chemical in air
that will continue to propagate a flame once initiated. The flame would propagate
at any concentration from the lower limit until it reaches an upper limit where the
fuel to air ratio is too rich and the flame is quenched.
n/a
n/a
400°C
TABLE 1. MAJOR INDUSTRIAL USES OF HYDROGEN [1-3]
Industry sector
Uses of H2
Metals
Petroleum refining
Pharmaceuticals
Chemicals
Glass and ceramics
Food and beverage
Electronics
Miscellaneous
In steel manufacturing, H2 is used, along with inert gases, to establish a reducing atmosphere.
This is required for heat treating steel and welding. H2 is used in annealing
stainless-steel alloys, sintering and copper brazing
* In petroleum refining, H2 is used to hydrogenate hydrocarbons to improve combustion
characteristics of fuels. H2 is catalytically combined with intermediate streams
to convert heavier, unsaturated compounds to lighter and more stable compounds
* H2 is also used to remove sulfur from crude oil fractions
H2 is used in the manufacture of pharmaceutical products, such as in asymmetric
catalytic hydrogenation reactions to create chiral centers
H2 is used as a raw material in the chemical synthesis of ammonia (Haber-Bosch synthesis),
methanol, hydrogen peroxide, polymers and organic solvents
In float glass manufacturing, panes of glass are made by depositing molten glass onto
a tin bath to obtain a smooth surface. H2 (with nitrogen) is used to create a positivepressure
atmosphere to prevent oxidation of the tin bath
H2 is used to hydrogenate unsaturated fatty acids in animal and vegetable oils, producing
solid fats for margarine and other food products
H2 can be a carrier gas for active trace elements in the manufacture of semi-conducting
layers in integrated circuits
* Power-plant generators can be cooled with H2, since it offers low frictional resistance
and high thermal conductivity
* Liquid H2 is used as a rocket fuel in the space industry
* H2 is used as a protective atmosphere in the fabrication of nuclear fuel rods
which are actually dust particles burning
briefly in the flame [2].
TABLE 2. SELECTED PROPERTIES OF HYDROGEN GAS [1,4]
Property
2.016 g/mol 2.016 g/mol
-252.8°C
446.0 kJ/kg
0.0696
H2 leak detection
Gas detection equipment can sense
a H2 leak before it ignites, increasing
the possibility of stopping a leak
before it causes a fire or explosion.
Catalytic bead (Pellistor) detectors
sense H2 in combustible concentrations
when it combines with oxygen
to produce heat. This sensor usually
consists of a matched pair of platinum
wire-wound resistors, one of which is
encased in a ceramic bead. The active
catalytic bead is coated with a
catalyst, while the reference
catalytic bead
remains untreated.
The resistors are then
enclosed behind
SI Units
H2
a
67.76 kg/m3
-259.2°C
flame-proof sinter or
porous filter. When
the combustible gas
comes in contact
with the active catalytic
bead surface,
the gas is oxidized
and heat is released,
which changes the
resistance of the wire.
The reference (passive)
bead maintains
the same electrical
resistance in clean
CHEMICAL ENGINEERING WWW.CHEMENGONLINE.COM DECEMBER 2020
air as the active bead, but does not
catalyze combustible gas. Combustible
gas concentrations are then determined
by comparing the difference
between the active and passive bead
circuits. Limitations include the susceptibility
to poisoning of the catalytic
beads and their inability to signal a
fault when they fail [2].
Hydrogen quickly floats upward
and disperses, so detectors should
be located close to and above spots
where a leak might occur (just above
a valve, for example).
Multi-spectrum NIR
If a flame ignites, multi-spectrum infrared
(MIR) flame detection has become
the preferred choice for detecting H2
flames in industrial settings. Some
MIR detectors are designed specifically
to detect the IR radiation from H2
flames. MIR flame detectors rely on a
combination of IR filters and software
analysis to detect flames and reduce
the potential for false alarms.
n
References
1. Rivkin, C., Burgess, R. and Buttner, W., Hydrogen Technologies
Safety Guide, National Renewable Energy Laboratory, NREL
Technical Report, January 2015.
2. Hosch, M.J. and Paterson, A., Detection Systems for Reducing the
Risk of Hydrogen Fires, Chem. Eng., April 2020, pp. 54-56.
3. Universal Industrial Gases Inc., Hydrogen Properties, Uses and
Applications, web resources, www.uigi.com/hydrogen, accessed
Nov. 2020.
4. Safety Data Sheet (SDS) for molecular hydrogen, 2018.
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