CACHE Modules on Energy in the Curriculum
Hydrogen as a Fuel
Module Title: Hydrogen Flammability
Module Author: Dr. Daniel A. Crowl
Author Affiliation: Michigan Technological University
Course: Stoichiometry, process design, process safety
Text Reference:
Felder and Rousseau, Elementary Principles of Chemical Processes, 3rd ed., New York: John
Wiley and Sons, 2005.
Crowl and Louvar, Chemical Process Safety, Fundamentals with Applications, 2nd ed., Upper
Saddle river, NJ: Prentice Hall, 2002.
Concepts: Flammability of gas.
Problem Motivation:
Fuels are flammable because we burn these fuels to produce energy. However, the property of
flammability also creates a hazard in that a leakage or evaporation of the fuel might produce a
flammable atmosphere when it is not expected. This may lead to a fire or explosion if an
ignition source is found.
This module will look at the issue of fuel flammability.
The flammable limits define the range of fuel concentrations in air over which the mixture is
flammable. If a flammable mixture is formed due to the leakage or evaporation of a fuel, a fire
or explosion is likely.
The flammable limits are defined at a temperature of 25oC and 1 atm. As the temperature
increases the flammability range typically increases – the lower limit decreases and the upper
limit increases.
The experimental procedure to determine the flammable limits is an empirical procedure. The
flammable limits do not have a theoretical basis like heat capacity. It is always recommended
that flammable limits be obtained as close as possible to actual process conditions.
Basic Definitions:
 Lower Flammable Limit (LFL)
The volume percent fuel in air below which combustion is not possible – the mixture is too
lean. The units are volume percent fuel in air.
 Upper Flammable Limit (UFL)
2nd Draft
D. A. Crowl
June 25, 2010
Page 1
The volume percent fuel in air above which combustion is not possible – the mixture is too
rich. The units are volume percent fuel in air.
Problem Information
Example Problem Statement:
The flammable limits for three fuels are provided below:
Hydrogen:
Gasoline:
Methane:
LFL
UFL
4.0%
1.3%
5.0%
75%
7.2%
15.0%
Molecular Weight
kg/mol
0.002
0.106
0.016
a. A storage room has dimensions of 4 m x 4 m x 4 m. Calculate the amount of each chemical
species, in kg, that must leak or evaporate into the room to form a flammable mixture.
b. A cylinder containing 200 SCF of hydrogen is stored in this room. If the cylinder leaks out
into the room, will it form a flammable mixture?
Assume a temperature of 30oC and a pressure of 1 atm.
Additional data:
Gasoline:
MW:
Density:
106
720 kg/m3
Example Problem Solution:
a. The room has dimensions of 4 m x 4 m x 4 m. The total volume is 4 x 4 x 4 = 64 m3.
Hydrogen: The lower flammable limit is 4% by volume hydrogen in air. Thus, the hydrogen
gas required to reach the LFL is
 0.04   64 m3   2.56 m3
The molar density of any ideal gas at 1 atm and 30oC = 303K is:
P
1 atm


 40.2 mol/m3
5
RgT  8.2057 10 m3 atm/mol K   303 K 
The total amount of hydrogen at the LFL is
 2.56 m  40.2 mol/m   102.9 mol
3
3
102.9 mol = 0.206 kg
Only 0.206 kg of hydrogen needs to leak out in order to form a flammable mixture.
Gasoline: The LFL is 1.3%. The total volume of gasoline vapor required to reach the LFL is
 0.013  64 m3   0.832 m3
2nd Draft
D. A. Crowl
Page 2
June 25, 2010
The total moles of gasoline in this vapor is
 0.832 m  40.2 mol/m   33.4 mol
3
3
33.4 mol = 3.54 kg
Methane:
The LFL is 5.0%. The total volume of gasoline vapor required to reach the LFL is
 0.05  64 m3   3.2 m3
The total moles of methane in this vapor is
 3.2 m  40.2 mol/m   128.6 mol
3
3
128.6 mol = 2.06 kg
b. Convert SCF to m3
 2.832 102 m3 
3
 200 SCF 
  5.66 m
3
1 ft


The volume fraction of fuel is thus
5.66 m3
 0.088  8.8 vol. % hydrogen
64 m3
Yes, the mixture is flammable.
2nd Draft
D. A. Crowl
Page 3
June 25, 2010
Home Problem Statement:
A storage room has dimensions of 4 m x 4 m x 4 m. Calculate the amount of each chemical
species, in kg, that must leak or evaporate into the room to form a flammable mixture. Perform
the calculation for the upper flammable limit.
2nd Draft
D. A. Crowl
Page 4
June 25, 2010