The limiting oxygen concentration and flammability limits of

gases and gas mixtures

Isaac A. Zlochower*,

*	 Corresponding author. Tel.: þ1 412 386 4960; fax: þ1 412 386 6595.

E-mail address: iaz0@cdc.gov (I.A. Zlochower).

Gregory M. Green

Pittsburgh Research Laboratory, National Institute for Occupational Safety and Health, Pittsburgh, PA 15236, USA

abstract

This paper presents data on the limiting (minimum) oxygen concentration (LOC), in the presence of added

N2, ofmethane (CH4), propane (C3H8), ethylene (C2H4), carbon monoxide (CO), and hydrogen (H2), and some

of their binary mixtures. It also addresses the issue of the flammable concentration (flammability) limits of

these pure gases in air. The study is based on spark ignited explosions in large, spherical laboratory vessels

(120-L and 20-L) using a 7% pressure-rise criterion for explosion propagation. The results of the study are

compared with the older values which used long flammability tubes with a diameter 25 cm together with

visual evidence of substantial upward propagation. They are also compared to results reported recently

using a 12-L spherical flask with a visual flame propagation criterion. Finally, they are compared to results

reported in Europe using more modest flammability criteria and smaller chambers.

The findings reported here show excellent agreement between the 120-L and 12-L results, good

agreement with the 20-L results, and reasonable agreement with the earlier flammability tube values for

the lower flammability limits. They disagree, however, with the more conservative European values.

These results and those from the 12-L experiments also feature lower LOCs than are given by traditional

flammability tubes. A model for the LOCs of such fuel mixtures based on the Le Chatelier mixture rule for

lower flammable limits is seen to reasonably fit the observed results on binary mixtures and can

accommodate more complex mixtures as well. One such set of ternary mixtures containing CH4 and 1:1

CO:H2 is well fitted by the model.

1. Introduction

Starting with basic definitions, the lower and upper flammability (or explosibility) limits (LFL and UFL, respectively) are the

limiting fuel concentrations in air that can support flame propagation and lead to an explosion. Fuel concentrations outside those

limits are non-flammable. The progressive addition of an inert gas

to a fuel–air mixture causes the narrowing of the flammability

range to the point where the two limits coincide. The limiting

oxygen concentration (LOC) is the minimum O2 concentration in

a mixture of fuel, air, and an inert gas that will propagate flame. In

this paper, the inert gas will be nitrogen. In practice, the limits

(LFL, UFL, and LOC) represent an average between the neighboring

concentrations inside and outside the experimental flammability

limits (ASTM International, 2008a, 2008b).

There is currently a significant difference of opinion between

American and European based standard-setting organizations as to

the prescribed test vessels and criteria for flammability and LOC

determinations (ASTM International, 2008a, 2008b; British and

European Standard, 2003, 2007). The traditional criterion used in

the US, which was the basis of an extensive database of flammability limits, required that flame and explosion propagation be

distinguished from ignition phenomena (Coward & Jones, 1952;

Kuchta, 1985). In order to demonstrate unambiguous flame propagation, U.S. standards required that flame be observed at some

distance from the ignition source and to have traveled through

a significant fraction of the enclosed volume. That requirement was

relaxed in a more recent US standard that featured the use of a 5-L

spherical glass flask, and mandated only that flame propagation be

established by evidence of horizontal as well as vertical flame travel

(ASTM International, 2008a). Where ambiguity resulted, the standard called for using a larger (12-L) flask with a more extensive

flame evolution – a flame cone with an arc spanning at least 90° at

the top of the flask, as measured from the point of ignition (ASHRAE, 2007; ASTM International, 2008a). The explosion overpressure

in a closed system was, correspondingly, mandated to be 7%,1 i.e.,

1 A 7% pressure increase represents a 1 psi increase starting at 1 bar (14.5 psia) or

1 atm (14.7 psia).
a significant percentage of the initial value (ASTM International,

2008b). Although the exact flame travel distance or overpressure

required for designation as a true flame propagation and explosion

is somewhat subjective and apparatus-dependent, any attempt to

minimize such requirements will lead to results that are subjective,

apparatus-dependent, and overly conservative. Adding further to

the problem, the flammability limits and LOCs when traditionally

determined have no built-in safety factors. In practice, therefore,

such factors must be imposed. For example, NFPA 69 requires that

the fuel concentration only reach 25% of the LFL value (or 60% of

the LFL for gaseous systems with automated in-line sensors and

controls) (NFPA 69, 2008). Imposing these factors has been the

long-standing practice in the US (MSHA, 1996; NFPA 69, 2008).

The current standard adopted by the European Union, based on

the earlier German standard (DIN 51649,1986), is a radical departure

from the above considerations. In that standard (EN 1839T), a separation of flame 10 cm above the 0.2–0.5 s induction spark in an

8 x 30 cm cylinder defines a flammable mixture. The same is true if

the flame extends to 24 cm without separation from the spark

electrodes. The resultis basically to substitute an ignitability criterion

for flammability. For a closed spherical or cylindrical system with

a minimum volume of 5-L and using such a centrally located induction spark, or fuse wire of 10–20 J nominal energy (EN 1839T), the

corresponding criterion is a 5% pressure rise above that given by the

source alone (British and European Standard, 2003). Other European

researchers go further and insist that any separation of flame from

the ignition spark bespeaks a flammable mixture (meaning ignitable). The same applies to an overpressure that is just measurable

(2%) (De Smedt et al., 1999). The LFLs and LOCs resulting from such

minimal flame propagation criteria are, understandably, lower than

those resulting from the traditional ones since they have a built-in

safety factor. Despite these concerns the primary purpose of this

article is not to critique the European methodology, but to alert the

readers to the current existence of incompatible flammability databases; the US database (Coward & Jones, 1952; Kuchta, 1985; NFPA,

2008; Zabetakis, 1965) and the European one (CHEMSAFE, 2007).

More refined techniques often lead to the revision of earlier data and

to progress in the field of study. Revisions based on new arbitrary

criteria, however, tend to produce confusion. Moreover, the incompatible databases may lead to the misapplication of traditional safety

factors to the already conservative European data. This caution has

already been issued by Britton (2002) and is echoed in this paper.

2. Experimental

The flammability data reported here were obtained in the

spherical 120-L and 20-L chambers used in earlier studies (Cashdollar, Zlochower, Green, Thomas, & Hertzberg, 2000). The internal

diameter of the 120-L chamber is 60 cm and its pressure rating is

69 bar. Instrumentation includes a sensitive strain-gauge pressure

transducer to measure the partial pressures as the gases are added

and mixed, and a higher capacity strain-gauge pressure transducer

to monitor the explosion pressure. The strain-gauges have

a response time of 1 ms. The pressure transducers were mounted on

the top and bottom flanges of the chamber. A fine wire thermocouple near the top of the chamber was used to record the steadystate temperature after the addition of each gaseous component.

There were also ports with ball valves for connection to a vacuum

pump and vent. An internal non-sparking fan at the bottom of the

vessel was used to mix the gases. The gases were added via

a manifold and controlled via manual switches in an isolated

control room where the additions, pressure monitoring, mixing and

spark ignition initiation, and PC recording were conducted.

The spark ignition system consisted of 3-mm diameter metal rod

electrodes with a durable plastic spacer for rigidity. The pointed

spark electrodes terminated below the center of the sphere and

were bent to face each other with a gap of 6 mm. The electrical

energy so delivered came from a 1300 mF capacitor charged to 300 V

which was then discharged through a transformer to generate

a strong spark. This spark was powered by a stored energy on the

capacitor of 58 J based on the equation: stored energy 1⁄4 1/2CE2

,

where C is the capacitance and E is the voltage. The actual electrical

energy in the spark gap was considerably less, however, because of

the low efficiency of the transformer circuit (Hertzberg, Conti, &

Cashdollar, 1985).

The calibrations of the pressure transducers were checked daily

using the internal shunt calibration resistors provided by the

manufacturer. Samples of the gas mixtures could be collected in

evacuated test tubes through a sampling needle on the side of the

chamber. These samples were then analyzed by gas chromatography

(GC). During the initial evaluation of the mixing efficiency in the

120-L chamber, samples of H2–air mixtures were collected after

the gases had been added to the chamber and after 2 and 5 min of

mixing by the fan. There was essentially no difference in the

measured concentrations of H2 for the gas mixtures over this time

period, showing that there was good mixing of the gases even before

the fan was turned on. The reported concentrations are in mole

(volume) percent, based on the partial pressure of the component

relative to the total pressure. It was noted that the addition of gas to

a fully or partially evacuated chamber led to an increase in

temperature due, presumably, to quasi-adiabatic compression of the

prior atmosphere. This temperature rise was allowed to fall to

a steady-state value. That temperature was then used to correct all

the component partial pressures to a common (final) temperature

using the ideal gas law.

The 20-L chamber, with a pressure rating of over 20 bar, is nearly

spherical, having a diameter of 30 cm and a height of 35 cm. The

ceramic encased electrodes were located slightly below the midheight of the chamber, and the exposed, pointed steel tips were

spaced 6 mm apart. The charging circuit was similar to that used for

the 120-L sphere except that an 800 mF capacitor was charged to

300 V, giving a nominal stored energy of 36 J. The actual thermal

energy deposited in the chamber, based on the observed pressure

increase due to the spark, was 0.5–1 J. The partial pressures of the

gaseous components were measured as above or via a sensitive,

temperature-controlled capacitance manometer that had a resolution of 0.1 torr (0.13 mbar) over a range of 1000 torr (1.33 bar). This

unit was calibrated by the manufacturer prior to use.

The data from the pressure transducers and the thermocouple

were recorded using a high speed analog to digital (A/D) board in

a personal computer (PC). This system can sample the data from

various instrument channels, usually at speeds less than 20 kHz per

channel. An in-house computer software program converted the

raw data to engineering units, plotted the data vs. time, and

allowed various data smoothing options. Maximum pressure and

maximum rate of pressure-rise values were obtained from the

pressure vs. time traces. The reproducibility of the flammability

data was checked by repeated tests over a period of months and

years.

The experimental determination of the flammability limits of

the gases studied was done by fully evacuating the chamber and

then admitting the fuel gas to the predetermined partial pressure

required to produce the desired final concentration in air. Dry,

oil-free air was admitted from a compressed gas cylinder to give the

desired final pressure of 1 bar (the ambient pressure at the elevation of the laboratory was always less than that final pressure). The

temperature in the chamber rose during the addition of both the

fuel and air, as previously noted, and the gas was allowed to come

to a steady-state temperature and pressure. The temperature of

the fuel gas was then used to correct the fuel pressure to that
corresponding to the final temperature of the mixture according to

the ideal gas law. The final concentration of the fuel gas was then

calculated from its corrected pressure. Next, the non-sparking

internal fan was started and run for at least 4 min to fully mix the

gases. The fan was shut off and the generated turbulence was

allowed to subside for 1 min. The capacitor spark ignition system

was then charged to 300 V and discharged through an ignition coil

to create the high voltage spark. The series of sparks thereby

produced lasted for about 0.1 s.

The initial concentrations of fuel gas chosen were based on the

literature values for the flammability limits or on prior determinations using such chambers. Once marginal explosion pressures

were produced, the concentrations were changed in small steps

(0.1–0.2%) for the LFL and UFL measurements of methane, propane,

ethylene, and CO. The concentration step for the LFL of hydrogen

was 0.5–1% owing to the low sensitivity of the explosion pressure to

concentration near the LFL (Cashdollar et al., 2000).

The LOC measurements were made starting at fuel concentrations near the LFL and using oxygen concentrations (from the air

and nitrogen additions) that were near the LFL multiplied by the

stoichiometric oxygen/fuel ratio (the ‘‘R’’ column in Table 1). The

oxygen concentration was varied in steps of 0.2% until the marginal

explosion conditions were delineated and verified. Then the fuel

concentration was changed and the minimum oxygen concentration

again determined. The process was repeated until a global minimum

oxygen concentration (LOC) with added N2 was established.

Table 1

Flammability/LOC: 120-L and 20-L closed vessel results vs. 12-L glass sphere and flammability tube.

Fuel (F) Vessel Stoichiometric equation Mole ratio (R) (O2/F) LFL (mol%) UFL (mol%) LOC (N2) (mol%) Explosion criterion

Hydrogen (H2)

Carbon monoxide

Methane (CH4)

Ethylene (C2H4)

Propane (C3H8)

(CO)

120-L

20-L

Flam.a tube

120-L

12-Lb

Flam.a tube

120-L

20-L

12-Lb

Flam.a tube

120-L

12-Lb

Flam.a tube

120-L

12-Lb

Flam.a tube

2H2 þ O2 1⁄4 2H2O

2CO þ O2 1⁄4 2CO2

CH4 þ 2O2 1⁄4 CO2 þ2H2O

C2H4 þ 3O2 1⁄4 2CO2þ2H2O

C3H8 þ 5O2 1⁄4 3CO2þ4H2O

0.5

0.5

2

3

5

7

6

4

12.2

12.2

12.5

5.0

4.9

4.9

5.0

2.7

2.7

2.7

2.0

2.0

2.1

75.9

75.0

72.0

72.5

74.0

15.8

15.9

15.8

15.0

31.4

31.5

36.0

9.8

10.0

9.5

4.6

4.7

5.0

5.1

5.5

11.1

10.7

11.3

12.0

8.5

8.6

10.0

10.7

10.5

11.5

Pressure

Pressure

Visual

Pressure

Visual

Visual

Pressure

Pressure

Visual

Visual

Pressure

Visual

Visual

Pressure

Visual

Visual

a

b

Flammability tube data summarized

The data from a 12-L spherical flask

by

by

Kuchta (1985).

Kondo et al. (2006, 2008).

3. Results and discussion: single fuels

Previous results (Cashdollar et al., 2000) for the LFL of H2 in the

120-L sphere using spark ignition showed a slow, linear increase in

explosion pressure with concentration below 7%, followed by an

accelerated increase above 7%. The pressure rise became steep at 8%

H2. The LFL of H2 is thus quite sensitive to the pressure criterion for

explosion propagation. The 7% pressure-rise criterion for spark

ignition yielded an LFL of 7% in this chamber. The explosion pressure

rise also occurred gradually with low H2 concentrations in the 20-L

chamber (below 8% H2), but with a steeper slope. In the 20-L

chamber, the 7% pressure-rise criterion was satisfied at 6% H2 in air.

With a meter-long upward flame propagation criterion in vertical

flammability tubes, the LFL was found to be 4%. The much shorter

30 cm tubes used in the EN 1839 (T) method gave an LFL of 3.6%

(Schroder & Molnarne, 2005). The tube values are based on the

buoyant rise of small flame kernels which represent only a small

fraction of fuel consumed and a correspondingly minimal pressure

rise. Measurements in a 14-L closed sphere gave an LFL of 4.2%

(Schroder & Molnarne, 2005), but the use of fuse wire ignition

rather than the weaker and more localized spark ignition is believed

to have unduly influenced the pressure rise associated with partial

propagation (5%). A more robust explosion pressure-rise criterion

(7%) for hydrogen in larger chambers would appear to be better

correlated with actual explosion hazards.

There is less data for the UFL in the 120-L sphere. However, the

sharp drop in explosion pressure beyond 75.7% hydrogen (the

pressure rise is near zero at 76%) argues against a significant

dependence of UFL on the above pressure criterion. The UFL is

estimated as 75.9% H2, the midpoint between these values. Previously, a value of 76.8% was reported by Cashdollar et al. (2000), but

that value was obtained in an 8-L chamber. The LFL in that chamber

was also found to be lower (5%) than in the larger (20-L and 120-L)

chambers. Similarly high UFLs were given in the tests run according

to the EN 1839 standard (76.6%, tube and 77.0%, closed sphere).

Given the sensitivity of hydrogen flammability to the experimental

conditions and chamber volume, the above relative difference of 1%

for the UFL is not surprising.

The data on explosion pressures at different oxygen vs.

hydrogen concentrations in air–nitrogen mixtures is given in Fig. 1,

where the legend symbols represent pressure-rise ranges. In other

words, Fig. 1 and the subsequent figures for LOC data are pressure

parameter plots for given O2 and fuel concentrations, with the

different symbols representing different pressure-rise levels. Fig. 1

shows a minimum oxygen concentration (LOC) of 4.6% that is

independent of hydrogen concentration in the range of 10–30%.

More typically, however, the minimum oxygen concentration for an

inert fuel–air mixture occurs at a narrow range of fuel concentrations. Those ‘‘worst-case’’ fuel concentrations define the LOC for the

fuel–air mixture containing the inert gaseous additive (e.g. N2).

The results for the LFL of CO are similar to that of H2, but feature

a steeper rise in pressure with concentration. They are also dependent on the amount of water vapor present in the mixture. In

contrast to the ordinary behavior of water as an effective inhibitor of

explosions due to its large heat capacity, here it is an accelerant – at

least at low concentrations. It functions, presumably, by supplying

the H and OH free radicals for the branching chain reactions needed
6.0 

>30 

15-30 

5.5 7-15 

3-7 

ygen, 

<3 

5.0 

xO

4.5 

4.0 

5 10 15 20 25 30	 

Hydrogen, 

Fig. 1. LOC data on H2 in air with added N2 that was obtained in the 120-L sphere.

Parametric pressure plot at different O2 vs. H2 concentrations, with the symbols

representing different pressure-rise ranges.

for combustion and for rapid flame propagation. The measured LFL is

12.2% when the mixture is nearly saturated with water. Similarly, the

UFL is 72% for near saturation. The results in the 12-L glass sphere for

CO in moist air with added N2 (LFL 1⁄4 12.2%, UFL 1⁄4 72.5%) that was

reported by Kondo, Takizawa, Takahashi, Tokuhashi, and Sekiya

(2008) agrees with the 120-L values (based on their listed uncertainty of 0.5% for the UFL). Fig. 2 shows an LOC in the 120-L sphere of

about 5.1% O2 for gas mixtures nearly saturated with water vapor.

That LOC occurs near a 20% CO concentration.

6.0 

> 30 7-15 <3 

10.0 15.0 20.0 25.0 30.0 35.0 

Carbon Monoxide, 

LOC = 5.1 

5.8 

5.6 

Oxygen, 

5.2
 

5.0
 

Fig. 2. 120-L data on the LOC of CO in nearly saturated moist air with added N2.

Parametric pressure plot at different O2 vs. CO concentrations, with the symbols

representing different pressure-rise levels.

5.4 

The above results represent atypical fuels that are lighter and

more diffusive than O2 (H2), or have branching chain reactions

associated with flame propagation that are more difficult to

establish (CO). Hydrogen, for example, is more concentrated at the

flame front (i.e., the real fuel concentration at the flame front is

higher than the nominal value) which clearly impacts the LFL value

(Cashdollar et al., 2000; Hertzberg, 1989). More typical flammable

gases – namely, the common hydrocarbons: methane (CH4), the

primary constituent of natural gas; propane (C3H8), the popular

cooking and heating fuel; and ethylene (C2H4), a basic chemical

feedstock for polyethylene and other plastics – were studied as well.

Cashdollar et al. (2000) have reported 120-L data on the LFL of

methane in air which show a gradual increase in pressure from 4.6

to 5.0%. There is little reason, however, to dispute the choice of 5.0%

as the LFL of methane. Both the pressure criterion of 7% and the

existence of a discontinuous change in slope yield the same LFL

value. The corresponding UFL is 15.8% based on the newer data. The

same value is the average of the values reported by Cashdollar et al.

(2000) for the 120-L and 20-L chambers (15.7 and 15.9%, respectively), while the LFL was 5.0% for both chambers. The 12-L glass

sphere gave an LFL of 4.9% and a UFL of 15.8% (Kondo, Takizawa,

Takahashi, & Tokuhashi, 2006).

Fig. 3 shows an LOC of 11.1% O2 for 5.2–5.8% CH4 in air–N2

mixtures in the 120-L sphere, but the LOC was found to be 10.7% O2

in the 20-L chamber. The 12-L sphere gave a value of 11.3%, as

reported by Kondo et al. (2006). The lower value given by the 20-L

chamber may indicate the need for a higher pressure-rise criterion

in chambers appreciably smaller than 120-L (Brandes & Ural, 2008).

15 

>30 

15-30 

7-15 

3-7 

<3 

Oxygen, 

5.0 5.2 5.4 5.6 5.8 6.0 

14 

13

12 

11
 

10
 

Methane, 

Fig. 3. 120-L data on the LOC of CH4 in air with added N2. Parametric pressure plot at

different O2 vs. CH4 concentrations, with the symbols representing different pressurerise ranges.

Past work (Cashdollar et al., 2000) gave an LFL value for propane

of 2.05% with a sharp rise in pressure with concentration. The

pressure drop with concentration near the UFL was more gradual. It

was found to be 9.8% for the 7% pressure criterion. Current results in

the 120-L sphere are consistent with the earlier findings. The LOC of

propane is seen to be about 10.7% % at 2.5–2.7% propane (Fig. 4). The

12-L glass sphere results of Kondo et al. (2006) gave an LFL of 2.0%,

a UFL of 10.0%, and an LOC of 10.5%, again in agreement with the 120­

L results.

Ethylene also features a sharp rise in pressure with concentration at low concentrations and a slower decline in pressure near the

UFL. The LFL is found to be 2.7% and the UFL is 31.4% using the 7%

criterion. The LOC is seen to be 8.5% O2 at 2.9–3.2% ethylene (Fig. 5).

The 12-L results of Kondo et al. (2006) yielded an LFL of 2.7%, a UFL

of 31.5%, and an LOC 1⁄4 8.6%.

The 120-L and 20-L data are summarized in Table 1 together

with the reported results from the 12-L sphere (Kondo et al., 2006,

2008) and flammability tubes (Kuchta, 1985). The reported results

from the 12-L glass sphere are essentially identical to the results

obtained here in the 120-L sphere despite the great difference in

volume and propagation criterion.

A good reflection of the difference between the US and European

standards is given by the LFLs of methane in air. Traditionally, this

value has been listed as 5.0% based on the results of flame propagation studies using a large upward propagation distance in tall,

wide containers (Britton, 2002; Coward & Jones, 1952), in closed,

larger spherical vessels (120-L and 20-L), and in a very large spherical

vessel (Cashdollar et al., 2000). The European Standard (Annex E)

using a 5% pressure-rise criterion in a small vessel (EN 1839B) gave

4.6% as the LFL. Using the flame detachment criterion in a small
10.8 

11.0 

10.6 

10.4 

10.2 

10.0 

>30 

15-30 

7-15 

3-7 

<3 

2.0 2.5 3.0 3.5 4.0 

Propane
 

LOC = 10.7
 

Oxygen, 

Fig. 4. The LOC of propane in air with added N2; 120-L data. Parametric pressure

plot at different O2 vs. C3H8 concentrations, with the symbols representing different

pressure-rise ranges.

10.0 

9.5 

9.0 

8.5 

8.0 

>30 

15-30 

7-15 

3-7 

<3 

2.0 2.5 3.0 3.5 4.0 4.5 5.0 

Ethylene, LOC = 8.5 

Oxygen, 

Fig. 5. The LOC of ethylene in air with added N2; 120-L data. Parametric pressure plot

for different O2 vs. C2H4 concentrations, with the symbols representing different

pressure-rise ranges.

cylindrical vessel (EN 1839T) yielded a value of 4.4% in this standard

(British and European Standard, 2003). A similar difference was

observed for the UFL measurements of methane in air. The European

standard methods gave 16.6% and 16.8% values for the tube and

‘‘bomb’’ methods, respectively – cited in Annex E, while both the

120-L and 12-L spheres gave 15.8%. A further confirmation of

the results reported here is given by the studies of Kondo’s group on

the effect of vessel size and shape on the experimental flammability

limits of methane and propane. They found that cylindrical vessels

did not yield values that were size independent until quite large

dimensions were used. Their largest cylinder (45 x 100 cm) with

a volume of 160-L gave values quite consistent with those reported

here for full upward propagation of a flammable mixture (methane

flammability limits 1⁄4 5.0–15.8%; propane limits 1⁄4 2.0–9.9%) (Takahashi, Urano, Tokuhashi, & Kondo, 2003).

The different values cited for the LFL of methane in the new

databases which are based on the European standards (CHEMSAFE,

2007) should not be interpreted as an argument for the retention of

older flammability values if new experiments in larger chambers

using comparable flammability criteria produce new values. That is

the case with experiments reported here, particularly in regards to

LOC values. These experiments now generate systematically lower

LOC values, as do the results in the 12-L glass sphere (Kondo et al.,

2006). This agreement between the pressure-rise criterion used in

the 120-L sphere and the visual criterion in the 12-L sphere lends

encouragement to the assumption that values have been determined that are reasonably independent of vessel size and flame

propagation criteria.
 

The above argument implies that the LOC values in the older
 

literature are not sufficiently conservative and should not be used to

generate safety protocols unless conservative safety factors are

incorporated. The problem with the older LOC values is also recognized in the 2008 NFPA 69 standard which requires subtracting 2%

(abs.)from the old values.While thatis probably a conservative stance,

it may involve unnecessary expense in the case of hydrogen – based

on the results obtained in the 120-L sphere. Additional measurements

using large chambers and appropriate flame propagation criteria will

be needed to form a reliable database of LOC values.

4. Fuel mixtures

The LFL of fuel mixtures is predicted adequately by the Le

Chatelier rule for fuel mixtures (Burgess et al., 1982; Coward &

Jones, 1952; Kondo et al., 2008; Liekhus, Zlochower, Cashdollar,

Djordjevic, & Loehr, 2000). That rule is less adequate, but still

generally conservative, in predicting the UFL of mixtures (Kondo

et al., 2008). The LOCs of such mixtures, however, have not been

extensively studied. Prior work on mixtures of hydrogen and

benzene (Thomas, 1996) established the utility of a calculation

scheme that will be derived in this paper and used to compare to

the experimental results on binary mixtures of hydrogen, CO, or

ethylene with methane. Mixtures of propane and methane are not

considered since their LOCs are too close to produce a significant

variation with composition. The experimental method chosen was

the same as for the single fuels, except that two different fuels were

mixed together with air and nitrogen to establish the stated

mixtures. Ternary mixtures of 1:1 hydrogen:CO with methane were

also studied. Water was not added to the mixtures containing CO

since the other fuel(s) had the H species needed for efficient CO

combustion. The results are summarized in Table 2.

Table 2 shows the experimental and calculated values of the LOC

of CH4 mixtures with H2 and/or CO, or with C2H4. The predicted

LOCs involving mixtures of H2 and/or CO with CH4 follow the

experimental values but fall to a maximum of 0.9% (abs.) below

them. The calculations to be described in the next section are

therefore seen as realistic but conservative. H2 and CO are unusual

fuels, however (high diffusivity or low reactivity) as noted earlier.

The deviation of the experimental results from the derived formula

is, therefore, not surprising. The one clear example of a mixture of

more typical fuels with widely different LOCs is the mixture of

methane and ethylene. The calculated values are very close to the

experimental ones with a maximum difference of 0.15% (abs.).

5. LOC of fuel mixtures

As noted previously, the lower flammability limit (LFL) for

mixtures of fuels is adequately given by the Le Chatelier rule:

X Lmixt: 1⁄4 1= xi=Li (1)

Where Lmixt is the LFL of the fuel mixture, Li is the LFL of fuel

component i, xi is the mole fraction of the fuel component.

The LOC of fuel mixtures can be derived from (1) by defining

L*

mixt and Li as, *

* L* 1⁄4 ðLOCÞi

=Ri; L* 1⁄4 ðLOCÞ (2) i mixt mixt=R

Where (LOC)i is the experimental value for LOC of component i,

(LOC)mixt is the calculated LOC for the fuel mixture, Ri is the
Table 2

LOC: fuel mixtures containing methane: CH4–H2; CH4–CO; CH4–C2H4; CH4–1:1 CO:H2: experimental vs. calculated.

Fuel: % CH4 Methane–H2 Methane–CO Methane–ethylene CH4–1:1 CO:H2

(LOC)exp (LOC)calc (LOC)exp (LOC)calc (LOC)exp (LOC)calc (LOC)exp (LOC)calc

0

10

20

25

35

40

50

60

75

80

90

100

4.6

6.5

7.7

8.3

9.1

10.3

10.9

11.1

4.6

5.6

6.9

7.65

8.64

10

10.7

11.1

5.1

6.1

7.8

9.5

10.9

11

11.1

5.1

6.1

7.4

9.0

10.2

10.8

11.1

8.6

8.6

8.9

9.3

10.05

11.1

8.6

8.7

9

9.45

10.1

11.1

4.8

7.3

8.7

9.7

10.5

11.1

4.8

6.7

8.2

9.4

10.3

11.1

stoichiometric molar ratio of oxygen to fuel i (see Table 1), R* is the

P stoichiometric ratio of oxygen to the fuel mixture, as given by xiRi.

P For L* 1⁄4 1= xi=L* (modified Le Chatelier rule), mixt i

substituting (2) into the modified Le Chatelier and rearranging

terms gives

ðLOCÞmixt 1⁄4 Xx X * : iRi= xi=Li 1⁄4 XxiRi=

XxiRi=ðLOCÞi

For a 2-component mixture (a,b):

 ðLOCÞmixt: 1⁄4 ðxaRa þ xbRbÞ= xaRa=ðLOCÞa þ xbRb=ðLOCÞb

For binary mixtures of methane (m) and hydrogen (h): Rm 1⁄4 2,
 

Rh 1⁄4 0.5, xh 1⁄4 1 - xm, LOCm 1⁄4 11.1, LOCh 1⁄4 4.6. The result is:
 

LOCm–h 1⁄4 (1.5xm þ 0.5)/(0.0715xm þ 0.109).
 

For binary mixtures of methane and CO: Rco 1⁄4 0.5, xco 1⁄4 1 - xm,
 

LOCco 1⁄4 5.1. The result is: LOCm–co 1⁄4 (1.5xm þ 0.5)/
 

(0.0821xm þ 0.0980).
 

For binary mixtures of methane and ethylene: Ret 1⁄4 3,
 

xet 1⁄4 1 - xm, LOCet 1⁄4 8.6. The result is: LOCm–et 1⁄4 (3 – xm)/
 

(0.349 – 0.169xm).
 

For ternary mixtures of methane with 1:1 CO:H2,
 

xco 1⁄4 xh 1⁄4 (1 - xm)/2. The result is: LOCm–co:h 1⁄4 (1.5xm þ 0.5)/
 

(0.0768xm þ 0.1034).
 

The above equations lead to the calculated LOC results shown in

Table 2. As noted previously, the calculated results for the mixture

of hydrocarbons accurately predict the experimental values. The

results for the unusual fuels, H2 and CO, are below the experimental

values, but barely outside experimental error. In any case, the

calculated results are conservative. It remains to be seen how this

calculation scheme will fit data on oxygenated and other nonhydrocarbon fuels.

6. Conclusions

The primary finding reported here is that the traditional flammability tube values for the LOCs are too high – at least for N2

inerting that was the subject of this study. The differences from the

120-L results are typically less than 1% (e.g. 12 vs. 11.1% for CH4, 11.5

vs. 10.7 for C3H8), but the difference is 1.5% for ethylene (10 vs.

8.5%). These results should be taken into account by the standardsetting organizations – or at least be subject to independent

verification. A start in this direction is currently being made by

standards committees based in the US (NFPA 69, 2008).

This study also generally confirms the results of the flammability tube measurements on the LFLs of these fuels (hydrogen

being exceptional). The excellent agreement between the 120-L

results using a 7% pressure-rise criterion with the reported results

in the 12-L glass sphere which uses a visual criterion supports

the contention that these results are realistic. The issue of the

conflicting American and European flammability standards needs

to be resolved so as to provide a consistent and reliable database for

flammability.

Acknowledgement

This is to acknowledge the important contributions of Kenneth

L. Cashdollar over the years to the flammability and LOC studies in

the 120-L and 20-L chambers, and to his helpful suggestions and

advice. Sad to say, Ken recently passed away and we will no longer

have the benefit of his leadership, humanity, and superlative

experimental and organizational skills. His many contributions to

our organization and the field of explosion prevention will be sorely

missed.

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