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IChemE SYMPOSIUM SERIES NO. 153 # 2007 IChemE

CHARACTERIZATION OF UPPER FLAMMABILITY LIMITS OF METHANE/AIRMIXTURES AT ELEVATED PRESSURES: GAS COMPOSITION MEASUREMENTAND FLOW VISUALIZATION

Jenq-Renn Chen, Ruei-Hsiung Jen, Hsaio-Yun Tsai and Huang-Jen Pan

Department of Safety, Health and Environmental Engineering, National Kaohsiung First University of Science & Technology,

1 University Rd, Yenchau, Kaohsiung, 824, Taiwan; e-mail: [email protected]

In this work, the UFL of methane/air mixtures were measured with the double containment

explosion testing apparatus developed by Chen and Liu (2003). An online gas chromatograph

system is developed and incorporating into the explosion testing apparatus for analyzing gas com-

position before and after ignition. The system comprises of a TCD detector and dual GC columns

that capable of separating and quantifying oxygen, nitrogen, methane, carbon monoxide, and

carbon dioxide from the test mixtures. For methane/air mixtures at a pressure of 0.6 MPa, it is

found that the previously reported cool flame near the upper flammability regime showed small

but non-quantifiable increase in the concentration of carbon monoxide and carbon dioxide. Only

those ignitions with strong pressure increase showed production of carbon monoxide and carbon

dioxide. Direct visualizations of ignition flame were also obtained with a modified test cell with

double-sided plexiglasses and a containment vessel with double-sided glasses. The visualization

found that only non visible flame near the cool flame while inside the UFL the flame generated

from the top of test cell, far away from the igniter, and then propagated downwards throughout

the test cell. Both results suggested that the cool flame in the flammability tests, usually character-

ized by small pressure rises, was a localized combustion around the fusing ignition wire rather than

a partial oxidation of methane. Thus, it is concluded that the cool flame can be excluded from the

definition of upper flammability.

KEYWORDS: flammability limits, methane, pressure, GC, visualization

INTRODUCTIONThe upper flammability limits (UFL) of hydrocarbon atelevated pressure are important operating data for thepartial oxidation of hydrocarbons. The oxidation wasusually operated in the fuel rich regime to avoid the poten-tial deflagration from unconsumed oxygen. The precise UFLis thus crucial for the safe operation of partial oxidation ofhydrocarbons. However, the UFL data at elevated pressureare scarce and direct measurements are needed.

Current measurement of flammability is done bypressure measurement in which flammable is defined byincrease in pressure after ignition. The pressure measure-ment is however affected by the testing configuration andthe spark from the igniter. Many criteria have been proposedfor defining the flammability boundary. For example, the7% and 3 % criteria in which if the pressure ratio, definedby the ratio of pressure after ignition to the initial pressure,is greater than 7% or 3%, respectively. Alternatively, atangent criterion, in which the intersection between thetwo tangent line from flammable pressure rise and zeropressure rise, may be use. None of these criteria produceconsistent results.

An online gas chromatograph system is developedand incorporating into the explosion testing apparatus foranalyzing gas composition before and after ignition. Thesystem comprises of a TCD detector and dual GCcolumns that capable of separating and quantifying

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oxygen, nitrogen, methane, carbon monoxide, and carbondioxide from the test mixtures. Direct visualizations ofignition flame were also obtained with a modified test cellwith double-sided plexiglasses and a containment vesselwith double-sided glasses. Both high speed video andcolor video camera are both used to record the flame.

The present investigation will help to resolve theambiguity in the definition of upper flammability and willalso be useful in developing a realistic physical model forpredicting the pressure dependence on flammability limits.

EXPERIMENTAL SETUP

ONLINE GAS CHROMATOGRAPHY SETUPGas chromatography has been widely used for measuringgas composition prior to explosion tests to confirm theinitial gas composition, e.g. Vanderstraten et al. (1997)and Pfahl (2000). However, it is not yet applied to themeasurement of gas after the explosion tests probably dueto the difficulty in separating and quantifying the oxidant –oxygen, the fuel – such as methane, and the combustionproducts such as carbon monoxide and carbon dioxide.

In this work, the UFL of methane/air were measuredwith the double containment explosion testing apparatusdeveloped by Chen and Liu (2003). An online gas chromato-graph system is developed and incorporating into theexplosion testing apparatus for analyzing gas composition

Figure 1. Sampling and injection for the gas chromatography system. (a) Sample loop fill. (b) Sample loop inject


before and after ignition. The system comprises of aTCD detector and dual GC columns that capable ofsimultaneously separating and quantifying oxygen, nitro-gen, methane, carbon monoxide, and carbon dioxide fromthe test mixtures.

Although it is possible to separate oxygen, nitrogen,from other component with only one column such as aCP-Silica PLOT column, it will require a very low tempera-ture down to 280 8C which is cumbersome as well as costly.Thus, two columns were used. Column 1, the PLOT-Qcolumn, is capable of separating the fuel gas such asmethane and but is unable to separate the air. Column 2,the PLOT-5A which has a durable molecular sieve 5Acoating, is capable to separate nitrogen and oxygen but issuspected to large molecules such as CO2. Connecting thetwo columns in serial will result in plugging of CO2 inthe PLOT-5A column. It is thus necessary to avoid CO2from entering the PLOT-5A column.

A ten port valve is used in directing the gas into twodifferent columns at different time before entering the detec-tor. The connection of the ten port valve is shown inFigure 1. During “Loop fill”, the sampled gas enters thesample loop through port 1.When the loop is filled andstabilized, the valve is then switched to “INJECT” inwhich the sampled gas will be carried into PLOT-Qthrough the Helium carrier gas from port 4. The sampledgas was separated into oxygen/nitrogen mixture, methane,CO, CO2, and other components. The separated gas thenflow through port 6 and port 5 into PLOT-5A column.

Figure 2. (a) Front view of test cell (b) side view of test cell (c)

testing system

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Before CO2 eluting from PLOT-Q column, the valve isswitched back to “LOAD” state such that the CO2 andhigher alkanes enters the detector directly without enteringPLOT-5A column. Thus, only oxygen/nitrogen mixture,methane, and CO entered the PLOT-5A column forfurther separation. The switching is done with a timed-actuator from VICI Valve Instrumentation Co. The gaseseluting from the PLOT-5A column enter PLOT-Q againbefore entering the detector. An analysis cycle is thus com-pleted. The sample loop is ready for another injection. Cer-tainly, if the gases mixture containing hydrocarbon largerthan methane, they may be overlapping with the gasesfrom the PLOT-5A column. However, it is always possibleto avoid overlapping by adjusting the switching time or thecolumn length. The current switching method gives a singlechromatogram which avoid the ambiguity from dual columnlinked in parallel which results in two sets ofchromatograms.

VISUALIZATIONTraditional gas explosion tests were carried out by pressurevessel that is capable to withstand the full overpressurefrom the explosion. Although stainless steel vessel can beeasily made to resist very high pressure by increasing thevessel thickness, the view port requires even thicker glasswhich renders the fabrication difficult and costly. Thus,typical gas explosion testing at elevated pressure such asVanderstraten et al. (1997) used vessel without any view

arrangement of shadowgraph and double contaminant explosion

Figure 3. Chromatogram for methane/air with methane concentration of 18.36% at an initial pressure of 0.602 MPa. Red line: after

ignition. Dark line: before ignition


port and visualization of gas explosion at elevated pressurewas not possible.

In the present work, the direct visualizations ofignition flame were also obtained with a modified test cellwith double-sided plexiglasses and a containment vesselwith double-sided glasses. Figure 2(a) and (b) shows thedesign of the test cell. The test cell is a rectangular vesselwith inner height of 8 cm, length and width are both 4 cm.Total volume of the test cell is 128 cm3. Wall thickness ofthe test cell is 1 cm. Two sides of the cell are open andsealed by plexiglass. The plexiglass has been tested toresist pressure up to 1 MPa. The test cell is placed in a con-taminant vessel that fitted with pressure-resistant glass thatresists pressure up to 10 MPa. The glass view port has alength of 13.5 cm and a width of 1.7 cm. Before theexplosion ignition, the contaminant vessel is pressurizedby nitrogen to the same pressure as the test cell. The con-taminant vessel has an internal volume of 4 L. When theplexiglasses are ruptured by overpressure, the overpressurewill be buffered and reduced by the contaminant vessel bya factor of about 30. As the normal peak overpressurereaches about 10 times of the initial pressure, the presentdouble contaminant system will allow test pressures up to

Figure 4. Chromatogram for methane concentration of (a) 18.50% a

of 0.600 MPa

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7.5 MPa. This is well above the pressures of most oxidationprocesses.

A shadowgraph system is setup in Figure 2(c) tomeasure the flame front during ignition and explosion.The shadowgraph shows the density fluctuation in the gasand can be a good indication of flame front or thermal con-vection wave. A high speed camera with 500 frames/secondis used to record the shadowgraph. In addition, a color videocamera is also used to record the flame directly from theview port. The color video recorded the flame structure,color and intensity at a speed of 30 frames/second.

RESULTS

GAS CONCENTRATION MEASUREMENTFigure 3 shows the chromatogram for methane/air withmethane concentration of 18.36% at an initial pressure of0.602 MPa. There are some significant decreases inmethane and oxygen peaks while carbon dioxide andcarbon oxide peaks are increased significantly. A smallwater peak is also identified. The current TCD detectorhas a poor sensitivity for water. Thus, only CO and CO2

peaks will be used for the measurement. Peak overpressure

t an initial pressure of 0.593 MPa (b) 19.27% at an initial pressure

Figure 5. Summarizes the results of pressure ratio and GC counts for CO and CO2 for methane concentration greater than 18.45%


recorded was 1.918 MPa, giving a pressure rise ratio of 3.18.Thus, this concentration is classified as flammable.

Figure 4(a) shows the chromatogram for methane/airwith methane concentration of 18.5% at an initial pressureof 0.593 MPa. Very minor carbon dioxide and carbonoxide peaks are found but their peak area are below theTCD quantifying detection limits of 0.1% for CO2 and0.5% for CO. Peak overpressure recorded was 0.609 MPa,giving a pressure rise ratio of 1.027. This is close to the tra-ditional 3% criterion for defining flammability. Figure 4(b)shows the chromatogram for methane/air with methaneconcentration of 19.27% at an initial pressure of0.600 MPa. The carbon dioxide and carbon oxide peaksare even smaller than those of 18.5% mixture and are alsobelow the TCD quantifying detection limits. Peak overpres-sure recorded was 0.6044 MPa, giving a pressure rise ratioof 1.007. This is smaller than the traditional 3% criterionfor defining flammability.

Figure 5 summarizes the results of pressure ratio andGC counts for CO and CO2 for methane concentrationgreater than 18.45%. Although all data in Figure 5 liesbelow the 3% criterion, there is a clear connectionbetween pressure rise and CO/CO2 GC counts. The

Figure 6. Sequence of frame captured by high speed camera for me

frame is differed by 30 ms

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pressure rises are always below 1% with methane concen-trations greater than 18.80%. CO and CO2 GC counts arealso proportional lower than those with methane concen-tration below 18.8%. The range of methane concentrationbetween 18.45 � 18.80% corresponds to the cool flameregion according to Vanderstraten et al. (1997). Coolflames are often referred to as the phenomena associatedwith partial or intermediate oxidation reactions. Aldehydes,carbon monoxide and other intermediate products areformed, rather than the water and carbon dioxide producedby normal combustion flames (Pekalski et al., 2002). Thepresent results however found both CO and CO2 even inthese so called cool flame region. It is interesting to visualizethese cool flame in the visualization tests.

FLAME VISUALIZATIONFigure 6 is the sequence of frame captured by high speedcamera for ignition of methane/air mixture with methaneconcentration of 19.05%. Initial pressure is 0.60 MPa.Each frame is differed by 30 ms. Figure 7 is the sequencecaptured by color video camera with each frame differedby 33.3 ms. Apparently, the ignition generated by the

thane/air mixture with methane concentration of 19.05%. Each

Figure 7. Sequence of frame captured by color video camera for methane/air mixture with methane concentration of 19.05%.

Each frame is differed by 33.3 ms

Figure 8. Sequence of frame captured by high speed camera for methane/air mixture with methane concentration of 18.00%.

Each frame is differed by 30 ms

Figure 9. Sequence of frame captured by color video camera for methane/air mixture with methane concentration of 18.00%.

Each frame is differed by 33.3 ms


Nichrome wire produces only a hot plume around the igniterwith no visible flame as seen by the color video. The mush-room type of the plume is a characteristic result of highRaleigh number convective flow driven by very largedensity difference. The recorded pressure transient is flatand shows no clear overpressure which is in consistentwith the visualization results.

Figure 8 is the sequence of frame captured by highspeed camera for ignition of methane/air mixture withmethane concentration of 18.00%. Initial pressure is0.60 MPa. Each frame is differed by 30 ms. Figure 9 is thesequence captured by color video camera with each framediffered by 33.3 ms. Initially, the ignition produced a hotplume and then when the plume reached top of the testcell a visible flame propagate downwards. Figure 10shows the comparison between pressure transient andflame propagation. It is clearly seen that pressure risesonly during flame propagation. Before the flame formedthe pressure rise is insignificant. After the flame propagationstopped, the pressure also stopped to rise and started to fall.It is still not clear when the flame did not generate right inthe fusing wire. Extensive efforts have been made to

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ensure that the gas mixture inside the test cell is perfectlymixed. Yet in most tests with concentration near the upperflammability limit, the flame initiated from top of the testcell when the hot plume reached. Flame initiated directlyfrom the igniter is possible when the methane concentrationis far below the UFL. The visualization results are also dif-fered from the visualization tests by Pfahl et al. (2000) atambient pressure. We also performed tests for methane atambient pressure and produced flame directly from theigniter as seen in Figure 11.

It is interesting to visualize the ignition near the so-called cool flame region which very similar to the non-flam-mable region. Figure 12 shows the pressure transient andflame propagation for methane concentration of 18.22%.The cool flame region in the visualization test cell wasfound to be slightly different from the normal test cell.Namely, 18.17 � 18.33% for the visualization cell verse18.45 � 18.80% for the normal cell. The difference maybe attributed to the cell geometry, namely rectangle versecylinder. Despite the difference, the visualization resultssuggest that the small pressure rise was a localized reactionof methane around the fusing wire and the reaction front or

Figure 10. Comparison between pressure transient and flame propagation for methane concentration of 18.00%

Figure 11. Test results for 13.5% methane at ambient pressure. Each frame is differed by 1/30 second

Figure 12. Comparison between pressure transient and flame propagation for methane/air mixture with methane concentration

of 18.22%


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the flame did not propagate to the test cell wall. Furtherinvestigations are needed in order to resolve the peculiarflame phenomena near the UFL.

CONCLUSIONSDespite the difference in cool flame region, both concen-tration measurement and visualization results suggest thatthe small pressure rise in the cool flame region was a loca-lized reaction of methane around the fusing wire and thereaction front or the flame did not propagate to the testcell wall. The presence of small amount of CO and CO2

also suggest that the cool flame was a localized reactionof methane around the fusing ignition wire rather than thepartial oxidation of methane. Thus, based on the presentvisualization and concentration measurement the coolflame, characterized by small pressure rise, can be excludedfrom the definition of flammability. The present investi-gation also revealed the peculiar phenomena at elevatedpressure in which flame generated far from the igniterrather than directly from the igniter. Such observation willalso be useful in developing a realistic physical model forpredicting the pressure dependence on flammability limits.

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ACKNOWLEDGEMENTSThis work has been supported by the National ScienceCouncil, Taiwan, through grants NSC 92-2214-E-327 -001and NSC 93-2214-E-327 -001.

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3. Pekalski, A. A., J. F. Zevenbergen, H. J. Pasman, S. M.

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