report on experimentally determined self-ignition .... no. 5 sit+idt...safekinex - deliverable 5 -...

Programme “Energy, Environment and Sustainable Development” Project SAFEKINEX: SAFe and Efficient hydrocarbon oxidation processes by KINetics and Explosion eXpertise Contract No. EVG1-CT-2002-00072

Report on experimentally determined self-ignition temperature and

the ignition delay time Deliverable No. 5 (BAM, TUD) Co-ordinating participant: Federal Institute for Materials Research and Testing (BAM)

SAFEKINEX - Deliverable 5 - Experimentally determined self-ignition temperature and the ignition delay time page 2 (74)

Table of Contents:

1 Introduction ......................................................................................................................... 3

2 Terms and Definitions ........................................................................................................ 3

3 Experimental work plan...................................................................................................... 6

4 Experiments in stainless steel vessel, effect of pressure (BAM) ................................... 8

4.1 Test apparatus ............................................................................................................. 8

4.2 Procedure and interpretation of results .................................................................. 11

4.3 Setup of conditions ................................................................................................... 13

4.4 Results ........................................................................................................................ 14

4.5 Discussion of results................................................................................................. 27 4.5.1 Influence of filling rate........................................................................................... 28 4.5.2 Slow oxidation phenomena................................................................................... 31 4.5.3 Explosion phenomena, pressure change ............................................................. 38 4.5.4 Pressure dependency of the ignition temperature ................................................ 40

5 Quartz vessel apparatus, effect of volume (TUD) .......................................................... 41

5.1 Introduction ................................................................................................................ 41 5.1.1 Nomenclature used............................................................................................... 41 5.1.2 Determination of the induction time τmax ............................................................... 42

5.2 Test apparatus and procedure ................................................................................. 44 5.2.1 Apparatus ............................................................................................................. 44 5.2.2 Procedure ............................................................................................................. 46

5.3 Test series .................................................................................................................. 47 5.3.1 Test series in 500 ml vessel ................................................................................. 47 5.3.2 500 ml vessel, with stirring.................................................................................... 49 5.3.3 Quiescent mixture................................................................................................. 51 5.3.4 Tests in 200 ml vessel .......................................................................................... 53 5.3.5 Tests in 100 ml vessel .......................................................................................... 58

6 Comparisons ..................................................................................................................... 61

7 Perspective ........................................................................................................................ 64

8 Literature............................................................................................................................ 66 APPENDIX ................................................................................................................................. 68


1 Introduction The present paper reports the results of SAFEKINEX task WP 2.1 according to the Detailed Project Plan (file dated 3 June 2003), entitled Experimental Determination of the Ignition Delay Time (Self Ignition and Cool Flame Temperature). It focuses on oxidation phenomena (e.g. self-ignition, cool flame) of gaseous hydrocarbon-air mixtures, which are of great interest for safety engineering purposes. Sources of influence such as chemical and physical influences of the test apparatus parameters, the determination procedure applied, the initial temperature, the initial pressure and others are not completely understood yet. Safety and efficiency of industrial proc-ess conditions of partial oxidation reactions suffer from a limited knowledge of safety indices at super ambient conditions (high temperature and pressure). Sub-optimal process costs therefore can be created either by unsafe conditions, which were believed to be safe, or on the other hand by too conservative safety margins. This deliverable report deals with the influence of initial pressure, initial temperature and vessel volume on oxidation phenomena and related characterising indices. Two different test set-ups have been used: a quartz glass vessel based on the standard equipment for determining the auto-ignition temperature [8] and a stainless steel vessel designed to investigate the effect of higher pressures. Induction times and ignition temperatures for methane, ethylene and n-butane at three different fuel gas-air compositions were investigated. Brief explanations for unexpected phenomena observed are presented, although a more extensive treatment of the complex reac-tion patterns and the resultant phenomena will be at a later stage. This will be under the model-ling task WP 3.4, Auto-ignition temperature, while further experimental evidence will be col-lected in task WP 4.2.3, Experiments needed for kinetic model development (Constant Volume Bomb). In a departure from common standards [4] and [8] the method applied for the purpose of this investigation deals with test mixtures of well-defined concentration. 2 Terms and Definitions Most of the terms and definitions used in the present report were already given in SAFEKINEX Deliverable no.2, entitled ‘Report on experimental factors influencing explosion indices determi-nation’ [1]. A review about the detailed definitions of safety related indices is given by the Euro-pean standard EN 1127 “Explosive atmospheres – Explosion prevention and protection, Part 1: Basic concepts and methodology” of 1997 [2]. Furthermore the database CHEMSAFE® [3] in-cludes a definition part for explosion indices. In other standards defined by DIN [29], BSI [24], ISO [30], NFPA [27] and ASTM [8] slightly different nomenclatures were introduced. Comparing existing standards and safety related literature the usage of different terms for the same phe-nomenon and identical terms with different meanings sometimes causes ambiguity. Since common standards usually focus on inhomogeneous mixture cases (leakages) it became nec-essary to further develop nomenclature with respect to oxidation phenomena observed when homogeneous mixtures are investigated. Additionally clear advice for terms and definitions ap-plicable for super-ambient experimental conditions is still missing. The nomenclature used for the purpose of the present report is listed below. Figure 2.1 present a visualisation of the SAFEKINEX classification approach. Combustion phenomena An explosion is defined in EN 1127 [2] as an abrupt oxidation or decomposition reaction pro-ducing an increase in temperature, pressure, or in both simultaneously. Self-Ignition (SI) is an explosion of a test mixture that has been exposed to a temperature that is identical to or above the ignition temperature of the test mixture.


Cool flame (CF) is an (partial) oxidation phenomenon manifested by a pale blue luminescence that might be seen in a dark room after eye accommodation and an increase in temperature. However, both the rate and the increase of measurable parameter (T or P) are far lower com-pared to explosion. Slow combustion (SC) is an (partial) oxidation phenomenon not associated with any visible luminescence, but accompanied by a change in pressure and an increase in temperature. The increase and the rate of increase of a measurable parameter (T or P) are lower compared to the cool flame phenomenon. Slow Oxidation Reactions (SOR) are oxidation phenomena manifested by any increase in temperature or change in pressure far lower than in case of explosion. SOR covers both, slow combustion (SC) and cool flame (CF). The temperature region between RTT (Reaction Thresh-old Temperature, see below) and IT (Ignition Temperature, see below) is called SOR region. Safety characteristics (measurable parameter) The Ignition Temperature (IT) is defined as the lowest temperature of the test vessel wall at which a mixture ignites [1]. The Minimum Ignition Temperature (MIT) is the IT of the most sensitive mixture. Thus MIT is the lowest IT observed when the concentration of the fuel is varied. (see Figure 2.1). The MIT is also named as Auto-Ignition Temperature, Autogenous Temperature, or Self-Ignition-Temperature and is denoted by acronyms like AIT or SIT (mostly by US based organisations e.g. ASTM or NFPA). Cool Flame Temperature (CFT) is defined as the lowest vessel temperature at which a given mixture is capable of generating luminous oxidation reactions (cool flames) [8]. Reaction Threshold Temperature (RTT) is the lowest vessel temperature at which any (lumi-nous or non-luminous) slow oxidation reaction is observed in the test mixture at given concen-tration. Induction time, τ, is defined as the time lag between the completed injection of the test mixture and occurrence of any exothermic phenomena. A detailed explanation about the methods of determination of the induction time from measured data is given in section 5.1.2. According to European prEN 14522 [4] and SAFEKINEX internal Standard Operating Procedure (SOP) [5] a special type of induction time is the Ignition Delay Time (IDT), defined as the time lag between the completed injection of the test mixture and occurrence of ignition.


Fuel concentration

Temperature

IT

CFT

RTT

SI

CFSC

SCMIT

Figure 2.1 Conceptual presentation of definition of terms related to oxidation phenomena with respect to tempera-ture depending on fuel concentration. Basically two areas can be distinguished. Above the top line repre-senting ignition temperatures, IT, is the area of self-ignition, SI. This shows conditions at which a mixture when instantaneously exposed to, will spontaneously self-ignite after a certain (short) induction time (igni-tion delay time) depending on temperature. In a given test set up the lowest temperature at which this happens at a certain fuel concentration, is the ignition temperature. The minimum ignition temperature, MIT, at optimal fuel concentration is indicated in the graph. Bottom dotted line represents the reaction threshold temperature, RTT, which is less sharply defined, since its value rather depends on the sensitiv-ity of the measuring devicesensor. In between is an area of slow oxidation reactions (or partial oxidation phenomena), indicated as slow combustion (SC). (Although in common use this term is somewhat mis-leading since the oxidation is partial and does not lead to the common combustion products). In a certain band of temperature and again after some induction time cool flame (CF) can occur. The lowest tempera-ture, at which as a function of composition cool flame is observed, is the cool flame temperature or CFT. Although in glass equipment cool flame can in principle be visually detected as a pale blue light, in the apparatus as used here cool flame was detected on the basis of a temperature spike of some tens of degrees. In the glass flask open to internal overpressure explosion can be heard and it produces a much higher and steeper temperature peak than cool flame. In steel it can be noticed by both a jump of the temperature and pressure sensor signal output. Slow combustion appears as a prolonged period of in-creasing and later decreasing temperature. The occurrence of cool flame requires accumulation of perox-ides. In an initially quiescent gas this will take place in a warming cloud, which will start rising by natural convection. The phenomenon is therefore affected by conditions of the temperature-time history, gas motion, heat transfer and confinement. So, just below a temperature at which explosion is produced, in stead of cool flame only slow combustion may be seen. For further background information, see [14] and [16].


3 Experimental work plan According to the SAFEKINEX detailed project plan, WP 2.2, the three fuel gases to be investi-gated are methane, ethylene and n-butane in air. For each of them three different fuel gas-air compositions were investigated. The test method applied, in general, covers the determination of certain oxidation phenomena such as cool flame and self-ignition occurring in a vessel that is uniformly heated. The tempera-ture of the test vessel is varied to find the lowest temperature (of the hot surface) and to map different combustion phenomena at various temperature. If a gas is contained in any vessel its molecules inevitably will collide with the vessel walls. Since the vessel surface may have catalytic activity that modifies the test output two different materials were used. Active surfaces may be able to initiate, propagate and/or terminate forma-tion of active species. Especially at higher temperatures, phenomena such as partial oxidation, cool flame, isomerisation, polymerisation and decomposition may be enhanced. However, be-cause the project concerns gas phase reaction kinetics, it is not a goal to investigate the surface activity since high catalytic activity is an undesirable phenomenon. Therefore, results obtained in vessels made from stainless steel (BAM) and quartz glass (TUD) are compared in order to examine the reproducibility of experimental results. The influence of the vessel size was investi-gated using quartz vessels of 100, 200, and 500 ml volume. The larger the vessel the less heat loss is expected to have an influence. Experiments performed followed the SAFEKINEX standard operating procedure (SOP) [5] since

• the European standard had to be modified for our project in order to cover super ambi-ent test conditions

• Number of repetition of tests had to be reduced to obtain a feasible workload. Brief summary of the SAFEKINEX SOP Ignition vessel: cylindrical stainless steel autoclave (or quartz glass flask) Vessel volume >0,2 dm3

Thermocouples: 1 inside the test flask (fine NiCr/NiAl) Temperature interval: <10 K Metering device: pressure transducer or mirror Mixture preparation: partial pressure of compounds; homogeneous Criterion: abrupt temperature and pressure rise Pex/Pi > 1.05 or visible flame Combustion indication: thermocouple located inside the test vessel Waiting time: >10 minutes Table 3.1 Overview concerning the equipment used at TUD and BAM and work scheme. BAM Volume 0,2 dm3

Fuel gas methane, ethylene, n-butane Number of different compositions 3 Initial pressure 1 bar(a) 5 bar(a) 10 bar(a) 30 bar(a) Vessel material stainless steel 1.4122 TUD Volume 0,1 dm3 0,2 dm3 0,5 dm3

Fuel gas n-butane Number of different compositions 1 Initial pressure atmospheric pressure Vessel material quartz


During the experimental investigations some additional experimental investigations were neces-sary to resolve some anomalies. At the work package 2 meeting in Karlsruhe on May 28th 2004, it was decided to reduce the workload concerning methane.


4 Experiments in stainless steel vessel, effect of pressure (BAM) 4.1 Test apparatus The test apparatus of BAM for the determination of IT and IDT at elevated pressures consists of six cylindrical 200-ml autoclaves made of stainless steel (material no. 1.4122). The inner diame-ter is 50 mm at a height of 115 mm. In order to assure an optimal heat distribution the auto-claves were placed inside an aluminium cylinder with an enclosed base. Its wall thickness was 16 mm. Cylinder length was 30 mm more than the autoclaves. An electrical heating sleeve of 1 kW was mounted on the aluminium cylinder. Microcomputer PID controllers were used to con-trol the cylinder temperature measured by PT 100 sensors. To reduce heat losses on this appa-ratus, a thermal insulation made from mineral wool shielded by aluminium foil was used. The equipment is installed in a special protection room suitable for explosion tests (Figure 4.2). Piezo-resistive pressure transducers (Keller, type PA-10, time resolution f>30 kHz) measure the pressure. Pressure transducers suitable for different maximum pressures (1, 10, 20, 50, 200 bar(a)) were used. The temperature of the gas mixture inside the ignition vessel is meas-ured by calibrated 1,5 mm thermocouples type "K" (NiCr/NiAl) class 2 according to DIN-IEC 584 mounted in the middle of the autoclave. Note: Thermocouples have a limited sensitivity and because they have a finite response time they do not provide the real flame temperature. It depends on how fast temperature change occurs. The response time of the thermocouples is the time to reach the 1/e fraction or 63% of the value of an imposed step increase in temperature. The response time is related to the mechanism of heat exchange between the sensor and its surrounding and is therefore a func-tion of several properties of the sensor and its surrounding, e.g. flow state of the fluid, heat ca-pacity of the measured gas, temperature, temperature gradient and others. The response time characteristics of the thermocouples used, was not measured. Analogue thermocouple amplifiers from Greisinger, type GTH 1200A were used. The pressure-time history and the temperature-time history were stored in a computer using a 12 bit A/D con-verter from Keithley, type DAS 1402. The initial temperature (Ti), the initial pressure (Pi) and the highest value of the pressure-time curve (explosion pressure, Pex) are calculated, displayed and stored. The operating initial pressure of the apparatus ranges from vacuum to 50 bar(a). The maximum operating initial temperature is limited to 500 °C. Remote control ball valves are protected by manually operated needle valves (see flow chart diagram, Figure 4.1), which can be adjusted according to the experimental conditions to be in-vestigated.

101TIC

101TIR

PIR201

102TIC

102TIR

202PIR

TIR103

103TIC

PIR203

104

104

TIC

TIR

205PIR

105

105

TIC

TIR

PIR205

106TIC

106TIR

206PIR

PI201

PI202

101TI

autoclave operating board

MV7 MV1 MV2 MV3 MV4 MV5

MV6

MV8

MV9 MV10

compressed air

Gas1

Gas 2

Exhaust

1 2 3 4 5

6

Gas 3

8

7

1-6 : ignition vessels 7 : stirred mixing vessel 8 : vacuum pump

Figure 4.1 Flow chart diagram of Minimum Ignition Temperature apparatus with six 0,2 dm3 stainless steel autoclaves (BAM)


Figure 4.2 Experimental set up of the MIT apparatus

Figure 4.3 Measurement equipment for the MIT apparatus


4.2 Procedure and interpretation of results The test mixture was produced separately in a mixing vessel by partial pressure method. Inside the mixing vessel a stirrer provides homogenisation of the mixture. The prepared mixture is passed into the evacuated (vacuum of better than 10-3 bar(a)) and heated test vessels. Needle valves were used to control the filling velocities so that filling times were between 5 and 30 seconds, depending on the initial pressure to be reached. Observations could be differentiated between four different categories of P-t- and ∆T-t-histories:

A. No reaction occurs, B. Slow oxidation reaction(SOR), C. Explosion occurs some time after the filling was completed, D. Ignition occurs during the filling procedure.

A. No reaction In case that no reaction occurred within the observation time, the shape of the pressure-time-curve and the temperature-time curve were as depicted in Figure 4.4a-b. During the filling pro-cedure, of course, small temperature increase was observed due to compression.

pressure

time

∆T

time

, ,

Figure 4.4a-b P-t- and ∆T-t-histories registered in case of an ignition attempt with no explosion occurred. Typical ∆T observed were in the range of 2 to 4 K due to compression heat produced during the filling. B. Slow oxidation reaction (SOR) Another case to be observed in this investigation, already described by several authors, see e.g. [10], [11] and [12], is the so called cool flame phenomena. Originally the term cool flame was related to the visible pale blue light emitted during slow oxidation reactions taking place. These reactions could be the same reactions that occur before an ignition. Therefore these par-tial oxidation reactions are also called pre-ignition reactions although not every small tempera-ture increase necessarily is accompanied by a visible pale luminescence. In ASTM E659 [8] a distinction is made between ignition, luminous (cool flame) and non-luminous reactions. The ignition temperature cannot be considered as a discontinuity in reactivity but even outside the ignition range transients such as cool flame(s) can occur. Intermediates formed by these partial oxidation reactions may be more reactive than the original substance and radical branch-ing processes may accelerate conversion even at lower temperature.

SAFEKINEX - Deliverable 5 - Experimentally determined self-ignition temperature and the ignition delay time page 12 (74) pressure ∆T

time time Figure 4.4c-d P-t- and ∆T-t-histories registered in case of an ignition attempt in which partial oxidation phenomena, here slow combustion was observed. Typical ∆T observed were in the range of 5 to 50 K. The problem with the term “cool flame” is that in hot closed steel bombs no simple optical ob-servation is possible. Temperature and pressure measurement have to be used to observe such (partial) oxidation phenomena. For identification of SOR the temperature difference ∆T = T(t)-Ti will be used. An example for an ignition attempt leading to slow combustion obser-vation is shown in Figure 4.4c-d. Partial oxidation phenomena can easily start, even during the filling process or some time after the filling is completed. Both occurrences were found (Section 4.4). All of the three fuel gases investigated disclosed slow oxidation phenomena to an extent depending on the type of fuel gas. C. Explosion after completed filling In other cases an explosion occurred after the filling procedure was completed. The P-t- and ∆T-t-curves registered in such cases are shown in Figure 4.5a-b.

IDTmax

time IDT

, ∆T ,

time

pressure

Figure 4.5a-b P-t- and ∆T-t-histories registered in case of an ignition occurring after the filling procedure was completed. The ignition delay time, IDT is marked in the diagram of the P-t-history. Typical ∆T observed were in the range of up to 700 K. There exists a significant experimental error in the determination of the IDT. This error is caused by the filling time. Faster filling of the vessel would lower the time error, but at the same time, due to compression, heat is introduced into the system, thus the error in the initial temperature would increase significantly. Therefore the filling rate was limited to cause a compression heat of not more than 5 K to be measured by the thermocouples when pure air is used instead of fuel gas air mixtures. In most experiments, compression heat observed gave a temperature rise of approximately 2 K.

SAFEKINEX - Deliverable 5 - Experimentally determined self-ignition temperature and the ignition delay time page 13 (74) D. Ignition during filling procedure Experiments at even higher initial temperatures led to a fourth case of experimental observation when ignition took place before the filling procedure was finished. According to the definition given above, in such cases an IDT of zero has to be reported. Due to the time error in this in-vestigation, the zero value does not necessarily represent the chemical nature of the substance. It is well possible that the delay time of the ignition process is shorter than the filling time of the vessel. In Section 4.4 numerous ignition attempts with an IDT -value of zero will be reported. Figure 4.5c-d shows an example of such behaviour.

, ∆T Pressure

time time Figure 4.5c-d P-t- and ∆T-t-histories registered in case of an ignition occurred during filling procedure. Typical ∆T observed were in the range of up to 400 K. 4.3 Setup of conditions One important point to take heed of, was keeping experiments performed on different fuel gases comparable. Therefore the parameter λ, an inverse equivalence ratio, widely applied in engine technology, e.g. for car engines, was used. It is based on the assumption that combustion fol-lows a stoichiometric reaction (see Table 4.1) and basically leads to total oxidation products water and carbon dioxide. Table 4.1 Reaction equation for complete stoichiometric combustion of methane,

ethylene and n-butane Methane CH4 + 2 O2 --> CO2 + 2 H2O

Ethylene CH2=CH2 + 3 O2 --> 2 CO2 + 2 H2O

n-Butane n-C4H10 + 13/2 O2 --> 4 CO2 + 5 H2O The composition of the fuel gas mixture is characterised by a parameter λ which indicates how far the actual mixture composition differs from an ideal stoichiometric mixture with respect to oxygen content of air. The definition of λ is

[ ][ ] tricstoichiome_

mixture_actual_

)fuel(x/)air(x)fuel(x/)air(x

=λ

where x means the mole fraction. Hence fuel rich mixtures have small value of λ. Stoichiometric reactions describe the total conversion of the fuel gas to water and carbon diox-ide. Thereby only the overall reaction path is considered although it is a well known fact that

SAFEKINEX - Deliverable 5 - Experimentally determined self-ignition temperature and the ignition delay time page 14 (74) every oxidation process of hydrocarbons is accomplished by formation of other intermediate and final products like carbon monoxide, aldehydes, ketones, soot and many others. The formation of partial oxidation products inside the reaction zone depends on the reaction conditions, e.g. on the combustion temperature. However this usefully leads to definition of reference composition, in order to obtain comparable experimental conditions for the different fuel gases. In Table 4.2 the fuel gas concentration and corresponding λ-value is given. (Whereas this way of expressing a combustible mixture composition is custom in the automotive community, in the combustion science the equivalence ratio, Φ, is more generally used. The equivalence ratio equals the re-ciprocal of the λ-value. For clarity the values of Φ are also given in the Table 4.2) Table 4.2 Molar fractions of the different fuel gas - air compositions.

λ = 1,00 1,50Φ = 1,00 0,67

stoichiometric fuel lean

Methane 0,0950 0,0654

Ethylene 0,0654 0,0446

n-Butane 0,0313 0,0211

0,1892

0,0972

0,303,33

fuel rich

0,2593

The purity of the fuel gases used for mixture preparation requested by the SOP is correspond-ing to a molar fraction of 99,8% or better. In case of n-butane and methane fuel gases of a lower purity (99,5 %; purity level 2.5 from Messer-Griessheim) were used at BAM. 4.4 Results In the SAFEKINEX project reaction models should be set up to describe the ignition processes by their chemical and physical nature. In order to make experimental values of IDT comparable to computer model prediction, it became necessary to take care of the experimental time error described in Section 4.2. Therefore, an additional definition is set up to characterize this error. In case of explosion, the IDTmax is the time between the beginning of the filling period and the explosion. IDTmax will therefore always be greater than zero. This is also shown in Figure 4.5a. If IDT>0, the difference between IDT and IDTmax is equal to the filling time. The IDT could be seen as a minimum value, IDTmax as a maximal value. As long as a model prediction of the IDTmodel value lies in between the two experimentally determined values,

IDT < IDTmodel < IDTmax then the model is still in agreement with the experiment. This idea gave reason to plot the de-pendency of the IDT as shown in Figure 4.6 to allow an easier further use of this data with re-spect to the modelling task.


300 310 320 3300

20

40

,I

IDTmax

IDT

Ti / °C

DT / s

Figure 4.6 IDT and its corresponding IDTmax at five different temperatures: At the highest two temperatures near 330 °C ignition has taken place before the filling of the vessel had been finished. At the lowest near 300 °C no ignition was observed, thus IDT equals infinity. The following Tables of Figures 4.7 to 4.14 present the experimental results concerning the de-termination of the ignition temperatures and the ignition delay times. Scaling of the diagrams shown below is done according to the temperature range investigated at the single experimental series at fixed initial pressure and mixture composition. One figure table consists of up to 12 small diagrams, numbered from a to l. Numbering runs from left to right within the single rows first, then down the columns. All diagrams of one diagram table belong to the same fuel gas-air mixtures composition. Always three diagrams shown in each of the four rows belong to one series of experimental conditions at constant initial pres-sure. The column left (diagrams number a, d, g, j) provides the temperature dependency of the IDT. The particular IT is obtained from the leftmost IDT bar. The middle column (diagrams num-ber b, e, h, k) shows the corresponding highest temperature differences to the initial tempera-ture observed in single experimental runs. The diagrams in the right column (diagrams number c, f, i, l) give the highest pressures Pex measured during the single experiments. Pressure trans-ducers used are suitable only up to operating temperatures of 100 °C. In order to protect them from being overheated they are connected to the autoclaves by a tube of 10 cm length and ¼ inch outer diameter, approx. 4 mm inner diameter. The pressure dynamics measurement therefore is erroneous, but with all of the experiments performed in the same equipment, the source of error created stays constant. Thus, the additional diagrams of Pex vs. Ti provide impor-tant information, too. From the temperature rise it could be easily identified whether any kind of reactions were observed, or not. There are a lot of tests where no explosion was observed. In such cases the value for IDT is missing while a temperature rise and a highest pressure value (nevertheless formally called explosion pressure Pex; in the order of the initial pressure) are re-ported. Some of the experimental conditions investigated were stopped earlier than originally planned. This is for example the Pi= 30 bar(a) step for n-butane and ethylene’s fuel lean mixtures. The reason for this is that those experiments were already covered by the results shown in the Pi= 10 bar(a) series. If some ignition takes place during filling, the IDT is equal to zero regard-less of filling is stopped when either 10 bar(a) or 30 bar(a) is reached. The missing experiments would have been redundant. Repetitions were performed only as reproducibility tests when they were thought to be necessary. This is the reason why the number of single experiments per-formed, differ from series to series.


Figure 4.7a-f Experimental results for the stoichiometric methane-air mixture at initial pressures of Pi = 10 bar(a) and 30 bar(a) λ = 1,0 IDT / s vs. Ti / °C (Tmax-Ti) / K vs. Ti / °C Pex / bar(a) vs. Ti / °C Pi = 1 bar(a)

Pi = 5 bar(a)

To be measured.

Pi = 10 bar(a)

Pi = 30 bar(a)

430 440 450 460 470 4800

100

,

430 440 450 460 470 4800

100

200

,

430 440 450 460 470 4800

20

40,

420 440 460 4800

100

200

,

420 440 460 4800

200

400

,

e.d.

c.b.a.

,

420 440 460 4800

50

100

150f.


Figure 4.8a-f Experimental results for the fuel rich methane-air mixture at initial pressures of Pi = 10 bar(a) and 30 bar(a) λ = 0,3 IDT / s vs. Ti / °C (Tmax-Ti) / K vs. Ti / °C Pex / bar(a) vs. Ti / °C Pi = 1 bar(a)

Pi = 5 bar(a)

To be measured.

Pi = 10 bar(a)

Pi = 30 bar(a)

420 430 440 450 460 4700

100

200

,

420 430 440 450 460 4700

50

100

150,

420 430 440 450 460 4700

10

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,

360 380 400 420 4400

100

200

,

360 380 400 420 4400

200

400

,

360 380 400 420 4400

50

100

,

f.e.d.

c.a. b.


Figure 4.9a-l Experimental results for the fuel lean ethylene-air mixture at initial pressures of Pi = 1 bar(a), 5 bar(a), 10 bar(a) and 30 bar(a) λ = 1,5 IDT / s vs. Ti / °C (Tmax-Ti) / K vs. Ti / °C Pex / bar(a) vs. Ti / °C Pi = 1 bar(a)

, ,

Pi = 5 bar(a)

Pi = 10 bar(a)

Pi = 30 bar(a)

390 400 410 420 430 4400

20

40

390 400 410 420 430 4400

0.5

1

1.5,

390 400 410 420 430 4400

2

4c.a. b.

300 350 400 4500

20

40

,

300 350 400 4500

50

100

150,

300 350 400 4500

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,

300 350 400 4500

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,

300 350 400 4500

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300 350 400 4500

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,

310 320 330 340 350 3600

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,

310 320 330 340 350 3600

200

400,

310 320 330 340 350 3600

50

100

150,

l.k.j.

i.g. h.

f.e.d.


Figure 4.10a-l Experimental results for the stoichiometric ethylene-air mixture at initial pressures of Pi = 1 bar(a), 5 bar(a), 10 bar(a) and 30 bar(a) λ = 1,0 IDT / s vs. Ti / °C (Tmax-Ti) / K vs. Ti / °C Pex / bar(a) vs. Ti / °C Pi = 1 bar(a)

, ,

Pi = 5 bar(a)

Pi = 10 bar(a)

Pi = 30 bar(a)

300 350 400 4500

20

40

300 350 400 4500

0.5

1

1.5,

300 350 400 4500

2

4c.a. b.

350 400 4500

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40

,

350 400 4500

50

100

150,

350 400 4500

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20

,

300 350 400 4500

50

,

300 350 400 4500

100

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300,

300 350 400 4500

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40

,

310 320 330 340 350 3600

500

,

310 320 330 340 350 3600

200

400

600,

310 320 330 340 350 3600

50

100

150,

l.k.j.

i.h.g.

f.e.d.


Figure 4.11a-l Experimental results for the fuel rich ethylene-air mixture at initial pressures of Pi = 1 bar(a), 5 bar(a), 10 bar(a) and 30 bar(a) λ = 0,3 IDT / s vs. Ti / °C (Tmax-Ti) / K vs. Ti / °C Pex / bar(a) vs. Ti / °C Pi = 1 bar(a)

, , ,

Pi = 5 bar(a)

Pi = 10 bar(a)

Pi = 30 bar(a)

420 440 4600

50

Pi = 1,3 bar(a)

420 440 4600

20

40

420 440 4600

5

10 c.a. b.

300 350 400 4500

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300 350 400 4500

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150,

300 350 400 4500

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300 350 400 4500

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300 350 400 4500

100

200

300,

300 350 400 4500

20

40

,

300 350 400 4500

500

1000,

300 350 400 4500

200

400

600,

300 350 400 4500

50

100

150,

l.k.j.

i.h.g.

f.e.d.

i = 32 bar(a)
P


Figure 4.11m-o Experimental results for the fuel rich ethylene-air mixture at initial pressures of Pi = 2 bar(a) λ = 0,3 IDT / s vs. Ti / °C (Tmax-Ti) / K vs. Ti / °C Pex / bar(a) vs. Ti / °C additional experiments Pi ≈ 2 bar(a)

,

340 360 380 400 420

0

20

40

340 360 380 400 4200

50

100, ,

340 360 380 400 4200

2

4o.m. n.


Figure 4.12a-l Experimental results for the fuel lean n-butane-air mixture at initial pressures of Pi = 1 bar(a), 5 bar(a), 10 bar(a) and 30 bar(a) λ = 1,5 IDT / s vs. Ti / °C (Tmax-Ti) / K vs. Ti / °C Pex / bar(a) vs. Ti / °C Pi = 1 bar(a)

,

Pi = 5 bar(a)

Pi = 10 bar(a)

Pi = 30 bar(a)

300 310 320 3300

20

40

300 310 320 3300

10

20

30,

300 310 320 3300

0.5

1

1.5,

c.a. b.

280 300 320 3400

20

40

,

280 300 320 3400

50

100

150,

280 300 320 3400

10

20

,

300 350 4000

20

40

,

300 350 4000

100

200

,

300 350 4000

50

,

200 220 240 260 2800

100

,

200 220 240 260 2800

200

400

,

200 220 240 260 2800

50

100

150,

l.k.j.

i.g. h.

f.e.d.


Figure 4.13a-l Experimental results for the stoichiometric n-butane-air mixture at initial pressures of Pi = 1 bar(a), 5 bar(a), 10 bar(a) and 30 bar(a) λ = 1,0 IDT / s vs. Ti / °C (Tmax-Ti) / K vs. Ti / °C Pex / bar(a) vs. Ti / °C Pi = 1 bar(a)

,

Pi = 5 bar(a)

Pi = 10 bar(a)

Pi = 30 bar(a)

300 320 3400

20

40

300 320 3400

5

10

15,

300 320 3400

0.5

1

1.5,

c.a. b.

270 280 290 300 3100

20

40

,

270 280 290 300 3100

100

200,

270 280 290 300 3100

10

20

30,

200 300 4000

20

40

,

200 300 4000

100

200

300,

200 300 4000

20

40

60,

200 2500

100

,

200 2500

200

400

600,

200 2500

50

100

150,

l.k.j.

i.h.g.

f.e.d.


Figure 4.14a-i Experimental results for the fuel rich n-butane-air mixture at initial pressures of Pi = 1 bar(a), 5 bar(a) and 10 bar(a) λ = 0,3 IDT / s vs. Ti / °C (Tmax-Ti) / K vs. Ti / °C Pex / bar(a) vs. Ti / °C Pi = 1 bar(a)

,

Pi = 5 bar(a)

Pi = 10 bar(a)

Note: a) of initial pr could not be investigated since at ixture conditions n-butane condenses in the mixture vessel.

300 320 3400

20

40

300 320 3400

20

40

60,

300 320 3400

1

2

3,

c.a. b.

270 280 290 300 310 3200

50

,

270 280 290 300 310 3200

50

100

150,

270 280 290 300 310 3200

10

20

,

300 350 400 4500

50

100

,

300 350 400 4500

100

200

300,

300 350 400 4500

20

40

60,

i.h.g.

f.e.d.

30 bar(
essure these m


The IT of each experimental series can be extracted from the diagrams shown above and are reported in Table 4.3. Figures 4.15 to 4.17 visualize the pressure dependency of ignition tem-peratures obtained. Table 4.3 Ignition temperature for methane, ethylene and n-butane at different

mixture compositions and initial pressures fuel gas λ Pi / bar(a) IT / °C IT / K Methane 0,3 10 430 700

0,3 30 400 675 1,0 10 450 725 1,0 30 415 690

Ethylene 0,3 1,3 470 740 0,3 2 430 705 0,3 5 360 630 0,3 10 315 590 0,3 30 275 550 1,0 5 430 705 1,0 10 385 660 1,0 30 315 590 1,5 5 440 715 1,5 10 395 670 1,5 30 335 610

n-butane 0,3 1 295 570 0,3 5 280 550 0,3 10 270 540 1,0 1 310 585 1,0 5 285 560 1,0 10 285 560 1,0 30 270 545 1,5 1 310 585 1,5 5 285 560 1,5 10 285 560 1,5 30 275 550

For convenience of comparison with other data the last column in Table 4.3 contains the IT-values expressed in K. Temperature values reported here were rounded according to the SAFEKINEX Standard Operating Procedure [5].


0 10 20 30200

300

400

500

600

IT /

°C

P i / bar(a)

λ = 1,0

λ = 0,3

Figure 4.15 Pressure dependency of the ignition temperature of methane-air mixtures on fuel gas concentration.

0 10 20 30200

300

400

500

600

IT /

°C

P i / bar(a)

λ = 1,0

λ = 1,5

λ = 0,3

Figure 4.16 Pressure dependency of the ignition temperature of ethylene-air mixtures on fuel gas concentration.

0 10 20 30200

300

400

500

600

IT /

°C

P i / bar(a)

λ = 1,5 λ = 1,0

λ = 0,3

Figure 4.17 Pressure dependency of the ignition temperature of n-butane-air mixtures on fuel gas concentration.


4.5 Discussion of results Note: As can be seen in the diagram tables above, in the temperature region above the IT the explosion pressure measured may suddenly decrease with further increasing temperature. This is some experimental artefact, since with increasing initial temperatures it becomes more and more difficult to close the valve before ignition occurs. Thus, with the vessel valve left open when ignition takes place during the filling procedure, the pressure measurement of course will lead to much smaller values. Such cases can be identified by comparing the IDT diagrams with the explosion pressure diagrams. With increasing initial temperatures IDT of an experimental series becomes zero (see Section 4.2 D). GENERAL OBSERVATIONS

• With all three fuels IT decreased significantly with increasing pressure. • With higher amounts of fuel gas in the mixture, IT is lower at the same initial pressure. • All of the three fuel gases investigated showed a region with slow oxidation reactions. • The ignition temperatures measured for stoichiometric mixtures (λ=1,0) are close to the ig-

nition temperatures of fuel lean mixtures (λ=1,5). This was observed for all of the three fuel gases investigated.

• When increasing the initial pressure Pi the maximum temperature difference observed dur-ing ignition always increased. Also, above 1 bar(a) pressure rise at the ignition temperature is always at or near maximum. This arises because the rate of heat exchange between the gas phase and the vessel wall is relatively slower than heat production rate during an ex-plosion. For the constant volume apparatus used, higher initial pressure means that the number of molecules is increased and therefore, complete conversion assumed, the amount of energy produced during ignition is higher.

• Self-ignitions near the stoichiometric fuel gas-air compositions produced the most violent explosions in terms of heat production as well as in terms of explosion pressures obtained.

• Regions of slow oxidation reactions become smaller with increasing initial pressure and in-creasing λ -value, hence leaner mixtures.

In Figures 4.7 – 4.14 it can be seen that the explosion pressures recorded at the ignition tem-perature are significantly higher than the initial pressure in case the initial pressure level is ex-tended above atmospheric. Mostly Pex is higher than Pi by a factor two to three. The atmos-pheric pressure record data show only small pressure jumps at IT. In very crude terms would one not expect the explosion pressure to initial pressure ratio to roughly scale with initial density? SPECIFIC OBSERVATIONS In the following paragraph some findings more specific to the substance investigated will be mentioned. There will be some kind of ordering focussing on:

• lowest / highest IT • shortest / most elevated induction times • smallest / highest temperature difference observed • weakest / most violent explosion / reaction • lowest / strongest temperature dependency of the induction times • lowest / strongest pressure dependency • influence of filling rate • smallest / widest SOR range observed

The three fuel gases clearly show different ignition temperature dependencies on the initial pressure, Figure 4.15-4.17. If experiments at fixed initial pressure are compared, the highest IT was determined for methane, the lowest IT was found for n-butane. Normal-butane revealed relatively short ignition delay times, always below 50 seconds. An in-duction time of more than 10 minutes was obtained for ethylene at most elevated initial condi-


tions (Figure 4.11j). Such an induction time value is neither covered by the European norm nor by the SAFEKINEX SOP. Remarkably long delay times before slow oxidation reactions can be seen for methane. Figure 4.19c-d shows such an experiment, just below IT. The reaction starts approximately eight minutes after the completion of the filling. The highest temperature increases were observed for the stoichiometric fuel gas-air mixtures of n-butane (Figure 4.13k) and ethylene (Figure 4.10k). At elevated initial pressures fuel rich mix-tures provided only slightly smaller ∆T while fuel lean compositions led to ∆T much smaller than was the case with stoichiometric mixtures. Experiments at atmospheric initial pressure disclosed the minimum in ∆T at stoichiometric composition for n-butane (compare Figures 4.12b, 4.13b, 4.14b). Ethylene gave the most violent explosions observed in investigations with closed volume bombs at BAM. Methane showed only relatively weak explosions. The widest variety of IDT was obtained at high initial pressures and fuel rich mixtures for both, n-butane and ethylene (Figures 4.11j, 4.14g). Methane is an exception. With respect to the lim-ited data available and the experimental uncertainties only small differences in the behaviour of the stoichiometric mixture compared to the fuel rich composition for methane could be found. Sometimes even a small deviation in initial pressure makes the difference and gives reason for slow oxidation reactions or ignition to be observed. An example for the fuel rich mixture of ethyl-ene-air is shown in Figure 4.22. At an initial pressure of 1 bar(a) no explosion but SOR occurred (Figure 4.22a-b) while with the initial pressure increased to 1,3 bar(a) explosion was observed (Figure 4.22c-d). Similar behaviour is observed at elevated initial pressure conditions (Figure 4.22e-h). n-Butane showed the weakest pressure dependency of IT. Ethylene exhibited a strong pressure dependency of the ignition temperature accompanied by remarkably high ignition temperature differences between fuel rich and fuel lean mixtures (Figure 4.16). Auto-ignition temperature of methane at atmospheric initial pressure should be 595 °C according to the ChemSafe database. Kong et al. [9] reported higher ignition temperatures for methane-air, determined at atmospheric pressure. They performed an investigation of the concentration dependency of IT and found ignition temperatures in the range of 635 to 690 °C. Thus methane, in principle, could be ex-pected to show behaviour similar to ethylene but, with trends shifted to lower initial pressures. In view of further use of the data for the validation of detailed kinetic models the IDT-information for the various cases is presented in an alternative, graphical form in the Appendix. For the fuel n-butane it could also be used for comparing induction times in the steel and glass vessels. This comparison will be made in Section 6. 4.5.1 Influence of filling rate Several conditions were found to produce ignition during the filling procedure indicated by the value of the IDT to be zero (see Figures 4.7 – 4.14). It seemed appropriate to consider the influ-ence of the filling rate. As soon as slow oxidation phenomena were discovered, evidence was available that several reaction pathways to different products could be involved, since in some cases different final pressures could be found for ignition compared to the SOR near the IT. Variation of filling time should result in the following behaviour:

1. With longer filling times ignitions become weaker since more time for heat exchange with the vessel’s wall and more time for slow reactions is available. Extended filling times fi-nally result in a nearly isothermal behaviour during filling. However since ignition is al-ways accomplished by the thermal effect of pre-ignition reactions it will not be possible to suppress pre-ignition reactions.

2. With extended filling times explosions can be completely suppressed, since slow oxida-tion reactions will consume the oxygen available during the filling procedure.

3. With the filling time near zero slow reactions will be suppressed, since there is not enough time left for them. While temperatures during rapid filling jump higher due to compression heat the time available for consumption of oxygen and fuel by slow reac-tions commenced is small. Thus, high concentrations of oxygen and fuel will lead to


spontaneous explosion in case of very short filling times. This will be just one single ex-plosion consuming at least almost all fuel at nearly adiabatic conditions.

4. When reasonably practical filling times are applied, ignition may be observed several times since pre-ignition reactions in pockets of hot gas may be fast enough to ignite the mixture present. With the filling procedure being continued to reach the initially planned pressure, further explosions may take place later on. At a certain filling speed a maxi-mum number of explosions will be found.

Figure 4.18 shows such experiments performed in order to explore the effect of filling time on the observed phenomena. The filling rate was varied between 33 to 580 mbar/s. With the appa-ratus available it was neither possible to adjust filling time to an infinitely small value nor to fill arbitrarily slowly, but the results obtained within the investigated range clearly show the trends predicted before. Multiple ignitions during the filling procedure were observed. Similar behaviour was found at different initial conditions and with all of the three fuel gases investigated. An addi-tional single experiment on ethylene will be shown later (Figure 4.22i-j). Figure 4.21g-j reveals a peculiar phenomenon with ethylene as fuel. When the initial tempera-ture was increased from 315 to 380oC ignition occurred during the filling procedure, but the valve couldn’t be closed in time. Additional fuel gas mixture was thus filled and led to an unique oscillatory pressure behaviour (Figure 4.21i-j). However, these oscillations are unlikely to have a chemical origin. Circumstances did not allow repeating this experiment. It is still unresolved whether this phenomenon could be interpreted as some kind of error or artefact, natural convec-tion or an oscillating reaction.


Figure 4.18a-j P-t and ∆T-t-histories for ethylene-air mixtures (λ = 0,3) at Pi=10 bar(a) and Ti=460 °C recorded by changing the filling rate

Filling rate

P / bar(a) vs. t / s ∆T / K vs. t / s

a. b. •V / mbar/s

580

c. d.

350

e. f.

120

g. h.

70

i. j.

33

0 20 40 600

5

10

15

0 20 40 600

50

100

0 10 20 30 400

5

10

15

0 10 20 30 400

50

100

0 20 40 60 80 1000

5

10

15

0 20 40 60 80 1000

50

100

0 50 100 1500

5

10

15

0 50 100 1500

50

100

0 100 200 300 4000

5

10

15

0 100 200 300 4000

50

100


4.5.2 Slow oxidation phenomena Methane provided the smallest SOR temperature range (RTT always approximately 10 K below IT, see Figures 4.7, 4.8 and 4.19 and compare with IT in Table 4.3). Duration of such reactions was quite extended, lasting for up to 30 minutes. The slow oxidation phenomena of methane-air mixtures (Figure 4.19) were accompanied by only small changes of the pressure, as observed with ethylene and n-butane, too. At high initial temperature (lower initial pressure) the tempera-ture-time history shows the filling procedure significantly superposed by reaction. As soon as no further compression heat is produced, this reaction ceases. After a remarkably long induction period, reaction starts again. In ethylene-air mixtures, Figures 4.20-4.22, first reactions were initiated by compression heat during the filling procedure. In general ethylene showed a different behaviour as compared to methane. For ethylene a wide SOR -region was determined. At an initial pressure of one bar(a) ethylene turned out to show no self ignition below approx. 500 °C, but even slightly higher initial pressure lead to explosion, see Figure 4.11a. This gave reason for additional experiments at an initial pressure of two bar(a), see Figure 4.11m-o, and it turned out that these were the condi-tions leading to an unexpected wide SOR -region. Slow oxidation reactions were observed even 90 K below the IT, see Figure 4.21a. The values of the difference between IT and RTT are re-ported in the Figures 4.19 - 4.23 and collected in Table 4.4. It is obvious that the SOR -region becomes wider when initial pressure is reduced. Lower initial pressures are accompanied by higher ignition temperatures. As a consequence of this, with the SOR -range being shifted to higher temperatures, reaction rates for the “slow reactions” appear higher as well. A maximum of the SOR temperature range was obtained with the stoichiometric mixtures while the lean mix-tures provided the most violent partial oxidation phenomena (see Figure 4.20d) observed in our investigation. Ethylene also showed multiple stage reaction phenomena, as can be identified from Figure 4.20a (2 stages). Sometimes small deviations in the filling procedure caused the valve to close too late and higher initial pressures than originally wanted were reached. Repeti-tion of experiments with slightly lower initial pressure sometimes leads only to pre-ignition reac-tions instead of ignition. Unfortunately, no analysis of partial oxidation products e.g. by gas chromatography was available since this was not included within this project task. Those ex-periments leading to slow reactions were never followed by an ignition. SOR -regions observed with n-butane-air mixtures were wider than with methane but smaller than those of ethylene (see Figure 4.23). Even at elevated conditions n-butane produced weak reactions, but its induction time could also easily exceed the 5 minute criterion (Figure 4.23k) given by future European norms [4]. Therefore it becomes evident that the waiting time in the AIT -standards applied to elevated pressures must be longer compared to the current standards at atmospheric pressure. The experimental findings presented here indicate that partial oxidation phenomena can serve as an unexpected ignition source on industrial scale. Common explosion protection standards focus on the ignition phenomena and the ignition temperature. SORs are not taken into consid-eration. Additionally, observation times are set up too small to cover the delay times of partial oxidation phenomena. This is a safety relevant gap. In Section 7 it will be discussed how to identify this unexpected risk.


Figure 4.19 a-j Partial oxidation phenomena observed for methane-air. Mixture P / bar(a) vs. t / s ∆T / K vs. t / s λ = 1,0 a. Pi = 10 bar(a) b. Ti = 440 °C IT = 450°C IT-RTT>10 K

0 200 400 6000

5

10

0 200 400 6000

5

10

λ = 1,0 c. Pi = 30 bar(a) d. Ti = 410 °C IT = 415°C IT-RTT>10 K

0 500 1000 15000

10

20

30

40

0 500 1000 15000

5

10

λ = 0,3 e. Pi = 10 bar(a) f. Ti = 425 °C IT = 430°C IT-RTT>10 K

0 500 10000

5

10

0 500 10000

5

10

λ = 0,3 g. Pi = 30 bar(a) h. Ti = 390 °C IT = 400°C IT-RTT>10 K

0 500 1000 15000

10

20

30

40

0 500 1000 15000

5

10

15

λ = 0,3 i. Pi = 30 bar(a) j. Ti = 380 °C IT = 400°C IT-RTT>10 K

0 100 200 3000

10

20

30

40

0 100 200 3000

2

4


Figure 4.20a-h Slow oxidation phenomena observed for ethylene-air. Mixture ∆T / K vs. t / s ∆T / K vs. t / s λ = 1,5 a. Ti = 400 °C b. Ti = 435 °C Pi= 5 bar(a) IT = 440°C IT-RTT>40 K

λ = 1,5 c. Ti = 310 °C d. Ti = 330 °C Pi= 30 bar(a) IT = 335°C IT-RTT>25 K

λ = 1,0 e. Ti = 370 °C f. Ti = 425 °C Pi= 5 bar(a) IT = 430°C IT-RTT>60 K

0 100 200 3000

5

10

0 50 100 150 2000

20

40

0 50 100 1500

20

40

0 50 100 150 2000

5

10 2 stages

0 200 400 6000

5

10

0 100 200 3000

20

40

λ = 1,0 g. Ti = 320 °C h. Ti = 380 °C Pi= 10 bar(a) IT = 385°C IT-RTT>65 K

0 100 200 3000

5

10

0 50 100 1500

20

40


Figure 4.21a-j Slow oxidation phenomena and ignitions observed for ethylene-air Mixture ∆T / K vs. t / s ∆T / K vs. t / s λ = 0,3 a. Ti = 340 °C b. Ti = 420 °C Pi= 2 bar(a) IT = 430°C IT-RTT>90 K

0 200 400 600 8000

5

10

0 100 200 3000

5

10


0 500 10000

5

10

0 100 200 3000

20

40


0 500 10000

5

10

0 200 400 6000

20

40

λ = 0,3 g. P / bar(a) vs. t / s h. Ti = 315 °C Pi= 10 bar(a) IT = 315°C IT-RTT>35 K

0 100 200 3000

100

200

300

λ = 0,3 i. P / bar(a) vs. t / s j. Ti = 380 °C Pi= 10 bar(a) IT = 315°C IT-RTT>35 K

0 50 1000

50

100

0 100 200 3000

20

40

0 50 1000

10

20

period ≈ 8,1 s


Figure 4.22a-j Slow oxidation phenomena and ignitions observed for ethylene-air Mixture P / bar(a) vs. t / s ∆T / K vs. t / s λ = 0,3 a. b. Ti = 470 °C Pi= 1 bar(a) IT = n.a. IT-RTT>n.a.

0 100 200 3000

1

2

3

0 100 200 3000

5

10

λ = 0,3 c. d. Ti = 470 °C Pi=1,3 bar(a) IT = n.a. IT-RTT>n.a.

0 50 100 150 2000

1

2

3

0 50 100 150 2000

10

20

30

λ = 0,3 e. f. Ti = 270 °C Pi= 30 bar(a) IT = 275°C IT-RTT> n.a.

0 500 10000

20

40

0 500 10000

20

40

λ = 0,3 g. h. Ti = 270 °C Pi= 32 bar(a) IT = n.a. IT-RTT>n.a.

0 500 10000

50

100

0 500 10000

200

400

prEN 14522

λ = 0,3 i. j. Ti = 390 °C Pi= 30 bar(a) IT = n.a. IT-RTT>n.a.

0 20 40 60 800

20

40

60

0 20 40 60 800

100

200

300


Figure 4.23a-h Slow oxidation phenomena observed for n-butane-air Mixture ∆T / K vs. t / s ∆T / K vs. t / s λ = 1,5 a. Ti = 270 °C b. Ti = 280 °C Pi= 5 bar(a) IT = 285°C IT-RTT>10 K

0 200 400 6000

5

10

0 50 1000

5

10


0 200 400 6000

5

10

0 100 200 3000

5

10


0 200 400 6000

5

10

0 200 400 6000

5

10

λ = 1,0 g. Ti = 270 °C h. Ti = 280 °C Pi= 10 bar(a) IT = 285°C IT-RTT> 10 K

0 200 400 6000

5

10

0 200 400 6000

5

10

λ = 1,0 i. Ti = 250 °C j. Ti = 260 °C Pi= 30 bar(a) IT = 270°C IT-RTT>20 K

0 100 200 300 400 5000

5

10

0 500 1000 15000

5

10


Figure 4.23k Slow oxidation phenomena observed for n-butane-air Mixture ∆T / K vs. t / s λ = 0,3 k. Ti = 265 °C Pi= 10 bar(a) IT = 270°C IT-RTT>n.a.

0 500 10000

5

10

prEN 14522

An overview over the SOR temperature range observed is given in Table 4.4. From the limited experi-ments performed, for some of the experimental conditions investigated (compare ignition temperature values reported in Table 4.3) a RTT is not available (n.a.) with sufficient accuracy, so far. Table 4.4 SOR temperature range for methane, ethylene and n-butane at

different mixture compositions and initial pressures fuel gas λ Pi / bar(a) IT − RTT / K Methane 0,3 10 10

0,3 30 10 1,0 10 10 1,0 30 10

Ethylene 0,3 1,3 n.a. 0,3 2 90 0,3 5 50 0,3 10 35 0,3 30 n.a. 1,0 5 60 1,0 10 65 1,0 30 n.a. 1,5 5 40 1,5 10 n.a. 1,5 30 25

n-butane 0,3 1 n.a. 0,3 5 n.a. 0,3 10 n.a. 1,0 1 n.a. 1,0 5 10 1,0 10 10 1,0 30 20 1,5 1 n.a. 1,5 5 10 1,5 10 10 1,5 30 n.a.


4.5.3 Explosion phenomena, pressure change Selected explosion experiments were recorded over an extended time range so that finally the vessel contents were almost at initial temperature. The aim of this procedure was to obtain in-formation about the change of the number of gaseous molecules from the pressure measure-ment. Several pressure-time history diagrams presented above show the final pressure reached after explosion and thermal equilibration is mostly higher than the initial pressure (e.g. Figures 4.21g, 4.22c and 4.22g). This is to be expected if the number of molecules changes during the course of reaction. Information about final pressure therefore might be useful for modelling pur-poses. The following Table 4.5 gives an overview about selected single experiments and their initial ignition conditions. Additionally, the end pressure calculated from the assumption of ther-modynamic equilibrium is reported, too. This calculation is a first rough estimate obtained from “GasEQ” [7]. Final pressures calculated with the assumption of thermodynamic equilibrium Pfi-

nal,eq. in case of fuel lean and stoichiometric mixtures were always in good agreement with pres-sures measured. In a later stage the calculations have been repeated with the programme Ex-plosion Pressure, developed and described as Deliverable No. 16 of the SAFEKINEX project. The agreement with measured final pressures was on average similar to the GasEQ results, see the column Pfinal,eq. / bar(a) Expl Press in Table 4.5. From the equilibrium calculations it could be concluded that fuel lean and stoichiometric mix-tures should not lead to a significant soot production while the fuel rich mixtures should. Regular inspection of the autoclaves indicated this to be in agreement with experimental observation. Prediction of final pressures only failed for some fuel rich mixtures when a significant deviation between Pfinal,eq and Pfinal was obtained. Possible explanation is that only a fraction of the initial mixture is converted, especially at fuel rich mixtures, where the flame is not able to propagate downwards against the buoyancy force [18]. Such kind of explosion phenomenon was observed in actual high speed observations of explosions induced by exploding wire ignition at elevated experimental conditions. In this case, the fuel gas conversion was incomplete. Deviations be-tween predicted equilibrium final pressures and the experimental observed final pressure be-come smaller the lower the initial pressure of the fuel rich experiment is chosen. The buoyancy force is proportional to density of the medium that is proportional to pressure and inversely to

temperature (for ideal gases PMRTρ = ) Low initial pressure, as already explained above,

leads to lower temperature difference ∆T hence smaller the buoyancy force (e.g. demonstrated in Figures 4.11a-l). Another possible explanation is soot formation and its proper representation in the equilibrium approach.


Table 4.5 List of initial and final conditions of selected experiments above IT Fuel gas λ Ti

/ K Pi

/ bar(a) Tfinal / K

Pfinal / bar(a)

Pfinal,eq. / bar(a)

Pfinal,eq./ bar(a) Expl Press

file name

methane 0,3 705 10,0 705 10,8 13,6 11,6 ME2E0013 0,3 716 10,0 715 11,2 13,6 11,5 ME2E0014 0,3 674 29,1 675 34,0 39,7 33,0 ME3E0003 0,3 684 29,6 685 34,4 40,4 33,7 ME3E0011 0,3 676 31,0 679 36,2 42,4 35,3 ME3E0032 1,0 726 10,0 725 9,95 9,99 10,1 ME2E0002 1,0 754 10,8 747 10,65 10,7 10,8 ME2E0005 1,0 689 30,0 689 29,7 30,0 30,2 ME3E0021 1,0 697 30,8 696 30,5 30,76 31,0 ME3E0023

ethylene 0,3 743 1,3 743 1,5 1,67 1,63 EE0E0019 0,3 708 2,0 704 2,2 2,5 2,5 EE0E0026 0,3 644 5,05 644 6,2 6,25 6,2 EE1E0015 0,3 634 5,2 632 6,2 6,4 6,3 EE1E0011 0,3 592 9,7 593 11,9 12,0 11,7 EE2E0089 0,3 588 10,2 588 12,6 12,6 12,3 EE2E0027 0,3 603 10,3 606 12,9 12,7 12,5 EE2E0030 0,3 548 32,0 543 37,9 39,3 37,9 EE3E0008 1,0 706 4,9 704 4,9 4,9 5,0 EE1E0046 1,0 676 9,7 691 9,95 9,9 10,0 EE2E0047 1,0 589 29,95 590 29,7 30,0 30,4 EE3E0054 1,0 596 30,1 598 29,6 30,2 30,6 EE3E0049 1,5 718 4,7 715 4,7 4,7 4,7 EE1E0060 1,5 672 10,2 669 10,1 10,15 10,18 EE2E0051 1,5 614 30,4 612 30,2 30,3 30,5 EE3E0070

n-butane 0,3 558 5,05 564 6,8 6,9 6,9 BA0E0051 0,3 552 5,1 565 6,75 7,1 7,05 BA0E0050 0,3 542 9,8 546 13,1 13,4 13,2 BA0E0104 0,3 545 10,2 542 13,7 13,8 13,6 BA0E0072 1,0 588 1,10 589 1,11 1,15 1,17 BA0E0022 1,0 562 5,0 589 5,2 5,5 5,5 BA0E0037 1,0 560 9,8 571 10,2 10,46 10,57 BA0E0069 1,0 549 30,2 546 31,2 31,45 31,7 BA3E0009 1,5 588 1,075 588 1,08 1,11 1,11 BA0E0010 1,5 560 4,95 552 5,2 5,04 5,04 BA0E0042 1,5 560 9,7 572 9,95 10,2 10,2 BA0E0059 1,5 552 29,9 548 30,3 30,7 30,7 BA3E0017


4.5.4 Pressure dependency of the ignition temperature The following will be focused on the ignition temperature of ethylene. A first trial will be de-scribed to explain IT dependency on initial pressure and initial temperature. Similar modus op-erandi based on the Semenov theory became common practice for interpolation of AIT as re-cently demonstrated by Caron et. al. [11] and Vandebroeck et. al. [13]. Corresponding to the Semenov thermal explosion theory an explosion will occur when the rate of heat production Q 1 exceeds the rate of heat losses Q 2. & &

Heat loss is assumed to occur to the wall, at fixed temperature Ti, at a rate that is governed by a heat transfer coefficient between the gaseous reactants and the vessel surface. The thermal capacity of the vessel is so high relative to that of the gaseous contents that it acts as an infinite heat sink, so the rate of transport of heat through the wall material is not relevant. On this basis, the rate of heat loss is a linear function of the difference between the gas and wall temperature. The heat production term follows the exponential Arrhenius dependencies and correlates engi-neering factors like volume and concentrations with kinetic parameters like activation energy, pre-exponential factor, reaction order and exothermicy of the overall reaction. In case that the temperature is just high enough to obtain ignition, the point where 1 is slightly higher than Q 2 is just reached, thus Q 1= 2 can be used as a criterion to obtain a mathemati-cal description of the form [6]:

Q&& & Q&

BTA

T

Pln

imi

ni +=⎟

⎟

⎠

⎞

⎜⎜

⎝

⎛ Only a single kinetic equation is used here that represents the overall reaction of the complex and multiple steps oxidation process. Such interpolation tool can be applied as the overall reac-tion step provided the reaction path does not change with changes of temperature, pressure and initial mixture composition. For limited range of changes of the above-mentioned variables such approximation is acceptable. Relation in case of ethylene is presented in Figure 4.24.

11

12

13

14

15

16

1,3 1,4 1,5 1,6 1,7 1,8 1,9

1000 / IT / K-1

ln(P

i)

λ = 1,5

λ = 0,3

λ = 1,0

Figure 4.24 Pressure dependency of IT for ethylene-air mixtures correlated by a Semenov correlation, experimental points according to Table 4.3, lines correspond to a Semenov model fit It has to be pointed out that such a simple single equation, first or second order kinetic expres-sion (formal kinetics) is far from chemical reality. Exponential factors given in the equation above can be chosen arbitrarily and always result in linear plots. With the limited data available in the present report, the Semenov plot can neither be applied to discriminate simple reaction order nor that heat explosion theory can explain SOR phenomena. Therefore, the reaction order of the kinetic expression used here is meaningless. Only the applicability of an Arrhenius type equation to fit our experimental data could be demonstrated


5 Quartz vessel apparatus, effect of volume (TUD) 5.1 Introduction Based on the results obtained at BAM presented in previous chapters and also on results of the present chapter the shortcomings of the international standards (ASTM, EU) become more evi-dent [1]. Presentation of results of TUD experiments hit different but similar difficulties as seen earlier. As denoted before, the ignition criterion seems to be not adequate to describe the com-plexity of combustion phenomena. For example a pressure rise generated by a cool flame may exceed the ignition threshold criterion of 5% pressure rise of the initial pressure (as used in some standards) even though rate of increase of pressure recorded in the CF regions are not of an “abrupt” nature. The same is true for a temperature rise [16]. The required experiment wait-ing times are too short to capture relevant phenomena. It is desirable to develop a renewed terminology in order to describe all main classes of the oxidation phenomena in a more precise and extensive way. This is a complex task because of the large variety of different types of fu-els, reaction conditions and phenomena to be observed and classified in case of oxidations. The nomenclature applied by TUD is slightly different due to other kinds of experimental limita-tions than discussed before. It is therefore explained in the next paragraph even though it is described in more detail elsewhere [16]. 5.1.1 Nomenclature used Depending on several parameters, at low temperatures, several oxidation phenomena have been observed, in a constant volume vessel, like a single cool flame, successive cool flames (up to seven [15]), two-stage ignition, multiple stage ignition [20] and self-ignition. The shapes of the pressure-time histories related to these observations are presented in Figure 5.1.1.

Time [arbitrary unit]

Pres

sure

[arb

itrar

y un

it]

a

b c

d e

f g

Figure 5.1.1. Combustion phenomena measured for iso-butane/oxygen mixtures (1:2) [15]. The shape of the traces designated by a-g is explained in the text and Table 5.1


Table 5.1 Nomenclature of combustion phenomena are illustrated in Figure 5.1.1. Oxidation phenomena Short description Example Slow combustion SC Figure 5.1.1 line b Cool flame CF Figure 5.1.1 line c Multiple cool flames MSC Figure 5.1.1 line a Two-stage ignition or Two-stage process (with intermediate pressure drop)*

2SI or 2SP Figure 5.1.1 line e

Two-stage ignition or Two-stage process (without intermediate pressure drop) *

2SI or 2SP Figure 5.1.1 line d and f

Three-stage ignition or Three-stage process* 3SI or 3 SP Figure 5.1.1 line g * Depending on an ignition criterion that classifies the ‘ignition’ stage as ignition or no-ignition For the pressure traces presented in Figure 5.1.1 a spherical pyrex glass vessel of volume 500 cm3 was used with a mixture of iso-butane and oxygen at a ratio of 1:2. Below 613 K (line a), the pressure rise generated by these multiple cool flames (MCF) is transient, and the pressure falls almost to its initial value after passage of each successive cool flame. Between 613 K and 623 K, the transient pressure pulses are smaller, almost unnoticeable, and are su-perimposed upon continuous pressure increase due to slow combustion (line b). Such behav-iour is designated as slow combustion (SC). Above 623 K, only one cool flame propagates (CF), its build up is much faster (line c) compared to the first case; line a. The so-called two-stage ignition (2SI) is illustrated by lines d, e, and f, with and without intermediate pressure decrease. It is observed above 623 K and initial pressure above ca. 51 kPa. If the pressure or temperature increase is too low to exceed an ignition criterion the phenomena is called two-stage process (2SP). In the first stage of these processes (2SI or 2SP) a cool flame occurs causing accumula-tion of reactive intermediates. Their concentration is high enough to cause a subsequent accel-eration of the reaction that ends with a normal hot ignition with a complete consumption of one of reactants and violent pressure and temperature increase. Additionally, a three-stage ignition (3SI) is observed at high pressure and temperatures starting from 583 K (line g). At tempera-tures higher than 683 K self-ignition occurred resulting in a fast monotonic pressure and tem-perature rise. Such process is named self-ignition (SI). The analysis of combustion phenomena might be performed with respect to pressure [15, 21, 22, 23] or temperature [20, 25, 26] traces of the mixture over time. In deviation from SAFEKINEX SOP [5] a single peak of temperature increase above 200 °C or pressure increase twice of the initial pressure combustion phenomena is classified as ignition, otherwise term cool flame is used. The same criterion is used to discriminate between two-stage ignition and two-stage process. 5.1.2 Determination of the induction time τmax Induction time, τ, is defined as the time lag between the completed injection of the test mixture and occurrence of oxidation phenomena, e.g. explosion. However, this definition is difficult to be applied to the vessels like recommended by EN, DIN or ASTM. This is because the experimental flask is open to the atmosphere and is equipped only with thermocouple(s). The same concerns the apparatus developed at TUD where the test flask is semi-open (closed by the weight of the lid). A pressure sensor was not installed on the test flask. Because of that it is not possible to define the moment when the (rather short) injection process of the sample gas is completed. However the beginning of the injection process is eas-ily observed (decrease of temperature due to injection of cold gas sample). Therefore the induc-tion time determined in this method is the time interval between the beginning of the injection process and occurrence of an exothermic reaction (similar to the definition of IDTmax given in section 4.2 C), denoted as τmax. It includes the experimental time error caused by the filling pro-cedure. Note: An accurate determination of time at which exothermic oxidation phenomena occurs gives practical problems due to complexity of oxidation phenomena and difficulty in accurate determi-


nation of the rate of the gas phase oxidation [28]. It was demonstrated that the rate of tempera-ture or pressure change depends on the thermal mode (e.g. isothermal and non-isothermal oxi-dation) and depends on heat losses of a test vessel. Depending on definition, the rate of the gas phase oxidation, needed as a criterion for determination of the moment of the combustion phe-nomena occurrence, might be determined based on changes in reactants conversion, interme-diate or stable species composition, temperature or pressure. The international standards [4, 8, 29, 30] refer to a visual determination of the spontaneous igni-tion, thus e.g. slow combustion phenomena might be overlooked (due to lack of light emission and open test vessel). The ASTM standard [8], however, in principle allows determination of oxidation phenomena based on temperature changes only. Common determination methods for the induction time, used in research, are defined with respect to pressure and temperature change (see point 5.1.1). The determination methods might be divided into tangential and per-pendicular ones. Both types of methods, with respect to pressure, are presented in Figure 5.1.2.

P

P=P0

τptan timeτp

per

Figure 5.1.2 Determination of the induction time according to tangential and perpendicular methods. From Figure 5.1.2 it can be concluded that the estimated time τp reveals an additional source of uncertainty: the example reaction has already made progress before the calculated time τp. The induction time determined by the perpendicular method (τp

per) is defined as the time from the moment of admission of the reactants into the test vessel to the moment the maximum rate of pressure rise is attained. The superscript per is used for the perpendicular method; the sub-script p is used to indicate the method with respect to pressure. The induction time determined by the tangential method (τp

tan) is defined as the time interval between the moment of admission of a mixture to a test vessel to the onset of the rapid pressure rise. This characteristic time is determined by the intersection of the line of initial pressure with the tangent at the point of maximum rate of pressure rise. The superscript tan is used to indicate use of the tangential method. This method has been used recently [23, 28, 31, 32, 33]. It is a preferred method be-cause the characteristic reaction time determined by the perpendicular method is not seen as a strict chemical time, according to Wilk [31]. Since τp

tan is the time until the beginning of the pres-sure rise, it is less dependent on the physical phenomenon, as the heat loss from the vessel in this stage of the reaction is very small. Thus τp

tan depends more on the overall chemical reaction rate. In a closed vessel pressure and/or temperature signal can be used for determination of the in-duction time, but with the set-up (see point 5.2.1) only temperature is an option. Use was made of the (dT/dt)max point of time. This can still be carried out in two ways: perpendicular or tangen-tial methods. The latter method was used.


5.2 Test apparatus and procedure 5.2.1 Apparatus The apparatus used was the spontaneous ignition apparatus from the explosion group of the TU Delft [14], [16]. This static reactor was designed based on the apparatus used in the ASTM E 659-78 [8] test method for determining spontaneous ignition temperature, but provided with a number of extensions and additions to enable more fundamental work. The basic design an idea of the apparatus originates from both standardised experimental apparatus [8] pre-sented in Figure 5.2.1, and test apparatuses used in the several self-ignition research work. The ASTM test vessel is a 500 ml borosilicate round-bottomed, short-necked open flask. The open end of the flask serves as inlet for solids and liquids. Four thermocouples are used to measure the temperature of the flask. One is mounted in the centre of the vessel, the three others are connected with the outer surface of the flask (at the top, middle and bottom).

Figure 5.2.1. ASTM spontaneous ignition apparatus [8]. The ASTM apparatus design was extended to allow different types of experiments to be con-ducted. All features of the ASTM test apparatus were preserved. The main alterations are:

• Ability to change the experimental flask volume (100, 200 and 500 cm3), • Usage of the covered flask (opened and closed flask experiments), • Possibility of performance of experiments at sub-ambient pressure, • Possibility of stirring the experimental mixture inside the flask at adjustable rotation

speed, • Insertion of two thermocouples inside the flask instead of one, • Adjustable location of the inside and outside placed thermocouples.

The cross sections of the experimental flask and the auto-ignition apparatus are presented in Figure 5.2.2. In the figure quartz window covers the flasks. All glassy parts are manufactured from quartz to enlarge the temperature operation range and durability of these parts.


Figure 5.2.2. Cross-section of the auto-ignition apparatus (left) and experimental flasks (right). The thermocouples inside the flask are of type K with an exposed measuring junction of 330 µm in diameter. A response time of such thermocouple under certain conditions of sudden expo-sure to hot gas (temperature, flow state, etc…) was measured to be about 5·10-2 seconds. The thermocouples located outside the flask are also of type K but with a grounded junction of 1.5 mm in sheath diameter. Their location is presented in Figure 5.2.3. All thermocouple signals are amplified to the range of 0-10 volts and acquired by a data acquisition programme on a computer. The voltage amplifier provides a signal step response (10-90%) in 11.5 ms. Thus for gradually changing signals as are generated during any experiment the amplification frequency is at least 87 Hz. The collected signals are displayed on the computer monitor during an ex-periment.

T5

T4

T1

T2

T3

T5

T4

T1

T2

T3

Figure 5.2.3. Position of the thermocouples in the flask vessel For experiments requiring stirring of the test mixture, a magnet and a magnetic stirrer coated by borosilicate glass was assembled. The magnet is placed just below the experimental flask and a step electrical motor rotates it at adjustable rotation causing rotation of the stirrer. The magnetic stirrer is placed inside the flask and attached to the inside thermocouples. The apparatus can operate up to 750 °C at atmospheric and sub-ambient pressure. The self-ignition temperature of gases, liquids and solids can be measured. The different volumes allow the effect of heat transfer from the gas to the surrounding flask to be investigated. The open end of the flask serves as inlet for samples to be investigated either


in gas or liquid phase. To eliminate hot gas escaping from the flask due to buoyancy force (temperature gradients due to start of pre-ignition reactions), a quartz cover is placed. It allows visual observation of the inside of the flask; however explosion gases can escape. This elimi-nates most of the uncertainty about the mixture composition in the flask. A magnetic stirrer is placed at the bottom of the test apparatus to facilitate mixing of the test sample (especially for liquid samples). A mirror is placed above the quartz window to allow a safe view into the centre of the flask. 5.2.2 Procedure Before the experimental procedure can be started, the experimental flask of the apparatus is brought up to the required temperature and must reach thermal equilibrium. Next, a fuel/oxidiser mixture is prepared in the buffer vessel, kept at room temperature, by the partial pressure method and left undisturbed for about 5 minutes to ensure good mixing. Such additions method allows good mixing of gases even of high density difference [19]. Before the sample is added to the flask its contents is evacuated at least till 0.3 bar(a). Before addition of the sample into the flask the data acquisition programme is started. Data recording lasts during the sample addition and required time interval of the experiment. A pre-mixed cold mixture is injected into the hot flask (valve V9) such that the added volume of the test mixture is at least ten times the volume of the flask. Valve V9 is a ball valve that allows a quick opening and easily reproducible addition of the test mixture to the flask. By standardising the operation the repeatability of the gas injec-tion attains a fine degree of control. After completion of the experiment the flask purged with nitrogen is cleaned with an air gun. Next, depending on the result, the entire procedure is re-peated. A flow scheme of the apparatus is presented in Figure 5.2.2.

Figure 5.2.2. Flow chart diagram of the TU Delft quartz Auto Ignition apparatus


5.3 Test series The combustion phenomena of a 9.5 % n-butane in air mixture (λ ≈ 0.3) were studied. The ef-fects of working in a closed versus an open vessel were investigated separately [17]. Common test standards like [8], [29], [30] deal with open flasks so that products and educts can escape. The experiments in an open flask were performed with a different hydrocarbon mixture (CH4, C2H4), but the results clearly showed that a closed system has to be preferred. Due to absence of the possibility of evacuating the flask before the start of the experiment, a method of exces-sive flushing (min 5 times the volume) was applied [2], [5]. Reproducibility of the experiments has been investigated quite thoroughly. By doing experi-ments at least twice, the reproducibility of the results could be shown. Some typical runs will be presented here to demonstrate this for three completely different types of oxidation phenomena, namely explosion, cool flame, and slow combustion. The results obtained will be shown for dif-ferent vessel volumes and in order of increasing ambient temperature. In the 500 ml vessel the effect of stirring on the gas sample has been investigated. 5.3.1 Test series in 500 ml vessel Figures 5.3.1 and 5.3.2 show the measured temperature profiles at Ta ≈ 605 K in a vessel with-out stirring. This type of profile is classified as multiple cool flame. Temperature jumps are in the range of 5 – 10 K. Both curves have similar shapes, although Figure 5.3.1 shows more oscilla-tions.

585

590

595

600

605

610

615

620

625

630

635

20 30 40 50 60 70 80 90 100

time [s]

Tem

pera

ture

[K]

TwallT4T5

Figure 5.3.1 n-Butane-air temperature-time profile in quartz flask, V = 500 ml, ynb= 0.095, yair= 0.905. Ta = 604 K, P0 = 1atm, exp340C_1.


585

595

605

615

625

635

20 30 40 50 60 70 80 90 100

time [s]

Tem

pera

ture

[K]

TwallT4T5

Figure 5.3.2 n-Butane-air temperature-time profile in quartz flask, V = 500 ml, ynb= 0.095, yair= 0.905. Ta = 607 K, P0 = 1atm, exp340C_2. Figures 5.3.3 and 5.3.4 show the reproducibility of the experiments in the 500 ml vessel at about 150 K higher temperatures, i.e. at 750 K. There is one distinct peak of about 100 K, fol-lowed by a subsequent smaller peak. Pressure relief and a visible flame accompanied the tem-perature peak.

690

710

730

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850

0 50 100 150 200 250 300

time [s]

Tem

pera

ture

[K]

TwallT4T5



720

740

760

780

800

820

0 50 100 150 200 250 300

time [s]

Tem

pera

ture

[K]

TwallT4T5

Figure 5.3.4 n-Butane-air temperature-time profile in quartz flask, V = 500 ml, ynb= 0.095, yair= 0.905. Ta = 747 K, P0 = 1atm, exp480C_1. 5.3.2 500 ml vessel, with stirring Reliable operation of the stirrer although feasible, was not easy. The combustion phenomena observed during the experiments were cool flames (CF), slow combustion (SC) and explosions (EXPL). Sometimes the borderline between oxidation phe-nomena was not sharply defined. An example is presented in Figures 5.3.5 and 5.3.6, where during the first experiment a rapid temperature increase followed slow combustion. In the sec-ond experiment however only a slow combustion was observed.

720

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0 20 40 60 80 100 120 140

time [s]

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pera

ture

[K]

TwallT4T5



700

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760

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800

0 20 40 60 80 100 120 140

time [s]

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pera

ture

[K]

TwallT4T5

Figure 5.3.6 n-Butane-air temperature-time profile in quartz flask, V = 500 ml, ynb= 0.095, yair= 0.905. Ta = 722 K, P0 = 1atm, n-but-0,095_500ml_stirr_450c _vac_4. Figure 5.3.7 illustrates the effect of the changing ambient temperature on the induction times. The open rhombi at Ta=722 K represent the induction times determined for the explosions at this temperature, whereas the other induction time points at the same temperature were more easy to interpret as slow combustion. Below this temperature slow combustion occurred, above this temperature explosions. When one reviews Figure 5.3.7 over the investigated temperature range, it is evident that induc-tion times first decrease with increasing initial temperature and then increase with increasing ambient temperature. Further temperature increase, beyond 730 K causes decrease in the in-duction times. The temperature region where induction times increase is observed is called the Negative Temperature Coefficient (NTC)-region. It is caused by changes in the main oxidation paths.

0

5

10

15

20

550 570 590 610 630 650 670 690 710 730 750 770 790

Ambient Temperature [K]

Igni

tion

Del

ay T

ime

[s]

EXPL.SCCF + SCCFT5

T4

Indu

ctio

n tim

e_m

ax [s

]

Figure 5.3.7 Induction times as function of the ambient temperature Ta, NTC-diagram, V = 500 ml, ynb= 0.095, yair= 0.905, P0 = 1atm, with stirring.


Inside the vessel are two thermocouples. One is placed in the centre (T4), the other at the top (T5). Figure 5.3.8 shows the maximum attained temperature during the experiments, measured with both thermocouples. p

550

600

650

700

750

800

850

900

950

1000

1050

550 570 590 610 630 650 670 690 710 730 750 770 790


Max

imum

Tem

pera

ture

[K]

stirr T4 max

stirr T5 max

Figure 5.3.8 Maximum attained temperature as function of the vessels ambient temperature, V = 500 ml, ynb= 0.095, yair= 0.905, P0 = 1atm, with stirring. Despite the stirring the temperature difference between T4 and T5 becomes larger when tem-perature increase is faster and more significant. The self-ignition therefore apparently takes place with significant inhomogeneity in the gas phase. Therefore it can be concluded that the particular stirrer is not able to homogenize the test mixture. . 5.3.3 Quiescent mixture In Figure 5.3.9 the induction times determined for the experiments in the 500 ml vessel with the stirrer switched off are presented. This graph also shows results from an earlier series of ex-periments without stirring. Because of a very good overlap of the experimental results it can be concluded that presence of non-rotating stirrer inside the vessel does not change the test output thus has no catalytic effect.

0

5

10

15

20

550 570 590 610 630 650 670 690 710 730 750 770 790


Igni

tion

Del

ay T

ime

[s] no stirr [pepijn]

no stirr [patrick]

CF CF + SC SC EXPL.

Indu

ctio

n tim

e_m

ax [s

]

Figure 5.3.9 Induction times as function of the ambient temperature, NTC diagram, V = 500 ml, ynb= 0.095, yair= 0.905, P0 = 1atm, without stirring.


The maximum attained temperature during the experiments is shown in Figure 5.3.10. p

550

600

650

700

750

800

850

900

950

1000

1050

550 570 590 610 630 650 670 690 710 730 750 770 790


Max

imum

Tem

pera

ture

[K]

no stirr T4 max

no stirr T5 max

Figure 5.3.10 Maximum attained temperature as function of the vessels ambient temperature, V = 500 ml, ynb= 0.095, yair= 0.905, P0 = 1atm, without stirring. A comparison between the induction times determined with the 500 ml vessel in the presence and absence of stirring is shown in Figure 5.3.11. One may conclude that stirring has no effect on the output of test thus either its efficiency is too low or within the investigated temperature range enhanced mass transfer has no effect. The slight difference in the values above 10 sec-onds indicates possible changes in the dominating phenomena compared to values with shorter induction times.

0

5

10

15

20

550 570 590 610 630 650 670 690 710 730 750 770 790


no stirr

stirr

Igni

tion

Del

ay T

ime

[s]

Indu

ctio

n tim

e_m

ax [s

]

Figure 5.3.11 Comparison of measured induction times as function of the ambient temperature from experiments with and without stirring, V = 500 ml, ynb= 0.095, yair= 0.905, P0 = 1atm. When we compare the maximum attained temperature during the experiments with and without stirring (Figure 5.3.12), not much difference can be found. The offset between T4 and T5 is lar-ger when a rapid increase of temperature was observed.


p

550

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700

750

800

850

900

950

1000

1050

550 570 590 610 630 650 670 690 710 730 750 770 790


Max

imum

Tem

pera

ture

[K]

stirr T4 maxstirr T5 maxno stirr T4 maxno stirr T5 max

Figure 5.3.12 Maximum attained temperature as function of the vessels ambient temperature from experiments with and without stirring of the mixture, V = 500 ml, ynb= 0.095, yair= 0.905, P0 = 1atm. Another way of data presentation (maximum temperature increase as a function of ambient temperature) is shown in Figure 5.3.13. The transition from the low and intermediate tempera-ture oxidation regions can be seen.

0

25

50

75

100

125

150

175

200

225

250

550 570 590 610 630 650 670 690 710 730 750 770 790


Tem

pera

ture

incr

ease

[deg

]

stirr T4(max) - Tostirr T5(max) - Tono stirr T4(max) - Tono stirr T5(max) - To

Figure 5.3.13 Maximum increase in temperature as function of the vessels ambient temperature, V = 500 ml, ynb= 0.095, yair= 0.905, P0 = 1atm. 5.3.4 Tests in 200 ml vessel For the experiments with the 200 ml test vessel the internal stirring system was removed; the thermocouples were mounted without a cross-link. The outer surface of the flask was wrapped in aluminium foil (according to the ASTM-standard [8]) to improve heat transfer and uniformity of the temperature. Figures 5.3.14, 5.3.15 and 5.3.16 show measured temperature profiles at Ta ≈ 585 K. Three consecutive cool flames were observed. Temperature peaks are in the range of 10–15 K. The obtained profiles have very similar shapes and reproducibility is quite well, although in the third


test temperatures reached are 5–10 K higher. In Figure 5.3.14 some noise in the curve from the wall temperature can be seen. This distortion occurred when the temperature signal was turned on, on the display of the controlling device during data acquisition.

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TwallT4T5


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[K]

TwallT4T5

Figure 5.3.16 n-Butane-air temperature-time profile in quartz flask, V = 200 ml, ynb= 0.095, yair= 0.905. Ta = 584 K, P0 = 1atm, exp316C_5. Figure 5.3.17 and Figure 5.3.18 show the measured temperature profiles at the higher tempera-ture of Ta ≈ 735 K. During the experiments a pressure relief was observed. Based on this obser-vation the phenomenon was classified as explosion. Temperature increased in the range 120–140 K. The curves demonstrate perfect reproducibility at higher temperatures.

730

750

770

790

810

830

850

0 20 40 60 80 100 120 140

time [s]

Tem

pera

ture

[K]

TwallT4T5

Figure 5.3.17 n-Butane-air temperature-time profile in quartz flask, V = 200 ml, ynb= 0.095, yair= 0.905. Ta=736, P0 = 1atm, exp460C_1.


730

750

770

790

810

830

850

0 20 40 60 80 100 120 140

time [s]

Tem

pera

ture

[K]

TwallT4T5

Figure 5.3.18 n-Butane-air temperature-time profile in quartz flask, V = 200 ml, ynb= 0.095, yair= 0.905. Ta=736, P0 = 1atm, exp460C_2. Figures 5.3.19 and 5.3.20 also show reproducible results at still higher temperature (Ta ≈ 797 K). Two distinct peaks could be observed, both with a 50 K lower temperature jump than the previous ones. At this time a flame was visible and was followed by a pressure relief.

770

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[K]

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910

0 10 20 30 40 50 60 70 80 90 100

time [s]

Tem

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[K]

TwallT4T5

Figure 5.3.20 n-Butane-air temperature-time profile in quartz flask, V = 200 ml, ynb= 0.095, yair= 0.905, Ta = 799 K, P0 = 1atm, exp520C. Figure 5.3.21 shows the NTC-diagram constructed from the experiments with the 200 ml vessel, for the 9.5 % n-butane in air mixture. The open rhombi (at Ta=729 K and Ta=740 K) represent experiments were explosions were observed. In the three experiments carried out at a set oven temperature Tset = 460 °C (733 K) and Tset= 470 °C (743 K), only one time an explosion was observed. The other two times slow combustion was found.

0

5

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15

20

25

550 570 590 610 630 650 670 690 710 730 750 770 790


Igni

tion

Del

ay T

ime

[s]

CF CF + SC SC EXPL.

Indu

ctio

n tim

e_m

ax [s

]

Figure 5.3.21 Induction times as function of the ambient temperature, NTC-diagram, V = 200 ml, ynb= 0.095, yair= 0.905, P0 = 1atm, without stirring. Due to the higher heat loss per volume with respect to the 500 ml vessel, the region where slow combustion takes places is broader. The determination of the induction times was difficult in the region from 690 to 720 K, due to the very slow combustion. The temperature increase during reaction is very small in this region, as


can be seen from Figure 5.3.23. The maximum attained temperature during the experiments is shown in Figure 5.3.22. The peak at Ta=730 K relates to the earlier described explosion.

550

600

650

700

750

800

850

900

950

1000

550 570 590 610 630 650 670 690 710 730 750 770 790


Max

imum

Tem

pera

ture

[K]

T4 max T5 max

Figure 5.3.22 Maximum attained temperature as function of the vessels ambient temperature, V = 200 ml, ynb= 0.095, yair= 0.905, P0 = 1atm, without stirring.

0

25

50

75

100

125

150

175

200

225

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550 570 590 610 630 650 670 690 710 730 750 770 790


Tem

pera

ture

incr

ease

[deg

]

T4(max) - To

T5(max) - To

Figure 5.3.23 Measured maximum temperature increase as function of the vessels ambient temperature, V = 200 ml, ynb= 0.095, yair= 0.905, P0 = 1atm, without stirring. 5.3.5 Tests in 100 ml vessel With the internal stirrer removed experiments have also been conducted in a 100 ml vessel. The outer surface of the vessel was wrapped in aluminium foil. Figure 5.3.24 shows the relationship between induction times and ambient temperature. For the experiments performed between 690 and 730 K, no delay times could be determined. The increases in temperature during these ex-periments were negligible. The delay times corresponding to Ta -values between 732 and 772 K were determined manually. The square markers in Figure 5.3.24 correspond to these experi-ments.


The data point at Ta = 813 K (exp-nostirr_550c_vac_1, marked with an open rhombus) repre-sents explosion-like behaviour. Other rapid increases of temperature were seen just starting from Ta = 832 K. This can also be seen in Figure 5.3.24.

0

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35

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50

550 570 590 610 630 650 670 690 710 730 750 770 790 810 830 850 870


Igni

tion

Del

ay T

ime

[s]

EXPL.SCCF + SCCF negligible

Indu

ctio

n tim

e_m

ax [s

]

Figure 5.3.24 Induction times as function of the ambient temperature, NTC-diagram, V = 100 ml, ynb= 0.095, yair= 0.905, P0 = 1atm, without stirring. Figure 5.3.25 and Figure 5.3.26 show respectively the maximum attained temperature and the increase in temperature during the experiments. The increase in temperature value in Figure 5.5.3 is partly below zero. This is due to the fact that the average initial temperature [(T1+T2+T3)/3, three thermocouples located on the outer surface of the flask] is subtracted from T4 and T5. The average temperature can be higher than T4 and/or T5. p

550

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950

550 570 590 610 630 650 670 690 710 730 750 770 790 810 830 850 870


Max

imum

Tem

pera

ture

[K]

T4 max

T5 max

Figure 5.3.25 Maximum attained temperature as function of the vessels ambient temperature, V = 100 ml, ynb= 0.095, yair= 0.905, P0 = 1atm, without stirring.


-20

-10

0

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550 570 590 610 630 650 670 690 710 730 750 770 790 810 830 850 870


Tem

pera

ture

incr

ease

[deg

]

T4 max

T5 max

Figure 5.3.26 Measured maximum temperature increase as function of the vessels ambient temperature, V = 100 ml, ynb= 0.095, yair= 0.905, P0 = 1atm, without stirring.


6 Comparisons The results for the experiments (without stirring) performed in different quartz vessel sizes were compared. Figure 6.1 shows the induction times determined in the vessels of 100, 200 and 500 ml. Distinct differences can be seen at higher temperature, i.e., above 710 K. g y T

0

5

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15

20

25

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550 570 590 610 630 650 670 690 710 730 750 770 790 810 830 850 870


500 ml

200 ml

100 ml

Igni

tion

Del

ay T

ime

[s]

Indu

ctio

n tim

e_m

ax [s

]

Figure 6.1 Induction time versus ambient temperature showing the Negative Temperature Coefficient (NTC)-diagrams for the 100, 200 and 500 ml flasks; ynb= 0.095, yair= 0.905, P0 = 1atm . In the graph the upper open squares at the right hand side between 730 and 770 K are test results with the 100 ml vessel, in which the induction time value had to be determined by hand in stead of by computer. With the low temperature oxidation mechanism active (LTOM), the induction times do not differ much as a function of vessel size. Also at the beginning of the region of negative temperature coefficient (NTC) the induction times are found almost similar for different vessel size. At higher temperatures there is more difference in the induction times. With smaller vessel size, the width of NTC and the amplitude of NTC increase. This can be explained by the fact that with an in-crease in heat loss (smaller vessel size yields a relative increase in heat loss) fast acceleration of the temperature is suppressed. Hence explosion is suppressed. With smaller vessel size the temperature region where slow combustion takes place, is much more extended. With larger vessel size the transition to explosion is faster. With the 100 ml vessel, heat loss compared to heat production is that large that with temperatures from 675 to 725 K no combustion phenom-ena were observed at all. The heat effect of the combustion phenomena that took place was not visible. All data with respect to temperature and vessel size are gathered in Table6.1. Table 6.1 Observed combustion phenomena in series of experiments with 9.5 vol% n-butane in air

at 1 atmosphere in quartz vessels with respect to temperature and vessel volume Vessel size [ml]

Temperature interval [K]

CF CF + SC SC EXPL 100 580 - 645 645 - 675 725 - 825 825 - … 200 570 - 600 600 - 680 680 - 745 745 - … 500 570 - 635 635 - 695 695 - 715 715 - … 500* 580 – 625 625 - 695 695 - 720 720 - …

* with stirring


Besides the comparison of tests done in the quartz vessel there is one test condition at which results can be compared of tests in the stainless steel vessel and in the quartz one. This is the 200 ml test at 1 bar(a) with 0,095 mole % n-butane (λ ≈ 0,3) in air, see Figure 4.14a for the steel and Figures 5.3.21 – 5.3.23 for the quartz vessel. The ignition in the steel vessel is at 568 K according to Table 4.3, IDT is between 20 and 35 s (Fig. 4.14a) the temperature rise is 20 K, the pressure rise less than 0,1 bar. According to the SOP definition of 5% of pressure rise the experiment was classified as explosion and consequently the starting temperature as the ignition temperature. The temperature at which in the 200 ml quartz vessel for the first time a cool flame occurs is 570 K, the IDT 25 s, the temperature rise is in the range of 25-50 K. The agreement is striking. However, the oxidation phenomenon was classified as cool flame. The difference in the tem-perature rise values is easily explained on the basis of the type of the measuring tip of the ther-mocouple and their response time. The thermocouple used at TUD is an open junction type with the measuring tip of 330 micrometers, compared to an insulated thermocouple. The former has shorter response time, thus should record a higher temperature rise. According to the quartz test apparatus explosion takes place at 730 K. In the steel vessel a sig-nificant jump in pressure occurs at 693 K. If one takes into account that the quartz flask is in principle open and the steel vessel tightly closed, it may well be that at this temperature the SOR behaviour changed into an explosion giving again good agreement. What becomes critical out of this comparison is the definition of ignition. What ignition criterion shall be used? In order to answer the question other data were reviewed with respect to the auto-ignition temperature of n-butane air mixture at atmospheric pressure. An assumption here is that that the AIT is not so much affected by a change in composition from the most sensitive mixture to the concentration used in the present research i.e. 9.5% or λ ≈ 0.3 or in other words this composition is in the neighbourhood of the most sensitive concentration. The AIT-values are given in Table 6.2. Table 6.2 AIT values of various sources for n-butane in air at standard pressure. AIT [°C] Organisation/company Source 287 NFPA [27] 287.8 Huntsman [34] >400 Conaco [34] 405 BP Amoco [35] 405 Praxair [34] 420 Air Liquide Canada [35] 430 Chevron Chemicals, Phillips Chemicals [35]

Because of the origin of data the most likely determination methodology is ASTM [8]. According to this standard the experimental flask has a volume of 500 ml. Table 6.2 reveals basically two values of the auto-ignition temperature: 287°C (560 K) and 405°C (678 K). These two tempera-ture values correspond to respectively the cool flame temperature and ignition temperature de-termined in the quarts flask (500 ml), 580 K and 720 K. The two significantly different values of the auto-ignition temperature reveal the ambiguity of the ignition criteria postulated by the standards. More precisely the ignition criterion, the cool flame criterion, and the slow combustion criterion with respect to measurable variables are not defined well. These variables are temperature and/or pressure rise and/or visual observation (excluded for experiments at elevated pressure because of the vessel strength). The question that can be formulated is, how much temperature and/or pressure rise should there be to distinctively clas-


sify various oxidation phenomena and ignition. This question has been already raised previ-ously, e.g. [16] but in connection with experiments on a different scale of equipment dimension. Therefore, with respect to the optical criteria for the quartz flask and the experimental results obtained in the quartz vessel as well as the experimental results obtained in the steel vessels the pressure criterion introduced in Chapter 3 could be modified. Based on the pressure signal and from the comparison of these results it could be suggested to further differentiate between various combustion phenomena, like:

• Ignition, mostly accompanied by an explosion pressure of at least two times the initial pressure,

• Cool flame, accompanied by explosion pressure values in the range of 1.05 to approxi-mately twice the initial pressure with a sudden pressure and temperature increase,

• Slow oxidation reaction (luminous and non-luminous), accompanied by a slight variation of the pressure signal and a slow increase of temperature.

Such classification is also in agreement with the experimental results presented by Pekalski et al. [16]. Caron et. al. [11] arbitrarily set up temperature and pressure based classification crite-ria. In contrast to [11], the criteria presented above

- are based upon comparisons of experiments performed in quartz and steel vessels and - in principle allow a classification of experiments with ∆T below 200 K to be considered

as explosion (in cases where a sudden pressure increase is observed). Note: Temperature based criteria always suffer of lag time effects of temperature sensors.


7 Perspective Because of the comprehensive experimental approach it was possible to demonstrate that the current international standard dedicated for determination of the Auto-ignition temperature at standard pressure cannot be applied without change to elevated pressure conditions (see point 4.5.2 and 5.1). The waiting time for experiments at elevated conditions must be longer than the longest waiting time at standard pressure i.e. 10 minutes. Additionally, on the basis of the results presented, it became possible to suggest an unambigu-ous criterion for distinction between various combustion phenomena like ignition, cool flame, and slow combustion. This interpretation scheme should be further tested and improved or modified in future investigations. A wide range of experimental initial conditions of auto-ignition processes was investigated. Nevertheless, in the SAFEKINEX project only a limited number of experiments could be per-formed. On the basis of the knowledge available concerning the combustion phenomena inves-tigated the following items of interest should be further subject of study:

• Further investigation to estimate more precisely the different temperature and/or pres-sure ranges for various combustion phenomena. One may also compare rate of changes of temperature and /or pressure.

• Cool flame can become an additional source of ignition and it was demonstrated that re-actions might appear far below the ignition temperature. Only a limited number of fuel gases at a limited range of experimental conditions have been investigated. From the point of view of industrial process safety (operations at elevated conditions) it is relevant to investigate this gap.

• The behaviour of fuel gases at even fuel richer concentration as well as fuel gas-oxygen mixtures (pure oxygen or oxygen-inert mixtures instead of air) is still unexplored.

• It would be interesting to discover the catalytic influence of different vessel materials. Vessel material’s influence is not only of practical interest but also essential in kinetic model development. The numerous articles in the field of reaction kinetics presented a wide variety of kinetic models or presented a variety of different kinetic parameter values for different reaction conditions. This implies that kinetic modelling is still a partly empiri-cal approach, especially with respect to heterogeneous reactions. Thus, identifying the change of the rate determining step in a kinetically based comparison of the variation in explosion behaviour due to different catalytic vessel materials should lead to a better understanding of the elementary reaction steps involved. Surprisingly the agreement be-tween the results obtained in the stainless steel and the quartz vessel is good implying that surface effects were negligible. Is that because of sufficient “ageing” of the vessel walls before a series of experiments or is the contact effect in 200 ml indeed small?

• The information about stable intermediate species formed in the SOR -region and in the post reaction mixture would contribute a lot of useful information on the kinetics of the oxidation phenomena. The information could be useful for strengthened the distinguish criterion between various combustion phenomena. This could be done e.g. by applying gas chromatography.

• Much effort has to be put into generating a sufficient amount of reliable experimental data. Only three gaseous hydrocarbons have been investigated, so far, and these con-cerns hydrocarbons. The results disclosed remarkable differences in combustion behav-iour. Other fuel gases are also of industrial interest, such as ammonia, nitrogen oxides,


fluorine or sulphur containing substances like tetra-fluorine-ethylene or hydrogen sul-phide, other hydrocarbons and their derivatives, e.g. ethylene oxide, propylene oxide, vapours (carbonyls, ethers like methyl-tertiary-butyl-ether), should be expected to show even more different behaviour due to their different functional groups. Different oxidizers like N2O or ozone used in the fine chemical industry also are of interest. Such knowl-edge could possibly be used to enlarge combustion understanding, allow process opti-misation and intensification (operation at higher pressure and/or temperature) and avoid possible upsets or even run-away(s) in industrial plants.

• It is clear that oxidation phenomena are complex and require good understanding when test results are scaled up.

• The present report is a first attempt to combine different types of classification criteria for the phenomena observed to get a consistent view on the subject. This is the basis for fu-ture investigations collecting additional knowledge for a further progress in the develop-ment of nomenclature and classification criteria (e.g. based on pressure, temperature, light intensity, etc.) to be applied in safety considerations and decision making.

However, even performing a small part of the workload described above would require a con-sortium of several partners co-operating in order to take this second step forward. On request of our industrial project partner BASF, an investigation on propylene in steel vessels will be performed within the SAFEKINEX project according to task WP 2.5.3, additional experi-ments for (industrial) end-users. This work is ahead of schedule and already in progress. Inves-tigations on propylene will be presented in a separate report later in the project.


8 Literature [1] SAFEKINEX, Deliverable No.2, Report on experimental influences on the determination of explo-

sion indices

[2] EN 1127, Explosive atmospheres – Explosion prevention and protection, Part 1: Basic concepts and methodology

[3] CHEMSAFE, Database of evaluated safety characteristics, Update 2002, DECHEMA, BAM und PTB, Frankfurt/M., Germany

[4] prEN 14522: Determination of the minimum ignition temperature of gases and vapours, document for enquiry (2002)

[5] SAFEKINEX, Standard Operating Procedures for the determination of auto ignition delay times, SOP/IDT

[6] Lewis, van Elbe, Combustion, flames and explosions of gases, Academic Press, New York-London 1961

[7] http://www.gaseq.co.uk.

[8] ASTM E 659 –78 (reapproved 2000), Standard Test Method for Autoignition Temperature of Liq-uid Chemicals

[9] Kong, D., Eckhoff, R. K., Alfert, F.: Autoignition of CH4/air, C3H8/air, CH4/C3H8/air and CH4/CO2/air using a 1 l ignition bomb; Journal of Hazardous Materials 40 (1995) 69-84

[10] A.A. Pekalski., J.F. Zevenbergen, H.J. Pasman, S.M. Lemkowitz, A.E. Dahoe, B. Scarlett The relation of cool flames and auto-ignition phenomena to process safety at elevated pressure and temperature, Journal of Hazardous Materials 93 (2002) 93–105

[11] Caron, M., Goethals, M., DeSmedt, G., Berghmans, J., Vliegen, S., Van’t Oost, E., van den Aarssen, A.: Pressure dependence of auto-ignition temperature of methane/air mixtures; Journal of Hazardous Materials, A65, 1999, 233-244

[12] Berger, R. J., Marin, G. B., Investigation of gas-phase reactions and ignition delay occurring at conditions typical for partial oxidation of methane to syntheses gas, Ind. Eng. Chem. Res. 38, 1999, 2582-2592

[13] Vandebroeck, L., Verplaetsen, F., Berghmans, J., van den Aarssen, A., Winter, H., Vliegen, G., van’t Oost, E., Auto-ignition hazard of mixtures of ammonia, hydrogen, methane and air in a urea plant, Journal of Hazardous Materials, 93, 2002, 123-136

[14] Ten Holder, G. P., The Influence of the Overall Heat Transfer Coefficient on Combustion Phe-nomena of n-Butane/air Mixtures, Internal Report, Explosion Group, Delft University of Technol-ogy, May 2003

[15] Luckett G. A., Pollard R. T., Combustion and Flame, 21 (1973), 265-247

[16] Pekalski, A.A., “Theoretical and experimental study on explosion safety of hydrocarbons at ele-vated conditions”, Ph.D. thesis Technical University of Delft, the Netherlands, November 2004, ISBN 90-9018842-8

[17] Blokland, M., Determination of the Explosion Indices at temperature Range within the Low Tem-perature Oxidation Regime, Internal Report, Explosion Group, Delft University of Technology, Dec 2003

[18] Pekalski A.A., Schildberg H.P., Smallegange P.S.D., Lemkowitz S.M., Zevenbergen J.F., Braithwaite M., Pasman H.J., ‘Determination of the explosion behaviour of methane and propene in air or oxygen at standard and elevated conditions’, Process Safety and Environmental Protec-tion Journal, 2004 or 2005 (in press). Or Pekalski A.A., Schildberg H.P., Smallegange P.S.D., Lemkowitz S.M., Zevenbergen J.F., Braithwaite M., Pasman H.J., ‘Determination of the explosion behaviour of methane and propene in air or oxygen at standard and elevated conditions’ 11th In-ternational Symposium on Loss Prevention in Process Industries, 31 May-3 June 2004, Praha, Czech Republic.


[19] Cashdollar K.L., Zlochower I.A., Green G.M., Thomas R.A., Hertzberg M., Journal of Loss Pre-vention in the Process Industries 13 (2000) 327-340.

[20] Griffiths J.F., Gray B.F., Gray P., 13th International Symposium on Combustion, (1971) 239-247.

[21] Fish A., Angew. Chem. Internat. Edn., 7 (1968) 45.

[22] Barnard J.A., Watts A., 12th International Symposium on Combustion, 1969, 365.

[23] Wilk R.D., Cohen R.S., Cernasky N. P., Ind. Eng. Chem. Res., 34 (1995) 2285-2297.

[24] B.S. 4056 : 1966, Method of test for Ignition Temperature of gases and vapours, British Standards Institution, London (1966)

[25] Knox J.H., Norrish R.G.W., Trans. Faraday Soc., 50 (1954) 928.

[26] Gross M., Diamy A., Ben-Aim R., 12th International Symposium on Combustion, 1967, 1107.

[27] NFPA (National Fire Protection Association), NFPA 325 ‘Guide to hazard properties of flammable liquids, gases, and volatile solids’ 1994 Edition.

[28] Wilk R.D., Cernansky N.P., Cohen R.S., Combust. Sci. and Tech., 52 (1986) 39.

[29] DIN 51794, ‘Prüfung von Mineralölkohlenwasserstoffen – Bestimmung der Zündtemperatur’.

[30] IEC 60079-4, ‘Electrical apparatus for explosive gas atmospheres – Part 4: Method of test for ignition temperature’, 1995.

[31] Wilk R.D., Cernansky N.P., Cohen R.S., Combust. Sci. and Tech., 49 (1986) 41.

[32] Battin-Leclerc et al., Chemical Engineering Science, 55 (2000) 2883.

[33] Warth V., Stef N., Glaude P. A., Battin-Leclerc F., Scacchi G., Come G. M., Combustion and Flame, 114(1-2) (1998) 91-102.

[34] www.msdsonline.com

[35] www.msds.com)

http://www.msdsonline.com/

http://www.msds.com/


APPENDIX Ignition Delay Times, closer consideration for kinetic model validation In Figures 4.7 to 4.14 IDT values measured in the steel test set-up have been plotted. Thorough attention was paid to the difficulty of doing the measurements because of the limitations adher-ing to the filling procedure. Final pressure was reached after a finite time. Speeding up gener-ates too much compression heat. As a measure of uncertainty for the ignition delay time a range has been indicated in the plots rather than a point. In the comparisons Figure App 1 given below for a better overview the range start and end points have been extracted and the values averaged. (This value is called “IDT mean value”. It has no strict physical meaning). The tem-perature dependencies of the ignition delay time at different mixtures compositions have been plotted with the objective to try to facilitate a validation of detailed kinetic models. In Figure App 1 the comparison is made between the rich n-butane air mixtures investigated in the 200 ml quartz vessel and the steel vessel at nominally the same conditions. In the quartz vessel the filling time is also finite but relatively short. The comparison is encouraging, although no abso-lute measure of accuracy can be given for either test set up. In the steel vessel IT was found at 570 K, while the first strong pressure jump occurred at 590 K. In quartz explosions were heard above 730-750 K, so it seems that confinement helps to get explosion earlier.

In Figures App. 2 to 8 the results of the tests in steel at higher pressures are presented. The points are connected by a line only for better visibility.

Figure App. 1 Comparison of what is called induction time_max to the occurrence of a cool flame measured in a 200 ml quartz vessel with 9.5% n-butane in air at 1 bar(a) and the averaged ignition delay time in a steel vessel of the same size with almost the same mixture composition (9.7% n-butane in air.).

Figure App. 2. Ignition De-lay Times as a function of Temperature for two meth-ane-air mixtures, rich and stochiometric respectively 25.9 and 9.5% methane in air or (λ = 1.0 and 0.3) at pressures of 10 bar(a) and 30 bar(a). Note: the de-crease of Ignition Tempera-ture with a rich mixture and a higher pressure.


Figure App. 3. IDT values for 9.7% n-butane in air (λ = 0.3) at three different pressures as a function of temperature. Note the de-crease in ignition tempera-ture with pressure and the lengthening of the ignition delay at the higher pressure. Since the 1 bar(a) line coin-cides with cool flame occur-rence in the (semi-open) quartz vessel, it is likely that it is the confinement of the steel vessel that creates the condition for the develop-ment of an explosion.

Figure App. 4. IDT values for stoichiometric n-butane in air (3.1% or λ = 1.0)

Figure App. 5. IDT values for lean n-butane in air (2.1% or λ = 1.5). It looks as if the actual mixture composition has not a strong influence on the ignition temperature range for the various pres-sures. The lowest tempera-tures are with the rich mix-ture in Figure App.2.


Figure App. 6. IDT values for 18.9% ethylene in air (λ = 0.3) at three different pressures as a function of temperature. At the a pres-sure of 1.3 bar(a) and at 2 bar(a) only one value of the ignition temperature has been tested. Note the de-crease in ignition tempera-ture with pressure and the lengthening of the ignition delay at the higher pressure.

Figure App. 7. IDT values for stoichiometric ethylene in air (6.5% or λ = 1.0)

Figure App. 8. IDT values for lean ethylene in air (4.5% or λ = 1.5). Trends are simi-lar as before. Note the shorter IDT axis in this case.


Table App 1. Data appendant to the IDT versus IT plots in Figures 4.7 - 4.14.

CH4 Ti Ti IDTmax IDT IDTmean

λ = 1.0 / °C / K / s / s / s Pi=10 bar(a) 450 723 95.9 81.4 88.7

460 733 72.9 58.5 65.7 470 743 28.5 13 20.8 480 753 17.7 3.7 10.7


λ = 1.0 / °C / K / s / s / s Pi=30 bar(a) 415 688 195.5 174.9 185.2

420 693 73.9 57.5 65.7 430 703 51.9 35.7 43.8 440 713 59.3 46.1 52.7

450 723 34.4 17 25.7 450 723 29.0 13.2 21.1 460 733 25.3 7.8 16.6 470 743 19.2 1.5 10.4 480 753 14.9 0.0 7.5


λ = 0.3 / °C / K / s / s / s Pi=10 bar(a) 430 703 183.3 168.0 175.7 435 708 174.6 156.8 165.7 440 713 71.8 55.6 63.7 450 723 36.0 15.6 9.3 460 733 23.5 9.2 16.4 470 743 15.9 0.0 8.0


λ = 0.3 / °C / K / s / s / s Pi=30 bar(a) 400 673 132.0 116.0 124.0 400 673 149.8 131.3 140.6 405 678 132.1 115.0 123.6 410 683 122.4 104.0 113.2 420 693 52.9 32.9 42.9 420 693 39.7 24.3 32.0 430 703 28.2 11.7 20.0 440 713 20.8 5.0 12.9 450 723 18.9 4.2 11.6

C4H10 Ti Ti IDTmax IDT IDTmean

λ = 0.3 / °C / K / s / s / s Pi=1 bar(a) 295 568 37.0 21.0 29.0

295 568 36.0 20.0 28.0 300 573 29.0 10.0 19.5 310 583 15.0 5.5 10.3 319 592 10.5 0.5 5.5 330 603 7.0 0.0 3.5

340 613 5.5 0.0 2.8


λ = 0.3 / °C / K / s / s / s Pi=5 bar(a) 280 553 31.0 21.5 26.3

285 558 21.0 11.0 16.0 290 563 16.0 7.0 11.5 295 568 17.5 7.5 12.5 295 568 13.5 5.0 9.3 300 573 11.2 4.5 7.9 300 573 11.7 2.0 6.9 305 578 9.6 2.5 6.1 310 583 8.7 2.0 5.4 315 588 7.2 0.2 3.7 315 588 7.0 1.0 4.0 320 593 6.3 0.0 3.2


λ = 0.3 / °C / K / s / s / s Pi=10 bar(a) 270 543 108.0 97.0 102.5 270 543 97.0 89.0 93.0 275 548 43.0 31.0 37.0 280 553 28.5 16.5 22.5 285 558 20.5 10.0 15.3 290 563 17.0 6.5 11.8 295 568 13.5 4.5 9.0 300 573 12.8 3.0 7.9 305 578 8.6 0.2 4.4 310 583 9.5 1.5 5.5 315 588 6.7 0.0 3.4 320 593 5.6 0.0 2.8 330 603 4.0 0.0 2.0 340 613 3.3 0.0 1.7

350 623 2.3 0.0 1.2 360 633 2.6 0.0 1.3 370 643 2.0 0.0 1.0 380 653 1.9 0.0 1.0

390 663 2.4 0.0 1.2 400 673 2.6 0.0 1.3 410 683 2.2 0.0 1.1 420 693 2.7 0.0 1.4 430 703 2.6 0.0 1.3 440 713 2.3 0.0 1.2 450 723 2.2 0.0 1.1 460 733 2.2 0.0 1.1



λ = 1.0 / °C / K / s / s / s Pi=1 bar(a) 310 583 17.0 7.0 12.0

320 593 11.5 1.5 6.5 330 603 9.0 0.0 4.5 340 613 7.0 0.0 3.5


λ = 1.0 / °C / K / s / s / s Pi=5 bar(a) 285 558 28.2 14.5 21.4

285 558 23.0 9.5 16.3 290 563 19.5 9.5 14.5 290 563 17.0 5.0 11.0 295 568 21.0 13.0 17.0 295 568 16.2 4.2 10.2 300 573 10.5 3.0 6.8 300 573 10.5 4.0 7.3 305 578 10.8 4.8 7.8 310 583 13.0 6.0 9.5 310 583 8.0 1.8 4.9 315 588 6.3 0.0 3.2


λ = 1.0 / °C / K / s / s / s Pi=10 bar(a) 285 558 18.0 9.0 13.5

290 563 17.0 10.0 13.5 290 563 14.0 5.5 9.8 295 568 12.1 4.6 8.4 300 573 11.0 3.5 7.3 300 573 10.1 2.7 6.4 305 578 8.5 1.5 5.0 305 578 8.0 0.8 4.4 310 583 8.0 0.0 4.0 320 593 5.1 0.0 2.6 330 603 3.6 0.0 1.8 340 613 3.2 0.0 1.6 360 633 2.4 0.0 1.2 410 683 3.3 0.0 1.7 460 733 3.3 0.0 1.7


λ = 1.0 / °C / K / s / s / s Pi=30 bar(a) 270 543 107.0 87.0 97.0

285 558 38.5 13.0 25.8


λ = 1.5 / °C / K / s / s / s Pi=1 bar(a) 310 583 16.0 5.0 10.5

320 593 10.5 1.0 5.8 329 602 8.5 0.0 4.3 330 603 7.0 0.0 3.5


λ = 1.5 / °C / K / s / s / s Pi=5 bar(a) 285 558 27.0 18.5 22.8

285 558 28.0 19.0 23.5 290 563 19.0 10.5 14.8 290 563 16.5 8.5 12.3 295 568 17.5 12.0 14.8 300 573 7.0 0.0 3.5 310 583 5.0 0.0 2.5 319 592 4.0 0.0 2.0 330 603 3.0 0.0 1.5 340 613 4.0 0.0 2.0


λ = 1.5 / °C / K / s / s / s Pi=10 bar(a) 285 558 22.0 14.0 18.0 290 563 23.0 16.0 19.5 290 563 15.0 7.0 11.0 290 563 15.0 8.0 11.5 295 568 13.0 5.5 9.3 295 568 11.8 5.0 8.4 295 568 11.0 3.5 7.3 300 573 13.5 5.0 9.3 300 573 10.5 2.0 6.3 300 573 10.5 3.3 6.9 305 578 9.2 1.4 5.3 310 583 7.9 0.0 4.0 315 588 5.5 0.0 2.8 320 593 4.6 0.0 2.3 330 603 3.6 0.0 1.8 340 613 3.2 0.0 1.6 360 633 2.4 0.0 1.2 410 683 3.9 0.0 2.0


λ = 1.5 / °C / K / s / s / s Pi=30 bar(a) 275 548 125.0 105.0 115.0 280 553 34.8 12.3 23.6 285 558 20.0 0.0 10.0



λ = 0.3 / °C / K / s / s / s Pi=1.3 bar(a) 470 743 20.2 12.0 16.1


λ = 0.3 / °C / K / s / s / s Pi=2 bar(a) 430 703 42.0 33.5 37.8

430 703 34.0 24.0 29.0


λ = 0.3 / °C / K / s / s / s Pi=5 bar(a) 360 633 39.7 29.0 34.4

360 633 26.4 15.0 20.7 365 638 43.8 33.0 38.4 365 638 39.8 30.0 34.9 370 643 24.8 14.0 19.4

375 648 20.5 11.0 15.8 380 653 21.7 11.7 16.7 380 653 17.5 6.5 12.0 385 658 17.1 8.0 12.6 390 663 14.7 4.3 9.5 395 668 14.3 4.0 9.2 400 673 12.0 1.5 6.8 405 678 12.5 3.0 7.8 405 678 11.5 1.5 6.5 410 683 12.9 4.2 8.6 410 683 10.8 0.0 5.4 410 683 10.2 0.0 5.1 415 688 9.2 0.0 4.6

420 693 9.1 0.0 4.6 440 713 7.3 0.0 3.7

470 743 5.7 0.0 2.9


λ = 0.3 / °C / K / s / s / s Pi=10 bar(a) 315 588 125.0 106.0 115.5

320 593 177.5 18.0 97.8 320 593 155.0 34.0 94.5 320 593 88.0 68.5 78.3 325 598 67.2 50.7 59.0 330 603 57.3 40.0 48.7 335 608 41.9 23.2 32.6 335 608 46.3 27.6 37.0 340 613 30.7 11.0 20.9 345 618 24.8 9.0 16.9 350 623 22.0 6.0 14.0 355 628 19.5 2.7 11.1 360 633 17.9 0.0 9.0 365 638 16.5 0.0 8.3 380 653 14.1 0.0 7.1 385 658 12.2 0.0 6.1 400 673 11.8 0.0 5.9


λ = 1.0 / °C / K / s / s / s Pi=5 bar(a) 430 703 37.0 27.5 32.3

435 708 24.4 16.4 20.4 435 708 23.6 15.0 19.3 440 713 19.4 11.5 15.5 445 718 15.1 7.0 11.1 450 723 11.1 2.1 6.6 450 723 15.5 6.0 10.8 450 723 14.5 6.5 10.5

455 728 11.7 2.5 7.1 460 733 12.4 3.2 7.8 460 733 12.3 1.8 7.1 470 743 10.4 0.4 5.4


λ = 1.0 / °C / K / s / s / s Pi=10 bar(a) 385 658 55.7 38.9 47.3

385 658 54.7 36.0 45.4 385 658 52.0 34.0 43.0 390 663 58.1 40.3 49.2 395 668 39.3 20.8 30.1 400 673 30.0 12.0 21.0 405 678 26.0 8.0 17.0 410 683 24.4 5.6 15.0 410 683 21.3 5.0 13.2 415 688 19.9 2.7 11.3 420 693 17.8 1.5 9.7 425 698 15.5 0.0 7.8 440 713 13.2 0.0 6.6 450 723 13.5 0.0 6.8


λ = 1.0 / °C / K / s / s / s Pi=30 bar(a) 315 588 600 590 595

320 593 185.6 162.4 174.0 330 603 50.3 30.3 40.3 330 603 44.0 22.5 33.3 340 613 40.4 18.0 29.2 350 623 29.1 5.0 17.1 360 633 20.8 0.0 10.4



λ = 0.3 / °C / K / s / s / s Pi=32 bar(a) 272 545 777.5 743.8 759.7 Pi=30 bar(a) 275 548 477.0 448.5 462.8

280 553 290.0 255.8 272.9 280 553 250.0 220.0 235.0 285 558 247.0 219.0 233.0 290 563 196.8 124.0 160.4 295 568 78.5 49.5 64.0 300 573 58.2 28.7 43.5 305 578 48.8 21.5 35.2 310 583 40.6 15.2 27.9 315 588 35.0 9.8 22.4 320 593 46.0 16.8 31.4 325 598 35.9 6.9 21.4 330 603 24.3 0.0 12.2 340 613 23.4 0.0 11.7 400 673 9.3 0.0 4.7 440 713 4.1 0.0 2.1 480 753 2.6 0.0 1.3


λ = 1.5 / °C / K / s / s / s Pi=5 bar(a) 440 713 40.5 32.0 36.3

440 713 34.0 25.0 29.5 445 718 33.0 24.0 28.5 450 723 20.7 12.2 16.5 460 733 15.3 6.3 10.8


λ = 1.5 / °C / K / s / s / s Pi=10 bar(a) 395 668 63.0 43.6 53.3

395 668 56.4 38.5 47.5 395 668 53.7 35.7 44.7 400 673 49.0 30.5 39.8 400 673 50.4 34.8 42.6 400 673 46.7 26.7 36.7 400 673 42.0 27.0 34.5 405 678 54.9 39.0 47.0 405 678 53 36 44.5 405 678 40.5 21.7 31.1 410 683 44.5 28.6 36.6 415 688 41.5 24.5 33.0 420 693 30.3 14.1 22.2 420 693 28.2 9.4 18.8 425 698 21.8 4.8 13.3 430 703 21.1 3.6 12.4 435 708 17.7 0.0 8.9 440 713 15.9 0.0 8.0 450 723 16.6 0.0 8.3


λ = 1.5 / °C / K / s / s / s Pi=30 bar(a) 335 608 57.0 35.5 46.3

335 608 40.0 21.7 30.9 340 613 81.3 62.0 71.7 340 613 49.8 29.3 39.6 340 613 50.7 32.7 41.7 350 623 35.8 20.0 27.9 360 633 23.9 8.6 16.3

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report on experimentally determined self-ignition .... no. 5 sit+idt...safekinex - deliverable 5 -...

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