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Title: Explosion behavior of CH4/O2/N2/CO2 and H2/O2/N2/CO2 mixtures

Article Type: Full Length Article

Keywords: oxy-fuel, explosion, effect of CO2, burning velocity

Corresponding Author: Dr. Almerinda Di Benedetto,

Corresponding Author's Institution:

First Author: Almerinda Di Benedetto

Order of Authors: Almerinda Di Benedetto; Francesco Cammarota; Valeria Di Sarli, PhD; Gennaro

Russo; Ernesto Salzano

Abstract: In this work, the explosion parameters (peak pressure, maximum rate of pressure rise,

burning velocity) for CH4/O2/N2/CO2 and H2/O2/N2/CO2 mixtures are experimentally evaluated in

a closed cylindrical vessel. Measurements are carried out with the O2/fuel ratio set to the

stoichiometric value. The molar percentage of CO2 in the mixture and the molar O2/(O2+N2) ratio

(i.e., the level of oxygen-enrichment) are varied in the (0 - 60 %) and (0.21 - 0.8) ranges,

respectively. All tests start from ambient pressure and temperature.

The PREMIX module of the CHEMKIN package, coupled to the GRI-Mech detailed reaction

scheme, is also used to calculate the un-stretched laminar burning velocity (Sl). For each level of

oxygen enrichment, simulations are performed increasing the CO2 content from zero up to the

value which gives Sl = 50 mm/s. The regions of flame extinction are then identified as a function of

the CO2 content.

Through ad hoc simulations, it is shown that the main role of CO2 is not kinetic, but rather thermal

in reducing the adiabatic flame temperature and thus the combustion rate.

Napoli, February 2, 2009

Dear Editor,

Please receive the manuscript:

Explosion behavior of CH4/O2/N2/CO2 and H2/O2/N2/CO2 mixtures

by Di Benedetto A., Di Sarli V., Salzano E., Cammarota F. and Russo G.

to be submitted for publication to the Int. Journal of hydrogen energy.

I thank you for your kind attention.

Sincerely yours,

Almerinda Di Benedetto

Cover Letter

Napoli, February 2, 2009

Dear Editor,

Please receive our suggestion for Reviewers:

Isabelle Sochet, ProfessorLaboratoire Energétique Explosions Structures, Ecole Nationale Supérieure dIngénieurs de Bourges (ENSIB)isabelle.sochet@ensi-bourges.fr

Hans J. Pasman ProfessorChemical Risk Management, Delft Universityhjpasman@gmail.com

Arief Dahoe, LecturerBuilt Environment Research InstituteUniversity of Ulsterae.dahoe@ulster.ac.uk

Sincerely yours,

Almerinda Di Benedetto

* Suggested Reviewers

1

Explosion behavior of CH4/O2/N2/CO2 and H2/O2/N2/CO2 mixtures

Di Benedetto A.1,, Di Sarli V.1, Salzano E.1, Cammarota F.1 and Russo G.2

1 Istituto di Ricerche sulla Combustione, CNR, Via Diocleziano 328, 80124, Napoli, Italy

2 Dipartimento di Ingegneria Chimica, Università degli Studi di Napoli Federico II, Piazzale

Tecchio 80, 80125, Napoli, Italy

Abstract

In this work, the explosion parameters (peak pressure, maximum rate of pressure rise, burning

velocity) for CH4/O2/N2/CO2 and H2/O2/N2/CO2 mixtures are experimentally evaluated in a closed

cylindrical vessel. Measurements are carried out with the O2/fuel ratio set to the stoichiometric

value. The molar percentage of CO2 in the mixture and the molar O2/(O2+N2) ratio (i.e., the level of

oxygen-enrichment) are varied in the (0 - 60 %) and (0.21 - 0.8) ranges, respectively. All tests start

from ambient pressure and temperature.

The PREMIX module of the CHEMKIN package, coupled to the GRI-Mech detailed reaction

scheme, is also used to calculate the un-stretched laminar burning velocity (Sl). For each level of

oxygen enrichment, simulations are performed increasing the CO2 content from zero up to the value

which gives Sl = 50 mm/s. The regions of flame extinction are then identified as a function of the

CO2 content.

Through ad hoc simulations, it is shown that the main role of CO2 is not kinetic, but rather thermal

in reducing the adiabatic flame temperature and thus the combustion rate.

Keywords: oxy-fuel, explosion, effect of CO2, burning velocity

Corresponding author. Phone: +39 081 7682947; fax: +39 081 7622915.

E-mail address: dibenede@irc.na.cnr.it (A. Di Benedetto).

* ManuscriptClick here to view linked References

2

1. Introduction

In order to reduce CO2 emissions, its capture from power plants and the subsequent sequestration in

geological storages has been proposed (Abraham et al., 1982; Payne et al., 1989; Ulizar et al.,

1997). The cost connected to the capture operation decreases on increasing the CO2 content in the

stream. To increase this content, the fuel mixture is previously mixed with recycled fuel gas.

Furthermore, if highly oxygen-enriched air (up to pure oxygen) is used, it is possible to reduce the

nitrogen concentration, thus favoring the CO2 separation process and also lowering NOx emissions.

When pure oxygen is used, the technology is termed “oxy-fuel combustion”. This is a zero-emission

technology, given that the combustion products contain only carbon dioxide and water vapor,

enabling the complete capture of the CO2 produced.

The use of oxygen-enriched air poses safety issues, since pure oxygen increases the laminar burning

velocity and enlarges the flammability range with respect to air (Scott et al., 1957). However, the

increment of CO2 concentration decreases the laminar burning velocity, reduces the flammability

range and the flame temperature of several hydrocarbons in air (Arpentinier et al., 2001).

Arpentinier et al. (2001) have compared the burning velocity of stoichiometric mixtures of methane

in air to those of stoichiometric methane in oxygen with CO2 and Ar in the same proportion as N2 in

air. They have shown that the burning velocity is correlated to the adiabatic flame temperature,

which is lower when using CO2 rather than Ar and/or N2. The effect of CO2 on the burning velocity

and flammability limits has been considered mainly as a thermal effect.

When using high CO2 concentrations coupled to high levels of oxygen (not as in air), the key issue

it to understand what the effect of CO2 is and to clarify whether CO2 may play a kinetic role.

Recently, Cuoci et al. (2008) have shown that, under the typical conditions of oxy-combustion, CO2

may participate to the kinetic mechanism reacting with H radicals according to the step

H+CO2 OH+CO.

3

The aim of the present paper is at studying the combined effects of CO2 and O2 on the explosion

parameters (peak pressure, maximum rate of pressure rise, burning velocity) of CH4/O2/N2/CO2 and

H2/O2/N2/CO2 mixtures.

To this end, measurements are carried out in a closed cylindrical vessel. The O2/fuel ratio is set to

the stoichiometric value, while the CO2 molar content in the mixture and the level of oxygen-

enrichment are varied in wide ranges.

The PREMIX module of the CHEMKIN package, coupled to the GRI-Mech detailed reaction

scheme, is also used to calculate the un-stretched laminar burning velocity. Through ad hoc

simulations, the kinetic role of CO2 on the combustion rate is studied separately from the thermal

role.

2. Experimental Set-Up

The experimental set-up, shown in Figure 1, was a cylindrical closed vessel (5 lt). The mixture

composition was obtained by the partial pressure method. The O2/fuel ratio was set to the

stoichiometric value for both CH4/O2/N2/CO2 and H2/O2/N2/CO2 mixtures. The molar percentage of

CO2 in the mixture and the molar ratio E=O2/(O2+N2) (i.e., the level of oxygen-enrichment) were

varied in the (0 - 60 %) and (0.21 - 0.8) ranges, respectively.

A neon electrical transformer Resinblock IP44, 220 - 230 V and 50 Hz with 3 kV and 25 mA

discharge current, was used for spark ignition. The spark gap was 1 mm.

For all tests, the initial pressure was P = 1 bar and the initial temperature was set to 300 K. Each run

was performed three times and the average value was taken.

The laminar burning velocity was obtained from the time pressure records of explosions occurring

in the closed vessel (Lewis & Von Elbe, 1934). In particular, we used the equations of Dahoe et al.

(1996) that link the flame radius, rf, and the laminar burning velocity, Sl, to the pressure time

history.

The flame radius was calculated from:

4

1/31/3 1/

max

max

31

4f

P PV Pr

P P P

(1)

The laminar burning velocity was calculated according to:

2/31/3 1/ 1/

max

max max

1 1 43 1

3 3l

P PP P dPS

P P V P P P P dt

(2)

In order to get correct measurements, Sl has to be calculated at flame radius greater than 6 mm

(Huang et al., 2006), to avoid the effects associated to the ignition energy, and lower than a critical

value at which the flame starts to feel the influence of walls and vessel shape. We adopted the

criterion proposed by Dahoe (2005), by calculating Sl in the range rf = 20 25 mm, thus ensuring

the stage of flame propagation in which ignition and walls play a negligible role.

As shown by Dahoe (2005), the values of Sl calculated by means of this methodology do not

significantly differ from the values reported in the literature and measured with more advanced

techniques, such as photographs of the propagating flame.

3. Model Description

The calculation of the laminar burning velocities for CH4/O2/N2/CO2 and H2/O2/N2/CO2 mixtures

was carried out by means of simulations of the one-dimensional, planar, adiabatic, steady, un-

stretched, laminar flame propagation.

The Sandia PREMIX module (Kee et al., 1985) of the CHEMKIN package was used coupled to the

detailed GRI-Mech (version 3.1) reaction scheme (Bowman et al., 1999).

5

The code, which adopts a hybrid time-integration/Newton-iteration technique to solve the steady-

state mass, species, and energy conservation equations, was set up to simulate a freely propagating

flame with mixture-averaged formulas.

The initial pressure and temperature were set to 1 atm and 300 K, respectively.

The adopted type of formulation requires an additional boundary condition for the mass flow rate

that was assigned by fixing the flame location and, in particular, the point at which the flame

temperature reaches the value equal to 400 K. To start the iteration, the temperature profile

estimation obtained by VanMaaren et al. (1994) for stoichiometric methane/air flame was adopted,

as suggested by Uykur et al. (2001). The temperature profile resulting from the first simulation step

was used for the next step.

In the computations, the windward differencing on both convective and diffusion terms was used.

The solutions were obtained with gradient and curvature values of 0.2. The total length of the

calculation domain, starting 2 cm upstream of the reaction zone, was chosen equal to 12 cm. Further

increases in mesh resolution and domain-size resulted in less than 1 cm/s difference in the

calculated flame speeds.

6

4. Results and Discussion

In the following, the experimental results are first presented. Successively, the model results for the

laminar burning velocity are compared to experimental data. With the validated model, ad hoc

simulations are then run to study the kinetic and thermal roles of CO2 on the combustion rate.

4.1 Experimental Results

In Tables 1 and 2, the mixture compositions used for the experimental runs are given.

The pressure histories as obtained for the CH4/O2/N2/CO2 and H2/O2/N2/CO2 mixtures are plotted in

Figures 2 and 3, respectively.

As shown in Figure 2, the pressure trends of CH4/O2/N2/CO2 mixtures strongly differ. Depending

on the contents of O2 and CO2, they exhibit only one pressure peak or two pressure peaks.

When the reactivity is relatively low, two peaks are found whose presence has to be addressed to

the coupling of the cylindrical shape of the reactor and the heat losses. Initially, the flame

propagation is spherical. However, when the flame approaches the cylinder walls, it preferentially

proceeds along the axial direction. When the flame is relatively slow in approaching the cylinder

walls, the heat losses become predominant with respect to the heat production rate and the pressure

decreases. When the axial propagation predominates, the flame propagation becomes faster, the

heat production rate overcomes the heat losses, and the pressure increases again.

In the case of H2/O2/N2/CO2 mixtures (Figure 3), only one pressure peak is found suggesting that

the heat losses are not relevant with respect to the heat production rate: hydrogen is indeed much

more reactive than methane.

In Tables 1 and 2, the results in terms of maximum pressure (Pmax) and its standard deviation ((P)),

burning velocity (Sl), maximum rate of pressure rise (dP/dtmax) and corresponding values of

adiabatic temperature (Tad) and pressure (Pad) are presented for the CH4/O2/N2/CO2 and

H2/O2/N2/CO2 mixtures, respectively. In these tables, stable flame refers to the case of ignited

7

mixture and stable flame propagation in the vessel. In Table 1, no ignition indicates a case in which

no flame propagation is observed for 3 consecutive attempts.

As a general trend, it appears that, on increasing the adiabatic temperature, the mixture reactivity

increases for both methane (Table 1) and hydrogen (Table 2).

This trend demonstrates the relevant role of CO2 in affecting the flame temperature and thus the

laminar burning velocity.

From the pressure time histories, we calculated the laminar burning velocity as described in Section

2. In Tables 1 and 2, the values obtained are reported. These values are also plotted in Figure 4 and

will be discussed later.

4.2. Model Results

In Figure 4, the calculated laminar burning velocity for methane (Fig. 4A) and hydrogen (Fig. 4B)

is plotted versus the CO2 molar fraction at different values of the oxygen-enrichment factor (E).

On increasing the CO2 content, the laminar burning velocity decreases down to very low values

which suggest that extinction occurs.

On increasing the oxygen content (i.e., on increasing E), higher values of the laminar burning

velocity are attained. In Figure 4, experimental data coming from the literature (Lewis and von

Elbe, 1987; Kim et al., 2002; Quiao et al., 2005) and from our experiments are also reported. A

good agreement between numerical and experimental results is found.

From the results of Figure 4, it is possible to find out the value of CO2 concentration at extinction

(CO2,ext). Following Westbrook (1983), we may assume as CO2,ext the value at which Sl = 50 mm/s.

These values of CO2,ext are given in Table 3.

In Figure 5, the CO2 content at extinction is plotted as a function of the oxygen-enrichment factor

(E) for both methane (5A) and hydrogen (5B) flames. This graph allows identifying the zones of

extinguished flame and stable flame, enabling the evaluation of the safe compositions. The stable

zone is larger in the case of hydrogen than in the case of methane. However, it is found that, when

8

oxy-flames are studied (E = 1), flame extinction occurs at the same values of the CO2 content

( ≈ 70%) for both methane and hydrogen.

4.3. The Role of CO2

We have shown that the model is able to describe, with a reasonable accuracy, the experimental

trend and values of the laminar burning velocity also in conditions of oxygen-enrichment for both

methane and hydrogen.

We then use the model to investigate and quantify the role of CO2 in affecting the flame

propagation.

4.3.1. Effect of CO2 on Kinetics

In order to understand whether the nature of the influence of CO2 on the flame reactivity is kinetic,

we performed calculations of the un-stretched laminar burning velocity (Sl) by suppressing the

following steps of the reaction mechanism in which CO2 is involved as reactant:

O+CO(+M) CO2 (+M) (1)

O+HCO H+CO2 (2)

O+CH2CO CH2+CO2 (3)

O2+CO O+CO2 (4)

OH+CO H+CO2 (5)

HO2+CO OH+CO2 (6)

9

CH + CO2 HCO + CO (7)

In Figure 6, the calculated laminar burning velocities are plotted as a function of the CO2 content at

different values of the oxygen enrichment for both methane (top) and hydrogen (bottom) as

obtained with the full mechanism and by artificially removing the kinetic steps 1-7.

It is worth noting that, when the kinetic role of CO2 is neglected, the laminar burning velocity for

both CH4 and H2 mixtures increases.

From a sensitivity analysis, we found that, among the CO2-involving kinetic steps (1-7), step 5 is

the controlling one.

The inhibiting role of CO2 can be then addressed to the consumption of H radicals through step 5.

The effect of CO2 is stronger in the case of methane combustion than in the case of hydrogen

combustion. Furthermore, in the case of CH4 combustion, the extinction values increase if CO2

plays a kinetic role.

In Table 4, the extinction values of CO2 are given as calculated by neglecting the CO2 kinetic role

and then steps 1-7. By comparing these data (Table 4) with those obtained by including all steps (1-

7) (Table 3), it results that no significant differences apply in the case of hydrogen, while in the case

of methane the kinetic role of CO2 is more pronounced.

4.3.2. Effect of CO2 on Thermodynamic Properties

The dilution of the reactive mixture with CO2 causes a decrease of the adiabatic temperature.

In Figure 7, the values of the adiabatic temperature (at constant pressure) are plotted versus the CO2

content for both methane and hydrogen, at different values of the air enrichment, E. At each value

of the enrichment value, a significant decrease of the adiabatic temperature is observed. In the case

of the oxy-fuel, a decrease of the adiabatic temperature from 3000 K up to 1800 K is observed when

adding 70 % of CO2.

10

If we also compare the adiabatic temperature as obtained for the oxy-fuel at CO2 = 20 % with the

value at CO2 = 0 for air mixture (E = 0.21), we may see the effect of N2 substitution with CO2, in

the air proportions: replacing CO2 with N2 decreases the adiabatic temperature.

For all the data of Figure 7 obtained by varying the oxygen content from air to an enrichment factor

E = 0.8, and CO2 content from 0 up to the extinction value, we plotted the obtained laminar burning

velocity as function of the adiabatic flame temperature, as shown in Figure 8.

All the data almost overlap and are mainly correlated by the flame temperature, whatever the CO2

content and the enrichment factor (E). This result confirms that the main role of CO2 is thermal in

affecting the heat capacity of the mixture, then the flame temperature and eventually the reaction

rate.

4.3.3. Prediction of the Effect of CO2 on the Burning Velocity

At the conditions investigated here, the role of CO2 in affecting the laminar burning velocity is

mainly thermal. We then calculated the burning velocity as function of temperature and

composition from the formula (Lewis von Elbe, 1987):

2/1

ul 12S

(8)

where is the air to fuel mass stoichiometric ratio, u is the un-burnt mixture density, is the mass

production rate, and is the thermal diffusivity given by:

PC

(9)

The thermal conductivity is assumed constant ( = 0.02 W/m K), while the density and specific heat

are updated as function of temperature and composition.

11

We adopted a single step reaction rate for the combustion of methane and hydrogen:

On

Fmo CC)RT/Eaexp(k (10)

In Table 6, the values of the kinetic parameters used for each fuel are given.

The kinetic parameters of methane were taken by Westbrook and Dryer (1981), while two set of

kinetic parameters were used for hydrogen (Marinov et al., 1995 and Mitami & Williams, 1980).

The calculations were performed at average temperature and concentration values in the flame

reaction zone.

In Figure 9, the theoretical values of the burning velocity are plotted versus the adiabatic

temperature at varying the CO2 content and the enrichment factor. It appears that the Sl formula of

Equation 8 is able to reproduce the trend both qualitatively and quantitatively.

In the case of H2, we found that, only implementing the kinetic parameters of Marinov et al. (1995),

we obtained good predictions for the laminar burning velocity, Sl.

5. Conclusions

The effect of CO2 addition to CH4/O2/N2 and H2/O2/N2 mixtures has been studied at stoichiometric

conditions and at different levels of oxygen-enrichment.

The presence of CO2 has been shown to significantly affect the laminar burning velocity up to flame

extinction.

At each value of the air enrichment factor (oxygen content), the CO2 value of extinction has been

calculated. From these data, the regions of extinguished and stable flame have been identified.

The simulations have shown that the main role of CO2 is to modify the heat capacity of the mixture,

then lowering the flame temperature. When the CO2 content decreases the flame adiabatic

temperature down to about 1500 K, the flame extinguishes. CO2 has been found to play also a

12

kinetic role. However, its main role has been found to be thermal in affecting the adiabatic flame

temperature.

All the obtained data (both experimental and numerical) of the laminar burning velocity have been

correlated by the theoretical formula of Lewis von Elbe (1987) implementing a single step reaction

rate. It has been found that this formula is able to capture all the values of the laminar burning

velocity at varying the oxygen and CO2 contents.

13

References

1. Abraham, B. M., Asbury, J. G., Lynch, E. P., and Teotia, A. P. S., Oil Gas J. March 15:68–

75 (1982).

2. Arpenteiner P., Cavani F. and Trifirò F., The Technology of Catalytic Oxidations, Safety

Aspects - Vol. 2; Editions Technip. Paris, 2001

3. Bowman CT, Frenklach M, Gardiner WC, Smith GP. The “GRI-Mech 3.0” Chemical

Kinetic mechanism 1999, www.me.berkeley.edu/gri mech/.

4. Dahoe, A.E. (2005). Laminar burning velocities of hydrogen-air mixtures from closed vessel

gas explosions. Journal of Loss Prevention in the Process Industries, 18, 152-166.

5. Dahoe, A.E., & de Goey, L.P.H. (2003). On the determination of the laminar burning

velocity from closed vessel gas explosions. Journal of Loss Prevention in the Process

Industries, 16, 457-478.

6. Dahoe, A.E., Zevenbergen, J.F., Lemkowitz, S.M., Scarlett, B. (1996). Dust explosions in

spherical vessels: The role of flame thickness in the validity of the ‘cube-root law’. Journal

of Loss Prevention in the Process Industries, 9, 33-44.

7. Egolfopoulos FN, Cho P, Law CK. Laminar flame speeds of methane/air mixtures under

reduced and elevated pressures. Combust Flame 1989; 76:375-91.

8. Huang, Z., Zhang, Y., Zeng, K., Liu, B., Wang, Q., & Jiang, D. (2006). Measurements of

laminar burning velocities for natural gas–hydrogen–air mixtures. Combustion and Flame,

146, 302-311.

9. Kee RJ, Grcar JF, Smooke MD, Miller JA. A FORTRAN program for modeling steady

laminar one-dimensional premixed flames. Sandia National Laboratories Report 1985,

SAND 85-8240.

14

10. Lewis, B., & Von Elbe, G. (1934). Determination of the speed of flame and temperature

distribution in a spherical bomb from time-pressure explosions records. Journal of Chemical

Physics, 2, 283-290.

11. Lewis, B., & Von Elbe, G. (1987). Combustion, flames and explosions. 3rd Edition,

Academic Press Inc.

12. Payne, R., Chen, S. L., Wolsky, A. M., and Richter, W. F., Combust. Sci. Technol. 67:1–16

(1989).

13. Cuoci A., Frassoldati A., Faravelli T., Ranzi E., Environmental Engineering Science,

Volume 25, Number 10, 1407 – 1422, 2008

14. Scott FE, Van Dolah RW, Zabetakis MG, Sixth Symposium (International) on Combustion,

The Combustion Institute, Pittsburgh, 540-545, 1957

15. Ulizar, I., and Pilidis, P., ASME J. Eng. Gas Turbines Power 119:612–616 (1997).

16. Uykur C, Henshaw PF, Ting, DS-K, Barron RM. Effects of addition of electrolysis product

on methane/air premixed laminar combustion. Int J Hydrogen Energy 2001; 26: 265-73.

17. VanMaaren A, Thung DS, DeGoey LPH. Measurement of flame temperature and adiabatic

burning velocity of methane/air mixtures. Combust Sci Tech 1994; 96:327-44.

18. Westbrook, C.K., Dryer, F.L., Simplified Reaction Mechanisms for the Oxidation of

Hydrocarbon Fuels in Flames, Comb. Sci. and Tech., Vol. 27, pp. 31-43, 1981.

15

Nomenclature

A flame area, m2

KG deflagration index, bar m/s

KG,max maximum deflagration index, bar m/s

L Markstein length, mm

lk Kolmogorov scale, mm

P pressure, bar

Pambient ambient pressure, bar

Pmax maximum (adiabatic) pressure, bar

Po initial pressure, bar

Req equivalent vessel radius, m

Req equivalent vessel radius, m

ReT turbulent Reynolds number

rf flame radius, m

Sf laminar burning velocity in Eqs. 2 and 3, m/s

SfL stretched burning velocity, m/s

SL laminar burning velocity, m/s

Su laminar burning velocity at atmospheric pressure and temperature, m/s

Sun unstretched burning velocity, m/s

t time, s

Tambient ambient temperature, K

To initial temperature, K

u' turbulent intensity (i.e., velocity fluctuation), m/s

V vessel volume, m

Greek symbols

stretch rate, s-1

16

exponent in Eq. (6)

exponent in Eq. (6)

u adiabatic coefficient of the unburned gas

exponent in Eq. (3), viscosity Pa s

exponent in Eq. (3)

density, kg/m3

17

Table 1. Experimental results for the explosion of methane with different O2, N2, CO2 composition, in terms of maximum pressure reached in the vessel, rate of pressure rise and laminar burning velocity. Standard deviation σ are also reported. RUN CH4

%CO2

%O2

%E=O2/(O2+N2) Pmax

kPaσ(P) kPa

Su

m/sdP/dtmax

kP /sTad

(K)Pad

(bar)notes

1 8 20 16 0.21 327 42 0.07 1511 2182 713 stable flame2 10 40 20 0.40 438 16 0.16 2221 2266 760 stable flame3 11 60 22 0.80 440 33 0.18 2578 2240 755 stable flame4 9.5 0 19 0.21 705 17 0.37 11920 2587 875 stable flame5 13 20 26 0.40 808 25 0.61 19824 2690 922 stable flame6 17 40 34 0.8 823 20 0.65 23565 2760 975 stable flame7 5.7 40 11 0.21 - - - - 1590 531 no ignition

Tables

Table 2. Experimental results for the explosion of hydrogen with different O2, N2, CO2 composition, in terms of maximum pressure reached in the vessel, rate of pressure rise and laminar burning velocity. Standard deviation σ are also reported. RUN H2

%CO2

%O2

%E=O2/(O2+N2) Pmax

kPaσ(P) kPa

Su

m/sdP/dtmax

kPa/sTad

(K)Pad

(kPa)notes

6-8 18 40 9 0.21 350 12 0.35 1873 1673 500 stable flame7-9 25 60 12.3 0.80 467 8 0.55 14055 1908 560 stable flame

8-10 27 40 13.4 0.40 546 73 0.85 17251 2170 630 stable flame9-11 24 20 12 0.21 580 22 1.5 25314 2210 650 stable flame

Table 3 – Extinction values of CO2 concentrationFuel CO2,ext (%)

@ air (E=0.21)

CO2,ext (%)

@E = 0.4

CO2ext (%)

@ E = 0.8

CO2,ext (%)

@ E = 1

CH4 20 45 65 70

H2 40 58 68 71

Table 4 – Extinction values of CO2 concentration without kinetic steps involving CO2 as reactant.Fuel CO2,ext (%)

@ air

CO2,ext (%)

@ E = 0.4

CO2,ext (%)

@ E = 0.8

CO2,ext (%)

@ E = 1

CH4 28 52 70 70

H2 45 62 70 72

Table 5 - = 0.02 W/m K-Fuel k° Ea (kJ/mol) m n

CH4 (Westbrook & Dryer, 1981) 2.119 1011 m1.5/kmol0.5 s 202.7 0.2 1.3

H2 (Marinov et al., 1995) 1.8 1013 cm3/mol s 146.4 1 0.5

H2 (Mitami & Williams, 1980) 2.196 1012 cm1.5/mol0.5 s 109.2 1 1

1

Central spark ignition

Electrical transformer, 220-230 V e 50Hz with 3.0 KV and 25/30 mA discharge

V = 5 litersMAWP = 200 barTest pressure = 400 bar

mechanical stirrer

Figure 1 - Experimental set-up.

Figure1

100

200

300

400

500

600

700

800

900

0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 2

t (s )

Pre

ssu

re, k

Pa

R U N1_E =0.21; C O2=20%

R U N2_E =0.40; C O2=40%

R U N3_E =0.8; C O2=60%

R U N4_E =0.21; C O2=0

R U N6_E =0.8; C O2=40%

R U N5_E =0.40; C O2=20%

Figure 2 - Pressure histories for the mixture CH4-O2-N2-CO2 explosion at different E ratio (E=O2/(O2/N2) and CO2 molar fraction.

Figure2

100

200

300

400

500

600

700

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1

t (s )

Pre

ssu

re, k

Pa

RUN8_E=0.21; CO2=40%

RUN11_E=021; CO2=20%

RUN10_E=0.4; CO2=40%

RUN9_E=0.8; CO2=60%

Figure 3 . Pressure histories for the mixture H2-O2-N2-CO2 explosion at different E ratio

(O2/(O2/N2) and CO2

Figure3

H2

CO2 (%)

0 20 40 60 80

Sl (

cm/s

)

1

10

100

CH4

Sl (

cm/s

)

1

10

100

1000air E=0.4 E=0.8

Experimental data of Quiao et al., 2005Experimental data of Kim et al., 2002

Oxyfuel

Experiments, E=0.8Experiments, E=0.4Experiments, air Lewis von Elbe data

Figure 4 – Laminar burning velocity vs. CO2 content in the mixture at different values of the enrichment factor.

Figure4

H2

E=O2/(O2+N2)

0.2 0.4 0.6 0.8 1.0

CO

2 (%

)

20

40

60

80

CHEMKIN

Theoretical from Sl

CH4

CO

2 (%

)

0

20

40

60

80

Theoretical from SlCHEMKIN Experimental_ NO IGNITION

Extinguisehd flame

Stable Flame

Extinguisehd flame

Stable Flame

Figure 5– Extinction map in the plane CO2 content – Oxygen enrichment (E)

Figure5

CH4

CO2 (%)

0 20 40 60 80

Sl (

cm/s

)

1

10

100

H2

Sl (

cm/s

)

10

100

1000

air E=0.4 E=0.8 Oxy_fuel

Figure 6–Laminar burning velocity as function of the CO2 content in the reactants mixture at varying the enrichment factor. Simulations with all the kinetic steps (solid lines); simulations

without the CO2 involved kinetic steps (dashed lines).

Figure6

H2

CO2 (%)

0 20 40 60 80

Ta

d (K

)

1500

2000

2500

3000

CH4

Ta

d (K

)

1500

2000

2500

3000 AirE=0.8E=0.4Oxyfuel

Figure 7 – Adiabatic temperature as calculated for CH4/O2/N2/CO2 and H2/O2/N2/CO2 at varying CO2 content and enrichment factor (E).

Figure7

H2

Tad (K)

1500 2000 2500 3000

Sl (

m/s

)

1

10

100

1000

E=0.4

E=0.8

Air

Oxyflame

AirE=0.8E=0.4

CH4

Oxyflame

Figure 8 – Calculated values of the burning velocity as function of the flame temperature for hydrogen and methane flames at varying oxygen enrichment and CO2 content

Figure8

Tad (K)

1600 2000 2400 2800 3200

Sl (

m/s

)

1

10

100

1000 H2

Tad (K)

1600 2000 2400 2800 3200

Sl (

m/s

)

1

10

100

1000

CH4Theoretical_AirTheoretical_E=0.8Theoretical_E=0.4

AirE=0.8E=0.4

E=0.4E=0.8Air

Theoretical_E=0.4_Marinov et al., 1995Theoretical_E=0.8_Marinov et al., 1995Theoretical_Air_Marinov et al., 1995

Theoretical_E=0.8_Mitami & Williams, 1980Theoretical_E=0.4_Mitami & Williams, 1980Theoretical_Air_Mitami & Williams, 1980

OxyfuelTheoretical_Oxyfuel

Oxyfuel

Theoretical_Oxyfuel

Figure 9 – Calculated and theoretical values of the burning velocity as function of the flame temperature for hydrogen and methane flames at varying oxygen enrichment and CO2 content.

Figure9