Di-tertiary-butyl Peroxide Decomposition and Combustion with Air

Dec 5, 2016 - The calculations reproduce this two-stage ignition behavior for fuel-rich flames, and the calculated flame structures reflect the intera...
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Di-Tertiary-Butyl Peroxide Decomposition and Combustion with Air: Reaction Mechanism, Ignition, Flame Structures, Laminar Flame Velocities Nadia Sebbar, Peter Habisreuther, Henning Bockhorn, Itsaso Auzmendi-Murua, and Joseph William Bozzelli Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.6b02201 • Publication Date (Web): 05 Dec 2016 Downloaded from http://pubs.acs.org on December 6, 2016

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Di-Tertiary-Butyl Peroxide Decomposition and Combustion with Air: Reaction Mechanism, Ignition, Flame Structures, Laminar Flame Velocities N. Sebbar1, P. Habisreuther1 , H. Bockhorn1 I. Auzmendi-Murua2, J. W. Bozzelli2 1

Karlsruhe Institute of Technology (KIT), Engler-Bunte-Institute, Combustion Technology Engler-Bunte Ring 7, D-76131 Karlsruhe, Germany 2

New Jersey Institute of Technology (NJIT), Chemical Engineering Department, Newark, New Jersey, USA This work is dedicated to Brian Haynes on the occasion of his 65th birthday

Abstract This study investigates the ignition/combustion of di-tertiary-butyl-peroxide (DTBP) over a wide range of fuel/air ratios using numerical flame calculations to gain insight into species profiles and ignition/combustion characteristics of DTBP/air mixtures. The results are flame zone and reaction zone structures as well as the corresponding laminar flame speeds and propagation speeds of reaction zones. Several detailed reaction mechanisms are evaluated in the flame simulations and sensitivity of the computed results to important reaction steps was evaluated. The ignition of DTBP in absence of oxygen belongs in the category of thermal runaway since there is no chain branching associated with the early stages of the reaction, i.e. decomposition steps lead to acetone and ethane. In the presence of oxygen, the development of a flame is more complex and exhibits a twostage ignition where the first stage, at lower temperatures, involves the thermal decomposition of DTBP to acetone, which further oxidizes to the final combustion products. This second high temperature ignition process involves chain branching reactions for the combustion of acetone and ethane and the usual chain branching reactions of the H2/O2 system. The calculations reproduce this two-stage ignition behaviour for fuel rich flames and the calculated flame structures reflect the interaction of decomposition of DTBP to acetone and ethane and the subsequent combustion. DTBP has a relatively weak RO—OR bond and readily undergoes dissociation at higher temperatures to two RO• radicals and it is widely used as a radical source, reaction initiator. Five reaction mechanisms (M1 to M5) were employed in this study for analysis of the reaction system, which differ mainly in the reaction rates for the initial DTBE decomposition reaction. When using relative high reaction rates for the initial decomposition reactions (M1 and M2), the flame velocities of DTBP/air mixtures exhibit two maxima over the entire range of mixture fraction. The first maximum is located at stoichiometric conditions while the second prevails under fuel rich conditions. Decelerating the primary decomposition reactions (M3, M4 and M5) in favour of the combustion reactions of the DTBP decomposition products, shows the second maximum disappearing as result of the relatively low propagation speed of the reaction zone. The first maximum is only slightly altered by varying the reaction rates, because the decomposition step of DTBP and the combustion of the primary decomposition products merge at stoichiometric conditions and temperatures are high enough to result in high decomposition rates. 1 ACS Paragon Plus Environment

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1. Introduction Di-tertiary-butyl peroxide (DTBP) is an organic peroxide containing two tertiary butyl groups, see Fig. 1. DTBP can be synthesized inexpensively by reacting isobutylene with tertiary-butyl hydro peroxide [1]. The weak O—O peroxide peroxide bond [2] (41 kcal mol-1) undergoes homolytic bond fission at temperatures above 400K, (CH3)3COOC(CH3)3 → 2(CH3)3CO•

(R667)

For this reason, DTBP is commonly used as a radical initiator in organic synthesis and polymer chemistry or as cross linking agent and for hardening of unsaturated polyesters. In the gaseous phase the tertiary-butoxy-radical undergoes further decomposition to ethane and acetone [3, 4], 2(CH3)3CO• (g) → C2H6 (g) + 2 CH3COCH3

(R666)

The overall decomposition of DTBE and formation of these primary decomposition products from the DTBP via reactions R667 and R666 is exothermic. This exothermic decomposition releases heat to the reaction system so that after mixing with ambient air, self-ignition temperatures of the resulting mixtures may be exceeded. The exothermic decomposition followed by the ignition of the primary decomposition products with air, attributes hazardous properties to DTBP, with a hazard level that is similar to that of other compounds from the class of organic peroxides. It is important to note that precautions have to be taken and preventive measures are necessary when handling DTBP in the process industry. If DTBP is released accidently during transport or storage the risk of pool fires is augmented which may affect the adjacent facilities. A study on decomposition of DTBP at high pressures to 255 atm and temperatures to 200°C has been reported by Buback and Lendle [5]. They report a reaction enthalpy to the initial products in R667 of 36 kcal mol-1 and reaction entropy of 31 cal mol-1 K-1. DTBP is also used in combustion engines under conditions where oxygen is limited [6,7] and has been recognized as an effective cetane improver [8,9,10,11]. It has a potential advantage compared to nitrates in reducing NOx emissions at comparable cetane levels [7]. Some studies recommend DTBP as improver for diesel ignition [12]. The complete combustion according to reaction R689 (CH3)3COOC(CH3)3 (g) + 11.5 O2 (g) → 8 CO2 (g) + 9 H2O (g)

(R689)

is exothermic by about 1187 kcal mol-1 which is higher by a factor of about 30 compared with the anaerobic decomposition corresponding to reaction R667 and R666.

Figure 1: Di-tertiary-butyl peroxide (DTBP) structure. Safety aspects come into play with regard to the use of DTBE in polymer chemistry or as ignition improver due to the hazardous properties of DTBP. It is of importance to know and understand the ignition and combustion behaviour of DTBP/air mixtures over a wide range of fuel/air ratios in order to evaluate is use in these applications. 2 ACS Paragon Plus Environment

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The decomposition and ignition behaviour of DTBP over a wide range of fuel/air ratios is evaluated in this study. The resulting reaction zones, flame structures and the corresponding propagation speeds of reaction zones and laminar flame speeds are investigated with the help of numerical flame calculations. Detailed reaction mechanisms are adopted for the simulations and the sensitivity of the computed results to crucial reaction steps is evaluated. The study starts with a set of decomposition reactions of DTBP initially proposed by Griffith et al. [13] combined with a C1- to C3–species combustion mechanism from Pichon et al. [14]. Supplementary reaction mechanisms for the parent DTBP and initial intermediates are compiled using structures and the thermochemistry of the radical intermediates, stable products, and transition state structures from the dissociation of DTBP reported in a previous study [2]. Finally, results employing a reaction mechanism developed by Wang et al. [15,16] for the numerical investigation of the effect of DTBP as additive on the reactivity of methanol and ethanol in combustion are reported and compared with the results from simulations with the other reaction mechanisms.

2. Applied methods and reaction mechanisms 2.1. Applied methods The flame and reaction zone structures, the corresponding laminar flame speed and the propagation speed of reaction zones were calculated from the mass- and energy balances for a flat, one-dimensional, isobaric, homogeneously premixed DTBP/air flame, using the PREMIX flame program from the CHEMKIN collection[17]. PREMIX accounts for finiterate chemical kinetics and multi-component molecular transport. The freely propagating planar flame model was used along with the mixture averaged transport properties evaluated using the CHEMKIN subroutine library. The solution is calculated on an adaptive mesh that takes into account a relative gradient and curvature of the solution vector. To ensure an accurate converged and grid independent solution, the adaptive mesh criteria for the largest relative gradient and curvature were toughly selected as 0.04 and 0.07. The calculations were performed over a wide range of DBTP/air ratios from pure DTBP to very lean DTBP/air flames. When available, thermodynamic and transport data required for the numerical simulations have been taken from literature. If not available, e.g. for species such as DTPB, intermediate 0 radicals and stable species from DTBP decomposition, entropies S 298 and heat capacities

C p (T ) have been estimated with the rigid-rotor-harmonic-oscillator approximation [18], which is based on vibration frequencies, moments of inertia, symmetry, spin degeneracy and optical isomers of the molecules. DFT optimized structures, calculated vibration frequencies and moments of inertia were used to calculate the contributions to entropy and heat capacity from vibration, translation, and external rotation (TVR) on the basis of formulas from statistical mechanics and by use of the SMCPS [19] program. Torsion frequencies for treatment of hindered internal rotators were calculated utilizing the ROTATOR program [20]. The transport data, not available from literature but needed in the PREMIX code for the homogeneously premixed flame calculations, have been estimated with the help of the TRANSCAL program [21]. This code is based on the Joback [22] method which is a group contribution method. Assuming that there are no interactions between the groups, Joback uses additive contributions only. In the TRANSCAL code basic structural information is used: list 3 ACS Paragon Plus Environment

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of functional groups for calculation of well depth e/kB, collision diameter, dipole moment and polarizability. Ref. [2] defines all calculation methods and reports the thermodynamics of DTBP. The rate coefficients used in the different mechanisms called M1 to M5 (see section 2.2) are obtained from the cited literature or by calculations using the Canonical Transition State Theory (TST) [23]. In this case the ThermKin code [19] is used to determine the elementary reaction rate coefficients and to express the rate coefficients in the modified Arrhenius form (see Table 1). From the kinetic analysis, high-pressure rate constants for each channel are obtained from the calculated energies, vibration frequencies and structures. The modified Arrhenius parameters are determined from regression analysis with application of the principle of least squares. Entropy differences between reactant and transition state are used to determine the pre-exponential factor A via canonical TST. Thermodynamic properties of reactants and transition states are obtained from previous computations [2] or from literature. ThermKin also converts thermodynamic properties to the NASA polynomial format required for simulations with CHEMKIN. 2.2. Reaction mechanisms used for numerical simulations Mechanism M1: The reaction mechanism M1 is based on a detailed C1- to C3–species combustion mechanism with 665 reactions and 118 species developed by Pichon et al [14]. This reaction mechanism, initially derived for combustion of dimethyl ether [24], was modified by the authors to include reactions of acetone. They also re-validated submechanisms for hydrogen-, carbon monoxide- and methane-combustion and revised results for ketene and acetone. To this reaction mechanism, the decomposition of DTBP to acetone has been added according to reactions R667 and R666 reported by Griffith et al. [13]. The corresponding rate coefficients are listed in Table 1. Table 1. Kinetic parameters for the primary decomposition reactions of DTBP from literature and this worka. Reactions k = A Tb exp (-E/RT) A

C8H18O2 ↔ C4H9O + C4H9O

C 4 H9 O

↔ CH3 + CH3COCH3

R 667 R 667a R 667b R 666 R 666a R 666b

Ref. 13 This worka Ref. 15 Ref. 13 This worka Ref. 15

2.00E+15 4.83E+08 1.81E+12 5.00E+09 9.69E+09 1.50E+11

b

0.0 1.6 0.0 0.0 1.1 0.0

E

36329.0 43127.0 0.0 10993.0 12903.0 11900.0

ATb (cm3 mol-1 sec-1), E (cal mol-1) a Reactions defined as “this work” have been deduced from the results reported in reference 2 using the methods explained in section 2.1. Molecular and thermodynamic properties for reactants, intermediates, and products for the C1to C3–species combustion mechanism have been taken from the literature [14, 24]. The thermodynamic data of C8H18O2 (DTBP) and C4H9O are extracted from [2]. Mechanism M2: The basis of the mechanism M2 is the above C1- to C3–species reaction system combined with the two DTBP decomposition reactions R667 and R666. A supplementary sub-mechanism has been added describing the major unimolecular dissociation reaction paths of DTBP resulting from RO–OR and R–OOR bond cleavages and several molecular elimination pathways as evaluated in a previous study [2]. Over 10 initial reaction 4 ACS Paragon Plus Environment

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paths were identified resulting in 21 new developed reactions for DTBP decomposition. This supplementary is implemented in the CHEMKIN suite together with the C1- to C3–species combustion mechanism. The 21 calculated reactions as well as the corresponding thermodynamic and transport properties of the species are available in CHEMKIN format in the supplementary material to this paper. The final mechanism M2 consists of 688 reactions and 146 species. Mechanism M3: As will be discussed in detail below, the crucial steps for the decomposition and combustion of DTBP are reactions R666 and R667. Calculations of the rate coefficient for the C4H9O—OC4H9 bond scission using the methods described briefly in section 2.2 yields values significantly different from the values reported by Griffith et al. [13]. The calculations show also higher activation energies for both reactions R666 and R667 when compared to reference [13]. M3 is a mechanism included in this study incukded for cimoarison, based on the C1- to C3–species combustion reaction system combined with the reaction pathways developed in [2]. However, for the decomposition reactions of DTBP the rate coefficients R667a and R666a, see Table 1, are used. The mechanism M3 consists also of 688 reactions and 146 species. Mechanism M4: A fourth mechanism included in this study has been developed by Wang et al. [15,16] for the numerical investigation of the effect of DTBP as additive on the reactivity of methanol and ethanol in mixtures with a primary reference fuel (PRF), which is a mixture of n-heptane and iso-octane. This mechanism contains the updated methanol and ethanol submechanisms from Metcalfe et al. [25], the intermediate C3- to C4-species sub-mechanism from the San Diego combustion mechanism [26], the C7- to C8–species reactions for PRF decomposition [16] and a DTBP reduced sub-mechanism. The Wang et al. mechanism is assigned M4 in this study. It should be noted that the two DTBP decomposition reactions R666b and R667b in M4 differ again from those of Griffith et al. [13], the rate coefficient of R666b comparable to the one of this work R666a, see Table 1. M4 consists of 349 reactions and 80 species. Mechanism M5: The last mechanism M5 integrates the C1- to C3–species reaction system and the 21 reactions of the supplementary mechanism from reference [2]. The two primary reactions for DTBP decomposition are taken from Wang et al. [15,16], R667b and R666b in Table 1. We note that the rate coefficients of the primary reactions again differ from those of Griffith et al. [13], compare Table 1. The pre-exponential factor A for the first reaction R666b is smaller with 1.81 1012 compared with 2.00 1015 from Griffith et al. For better understanding, a summary of the five reaction mechanisms is displayed in Table 2. Table 2. Mechanisms used for the flame calculations in this work. M1 M2 M3 M4 M5

Pichon et al. [14] + R667 and R666 from Griffith [13] 667 reactions 121 species Pichon et al. [14] + R667, R666 from Griffith [13] + 21 new reactions from [2] 688 reactions 146 species Pichon et al. [14] + R667a, R666a + 21 new reactions from [2] 688 reactions 146 species Wang et al. [15,16] 349 reactions 80 species Pichon et al. [14] + R667b, R666b + 21 new reactions from [2] 688 reactions 146 species 5 ACS Paragon Plus Environment

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3. Results and Discussions In the following sections results from detailed numerical simulations of species concentrations, temperatures, propagation speed of decomposition reaction zones and flame velocities for one-dimensional, isobaric, reacting mixtures for a wide range of DTBP/air ratios are discussed. The results are presented for the self-decomposition of pure DTBP and DTBP to which air is gradually added. The DTBP combustion behavior is investigated under rich, stoichiometric and lean conditions. The results reported are given in terms of the mixture fraction ξ of DTBP which is defined in Eq. (1), mi being the masses of fuel and air: 



(1)

 

ξ is also expressed in term of equivalent ratio as: ∅

/ 

(2)

/ 

The stoichiometry is defined according to R689. 3.1. Calculations based on mechanism M1 3.1.1. Self-decomposition of DTBP The decomposition of DTBP can result in a thermal runaway since the reactions of the initial product are exothermic. The initial decomposition has the peroxy group O—O bond cleave to form two tertiary-butoxy radicals (C4H9O), which undergo exothermic decomposition to acetone and methyl radicals. In addition, two methyl radicals can combine forming a new carbon-carbon bond, 90 kcal mol-1. This exothermicity is reflected by the temperature profile for the decomposition of DTBP given in Fig. 2. The temperature (from the initial temperature of 365 K) increases in the reaction zone and approaches the final temperature at about 910 K which is sufficiently high to ignite the primary decomposition products when being mixed with some quantities of air (presence of O2). In the presence of molecular oxygen, the methyl radicals can dissociate or react with O2, where both processes are exothermic. The overall oxidative decomposition is exothermic as illustrated in the introduction.

Figure 2: Calculated flow speed and temperature profile for DTBP decomposition (ξ = 1.0, ∅ = ∞). 6 ACS Paragon Plus Environment

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Fig. 2 contains additionally the profiles of the flow velocity in the one-dimensional reaction zone. If a propagation speed of the decomposition reaction zone is defined in analogy to the laminar flame speed, which is the velocity at which an unstretched laminar flame propagates through a quiescent mixture of unburned reactants, an inlet velocity of the “decomposition flame” of little less than 20 cm/s can be extracted from Fig. 2. It has to be mentioned, that the value of location shown in the Figures is only significant in the sense of specifying a position relative to the reaction zone or flame. The mass fraction profiles displayed in Fig. 3 reveals that under this condition the main reaction components and products are DTBP, C2H6 and CH3COCH3. All other species being involved in the reaction mechanism M1 are present in negligible mass fractions. Particularly, all the oxygen of the system is contained in CH3COCH3 and there is no free O2 for further oxidation of the primary decomposition products. The rapid decomposition of DTBP into tertiary-butoxy-radicals C4H9O is followed by an equally rapid decomposition of this species into CH3COCH3 and CH3. These methyl radicals are not oxidized due to the lack of oxygen and the recombination reaction of the methyl radicals leads to ethane as another main product of the decomposition. Due to the rapid decomposition of C4H9O into C2H6 and CH3COCH3 the mass fraction of methyl radicals attains only very small values.

Figure 3: Mass fraction profiles of the species for DTBP decomposition (ξ = 1; ∅ = ∞). The rapid increase of the temperature as displayed in Fig. 2 is due to the heat release from the different reactions taking place during the decomposition of DTBP. This heat release can be traced back to single reactions of the employed mechanism M1 by Eq. (3): .

Q j = rj ⋅ ∆H rxn j

(3)

In Eq. (3) rj means the reaction rate of the reaction j and ∆Ηrxn j the respective heat of reaction. Because the reaction rate changes dependent on position within the reaction zone, it is necessary to find a particular point at which significant chemical conversion occurs. The point at which the temperature gradient is at its maximum is selected for characterizing the heat release for the decomposition of DTBP as illustrated on the right cutout of Fig. 4.

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Figure 4 reveals that the dissociation of DTBP to C4H9O and the further dissociation of C4H9O to CH3COCH3 and CH3 are both endothermic. On the other side, four reactions involving CH3 show an important heat release which over compensates the heat of reaction for the decomposition reactions. The most exothermic reactions are the recombination reactions CH3 + CH3O → CH3OCH3 and CH3 + CH3O → CH2O + CH4, both forming stable species. Among the two other exothermic reactions, we note the formation of a methyl-peroxy radical CH3OO from CH3 + O2. As already demonstrated in the literature and in previous studies [27, 28] the formation of an energized peroxy radical is exothermic by about 31 kcal mol-1 for the formation of CH3OO. The fourth exothermic reaction recombines the methylperoxy with a methyl radical to form methoxy radicals CH3O. We point out that at ξ = 1.0 (∅ =∞), the methyl recombination to ethane is not important in terms of heat release in comparison to the reactions indicated in Fig. 4. The self-decomposition of DTBP is examined here, hence the oxygen involved in the exothermic reactions must be provided by acetone decomposition following the DTBP dissociation. The C1- to C3– species combustion mechanism [14] used for the present calculations includes further reactions of CH3COCH2 and CH3CO forming CH2CO, CH3 or CO which are a source of oxygen.

→ → 1000 900

→

800 700 Temperature (K)

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→

→

600 500 400 300 200 100

→

0 -4.735

-4.73

-4.725

-4.72

-4.715

-4.71

Figure 4: Reactions with most important contribution to the heat release (kcal cm-3 s-1) for ξ = 1.0 (∅ = ∞); position in the reaction zone as indicated in the right cutout. The heat release analysis identifies reactions of CH3 as the most important for the heat release during decomposition of DTPB. The exothermic reactions convert CH3 to stable species, so that a possible chain carrier for further decomposition reactions is deactivated. To determine which reactions from the reaction mechanism M1 are important for the formation/oxidation of the methyl radicals a sensitivity analysis has been performed. Fig. 5 gives the maximum values throughout the reaction zone of sensitivity coefficients for the mass fraction of CH3 with respect to the preexponential factors of the most important reactions. The sensitivity coefficients are defined as

Sij = ∂ ln xi / ∂ ln Aj

(4) where xi are the mass fractions of species i and Aj are the preexponential factors in the rate coefficients of reaction j. The sensitivity coefficients reflect a relative change of the model response x, i.e. calculated mass fractions of species i, caused by a relative change of the parameter A, i.e. preexponential factor of rate coefficient of reaction j, and can be extracted from the flame calculations with the CHEMKIN code. 8 ACS Paragon Plus Environment

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→ → → →

→

Figure 5: Sensitivity analysis for the mass fractions of methyl radical CH3 at ξ = 1.0 (∅ = ∞). The results indicate that the reactions that have a large impact on the formation of CH3 are the decomposition of DTBP to C4H9O which further decomposes to CH3COCH3 + CH3 and three reactions for CH3 formation: C2H5 + CH3 → C3H8, C2H6 → CH3+ CH3 and when oxygen is present CH3 + O2 → CH3OO. An important conclusion here is that the DTBP decomposition is the key reaction for the formation of other species responsible for energy release and it is of consequential importance to investigate in more detail this decomposition reaction and other possible pathways for DTBP oxidation/decomposition, see the discussion below.

3.1.2. Oxidation of DTBP The following discussion about the oxidation of DTBP is focused on details of mostly three compositions of the ignitable mixture with ξ = 0.3 (∅ = 4.6) (fuel rich), ξ = 0.085 (stoichiometric, ∅ = 1) and ξ = 0.049 (∅ = 0.55) (fuel lean). Results from calculations of different mixture compositions are also reported in terms of flame speeds. In the presence of oxygen the development of the decomposition zone and flame is more complex and exhibits a two-stage ignition where the first stage at lower temperatures is responsible for the thermal decomposition of DTBP to acetone and ethane. In the second high temperature ignition process the primary decomposition products are then oxidized to final combustion products. The high temperature ignition process involves chain branching reactions for the combustion of acetone and ethane and the usual chain branching reactions of the H2/O2 system. The self ignition temperatures are reported to be at 453 K for the first ignition step, however, referring to Fig. 2 an initial fuel temperature of even 365 K leads also to thermal decomposition. Self ignition temperature for the second ignition step is given with 757 K [29, 30].

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Table 3. Measured and calculated flame velocities of DTBP in air. ξ and ∅ of DTBP

ξ = 0.049 – ∅ = 0.55 (lean) ξ = 0.085 – ∅ = 1 (stoichiometric) ξ = 0.211 – ∅ = 2.88 (rich)

Simulated Laminar Flame Speed

Experimental Laminar Flame Speed [32]

15.58 cm/s

20 cm/s

56.1 cm/s

61 cm/s

20.2 cm/s

17 cm/s

Figure 6 reveals that in fuel rich flames the two stage ignition is clearly locally separated. At ξ = 0.3 (∅ = 4.6) the first ignition starts at about 10 mm and is caused by the decomposition of DTBP to acetone and methyl reaching a temperature plateau at about 910 K. This is followed by the second ignition at about 100 mm which leads to fuel rich combustion of the primary decomposition products and a final temperature of about 1400 K. The resulting flame speed is about 30 cm s-1, see Fig. 15. Reducing the mixture fraction to stoichiometric and lean conditions leads to a merging of the two ignition steps which is mainly caused by a faster consumption of the primary decomposition products by oxygen. The calculated laminar flame speeds compared with experimental values from [32] are given in Table 3.

Figure 6: Calculated temperature profiles for different DTBP/air flames with ξ = 0.3 (∅ =4.6), ξ = 0.085 (∅ =1) and ξ = 0.049 (∅ =0.55). The mechanism M1 includes reactions which describe the decomposition of DTBP to C4H9O followed by CH3 elimination to form CH3COCH3 and the oxidation of the primary decomposition products. Table 4 summarizes the major and minor intermediates and products present in the system. They are classified in three categories, according to the appearing mass fractions during combustion: the major species, minor and trace species, which mass fractions are around 10-1 to 10-2, 10-3 and 10-4 g cm-3, respectively. We note that the mass fraction of C4H9O in the system is very small (mass fraction in the order of ~10-4 g cm-3) indicating that it decomposes rapidly to CH3 and CH3COCH3. 10 ACS Paragon Plus Environment

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Table 4: Major, minor and trace species during combustion of DTBP. Primary Species (~10-1-10-2 g cm-3) C8H18O2 CH3COCH3 O2 H2O CO CO2

Secondary Species (~10-3 g cm-3 ) H2 O OH CH3 CH4 C2 H 4 C2 H 6 CH3OCH3 CH2O CH2CO HCCO CH3OH CH3O2

Minor Species (~10-4 g cm-3) H HO2 C2H2 H2O2 CH3CHO HCO HOCHO C3H8 CH3O CH3O2H CH2 CH3COCH2O2 C4H9O

In Figures 7 to 9 mass fraction profiles of the major and minor species for the three DTBP flames are given. Under fuel rich conditions the rapid decomposition of DTBP into C4H9O is followed by an equally rapid decomposition of this species into CH3COCH3 and CH3. The more stable decomposition products are buffered before the rapid combustion in the second step takes place. In the flame with ξ = 0.3 (∅ =4.6), see Fig. 7, acetone is consumed rapidly after the second ignition step. In addition, due to the comparatively high oxygen content in the flame, the concentrations of CH4 and C2H4 appear to reach a maximum value where C2H6 is consumed. The decomposition zone of DTBP and the combustion zone of the primary decomposition products are clearly separated as indicated in the cutout in Fig. 7.

Figure 7: Mass fraction profiles of the species for rich fuel DTBP flames (ξ = 0.3;∅ = 4.6), The flux analysis for the consumption of CH3COCH3 at ξ =0.3 (∅ =4.6) shows that two reactions are mainly responsible for the oxidation of CH3COCH3, both resulting in the formation of CH3COCH2. For the further oxidation of CH3COCH2 the most prevalent and chain propagation reaction channel is CH3COCH2 → CH2CO + CH3 which has relative high activation energy. 11 ACS Paragon Plus Environment

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Figure 8: Mass fraction profiles of the species for stoichiometric DTBP flames (ξ = 0.085; ∅ =1). At stoichiometric conditions, see Fig. 8, the primary decomposition zone of DTBP and the oxidation zone of the primary decomposition products merge and DTBP is completely consumed while some oxygen is remaining. Compared to lean conditions, the formation of ethane in comparison with CH4 and C2H4 is noticeably higher. CH4 and C2H4 at stoichiometric conditions are totally consumed. On the other side, under these conditions, the mass fraction of OH is markedly higher than at rich conditions. The formation of ethane increases noticeably at stoichiometric

condition where there is less oxygen to consume the methyl radical. Reaction flux analysis exhibits that acetone formation through the dissociation reaction of C4H9O to CH3COCH3 + CH3 is the most important reaction under these conditions. The methyl formed then recombines to form C2H6 explaining the markedly increase shown in Fig. 8. It is important to point out that at stoichiometric conditions the temperature reaches its maximum value as shown in Figure 6 and, therefore, accelerates considerably the decomposition and combustion of the primary decomposition products.

Figure 9: Mass fraction profiles of the species for lean DTBP flames (ξ = 0.049, ∅ = 0.55). 12 ACS Paragon Plus Environment

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Figure 9 illustrates the mass fraction profiles for major and minor species through a lean flame (ξ = 0.049, ∅ = 0.55). At fuel lean conditions, oxidation of the primary decomposition products is decelerated due to the lower temperature. However, oxidation rates are high enough to result in a coalescence of the first and second ignition step. The first ignition step is not pure decomposition of DTPB, but combines decomposition with oxidation. While compared with the stoichiometric case less consumption of oxygen and less formation of water are observed, carbon monoxide and carbon dioxide are obtained during the decomposition step, conversion to secondary products such as CH4, C2H6 and H2 can also be observed. Due to the excess of oxygen the products formed in the first ignition step are further oxidized leading to the formation of OH and O in the second ignition step when compared to the carbon compounds. There are competing reactions for the formation/destruction of C2H6 and CH4. In the fuel lean flame both species are formed and consumed while under stoichiometric conditions ethane formation is markedly favored. The molar flux analysis for acetone at ξ = 0.049 (∅ = 0.55) shows the rapid decomposition of acetone, mainly through two reactions because of the presence of radicals (H and OH) which abstract a hydrogen from CH3COCH3 to form a CH3COCH2 radical. Heat release (kcal cm-3 s-1) -0.007 -0.006 -0.005 -0.004 -0.003 -0.002 -0.001

0

0.001 0.002 0.003 0.004 0.005 0.006 0.007

c8h18o2 →c4h9o+c4h9o c4h9o → ch3+ch3coch3 ch3coch2o2h+o2 →ho2+ch3coch2o2 ch3coch2+o2

→ch3coch2o2

ch3o2h+o2 →ch3o2+ho2 ch3o2+ch3 → ch3o+ch3o ch3+o2 → ch3o2 ch3o+ch3o → ch3oh+ch2o ch3o+o2 →ch2o+ho2 hco+o2 →co+ho2 ho2+ho2 → h2o2+o2

Figure 10: Reactions with important contribution to the heat release (kcal cm-3 s-1) at ξ = 0.3 (∅ = 4.6), first ignition; position in the reaction zone as indicated in the right cutout. For flames with mixture fraction ξ = 0.3 (∅ = 4.6) the contributions of the different reactions to the heat release at the distinct two ignition-stages are illustrated in Figures 10 and 11. At the first ignition point, it appears that one important reaction is the endothermic DTBP decomposition to C4H9O which further dissociates to acetone and CH3. At the first ignition step eight reactions contribute in a significant way to the heat release. The most important reactions are related to the methyl group formed from the dissociation of C4H9O. The recombination of CH3 with the available O2 forms a methyl peroxy radical CH3OO with an important heat release. The most exothermic reaction here is the reaction of two methoxy radicals, CH3O, to form two stable species, methanol CH3OH and formaldehyde CH2O.

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→ → → → → → → → → → → →

Figure 11: Reactions with important contribution to the heat release (kcal cm-3 s-1) at ξ = 0.3 (∅ = 4.6), second ignition; position in the reaction zone as indicated in the left cutout. In the second ignition step, the overall dominant exothermic reaction is the recombination of two methyl radicals to form a stable ethane with a relatively high heat release. We note also that CH3 reacts with HO2 and forms CH3O + OH with some heat release. Several reactions that contribute to the heat release are oxidation reactions with the remaining molecular oxygen. It is important to point out that at a mixture fraction of ξ = 0.3 (∅ = 4.6) no oxygen is left, meaning that the part of the available oxygen is provided from the decomposition reactions of DTBP.

→ →

→ → → → → →

→ → → →

2200 2000 1800 1600

Temperature (K)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

1400 1200 1000 800 600 400 200 0 -4.74

-4.73

-4.72

-4.71

-4.7

-4.69

-4.68

-4.67

-4.66

Figure 12: Reactions with important contribution to the heat release (kcal cm-3 s-1) ξ = 0.085 (∅ = 1); position in the reaction zone as indicated in the left cutout. For the stoichiometric case (ξ = 0.085) the heat release analysis performed at the point indicated in Fig. 12 reveals that the recombination of the two methyl reaction to the formation of ethane CH3 + CH3 → C2H6 is the most important reaction in terms of heat release followed by two oxidation reactions HCCO + O2 → CO2 + HCO and HCO + O2 → CO + HO2. Among the reactions which contribute to heat release, we note the formation of a methyl-peroxy radical CH3OO from CH3 + O2. As already mentioned above [27, 28], the formation of an 14 ACS Paragon Plus Environment

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energized peroxy radical is exothermic. Under stoichiometric conditions the flame velocity is 56.1 cm s-1, see Table 4, which is almost the maximum flame speed achievable in DTBP/air flames. The maximum flame speed is attained at slightly rich conditions with ξ=0.094 mixture fraction and 59 cm s-1. The results presented so far demonstrate, that the combustion of DTBP with air is initiated by the decomposition reaction of DTBP to tertiary-butoxy radicals, which then react to acetone and ethane. These two steps of reactions each depend on the reaction conditions, which result from the respective mixture fraction. To demonstrate this behaviour, the sensitivity coefficients for the mass fraction of CH3COCH3 and C2H6 with respect to the pre-exponential factors of the most important reactions have been calculated and displayed in Figures 13 and 14. The reactions corresponding to the numbers shown on the Y-axis of Figures 13 and 14 are listed in supplementary material. The sensitivity coefficients reflect the maximum values throughout the flame and are calculated for ξ = 1.0 (only fuel, ∅ = ∞), 0.151 (fuel rich, ∅ = 1.91), 0.085 (stoichiometric, (∅ = 1) and 0.049 (fuel lean, (∅ = 0.55). For better comparison, the sensitivity coefficients of each mixture fraction represented in Figs. 13 and 14 is identified by a colour. The bars represented in each figure are not overlapped but added one after the other. We note that under lean conditions (red bars) the sensitivity coefficients as defined by Eq. (3) are largest. For acetone at ξ = 0.049 (∅ = 0.55) many reactions have a large impact on the formation and oxidation of CH3COCH3 and C2H6. In detail, it is reaction R667 (C8H18O2 → C4H9O + C4H9O) which has a negative sensitivity coefficient, while R309 (CH3COCH3 + H → CH3COCH2 + H2) shows a larger positive sensitivity coefficient. R9 shows the larger positive sensitivity coefficient (H + O2(+M) → HO2(+M)) but R1 (H + O2 → O + OH) has an overall large negative coefficient. Other reactions like R24 (CO + OH → CO2 + H), R26 (HCO + M → CO + H + M), R105 (CH3 + OH → CH2(s) + H2O) and R109 (CH3 + HO2 → CH3O + OH) are also of importance. The above analysis exhibits on the one hand, in which direction the mass fraction of CH3COCH3 would change, if the pre-exponential factors of the respective rate coefficients would vary. On the other hand, it can be stated, that under fuel lean conditions the mass fraction of CH3COCH3 is affected most by the chain propagating and branching reaction, which occur during hydrocarbon combustion. At stoichiometric conditions the mass fraction of CH3COCH3 is sensitive adverse the rate coefficients of only three reactions, R24, R26 and R109, and the sensitivity coefficients are much smaller compared with the lean case. At fuel rich conditions, ξ = 0.151 (∅ = 1.91) (yellow), the sensitivity coefficients are further diminished and the rate coefficients of only few reactions, among them R109, affect the mass fraction of acetone.

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Figure 13: Sensitivity analysis for the mass fraction of CH3COCH3 for different DTBP/air mixture fractions (the bars are not overlapped, they are added one after the other). The reactions corresponding to the numbers shown on the Y-axis can be found in supplementary material. Figure 14 illustrates the sensitivity analysis for the mass fractions of ethane and reflects similar behaviour. Noticeable sensitivity coefficients are obvious for only two DTBP/air mixture fractions, i.e ξ = 0.049 (red, ∅ = 0.55) and ξ = 0.085 (black, ∅ = 1). This means, that under lean to stoichiometric conditions the formation/decomposition of ethane is strongly dependent on the reaction rates of a number of reactions, while at rich fuel conditions none of the reactions present in the mechanism shows comparable impact on the formation and oxidation of ethane. The sensitivity analysis reveals that the highest sensitivity coefficients at stoichiometric conditions are calculated for R1 (H + O2 → O + OH) with a negative coefficient. Among others, Figure 14 reveals that R24 (CO + OH → CO2 + H) and R118 (CH3O2 + CH3 → CH3O + CH3O) have positive sensitivity coefficients at lean conditions and negative ones at stoichiometric conditions. The sensitivity coefficients for R26, R105 and R111 (CH3 + O → CH2O + H) are noticeable as well. The sensitivity analysis shows that under fuel lean and stoichiometric conditions the mass fractions of CH3COCH3 and C2H6 are most affected by the chain propagating and chain branching reactions, which occur during hydrocarbon combustion. The “combustion environment” decides the further fate of DTBP and the primary decomposition products once they are formed.

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Figure 14: Sensitivity analysis for the mass fractions of C2H6 for different DTBP/air mixture fractions (the bars are not overlapped but added one after the other). The reactions corresponding to the numbers shown on the Y-axis can be found in supplementary material. 3.1.3. Flame velocity-mixture fraction correlation Results of unstreched laminar flame velocities as a function of mixture fractions at normal pressure (P = 1 atm) and Tfuel = 365 K are represented in Figure 15. The DTBP/air mixture fractions have been varied in the range 0 < ξ < 1.0. The present simulations show the highest flame speed for combustion of DTBP under near stoichiometric conditions. Contrary to flames of other fuels [31], the flame speed exhibits a second maximum when moving from lean to rich conditions. Here, “lean” and “rich” regions are defined as a range of fuel/oxidizer mixtures in which lean means less fuel than oxidizer and rich, more fuel than oxidizer as commonly used in combustion. As expected from 0 < ξ < 0.15 (lean region) the flame speed increases to reach a maximum of 59 cm s-1 at ξ = 0.094 (∅ = 1.1), slightly above the stoichiometric mixture ξ = 0.085 (∅ = 1), and then decreases. For the mixture fraction range 0.15 > ξ > 1 (rich region) the calculations show a second broad maximum for a DBTP mixture fraction at 0.56 with about 37 cm s-1. The calculated flame velocities are compared with experimental data available in the literature [32] and summarized in both Table 3 and Fig. 15. The data in Table 3 show that the calculated laminar flame velocities vary from near 16 cm s-1 in the lean region, up to 56 cm s-1 at stoichiometric conditions and down to some 20 cm s-1 at very rich conditions. The agreement between experimental flame velocities and calculated values using the reaction mechanism M1 is noticeably good for mixture fractions, below and at 0.2, investigated and reported by Schälike et al. [32]. Unfortunately no data we are aware of are available for mixture fractions greater than 0.2. 17 ACS Paragon Plus Environment

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The laminar flame speed was also calculated at elevated higher pressure of P = 5 atm. As expected, the results show a decrease of the laminar flame velocities with pressure. The same behavior was obtained from different experiments on different flames [33, 34]. The flame speed of DTBP/air flame at P = 5 atm, shows the same trend as at 1 atm, a narrow maximum at stoichiometric conditions and then broad maximum at fuel rich mixtures. For mixture fractions ranging between 0.15 and 0.4, the pressure has a minor effect. A difference is then noticeable for mixture fractions larger than ξ = 0.4 (∅ = 7.2). We note that the maximum flame speeds at P = 1 and 5 atm are 60 cm-1 and 40 cm-1 respectively.

Figure 15: Calculated and experimental laminar flame speeds for different DTBP/air mixture fractions, Tfuel initial = 365 K and P = 1 and 5 atm; experimental values from [32]. 3.2.

Calculations based on mechanism M2 - comparison with M1

The mechanism M1 describes the primary decomposition reactions of DTBP by only two reactions, R667 and R666, whereas the further decomposition and oxidation of the primary decomposition products is formulated with the help of 665 reaction and 118 species. Harris and Peters [9] pointed out that at higher pressure a second reaction path for the decomposition of the primary decomposition product C4H9O becomes important and has to be considered. This second reaction path is the abstraction of H atoms by C4H9O forming tertiary-butyl alcohol: C4H9O + RH → C4H9OH + R. Although relatively little data are available for this process, Yip and Pritchard [4] found approximately equal yields of C4H9OH (tertiary-butyl alcohol) and CH3COCH3 at 400 K when DTBP was decomposed in the presence of propane at 30 atmospheres. Additionally, further reactions have to be considered like the intramolecular H abstraction of C4H9O to form an alcohol radical (CH3)3CO → (CH3)2(CH2)COH) or a hydroperoxy compound (CH3)3COO) from the OO—C fission reaction. These peroxy radicals may be the origin of a number of new important reactions for the chemistry in the DTBPflame. A number of further plausible thermal decomposition pathways for C4H9O have been evaluated [2] and have been added to the mechanism M1. The rate coefficients of these reactions have been calculated in this work and are given as supplemental material to this paper and the extended mechanism is denoted M2. Note that M2 also contains the unchanged reactions R666 and R667. Unstreched laminar flame velocities as a function of mixture 18 ACS Paragon Plus Environment

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fractions at normal pressure (P = 1 atm) and Tfuel = 365 K are calculated with M2 and illustrated in Figure 16 in comparison to the results with M1. 70

60 M1

Flame velocity cm/s

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M2

50

Experiment 40

30

20

10

0 0

0.2

0.4

0.6

0.8

1

Mixture fraction

Figure 16: Laminar flame speed for DTBP/air mixtures calculated with mechanisms M1 and M2, Tfuel initial = 365 K and P = 1 atm. Figure 16 illustrates that fuel lean mixtures result in faster flames than fuel rich mixtures. The flame velocity of DTBP/air mixtures calculated with M2, exhibit two maxima, similar to the results obtained with M1. At near stoichiometric conditions the flame speed calculated with M2 is slightly higher than that calculated with M1. This can be explained by the additional reaction channels opened for the decomposition of C4H9O that accelerate the formation of primary decomposition products which are then oxidized rapidly under fuel lean or stoichiometric conditions. The calculated values for the flame speed at stoichiometric conditions are 56.1 cm s-1 with M1 versus 62.2 cm s-1 with M2. We note a better agreement of the results from M2 with the experimental value of 61 cm s-1, see Table 3. In fuel rich mixtures, 0.15 > ξ > 1, the calculations with M2 show also a second maximum with 31.1 cm s-1. In the fuel rich region the calculated flame velocity is significantly lower compared with the results from M1. The second maximum is connected with the ignition and oxidation of acetone. With M2 less acetone is formed and other reactions are competing with its formation. Table 5. Calculated flame velocities (in cm/s) of DTBP/air with different reaction mechanisms. Mechanism M1 M2

ξ = 1.0 ∅=∞ 18.5 12.5

Rich mixtures ξ = 0.628 ξ = 0.3 ∅ = 18.2 ∅ = 4.6 35.9 29.6 29.4 28.2

ξ = 0.151 ∅ = 1.91 15.1 15.8

Lean mixtures ξ = 0.085 ξ = 0.049 ∅=1 ∅ = 0.55 56.1 15.6 62.2 18.4

In Table 5 values of the flame velocities calculated with M1 and M2 are compared. It can be seen that all flames in fuel rich mixtures are slower when calculated with M2, while under fuel lean conditions no significant change is calculated. Apparently, in the fuel rich mixtures flame velocities are dependent on the additional decomposition reactions of C4H9O. 19 ACS Paragon Plus Environment

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3.3.

Calculations based on mechanisms M3/M4/M5

3.3.1 Flame speed calculated with mechanisms M3/M4/M5 The flame velocities calculated with mechanisms M3, M4 and M5 show a different behavior than those estimated with M1 and M2 as illustrated in Fig. 17. The mechanism M3 differs from M2 in the rate coefficients of the two primary reactions only. For the first dissociation reaction (RO—OR) the calculated pre-exponential factor used in M3 is seven orders of magnitude lower and, therefore, the reaction is much slower. Griffith et al. [13] report A = 2x1015 versus A = 4.83x108 used in M3 and calculated in this work using the methods described briefly in section 2.1., see Table 1. 70

60

Flame velocity cm/s

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M3 M4 M5 Experiment

50

40

30

20

10

0 0

0.2

0.4

0.6

0.8

1

Mixture fraction

Figure 17: Laminar flame speed for DTBP/air mixture calculated with different mechanisms, Tfuel initial = 365 K and P = 1 atm. The primary dissociation reaction plays a decisive role in the reaction mechanism M3 and the formation of acetone is strongly affected. Using M3 the primary decomposition reaction R666a is still the dominant path, but it is now in competition with the new added decomposition reactions from [2]. Therefore, less acetone is formed in favor of other intermediates. The flame velocities obtained with M3, therefore do not exhibit the second broad maximum as illustrated in Fig. 17. The use of the mechanism M4 of Wang et al. [15,16], results in the same flame velocity without the second maximum for fuel rich mixtures. The maximum flame velocity calculated with M4 is 56.2 cm s-1 versus 58.3 cm s-1 with M3. In their study Wang et al. discuss that acetone is relative stable and can only be consumed at high temperatures. They further focus on CH3 as a reactive radical that undergoes oxidation reactions, provided sufficient O2 is available under lean conditions. The oxidation of CH3 further provides OH, HO2 and H radicals acting as chain branching species. Simulations with the reaction mechanism M5, has been conducted to include the oxidation reactions of acetone in more detail. M5 includes the C1- to C2- species combustion mechanism and the new reactions from [2] combined with the two primary reactions of DTBP reported by Wang et al. The results computed with mechanism M5 exhibit the same characteristics as the ones calculated with M3 and M4, see Fig. 17.

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Table 6. Calculated flame velocities (in cm/s) for DTBP/air with different reaction mechanisms. Mechanism

Rich mixtures ξ = 0.628 ξ = 0.3 ∅ = 18.2 ∅ = 4.6 1.8 4.38 4.3 7.1 3.3 6.3

ξ = 1.0 ∅=∞ 0.13 0.32

M3 M4 M5

ξ = 0.151 ∅ = 1.91 11.8 11.4 12.1

Lean mixtures ξ = 0.085 ξ = 0.049 ∅=1 ∅ = 0.55 57.8 17.9 55.4 17.6 58.71 18.6

The flames velocities are identical to those calculated by M1 and M2 in fuel lean mixtures and decrease strongly for fuel rich mixtures. It appears that the double ignition is not only dominated by the kinetics of the primary reactions but also by the temperature of the system which is lower in the fuel rich region. It may confirm the hypothesis of Wang et al. who suggest that acetone is stable until the system reaches high temperature.

3.3.2 Species concentration profiles calculated with M3, M4 and M5 Plots of species mole fractions of acetone, CO, CO2 and H2O at ξ = 0.3 (∅ = 4.6) and ξ = 0.085 (∅ = 1) through the flames are illustrated in Fig. 18. These results displayed calculated with M3 (left), M4 (middle) and M5 (right). 0.2

DTBP - ξ = 0.3

DTBP − ξ = 0.3

Mole Fractions

0.15

DTBP - ξ = 0.3

0.15

0.1

CO

CO

CO2

CO2

H2O

H2O

CH3COCH3

CH3COCH3

M3

0.05

CO CO2 H2O CH3COCH3

0.05

M4 M5 0 0

0.5

1

1.5 0

0.5

Location (cm)

1

1.5 0

0.5

Location (cm)

1

1.5

Location (cm)

0.15

DTBP - ξ = 0.085

DTBP - ξ = 0.085

Mole Fractions

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DTBP - ξ = 0.085

0.1 CO CO2 H2O CH3COCH3

CO

CO

CO2

CO2

H2O

H2O CH3COCH3

CH3COCH3

0.05

M5

M4

M3 0 0

0.1

0.2

0.3

0.4

Location (cm)

0.5

0.6 0

0.1

0.2

0.3

0.4

0.5

0.6 0

Location (cm)

0.1

0.2

0.3

0.4

0.5

0.6

Location (cm)

Figure 18: Mole fraction profiles of the species for rich fuel DTBP flames ξ = 0.3 (∅ = 4.6) and stoichiometric ξ = 0.085 (∅ = 1). Left calculated with M3, middle M4, right calculated with M5. Tfuel initial = 365 K and P = 1 atm. 21 ACS Paragon Plus Environment

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In the fuel rich mixture (ξ = 0.3 - ∅ = 4.6) Fig. 18 shows that for all three mechanisms there is less acetone formation compared with M1 (see Fig. 7). Mechanism M4 shows that acetone tends to stabilize around the second ignition and is then smoothly consumed. Calculations with M3 and M5 show both a rapid formation followed by a slow consumption. We observe that there is a double ignition in the mole fraction profiles of CO, CO2 and H2O with M4 and M5. The slow primary reaction used in M3 is responsible for the absence of the double ignition, because M3 and M5 differ only in the two primary reactions. It appears that the combination of slow reaction and the availability of oxygen at ξ = 0.3 ∅ = 4.6) results in the oxidation of acetone and of other radicals like CH3. The same behavior is observed for flames calculated for stoichiometric conditions. Since enough oxygen is available, the intermediately formed acetone is immediately consumed for M3, M4 and M5 and no apparent difference can be seen among the three mechanisms. This suggests that, when oxygen is in excess the primary reactions are not as dominant and that further oxidation reactions are of importance to the overall system behavior. Results of the sensitivity analysis confirm this, see section 3.1.2 for mechanism M1, and Figs. 13 and 14. The main result of this sensitivity analysis shows that, at ξ = 0.049 (∅ = 0.55) many oxidation reactions have a great impact on acetone formation/oxidation. 4. Conclusions The decomposition and ignition/combustion of (DTBP) over a wide range of fuel/air ratios have been studied with the help of numerical flame calculations. Flame structures and reaction zones structures as well as the corresponding laminar flame speeds and propagation speeds of reaction zones are presented. Detailed reaction mechanisms are adopted/developed and utilized for the flame simulations. Essential reactions concerning heat release and the sensitivity of the computed results to crucial reaction steps are evaluated. The decomposition of DTBP in absence of oxygen belongs to the category of thermal runaway since there is no chain branching associated with the early stages of the reaction, i.e. decomposition to acetone and formation/decomposition of ethane. The methyl radicals formed during the decomposition of C4H9O recombine to give ethane under this condition. Development of the flame is more complex in the presence of oxygen, and the system exhibits a two-stage ignition. The first stage occurring at lower temperatures involves the thermal decomposition of DTBP to acetone, which further oxidizes to the final combustion products. The second high temperature ignition process involves chain branching reactions for the combustion of acetone and ethane and the usual chain branching reactions of the H2/O2 system. In fuel rich mixtures, the two-stage ignition is locally separated and the calculations reproduce this two-stage ignition behaviour for fuel rich flames. The calculated flame structures reflect the interaction of decomposition of DTBP to acetone and ethane and the subsequent combustion. In fuel-lean mixtures the two ignition zones merge and the decomposition of DTBP to the primary decomposition products is overlapped by the combustion of the primary decomposition products. The two-step ignition behaviour of DTPB/air mixtures can be well reproduced with the computations. Calculations using five reaction mechanisms, which differ mainly in the reaction rates for the initial decomposition reactions of DTBP, have been employed. When using relative high reaction rates for the initial decomposition reactions (reaction mechanisms M1 and M2), the flame velocities of DTBP/air mixtures exhibit two maxima over the entire range of mixture fraction. The first maximum is located at stoichiometric conditions while the second prevails under fuel rich conditions. Decelerating the rates of primary decomposition reactions (reaction mechanisms M3 to M5) results in amplifying the combustion reactions of the 22 ACS Paragon Plus Environment

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decomposition products of DTBP, the second maximum disappears on account of relative low propagation speed of the reaction zone. The first maximum is only little affected by varying the reaction rates because under stoichiometric conditions the decomposition step of DTBP and the combustion of the primary decomposition products merge and temperatures are high enough to yield high decomposition rates. Experimental data available in the literature at three mixture fractions can be well reproduced. Further studies on the unimolecular primary dissociation rate constant kinetics of DTBE are needed.

Acknowledgment This work has been partially supported by the "Arbeitsgemeinschaft Industrieller Forschungsvereinigungen” (AIF). We are thankful for that. References (1) (2) (3) (4) (5) (6)

(7) (8) (9) (10) (11) (12) (13) (14) (15) (16) (17)

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