The ability to predict the inter-conversion of poly-aromatic hydrocarbons (PAHs) of different toxicities and emissions of fine carbon-based particles from Diesel engines are of increasing relevance given their harmful effects. The matter is complicated by the complexity of Diesel fuels and model fuel blends have to be used in numerical simulations of practical engines. The use of aromatic fuel component(-s) in such blends provides a route towards the modulation of the propensity of a fuel to produce such emissions provided the chemistry is sufficiently well understood. The current work extends past efforts related to the oxidation of 1-methyl naphthalene, which has been identified as a potential key component of surrogate Diesel fuels. Specifically, 1-methyl naphthalene may be used to modulate sooting tendencies and the methyl groups on aromatic rings (e.g. xylenes and tri-methyl benzenes) have also been identified as important in the context of fuel reactivity.
Past work on the oxidation of single-ring aromatics include the studies by Emdee et al. [1], Lindstedt and Maurice [2] and Klotz et al. [3]. Studies of two-ringed structures are less prevalent. However, Shaddix [4] investigated the oxidation of naphthalene and 1-methyl naphthalene under turbulent flow reactor (TFR) conditions and Pitsch [5] proposed a kinetic mechanism for the oxidation latter species. Lindstedt et al. [6] considered a wide range of PAH formation paths and Dagaut and coworkers [7,8] studied the oxidation of m-xylene and 1-methyl naphthalene under jet-stirred reactor (JSR) conditions. The chemistry of soot/PAH growth and oxidation tends to be slow compared to flow time-scales and detailed studies of turbulent flames are less prevalent. Lindstedt and Louloudi [9] reported a study of the formation and oxidation of soot in turbulent diffusion flames using a transported PDF method combined with the method of moments and with a soot surface oxidation analogy based on naphthalene. The explicit functional form of the latter was obtained using the systematic reduction technique of Peters (e.g. Peters and Rogg [10]). The chemistry of naphthalene and indene were subsequently explored by Lindstedt et al. [11] in an effort to further evaluate the ability of a fixed sectional method [12] to compute soot particle size distributions in the size range from < 1 nm to 100 nm. Critical reaction steps in the oxidation process were identified and subjected to detailed investigations via quantum mechanical methods using Gaussian-03 [13] with rate constants determined from the potential energy surfaces using variable transition state theory and RiceRamsperger-Kassel-Marcus/master equation approaches. The critical reaction paths included C9H7 + HO2/O2 channels and the linkage of C5 and C6 rings as part of the oxidation process [11]. The current work further assesses the progress made in the understanding of the associated reaction paths for two-ringed aromatics. In particular, attention is given to 1-methyl naphthalene which has been identified as a means of modulating the sooting propensity of surrogate Diesel fuels. The applied detailed chemical reaction mechanism was initially created from reaction classes derived from studies of the oxidation and pyrolysis of toluene, benzene and cyclo-pentadiene. Accurate thermodynamic data is particularly important given the large number of isomerisation reaction present in detailed reaction sequences for PAH formation/oxidation. As part of the present work, earlier estimates, often obtained on the basis of variants of Benson's additivity method, were replaced by data derived from quantum mechanical methods using Gaussian-03 (at the G3MP2B3 level) in combination with density functional theory (DFT) analysis for internal rotations. The validation of the derived mechanism was achieved by comparison with experimental data from jet stirred and turbulent flow reactors.
The 1-methyl naphthalene chemistry was initially tested under JSR conditions using data from Mati et al. [8] for the conditions shown in Table 1.
The data is suitable for clarifying the decomposition channels of the fuel and provides extensive information on stable species.
Φ 0.5 1.0 1.5
P (atm) 1.0 1.0 1.0
T (K) 1097-1290 1094-1400 1147-1440
O2
Shaddix [4] performed gas-phase sampling to study the oxidation of 1-methyl naphthalene in a
TFR and obtained time-dependent concentration
1.5
1.0
1166
profiles for major species under the conditions A rate analysis was performed at a temperature of
shown in Table 2. In the current study, computa- 1202 K to highlight key reaction pathways. The
1tions were performed corresponding to all the ex- methyl naphthalene oxidation is overall controlled
perimental conditions. The focal point of the cur- by reactions (
In contrast to other studies, the developed
mechanism does not feature any global reaction steps
and generally reasonable agreement was obtained
between computations and the experimental data
obtained by Mati et al. [8] as exemplified for fuel
rich oxidation shown in Fig. 1.
Methane is produced by reactions (
Reaction (
C11H10 + CH3 = C11H9 + CH4 C11H10 + CH3 = C11H9P + CH4
CH3 + HO2 = CH4 + O2
The CH3 radical pool is predominantly formed via
reactions (
C9H7 + CH3 = C9H7CH3
C11H10 + H = C10H8
+ CH3
(
(
The current work has shown that the breakdown products resulting from the oxidation of 1methyl naphthalene can be modeled with reasonable accuracy. The actual distribution of the primary two-ring and secondary single-ring aromatics is important as it can be expected that the product distribution will influence the distribution of subsequent PAHs formed during the fuel oxidation process. Irrespective of such considerations, the ability to represent the chemical kinetics of a key surrogate fuel component represents an important step towards the creation of increasingly accurate surrogate Diesel fuel blends for the use in practical engine simulations following the application of simplification techniques.
The authors are grateful for the financial support of EOARD under award FA8655-06-1-3052 and BP Global Fuels Ltd.