(167a) Unravelling the Intial Decomposition Chemistry of Renewable Fuels

In comparison to fossil derived hydrocarbons the chemistry of combustion and pyrolysis of oxygen containing renewable fuels is quite poorly understood[1]. The available kinetic models developed for combustion of mixtures of hydrocarbons and oxygenated compounds do a reasonable job in predicting auto-ignition, flame propagation parameters, and the formation of the main regulated pollutants. However they do quite poorly for predicting the formation of minor products (alkenes, dienes, aromatics, aldehydes) and soot, which have a deleterious impact on both the environment and on human health. For example most of the kinetic models simply do not include formation of aromatics, let alone soot. Moreover, the validation for toxic aldehydes such as formaldehyde is often impossible because these components are quite difficult to measure quantitatively.

            In this contribution we will evaluate the decomposition chemistry of different candidates of renewable fuel (additives) ranging from longer alcohols (butanols, pentanols) to esters (saturated, unsaturated, cyclic) and furans both experimentally and theoretically.New experimental data have been acquired on a new setup that is both equipped with a plug flow reactor made out of quartz and Incoloy800HT. The latter allows to determine valuable kinetic data and at the same time evaluate the importance of surface reactions on the decomposition chemistry which could be important for real engines. The analysis section of the bench scale pyrolysis set-up has been discussed at length previously [2-6]. The effect of temperature on the product distribution was determined for two dilution regimes. During the currently discussed set of experiments the reactors are operated almost isothermally. The pressure drop over the reactor was found to be negligible. The analysis section of the pyrolysis set-up enables on-line qualification and quantification of the entire product stream, i.e. a wide boiling mixture containing permanent gasses (H₂, CO, CO₂, etc.), alcohols, aldehydes and ketones, esters, and hydrocarbons ranging from methane to polyaromatic hydrocarbons (PAH). As discussed previously[4], three different gas chromatographs are required: a refinery gas analyzer (RGA), a light oxygenates analyzer (LOA) and a GC×GC-FID/(TOF-MS). Response factors (RF_i) of all permanent gasses and light hydrocarbons (C₂-C₄) were determined with the aid of a gaseous calibration mixture provided by Air Liquide. The response factors of all C₅₊ hydrocarbons and methyl esters were determined using the effective carbon number concept, relative to methane.

The application Gaussian-09 [7] was used to carry out the computation of geometries, frequencies and energies of all the species. The model chemistries CBS-QB3, CBS-APNO and G3 were employed to determine the species enthalpy, H, and free energy, G, at 298.15 K giving a reasonable compromise between accuracy and computational expense [8-12]. The use of three distinctly different composite methods is crucial to an appreciation of the uncertainties with which energies can be determined[13, 14]. The three model chemistries differ substantially in their geometry optimization and vibrational frequency calculation procedures and in their corrections to the electronic energy. Visualization and manipulation of the Gaussian output was provided by Chemcraft[15]. The vibrational frequencies and moments of inertia of each molecule were computed at the B3LYP/6-311G(2d,d,p) level of theory (this is the default optimiser in the CBS-QB3 composite method) together with the keyword Freq=HinderedRotor. The latter choice forces an analysis of the vibrational modes and selects those that are more appropriately considered to be hindered rotors at the temperature at which the calculation is being carried out. The net effect is that primary statistical thermodynamic data alters as a vibration is re-classified as a hindered rotor. Choosing a different functional and basis set makes very little difference to the computed entropy; for example, using BMK/6-31+G(2df,p) with an entropy-specific scaling factor reduces the entropy of 5-methyl-2-acetylfuran by approximately 1% from the default B3LYP/6-311G(2d,d,p) values. In order to verify the above mentioned method the Thermo module of Multiwell [16] was used to calculate thermodynamic parameters under the rigid rotor harmonic oscillator approximation. In appropriate cases the barriers to internal rotations were computed by relaxed dihedral angle scans at the B3LYP/6-311G(2d,d,p) level and the results treated as 1-dimensional hindered rotors. The ab-initio calculations will be used to show the differences in how these different renewable components decompose under our conditions.

References

1.            Metcalfe, W.K., J.M. Simmie, and H.J. Curran, Ab Initio Chemical Kinetics of Methyl Formate Decomposition: The Simplest Model Biodiesel. Journal of Physical Chemistry A, 2010. 114(17): p. 5478-5484.

2.            Harper, M.R., et al., Comprehensive reaction mechanism for n-butanol pyrolysis and combustion. Combustion and Flame, 2011. 158(1): p. 16-41.

3.            Van Geem, K.M., et al., Accurate High-Temperature Reaction Networks for Alternative Fuels: Butanol Isomers. Industrial & Engineering Chemistry Research, 2010. 49(21): p. 10399-10420.

4.            Pyl, S.P., et al., Rapeseed oil methyl ester pyrolysis: On-line product analysis using comprehensive two-dimensional gas chromatography. Journal of Chromatography A, 2011. 1218(21): p. 3217-3223.

5.            Chen, Q. and G.F. Froment, Thermal-Cracking of Substituted Aromatic-Hydrocarbons .1. Kinetic-Study of the Thermal-Cracking of I-Propylbenzene. Journal of Analytical and Applied Pyrolysis, 1991. 21(1-2): p. 27-50.

6.            Chen, Q. and G.F. Froment, Thermal-Cracking of Substituted Aromatic-Hydrocarbons .2. Kinetic-Study of the Thermal-Cracking of Normal-Propylbenzene and Ethylbenzene. Journal of Analytical and Applied Pyrolysis, 1991. 21(1-2): p. 51-77.

7.            M.J. Frisch, et al., Gaussian 09, 2009: Wallingford, CT.

8.            Curtiss, L.A., et al., Gaussian-2 Theory for Molecular-Energies of 1St-Row and 2Nd-Row Compounds. Journal of Chemical Physics, 1991. 94(11): p. 7221-7230.

9.            Curtiss, L.A., et al., Assessment of complete basis set methods for calculation of enthalpies of formation. Journal of Chemical Physics, 1998. 108(2): p. 692-697.

10.          Curtiss, L.A., et al., Gaussian-3X (G3X) theory: Use of improved geometries, zero-point energies, and Hartree-Fock basis sets. Journal of Chemical Physics, 2001. 114(1): p. 108-117.

11.          Ochterski, J.W., G.A. Petersson, and J.A. Montgomery, A complete basis set model chemistry .5. Extensions to six or more heavy atoms. Journal of Chemical Physics, 1996. 104(7): p. 2598-2619.

12.          Montgomery, J.A., et al., A complete basis set model chemistry. VII. Use of the minimum population localization method. Journal of Chemical Physics, 2000. 112(15): p. 6532-6542.

13.          Simmie, J.M., et al., Enthalpies of Formation and Bond Dissociation Energies of Lower Alkyl Hydroperoxides and Related Hydroperoxy and Alkoxy Radicals. The Journal of Physical Chemistry A, 2008. 112(22): p. 5010-5016.

14.          Simmie, J.M. and H.J. Curran, Formation Enthalpies and Bond Dissociation Energies of Alkylfurans. The Strongest CøXX Bonds Known? The Journal of Physical Chemistry A, 2009. 113(17): p. 5128-5137.

15.          G.A. Zhurko and D.A. Zhurko, chemcraft.

16.          Barker, J.R., Multiple-well, multiple-path unimolecular reaction systems. I. MultiWell computer program suite. International Journal of Chemical Kinetics, 2001. 33(4): p. 232-245.