46751-AC9
Biodiesel Ignition Experiments and Chemical-Kinetic Modeling
Objective This project focuses on studying the ignition properties of biodiesel. Methyl butanoate is chosen as a surrogate biodiesel fuel because it has a similar molecular structure and a combustion model has been proposed [1]. The auto-ignition of methyl butanoate is studied by determining ignition delays behind reflected shock waves in a shock tube. The ignition delays are determined from end-wall pressure histories and chemiluminiscence of the CH combustion intermediate. These data are used to test predictions of a chemical kinetic model in order to improve and validate it. Approach A shock tube facility has been constructed to study the auto-ignition characteristics of biodiesel surrogate fuels. In order to determine the thermodynamic conditions behind the reflected shock, the shock tube was characterized with non-reactive shots. It is known that non-ideal effects in a shock tube result in attenuation of the shock velocity [2]. The shock attenuation of the present facility was investigated and found to lie within acceptable limits. Accomplishments Initial ignition studies of methyl butanoate/air mixtures have been carried out at pressures of 1 and 5 atm using measured pressure histories. The results were presented in a Work-in-Progress Poster at the 32nd International Symposium on Combustion, 2008. In addition to pressure histories, CH chemiluminiscence observed using a photo multiplier tube has been added in recent experiments. An undergraduate project has been completed that designed an improved test section for the shock ignition facility. Presently, an undergraduate honour thesis student and a PhD student are working with the principal investigator on the project. These students have acquired sound training in combustion experiments and modeling. Experimental studies of methyl butanoate/air ignition at pressures of 1 and 5 atm have identified the influence of pressure, equivalence ratio and temperature on methyl butanoate ignition. The available combustion model has been observed to predict ignition delays that are shorter than experimental measurements [1, 3]. Comparing the preliminary results obtained in this work with the original model [1] confirms that the model under predicts ignition delays. The discrepancy is thought to be linked to uncertainties in rate parameters of key reactions with the fuel molecules. Sensitivity analysis has been performed to identify the controlling reactions in the model. It is found that some of the reactions with the most significant influence on ignition are radical reactions with methyl butanoate as well as well-established and reliable reactions from hydrogen and methane oxidation sub-mechanisms. The reactions specific to methyl butanoate were originally suggested based on analogy and group additivity rules [1]. These rate parameters have been compared with their straight chained alkane analogues, suggesting that the bond additivity and similarity methods may not be sufficient. Modifications to the original mechanism have been undertaken by other researchers [3]. The H-abstraction reactions of the original fuel molecules have most often been targeted for optimization to obtain agreement with experimental results. These modifications are presently being examined and compared to the new experimental data obtained in this work. Future work - Refinement of measurement techniques in order to further reduce the experimental uncertainty. - Further studies on methyl butanoate with a focus on mechanism validation. The modified versions [3] of the Fisher et al. mechanism [1] will be studied and validated. - Methyl butanoate ignition will be compared to other short-carbon-chain methyl esters, such as methyl methanoate and methyl acetate, in order to establish the influence of the length of the alkyl group on methyl ester ignition. This is important if the results are to be extended to the long-chain methyl esters that make up true biodiesel fuels. - Comparative studies of methyl butanoate, butane, butylaldehyde, butanone and butanol to establish the influence of the methoxy group on ester ignition. References [1] E.M. Fisher, W.J. Pitz, H.J. Curran, and C.K. Westbrook (2000) Proc. Combust. Inst, 28:1579-1586. [2] E.L. Petersen, R.K. Hanson (2001) Shock Waves, 10:405-420. [3] Metcalfe, W.K, Dooley, S., Curran, H.J., Simmie, J.M., El-Nahas, A.M. and Navarro, V.M (2007) J. Phys. Chem. A 111:4001-4014.
