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Effect of Combustion Mode on Performance and Startup of Microburners

Niket Kaisare, Chemical Engineering, University of Delaware, 355, Colburn Laboratory, Newark, DE 19716, Soumitra Deshmukh, Velocys Inc., 7950 Corporate Blvd, Plain City, OH 43064, and Dionisios G. Vlachos, Department of Chemical Engineering, University of Delaware, 150 Academy Street, Newark, DE 19716.

An understanding of the performance and stability limits of homogeneous and catalytic combustion is critical in design and operation of microburners used in portable power generation applications. Due to the high surface area to volume ratio and close coupling between the solid and gas phases, the reactor solid structure plays a significant role in microreactors. Consequently, reactor dimensions (length, gap width and wall thickness) and wall physical properties strongly affect the performance of both homogeneous and catalytic microcombustion.

A transient one-dimensional (1D) model with gas as well as catalytic reactions in a parallel plate microreactor with axial symmetry is developed in this work. The model described in our earlier paper [1] is modified to include catalytic reactions as well. The kinetic expression for catalytic combustion is obtained by systematic model reduction of a thermodynamically consistent microkinetic model recently developed in our group [2], whereas the one-step chemistry developed by Westbrook and Dryer [3] is used for homogeneous combustion. Simulations of 2D computational fluid dynamics (CFD) models are used to estimate Nusselt and Sherwood numbers used in the model.

We use the aforementioned model to compare with recently published experimental data from our group [4] and found reasonably good agreement. Next we study the limits of stable combustion in microreactors and to compare the operation of homogeneous combustion with that of catalytic combustion. The burner wall structure is responsible for both heat losses to the surroundings as well as heat recirculation through the solid. Additionally, catalytic reactions occur on the catalyst deposited on the wall surface. We study the effect of reactor dimensions, wall conductivity and inlet velocities on homogeneous and catalytic combustion of propane-air and methane-air mixtures. It is shown that the combustion mode has a substantial effect on the stability of microburners and the mechanism by which stability is lost. By-and-large, catalytic microburners are much more robust and exhibit lower operating temperatures, features which make them attractive for certain portable power generation applications. Next, the transient model is also used to study ignition characteristics in microburners and to develop strategies for fast device startup.

[1] N.S. Kaisare, D.G. Vlachos, Proc. Combust. Inst. 31 (2006) accepted [2] A.B. Mhadeshwar, Ph.D. Thesis, University of Delaware, June 2005 [3] C.K. Westbrook, F.L. Dryer, Combust. Sci. Technol. 27 (1981) 3143 [4] D. G. Norton, E. D. Wetzel, and D. G. Vlachos, Ind. Eng. Chem. Res. 45 (2006) 76-84