(181a) Heat Recuperation Strategies for Microreactors: Thermal Management in Gas-Phase and Catalytic Propane/air Oxidation

There has been increasing interest in the development of new portable power generation devices. Based on the relatively high gravimetric energy density of liquid fuels [1], an integrated thermoelectric/microreactor system with relatively low chemical to electrical energy conversion efficiency has an overall energy density on par with conventional batteries. It has recently been suggested that effective thermal management of the released heat can improve the efficiency of these systems so that their energy density exceeds that of traditional batteries[2]. Based on the increased area to volume ratio, heat losses play in important role in the overall stability of these microsystems. In addition, the thermal efficiency of the microreactor must be optimized, as enthalpy is lost in the exiting gases. One thermal management strategy to overcome these loses is the utilization of ?excess enthalpy? burners, suggested first by Lloyd and Weinberg [3], where the hot exiting products are used to exchange heat with the incoming cool reactants. Heat recirculation is an example of these ?excess enthalpy? reactors and was used in this study of the enhancement of catalytic microcombustor stability with respect to single channel systems.

The heat loss stability of both gas-phase [4] and catalytic [5] microcombustion of propane/air mixtures was studied in single channel and heat recirculation reactors. Using the commercial, computational fluid dynamics (CFD) package Fluent, the effect of wall thermal conductivity on the critical heat loss coefficient was determined. It was observed in both gas-phase and catalytic systems that the heat recirculation benefit is only observed in the limit of lower thermal conductivity walls. This is based on the recirculated gases providing additional preheating of the reaction zone, when the axial conductive heat transfer via the walls is insufficient. The heat recirculation effect on the microsystem stability with respect to inlet velocity is also discussed. Comparison of the stability of homogeneous and catalytic microreactors is also done. Select simulation results were tested experimentally. Specifically, stainless steel, single channel and heat recirculation microreactors were fabricated and tested with Pt/anodic alumina catalysts. Consistent with CFD results, the conductive wall heat recirculation system was not observed to demonstrate a significant increase in stability.

References:

1. Sitzki, L., et al. Combustion in Microscale Heat-Recirculating Burners. in The Third Asia-Pacific Conference on Combustion. 2001. Seoul, Korea.

2. Federici, J.A., et al., Catalytic microcombustors with integrated thermoelectric elements for portable power production. J. Power Sources, 2006. 161(2): p. 1469-1478.

3. Lloyd, S.A. and F.J. Weinberg, A burner for mixtures of a very low heat content. Nature, 1974. 251: p. 47-49.

4. Federici, J.A. and D.G. Vlachos, A Computational Fluid Dynamics Study of Propane/Air Microflame Stability in a Heat Recirculation System. Comb. and Flame, 2007. Accepted: p. doi:10.1016/j.combustflame.2007.09.009.

5. Federici, J.A. and D.G. Vlachos, Catalytic Combustion of Propane/Air Mixtures in Single Channel and Heat Recirculation Microburners: An Experimental and Computational Fluid Dynamics Modeling Comparison of Stability. 32nd International Symposium of Combustion, 2007. Submitted.