(581d) Enhancing Microburner Stability for Portable Power through Heat Recuperation

Kaisare, N. - Presenter, University of Delaware

Microcombustion will be central to portable power and/or hydrogen generation for distributed energy production. A major obstacle in using microcombustion is the limited range of operation. The idea of using the heat of combustion effluents to preheat the incoming reactants was pioneered by Weinberg and coworkers [1,2]. Lloyd and Weinberg [2] provided several different configurations of such ?excess enthalpy burners?. Driven by the demands of using hydrocarbon-based portable energy devices, there is a growing interest in studying means to enhance stability of microburners through heat recuperation.

First, we investigate the use of reverse-flow (RF) operation to increase the limits of stable self-sustained homogeneous combustion in a microreactor, equipped with four control valves that are used to periodically reverse the flow direction. Such RF burners are called heat regenerative burners. The energy stored in the solid structure in one cycle is released to the cold inlet stream in the next cycle, resulting in heat recirculation. The RF operation improves the region of autothermal operation through a ?heat trap? effect [3]. While the RF operation has been used in catalytic reactors for autothermal combustion of lean mixtures of volatile organics [4], we extend this concept to homogeneous micro-combustion of a stoichiometric propane-air mixture in this work. To this end, we develop a transient one-dimensional (1D) model, which explicitly accounts for transport, diffusion and reaction in the gas phase, and axial thermal conduction in the solid and gas phases.

The next idea is to carry out combustion in a counter current heat exchanger arrangement, wherein the exit hot gases in one channel are used to preheat the cold reactants in the other channel. Such devices are called heat-recirculating burners [5]. The aim of this work is to compare and contrast the performance of heat recirculating burners and periodically operated heat regenerative burners. We study the effect of design and operating conditions on the stability of these devices. Finally reactor designs that maximize the amount of energy generated per unit reactor, width while maintaining the maximum device temperature at reasonable values, are proposed.

[1] A.R. Jones, S.A. Lloyd, F.J. Weinberg, Proc. Roy. Soc. Lond. A 360 (1978) 97-115. [2] S.A. Lloyd, F.J. Weinberg, Nature 251 (1974) 47-49. [3] R.E. Hayes, Chem. Engng Sci. 59 (2004) 4073-4080. [4] G. Kolios, J. Frauhammer, G. Eigenberger, Chem. Engng Sci. 55 (2000) 5945-5967. [5] P.D. Ronney, Combust. Flame 135 (2003) 421-439.