Optimization of a catalytic combustor using electrosprayed liquid hydrocarbons for mesoscale power generation

A detailed study on the performance of a combustor to be used as a portable power source for mesoscale applications is presented. The burner operation is based on the combination of liquid fuel electrospray injection with combustion through a stack of catalytically coated grids, for the delivery of ≈ 100 W of thermal power. The main design challenges relate to emission minimization, versatility for the coupling to power conversion modules, thermal management, and miniaturization. Combustion efficiency and emission reduction were pursued through catalyst optimization. Using two-dimensional infrared temperature measurements and gas chromatography/mass spectrometry/flame ionization detection exhaust gas analysis, we established a catalyst formulation which provides in excess of 99% combustion efficiency, based on the conversion of the parent hydrocarbon and air to CO2 and H2O. Remarkably, reliable catalyst operation was achieved even using the notoriously polluting JP8, with as many as 1200 ppm of sulfur naturally present in the fuel. CO emission is undetectable and catalytic surface temperatures fall in the 900–1500 K range, which is appropriate for coupling with thermal-to-electric energy conversion systems, such as thermoelectric and Stirling engines. The burner was tested for prolonged operation (500 h) for catalyst stability and aversion to coking, even under conditions of high air inlet temperature, to simulate conditions of heat recuperation that are indispensable to the design of high efficiency mesoscale devices. Droplet sizes reveal the need for fuel distributor multiplexing to minimize vaporization time and therefore the size of the necessary preheat chamber. The results of the characterization of a prototypical device led to an improved design utilizing multijet electrospray injection from a single fuel source, an electrospray ring extractor, and whirl, sideport air injection. In addition to reduced emissions and better temperature uniformity, this improved design relying on conventional fabrication resulted in optimal performance in a volume on the order of 10 cc.  2004 The Combustion Institute. Published by Elsevier Inc. All rights reserved.