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  • 909

    Proceedings of the Combustion Institute, Volume 29, 2002/pp. 909916

    DEVELOPMENT OF A MICROREACTOR AS A THERMAL SOURCE FORMICROELECTROMECHANICAL SYSTEMS POWER GENERATION

    J. VICAN,1 B. F. GAJDECZKO,1 F. L. DRYER,1 D. L. MILIUS,2 I. A. AKSAY2 and R. A. YETTER31Department of Mechanical and Aerospace Engineering

    2Department of Chemical EngineeringPrinceton University

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    Princeton, NJ 08544, USA3Department of Mechanical and Nuclear Engineering

    Propulsion Engineering and Research CenterThe Pennsylvania State UniversityUniversity Park, PA 16802, USA

    An alumina ceramic 12.5 12.5 5.0 mm microreactor was constructed using a modified stereolith-ography process. The design was based on a Swiss roll concept of double spiral-shaped channels tofacilitate a high level of heat transfer between the reactants and combustion products and wall surfacecontact of the flow through the microreactor body. Self-sustained combustion of hydrogen and air mixtureswas demonstrated over a wide range of fuel/air mixtures and flow rates for equivalence ratios from 0.2 to1.0 and chemical energy inputs from 2 to 16 W. Depositing platinum on gamma alumina on the internalwalls enabled catalytic ignition at or near room temperature and self-sustained operation at temperaturesto 300 C. Catalyst degradation was observed at higher operating temperatures and reignition capabilitieswere lost. However, sustained operation could be obtained at wall temperatures in excess of 300 C,apparently stabilized by a combination of surface and gas-phase reaction phenomena. A global energybalance model was developed to analyze overall reactor performance characteristics. The reactor designand operating temperature range have potential applications as a heat source for thermoelectric and py-roelectric power generation at small scales compatible with microelectromechanical systems applications.

    Introduction

    With the successful development of microelectro-mechanical systems (MEMS) devices such as micro-sensors and actuators, there has been considerableinterest in developing chemically fueled micropowergeneration systems that could produce 300 mW orless, with footprints sufficiently small to colocatecommunications, sensors, actuators, and power sys-tems all on the same chip. Chip level integration thatincludes power generation would eliminate inter-connects and benefits applications involving multi-point distributed sensing, communications, and re-sponse. Depending on the battery technologyassessment source [1,2], liquid hydrocarbons arefrom 12 to more than 50 times more energy-dense(by volume) than advanced electrochemical sources,provided that ambient air is available as the oxidizingagent. Thus, overall conversion efficiencies to equiv-alent electrical output as low as 10% would suggestchemical energy conversion could be preferred interms of useful life.

    Conventional gasoline or diesel fuels are not likelyto be useable in MEMS chemical conversion sys-tems, because of fuel chemical instability (property

    degradation) in long-term storage, resulting in par-ticulates and deposits. However, liquefied gaseousfuels (propane, butane, and their conjugate olefins)or hydrocarbon liquids designed specifically for longshelf life (and even higher energy densities) are rea-sonable candidates for MEMS chemical energy con-version devices. Chemically driven power-genera-tion modes may also be advantageous for specificapplications such as complementing battery sourcesfor high-power output or power peaking.

    A myriad of chemical-to-electrical conversion ap-proaches are, in principal, open to consideration atthe MEMS level. Current research efforts using me-chanical motion to generate power include micro-gas turbines, micro-Wankel engines, and linear, mi-cropulsating systems based on oscillating electricalfield and piezoelectric deflections/oscillations [3].Direct energy conversion concepts without movingparts that are being considered include fuel cells andthermal reactors coupled to thermoelectric, photo-voltaic, and pyroelectric devices. Highly efficientfuel cells presently operate only on pure hydrogen.However, hydrogen storage technologies producingvolumetric energy densities approaching liquefiedgaseous fuels do not yet exist. On-board hydrogen

  • 910 NEW CONCEPTS IN COMBUSTION TECHNOLOGYMicro-Power Generation

    Fig. 1. A conceptual power supply with an excess en-thalpy (Swiss roll) catalytic reactor.

    Fig. 2. Cross-section of exemplar microreactor designfabricated by stereolithography. Top flow channel (same onbottom) is visible, spiraling into the center of the structure.

    generation significantly increases the complexity ofthe system, and as a result, may eliminate the overallsystem efficiency advantage and negatively affect re-liability and operating lifetime.

    Thermal microreactors of various types, used withdirect thermal to electrical energy conversion orcoupled with external heat engine concepts offer theadvantages of simplicity, longer-term compatibilitywith various hydrocarbon fuel types, and flexibilityin decoupling system chemical conversion and en-ergy extraction time scales. Of primary importanceto these concepts is a chemical to thermal energyconversion reservoir coupled to methodologies toconvert thermal to electrical energy (Fig. 1). Alongwith us, other groups, for example, Ref. [4], are de-veloping thermal microreactor designs based on theconcept of excess enthalpy combustion [5]. As a re-sult of regeneratively preheating the reactants, com-bustor temperatures can achieve super adiabatictemperatures, increasing combustor efficiency, ex-tending fuel/air mixture limits for which gas-phasekinetics remain explosively chain branched, and re-ducing wall quenching.

    Since 1998, we have been developing thermal mi-croreactors on small scales that would achieve op-erating temperatures below 1100 K that could even-tually incorporate excess enthalpy operating

    principles, that are compatible with eventual use ofliquefied gaseous hydrocarbons as fuels, and thathave potential for being scaled to even smaller di-mensions. The microreactor design described in thispaper (see Fig. 2) is intended to provide a favorableenvironment for microscale combustion of gaseousfuel and air mixtures. A layer of catalyst (platinum)is deposited on the internal walls of the channels tolower the range of the operating temperatures. Thespiral Swiss roll design shown provides a numberof desirable features, including: (1) thermal energytransfer mechanism to preheat the reactants usingthe exhaust heat combustion volume, (2) relativelylarge internal surface/volume ratio needed for theheat transfer through the walls and for effective ac-tion of the catalyst, and (3) large top- and bottom-face surface area for the heat transfer to externaldevices.

    In the present paper, we describe the reactor andthe fabrication process, develop an analytical modelto characterize the performance of the reactor, andpresent results on the operation of the reactor withmixtures of hydrogen and air. In addition, the reactoris coupled to commercial thermoelectric modules todemonstrate chemical to electrical energy conver-sion at the length scales of the present reactor. Ad-vancements in reducing the scale of the reactor, im-proving heat management, and implementinghydrocarbon fuels will be discussed in forthcomingpapers.

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    Microreactor Fabrication

    Three-dimensional stereolithography was utilizedto manufacture the microreactors. A three-dimen-sional virtual solid body produced by computer-aided design is sliced finely (0.075 mm) along theZ-axis, creating a build file consisting of a stack ofXY layers. During the build, sublayers are succes-sively added to the part by raster scanning an UVlaser across its XY surface, wetted by a photocura-ble bath of liquid resin [6]. The photocured, solidi-fied layer is moved down by one layer thickness in-side the liquid bath, and the patterned curingprocess is repeated. The resin used is a highly con-centrated colloidal dispersion prepared by dispers-ing alumina powder in an aqueous solution of ultra-violet curable polymers. The ceramic green bodymanufactured is subsequently dried, the photocur-able binder is burned out, and the part is sintered tofull density.

    The ceramic microreactor part (Figs. 2 and 3) canbe generated in about 4 hr and is an alumina mon-olith with two distinct approximately 800 lm2 chan-nels (one each side) that spiral inward, joining nearthe center of the body. The approximate length ofthe internal path was 0.1 m. A larger central hole isadded to the structure to accommodate the posi-tioning of an electrical resistance heating wire

  • MICROREACTOR FOR MEMS POWER GENERATION 911

    Fig. 3. Stereolithography process with examples of fab-ricated parts. The finished reactor, showing inlet and outletholes on the front surface, is shown in the center.

    through the center of the body to provide sufficientheat to initiate catalytic reactions of a premixed fuel/air mixture flowing through the spiral passages.

    Following stereolithographic fabrication, the en-trained liquids are removed from the unsintered part(green body) and the polymers are burned out of thestructure at low temperatures using a stepwise heat-ing program raising the part temperature to 425 Cover 6 hr. The green body is then sintered at hightemperatures (1550 C for 2 hr) to full density. Post-sintering processing includes the addition of a plat-inum catalyst supported on a gamma alumina washcoat deposited within the microreactor channels.The catalyst is added by injecting hydrogen hexa-chloroplatinate into the channels and heating thedried microreactor to 350 C for 2 hr in air and thenfor 8 hr in hydrogen.

    Microreactor Model Analysis

    As a prelude to detailed computational fluid dy-namics and thermal stress analyses, a simple energybalanc

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