Wobbe Index meter is widely used to reflect the energy content and gas quality of a fuel gas mixture. It is highly in demand to downscale from the conventionally large Wobbe Index facility to a miniaturized Wobbe Index meter. Therefore spontaneously combustion of the fuel gas/air mixtures on a MEMS-based combustor can provide in-line temperature monitoring of the combustion produced heat. However, flame can be quenched by the channel walls when the walls are closely spaced. Therefore, it is crucial to design a micro-combustor with good thermal management and strong mechanical stability. MEMS-based Trench-Assisted Surface Channel Technology is designed and developed to realize these large-volume suspended combustor structures

de Boer, Meint J.

Groenesteijn, Jarno

Lötters, J.C.

Veltkamp, Henk-Willem

Wiegerink, R.J.

Zeng, Yaxiang

Zhao, Yiyuan

English

Wiegerink, Remco J.

Lötters, Joost Conrad

University of Twente Research Information

Design principles and fabrication method for a miniaturized fuel gas combustion reactor

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Sensors PosterDESIGN PRINCIPLES AND FABRICATION METHOD FOR A MINIATURIZED FUEL GASCOMBUSTION REACTORY. Zhao1, H.-W. Veltkamp1, M.J. de Boer1, Y. Zeng1, J. Groenesteijn2, R.J. Wiegerink1, and J.C. Lötters1,21 MESA+ Institute for Nanotechnology, University of Twente, Enschede, The Netherlands2 Bronkhorst High-Tech BV, Ruurlo, The NetherlandsABSTRACTWobbe Index meter is widely used to reflectthe energy content and gas quality of a fuel gasmixture. It is highly in demand to downscalefrom the conventionally large Wobbe Index facilityto a miniaturized Wobbe Index meter. Thereforespontaneously combustion of the fuel gas/air mixtureson a MEMS-based combustor can provide in-linetemperature monitoring of the combustion producedheat. However, flame can be quenched by thechannel walls when the walls are closely spaced.Therefore, it is crucial to design a micro-combustorwith good thermal management and strong mechanicalstability. MEMS-based Trench-Assisted SurfaceChannel Technology is designed and developedto realize these large-volume suspended combustorstructures.KEYWORDSMicro-Combustion; Exothermic Microreactor;Micro-flame Quenching; Trench-Assisted SurfaceChannel Technology.INTRODUCTIONGas exchangeabilityThe composition of natural gas originating fromdifferent countries differs substantially. In theNetherlands, real-time information of the natural gascomposition is getting more important since the Dutchgas grid will change with the introduction of biogas,hydrogen and foreign natural gas.What’s more, thetransition of natural gas payment from per cubic meter toper amount of heat generated also requires precise in-linegas quality monitoring. Gas blends need to be tunedto deliver a constant energy content to the end-users.Wobbe Index is widely used in many countries to reflectthe energy content of fuel gases.[1] For any givenorifice, all the gas mixtures that have the same WobbeIndex will deliver the same amount of heat and areinterchangeable.[2]Wobbe IndexWobbe Index is calculated by the Higher HeatingValue and the square root of the Specific Gravity of thefuel gas, as shown in equation (1).Wobbe Index =Higher Heating Value√Specific Gravity(1)Heating values [MJkg−1] of a fuel are used toquantify the maximum amount of heat that can begenerated by the fuel combustion with air at the standardconditions (25 ◦C and 101.3 kPa). The amount of heatrelease from the fuel combustion will depend on thephase of water in the products. If the product water isin the gas phase, the total heat release value is denotedas the Lower Heating Value (LHV). When the watervapor is condensed to liquid, additional energy equalsto the latent heat of water vaporization can be extracted.The value of the total energy release is called the HigherHeating Value (HHV). The value of the LHV and HHVis related by the amount of energy released during thephase change of water between the vapor and liquid, asshown in equation (2):[3]LHV = HHV − NH2O,P ×MH2O × hfgNfuel ×Mfuel(2)where NH2O,P is the number of moles of water inthe products and MH2O,P is the molecular weight ofwater. Latent heat for water at standard conditions ishfg = 2.44 MJkg−1. Nfuel is the number of moles offuel burned and Mfuel is the molecular mass of the fuel.Specific Gravity, also called relative density, isdefined as the density of the fuel gas divided by thedensity of dry air of standard composition at the samespecified conditions of pressure and temperature.[3]Higher Heating Value from a constant-volumereactorFor a closed system of the constant-volume reactor,at the standard conditions, the Higher Heating Value ofThe 3rd Conference on MicroFluidic Handling Systems, 4–6 October 2017, Enschede, The Netherlands37a fuel is calculated as shown in equation (3):[3]HHV =−Q0rxn,v − (∑iNi,P −∑iNi,R)× R̂u × T0Nfuel ×Mfuel(3)where Ni,P and Ni,R are the number of moles ofspieces i in the product and reactant, respectively. R̂uis the universal gas constant and T0 is the standardtemperature. Nfuel is the number of moles of fuel burnedand Mfuel is the molecular mass of the fuel. The amountof heat transfer −Q0rxn,v flows from the combustionsystem to the environment can be estimated dependson the specific heat exchange mechanism of the reactor,namely convective heat transfer, conductive heat transferand radiative heat transfer.DESIGN PRINCIPLES FOR ASMALL-SCALE WOBBE INDEX METERMiniaturization of Wobbe Index meterFor the central heating systems and fuel gassupplies industries, conventionally available WobbeIndex facilities have large volume of around 1m3.Moreover, they cost more than 50, 000e and consumeslarge quantities of gas sample to measure the WobbeIndex of the fuel gas. For in-line gas quality monitoringand easy operations in the remote areas, an on-chipWobbe Index meter[4] shows promising benefits ofdownscaling both the device size and the energyconsumption when comparing to the currently availablesystems.Our envisioned integrated Wobbe Index meterconsists of a gas density sensor and a small-scaledcombustor fabricated by the same MEMS technology.Proof-of-concept for a density sensor has beendemonstrated from the micro-Coriolis flow sensor.[5]Here we will discuss the design principles and proposea fabrication technology to realize the small combustor.Future work is planned to design and fabricate a gasdensity sensor by the same technology.Our approach to experimentally obtain the HHVvalue of the fuel gases is by performing completecombustion of the fuel gases on-chip with precisetemperature monitoring and in a controlled volumecombustion chamber. The combustor chip designconsists of mainly three functional parts, namely themixer, preheater and combustor. Stoichiometric fuel gasand air are first sufficiently mixed in a diffusion-drivenmixing channel. Then the well-mixed combustible gasmixture can be preheated to just below the auto-ignitiontemperature of the fuel gas. Once the completecombustion takes place in the designated combustionchamber, the flame temperature and flame burningvelocity can be measured and deduced by the localtemperature sensor. Finally, HHV of fuel gases can befurther calculated by thermodynamic relations dependson the mechanism of the heat transfer from the hotcombustor to the temperature sensor.However, down-scaling of the combustor sizesintroduces new problems, namely the micro-flamequenching phenomena.Flame propagationFuel gas complete combustion with thestoichiometric or excess air is a highly exothermicreaction. The reaction has a self-propagating reactionzone which is also known as flames. The burningvelocity and the flame temperature are the twocharacteristic properties of flames. Flames willpropagate through the stationary unburnt gas mixture ata characteristic burning velocity. The velocity of thiswave is controlled by both the diffusion of heat and thediffusion of active radicals. In details, a larminar flamepropagation in premixed gases has two mechanisms:firstly, the unburnt mixture is conductively heated bythe burnt zone and become sufficiently rapid to startself-propagating. Secondly, the active radicals diffusefrom the burnt reaction zone to the unburnt mixture andignite the exothermic reaction. Therefore, both massand heat diffusion of the active radicals contribute tothe propagation of the reaction zone and determinesthe burning velocity and flame temperature. However,down-scaling of the combustor channel dimensionsfrom macro-sizes to micro-sizes will probably causeflame quenched by the closely-spaced walls.Down-scaling caused flame quenchingFuel gas combustion in small-scale channels hasbeen reported to suffer from flame extinction.[6] Inthe microchannels where the surface-to-volume ratiois so large that the heat loss through the channel wallsurfaces to the environment becomes substrantial. Oncethe heat loss to the surroundings is even larger than theheat generated from the exothermic combustion reaction,the flame or the reaction zone stops propagating in thechannel and extinguishes,this is known as the thermalquenching of the micro-flames.[6] Therefore, in orderto maintain continuous flame propagation, there exists aThe 3rd Conference on MicroFluidic Handling Systems, 4–6 October 2017, Enschede, The Netherlands38threshold where the characteristic channel dimensionmust be larger than the critical quenching diameterof the fuel gas.[7] Good thermal management of thecombustion reactor is crucial to surpress the thermalquenching of the micro-flames. Promising results fromthe Swiss-Roll combustors[8] have shown that the excessenthalpy or heat recirculation can extend the flameflammability limit to even under the traditionally knowncritical quenching distance for the fuel gas.Another down-scaling effect taking place in thesmall combustion reactors is the radical quenching.When the characteristic channel diameter is not muchlarger than the mean free path of the reactants molecules,a substantial amount of active radicals would easilydiffuse from the homogeneius combustion zone to thesurface of the inner walls. These heterogeneous radicalsadsorption and recombination at the inner wall surfacewould cause the flame quenched even when there islittle heat loss to the environment.[7] A straightforwardsolution to minimize the radical quenching is to choosechemically inert channel inner wall materials. Also,temperature of the channel walls plays a major role tocontrol the kinetics of the adsorption, recombination anddesorption of the radicals and molecules.Summary of the combustor design principlesIn order to fabricate a small-sized combustor toburn the fuel gas on chip and measure the elavatedtemperature from the combustion produced heat, it isnecessary to maintain a continuous flame propagationwithout flames extinction induced by thermal quenchingor radical quenching. The most important principles fordesigning a micro-combustor are as follows: Firstly,design a combustion channel with sufficiently largecross-sectional area is the key principle for sustaininga continuous flame propagation. In addition, excessenthalpy can be supplied to the system by heating upthe combustor channel walls and preheat the unburntgas mixtures. Moreover, chemically inert wall materialand channel wall temperature tuning is key to reduceradical quenched by the wall. Finally, a reliable andaccurate local temperature sensor is essential for in-linemonitoring the flame location and burning speed.TRENCH-ASSISTED SURFACE CHANNELTECHNOLOGYWe developed a micromachining fabrication processcalled Trench-Assisted Surface Channel Technology(TASCT) to realize such a micro-combustor with a largeinternal volume. TASCT is evolved from the conceptof the Surface Channel Technology[9][10] developedfor the micro-Coriolis chips, where the silicon waferis isotropically etched through the small-slit pattern tocreate channels right underneath the surface of the wafer.These surface channels have the cross-sectionalshape of a partially circular channel with a flat top, thechannel size and shape are determined by the designedslits pattern. Channels with diameter around 300 µmcan still be made by the Surface Channel Technology.However, the mechanical stability are likely to decreaseonce the channel diameter becomes larger. In orderto make channels with larger cross-sectional area, weintroduce new features of high aspect-ratio trenches intothe Surface Channel Technology concept.The TASCT process requires a SOI wafer and allthe channels are realized in the device layer. Thefabricated channels and chambers have a rectangularcross-section, as illustrated in Figure 1 of a SOI waferwith cross-sectional view. The height of the channelside walls is 50 µm, which is defined by the device layerthickness of the SOI wafer. Surface channels of anydesired planar shape and dimension can be achievedthrough the combination of the high aspect-ratio refilledtrenches and the Surface Channel technology. SOI waferis heavily doped and the silicon heaters are defined bythe 3 µm wide trenches to heat up the chamber side walls.Platinum heaters and temperature sensors are placed ontop of chamber to heat up the chamber roof membraneand sense the temperature variation. To provide thermalisolation, large cavities can be made underneath and tothe sides of the combustion chamber. Flexure structuresare made of channels across the side cavities and theyare not depicted here.Figure 1: Schematical cross-sectional view of a suspendedlarge-volume combustion chamber using TASCT fabricationprocess, key features are indicated in the illustration.A more detailed description of TASCT can befound in the MFHS extended abstract submissionThe 3rd Conference on MicroFluidic Handling Systems, 4–6 October 2017, Enschede, The Netherlands39from H.-W. Veltkamp with the title ”Fabricationof Large-Volume Rectangular Channels usingTrench-Sidewall Technology and A SOI Substrate”.CONCLUSION AND FUTURE WORKIn conclusion, we discussed the motivationof developing a miniaturized Wobbe Index meter.Flame quenching phenomena in the small channelsdrive us to discover the principles when designingthe micro-combustor suitable to have continuouscombustion of the fuel gas. We also developed aninnovative fabrication method TASCT to realize alarge-volume combustor channel with good thermalmanagement and strong mechanical strength from a SOIwafer. Future work will be continuing the fabrication ofa complete Wobbe Index meter with the integration of allthe fluidic connections and heat management electronicsto perform fuel gas combustion on the chip.ACKNOWLEDGEMENTThis work is part of the research programmeIntegrated Wobbe Index Meter with project number13952, which is (partly) financed by the DutchTechnology Foundation STW, which is part of theNetherlands Organisation for Scientific Research(NWO).REFERENCES[1] Union, I. G. (2011). Petroleum B. guidebook togas interchangeability and gas quality.[2] Zachariah-Wolff, J. L., Egyedi, T. M., andHemmes, K. (2007). From natural gas to hydrogenvia the Wobbe index: The role of standardizedgateways in sustainable infrastructure transitions.International journal of hydrogen energy, 32(9),1235-1245.[3] McAllister, S., Chen, J. Y., and Fernandez-Pello, A.C. (2011). Fundamentals of combustion processes.New York: Springer.[4] Lötters, J. C., Lammerink, T. S., Pap, M. G.,Sanders, R. G., de Boer, M. J., Mouris, A. J.,and Wiegerink, R. J. (2013, January). Integratedmicro Wobbe index meter towards on-chip energycontent measurement. In Micro Electro MechanicalSystems (MEMS), 2013 IEEE 26th InternationalConference on (pp. 965-968). IEEE.[5] van der Wouden, E., Groenesteijn, J., Wiegerink,R., and Lötters, J. (2015). Multi parameter flowmeter for on-line measurement of gas mixturecomposition. Micromachines, 6(4), 452-461.[6] Ju, Y., and Xu, B. (2005). Theoretical andexperimental studies on mesoscale flamepropagation and extinction. Proceedings of theCombustion Institute, 30(2), 2445-2453.[7] Maruta, K. (2011). Micro and mesoscalecombustion. Proceedings of the CombustionInstitute, 33(1), 125-150.[8] Ju, Y., and Choi, C. W. (2003). An analysisof sub-limit flame dynamics using oppositepropagating flames in mesoscale channels.Combustion and Flame, 133(4), 483-493.[9] Groenesteijn, J., de Boer, M. J., Lötters, J. C., andWiegerink, R. J. (2017). A versatile technologyplatform for microfluidic handling systems, partI: fabrication and functionalization. Microfluidicsand Nanofluidics, 21(7), 127.[10] Dijkstra, M., De Boer, M. J., Berenschot, J.W., Lammerink, T. S. J., Wiegerink, R. J.,and Elwenspoek, M. (2007). A versatile surfacechannel concept for microfluidic applications.Journal of Micromechanics and Microengineering,17(10), 1971.CONTACT* Yiyuan Zhao, y.zhao-5@utwente.nlThe 3rd Conference on MicroFluidic Handling Systems, 4–6 October 2017, Enschede, The Netherlands40

Design principles and fabrication method for a miniaturized fuel gas combustion reactor

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