Catalytic Micro-combustor Research Articles

Analyzing the combustion characteristics of premixed methane-oxygen with different hydrogen addition ratios in a catalytic micro-combustor

Experimental and numerical study of premixed methane-oxygen combustion characteristics in a catalytic micro-combustor with different hydrogen addition ratios has been presented in this paper. The objective is to examine the effects of platinum catalysts and hydrogen additions on methane and oxygen combustion. The simulation of a three-dimensional micro rectangular model was carried out with a gas phase reaction mechanism combined with a surface reaction mechanism, and the model was validated with experimental results with a maximum deviation of just 1.56 percent. In the experimental phase, the effects of flammability limit, equivalence ratio, and inlet velocity were studied, while in the numerical phase, the effect of hydrogenation on combustion characteristics was investigated. Experimental results show that adding 10 % hydrogen to a premixed methane-oxygen mixture significantly increases its flammability limit. The maximum external wall temperature along the centerline was observed at an equivalent ratio of 1. Additionally, an increase in the inlet velocity leads to a corresponding increase in the external wall centerline temperature. Catalytic reaction wall temperatures show a gradual decline, with the highest temperatures located at the inlet. An observed trend revealed a decrease in the external wall centerline temperature as the hydrogen addition increased from 0 % to 40 % due to the reduced total heating value of the blended fuel. Due to its higher reactivity and ability to promote complete combustion, hydrogen also plays an important role in CH4 and O2 conversion. In addition, CO and CO2 emissions decreased with increased hydrogen percentage in fuel mixtures. Furthermore, hydrogen addition can reduce external wall heat losses as well. After adding 0–40 % hydrogen to a premixture of (CH4–O2) fuel, the total heat loss of the external wall was reduced from 19.48 W to 17.11 W.

Significant catalyst savings and reaction kinetics enhancement for catalytic micro-combustors via non-uniform catalyst segmentation

A recent prioritization of energy-efficient and sustainable technologies has fueled an ongoing demand for improving the heat sources necessary to drive thermally dependent processes in portable and small-scale devices. Previous efforts have explored hydrocarbon-based catalytic combustion (and its kinetics and fuel usage efficiency) for reliable heating in microscale devices (TPVs, TEGs, etc.). However, for mesoscale systems that require a larger length scale combustor, fuel management and the ability to maintain consistent heat flux across the combustor become challenging due to kinetics and mass diffusion limitations, which are extrinsic to microscale combustors. In this work, we present the design and experimental evaluation of a compact parallel plate catalytic micro-combustor with non-uniform, segmented catalytic zones that allow for direct control of heating, spatially. Specifically, we performed a comparative FEM study of various catalyst segmentation patterns to further optimize catalyst bed spacing for decreased fuel consumption and improved combustion reaction kinetics. Due to these enhancements, the proposed patterning scheme can achieve similar temperature profiles to those of a fully-coated surface while using 77.8% less catalyst and reducing extreme thermal gradients during the early transient combustion phase. Finally, we demonstrated the capabilities of the proposed catalytic combustor by testing the combustor in a controlled environment.

High energy density entrainment-based catalytic micro-combustor for portable devices

The increasing demand for low-cost, high energy density heat sources has motivated the development of compact and lightweight combustion-based devices. In this work, we developed an energy-dense (≈236 MW/m3) entrainment-based catalytic micro-combustor for heating portable systems. The multichannel micro-combustor (coated with Pt/Al2O3 catalyst) leverages a copper-nichrome wire to enable quick and localized ohmic preheating durations (2–3 mins). Furthermore, we demonstrated low ignition temperature (108–125 °C), which facilitates low energy consumption (∼1948 J). In addition, an optimal fuel flow rate (3.09 × 10-8 m3/s) was determined via FEM simulations and experiments to enable fuel savings (high fuel conversion) while achieving high heat fluxes by analyzing the reaction kinetics and species transport behavior in the microchannels. Through independent testing, we established the micro-combustor’s ability to maintain long-term autothermal combustion at a high saturation wall temperature (585 °C), which was attained at short timescales to enable fast heating/cooling cyclability. The successful cyclic heating demonstration of large thermal mass additions (at least 41 times the micro-combustor’s mass), coupled with the combustor’s high energy density, shows promise for device-level implementation for a range of commercial, defense, and energy conversion applications.

Kinetics of catalytic oxidation of oxygenated fuels on Pt/ZSM-5 catalyst

To build a set of complete kinetic parameters of oxygenated fuels kinetic model on Pt catalyst, methanol was used as an example to carry out the catalytic oxidation kinetics experiment of oxygenated fuels on Pt/ZSM-5 catalyst. The Power law model and Langmuir–Hinshelwood (L–H) model were established to characterise the catalytic oxidation reaction of methanol. Then the oxidation kinetics of methanol, ethanol, dimethyl ether (DME) and n-butanol on Pt/ZSM-5 was studied under the same experimental conditions. It was found that the reaction orders of fuel molecules (methanol is −0.14) were much less than that of oxygen (1.23) in Power law model. The adsorption constants of fuel molecules were higher than that of oxygen in L–H model. The adsorption characteristics of alcohols on Pt were similar, but the reaction orders of alcohols were not consistent. The adsorption constants and adsorption heat of dimethyl ether were much larger than that of alcohols. The intrinsic reaction rates of four oxygenated fuels on Pt/ZSM-5 were compared at the same input power: r methanol r DME r ethanol r n-butanol. In general, methanol is a suitable oxygenated fuel in the design and development of catalytic micro-combustor.

The Effect of Differentiating the Thermal Conductivity between Inner and Outer Walls on the Stability of a U-Bend Catalytic Heat-Recirculating Micro-Combustor: A CFD Study

The effect of differentiating the thermal conductivity between inner and outer walls on the stability of a U-bend catalytic heat-recirculating micro-combustor was investigated. To this end, a two-dimensional computational fluid dynamics (CFD) model was developed using the commercial code ANSYS Fluent (release 2020 R1) and, for different combinations of values for the inner and outer thermal conductivities, simulations of lean pre-mixed propane/air combustion were performed by varying the inlet gas velocity. Numerical results have shown that extinction is mainly ruled by the inner wall, whereas the outer wall controls blowout. Differentiating the thermal conductivity has been found to be an effective strategy to jointly exploit the better extinction resistance of low-conductive (i.e., insulating) materials, required by the inner wall, and better blowout resistance of highly conductive materials, required by the outer wall, thus enlarging the stable operating window of the catalytic micro-combustor compared to the use of the same material for both walls.

Modeling of Thermal Integration of a Catalytic Microcombustor with a Thermoelectric for Power Generation Applications

Power generation from coupled devices employing combustion in microreactors is generating interest for portable or decentralized energy demands. We develop a model framework to predict the performance of an integrated catalytic microreactor–thermoelectric generator (TEG) device, using CFD. A strategy for modeling a standalone TEG module is proposed and validated first, followed by thermally integrating it with a microreactor. The concepts of 1D global energy conservation with lumped treatment of the electrical properties are used to develop the TEG model. Multiple thermocouples, consisting of p- and n-type semiconductors connected in series, are modeled as a single block. The properties of the Seebeck coefficient, internal resistance, and thermal conductance are aggregated for the entire block and estimated as polynomial functions of temperature. These parameters can be estimated from manufacturer data. The source terms for Seebeck, Peltier, Thomson, and Joule’s effects are incorporated into the energy conservation equation to model the TEG at steady state. The TEG model is validated with experiments in the literature. The catalytic combustion of hydrogen in a microreactor is validated independently and is subsequently used for modeling a catalytic microreactor integrated with a TEG module for power generation. The power extracted by various load resistances from the coupled device, at multiple values of inlet flow rates, is calculated and shows a good comparison with experimental data in the literature.

A Novel Catalytic Micro-Combustor Inspired by the Nasal Geometry of Reindeer: CFD Modeling and Simulation

A three-dimensional CFD model of a novel configuration of catalytic micro-combustor inspired by the nasal geometry of reindeer was developed using the commercial code ANSYS Fluent 19.0. The thermal behavior of this nature-inspired (NI) configuration was investigated through simulations of lean propane/air combustion performed at different values of residence time (i.e., inlet gas velocity) and (external convective) heat transfer coefficient. Simulations at the same conditions were also run for a standard parallel-channel (PC) configuration of equivalent dimensions. Numerical results show that the operating window of stable combustion is wider in the case of the NI configuration. In particular, the blow-out behavior is substantially the same for the two configurations. Conversely, the extinction behavior, which is dominated by competition between the heat losses towards the external environment and the heat produced by combustion, differs. The NI configuration exhibits a greater ability than the PC configuration to keep the heat generated by combustion trapped inside the micro-reactor. As a consequence, extinction occurs at higher values of residence time and heat transfer coefficient for this novel configuration.

Development of portable power unit with catalytic micro-combustor

A prototype of the portable power unit with coupling catalytic microcombustor, thermo-electric modules and air/fuel supply devices was developed. The developed power unit was completely self-standing; the air supply system was driven by a part of electricity generated at the generator unit. When the thermal input from the combustor was 12W, the total electricity is 330 mW and the net output electricity was 225 mW, which corresponded to the back work ratio of 32%. The final conversion efficiency of the total system reached 1.87%, which was order of world record.

Self-Similar Nonasymptotic Solution of Multireaction Stationary Flow in Catalytic Microcombustor

The present analytical work studies the modeling of a catalytic microcombustor with the stationary flame. In this model, the nonasymptotic solution of energy and mole fraction equations with self-similar matching condition is made at preheat, reaction, and postflame zones, where comparison between multi- and single-reaction combustion and calculations of noncatalytic and catalytic condition is achieved. The results are checked by experimental published data to ensure these comparisons show acceptable agreements. The investigations show that the position of noncatalytic maximum temperature occurs at the larger value of coordinates than its catalytic condition, where this issue is because of the occurrence of mass conversion in a larger length for the catalytic condition than noncatalytic one. At low Peclet number, the single- and multireaction curves are coincidence, whereas at high Peclet number, the multireaction values are nearly 5% smaller than single-reaction ones.

Effect of gas-phase reaction on catalytic reaction for H2/O2 mixture in micro combustor

The catalytic reaction characteristics of the hydrogen and oxygen mixture in the catalytic micro-combustor were detected by thermocouple, infrared thermal imager and OH-PLIF. The equivalence ratio of transition stage from coupling reaction (coupled homogeneous-heterogeneous combustion) to pure catalytic reaction (heterogeneous combustion) was obtained. The three-dimensional pure catalytic reaction model with detailed catalytic reaction mechanism was established in a rectangular catalytic micro-combustor. The simulation model was validated with experimental data. The effects of the intermediate and final products from gas-phase reactions on the pure catalytic reaction were discussed as well as the effects of gas-phase reactions on the catalytic reaction in the process of coupling reactions were studied. The intermediate product OH radical can improve the hydrogen conversion of the surface reaction, and the effect of O radical is not obvious. The final product H2O has an inhibitory effect on the surface reaction. Since the mass fraction of H2O is much higher than other gas-phase reaction products, the dominant effect of the gas-phase reaction on the catalytic reaction is suppression. In the coupling reactions, the fuel consumed by the gas-phase reaction weakened the catalytic reaction.

Interaction between heterogeneous and homogeneous reaction of premixed hydrogen–air mixture in a planar catalytic micro-combustor

Hetero-/homogeneous combustion of hydrogen–air mixture in a platinum-coated micro channel was studied by means of three-dimensional (3D) Computational Fluid Dynamics (CFD) model using detailed reaction mechanisms for homogeneous (gas phase) and heterogeneous (catalytic) reactions. The influence of heterogeneous reaction on homogeneous reaction was obtained by discussing the hetero-/homogeneous reaction characteristics, the process and competitiveness of heterogeneous reaction. Intense depletion of fuel and product formation near the inlet has shown that fuel absorption reaction was predominant whiles there was no homogeneous reaction. The free radicals from homogeneous reaction near the catalytic surface were drained, resulting in the interruption of chain reaction. The desorbed product as the third-body promoted the chain terminating reaction to deactivate free radical. The competitiveness value of heterogeneous reaction for capturing fuel was 14.85%, indicating a weak reaction intensity in the catalytic combustor. The heterogeneous reaction presented an inhibition effect on homogeneous reaction but greatly enhanced the combustion efficiency in the micro channel.

Erratum to: Analytical two-dimensional modeling of hydrogen–air mixture in catalytic micro-combustor

Erratum to: Analytical two-dimensional modeling of hydrogen–air mixture in catalytic micro-combustor

Methanol catalytic micro-combustor with pervaporation-based methanol supply system

A catalytic micro-combustor using a pervaporation membrane as the passive methanol vaporizer is developed for direct methanol fuel cell (DMFC) applications, which is able to realize self-starting in an ambient environment. The methanol feed rate and the membrane diffusion coefficient related to the permeation flux are experimentally evaluated. Results indicate that the air feed rate (AFR) has positive effects on enhancing the methanol production and also increasing the catalyst temperature. A higher AFR promotes vapor generation and improves the combustion rate, thereby facilitating a higher start-up performance of the combustor. The infrared thermal analysis verifies the positive effect of increasing the AFR on the status of heat production and distribution. Results also validate the feasibility of changing the operating orientation of the combustor, which illuminates its potential use for portable applications. The online fuel cell tests validate that this combustor is viable to be used as a heat generator for a vapor-feed DMFC.

Analytical two-dimensional modeling of hydrogen–air mixture in catalytic micro-combustor

This paper investigates the combustion phenomenon in catalytic micro-combustor by using a two-dimensional model. In solution of energy and mass equations, the temperature and mass dependant reaction rates are considered with an iterative procedure where the series matching conditions are exerted between neighbor zones. The reaction zone thickness is considered as a variable parameter and predicted by the solution results of the present study. The study of main parameters effects such as normalized wall temperature, Peclet number and equivalence ratio is also attained. The limitations of low and high Peclet number are determined where at high Peclet numbers the maximum temperature is decreased and the trend is inversed at low Peclet numbers. In order to validate this model the results are compared with published experimental data, showing an acceptable agreement and confirming the accuracy of the predicted data.

Characterization of Wet Impregnation Method for Catalytic Microcombustor by Using Platinum

The wet impregnation method for catalytic microcombustor was characterized by using platinum as a catalyst. The main purpose of this study is to increase the surface porosity of the catalyst support. A high surface porosity indicates that a high amount of catalyst was deposited within the surface areas. The performance of the catalytic microcombustor improves with increasing catalytic surface area. The stainless steel catalyst support was treated with sulfuric acid solution containing polyvinyl (3.89 wt%) and propargyl alcohol (1.48 wt%). Combustion test was performed using LPG-air to test the performance of the catalyst. The surface support treated with polyvinyl (PVA) showed a higher surface porosity and combustion blow-out limit compared with propargyl alcohol. The combustion mode changes from surface to submerged combustion after the catalyst was deposited in the support surface.

Pt-Based Catalytic Microcombustor for Micropower Generation

This paper presents the fabrication and characterization of platinum-based catalytic microcombustor. The platinum catalyst was deposited onto type-304 stainless steel using the wet impregnation method. The stainless steel undergoes controlled conversion coating treatment in sulfuric acid solution to increase the porosity of its surface before the deposition of the platinum catalyst. The scanning electron microscopy result showed that the porosity on the stainless steel surface will depend on the length of treatment time in the sulfuric acid solution. The surface porosity increased as the treatment time increases. The stainless steel surface morphology changed from smooth to ‘cracked-mud’ morphology after treatment in sulfuric acid solution. The treatment time also provide significant effect to the amount of platinum deposited on the stainless steel surface. The energy dispersive x-ray analysis showed that the amount of deposited platinum for 10 seconds of treatment time was 0.68 wt%, whereas those for 20 and 30 seconds were 0.87 wt% and 1.10 wt%, respectively. Liquefied petroleum gas-air combustion result showed that the flame completely submerged inside the microcombustor with a catalyst, whereas portions of flame can be observed at the exhaust for the microcombustor without a catalyst. The minimum air-to-fuel ratios before the combustion blow-out for 10, 20, and 30 seconds of treatment time was 0.5, 0.4, and 0.3 respectively.

Performance of a catalytic micro-combustor based on Pt/Al2O3/Ni for methanol fuel cell application

Abstract This work focuses on the performance of a catalytic micro-combustor to be used as a heat source for a direct methanol fuel cell. This combustor uses Pt/Al 2 O 3 /Ni to provide catalysis for methanol combustion. It is proved that the use of a higher catalyst loading mostly promotes a higher methanol conversion rate. The catalyst distribution has a dominant effect on the reaction performance. Results also verify the necessity of optimizing the amount of Al 2 O 3 loading. Three models based on different structural design and operating mechanism are evaluated. It is shown that adding a preheating layer helps gain a uniform temperature distribution in the combustion chamber. Compared with methanol injection, the bubbling method for methanol supply promotes a faster start-up of the combustion reaction. Besides, the feasibility of using a heat recirculation layer to realize self-combustion at room temperature is also validated. All these issues are of practical significance to the combination of combustor and fuel cell.

Steady-State Multiplicity in Catalytic Microcombustors

In this work, two-dimensional computational fluid dynamics (CFD) simulations were run to investigate the possibility of a link between the initial conditions and the occurrence of blowout for a parallel-plate catalytic microcombustor. The results show that steady-state multiplicity occurs: Depending on the initial conditions, the range of inlet gas velocities at which stable operation is attained can be enlarged to avoid blowout. It is concluded that investigations into the thermal behavior of catalytic microcombustors have to deal with appropriate and aware choices of the initial conditions.

Catalytic combustion of alcohols for microburner applications

The combustion of energy dense liquid fuels in a catalytic micro-combustor, whose temperatures can be used in energy conversion devices, is an attractive alternative to cumbersome batteries. To miniaturize the reactor, an evaporation model was developed to calculate the minimum distance required for complete droplet vaporization. By increasing the ambient temperature from 298 to 350 K, the distance required for complete evaporation of a 6.5 μm droplet decreases from 3.5 to 0.15 cm. A platinum mesh acted as a preliminary measurement and demonstrated 75% conversion of ethanol. We then selected a more active rhodium-coated alumina foam with a larger surface area and attained 100% conversion of ethanol and 95% conversion of 1-butanol under fuel lean conditions. Effluent post-combustion gas analysis showed that varying the equivalence ratio results in three possible modes of operation. A regime of high carbon selectivity for CO 2 occurs at low equivalence ratios and corresponds to complete combustion with a typical temperature of 775 K that is ideal for PbTe thermoelectric energy conversion devices. Conversely for equivalence ratios greater than 1, carbon selectivity for CO 2 decreases as hydrogen, olefin and paraffin production increases. By tuning the equivalence ratio, we have shown that a single device can combust completely for thermoelectric applications, operate as a fuel reformer to produce hydrogen gas for fuel cells or perform as a bio-refinery for paraffin and olefin synthesis.

A micro-fuel processor with trench-refilled thick silicon dioxide for thermal isolation fabricated by water-immersion contact photolithography

This paper presents a micro-fuel processor to produce hydrogen for micro-fuel cells. The micro-fuel processor has an original design for a heat source and a reforming chamber to improve thermal efficiency. To improve the mechanical strength as well as to maintain the effectiveness of thermal isolation, the reaction area is suspended by trench-refilled thick SiO2. When the reaction area was heated at 200 °C, the outer silicon frame was at 100 °C, confirming the effectiveness of thermal isolation. To fabricate the trench-refilled thick SiO2 using a contact aligner, water-immersion contact photolithography was used and successfully demonstrated to produce fine patterns with line/space of 2 µm/1 µm (a pitch of 3 µm). The highest temperature could be controlled by tuning the equivalent ratio of H2–air mixture fed into the catalytic micro-combustor. The conversion rate of methanol to hydrogen by steam reforming was lower than 1% due to uneven temperature distribution and channeling effect.