Combustion in microspaces and its applications

PhD research can be divided in three main parts: part one and two related to the development of some of the most important aspects of the catalytic combustion in microspaces, part three related to a possible application of the catalytic combustion in microspaces. Part 1: The combustion of gaseous HC fuels in a small confined space could represent an alternative way to produce thermal and electrical energy. The combustion of CH4 and its lean mixtures with H2 on catalytic monoliths was studied and optimized. 2% Pd/(5% NiCrO4), 2% Pd/(5% CeO2ZrO2), 2% Pd/(5% LaMnO3ZrO2) and 2% Pt/(5% Al2O3) catalysts, suitably developed, were deposited on SiC monoliths via in situ SCS and tested in a lab-scale microreactor by feeding only CH4, only H2, and three lean CH4/H2 mixtures with increased content of H2 and constant thermal power density of 7.6 MWth m-3. Monolith with 2% Pt/(5% Al2O3) was very appropriate for the combustion of only CH4 or H2, but its performance worsen when H2 was added to the reactive mixture. On the contrary, the Pd-based catalysts were most suitable for the combustion of the CH4/H2 lean mixtures, with the best behavior shown by 2% Pd/(5% NiCrO4) followed by 2% Pd/(5% CeO2ZrO2). Monolith coated with 2% Pd/(5% LaMnO3ZrO2), instead, showed the worse performance, both in terms of CH4 combustion only and of the various mixtures; moreover, it displayed quite high CO emissions, not compatible with the environmental issues. In particular, the catalytic reactivity towards CH4 combustion of the Pd- based raised by increasing the H2 content in the reactive mixture. The observed enhancement in reactivity of the mixture when the CH4 fuel was enriched with H2 could be explained by an increase of the OH• radicals in the gas mixture. Part 2: The present work deals with the investigation on the performance of catalyst 2% Pd/ 5% LaMnO3•ZrO2 (PLZ), lined on silicon carbide (SC, with thermal conductivity of 250 W m-1 K-1) or cordierite (CD, with thermal conductivity of 3 W m-1 K-1) monoliths, for the CH4/H2/air lean mixtures oxidation. The bare and coated monoliths were tested into a lab- microreactor designed to provide a favorable environment for microscale combustion of CH4/H2/air lean mixtures to reach high power density (7.6 MWth m-3; GHSV 16,000 h-1). Various CH4/H2 mixtures were tested in heating and cooling phases on the various monoliths, by studying both the homogenous and heterogeneous reactions. The relative percentages of methane and hydrogen were mutually varied (maintaining the sum of the two fuels equal to 100%), in order to always assure a constant power density. The air was always fed with  equal to 2. The main aim of the catalytic combustion tests was to select the best settings to achieve at the minimum temperature full CH4 conversion with the minimum H2 concentration in the reactive mixture, accompanied by the lowest possible CO concentration. Depending on the thermal conductivity of the tested monoliths, the existence of a steady-state multiplicity was verified, mainly when the hydrogen concentration was quite low. Basically, microburners with low wall thermal conductivity (CD monoliths) exhibited shorter ignition times compared to the higher thermal conductivity ones (SC monoliths) due to the formation of spatially localized hot spots that promoted catalytic ignition. At the same time, the CD material required shorter times to reach steady-state. But SC materials assured longer time on stream operations. The presence of the catalyst lined on both monoliths allowed reaching lower CO emissions. The best results belonged to the catalytic SiC monolith, with a low hydrogen concentration in the fed mixtures. Part 3: The idea was to realize an autothermal steam reforming reaction. This was made by coupling a combustion reaction (exothermic), which provided the heat necessary, with a steam reforming reaction (endothermic) in a same specific built micro reactor. The total reagents chosen for the two reactions were methane (used both as fuel and as a reactant for the steam reforming), air and steam (produced by heating water). The main advantage of this system: producing enough energy, for example, to power auxiliary transportation of vehicles, reducing consumption and pollutant emissions; at the same time, because of the overall limited dimensions, reducing the risk of explosion if compared to the hydrogen "on board " storage. The development was a stainless steel reactor consisting of two plates with microchannels, containing the catalyst (Pt/AlO3), in which the reactions took place. These plates were placed in indirect contact, separated by a middle plate made of stainless steel, so to conduct the heat from the combustion side to the steam reforming, and also to avoid the mixing of the fluids. The sealing of both sides were ensured by two ceramic gaskets, suitable to withstand high temperatures. The sizing was performed first theoretically assuming a S / C = 4 (Steam to Carbon), and taking into account the maximum flow rates that could be set to the mass flow controllers. It was then calculated the theoretical thermal power necessary to sustain the steam reforming process, and then calculated the flow of methane and air to be sent to the combustor, to obtain an autothermal reforming. The catalyst used was chosen because of its catalytic activity for both types of reaction. Once it was determined the best side for the steam reforming, it was decided to experiment the coupled reactions. After having reached 900 °C in oven, with complete methane combustion, oven heat was no more provided: combustion was able to be sustained because of a mixture of 7% CH4 in air (inside the flammability limit) and reagents for the steam reforming were sent in a steam/carbon 4:1 replacing nitrogen flow. Results show how the performance of the reactor was affected by thermal dissipation; hence the material used as insulating, in order to wrap up the reactor, plays a key role for performing tests. Tests were carried out increasing thermal power from combustion side to balance the heat dissipations, so to obtain a balance between heat generated and used by the reaction of steam reforming and the heat lost in the environment. It has been showed the way for producing good quality data on coupling combustion and steam reforming reactions in this reactor. In a future, it could be possible using a GC instead of the ABB analyzer in case of new tests with high CH4 not reacted, or of course improving methane conversion choosing a better catalyst for steam reforming, composing a reactor with multiple plates for optimizing the process as shown in Vlachos' simulations, and trying to run flows in either concurrent or countercurren

Experimental Insights into the Coupling of Methane Combustion and Steam Reforming in a Catalytic Plate Reactor in Transient Mode

The microstructured reactor concept is very promising technology to develop a compact reformer for distributed hydrogen generation. In this work, a catalytic plate reactor (CPR) is developed and investigated for the coupling of methane combustion (MC) and methane steam reforming (MSR) over Pt/Al2O3-coated microchannels in cocurrent and counter-current modes in transient experiments during start-up. A three-dimensional (3D) computational fluid dynamics (CFD) simulation shows uniform velocity and pressure distribution profiles in microchannels. For a channel velocity from 5.1 to 57.3 m/s in the combustor, the oxidation of methane is complete and self-sustainable without explosion, blow-off, or extinction; nevertheless, flashbacks are observed in counter-current mode. In the reformer, the maximum methane conversion is 84.9% in cocurrent mode, slightly higher than that of 80.2% in counter-current mode at a residence time of 33 ms, but at the cost of three times higher energy input in the combustor operating at ∼1000 °C. Nitric oxide (NO) is not identified in combustion products, but nitrous oxide (N2O) is a function of coupling mode and forms significantly in cocurrent mode. This research would be helpful to establish the start-up strategy and environmental impact of compact reformers on a small scale

Numerical and experimental analyses of single and two-phase microfluidic flows with implications in microreactors

Aquesta tesi centra els seus esforços en l'àmbit de la microfluídica, un camp relativament recent dins de la Mecànica de Fluids, amb un futur prometedor i amb un ritme d'investigació intens en les seves diferents especialitzacions. En aquest sentit, la tesi presenta dos aportacions científiques principals. Primer, aporta una eina numèrica d'elaboració pròpia per realitzar simulacions de fluxos reactius en microcanals. Eina que s'aplica satisfactòriament a la identificació dels principals processos de transport involucrats en la oxidació parcial del metà per a produir gas de síntesi, i a l'estudi de l'efecte que tenen alguns paràmetres d'operació en aquest procés reactiu. Segon, estén el coneixement dels fluxos multifàsics en microunions en T, estudiant experimentalment fluxos de dues fases amb fluids principalment miscibles i en condicions supercrítiques, que son portats al seu equilibri vapor-líquid. Durant aquest estudi, a més, reporta un succés inesperat que presenta futurs reptes en l'aplicació d'aquest tipus de fluxos multifàsics.The present thesis focuses on microfluidics, a relatively recent field of Fluid Mechanics with promising expectations and with an intense scientific interest on its different areas. In this regard, the thesis aims to provide two main scientific contributions. First, it presents an in-house numerical tool to carry out simulations of reactive flows within microchannels. The tool is successfully applied to the identification of the main transport phenomena involved on the partial oxidation of methane to produce synthesis gas, and to the analysis of the effect of several operating parameters on this reactive process. Second, it extends the knowledge on multiphase flows in microfluidic T-junctions with an experimental study of two-phase flows of mixtures of potentially miscible fluids, in supercritical conditions and in vapour-liquid equilibrium. In this study it is also reported an unexpected phenomenon, which brings new challenges to the application of these kind of multiphase flows

Controlling parameters of excess enthalpy combustion

textExcess enthalpy combustion utilizes heat recirculation, in which heat is transferred from hot products to cold reactants to effectively preheat the reactants, in order to achieve improved combustion performance through the extension of flammability limits and increased burning rate. This research examines the effect of key parameters in excess enthalpy combustion on combustion stability, fuel conversion, and product species production through experimental and numerical investigation. Operating condition parameters that are studied include inlet reactant equivalence ratio and inlet velocity, and reactor geometry parameters that are studied include reactor channel height and length. Premixed reactants, including gaseous and liquid fuels, are investigated at rich and lean conditions. The examination of liquid fuels and the ability of a reactor to support rich and lean combustion of both gaseous and liquid fuels is a significant demonstration of a reactor’s flexibility for practical applications. This research experimentally and numerically examines excess enthalpy combustion in a counter-flow reactor. First, the counter-flow reactor, previously used for thermal partial oxidation of gaseous hydrocarbon fuels, is used in experiments to reform a liquid hydrocarbon fuel, heptane, to syngas. The effect of inlet operating conditions, including reactant equivalence ratio and inlet velocity, on combustion stability and product composition is explored. Second, lean combustion is demonstrated through experiments in the same counter-flow reactor previously used in reforming studies. The effect of inlet operating conditions, including reactant equivalence ratio and inlet velocity, on combustion stability and pollutant concentrations in combustion products is studied. An analytical model, previously developed for rich combustion, is adapted to qualitatively predict the behavior of the counter-flow reactor in response to changes in lean operating conditions. Third, lean combustion in the counter-flow reactor is further studied by examining the combustion of increasingly complex gaseous and liquid fuels. Again, the effect of inlet operating conditions, including reactant equivalence ratio and inlet velocity, on combustion stability and pollutant concentrations in combustion products is studied. Fourth and finally, a computational scaling study examines the impact of counter-flow reactor channel geometry on combustion stability, temperature increase above adiabatic values, heat recirculation, and fuel and product species conversion efficiency.Mechanical Engineerin

Photocatalytic Reaction in Monolithic Optical Fiber Reactor with Inverse Opal Catalyst

The development of photocatalytic reactor is essential for the successful application of heterogeneous semiconductor in environmental study, which has been shown to be photoactive and effective to oxidate organic pollutant and photoreduce CO2 to useful compounds. In this dissertation, a monolithic optical fiber reactor (MOFR) coated with inverse opal titania, which uses optical fibers as light-transmitting conductor and support of catalyst, was developed for both photodegradation and photoreduction. 1,2-dichlorobenzene, a volatile organic compound (VOC), was selected as the organic pollutant. This configuration of reactor and catalyst provides a high surface area, enhances mass transfer within the catalyst, manipulates photons transmission within fibers, and provides higher quantum efficiency. The effects of flow rate, UV intensity, humidity (water vapor pressure) and temperature were investigated for the photodegradation in gaseous phase. The results show that flow rate and UV intensity determine the reaction regime simultaneously. Higher humidity can significantly decrease the photoreaction. Inverse opal titania shows higher quantum efficiency than conventional P25 catalyst in this study. This configuration can also work in an aqueous phase to degrade organic compounds. With inverse opal titania doped with Cu, MOFR can be used to photoreduce CO2 to methanol at mild experimental conditions. The effects of water vapor pressure, flow rate and UV intensity were investigated in detail and optimized. The results show there is an optimal value for the water vapor pressure in this study. In addition, inverse opal catalyst shows higher quantum efficiency for reduction. A three-dimensional model was developed to simulate the process of photodegradation both in gaseous phase and aqueous phase. A convection diffusion model, reaction kinetics model and UV radiance model in optical fiber were incorporated. Reasonable agreement between experimental results and model-predicted results was found. This model certainly explains the experimental results. So it is used to select optimal value for each experiment parameters in MOFR