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 countercurrent.