High pressure catalytic combustion
Barbato, Paola Sabrina (2010) High pressure catalytic combustion. [Tesi di dottorato] (Inedito)
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The study of catalyst behavior at pressure up to 12 bar during CH4, H2, CO and their mixtures combustion is the main purpose of this PhD thesis. Actually the interest towards catalytic combustion as an alternative route to produce electric power is renewed due to the use of Low-Btu fuels. Therefore the research activity was focused notably on CH4 but also on H2 and CO combustions and on the effect of their addition on methane combustion at variable pressure. At this purpose it was necessary to design and realize an innovative lab scale plant which operates at temperatures up to 1000°C and pressure up to 12 bar and, with the proper reactor configuration and operative conditions, in two different operating modes: isothermal and auto-thermal. The active phases considered in this experimental activity are a conventional Pt catalyst (1%wt), and a more thermally stable catalyst, a supported perovskite (20%wt LaMnO3), and a bi-functional Pt-perovskite catalyst. Perovskites are cheap and show an activity only slightly lower compared to noble metals at condition relevant for GT engines. Moreover their behavior under pressure is quite unknown. The need for low combustor pressure drops makes necessary the use of an appropriate substrate. For this reason particular attention was devoted to deposit efficaciously the catalysts powders over appropriate planar (α-Al2O3) and honeycomb monolithic (cordierite) substrates. The materials used in this thesis were completely characterized by means of temperature programmed reductions of the different catalysts under H2 or CO flows. Results revealed that the reducibility of the catalysts, characteristic temperatures and reduction degrees strongly depends on the reducing agent. In particular, H2 is the most reducing agent for Pt, while perovskite preferentially interacts with CO. The bi-functional Pt-perovskite catalyst show intermediate properties with respect to the single phases. Since the availability of reliable heterogeneous kinetic data is necessary for the correct description of the catalytic processes, CH4, CO and H2 combustions under isothermal conditions have been separately studied on the perovskite and the noble metal catalyst. Particular attention was devoted to study the fluid dynamics of the reactor and to characterize the mass transfer properties of the systems in order to find the conditions free from diffusion limitations. Moreover a proper reactor model was developed in order to find the best kinetic models. Concerning the Pt catalyst, H2 combustion apart, in all cases it was possible to derive a simple reaction rate well fitting all experimental data; fractional rate expressions, derived from models including both fuel and oxygen adsorption, provided the best description of the experimental results. With regard to the Perovskite catalyst, in the investigated temperature range methane combustion rate can be expressed with a single fractional equation taking into account only methane adsorption. An apparent linear reaction rate could be used to fit the data only at atmospheric pressure. As a consequence, the extension of such kinetics at higher pressures leads to an overestimation of the reaction rate. The evidence that oxygen dependence is negligible is in agreement with literature data and is due to the occurrence of the reaction with lattice oxygen. On the contrary, both CO and H2 combustions on perovskite are influenced by changes of oxygen partial pressure. In both cases, the best models suggest the reaction of at least a fraction of the fuel with α-oxygen, generally weakly bonded to the catalyst surface. Moreover, according to the strong CO affinity with perovskite the CO combustion rate must take into account the negative effect of CO accumulation on the surface, leading to a less than linear reaction order with respect to the fuel. As a general conclusion, excluding some conditions of H2 combustion on Pt, the effect of pressure on the combustion kinetics is positive even if less than linear. Concerning the effect of the pressure under autothermal conditions, it was found that methane can be ignited simply by increasing the pressure, due to two concomitant effects: higher reaction rates, according to the conclusions of the kinetic study, and longer contact times, due to the reduction of the flow velocity. Moreover, once ignited, the pressure can be lowered without the occurrence of quenching phenomena, i.e. keeping stable operation. A positive effect of Low BTU fuels co-feeding on methane light off has been detected on perovskite-based catalysts, eventually doped with Pt. As a matter of fact, lower pre-heating temperatures are needed in order to ignite methane. Ignition occurrence could be obtained by changing the operating pressure too. The main reason of such effect is due to thermal causes. As a matter of fact, depending on the catalyst formulation, low BTU fuels can be easily converted in the first part of the reactor and the produced heat increases the temperature (and consequently the kinetics) downstream up to the imbalance between generated and exchanged heat is reached.
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