Combustion

La combustion est une réaction exothermique d'oxydoréduction. Lorsque la combustion est vive, elle se traduit par une flamme ou par une explosion (déflagration, voire détonation si le front de flamme dépasse la vitesse du son). La combustion de la biomasse et des carburants est la principale source de pollution de l'air, avec des effets cancérigènes, reprotoxiques et cardiovasculaires notamment. vignette|upright|Combustion du bois : vive avec flammes et lente avec braises incandescentes. Triangle du feu Avant 1980 on considérait que la réaction chimique de combustion ne peut se produire que si l'on réunit trois éléments : un combustible, un comburant, une énergie d'activation en quantités suffisantes. C'est pourquoi on parlait alors du « triangle du feu ». Depuis les années 1980 on a découvert qu'une des étapes indispensable de la réaction chimique est la production de radicaux libres ; cette étape est nécessaire pour que la combustion s'entretienne et que l'on pui

NUMERICAL FLOW SIMULATION OF A NATURAL GAS ENGINE EQUIPPED WITH AN UNSCAVANGED AUTO-IGNITION PRECHAMBER

Daniel Favrat, Stefan Heyne, Jan-Bernhard Vos, Dirk Wunsch

This work is part of ongoing research on stationary natural gas engines equipped with unscavanged prechambers. Spark ignition inside the prechamber has been successfully proven with natural gas as well as biogas, reducing emissions while keeping good efficiencies. The conversion of the prechamber system from spark ignition to auto-ignition will lead to longer maintenance intervals in the case of stationary natural gas engines and to a probable more efficient and cleaner combustion. The latter expectation is based on the combustion mode: HCCI like combustion inside the prechamber and faster combustion in the main chamber due to an earlier arrival of the flame front, compared to prechamber spark ignition. To the authors knowledge no work following this approach has been conducted. From this new approach one of the major difficulties arises. While for standard prechamber systems either the spark plugs or injecting gas directly into the prechamber creating rich conditions guarantee to well know the point of ignition, auto-ignition of a premixed gas does not do so. The basic idea of this prechamber is to create as homogeneous conditions as possible. Then, the reacting mixture streams through the orifices into the main chamber. The difficulty here is to create a state that triggers auto-ignition only inside the prechamber. As temperature is the main influencing factor on the reaction chemistry the prechamber walls are heated, aiming at higher gas temperatures inside the prechamber. However, auto-ignition will take place wherever the energy level is sufficiently elevated. In particular, this causes difficulties in the region of the exhaust valve. In this work the three- dimensional flow conditions in a mono-cylinder test engine were numerically simulated in order to better understand the conditions inside the engine, with the new prechamber configuration. Therefore, a geometric model was created, disregarding intake and exhaust valves to simplify matters. A suitable multi-block mesh for parallel computing was generated and models for the initial and boundary conditions derived. The chemical reactions were incorporated to the flow field, using a 54 species mechanism developed for natural gas combustion by two approaches. In a first step the flow field was superposed with zero-dimensional Chemkin calculations whereas in a second step the species conservation equations were solved together with the Navier Stokes equations, which is much more expensive in calculation time. Finally, the numerical results were compared to experimentally gained data. Results show the sensitivity of the computations to varied initial and boundary conditions and provide a guideline for modifications in order to enhance the prechamber configuration. Numerical results show the risk of auto-ignition in the main chamber. Experimental results could be reproduced numerically in regard to the time dependency of first ignition.

2007

Promotion électrochimique des catalyseurs à base de rhodium et d'iridium

Justyna Eaves

In this thesis, the electrochemical promotion of lr and Rh based catalysts was studied for the complete combustion of ethylene and propylene. The study was performed by using two types of electrocatalytical cells: a classical, pellet type cell and a tubular cell. This work has two major goals. The first one is the optimization of the employed electrocatalytical systems from electrochemical point of view. The second one concerns the study of the mechanism of the electrochemical promotion. The major part of this work concerns the electrochemical characterization of two cell types: a pellet type cell and a tubular cell. On the basis of the obtained results the modélisation of the potential and current distribution was performed, providing key parameters for the development of more efficient cell configurations. On one hand, these configurations ensure an uniform current distribution on the working electrode and enable correct catalyst potential measurement. On the other hand, it minimizes the current bypass in the bipolar tubular cell. The aim of the investigation in a two-compartment reactor was to evaluate and to compare the Au and Ir electrodes used in this work. Under 400°C, gold was found to be much worse material than lrÛ2 both as reference and counter electrode. Its potential was affected by oxygen and propylene adsorption and no equilibrium of electrochemical reaction was obtained at such low temperature. On the base of this knowledge, several materials better adapted to electrochemical promotion conditions have been proposed. Using the two-compartment reactor potential measurements under steady state conditions allowed to verify the mechanism of the propylene combustion on Rh and that of ethylene combustion on lr based catalysts proposed in the literature. Ameliorations of the existent kinetic models were proposed. The kinetic measurements were then analyzed with the additional help of ex situ XPS analysis. This study allowed to link the oxidation state of the catalyst surface with the extent of electrochemical promotion of the catalysts. It also allowed the ex situ observation of the appearance of new oxygen (oxide) species on the gas exposed surface of Ir catalyst. This new type of oxygen was different from the oxygen of Ir02 oxide, and it can represent the promoting species, according to backspiïïover theory. The same oxygen species was observed on well oxidized Ir02 catalyst surface, even before the polarization of this catalyst. Its presence was associated to the catalyst-support effect. It explains the high activity of Ir02 catalyst by comparison with the catalytic activity of lr metal. XPS analysis of Rh based catalysts indicated the possibly formation of an inferior Rh oxide (close to RhO) due to polarization. This may be a possible explanation of the observed permanent electrochemical promotion. Voltammetric charge measurements in very narrow potential window were performed while changing the catalyst thickness. The charge was demonstrated to be proportional to the catalyst thickness, suggesting the presence of the charged species on the gas exposed catalyst surface. The analogy between this situation and that of the emerged electrodes in liquid state electrochemistry was emphasized. The voltammograms obtained in this study were interpreted in the quite new, original way, which allowed to calculate the capacity of the different catalyst interfaces, the gas/catalyst interface, and the triple phase boundary, in good agreement with reference values from impedance spectroscopy measurements. Finally, the reaction rate transients during galvanostatic polarization of lr based catalyst have been moralized. The model fits well the experimental results.

EPFL2004

Promotion électrochimique des catalyseurs à base de rhodium et d'iridium

Justyna Eaves

EPFL2004

NUMERICAL FLOW SIMULATION OF A NATURAL GAS ENGINE EQUIPPED WITH AN UNSCAVANGED AUTO-IGNITION PRECHAMBER

Daniel Favrat, Stefan Heyne, Jan-Bernhard Vos, Dirk Wunsch

2007

Electrochemical promotion of RuO2 catalysts for the gas phase combustion of ethylene

NUMERICAL FLOW SIMULATION OF A NATURAL GAS ENGINE EQUIPPED WITH AN UNSCAVANGED AUTO-IGNITION PRECHAMBER

Promotion électrochimique des catalyseurs à base de rhodium et d'iridium

Promotion électrochimique des catalyseurs à base de rhodium et d'iridium

Electrochemical promotion of RuO2 catalysts for the gas phase combustion of ethylene

NUMERICAL FLOW SIMULATION OF A NATURAL GAS ENGINE EQUIPPED WITH AN UNSCAVANGED AUTO-IGNITION PRECHAMBER