Application des modèles mécanistiques de cinétique chimique aux combustions industrielles. Illustration par la fabrication du gaz de synthèse
Application of Mechanistic Models of Chemical Kinetics to Industrial Combustion. Illustration by Synthetic Gas Manufacturing
Institut Français du Pétrole
En combustion, la formation d'espèces mineures clés, comme les polluants, peut être interprétée par des modèles mécanistiques de cinétique chimique. Les informations que fournissent ces modèles, même s'il ne s'agit que de tendances, sont suffisamment fiables pour définir des choix technologiques. Toutefois, compte-tenu de la complexité des phénomènes traités, leur emploi fait appel à une méthode indirecte décrite dans cet article et illustrée par la conception d'un réacteur autotherme destiné à la préparation d'un syngaz (gaz de synthèse). Dans l'exemple proposé comme application, l'objectif est de faire fonctionner à l'air un réacteur opérant actuellement à l'oxygène pur. Le modèle mécanistique choisi établit très clairement les contraintes imposées par ce choix.
During the development of a partial combustion reactor for natural gas , Institut Français du Pétrole (IFP) has made use of a mechanistic model to determine the impact of operational parameters on the formation of soot. The model we chose deals with the oxidation and pyrolysis of light hydrocarbons by several hundred elementary reactions, some of which are shown in Table 1. All the species taken into consideration as well as their linking are shown in the flowchart in Fig. 3. Our data mainly came from References  and , from which we took all the reactions of species having three carbon atoms or less as well as the pyrolysis reactions of hydrocarbons with four carbon atoms. In this database, the kinetic coefficients of reactions between CH4 and the C2H5, C2H3 and C2H radicals were replaced by the values published in Reference . This set of reactions is not sufficient to analyze the formation of soot, and so we added on the pyrolysis reactions of acetylene from Reference . The model assimilated the total mass of carbon contained in C5 and C6 hydrocarbons with a soot number assumed to be proportional to the mass of soot formed by the actual process simulated. This procedure was first checked by applying the model to various published experimental results. Mechanistic models are not calibrated, which opens up the possibilities of their use in very wide fields. On the other hand, to save computer time, they can be used only to deal with ideal reactors such as plugflow reactor and stirred reactor as well as with one-dimensional flames. It is thus important to know the similarities and differences between reactors of this type and actual combustion. To do this, reference is made to Figs. 4 and 5. As a geometric parameter, the model can take only the volume of the combustion chamber into consideration. Despite these restrictions, we wanted to know whether a stirred reactor could represent the behavior of a partial-oxidation industrial reactor. For this we simulated tests performed in a pilot plant having the characteristics shown in Table 2. Figs. 8 and 9, which compare the measurements and computing, give encouraging results, especially for evolution of soot with the 02/C ratio. As an applied exercise, we dealt with the partial oxidation of natural gas in air. The partial oxidation of oxygen is applied industrially and served as a reference, even though there is no commercial process operating in air. The model indicates that such a process must operate with a very unusual preheating level, which a standard heat exchanger cannot reach. We thus show how important it is to undertake such a procedure before starting an experimental program.
© IFP, 1991