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Theses

Auto-échauffement d'un lit ventilé de matériaux carbonés : cas du bois torréfié

Abstract : Torrefaction is one of the thermo-chemical pretreatment processes of lignocellulosic biomass that facilitates both the storage and transport of the material and increases the energy value of the product. However, as the torrefied substrate is more reactive, it is more prone to spontaneous exothermic mechanisms that can lead to self-heating of the material. This issue is not well investigated in the case of torrefied wood since its industrial application is mainly in the test phase. For this reason, this topic is further studied throughout this thesis. Indeed, the aim was to understand the phenomena responsible for the self-heating of a bed of biomass ventilated with oxidizing gas at low temperature. To do this, self-heating scenarios of torrefied wood chips were created under an oxidizing atmosphere. Pilot-scale experiments were conducted in a 12 L fixed-bed reactor. During these tests, we demonstrated that self-heating is intensified when the oxidizing gas flow rate is low and under a high oxygen fraction. In addition, the heat produced during the self-heating of the wooden bed was estimated on the basis of a heat balance and thermal data. Then, the source term was correlated to the oxygen fraction and temperature in a simplified model. The apparent kinetic parameters and heat of reaction associated with self-heating were derived from this. On the other hand, in order to understand the exothermic phenomena characterizing self-heating, low temperature oxidation tests are carried out on a small scale (ATG/ATD). On the basis of these analyses, kinetic models were developed to distinguish and quantify the mechanisms identified experimentally. These two approaches have made it possible to highlight three main mechanisms involved in low-temperature oxidation: chemical adsorption of oxygen on the reagent, decomposition of the oxygen complexes formed during adsorption and a direct oxidation reaction. In a more problem-oriented approach to industrial-scale self-heating, a numerical model coupling chemical kinetics and mass and heat transfers was designed at the scale of the particle bed. This model provided a reasonable prediction of the thermal performance of the torrefied wood bed under high ventilation flow. It was then extrapolated to an industrial scale to simulate the thermal behaviour of a storage silo undergoing self-heating.
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Amina Bouzarour. Auto-échauffement d'un lit ventilé de matériaux carbonés : cas du bois torréfié. Génie des procédés. Ecole des Mines d'Albi-Carmaux, 2019. Français. ⟨NNT : 2019EMAC0012⟩. ⟨tel-02903159⟩

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