Textural characterization of media composed of compacted pieces of cardboard and polyethylene using a gas tracer method

International audienceThe aim of this work is the experimental determination of effective transport properties of porous media consisting of compacted pieces of cardboard and polyethylene (PE). The proposed method itself is more general and can be applied to many different materials and contexts. Three major transport properties were determined: porosity, tortuosity factor and permeability. Three parameters characterizing the media were varied over a wide range: the bulk density, the size of the elements entering the mix, and the proportion of cardboard and PE in the mix. The properties were measured by means of a specially designed experimental device based on miscible gas tracing. The porosity and tortuosity factor were simultaneously determined by parametric identification, based on the experimental sample output response to an inlet gas concentration step change compared to the results of a direct numerical model. Permeability was calculated in the standard way from the measurement of the pressure drop across the sample. The reproducibility of the measurements was very good. It was found that changing the material density of the medium significantly affects all three structural properties. When the bulk density is varied between 300 and 900 kg m À3 , the tortuosity factor varies in a range as large as 18-8 and the permeability decreases by a ratio of 2-3. The tortuosity factor shows unusual variation, characterized by a decrease when density is increased above 500 kg m À3. The size of the elements does not significantly affect the structural properties of the medium in the range of parameters studied

Isoconversional and Distributed Activation Energy Models for the Kinetic Study of Biomass Fast Devolatilization Based on MS Data

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Darcy Scale Modeling of Smoldering: Impact of Heat Loss

Modelling the propagation of smoldering fronts with forced air feeding in a porous medium remains a challenge to science. One of the main difficulties is to describe the carbon oxidation reaction that supports this self-sustained process. Pore scale approaches are required to tackle this complex coupled heat and mass transfer problem with chemistry. They nevertheless require high computation effort and still miss experimental validation. Furthermore, the heat loss at the walls of the cells inherent to every laboratory scale system adds another level of complexity in the understanding of the coupling between the phenomena at stake. Indeed, it induces a non homogeneous temperature field throughout the system. In this paper, a 2D Darcy scale model is developed and validated by confrontation with experimental results from the literature, covering wide ranges of carbon content of the medium and forced air velocity. A reasonable description of the front temperature, velocity and non-consumption oxygen amount is reached. The model finally enables to bring understanding of the impact of heat loss which controls front shape and stability near the system walls

Reaction rates for the oxidation of highly sulphurised petroleum cokes : the influence of thermogravimetric conditions and some coke properties

International audienceThe reaction with air of a large number (22) of high-sulphur petroleum cokes was studied by temperature-ramped thermogravimetric analysis. The kinetic parameters for each coke were established, based on BET surface areas. The oxidation rates (kg C m 22 s 21 atm 21) were found to vary with sample mass. This was a result of limitations on oxygen transfer, despite the small masses and low heating rates used. Limitations were present both externally (from the crucible mouth to the bed surface) and internally (from the sample surface to the bed interior). A method to take these effects into account was adopted, based on an analysis of the relevant diffusion rates. Application of this method reconciled the rate data for four different sample masses, except at high temperatures. The formation of a partially fused ash crust is believed to be the reason for this latter effect. The activation energies of the cokes varied between 195 and 280 kJ mol 21 , and the absolute rates varied by a factor of 10. They were between 1000 and 10,000 times higher than the average reactivity of carbon as reported in the literature. The elevated apparent rates are believed to have two causes, one in the combustion process and the other in the interpretation of the results. The first cause is the strong catalytic effect of the inorganic components, although the ash contents ranged only from 0.3 to 1.5%. The most active metal is vanadium, which is present in significant concentrations. The effectiveness of V 2 O 5 as a gasifying catalyst is believed to be due to its low melting point. Increasing sulphur content in the cokes produces no perceptible change in the combustion rates. The second cause for poor combustion correlation is the inadequacy of BET surface area for expressing combustion rates

The high temperature reaction of carbon with nitric oxide

International audienceFour pyrolyzed carbonaceous materials were reacted with 950 ppm of nitric oxide (in N 2) in an entrained flow furnace at temperatures between 800 and 1,000°C. Their apparent reactivities, corrected for mass transfer, when calculated on a BET area basis displayed a wide spread of values. However, when recalculated for the surface area estimated for pores greater than 20 nm in size, as measured by mercury porosimetry, far better agreement was obtained. The results are consistent with other data in the literature, for which equivalent surface areas were available

Modelling of non-consolidated oil shale semi-coke forward combustion: influence of carbon and calcium carbonate contents

A one dimensional (1-D) numerical model to describe forward filtration combustion in a porous bed is proposed. The numerical model is based on mass and momentum conservation law (generalized Darcy’s law). We assume local thermal equilibrium between gas and solid phases. The effect of carbon and calcium carbonate content on the propagation of the high temperature combustion front has been investigated. A simple carbon oxidation reaction, producing CO and CO2, describes the combustion. We found that increasing the carbon content of the bed increases the peak temperature. However, the combustion front velocity versus carbon content does not show a monotone behaviour. The front velocity increases while carbon content increases up to a certain value and then decreases. Also, we observed that higher the temperature is, stronger the calcium carbonate decomposition is. Consequently, the calcium carbonate decomposition is closely linked to the peak temperature. Moreover, increasing the calcium carbonate content of the porous bed resulted a decrease of the peak temperature. These results as well as the composition of produced gases are consistent with the previous published experimental study. Results of this paper show that using a 1-D model with a simple reaction scheme for combustion and for calcium carbonate decomposition produces satisfactory results for simulation of filtration combustion process

Torréfaction du bois et de ses constituants (expériences et modélisation des rendements en matières volatiles)

Actuellement, l industrialisation de la torréfaction de biomasse se heurte notamment à un manque de connaissances de la nature et de la quantité des matières volatiles produites en fonction des conditions opératoires et de la matière première. L objectif de ces travaux est donc de mieux comprendre comment s opère la torréfaction de la biomasse, en se concentrant sur l étude de la perte de masse du solide et des rendements en matières volatiles. La torréfaction est considérée à partir de bois sec, sous atmosphère inerte et suivant un palier à une température comprise entre 200C et 300C. Lors d une étude expérimentale, du hêtre et ses constituants, à savoir cellulose, xylane et lignine, ont été torréfiés, en régime chimique, dans une thermobalance et dans un pilote de torréfaction à échelle laboratoire. Le bilan matière boucle entre 97% et 104%. Les principales matières volatiles émises par la torréfaction de ce bois sont l eau, le formaldéhyde, l acide acétique et le CO2. De l acide formique, du CO, du méthanol et du furfural sont aussi mesurés en quantité moindre. Certaines de ces espèces ne sont pas produites par tous les constituants du hêtre. Il semble en particulier que l acide acétique soit produit à partir de la dégradation des acétates contenus dans les hémicelluloses. Par ailleurs, il apparaît en première approximation que la transformation peut être correctement représentée par la loi d additivité jusqu à 250C. Cela n est plus le cas à 280C et 300C, du fait d interactions entre la cellulose et les deux autres constituants du bois. Celles-ci ralentissent la vitesse de torréfaction de la cellulose. A partir de ces résultats expérimentaux, a été développé dans ces travaux un modèle de torréfaction du bois, basé sur la superposition de sous-modèles décrivant chacun la torréfaction d un constituant du bois. Ce modèle, qui présente comme originalité de prévoir en fonction de la proportion du bois en cellulose/hémicelluloses/lignine à la fois le rendement en solide et en huit espèces volatiles, et de prendre en compte les interactions à l aide d un facteur empirique, a été validé sur les expériences de torréfaction du hêtre entre 220C et 300C. Son utilisation a mis en évidence l influence significative des contenus en hémicelluloses et cellulose sur les rendements en produits de la torréfaction.The industrialization of the biomass torrefaction process requires better knowledge of the volatile species release versus operating conditions and feedstock. In this context, the present work aimed at studying solid mass loss and volatile species yields during biomass torrefaction. This transformation was considered on dry wood, at a temperature plateau between 200C and 300C and under inert atmosphere. First, torrefaction experiments were conducted under chemical regime on beechwood and its constituents cellulose, lignin and hemicelluloses in a thermobalance and in a lab-scale device. The mass balance closure was achieved with values ranging from 97 and 104%. The main volatile species measured were water, formaldehyde, acetic acid and CO2. Smaller amounts of methanol, CO, formic acid and furfural were also quantified. All those gas species were not produced by the three biomass constituents. In particular acetic acid seems to be produced by the degradation of the acetate groups contained in hemicelluloses. The results showed that in a first approximation torrefaction can be described by the additive law up to 250C. But this law is not valid at 280C and 300C because of interactions between cellulose and the two other wood constituents. These interactions lead to a decrease in the torrefaction rate of cellulose. Based on these experimental results, a model of wood torrefaction was developed. It consists in the superposition of sub-models describing the torrefaction of each wood constituent. The originality of this model lies in its ability to predict both solid yield and eight volatile species yields depending on cellulose/hemicellulose/lignin wood composition, and to take into account interactions by means of an empirical factor. It was validated on beechwood torrefaction experiments between 220C and 300C. Finally, this model highlighted the significant influence of the proportion of hemicellulose and cellulose on torrefaction product yields.TOULOUSE-INP (315552154) / SudocSudocFranceF

Contrôle de la température d'un front de combustion propagé dans un milieu poreux contenant du carbone et des carbonates afin de limiter les émissions de CO2

La thèse s intéresse au contrôle de la température d un front de combustion propagé dans un milieu poreux contenant du carbone fixe et des carbonates (CaCO3). L objectif principal est de réduire la température, in situ (récupération d huile ou production de gaz à partir d un schiste bitumineux) ou dans un procédé (combustion de semicoke), afin de limiter la décarbonatation du milieu et les émissions induites de CO2. Le milieu réactif retenu pour réaliser les expériences en laboratoire est un schiste bitumineux préalablement broyé (0.5 à 2 mm) et pré-pyrolysé, appelé semicoke. Le front est propagé en co-courant. La première technique testée expérimentalement est l ajout au semicoke d un matériaux inerte (sable) et/ou d un matériaux réactif (CaCO3) afin de faire varier le taux de carbone fixe et le taux de CaCO3 et ceci indépendamment. Nous montrons que l augmentation de CaCO3 permet de baisser la température à 800 C, mais pas en dessous, ce qui ne permet pas d éviter la décarbonatation. Faire chuter le contenu en carbone fixe permet de baisser la température du front, voire d atteindre l extinction. Aux températures de propagation les plus basses, la décarbonatation est fortement limitée. En revanche le front ralentit car il n utilise plus tout l oxygène alimenté. La deuxième technique originale consiste à ajouter du CO2 (20 %molaire) dans l air de combustion. Nous montrons que dans le cas d un front chaud, ceci permet de réduire le taux de décarbonatation de 100% à 70%, et d augmenter en parallèle la production de CO résultant de l oxydation du carbone fixe, ce qui augmente le PCI du gaz produit. Sur un front plus froid, la décarbonatation qui était de 20% est totalement évitée par l ajout de CO2. Enfin, des expériences sont proposées dans le mode de combustion reaction trailing , très peu connu et mis en oeuvre. Ce mode a l intérêt majeur d éviter les réactions de Lower Temperature Oxidation préjudiciables au rendement en huile ou en gaz d un process in situ. Des expériences stables et répétables sont réalisées avec différents pourcentages d oxygène dans le gaz alimenté. La température du front est directement liée à ce paramètre ; la décarbonatation est clairement limitée dans ce mode de propagation. Deux types de modélisation sont proposés. Un bilan de matière et d énergie basé sur des expressions analytiques simples permet d évaluer la température du front et sa vitesse de propagation. Un modèle numérique développé par l IMFT se base sur des équations de transfert convectif/diffusif de chaleur et de matière, couplées aux réactions d oxydation du carbone (en CO et en CO2) et de décarbonatation de CaCO3. Il décrit de façon très satisfaisante les expériences en mode reaction leading avec variation de la composition du milieu (première technique).This PhD thesis focuses on the control of the smoldering front propagating in a porous medium containing fixed carbon and carbonates (CaCO3). The main objective is to reduce the front temperature, in situ (oil recovery or gas production from oil shale) or in process (combustion of semicoke), in order to limit the medium decarbonation and the resulting CO2 emissions. The reactive porous medium retained to realize the laboratory experiments is a crushed (0.5 to 2 mm) and pre-pyrolyed oil shale, called semicoke. The front propagates in co-current. The first technique experimentally tested is the addition to the semicoke of an inert material (sand) and/or a reactive material (CaCO3) to vary the contents of fixed carbon and of CaCO3, independently. We show that the increase of the CaCO3 content enables to reduce the temperature to 800 C, but not below; this does not allow to avoid decarbonation. Bringing down the fixed carbon content enables to reduce the front temperature, see even to reach extinction. In the lowest temperatures of propagation, the decarbonation is strongly limited. On the other hand, the front slows down because it does not use all of the fed oxygen. The second original technique consists in adding CO2 (20 mol.%) to the oxidizer air. We show that for a hot front, the decarbonated fraction is reduced from 100% down to 70%, and the CO production at fixed carbon oxidation is increased; this leads to increase the LCV of the produced gas. For a cold front, the decarbonation which was 20%, is totally avoided by adding CO2. Finely, experiments are proposed in the reaction trailing combustion mode, little known and implemented. This mode has the major interest to avoid the reactions of Lower Temperature Oxidation prejudicial for oil or gas yields in in situ process. Stable and repeatable experiments are realized with different oxygen fractions in feeding gas. The front temperature is directly linked to this parameter; the decarbonation is clearly limited in this mode of propagation. Two types of modeling are proposed. A mass and thermal balance based on simple analytical expressions enables to evaluate the front temperature and velocity. A numerical model developed by IMFT is based on convective/diffusive heat and mass transfer equations coupled with the oxidation reactions (into CO and CO2) and CaCO3 decarbonation is proposed. It describes in a very satisfactory way the experiments in the reaction leading mode with variation of the medium composition (first technique).TOULOUSE-INP (315552154) / SudocSudocFranceF