Particle Size Distribution in a Godbert-Greenwald Furnace: Experiments and Modelling

During a dust dispersion, the particle size distribution (PSD) depends on several factors such as the turbulence, the initial particle size and shape as well as the dust concentration. As a consequence, when determining safety parameters using standard procedures, its potential evolution should be considered. Different powders were chosen: glucose, starch, ascorbic acid, glass beads and cellulose. A Godbert-Greenwald furnace was used to disperse the powders and determine their minimum ignition temperature (MIT) according to ISO/IEC 80079-20-2:2016 standard. The PSD of each powder was determined in-situ at different locations using a laser diffraction sensor. Some powders showed clear signs of breakage, as for glucose whose mean diameter decreases from 166 to 76 µm during its dispersion. On the contrary, many samples tended to agglomerate, e.g. starch and cellulose. For instance, the d90 of starch can even be quadrupled under certain conditions. Agglomeration occurs especially for fine dusts due to strong inter-particles forces (e.g. starch) or for elongated fibres due to entanglement phenomenon (e.g. cellulose). During a powder dispersion in the Godbert-Greenwald furnace, the PSD evolves not only as a function of time but also along with its location. The impact of the glass elbow on PSD variation has notably been highlighted by placing the G-G furnace horizontally. For powders showing strong tendency to agglomeration or breakage, the influence of the dispersion pressure has also been studied. The role of such PSD modification on the MIT has been measured and, depending on the dispersion procedure, temperature differences of more than 50°C have been observed. The agglomerate strength was assessed using three models (from Rumpf, Weiler and Kendall works) and compare to the deagglomeration stress exerted on the powders. In the case of cohesive powders, fibres or brittle dusts, attention should be paid to the PSD evolution during MIT determination

Comparative analyses of three olive mill solid residues from different countries and processes for energy recovery by gasification

International audienceBiomass is a renewable energy source which may provide a significant contribution to the reduction of fossil fuels consumption and the associated environmental impacts. The use of agricultural or agro-industrial waste such as solid residues from olive oil production is particularly relevant since it may combine several benefits. Gasification is a promising waste-to-energy technique for this type of lignocellulosic residues. The technology however is adapted to a relatively limited panel of solid waste fuels of defined specifications, which must therefore be characterized properly to assess their adaptation. The purpose of this research was to analyze and compare three different olive mill solid residues by complementary techniques such as Fourier transform infrared spectroscopy (FTIR) and thermochemical methods, in order to characterize these residues as potential fuels for gasification. The results obtained underlined the complex nature of the residues and indicated that they were mainly organic, with very little mineral matter. In addition to the major organic components (cellulose, hemicelluloses and lignin), the presence of several minor organic constituents was shown by thermogravimetry coupled to differential scanning calorimetry and FTIR. The gas produced from pyrolysis was analyzed by gas chromatography and mass spectrometry. It was found to contain several degradation products from lignocellulosic material and olive oil, such as hydroxyacetone, furfural and methoxyphenols. The influence of the olive oil extraction process (two-phase or three-phase) was also demonstrated. It was shown that the thermochemical degradation of olive mill residues followed a complex pathway but the composition of the residues met the requirements for gasification for most parameters

Mécanismes impliqués dans la combustion rapide des poudres organiques : application aux explosions de poussières

Le risque industriel lié aux explosions de poussières est communément accepté comme élevé par la communauté scientifique, et il doit être réduit au minimum pour améliorer les conditions de travail. Cependant, la méconnaissance des phénomènes impliqués, combinée à une faible prise de conscience générale de nombreuses réalités industrielles, transforme les explosions de poussières en scénarios accidentels profondément dangereux. La manipulation des poudres est une brique fondamentale de la réalité industrielle contemporaine, et aucun raccourci ne peut être choisi pour la contourner. Pour cette raison, les explosions de poussières se confondent avec une grande variété de processus et doivent être étudiées et bien comprises. Dans la seconde moitié du XXe siècle, une transition énergétique a déstabilisé le secteur des énergies fossiles, le rendant de plus en plus vert. En conséquence, la biomasse lignocellulosique était et est toujours l'une des sources de bioénergie les plus populaires dans le monde. En outre, son énorme polyvalence et les progrès technologiques importants ont permis d'entamer la concurrence avec les combustibles fossiles. Le développement du secteur de la bioénergie et l'explosion de poussières sont les deux acteurs de ce travail. Par conséquent, les explosions de poussières organiques ont été choisies comme sujet d'étude, comprises et caractérisées pour contribuer à la connaissance scientifique de ces phénomènes. Elles peuvent être traitées comme des systèmes transitoires, turbulents et chargés de particules, ce qui les rend complexes et loin d'être simples à comprendre. Plusieurs phénomènes physico-chimiques sont impliqués dans le global. Tout d'abord, une étape de dispersion est inévitable dans une explosion de poussières. Dans cette phase, les caractéristiques de la poudre peuvent changer et avoir un impact important sur son comportement. Ensuite, les interactions entre particules sont responsables de l'agglomération et de la désagglomération dans le nuage de poussière ainsi formé. Là encore, elles peuvent modifier les caractéristiques du système, influençant son comportement thermique. Les forces cohésives peuvent conduire à une forte agrégation des particules, ce qui a notamment une incidence sur l'échelle de temps de chauffage des particules. Une large sélection de poudres organiques a été testée, et leur tendance à s'agglomérer en un nuage de poussière a été quantifiée. Troisièmement, une brique fondamentale des explosions de poudres organiques est l'étape de pyrolyse, qui peut être considérée comme une dévolatilisation responsable de la création d'une atmosphère inflammable au sein du nuage de poussière. C'est souvent l'étape limitant la vitesse du processus global, et elle doit donc être bien analysée. Ensuite, la synergie potentielle entre les phases gazeuse et solide peut conduire à un système réactif différent de la somme des deux. De nouveaux phénomènes pourraient en découler. Par exemple, les transferts radiatifs de chaleur sont des acteurs majeurs dans la phase de propagation de la flamme, fortement dépendants de la concentration en poussière et de ses propriétés optiques. En conclusion, désireux de développer une méthode originale pour déterminer la vitesse de flamme laminaire des poudres organiques, trois appareils ont été sélectionnés et comparés. Les avantages et les inconvénients de chacun ont été listés et utilisés comme critères de choix.The industry's risk associated with dust explosions is commonly accepted as high by the scientific community, and it must be reduced to a minimum to improve working conditions. However, the lack of knowledge of the phenomena involved, combined with a scarce general awareness of numerous industrial realities, transforms the dust explosions into profoundly dangerous accidental scenarios. Powder manipulation is a fundamental brick of contemporary industrial reality, and no shortcut can be chosen to bypass it. For this reason, dust explosions are merged with a large variety of processes and need to be studied and well understood. In the second half of the XX century, an energetic transition destabilised the fossil energy sector, painting it greener and greener. As a result, lignocellulosic biomass was and still is one of the most popular worldwide bioenergy sources. Moreover, its enormous versatility and significant technological progress made it possible to start the competition with fossil fuels. The development of the bioenergy sector and the dust explosion are the two actors of this work. Therefore, organic dust explosions were chosen as a study subject, comprehended and characterised to contribute to the scientific knowledge of these phenomena. They can be treated as transient, turbulent, particle-laden systems, which makes them complex and far from being straightforward to understand. Several physicochemical phenomena are involved in the global one. First, a dispersion step is unavoidable in a dust explosion. In this phase, the powder's features may change and strongly impact its behaviour. Second, particle-to-particle interactions are responsible for agglomeration and deagglomeration in the dust cloud thus formed. Again, it can modify the system's characteristics, influencing its thermal comportment. Cohesive forces may lead to strong aggregation of the particles, which has notably a mark on the particle heating time scale. A large selection of organic powders was tested, and their tendency to agglomerate in a dust cloud was quantified. Third, a fundamental brick of organic powder explosions is the pyrolysis step, which can be seen as a devolatilisation responsible for creating a flammable atmosphere within the dust cloud. It is often the rate-limiting step of the global process, and it needs thus to be well analysed. Then, the potential synergy between the gaseous and solid phases can lead to a reactive system different from the sum of the two. Some new phenomena might be generated from this. For instance, radiative heat transfers are major players in the flame propagation phase, strongly dependent on the dust concentration and its optical properties. To conclude, willing to develop an original method to determine the laminar flame speed of organic powders, three apparatuses were selected and compared. The benefits and drawbacks of each were listed and used as choosing criteria

Mécanismes impliqués dans la combustion rapide des poudres organiques : application aux explosions de poussières

The industry's risk associated with dust explosions is commonly accepted as high by the scientific community, and it must be reduced to a minimum to improve working conditions. However, the lack of knowledge of the phenomena involved, combined with a scarce general awareness of numerous industrial realities, transforms the dust explosions into profoundly dangerous accidental scenarios. Powder manipulation is a fundamental brick of contemporary industrial reality, and no shortcut can be chosen to bypass it. For this reason, dust explosions are merged with a large variety of processes and need to be studied and well understood. In the second half of the XX century, an energetic transition destabilised the fossil energy sector, painting it greener and greener. As a result, lignocellulosic biomass was and still is one of the most popular worldwide bioenergy sources. Moreover, its enormous versatility and significant technological progress made it possible to start the competition with fossil fuels. The development of the bioenergy sector and the dust explosion are the two actors of this work. Therefore, organic dust explosions were chosen as a study subject, comprehended and characterised to contribute to the scientific knowledge of these phenomena. They can be treated as transient, turbulent, particle-laden systems, which makes them complex and far from being straightforward to understand. Several physicochemical phenomena are involved in the global one. First, a dispersion step is unavoidable in a dust explosion. In this phase, the powder's features may change and strongly impact its behaviour. Second, particle-to-particle interactions are responsible for agglomeration and deagglomeration in the dust cloud thus formed. Again, it can modify the system's characteristics, influencing its thermal comportment. Cohesive forces may lead to strong aggregation of the particles, which has notably a mark on the particle heating time scale. A large selection of organic powders was tested, and their tendency to agglomerate in a dust cloud was quantified. Third, a fundamental brick of organic powder explosions is the pyrolysis step, which can be seen as a devolatilisation responsible for creating a flammable atmosphere within the dust cloud. It is often the rate-limiting step of the global process, and it needs thus to be well analysed. Then, the potential synergy between the gaseous and solid phases can lead to a reactive system different from the sum of the two. Some new phenomena might be generated from this. For instance, radiative heat transfers are major players in the flame propagation phase, strongly dependent on the dust concentration and its optical properties. To conclude, willing to develop an original method to determine the laminar flame speed of organic powders, three apparatuses were selected and compared. The benefits and drawbacks of each were listed and used as choosing criteria.Le risque industriel lié aux explosions de poussières est communément accepté comme élevé par la communauté scientifique, et il doit être réduit au minimum pour améliorer les conditions de travail. Cependant, la méconnaissance des phénomènes impliqués, combinée à une faible prise de conscience générale de nombreuses réalités industrielles, transforme les explosions de poussières en scénarios accidentels profondément dangereux. La manipulation des poudres est une brique fondamentale de la réalité industrielle contemporaine, et aucun raccourci ne peut être choisi pour la contourner. Pour cette raison, les explosions de poussières se confondent avec une grande variété de processus et doivent être étudiées et bien comprises. Dans la seconde moitié du XXe siècle, une transition énergétique a déstabilisé le secteur des énergies fossiles, le rendant de plus en plus vert. En conséquence, la biomasse lignocellulosique était et est toujours l'une des sources de bioénergie les plus populaires dans le monde. En outre, son énorme polyvalence et les progrès technologiques importants ont permis d'entamer la concurrence avec les combustibles fossiles. Le développement du secteur de la bioénergie et l'explosion de poussières sont les deux acteurs de ce travail. Par conséquent, les explosions de poussières organiques ont été choisies comme sujet d'étude, comprises et caractérisées pour contribuer à la connaissance scientifique de ces phénomènes. Elles peuvent être traitées comme des systèmes transitoires, turbulents et chargés de particules, ce qui les rend complexes et loin d'être simples à comprendre. Plusieurs phénomènes physico-chimiques sont impliqués dans le global. Tout d'abord, une étape de dispersion est inévitable dans une explosion de poussières. Dans cette phase, les caractéristiques de la poudre peuvent changer et avoir un impact important sur son comportement. Ensuite, les interactions entre particules sont responsables de l'agglomération et de la désagglomération dans le nuage de poussière ainsi formé. Là encore, elles peuvent modifier les caractéristiques du système, influençant son comportement thermique. Les forces cohésives peuvent conduire à une forte agrégation des particules, ce qui a notamment une incidence sur l'échelle de temps de chauffage des particules. Une large sélection de poudres organiques a été testée, et leur tendance à s'agglomérer en un nuage de poussière a été quantifiée. Troisièmement, une brique fondamentale des explosions de poudres organiques est l'étape de pyrolyse, qui peut être considérée comme une dévolatilisation responsable de la création d'une atmosphère inflammable au sein du nuage de poussière. C'est souvent l'étape limitant la vitesse du processus global, et elle doit donc être bien analysée. Ensuite, la synergie potentielle entre les phases gazeuse et solide peut conduire à un système réactif différent de la somme des deux. De nouveaux phénomènes pourraient en découler. Par exemple, les transferts radiatifs de chaleur sont des acteurs majeurs dans la phase de propagation de la flamme, fortement dépendants de la concentration en poussière et de ses propriétés optiques. En conclusion, désireux de développer une méthode originale pour déterminer la vitesse de flamme laminaire des poudres organiques, trois appareils ont été sélectionnés et comparés. Les avantages et les inconvénients de chacun ont été listés et utilisés comme critères de choix

Fast and tiny: A model for the flame propagation of nanopowders

International audienceTo avoid the influence of external parameters, such as the vessel volume or the initial turbulence, the explosion severity should be determined from intrinsic properties of the fuel-air mixture. Therefore, the flame propagation of gaseous mixtures is often studied in order to estimate their laminar burning velocity, which is both independent of external factors and a useful input for CFD simulation. Experimentally, this parameter is difficult to evaluate when it comes to dust explosion, due to the inherent turbulence during the dispersion of the cloud. However, the low inertia of nanoparticles allows performing tests at very low turbulence without sedimentation. Knowledge on flame propagation concerning nanoparticles may then be modelled and, under certain conditions, extrapolated to microparticles, for which an experimental measurement is a delicate task. This work focuses on a nanocellulose with primary fiber dimensions of 3 nm width and 70 nm length. A one-dimensional model was developed to estimate the flame velocity of a nanocellulose explosion, based on an existing model already validated for hybrid mixtures of gas and carbonaceous nanopowders similar to soot. Assuming the fast devolatilization of organic nanopowders, the chemical reactions considered are limited to the combustion of the pyrolysis gases. The finite volume method was used to solve the mass and energy balances equations and mass reactions rates constituting the numerical system. Finally, the radiative heat transfer was also considered, highlighting the influence of the total surface area of the particles on the thermal radiation. Flame velocities of nanocellulose from 17.5 to 20.8 cm/s were obtained numerically depending on the radiative heat transfer, which proves a good agreement with the values around 21 cm/s measured experimentally by flame visualization and allows the validation of the model for nanoparticles

A flame propagation model for nanopowders

International audienceThe determination of explosion severity should be made from intrinsic properties of the fuel-air mixture in order to avoid the influence of external parameters, such as the vessel volume or the initial turbulence. To overcome such limitations, the flame propagation of gaseous mixtures is often studied in order to estimate their laminar burning velocity, which is independent of external factors and is a useful input for CFD simulation and for the sizing of protective devices. Experimentally, this parameter is difficult to evaluate when it comes to dust explosion due to the inherent turbulence during the dispersion of the cloud. However, the low inertia of nanoparticles allows performing tests at very low turbulence without sedimentation. Knowledge on flame propagation concerning nanoparticles may then be modelled and, under certain conditions, extrapolated to microparticles, for which an experimental measurement is a delicate task. This work then focused a nanocellulose with primary fiber dimensions of 3 nm width and 70 nm length. A one-dimensional model was developed to estimate the flame velocity of a nanocellulose explosion, based on an existing model already validated for hybrid mixtures of gas and carbonaceous nanopowders similar to soot. Due to the fast devolatilization of organic powders, the chemical reactions considered are limited to the combustion of the pyrolysis gases. The finite volume method was used to solve the mass and energy balances equations and mass reactions rates constituting the numerical system. Finally, the radiative heat transfer was also considered, highlighting the influence of the total surface area of the particles on the thermal radiation. Flame velocities of nanocellulose from 17.5 to 20.8 cm.s-1 were obtained numerically depending on the radiative heat transfer, which proves a good agreement with the values around 21 cm.s-1 measured experimentally by flame visualization and allows the validation of the model for nanoparticles

Study of flash pyrolysis and combustion of biomass powders using the Godbert-Greenwald furnace : an essential step to better understand organic dust explosions

An organic dust explosion is a heterogeneous system on a space and time scale. Predicting the parameters characteristic of its severity needs experimental and theoretical approaches to find the optimal compromise between consistency with reality and modelling time. A hybrid method is proposed to study flash pyrolysis and combustion of several organic powders (cellulose, wheat starch, oak wood, Douglas fir and olive pomace). A Godbert-Greenwald furnace was employed to perform the experiments to mimic the fundamental characteristics of a dust explosion: high particle heating rate, high reaction temperature and short residence times. At 973 K, the residence time is a critical parameter: the large particles of cellulosic compounds (wood, cellulose) do not reach their pyrolysis temperature and only fibres smaller than 20 or 30 µm are fully converted. As the particle size distribution of starch is smaller, heat transfer is not directly the limiting phenomenon but rather the strong tendency for powders to agglomerate during pyrolysis. At higher temperatures, secondary reactions of primary tars are evidenced, stressing the influence of the pyrolysis stage and leading to heterogeneous combustion. The composition of the pyrolysis gases as a function of the nature of the powder and the temperature was also determined. A lumped-kinetic model adapted to dust explosion was developed and validated for cellulose. The kinetics constants corresponding to levoglucosan to permanent gases and cellulose to char and water reactions are significantly different from those proposed by the literature, demonstrating that dust explosion kinetic parameters must be obtained under conditions consistent with such phenomenon

Caractérisation de grignons d’olives en vue d’une valorisation thermochimique par gazéification

International audienceLa biomasse est une source d'énergie renouvelable qui peut contribuer de manière significative à la réduction de la consommation de combustibles fossiles. L'utilisation de déchets agricoles ou agro-industriels tels que les grignons d’olives est particulièrement pertinente. La gazéification est une technique prometteuse de valorisation énergétique des déchets pour ce type de résidus lignocellulosiques. La technologie est cependant adaptée à un panel relativement limité de combustibles solides ayant des spécifications définies, qui doivent donc être caractérisées correctement. Le but de cette étude était d'analyser et de comparer des grignons d’olives de 3 origines différentes par des techniques complémentaires telles que la spectroscopie infrarouge à transformée de Fourier, la thermogravimétrie couplée à l'analyse calorimétrique différentielle et la pyrolyse suivie d'une chromatographie en phase gazeuse / spectrométrie de masse (Py-CG/SM). Les résultats soulignent la nature complexe des grignons qui sont principalement organiques. En plus des principaux composés organiques (cellulose, hémicelluloses et lignine), la présence de plusieurs constituants organiques mineurs a été montrée. La Py-CG/SM a montré que le gaz produit par pyrolyse contient plusieurs produits de dégradation de la matière lignocellulosique et de l'huile d'olive. L'influence du procédé d'extraction de l'huile d'olive (à deux phases ou à trois phases) a également été mise en évidence. La dégradation thermochimique des grignons suit un mécanisme complexe mais la composition des grignons répond à la plupart des spécifications requises par la gazéification