Oscillating Ponomarenko dynamo in the highly conducting limit

This paper considers dynamo action in smooth helical flows in cylindrical geometry, otherwise known as Ponomarenko dynamos, with periodic time dependence. An asymptotic framework is developed that gives growth rates and frequencies in the highly conducting limit of large magnetic Reynolds number, when modes tend to be localized on resonant stream surfaces. This theory is validated by means of numerical simulations.Comment: 12 pages, 4 figure

Parametric instability of the helical dynamo

We study the dynamo threshold of a helical flow made of a mean (stationary) plus a fluctuating part. Two flow geometries are studied, either (i) solid body or (ii) smooth. Two well-known resonant dynamo conditions, elaborated for stationary helical flows in the limit of large magnetic Reynolds numbers, are tested against lower magnetic Reynolds numbers and for fluctuating flows (zero mean). For a flow made of a mean plus a fluctuating part the dynamo threshold depends on the frequency and the strength of the fluctuation. The resonant dynamo conditions applied on the fluctuating (resp. mean) part seems to be a good diagnostic to predict the existence of a dynamo threshold when the fluctuation level is high (resp. low).Comment: 37 pages, 8 figure

Fast pyrolysis bio-oil production in an entrained flow reactor pilot

Bio-oil produced from biomass fast pyrolysis could constitute an alternative to fossil liquid fuels, especially to be combusted for local district heating. So far, only few studies have dealt with bio-oil production by biomass fast pyrolysis in an entrained flow reactor [1], yet it could constitute an alternative to the better-known fluidised bed pyrolysis process. In the context of the BOIL project with the CCIAG Company (Grenoble district heating), a new pilot based on an entrained flow reactor concept has been designed [2]. The pilot design has been carried out on the basis of woody biomass fast pyrolysis experiments and modeling performed in a drop tube reactor as a first step laboratory-scale study, and also CFD modeling [2-3]. The facility is composed of a biomass injection system with a hopper and a feeding screw, an electrically heated pyrolysis reactor, a cyclone to separate gas and char, 3 heat exchangers to cool the gas (at 30°C, 0°C and 0°C respectively) and condense bio-oil, and a post-combustion unit to burn the incondensable species. Gas temperature is maintained at 350°C from the reactor outlet to the entrance of the first heat exchanger in order to avoid bio-oil condensation. Several conditions were tested in 14 runs: 3 different biomass feedstocks, varying biomass feeding rates from 2 to 9 kg/h and two reactor temperatures 500°C and 550°C. 85 kg of bio-oil has been produced for combustion tests. Recovered bio-oil mass yield is on average 50%, its LHV is about 15 MJ/kg, its water content 26%w and its pH 2.15. We identified three main difficulties during the runs: about 15% of the bio-oil go through the heat exchanger, some char particles go through the cyclone which causes regular plugging of the first heat exchanger. Detailed analyses of the bio-oil produced have been done and the chemical and physical bio-oil characteristics have been compared to the European Standard recommendations [4]. With a regularly cleaning of the first heat exchanger, we successfully produce bio-oil with physical and chemical properties in agreement with the European Standard recommendations. Combustion tests of the bio-oil produced have been carried on by the CIRAD. They succeeded in obtaining a stable flame (without the use of a pilot flame) in a 50 kW burner and a 250 kW combustion chamber. However the physical and chemical characteristics of the bio-oil involve the use of specific pump and pulverization system adapted. In perspective for future projects, it would be interesting to perform pilot modifications in order to increase bio-oil yield and to minimize heat exchanger cleaning, and to test other resources like agricultural biomass or solid recovered fuels. Bibliography 1. J.A. Knight, C.W. Gorton, R.J. Kovac, Biomass 6, pp. 69-76, 1984. 2. Fast pyrolysis reactor for organic biomass materials with against flow injection of hot gases - US 20170166818 A1 3. Guizani, S.Valin, J.Billlaud, M.Peyrot, S.Salvador, Fuel, 2017, 207, pp.71-84. 4. C.Guizani, S.Valin, M.Peyrot, G.Ratel S.Salvador, Woody biomass fast pyrolysis in a drop tube reactor - Pyro2016 conference 5. Fast pyrolysis bio-oils for industrial boilers – Requirements and test methods – EN 1690

Biofuels from waste to road transport

Biofuels from Waste to Road (WASTE2ROAD) is an EU funded project under the Grant Agreement No. 818120 within the LC-SC3-RES-21-2018 call, “Development of next generation biofuels and alternative renewable fuel technologies for road transport”, as a Research and Innovation Action of the European Union’s Horizon 2020 Programme. The project started in the fall 2018 and will run for 4 years. In 2014, total waste production in the EU amounted to 2.5 billion tons. From this total only a limited (albeit increasing) share (36%) was recycled, while the rest was landfilled or burned, of which some 600 million tons could have been recycled or reused. Conversion of all sustainably available biogenic wastes and residues to biofuels could provide 27% of total transport fuel by 2050, achieving around 2.1 gigatons of CO2 emission reductions per year. The increasing demand for biofuels[1] implies the need for the transformation of diverse bio-resources into liquid fuels, and includes transformation of the biogenic part of municipal and industrial wastes into such biofuels. This clearly is a stepping stone to achieve the European goals[2] but it also poses challenges, such as 1) diversity and inhomogeneity of wastes throughout Europe (variable composition depending on the type of waste and geographical location), 2) the complexity of the conversion of wastes compared to fossil oils, 3) the technological aspects of co-refining and 4) high overall costs with moderate process performance. [1] https://www.iea.org/publications/freepublications/publication/Biofuels_Roadmap_WEB.pdf [2] https://europeanclimate.org/wp-content/uploads/2014/02/WASTED-final.pdf Please click Additional Files below to see the full abstract

Instabilité paramétrique de la dynamo de Ponomarenko

We have studied the influence of large scale fluctuations on the dynamo threshold. For that purpose, we have solved the kinematic problem for a helical flow, to the stationary part of which, we have added a periodic and also helical time modulation. For a helical stationary flow, previous studies have shown that for high magnetic Reynolds numbers, the magnetic field is generated on a resonance surface characterised by a resonant condition on the velocity flow. For a modulated flow, we have shown that, for low amplitude of modulation, it is the resonant condition of the stationary part, that controls the generation of the magnetic field. For large amplitude of the fluctuations, it is the resonant condition of the fluctuating part of the flow that governs the dynamo efficiency as well as the dynamo threshold. In most cases and if the resonant condition is satisfied for both parts of the flow, we find that the threshold increases with the intensity of the fluctuations, and then decreases, while remaining higher than the threshold corresponding to the stationary threshold of the same geometry. If the resonant condition is not verified for the modulation, then the threshold increases drastically with its intensity.These results show that the optimization of the dynamo experiments may not only depend on the stationary part of the flow but also on its non stationary large scale part. If the fluctuations are not optimized, then the threshold may increase drastically, even if the amplitude of these fluctuations is low.Nous avons étudié l'influence de fluctuations de grandes échelles sur le seuil de l'instabilité dynamo. Pour cela, nous avons résolu le problème cinématique pour un champ de vitesse hélicoïdal, auquel nous avons rajouté à sa partie stationnaire, une modulation périodique dans le temps, également hélicoïdale. Pour un champ de vitesse hélicoïdal stationnaire des études précédentes ont montré que pour de grands nombres de Reynolds magnétique le champ magnétique était généré au niveau d'une surface caractérisée par une condition de résonance sur le champ de vitesse. Pour un champ de vitesse modulé, nous avons montré que pour une faible amplitude de modulation, c'est la condition de résonance portant sur la partie stationnaire qui gouverne la génération du champ magnétique. Pour une grande amplitude de modulation, c'est la condition de résonance portant sur la modulation qui contrôle la génération du champ magnétique. Dans la plupart des cas et si la condition de résonance est vérifiée pour les deux parties du champ de vitesse, on trouve que le seuil augmente en fonction de l'intensité de la modulation, puis diminue tout en restant supérieur au seuil de l'écoulement stationnaire de même géométrie. Si la condition de résonance n'est pas vérifiée pour la modulation, alors le seuil augmente drastiquement avec son intensité.Cette étude suggère que l'optimisation des expériences dynamo dépend non seulement de la partie stationnaire du champ de vitesse, mais aussi de ses fluctuations de grande échelle. Si ces dernières ne sont pas optimisées alors le seuil dynamo peut augmenter drastiquement, même si celles-ci sont de faible intensité

Instabilité paramétrique de la dynamo de Ponomarenko

Nous avons étudié l'influence de fluctuations de grandes échelles sur le seuil de l'instabilité dynamo. Pour cela, nous avons résolu le problème cinématique pour un champ de vitesse hélicoïdal, auquel nous avons rajouté à sa partie stationnaire, une modulation périodique dans le temps, également hélicoïdale. Pour un champ de vitesse hélicoïdal stationnaire des études précédentes ont montré que pour de grands nombres de Reynolds magnétique le champ magnétique était généré au niveau d'une surface caractérisée par une condition de résonance sur le champ de vitesse. Pour un champ de vitesse modulé, nous avons montré que pour une faible amplitude de modulation, c'est la condition de résonance portant sur la partie stationnaire qui gouverne la génération du champ magnétique. Pour une grande amplitude de modulation, c'est la condition de résonance portant sur la modulation qui contrôle la génération du champ magnétique. Dans la plupart des cas et si la condition de résonance est vérifiée pour les deux parties du champ de vitesse, on trouve que le seuil augmente en fonction de l'intensité de la modulation, puis diminue tout en restant supérieur au seuil de l'écoulement stationnaire de même géométrie. Si la condition de résonance n'est pas vérifiée pour la modulation, alors le seuil augmente drastiquement avec son intensité. Cette étude suggère que l'optimisation des expériences dynamo dépend non seulement de la partie stationnaire du champ de vitesse, mais aussi de ses fluctuations de grande échelle. Si ces dernières ne sont pas optimisées alors le seuil dynamo peut augmenter drastiquement, même si celles-ci sont de faible intensité.We have studied the influence of large scale fluctuations on the dynamo threshold. For that purpose, we have solved the kinematic problem for a helical flow, to the stationary part of which, we have added a periodic and also helical time modulation. For a helical stationary flow, previous studies have shown that for high magnetic Reynolds numbers, the magnetic field is generated on a resonance surface characterised by a resonant condition on the velocity flow. For a modulated flow, we have shown that, for low amplitude of modulation, it is the resonant condition of the stationary part, that controls the generation of the magnetic field. For large amplitude of the fluctuations, it is the resonant condition of the fluctuating part of the flow that governs the dynamo efficiency as well as the dynamo threshold. In most cases and if the resonant condition is satisfied for both parts of the flow, we find that the threshold increases with the intensity of the fluctuations, and then decreases, while remaining higher than the threshold corresponding to the stationary threshold of the same geometry. If the resonant condition is not verified for the modulation, then the threshold increases drastically with its intensity.These results show that the optimization of the dynamo experiments may not only depend on the stationary part of the flow but also on its non stationary large scale part. If the fluctuations are not optimized, then the threshold may increase drastically, even if the amplitude of these fluctuations is low.GRENOBLE1-BU Sciences (384212103) / SudocSudocFranceF

Parametric instabilities of the Ponomarenko dynamo

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Soot formation and oxidation during bio-oil gasification : experiments and modeling

SCIE Article le 17/12/13International audienceA model is proposed to describe soot formation and oxidation during bio-oil gasification. It is based on the description of bio-oil heating, devolatilization, reforming of gases and conversion of both char and soot solids. Detailed chemistry (159 species and 773 reactions) is used in the gas phase. Soot production is described by a single reaction based on C$_2$H$_2$ species concentration and three heterogeneous soot oxidation reactions. To support the validation of the model, three sets of experiments were carried out in a lab-scale Entrained Flow Reactor (EFR) equipped with soot quantification device. The temperature was varied from 1000 to 1400℃ and three gaseous atmospheres were considered: default of steam, large excess of steam (H$_2$O/C=8), and the presence of oxygen in the O/C range of 0.075-0.5. The model is shown to accurately describe the evolution of the concentration of the main gas species and to satisfactorily describe the soot concentration under the three atmospheres using a single set of identified kinetic parameters. Thanks to this model the contribution of different mechanisms involved in soot formation and oxidation in various situations can be assessed

Influence of H2O, CO2 and O-2 addition on biomass gasification in entrained flow reactor conditions: Experiments and modelling

International audienceBiomass gasification in Entrained Flow Reactor (EFR) is both studied with experiments in a drop tube reactor and modelling with a 1-D model (GASPAR). Operating conditions are chosen thanks to results of a preliminary modelling of an industrial EFR. Influence of addition of steam (0.55 g/g db), carbon dioxide (0.87 g/g db) and oxygen (Equivalent Ratio: 0-0.61) is investigated between 800 and 1400 degrees C with beech wood particles sieved between 315 and 450 mu m as feedstock. The model takes into account pyrolysis reaction, gas phase reaction with a detailed chemical scheme (176 species, 5988 reactions), char gasification by steam and CO2 and soot formation. H2O or CO2 addition has no influence on gasification product yields at 800 and 1000 degrees C, while at 1200 and 1400 degrees C the char gasification is significantly enhanced and soot formation is certainly inhibited by OH radical which reacts with soot precursors. The modification of output gas phase composition is mostly due to WGS reaction which reaches thermodynamic equilibrium from about 1200 degrees C. As expected, O-2 has a significant influence on gas and tar yields through combustion reactions. Char and soot yields decrease as ER increases. The GASPAR model allows a good prediction of gas and char and gives relevant evolution of soot and tar yields on the large majority of conditions studied

The Heat Treatment Severity Index: A new metric correlated to the properties of biochars obtained from entrained flow pyrolysis of biomass

International audienceThe properties of biochars produced in biomass pyrolysis processes highly depend on the pyrolysis conditions, specifically the pyrolysis temperature and thermal treatment duration. The higher the temperature and the duration, the more severe is the heat treatment. The present work proposes a new Heat Treatment Severity Index (HTSI) for the quantification of the heat treatment severity during the pyrolysis reaction. This metric takes into account both effects of reactor temperature and heat treatment time inside the reactor. The relevance of this HTSI is assessed through analyzing the evolution of some properties of biochars obtained after pyrolysis of 370 µm beech wood particles in an entrained flow reactor. These biochars were characterized for their chemical composition (elemental analysis), structure (Raman spectroscopy) and reactivity towards oxygen (thermogravimetric analysis). These properties were well correlated with the HTSI following remarkable mathematical relationships. This finding demonstrates the possibility to engineer biochars with controlled properties by a careful mastering of the experimental conditions of the pyrolysis process in terms of temperature and heat treatment time