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This paper presents an experimental and thermodynamic contribution about the role of inorganics in ash-bed material interactions during thermal conversion of miscanthus in fluidized bed. The objectives are (1) to describe the transformation of inorganics at high temperature, (2) to reveal their role in the agglomeration and (3) to provide recommendations for miscanthus gasification in fluidized bed. The main ash forming elements in miscanthus are K, Si, Ca, Mg, P, S and Cl. The ashes are composed of silica, carbonates and salts. The carbonates and salts decompose and volatilise at 700 °C. At elevated temperature, the dominant solid phases are Ca and Mg silicates. The liquid phase is composed of SiO2, K2O, CaO, MgO regardless of the atmosphere. The accuracy of thermodynamic prediction tool is evaluated with the experimental results. The ash-bed interactions show that the wetting of bed material by molten ashes is one of the key parameters of the agglomeration. The adhesion of particles increases in the order of silica sand, olivine, calcined olivine. There is no significant difference in the agglomeration mechanism in oxidizing or reductive atmosphere. However, in reductive atmosphere, two immiscible liquid phases can occur. The parametric investigation shows that the operating temperature has a significant effect on the agglomeration ratio and the addition of kaolin or dolomite is the most effective tool to reduce agglomeration risks.

Olivine is one of the refractory materials well-suited for fluidized-bed reactor technology. However, this material agglomerates at high temperatures due to the presence of sticky molten ash. The aim of this work is to investigate the mechanism of olivine agglomeration in fluidized-bed reactors and to determine the risk factors for agglomeration. A laboratory fluidized-bed reactor was designed to study the agglomeration effect between the ash and the refractory bed material. A systematic experiment was performed to determine the agglomeration ratio as a function of different parameters (operating time, bed materials, ash content, temperature, gas flow and additives). The mechanism of adhesion between the molten ash and the bed material is described, and the optimization of parameters to prevent this agglomeration is determined.

Raw materials of mineral industries are mainly in the form of ore, which generate fine mineral particles during the extraction, comminution and transport which are primarily regarded as non-recyclable waste. The aim of this paper is to develop and characterize materials using recycled fine particles of bauxite sintered at high temperature at a laboratory scale for the calcium aluminate industry. To use these recycled materials as raw materials, the physico-chemical transformations during high-temperature processing are described. Phase transformations as a function of temperature were determined by XRD, and optical and electronic microscopies were utilized to observe microstructures. Materials with different particle size distributions showed different chemical reactivity as a function of particle size. The optimization of particle size distribution makes it possible to substitute synthetic bricks for bauxite ore using recycled fine particles of bauxite.

One of the main concerns about biomass fluidized-bed gasification and combustion is the risk of bed particle agglomeration because of ash melting. Although many studies have been conducted about the agglomeration mechanism using silica sand, olivine is mostly mentioned as an alternative bed material for tar decomposition, and its interaction with biomass ash has not yet been fully understood. The aim of this work is to investigate the agglomeration of Miscanthus ashes, focusing on thermophysical and thermochemical aspects. Three different bed materials {silica sand (SiO2) and raw and calcined olivine [(Mg,Fe)2SiO4]} and an additive to prevent agglomeration {dolomite [CaMg(CO3)2]} were tested. The effects of atmosphere and Miscanthus harvest time were also investigated. It was found that the key parameter of agglomeration is the wettability of bed particles by molten ashes. In contact with ashes, all three bed materials showed good wetting tendencies, while dolomite had non-wetting properties. The adhesion...

International audience; This publication presents new advances in the field of refractories characterization. These laboratory methods that combine experiments and numerical analyses and concern both the thermomechanical and thermochemical behaviour are illustrated through different examples: identification of asymmetrical creep, determination of elastic and inelastic properties, measurements of macroscopic deformation, phase transformations or corrosion kinetics. These advanced techniques offer the refractory community new opportunities to improve the knowledge and the prediction of the phenomena of degradation of the refractories.

A framework is presented to define the formation and migration mechanisms of oxides in olivine during their phase transformations at high temperature. The behaviour of olivine particles has been investigated at 1400 °C at two different residence times: 4 h and 115 h. The structural and vibrational properties of the resulting materials were analysed using both X-ray diffraction (notably, in situ X-ray diffraction) and Raman spectroscopy including its mapping mode. The measurements have shown the migration of magnetite and enstatite from the inner part of olivine particles towards the surface, which results in the alteration of the solid solution and the intergranular reaction of particles. These results contribute to a better knowledge of olivine material science, in particular, in the areas of catalysis, refractories and engineering.

This work aims at studying the volume change of bauxite corroded by a molten slag. Cylindrical samples were prepared by mixing ground bauxite with slag. Optical measurement at high temperature (1450 °C) of deformation with a high-resolution camera has been developed. Image processing allowed for determining the change in diameter of the sample. We showed that the deformation was induced by the precipitation of new expansive crystallised phases observed by SEM-EDS analyses. Adding pellets of the same slag upon the samples allowed to emphasize the effect of the slag amount on the size change. The change in diameter significantly increased in the impregnated area.

This work focused on the transformation of inorganics in Miscanthus ash and the interaction of ashes with silica sand, olivine and calcined olivine. The samples were heat-treated in a laboratory furnace without the fluidization of the bed particles. The transformation of Miscanthus ash was studied with experimental methods (chemical fractionation analysis, ICP-MS, XRD, SEM-EDX) and with thermodynamic calculations (FactSage) between 400 °C and 1400 °C. The ash-bed material interface was analyzed by scanning electron microscope (SEM-EDX). The effect of the harvest time and the atmosphere were investigated. The characterization of the ashes (winter harvest) revealed the presence of alkali salts, carbonates and silica at low temperature and the formation of Ca and Mg-silicates, Ca-phosphate and molten K-silicate above 750 °C. In oxidizing atmosphere the S formed K2SO4, while in reducing atmosphere the S left in the gaseous phase. The ash of the summer harvest Miscanthus contains more K and less Ca and Si, therefore carbonates and salts are formed both at low and high temperature. The interaction tests revealed only physical adhesion between the molten ash and the silica sand, while the olivine played an active role in the agglomerate formation via the diffusion of Fe into the molten ash.

Biomass gasification is more suitable than combustion for Combined Heat and Power (CHP) applications. In this field, fluidized bed reactor technology ensures good mixing of continuously fed fuel and provides homogeneous temperature distribution in the reactor. The aim of the gasification is to produce clean syngas, thus it is important to investigate the influence of various operating conditions on the property of gas products and tar evolution. Usually, temperature and catalytic bed composition are the most studied parameters. In spite of the advantages of fluidized beds, biomass gasification suffers from technical problems such as slagging and fouling due to the high ash content and the formation of low melting point compounds and eutectic composition. The understanding of bed agglomeration requires the investigation of natural phases of biomass mineral matter and bed material. Due to the abundance of ash in biomass and its low degree of crystallization, supplementary methods have to be used beside XRD, LA-ICP-MS and SEM–EDX analysis to characterise interactions between ashes and fluidized bed material.

Steam gasification of Miscanthus X Giganteus (MXG) at high heating rate in a fluidised bed reactor with the use of olivine as catalyst was investigated. The effects of temperature (815–880 °C) on the yields and the compositions of syngas and tars were determined. The experimental results show that the gas yields and the content of H 2 increase with the temperature, while the yields of tar, char and the content of CO, CO 2 and CH 4 in the product gas decrease. Noteworthy is that about 1.1 m 3 of dry gas (at ambient conditions) per kg of dry ash free biomass were obtained with about 46% of H 2 and 24% of CO by volume at 880 °C. The tars composition was determined by FTIR, NMR and GC/MS. The identification of different compounds shows mainly the presence of simple molecules. This may be facilitating the possibility of complete tar reforming process (hot gas cleaning), to improvement of the syngas yield and the decrease of the formation of pollutants.

The Miscanthus X giganteus (MXG) presents many advantages (high yield, perennial crop, easy harvesting…) so it can be considered as a good candidate in terms of renewable energy sources. Several works have been carried out and were devoted to the MXG, especially in the agricultural field, but this study is the first which deals with gasification in order to produce syngas. The catalytic steam gasification of MXG in a fluidised bed reactor into presence of olivine based catalysts was investigated. Three parameters were studied, the temperature (800 °C and 900 °C), the pellets size (6 mm and 8 mm) and the nature of catalyst (olivine and Ni/olivine). Noteworthy is the efficiency shown by the Ni/olivine at 800 °C, which leads to the production of 1.7 m 3 kg − 1 daf of gas, containing 50% of H 2 . Ni/olivine catalyst was characterised by XRD, TPR and SEM-EDX in order to monitor its structural changes during the process. Moreover, a solvent system of tar recovery was tested, which allows to obtain a more representative set of the whole tars. Then, the tars composition was determined by GC/MS. The identification of different compounds shows the presence of different PAHs, in majority naphthalene.

Miscanthus × Giganteus is an excellent candidate for energy cultivation. Here we report, for the first time, the results of the pyrolysis of Miscanthus × Giganteus straw or pellets both in tubular reactor (3–6 g) and in rotary kiln (10–30 g). At 400–600°C the fractions obtained from both reactors are: solid 16–25 (wt.%); liquids 25–40; water 15–20 and gases 15–50. GC-MS analyses of pyrolysis liquids reveal the occurrence of phenolic derivatives and ethanol from lignin, furanic and linear oxygenated compounds from cellulose and hemicellulose. Finally the chars produced by the pyrolysis of M×G pellets in rotary kiln present good calorific values close to 29,000 J/g. Additionally, activated carbons with a BET surface area as high as 800–900 m2/g are produced from pellets. These results indicate that chars have a good potential either for energy production, e.g. briquetting, or as adsorbents precursors.

International audience; At high temperature, biomass steam gasification generates tars and more particularly Polycyclic Aromatic Hydrocarbons (PAHs). To assess the ability of olivine-based catalyst to eliminate the tars by steam reforming, alpha-methylnaphthalene (MNP) was chosen as model compound. The Ni/olivine has shown a good ability to reform the MNP, much better than olivine alone. Moreover, catalyst water pre-treatment has proven its efficiency. Thereby, this investigation has emphasized secondary reactions such as water gas shift reaction, and an overall mechanism has been proposed, taking into consideration the important role of adsorbed oxygen species coming from water catalytic dissociation over nickel metal. In addition, catalyst activity and stability have been discussed on the basis of X-ray diffraction (XRD) and scanning electron microscopy (SEM) coupled with EDX analysis, carried out on the samples before and after experiments. On the prepared fresh catalysts, we have shown the presence of NiO-MgO solid solution, formed on the surface of olivine support. Then, after the experiment, both Ni metal (Ni-0) and Ni-Fe alloys have been observed.

Olivine is one of the well-suited materials for fluidized bed reactor technology. After calcination at high temperature, olivine undergoes phase transformations resulting in dehydration and oxidation of fayalite to hematite and magnetite. The transformation mechanisms of olivine subjected to a calcination process at 1400 °C for 4 h are studied. Calcined olivine is characterized by X-Ray Diffraction (XRD), in situ XRD at varying temperatures and Raman spectroscopy. This paper explains the contribution of Raman spectroscopy to the study of iron oxide with regard to XRD. The heterogeneous distribution of hematite, magnetite and forsterite in the calcined material is exhibited by Raman mapping.

The continuous increase in CO2 content in the atmosphere, mainly due to the combustion of fossil fuels, is predicted to lead to the global warming. Biomass is considered as a promising renewable energy source. The conventional method for the production of synthesis gas from biomass is gasification. The gasification of biomass provides the most attractive solution for the introduction of biomass in decentralised power production. Conversion of biomass to synthesis gas becomes more and more important in terms of renewable energy sources. Moreover, in the different processes of gasification, the main problems remain the total removal of tars and light hydrocarbons like methane formed during the thermal treatment. In fact, it is necessary to obtain a very pure and stoichiometric mixture of CO and H2 that can be used for the preparation of hydrocarbons via the Fischer-Tropsch synthesis. To solve this problem, beside temperature reaction process, it is also necessary to select an appropriate and efficient catalyst. Among the factors affecting the activity of the tar decomposition reactions, the positive catalytic role of iron has been demonstrated 1,2 . This good catalytic behaviour is exhibited by olivine, an iron-containing mineral, in a systematic study of bed inventories for the biomass gasification process. As reported extensively in the literature, biomass steam gasification (performed in main cases in fluidised bed reactors) results in the conversion of carbonaceous materials to permanent gases (H2, CO, CO2, CH4, and light hydrocarbons), chars and tars 1 . The addition of steam water as gasifying agent and catalyst in gasification process makes it possible to obtain high-grade product 3 .

Correction for ‘Investigating the mechanism of phase transformations and migration in olivine at high temperature’ by R. Michel et al., RSC Adv., 2014, 4, 26645–26652.

Biomass is attractive substitute to fossil fuels but its inorganics can interact with bed material during fluidized bed gasification and lead to agglomeration and defluidization [1, 2]. The aim of this study is to evaluate inorganic phase transformation and interaction with bed material during thermal processes of miscanthus. The fundamental aspects of this work can help to fulfil the gap in predicting methods and to cope with the difficulty of detecting and quantifying the different chemical phases during ash formation and gasification. The main inorganics in ash are Si, K, Ca, Mg, P, S and Cl. Difficulties on the characterisation are, among others, the small quantity of available sample, the coexistent of vitreous and crystalline phases and the fact that the inorganics are also present in the form of phosphate, sulphate, chloride and silicate besides oxide. Samples were analysed by TGA, ICP-MS, XRD and SEM-EDS. Each analytical technique gives important but not complete information about the transformation of minerals. Their combination with thermodynamic calculation can bring us closer to the understanding of the complex phenomena of ash melting.

For over 30 years researchers have been investigating biomass ash transformation during different thermal processes to better understand slagging, fouling and agglomeration phenomena. During the thermal conversion of biomass, inorganics of the plant can transform or volatilise, condense in the flue gas channel and on the surface of heat exchangers resulting in slagging and fouling. It is also possible that part of the volatilised inorganic and other non-volatile inorganics react with the bed material in the reactor leading to agglomeration. Elements which are in ionic form in the plant (alkaline, S, Cl) can leave the reactor with the flue gas (fly ash) and form deposit of salts, while silicates mostly remain in the reactor (bottom ash) and they can form viscous melts of oxides and agglomerates [1]. Therefore, it is important to predict the temperature of ash fusion and volatilisation of a given biomass before considering it as a biofuel. Energy crops such as miscanthus are good candidates due to their high heating value and resistance to different climates, though their high silica and potassium content has to be taken into account as these compounds can form low melting point silicates at relatively base temperature (< 800oC). Ash fusibility test is a widely used method in industry to determine the phase transformation of the ash. Besides, many studies have been carried out about predicting the inorganic phase transition by calculating thermodynamic equilibrium [2-4]. However, these results must be interpreted with care as equilibrium state usually cannot be reached in industrial applications as the transformation time is too short 2 . Identifying and quantifying the inorganics in miscanthus samples involves certain challenges due to the small quantity of ash forming elements in the plant, the variety of inorganic compounds, and their appearance both in crystalline and amorphous form. The aim of this study is to compare the thermodynamic model calculations with laboratory tests as a first step to evaluate their accuracy for describing phase transition and ash melting of miscanthus. 2 Experiments

Agglomeration and defluidisation processes in fluidised-bed reactors due to ash melting are a real difficulty encountered when gasifying feedstock with high ash content. This work aims to investigate the influence of agglomeration on the hydrodynamic behaviour of a bubbling fluidised-bed reactor. The identification of the main parameters involved in agglomeration and defluidisation and the study of their influence on these processes are carried out inside both a laboratory fluidised-bed operating at high temperature (700–1000 °C) and a cold-flow fluidised-bed at ambient temperature. Results are then compared to a gasification test performed in a pilot-scale facility (800 °C). Results show that hydrodynamic disturbances and defluidisation are the result of agglomerates segregation. Once formed, the agglomerates tend to segregate at the bottom of the bed causing the formation and the build-up of a fixed bed crossed by multiple channels. In addition, the hydrodynamic behaviour is highly sensitive to the in-bed liquid amount. Complete defluidisation was seen to occur for very low liquid amount (few vol%). Based on these results, a simplified modelling approach is given in order to predict the operating time before the complete defluidisation of the reactor.