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.

This article presents a novel air-blown bubbling fluidized bed device that has the ability to sample gas and bed materials at various axial positions during the gasification experiments. The reactor was operated with olivine, as bed material, and Miscanthus, a biomass rich in potassium and silica, and thus prone to bed agglomeration. The comparison of gas and char axial profiles along the bed allows a better understanding of the biomass gasification: it shows in particular that 02 consumption and CO, production at the bottom of the bed are mainly due to char oxidation, even if few pyrolysis gases may also be produced and oxidized near the grid. Regarding bed defluidization, the agglomerate fraction is followed by taking bed samples during the operation: it is shown that the rate of agglomeration is linear while defluidization signs appear when the agglomerate fraction reaches 6% near the grid. Small agglomerates are observed on the top of the bed, whereas big agglomerates are segregated near the grid. The SEM-EDX analysis shows that the layer that sticks olivine particles together does not strictly correspond to biomass ashes melt: it contains also particles and atoms that come from the erosion of olivine and the stainless steel wall.

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. (C) 2015 Elsevier Ltd and Techna Group S.r.l. All rights reserved.

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)(2)SiO4]} 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 between bed materials and molten ashes increases in the order of silica, olivine, and calcined olivine. While, in the case of silica sand, only physical adhesion occurred, the diffusion of iron oxide into the molten ash was observed using olivine. Calcined olivine has a roughened surface, which further increased the adhesion. The atmosphere did not influence the mechanism of the ash/bed material interaction. On the other hand, Miscanthus harvest time had a significant effect on ash reactivity and interaction with raw and calcined olivine.

The aim of this work is to describe the inorganic phase transformation of Miscanthus x giganteus ash at a high temperature taking into account the effect of oxidizing/reducing atmospheres and different harvest periods. A double approach was used: thermodynamic calculations and experimental validation in laboratory devices. The samples were characterized by scanning electron microscopy energy-dispersive X-ray spectroscopy (SEM-EDX) and X-ray diffraction (XRD). Below 800 degrees C, the inorganics are mainly present as salts [KCl, K2SO4, and Ca-3(PO4)(2)], carbonates (CaCO3), and SiO2, while at higher temperatures (>900 degrees C), the solid phase is enriched in CaSiO3 and Ca-3(PO4)(2). The ash starts to melt around 750 degrees C, and the liquid phase is composed of SiO2, K2O, and CaO. The main difference between oxidizing and reducing atmospheres is the form of sulfur. In an oxidizing atmosphere, K2SO4 is formed, while in a reducing atmosphere, only traces of organically attached S were detected by SEM-EDX and most of the S has volatilized. The thermodynamic calculations were performed using the software package FactSage 6.4 with two different oxide databases: FToxid and GToxid. GToxid database predicts a higher solidus temperature (T-s > 900 degrees C) but greater liquid/solid ratio than FToxid (T-s approximate to 740 degrees C). Both databases predict the formation of K silicates or K-Ca (Mg) silicates below 800 degrees C, while the main solid phase observed by XRD was SiO2. Reasons are linked to uncertainties in the initial mass balance of ashes, database uncertainties, and limitations of the diffusion and chemical reaction of elements. The composition of the liquid phase is well-described in both databases, although GToxid gives somewhat closer results to the SEM-EDX analysis with the incorporation of Ca and Mg into the liquid phase.

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

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.

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.