11A key issue associated with Fluidized Bed Combustion of biomass is agglomeration. The presence of 12 high quantities of alkali species in biomass ash leads to the formation of sticky alkali-silicate liquid 13 phases during combustion, and consequently the adhesion and agglomeration of bed material. This 14 review principally examines probable mechanisms of agglomeration and the effects of operational 15 variables in reducing its severity. Additionally, an overview of monitoring and prediction of 16 agglomerate formation is given. Two key mechanisms of agglomeration are apparent in literature, 17 and both may occur concurrently dependent on fuel composition. Coating-induced agglomeration is 18 defined by the interaction of alkali metals in fuel ash with the bed material, commonly silica sand, to 19 form an alkali-silicate melt. Melt-induced agglomeration is defined by the presence of sufficient 20 amounts of both alkali compounds and silica liquid phases sourced from the fuel ash to form a 21 eutectic mixture. Physical mechanisms, such as tumble agglomeration and sintering, may further 22 enhance either of the coating-induced or melt-induced mechanisms. Of the operational variables volumes of non-combustible contaminants, high moisture contents and, in most applications, 63 requires a large amounts of pre-processing/pre-treatment with specialised transportation. Due to 64 these challenges, technologies such as fluidized bed combustion (FBC) boilers have been employed. 65 FBC offers a number of advantages, such as combustion of different fuel types, blends, and ranges of 66 qualities, features commonly referred to under the umbrella term of [5]. Hundreds 67 Page 4 of 75 of full-scale bubbling fluidized bed (BFB) [6, p. 7] and circulating fluidized bed (CFB) [6, p. 8] boilers 68have been deployed around the world [7, 8] for power generation and/or steam sales to industrial or 69 chemical plant sites. However, each FBC plant development has to overcome slagging, fouling, 70 corrosion and, most significantly, agglomeration issues resulting from the composition and 71 behaviour of the biomass fuel stock [9]. 72

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ReuseUnless indicated otherwise, fulltext items are protected by copyright with all rights reserved. The copyright exception in section 29 of the Copyright, Designs and Patents Act 1988 allows the making of a single copy solely for the purpose of non-commercial research or private study within the limits of fair dealing. The publisher or other rights-holder may allow further reproduction and re-use of this version -refer to the White Rose Research Online record for this item. Where records identify the publisher as the copyright holder, users can verify any specific terms of use on the publisher's website. TakedownIf you consider content in White Rose Research Online to be in breach of UK law, please notify us by emailing eprints@whiterose.ac.uk including the URL of the record and the reason for the withdrawal request. AbstractTurbulent fluidization is now widely recognized as a distinct flow regime and is commonly utilized in industrial fluidized-bed reactors. However, relatively fewer attempts have been made to rigorously model these systems in comparison to bubbling and circulating fluidized beds. In this work, we have rewritten the original bubble based EMMS model in form of a mixture to apply it to turbulent fluidization. At microscale this mixture is composed of gas and particles whereas voids and gas-particle suspension make up this mixture at mesoscale level. Subsequently, all the system properties are then calculated in terms of mixture rather than individual phases. With the minimization of the objective function for the bubbling mixture, the set of equations is then solved numerically. The objective function, used to close the system of equations, is composed of the energy consumption rates required to suspend gas-particle suspension and the energy consumed due to interaction between suspension and voids. The model is then applied to simulate gas-solid turbulent fluidized beds. Simulation results are encouraging as the model is able to predict the dense bottom and dilute top zones along the height of the bed. Comparison of results with experimental data and homogeneous drag model has been made for validation purposes.

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This paper presents experimentally validated three-dimensional numerical simulation of a 350 kW pilotscale bubbling fluidized bed combustor, which has been developed by using commercial CFD software package, Fluent 14.5. The solid particle distribution has been simulated by using the multiphase Euler-Euler Approach. The gas-solid momentum exchange coefficients were calculated by using Syamlal and O'Brien drag functions. The CFD model is created as the realistic representation of the actual pilot-scale bubbling fluidized bed. All simulations are performed in transient mode for an operation time of about 350 s. The experimental study is performed with silica sand particles with mean particle size of 0.6 mm and density of 1639 kg/m 3 . The bed was filled with particles up to a height of 0.30 m. The same conditions are used for the simulations. The present work combines both experimental and computational studies, where the CFD-Simulation results are compared to those obtained by experiments. The predicted simulation results of minimum fluidization velocity and pressure drop values of the pilot-scale bubbling fluidized bed combustor have good agreement with the experimental measurements.