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Devolatilization of a single wet wood particle in a hot atmospheric fluidized bed is studied through a mathematical model and experiments. A typical tropical woody biomass, “Casuarina Equisetifolia”, is used in the study. A three-dimensional (3D) transient model with detailed consideration of (i) fuel anisotropy, (ii) fuel moisture, (iii) shrinkage during drying and devolatilization, (iv) heat generation, and (v) variable properties with a suitable reaction scheme is developed to determine the devolatilization time (τd) and char yield (Yc) of cubic/cuboidal wood particles at various initial fuel moisture contents and bed temperatures. Experiments are conducted for 10, 15, 20, and 25 mm cube-shaped wood particles at bed temperatures of 1023, 1123, and 1223 K. The model predictions agree well with the measured data (present experiments and those reported in the literature) within ±10% for the devolatilization time and ±11% for the char yield, supporting the validity of the overall structure of the model. A ...

This work presents the results of experiments conducted to determine the mass loss characteristics of a cylindrical wood particle undergoing devolatilization under oxidation conditions in a bubbling fluidized bed combustor. Cylindrical wood particles having five different sizes ranging from 10 to 30 mm and aspect ratio (l/d = 1) have been used for the study. Experiments were conducted in a lab scale bubbling fluidized bed combustor having silica sand as the inert bed material and air as the fluidizing medium. Total devolatilization time and mass of wood/char at different stages of devolatilization have been measured. Studies have been carried out at three different bed temperatures (Tbed = 750, 850 and 950 �C), two inert bed material sizes (mean size dp = 375 and 550 ?m) and two fluidizing velocities (u = 5umf and u = 10umf). Devolatilization time is most influenced by the initial wood size and bed temperature. Most of the mass is lost during the first half of the devolatilization process. There was no clear influence of the fluidization velocity and bed particle size on the various parameters studied. The apparent kinetics estimated from the measured mass history show that the activation energy varied narrowly between 15 and 27 kJ/mol and the pre-exponential factor from 0.11 and 0.45 s-1 for the wood sizes considered. � 2009 Elsevier Ltd. All rights reserved.

This work proposes a transient heat transfer model to predict the thermal behaviour of wood in a heated bed of sand fluidized with nitrogen. The 2-D model in cylindrical coordinates considers wood anisotropy, variable fuel properties, fuel particle shrinkage, and heat generation due to drying and devolatilization. The influence of initial fuel moisture content, thermal diffusivity, particle geometry, shrinkage, external heat transfer coefficient, chemical reaction kinetics and heats of reaction on temperature rise is presented. The cylindrical wood particles chosen for the study have length (l) = 20�mm, diameter (d) = 4�mm and l = 50�mm and d = 10�mm, both having an aspect ratio (l/d) of 5. The bed temperature is 1123�K. The model prediction is validated using measurements obtained from literature. The temperature rise in the wood particle is found to be sensitive to changes in the moisture content and thermal diffusivity and heat of reaction (in larger particles) while it is less sensitive to the external heat transfer coefficient and chemical kinetics. Also shrinkage is found to have a compensating effect and it does not have any significant influence on the temperature rise. Beyond an aspect ratio of three, the wood particle behaves as a 1-D cylinder. � 2008 Elsevier B.V. All rights reserved.

A mathematical model capable of estimating the temperature and stress distribution in a devolatilizing cylindrical wood particle is proposed. The model assumes wood as orthotropic and comprises a thermal submodel and a stress submodel. It takes into account the contributions to stress by the temperature rise, shrinkage during drying and devolatilization, and mechanosorption. The thermal submodel estimates the temperature distribution and shrinkage as a function of space and time to be used as inputs to the stress submodel. Calculations have been carried out for cylindrical wood particles of aspect ratio l/d = 1 and with dimensions of 10, 20, and 30 mm. The model predictions compared with the results of an available drying model are found to be satisfactory. During devolatilization of the wood particle, the model predicts high compressive stresses at the center while those at the surface are tensile in nature and moderate in magnitude. A modified von Mises criterion predicts failure at the axial center of the cylinder, with the size of the crack formed by the failure increasing from 0.4 to 1.4 mm in diameter as the size of the wood particle increases from 10 to 30 mm. The predicted crack radius and the time of crack initiation are compared with the measurements carried out on wood particles undergoing devolatilization in a fluidized bed combustor. The predicted crack radius and time of crack initiation are found to be sensitive to the magnitude of the mechanical properties. � 2008 American Chemical Society.

Thermal behavior and stresses developed in a wood particle are predicted using a mathematical model formulated for drying in fluidized bed. Drying experiments have been carried out on 10 mm long and 30 mm diameter cylindrical disks at a bed temperature of 403 K (130°C). Drying time, mass reduction, and shrinkage during drying have been measured and used to validate the thermal model. The stress model is validated with an analytical solution available in literature. The drying time is experimentally found to be 630 s. The wood particle shrinks by 4.6% in the radial direction and by 1.0% in the longitudinal direction. The model indicates splitting failure at the circumference, which was also noticed during experiments.

Casuarina equisetifolia, a hard wood, and a popular energy crop in many tropical countries, was investigated experimentally for its char fragmentation in a laboratory scale atmospheric bubbling fluidized bed combustor. The effect of fuel shape and size on wood char fragmentation was studied. Wood particles of spherical, cylindrical (aspect ratio of 1), and cubical shapes of different sizes ranging from 10 to 25�mm were used in the experiments. Fragmentation of wood char was quantified in terms of various parameters, such as Number of Fragments (NF), Percentage of Fragmentation Events, Frequency of Fragmentation, Timing interval of Fragmentation, Size distribution of char and Fragmentation Index (FI). Also, qualitative observations on the evolution of char in terms of deformation, cracks and surface texture are discussed. It was observed that Casuarina equisetifolia wood of sizes greater than 15�mm, of all shapes undergoes primary fragmentation during the devolatilization phase. Furthermore, chars fragment at the early stages (1st or 2nd quarter) of the char combustion phase, underscoring the significance of the phenomenon in fluidized bed combustion. For all the shapes of wood considered, there appears to be a cut-off size of the initial wood, below which its char certainly undergoes fragmentation. It was observed that the average char particle size at any instance during its combustion falls in a narrow range of 3.7-6.9�mm, 3-6.6�mm and 3-9.5�mm for spherical, cylindrical and cubical wood particles, respectively. Wood of initially cylindrical shape undergoes extensive fragmentation when compared with spherical and cubical shapes. � 2008 Elsevier B.V. All rights reserved.

A two-dimensional model is developed for the determination of devolatilization time and char yield of cylindrical wood particles in a bubbling fluidized bed combustor. By using the concept of shape factor, the model is extended to particles of cuboid shape. The model prediction of the devolatilization time agrees with the measured data (present and those reported in the literature) for cylindrical and cuboidal shaped particles within �20% while the char yield is predicted within �17%. Influence of some important parameters namely, thermal diffusivity, external heat transfer coefficient and shrinkage, on the devolatilization time and char yield are studied. Thermal diffusivity shows noticeable influence on devolatilization time. The external heat transfer coefficient shows little influence beyond a value of 300 W/(m2 K). However particle shrinkage shows negligible effect on the devolatilization time but has a significant influence on the char yield. � 2008 Elsevier Ltd. All rights reserved.

In some large capacity Circulating Fluidized Bed (CFB) boilers with water walls, additional heat transfer surfaces are necessary to control the furnace temperature while restricting the boiler height to about 40 m due to commercial considerations. These surfaces could be in the form of (1) External Heat Exchanger (EHE) in the solids recycle loop, (2) wing wall and omega panels at the very top of the furnace, or (3) a division or a curtain wall running through the core region of the upper dilute zone of the boiler. It is necessary to know the heat transfer characteristics for these core surfaces from design and operating considerations. In this context, experimental heat transfer studies have been conducted on a 0.6-m high single vertical tube, 0.5- and 1.0-m high U-tubes, and a 0.5-m high finned U tube, located in the top 10% height of a 100 mm?100 mm cross-section and 5.5-m tall CFB riser using silica sand of three mean sizes in the range of 143 to 363 ?m as the bed material. The fluidizing air velocity and solid circulation flux were varied in the range of 4-8 m/s and 13-80 kg/m2/s, respectively. The resulting cross-sectional average suspension density and measured total (gas convective plus particle convective) surface-average convective heat transfer coefficients for U-tubes were in the range of 9 to 25 kg/m3 and 30 to 105 W/m2/K, respectively. For single vertical tube at axis, they were in the range of 10 to 20 kg/m3 and 60 to 128 W/m2/K, and at off-axis positions, they were in the range of 10 to 20 kg/m3 and 70 to 140 W/m2/K, respectively. The results confirmed the increasing trend of the heat transfer coefficient from the axis towards the riser wall, thus reflecting the core-annulus flow structure of the CFB riser. It was observed that the length of the tube influences the heat transfer coefficient and fins augmented the heat transfer rate. Gas convective contribution was found to be substantial especially at high gas velocities and for large particles. Experimental data were compared with published heat transfer results under somewhat similar conditions. Using the experimental data points for the U-tubes, two empirical heat transfer correlations were developed in terms of the important design and operating parameters. Copyright ? 2002 Elsevier Science B.V.

Horizontal pressure gradient fluctuations caused by a bubble passage in the neighbourhood of the probe are simulated using Davidson's pressure field around a circular bubble. The salient features of these fluctuations are studied with regard to their sensitivity to the obliquity and randomness of the bubble traversing path. The utility aspects of horizontal pressure differential measurements over vertical pressure differentials are highlighted and a qualitative comparison between the simulated and those experimentally obtained from a two-dimensional bed, showing strong similarities are presented.Horizontal pressure gradient fluctuations caused by a bubble passage in the neighbourhood of the probe are simulated using Davidson's pressure field around a circular bubble. The salient features of these fluctuations are studied with regard to their sensitivity to the obliquity and randomness of the bubble traversing path. The utility aspects of horizontal pressure differential measurements over vertical pressure differentials are highlighted and a qualitative comparison between the simulated and those experimentally obtained from a two-dimensional bed, showing strong similarities are presented.

Whether or not the maximum in the differential pressure fluctuation due to bubble passage occurs when the bubble boundary touches the probe is examined, based on the Davidson's theoretical model for the pressure field around a bubble. The predictions based on the various inflection points in a differential pressure fluctuation record are examined carefully with regard to the characterization of apparent bubble pierced length. Experimental evidence is presented to substantiate these. The use of point pressure fluctuations in retrieving the true bubble pierced length from the apparent bubble pierced length is illustrated. The effect of practical limiting factors in probe design on the estimation of bubble parameters is studied in order to obtain a sound interpretation of the resultant probe response.

In this paper, an emulsion layer model is presented which predicts the thickness of a downward-moving emulsion layer along the wall of a circulating fluidized bed, the mean solids velocity, and the solids flux in the layer. Also presented is a heat transfer model which, in combination with the emulsion layer model, predicts the low-temperature data very well. An alternate slab model proposed for the radiative component in a high-temperature circulating fluidized bed agrees well with experimental data. The heat transfer predictions of the overall model for such operating parameters as solid circulation flux, suspension temperature, length of the heat transfer surface, superficial gas velocity, and mean particle size are in good agreement with the published data for long surfaces.