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.

B. V.S.S.S. Prasad

Department of Mechanical Engineering

Ajit Kumar Kolar

Fluidized bed combustors

Heat transfer

Mathematical models

CHAR YIELD

Devolatilization

PARTICLE SHRINKAGE

Wood

IIT Madras is a public technical and research university located in Chennai, Tamil Nadu. Founded in 1959, it is recognised as an Institute of National Importance.

IIT Madras has been ranked as the top engineering institute in India for four years in a row by the National Institutional Ranking Framework of the MHRD

It currently offers undergraduate, postgraduate and research degrees across 16 disciplines in Engineering, Sciences, Humanities and Management. About 596 faculty belonging to science and engineering departments and centres of the Institute are engaged in teaching, research and industrial consultancy.

IIT Madras

Fuel

Digital X-ray Radiography (DXR), a non-destructive technique (NDT) is used to visualize and characterize the dynamics of the thermo-physical behaviour of wood during devolatilization in a hot Bubbling Fluidized Bed (BFB). X-ray radiographs of cube shaped Casuarina Equisetifolia wood of size ranging from 10 to 25 mm is used to visualize, interpret and quantify the thermal behaviour in terms of location and propagation of char front, intra-particle conversion, "true" char formation time (CFT), and physical behaviour in terms of temporal and terminal shrinkage (transverse, longitudinal and volumetric). Also, initiation and orientation of sub-surface fissure in a devolatilizing particle, leading to fragmentation is identified and qualitatively discussed. A three dimensional (3-D) mathematical model has been used to predict the progress of intra-particle conversion and the predictions are compared with the experimental data obtained from DXR. It is found that the "true" CFT inferred from DXR technique is 12% and 26% lower than that obtained by the Flame Extinction Time (FET) technique for a 10 and 15 mm particle, respectively. The terminal mean longitudinal, transverse and volumetric shrinkage are found to be in the range of 12-17%, 22-23% and 44-47%, respectively for all the particle sizes studied. The terminal volumetric shrinkage falls well within the range of 40-65% reported in the published literature. In contrast to the popular assumption in the devolatilization models, the variation of shrinkage with conversion is found to be non-linear in along-the-grain direction. From the present study it is verified/established experimentally (visually) for the first time the difference in rate of propagation of char front across and along the grain direction of the wood particle. The predictions of the 3-D model on progress of conversion compare well with the experimental findings of DXR technique. This technique can also be applied to other solid fuels. © 2013 Elsevier Ltd. All rights reserved.

Visualization and characterization of thermo-physical behaviour of wood during devolatilization in a hot fluidized bed using X-ray radiography technique

Considerable amount of investigation on the subject of devolatilization of wood is found in the open literature. However, a systematic study of the effect of initial particle size and shape, and bed temperature on devolatilization time and char yield of wood in a hot fluidized bed is still missing. This paper attempts to fill this gap through a systematic experimental investigation to determine the devolatilization time and char yield of a typical woody biomass, "Casuarina equisetifolia" particles of different initial sizes and shapes at various fluidized bed temperatures. Experiments are conducted using 10, 15, 20, and 25 mm Casuarina wood particles of three shapes, namely, cube, cylinder, and sphere at bed temperatures of 1023, 1123, and 1223 K. It is found that the initial wood particle size has the strongest influence on devolatilization time followed by the shape of initial wood particle and the bed temperature. Correlation for devolatilization time (τd) as a function of initial wood particle size (deq), sphericity (φ), and bed temperature (Tb), is developed using 573 experimental data points exhibiting a correlation coefficient of 0.96 and predictions falling well within a deviation band of ±20%. The predictions of the present correlation are compared with the predictions of the existing correlations in literature for conditions also out of the present study and the deviation is found to be ±30%. Char yield, defined as the ratio of the residual mass at the end of devolatilization process to the initial mass of the wood particle is found to be in the range of 9-14% for all sizes, shapes, and bed temperatures. Char yield does not depict any definite trend with the variation of initial particle size, shape and bed temperature. © 2011 Elsevier B.V. All rights reserved.

Journal of Analytical and Applied Pyrolysis

Experimental investigation of the effect of initial fuel particle shape, size and bed temperature on devolatilization of single wood particle in a hot fluidized bed

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 μm) 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.

Mass loss and apparent kinetics of a thermally thick wood particle devolatilizing in a bubbling fluidized bed combustor

The shrinkage characteristics of Casuarina wood in terms of the effect of fuel particle shape and size on the longitudinal, transverse, and volumetric shrinkage during devolatilization in a laboratory scale bubbling fluidized bed combustor are presented. Shrinkage of single fuel particles was measured in the longitudinal and transverse directions (with respect to the wood fibre) for various fuel particle shapes-like disc (l/d=0.2-0.67), cylinder (l/d∼1) and rod (l/d=2-10). The fuel particle dimensions ranged from 5 to 100 mm. The effect of the bed temperature on the shrinkage was studied by varying the bed temperature in the range of 650-850°C. Fuel particle shape and size were found to influence the shrinkage in the two mutually perpendicular directions. The variation in the fuel particle heating rates for various shapes and sizes was found to be the cause of the variation in the shrinkage values. For all the shapes and sizes considered, the longitudinal shrinkage was found to be in the range of 6.5-24%, the transverse shrinkage from 14% to 29%, and the volumetric shrinkage from 35% to 58%. The average volumetric shrinkage was estimated to be 47% with a standard deviation of ±3.8%. Shrinkage increased negligibly with the increase in bed temperature. Increase in fuel particle density led to a decrease in volumetric shrinkage, however, this effect was not conclusive because of the effect of other factors-like chemical composition and wood type. Correlations for estimating the shrinkage coefficients in the two principal directions are presented.

Biomass and Bioenergy

Shrinkage characteristics of Casuarina wood during devolatilization in a fluidized bed combustor

Simplified model for calculation of devolatilization in fluidized beds

Industrial & Engineering Chemistry Research

Biomass Pyrolysis in a Fluidized Bed Reactor. Part 2:  Experimental Validation of Model Results

Biomass Pyrolysis in a Fluidized Bed Reactor. Part 1:  Literature Review and Model Simulations

Decomposition of Wood Particles in Fluidized Beds

Modelling and experimental investigation of devolatilizing wood in a fluidized bed combustor

Journal	Fuel
ISSN	00162361
Open Access	No