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.

Ajit Kumar Kolar

Combustors

Fluidized bed combustion

Shrinkage

Temperature distribution

vaporization

CASUARINA WOOD

Devolatilization

Shape

size

Forestry

CASUARINA

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

Biomass and Bioenergy

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.

Fuel

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

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 sensitivity analysis is carried out using the 3D model to identify the important parameters influencing the devolatilization time and char yield and the level of confidence (degree of uncertainty) in using them as model inputs. The important parameters considered include the initial wood particle density, thermal conductivity, external surface heat-transfer coefficient, and various kinetic schemes available in the literature. Invariably, the relative influence of the parameters on the devolatilization time and char yield is found to be predominant in larger particle sizes. The initial particle density of wood has the strongest influence on the devolatilization time, followed by the thermal conductivity and specific heat capacity. The devolatilization time increases considerably with an increase in the initial wood density and decreases moderately with an increase in the thermal conductivity of wood, while the char yield remains negligibly influenced by the initial density and thermal conductivity of wood. The devolatilization time decreases considerably when the external surface heat-transfer coefficient is varied from 100 to 500 W m -2 K-1, beyond which the influence is negligible. The prediction of the devolatilization time and char yield is found to be very sensitive to the reaction kinetic parameters (rather than the wood properties), and hence due care must be taken in adopting the right kinetic parameters available in the literature to model. © 2010 American Chemical Society.

Energy and Fuels

Transient three-dimensional mathematical model and experimental investigation of a wet devolatilizing wood in a hot fluidized bed

A two-dimensional (2D) mathematical model in cylindrical coordinates has been formulated and used to predict the progress of conversion in a cylindrical wood particle undergoing pyrolysis in a fluidized bed. The transient model takes into account the heat consumed and shrinkage during drying and pyrolysis reactions in addition to anisotropy and changing thermal properties during the conversion. Predicted centre temperature has been compared with measurements found in the literature for validating the model. Calculations have been carried out for a 10 mm long and 10 mm diameter wooden cylinder in a bed of sand at 1107 K and fluidized by nitrogen. Developing profiles of temperature, solid-phase density, moisture content and degree of conversion at various stages have been presented. It is found that the conversion progresses faster and the gradients are steeper across the grains than along them. A comparison with a 1D model shows that thermal wave propagation predicted by the 2D model is faster than that of 1D. © 2008 Elsevier B.V. All rights reserved.

Progress of conversion in a shrinking wet cylindrical wood particle pyrolyzing in a hot fluidized bed

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.

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

Modeling the combined impact of moisture and char shrinkage on the pyrolysis of a biomass particle☆

Pyrolysis of large wood particles: a study of shrinkage importance in simulations☆

Chemical Engineering Science

Modeling the impact of shrinkage on the pyrolysis of dry biomass

Birch wood particle shrinkage during rapid pyrolysis

Wood Science and Technology

Predicting anisotropic shringkage of softwood Part 1: Theories

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

Journal	Biomass and Bioenergy
ISSN	09619534
Open Access	No