Biomass Conversion and Biorefinery

, Volume 8, Issue 1, pp 211–223 | Cite as

The effects of air velocity, temperature and particle size on low-temperature bed drying of wood chips

  • Niranjan Fernando
  • Mahinsasa Narayana
  • W. A. M. K. P. Wickramaarachchi
Review Article


This paper describes a mathematical model for wood chip packed bed drying process with the effects of hot air flow velocity, temperature and particle size. A single-particle drying model was developed by considering impacts of external and internal parameters. External parameters are hot air flow velocity and air temperature. Internal parameters are porosity and particle size. These parameters are incorporated to the present model by introducing two mass transfer coefficients. The model was fine-tuned by comparing simulation results and experimental data. Effects of a factor relating to internal mass transfer coefficient were found for three wood types, and a functional dependence of internal mass transfer on temperature was suggested in this study. The model was implemented in computational fluid dynamics (CFD) to evaluate spatial variation of moisture in the packed bed drying process. The CFD model was validated by results of lab-scale packed bed. Drying performance of the packed bed was estimated by CFD simulations for variations of external hot air flow velocity, flow temperature and particle size. Sensitivity of these parameters for dying performance was evaluated by design of experiment (DOE) method. It was clarified that air temperature is most critical for the drying process. Interaction of between external hot air flow velocity and particle size for dying performance is also significant.


Wood chip drying Drying kinetics Modelling optimisation Moisture content 



The specific surface area (m−2)


Surface area of wood particle (m−2)


Sutherland’s constant


Diffusivity coefficient of gas species i (m2 s−1)


Overall heat transfer coefficient based on wood particle surface area (W/m2K)


Thermal conductivity of gas phase (W m−1 K−1)


Thermal conductivity of biomass (W m−1 K−1)


Characteristic length of biomass particle (m)


Molar mass of air (kg mol−1)


Nusselt number


Prandlt number


Gas constant


Reynolds number


Sherwood number


Schmidt number


Air pressure (Pa)


Stephan Boltzmann constant

Emissivity of solid particles


Moisture evaporation flux (kg m−3 s−1)


Heat capacity of gas phase (J kg−1 K−1)


Heat capacity of solid phase (J kg−1 K−1)


Saturation vapour pressure


Reference temperature (K)


Gas phase temperature (K)


Solid phase temperature (K)


Expected data from the theory


Gas phase thermal conductivity (W m−1 K−1)


Solid phase thermal conductivity (W m−1 K−1)


Internal mass transfer resistance (m s−1)


External mass transfer resistance (m s−1)


Mass transfer coefficient (m s−1)


Dry mass of biomass (kg)


Observed data


Gas phase moisture fraction within wood particle


Gas phase moisture fraction


Moisture fibre saturation point of wood particle


Volume fraction of gas phase


Volume fraction of solid phase


Dynamic viscosity of gas phase (Pa s)


Density of gas phase (kg m−3)


Cell density of species j (kg m−3)


Density of solid phase (kg m−3)

σi , g

Average collision diameter (A)


Evaporation enthalpy of water



The authors would like to thank the senate research committee of University of Moratuwa for providing the financial support for the research project (SRC/CAP/14/06). The authors would also like to thank both Mr. P. W. Vidanage and Mr. U. R. Amarasinghe for their contribution to the experimental study of this research work for validation of the presented mathematical models.


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© Springer-Verlag Berlin Heidelberg 2017

Authors and Affiliations

  1. 1.Department of Chemical and Process EngineeringUniversity of MoratuwaMoratuwaSri Lanka

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