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ABSTRACT
A transient 2D kinetic-heat transfer model incorporating primary and secondary reactions is proposed to investigate the effect of reactor temperature and particle length/diameter (L/D) ratio on the pyrolysis behavior and validate it with the experimental results. The RMS error between the experimental and the model predictions are found to be 0.36–1.89% for 2D model and 13.00–13.11% for 1D model at reactor temperatures 723–798 K and L/D of 1.0. This uncderscores the necessity for the use of the 2D model in place of the 1D model, particularly at the lower L/D ratio. Temporal propagation of temperature and mass fractional residue profiles at different locations of the pyrolyzing casuarina (Casuarina equisetifolia) biomass particle is analyzed. At a high L/D (~6), two symmetric hot spots are found to appear at two axial end faces, which gradually merge to form a dumbbell-shaped hot spot near the central core. For a low L/D (~1), a symmetrical, concentric, elliptical, shell-shaped hot spot appears at a short distance from the outer surface, which quickly propagates toward the center, eventually merging into a sphere at the particle center. It is estimated that for a 0.0254 m diameter particle at the reactor temperature of 673 K, the optimal residence time is 10 min for L/D = 1, while it is 17 min for L/D = 2.
Nomenclature
| B | = | mass fraction of virgin biomass |
| C1, C2 | = | mass fraction of char |
| Cps | = | heat capacity of solid (J kg−1 K−1) |
| Ei | = | activation energy (J mol−1) |
| G1, G2 | = | mass fraction of volatiles |
| hc | = | convective heat transfer coefficient (J m−2 s−1 K−1) |
| kci | = | reaction rate constant (s−1) for char formation |
| kvi | = | reaction rate constant (s−1) for volatile formation |
| kci0 | = | pre-exponential factor (s−1) for char formation |
| kvi0 | = | pre-exponential factor (s−1) for volatile formation |
| n1, n2 | = | reaction order |
| Pr | = | Prandtl number |
| r | = | radial coordinate (m) |
| Re | = | Reynolds number |
| Rg | = | universal gas constant (J mol−1 K−1) |
| t | = | time (s) |
| T | = | temperature |
| T0 | = | initial temperature |
| Tf | = | bulk temperature |
| W | = | residual mass fraction |
| X | = | Moisture mass fraction in biomass |
| Z | = | axial coordinate (m) |
| Greek letters | = | |
| ΔH | = | heat of reaction (J kg−1) |
| δ | = | deposition coefficient |
| ρs | = | solid density (kg m−3) |
| ρs0 | = | initial solid density (kg m−3) |
| λe | = | thermal conductivity (J m−1 s−1 K−1) |
| ε | = | emissivity coefficient |
| σ | = | Stefan – Boltzmann constant (J m−2 s−1 K−4) |
| Subscripts | = | |
| i | = | reaction index |
| 1 | = | primary pyrolysis products |
| 2 | = | secondary pyrolysis products |
| LIST OF ABBREVIATIONS | = | |
| 1D | = | One-dimensional |
| 2D | = | Two-dimensional |
| CFD | = | Computational Fluid Dynamics |
| GBEP Global bioenergy partnership | = | |
| GHG | = | Greenhouse gases |
| H/C | = | Hydrogen to carbon |
| L/D | = | Length diameter ratio |
| O/C | = | Oxygen to carbon |
| RMS | = | Root mean square |
| TGA | = | Thermogravimetric analysis |
Acknowledgements
The authors acknowledge the infrastructural and financial support of the National Institute of Technology, Durgapur and the FIST program of DST, Government of India and the National Institute of Technology Durgapur for this work.
Disclosure statement
No potential conflict of interest was reported by the author(s).