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Journal of Thermal Analysis and Calorimetry

, Volume 111, Issue 1, pp 183–192 | Cite as

Mass transfer limitation in thermogravimetry of biomass gasification

  • Benedikt Nowak
  • Oskar Karlström
  • Peter Backman
  • Anders Brink
  • Maria Zevenhoven
  • Severin Voglsam
  • Franz Winter
  • Mikko Hupa
Article

Abstract

In order to determine the intrinsic reactivity behavior from thermogravimetry studies, the experimental conditions should be such that the reactions are not mass transfer limited. Biomass char usually has a higher reactivity than coal chars. Therefore, mass transfer limitations may be more problematic when studying biomass char reactivity. Chemical reaction kinetics and mass transfer processes present in thermogravimetry are used for modeling the overall reaction rate for spruce bark CO2 gasification. Thermogravimetric experiments are carried out between 700 and 900 °C, and the CO2 concentration is varied between 10 and 90 vol%. The intrinsic activation energy is found to be 120 kJ mol−1. The transition temperature between regimes I and II is here defined when the fraction apparent to true activation energy equals 0.75. Higher external mass transfer (e.g., by decreasing the diffusion path through the crucible’s freeboard), decreasing the sample amounts, and higher CO2 partial pressures for the Langmuir–Hinshelwood reaction type increase the transition temperature. The results show that the transition temperature between regimes I and II conditions is approx. 1,030 °C for 90 vol% CO2.

Keywords

Spruce bark Char CO2 gasification Kinetic regime Model 

List of symbols

Variables

A

Pre-exponential factor (s−1)

c

Concentration (mol m−3)

d

Diameter (m)

D

Diffusion coefficient (m2 s−1)

E

Activation energy (J mol−1)

h

Height (m)

J

Flow (mol s−1)

k

Reaction rate constant, first-order reaction (s−1)

k1

Reaction rate constant, Langmuir–Hinshelwood mechanism (s−1)

k2, k3

Constant, Langmuir–Hinshelwood mechanism (m3 mol−1)

m

Mass (kg)

M

Molar mass (g mol−1)

N

Number

p

Pressure (Pa, bar)

r

Radius (r = 0,…,R) (m)

R

Radius (R = d P/2) (m)

Re

Reynolds number (=ud/v)

\( \mathcal{R} \)

Universal gas constant (J mol−1 K−1)

S

Surface (=d 2π) (m 2)

Sc

Schmidt number (=ν/D)

Sh

Sherwood number (=βd/D)

t

Time (s, min)

T

Temperature (K, °C)

v

Diffusion volume

X

Conversion

Greek symbols

β

Mass transfer coefficient (m s−1)

γ

Abbreviation Eq. (23) (m)

δ

Abbreviation Eq. (25) (m3 s−1)

ε

Porosity

ν

Kinematic viscosity (m2 s−1)

φ

Abbreviation Eq. (27) (m−1)

ρ

Density (kg m−3)

ϱ

Reaction rate (mol m−3 s−1)

τ

Tortuosity

Indices

a

Ash

A

True (activation energy)

app

Apparent

B

Bed

c

Char (including ash)

F

Freeboard

i

Counting index

P

Particle

T

Free flow in thermogravimetric device

Notes

Acknowledgements

The authors wish to thank the COST (European Cooperation in Science and Technology; Action CM901: Detailed Kinetic Models for Cleaner Combustion) for funding the scientific exchange of B. Nowak at Åbo Akademi University. Additional funding by Tekes within the EraNet-Bioenergy project Science Tools for Fuel Characterization for Clean and Efficient Biomass Combustion (SciToBiCom) is also gratefully acknowledged.

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Copyright information

© Akadémiai Kiadó, Budapest, Hungary 2012

Authors and Affiliations

  • Benedikt Nowak
    • 1
  • Oskar Karlström
    • 2
  • Peter Backman
    • 2
  • Anders Brink
    • 2
  • Maria Zevenhoven
    • 2
  • Severin Voglsam
    • 1
  • Franz Winter
    • 1
  • Mikko Hupa
    • 2
  1. 1.Institute of Chemical EngineeringVienna University of TechnologyViennaAustria
  2. 2.Process Chemistry CentreÅbo Akademi UniversityTurkuFinland

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