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Influence of the heating conditions on the thermal and chemical behaviour of solid particles undergoing a fast endothermic decomposition

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Abstract

This paper deals with models describing the thermal and chemical behaviour of solid particles undergoing fast endothermic reactions under the influence of an external heat flux. The heat source temperature is supposed to be constant, to increase with time, or to deliver a simple thermal flash. It is shown that the pyrolysis conditions (reaction temperature, conversion, etc.) depend on the chemical characteristics of the reaction and also to a large extent on the external heating conditions. Relationships are proposed to take into account these parameters. The results are applied to the thermal decomposition of NaHCO3. The pyrolysis of cellulose is finally chosen in order to show how these operating parameters can also affect the selectivity of a more complex reaction.

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Abbreviations

A,A a,A b,A c :

Preexponential factor of the Arrhenius law equation and of processes a, b, c / s−1

A o :

RatioA c/A b

B :

Dimensionless parameter describing the evolution ofρ andC p as a function ofX

BiT :

Thermal Biot number

C p :

Heat capacity of the solid at conversionX/J·kg−1·K−1

C p1 :

C p2 Heat capacity (of solid S1; of solid S2)/ J·kg−1· K−1

d p :

Particle diameter/m

E, E a,E b,E c :

Activation energy of processes a, b, c / J·mol−1

E :

Ratio (E b−E c)/R g/ K

h :

Heat transfer coefficient / W·m−2·K−1

h w :

Radiation heat transfer coefficient / W·m−2·K−1

H :

Thermicity criterion

k :

First order kinetic constant / s−1

L :

Characteristic particle size (volume to external surface area ratio, =d p/6 for a sphere) / m

m o :

Initial mass of solid S1/kg

m 1 :

Mass of solidS 1 remaining at conversionX/kg

M 1 :

Molecular weight of solidS 1/kg·mol−1

M 2 :

Molecular weight of solid S2/ kg·mol−1

r, r W :

Rate of chemical reaction atT s andT w / kg·m−3·s−1

R g :

Gas constant / J·mol−1·K−1

S 1,S 2 :

Solid reactant and product

t :

Time / s

t e :

Heat transfer time constant / s

r W :

Chemical time constant / s

T So,T S,T Si,T R :

Particle temperature (initial; at timet; at the time of quenching; when the reaction starts) / K

TW :

Heat source temperature / K

X o,X,X i, Xc :

Conversion (initial; at timet; at the time of quenching; in the pyrolysis of cellulose)

y :

Reduced temperature

α :

Heating rate of the heat source / K·s−1

ΔH :

Reaction enthalpy / J·kg−1

ε :

Emissivity

ϕ :

Instantaneous yield of tar

γ :

Activation criterion

λ1, λg :

Thermal conductivity (of solidS 1; of the carrier gas) /

ρ :

Density of the solid at conversionX/ kg·m−3

ρ1, ρ2 :

Density (of solidS 1; of solidS 2)/kg·m−3

σ :

Boltzmann constant (5.67·10−8 W·m−2·K−4)

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Authors and Affiliations

  1. Laboratoire des Sciences du Génie Chimique, CNRS-ENSIC, 1, rue Grandville, BP 451, 54001, Nancy, Cedex, France

    J. Lédé

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  1. J. Lédé
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Lédé, J. Influence of the heating conditions on the thermal and chemical behaviour of solid particles undergoing a fast endothermic decomposition. Journal of Thermal Analysis 46, 67–84 (1996). https://doi.org/10.1007/BF01979948

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  • DOI: https://doi.org/10.1007/BF01979948

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