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Modelling of a Traveling Bed WASTE Gasifier

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Abstract

This work deals with the modelling of a travelling bed waste gasifier. This gasifier is a part of a more general process devoted to the production of electricity from waste and refused derived fuel. In a first stage the load is dried and gasified in a travelling bed gasifier using air with an equivalence ratio of 0.3. This product gas contains light hydrocarbons as well as tars. It is converted into a syngas free from tars in a specific reformer (Turboplasma©) that uses plasma technology in order to raise the temperature of the incoming gas to a value compatible with thermal cracking of tars. The model of the travelling bed gasifier is based on the coupling between a software devoted to the description of the chemical and physical processes occurring within the travelling bed, and a CFD package (Fluent™) that allows for the description of the homogenous gas reaction and radiation occurring within the freeboard of the bed. The first software is a “home made” software based on the conservation equations of the solid, liquid and gas phases as well as of the energy. The balance equations are firstly written at the “phase scale”, and then, using a homogenisation technique (volume averaging) balance equations are derived at a representative volume scale. In this first work, this model is written using a one dimensional formalism along the axis of the travelling bed. Wood is used as a case study material with a pyrolysis mechanism that uses three parallel reactions leading to the formation of gas, tars and char. The gas and tars produced during the pyrolysis step can be converted within the bed itself but also over the freeboard of the bed. Hence, the free board of the bed is used as a boundary condition of the CFD domain, with known local mass flow-rate, composition and temperature. This study allows for a complete and precise description of the processes occurring within the gasifier under consideration.

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Abbreviations

aw :

Water activity

C:

Heat capacity [J kg−1 K−1]

Cp:

Constant pressure heat capacity [J kg−1 K−1]

D:

Diffusivity in the air [m2 s−1]

\( \underline{\underline{\text{D}}}_{\text{ieff}} \) :

Diffusion tensor of i gaseous species in porous medium [m2 s−1]

\( \underline{\underline{\text{D}}}_{\text{b}} \) :

Diffusion tensor of bound water in porous medium [m2 s−1]

e j :

Unit vector in the j direction of space

f, f :

Scalar and vector functions

Fmi :

i species mass flux [kg m−2 s−1]

g :

Gravity vector [m s−2]

H:

Height of the bed [m]

hi :

Intrinsic enthalpy of i gaseous species [J kg−1]

\( {\bar{\text{h}}}_{\text{b}} \) :

Intrinsic averaged enthalpy of bound water [J kg−1]

hα :

Intrinsic averaged enthalpy of free water [J kg−1]

∆Hb :

Heat of desorption [J kg−1]

\( \Updelta {\text{H}}_{\text{v}}^{\text{ref}} \) :

Latent heat of vaporisation at the reference temperature Tref [J kg−1]

\( \underline{\underline{\text{k}}} \) :

Intrinsic permeability tensor [m²]

\( \underline{\underline{\text{k}}}_{{{\text{r}}\alpha }} \) :

Relative permeability of the α phase in the porous medium [m²]

l:

Width of the porous bed [m]

L:

Length of the porous bed [m]

m:

Molar mass [kg]

q :

Conduction heat flux [W m−2]

n :

Outer unit normal to the product

P:

Pressure [Pa]

ri :

Chemical reaction rate of i species [kg m−3 s−1]

FQ :

Heat flux [W m−2]

T:

Temperature [K or °C]

t:

Time [s]

t :

Unit tangential vector to the product

V :

Velocity [m s−1]

W:

Moisture content (in dry basis)

Ea:

Energy of activation [J mol−1]

N:

Number of gaseous species present in the reactor

qray :

Radiation heat flux [W m−2]

I:

Intensity of the radiation heat flux [W m−2]

ki :

Pre exponential factor in formulae of the homogeneous chemical reaction rate [s−1]

w :

Interface velocity vector [m]

R:

Perfect gas constant

λ:

Thermal conductivity [W m−1 K−1]

\( \underline{\underline{\lambda }}_{\text{eff}} \) :

Effective thermal conductivity tensor [W m−1 K−1]

ρ:

Density [kg m−3]

μ:

Viscosity [kg m−1 s−1]

\( \bar{\omega }_{\text{i}} \) :

Mass fraction of i species in the porous medium

ωi :

Mass fraction of i species in the surroundings

εα :

Volume fraction of the α phase

φ:

Conservative variable

Γ :

Surface flux corresponding to the conservative variable φ

Π:

Production/loss volumetric rate of the conservative variable φ

α:

Phase α

i:

Gaseous species i

b:

Bound water

s:

Solid

c:

Capillarity

sat:

Bound water saturation point

fo:

Formation

g:

Gas

j:

Direction of space

L:

Liquid

OM:

Organic matter

I:

Inorganic matter

C:

Char

vsat:

Vapour saturation

v:

Vapour

hom:

Homogeneous

het:

Heterogeneous

gasi:

Gasification

pyr:

Pyrolysis

com:

Combustion

0:

Frontier of the porous medium corresponding to x2=0

H:

Interface between the porous medium and the gaseous environment in the reactor

bed:

Porous medium

1:

Relative to the axial direction of the bed (Ox1)

2:

Relative to the transverse direction of the bed (Ox2)

bed:

Relative to the bed

foi:

Formation of the i component

decar:

Decarbonation

L:

Liquid phase

g:

Gas phase

α:

α phase

ref:

Reference

s:

Solid

waste:

Waste

−:

Average

−α :

Intrinsec average to the α phase

\( \nabla \) :

Gradient operator

\( \nabla \cdot \) :

Divergence operator

\( \underline{\underline{\left( \cdot \right)}} \) :

Second order tensor

\( \frac{\text{D}}{\text{Dt}} \) :

Material derivative

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Authors and Affiliations

  1. Laboratoire de Thermique Energetique et Procedes, Université de Pau et des Pays de l’Adour, Rue Jules Ferry, B.P. 7511, 64075, Pau cedex, France

    P. Bernada, F. Marias, A. Deydier & F. Couture

  2. Europlasma, 21 rue Daugère, 33520, Bruges, France

    A. Deydier & A. Fourcault

Authors
  1. P. Bernada
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  2. F. Marias
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  3. A. Deydier
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  4. F. Couture
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  5. A. Fourcault
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Correspondence to P. Bernada.

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Bernada, P., Marias, F., Deydier, A. et al. Modelling of a Traveling Bed WASTE Gasifier. Waste Biomass Valor 3, 333–353 (2012). https://doi.org/10.1007/s12649-012-9115-9

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