THERMOCHEMICAL CONVERSION: (CO)GASIFICATION AND HYDROTHERMAL GASIFICATION - Biomass as a Sustainable Energy Source for the Future - page 312

It is commonly agreed (Di Blasi, 2009; Hurt, 1998; Winter et al., 1997) that three main
regimes can be distinguished during solid particle conversion. These are based on the
Thielemodulus (a parameter indicating the ratio of the overall reaction rate to the internal
diffusion rate) and the effectiveness factor (the ratioof the actual reaction rate to thatwhich
would occur if all the surface throughout the internal pores were exposed to the gaseous
reactant at the same conditions as that existing at the external surface of the particle):

Regime I . Kinetic control prevails when the reaction occurs at low temperature
with small char particles. Under these conditions, the value of the Thiele modulus
is small and the effectiveness factor approaches unity. As a consequence, conver-
sion occurs throughout the particle, which shows changes in density, but its particle
size remains constant.
Regime II . Intraparticle mass transfer control takes place when the particle size is
larger. This in combination with a low porosity of the particles leads to a limited
gaseous reagent penetration into the char surface. Given these conditions, the
Thiele modulus is much greater than unity, and the effectiveness factor much less
than unity. Consequently, conversion occurs practically only at the particle s exte-

rior surface. Therefore, the particle size decreases without much change in density.
Regime III . External mass transfer control, finally, occurs when the gas

'

solid
reaction takes place at high temperature with even larger char particles present.
In this situation, the reaction rate is proportional to the external surface of the
particle and thus depends on its size.

-

Furthermore, heat transfer may also affect char conversion due to heat release and
during combustion and heat absorption during gasification. This may enhance the tem-
perature gradient between the surface and the core of the particle. Both mass and heat
transfer effects are enhanced by high temperatures. To estimate the effects of mass and
heat transfer during char reactions, various criteria are available in the literature. For
instance, the Mears criterion (Mears, 1971) is widely used to estimate nonnegligible
effects of external mass transfer Equation (10.16) and intraphase heat transfer Equation
(10.18). The Weisz Prater criterion (Fogler, 1999) is normally used to determine

whether limitation by internal mass transfer occurs Equation (10.17). When these
Equations (10.16

-

10.19) are satisfied, the effects of external mass transfer, internal
mass transfer, and intraphase heat transfer effects can be neglected. In Example
10.2, alternative formulations for the first two criteria mentioned will be given.
Negligible external mass transfer:

-

R 0 i ρ p 1

− ð − ε bed Þ r p n

<0 : 15 ð Eq : 10 : 16 Þ

k g c i , b

η

Negligible intraparticle mass transfer limitation (with being the internal effective-

ϕ 1 the Thiele modulus):

ness factor of a particle and

ηϕ 1 2 =3 ð ϕ 1 coth ϕ 1 − 1 Þ <<1 ð Eq : 10 : 17 Þ