THERMOCHEMICAL CONVERSION: (CO)GASIFICATION AND HYDROTHERMAL GASIFICATION - Biomass as a Sustainable Energy Source for the Future - page 310

k 3 CO+ C f s

CO ðÞ! ðÞ ð RX : 10 : 3c Þ

k 4f

+H 2 !

C f s ðÞ CH ðÞ 2 ð RX : 10 : 3d Þ

k 4b

k 5f

2H 2 !

C f s ðÞ +1 = CH ðÞ ð RX : 10 : 3e Þ

k 5b

The oxygen exchangemodel is based on reactions (RX. 10.3b) and (RX. 10.3c), with
the latter being irreversible. The traditional hydrogen inhibitionmodel is an extension of
the oxygen exchange model with reaction (RX. 10.3d). A second version of the hydro-
gen inhibition model consists of reactions (RX. 10.3b), (RX. 10.3c), and (RX. 10.3e).
The rate expression relating to thismechanismis againof Langmuir

- Hinshelwood type:

k 1f p H 2 O

r = ð Eq : 10 : 10 Þ

1+ k 1f

k 3 p H 2 O +f p H ðÞ

p H 2

k 1b
k 3

with f p H ðÞ = ½ oxygen exchangemodel

p H 2

k 4f
k 4b

f p H ðÞ = ½ hydrogen inhibitionmodel, traditional

p H 2

k 5f
k 5b

p

f p H ðÞ = ½ hydrogen inhibitionmodel, second version

Often, a further simplification to n th -order kinetics is applied:

r =k p H 2 O

ð Eq : 10 : 11 Þ

It should be noted, however, that the heterogeneous reaction rate can be expressed in
different ways. The char consumption rate during reaction can be directly measured,
e.g., using a thermogravimetric analysis (TGA) (see Chapter 2), and the reaction rate
can be expressed as

m ∂

1 m

∂ − X ∂

1 X

∂

r char = −

t =

ð Eq : 10 : 12 Þ

1 t

with

X = m t −

m 0

ð Eq : 10 : 13 Þ

m 0 − m

∞

where m 0 and m ∞ are the char sample ' s initial and final mass values and

d X
dt = r char T,cðÞ

, d X c i R s X ðÞ , e : g :

dt =kf X

ðÞ ð Eq : 10 : 14 Þ