Environmental Engineering Reference
In-Depth Information
The supply of heat to accomplish the main endothermic gas-generating reactions can
be provided
in situ
by exothermic partial oxidation (autothermal operation) or indi-
rectly by integration of the reaction process with an external exothermic process,
which transfers heat to the main gasification reactor.
10.1.3 Characteristic Process Parameters
Important for the generation of a good quality product gas is the ratio of oxidizer to
fuel. A parameter that characterizes this is the stoichiometric oxygen ratio,
λ
(lambda
value), which is defined as
external O
2
supply
=
fuel supply
λ
=
ð
Eq
:
10
:
1
Þ
stoichiometric O
2
requirement
=
unit of fuel input daf basis
ð
Þ
Different
“
oxidation regimes
”
can be distinguished for thermochemical biomass
conversion depending on
λ
. Processes with
λ
> 1 refer to combustion, processes
with
< 1 to gasification. In literature,
one also finds the equivalence ratio (ER), which is the inverse of
λ
= 0 to pyrolysis, and processes with 0 <
λ
λ
λ
values
signify higher concentrations of the targeted main gasification constituents, H
2
and
CO, in the produced gas. What needs to be considered though is that with decreas-
ing
.Lower
λ
values, the endothermic reactions will increasingly prevail so that tempera-
tures might decrease to values that are too low to sustain the gasification
process. This can be overcome by external heat supply (indirect gasification), or
the
λ
value can be increased so that heat is generated
in situ
by partial oxidation
(autothermal gasification).
When steam is used as a component of the oxidizer flow supplied to the gasifier, the
steam-to-biomass ratio plays an important role in determining the final gas composi-
tion. It can be defined in two ways, as presented below:
SB =
steammass flow
fuel feed rate
:
:
ð
Eq
10
2
Þ
SB
∗
=
steammass flow+ fuel moisture flow
fuel feed rate
ð
Eq
:
10
:
3
Þ
Important output parameters for the evaluation of a gasification process are the
carbon conversion (CC) and the cold gas efficiency (CGE), respectively. They are
defined as
!
−
φ
m
,
C
,
residue
φ
m
,
C
,
feed
CC = 1
ð
Eq
:
10
:
4
Þ
X
!
φ
m
,
i
LHV
i
CGE =
ð
Eq
:
10
:
5
Þ
φ
m
,
fuel
LHV
fuel
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