Environmental Engineering Reference
In-Depth Information
1
0.9
0.8
0.7
H 2 (g)
CH 4 (g)
N 2 (g)
O 2 (g)
H 2 O(g)
CO(g)
CO 2 (g)
C(s)
0.6
0.5
0.4
0.3
0.2
0.1
0
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1
Stoichiometric oxygen ratio λ ( )
FIGURE 10.2 Chemical equilibrium for gasification of dry wood at T = 850 C and
p = 0.1 MPa.
compounds are formed when full equilibrium would be reached (after infinite time at
given temperature and pressure). This helps to understand trends in the formation of
gaseous components as a function of the applied oxidizer stoichiometry and to eluci-
date which role catalysis might play, e.g., to reduce often unwanted hydrocarbon
concentrations under the proposed process conditions.
Figures 10.2 and 10.3 illustrate a few typical thermodynamic equilibrium calcula-
tions based on air-blown gasification under adiabatic conditions. A representative
temperature of 850 C has been chosen as well as two pressure levels, atmospheric
pressure and a pressure of 2.0 MPa. As the dry biomass composition, a typical wood
composition of CH 1.4 O 0.6 was used, and the air-to-biomass stoichiometric ratio,
,
was varied between 0 and 1; the values of the heat of formation of the components
(
λ
H f 0 ) were obtained from Zainal (2001). As components in the calculation proce-
dure, the following ones were selected: H 2 , CO, CO 2 ,CH 4 ,N 2 ,H 2 O, O 2 , and
C(s). C(s) was taken as graphite. Minimization of the Gibbs energy was applied to
calculate the chemical equilibrium. This procedure to calculate chemical equilibrium
has already been explained in details in Chapter 5.
What can be observed is that when the value of
Δ
decreases from 1 (the point of
exact stoichiometric combustion), the composition of the gas changes in such a
way that the complete oxidation products CO 2 and H 2 O decrease in concentration
and that the CO and H 2 concentrations increase. Furthermore, it can be observed that
the values of the CH 4 concentrations remain very low under atmospheric conditions,
but at increased pressure, they substantially increase. Important to note is that when
decreasing
λ
λ
, at a certain value of this parameter, solid carbon starts to form. This is
called the
carbon boundary,
and the value at which it occurs corresponds to the
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