Environmental Engineering Reference
In-Depth Information
TABLE 10.6 Composition of the gas produced in the PDU (see
Figure 10.10) by SCWG of 5 wt% glycerol in water with and without
addition of NaOH.
p
= 27.0 MPa; T = 580
C
Concentration (vol.%)
0.0075 wt% NaOH
Component
Concentration (vol.%)
Hydrogen (H
2
)
29
60
Carbon monoxide (CO)
30
0.5
Carbon dioxide (CO
2
)
13
21
Methane (CH
4
)
15
12
Ethene/ethane
11
5
Propene/propane
2
1
LHV [MJ m
n
−3
]
21
15
Biomass gasification in supercritical water (SCW) (600
C, 30.0 MPa) is a novel
process under development after the pioneering work of, e.g., Modell and Antal
and coworkers since the mid-1980s (Antal et al., 2000). In the laboratory and in
pilot-plant work that has been carried out so far, stirred autoclaves and tubular reactors
are used frequently in connection with a shell and tube heat exchanger. Not much
attention has been paid to practical design aspects of the supercritical water gasifica-
tion (SCWG) process (including heat exchanger and reactor). Figure 10.10 shows a
possible flow sheet of the process, and typical results for glycerol are listed in
Table 10.6.
10.4.2 Supercritical Water Properties
The properties of SCW are quite different from those of the normal liquid or steam at
atmospheric pressure. For instance, at the critical point, the density is around 300 kg
m
−3
m
−3
for liquid water at ambient conditions), the dielectric constant is
5 (vs. 80 for liquid water at ambient conditions), and the ion product Kw = [H
+
]
[OH
−
]is10
−11
(vs. 10
−14
for liquid water at ambient conditions). The behavior of
water near and above the critical point has been studied extensively. In fact, all
relevant physical properties have been determined experimentally and have been
tabulated by the US National Institute of Standards and Technology (NIST)
(tinyurl.com/9s23f ). The most striking feature of SCW is the possibility to manipulate
and control its properties around the critical point by tuning the temperature and pres-
sure. In the vicinity of the critical point where the ion product is high (10
−11
), the H
+
concentration is about 30 times higher than at ambient conditions, offering increased
opportunities for acid-catalyzed reactions. As an important consequence of the change
in the dielectric constant, SCW behaves like a nonpolar solvent and exhibits a high
solubility for nonpolar organic compounds like benzene. Gases like oxygen, nitrogen,
carbon dioxide, and methane are completely miscible in SCW. On the contrary, the
solubility of inorganic salts like NaCl is decreased to very low values.
(vs. 1000 kg
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