Hydrothermal Gasification of Biomass - Biomass Gasification, Pyrolysis and Torrefaction - page 320

1400 -10

-12
-14
-16
-18
-20
-22
-24
-26
-28
-30

Ionic product

1200

1000

Density

800

600

24MPa
38MPa
Drop of physical properti es
at 24MPa
at 38MPa

400

200

0

0 200 400 600

Temperature (°C)

FIGURE 9.3 At a given temperature, both density and ion product of water increase with pres-
sure. Data plotted for 24 MPa show that these values reduce fast at the critical temperature, but
that at 38 MPa it is more gentle. (Note: ion product is plotted as negative log kw, from Kritz
(2004).)

compounds but a poor one for strongly polar inorganic salts. SCW can be
a solvent for gases, lignin, and carbohydrates, which show low solubility
in ordinary (subcritical) water. Good miscibility of intermediate solid
organic compounds as well as gaseous products in liquid SCW allows
single-phase chemical reactions during gasification, removing the inter-
phase barrier of mass transfer.

SCW has a high density compared to subcritical steam at the same tem-
perature. This feature favors the forward reaction between cellulose and
water to produce hydrogen.

10 2 11 Near its critical point, water has higher ion products ([H 1 ][OH 2 ] B

(mol/l) 2 ) than it has in its subcritical state at ambient conditions ( 10 2 14

(mol/l) 2 )( Figure 9.3 ). Owing to this high [H 1 ] and [OH 2 ] ion, the water
can be an effective medium for acid- or base-catalyzed organic reactions
(Serani et al., 2008). Above the critical point, however, the ion product
drops rapidly (

B

10 2 24 (mol/l) 2 B at 24 MPa), and the water becomes a

poor medium for ionic reactions.
Most ionic substances, such as inorganic salts, are soluble in subcritical
water but nearly insoluble under typical conditions in SCW. As the tem-
perature rises past the critical point, the density as well as the ionic prod-
uct decreases ( Figure 9.3 ). Thus, highly soluble common salt (NaCl)
becomes insoluble at higher temperatures above the critical point. This
tunable solubility property of SCW makes it relatively easy to separate
the salts as well as the gases from the product mixture in an SCW
gasifier.

Gases, such as oxygen and carbon dioxide, are highly miscible in SCW,
allowing homogeneous reactions with organic molecules either for