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Water for Energy and Fuel Production
5. High pressure used in the supercritical gasification helps downstream operations such as storage and transportation of the product gases, carbon capture,
and purification of the product gases by reforming or pressure swing adsorption.
6. As shown by Savage and others [1–13] (Aljishi et al., 2010, pers. comm.), SCW
provides a homogeneous medium to carry out numerous organic chemical
reactions such as decomposition, partial and complete oxidation, hydration/
dehydration, hydrogenation/dehydrogenation, hydrolysis, elimination and
rearrangement, and C–C bond formation with ease in which water acts as a
benign medium, a reactant, or a catalyst.
7. With the use of a suitable catalyst, SCW gasification can be easily accompanied by a reforming reaction.
The major disadvantages deal with the operational issues such as the use of highpressure water, which may carry some toxic and corrosive substances. The processing of supercritical operations may require the use of special materials that may be
expensive and demand substantial maintenance and replacements costs. The capital
and operating costs associated with high-pressure operations may be considerably
larger than those for low-pressure gasification and pyrolysis operations. In recent
years, however, the prices of high-pressure equipment have come down.
10.2 PrOPerties OF sCW
Hydrothermal treatment of carbonaceous materials in supercritical conditions has
taken a significant momentum ever since the pioneering work of Modell and
coworkers from the Massachusetts Institute of Technology (MIT) in the late 1970s
[1–14] (Aljishi et al., 2010, pers. comm.). Figure 10.1 illustrates the thermodynamic
region (in terms of pressure–temperature diagram) of SCW treatment of the carbonaceous materials. The three regions shown in the figure take advantage of substantial changes in the properties of water that occur in the vicinity of its critical
point at 374°C (T c ) and 22 MPa (P c ). The behavior of the important properties of
water such as density, ion dissociation constant, dielectric constant, and solubility
limits of various salts as a function of temperature was described in Chapter 5 and
will not be repeated here [1–5]. In that chapter, we examined the role of water as
a chemical reactant under subcritical conditions. In this chapter, we focus on the
role of SCW for carrying out various organic chemical reactions. In SCW, more
chemically and energetically favorable pathways to gaseous and liquid fuels can
be achieved by better control of the rate of hydrolysis and phase partitioning and
solubility of components.
Water at ambient conditions (25°C and 0.1 MPa) is a good solvent for electrolytes
due to its high dielectric constant [1–10] (Aljishi et al., 2010, pers. comm.), whereas
most organic matter are sparingly soluble [1–10] (Aljishi et al., 2010, pers. comm.). As
water is heated, the H-bonding starts weakening, allowing dissociation of water into
acidic hydronium ions (H 3 O + ) and basic hydroxide ions (OH − ). The structure of water
changes significantly near the critical point because of the breakage of infinite networks of hydrogen bonds and water exists as separate clusters with a chain structure.
In fact, the dielectric constant of water decreases considerably near the critical point,