Hydrothermal treatment of waste materials in supercritical conditions has gained a great deal of momentum ever since the pioneering work of Modell and coworkers from M. I.T. in the late 1970s. As indicated earlier, Figure 6.8 illustrates the region of supercritical water gasification for waste treatment in the pressure — temperature diagram. The three regions shown in the diagram take advantage of substantial changes in the properties of water that occur in the vicinity of its critical point at 374°C (Tc) and 22 MPa (Pc). The behavior of the important prop­erties of water such as density, ion dissociation constant, and dielectric constant with respect to temperature is illustrated in Figure 6.9 [76]. In supercritical con­ditions, 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 in supercritical water.

Water at ambient conditions (25°C and 0.1 MPa) is a good solvent for elec­trolytes because of its high dielectric constant [76], whereas most organic matter is sparingly soluble [76]. As water is heated, the H-bonding starts weakening, allowing dissociation of water into acidic hydronium ions (H3O+) 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

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point, which causes a change in the dynamic viscosity and also increases the self-diffusion coefficient of water [76].

Supercritical water has liquid-like density and gaslike transport properties, and behaves very differently than water at room temperature. For example, it is highly nonpolar, permitting complete solubilization of most organic compounds. The resulting single-phase mixture does not have many of the conventional transport limitations that are encountered in multiphase reac­tors. However, the polar species present, such as inorganic salts, are no longer soluble and start precipitating. The physicochemical properties of water, such as viscosity, ion product, density, and heat capacity, also change dramatically in the supercritical region with only a small change in the temperature or pressure, resulting in a substantial increase in the rates of chemical reactions.

It is important to mention that the dielectric behavior of 200°C water is similar to that of ambient methanol, 300°C water is similar to ambient ace­tone, 370°C water is similar to methylene chloride, and 500°C water is simi­lar to ambient hexane [76]. In addition to the unusual dielectric behavior, the transport properties of water are significantly different than the ambient water as shown in Table 6.6.

Supercritical processes are often not considered to be economical because of high capital costs associated with high-pressure equipment and high operating costs associated with the compression or pumping of supercriti­cal media, however, the above-described unique properties of supercritical fluids offer some very interesting possibilities. Furthermore, in recent years the prices of high-pressure equipment has come down. In conjunction with catalysis, a supercritical fluid can dissolve unwarranted hydrocarbons from the catalyst surface into the supercritical fluid phase. Supercritical fluids have a better capacity to handle heat due to high capacities. The adsorption/ desorption phenomena can be better handled in a supercritical fluid due to higher solubility. The oligomeric coke precursors or sulfur species can be easily dissolved by the supercritical fluids.

An excellent review of supercritical water (SCW) gasification of bio­mass and organic wastes was recently published by Guo, Cao, and Liu [91]. Numerous studies have examined supercritical water partial oxidation [92,

93]. These and other studies found that the yields of H2O and CO increased with increasing water density. Yields of H2 were 4 times better with NaOH and 1.5 times better with ZrO2 compared to reaction without a catalyst. Supercritical fluids gave increased pore accessibility, enhanced catalyst abil­ity to coking, and increased desired product selectivity.

An extensive amount of work on supercritical water gasification of organic wastes has been reported in the literature [78, 79, 94-96]. The studies have shown that gasification generally produces a hydrogen and carbon dioxide mixture with simultaneous decontamination of waste. The homogeneous solution of waste and water makes it easy to pump to the high-pressure reactor without pretreatment. Xu and Antal [97] studied gasification of 7.69 wt% digested sewage sludge in supercritical water and obtained gas that largely contained H2, CO2, a smaller amount of CH4, and a trace of CO. Other waste materials show similar behavior. The equilibrium yields as functions of temperature and pressure for SCWG of 5% sawdust reported by Guo et al. [91] indicate the main products to be hydrogen, carbon diox­ide, and methane at low temperatures and hydrogen and carbon dioxide at high temperatures.

An increase in pressure significantly decreases the product concentration of carbon monoxide and slightly decreases the product concentration of the hydrogen. The pressure change has very little effect on the product concen­trations of carbon dioxide and methane. In addition to temperature and pres­sure, other parameters that affect the gas yield are feedstock concentration, oxidant, reaction time, feedstock composition, inorganic impurities in the feedstock, and biomass particle size. Several catalysts such as alkali (NaOH, KOH, Na2CO3, K2CO3, Ca(OH)2), an activated carbon, metal oxide, and met­als also affect the conversion and gas yields. The last two are important for reforming under supercritical conditions. Although high-temperature super­critical water gasification produces hydrogen and carbon dioxide, Sinag, Kruse, and Schwarzkopf [98] showed that a combination of two technologies, supercritical water and hydropyrolysis on glucose in the presence of K2CO3, produces phenols, furfurals, organic acids, aldehydes, and gases.

The generation of hydrogen from waste has long-term and strategic impli­cations inasmuch as hydrogen is the purest form of energy and is very useful for product upgrading, fuel cells, and many other applications. Hydrogen can be produced from waste via numerous high-temperature technologies such as conventional or fast pyrolysis (e. g., olive husk, tea waste, crop straw, etc.), high-temperature or steam gasification (e. g., bionutshell, black liquor, wood waste, etc.), supercritical fluid extraction (e. g., swine manure, orange peel waste, crop grain residue, petroleum basis plastic waste, etc.), and supercritical water gasification (e. g., all types of organic waste, agricultural and forestry waste, etc.), as well as low-temperature technologies such as anaerobic digestion and fermentation (e. g., manure slurry, agricultural resi­due, MSW, tofu wastewater, starch from food waste, etc.).

For high-temperature technologies, supercritical water gasification gener­ates more hydrogen at a lower temperature than pyrolysis or gasification [99, 100]. Supercritical water gasification also does not require drying, sizing, and other methods of feed preparation thereby requiring less expense for the overall process. The temperature of the pyrolysis and gasification processes can be reduced if the gases coming from them are further steam reformed. This, however, adds to the overall cost. The rates for the low-temperature processes such as anaerobic digestion and fermentation can be enhanced with the use of suitable microbes and enzymes. The development of a future hydrogen economy will require further research in the improvement of these technologies.