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Energy, especially electricity, plays a vital role in our modern industrial society and in our current civilization. In order to increase the power generation efficiency of fossil fuels, supercritical and ultra-supercritical power plants, using supercritical water (SCW) as the working medium, have been developed over the past century [1].

Sewage sludge (SS) with a moisture content of about 80 wt% is a byproduct of wastewater treatment. With the development of the urbanization process, this amount has been increasing with the construction and expansion of wastewater treatment plants. Containing complex organic matters as well as heavy metal and viruses, SS is not only a pollutant but also a wet resource.

Above the critical point (T = 374.15 °C, P = 22.1 MPa), water exists as a single phase and has analogous transportation properties of gas and solvent properties like liquid, which is known as supercritical water (SCW) (Kutney in thermodynamic and transport property modeling in super critical water. Massachusetts Institute of Technology, 2005 [1]; Shaw et al. in Chem. Eng. News 69:26–39, 1991 [2]). SCW is a non-polar solvent that can be completely miscible with organic compounds and gases such as hydrogen, nitrogen, and oxygen.

Today’s commercial operation of power systems, such as Supercritical Generating Unit and IGCC, presents some congenital problems [1]. First, both the investment and operation cost of the desulfurization and denitration equipment are very expensive. Second, the carbon capture is not only inefficient but also leads to the power generation efficiency reduce exceeding 8%.

Hydrogen is defined as an attractive energy carrier due to its potentially higher energy efficiency and lower generation of pollutants, which can replace conventional fossil fuels in the future. The governments have invested huge funds and made great efforts on the research of hydrogen production. Among the various options, supercritical water gasification (SCWG) is the most promising method of hydrogen production from biomass.

Supercritical water (SCW: T > 374.15 °C, P > 22.1 MPa) possesses specially physical and chemical properties such as high diffusivity, low viscosity, low dielectric constant, and a small amount of hydrogen bonds. Organic matter and oxygen can be dissolved in SCW with any ratio under high enough pressure condition while the solubility of inorganic salt in SCW is extremely low. Therefore, these properties are beneficial to reduce resistance of mass and heat transfer and to separate salts from SCW. As an attractive media for regulating chemical reaction process and performing salt separation process, SCW has attracted more attentions since the early 1980s. The definition of supercritical water oxidation (SCWO) is that the organic matter is oxidized into harmlessly small molecular compounds such as carbon dioxide, nitrogen, water, and inorganic salt by excess oxidant in SCW. Supercritical water gasification (SCWG) means that organic matter undergoes hydrolysis, pyrolysis reaction and so on in SCW to form a hydrogen-rich mixing gas.

The potential of SCWO lies in the complete destruction of persistent, organic waste materials. The main obstacles in the technical application of SCWO include the construction materials degradation triggered by high-temperature and high-pressure conditions and some highly aggressive species (such as oxygen, acids, alkali, and some salts), the precipitation of salts and insoluble materials, especially piping plugging issues resulting from them, and high running cost. In the following, the above three key problems encountered in designing systematic processes and their potential solutions are reviewed roughly, more information is available in the above sections of this book.

Nanomaterials are always at the leading emerging area of material and nanotechnology. Especially in this age, the ever-increasing population in the world makes energy, chemicals and other resource efficiency particularly important in sustaining concept [1]. The significant reduction in size (1–100 nm) often improves their electronic and other properties due to quantum confinement effects.