In this chapter the authors seek to acquaint the reader with unusual properties of supercritical fluids, and how these properties are used for various applications in the synthesis of biofuels and the intensification of energy processes and thermal cycles. The review includes the studies of leading scientists from around the world related to effective and environmentally friendly methods and technologies of energy acquisition and conversion involving the synthesis of motor fuels, materials and chemicals, solar energy conversion and thermal cycle efficiencies using supercritical fluids. A special place in the chapter is occupied by studies of the transformation of vegetable oils, algae and wood in supercritical alcohols, esters and water to produce biodiesel fuels and hydrogen.
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The intensification of energy processes while preserving the quality of the environment is an integral component of global economic and social development. Energy and the technology of synthesizing motor fuels, hydrogen and chemicals occupy a special place among these processes. While there are many approaches and methods for achieving the above objectives, this review presents the results of research, practical applications and chemical processes carried out in supercritical fluids.
In this review, the authors seek to acquaint the reader with unusual properties of supercritical fluids, and how these properties are used for various applications in the synthesis of biofuels and the intensification of energy processes. The authors do not discuss in detail herein the structure of supercritical fluid nor its thermodynamic and thermal properties near the critical point, as these topics have been addressed in previously published papers (Schmidt et al., 1946; Eckert et al., 1996; Kiran, 2000; Sengers & Levelt, 2000).
Due to their unique properties, supercritical fluids (SCFs) offer an attractive environment for various types of chemical reactions and physical processes involving transformations in the underlying energy cycles. Many physical and chemical properties of SCFs lie between those of liquids and gases. SCFs have also unique solubility. While many chemical components are not soluble in fluid under normal conditions, these become almost completely soluble under supercritical conditions. Conversely, some substances (generally salts) that are readily soluble in liquid under normal conditions, they are not very soluble in this medium under supercritical conditions. Unlimited solubility effectively allows for a reduction in heat and mass limitations and increases the mass transfer rate, e.g., it significantly increases the rate of chemical reactions in catalyst grains.
The practical application of SCF technologies includes waste recycling and decomposition, biomass and coal conversion processes, synthesis of new materials, and synthesis of bio-fuels and hydrogen, among many others.
Among the chemical reactions performed in SCFs, the oxidation reactions carried out in supercritical water are of the greatest practical interest today. These form the basis of almost all known processes and technologies for processing different type of wastes, including the destruction of dissolved dangerous and harmful organic substances, biological wastes and military production (Harradine et al., 1993; Savage et al., 1995; Pisharody et al., 1996; Anikeet et al., 2006; Kritzer & Dinjus, 2001; Vadillo et al., 2014).
Technologies using supercritical conditions allow the conversion of wastes at a high rate and significantly lower temperature; therefore, non- NOx, and SO2, metal oxides require little or no energy supply. Organic waste components and oxygen dissolve almost completely in the supercritical solvent near its critical point, forming a single-phase environment conducive to rapid oxidation of organics into CO2, H2O and N2. In SCF in the presence of oxygen, practically all hydrocarbons are oxidized to CO2 and water, and compounds containing nitrogen are oxidized to form N2 or N2O. If air is used as the oxidant, nitrogen under these conditions (temperature for supercritical water reaction requires slightly more than 380oC) is not oxidized, nor does NOx form.