Supercritical water (SCW) means the water in a special state that temperature and pressure are both higher than the critical points (T=374.15 DEG C, P=22.12 MPa). In this state, only a small amount of hydrogen bonds exist; the dielectric constant of the water approximates to that of an organic solvent; and the water has a high diffusion coefficient and low viscosity. Organic matter and oxygen can be dissolved in any proportion with the SCW converting heterogeneous reaction to homogeneous reaction, thereby reducing the resistance of mass transfer and heat transfer greatly. The solubility of inorganic matter, especially salts, is extremely low in the SCW, so that the inorganic matter can be separated easily.
The supercritical water treatment technology of organic wastewater comprises supercritical water oxidation (SCWO) technology, supercritical water gasification (SCWG) technology and supercritical water partial oxidation (SCWPO) technology.
SCWO makes use of the special features of water in a supercritical state to completely decompose organic matter through quick oxidation reaction of the organic matter and an oxidizing agent in the supercritical water, thereby realizing the harmless treatment on the organic matter. SCWG makes use of the special features of water in a supercritical state, wherein without an oxidizing agent, reactions such as hydrolysis and pyrolysis of the organic matter take place in the supercritical water to generate flammable gas which mainly comprises hydrogen. SCWPO makes use of the special features of water in a supercritical state to decompose the organic matter to generate flammable gas which mainly comprises hydrogen on the premise of providing partial oxidizing agent. Due to the addition of the oxidizing agent, the matter with decomposition difficulty in gasification previously can be decomposed so as to improve gasification rate. Meanwhile, the oxidizing agent inhibits the generation of tar so as to decrease the blockage of the reactor.
Although great progress has been made in supercritical water treatment technology, multiple problems in reactor design still remain to be solved, specifically:
1) The reaction conditions of SCWO, SCWG and SCWPO are rigorous, requiring higher temperature and pressure (the temperature is greater than 374.15 DEG C, and the pressure is greater than 22.12 MPa), so that the requirements on reactor material are high on one hand, and on the other hand, the reliability of the safe operation of the reactor is reduced. Currently, there exist transpiring wall reactors and water-cooled wall reactors, wherein by leading into a stream of cold fluid to isolate the bearing wall of the reactor and the high temperature fluid to achieve the purpose of separating the high-temperature area and the high-pressure area, thereby reducing the requirements on the reactor material and improving the reliability of the safe operation of the reactor, however, the two kinds of reactors have the defects of low energy utilization efficiency, complex control and others.
2) In the supercritical water environment, high temperature, high pressure, dissolved oxygen and some free radicals and ions generated in the reaction can accelerate the corrosion rate of corrosion-resistant material. Tests have proved that stainless steel, nickel-based alloys, titanium alloys and other high corrosion-resistant materials are subjected to corrosion in varying degrees in a supercritical water treatment system, especially in an SCWO system. The corrosion of the materials can lead to the decline of the service life of the reactor, resulting in safety accidents.
3) In the supercritical water oxidation treatment, the oxidation of ammonia nitrogen is more difficult. Due to the utilization of a catalyst in the supercritical water reaction, the conversion rate of the reactants including ammonia nitrogen can be obviously improved, the reaction time is shortened, and the reaction temperature and the reaction pressure are lowered. Presently, the loading and the replacement of the catalyst in a sealing reaction system are more difficult, and the problems such as the loss and inactivation of the catalyst still exist.
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