| Currently, energy crisis and environmental pollution have become serious problems to humanity. More attention is paid on organic waste treatment without pollution and renewable energy development. Supercritical water(SCW) is an environmentally friendly reaction medium. It can dissolve organic matters and the reaction is fast and efficient. The technology of organic waste treatment in SCW has drawn attention at home and abroad.There are two kinds of reaction methods:oxidation and pyrolysis taking place in supercritical water. In this paper, several kinds of representative organic matters were selected for experiments on pyrolysis and oxidation in supercritical water. Influencing rule of different reaction conditions on reaction process and products was obtained and migration of several key elements was discussed. Kinetic models of several reactions were generalized. The results would offer basic data for practical use. Corresponding measures were taken to solve corrosion and precipitation during supercritical reactions. System of energy utilization during supercritical water oxidation was designed and application prospect was analyzed.The physical and chemical properties were analyzed in detail. According to IAPWS formulation 97, fitted quadratic equations about density, thermal conductivity, viscosity, specific heat varying with temperature in supercritical water were obtained. Continuity equation, momentum equation and energy equation were generalized.Pyrolysis efficiency and according influencing factors of wood dust and waste tires in SCW were studied.Organic matters can be well decomposed in SCW. Raising temperature and pressure, extending residence time can promote pyrolysis reaction. Reaction temperature and residence time played important role in pyrolysis reaction while influence of pressure was unessential. So the pressure should not be too high after above the critical point to protect the equipments.Elemental analysis of solid products after pyrolysis in SCW was carried out. Wood dust has been nearly completely decomposed in sub-critical state. And with the increase of temperature and pressure, the pyrolysis was more complete and carbon content in residual solid was higher, but the speed was gradually slower. The efficiency of waste tires was only 40% in sub-critical state. With the increase of temperature and pressure, the reaction continued and nearly completed at 550℃,22.2MPa. Carbon content of the solid products first increased and then decreased with the increase of temperature and pressure. The average carbon content was over 80%. The influence of pressure around the critical point was important.The gas products of wood dust pyrolysis in supercritical water were CO2, H2, CO, CH4 etc. with the increase of temperature, the content of CO2 first increased and then decreased, while that of CO is low and stable and the content of CH4 always increased. With the increase of pressure, the content of CO and CH4 decreased but the content of CO2 and C2-C6 hydrocarbon increased. CH4 and H2 were main gas products of waste tires pyrolysis in SCW, the content of which increased quickly with increase of temperature.400℃-500℃was the main temperature period for CH4 generation. Also the content of CO of waste tires pyrolysis in SCW was low and increased slightly between 450℃-500℃.There was no SO2, NOx in gas products in supercritical water pyrolysis. Due to high content of carbon, solid product could be utilized as activated carbon after treatment. Pyrolysis oil and gas product could be used as fuel. As technology dealing with wastes and recovering energy, supercritical water pyrolysis presents broad prospects.Paper-making black liquor with high concentration was chosen to be oxidized in SCW. The effects of temperature, residence time, pressure on oxidation were studied. And the result showed that 99% of waste water could be decomposed during supercritical water oxidation. The main gas product was CO2. Higher temperature, longer residence time and higher pressure led to higher reaction efficiency of waste water in supercritical water oxidation. Among these three influencing factors, temperature played more important role than residence time and pressure. Alkalesence condition was helpful for paper-making black liquor oxidation in supercritical water.Supercritical water oxidation of domestic garbage was studied and effects of temperature, pressure, carbon content in adding activated carbon and excess oxygen coefficient was discussed. The results demonstrated that the conversion efficiency of domestic garbage to CO2 was over 80% in condition of above temperature of 400℃, pressure of 23MPa, residence time of lmin and excess oxygen coefficientα=3. Raising carbon content in garbage could be helpful to higher conversion efficiency, which proved the catalytic role of activated carbon. Reaction temperature is the key influencing factor.Migration mechanism of elements was investigated during garbage oxidation in supercritical water. With the increase of temperature, yield of carbon in gas products got higher, while that in solid products got lower and that in liquid products kept almost constant. There was nearly no NO, NO2, NH3, HCN production and part of nitrogen element was converted to N2O. About 5%-30% of sulfur element was kept in liquid phase in the form of sulfate, and no SO2 and SO3 were found in gas products. Adding activated carbon could not only promote the oxidation but also reduce N2O production.Dynamics of organic matters oxdation in SCW was derived. The relationship of the SCW P-V-T was described in equation of state.The paper presented and analyzed the technology of energy conversion in supercritical water oxidation. The prospect and industrialization were dicussed. Energy can be recovered to satisfy energy supply of reaction or used to heat supply. And it could reduce treatment cost of the technology. The process can be used widely in regions of low pollution and high-grade energy required, especially applicable in regional heat supply in cities and dealing with wastewater or rubbish simultaneously. Technology of energy transformation in supercritical water oxidation is applicable for various fuels, such as coal powder, biomass, industrial organic wastes, organic waste water and so on. During the process, no NOx, SOx and dioxin produced. CO2 with high concentration could be separated and recovered. Transpiring wall reactor was designed to solve corrosion and precipitation which were main problems in supercritical water oxidation.Based on mass balance equation and energy balance equation, in energy transform of SCWO the effective energy efficiency was 56.4%. Dealing with waste water(COD=40000mg/L), the cost is 33yuan/t. The cost of oxygen is 71.8%. Improving the process can reduce cost.Supercritical water pyrolysis and oxidation is a new technology to deal with wastes for utilization and energy recovery. It has drawn much attention at home and abroad. Researches on basis and application were carried out and some results were obtained. But more researches such as catalytic oxidation, characteristics and roles of supercritical water, industrialization etc. should be done. Reaction mechanism, dynamics, thermodynamics and optimization of energy utilization were still not distinct and a great deal of basic data is needed. |
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