Design And Technical Economic Analysis On Heat Supply System By Supercritical Water Oxidation Energy Conversion

At present,mankind is confronted with a severe trail caused by energy crisis and environmental pollution.More attention is paid for the treatment and reuse of organic waste.At the same time,the requirements of pollutant discharge are stringent increasingly.The technology of supercritical water oxidation,SCWO for short,is presented at this background.During the SCWO process,the organic compounds react completely with the oxidant-mostly oxygen-in a single-phase reaction forming CO₂ and H₂O;Organic bounded nitrogen predominately forms N₂ and small amounts of N₂O;The hetero-atoms chlorine,sulfur,or phosphorus presenting in the organic wastes are transformed into the mineral acids respectively.Reaction time,at which a complete conversion is observed,seldom exceed some minutes.Energy is generated in the process,and the reaction could be self-sufficient at a certain organic concentration. This technology is suitable for organic waste treatment which is toxic,hazardous and high concentration of refractory organic matters particularly.Using of transpiring water reactor is an important way for reducing the risk from corrosion and salt-plugging.In addition,the recovery of energy and CO₂,which are generated in the SCWO process,can not only improve the economy of organic waste treatment,but also realize the reuse of the product from waste treatment.A system which recoveries the reaction energy for heat supply is constructed, referring to the other system in existence and combining with the purpose of our own subjects.The system has a capacity of 20L/h,and the type of reactor is Transpiring Water Reactor.The simulative organic wastewater is 2-10wt%glucose,temperature at the inlet of the reactor is 300-350℃;Pressure of the system is 23-30MPa;the residence time of the reaction is 30-150s;transpiring water is divided into three parts,their temperature at the inlet of the reactor is 350/150/20℃separately,and the proportion of the flow is 4:3:3;the oxidant is oxygen,and the excess ratio of the oxygen is 1-3; after the reaction,temperature at the outlet of the reactor is 300℃.The design of the system is based on the mass and energy balance of the whole system.The main job concludes the structural design and selection of the reactor,pump,heat exchanger, electric heater,valves,measuring devices,energy recovery devices,as well as other parts of the system.Economic analysis of supercritical water oxidation system which has capacity of 300m³/d and contains transpiring water reactor is presented,and the analysis was based on mass-energy balance equations and combined with the theory of the economic analysis.The result shows that the rate of energy utilizable for the system can reach 56.4%and the cost for organic waste treatment is around 33.05 Yuan/t,the cost of oxygen is 71.8%for the whole cost.If the process could be improved,the cost of oxygen could be reduced or a new oxidant and using methods could be found,the operating cost of the system must be reduced significantly.For a given supercritical water oxidation systems,among all the operating parameters,the most obvious factor for the cost of wastewater treatment is oxygen excess ratio.As the oxygen excess ratio increases,the rate of energy utilizable for the system decline and the cost grows up rapidly;The second factor is the concentration of organic compounds in waste water,the rate of energy utilizable and cost of wastewater treatment both increase with the a higher COD values of the waste water, but cost of COD unit declines.When the capacity of the system becomes bigger,the rate of energy utilizable of system will not change,but the cost of wastewater treatment reduces.As the pressure of the system raises,the cost of wastewater treatment will also grows up.Lengthening of residence time will require larger size of the devices,thus the capital input of device must be needed more.In a word,in the process of organic waste treatment using SCWO technology,we are preferred low excess;oxygen ratio,low pressure and short residence time under the premise of COD removal rate.The cost of organic waste treatment will reduce when dealing with large flow and high concentration waste.Energy and CO₂ generated in the process of organic waste treatment using SCWO technology can be recovered.In this way,the economy of the process could be improved,and the emission of greenhouse gas also be reduced.SCWO technology is economically viable and has environmental benefit;it must have a bright future in the field of organic waste treatment.