Design And Simulation Research Of Waste Heat Recovery System In Waste-to-energy Plant

In recent years,as the development of living standard,the amount of municipal solid waste（MSW）increases quickly.Combustion become the major method to deal with MSW because it can realize waste to energy.However,the energy efficiency of waste-to-energy plant（WTE）is limited and the economic cost is high.One of the most important reasons is that there is a large amount of energy loss in WTE caused by the direct heat release of flue gas and MSW incineration bottom ash.Therefore,based on the principle of temperature matching and cascade utilization,proper waste heat recovery cascade systems are proposed,which consider both the heat source and the advantages of different waste heat recovery technology in different temperature ranges.This research can provide certain positive meaning to improve the energy efficiency and economic performance of WTE.First of all,in order to treat the problem of mismatching in energy utilization and heat loss of low-temperature exhaust gas,a novel system consisted of organic Rankine cycle（ORC）and heat pump cycle is designed with the regard of energy requirement.Thermodynamic analysis and exergoeconomic analysis are performed to study the thermodynamic and economic performance of the WTE plant integrated with the waste heat recovery system.The results show that the total electrical efficiency of the WTE increases to 28.4%,with an improvement of 6.5%when the high-grade energy is used to generate electricity.The payback period of the combined system is approximate to 0.5-0.6 years.The economic analysis demonstrates that the combined system has a good economic performance.Besides,very few studies are done on the feasibility of recovering the sensible heat of high-temperature bottom ash solid particles which come from waste incineration.In order to realize the cascade recovery of high-temperature solid particles and low-temperature flue gas waste heat,a combined power and refrigeration system is innovatively proposed,which is consisted of a steam Rankine cycle（RC）,an ORC,and a lithium bromide absorption refrigeration cycle（ARC）.A detailed modeling and analysis of the system is conducted.The net present value（NPV）method is used to evaluate the benefits and dynamic payback period（DDP）of the WTE after adding the proposed system.Based on the sustainability factor（SI）and ecological efficiency coefficient（EE）,the environmental performance of the WTE is studied.Results show that,by recovering the waste heat of high-temperature solid particles and low-temperature flue gas,the energy efficiency and exergy efficiency of the system increase by37.66%and 35.65%,respectively.From the perspective of economy,the NPV of the WTE increase by 12.91 M$and the DDP decrease by 4.79 years.Besides,the EE of the system increase by 11.28%.Finally,because RC system causes a large heat loss in waste heat recover from high temperature solid particles,the supercritical CO₂ Brayton cycle（S-CO₂）is introduced as its superiority in high temperature waste heat recovery.Therefore,the S-CO₂ system is adopted to harness the waste heat of high-temperature solid particle instead of the RC system and district heating is added to the combined system.The novel combine system can realize the cogeneration of cooling,heating,and power.Results reveal that the energy efficiency and exergy efficiency of the WTE integrated with the modified system have increased by 16.92%and16.02%,respectively.The results of economic analysis show that the maximum NPV of the WTE has increased by 10.75 M$and the DDP has been shortened by0.68 years.Sensitivity analysis is also carried out on the system to determine the influence of parameters on system performance.Based on the sensitivity analysis results,NSGA-Ⅱ is used for multi-objective optimization.In conclusion,recovering the waste heat of the WTE can not only improve the energy efficiency of the system,but also enhance the economic and environmental performance.Because there exist various ranges temperature waste heat in the WTE,designing proper energy cascade recovery system by taking the advantages of each waste heat recovery technology can significantly heighten waste heat recovery rate and reduce the exergy loss of heat transfer process.