| With the rapid growth in municipal solid waste(MSW)generation and the leading energy and environmental issues,waste management has been facing increasingly severe challenges,of which the traditional emphasis on MSW end-of-pipe treatment can no longer meet the needs of sustainable development;waste management should be guadually shifted to the whole process management.This requires not only technological innovation to achieve cleaner and more efficient MSW treatment,but also integrated management for energy-environment-economy multi-objective optimization of the whole process.Currently,thermal treatment has become an important approach for the energy utilization of MSW,especially,compared to traditional MSW direct incineration,pyrolysis and gasification has received extensive concerns due to the advantages of dioxins reducation and heavy metals control as well as the potential to achieve a higher energy recovery efficiency.However,the development of MSW pyrolysis and gasification in China is severely constrained due to the lack of basic and application researches,thermal characteristics and the pollutants migration and transfer mechanism under controlled atmosphere is still unclear.In addition,the existing waste management assessment methods also exhibit shortcomings in methodological development and practical application.In view of the facts,the goal of the present study is to develop an efficient and clean MSW energy utilization approach.Different units in entire waste management system,including pyrolysis/gasification thermal conversion module,emission control module,source pre-treatment and quality upgrade module,and products efficient-utilization module,are investigated via both experimental and evaluation methods.In terms of thermal conversion module,pyrolysis and gasification of mixed MSW are investigated in a fluidized bed.Results reveal that a rising equivalence ratio(ER)increases the syngas yield,but the gas quality is degraded as a result of the continuously enhanced oxidation reactions.A higher temperature favors pyrolysis reactions but causes an evident decrease in the syngas heating value during gasification.The increase of the MSW moisture content is effective for stimulating H2 production;nevertheless the energy conversion efficiency declines.The optimal operating performance is obtained at ER of 0.4,temperature of 650?,and MSW moisture content around 20-25%.In terms of emission control module,both the partitioning of heavy metals and dioxin formation during pyrolysis,gasification and incineration of MSW are experimentally investigated.It is found that a reductive atmosphere favors the evaporation of Cd and Zn but refrains Cu,Ni,and Cr volatilization,since metals are mainly reduced to their elemental form or sulfide.Most of the vaporized metals are transferred to the cyclone by an evaporation and condensation mechanism;but entrainment is also a determining factor for less-volatile metals,especially considering the high gas velocity in fluidized bed reactor.On the other hand,results also reveal that a reductive atmosphere effectively reduces the formation of dioxins.At ER of 0.6,the dioxin generation in the gas phase and fly ash reaches a bottom value at 3.04 ng/g-MSW and 0.97 ng/g-MSW,respectively.The total dioxin generation under pyrolysis and gasification conditions can reaches an order of magnitude lower than incineration conditions.In order to evaluate the waste management system a systematically and to search for optimization measurement,a life cycle Energy-Environment-Economy(3E)multi-objective assessment model is established.The model is based on the traditional life cycle 2E(energy and environment)assessment method,and life cycle cost is adjusted to evaluate economic performance.Afterwards,multi-criteria decision making is improved to integrate 3E factors.Besides,a two-step weight factor analysis is added to adopt different preferences proposed by different stakeholder groups to test the robustness of the model.This novel 3E model is then applied for the comparison of current MSW treatment technologies.Results show a highly credibilityand feasiblity of the model.The source pre-treatment and quality upgrade module and products efficient-utilization module are assessed.Results from source pre-treatment and quality upgrade module shows that recycling activities especially plastic not only allow the reduction of virgin materials production,but also reduce the Cl content of MSW to decrease acid gas emissions.The separation of food waste is effective to increase the heating value of MSW,thus to increase the energy recovery potential.On the other hand,seven integrated systems coupled with different thermal conversion processes and different products utilization approaches are assessed.Results show that compared to direct waste incineration,pyrolysis and gasification exhibits lower environmental impacts due to the decrease of NOx,HCl and SO2.Besides,syngas generated from pyrolysis and gasification also show potential to be utilized in gas turbine and gas engine,and the life cycle energy conversion efficiency could be increased significantly.Gasification scenarios exhibit better energy and environmental performance than pyrolysis scenarios,and "two-step combustion",i.e.,gasification and then used by steam turbine,is particularly recommended.Coupling the results from pyrolysis/gasification thermal conversion module,emission control module,source pre-treatment and quality upgrade module,and products efficient-utilization module,an innovative MSW thermal treatment option based on pyrolysis and gasification is put forward.A pilot-scale test platform is constructed and verification test is conducted.Results show that under gasification test condition,the concentration of conventional pollutants and heavy metals can meet the emission standards;the concentration of dioxins in flue gas is only 0.19 ng I-TEQ/Nm3 before purification device,which is significantly lower than incineration condition.Leaching toxicity of heavy metals in both bottom ash and fly ash are in line with national limit standards.Based on the pilot-scale environmental data,a life cycle 3E comparison is conducted between the gasification system and the traditional MSW direct incineration system,with result revealing that the former one is superior in all the energy,environmental and economy factors.Overall,the innovative MSW thermal treatment option can be utilized as an alternative to substitute the traditional waste direct incineration,in order to achieve clean and efficient energy utilization of MSW.The present study is useful to be served as theoretical basis and practical experience to facilitate its further development in China. |
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