| Because of large quantity and wide distribution,the disposal of sludge is a typical environmental problem in China.And The implementation of national ecological civilization strategy must deal with the problem.Current disposal technologies,such as landfill,composting,incineration,pyrolysis and building materials,are constrained by cost or heavy metal problems.Therefore,it is urgent to develop the technology to make sludge value-added utilization.Value-added utilization of sludge can more effectively promote environmental protection,as well as social green,low-carbon and recycling development.Because pyrolysis can produce sludge-based biochar which can fix carbon and heavy metals,and can be used for adsorption,catalysis or soil improvement.Thus,it is considered as a potential way to realize the value-added utilization of sludge.However,it is unable for current pyrolysis technology to improve the performance of biochar,stabilize heavy metals and effectively control energy consumption at the same time.This Ph.D.project proposed a technical approach of the stepwise microwave synergistic pyrolysis（SMSP）to dispose of sewage sludge.The SMSP approach includes a pre-pyrolytic stage under conventional heating and a sequential microwave heating stage.The purpose of the pre-pyrolytic stage is to alter temperature rising behavior of sludge feedstock under microwave irradiation,which is done by partially carbonizing them to obtain pre-pyrolytic sludge（PPS）biochars with optimum dielectric property.In principle,either insufficient or excessive carbonization should be avoided at pre-pyrolytic stage.The purpose of the subsequent microwave heating stage is to rapidly heat up the PPS biochar to solidify heavy metals via vitrification and simultaneously improve its microstructure.The high-energy site effect caused by the interaction between microwave and PPS biochars would play a significant role.No extra microwave absorbers need to be added in the SMSP approach.The approach can take full advantage of the selectivity,rapidity and integrity of microwave heating,and improve the overall energy efficiency through the efficient thermal conversion of microwave energy.This project can open a superior treatment route for the value-adding of sewage sludge,which can collectively realize the quality upgrading,heavy metal solidification and energy penalty control.In this paper,experimental methods were adopted.Research ideas and technical routes were conducted from feasibility analysis to mechanism study,and then to process energy consumption analysis as well as application potential exploration of biochar products.The key influencing factors and corresponding mechanism of the SMSP approach of sludge were systematically discussed and studied.The main contents and results are as follows:First,the feasibility and advantages of the stepwise microwave synergistic pyrolysis（SMSP）approach was studied,which was conducted by comparing the specific surface area and the solidification effect of heavy metals of sludge-based biochar with conventional pyrolysis route.The results show that the pore structure and the solidification effect of heavy metals of sludge-based biochar produced from the SMSP approach（SC-SMSP）are all good,and the SMSP approach is feasible.The specific surface area of the optimum SC-SMSP is about 65 m2/g,which is about 30%higher than that of the single conventional pyrolysis and about 150%higher than that of the single microwave pyrolysis.And the SC-SMSP has advantages of enhancing solidification effect of heavy metals and reducing the leaching ability.The residual rate of heavy metals in conventional pyrolysis biochar is about 40-75%,and that of microwave pyrolysis biochar is about 40-50%;while the optimum SC-SMSP has a heavy metal residue of about 80%,and the leaching amount of heavy metals is the minimum.By controlling and reducing the temperature,time or microwave power,the SMSP approach has ability to control and reduce the overall energy consumption.Based on the feasibility of the SMSP approach,the evolution of the dielectric properties and the temperature rising behavior under microwave irradiation of sludge with different carbonization levels were studied,which was tried to find the optimum matching conditions of the SMSP approach.The results show that the optimized parameters of the temperature and time,microwave irradiation power and time are as follows:the pre-pyrolysis temperature is about 600-700℃,the pre-pyrolysis time is about 5-20 min,the microwave irradiation power is about 700-1000w,and the microwave irradiation time is about 3-6 min.The pre-pyrolysis temperature is a key factor,which affects the dielectric properties and temperature rising behavior of sludge under microwave irradiation.The higher pre-pyrolysis temperature,the higher dielectric properties and temperature rising behavior.The order of influencing factors are pre-pyrolysis temperature>power and time of microwave irradiation>pre-pyrolysis time and weight.In order to obtain pre-pyrolytic sludge（PPS）biochars with optimum temperature rising behavior,pre-pyrolysis temperature of about 600℃is the key guarantee condition.The mechanism of carbonization level on the dielectric properties of sludge is mainly due to the fact that the chemical composition and microstructure of the PPS biochar can be obviously changed by the pyrolysis temperature.Based on the optimum matching conditions of dielectric property and temperature rising behavior of the SMSP approach,the matching conditions of the SMSP approach were screened from the perspective of optimizing the physical and chemical qualities of biochar products（SC-SMSP）.The results show that here the optimum matching parameters of the SMSP approach are as follows:pre-pyrolysis temperature is about 700℃,pre-pyrolysis time is about 10-15 min,microwave irradiation power is about 700-900W,and microwave irradiation time is about 4-5 min.The order of influencing factors to biochar qualities are pre-pyrolysis temperature>power and time of microwave irradiation>pre-pyrolysis time and weight.When pre-pyrolysis temperature increased from 400℃ to 700 ℃,the carbonization level and stability of SC-SMSP were significantly improved,in which the molar ratio H/C decreased by about 62%,the fixed carbon content increased by about 75%,and the pH value increased from about 7.5 to 11.5.The pore structure of SC-SMSP is more developed,in which the specific surface area increases by about 432%,the average pore size is mainly mesoporous and decreases by about 49%,and the total pore volume increases by about 218%.And it is more conducive to change the mineral phase form of SC-SMSP,and generate micro or nano sized vitrified spherical particles and new crystal structure.Compared with the optimum sludge-based biochar obtained by conventional pyrolysis route,the quality of the optimum SC-SMSP is better.It is mainly attributed to the high-energy sites effect induced by microwave irradiation,which is conducive to strengthening the vitrification of ash and the volatilization of volatiles to improve the pore structure.Furthermore,based on above researches of optimum matching conditions of the SMSP approach,matching conditions were further screened from the perspective of optimizing the solidification effect of heavy metals,and heavy metals of Cu,Zn,Ni and Pb in sludge were selected as the research objects.The results show that the optimum matching conditions are as follows:pre-pyrolysis temperature is about 600700℃,pre-pyrolysis time is about 10-20 min,microwave irradiation power is about 700-1000 W,and microwave irradiation time is about 3-6 min.The order of influencing factors to solidification effect of heavy metals are pre-pyrolysis temperature>pre-pyrolysis time>power and time of microwave irradiation>pre-pyrolysis weight.When the pre-pyrolysis temperature increases to about 600-700℃,and the pre-pyrolysis time and microwave irradiation time are shortened as much as possible,it is helpful to obtain SC-SMSP with high residual rate of heavy metals,low leaching rate,high proportion of residual（F4）and low risk.Moreover,compared with the optimum sludge-based biochar obtained by conventional pyrolysis route,the optimum SC-SMSP has better solidification effect of heavy metals.The mechanism is mainly attributed to two aspects:one is the reduction of carbonization time which reduces the volatilization probability of heavy metals and then reduces the loss;the other one is that high-energy effect generated by microwave with enhanced PPS biochars,which leads to micro vitrification of SC-SMSP and reduces the leaching ability of heavy metals.Finally,based on the above results of experimental and mechanism studies,this paper tried to establish the initial stepwise microwave synergistic pyrolysis（SMSP）process,and its energy consumption characteristics were analyzed.The SC-SMSP was also used as a carbon-based catalyst,which aims to explore the practical potential of the SMSP process and biochar product.The results show that the energy consumption difference between the SMSP process and the conventional pyrolysis process is mainly reflected in the microwave induction stage of the SMSP process.At this time,the heat loss of the equipment surface has the potential to reduce significantly with the decrease of pyrolysis time.Compared with the conventional pyrolysis process,the SMSP process has about 20%or more energy-saving potential to produce biochar with similar qualities,and the shorter the microwave induction time,the more significant the energy-saving ability.The project provides a fundamental process for the preparation of cheap and efficient char-based catalysts. |
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