Study On Melting Characteristics And Self-Solidifying Mechanism Of Sludge-Rice Husk Co-combustion Ash

China's "13th Five Year national urban sewage treatment and recycling facilities construction plan" clearly put forward "sewage sludge from urban sewage treatment facilities stabilization,decrease decrement,innocuity and resource should be encouragd.”Due to high rate of reduction,processing speed,and destroying all the organic pollutants,sludge incineration disposal had become the mainstream sludge disposal.However,sewage sludge with high water content has a low calorific value,which makes it necessary to accumulate a large amount of fossil fuels to maintain stable combustion.Meanwhile,China's“13th Five Year renewable energy development plan" establishes the research direction of biomass energy.Rice husk as a typical agricultural biomass waste has high calorie and low nitrogen and sulfur content is,so it is widely used in the field of biomass power generation.If the existing incinerator can be used to dispose sludge and rice husk,and make full use of the heat generated during the burning process to generate electricity,it will be an ideal way.It not only solves the problem of sludge disposal,but also reduces energy consumption,while achieving the energy utilization of solid waste.In this paper,the preprocessed sewage sludge and rice husk were studied.The combustion and melting characteristics of sludge-rice husk mixture were studied.The physical and chemical properties of the byproduct co-combustion ash and the migration and transfer of heavy metals were investigated.The reaction mechanism of the self-solidifing/stabilizing of the ash was revealed,the main content and results of research are as follows:(1)The co-firing characteristics of sludge and rice husk were studied by thermal analysis method.The interaction between sludge and rice husk was discussed,and the"Two pseudo-component four-stage semi-global reaction apparent kinetics model"was established.The results show that,the mixture of sludge and rice husk has a low burnout temperature between 490-710 ?,that is,through the low temperature incineration(<900 ?),the complete destruction of combustible,.organic pollutants and pathogenic microorganisms can be reached.In terms of energization,the amount of sludge in the mixture can reach 30%.The kinetic control mechanism of the combustion process is D1?F3?D1?D3.(2)Sewage sludge-rice husk co-combustion ash was prepared at different ashing temperature.The physical and chemical properties of ash were analyzed.The effects of chemical composition and mineral composition on the melting properties of ash were discussed.Combined with ternary phase diagram method and sintering index method,and reference to the conventional slagging performance index,it can be predicted whether the mixture of sludge and rice husk will melt at low temperature.The results show the chemical compositions of co-combustion ash are SiO2,CaO,Al2O3.The main mineral compositions of the ash are amorphous,quartz and gismodine.Known from the microscopic morphology of the ash,the co-combustion ash has the both characteristic of RHA and SSA.The chemical composition and mineral composition of ash are the internal factors that affect the ash melting characteristics.The increase of SiO2 content increases the melting point of the ash,while the increase of CaO,Al2O3 reduces the the melting point.The increase of the vitreous content increases the melting point,while the increase of the crystal content reduces the melting point.Synthesizing the results of the ternary phase diagram method,the sintering index method,and the conventional slagging performance index,we can speculate that the ash did not melt when the mixture of sewage sludge and rice husk combust at the lower temperature.(3)Through the experimental study of the indicators of heavy metals,the migration and transformation of heavy metals were discussed.The synergetic effect between sewage sludge and rice husk on migration and conversion was discussed.And the the ecological risk of of co-combustion ash was evaluated.The results show that the contents of target of heavy metal elements in the co-combustion ash are high,and the total content increases with the increase of initial sludge content.The cumulative leaching concentration of Mn,Zn,Cu in the ashes exceeds "Integrated wastewater discharge standard".Therefore,co-combustion ash(H and W)belongs to class ? general industrial solid waste.The contents of Cr,Fe and Ni in the ash mainly exist in the residual mineral lattice.While the Mn and Zn elements mainly exist as the acid soluble,that is mainly exiting in the form of adsorption/precipitation/coprecipitation.Therefore,the migration ability of Cr,Fe and Ni is weak,and Mn and Zn have strong mobility.The synergistic effect of sludge and rice husk promotes heavy metals remaining in the solid phase.And co-combustion ashes show higher risk to ecological environment.(4)The solidification/stabilization of the ash was carried out.The self-solidification/stabilization mechanism of the co-combustion ash was studied,and the long-term leaching behavior was dicussed.The results show the hydration reaction of the solidified body is different at different reaction stages:the early stage is controlled by the NG process and the interim process is controlled by the D process.The addition of co-combustion ash reduces the diffusion coefficient of slurry and hinders the early hydration.Therefore the addition of co-combustion ash reduces the early compressive strength of the self-curing body.With the increase of curing age,the microstructure of the solidified body is gradually dense,and the ash is involved in the hydration and hardening process,which promotes the formation of stable C-S-H and other hydration products in the self-curing body.The addition of co-combustion improves the compressive strength in the middle and late stages.Compared to the ash,the decrease of acid soluble heavy metals in the self-curing body indicates that the leaching of heavy metals is weakened.However the leaching value of Mn in the self-curing body exceeds "Integrated wastewater discharge standard".Therefore,the self-curing body itself belongs to class II general industrial solid waste.The leaching concentration/long leaching concentration of Ba,Ni and Zn in the organic-inorganic composite solidified body is lower than that of the grondwater standard(?).The long leaching concentration of Mn is higher than that of the groundwater standard(?),but still far below the "Integrated wastewater discharge standard".It is shown that the cement-asphalt composite solidification/stabilization can effectively prevent the migration of heavy metal to the environment in the long-term scale.