Study Of The Co-Drying Characteristics Of Sewage Sludge Mixed With Biomass

With the quick acceleration of China’s urbanization and the increasing continuously handling capacity of municipal sewage, large quantities of sewage sludge are needed to be disposed safely and efficiently. Thermal drying is an efficient and environmentally friendly method for sludge treatments. Dried sludge can be further land-filled, made into agricultural fertilizers which used in garden industry and building materials, or incinerated directly, to achieve the requirements of reduction, stability, reclamation and harmlessness of sludge safety disposals. Hot air convection drying is a main method of thermal drying of sewage sludge; But in the drying process, sludge is easy to agglomerate and shrink which increases the resistance of internal moisture diffusing to the surface. This fact causes a drying process with a long drying time and large energy consumption.To overcome the technical difficulty thatthe drying shrinkage of sludge increased internal mass transfer resistance, an possible method was proposed in this thesis, to improve the hot air drying performance of sewage sludge by adding porous biomass:The porous biomass were mixed uniformly with the wet sludge to modify the inner construction of sludge, increase its porosity, add its internal water diffusion channels and thus decrease mass transfer resistance. Furthermore, the addition of biomass will increase the heating value of wet sludge which initially contains a low calorific value, to achieve a self-sustained combustion.Aiming to improve the understanding of the co-drying process of the biomass and sludge mixture, this thesis focused on the co-drying characteristics of biomass and sludge mixture, the effect of the operating parameters on its drying performance, establishment of drying mathematical models, and the pyro lysis and incineration characteristics of dried mixture. The main work of this thesis includes:1) The co-drying characteristics of the sewage sludge and biomass mixture. The drying experiments of sludge and biomass mixtures were carried out using the self-made hot air convection drying facility and experimental data was collected/analyzed. It was found that sawdust and bran adding can reduce effectively the sludge drying time. Under the experimental condition, sawdust adding can half the drying time.2) The establishment of the drying mathematics models of the sewage sludge/biomass mixtures. By considering the volume shrinkage, the drying kinematics and dynamical models are established respectively; and it was found that the biomass addition such as sawdust can enhance the diffusion coefficient and reduce the drying activation energy.3) The pyrolysis and combustion characteristics of the sewage sludge/biomass mixture. The pyrolysis and combustion curves of mixture were measured respectively under the nitrogen and air atmosphere using the TA thermogravimetric analyzer and then, the combustion index of mixture were calculated and analyzed. It was found that sewage sludge reduces the ignition temperature of biomass Ti; biomass increases the flammability index Cb、combustion stability index G and combustion index S of the sewage sludge.