| With rapid development of industrialization and urbanization,China's sludge production has increased significantly with the increase of sewage treatment facilities.However,current sludge treatment capacity of China is not sufficient yet,the situation of sludge disposal is severe.The traditional methods of sludge disposal in China have certain limitations,the level of resource utilization is not high,and there is a risk of secondary pollution.With increasingly stringent environmental standards and increasing requirements for energy recovery and utilization,it is of great significance to explore and develop technologies that make sludge harmless and resource utilization.In this paper,the characteristics of thermal plasma in the harmless treatment and resource utilization of sludge are studied.The main research contents and conclusions are as follows:1.Multiphysics numerical simulation of thermal plasma was carried out based on the finite element analysis method.Based on the establishment of the transferred arc thermal plasma model,the effects of argon,helium,nitrogen,air,carbon dioxide on the basic characteristics of thermal plasma were studied.The influence of working parameters,electrode shape and nozzle structure on the characteristics of thermal plasma were also studied.Furthermore,the non-transfered arc plasma and jet model was established for studying the influence of working parameters on the distribution characteristics of temperature field.Based on the simulation results,gas selection suitable for thermal plasma treatment of sludge,parameter design of plasma torch were discussed,and the design of thermal plasma sludge treatment reactor was carried out.2.The experimental study on the basic characteristics of thermal plasma was carried out,mainly including volt-ampere characteristics and its influencing factors,emission spectroscopy diagnosis and jet characteristics research.The experimental results showed that thermal plasma exhibited the volt-ampere characteristic of a typical arc discharge.Therefore,the thermal plasma output power can be increased by increasing the current or increasing the gas flow rate.The study also founded that for the same current and gas flow,the corresponding voltage and power were:carbon dioxide>nitrogen>argon.The results of thermal plasma emission spectroscopy showed that nitrogen and carbon dioxide as working gases had higher temperature and electron density than argon.At 100 A and 10 L/min,core temperature of nitrogen and carbon dioxide thermal plasma jets can reach above 10 000K,electron density can be up to 1023 m-3.That was,thermal plasma reached local thermodynamic equilibrium state.The influence parameters of thermal plasma jet shape showed that,under the same condition,nitrogen and carbon dioxide had longer thermal plasma jets than argon.At100 A and 5 L/min,nitrogen and carbon oxide jets can be up to 35 cm,which maintained stable and long jets.The operating parameters such as current and gas flow can effectively adjust the shape of the thermal plasma jet.3.Experimental study of thermal plasma treatment of sludge.Thermal plasma treatments of sludge samples were carried out by using transferred arc thermal plasma and non-transferred arc thermal plasma,respectively.The effects of moisture content,working gas and current on gas products and basic characteristics of solid residue after thermal plasma treatment were investigated.When transferred arc thermal plasma was used to treat coking plant sludge with carbon dioxide as working gas,gas components were mainly CO and H2,and no complex gas compounds were detected.The calorific value was 8.43 MJ/m3,which had the value of energy recycling.Further,non-transferred arc thermal plasma was used to treat sewage sludge,the calorific value of gas products can reach 5.10 MJ/m3 when nitrogen was used as working gas,while calorific value can reach 9.20 MJ/m3 when carbon dioxide was used as working gas.It is noted that thermal plasma can effectively treat sludge with high moisture content,and the increase of moisture content can effectively promote the formation of syngas and increase the ratio of H2/CO.When the water content increased to65%,H2/CO reached 2.0,which can expand the range of the syngas resource utilization.By comparison of the gas product characteristics of the two thermal plasmas,both of them had practical engineering application value.However,the transferred arc thermal plasma was subject to some restrictions in practical applications,non-transferred arc plasma had the advantage of simple operation,less working gas flow rate,flexible and controllable working parameters,applicability of sludge treatment with high moisture content,which meaned more practical application value in the actual process.The analysis of solid residue after thermal plasma treatment showed that the heavy metal leaching concentration of the solid residues were much lower than“Identification of hazardous waste leaching toxic identification standard"?GB 5085.3-2007?.The addition of silicon dioxide helped to reduce the heavy metal leaching concentration of the solid residue.The solid residues after treatment were in an amorphous structure,and were dense and smooth in microscopic surface.The specific surface area was significantly reduced,and the density was significantly increased after treatment.The specific surface areas of sewage sludge solid residue and coking sludge solid residue were reduced to 12.90%and40.86%,respectively,and the densities were increased to 2.33 and 1.65 times,respectively.The capacity reduction rates were 57.08%and 39.40%,respectively.The above results show that thermal plasma technology can effectively realize the harmless treatment and energy utilization,and has obvious volume reduction effect.Thermal plasma can effectively treat sludge with high water content,and can expand the scope of utilization of syngas resources,providing a new way for sludge disposal. |
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