Thermalanalysis And Gas Purification During The Pyrolysis Of Municipal Sewage Sludge

With the increase of annual sewage sludge output and environmental standard,the problem of treatment and disposal of sludge is becoming increasingly serious.The pyrolysis technology of sludge has many advantages,such as high energy efficiency,without dioxins,high heat value production,so it has been favored by researchers around the world recently.To solve the problem of the disposal of mechanical dewatering sludge cake,systematic study was conducted on products features,kinetic analysis,pyrolysis mechanism,heat-mass transfer characteristics,energy balance,carbon emissions and the purification of pyrolysis gas by experimental and numerical methods.（1）Experimental study on sludge cake pyrolysis was carried out using tube furnace pyrolyzer.The effects of process parameters on pyrolysis characteristics were analyzed,and the different products were characterized and discussed.Sludge pyrolysis is mainly the decomposition of phenols,ketones,aliphatic and other organic maters.The pyrolysis temperature for the purpose of pyrolysis gas should be above 800℃.The optimum sludge particle size range and water content are<10 mm and 20%,respectively.Pyrolysis gas contains high combustible components such as:H₂,CH₄,CO,C₂H₄,etc.,and their total content exceed 60%.Pyrolysis oil is complex and it contains a certain amount of polycyclic aromatic hydrocarbons（PAHs）;Fe₂O₃ can be added to promote gas production,and it will also increase the PAHs content in pyrolysis oil.However,there is no obvious promoting effect on the content of combustible gas components.（2）The thermogravimetric analyzer was used to study the pyrolysis characteristics of sludge,sawdust and their mixture.By using Coats-Redfern method,Flynn-Wall-Ozawa method,Kissinger-Akahira-Sunose method and double extrapolation method,the kinetic parameters and the most probable mechanism function of pyrolysis were analyzed.The increase of the sawdust ratio enhances the characteristics of volatilization.The addition of a small amount of sawdust resulted in a certain synergistic effect.It weakened and turned into inhibitory effect with increase of the addition proportion of sawdust.The Nucleation-Growth model with higher order reaction series can describe the pyrolysis process of samples.There is a dynamic compensation effect between apparent activation energy and frequency factor at different heating rates.（3）By considering the effect of sawdust and moisture content,the heat-mass transfer model in pyrolysis process was established by coupling the reaction kinetic equation with the energy-mass balance equations.The calculation results are in good agreement with experimental results,and the model has a higher accuracy for sludge particles within 10mm.The increase of pyrolysis temperature will increase the rate of temperature rise and weightlessness,and the peak value is ahead of schedule.However,the decrease of sample size will reduce the maximum weightlessness rate.The increase of water content will sharply prolong the pyrolysis time,increase the internal temperature gradient and decrease the maximum weightlessness rate.（4）The complete process of dehydration-desiccation-pyrolysis of wet sludge was improved,and the energy balance and carbon emissions in the sludge pyrolysis stage were analyzed.The sensible heat of water analysis is much greater than that of the product.When the heat utilization rate reaches 75%,the chemical heat generated by the pyrolysis gas can maintain the minimum pyrolysis energy required.The total amount of carbon emissions in the mechanical dewatering-drying stage and the pyrolysis stage are 102.98and 168.47 kg/t wet sludge,respectively.The carbon emission during the pyrolysis stage is58.87 kg/t wet sewage.（5）Corona discharge experiments of sludge pyrolysis gas and ESP collection experiments of pyrolysis fly ash were carried out,besides,the transient numerical simulation of the possible explosion process was simulated by using dynamic grid numerical technique.Stable corona can be established in sludge pyrolysis gas.The increase of H₂ and CH₄ content will increase corona current and shorten working voltage range.The collection efficiency of pyrolysis fly ash exceed 95%.The venting time increases linearly with the internal pressure,and decreases nonlinearly with the increase of valve cover stroke.But it will be significantly prolonged with the decrease of medium temperature.Pyrolysis technology can efficiently realize sludge resource utilization,and the research on pyrolysis heat transfer model,process analysis and dust removal technology can provide theoretical basis for design of pyrolysis equipment and system.