Research On Heat Transfer And Reaction Coupling During Waste Tire Pyrolysis In Rotary Klin And Optimization Of Reactor

With the increasing production of waste tire,it has brought many environmental and safety problems.It is urgent to properly dispose of them.Pyrolysis is an efficient waste tire resource treatment technology.The commonly used pyrolysis reactor is rotary kiln,which has attracted much attention because of its large processing capacity and strong raw material adaptability.However,the current design of rotary kiln mainly relies on production practice and empirical formula.Due to the lack of in-depth research on the coupling and matching relationship between heat transfer and reaction in the reactor,lead to the reactor design is unreasonable,and the heat transfer and reaction in the pyrolysis process do not match.There are disadvantages such as huge cracking equipment and high process energy consumption.In view of the above problems,this paper studied the heat transfer characteristics of waste tire pyrolysis,and obtained the law of thermal resistance change during pyrolysis process.A pilot plant of 100-ton rotary kiln was built to analyze and experimentally study the heat transfer process of waste tire pyrolysis in the reactor,and the experimental correlation formula of comprehensive heat transfer coefficient was obtained.The movement and heat transfer process of waste tire particles in the rotary kiln reactor were studied by numerical simulation,and the pyrolysis reaction kinetic equation was coupled to establish the coupling relationship model between heat transfer and reaction of waste tire pyrolysis in rotary kiln.On this basis,the relationship between the cracking degree of waste tire and the size of the reactor was studied,and the optimal design criterion of the reactor was formed.The structure of the 10,000-ton rotary kiln was optimized to reduce the size of the pyrolysis equipment and improve the heat utilization efficiency of the reactor.Due to the poor thermal conductivity of waste tire,the speed of heat transfer controls the reaction rate of the whole pyrolysis process.Based on this,the heat transfer characteristics of waste tire pyrolysis process were investigated through theoretical analysis and experimental study.The heat transfer process of transferring external heat to particles was divided into three main heat transfer links:heat transfer between reactor wall and material bed,heat transfer in the material bed and heat transfer in the particle.Thermal resistance analysis and heat transfer measurement experiment were carried out respectively.The results showed that the heat transfer between the reactor wall and the particles is mainly affected by the thickness of the ash layer.When the thickness of the ash layer increases from 2 mm to 14 mm,the thermal resistance between the reactor wall and the particles increases by 8 times,and the material heating rate is only 0.05℃/s,indicating that the heat transfer from the reactor wall to the inside of the material bed has been less.There are many factors affecting the heat transfer process in the material bed,among which the temperature of the external heat source has the greatest influence.When the temperature rises from 400℃to 600℃,the thermal resistance in the material bed decreases with the increase of the temperature.When the temperature reaches 500℃,the thermal resistance in the material bed decreases significantly by 46%,and the thermal resistance in the material bed is no longer significantly reduced when the temperature is increased.The particle size of waste tire is the main factor that affects the heat transfer in the particles.The larger the particle size,the greater the thermal resistance in the particles,and the lower the heating rate at the center of the particle.When the particle size increases from 4 mm to 20 mm,the thermal resistance inside the particle has increased by 14 times,and the heating rate of the center of the particle was only 0.06℃/s,indicating that it is difficult for the external heat to transfer to the center of the particle.According to the thermal resistance analysis results of waste tire pyrolysis heat transfer process,when the heat source temperature is 500℃,the particle size of waste tire is less than 20 mm,and the ash layer thickness of reactor wall is less than 14 mm,the thermal resistance in the material bed>>the thermal resistance in the particle>the thermal resistance between reactor wall and material bed,and the thermal resistance in the material bed accounts for more than 80%of the total thermal resistance.In order to study the heat transfer process in the material bed during the pyrolysis of waste tire in rotary kiln,an experimental device of 100-ton rotary kiln was built in this paper,and the temperature change law of the material entering the reactor under different working conditions was investigated.The results show that the rotary kiln wall temperature has the greatest influence on the initial heating rate of materials after entering the reactor.With the increase of the rotary kiln wall temperature from 400℃ to 600℃,the greater the temperature difference with the material,the greater the initial heating rate of materials after entering the reactor.When the wall temperature reaches 500℃,the initial heating rate of materials increases by 2 times,and the initial heating rate does not increase obviously when the wall temperature is further increased.The dimensional analysis method is used to analyze the relevant physical quantities in the process of waste tire pyrolysis in rotary kiln,and then the dimensionless quantities and other basic quantities were obtained by using the principle of dimensional harmony.The dimensionless quantities were arranged into the power exponent function form commonly used in the experimental correlation formula in heat transfer.The comprehensive heat transfer coefficient experimental correlation((?))for the waste tire pyrolysis in rotary kiln in the temperature range of 400-600 ℃is obtained.Based on the results of temperature change during the waste tire pyrolysis in rotary kiln under different working conditions,a database was established to analyze the function relationship,and the specific comprehensive heat transfer coefficient experimental correlation formula((?)(?))was obtained.Based on the analysis of the movement and heat transfer process of waste tire particles in rotary kiln reactor,the coupling relationship model between heat transfer and reaction of waste tire pyrolysis in rotary kiln was established by introducing the experimental correlation equation of comprehensive heat transfer coefficient and coupling the pyrolysis reaction kinetic equation.The experimental verification of the established model was carried out on the 100-ton pilot plant and the 10,000-ton demonstration plant.The results showed that the experimental results of temperature in the pilot plant agree well with the calculated results with the maximum error of 5.5%,but the error of temperature in the demonstration plant is more than 17.0%,indicating that there is significant amplification effect in the process of reactor amplification.Numerical simulation method was used to study the enlargement law of the reactor with different scales.It was found that with the increase of the reactor scale,there was a non-linear relationship between the heat transfer area of the material bed in the reactor and the enlargement factor,resulting in a large error in the temperature results calculated by the model.The maximum temperature difference in the material bed increased from 27℃ to 54℃ when the scale of the reactor was enlarged from 100 to 10,000 tons.The model was modified according to the results of experiment and simulation.The temperature difference between the modified model calculation result and the experimental result of temperature measured in the demonstration plant was less than 23℃,and the error was less than 4.8%.Based on the modified model,the heat and mass transfer process of waste tire pyrolysis in rotary kiln was analyzed.According to the relationship between the pyrolysis conversion rate of the waste tire and the reaction time,the analysis and evaluation method of the pyrolysis degree was established.The influence of the pyrolysis temperature and reaction time on the pyrolysis degree of the waste tire was investigated.The results show that the pyrolysis temperature has the greatest influence on the pyrolysis degree of the waste tire,and the waste tire can be fully cracked only when the temperature reaches 500℃ and the appropriate reaction time is maintained.At the pyrolysis temperature of 500℃,when the reaction time reaches 40 min,the pyrolysis degree of waste tire reaches 91%,which has been basically completely cracked.In the rotary kiln pyrolysis reactor,the reactor length mainly affects the reaction time of waste tire pyrolysis and then the pyrolysis degree,while the diameter mainly affects the reactor capacity.Through experimental research and characterization,the relationship between waste tire pyrolysis degree and reaction time was obtained φ=102.3-99.8/(1+t/23.5)4.6,and the relationship between reactor diameter and processing capacity was obtained G=181.82D2.74.The effective length of the reactor required for the waste tire to reach a specific degree of pyrolysis could be designed and calculated L=dωt,the effective diameter of the reactor could be designed and calculated within the prescribed treatment capacity.On this basis,the reactor optimization design guidelines were formed,and specific technical solutions were proposed for the structural optimization design of the 10,000-ton waste tire rotary kiln pyrolysis reactor.On the premise of ensuring sufficient pyrolysis of waste tire and the reactor processing capacity.The unit volume capacity of the 10,000-ton rotary kiln pyrolysis reactor will increase by 2.3 times.