| Sludge drying is a prerequisite to realize its harmless resource utilization,but the high energy consumption of drying equipment is an urgent problem to be solved.Indirect heat transfer dryer is currently more commonly used sludge drying equipment,currently facing two problems.First,the sludge viscosity will lead to a decline in the heat transfer coefficient of the dryer,so that the dryer’s processing capacity can not reach the design value.Second,the residual heat in the drying exhaust gas due to the low grade of energy,which is generally not utilized,resulting in a large energy loss.In this paper,a new type of elliptical blade dryer is designed to improve the heat exchange efficiency of the viscous sludge and also it also studies on technology of recovering waste heat from exhaust gas from low temperature heat pipe.for the utilization of waste heat in the drying tail gas.On one hand,the paper has built experimental platforms for intermittent and continuous biaxial elliptical blade dryer to study the drying performance of new dryer,At the same time,a drying model that can describe sludge drying in the new dryer is established.On the other hand,a mass-energy balance model of a sludge drying-incineration project is built to analyze the working datas of drying tail gas.At last,according to the analysis,an industrial experimental study on the recovery of waste heat from tail gas using low-temperature heat pipe is carried out.The paper began with an experimental study on the ability of breaking and selfcleaning capabilities of the elliptical blade dryer developed in this paper for viscous sludge.Compared with other materials,sludge drying viscous problem was an important factor affecting sludge drying.In order to compare the drying characteristics of sludge in elliptical blade dryer and wedge-shaped paddle dryer,an experimental platform of intermittent elliptical blade and paddle dryer was built.It was found that under the same working conditions,the particle size of sludge dried in the elliptical blade dryer was smaller than that of the paddle dryer and the drying rate was faster.Also,the shaft power of the rotor shaft was lower,especially when the moisture content of sludge was in the range of 55%～75%.When the sludge water content decreased into the viscous zone,the fluctuation of the shaft power of the elliptical blade dryer was more obvious,meaning that the reaction force of the sludge on the blades was greater.The phenomenon indicated that the elliptical blade had higher sludge crushing and selfcleaning ability compared with the wedge-shaped paddle.In order to prove the heat transfer mechanism and design method of the elliptical blade dryer,a drying heat transfer model suitable for the elliptical blade dryer was established.Parameters such as the mixing number of the drying heat transfer model were determined through experiments.Comparison of the experimental and simulation results showed that the established model can simulate the drying process of sludge in the intermittent elliptical blade dryer well.The effects on sludge drying of the two important parameters,heat source temperature and shaft rate were analyzed by the model.Increasing the heat source temperature could effectively accelerate the drying rate of sludge,while increasing the shaft rate had less effect on the sludge drying process.In order to study sludge flow in a real continuous dryer,a two-dimensional Markov chain model was developed to simulate the flow characteristics of sludge in a continuous biaxial elliptical blade dryer.At the same time,sludge drying experiments were carried out with potassium iodide(KI)as a tracer for validation.By comparing the actual residence time distribution density curves(RTD curves)with the simulated RTD curves,it can be seen that the simulated RTD curves matched the actual RTD curves well,which proved the validity of the model.The effects of three important parameters including axial recirculation coefficient,transverse recirculation coefficient and sludge residence time on the drying process were investigated.It was found that the recirculation coefficient had a great effect on sludge flow but a small effect on the residence time of sludge in the dryer.Also,the sludge residence time had a small effect on the sludge flow.The Markov chain model coupled with penetration model to study the flow and heat transfer in a continuous elliptical blade dryer,and the influence laws of working condition parameters on the sludge drying process were explored.The results showed that the higher the heat source temperature of the dryer,the faster its drying rate and the lower the water content along the dryer was.The dryer rotary rate had little influence on the sludge water content along the dryer.The increase of sludge flow rate lead to the increase of outlet sludge water content.When overflow height increased,the outlet sludge water content decreased.The drying of sludge was more sensitive to the tilting angle of the dryer.When the tilting angle of the dryer decreased,the effect of sludge drying was better and the sludge water content along the dryer was lower.In addition,the drying experiments and simulation results carried out in the wedge blade and elliptical blade dryer under the same working condition showed that the drying rate per unit area of the elliptical blade dryer was higher than that of the wedge blade dryer,and the rotor power and electricity consumption were lower.Finally,taking the drying system of asludge drying and incineration project as the research object,the mass-energy balance model of sludge drying system was established and the energy loss of each equipment of the system was analyzed.The results of the analysis showed that the main energy losses of the system were discharge loss of the dryer tail gas scrubber,discharge loss of flue gas scrubber tower,dryer heat loss,incinerator and waste heat utilization system heat loss,flue gas scrubber tower heat loss and exhaust heat loss.Among them,discharge loss of the dryer tail gas scrubbing,was about 45% of the total system energy loss.To address this problem,this paper proposed using low-temperature gravity heat pipe to recover the waste heat of sludge drying tail gas.A pilot-scale aluminum-ammonia heat pipe designed independently was tested in the field.The results showed that the heat pipe waste heat recovery was good and the heat loss of the heat transfer process was 4.5%.The efficiency of the heat pipe heat exchanger was 61%. |
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