| In order to study sludge drying mechanism in paddle dryers and optimal energy-efficient operation mode of sludge drying and incineration system,three lab-scale sludge drying test systems with a batch-type paddle dryer,a uniaxial continuous paddle dryer and a biaxial continuous paddle dryer were developed,respectively,and correspondingly,an intermittent,a one-dimensional continuous and a two-dimensional continuous sludge drying model and mass and energy balance model of sludge drying and incineration system were established.Experimental and theoretical study was carried out based on sludge drying test systems and sludge drying models.First of all,considering the results of sludge drying test and reliability of calculation,the penetration model was modified and the modified model can simulate sludge drying process in a batch type dryer well.Then the model was used to do sensitivity analysis of drying parameters on sludge drying process.The increase of dryer wall temperature can rise sludge drying rate and sludge temperature.However,when the sludge moisture content is higher than 30%,the sludge temperature can only go up to a range of 90-100 ?.The increase of stirrer speed has almost no influence on sludge drying rate and sludge temperature within the examined range.Only the sludge drying rate will increases slightly.Secondly,the coupling model of penetration model and one-dimensional Markov chain model was used to simulate the sludge drying process in a uniaxial continuous paddle dryer and the simulation results were good.The sludge drying process in a uniaxial continuous paddle dryer can be divided into three stages:the pasty phase,the sticky-granular phase and the complete-granular phase.The influences of the drying parameters on the sludge drying kinetics in a uniaxial continuous paddle dryer were investigated.In the examined range,dryer wall temperature has a positive influence on sludge drying rate per unit area,dryer slope and overflow height have negative and positive influences on effective contact area,respectively,sludge flow rate has a positive influence on wet sludge quantity per unit effective contact area and stirrer speed has little influence on the sludge drying kinetics.The hold-up of the dryer in a steady state(Hu,for short)has a remarkable influence on mean residence time of sludge in a dryer and final moisture content at the outlet of the dryer.Within the examined range,sludge flow rate,dryer wall temperature and stirrer speed have little influence on Hu and dryer slope and overflow height have negative and positive influences on Hu,respectively.Thirdly,based on the one-dimensional Markov chain model,the two-dimensional Markov chain model was developed to simulate sludge flowing in a biaxial continuous paddle dryer.Validation test of the two-dimensional Markov chain model was carried out in a self-designed biaxial continuous paddle dryer and the validation result is good.Then the model was used to do sensitivity analysis of parameters on sludge flowing characteristic.The result shows that the backmixing coefficients R and r are critical for the sludge flowing uniformity(the shape of RTD curve)while geometric residence time of dried sludge in a cell ? controls mean residence time of sludge(mean residence time of RTD curve)in a dryer,which means that Hu controls the final moisture content at the outlet of the dryer when the sludge flowing rate is fixed.Fourthly,the coupling model of penetration model and two-dimensional Markov chain model was developed to simulate the sludge drying process in a biaxial continuous paddle dryer and the coupling drying model was modified by introducing the phenomenon of adhering on the wall and crusting and the concept of sticky phase.The modified model can simulate sludge drying process well.The sludge drying process in a biaxial continuous paddle dryer can be classified into two types according to whether the phenomenon of adhering on the wall and crusting happens.When the phenomenon does not happen,the sludge drying process can be divided into three stages:the pasty phase,the sticky-granular phase and the complete-granular phase.And when the phenomenon happens,the sludge drying process can be divided into four stages:the pasty phase,the sticky phase,the granular phase and the complete-granular phase.Hu has a positive influence on the occurrence of the phenomenon of adhering on the wall and crusting.The damping of the sludge drying process has a positive influence on the demarcation point for the occurrence of the phenomenon.The occurrence of the phenomenon of adhering on the wall and crusting will weaken the heat transfer during sludge drying process,especially for the heat transfer in the sticky phase.The influences of the drying parameters on the sludge drying kinetics in a biaxial continuous paddle dryer were investigated.Without considering the influence of the phenomenon of adhering on the wall and crusting,dryer slope and stirrer speed has little influence on sludge drying process and the decrease of sludge flow rate and the increase of overflow height and dryer wall temperature will accelerate the decline rate of sludge moisture content along the dryer within the examined range.Meanwhile,the decrease of sludge flow rate,dryer wall temperature and overflow height and the increase of stirrer speed and dryer slope will reduce Hu within the examined range.Then the comparison of the coupling drying model and lumped parameter method,which is the existing common method for designing and calculation of sludge dryer,was conducted.The result shows the error of the results of lumped parameter method is bigger than that of the coupling drying model.The accuracy of lumped parameter method strongly depends on the experience of assigning a value for the comprehensive heat transfer coefficient of dryers.But the coupling drying model can simulate the sludge drying process well through theoretical calculation.Therefore,the development of the coupling drying model will benefit the theorization of practical heat transfer calculation and design of sludge paddle dryer.Furthermore,it will provide theoretical support for more effective and accurate calculation of actual working condition and design of dryer.Fifthly,a mass and energy balance model of sludge drying and incineration system was established to analyze the mass and energy balance and energy consumption of one actual sludge drying and incineration project and the optimum sludge moisture content before incineration(OSMCI,for short)is determined according to the principle of lowest system total energy consumption(LSTEC,for short).In this condition,the maximum energy loss is the latent heat of water which is taken away by carrier gas during the sludge drying process and the second maximum energy loss is thermal energy which is absorbed by cooling water in the flue gas cooling tower.Then,the influences of the fluctuations of the system operation load,sludge calorific value and initial moisture content on system operation were researched.When the system operation load and the initial moisture content fluctuates in the range of study,OSMCI should be kept stable to ensure the LSTEC.However,when the sludge calorific value increases(decreases),OSMCI should increase(decrease)linearly to ensure the LSTEC.This balance model can provide guidance for design,operation,lowering cost and improving efficiency of the sludge treatment projects which use auxiliary fuel(like natural gas,coal,oil,etc.),indirect dryers and incinerators. |
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