Study On Simulation Of Sewage Sludge Pyrolysis Process And Energy Balance In Pyrolysis

Sludge pyrolysis is becoming an emerging technology for sludge treatment and disposal.It can produce biochar,tar,and syngas,as well as reduce sludge volume massively.Therefore,it has been comprehensively studied in recent years.However,due to its complex process and unavailability of mechanism information,methods for validating and predicting the sludge pyrolysis remained less clear.In this study,artificial neural network model and computational fluid dynamic model are applied to simulate the distribution of pyrolytic products,predict the higher heating value of syngas,and calulate the related energy balance.We expect that the experimental efficiency and energy recovery can be enhanced after optimizing the parameters of sludge pyrolysis.1.Prediction of Municipal Sludge Pyrolysis Based on Artificial Neural Network ModelIn this study,a three-layer backpropagation neural network was developed to predict the higher heating value（HHV）of syngas produced from sewage sludge during the pyrolysis process.C,H and O,volatile matter（VM）,dosage of Fe₂O₃ and Al₂O₃,reaction temperature（T）,and moisture content（MC）were selected as input variables,while HHV was selected as the output variable.The number of neurons in the hidden layer and the initial value of weights and bias were optimized by using RSM.The trained artificial neural network（ANN）model showed a good prediction accuracy（MSE of 0.0060 and R² of 0.9766）.The trained ANN model showed superiority over MLR and PCR models for prediction of HHV in both training and validation.The reaction temperature and moisture content were the most two influential factors affecting HHV of syngas in the pyrolysis of sewage sludge.2.Simulation of the Distribution of Pyrolytic Products Based on Computational Fluid Dynamics ModelA geometric model of the tube furnace was established based on a laboratory-scale horizontal tube furnace.The kinetic model of sludge pyrolysis was established based on the pyrolysis mechanism of sewage sludge.The distribution of flow field,temperature and the concentration of syngas（which were difficult to measure）were simulated.Results showed that the fast pyrolysis reaction occured rapidly at a high reaction temperature.Due to the reaction between gas phases,the generation of H₂ and CO tends to stabilize after 7 seconds.It may be beneficial to obtain a high calorific value syngas at this time.The initial moisture content of cake would enhance the production of hydrogen-rich gas during pyrolysis.With the increase of the moisture content（after 30 wt.%）,this improvement became less significant.There were indiscernible differences of the mass fractions on H₂ and CO when the moisture content ranged from 40 to 50 wt.%.The increased moisture content may lead to a high flow rate of gas in the tube furnace,which resulted in a decrease in the residence time of syngas,thus leading to a transient reaction time.The reaction temperature had an important influence on the distribution of gas products.The pyrolysis reaction rate increased with the temperature incresed,and more high-calorific gases could be produced.When temperature was increased from 600 to 900 ^oC,the yield of H₂ increased from 0.05 to 0.35L/g DS,and the yield of CO increased from 0.21 to 0.33 L/g DS.3.Study on Energy Distribution of Municipal Sludge PyrolysisThe energy of sludge pyrolysis process includes energy consumption and product calorific value,and therefore can be divided into energy consumption for drying,heating and pyrolysis reaction.The results showed that the energy consumption for drying was the most important part of the energy consumption in the pyrolysis process and it was closely related to the initial moisture content of the sludge cake.Compared with the traditional centrifugal dewatering technology,the deep-dewatering of Fenton-red mud reagents could reduce the initial moisture content of the mud c ake（80 to 60 wt.%）,and greatly reduced the energy consumption for drying.The energy consumption for drying reduced to 7.64 MJ/kg_dried,SS,with a reduction percentage of 65%.Red mud could promote the increase of pyrolysis gas production as a catalyst during pyrolysis.The calorific value of gas products was increased with a ratio of 30-40%（at 900 ^oC）.Therefore,in terms of energy consumption,deep dewatering technology was superior than the traditional sludge dewatering with centrifugation.