| Due to the large quantity,low cost,clean and renewable characteristics of biomass resources,more and more attention has been paid by governments and research institutions.Pyrolysis,as a means of thermochemical treatment,has the highest efficiency,and product diversification is more in line with market energy and chemical needs.Domestic and foreign researchers have researched a lot on biomass pyrolysis kinetic parameters,pyrolysis product characteristics,pyrolysis process and mechanism.However,relatively few studies have been conducted on the heat and mass transfer characteristics of pyrolysis process.The research content and main results of this paper are as follows:TG-DSC method was used to study the pyrolysis characteristics and kinetics parameters of eucalyptus.Eucalyptus pyrolysis can be divided into three stages,respectively drying and heating,fast pyrolysis and charring stage.Pyrolysis reaction was easier to happened with the increase of heating rate,where the pyrolysis reaction coefficient Q was increased from 1.01×10-6at 5℃/min to 2.42×10-6at 30℃/min.Ozawa-Flynn-Wall and Friedman method were used to obtain the activation energy and pre-exponential factor of the first and second stages of pyrolysis.The results showed that the activation energy of the two stages derived from integral and differential method are 60.67 k J/mol and82.33k J/mol,134.33 k J/mol and 148.50 k J/mol.Meantime,the pre-exponential are 6.87 s-1and 10.43 s-1,9.27 s-1 and 10.77 s-1.The random nucleation and subsequent growth was introduced to be the best reaction mechanism for the first two stages by the means of Malek method.The reaction equation was sloved by data fitting method due to the fact that the third stage of pyrolysis can’t be sloved by the aforementioned conventional methods.At 20℃/min,eucalyptus were pyrolyzed using fixed bed.The solid yield,composition,microstructure and thermophysical parameters were analyzed systematically.With the increase of pyrolysis temperature,the solid yield and volatile matter content were deceased,while the ash and fixed carbon contents were increased.The internal char pore gradually developed to promote the heat and mass transfer during pyrolysis processes and the density of char was deceased first and then incrased.Moreover,the heat conductivity coefficient was slightly decreased first and then index-type increased.TG-DSC method was used to investigate the specific heat and pyrolysis reaction heat.The results showed that the specific heat was first decreased and then increased like a"U-type",while the reaction heat was decreased first and then increased with the increase of pyrolysis temperature.3D cylindrical shape model（f20′40mm）was established using ANSYS software to investigate the pyrolysis process under heat rate of 20℃/min.The results from Newmark progressive integral method showed that heat was conducted to inner layer from the outside one.The particles during pyrolysis process can be divided into charring,pyrolysis,evaporation and raw layer.The influences of model dimension,heating rate and reaction heat,particle length-diameter（l/d）ratio,specific heat and heat conductivity coefficient on the temperature distribution inside biomass during pyrolysis were studied.The results showed that 3D model was better than 2D model on the heat transfer due to the nimby effect.The heating rate,specific heat and heat conductivity coefficient of the model had a great influence on the temperature distribution.Due to the internal heat transfer lag effect,the radial model temperatures were diversed more with the increase of heating rate when the outer wall temperature was under 350℃.At 350℃,the difference of radial model temperatures were smaller,and keep the difference in the process of subsequently rise.Then the whole model temperature increased to 850℃gradually.When the specific heat was constant,the model temperature heating of each sampling point varied.Therefore,ignoring the change of the specific heat would lead to the inaccurate of heat transfer model.When the heat conductivity coefficient was a constant value of 0.05 w/（m·℃）,the initial period of the model was heated faster,while the end perild showed the opposite trend.The reaction heat and particle size have no obvious influence on the model.By comparing the simulation data of the pyrolysis model with the thermogravimetric experimental data,it is proved that the pyrolysis model established in this paper can simulate the temperature distribution and weight losing process of eucalyptus sawdust in the pyrolysis process. |
