Biomass Pyrolysis Kinetics And Pelletization Constitutive Properties

Biomass is a major kind of renewable energy characterized by wide availability,renewability and CO₂ neutrality,which will play an important role in achieving the“carbon emission peak”and“carbon neutrality”goals of China.Thermochemical conversion and physical conversion are the main approaches of biomass utilization while sawdust is the most widely used biomass material.Rapid development of biomass energy has resulted in an insufficient biomass supply.Exploration of alternative biomass materials is urgently needed.In this work,fuel properties of five biomass materials were investigated,including:（1）Pinus sylvestris var.mongolica Litv.sawdust（PS）,regarded as the relative standard material for its wide utilization,（2）Eucalyptus urophylla sawdust（ES）,considered as a representative of fast-growing tree,（3）Flammulina velutipes spent substrates（FVSS）,chosen as the typification of edible fungi cultivation residue,（4）Arundo donax var.versicolor（AD）,treated as the residue of wetland plant,and（5）herbal residue（HR）,used as the residue of Chinese medicine production.ES,FVSS,AD and HR are considered as potential alternatives to sawdust such as PS.This work mainly focuses on the properties of these five materials during thermochemical and physical conversions.Special attention are paid to kinetics and thermodynamics of slow pyrolysis,solid product properties of torrefaction,pelletization mechanism,and sodium lignosulfonate（SL）bonding mechanism.The results of this paper are expected to lay theoretical foundations for utilization of potential alternative biomass fuels.（1）Slow pyrolysis tests were carried out in a thermal analyzer.Simultaneous degradation of different components formed the overlapped peaks.A deconvolution technique was applied to distinguish an overlapped peak into three individual peaks corresponding to material components of hemicellulose,cellulose,and lignin,to solve the inadaptability of the iso-conversional method to computing kinetic parameters.In particular,the asymmetric function of Bi-Gaussian was employed to capture the asymmetric peak shapes.After deconvolving,the Starink method was used to estimate the apparent activation energy（E_α）.The E_αvalues of hemicellulose,cellulose and lignin correspond to ranges of 147.08-205.04,188.11-323.02,and 121.46-265.36k J·mol^-1,respectively.The equation of Kissinger was adopted to assess the pre-exponential factor（A）,which are all higher than E+9 s^-1.Slow pyrolysis is a process in which some complex chemical reactions occur.The master-plots approach was applied to determine the kinetic mechanism function.The F2 model dominates the degradation of PS cellulose,FVSS hemicellulose and AD cellulose.The F4 model governs the degradation of ES hemicellulose,and the D3 model controls the degradation of HR hemicellulose.Some known general equations were employed to compute the thermodynamic parameters of enthalpy change,entropy change,and Gibbs free energy change.Slow pyrolysis is an endothermic,disorder-like and nonspontaneous process.（2）The torrefaction experiments were conducted on a horizontal tubular reactor.The solid product properties like ultimate analysis,proximate analysis and higher heating value were studied.The results show that C_d,ash content,fixed carbon,and higher heating value tend to rise,while H_d,O_d,and volatile matter tend to lessen,and N_d and denitrogenation efficiency fluctuate to lower first and then higher with increasing torrefaction temperature or torrefaction time.These compounded influences reduce the solid yield and energy yield.Nevertheless,the energy yield is higher than the solid yield,contributing to the increase of energy density.Overall,the torrefaction temperature has a larger influence than torrefaction time on solid product properties,which is more obvious under higher torrefaction temperature（290,320℃）.The properties of AD solid products displayed the largest variation,as proven by the lowest solid yield and energy yield and the highest denitrogenation efficiency.（3）Pelletization experiments were conducted using an electronic universal compression apparatus.During the elasto-visco-plastic deformation stage,material particles manifest themselves in a complex mechanical way.In this work,a non-linear constitutive model was established based on the theory of rheology.The correlations between model coefficients（elastic modulus E,plastic modulus R,strain hardening exponent n,viscous coefficientη,and frictional loss factorσ_C）and pellet properties were revealed to solve the inapplicability of the present models to the complexity of particle deformation.The model coefficients are a function of increased pressure.The elasto-visco-plastic deformation stage originates at a pressure of approximately 1 MPa.The structure of biomass transforms from a loose powder body to a solid compact.Close connections between model coefficients and pellet properties were found.Density and mean volume expansion rate of pellet products are inversely proportional to plastic modulus and elastic modulus,respectively.Meyer hardness is inversely proportional to viscous coefficient absolute values.A higher elastic modulus value,lower plastic modulus value,and higher viscous coefficient absolute value for each material corresponding to better pellet properties were obtained at a lower moisture content of 8 wt.%and higher temperature of 120℃.（4）Pelletization experiments with SL addition were conducted using the same electronic universal compression apparatus.In this work,changes in model coefficients with different SL additions were obtained according to the established constitutive model to reveal the bonding mechanism of SL from the perspective of particle mechanical evolution.The addition of SL improved the unit density and Meyer hardness and lowered the mean volume expansion rate of the pellets.SL increased the elastic modulus value and the viscous coefficient absolute value as well as reduced the plastic modulus value under certain pelleting conditions.Nevertheless,the change amount of the model coefficients corresponding to the improvement of the pellet qualities is correlated to the material itself.The results obtained from this work can provide some theoretically new and basic cognitions for the conversion of these biomass materials and offer some guidance for the actual conversion process.