Investigation On The Kinetic Behavior And Optimization Of SCE Algorithm For Typical Biomass

With the continuous development of the society,the traditional fossil energy will gradually be moved out from the future energy structure because of its limited resource and the pollution to the environment.As a huge resource and clean renewable energy,the biomass resources will play an increasingly important role in future.The main components of biomass include C,H and O while N and S are very low,which ensures the clean and safty.In China,the biomass energy conversion technology has been being immature and the most of biomass were burned directly.It resulted in low energy efficiency,serious atmospheric pollution and damage to the ecological environment.The biomass pyrolysis is not only an efficient method of energy conversion,but also a phenomenon that occurs in the process of gasification,liquefaction and combustion.In this paper,the thermogravimetric experiment of the four typical biomass materials and the optimization of the related thermodynamic parameters were carried out.Bermuda grasses(herbaceous plants),soybean stalks(straw),basswood(soft wood)and ashtree(hard wood)are typical fiber biomass materials,whose main components are cellulose,hemicellulose and lignin.The pyrolysis of four typical biomass materials was investigated based on thermogravimetric analysis in nitrogen atmosphere.At different heating rates(10,20,30 and 40 K/min),the pyrolysis of biomass materials can be divided into three stages.Firstly,the evaporation of residual water in the sample and a phenomenon which called “glass transition” can be found in the pyrolysis process.Then,the sample pyrolysis produces a large amount of volatile matter at this stage and result in an apparent weightlessness.Among them,hemicellulose pyrolysis occurs first and then cellulose start pyrolysis,lignin pyrolysis will take place in the whole pyrolysis process.Finally,the residues are slowly thermally degraded to produce coke.Two characteristic peaks appear in the DTG-T curves of Bermuda grass and basswood,which indicates that the content of hemicellulose is relatively high.However,there is only one obvious characteristic peak in the DTG-T curves of soybean stalk and ashtree,implying that the content of hemicellulose is relatively low.In addition,the maximum pyrolysis rate increase and the corresponding temperature increase with the heating rate increasing.The TG-T curves and DTG-T curves are shifted to the high temperature side as a whole.Two model-free methods of Flynn-Wall-Ozawa(FWO)and Kissinger-Akahria-Sunose(KAS)were employed to estimate the values of activation energy under different conversion rates.The results showed that the pyrolysis of Bermuda grass could be divided into two regions.The conversion rate at first region is from 0.10 to 0.40 while that at the second one is from 0.40 to 0.75.The pyrolysis of basswood is also divided into two regions and the conversion rate at first one is between 0.10 and 0.50 while that at the second one is between 0.50 and 0.85.The pyrolysis process of soybean stalk and ashtree was considered as a whole.Then,a model-fitting method called Coats-Redfern(CR)was used to predict the reaction mechanism for pyrolysis of biomass and determine the values of pre-exponential factor.For Bermuda grass,the diffusion-Jander model is the optimal reaction mechanism function to describe the pyrolysis at first region and the four-order reaction model can be used to describe the pyrolysis at second region.For the whole pyrolysis process of soybean stalk,the four-order reaction model is the optimal reaction mechanism function.For the two regions of pyrolysis of basswood,the optimal reaction mechanism function is the diffusion-Jander model.The five-order reaction model is the optimal reaction mechanism function to describe the pyrolysis process of ashtree.A multi-component pyrolysis reaction mechanism should be established because of the complex composition of biomass resources.Therefore,it is important to determine the kinetic parameters of each component.In this paper,the SCE(Shuffled Complex Evolution)global optimization algorithm is applied to the analysis of thermogravimetric curves.A series of parameters,such as proportion of components,activation energy,pre-exponential factor,reaction order and carbon yield were optimized.Then,the more universal kinetic parameters were obtained which provides the basic data and theoretical basis for the design of high-efficiency biomass resources converter and numerical simulation of biomass pyrolysis.