Study On Pyrolysis Mechanism And Reaction Kinetics Of Agricultural And Forestry Wastes

As the world’s largest agricultural production country,the annual production volume of agricultural and forestry waste in China is considerable.At present,pyrolysis technology is a high-value utilization method to promote the migration and transformation of organic matter components in raw materials so as to obtain value-added products.However,in industrial practice,the problems such as low utilization efficiency of raw materials,low optimization effect of reactor structure and poor product quality are often exposed.This is mainly caused by unclear understanding of the pyrolysis mechanism,unreliable sources of simulation data and incomplete grasp of the changing laws of product properties.In view of the above problems,this paper summarized the pyrolysis mechanism and reaction path of raw materials by combining the gas release law and residual coke surface characteristics.On this basis,different analytical methods were used to conduct a comprehensive kinetics of the whole pyrolysis process.At last,as for the real industrial process,the product change rule of the raw materials was further investigated under the condition of rapid pyrolysis.The pyrolysis behavior of poplar sawdust,almond shell and wheat straw was investigated by TG-MS analyzer and in-situ Fourier transform infrared spectrometer（FTIR）.It was found that the increase of heating rate would make the thermal hysteresis more serious,and the wood chips of poplar were more easily decomposed than the other two materials.In the initial stage of pyrolysis,the hydrogen bonding network within the biomass macromolecular structure was broken and water molecules were separated.At250-350℃,the reaction of decarbonylation and decarboxylation gradually deepened to the removal of methylene and benzene ring substituents,resulting in the generation of a large amount of CO,CO₂ and acetic acid.Dehydrogenation reaction,ring opening reaction and cracking reaction mainly occurred at 360℃.After 400℃,aliphatic hydrocarbon aromatization and benzene ring polycondensation began.Demethylation mainly took place at about 420℃.The polycondensation dehydrogenation reaction of coke mainly occurred between 500-600℃.Secondary reactions such as carbonate decomposition,methane/tar steam reforming and tar pyrolysis would be carried out in the high temperature section above 650℃.Friedman’s method,Master-plots method and multi-Gaussian-DAEM-reaction model were used to analyze the kinetics of the pyrolysis process of raw materials.It was found that the activation energy of the three types of biomass was generally distributed between 120-220k J/mol,and all decomposition reactions could be described by F_n model.However,under the influence of different composition of raw materials,the variation trend of corresponding characteristic parameters of each sample（ln[A_αf（α）]and E_α）with the conversion rate was more obvious.Combined with the understanding of the reaction mechanism of raw materials pyrolysis,the basic information of the range division,peak area and full width at half maxima（FWHM）of the fitting peaks of each experimental result could be basically determined,so as to achieve the high-precision peak fitting of the experimental data.In addition,this study verified the feasibility and applicability of the multi-Gaussian-DAEM-reaction model in the pyrolysis process of mixture from three aspects.That is,the programming results were highly consistent with the experimental data;the activation energy distribution interval was reasonable;and the fitting level was high.Meanwhile,the kinetics parameters of each single peak could correspond to the characteristic reaction intensity,which further reflected the advantage of the method in the combination of mechanism analysis and parameter solution.Finally,in order to get closer to the process practice,the product distribution and quality changes of three raw materials under rapid pyrolysis were summarized in the downstream fixed bed reactor based on product yield and component analysis.The results showed that the yield of pyrolysis gas and the proportion of combustible gas in pyrolysis gas both increase continuously with the increase of pyrolysis temperature.The gas production efficiency of fast pyrolysis technology is higher than that of programmed heating.The yield of bio-oil was the highest at about 500-600℃,and its composition changes were mainly divided into three stages:continuous and stable precipitation of each component,alkane cracking and aromatization reaction,and polycyclic condensation of aromatic hydrocarbons.The yield of solid products continued to decrease with the increase of pyrolysis temperature.The carbon-based microcrystalline structure of the char did not change significantly before 600℃,ant it began to develop towards highly ordered graphitization after 600℃.