Study On The Low Temperature Pyrolysis Mechanism Of Corn Stalk And Densification Characteristic Of Biochar

As one of the three staple crops,the scale and yield of corn are huge in China.The corn production increased from 31.01 million tons to 259.23 million tons from 1967 to2017 and the planting scale increased from 16.02 million hectares to 42.43 million hectares.With the large-scale cultivation of corn in China,the stalk burning has always been a problem affecting the rural environment.Corn stalk is an representative type of biomass resources.The ways of stalk utilization mainly include transforming them into fertilizer,fodder,fuel,base stock,and raw chemicals.Low temperature pyrolysis is one of the main methods of biomass thermochemical transformation,which can improve biomass energy density and enhance the competitiveness of solid biomass resources.In this study,the low temperature pyrolysis mechanism of corn stalk,products characteristics,and the combustion process of biochar were systematically studied.The main research contents and results are as follows:（1）Study on the physicochemical properties of the corn stalk.From the perspective of fuel,the basic physicochemical characteristics of corn straw were tested to realize the quality of fuel and its main components content.The loose density,the tap bulk density,proximate analysis,organic element analysis,cellulose analysis,and the HHV were measured.In addition,hemicellulose,cellulose and lignin in corn stalk were separated and characterized.The separated hemicellulose preserved theβ-D-Xplp andα-L-Araf units.Celllulose mainly contained cellulose I_α,I_β.It was detected Aryl ether bond,C1-C which linked to a side chain in an aryl structure,C2-H/C6-H in Syringyl structure in separated lignin.It illustrated that hemicellulose,cellulose,and lignin preserved their specific functional groups well during the separation.（2）Effects of process parameters on the properties of the product by low temperature pyrolysis.By changing the heating petterns of corn stalk during the process of low temperature pyrolysis,the effects of variable heating rate and constant heating rate on the pyrolysis products were studied.The results showed that the proportion of biochar and pyrolysis gas in the product could be effectively increased by decelerating heating rate.Moreover,there are fewer polycyclic aromatic hydrocarbons in the bio-oil.29.82%of biochar and 27.49%of pyrolysis gas can be obtained by decelerating heating rate.In addition,it was found that CO₂ and CO escaped firstly,the gas peak of CH₄ was in the range of 430℃⁵20℃.The gas peak of H₂ was in the high-temperature zone.The final pyrolysis temperature（A）,holding time（B）and raw material particle size（C）was taken as the factors in the design of regression of quadric orthogonal revolving combination.Biochar yield,fixed carbon content,high calorific value and energy recovery efficiency were the response values.The results showed that the influencing degree of the final temperature of pyrolysis is obviously higher than the holding time and the particle size of raw material.With high calorific value as the response,the calculated model of the process was:551℃of pyrolysis temperature,150 min of holding time,0.8¹.0 mm of the particle size range.The HHV of biochar of the verification test was 30.78 MJ/kg which was only3.60%of deviation with a predicted value of the model.（3）Study on pyrolysis behavior of low temperature pyrolysis of the corn stalk.The changing of the organic element,the main component,and apparent forms were detected by many test methods.It was found that the carbon accumulated and oxygen and hydrogen released during low temperature pyrolysis.The corn stalk retained the original skeleton structure while the surface was rougher than raw materials.The higher pyrolysis temperature leads to the decrease of amorphous carbon in corn stalk and the degree of graphitization increased.The characteristic functional groups of hemicellulose and cellulose were also lost.The weight loss of hemicellulose is mainly concentrated in the range of 200℃³50℃,its main pyrolysis products are acetic acid,acetone,propionic acid,butanone,isopropyl vinyl ether,3-methyl-cyclopentene,cycloheptaene and other small molecular organic compounds.The weight loss of cellulose is mainly concentrated in the range of 330℃⁴00℃,its main pyrolysis products are glyoxal,acetic acid,propionic acid,butanone,succindialdehyde and so on.The degradation of lignin occured within a wide pyrolysis temperature range,and the products contain a lot of alkyls and alkoxylated phenolic compounds.（4）Study on reaction kinetics of low temperature pyrolysis of the corn stalk.In the study of reaction kinetics,it was found that heating rate had a great influence on the comprehensive pyrolysis characteristic index.The kinetic equation of pyrolysis was treated by the method of Friedman-Reich-Levi,Flynn-Wall-Ozawa,and Kissinger-Akahira-Sunose.It was found that the change rule of activation energy with the conversion rate was very consistent.And the average reaction activation energy obtained by these three models were282.88 kJ/mol,267.19 kJ/mol and 271.54 kJ/mol,respectively.The reaction activation energy obtained by Kissinger differentiation method was 245.00 kJ/mol.Johnson-Mehl-Avrami equation was selected as the most probable mechanism function of low temperature pyrolysis of corn stalk by Li Chung-Hsiung integral method.The reaction activation energy was 263.22 kJ/mol and the most probable mechanism of low temperature pyrolysis of corn stalk is random nucleation and subsequent growth.（5）Study on the co-densification of biochar and agricultural waste.The types,content of agricultural waste and molding pressure had an effect on the indexes of mixed particles.Higher agricultural waste content and higher molding pressure will increase the mechanical strength of the particles.Taken the durability as the only index,optimum mixture ratio and molding pressure of co-densified particles were found.Through the overall analysis of indexes such as durability,energy density and comprehensive combustion characteristic index,it was found the corn stalk and apple tree branch as the solid adhesive could guarantee the mechanical strength and combustion quality.Biogas slurry can ensure the durability of co-densified particles pellets.Its comprehensive combustion characteristic index was higher than that of biochar.