Study On Formation Mechanism Of Bio-chars During Pyrolysis

China has abundant biomass resources and pyrolysis is an important way for biomass conversion.The bio-char produced by the pyrolysis process has developed pore structure,high specific surface area and rich surface chemical properties,so it can be used as fertilizer in agricultural production,precursors of porous carbon materials and combustion fuels.Although the bio-char is widely used,the formation mechanism and evolution of physical and chemical properties in bio-chars are still unclear.Therefore,on the basis of previous studies,this paper starts from the principle of free radical reaction in the process of biomass pyrolysis,the effects of pyrolysis temperature,heating rate,residence time and inert gas flow rate on the formation of bio-chars were studied in order to obtain a further understanding of the formation mechanism of bio-chars.The main results are as follows:(1)The pyrolysis behavior of biomass at 400~800 ? under N2 atmosphere was studied,and it was related to the free radical reaction triggered by covalent bond breakage.The results showed that the formation of bio-chars is mainly divided into two stages: the volatile content is mainly released and the free radical concentration of bio-chars is continuously increasing at temperature less than 600 ?;the free radical condensation reaction is main and the free radical concentration of bio-chars is continuously decreasing at temperature more than 600 ?.When the pyrolysis temperature reaches 800 ?,the free radical signal of bio-chars completely disappears.(2)Analysis of the physicochemical properties of bio-chars during the pyrolysis process showed that: with the pyrolysis temperature increasing,the content of quinine group increases gradually,the contents of ester,anhydride and hydroxyl groups decrease,the content of carboxyl group is almost unchanged,which indicates that the quinone group can be used as a marker for the oxygen-containing functional groups of bio-chars tending to be stable;the graphite microcrystals in bio-chars increases at temperature less than 600 ?,the graphite microcrystals in bio-chars is almost unchanged at temperature more than 600 ?;the bio-char is dominated by micropores,and the pore volume and specific surface area of micropores have a positive linear relationship with pyrolysis temperature.(3)The yield and physical properties of bio-chars are closely related to pyrolysis conditions.The yield,free radical condensation and quinone contents of bio-chars are slightly higher in slow pyrolysis,but the bio-char obtained by fast pyrolysis has a higher degree of graphite microcrystalline at low temperature(400 ?).The change of N2 flow rate has little effect on the bio-char yield,free radical concentration and thermal stability.The extension of residence time can promote the release of volatiles,generate more free radical fragments,reduce the yield of bio-char and enhance the thermal stability.