Study On Reaction Behavior And Mechanism Of Biochars Formation In Biomass Pyrolysis

As a renewable resource,biomass is widely concerned by people due to its rich reserves and broad application prospects.Pyrolysis of biomass into bio-chars is one of the important directions of its conversion and utilization.At present,researchers have done a lot of research on the pyrolysis conditions,bio-char yields,pore structure and other parameters of single biomass,but lack of in-depth understanding of the differences and common laws between different bio-chars.Therefore,this paper selects four kinds of biomass for pyrolysis to prepare bio-chars,and analyzes the formation process of bio-chars from the aspects of biomass differences,pyrolysis conditions and free radical reaction behavior.The main research contents are as follows:（1）Comparing different biomass at 400-800 ^oC,the carbon yield,free radical concentration,surface functional groups and pore structure all show the same change law.It is found that K element can promote the polycondensation reaction between free radicals.The more K element exists,the more conducive to the formation of bio-chars.Lignin has a wide pyrolysis range,and the more content it has,the more free radicals it produces.Due to the different composition of raw materials,there are differences in surface functional groups and pore structure,but they show the same change law.In C1s spectral analysis,when the pyrolysis temperature increased,the content of C=C and C-C firstly increases and then decreases,while carbonyl,quinine,carboxyl and aliphatic groups in alcohol phenol ether showed the opposite trend.In O1s spectral peak analysis,with the increase of pyrolysis temperature,the content of quinone groups showed an upward trend,while the content of C=O and C-O in hydroxyl,ester and anhydride showed a downward trend.At the initial stage of pyrolysis reaction,the specific surface area of biochar increased greatly with increasing temperature,and then gradually stabilized.At the same time,the abundant pore structure and huge specific surface area of biochar are produced by the decomposition and release of oxygen-containing functional groups.Therefore,the more surface oxygen-containing structure biomass have,the more conducive to the pore development of biochar.（2）The effects of N₂ flow rate,residence time and heating rate on the formation of biochar at 500 ^oC were studied.The results showed that N₂ flow rate have no effect on the formation of bio-chars,while residence time and heating rate would reduce the carbon yield.The reason is that the release of volatilization is not a transient process,with the extension of residence time,the more fully the covalent bond breaks,the carbon yield decreases at a certain rate and the free radical concentration increases.The speed of heating rate will lead to the temperature difference of biomass.Too fast heating rate will make the volatile lose the conditions for deposition and aggravate the cracking of gas-phase volatile,resulting in the decrease of carbon yield and the increase of free radical concentration.The above results are proved by studying the effect of biochar particle size on biochar formation.（3）The physicochemical properties of biochar at different heating rates were analyzed.It was found that the content of quinone groups on the surface of biochar decreased with the heating rate,while the content of C=O in hydroxyl,ester and anhydride increased with the heating rate.This is mainly because the free radical fragments produced by pyrolysis are usually centered on aromatic units.With the acceleration of heating rate,the benzene ring components in quinone groups form free radicals,migrate to the tar macromolecules of volatile components and release them,resulting in the decrease of quinone groups content in biochar.（4）The effect of particle size on pyrolysis process depends on the effect of heating rate.In the process of pyrolysis reaction,the covalent bond in biomass began to break to form free radicals,and some of them were released with volatiles;Another part of free radicals that are easy to deposit polycondensate with each other to form solid particles and deposit on the surface of biochar.Due to the temperature difference between the inside and outside of biomass particles with different particle sizes,when the biomass particles are large,some free radicals that are easy to deposit will escape through the pores and encounter a high-temperature environment,so they cannot be polymerized and deposited,and will be released with volatile matter,resulting in the decrease of carbon yield and the increase of free radical concentration.