Effect Of Potassium Salts On Bio-char Formation In Biomass Pyrolysis

Bio-char is a porous carbon material prepared by biomass pyrolysis and has been widely used in energy,wastewater treatment,agricultural production,electrode materials and other fields.Therefore,the preparation of bio-char has gradually become a research hotspot.Alkali metals are the key factors affecting bio-char formation.However,most of the current studies focus on the migration and transformation of alkali metals during pyrolysis,lacking systematic research on the effects of alkali metals on bio-char formation.In this paper,walnut shells were used as the raw material to explore the effect of adding KCl on the yield and physicochemical properties of bio-chars,which was related to the mechanism of free radical reaction,and to reveale the mechanism of KCl in the bio-char formation process.Furthermore,the effects of potassium salts（KCl,K₂CO₃,KOH,KAc,K₂SO₄）on bio-char formation were compared.The main conclusions are as follows:（1）The bio-chars of acid-washed samples and KCl-impregnated samples were analyzed to study the effect of adding KCl on its physicochemical properties.The results showed that there were two states of KCl in bio-char.The KCl on the surface was removed by water washing,and the KCl embedded in the carbon layer still existed;the addition of KCl increased the carbon content of the amorphous structure and reduced the ordered carbon content,which was not conducive to the improvement of the degree of graphitization;the C-C/C=C content reduced and the C=O content increased with the addition of KCl,and the change trend was more obvious with the increase of KCl;the specific surface area of the bio-chars added with 1 wt%KCl is higher than that of the sample added with 5 wt%,indicating that the specific surface area of bio-chars decreased with the increase of KCl.（2）The acid-washed samples and KCl-impregnated samples were slowly pyrolyzed to investigate the effect of adding KCl on bio-char formation.The results showed that the addition of KCl could improve the bio-char yields and ash-free yields at 400～800℃,and effectively increase the ash-free yield increments at 400～600℃,which was related to the KCl embedded in the carbon layer,and its content had a good linear relationship with the increment.The promotion path of KCl at 400～600℃was determined in connection with the free radical reaction mechanism.KCl could promote the deposition of volatile free radicals,resulting in the formation of more bio-char by polycondensation,and the number of free radicals deposited gradually increased with the increase of KCl,thus increasing the yield of bio-char.（3）The effects of potassium salts on bio-char formation were compared.The results showed that the ash-free base yield increments of bio-char increased at 400～600℃with the addition of potassium salts,and the order was:KCl>K₂CO₃>KOH>KAc>K₂SO₄,indicating that potassium salts differed in the promoting ability,but followed the same reaction process.In addition,potassium salts existed in different states during the pyrolysis process.The crystal structures of K₂CO₃ and KAc changed during the pyrolysis process,while KCl,KOH and K₂SO₄ remained unchanged.