Research On The Transformation Of Inherent Potassium During The Thermal Conversion Process Of Biomass

Biomass has been attracting more attention owing to its renewability and carbon neutrality.However,during biomass thermal conversion,the release and transformation of inherent potassium can cause serious technical problems,such as fouling and slagging in thermal utilization systems.This thesis focuses on the migration behaviors and mechanisms of K under various thermal conversion conditions,aiming to provide basic support for solving the technical problems casued by K migration.For that purpose,a fixed-bed experimental system and an in-situ K concentration and particle temperature measurement system were established.Firstly,the migration of K during biomass pyrolysis and combustion was studied,using willow wood and rice straw to represent typical woody and herbaceous biomass,respectively.The experimental results revealed that K existed in willow wood mainly as organic K.During willow wood pyrolysis at different temperatures,only a small amount of organic K was released during devolatilization stage,and the remaining K in char was mainly transformed to char-K.During wiloow wood combustion,a small amount of organic K was released during devolatilization stage;during char combustion stage,charK was mainly converted to K2CO3 with a small amount of K2CO3 release;during ashcooking stage,all of the remaining K2CO3 was released to the gas phase.The K in rice straw mainly existed as organic K and KCl.During straw pyrolysis,the release raio of K increased significantly with temperature.During devolatilization stage,K was released as organic K and KCl,and the proportions of KCl release exceeded organic K gradually with increasing temperature.KCl and char-K can be released to gas during char pyrolysis stage.During straw combustion process,the release rate of K increased rapidly first and then decreased gradually.During devolatilization stage,K was mainly released as KCl;during char combustion,the release of KCl,the transformation of char-K,and the K-Si reaction forming K-silicates mainly occurred;during ash-cooking stage,K was released slowly from K-silicates.Secondly,the interaction of KCl with organic groups during pyrolysis was studied using KCl-loaded cellulose as model compounds.The experimental results revealed that during pyrolysis at 500 °C,the interaction of KCl with organic groups caused the release of K and Cl,as well as the formation of char-K and char-Cl.Limited by the amount of organic functional groups,the amounts of Cl release and char-K formation increased first and then remained nearly stable with the increase of KCl loading content,thus KCl became the main occurrence forms of K and Cl.After pyrolysis at 900 °C,all the KCl and char-Cl were released to gas.Ca promoted the reaction of KCl with organic functional groups,increasing the formation amount of char-K.Lastly,the migration characteristics of char-K and inorganic K during char reaction stage were studied respectively.The results revealed that during char pyrolysis,significant amount of char-K was released and the release rate increased with pyrolysis temperature,while the final remaining amounts of char-K at different temperatures were similar.Under oxidizing atmosphere,char-K was mainly transformed into K2CO3.At temperatures above 900 °C,significant release of K2CO3 was observed.The steam atmosphere significantly promoted the release rate of K by reacting wih K2CO3 and forming KOH.The reaction rate of K2CO3 with SiO2 forming K-silicates is significantly faster than the release rate of K2CO3,thus inhibiting the K release.The steam atmosphere promoted the reaction of K2CO3 or KCl with SiO2.HCl can react with char-K and other AAEMs elements in char forming volatile chloride salts,thus promoting the release of AAEMs at high temperatures.