Research On The Multiphase Transformation Of Chlorine And Potassium During Biomass Pyrolysis And Combustion

The energy utilization of biomass can effectively cope with energy depletion and environmental degradation,providing a strong support for achieving energy transition and the carbon peaking and carbon neutrality goals.However,biomass contains large amounts of chlorine（Cl）and potassium（K）,the release of which can lead to various ash-related issues such as corrosion and ash deposition,limiting the utilization of biomass to some extent.This dissertation investigates the migration mechanisms of chlorine and potassium including release and multi-phase conversion during biomass pyrolysis and combustion.It can provide a theoretical support for the clean utilization of biomass.Firstly,based on the fixed-bed pyrolysis system,the Cl in all pyrolytic products has been accurately quantified using various methods including improved Eschka and in-situ combustion of volatiles.Based on the drop-tube furnace（DTF）system,the K reactivity of reactor tubes made of silicon carbide,corundum and mullite in the combustion experiment has been investigated.By quantifying the K retained in the reactor tube,it is directly demonstrated that the K reactivity follows the order of mullite tube>corundum tube>silicon carbide tube.According to the emission characteristic of particulate matter,it is found that the enhanced K reactivity results in a significant decrease in the contents of K and Cl in PM₁₀ and the yield of PM₁,which indirectly demonstrates that using silicon carbide tube can obtain more accurate experimental data about the transformation of K.Secondly,the poplar woods loaded with KCl,Mg Cl₂ and ion-exchanged K have been pyrolyzed in the fixed-bed system to investigate the release characteristic of Cl and K in different chemical form.The results show that Cl is mainly released as HCl and CH₃Cl with a competitive relationship.Mg can destroy the hydrogen bond network and promote the breakage of oxygen-containing functional group,resulting in a significant increase in the release rate of Cl and the proportions of the Cl in oil and the water-insoluble Cl in char.In contrast,K can catalyze the depolymerization of lignin and promote the methylation of HCl.During pyrolysis,the release rate of KCl is higher,while ion-exchanged K tends to convert into insoluble K and water-soluble K with a high melting point.The HCl and CH₃Cl promote the conversion of ion-exchanged K and insoluble K into KCl and lead to an increase in the release rate of K during pyrolysis.Next,the secondary transformation of released Cl in vapor phase and vapor-solid interaction during biomass pyrolysis has been systematically investigated.The vapor-phase reaction of HCl with volatiles produces 200-940μg of CH₃Cl during the pyrolysis of per gram of alkali lignin,pectin and acid-washing poplar.Increasing temperatures of both pyrolysis and vapor-phase reaction promotes the generation of CH₃Cl.The solventisation of HCl and the protonation of compounds containing methoxyl group are key steps in the methylation reaction.The vapor-solid interaction of HCl with the pyrolyzing biomass/char causes a significant increase in the production of CH₃Cl,which is caused by the combination of HCl with methyl radicals generated in the early stages of pyrolysis.The addition of K₂CO₃ promotes the generation of both compounds containing methoxyl group and methyl radicals,thereby enhancing the methylation of HCl.Subsequently,the effects of temperature,gas composition and Cl content on K transformation have been investigated by combusting wheat straw and sugarcane bagasse in the DTF system.The results show that the release rate of K during the combustion of wheat straw is higher,while the water-soluble K in sugarcane bagasse is more easily converted to acid-soluble K.Then,as the combustion temperature increases,the acid-soluble K can be converted to insoluble K.In the vapor phase,the main K-containing compound is KCl.Increasing the temperatures of both combustion and vapor-phase reaction or introducing CO₂ and H₂O result in the conversion of KCl and K₂SO₄ to other forms such as KOH.High Cl content in biomass usually promotes the release of K as KCl and inhibits the formation of K-silicoaluminates and the sulfation of K in the vapor phase.Finally,the effect of Cl release during the low-temperature pyrolysis of wheat straw on the release and sulfation of K during char combustion has been further investigated.The release ratio of Cl during pyrolysis at 250-400°C ranges from 11.7%to 35.4%,accompanied by the increase in the proportion of ion-exchanged K from 15.3%to 27.5%,which causes the retention rate of K in ash and the sulfation rate of K in the vapor phase increasing by a maximum of 7.0%and 15.5%.In addition,it also increases the capture rate and sulfation rate of K by additives by a maximum of 17.1%and 25.2%.