Migration Mechanisms Of Chlorine And Potassium During Torrefaction And Subsequent Pyrolysis Of Rice Straw

Biomass generally contains high chlorine and alkali contents.The release of chlorine and alkali during biomass thermochemical conversion would bring about some equipment operation and environmental problems.This thesis investigates the migration behaviors of chlorine and potassium during biomass torrefaction.Biomass material include straw,sawdust,sargassum and cornstalk.The effects of torrefaction on the physicochemical properties of high temperature pyrolysis straw char are investigated.Furthermore,the impact mechanisms of torrefaction pretreatment on the migration behaviors of chlorine and potassium during straw high temperature pyrolysis are studied.Main results for this study are as follows:(1)Release and speciation transformation mechanisms of chlorine and potassium during biomass torrefaction are studied.Besides,the chemical composition,surface morphology and pore structure of torrefied biomass are also investigated.The result shows that the influence of torrefaction below 250 ? on the pore structure evolution of biomass is limited.Torrefaction at 300 ? results in a slight increase of mesopore and macropore specific surface area and a significant increase of micropore specific surface area.The release of chlorine was synchronous with devolatilization during torrefaction.Some part of chlorine in inorganic salts would transfer into organochlorine remained in the torrefied biomass.Release ratio of potassium is lower than that of chlorine during torrefaction and some part of water-soluble species would transfer into ion-exchanged species.Particle size has limited influences on release and transformation behaviors of potassium.There are particulates rich in potassium and chlorine dispersedly adhering to the surface of torrefied straw.(2)The influence of torrefaction pretreatment on the surface morphology,pore structure and graphitization of char during straw pyrolysis at 800-1200 ? is studied.The gasification activity of pyrolysis char of torrefied straw is also investigated.The result shows that torrefaction pretreatment results in a decrease of specific surface area of pyrolysis char.The more volatile matter removed during torrefaction leads to the lower specific surface area of pyrolysis char.Torrefaction at 300 ? results in a higher specific surface area during water washing straw pyrolysis.At the torrefaction temperature>250 ?,torrefaction leads to a lower graphitization degree of char during straw pyrolysis.Torrefaction at higher temperature would bring about a higher graphitization degree of char during pyrolysis of water washing straw.For both raw and water washed straw,torretaction leads to a decrease of gasitication activity of pyrolysis char.(3)Release and speciation transformation behaviors of chlorine and potassium during pyrolysis of raw and torrefied straw at 800-1200 ? is investigated.The impacts of torrefaction on total release ratios of chlorine and potassium during straw utilization with torrefaction and high temperature pyrolysis are discussed.The result shows that release ratio of chlorine during pyrolysis has a general trend of decreasing with the increased torrefaction temperature.Torrefaction increases the total release of potassium during pyrolysis below 900 0C.When the pyrolysis temperature higher than 1000 ?,total release ratio of potassium for torrefied straw is similar to that for raw straw.Torrefaction influences the release and transformation of chlorine and potassium during high temperature pyrolysis through five pathways as follows.First,organochlorine forms during torrefaction.Second,water-soluble potassium transfers into the ion-exchanged potassium.Third,torrefied straw contains lower volatile contents than raw straw.Fourth,torrefaction diminishes the porosity of pyrolysis char.Fifth,particulates rich in potassium and chlorine occur on the surface of torrefied straw.