Migration and speciation transformation of K and Cl caused by interaction of KCl with organics during devolatilization of KCl-loaded model biomass compounds

To better understand migration of K and Cl caused by interaction of KCl with organics, KCl-loaded model compounds (cellulose, xylan, lignin and pectin) were pyrolyzed within 300-800 degrees C using a fixed-bed apparatus. The amounts of different chemical occurrences of K and Cl in chars were quantitatively determined by sequential chemical fraction analysis combined with ICP-OES and IC. Morphologies and occurrence modes of Cl and K were further examined via XPS and SEM-EDS. Big differences in K and Cl migration were observed among four samples, and the distinction was closely related to types of organic functional groups. At temperature of below 500 V, a low proportion of K was released from four samples, while amounts of water-soluble K was transformed into organically-bounded form via different ion-exchange reaction mechanisms, thereby promoting Cl release. About 46.93-53.31% and 32.47-41.75% of organic-K, presumably formed by reaction of KCl and methoxyls, were detected in lignin and pectin chars, apparently higher than that in xylan and cellulose chars. At the same time, amounts (71.98-81.83%) of Cl was released from doped pectin, followed sequentially by lignin (35.12-52.34%), cellulose (18.56-35.68%), and xylan (10.67-18.65%). Part of released Cl was recaptured by active carbon sites, thereby forming organochlorine, and C-Cl generation followed a sequence of lignin > xylan > cellulose > pectin. As pyrolysis proceeded, cellulose exhibited highest K release due to decomposition of organic-K, while a considerable of char-K with high thermal stability still retained in other samples. Mechanism on migration of K and Cl caused by reaction of KCl with organics was proposed.