Transformation of HCl during pyrolysis of biomass and its model compounds

The release behavior of chlorine (Cl) during biomass pyrolysis has not been fully understood yet due to the complex transformation of hydrogen chloride (HCl). Here, we systematically investigate the transformation of HCl in vapor phase and its interactions with pyrolyzing biomass/char particles during biomass pyrolysis. Four biomass model compounds (cellulose, xylan, pectin, and alkali lignin) and an acid-washed wood were pyrolyzed in a fixed-bed reactor at 300–600 ℃ to produce Cl-free volatiles that reacted with the HCl injected downstream of the reactor. To explore the vapor–solid interactions of HCl with pyrolyzing biomass/char particles, the acid-washed wood was also pyrolyzed in a HCl atmosphere. The role of potassium (K) in the transformation of HCl was revealed via pyrolyzing a wood loaded with potassium carbonate (K2CO3). For all the experiments, the Cl distributed in char, heavy oil (condensed at 110 ℃), light oil (condensed at 0 ℃), and pyrolytic gases (as CH3Cl) was quantified. The results demonstrate that the vapor-phase reactions of HCl with the volatiles from pectin, lignin, and the acid-washed wood generate considerable amounts of methyl chloride (CH3Cl) but do not contribute to organic Cl in bio-oil. Increasing the temperatures of both pyrolysis and vapor-phase reaction promotes the generation of CH3Cl. The vapor–solid interactions of HCl with pyrolyzing biomass/char particles substantially enhance the formation of CH3Cl during the pyrolysis of the acid-washed wood at 300–500 ℃ because of the possible combination of HCl with the methyl radicals released in the initial stage of lignin pyrolysis. In addition, the vapor–solid interactions cause 120–490 μg/g feedstock of water-soluble Cl and 410–1120 μg/g feedstock of water-insoluble Cl being retained in the chars, which are produced from the capture of HCl by pores and carbon active sites in chars, respectively. The addition of K2CO3 catalyzes the cracking of lignin and thereby enhances the methylation of HCl in both vapor-phase reactions and vapor–solid interactions. It also increases the retention of Cl in the chars due to its capture by K.