Effects of Aaems Removal and Addition in Biomass on Co-Pyrolysis Characteristics and Product Distribution of Coal and Biomass

Abstract

The co-pyrolysis of coal and biomass, recognized as an effective carbon reduction technology, has attracted considerable attention. Thermogravimetric (TG) analysis and fixed-bed reactors were employed to investigate the effects of alkali and alkaline earth metals (AAEMs) on the pyrolysis characteristics and product distribution during the co-pyrolysis of coal and biomass. AAEMs not only reduced the pyrolysis temperature and decreased the maximum weight loss rate, thereby enhancing char yield, but they also promoted the catalytic cracking of the organic matrix. The TG results indicated that the enhancing effect of AAEMs on char yield followed the order: K > Na > Mg > Ca during the pyrolysis of biomass alone. However, during the co-pyrolysis process, the order shifted to Ca > K > Na > Mg. In the fixed-bed co-pyrolysis experiments, the observed order was Ca > Mg > K > Na. Compared to the pyrolysis of biomass alone, Ca significantly enhanced char yield during the co-pyrolysis process. This result was attributed to the formation of Ca(Al2Si2O8). The catalytic cracking effects of AAEMs on the organic matrix were influenced by the metal content and their activity on the char surface. Energy Dispersive Spectroscopy (EDS) results indicated that Na exhibited the highest content and activity, thereby exerting the strongest catalytic cracking effect on the organic matrix. Despite the high concentration of Ca, the formation of Ca(Al2Si2O8) resulted in an increased char yield. AAEMs were effective in promoting decarboxylation and dehalogenation reactions. During decarboxylation, Na activated hydrogen atoms stabilize alkyl radicals. In the dehalogenation reaction, Na activated iodine atoms enhance the process. The promotional effect of Na on dehalogenation reactions was stronger than its effect on decarboxylation reactions. The alkane content in the liquid-phase products increased as a result of both the dehalogenation and decarboxylation reactions.

Keywords: Coal and Biomass, AAEMs, pyrolysis characteristics, Product Distribution, Decarboxylation, Dehalogenation