Influence of biochar with loaded metal salts on the cracking of pyrolysis volatiles from corn straw

In this study, a two-section pyrolysis reactor was used for the pyrolysis of corn straw at 550°C. The pyrolysis of corn straw was performed in the first section, while a biochar sample (untreated biochar, acid-washed biochar, or biochar loaded with NaCl, KCl, MgCl2, or CaCl2) was placed in the second section. The effects of the biochar and the metal salts it contained on corn straw pyrolysis were researched. The results showed that acid-washing pretreatment removed most of the metal ions (99.1% of K, 95.2% of Ca, 94.3% of Na, and 95.3% of Mg) in biochar. As the amount of acid-washed biochar added raised from 0 to 3 g, the bio-oil yield declined from 51.2% to 45.3%, the gas yield raised from 21.9% to 27.1%, and volume fraction of CO and H2 increased from 33.5% and 10.8% to 35.6% and 13.1% respectively. In addition, the acid-washed biochar promoted the decomposition of acid substances in the pyrolysis volatiles and the formation of phenolic substances. After loading with various metal salts in biochar, the bio-oil yield is further reduced while gas yield is further increased. Among the metal salts, KCl exhibited the strongest effects on the enrichment of phenolics and decomposition of acids. In addition, MgCl2 clearly promoted CO formation. The results in this study confirm that both biochar and its constituent metal ions play important roles in the secondary cracking of pyrolysis volatiles, and thus biochar has potential for application as a catalyst in biomass pyrolysis.