Role of neutral extractives and inherent active minerals in pyrolysis of agricultural crop residues and bio-oil formations

The work reports a comparative study on the pyrolysis of five agricultural crop residues (peanut straw, cotton straw, sorghum stalk, corn stalk and reed) using a gas-sweeping fixed bed reactor. The study aims at clarifying the role played by the neutral extractives and inherent active minerals in the pyrolysis characteristics, product distributions and bio-oil compositions. Among five crop residues, reed showed the highest bio-oil yield but with the highest oxygen content and the lowest HHV (higher heat value), in relation to the least alkali and alkaline earth metals (AAEMs) in it. Peanut straw had a content of neutral extractives as high as 47.4%, which contributed to the formations of a significantly higher proportion of long chain aliphatic hydrocarbons and liquid nitrogenous compounds, distinct from four other crop residues. Peanut straw was also highly enriched with AAEMs, which was responsible for a strong catalytic activity for the selective deoxygenation of bio-oil. Neutral extractives in sorghum stalk and corn stalk formed some long chain fatty oxygenates and less nitrogenous compounds. The bio-oils derived from these two biomasses contained oxygenates as dominant compounds, however, they had a lower fraction of heavy constituents, lower oxygen contents, and higher HHVs than those of reed and cotton straw. This result was attributed to the catalyzed bio-oil deoxygenation by the relatively abundant AAEMs in sorghum stalk and corn stalk.