Exploring the effects of calcium bicarbonate on endogenous phosphorus conversion and pyrolysis product distributions of oil crop residues

Phosphorus-rich oil crop residues hold significant potential for use as phosphate fertilizers. Pyrolysis can enhance the enrichment of phosphorus in bio-char, although the transformation process remains unclear. To improve the availability of phosphorus and optimize its forms in bio-char, calcium bicarbonate (Ca(HCO₃)₂) was introduced in this study to analyze its effects on the transformation of phosphorus forms and the distribution of pyrolysis products. The addition of Ca(HCO₃)₂ increased the carbon monoxide (CO) content in the pyrolysis gas. Meanwhile, Ca(HCO₃)₂ reduced the acidity of bio-oil while increasing the aliphatic hydrocarbons content. The introduction of Ca(HCO₃)₂ promoted the formation of calcium-phosphorus (Ca-P) compounds. When the addition of Ca(HCO₃)₂ was at 10 %, the content of hydrochloric acid-soluble phosphorus (HCl-P) reached its maximum at 74.3 %, which was advantageous for phosphorus utilization. However, excessive amounts of Ca(HCO₃)₂ can lead to the conversion of HCl-P into residual phosphorus (RP), making it essential to maintain an appropriate proportion. Increasing the pyrolysis temperatures enhanced the lower heating value (LHV) of the gas due to the increased concentration of combustible gases, such as hydrogen (H₂), methane (CH₄), and carbon monoxide (CO). Additionally, the levels of aromatic and aliphatic hydrocarbons in bio-oil increased with rising temperatures, which is significant for subsequent high-value applications. Furthermore, elevated temperatures promoted an increase in the total phosphorus content in bio-char. However, lower temperatures (≤500 ℃) facilitated the reaction between Ca(HCO₃)₂ and organic phosphates, leading to the formation of HCl-P. In contrast, higher temperatures (≥600 ℃) resulted in the conversion of HCl-P to RP.