Pyrolysis of pig waste from intensive farming operations: Kinetics, product distribution, and transformation of endogenous heavy metals

Abstract

This study addresses the critical challenge of safe disposal and resource utilization of pig waste from intensive farming by investigating its pyrolysis behavior, product mechanisms, and heavy metal immobilization. The pyrolysis process was divided into four distinct stages, with kinetic parameters determined using the model-free FWO and KAS methods, ensuring accuracy without prior assumptions, complemented by the master plot method to elucidate reaction mechanisms. The optimal kinetic models identified as $f\left(\alpha \right)=4{\alpha }^{\frac{3}{4}}$, $f\left(\alpha \right)=3(1-\alpha ){[-\ln (1-\alpha \left)\right]}^{\frac{2}{3}}$, $f\left(\alpha \right)=3{\alpha }^{\frac{2}{3}}$, $f\left(\alpha \right)=2{\alpha }^{\frac{1}{2}}$. The volatile products, identified via TG-FTIR and Py-GC/MS, included CO₂, H₂O, CH₄, CO, carbonyl compounds, while pyrolysis oils were enriched with nitrogen-containing organics and ketones. Biochar produced at 450–750 °C demonstrated rich porosity, aromaticity, and high biochemical stability, with heavy metals predominantly immobilized in oxidizable and residual fractions (F3 +F4). FactSage 7.1 simulations provided novel insights into the phase transformations and migration trends of Zn, Mn, Cu, and Cr, highlighting the formation of stable slag or solid compounds at elevated temperatures, consistent with experimental findings. TCLP tests and ecological risk assessments confirmed the environmental safety of pig waste biochars, emphasizing the potential of pyrolysis for heavy metal stabilization. This work advances the understanding of pyrolysis mechanisms for pig waste from intensive farming and offers innovative solutions for sustainable pig waste management.