The carbon stability and energy characteristics of tea waste-derived biochar: Effects of pyrolytic temperature and co-pyrolysis with nanoscale zero-valent iron

Pyrolytic temperature and Fe addition are two typical factors widely used for modifying the characteristic of biochar, however, their co-effect on the C emission reduction (enhancing C stability) and fuel features of tea waste biochar remain unclear. Hence, this study systematically investigated the effects of pyrolytic temperature (300–900 °C) and nanoscale zero-valent iron (Fe) co-pyrolysis on the C stability and fuel features of TBC. Herein, H/C, (O + N)/C, FTIR spectrum, XRD spectrum, I_g/(I_d + I_g) and DOC release suggested that pyrolytic temperature improvement decreased the aliphaticity, polarity and DOC content, but increased the aromaticity and graphitic degree for TBC. Meanwhile, Fe co-pyrolysis decreased the polarity and enhanced the graphitic degree of TBC at 800–900 °C. Thermogravimetric analysis indicated that Fe co-pyrolysis lowered the thermostability of C. Differently, H₂O₂ oxidization method indicated Fe co-pyrolysis significantly enhanced the chemical stability of C. Furthermore, Uv–vis and fluorescence spectrum indicated that pyrolytic temperature improvement decreased the aromaticity and molecular size of biochar-derived DOC. Fe co-pyrolysis increased the release of large molecular humic-like matters rather than small molecular protein-like matters. All the results suggested high pyrolytic temperature and Fe co-pyrolysis could improve the environmental (physico-chemical) C stability of TBC and the C emission reduction. Additionally, TBC presented a considerable energy densification ratio (EDR) ranges (1.355–1.450) of wood- and straw-derived biochars, indicating TBC could be a potential high-performance biofuel to alleviate the energy crisis. This study provides important information to optimize pyrolysis conditions to re-use of tea waste for C emission reduction and fuel substitute.