Pilot-Scale Slow Pyrolysis, Post-Heat Treatment, and Self-Heating Performance of Biochar Fuels Derived from Construction, Renovation, and Demolition (CRD) Wood Waste

The accumulation of non-recyclable construction, renovation, and demolition (CRD) wood waste necessitates sustainable management strategies, for which thermochemical valorization is a promising option. Slow pyrolysis is particularly suitable due to its high biochar yields and potential to partially replace fossil coal in energy, metallurgical, construction, and environmental applications. In this study, end-of-life CRD wood was converted into biochar using a pilot-scale vertical retort–kiln system at furnace set-point temperatures of 600 °C and 800 °C for 4 h. The biochar produced at 800 °C, which exhibited superior characteristics, was subsequently subjected to post-heat treatment at 600 °C for 30–90 min in the presence of nitrogen within a tightly sealed rotary retort-kiln assembly. Self-heating behavior was evaluated using adiabatic oven tests at 120–140 °C. Biochar properties were characterized by proximate and elemental analysis, TGA/DTG, R50, FTIR, and SEM–EDX. Increasing the pyrolysis temperature to 800 °C increased carbon content from 49.88% in the raw feedstock to 85.11% in biochar, while oxygen and hydrogen contents decreased to 5.91% and 1.52%, respectively. Van Krevelen ratios (H/C = 0.21; O/C = 0.05) indicated enhanced carbon stability, with the higher heating value reaching 30.81 MJ/kg. The thermostable fraction reached 75.18%, R50 recalcitrance index 0.57, fixed carbon 70.59%, volatile carbon 23.31%, pH 8.9, and surface area 188.33 m2/g. Post-heat treatment further enhanced aromaticity (H/C = 0.18; O/C = 0.02) of this higher pyrolysis temperature biochar, increasing its fixed carbon and stability, and reducing volatile content. Extending treatment time from 30 min to 90 min raised fixed carbon to 77–78% and thermostability to 84–85%, while volatile carbon decreased to 13–15%. Microporosity peaked at 350–380 m2/g by 75 min before declining due to pore widening. SEM and EDX analyses confirmed this structural evolution, increased carbon content, reduced oxygen, suppressed alkali metals, and enrichment of alkaline earth metals. Yield loss was highest at 90 min (20–21%), highlighting the need to balance treatment severity and biochar product yield. Both the 800 °C biochar and its post-heat-treated forms passed self-heating tests, confirming improved oxidative stability for energy and environmental applications.