Development of a Multi-Stage Composite Retardant for Coal Spontaneous Combustion Using OPC-Modified Hydrogel and Ammonium Polyphosphate

Coal spontaneous combustion is a major hazard threatening mine safety and environmental protection. A green composite retardant was developed by grafting oligomeric proanthocyanidins (OPC) from grape seeds into a sodium-alginate/acrylic-acid (SA/AA) hydrogel and combining it with ammonium polyphosphate (APP). HPLC, FTIR, TG/DTG, programmed-temperature gas analysis, iso-conversional kinetics (KAS and FWO), and DFT calculations were employed to evaluate its performance. OPC grafting improved water retention and thermal stability of the hydrogel. The treated coal exhibited increased apparent activation energies (≈12.15 kJ·mol−1 at low conversion and ≈26.20 kJ·mol−1 at higher conversion), a maximum CO inhibition rate of 86%, and a crossover point temperature shift from 150.7°C to 170.5°C. DFT analysis revealed a low O – H bond dissociation energy (~68.05 kcal·mol−1), confirming effective radical scavenging. This study proposes a multistage inhibition strategy in which OPC, the hydrogel network, and APP play complementary roles at different temperature ranges, thereby achieving continuous suppression of coal spontaneous combustion from low-temperature radical scavenging to high-temperature char formation.