Preparation and characterization of a lignin based hydrogel inhibitor on coal spontaneous combustion

Abstract Although organic hydrogel fire-fighting materials have excellent properties in fire prevention and extinguishing, the harsh experimental conditions, combined with expensive synthetic raw materials, make organic gel fire-fighting marginally visible. This issue could be addressed by introducing lignin to the network of gel. Lignin is a macromolecular organic matter, which consists of phenyl propanol units and aromatic rings, and has excellent thermal stability and biocompatibility. The addition of lignin would improve the thermal stability of the hydrogel, increase the water absorption capacity, and make the hydrogel more porous. Copolymerization of lignin, N-isopropyl acrylamide (NIPAm) and acrylic acid (AA) through stirring at room temperature, followed by Fourier transform infrared spectrometry (FT-IR), thermogravimetry and differential scanning calorimeter (TG-DSC) analysis of the effect of lignin-based hydrogel on coal oxidation, the consumption of aliphatic hydrocarbons on the surface of coal slowed down, and the characteristic temperatures including critical temperature, initial exothermal temperature and the ignition temperature all increased significantly, while releasing less heat during the process of coal oxidation. The distributed activation energy showed that the presence of lignin-P(NIPAm-co-AA) increased the activation energy barrier throughout the entire oxidation process, with maximum increase of 37 kJ/mol, suggesting that lignin-P(NIPAm-co-AA) inhibited the coal oxidation effectively. Thus, the incorporation of lignin into P(NIPAm-co-AA) hydrogel represents positive benefits in terms of realizing high-value utilization, reducing the cost of hydrogel, and improving the fire extinguishing properties of hydrogel on coal oxidation.