Lignin-Based Hydrogel Reinforced Dry Water as Inhibitor for Coal Spontaneous Combustion

Dry water is a green fire extinguisher agent with good cooling and fluidity that is composed of hydrophobic silica and tiny liquid droplets. However, the structure of dry water is unstable and easily broken during production, transportation and storage. In this study, a lignin-based hydrogel was employed as the core material and hydrophobic silica as the wall material to develop a hydrogel reinforced dry water suitable for preventing coal spontaneous combustion. The bulk density, fluidity, stability and particle size distribution of the synthesized hydrogel reinforced dry water were all measured. Compared to traditional dry water, the lignin-based hydrogel reinforced dry water had a better angle of repose and outflow velocity, measuring 19.53° and 9.40 mL/s, respectively. Additionally, the bulk density and water retention in an open environment were 2.6% and 5.2% higher than traditional dry water, respectively. The particle size distribution of the lignin-based hydrogel reinforced dry water is likewise more uniform. Thermal analysis was used to examine the effect of the reinforced dry water on coal oxidation. The results showed that the reinforced dry water reduced the mass increase caused on by oxygen adsorption, hence inhibiting coal oxygen chemisorption. Further investigation revealed that addition of reinforced dry water improved the inhibitory performance of coal spontaneous combustion by decreasing coal oxidation heat release by 4.2% and 11.2%, respectively, in comparison to raw PING and YZ coal, and by increasing coal oxidation activation energy, with maximum increases of 23 and 265 kJ/mol for PING and YZ, respectively.