Formation mechanism and the extinguishment performance of gel-stabilized foam for inhibiting coal spontaneous combustion

The main ingredients of the novel gel-stabilized foam including foaming agent, thickening agent, and crosslinking agent, was prepared in this study. And the formation mechanism, microstructure, and fire extinguishment characteristics were analyzed in the study. The results shows that the foaming agent prepared from sulfonate anionic surfactant and betaine amphoteric surfactant could eliminate the effects of crosslinking agent ions and finally enhance the viscoelasticity of the foam film. The thickening agent was a polymer compound prepared through a synergistic reaction between polymer X and polymer H, with a mass ratio of 4:6. Moreover, the organic crosslinking agent was obtained using the polyhydroxy compounds as organic ligands (LGD) to react with the inorganic A compound. The experimental data showed that the foam stability became higher significantly with increasing crosslinking agent concentration, but it also weakened the foam expansion ratio ascribed to the improved solution viscosity. For considering the comprehensive performance of the foam, the optimal ingredient ratios were determined to prepare gel-stabilized foam, including 3 g/L for crosslinking agent, 4 g/L for thickening agent, and 3 g/L for foaming agent. Microstructure analysis exhibited slower coarsening and gas diffusion rate between adjacent bubbles in gel-stabilized foam compared to traditional aqueous foam, which was ascribed to the thick viscoelastic shell formed in foam film, finally enhancing the foam stability. During the formation gel-stabilized foam, the A ions ionized out from the crosslinking agent would react with the cis-hydroxy bonds –OH in the molecular structure of the thickening agent to form gel in the foam film. This significantly increased the foam viscosity and the film thickness to improve the water-retaining capacity, which provided larger resistance for foam to retard the liquid flow in bubble film dominated by gravity. The excellent stability was beneficial for gel-stabilized foam to cover the burning coal for enough time to reduce temperature and control the fire. Thus, coal fire extinguishment test showed that gel-stabilized foam could completely cover the ignited coal within 30 min, and decreased the temperature to 36.1 °C, far below the coal temperature (158.2 °C) treated by traditional aqueous foam. The results indicated more superior performance in controlling coal fires for gel-stabilized foam due to its longer stability time and superior ability to hold water compared to traditional aqueous foam.