Preparation of a fluorine-free foam for the prevention and control of spontaneous combustion of coal and its flame-retardant properties

Abstract

The application of fluorocarbon surfactants is limited because of their difficulty in biodegradation and serious environmental hazards. On this basis, an eco-friendly fluorine-free foam extinguishing agent was developed. A polyether-modified polydimethylsiloxane (Silok 8022), α-olefin sulfonate (AOS) and sodium lauroamphoacetate (LAMC) were used as composite foaming agents, and Mg-Al hydrotalcite (LDHs) was used as the inhibitor. An interfacial rheometer, a foam scanner and a cone calorimeter were used to study the compounding scheme and dosage of surfactants. Results revealed that the foam formed exhibited the best performance when the mass concentrations of Silok 8022, AOS and LAMC were 0.03 %, 0.1 % and 0.075 %, respectively, and that of the LDHs was 1 %. Then, the wettability of fluorine-free foam for coal and its rheological properties were systematically analysed via an interfacial rheometer and rotary rheometer. Compared with the surface free energy (42.52 mJ/m²) of the water-treated coal samples, the surface free energies of the coal samples treated with the coal fly ash three-phase foam, fluorine-containing aqueous film-forming foam and fluorine-free foam were 46.72, 48.88 and 52.83 mJ/m², respectively. The fluorine-free foam achieved the best wettability for coal. Three-stage low–high–low shear tests and amplitude scanning tests revealed that the shear structure recovery rate of fluorine-free foam was 94 %, and the linear viscoelastic region was 0.01 %–1 %. The fluorine-free foam had the strongest self-repair ability and deformation resistance. The inhibition performance of fluorine-free foam was studied by cone calorimeter experiment. It is concluded that fluorine-free foam effectively inhibits the combustion of coal. The flame retardant effect is remarkable. Finally, an experimental platform for small-scale fire extinguishment was constructed to research the fire-quenching ability of fluorine-free foam. Fluorine-free foam has better fire extinguishing and cooling effects on coal pile fires than fluorine-containing aqueous film-forming foam, which consistently and evenly covers the coal surface and rapidly permeates the coal. In addition, the coal body is not reignited, confirming its excellent fire-quenching and flame-retardant performance.