The improvement effect and mechanism of surfactants on the fluidity of coal tar

Abstract

During in-situ underground coal pyrolysis, the tar generated ascends from the high-temperature zone toward the surface and cools naturally. This cooling induces a sharp viscosity increase that blocks the extraction channel, impeding engineering implementation. To address this obstruction, this study systematically evaluates the viscosity-reduction performance of five chemical agents—sodium dodecyl sulfate (SDS), sodium dodecyl benzene sulfonate (SDBS), anhydrous betaine (BA), polyvinyl alcohol (PVA), and Tween 80—at 30 °C, simulating extraction conditions. Reduction ratios are measured across 1–9 wt% dosage. Contact angle, surface tension, and polarized-light microscopy reveal the underlying mechanisms. Results identify BA as the optimal agent, delivering a maximum viscosity reduction of 34.13% at 5 wt% dosage, representing a 2.2-fold improvement over simple water dilution (15.73%). This value is comparable to the ∼30% reduction reported for heavy oils using similar nonionic surfactants. Its superior performance originates from a marked decrease in oil–water interfacial tension and the formation of uniformly dispersed, near-spherical oil droplets. PVA, in contrast, increases system viscosity because of incompatibility with the tar. This work presents the first systematic comparison of anionic, zwitterionic, and nonionic surfactants for tar viscosity reduction under in-situ pyrolysis extraction conditions. The findings provide directly transferable guidance for field agent selection and dosage optimization.