Effects of mineral composition on tar yield and pyrolysis behavior of tar-rich coal from the Ningtiaota Coal Mine, Northern Shaanxi

Mineral matter can substantially affect the tar-generation potential of tar-rich coal, but how mineral-specific effects are reflected in natural mineral assemblages remains insufficiently constrained. In this study, samples from the Ningtiaota 4⁻¹ coal seam were characterized using X-ray diffraction, Gray–King low-temperature distillation, controlled mineral-addition experiments, gas chromatography, and Fourier-transform infrared spectroscopy of semi-coke. Quartz, kaolinite, calcite, and pyrite were identified as the dominant mineral phases in the studied coal samples. The controlled addition experiments showed that mineral type and loading level exerted distinct influences on pyrolysis product distribution. At 20 wt% loading, kaolinite decreased the tar yields of clarain and durain from 10.99% and 8.50% to 10.04% and 8.05%, respectively, while calcite reduced them to 10.29% and 8.25%. Pyrite produced only a limited increase in tar yield, whereas quartz had little effect under the present experimental conditions. Gas-product evolution and Fourier-transform infrared parameters indicate that mineral addition influenced the release and transformation of aliphatic structures during pyrolysis. Correlation analysis of natural coal samples further showed that variations in tar yield were closely associated with mineral assemblages, especially calcite-rich assemblages. Overall, these results suggest that mineral composition is an important factor influencing tar generation in the studied coal seam, with mineral-associated inhibitory effects being more evident than promotional effects under the present experimental conditions.