Ash-yield correlations of SAXS-derived apparent nanopore parameters in low-rank coals

Ten low-rank coal samples from different regions of China were selected to investigate their apparent nanopore structures by synchrotron small-angle X-ray scattering (SAXS). The relationships between ash yield and SAXS-derived apparent pore structure parameters were discussed in combination with proximate and ultimate analysis results. The results show that all ten samples exhibit positive Porod deviation. After correction, porosity, specific surface area, and pore-size distribution parameters were obtained. The average pore radius of the samples ranges from 6.70 to 12.26 nm, and the main peak radii are concentrated at 0.5 and 1.5 nm. The SAXS-derived apparent porosity and specific surface area are 0.00692–0.04747 and 34.8003–78.5669 m2/cm3, respectively, and both generally increase with increasing ash yield A(d). Pearson correlation analysis shows that the correlation coefficients of ash yield A(d) with porosity and specific surface area are 0.961 and 0.955, respectively, which are the highest among the physicochemical parameters considered. No clear correlation is observed between average pore radius and ash yield A(d). These results provide cross-sample evidence for apparent correlations between ash yield and SAXS-derived apparent porosity/specific surface area in untreated natural low-rank coal samples.