Evidence of reciprocal organic matter-pyrite interactions affecting sulfur removal during coal pyrolysis

Abstract

The mutual influences of the organic matter and pyrite on the removal of sulfur during coal pyrolysis were studied. Several high-sulfur, low-rank Spanish coals with different organic and pyritic sulfur contents were pyrolysed in a swept fixed-bed reactor to temperatures up to 800 °C at 5 K min −1. The coals and chars were demineralized with HF-HCl and examined by SEM-EDX and X-ray diffraction. A calibration curve of organic sulfur concentration vs. peak/background ratio was established for demineralized coals. The results indicated that organic sulfur tends to decrease with increasing pyrolysis temperature for coal samples containing very little or no pyrite. However, starting from 500 °C the organic sulfur content of chars from pyrite-containing coals increased, and the degree of dispersion of organic sulfur also increased with temperature. These findings indicated that H 2S released from pyrite decomposition can be reincorporated in char as organic sulfur. The influence of the organic matter on pyrite decomposition was investigated using float-sink fractions with pyrite contents of 0.4–33 wt% and organic matter contents of 20–80 wt%. Samples with higher pyrite contents released very little sulfur as H 2S at >630 °C, whereas samples with lower pyrite contents and hence higher organic matter contents evolved more H 2S. SEM-EDX and XRD showed that at 600 °C and with low organic matter contents, pyrite remained practically unaltered and pyrrhotite was very scarce. At 800 °C the transformation was incomplete, resulting in a pyrite core surrounded by a pyrrhotite shell. On the other hand, with high organic matter contents, pyrite particles showed evident signs of decomposition and were almost entirely transformed into pyrrhotite at 600 °C. These results show that only partial desulfurization of high-sulfur coals can be achieved by pyrolysis, owing to organic matter-pyrite interactions.