The effect of low molecular weight compounds in coal on the formation of light aromatics during coal pyrolysis

- Low molecular weight compounds in coal can promote the formation of BTEXN.
- Straight aliphatic hydrocarbons of extracts are the main hydrogen donors.
- The existing form of low molecular weight compounds has a significant effect on its H-donor activity.

The effect of low molecular weight compounds in KZ (Kazakhstan) brown coal on the formation and distribution of light aromatics such as benzene, toluene, ethylbenzene, xylene and naphthalene (BTEXN) during coal pyrolysis was investigated in this study online by Py-GC/MS. The results show that the BTEXN amount obtained from the pyrolysis of acid-washed KZ coal residues (AWKZR) by pyridine extraction is 33% lower than that from acid-washed KZ coal (AWKZ), indicating that low molecular weight compounds can promote the formation of BTEXN. The straight aliphatic hydrocarbons in low molecular weight compounds are the main hydrogen donor during the formation of BTEXN. The BTEXN amount from the pyrolysis of acid-washed KZ coal extracts (AWKZE) is only 14% of that from that of AWKZ, and thus the thermal cracking of coal macromolecular skeleton may be the main source of BTEXN. Compared with the KZ coal, the BTEXN amount obtained from the pyrolysis of the blends of KZ coal extracts (KZE) and residues (KZR) at 700 °C is reduced by 17%, thus indicating that the existing form of low molecular weight compounds, their bond with coal skeleton and spatial location have a significant effect on BTEXN formation during coal pyrolysis.