The effect of acid washing on the pyrolysis products derived from a vitrinite-rich bituminous coal

• Acid washing decreases water and tar yields of the coal.
• Acid washed coal derived gas contains more H2, CH4, CO2, C2H4, C2H6, C3H4, C3H6 and C4s.
• Acid washed coal derived tars are more aromatic than raw coal tars.
• Coal tars contain lighter boiling point species than acid washed coal tar.

Extensive characterization of the pyrolysis products, derived from raw and acid washed samples of a vitrinite-rich South African bituminous coal is reported. Pyrolysis experiments were carried out with use of a modified Fischer Assay setup at 520, 750 and 900 °C. Gaseous products were analyzed by gas chromatography (GC); tar yields by simulated distillation (SimDis), gas chromatography mass spectrometry and –flame ionization detection (GC–MS and –FID) and size exclusion chromatography (SEC-UC), and the char yields by proximate, ultimate and Brunauer–Emmett–Teller (BET) CO2 adsorption analyses. The water and tar yields of the acid washed coal fraction (AW TWD) was found to be lower, whilst the gas yields were found to be significantly higher than that of the raw coal fraction (TWD). The char yields were not significantly affected by acid washing. Some of the differences in pyrolysis product yields can be related to increased porosity of the acid washed coal fraction. GC analysis of the derived pyrolysis gases indicated that the AW TWD derived gas contained higher yields of H2, CH4, CO2, C2H4, C2H6, C3H4, C3H6 and C4s when compared to the gas derived from the TWD fraction, whilst the CO yield from the TWD fraction was greater at all final pyrolysis temperatures. Analyses of the tar fraction by means of SimDis, GC–MS and –FID and SEC-UV indicated that the acid washed coal derived tars where more aromatic in nature, containing more higher boiling point components, which increased with increasing final pyrolysis temperature. On the other hand, the tars derived from the TWD coal contained lighter boiling point components with increasing final pyrolysis temperature. The changes in the pyrolysis behavior of the bituminous coal are due to the removal of the mineral matter and also the influence of the acid washing process. This study confirmed that acid washing of coal changes the pyrolysis products composition of a bituminous coal.