Microwave-assisted and analytical pyrolysis of coking and non-coking coals: Comparison of tar and char compositions

Abstract

Pyrolysis conditions and mechanism greatly influence the quality of tar and char from coal, which eventually determine their further usage. This work investigates the pyrolysis of three coking coals and a non-coking high ash Indian coal via microwave-assisted and conventional heating, and the influence of heating rate, coal type, and heating mechanism on product yields and tar composition. Microwave pyrolysis experiments were conducted in a home-built microwave reactor, and analytical pyrolysis experiments were conducted in a Pyroprobe® interfaced with gas chromatograph-mass spectrometer (Py-GC/MS). The tar yields from microwave pyrolysis of coking coals were similar (∼7 wt.%), while it was slightly high from high ash Indian coal (∼11 wt.%). Char yields from both microwave and analytical pyrolysis were similar (78 ± 1.5 wt.%). Microwave pyrolysis tars were rich in naphthalene derivatives and polyaromatics with more than 45% selectivity, while analytical pyrolysis tar was rich in monoaromatics (>30%). There was no significant variation in the selectivities of other organic fractions. High selectivity of polyaromatics in microwave pyrolysis tar is due to the generation of localized hotspots, which led to excessive cracking of primary pyrolysis vapors. The higher heating value (HHV) of microwave pyrolysis tar was 32–33 MJ kg−1, while that of analytical pyrolysis tar was higher (35–36 MJ kg−1), which is attributed to the presence of more monoaromatics in the latter. Hydrogen and methane were the major components in the gaseous fraction. Coal chars were characterized using different techniques, and they were rich in carbon (≈80 wt.%) with high energy content (29 MJ kg−1).