Catalytic fast pyrolysis of coal tar asphaltene over zeolite catalysts to produce high-grade coal tar: An analytical Py-GC/MS study

Coal tar asphaltene (CTA) is a low-grade fuel with a high content of oxygen, high-viscosity and high-toxicity. Three kinds of zeolite catalysts ZSM-5, HY and USY were used for catalytic fast pyrolysis (CFP) of CTA to upgrade its pyrolysis vapors by using pyrolysis-gas chromatography/mass spectrometry (Py-GC/MS). The effects of the heating rate (1 degrees C/ms, 5 degrees C/ms, 10 degrees C/ms and 20 degrees C/ms), the pyrolysis time (20 s, 50 s, 80 s and 100 s), the catalyst types and the mass ratio of CTA-to-catalyst (MRCTC, 4:1, 2:1, 1:1 and 1:2) on the pyrolysis product distribution were investigated. Results show that a higher heating rate (20 degrees C/ms) and a shorter pyrolysis time (20 s) are more beneficial to improve the selectivity of aromatic hydrocarbons (ArHs). Moreover, Compared with the non-CFP of CTA, ZSM-5, HY and USY promote the decomposition of organic oxygen compounds (OOCs) and the generation of light ArHs (L-ArHs, e.g. benzene, toluene, xylene and naphthalene (BTXN)) from polycyclic ArHs (P-ArHs). The highest yield of BTXN has been obtained at the MRCTC of 1:1 over HY, which is 23 times than that obtained from fast pyrolysis of CTA. Viewed as a whole, the catalysis over these catalysts are capable to convert these undesirable pyrolysis products (containing OOCs, P-ArHs or heteroatom compounds) into BTXN through various reactions, and thus to decrease the oxygen content, aromaticity and toxicity of CTA pyrolysis products. Furthermore, the catalytic mechanisms of "Hydrocarbon pool" and "Phenolic pool" were also proposed.