Modification of USY by leaching and Zr, Ni loading for upgrading pyrolysis volatiles from coal tar residue

Pyrolysis is considered as an effective method to convent coal tar residue (CTR) with carcinogenic polycyclic aromatic hydrocarbons into high value-added products. However, high contents of heavy components in tar restrict the efficient utilization of CTR. In this paper, USY zeolite was leached by HNO and subsequently loaded by Zr and Ni for upgrading of CTR pyrolysis volatiles. The effects of the modified catalysts on the distribution of pyrolysis products, the yield and compositions of light tar (boiling point below 360 °C) were investigated. Results showed that compared with raw USY, the modified USY with HNO leaching for 2 h (UN2) improved light tar fraction up to 63.55 % and naphthalene content in tar increased by 96.01 %. The contents of toluene, xylene and naphthalene in tar were 32.00 %, 32.89 % and 11.57 % higher than that of UN2 and light tar fraction was improved up to 66.00 % when 10 wt% Zr was introduced into UN2 (10Z-UN2). Ni loading resulted in further upgrading of the tar. The fraction of light tar was up to 73.13 % at Ni content of 8 wt% (8Ni-10Z-UN2), and toluene content in upgraded tar was 3.56 times that over 10Z-UN2. To further improve the upgrading performance, steam was introduced in the action of 8Ni-10Z-UN2. The highest yields of tar (41.56 wt%) and light tar (31.07 wt%) were obtained at the mass ratio of steam to material (S/M) being 0.28, and the average molecular weight of tar was decreased to 219 amu from 744 amu before upgrading. Isotope tracing using DO as tracer was coupled with the cracking of cetane, -cresol and 1-methylnaphthalene as tar model compounds over 8Ni-10Z-UN2 to reveal the mechanism for high tar yield under steam. It was confirmed that •H produced by reaction of HO with model compounds participated in tar formation over 8Ni-10Z-UN2, and the contents of benzene and naphthalene were obviously enhanced. This work provides a sample route to upgrade the CTR pyrolysis volatiles into aromatic hydrocarbons.