Catalytic upgrading of coal pyrolysis tar over char-based catalysts

•Dual-stage fixed bed was used to study catalytic upgrading of coal pyrolysis tar.•Tested catalysts were char and char impregnated with CoCl2, NiCl2, CuCl2 and ZnCl2.•Char catalyst leaded to higher light tar fraction and little changed light tar yield.•Metal-char catalyst was more effective than char for upgrading coal pyrolysis tar.•Of the tested catalysts the Ni-char catalyst showed the best tar upgrading effect.

Catalytic upgrading of coal pyrolysis tar was investigated in a dual-stage reactor over char and metal-impregnated char (Co-char, Ni-char, Cu-char, Zn-char). The catalytic upgrading caused the lower total tar yield and the higher non-condensable gas yield but the fraction of light tar (boiling point < 360 °C) obviously increased to allow slightly higher total yield of light tar. When the catalytic upgrading was at 600 °C over a layer of char having a mass of 20% of the tested coal, the resulting light tar fraction in the tar increased by 25% in comparison with the coal pyrolysis only at 600 °C. Over the metal-impregnated char, which was 5% of the tested coal in mass, good upgrading effect was obtained at 500 °C. The catalytic tar-upgrading activity decreased in an order of Co-char > Ni-char > Cu-char > Zn-char, and over Ni-char the realized light tar yield and its content in the tar increased by 17.2% and 32.7%, respectively. The upgrading effect also lowered the contents of element N and S in the resulting tar by 45.6% and 43.5%, respectively. NH3-TPD clarified that the order in acidity of the char-based catalysts was the same as for the upgrading activity shown above.

Graphical abstractThis figure compares the fraction and yield of light tar (boiling point < 360 °C) obtained from coal pyrolysis under the conditions without (Blank) and with secondary catalytic upgrading over various char-based catalysts. In comparison with the pyrolysis without secondary upgrading, the adoption of the secondary upgrading all evidently increased the light tar fraction, while only the catalysts impregnated with metal species caused obvious increase in the light tar yield. The Ni-char catalyst showed the best upgrading effect, and next to this was the Co-char catalyst. These results demonstrated that the secondary catalytic upgrading greatly improved the tar quality, and this would greatly facilitate the downstream treatment for the tar.Figure optionsDownload full-size imageDownload as PowerPoint slide