Effect of torrefaction on the catalytic post-treatment of sewage sludge pyrolysis vapors using γ-Al2O3

This work examines the influence of sewage sludge pre-treatment by torrefaction on the distribution and properties of the products obtained from its pyrolysis followed by catalytic post-treatment of the hot pyrolysis vapors. Sewage sludge was first torrefied in an auger reactor under two different conditions: temperature of 250 °C and 275°C and average solid residence times of 13 min and 24 min, respectively. Pyrolysis was conducted at 530 °C in a fluidized bed reactor. The catalytic post-treatment of the hot pyrolysis vapors was carried out in a fixed bed reactor using gamma-alumina (γ-Al2O3) as a catalyst. The experimental results showed that the combination of the torrefaction pre-treatment with the catalytic post-treatment of the hot pyrolysis vapors did not noticeably improved the properties of the liquid organic phases for use as a fuel, in terms of O/C molar ratio and nitrogen content, compared to pyrolysis of sewage sludge with the same catalytic post-treatment of the hot pyrolysis vapors but without the torrefaction pre-treatment. Generally, the torrefaction did not have effect on the type of chemical families identified in the liquid phases and had little effect on the proportion of these families. The advantages of the combination of treatments were, on the one hand, the catalyst saving resulting from the lower amount of vapors generated by the pyrolysis of torrefied sewage sludge and, on the other hand, the slight decrease in the coke yield.