Microwave-assisted pyrolysis of textile dyeing sludge using different additives

- CaO and Fe increase H2 content up to 64.17 v%, and decrease CO2 content.
- Ca-bentonite and kaolin decrease H2 contents and non-condensable gas yield.
- CaO, Fe, Ca-bentonite reduce aromatics and increase oxygenates contents.
- CaO and Ca-bentonite enhance sulfur attention and some heavy metal immobilization.
- Maximum BET surface area of 122.6 m2/g is observed for SC with 20 wt% kaolin.

This paper investigated an auger pyrolyser under microwave (MW) irradiation for pyrolysis of textile dyeing sludge (DS). It showed that the sludge char (SC) yield decreased and the yield of non-condensable gas gradually increased with pyrolysis temperature. The condensate and oil yields reached maximum (i.e. 12.9 wt% and 0.8 wt%, respectively) at 650 °C. Addition of CaO and Fe increased the SC yield and H2 contents, and decreased CO2 content in the non-condensable gas. The H2 content increased from 25.4 v% without additives to 64.2 v% with addition of 30 wt% CaO at 650 °C. Whereas Ca-bentonite and kaolin decreased H2 contents with the latter decreasing SC yields and increasing condensate yields. The content of C, H and N in SC decreased with temperature while S increased. CaO and Ca-bentonite were beneficial for sulfur retention and some heavy metal immobilization for microwave pyrolysis of DS. The maximum BET surface area of 123 m2/g was observed for SC with addition of 20 wt% kaolin, consistent with SEM observations. The main components of the pyrolysis oil were analyzed by GC-MS. High proportions of aromatic compounds were found in the pyrolysis oil from different pyrolysis temperatures. Oxygenates were mainly formed in the temperature range of 350-650 °C. The content of aromatic compounds in the oil increased as temperature increased, up to 88.4% (GC-MS area) at 750 °C. Heterocyclic aromatic compounds containing nitrogen accounted for 30% ∼ 50% of the pyrolysis oil. CaO, Fe, Ca-bentonite considerably reduced the aromatics content in the pyrolysis oil. The additives used in this study acted as microwave absorbers (MWAs), catalysts, sorbents and reactants in the microwave assisted pyrolysis of DS. The combined functions of these additives and unique characteristics of MW heating contributed to different product yields, and various product compositions and product properties.