Microwave pyrolysis of textile dyeing sludge in a continuously operated auger reactor

Textile dyeing sludge (DS) was pyrolysed using an auger pyrolyser under microwave irradiation. It showed the non-condensable gas yield was increased as reactor temperature increased and the char yield decreased with temperature. The condensate and oil yields reached maximum at 650 °C (i.e. 12.86 wt% and 0.84 wt%, respectively). The content of C, H and N in DS and sludge char (SC) decreased with temperature while S increased. The SC and sorbents (e.g. CaO) showed positive effects on sulfur retention. The maximum BET surface area of 91.9 m2/g was observed for SC at 550 °C. Addition of catalysts such as CaO and Fe decreased pyrolysis oil yield and increased the H2 yield. Oxygenates were mainly formed at 450-650 °C. The relative content of aromatic compounds in the pyrolysis oil increased as temperature increased, up to 88.38% (GC-MS area) at 750 °C. Heterocyclic aromatic compounds containing nitrogen accounted for 20%-58% of the pyrolysis oil. The H2 content increased from 25.39 vol% without catalyst to 64.17 vol% with addition of 30 wt% CaO. As concentration in DS and SC was beyond the control standards for forestland use in China and should be further treated and disposed. The electricity consumption for pyrolysing DS of 60 wt% water content were 0.76-2.59 kWh/kg wet DS in the temperature range of 450-750 °C and auger speed range of 1-9 rpm. Higher pyrolysis temperature and lower auger speeds increased electricity consumptions.