Effect of the heating rate on the thermochemical behavior and biofuel properties of sewage sludge pyrolysis

Treatment of sewage sludge is a major environmental concern for society. One treatment option that converts wastes into valuable biofuels is pyrolysis. The aim of this study was to investigate the effect of heating rates (10, 40, 70, and 100 °C/min) on the thermal behavior and biofuel properties of sewage sludge pyrolysis from room temperature to 1000 °C. Thermogravimetric analysis indicated three main reaction zones in respective temperature ranges of <180, 180-600, and >600 °C for the sewage sludge pyrolysis. The most intense mass loss rate took place at 210-600 °C, with a maximum value of 0.2 wt %/°C at around 330 °C. Increasing the heating rate resulted in shifting of the mass loss peaks to higher temperatures. Computer-aided thermal analysis was applied to identify the specific heat and reaction heats of sludge during heating. The endothermic reaction peaks at 150-300 °C and exothermic peaks at 500-600 °C shifted to higher temperatures when increasing the heating rate. Gas chromatography results showed that CO₂ was the primary biogas species evolved in the low-temperature range (<600-700 °C), while CO dominated in the high range (700-1000 °C). Generally, the peak values of evolution rates of CO₂ and CO increased at higher heating rates. Fourier transform infrared results showed that functional groups in the raw sewage sludge with wavenumbers above 1200 cm^-1 disappeared or reduced after the sludge was subjected to pyrolysis. The investigated heating rate did not have obvious effects on the functional groups in the pyrolysis biochars. Gas chromatography-mass spectrometry analysis was also conducted to identify the changing trend of compound contents of produced bio-oils obtained at investigated heating rates. Understanding the thermochemical behavior and biofuel properties of sewage sludge pyrolysis at different heating rates in this work will facilitate more elaborate control of sewage sludge pyrolysis to produce biofuels.