Two-stage pyrolysis of municipal sewage sludge: Effect of cracking temperature

Sewage sludge samples were pyrolyzed in a two-stage fixed-bed reactor. The primary volatiles derived from sewage sludge pyrolysis were secondarily cracked through a tubular reactor with and without a char catalyst at predetermined temperatures. The objective of this research was to study the effects of cracking temperature on the three-phase products of sludge pyrolysis. Experimental results demonstrated that cracking temperatures above 600 °C significantly increased the gas yield and reduced the liquid yield. When the cracking temperature reached 700 °C, compared with one-stage pyrolysis, the cumulative yields of all types of main gaseous products increased, especially that of CH 4 , with its yield increasing by a factor of 5.98. The conversion of the pyrolysis liquid to gas products mainly depended on the decomposition of the aliphatic compounds contained in the liquid. In the pyrolysis liquid, monoaromatic and long-chain aliphatic nitrogen compounds decreased with increasing cracking temperature, whereas polycyclic aromatic hydrocarbons (PAHs) increased dramatically. Aliphatic ketones decomposed rapidly with increasing temperature and almost disappeared at 700 °C. The presence of the char catalyst not only further promoted the generation of gaseous products and reduced the yield of liquid products but also promoted the decomposition of long-chain aliphatic nitrogen compounds, monoaromatics, and PAHs. • The effect of cracking temperature was examined in a two-stage pyrolysis experiment • A high cracking temperature significantly increased the gas yield and decreased the liquid yield • The yield conversion mainly depended on the decomposition of aliphatic compounds • Char promoted the decomposition of aliphatic nitrogen compounds, monoaromatics, and PAHs