Temperature Influence and Distribution in Three Phases of PAHs in Wet Sewage Sludge Pyrolysis Using Conventional and Microwave Heating

Pyrolysis of wet sewage sludge with a moisture of 86.47 wt % was conducted using electric heating (conventional pyrolysis, CP) and microwave heating (microwave pyrolysis, MWP) separately on two sets of batch reactors. Sixteen polycyclic aromatic hydrocarbons (PAHs) characterized by United States Environmental Protection Agency (U. S. EPA) as priority pollutants were determined by gas chromatography/mass spectroscopy (GC/MS) in gas, liquid, and solid phases of pyrolysis products to get detailed transmission characteristics of PAHs in sewage sludge pyrolysis. Temperatures of 400-900 degrees C and microwave power of 400-900 W were investigated. Production yields in MWP and CP behaved similarly for different power in MWP and for different temperatures in CP, whereas liquid yielded more than 75% of the total weight for the high content of water in wet sewage sludge. Liquid yield went through a high peak at 500 W for MWP and 700 degrees C for CP. Total yield of PAHs had trend of increase in MWP, whereas it peaked at 700 degrees C in CP. Naphthalene, fluorene, phenanthrene, and anthracene contributed most of the yield change, which can be explained by integration of the secondary pyrolysis of solid, decomposition of PAHs and synthesis of gas in different temperature. CP yielded more PAHs than MWP, which is more obvious in gas fraction than oil fraction. Distillation effect had great influence on PAHs distribution between gas and oil fraction. PAHs yield in solid for MWP decreased with increase of power, which might be attributed to the special effect of microwave on the microscope characteristic of solid, whereas PAHs in solid for CP were even lower and would cause fewer environmental problems. The results also indicated "nonthermal" impacts of microwave on the process.