Characterization of nitrogen transformation during microwave-induced pyrolysis of sewage sludge

• Mass loss in the microwave pyrolysis was faster than in conventional pyrolysis.
• Microwave pyrolysis gave rise to more bio-gas productions (above 10%).
• Evolution of nitrogenous compounds in the char, tar and gas fractions was examined.
• Amines, nitriles and N-heterocycles were major intermediates in the tar and char.
• HCN yields accounted for about 50% of those of NH3 during the microwave pyrolysis.

The distribution of pyrolysis products and transformation of nitrogen in the char, tar and gas fractions at different temperatures were investigated during microwave-induced pyrolysis of sewage sludge. The results showed that under the temperature ranges investigated, the thermal decomposition of sewage sludge was faster in the microwave pyrolysis than in the conventional pyrolysis. At temperatures above 500 °C, microwave pyrolysis gave rise to a larger yield of bio-gas fraction (above 10%) and less bio-tar yield (below 15%) compared with those in the conventional pyrolysis. In addition, NH3 and HCN were the main nitrogenous gas during microwave pyrolysis, in which HCN yields were nearly half of those of NH3. FTIR and GC–MS analysis revealed that the pyrolysis of sewage sludge produced three nitrogen-containing compounds, including the amine/amide, heterocyclic-N and nitrile compounds, in the char and tar products. The pyrolysis temperature played significant roles on the transformation of nitrogen in the char, tar and gas products during microwave pyrolysis.