Nitrogen transformation among char, tar and gas during pyrolysis of sewage sludge and corresponding hydrochar

•Effect of HTC on nitrogen distribution during sewage sludge pyrolysis was evaluated.•HTC lowered nitrogen content and improved its stability.•Char-N and tar-N from hydrochar pyrolysis were more stable than those from sludge.•The total fraction of (NH3 + HCN)-N from hydrochar pyrolysis was much lower.•HTC reduced by 51.8-64.4% of NOx precursors from sewage sludge pyrolysis.

Hydrothermal carbonization (HTC) is a promising thermochemical pretreatment to convert sewage sludge into the hydrochar prior to combustion or pyrolysis for energy generation. To investigate the effects of HTC on nitrogen transformation behaviors, pyrolysis of sewage sludge and the corresponding hydrochar were conducted. The results showed nitrogen in the hydrochar existed in more stable forms, making more nitrogen retained in the char for the hydrochar pyrolysis. Amine-N, heterocyclic-N and nitrile-N were detected in significant amounts for sewage sludge pyrolysis tar, which contributed to predominant (NH3 + HCN)-N emission. Lower amine-N and heterocyclic-N content but higher alkyl nitrile-N content were observed in the hydrochar pyrolysis tar, which might explain the increase in HCN/NH3 ratio. More importantly, the (NH3 + HCN)-N yield from the hydrochar pyrolysis was considerably lower than that from sewage sludge pyrolysis, especially at temperatures higher than 650 °C. Taking account of the much lower nitrogen content, the total (NH3 + HCN)-N emission from the hydrochar pyrolysis was only 35.6-48.2% of that from sewage sludge pyrolysis. The results suggested that HTC pretreatment of sewage sludge for energy production has the additional environmental benefit of mitigating nitrogenous pollutants emission.

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