Effect of Hydrothermal Temperature on the Steam Gasification Performance of Sewage Sludge: Syngas Quality and Tar Formation

Hydrothermal pretreatment enhances both dewaterability and energy density of the municipal sewage sludge before steam gasification. Nevertheless, the effect of treatment condition on syngas quality and tar production was not clear. In this study, the hydrochars derived at different hydrothermal temperatures of sewage sludge were characterized by scanning electron microscopy, Fourier transform infrared spectroscopy, and BET analysis. Then, the gasification experiment of raw sludge and hydrochars was conducted in a fixed bed reactor to evaluate key parameters of syngas and tar including yield, chemical composition, and higher heating value. The results showed that the treatment substantially improved the diffusion condition on the sludge surface, while raising the temperature led to a certain extent of pore collapse. The surface area and pore volume of the raw sludge was increased by 7.66 and 1.73 times after treatment at 200 °C, leading to 4.2% increase in H2 content in syngas from gasification of derived char despite the apparent loss of volatile species. The lowest tar formation was also from hydrochar produced at 200 °C due to the enhanced in situ reforming. In addition, the treatment did not favor the reduction of PAH content in tar. This study provided basic data for the determination of hydrothermal temperature of sewage sludge targeting high-quality syngas and tar inhibition.