In this study, the volatile from sewage sludge (SS) pyrolysis is passed through the hot char from the same source in a fixed bed reactor. The reforming effect of the sewage sludge char (SSC) on the quality and energy distribution of liquid and gas products is studied. In comparison to dolomite, SSC presents a better tar-eliminating effect from 450 to 650 °C. The production of combustible products, including CH4, H2, and CO, is significantly increased by both the catalytic cracking on the SSC surface and the volatile–char reaction. After the reforming effect at the pyrolysis temperature of 650 °C, the gas yield is increased from 17.19 to 34.27 wt % and the fraction of combustible components is increased by 54.28 vol %. The quality of the oil is also upgraded by reforming of SSC, with the escalation of the light components, and nitrogen-containing and oxygenated fractions are greatly reduced. When the pyrolysis temperature is fixed at 650 °C and the reforming temperature is increased from 450 to 650 °C, the liquid yield is decreased from 18.76 to 10.67 wt %. In addition, the combustible fraction in the gaseous product is greatly increased. The calculation of the energy distribution indicates that, via the reforming process, a lot more heating value will concentrate on the gas product and the overall high heat value of the pyrolysis volatile and SSC will be significantly promoted.

Sewage sludge is an important pool of phosphorus (P), while thermal disposal results in the enrichment of P into a final solid product. Hydrothermal treatment combined with steam gasification is an effective method to produce high-quality syngas from sewage sludge. This study discussed the transformation of P during this integrated thermochemical process based on thermodynamic calculation and the Standards, Measurements and Testing Programme analysis of solid products. The results showed that P was enriched in hydrochar with the concentration above 14 mg/g. By the hydrothermal pretreatment, organic phosphorus (OP) was significantly removed from the solid as a result of destruction, while inorganic phosphorus (IP) was mostly recovered. Part of OP was converted to IP in disposal from 220 to 240 °C. At 260 °C, destructed OP was emitted to the gaseous phase in the form of phosphorus oxide and, thus, not captured by minerals. In addition, inorganic phosphorus associated with oxides and hydroxides of Al, Fe, and Mn (NAIP) tended to be transformed into Ca-associated forms (AP) with the increase of pH, and CaO addition significantly promoted this transformation. In the gasification process, OP was completely removed, while IP was almost recovered with the substantial conversion from NAIP to AP. The highest total P recovery (84.92%) was achieved from gasification ash of hydrochar pretreated at 200 °C with CaO addition, while P was the most concentrated in gasification ash of hydrochar pretreated at 240 °C (30.39 mg/g). This study provided basic data for P recovery from sewage sludge using thermochemical conversion.

Hydrothermal treatment not only reduces the cost of moisture removal but also improves the quality of the syngas from steam gasification for sewage sludge. However, the effect of the treatment on the gasification behavior, including the reactivity and emission of gaseous N compounds, was not clear. In this study, the kinetic analysis based on a thermogravimetric (TG) experiment was used to examine the reactivity of the hydrochars. In addition, the evolution of N functionalities in hydrochar with treatment conditions and the discharge of NH 3 and HCN during gasification were also investigated. The result showed that the treatment lowered the activation energy of sludge in gasification of remaining char and effectively reduced the discharge of NH 3 and HCN. CaO addition into raw sludge before treatment caused the shift of the peak temperature to the lower range in remaining char gasification and promoted the conversion of HCN to NH 3 .

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.