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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 NH3and 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 NH3and 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 NH3.

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.