Hydrogen-rich gas production by steam gasification of hydrochar derived from sewage sludge

• Hydrothermal carbonization (HTC) of sewage sludge to produce hydrochar.
• Steam gasification characteristics of raw sludge and hydrochar were investigated.
• Compared to raw sludge, hydrochar had a higher hydrogen yield.
• Steam gasification characteristics of raw sludge were improved by HTC pretreatment.

Hydrothermal carbonization is an effective pretreatment for further thermal conversion of high moisture biomass without a high cost of a dehydration process. The current paper concerns the properties of hydrochar derived from hydrothermal carbonization of sewage sludge, and the feasibility of steam gasification of hydrochar for hydrogen-rich gas production was investigated. Sewage sludge derived hydrochar was characterized using scanning electron microscopy, Fourier transform infrared spectroscopy, and inductively coupled plasma atomic emission spectroscopy to evaluate its feasibility for gasification application. The effect of reaction temperature, steam to biomass mass ratio, and addition of alkali catalysts on steam gasification characteristics of raw sewage sludge and corresponding hydrochar were evaluated, in terms of major composition of the produced gas, gas yield, gasification efficiency and energy density. The results showed that sewage sludge derived hydrochar was rich in hydrophilic functional groups and increased Fe, Ni, alkali and alkaline earth metals (i.e. K, Na, Ca, Mg), resulting in a higher hydrogen yield and energy efficiency than direct steam gasification of sewage sludge under identical conditions. In addition, hydrogen-rich gas production was also favored with the presence of alkali catalysts, especially for the hydrochar. The present study demonstrates that hydrothermal carbonization provides an effective pretreatment of sewage sludge for production of hydrogen-rich gas via steam gasification.