Gasification characteristics of hydrochar and pyrochar derived from sewage sludge

• Low temperature pyrolysis (LTP) of sewage sludge to produce the pyrochar.
• Hydrothermal carbonization (HTC) of sewage sludge to produce the hydrochar.
• The hydrochar is more hydrophobic than that of the pyrochar.
• The hydrochar enhanced hydrogen production from sewage sludge by gasification.

Two types of the biochars, pyrochar and hydrochar derived from low temperature pyrolysis (LTP) and hydrothermal carbonization (HTC) of sewage sludge were prepared and characterized. Their gasification properties were further experimentally evaluated. The results showed that the hydrochar was more hydrophobic than the pyrochar. The hydrochar was rich in nitrogen-containing functional groups and increased nickel, iron, alkali and alkaline earth metallic species compared to the raw sludge and pyrochar. It enhanced the interactions between the carbon surface and hydrogen bonding as well as gasification reactivity of the hydrochar, thus resulting in a higher hydrogen concentration and yield than the pyrochar under identical conditions. Additionally, the hydrochar had a more porous structure on the surface, facilitating the pores better accessible for condensable hydrocarbon molecules and thus improved the gas production and gasification efficiency. Although the energy recovery efficiency of LTP-gasification method was higher than that of HTC-gasification approach, the total energy consumption during the HTC pretreatment combined with subsequent gasification was lower than the energy evolved in the corresponding product gas in most gasification conditions. This study demonstrates that the integration of HTC pretreatment and subsequent gasification has promising potential for hydrogen-rich syngas production from sewage sludge.