Nitrogen Removal and Energy Recovery from Sewage Sludge by Combined Hydrothermal Pretreatment and CO₂ Gasification

Disposal of nitrogen-containing sewage sludge (SS) via gasification leads to the formation of NOx precursors and NOx and ammonia emissions. Pretreatment of SS via hydrothermal carbonization (HTC) was shown to improve coalification, desulfurization, and deamination. In this study, we propose a combined hydrothermal pretreatment and CO2 gasification process for removal of nitrogen from SS and production of energy via the gasification of the pretreated SS. In the HTC process, around 50% of nitrogen contained in the sludge was removed. The derived hydrochar (HC) was subsequently fed for gasification using CO2 as a gasifying agent, which was characterized by the expected reduction in the formation of ammonia. We further improved the process by establishing cogasification of hydrochar with waste leaves. The improvement is believed to be due to the catalytic effect of Ca, present in plant leaves, on the gasification. Cold gas efficiency of the cogasification process of a 1/1 hydrochar/leaves blend by mass was enhanced to 66%. The combination of hydrothermal conversion of sewage sludge with CO2 cogasification of hydrochar and waste leaves was shown to be a clean and efficient method for SS management and energy recovery