Speciation and transformation of nitrogen in the hydrothermal liquefaction of wastewater-treated duckweed for the bio-oil production

Abstract

Bio-oil production from wastewater-treated duckweed by hydrothermal liquefaction (HTL) is a sustainable development mode. In this study, the effect of reaction conditions on nitrogen transformation during HTL of wastewater-treated duckweed for bio-oil production was investigated. GC-MS, FT-ICR-MS and XPS were used to characterize the nitrogen speciation in each product. The highest bio-oil yield of 34.7 wt% was obtained at 360 °C, 60 min. The maximum high heating value (HHV) and energy recovery of bio-oil separately reached 36.41 MJ/kg and 86.11%. As the temperature increased (240 °C–360 °C), more nitrogen (21.14%–35.50%) migrated to the bio-oil and mainly existed in the form of nitrogen-containing heterocycles and amides. Higher temperatures favored the acylation reaction to produce more amides, while the Maillard reaction was hindered resulting in a decrease in the relative content of nitrogen-containing heterocycles. N2 species such as pyrazines, imidazole were the dominant organic nitrogen species. In the aqueous phase, the nitrogen recovery decreased from 44.7% to 39.3%, and pyridine, pyrimidine and pyrrolidine were the most abundant organic nitrogen forms. The nitrogen distributed to the solid residue decreased from 21.1% to 5.1%, with pyridine-N becoming the dominant nitrogen species at temperatures above 320 °C.