Speciation and transformation of nitrogen for swine manure thermochemical liquefaction

The nitrogen conversion mechanism of swine manure by thermochemical liquefaction with ethanol as solvent was investigated at a lower temperature range (180–300 °C). The fate of nitrogen in liquid phase products, bio-oil and biochar was evaluated by XPS, GC–MS and other methods. After thermochemical liquefaction, most of the nitrogen in swine manure was transferred to biochar (63.75%). As the temperature increased to 220 °C, the biochar-N yields decreased to 43.29%, accompanied by an increase in bio-oil-N and liquid phase product-N by 7.99% and 1.26% respectively. The results indicated that increasing the temperature could facilitate solid nitrogen structure cracking into bio-oil-N. Amines and heterocyclic nitrogen from protein peptide bond cracking and Maillard reactions made up the main nitrogen compounds in bio-oil, and high temperatures favored the further cyclization and condensation of heterocyclic nitrogen (e.g., indole, quinoline). In the case of biochar, the inorganic nitrogen disappeared at 260 °C and was obviously transformed into liquid phase products. The rising temperature promoted the polymerization of pyridine nitrogen and pyrrole nitrogen, which formed more stabilized nitrogen formation (such as quaternary nitrogen). Nitrogen conversion and possible reaction schematics during swine manure thermochemical liquefaction were explored in this study.