Effects of low-temperature hydrothermal pretreatment of high-protein Chlorella sp.on N distribution and thermal degradation of solid residue

Decomposition behavior of Chlorella sp. during low-temperature hydrothermal pretreatment (HTP) were studied. Distribution of various product yields, element components, energy recovery ratio and key elements (i.e., C and N) along with temperature (125-200℃) were investigated. The results show that amounts of C and N are enriched into aqueous phase, and high content of NH₃-N is detected due to deamination reaction above 175℃. During the HTP process, N distribution in oil product first increases gradually and then rapidly increases above 175℃. During the whole low-temperature HTP process, the yield and energy recovery ratio of solid residue decreases continuously. The N/C and O/C ratio of the solid residue also decreases, indicating HPT would promote property of the solid residue. The functional structure and thermal-degradation of Chlorella sp. and its solid residue are then comparatively examined by various techniques including FT-IR, XPS, TG-FTIR-MS and Py-GC/MS. The results show that functional structure of the solid residue is distinguished from that of the raw sample. The relative content of C-C bond increases while that of C-N and C-O bonds decreases. In addition to protein-N and quaternary-N, a low fraction of pyridine-N is also detected in the solid residue. Compared with these from raw material, less NH₃ and HCN are released from solid residue, and less N-containing heterocyclic compounds are generated during rapid pyrolysis.