Comparative study on the evolution of physicochemical characteristics of biochar produced from bio-oil distillation residue under different induction atmosphere

In the study, biochar was easily produced from bio-oil distillation residue through a tube furnace at a broad induction temperature range of 200-700 °C under nitrogen and carbon dioxide. The comparative evolution of biochar was characterized by evaluating its physicochemical properties including elements, calorific values, functional groups, graphitization degree and thermal stability. The results showed that compared with nitrogen atmosphere, carbon dioxide first contributed to biochar yield. After 600 °C, biochar yield in carbon dioxide dropped rapidly, while nitrogen had little effect on biochar yield. Through bomb calorimeter, the maximum calorific value of biochar was 34.13 MJ·kg−1 at 200 °C under carbon dioxide. Based on elements analysis, O/C, H/C and N/C of biochar induced by nitrogen were higher than those of carbon dioxide. Fourier transform infrared spectrometry (FTIR) indicated that induction temperature and atmosphere selectively affected the evolution of biochar functional groups. CO stretching vibration in carbon dioxide was more durable than that in nitrogen, whereas the stretching vibration of CH2 and CC had opposite phenomenon. In addition to 200 °C, the biochar under nitrogen possessed less graphitization with temperature increasing by Raman spectra analysis. Furthermore, the biochar induced by nitrogen was more stable than that induced by carbon dioxide according to the thermogravimetric analysis.