Rape straw as a source of bio-oil via vacuum pyrolysis: Optimization of bio-oil yield using orthogonal design method and characterization of bio-oil

• Rape straw is a potential pyrolysis material for bio-oil preparation.
• Effects on yield: heating rate > pyrolysis temperature > reactor pressure > holding time.
• Heating rate and pyrolysis temperature have the interactive effects on bio-oil yield.
• Polymerization contributes to the higher average molecular weight distribution.
• Light aromatics are the dominant component contributing to the lower polydispersity.

Rape straw from China rural area was treated by vacuum pyrolysis for bio-oil preparation. The orthogonal design method was employed to minimize the number of experiments. The effects of the factors including pyrolysis temperature, reactor pressure, heating rate and holding time on bio-oil yield were analyzed. The optimal conditions were obtained by using SPSS Statistics 20.0 (IBM, USA) and Matlab 7.12.0 (MathWorks, USA). Furthermore, the physicochemical properties of the bio-oil obtained at optimal conditions were evaluated. The composition was examined using gas chromatograph/mass spectroscopy (GC/MS). The thermo-gravimetric analysis (TGA) of the bio-oil was investigated within N2 and O2, respectively. The results showed that the order of the effects of the factors on bio-oil yield was heating rate > pyrolysis temperature > reactor pressure > holding time. Heating rate and pyrolysis temperature had a significant interaction. Optimal conditions were obtained at pyrolysis temperature of 495.5 °C, heating rate of 19.4 °C/min, reactor pressure of 5.0 kPa and holding time of 50 min. Confirmation runs gave 43.15% and 43.44% of bio-oil yield compared to 43.62% of predicted value. The water content, carbon residue, density, pH value, dynamic viscosity and higher heat value of bio-oil were 34.20%, 4.03, 1.14 g/cm3, 2.22, 4.28 mm2/s and 18.72 MJ/kg, respectively. Polymerization in the condensation process contributed to the higher average molecular weight distribution than pyrolysis vapors. Light aromatics (mainly including light phenols) were the dominant component for the bio-oil, which contributed to the relatively low polydispersity with a value of 1.25. The direct liquefaction bio-oil was unstable and corrosive due to the high reactivity of carbonyl extensively existing in acids, aldehydes and ketones. High oxygen content made the behavior of decomposition in O2 be similar to that in N2 before 450 °C. Further study on upgrading of the bio-oil should be performed to make it become an alternative fuel.