Bio-oil production from eight selected green landscaping wastes through hydrothermal liquefaction

Abstract

This study investigated the potential of eight types of green landscaping waste as feedstock to produce bio-oil through hydrothermal liquefaction (HTL). The eight selected plants differed in terms of botanical classification, morphology, leaf state, and growth habit. Leaves and branches as waste from these plants were separately subjected to HTL in a high-pressure batch reactor at 300 °C for 0.5 h. Results indicated the bio-oils and biochars of leaves obviously differed from those of branches in terms of yields and higher heating values (HHVs). However, less difference in yields and HHVs was found for HTL products within the eight leaves even though they were different in composition components such as cellulose, hemicelluloses, and lignin. The same was observed for branches. The average bio-oil yields of the leaves and branches were 33.74 and 43.22 wt%, respectively. The optimal bio-oil yield was 50.44 wt%, which was obtained when Cinnamomum camphora branches were used as feedstock. The average HHVs of light and heavy oils in the leaves were 25.13 and 31.27 MJ kg⁻¹, respectively. These HHVs were higher than those of light and heavy oils in the branches (21.51 and 28.71 MJ kg⁻¹, respectively). Among the oil products, the heavy oil derived from Salix alba leaves yielded the optimum HHV (35.63 MJ kg⁻¹). The mean HHV of biochar was 24.17 MJ kg⁻¹, which was considerably higher than that of feedstock (17.21 MJ kg⁻¹). Gas chromatography-mass spectrometry and Fourier-transform infrared spectrometry revealed the presence of value-added chemicals, such as phenolics, ketones, esters, acids, and alcohols, in bio-oils. The amounts of alkanes, alkenes, and alkynes in the bio-oils derived from the leaves were higher than those in the bio-oils derived from the branches. These results indicated the feasibility of using different types of green landscaping waste as feedstock to produce bio-oils with high HHV and yield through HTL.