Aging and emulsification analyses of hydrothermal liquefaction bio-oil derived from sewage sludge and swine leather residue

Hydrothermal liquefaction bio-oil is a renewable and potential alternative to fossil fuels. To date, however, the study of the aging and emulsification phenomena of bio-oils derived from sewage sludge and swine leather residue is still absent, which is important for the further applications of the bio-oils. In this study, bio-oils from the hydrothermal liquefaction of sewage sludge and swine leather residue were produced from two bio-oil separation methods, namely, extraction and distillation. It was found that the distilled bio-oil had less nitrogen-containing compounds and more low-boiling-point compounds than the extracted bio-oil, and thus possessed lower calorific value. The complicated composition and high oxygen content in the bio-oils triggered the aging reactions during storage, which was analyzed by measuring their water contents, total acid numbers, viscosities, and higher heating values. The aging behavior depended on the feedstocks and separation methods. The total acid number and water content of the bio-oil from swine leather residue had more vigorous change than those from sewage sludge. However, the variation in the higher heating values of the three bio-oils with storage time was not obvious owing to the trade-off among a variety of reactions. These phenomena could be explained by dehydration, acetyl, polymerization, condensation reactions in the bio-oils. Furthermore, the emulsification of 10 wt% bio-oil and 90 wt% diesel was evaluated by detecting the functional group spectra at different retention times. It suggested that the case of bio-oil/emulsifier = 1 could yield a stable and effective emulsion within 30 days storage, showing the promising emulsification applications of hydrothermal liquefaction bio-oil.