Stability of emulsion fuels prepared from fast pyrolysis bio-oil and glycerol

This study investigates the possibility of emulsifying bio-oil with crude glycerol to produce emulsion fuels. The focus is on the stability of bio-oil/glycerol (BG) emulsions under various emulsification conditions and the effect of major impurities in crude glycerol. The results suggest that temperature and surfactant play more important roles in BG emulsion stability than stirring speed and time. To produce stable BG emulsion, temperature over 45 °C and Span 80 of concentration <1% is preferred. High speed stirring over 3500 rpm is not necessary as it leads to rapid separation and also high energy consumption. It was also found that ageing of the BG mixture before emulsification is beneficial to the emulsion stability. Prolonged ageing at both relatively low temperature (e.g. 45 °C for 21 h) and high temperature (e.g. 65 °C for 1 h) resulted in BG emulsions with stable time over 12 h. The results suggest that the stability of BG emulsions may be governed by the breakdown and reconstruction of network structure of bio-oil and interaction of surfactant with dispersed bio-oil components and glycerol. Further results show that impurities in crude glycerol such as water, salt and alkaline catalyst lead to enhanced bio-oil phase separation and severely worsen the stability of BG emulsions to be below 1 h. Although soap in crude glycerol is a surfactant, a high content of soap (20%) in crude glycerol can also shorten the stable time of BG emulsions to be below 1 h at a high glycerol-soap loading level (25%) in the BG emulsions, likely due to micelles formation at excessive surfactant loading. Considering the positive effect of methanol on emulsion stability, emulsions of bio-oil/glycerol-impurities/Span 80 with stable time of 4.0-8.0 h can be prepared if the content of these adverse impurities were kept low (5% for soap or water; 2% for salt or alkaline catalyst) and with retention of 10% methanol in crude glycerol.