Effectiveness and mechanisms of crude glycerol on the biofuel production from swine manure through hydrothermal pyrolysis

A series of laboratory experiments were performed in a high-pressure batch reactor, fed with mixtures of swine manure and crude glycerol at various ratios to produce bio-oil. At 340 °C and 15 min retention time, the oil yield increased dramatically by adding crude glycerol as the co-substrate. At a swine manure to crude glycerol ratio of 1:3 (weight ratio), a maximum oil yield of 68% was obtained. In order to figure out the key component in crude glycerol that could lead to enhancement of the oil yield, crude glycerol was separated into three fractions—free fatty acid, glycerol/water/methanol and salts—by adding 85% phosphoric acid. Then the three fractions were added to manure individually at a 1:1 weight ratio of glycerol to manure, and the corresponding oil yield and quality were evaluated. This resulted in the discovery that the free fatty acid increased oil yield dramatically from 23.9% to 70.92%. Moreover, the oil quality was also improved, having a lower density and viscosity. The results indicate that co-hydrothermal pyrolysis of manure with crude glycerol or free fatty acid makes it possible to obtain a relatively high yield of bio-oil at a moderate high temperature.