Fast and Isothermal Hydrothermal Liquefaction of Polysaccharide Feedstocks

We investigated the isothermal and fast (short time, rapid heating) hydrothermal liquefaction (HTL) of six different polysaccharides abundant in food waste. Pectin was the polysaccharide most difficult to convert, and the highest biocrude yield it provided from isothermal HTL (350 °C, 31.6 min) was 13 wt %. The highest biocrude yields from isothermal HTL for the other polysaccharides were about 20 wt %. Increasing the reaction time during isothermal HTL at 350 °C increased the biocrude yields. Fast HTL converted all the different polysaccharides into higher yields of biocrude than isothermal HTL did. Pectin was again the most difficult to convert, but fast HTL (500 °C set point, 1 min) provided a yield of 17 wt %. The yields from the other polysaccharides were about 6–7 wt % higher. A biocrude from fast HTL had a smaller fraction of higher boiling species than the biocrude from isothermal HTL for the corresponding feedstock did. The biocrudes from fast HTL often had a higher heteroatom content, however, indicating a trade-off between biocrude yield and quality at different HTL conditions. Taken collectively, the present results show that fast HTL can valorize a variety of different polysaccharides and do so more quickly and more productively than conventional isothermal HTL can.