Effect of supercritical water temperature and Pd/C catalyst on upgrading fuel characteristics of gumweed-derived solvent-extracted biocrude

Grindelia squarrosa, known as gumweed, is a shrub that grows in arid lands in North America with up to 8% biocrude (or bio-oil) content that can be extracted by using a common solvent extraction method. This solvent-extracted biocrude (SEB) has nearly 24 wt% oxygen, which is not suitable for direct combustion as a bio-oil or biodiesel. In this study, supercritical water (SCW) upgrading is used to reduce the oxygen of the SEB and increase the fuel value by creating lighter oil fractions. Treatment for 30 min reduces the oxygen content from 24.1 ± 1.6 (for raw SEB) to 7.2 ± 0.7%, 7.1 ± 0.4%, and 6.9 ± 0.6% at 380, 400, and 420 °C respectively. The addition of palladium on carbon (Pd/C) catalyst at 400 °C further reduces the oxygen content to 4.8 ± 0.7%. This removal in oxygen content is shown in the reduction of carboxylic acid compounds in the oils. Without having to use hydrogen, such as with the traditional hydrodeoxygenation process, the SCW upgrading has rather high energy yields for the oils which all have higher heating values between 41.0 and 42.8 MJ kg−1 (with no significant effect of temperature or catalyst). Treatment at 400 °C had the highest energy yield at 76.4 ± 1.7%, and treatment with the Pd/C catalyst at 400 °C had the lowest at 61.4 ± 2.8%. These energy yields, higher heating values, and low oxygen contents show that SCW can effectively be used to convert gumweed-derived SEB into a valuable bio-oil.