Reduction of inorganics from macroalgae Laminaria digitata and spent mushroom compost (SMC) by acid leaching and selective hydrothermal liquefaction

Hydrothermal liquefaction (HTL) is a promising route for producing bio-crude from various biomass feedstocks. However, high content of inorganic constituents in biomass like macroalgae Laminaria digitata and spent mushroom compost (SMC) affect the conversion process and the resulting fuel products. This research studied the effects of different acid leaching treatments on such feedstocks, subsequent HTL, and bio-crude properties. Leaching treatments were performed using five different agents: deionized water, acetic acid, citric acid, sulfuric acid, and hydrochloric acid. Performance of leaching was evaluated by analyzing both leached biomass and HTL products by elemental analysis, ash content, inductively coupled plasma (ICP) analysis, and X-ray diffraction (XRD) analysis. Catalytic and non-catalytic HTL of both feedstocks before and after treatment were performed in a 10-mL microreactor at 400 °C with a holding time of 15 min and pressures of 27–30 MPa. For macroalgae, sulfuric acid and hydrochloric acid were found the most effective in reducing the ash content from 30.42 to 20.45 and 20.87%, respectively, followed by acetic and citric acid treatment that could reduce the ash content to 21.5 and 22.15%, respectively. Similarly for SMC, citric acid and acetic acid were found the most effective in reducing the ash content from 50.34 to 37.04 and 39.94%, respectively. Citric acid did not show significant leaching of organic components such as carbohydrates and proteins and represented a less toxic and hazardous option for the leaching. The results from HTL of untreated and citric acid-treated biomass showed that the acid leaching resulted in an increase in bio-crude yields from 20.7 to 29.2% (dry ash-free basis) for macroalgae and from 22.9 to 25.1% for SMC.