Processing of Rice Straw by Continuous Steaming and Atmospheric-Pressure Boiling-Water Extraction─A Novel Approach

The agricultural herbaceous biomass has high ash and potassium/chlorine contents. The present study proposes a novel mild hydrothermal treatment method to convert herbaceous biomass into solid biofuel with reduced ash, potassium, and chlorine contents, as well as liquid fermentable sugars. This two-step process involves continuous steaming under mild saturated steam followed by extraction using boiling water at atmospheric pressure. Experimental results indicate that continuous steaming of rice straw in saturated steam at temperatures between 150 and 210 °C leads to significant softening and pore-opening effects on the biomass cell structure. Consequently, subsequent atmospheric-pressure boiling-water extraction achieves high removal rates of potassium (56.9–77.4%), chlorine (82.2–97.0%), and sulfur (81.3–87.5%). Successful cultivation of Aspergillus oryzae was achieved using the liquid products obtained from the steaming at temperatures of 150 and 180 °C, resulting in fermented residues with high total protease activity.