The influence of alkali precipitation on supercritical water gasification of glucose and the alkali recovery in fluidized-bed reactor

• The precipitated alkali in SCW still had catalytic effect.
• Alkali accumulation improved the catalytic effect in the earlier gasification stage.
• The results from SCWG with different NaOH content at steady state were similar.
• Above 81% Na fed in the reactor was recovered after gasification.
• Fluidized-bed reactor was beneficial for alkali recovery.

The inorganic substances precipitate in the reactor for its low solubility in supercritical water, which may influence the performance of supercritical water gasification (SCWG). In the present study, we studied the influence of alkali precipitation on the gasification of glucose with NaOH as catalyst at 500 °C, 25 MPa in a fluidized-bed reactor. The presence of NaOH increased H2 fraction by accelerating water–gas shift reaction and also increased the gas yield. The catalytic effect was further improved with longer reaction time due to the alkali accumulation in the reactor. The precipitated alkali in the reactor still showed high catalytic effect on the subsequent gasification of glucose without further addition of alkali. The catalytic activity was slightly reduced for the transformation of NaOH to Na2CO3 and NaHCO3. The precipitated alkali flowed out of the reactor with water when the temperature in the reactor decreased to near the critical temperature because of the increasing solubility. Most of the sodium fed into the system (81.07%) was recovered in this process.