Direct gasification of dewatered sewage sludge in supercritical water. Part 1: Effects of alkali salts

In the present study, the catalytic effects of alkali salts [NaOH, KOH, K2CO3, Na2CO3 and Ca(OH)2] on the direct gasification of dewatered sludge in supercritical water were investigated by using a high-pressure autoclave at a constant temperature of 450 °C and a residence time of 30 min. The hydrogen yield increased in the presence of the alkali salts, except for Ca(OH)2. Specifically, the hydrogen yield increased from 0.68 to 3.45 mol/(kg OM) as the K2CO3 concentration increased from 0 to 8 wt%. Although Ca(OH)2 did not significantly impact the catalytic effect on the hydrogen yield, it did impact the CO2 yield. Generally, the addition of alkali salts did not affect the organic matter or total phenol concentrations in the liquid residue. Moreover, char formation was considerably suppressed by the alkaline catalyzed hydrolysis of the dewatered sludge [except in the case of Ca(OH)2].