Hydrogen production and phosphorus recovery via supercritical water gasification of sewage sludge in a batch reactor.

In this study, gasification of sewage sludge in supercritical water using a batch reactor was investigated. The effects of temperature, retention time, and the oxidation coefficient on gas composition, gas yield, total organic carbon removal efficiency (XTOC), gasification efficiency (GE), carbon gasification efficiency (CE), and phosphorus release rate (Xp) were investigated. The experimental results indicated that the yields for hydrogen, methane, and carbon dioxide increased with the increase in temperature from 380 °C to 460 °C. A maximum hydrogen molar fraction of 55.72% and a yield of 19.86 mol/kg were obtained at 460 °C and 27 MPa after 6 min. The GE, CE, XTOC, and Xp also increased with the increase in temperature. An extension of the retention time promoted the gasification of sludge, thereby resulting in an increase in the hydrogen and methane molar fraction, yield, GE, CE, XTOC, and Xp. Under the conditions of 420 °C and 27 MPa after 6 min, with an increase in the oxidation coefficient from 1.5 to 2.5, the oxidation reaction became dominant in the supercritical water. Hydrogen and methane were converted to carbon dioxide and water with an excess of hydrogen peroxide, which resulted in a lower hydrogen yield. However, the decomposition of organic compounds in the sludge was promoted with the addition of hydrogen peroxide, thereby resulting in an increase in the GE, CE, XTOC, and Xp. When the oxidation coefficient reached 2.5, a maximum GE of 131.6% and Xp of 98.74% were obtained.

Supercritical water gasification of landfill leachate for hydrogen production in the presence and absence of alkali catalyst.

Gasification of landfill leachate in supercritical water using batch-type reactor is investigated. Alkali such as NaOH, KOH, K2CO3, Na2CO3 is used as catalyst. The effect of temperature (380-500 °C), retention time (5-25 min), landfill leachate concentration (1595 mg L-1-15,225 mg L-1), catalyst adding amount (1-10 wt%) on hydrogen mole fraction, hydrogen yield, carbon gasification rate, COD, TOC, TN removal efficiency are investigated. The results showed that gaseous products mainly contained hydrogen, methane, carbon dioxide and carbon monoxide without addition of catalyst. However, the main gaseous products are hydrogen and methane with addition of NaOH, KOH, K2CO3, Na2CO3. In the absence of alkali catalyst, the effect of temperature on landfill leachate gasification is positive. Hydrogen mole fraction, hydrogen yield, carbon gasification ratio increase with temperature, which maximum value being 55.6%, 107.15 mol kg-1, 71.96% is obtained at 500 °C, respectively. Higher raw landfill leachate concentration leads to lower hydrogen production and carbon gasification rate. The suitable retention time is suggested to be 15 min for higher hydrogen production and carbon gasification rate. COD, TOC and TN removal efficiency also increase with increase of temperature, decrease of landfill leachate concentration. In the presence of catalyst, the hydrogen production is obviously promoted by addition of alkali catalyst. the effect of catalysts on hydrogen production is in the following order: NaOH ＞ KOH ＞ Na2CO3 ＞ K2CO3. The maximum hydrogen mole fraction and hydrogen yield being 74.40%, 70.05 mol kg-1 is obtained with adding amount of 5 wt% NaOH at 450 °C, 28 MPa, 15 min.