Influence of oxidation coefficient on product properties in sewage sludge treatment by supercritical water

Oxidation coefficient plays a significant role in sewage sludge treatment in supercritical water, such as supercritical water oxidation (SCWO) and supercritical water partial oxidation (SWPO). In this work, the influences of oxidation coefficient (n) on sewage sludge treatment in supercritical water are investigated systematically and the corresponding reaction mechanisms are discussed objectively. Moreover, corrosion properties of stainless steel 316 considered as reactor material are also explored. The results show that H2 yield first rises and then decreases with an increase in n. Its maximum value is approximately 190.4 ml/L when sewage sludge is disposed at 450 °C, 25 MPa, n = 0.6 and a residence time of 2.5 min. Under the reaction conditions of 540 °C, 25 MPa, n = 2.0 and 2.5 min, liquid product COD (Chemical Oxygen Demand) removal ratio, TOC (Total Organic Carbon) removal ratio and ammonia removal ratio of liquid product can reach up to 99.95%, 99.8% and 99.7%, respectively. However, if coupling SWPO and SCWO, we can obtain a certain amount of H2 and CH4, achieve the above removal ratios even at 450 °C and a lower total n (0.74), and meanwhile liquid products can meet corresponding discharge standards. However, special attention should be paid to reactor material corrosion in the above SWPO. We also confirm that stainless steel 316 undergoes more severe pitting corrosion in the absence of oxygen, compared with general corrosion in the excessive oxygen environment.