Supercritical water gasification of sewage sludge for power generation– thermodynamic study on auto-thermal operation using Aspen Plus

Abstract

Sewage sludge conversion through supercritical water gasification presents a viable pathway for waste management and power generation which would allow the waste treatment process to be incentivized economically rather than by policy aspiration. In this conceptual study, two system configurations were developed using Aspen Plus. In the first configuration, limitation on the preheating of dewatered sewage sludge was imposed to prevent biomass degradation and tar formation which may lead to plugging in heat exchanger. The second configuration processed sewage sludge and the constraints on feed preheating was not imposed due to the higher water content of feed stream. Supplementary liquid feedstock was used to achieve auto-thermal operation in the second configuration. The supplementary liquid feedstock considered in this study included (i) waste cooking oil, (ii) heavy oil and (iii) crude glycerol, which are underutilized commercially. Feed concentration required in order to operate the gasifier auto-thermally in the temperature range of 500–700 °C were analyzed using sewage sludge data from various sources. Feed concentration required for auto-thermal operation was found to be significantly dependent on the heating value of sewage sludge and minimal lower heating value of about 12.63 MJ kg−1 was recommended in the first configuration as mechanical dewatering in real operation may only thickened the sludge up to 25% dry matter content. Higher system efficiency was achieved at higher gasifier temperature and feed concentration, up to the concentration that achieved auto-thermal operation. Further increment on feed concentration would reduce the system efficiency as the surplus syngas was not collected. The use of supplementary liquid fuel in the second configuration enabled the treatment of sewage sludge with lower energy content and more optimal feed preheating allowed the system to reach up to 21.9% in energy efficiency, markedly higher than first configuration which achieved up to 18.8%.