Evaluation of the combined processes of supercritical water oxidation and gasification for lurgi coal gasification wastewater treatment

The supercritical water oxidation (SCWO) technology has shown high efficiency in Lurgi coal gasification wastewater treatment. However, the cost of oxygen and energy consumption become the major limiting factors for the industrial operation as the oxygen used is always excessive. In this paper, we proposed two schemes that combined the SCWO and gasification processes to recover the excessive oxygen and heat. The combined processes were simulated by Aspen Plus software and their economic values were also analyzed. In scheme (I), the SCWO and gasification processes were combined by a pressure reducer. The depressurized SCWO effluent was directly used as a gasification agent. In this scheme, zero wastewater discharge can be realized and all the excessive oxygen and heat were completely recovered. In scheme (II), the SCWO and gasification processes were combined by a heat exchanger. The heat of the SCWO effluent was recovered through the heat exchange with another introduced cold fluid. The oxygen was recovered in the oxygen recovery section set behind the SCWO process, in which a byproduct of industrial grade liquefied CO₂ was produced. The recovery efficiencies of oxygen and heat were 99.35% and 90.76%, respectively. For a single gasifier with steam of 27th^-1 (673.15 K) and oxygen of 6.73th^-1 (373.15 K) as gasification agents, the investment in scheme (I) and scheme (II) were 8500 and 103500 €, respectively, and the recovered profit of materials and heat were 730 and 691 € h^-1, respectively.