Thermodynamic Analysis of a Self-Sufficient and Nearly Emission Free Wastewater Treatment Plant

Effluent standards in wastewater treatment plants have been very strict due to a dramatic increase in the amount of contaminants of household and industrial wastewater. For this reason, wastewater treatment plants consume a significant amount of power in order to treat wastewater throughout the world. Since most of the power in the United States is produced by petroleum products and natural gas, greenhouse gas emission has been a severe problem. In this study, a multigeneration energy system has been developed numerically to create an energetically self-sufficient wastewater treatment plants (WWTP) with zero emission. Since the WWTP part has already been investigated in another study [1], only the oxy-fuel power generation system, which is a combination of an oxy-fuel combustion cycle with a Rankine cycle, has been modeled. In the oxy-fuel combustor, the biogas generated by anaerobic digestion process is combusted with pure oxygen and recycled CO₂ to produce hot exhaust gas. The system has been developed and modelled using Engineering Equation Solver and several parameters in the power cycle have been varied so as to investigate their effects on thermodynamic efficiencies and self-sufficiency ratio. The parameters in this sensitivity study are compression ratio, turbine inlet temperature, CO₂ return ratio, recycled CO₂ temperature, and temperature difference in the regenerator. Energy and exergy efficiencies of the parametric study have been calculated along with the self-sufficiency ratio. The maximum results for energy and exergy efficiencies shown to be 53.7% and 52.2%, and for self-sufficiency ratio to be 130.5%. The results show that this concept has a significant potential for future deployment.