Energy, exergy, economic and environmental analysis of a running integrated anaerobic digester-combined heat and power system in a municipal wastewater treatment plant

Abstract

4E (energy, exergy, economic, and environmental) analysis of a municipal wastewater treatment plant (WWTP) including an integrated system with anaerobic digesters and combined heat and power units (AD-CHP) is performed here. This type of analysis was required for AD-CHP system at Tehran WWTP to estimate the plant energy/exergy efficiency, to estimate annual profit and payback period, as well as to study these parameters with change in plant operating conditions. This necessary type of analysis was not found in literature and in previous studies. The results showed that the integrated system generated 4.6 MWel,net and 2.1 MWth,net with 3100 m3/day of total mixture of primary and secondary sludge (4.5% total solids and 81.2% volatile solids fraction). In this regard, the system energy efficiency was 24.3%. The flow exergy values throughout the plant were also estimated and demonstrated in two Sankey and Grassmann diagrams which showed the plant overall exergy efficiency of 17.3%. The integrated system also reduced CO2 emissions by 64% compared to that for a traditional energy supply system (buying electricity from the grid and producing hot water by burning biogas produced from anaerobic digesters). In economic analysis, a selling incentive scenario employed in which generated electricity was sold to the grid at price of 0.125 $/kWh while the price of electricity purchased was 0.081 $/kWh. The payback period was 2.4 years in this case which was significantly lower (57%) than that for the case with no-selling electricity to the grid (5.6 years). Moreover, the levelized cost of electricity (LCOE, which is the minimum selling price of electricity required by the integrated system or the point at which cost and revenue are equal) was 127 $/MWh, which was within the acceptable range reported by the International Renewable Energy Agency for the biomass-power generation technologies.