Technical and economic eva‎luation of a power plant for recovering energy from municipal sewage sludge with co-digestion and sludge cake gasification processes

4/15/2024 12:00:00 AM

In the present thesis, in the first step, a real biogas plant located at Tehran South Wastewater Treatment Plant (WWTP) is comprehensively eva‎luated from the technical and economic point of view. This is carried out with the help of 4E (energy, exergy, economic and environment) analysis. In the second step, with the aim of increasing biogas production and improving the output power of the biogas plant in off-design conditions, the co-digestion process in anaerobic digesters is eva‎luated theoretically for digesting sewage sludge alongside a co-substrate (including grease, biowaste, and manure). To this end, the ADM1 model, which is used for modeling a mono-digestion process, is modified to be applicable for modeling a co-digestion process. The modified model is then calibrated and validated using real operating data from the anaerobic digesters of Tehran South WWTP. In the final step, by technically modeling the gasification process in two different gasifier technologies, attention is focused on the disposal of digested sludge output from anaerobic digesters which creates many problems due to its large volume in WWTPs. The results of 4E analysis show that in design conditions, Tehran biogas plant has an overall energy efficiency of 24.3% and an overall exergy efficiency of 17.3%. Additionally, the levelized cost of electricity production for the plant was calculated to be 127 $/MWh. In the second step of this study, the operational data of three anaerobic digesters in Tehran biogas plant were analyzed over a period of 140 days (in off-design conditions). By modeling the co-digestion process integrated with a co-substrate feeding strategy, it was demonstrated that feeding grease and biowaste increased the digesters COD removal efficiency from 37% (in mono-digestion) to 58% and 54% (in co-digestion), respectively, feeding manure resulted in a decrease in COD removal efficiency up to 31%. Finally, the results of sludge cake gasification (with 25% solids) showed that the overall energy efficiency of the supercritical gasifier system (45.1%) is significantly higher than that of the dryer-dual fluidized bed gasifier system (1.6%). Since efforts were made to provide energy required for gasification and drying units independent of external fuel sources in both systems, design variables for the cleaning system were determined so that not only overall energy efficiency of gasification systems is maximized but also H2S concentration in cleaned syngas is less than 250 ppm to meet allowable limit for using syngas in gas engines. Furthermore, wastewater exiting from absorption column of water scrubber was cleaned by stripper column so that NH3 concentration is less than 20 ppm according to environmental regulations.

لجن فاضلاب شهري , نيروگاه بيوگاز , فرآيند كودايجسشن , فرآيند گازسازي

Municipal sewage sludge , Biogas power plant , Co-digestion process , Gasification process

Mohsen Yazdani

Dr. Sepehr Sanaye