Biogas Cogeneration System Sizing and Payback Based on Weekly Patterns of Anaerobic Digestion and Biosolids Dryer Operation

Abstract

Pinellas County Utilities (PCU) is implementing digestion improvements at its 33 mgd South Cross Bayou Water Reclamation Facility (SCBWRF) that will allow the codigestion of sludge and fats, oil and grease (FOG) feedstocks. Based on the expected high biogas production due to the codigestion of FOG and also due to a limit in the biogas utilization by the SCBWRF dryer/pelletizer, significant biogas quantities are expected to remain available after the pelletizer's biogas needs are met. The economic feasibility of implementing combined heat and power (CHP) generation at SCBWRF was analyzed in order to maximize lifecycle benefits and avoid energy wastage through biogas flaring. Analysis of various gas production scenarios considered the amount of available FOG as well as the weekly pattern of biogas production and consumption, presuming that the FOG would be fed to the digesters only during weekdays when the pelletizer would also be in operation. Internal combustion engine (ICE) based cogeneration provided the maximum lifecycle benefit among cogeneration alternatives. Based on the target biogas scenario, the payback period of a 1,400 kW (electrical) ICE cogeneration system and associated biogas treatment was estimated to be 7.3 years, with a projected 20-year lifecycle benefit of $5,400,000. The sensitivity of these benefits to key assumptions was assessed by varying uncertain technical and financial parameters.

Making Anaerobic Digestion Work with High Strength Wastes: Lessons Learned from Pilot to Full Scale

“…Following extensive laboratory experimentation, Pinellas County has proceeded with the design of digestion improvements to be able to significantly increase the amount of codigested FOG. A recent analysis (Kabouris et al, 2011) underscored the potential benefits from the codigestion of thickened FOG, with the projected biogas quantities using the improved digestion system potentially exceeding the biogas quantity used in fueling the on-site pelletizer. This excess biogas would be used for cogeneration and the dryer's waste heat would be used for the heating of digesters.…”