Economic and Environmental Sustainability Via On-Site Digester Gas Energy Production
Authors:
- C. Michael Bullard, P.E , Bryan R. Lisk, P.E. Scott A. Hardy, P.E. - Hazen and Sawyer
Anaerobic digestion is commonly utilized for wastewater residuals stabilization and the resultant methane rich digester gas (biogas) stream is generally utilized for process heating. It is estimated that of the 16,000 centralized wastewater treatment facilities in the United States approximately 3,500 utilize anaerobic digestion for residuals stabilization. However, only about three percent (~100) of those facilities are currently utilizing biogas to produce electricity and thermal energy in combined heat and power systems (USEPA Combined Heat and Power Partnership, 430R07003, April 2007).
Increasingly, wastewater treatment facilities that utilize anaerobic digestion are examining biogas beneficial use projects for energy recovery that transcend the current, and most common, practice of only capturing heat energy for process heating and flaring surplus biogas. Evaluating biogas-to-energy beneficial utilization projects must account for a wide range of site specific criteria in determining the quantity of usable energy that can be extracted from the biogas while simultaneously balancing process heating demands which are essential to anaerobic digestion process stability. In many cases these site specific criteria will determine the economic viability of converting digester gas to electrical/mechanical energy in a combined heat and power system.
Specifically, this paper will present selected results from five biogas utilization studies and designs projects conducted for facilities ranging in size from 15-MGD to 75-MGD covering a wide range of site specific considerations. These case studies will identify factors which contribute to both the economic viability and environmental sustainability of combined heat and power (CHP) projects. The goal is that by identifying these factors utilities considering CHP projects can make reasoned choices with regard to digester gas utilization.
The paper and presentation will cover the following major topics:
Estimating Digester Gas Production Rates
Siloxane and Micro-Constituent Impacts on Biogas Beneficial Use
Combined Heat-and-Power Energy Recovery System Configurations
Combined Heat and Power Lifecycle Cost Assessment Methodologies
Environmental and Climate Change (Greenhouse Gas Reduction) Benefits
Case Studies – Facility Locations
Moores Creek WWTP (Charlottesville, VA)
Roanoke WWTP (Roanoke, VA)
Henrico County WWTP (Richmond, VA)
F. Wayne Hill WWTP (Gwinnett County, GA)
Cape Fear Utility Authority Northside WWTP (Wilmington, NC)
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