On-site Energy Production With Combined Heat and Power Systems


  • Charles Bullard, Scott Hardy and Brian Lisk - Hazen and Sawyer

Anaerobic digestion is commonly utilized for wastewater residuals stabilization and the resultant methane rich digester gas (biogas) stream is commonly utilized for digestion 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. These on-site criteria include primary and secondary sludge mass fractions, digester residence time, seasonal heating demands, and local electrical energy costs. In many cases these on-site criteria will determine the economic viability of converting digester gas to electrical/mechanical energy in a combined heat and power system.

Specifically, we will present selected results from several biogas utilization studies conducted for facilities ranging in size from 15-MGD to 75-MGD covering a wide range of site specific operational criteria. 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:

• Typical Site Specific Demands and Constraints for Gas Utilization
• Typical Digester Gas Energy Recovery Systems
• Siloxane and Micro-Constituent Impacts on Biogas Beneficial Use
• Combined Heat-and-Power Energy Recovery System Process Configurations
• Lifecycle Cost Assessment with Variable Biogas Production
• Economic Benefits Assessment and Cost Recovery
• Environmental and Climate Change (Greenhouse Gas Reduction) Benefits

To request a copy of the full paper, please contact the author at mbullard@hazenandsawyer.com.

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