Seasonal and Lifecycle Cost Considerations In Evaluating Beneficial Utilization of Digester Gas

Authors:

  • C. Michael Bullard, PE, Hazen and Sawyer, P.C.
  • Kenneth L. Vogt, PE, Cape Fear Public Utility Authority
  • Craig Lundin, PE, Cape Fear Public Utility Authority

Anaerobic digestion is commonly utilized for wastewater residuals stabilization and the resultant methane rich digester gas 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 and only about two percent (~70) of those facilities are currently utilizing digester gas to produce electricity (WE&T, January 2008, pg. 34). Increasingly, wastewater treatment facilities are examining digester gas beneficial use projects for energy recovery that transcend the current, and most common, practice of capturing heat energy for process heating and flaring surplus digester gas.

Methodologies utilized for evaluating digester gas beneficial utilization projects must account for a wide range of site specific operational criteria in determining the quantity of usable energy that can be extracted from the digester gas while simultaneously balancing the process heating demands which are essential to anaerobic digestion process stability and the production of the digester gas energy resource. Specifically, this paper will present a process and economic modeling approach for considering site specific operational criteria when utilizing digester gas. The process and economic model will be developed and presented in the form of a case study at the Cape Fear Public Utility Authority’s (CFPUA) 16-million gallon per day James A. Loughlin Wastewater Treatment Facility located in Wilmington, North Carolina.

The process and economic model presented in this case study will consider the following site specific issues:

Seasonal Heating Demands –
Process heating demands for the anaerobic digestion process fluctuate seasonally as incoming liquid sludge temperatures change and as digester maintenance heat losses vary from the seasonal high requirements in winter to seasonal low requirements under summer operating conditions. A process model will be presented which considers seasonal fluctuations in liquid and maintenance heating demands relative to heat energy available from the digester gas to determine optimal configurations for balancing heat energy production with heat energy demands. A method for evaluating process and economic considerations associated with the varying seasonal process heating demands will be presented when evaluating different digester gas energy recovery systems.

Differential Digester Gas Energy Recovery System Benefits –
Three energy recovery system (ERS) configurations will be evaluated and presented in the case study. The base case will be utilization of a hot water boiler for process heating with flaring of excess digester gas. An internal combustion gas engine-generator (ICGE-G) burning excess digester gas without a combined heat and power configuration (i.e., “non-combined”) will be evaluated in comparison with a full combined heat and power ERS utilizing all the digester gas and with seasonal supplemental heating with purchased natural gas fuel. Usable energy recovery will be calculated for each of the systems and presented graphically.

Lifecycle Cost Assessment with Variable Digester Gas Production
A lifecycle cost assessment (LCA) model will be developed and presented which addresses the issue of variable digester gas production rates across the 20-year lifecycle period. Consideration will be given to increased digester gas production rates (SCF/MG treated) during the early phase of the assessment due to increased digester residence times compared to reduced digester gas production rates (SCF/MG treated) during later stages. As shown in the attached figures (Figure 4 and Figure 5) digester gas production rates fall as flow rate grows; however, total digester gas energy increases as influent flow increases.

Differential Economic Benefits Between Energy Recovery System Configurations
Economic benefits attributed to differential energy recovery effectiveness between the three energy recovery systems will be presented in the study for operating conditions across the full range expected during the lifecycle of the project (i.e., flow range from 10-MGD to 16-MGD). Estimated annual power production for the “non-combined heat and power” (n-CHP) system will be shown to range from 1,029 MWH/year to 1,729 MWH/year, respectively, for the 10-MGD and 16-MGD operating conditions. Estimated annual power production for the full combined heat and power (CHP) system will be shown to range from 1,960 MWH/year to 2,921 MWH/year, respectively, for the 10-MGD and 16-MGD operating conditions. Net present costs (and benefits) are calculated for both of the energy recovery system options and will be presented (see Table 1, attached). The net present costs (benefit) will be compared to the estimated capital costs to develop the ICGE-G capital costs and a benefit-to-cost ratio will be calculated and presented.

In addition to the information presented regarding the process and economics model results from digester gas testing for major constituents and siloxanes will be presented. A full discussion will be provided in the technical paper on the results of this analysis. Additionally, a discussion will be presented on the evaluation of ICGE-G and micro-turbines as potential energy recovery devices.

Lastly, a discussion will be presented on how the quality of the digester gas influenced the selection of an internal combustion engine as the preferred energy recovery system prime mover and implications for gas pretreatment to meet manufacturer’s stated gas quality requirements. Lastly, estimates of greenhouse gas emissions will be presented for each of the ERS configurations with consideration to “on-site” emissions and reduced “off-site” emissions due to the production of electrical power. Impacts of “renewable energy credits” (REC’s) and GHG emissions reductions on economic analysis will also be presented and discussed in the technical paper.

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

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