Understanding the Impact of Variability on Energy Recovery from Digester Gas


  • Hunter Long, Michael Bullard - Hazen and Sawyer
  • Chris Shamel, Frank Crump - City / County Utilities Commission (CCUC), Winston Salem, North Carolina

This technical paper and presentation will share results from an evaluation of digester gas utilization at the Muddy Creek Wastewater Treatment Plant (MCWWTP), Winston-Salem, NC. The evaluation revealed the importance of considering daily variation in digester gas production on the economic viability of on-site energy production.

The MCWWTP uses digester gas to heat boilers which produce hot water for anaerobic digestion process heating and facility heating. Excess digester gas not required for process heating may be used to fuel two (2) reciprocating internal combustion engine (RICE) driven blowers. The RICE driven blowers are preferentially operated during week day on-peak demand periods to offset blower electricity use. The facility’s electricity bill is divided into distinct on and off peak periods for the winter and summer with separate charges for energy consumption and monthly peak electricity demand. The on-peak demand charge has accounted for approximately 30 percent of the plant’s monthly electricity bill over the past three years and provides a strong incentive for the facility to reduce electricity costs by running the RICE driven blowers during the whole of the on-peak demand period. At the time of this evaluation, one of the RICE driven blowers was not operating and required an estimated $125,000 capital investment to return the RICE to service. The purpose of this study was to evaluate the electricity savings associated with operating one or two RICE driven blowers and the potential savings from installing heat recovery equipment.

Historical data for digester heating demand and digester gas production was fit to cumulative distribution curves for each month using CRYSTAL BALL® software. A Monte Carlo simulation analysis was then prepared for each of the digester gas utilization scenarios to determine the electricity savings for each scenario.

Had the average monthly digester gas production been used in the economic analysis rather than cumulative distribution curves, the analysis would have predicted an additional $48,000 annual electricity savings (an increase in savings of 50%) due to the ability to consistently off-set the on-peak demand charge. This additional savings would have a significant impact on the facility’s decision to invest in new, or recapitalize existing, digester gas utilization equipment. This paper and presentation will; address additional long-term biogas utilization scenarios at the MCWWTP, expand on the causes and effects of variation in digester gas production, and discuss the importance of using Monte Carlo type simulation to address the variability inherent in anaerobic digestion and digester gas utilization.

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

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