Sometimes Average isn’t Good Enough: Understanding the Impact of Variability on Energy Recovery from Digester Gas

摘要

This technical paper and presentation will share results from an evaluation of digester gasutilization options at the Muddy Creek Wastewater Treatment Plant (MCWWTP), Winston-Salem, NC. The evaluation revealed the importance of considering daily variation in digester gasproduction on the economic viability of on-site energy production. The MCWWTP uses digestergas to heat boilers which produce hot water for anaerobic digestion process heating and facilityheating. 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 arepreferentially operated during week day on-peak demand periods to reduce the MCWWTPelectricity consumption and operating costs. The facility’s electricity rate contains a significantdemand charge, $13.69 and $7.57 per kW each month, for the on-peak demand periods duringthe summer and winter respectively. However, the electricity rate has a relatively low energycharge, $0.0626 and $0.0382 per kWh during the on and off peak periods, respectively. Theevaluation identified $48,000 of annual demand reduction savings when considering the annualaverage digester gas production. However, when historical daily variability in digester gasproduction was considered the facility was not expected to achieve any of the demand reductionsavings. This paper will expand on the effect of digester gas variability on the MCWWTP’s longterm biogas utilization plan.