Biogas Action Planning in Tallahassee--Plans for the Future with Improvements for the Now

INTRODUCTION The City of Tallahassee developed a biogas and energy optimization plan for its Thomas P. Smith Water Reclamation Facility (TPS WRF). The goals of this plan included optimizing imports of organic wastes to anaerobic digesters and maximizing biogas energy use in a way that is operationally sensible, economically beneficial, and promotes sustainability. The cornerstone of this plan was the development of a Solids and Energy Flow Model a tool that dynamically and holistically tracks flows of solids and energy in its various forms throughout the TPS WRF. Using this methodology, a multi-phased improvement plan was developed to address immediate issues with biogas blowers and improve short term biogas utilization rates, while also laying out a long-term path for a more sustainable future. The identified improvements included: -Replacing existing digester blowers to allow short term use of biogas in onsite thermal dryers. -Implementation of new FOG receiving with feeds to individual digesters. -Implementation of Renewable Natural Gas (RNG) to fuel City owned vehicles. BIOGAS BLOWERS TO FUEL THERMAL DRYER A previous plant upgrade had installed new digesters and a new rotary drum thermal dryer. Biogas blowers intended to be used to fuel the dryer failed during this project's startup and were never fully put into service. Recurring failures in the blower seals were observed within weeks of operation. Due to these failures the system was never started up on biogas and the dryer has been run on natural gas. Detailed investigations into the blower failures revealed that moisture buildup due to the specific blower location and elevation were the cause of recurring failures. Blowers were located in close proximity to the raw biogas draw off from digesters, leaving little pipe length to cool gas and remove condensate (Figure 6). Blower suction elevations were also located at the low point in the piping system which exacerbated the buildup of condensate in the blowers. Improvements included reconfiguring biogas piping to include moisture knockouts while installing new blowers with higher suction elevations (Figure 10). This upgrade will allow biogas to be used in the dryer as intended saving significant amounts of natural gas use (Figure 9) in a short-term timeline. This project was designed and is currently in the construction phase. During design it was discovered that the rotary drum dryer controls were never officially configured to use biogas. The Design Team worked with the dryer manufacturer to devise comprehensive and unified control strategies for the entire biogas system including coordinating dryer firing rates based on digester dome pressure. NEW FOG RECEIVING AND RNG The longer-term improvements identified included upgrades to the FOG receiving and sludge pumping systems as well as the implementation of RNG. Currently the plant receives FOG and septage in a single receiving station. Staff reported O&M issues with this station being undersized and having long piping runs to connect to the Blended Sludge Tank for distribution to digesters (Figure 3). An item identified during investigations was that FOG is currently blended with sludge in the Blended Sludge Tank (Figure 11) making it impossible to segregate FOG to individual digesters. This is important because the plan called for segregating FOG feed to individual digesters to maintain D3 RIN classification for a portion of the biogas directed to a future RNG system. The plan included a new trucked waste receiving station dedicated only to FOG. This new station would be located in closer proximity to the digesters and would provide individual feed pumps to send FOG to individual digesters (Figure 13). It was determined that Digester #3 that is currently being rehabbed could accept all the existing FOG loads received while leaving the remaining 3 plant digesters 'untainted' by FOG and able to produce D3 RINs. The plan called for specific enhancements to Digester #3 feed piping and pumping allow this unit to become the 'FOG heavy' digester. RNG was targeted as the long-term biogas utilization strategy at the TPS WRF due to several beneficial circumstances. First is the fact that the City of Tallahassee owns the natural gas local distribution utility pipeline. City NG pipeline staff have expressed high interest and willingness to facilitate a pipeline interconnection with the TPS WRF at a reasonable cost and terms while avoiding lengthy negotiations with 3rd party pipeline operators. The second is that the City already owns and operates a CNG sanitation fleet, giving the project a captive vehicle user for the RNG again without the need to negotiate terms with a 3rd party. The plan examined projected RNG production rates and compared them to the City fleet fuel use in Diesel Gallon Equivalents (DGE) per day. The fleet was found to have a steady consumption rate on a weekly and yearly basis that approximately matched the production of the proposed RNG system. With the benefits of these City owned partner utilities the RNG project was found to have significant long-term economic and sustainability benefits over using the biogas in the thermal dryer. CONCLUSION The plan provided Tallahassee with both immediate action items for improving their biogas system while laying the foundation for a more sustainable future. The biogas blowers to fuel the dryer could be implemented on a short timeline (~1 year) at a small cost (~$1 million) and would save an estimated 60,000 mmbtu/year of NG use at a savings of $570,000/year. This project is already in construction to provide short term benefits and operational flexibility while the larger and long-term projects for FOG receiving and RNG production are being developed.