Alternative Approach to Accelerate Beneficial Biogas Utilization and RNG Production

Charlotte Water (CLT Water) is the largest public water and wastewater utility in the Carolinas, serving more than one million customers in the City of Charlotte and greater Mecklenburg County. Charlotte Water currently owns and operates five major water reclamation facilities (WRFs), treating an average of 325 million liters per day (ML/d) or 86 million gallons per day (MGD) of wastewater and is in design for a sixth plant, the Stowe Regional Water Resource Recovery Facility (WRRF). Stowe Regional WRRF will treat flows from the western portion of Mecklenburg County and will also accept and treat flow from two neighboring towns in a bordering county. The Stowe Facility will have the capacity to process 68 ML/d or 15 MGD of wastewater, which will eventually grow to 114 ML/d or 25 MGD. CLT Water has a phased regionalization program in place for consolidation of biosolids treatment at McAlpine Creek Wastewater Management Facility (McAlpine Creek WWMF) as shown in Figure 1, which includes solids conveyance projects and the design and construction of a thermal hydrolysis process (THP) system for pre-treatment of solids prior to anaerobic digestion. CLT Water completed a Biogas Production and Utilization Study for the McAlpine Creek WWMF in September 2020. This study projected expected biogas production at McAlpine Creek WWMF taking into account the regionalization of solids treatment and the installation of THP. The combined regionalization and digestion improvements are anticipated to increase biogas production to approximately 1,930 normal cubic meters per hour (Nm3/h) or 1,200 standard cubic feet per minute (scfm). These projected biogas production values were used to generate a bioenergy model to evaluate multiple alternatives for beneficial biogas utilization including Combined Heater and Power, Steam Generation for THP, Compressed Natural Gas (CNG) fueling, and Renewable Natural Gas (RNG) pipeline injection. The bioenergy model, shown in Figure 2, considered capital and lifecycle costs of each alternative and produced dynamic comparisons of the different alternatives based on varying assumptions and user inputs. A non-economic evaluation was also used to compare the biogas utilization options, including safety, risk and O&M considerations, bioenergy utilization, private development opportunity, and public relations. The outcome of the study was a recommendation to implement RNG pipeline injection as this would allow for the largest percent utilization (99%) of the biogas and the most flexibility to operations, while aligning closely with the City of Charlotte's Strategic Energy Action Plan (SEAP). Following completion of the Study, CLT Water proceeded with a competitive sale process, viewing the biogas as a personal property to be sold as a commodity to a buyer to be converted to RNG and soliciting sealed bids from project teams to implement the RNG strategy. As a condition of the sale, the potential buyer was required to provide a 'turn-key' solution for the utilization of the biogas by providing the buyer's own capital, facilities, design, construction, operation, management, and permitting services necessary to convert the biogas into RNG. The buyer will perform conditioning of the biogas and resell the resulting product gas as RNG, which will create Renewable Identification Numbers (RINs) that can be monetized via the federal RIN market as well as state level credit markets. The buyer will have the rights to all environmental credits/attributes such as renewable energy certificates, low carbon fuel standard credits, and other similar entitlements associated with the RNG. Figure 3 shows an arrangement of the proposed biogas utilization program. CLT Water conducted the following two solicitation steps in order to enter into an agreement for the sale: 1)Request for Proposals (RFP): Utilized to short-list qualified companies taking into consideration vendor qualifications and experience, demonstration of safe RNG operation practices, quality, delivery, workmanship, and any applicable environmentally preferable attributes associated with the company's proposed solution for RNG. The short-listed companies were qualified to receive an Invitation to Bid. 2)Invitation to Bid (ITB): sealed bid sale where short listed companies have an opportunity to provide CLT Water their best price for the purchase of unconditioned biogas within a defined cost framework. This procurement approach encouraged competition in the determination of the biogas value and efficiency of proposed solutions. CLT Water has completed the RFP and ITB stages, selected the highest responsive, responsible bidder, and are currently undergoing final contract negotiations which include a land license for the facility construction inside CLT Water property and an offtake agreement for the delivery of biogas. Design of the facility is expected to commence in Spring 2025 and is anticipated to be sized for 1,930 Nm3/h (1,200 scfm). The system will be capable of removing hydrogen sulfide (H2S), volatile organic compounds (VOCs), siloxanes, and carbon dioxide (CO2) to meet pipeline specifications. The presentation will summarize the history of the biogas utilization study, development of the RNG implementation approach, and highlights of design efforts completed at the time of the conference.