Aligning Cogeneration Sizing With Everyone's Goals (Big WRRF Edition)

Energy recovery and sustainability projects at water resource recovery facilities (WRRF) are typically discretionary and require an attractive return on investment to justify the capital expense. The Inflation Reduction Act (IRA), which provides federal funding for projects that produce renewable energy, dramatically changed financial considerations for energy projects. The certainty of the IRA to provide up to a 50% rebate on qualified capital expenditures means WRRF bioenergy projects that otherwise would not have been attractive have moved forward across the United States. To qualify, construction had to begin before December 31, 2024, creating increased urgency to accelerate project schedules that is novel to our industry. Many WRRFs pursued eligibility by implementing biogas fueled cogeneration (also known as combined heat and power, or CHP) systems to produce renewable electricity and heat on-site. With greater urgency in project schedule comes greater potential for overlooking big picture and fine details that are critical to a successful cogeneration system. Even with IRA rebate, cogeneration can be a big capital investment that hinges on the upside of its O&M benefit, so when they are not designed properly, the consequences can be severe for the owner. This is key to our industry's future, because interest in beneficial use of biogas is surging nationally as more utilities and municipalities implement voluntary climate action plans to reduce near-term greenhouse gas (GHG) emissions. We will see more facilities pursuing energy recovery technologies like cogeneration. OBJECTIVE This manuscript will summarize: 1.The cogeneration system sizing approach recently taken by a large WRRF that all stakeholders can learn from, including: owners, consultants, and plant staff. 2.Evidence for why it's important to size cogeneration systems properly. 3.The array of site-specific factors that drive the sizing and configuration of a cogeneration system. The intent is to give the audience an idea of what questions one should ask if interested in adding cogeneration to their facility. The session will use an evaluation done for the City of Columbus, Ohio (City) Southerly WWTP (SWWTP) Bioenergy Project that includes: 6 MG of digestion, fats oils grease (FOG) receiving facility for co-digestion, and 6 MW cogeneration system, as a case study showcasing the influence of IRA funding on cogeneration system alternatives development. Drivers that are client site-specific include: 1.The City has a climate action plan with aggressive goals to slash GHGs. 2.City staff have familiarity owning and operating a cogeneration system because their sister plant recently installed engines. 3.Life cycle economics are significantly better over the status quo. 4.Adding cogeneration to the Bioenergy Project makes it eligible for IRA Credit. To determine how to best leverage IRA funding, the City considered biogas conditioning and biogas upgrading paired with internal combustion engine or turbine technology (also referred to as 'prime mover'). Although very uncommon, pairing biogas upgrading to renewable natural gas quality with cogeneration was investigated to evaluate the reduced O&M requirements of the engines or turbines and simplification of fuel blending. SIZING APPROACH Goals and objectives must be established, and understanding how they differ helps to manage expectations and decision making as the design progresses. Goals serve as long term aspirations while objectives are requirements that must be met for the system to be deemed functional. Objectives SHOULD be weighted heavier, and oftentimes these aren't identified until the project team drills down into what's required to achieve goals. For the City it was more important that the system had high uptime, but it wasn't a necessary to keep the prime mover loaded 100% 24/7. The City wanted turndown for startup through year 20 biogas production which includes ramping up the FOG program - but they could tolerate some flaring. The heat recovery performance was weighted heavy because this will have a large impact on reducing the natural gas bill and meeting City climate action plan goals. The manuscript and presentation will do a deep dive into the details of matching the cogeneration system's size to the facility's biogas production and heat demands, which include: - Know Your Gas: having a firm handle on biogas production is going to bracket the cogeneration system's operation. The design biogas flow is estimated to be nearly 2,000 SCFM, while the startup minimum flow could be as low as 650 SCFM. - Utilize Your Gas, Big Picture: establish the universe of prime mover size alternatives that could fit the amount of biogas produced (Figure 1). Smaller sizes will be fully loaded early in the design life but may be overtaken by peak biogas flows in future years. Larger sizes can utilize all biogas flows through the design year, but the impacts of turn-down limitations need to be carefully considered. - Dynamic Conditions: understand the distribution, or bell curve, of biogas production, so that we can be confident of how much and how frequently the cogeneration system is loaded. When biogas production falls within the 'gaps' of the cogeneration system's operating bands, this biogas may need to be flared (Figure 2). - Natural Gas Blending: natural gas can be blended with biogas to keep the prime mover loaded in its acceptable operating range, but its not necessarily a 'silver bullet'. Owners that seek to reduce GHGs may not tolerate elevated natural gas consumption. - Heat Recovery: consider how the prime mover heat recovery fits the needs of the facility. Primer mover heat recovery performance can fluctuate throughout the year due to changes in climatic conditions impacting efficiency. EXPORT POWER? Cogeneration systems cannot be sized solely based on estimate biogas production. It needs to be coupled with the facility's electrical demand to capture the full picture of possible restrictions on the cogeneration system's daily operation and overall output. Depending on the frequency and magnitude of electrical demand fluctuations, there can be periods where surplus power is produced, therefore its critical to engage the power utility very early in the design to understand the implications of sending power back to the utility grid. Power utilities may require electromechanical safeguards that increase project cost and complexity. Permits required to export power can be logistically burdensome on the owner and are known to take years for approval. CONCLUSION AND KEY TAKEAWAYS The manuscript and presentation will tell the story of Columbus's cogeneration system sizing approach. Facilities of this size implementing cogeneration are limited, consequently there are invaluable lessons learned and considerations that all stakeholders can learn from. Carefully consider how your digester gas production lines up with the turn-down of the prime mover: this is going to drive operating costs and its especially important if utilizing all produced biogas is a must. The manuscript will include simulated 20-year biogas variability using daily biogas flow data from the SWWTP and similar size facilities to reflect the dynamic operating conditions and turn-down requirements of the cogeneration system. For some owners the recoverable heat can be more valuable than the electricity. For ALL owners, meeting the heat demands of your process IS the first priority. The manuscript will include an analysis that illustrates how SWWTP's heat demands vary season to season and compare prime mover technology heat recovery performance. For the plant-wide electrical system: the devil is in the details. The manuscript will include a deep dive into SWWTP's electrical data (Figure 3) and highlight key considerations of the plant-wide electrical infrastructure that are often overlooked during design. The ultimate outcome is confidence the cogeneration system design provides the City the maximum benefit. For the audience, an inside look at a sizing approach framework that can help our industry pursue successful energy recovery projects.