Next Evolution of Biogas Upgrading -- RNG System with Heat Recovery

Over the last fifty years, anaerobic digestion has become common practice at medium to large water resource recovery facilities (WRRFs) in the United States, providing a sustainable source of biogas while achieving significant improvements in sludge stabilization. With more installations, facilities are being presented with a larger array of biogas end use choices to meet their cost and sustainability goals. After four decades of operating a cogeneration facility to produce electricity and heat, Metro Water Recovery (Metro) and Carollo Engineers, Inc. (Carollo) completed a Biogas Utilization Study in 2021 to consider new beneficial uses of biogas at the Robert W. Hite Treatment Facility (RHWTF) that serves Colorado's Denver metropolitan area. The study compared replacing the existing cogeneration system with new cogeneration equipment versus installing biogas upgrading equipment to generate renewable natural gas (RNG) for pipeline injection. Through a detailed comparison of the biogas utilization alternatives that included financial analysis, operational considerations, and sustainable attributes, Metro selected RNG as the biogas utilization pathway which would provide the highest social, environmental, and financial benefit to Metro. In 2022, Metro and Carollo began design of the Biogas Utilization Project, including a biogas upgrading system for pipeline injection of RNG and a Boiler Mechanical Facility to replace the heat currently supplied by the cogeneration system. The initial phase of this project included further analysis into the available technologies for biogas upgrading. Most WRRFs in the United States have selected membrane or pressure swing adsorption technologies for the separation of methane and carbon dioxide due to their cost effectiveness at relatively small biogas flow rates. Although these approaches do not have technical limitations at higher flow rates, amine wash and water wash were determined to be more appropriate from a financial and physical layout perspective at Metro's design biogas flow of 3,000 standard cubic feet per minute (scfm). As part of the preliminary design effort, the team collected information on these two systems and conducted a triple bottom line alternatives analysis to support an informed technology selection. The detailed information included: - Equipment budgetary proposals (including capital and operating costs, equipment layouts, and equipment lead times) from multiple manufacturers. - Site visits to operating equipment installations. - Phone interviews with past and current RNG system operators. - Conceptual system layouts. - Conceptual design and construction costs. - Greenhouse gas (GHG) emissions. - Local natural gas utility's RNG quality requirements. A comparison of the water wash and amine upgrading systems is summarized in Table 1. Based on Metro's evaluation of the two technologies, site visits, and feedback from reference installations, the design team determined that both the amine and water wash upgrading systems could reliably produce RNG that meets the natural gas utility's quality requirements. While water wash systems contain fewer mechanical components and higher levels of redundancy (based on the equipment manufacturer's two-train proposal), the amine system offers slightly higher methane capture, a higher purity tail gas stream that may be more amenable to future beneficial use, lower GHG emissions, and the ability to recover a significant portion of the input heat to offset natural gas required for heating the digesters and buildings on site. Based on this evaluation, Metro selected the amine upgrading system for final design. In an amine upgrading system, raw biogas is boosted and conveyed into a counter-flow reactor tower where the carbon dioxide is solubilized into the amine solution at relatively low pressure (12 pounds per square inch). The methane flows out the top of the reactor tower while the carbon dioxide-rich amine solution is pumped to a stripping tower. Heat is added to the stripping tower to break the bond between the carbon dioxide and amine in solution, releasing carbon dioxide gas and regenerating the amine solution, which is pumped back to the reactor tower. The heat used by the amine upgrading system will be supplied by new hot water boilers that are being constructed as part of the project to heat the digesters and buildings (replacing heat that was previously supplied by the cogeneration system). The carbon dioxide separation process requires a high temperature to regenerate the amine solution. While the heat is often supplied by steam, in this case, the design includes superheated water pressurized to 125 pounds per square inch gauge (psig) and heated to 275 degrees, avoiding separate steam boilers, reducing overall system complexity, and improving redundancy. Up to 90 percent of the heat required by the amine upgrading system can be recovered and used elsewhere on the plant-site. However, the recoverable heat is 'low grade' in that it is not hot enough to be usable at the temperatures supplied from the amine skid for traditional digester process or building heating applications. To utilize heat recovery from this system, Metro and Carollo evaluated the use of industrial water-to-water heat pumps to upgrade the low-quality recovered heat to higher quality heat for use in the digestion process, the largest heat demand at the RWHTF. To better understand the costs and benefits of a heat pump heat recovery system, the team developed a 20-year net present value and estimated GHG emissions reductions associated with the heat recovery system. The capital cost of the heat pump system was developed by the construction manager at risk (CMAR) contractor, Moltz Construction, Inc. The total construction cost for the heat recovery system, including the building it is housed in, is estimated to be $9.2 million. If this system were to qualify for Inflation Reduction Act (IRA) funding (which is anticipated based on discussions with Metro's tax advisor), the capital cost would be reduced to approximately $6.4 million. While the heat pump system has a significant electrical demand, its high coefficient of performance means the energy output is more than five times the electrical energy input. The heat pumps will directly offset approximately $1.5 million in annual natural gas usage in the boilers and reduce GHG emissions by 30 percent compared to solely using natural gas for digester heating (Figure 1). The calculated 20-year NPV of the heat recovery system is approximately $3.1 million with a payback period of roughly 15 years. With the IRA's capital cost reduction (assumed to be a 30 percent discount) the 20-year NPV increases to $5.9 million and the payback period decreases to approximately 10 years. A graph comparing the 20-year NPV with and without the IRA reduction is shown in Figure 2. Construction of Phase 1 of the Biogas Utilization Project began in October 2024. Phase 1 includes construction of the Boiler Mechanical Facility, which will house the hot water boilers required to supply heat to the digesters, the amine upgrading system, and other process and building heating demands at the RWHTF. Construction of Phase 2 will begin in 2026 and includes demolition of the cogeneration facility and installation of the amine upgrading and heat recovery systems. At an inlet design flow rate of 3,000 scfm, the RWHTF's biogas upgrading system will be the second largest RNG system and the first installation of an amine upgrading system at a WRRF in the United States. Figure 3 presents a rendering of the proposed facilities. WRRFs are under increasing pressure to recover resources and make sustainable decisions related to energy use. This project showcases how a large regional WRRF pivoted from cogeneration to pipeline injection of renewable natural gas with a focus on maximizing resource recovery and minimizing natural gas use. This novel application of amine upgrading paired with a heat pump system provides a road map for other utilities to follow. The project's use of triple bottom line evaluations provides a solid business case for decision making.