Energy Management at WRRFs - Is Hydrogen a Viable Alternative?

APPLICABILITY Green hydrogen, that is hydrogen produced with renewable energy, is a likely energy source to decarbonize heavy transportation and other industries. The production of hydrogen through electrolysis produces pure oxygen and thermal energy as green byproducts, which can be used at Water Resources Recovery Facilities (WRRFs). Two case studies will explore the viability of integrating hydrogen production at WRRFs, the full-scale demonstration project at St Cloud, MN and the feasibility study at Cedar Rapids, IA. DEMONSTRATED RESULTS/CONCLUSIONS Co-locating hydrogen electrolyzer systems powered by renewable energy in proximity of existing WRRFs allows for the concurrent decarbonization of local industries and wastewater treatment operations. From the viewpoint of WRRFs, the byproduct green oxygen can be beneficially to oxygenate aeration tanks in an activated sludge process which would reduce the demand for grid electricity to power blowers. A second waste by-product is thermal energy resulting from cooling of the electrolyzer. The hydrogen could be used either on-site at the WRRF or off-site by local industries as a zero-carbon fuel source. For the past 10 years, several WRRFs have implemented pilot systems to evaluate the feasibility of incorporating electrolyzers into their process, primarily in Europe and Australia. The City of St. Cloud, MN will be the first United States installation of a full-scale electrolyzer system for green hydrogen production using renewable energy sources, with concurrent oxygen and heat recovery. A focus of the St. Cloud project is green hydrogen production for utilization by local industries, including a bus manufacturing company and a bus transport company. The City of Cedar Rapids, IA is conducting a feasibility study for incorporating an electrolyzer into their treatment facility as a replacement of an aging cryogenic oxygen production facility to produce the pure oxygen required by the City's WRRF. Potential revenue from the green hydrogen and beneficial use of the waste thermal energy are also being investigated. St. Cloud Demonstration Project The City of St. Cloud is striving to decarbonize its energy profile and develop additional marketable products. The City's Nutrient, Energy, and Water Recovery Facility (NEWRF) currently incorporates a significant amount of energy production and resource recovery through combined heat and power production, on-site solar production, and a waste to energy program with trucked-in high-strength wastes. The current project focuses on further decarbonizing energy production to include converting biogas to renewable natural gas for pipeline injection and construction of additional on-site solar capability as the primary energy source for the full-scale green hydrogen electrolyzer demonstration. Key objectives of the demonstration project include: (1) evaluating off-site markets and on-site uses for the produced hydrogen and oxygen gases, (2) developing strategies for the utilization of the produced oxygen within the existing NEWRF infrastructure, (3) assessing the need for storage of the renewable energy, hydrogen, and oxygen, and (4) optimizing the return on investment (ROI) using the renewable energy portfolio of the NEWRF. The full-scale demonstration project is being designed around a commercially available 1-megawatt (MW) hydrogen electrolyzer. Figure 1 presents a process flow diagram for integrating the electrolysis system into the NEWRF. As illustrated, a separate solar array will be installed for the electrolyzer with additional electrical energy provided by the existing cogeneration system and a procured Solar Garden subscription. The primary market for the green hydrogen will be for commercial sale to local transportation industries requiring compression and short-term storage facilities. Alternatives for utilization of the pure oxygen include blending it with aeration air downstream of the blowers and setting up separate zones with an independent aeration system for introduction of the pure oxygen. Excess thermal energy captured from the cooling loop of the electrolyzer will be either integrated into the existing process heating loop or used as a heating source for dedicated buildings. Cedar Rapids Feasibility Study. The City of Cedar Rapids Water Pollution Control Facility (WPCF) treats approximately 45 mgd of combined high-strength-industrial and domestic wastewaters for organic matter, solids, and ammonia removal. The heart of the secondary treatment process is a pure-oxygen UNOX system with the oxygen produced on-site by an 80-ton per day cryogenic system. The cryogenic system is nearing the end of its useful life and is being evaluated by the City for repair or replacement of the UNOX system. The feasibility study will evaluate the renewable power requirements, footprint, required equipment and facilities, required water treatment facilities, hydrogen production, and thermal energy production for an electrolyzer system capable of producing 80 tons per day of pure oxygen to replace the cryogenic production facility. A preliminary assessment indicates a 25-30 MW electrolysis system will be needed. Results The St Cloud Demonstration Project is currently in detailed design and scheduled to be completed in 2024. In addition to facilities layout, process connections, and required utilities, the four critical areas to be addressed for an electrolysis project to be successful at a WRRF include: (1) Securing renewable energy sources. The primary source of electrical energy for the electrolyzer will be a new 1 MW solar array to be constructed on-site. However, as illustrated in Figure 2, the dedicated solar array can only provide electricity for part of each day. To optimize the hydrogen and oxygen production, supplemental energy sources are needed to maintain a baseload of electricity for the electrolyzer. Maintaining a minimum level of 30% of capacity will produce nearly 2 times as much green hydrogen and green oxygen produced without this energy baseload. (2) Marketing green hydrogen. An aspect of the demonstration project is to blend some of the hydrogen produced with the natural gas feed to the cogeneration units. The target quantities and required blending equipment will be identified during the design phase in conjunction with the cogeneration system manufacturer. The primary commercial target markets for the green hydrogen will be the local bus transportation company and a bus manufacturer that is developing hydrogen-fuel cell powered buses. (3) Pure oxygen utilization. The oxygen produced will be used by the WRRF in its activated sludge process. Initially, the oxygen will be bled into the aeration header providing air to aeration basins which will increase the oxygen concentration of the air stream slightly. The pure oxygen produced by the 1 MW electrolyzer will provide approximately 5% of the total oxygen demand of the BNR treatment facility. To increase the beneficial use of the pure oxygen stream, pilot testing of aeration systems suitable for pure oxygen, such as membrane aerated biofilm reactors (MABR) and Speece cones are planned for the future. (4) Excess heat utilization. The electrolysis process produces heat that is typically removed through a water-based cooling loop and transferred via a heat exchanger to a glycol-based cooling loop and vented to the atmosphere through an outdoor air-cooled heat changer. Capturing the estimated 23,000 therms/yr of waste heat for beneficial use during the colder months of the year is an objective of the project. The Cedar Rapids feasibility study will be completed in the spring of 2024. An assessment of the existing UNOX system will be conducted to identify equipment and facilities that could be retrofitted for use with electrolyzer-produced pure oxygen. Compatibility between the existing oxygen equipment and the electrolyzer-produced oxygen will also be assessed. Recommended equipment redundancy needs will be presented. The amount of thermal energy electrolysis system will be quantified, and opportunities identified for integrating this heat energy into the WPCF heating systems. An assessment of renewable energy sources available to supply electrical energy to the electrolysis facility will be conducted including on-site solar and wind, on-site generation from WPCF produced renewable natural gas, and off-site purchased energy agreements. The feasibility study will include an assessment of potential markets for the produced hydrogen in addition to on-site use at the WPCF to include regional natural gas utilities, industrial users, transportation users. RELEVANCE The St. Cloud project is focused on the commercial production of hydrogen, whereas the Cedar Rapids study is focused on pure oxygen production. Both projects include the beneficial use of thermal energy and oxygen byproducts. These projects will provide bookends for the assessment of integrating electrolyzers into WRRFs.