San Jose Cogeneration Facility Project Case Study

Abstract: Introduction: The San José-Santa Clara Regional Wastewater Facility (RWF) provides secondary and advanced tertiary wastewater treatment for the cities of San José and Santa Clara, California and multiple tributaries. The RWF is co-owned by the cities of San Jose and Santa Clara, and is operated by the City of San Jose (City). The RWF has a rated capacity of 167 million gallons per day (MGD) and processes wastewater solids using anaerobic digestion. The RWF has used cogeneration engines for over 30 years to provide heat and power for the facility. However, the existing cogeneration system reached the end of its useful life. The City decided to replace the existing cogeneration engines with an updated system to continue providing the RWF's heat and power demands as part of its commitment to sustainability. The San Jose Cogeneration Facility Project was designed and constructed through a progressive design-build process. The design was centered around four 3.5 MW internal-combustion, engine-driven generator units. The Cogeneration Facility is shown in Figure 1 and the engine room in Figure 2. Table 1 is a project fact sheet with key specifications. The 14 MW system has the capacity to meet the projected RWF electrical demand of 13 MW in 2040. Space for a fifth 3.5 MW unit is provided to have a firm capacity of 14 MW in the future with one unit out of service. Experience throughout the planning, design, construction, and commissioning of the project is shared. Insights discussed: -Early selection and procurement of cogeneration units -Integration of cogeneration with standby power systems -Biogas treatment and management -Fuel blending of biogas with natural gas (and provisions for future landfill gas) -Emission requirements met with post-combustion treatment -Heat recovery and management -Business-case decision making throughout the project -Startup considerations Discussion: The project included early selection of the cogeneration and gas treatment systems in the initial design stage. By selecting the equipment early in the design, subsequent design stages were tailored to known specifications. The cogeneration units were selected through an evaluated bid process that quantified the value of electrical efficiency, fuel blending capability, gas compression requirements, cost of parts, and maintenance requirements (triple bottom line analysis). The cogeneration facility is integrated with two separate utility power feeds and standby generators. This integrated power system provides a reliable power supply for the RWF. Protective relays were included to meet the electric utility interconnection agreement and allow for excess power to be supplied to the utility grid if necessary. The design provides for supplemental natural gas to be replaced with landfill gas in the future. Because of the significant amount of natural gas required (as much as 50 percent of the fuel energy content), prior to landfill gas availability, to meet the plant power demand, these cogeneration units need to meet the stringent Bay Area Air Quality Management District (BAAQMD) emission requirements of a natural gas fueled cogeneration system. Digester gas treatment protects both the engines and the engine exhaust gas treatment equipment. The biogas treatment system is shown in Figure 3. Biogas treatment includes hydrogen sulfide removal (initially iron sponge, now a proprietary media), moisture reduction, siloxane removal via activated carbon, and particulate removal. In this case, gas treatment followed gas compression and moisture removal. Consequently, the gas treatment occurs at a relatively high pressure of 50 psig. Provisions for the incorporation of future landfill gas were also included in the piping design. Each engine is equipped with a dedicated gas blending system to allow operation using any blend of biogas and natural gas. The gas blending system responds to changes in digester gas production (via gas holder level) to achieve a power output setpoint without requiring a system shutdown, though the peak power output of a cogeneration unit reduces as the natural gas quantity increases. Exhaust gas treatment was provided to meet BAAQMD emission limits. Exhaust gas treatment consisted of selective catalytic reduction (SCR) for nitrogen oxide (NOx) reduction and an oxidative catalysis for carbon monoxide (CO) and volatile organic carbon (VOC) reduction. The most difficult contaminant to treat was NOx. The BAAQMD requirements limited the facility to 0.124 g/bhp-hr, compared to federal requirements of 1.0 and 2.0 g/bhp-hr for natural gas and biogas fueled engines respectively. Heat generated by the cogeneration units is recovered from the engine and exhaust gas to provide process heating demands as well as selected building heating. New dual-fuel auxiliary boilers provide supplemental and backup heating for the RWF. Rejection of low temperature heat that can't be beneficially used is provided by cooling towers. The anaerobic digestion system at the RWF is being converted from mesophilic anaerobic digestion to a temperature-phased anaerobic digestion (TPAD) system. The heat supply system was designed to accommodate the higher temperatures and heat requirements for the TPAD system. The heat recovery and hydronic heat supply system was coordinated with the TPAD project team to meet the heating demands both in terms of temperature and amount of heat while maximizing heat recovery from the engines. Using the progressive design-build delivery method enabled collaboration between the Owner and the Design-Builder, particularly during the early design stages. Approximately halfway through preliminary design, it became clear that the project required value engineering to reduce costs within the City's budget. Approximately 20 percent of the project cost was reduced through a value engineering effort with the City and the Design-Builder. Business case evaluations were conducted throughout the project for decision-making based on meeting multiple objectives of economic, environmental, social, and operational categories. Life cycle cost to benefit ratios were compared for different options to guide selecting the best option. There were several startup procedures that could be followed on other projects. The biogas treatment system was tested and commissioned using the boilers prior to sending treated gas to the engines. The engines were started on natural gas first to test ancillaries prior to running on biogas. Lessons Learned: -Early selection of cogeneration units and gas treatment system enables an efficient design process -Integrating the utility power feeds with the cogeneration units and standby generators results in a reliable plant power supply -Additional exhaust gas treatment (such as oxidation catalysts and selective catalytic reduction) can be used with comprehensive biogas treatment to meet strict air quality management districts. -A fuel blending system for biogas and natural gas, provides operating flexibility -Heat recovery from the engines can be designed to be compatible with a higher temperature thermophilic digestion process. -Costs can be controlled during design with a progressive design build procurement and value engineering.