Lessons Learned From The San Jose Cogeneration Facility Project

Introduction:
Biogas-fueled cogeneration systems play a key role at water resource recovery facilities (WRRFs) recovering energy and moving towards heat and power self-sufficiency. Managers of WRRFs that are considering installing, expanding, or upgrading a biogas-fueled cogeneration system will benefit from lessons learned from the Cogeneration Facility Project at the San Jose-Santa Clara Regional Wastewater Facility in San Jose, California. The City of San José (City) Santa Clara Regional Wastewater Facility (RWF) provides secondary and advanced tertiary wastewater treatment for the cities of San José and Santa Clara and multiple tributaries. The RWF has a rated capacity of 167 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. The San Jose Cogeneration Facility was designed and constructed through a progressive design-build process. The design was centered around four 3.5 MW 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 of cogeneration units - Integration of cogeneration with standby power systems - Biogas treatment and management - Fuel blending of biogas with natural gas and landfill gas - Emission requirements met with post-combustion treatment - Heat recovery and management - Business-case decision making throughout the project - Startup considerations
Findings: 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. 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. The design provides for supplemental natural gas and using landfill gas in the future. Because of the significant amount of natural gas required (as much as 50 percent of the fuel supply) to meet the plant power demand, these cogeneration units need to meet the 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 via iron sponge, 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. Each engine is equipped with a dedicated gas blending system to allow operation using any blend of biogas (digester gas and landfill gas) and natural gas. The gas blending system responds to changes in digester gas production to achieve a power output setpoint without requiring a system shutdown. Engine timing and air/fuel ratio are automatically controlled based on the blend of digester gas and natural gas. Peak power output of a cogeneration unit varies with the amount of natural gas in the fuel blend. Exhaust gas treatment was provided to meet Bay Area Air Quality 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. Extensive exhaust gas treatment was required because of the significant amount of supplemental natural gas used (greater than 10 percent of the fuel supply). Heat generated by the cogeneration units is recovered from the engine and exhaust gas to provide building and process heating demands. New 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 because of 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 treated gas was used in the boilers prior to sending it to the engines. The engines were started on natural gas first to test ancillaries prior to running on biogas.
Significance: The significance of this project come from several lessons learned including:
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.