Emissions Control for Digester Gas-fired Internal Combustion Engines at Wastewater Treatment Plants

The Orange County Sanitation District (OCSD) owns and operates two wastewater treatment plants (WWTPs) with large digester gas-fired internal combustion (IC) engines. Plant 1 has three (3) 2.5 megawatt (MW) IC engines and Plant 2 has five (5) 3 MW IC engines, fueled primarily by digester gas (biogas) and supplemented by natural gas. The IC engines are required to comply with South Coast Air Quality Management District (SCAQMD) Rule 1110.2 for stationary IC engines. On Feburary1, 2008, SCAQMD amended Rule 1110.2 to require biogas gas fired engines to meet stringent emission limits for NOx (11 ppmvd), CO (250 ppmvd) and VOC to (30 ppmvd) at 15 % oxygen by July 1, 2012.OCSD is performing a full scale demonstration project to determine the technical and cost feasibilities of meeting the future emission limits. The project includes a digester gas cleaning system using carbon media for removal of siloxanes and other harmful contaminants from the digester gas and a post combustion control using an oxidative catalyst for controlling of CO and VOCs emissions and a selective catalytic reduction (SCR) system with urea injection for controlling of NOx emissions. Emissions of CO, NOx, VOCs and air toxics are monitored at the inlet and outlet of the catalytic oxidizer/SCR system. The full scale system commenced operation in April 2010 and is being monitored for overall performance.SCAQMD provided partial funding for this demonstration project since the information will be used to support rulemaking decisions. SCAQMD will use the findings of this study along with other demonstration projects to finalize the Rule 1110.2 requirements for biogas fired IC engines.The results of the data collected for the demonstration project showed that the system can meet the average emissions limits of NOx, CO and VOC of 11, 250 and 30 ppmvd at 15% oxygen, respectively. Further evaluations are being performed to determine if the control system can meet the emission limits under all operating scenarios such as start up, shut down, changing loads and fuel blends.Currently, the control system is operating on the initial catalysts installed during startup and no catalyst has been replaced. Cost effectiveness of the project was developed after the system has operated for over one year.Depending on SCAQMD final rulemaking decisions, the results of the overall demonstration project may have a significant cost impact to operators of biogas fired IC engines in southern California.