Meeting Stringent Emissions Limits for Biogas-Fired IC Engines

Under federal emissions standards, i.e., New Source Performance Standards (NSPS) and National Emission Standards for Hazardous Air Pollutants (NESHAPS), facilities and manufacturers are required to provide combustion units that meet specific emissions limits for nitrogen oxides (NOx), non-methane hydrocarbons (NMHC), carbon monoxide (CO), and particulate matter (PM). The United States Environmental Protection Agency (USEPA) requires specific emissions requirements based on size, type, usage and date of manufacture (tiers). In addition USEPA and individual states are required to set more stringent emissions limits in areas where ambient air quality standards are not being met. In order for these nonattainment areas to eventually meet the ambient air quality standards, states and local municipalities are required to put in place regulations and impose strict emission limits in their State Implementation Plans (SIPs). In the case of ozone nonattainment, internal combustion (IC) engines have been targeted as potentially one of the major emitters of NOx emissions. Since NOx (along with volatile organic compounds (VOCs)) is a precursor to ozone formation, regulations in ozone nonattainment areas have established NOx emission limits for IC engines.Based on our survey, the South Coast Air Quality Management District (SCAQMD) issued the most stringent emission limits for stationary IC engines in the country. SCAQMD Rule 1110.2 for stationary IC engines was amended on Feburary1, 2008, to require biogas gas fired engines to meet stringent emission limits for NOx (11 ppmvd), CO (250 ppmvd) and VOC (30 ppmvd) at 15% oxygen.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. OCSD performed 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 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 over one year of data was collected and monitored for overall performance.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. 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 evaluated after the system was operated for over one year.