Air Pollutants And Odor Reduction System For Digester Gas-Fired Engines

The Orange County Sanitation District (OCSD) owns and operates two wastewater treatment plants (WWTPs) in Southern California with major power needs that are met by large digester gas-fired internal combustion engines (ICEs). The Central Power Generation Systems (CGS) at Reclamation Plant No.1 (Plant 1) and Treatment Plant No. 2 (Plant 2) are comprised of three (3) 2.5 megawatt (MW) engines and five (5) 3 MW engines, respectively and are primarily fueled by digester gas (biogas) and supplemented by natural gas. The ICEs are subject to the Federal Source Performance Standards for Criteria Pollutants, Air Toxics emissions and the applicable rules of the South Coast Air Quality Management District (SCAQMD) which are typically more stringent than the similar Federal regulations. SCAQMD Rule 1110.2 (R1110.2) – Emissions from Gaseous- and Liquid–Fueled Engines, last revised in September 2012 requires biogas fired engines to meet stringent emission limits for NOx (11 ppmvd), CO (250 ppmvd) and VOC (30 ppmvd) at 15 % Oxygen by January 1, 2016. These new limits represent 15-25% of the currently permitted emission limits. In addition, other SCAQMD rules and the California Air Resources Board’s (CARB) regulations direct the facilities to reduce adverse health impact to surrounding communities necessitating a substantial decrease in emissions of major pollutants (e.g. Toxic Air Contaminants such as Formaldehyde, Benzene, Chloroform).Odor is a major source of concern for the operators of WWTPs. Odorous emissions which could cause public nuisance are easily detectable and as a result are subject to frequent complaints to the regulators, other public officials, and to the facilities. While odors from WWTPs are seldom associated with combustion sources, it has been established that some compounds that are commonly present in combustion by-products at sufficient concentration (e.g. Formaldehyde, Sulfur compounds) do create odor that might cause a negative impact to the plants’ neighbors and other residential and business receptors.For the last several years (since 2010) OCSD has conducted a full scale demonstration project at both Plants 1 and 2 to determine the technical and economic feasibility of meeting SCAQMD’s approaching emission limits established in SCAQMD R1110.2. The full scale pilot system at Plant 1 includes a digester gas cleaning system (DGCS) using carbon-based media for removal of siloxanes and other harmful contaminants commonly found in digester gas. It also includes post combustion controls using an oxidation catalyst (CatOx) for controlling CO and VOC emissions (including air toxic emissions) and a selective catalytic reduction (SCR) system with urea injection for controlling NOx emissions. Emissions of CO, NOx, VOCs and air toxics are monitored at the inlet and outlet of the DGCS as well as at the catalytic oxidizer/SCR system. The system commenced operation in 2010 and is currently being monitored for overall performance. At Plant 2, an oxidation catalyst system designed for controlling CO, VOC and air toxic emissions has been in operation since 2009.This paper provides emission information and cost effectiveness data for installing and operating the CatOx and SCR system. Three (3) years of system’s operation demonstrated that the accepted construction performs well with no major deviations. The DGCS performed in accordance with technical requirements and the catalyst never required replacement. The objectives of emission reduction to the levels established in R1110.2 have been met. The detailed results of the overall performance are included in the presented paper. The presentation also contains comparative information on odorous emissions from the engines with and without the emission reduction system. Preliminary results demonstrate a large potential for the engine emissions control systems to reduce overall odors from the WWTPs. Also, the cost efficiency review may have a significant impact to strategic decisions of the operators of biogas fired ICE in Southern California.Authors acknowledge a significant role of OCSD consultant Kit Liang (formerly with the Malcolm Pirnie – Arcadis) in the development of the presented system.