Removing the Silica: Piloting Beneficial Anaerobic Digester Gas Utilization

Two Caterpillar blower engines equipped with catalytic oxidizers to control VOC emissions were installed at the Bergen County Utilities Authority located in Little Ferry, New Jersey. Upon startup, the engines and associated air pollution control devices became damaged by silica dust. This paper will describe the steps taken to identify and address the problems associated with the engines and air pollution control devices, by installing a digester gas cleaning system to remove the volatile methylsiloxanes (VMSs) and other deleterious compounds in the anaerobic digester gas. The project required using an emerging technology along with conventional air pollution control equipment such as a catalytic oxidizer to control VOC and air toxics emissions.A feasibility study was performed: to determine the level of VMSs in the digester gas, to identify and evaluate technologies that are capable of providing the level of removal required; and to develop a strategy to implement the selected gas cleaning system. The digester gas and the oxidation catalyst were analyzed for composition and various performance parameters to determine the cause and extent of the problem. Emerging technologies including carbon adsorption, refrigeration, synthetic resins, liquid adsorbents, and membranes were evaluated to remove VMSs from digester gas.A pilot testing study was performed to test the performance of a carbon adsorption system using a graphite based carbon media, the most promising technology selected from the technology evaluation. Weekly VMS sampling was performed during the pilot testing to monitor the removal performance, and daily reading of operating parameters (temperature, pressure drop, flowrate) were recorded. The pilot testing study was successful in demonstrating that graphite based carbon media adsorption was able to remove the VMS from the anaerobic digester gas stream.