To Treat, Or Not To Treat (Post Combustion Exhaust), That Is The Question

Digester gas, produced from the anaerobic stabilization of sewage sludge, is largely comprised of methane. As such, digester gas has long been used as a fuel for combined heat and power (CHP), also known as cogeneration systems that provide process heat and produce renewable electricity. Internal combustion engines are the most mature and common prime mover used for digester gas fueled CHP applications. One of the challenges with utilizing digester gas as fuel is the need to treat the gas to remove impurities in order to meet the engine manufacturer’s fuel quality requirements. Air emissions present another challenge for digester gas fueled CHP systems, especially for internal combustion engines.Air emission limits vary by geographic jurisdiction. They are constantly evolving and generally become more stringent as emission control technologies continue to improve. While exhaust quality is an important design and air permitting consideration for any digester gas fueled combustion system, this paper will focus solely on exhaust treatment for lean-burn internal combustion engines. In just the last few years it has become more common for digester gas fueled internal combustion engines to require some form of post combustion exhaust treatment in order to comply with air emission limits.Selective catalytic reduction (SCR) systems and oxidation catalysts are commonly used to treat engine exhaust. These exhaust treatment systems operate according to the principal of catalysis to reduce the concentration of harmful and regulated air pollutants found in combustion exhaust gases.Despite the best efforts to treat digester gas for use as engine fuel, operating post combustion exhaust treatment equipment will eventually result in the catalysts becoming deactivated and ineffective. Due to the sensitive nature of the catalyst equipment, more thorough digester gas treatment is required than what is normally needed to meet typical fuel quality requirements. A robust and properly operating digester gas treatment system should prolong SCR system and oxidation catalyst life.In addition to providing a detailed description of SCR systems and oxidation catalyst technologies, this paper will also present case studies of two water resource recovery facilities (WRRFs) that are in varying stages of modifying existing CHP systems. Air permitting requirements for both projects will be presented as well as the system modifications required to meet the more stringent permit limits. Digester gas treatment to meet engine fuel requirements and to protect the new catalysts will be presented.