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San Joaquin County Community Diesel Engine Emission Control Technologies Development Department NOx Control Strategies Although not as mature as PM control strategies in general, significant research into NOx control strategies that may be suitable for retrofit use is being conducted and a number of NOx control strategies for diesel engines are nearing commercial readiness. A sampling of NOx control technologies are briefly described below. As noted in the staff report,the verification procedure is appropriate for verifying NOx reductions equal to or greater than 15 percent. Exhaust Gas Recirculation Exhaust gas recirculation(EGR)is one of the most effective engine control methods for reducing NOx emissions. Spent combustion gases recirculated back into the intake system serve as a diluent to lower the oxygen concentration and to also increase the heat capacity of the air/fuel charge. Cooling the exhaust gas that is to be recirculated can used to minimize combustion temperatures. This reduces peak combustion temperature and the rate of combustion,thus reducing NOx emissions. Typical NOx reductions are about 50 percent. However,PM emissions may increase and fuel economy may decrease. The proper balance of EGR and temperature may provide the characteristics necessary for decreasing NOx emissions without increasing PM emissions. It is anticipated that cooled EGR would be an integral part of the engine manufacturers' effort to meet the lower NOx emission requirements in October 2002. Recently,hundreds of EGR systems which include diesel particulate filters have been successfully installed on existing Swedish urban buses, giving 50 percent NOx reductions and over 90 percent PM reductions(STT Emtec product literature). Selective Catalytic Reduction Systems Selective catalytic reduction(SCR) systems use a reductant,usually ammonia or urea,to convert NOx to nitrogen and oxygen. These systems are common in stationary sources and are also used on a few mobile sources in Europe. In this system, the reductant is injected into the exhaust upstream of the catalyst. As the exhaust gases,along with the reductant,pass over a catalyst applied to either a ceramic or metallic substrate,NOx emissions can be reduced by more than 70 percent(Manufacturers of Emissions Controls Association 2000). In addition, staff estimates that PM emissions could be reduced by 25 percent and HC r. emissions by 50 to 90 percent in SCR systems. SCR retrofit systems are expected to be available for urban bus applications within two to three years. NOx Adsorbers NOx adsorbers, also called NOx traps,are one of the newest emission control strategies under development. They employ catalysts to which NOx in the DeSilva Gates Quarry Project Draft Environmental Impact Report L_t t As 05105.05