Currently, there are several methods available for reducing the particulate matter emissions. These include exhaust after-treatment emission controls, such as diesel particulate filters (DPFs), diesel oxidation catalysts (DOCs), and flow through filters (FTFs), cleaner fuels, engine design modification or re-powering, and reducing hours of operation. Add-on control devices are technologically feasible and are available commercially. A CARB program
to verify the emission reduction capabilities of diesel engine control technologies has certified several DPFs and DOCs capable of meeting the proposed emission standards. Information on applicable control technologies is provided below.
Diesel Particulate Filter: DPFs allow exhaust gases to pass through but trap diesel particulate matter. Particulate emission reductions of greater than 85% can be achieved with DPFs, depending on engine baseline emissions, fuel sulfur content, and emission test method or duty cycle.
Diesel Oxidation Catalyst: DOCs rely on platinum group metals, such as platinum, iridium, osmium, palladium, rhodium, and ruthenium to reduce diesel engine particulate emissions. DOCs are capable of reducing particulate matter emissions by around 30%. Reduced sulfur (15 ppm) diesel fuel allows DOCs to achieve maximum effectiveness.
Flow Through Filters: FTFs are capable of reducing emissions of particulates alone, or in conjunction with a fuel-borne catalyst. The particulate matter control efficiency of a FTF is lower that that of a DPF, but the FTF is much less likely to plug under unfavorable conditions (e.g., high particulate matter, low exhaust temperatures, and emergency circumstances). FTFs provide a possible alternative to DPFs in applications that are unsuitable for DPFs.
Cleaner Fuels: Diesel particulate emissions can also be reduced through the use of cleaner diesel fuel, alternative diesel fuels, or alternative fuels (e.g., compressed natural gas). The CARB ATCM requires all stationary diesel-fueled engines to use low-sulfur diesel fuel, which would result in modest particulate matter reductions in addition to enabling the use of advanced exhaust after-treatment emission controls. Alternative diesel fuels (e.g., biodiesel, emulsified fuels, Fischer-Tropsch fuel, or a combination of these fuels with low sulfur diesel fuel) can be used in diesel engines without requiring engine or fuel system modifications. Alternative fuels, such as natural gas, propane, ethanol, or methanol can reduce emissions from diesel engines. Natural gas can also be used in some emergency standby applications. Natural gas would be inappropriate in safety situations to human life, where an on-site storage tank would be required.
Engine Design Modification and Re-powering: Certain engine modifications can be employed, generally at the time of engine rebuilding, to reduce particulate matter emissions. Re-powering, or replacing an older engine, can be a viable and cost-effective way to reduce emissions from older uncontrolled diesel engines. Compared with even recently manufactured diesel engines, current technology diesel engines are significantly cleaner and can provide significant NOx and particulate matter benefits. Re-powering with a fuel cell and/or hydrogen-fueled spark-ignition engines could be used as alternatives, depending on actual applicability to the specific engine. Re-powering is especially cost effective in instances where the engine would have been removed for a rebuild.
Reducing Hours of Operation: Reducing the number of operating hours, when feasible, can be an effective means to mitigate impacts. In cases where alternative fuels, exhaust after-treatment emission controls, engine design modification, and re-powering are not practical, owners of emergency standby engines may consider reducing the hours of operation for maintenance and testing to reduce emissions.
Under the current CARB’s proposal, “Diesel Engine-Only” facilities that reduce their total operating hours and amend their permits for their diesel engines to less than 20 hours per year combined total for all engines (for non-emergency operations) are not subject to the AB2588 requirements if the AQMD staff determines that the facility does not impact public health.