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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.
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