Replacement
Through the Clean School Bus USA program, EPA
is advocating that buses manufactured prior to 1990 be replaced with new,
cleaner buses equipped with pollution control devices.
Clean Diesel Technology
Compressed Natural Gas
Diesel Oxidation Catalysts
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Clean Diesel Technology
The combination of a diesel particulate matter filter
and ultra low sulfur diesel fuel is often referred
to as Clean Diesel Technology.
Diesel Particulate Matter Filters
Diesel particulate
matter (PM) filters are ceramic devices that collect PM in the exhaust stream. The high temperature of the exhaust
heats the ceramic structure and allows the particles inside to break
down (or oxidize) into less harmful components. They can be installed
on new and used buses, but must be used in conjunction with ultra-low
sulfur diesel (ULSD) - fuel with a sulfur content of less than 15
parts per million. The combination of PM filters and ULSD can reduce
emissions of PM, hydrocarbons
(HC), and carbon
monoxide (CO) by 60 to 90 percent.
PM filters come in a kit that includes mounting brackets and an electronic
monitoring device. The cost of the kit can range from $5,000 to $10,000.
Some buses may need the more expensive filters to compensate for lower
exhaust temperatures. PM filters work best on engines built after 1995.
As a result, knowing the age and type of each engine in the fleet as
well as the drive cycles of the buses is an important part of any retrofit
project. These factors determine whether or not buses are candidates
for filters. Although installation time can vary, field experience suggests
it takes about 6 to 8 hours to install the filter.
Ultra-Low Sulfur Diesel (ULSD)
ULSD will be available nationwide in June 2006, but currently is available
in certain parts
of the country . The primary purpose of ULSD is to enable or improve
the performance of aftertreatment technologies such as a PM filter.
The quantity of emissions reductions from the use of ULSD alone will
vary depending on the application, level of sulfur reduction, and other
fuel characteristics of the replacement fuel (e.g., cetane number, aromatics,
PNA).Some case studies suggest that the use of ULSD alone can reduce
emissions of PM between 5 and 9 percent. While ULSD-only emission reductions
for PM are relatively modest on a per-vehicle basis compared to aftertreatment
retrofit, the emission reductions can be significant if an entire fleet
is fueled with ULSD. The price differential between ULSD and regular
diesel fuel varies by location but ranges between 8 and 25 cents per
gallon. In 2006, when ULSD is available nationwide, the cost differential
will be much less.
Compressed Natural Gas
Another replacement option is to purchase new buses that run on compressed
natural gas (CNG) and have been fitted with an oxidation
catalyst to reduce pollution.
- CNG is a mixture of hydrocarbons,
mainly methane, and is produced either from gas wells or in conjunction
with crude oil production. Vehicles powered by natural gas perform just
like vehicles powered by diesel fuel. The majority of natural gas consumed
in the United States comes from sources within North America. Increased
use of clean, domestically produced fuels helps reduce our reliance
on oil from overseas, which is good for our nations energy security.
Natural gas buses can reduce emissions of particulate
matter (PM) by about 70 to 90 percent if they meet Clean
Fueled Fleet (http://www.epa.gov/otaq/cff.htm)
requirements or have catalysts.
- CNG engines that do not have catalysts,
may have higher formaldehyde, hydrocarbon, and ultra-fine PM emissions
than Clean Diesel engines meeting EPA's 2007 emission standard for PM.
Overall the cost of natural gas and diesel fuel are very similar but in some areas of the country natural gas fuel may be more economical. However, a new compressed natural gas school
bus costs about $30,000 more than a new diesel school bus (currently, only the largest school buses – Type D – are available with CNG engines). Additionally,
CNG buses require special refueling facilities as well as a special
maintenance facilities.
Diesel Oxidation
Catalysts
Purchasing new buses equipped with oxidation catalysts is a
cost-effective bus replacement strategy.
- Diesel oxidation catalysts are devices that use a chemical process to
break down pollutants in the exhaust stream into less harmful components.
Diesel oxidation catalysts can reduce emissions of PM by 20 percent and HC by 50 percent and CO by approximately 40 percent. Oxidation catalysts cost
about $600 to $2,000, can be installed on any new or used bus, and
run on regular diesel fuel. Although installation time can vary, field
experience suggests it takes about 1 to 3 hours to install an oxidation
catalyst.
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