Particulate traps:
An
exhaust treatment technology that substantially reduces diesel engine
particulateemissions is the trap oxidiir. A temperature-tolerant filter or trap
removes the particulate material from the exhaust gas; the filter is then
"cleaned off" by oxidizing the accumulated particulates. This
technology is difficult to implement because: (1) the filter, even when clean,
increases the pressure in the exhaust system; (2) this pressure increase
steadily rises as the filter collects particulate matter; (3) under normal
diesel engine operating conditions the collected particulate matter will not
ignite and oxidize; (4) once ignition of the particulate occurs, the burnup
process must be carefully controlled to prevent excessively high temperatures
and trap damage or destruction. Trap oxidizers have been put into production for
light-duty automobile diesel engines. Their use with heavy-duty diesel engines
poses more difficult problems due to higher particulate loading and lower
exhaust temperatures.
Types of
particulate filters include: ceramic monoliths, alumina-coated wire mesh,
ceramic foam, ceramic fiber mat, woven silica-fiber rope wound on a porous
tube. Each of these has different inherent pressure loss and filtering
efficiency. Regeneration of the trap by burning up the filtered particulate
material can be accomplished by raising its temperature to the ignition point
while providing oxygen-containing exhaust gas to support combustion and carry
away the heat released. Diesel particulate matter ignites at about 500 to 600•‹C. This is above the normal
temperature of diesel exhaust so either the exhaust gas flowing through the
trap during regeneration must be heated (positive regeneration) or ignition
must be made to occur at a lower temperature with catalytic materials on the
trap or added to the fuel (catalytic regeneration). Catalytic coatings on the
trap reduce the ignition temperature by up to 200°C.
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