Page 185 - Advanced Mine Ventilation

Diesel Exhaust Control 165

The core of a catalytic converter is an open-channel ceramic monolith or a metallic
honeycomb substrate that provides support to the catalyst. Cordierite (2MgO-2Al 2 O 3 -
5SiO 2 ) is the most popular ceramic material used for the substrate. It has many good
characteristics, such as high surface area, large open frontal area, low heat capacity,
low thermal expansion coefficient, and good mechanical strength [7].
Metallic substrates are made of metal, silica, iron, chromium, and aluminum alloys.
They have higher surface areas and low-pressure drop but are more expensive. Both
types of substrates are coated with catalysts formulated with noble metals, such as plat-
inum, palladium, and rhodium. The catalyst application is a two-step process. The first
step is a wash-coat of aluminum, silica, titanium, cerium, and other compounds. In the
second step, the noble metal catalysts are impregnated on to the wash-coat.
The catalytic converter oxidizes all gases such as CO, NO, and HC to CO 2 ,NO 2 ,

and CO 2 and water. The catalysts need to have a temperature above 300 C for efficient
performance. This is the reason they are installed next to the engine. The catalytic con-
verter usually lasts a long time. Five thousand hours of life is common when ultralow
sulfur fuel is used. It is always designed to match the engine exhaust without creating
too much back pressure. The current cost varies from $5000 to $10,000 depending on
the size of the equipment. The surface of the catalytic converter is always coated with

insulating material to keep the surface temperature below 302 F. For permissible en-
gines, they are water-jacketed.

11.3.4 Diesel Particulate Filters
To collect the DPM and further oxidize the diesel exhaust components (in some cases),
a diesel particulate filter is used on most diesel equipment except for very small en-
gines working in outby areas that do not need it to meet legal requirements.
DPM filters can be broadly classified as (1) low-temperature filters and (2) high-
temperature filters depending on the design.
The low-temperature filters are suitable for nonpermissible heavy-duty diesel
equipment working outby in a mine or permissible equipment working in the face
areas, such as shuttle cars, front-end loaders. The high-temperature filters have a
universal appeal and it can be found on all diesel equipment except those that are
permissible equipment. The latter can work safely even if the mine air contains 1%
methane by volume. This is a legal limit for methane in most mines.

11.3.4.1 Low-Temperature Filters

These are basically designed to collect DPM and cool the exhaust to a safe level.
Fig. 11.1 shows a schematic of a permissible filtration system. The hot exhaust
from the engine goes to a water-jacketed catalytic converter. Next it goes through
either a water bath where gases mix with water (as shown) or a water cooler without
mixing with water that only cools the exhaust. The cooled gas goes through a water/
flame trap before it enters the filtration housing. Mostly disposable paper or synthetic
paper is used for DPM collection. The filter has to be replaced periodically to keep the
intake air pressure generally below 15 in. of water gauge.