Catalytic Converter
A well-designed catalytic converter placed in the exhaust system next to the engine can drastically reduce all gaseous emissions, including carbon monoxide, unburnt hydrocarbons, and soluble organic fractions (containing polynuclear aromatics). It is a compact, durable, reliable, and a low-cost component. It should, therefore, be an integral part of all diesel engines approved for underground coal mines. Table 11.9 shows the characteristics of a well-designed catalytic converter.
Table 11.9. Characteristics of a Well-Designed Catalytic Converter
| CO | Reduced by 80%–95% |
| HC | Reduced by 85%–90% |
| DPM | Reduced by 25%–35% |
| Odor control | Very good |
| Influence on engine | Low-pressure drop; no fuel penalty |
| Reliability | Very good |
| Durability | ≥5000 h |
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-2Al2O3-5SiO2) 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 platinum, 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 CO2, NO2, and CO2 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 converter usually lasts a long time. Five thousand hours of life is common when ultra low 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 engines, they are water-jacketed.