NETT Diesel Oxidation Catalysts
Nett® Diesel Exhaust Catalytic Purifiers are available with two different catalyst systems:
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Standard catalyst
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D-Series catalyst
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The standard diesel oxidation catalyst yields a balance of high hydrocarbon, carbon monoxide, and particulate matter (SOF) conversion with low sulfate formation.
The D-Series diesel oxidation catalysts include zeolite-based hydrocarbon traps for improved low temperature activity and diesel odor control. They are capable of 40-50% HC conversion and effective diesel odor control at very low exhaust temperatures.
Nett Technologies offers several D-series catalyst coatings which are engineered for different applications. D-series catalysts incorporating sulfate suppressants for good high temperature DPM conversion, what makes them suitable for heavy duty applications, are also available. Request the Nett D-series Diesel Oxidation Catalysts Data Sheet for more information.
The exact emission performance of either catalyst type depends on the catalyst size, exhaust temperature, and raw exhaust composition. A minimum temperature of about 180°C (360°F) is required for conversion. Best catalyst performance occurs at temperatures above 250-300°C (480-570°F) when the conversion of carbon monoxide exceeds 90%. Typical CO and HC performance curves of the standard Nett® diesel purifier are shown on our Emissions FAQ page. Conversion of diesel particulate matter in the catalyst depends on the composition of the particulates and the sulfur content of the fuel. Achievable particulate matter reductions amount to 30-50%. Low sulfur diesel fuel is strongly recommended for best catalyst performance.
The D-Series catalysts show a significantly improved hydrocarbon performance at low exhaust temperatures. An example performance chart is shown in the Emissions FAQ page. It is important to understand that the low temperature HC activity is due to the adsorption on the HC trapping material rather than due to catalytic oxidation. Since the adsorption capacity is always limited and the HC traps have to be periodically regenerated by higher exhaust temperatures, this type of catalyst system is effective only at transient engine operation. The amount of HC trapping material in the Nett® D-Series catalysts is sufficient for a maximum of 15 - 30 minutes of continuous low temperature HC and odor control.
Catalyst Construction
Standard Diesel Oxidation Catalyst
The standard line of Nett® diesel exhaust purifiers utilizes metallic monolith catalyst supports. The supports are made with a corrugated, high temperature stainless steel foil. Packages of several foil layers are fitted in stainless steel housings and secured in place by stainless steel rings. A special herring-bone foil corrugation pattern creates a mixed flow cell structure. Exhaust gases are forced into the turbulent flow regime resulting in better contact between gas and catalyst, enhanced mass-transfer conditions, and higher conversion efficiency.
The catalyst is deposited onto the foil prior to forming the substrate. A special foil washcoating process provides unequaled control of washcoat uniformity, adhesion, and efficient catalyst use. Thick washcoat concentrations in cell corners which are inherent for other designs of metallic substrates completely disappear with the precoated foil technology.
Metallic |
Ceramic |
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|---|---|---|---|---|---|---|
Cell Density, (cpsi) |
160 |
240 |
320 |
200 |
300 |
400 |
Wall Thickness, (mm) |
0.05 |
0.05 |
0.05 |
0.30 |
0.21 |
0.18 |
Geometric Surface Area, (cm2/cm3) |
19 |
23 |
26 |
18.5 |
23.6 |
27.2 |
Open Frontal Area, (%) |
94 |
92 |
91 |
69 |
74 |
74 |
Bulk Density, (g/cm3) |
0.43 |
0.53 |
0.59 |
0.53 |
0.45 |
0.45 |
D-Series Catalyst
The D-Series catalysts are available on ceramic substrates. Round cordierite substrates with square cell geometry are used in all D-series catalysts.
Catalyzed substrates are wrapped in special packaging mat and canned into a steel container using the tourniquet packaging technology. Tourniquet is known as the best catalytic converter canning technology, producing the most rugged and durable converters.
Ceramic substrates produce somewhat higher pressure drop than metallic substrates of the same dimensions, due to their thicker walls. However, for most applications the D-series catalysts have to be sized larger than the standard oxidation catalysts, in order to provide sufficient volume of the HC trap. By using D-series catalyst substrates of larger diameter and larger frontal area, it is possible to achieve comparable pressure losses for the standard catalyst and the D-series catalyst installed on the same engine.
Most pressure drop comparisons between the ceramic and metal catalyst supports are based on bare (uncoated) substrates. While the uncoated ceramic supports do have thicker walls, the difference in wall thickness decreases after the catalyst coating is applied. This is explained by the inherent porosity of ceramic substrates, which "soak in" a portion of the catalyst coating into the wall pores. Since metallic substrates are not porous, the entire catalyst coating stays at their surface. Therefore, when the same loading of catalyst material is applied to a ceramic and a metallic substrate, it produces a thicker coating layer and more flow restriction in the metallic support.
Several advantages of Nett® catalytic converters make them the superior choice as a generic, retrofitting converter for all diesel engine applications:
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High mechanical durability. Metallic substrates will not crack or disintegrate even under harsh operating conditions and offer best thermal durability. The ceramic substrates are canned using the tourniquet technology, which produces the most rugged and durable catalytic converters.
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Easy Maintenance. The cell densities of the catalytic monoliths are carefully selected for each application. There is very little risk of clogging by diesel particulates. Most applications are virtually maintenance-free.
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Low Pressure Drop. Due to thin walls and large frontal area the catalyst substrates have low pressure drop and do not cause any fuel penalty.
Catalyst Durability
The rugged construction of Nett® diesel exhaust purifiers, using either metallic or ceramic supports, provides excellent mechanical durability. The catalyst substrate is designed to last for the entire life-span of the engine. Good engine maintenance, however, is necessary to sustain the catalyst activity for similar period of time. The two major causes of catalyst deactivation are (1) high temperature and (2) poisoning.
The Nett® catalyst may suffer thermal degradation when exposed to temperatures above 650°C (1200°F) for prolonged periods of time. Diesel engines have intrinsically cool exhaust gases and thermal catalyst deterioration is not likely to take place under normal operating conditions. Catalyst overheating may occur only as a result of serious engine malfunction, such as leaking injectors, when unburned fuel is oxidized in the catalyst and excessive temperature rise is observed due to the exothermic reaction.
Several chemical elements have the ability to either selectively poison the catalyst or to mask the catalytic surface, thus, preventing the contact between exhaust gases and active catalyst sites. Substances to be avoided include phosphorus, zinc, heavy metals, lead, arsenic, vanadium, and silicone. Some of these elements (e.g., phosphorus) may be contained as additives in the engine lube oil. Low lube oil consumption and the use of low-phosphorus oils are guidelines for extended catalyst life. Leaks of lube oil into the exhaust system are very detrimental and can irreversibly deactivate the catalyst within a few hours or days.
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