DIESEL OXIDATION CATALYST - DIESEL PARTICULATE FILTER / CATALYTIC DIESEL FILTERS

 What Is a Diesel Oxidation Catalyst?

Diesel Oxidation Catalysts (DOC) are catalytic converters designed specifically for diesel engines and equipment to reduce Carbon Monoxide (CO), Hydrocarbons (HC) and Particulate Matter (PM) emissions. DOC's are simple, inexpensive, maintenance-free and suitable for all types and applications of diesel engines.

Figure 1. How Diesel Oxidation Catalyst (DOC) Works

Modern catalytic converters consist of a monolith honeycomb substrate coated with platinum group metal catalyst, packaged in a stainless steel container. The honeycomb structure with many small parallel channels presents a high catalytic contact area to exhaust gasses. As the hot gases contact the catalyst, several exhaust pollutants are converted into harmless substances: carbon dioxide and water.

The diesel oxidation catalyst is designed to oxidize carbon monoxide, gas phase hydrocarbons, and the SOF fraction of diesel particulate matter to CO2 and H2O:

Diesel exhaust contains sufficient amounts of oxygen, necessary for the above reactions. The concentration of O2 in the exhaust gases from diesel engine varies between 3 and 17%, depending on the engine load. Typical conversion efficiencies for CO and HC in the Nett? diesel oxidation catalyst are given in Figure 2. The catalyst activity increases with temperature. A minimum exhaust temperature of about 200°C is necessary for the catalyst to "light off". At elevated temperatures, conversions depend on the catalyst size and design and can be higher than 90%.

Figure 2. Catalytic Conversion of Carbon Monoxide and Hydrocarbons

Conversion of diesel particulate matter is an important function of the modern diesel oxidation catalyst. The catalyst exhibits a very high activity in the oxidation of the organic fraction (SOF) of diesel particulates. Conversion of SOF may reach and exceed 80%. At lower temperatures, say 300°C, the total DPM conversion is usually between 30 and 50% (Figure 3). At high temperatures, above 400°C, a counterproductive process may occur in the catalyst. It is the oxidation of sulfur dioxide to sulfur trioxide, which combines with water forming sulfuric acid:

A formation of the sulfate (SO4) particulates occurs, outweighing the benefit of the SOF reduction. Figure 3 shows an example situation, where at 450°C the engine-out and the catalyst total DPM emissions are equal. In reality the generation of sulfates strongly depends on the sulfur content of the fuel as well as on the catalyst formulation. It is possible to decrease DPM emissions with a catalyst even at high temperatures, provided suitable catalyst formulation and good quality fuels of low sulfur contents are used. On the other hand, diesel oxidation catalyst used with high sulfur fuel will increase the total DPM output at higher temperatures. This is why diesel catalysts become more widespread only after the commercial introduction of low sulfur diesel fuel.

Figure 3. Catalytic Conversion of DPM

The diesel oxidation catalyst, depending on its formulation, may also exhibit some limited activity towards the reduction of nitrogen oxides in diesel exhaust. NOx conversions of 10-20% are usually observed. The NOx conversion exhibits a maximum at medium temperatures of about 300°C.

DOC’s reduce Carbon Monoxide (CO) and Hydrocarbons (HC) including the Soluble Organic Fraction (SOF) of Diesel Particulate Matter (DPM). DOC’s are simple, inexpensive, maintenance free and suitable for all types and applications of diesel engines

M-Series (Metallic) Diesel Oxidation Catalysts (DOC)

M-Series (Metallic) Diesel Oxidation Catalysts (DOC)

• Metallic DOC technology
• EPA verified for the stationary applications
• Maintenance-free, reliable and cost-effective emission control solution
• Available in both universal and direct-fit designs (Nett Technologies has a database of over 10000 direct-fit muffler designs)
• Mufflers are built entirely from corrosion resistant materials: aluminized and stainless steels
• Installation time and cost are reduced to a minimum
• DOC Catalytic Mufflers match or surpass the original muffler in sound attenuation and back pressure characteristics with addition of superior emissions performance
• Great at reducing deadly emissions, such as Carbon Monoxide (CO) and Hydrocarbons (HC)

M-Series – Technology

The M-Series line of Nett diesel exhaust catalytic converters utilizes metallic monolith catalyst supports. The supports are made from 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 herringbone foil corrugation pattern creates a mixed flow cell structure. Exhaust gases from the Diesel Oxidation Catalyst (DOC) are forced into the turbulent flow regime resulting in better contact between gas and catalyst, enhanced mass-transfer conditions, and higher conversion efficiency. Selected physical properties of Nett metallic monoliths are listed in the table below.

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.

D-Series (Ceramic) Diesel Oxidation Catalysts (Doc)

• Ceramic DOC technology with Zeolites designed to extend the performance of diesel catalytic converters into the low temperature range
• Maintenance-free, reliable and cost-effective emission control solution
• Available in both universal and direct-fit designs (Nett Technologies has a database of over 10000 direct-fit muffler designs)
• Mufflers are built entirely from corrosion resistant materials: aluminized and stainless steels
• Installation time and cost are reduced to a minimum
• DOC Catalytic Mufflers match or surpass the original muffler in sound attenuation and back pressure characteristics with addition of superior emissions performance
• Great at reducing deadly emissions, such as Carbon Monoxide (CO) and Hydrocarbons (HC)

D-Series – Technology

The D-Series Diesel Oxidation Catalysts (DOC) are available on ceramic substrates. Round cordierite substrates with square cell geometry are used in all D-Series catalysts. Selected properties of the substrates are listed in Table 1 below.

Catalyzed substrates are wrapped in special packaging mat and packed into a steel container using the tourniquet packaging technology. Tourniquet is known as the best catalytic converter packaging 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 are sized larger than M-Series Diesel Oxidation Catalysts (DOC) are, 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 M-Series and D-Series catalysts when installed on identical engines.

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 (Table 1), 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.

Diesel Particulate Matter Filtration

Diesel particulate filters operate by trapping soot particles from the engine exhaust, preventing them from reaching the environment. Unlike a catalytic converter which is designed to reduce gas-phase emissions flowing through the catalyst, the particulate filter is designed to trap and retain the solid particles until the particles can be oxidized or burned in the DPF itself, through a process called regeneration.

The most common diesel particulate filters in widespread use are cellular ceramic honeycomb filters with channels that are plugged at alternating ends, as shown in Figure .The ends of the filter, plugged in a checkerboard pattern, force the soot-containing exhaust to flow through the porous filter walls. While the exhaust gas can flow through the walls, the soot particles are trapped within the filter pores and in a layer on top of the channel walls. The honeycomb design provides a large filtration area while minimizing pressure losses, and has become the standard, so-called wall-flow filter for most diesel exhaust filtration applications. Ceramic materials are widely used for particulate filters, given their good thermal durability, with the most common ceramic materials being: cordierite, silicon carbide, and aluminum titanate.

Diesel particulate filter for heavy-duty vehicle (a), cross-section viewing showing filtration processes within several DPF channels (b), and close-up view of particle capture and build-up on the channel walls (c).

The details of the filtration process are illustrated in Figure 1(b), which shows the soot particles trapped along the inlet channel, which is open at the front end but plugged at the back end. DPFs contain several hundred channels or cells per square inch (cpsi), with the most common being 200 cpsi. Since half of the channels are plugged at the front of the DPF and the other half are plugged at the back of the filter, only half of the filter channels accumulate soot or ash. That is, only the channels open on the inlet side are exposed to the “dirty” exhaust flow, while the channels open to the outlet side remain clean. Given the small pore size and design of the honeycomb filters, DPFs can achieve a particle trapping efficiency of 99% or greater . Due to the high trapping efficiency and DPF cell design, no visible soot or ash should pass through the filter walls. Black streaks or visible soot in the outlet channels are a sure sign of filter failure.

Soot particles are captured and retained in the DPF through a combination of depth filtration inside the filter pores and surface filtration along the channel walls. The inset in Figure 1(c) shows these two processes, where a small fraction of the soot initially accumulates in the filter pores (1) and then subsequently builds a layer along the channel walls (2). As the soot load in the filter increases, so too does the filter’s trapping efficiency, as the accumulated soot provides an additional layer to trap incoming particles. The specific soot filtration mechanisms, whether in the pores on the surface of the walls, plays an important role in determining the overall increase in exhaust back pressure (or pressure drop across the filter), shown in Figure 2.

Figure 2. Pressure drop evolution with soot accumulation in the DPF showing rapid initial rise in pressure drop due to soot accumulation in the filter pores (1) followed by a gradual increase as soot builds a layer along the walls (2).

The porosity of most commercial DPFs ranges from around 40% to 60%. The walls of these filters contain a complex network of pores in the range of 10 – 30 micrometers (microns) in diameter [2]. In a new or clean DPF, the surface of the filter is exposed to the exhaust flow and soot rapidly accumulates in the surface pores. Although only a small fraction of the total soot accumulates in the filter micro-pores, it contributes to a steep rise in filter pressure drop shown in Figure 2. Subsequent soot accumulation in the DPF forms a layer (cake layer) along the walls of the channel, and results in a slower and more gradual rise in filter pressure drop . Depending on the soot loading level and filter type, the pore accumulation can account for 50% of the filter pressure drop, or more, in some cases. The non-linear response of the DPF to material accumulation complicates the determination of filter soot or ash loading levels based on pressure drop alone

Filter Regeneration

In order to reduce filter pressure drop due to soot accumulation, the filter is regenerated through a processes the burns off (oxidizes) the soot. There are two broad categories of regeneration processes, although most commercial applications use some combination of the two. This is particularly true with vehicles or equipment experiencing extended periods of low exhaust temperature operation, such as long periods of idle or low speed/load operating cycles.

Active Regeneration requires the addition of heat to the exhaust to increase the temperature of the soot to the point at which it will oxidize in the presence of excess oxygen in the exhaust. The combustion of soot in oxygen typically requires temperatures above 550 °C (1,000 °F). Since these high temperatures generally DO not occur during normal engine operation, a number of strategies are used to actively increase the exhaust temperature. Active regeneration systems may include the use of a diesel burner to directly heat the exhaust entering the DPF or the use of a diesel oxidation catalyst (DOC) to oxidize diesel fuel over the catalyst as a means for increasing the DPF temperature. Use of the DOC also requires excess diesel fuel in the exhaust, which may be accomplished through a fuel injector (hydrocarbon doser) mounted in the exhaust upstream of the DOC, or through late in-cylinder post injection strategies. Other forms of active regeneration include the use of electrical heating elements, microwaves, or plasma burners.

The use of a DOC in combination with some form of exhaust fuel dosing is the most common active regeneration strategy currently used for on- and off-highway applications. The duration of an active regeneration event generally ranges from 20 to 30 minutes on average, under normal operating conditions. In some cases, such as severe DPF soot plugging, a parked regeneration may be required, which can last up to several hours to slowly burn off the soot under more controlled conditions. Regardless of the specific strategy, active regenerations always require additional energy input (additional fuel) to heat the exhaust and the DPF to the required temperature.

Passive Regeneration, as the name implies, does not require additional energy to carry out the regeneration process. Instead, this strategy relies on the oxidation of soot in the presence of NO2, which can occur at much lower temperatures in the range of 250 °C to 400 °C (480 °F to 750 °F). A catalyst is used to convert NO present in the exhaust to NO2. These catalysts require the use of precious metals to facilitate the reaction, platinum (Pt), in particular, which adds additional cost to the system. In some cases the catalyst coating is applied directly to the DPF, as with a catalyzed DPF (C-DPF), or an upstream oxidation catalyst (DOC) may also be used . Many commercial systems utilize a combination of a DOC and C-DPF. Use of the catalysts allows NO2 to be produced and soot to be oxidized at temperatures which occur during normal engine or vehicle operation.

In an ideal case, if engine operation results in a certain amount of time spent within this passive regeneration “temperature window” then active regeneration may not be needed. In reality however, low temperature operation may occur for extended periods of time, such as long periods of idle or low load operation, particularly in cold climates, and some active regeneration may still be needed. In the absence of active regeneration, periods of low temperature operation may be supplemented by periods of high temperature operation (such as extended highway driving) to induce passive regeneration.

In order to reduce fuel consumption, passive regeneration is preferred, although most commercial systems still use active regeneration to varying degrees, depending on the drive cycle and operating conditions. Regardless of the regeneration method, the oxidation of soot (whether active or passive) results in incombustible material, or ash, which can not be burned, and remains in the DPF. Understanding the key differences between ash and soot, as well as their impacts on DPF performance is important when selecting the most appropriate cleaning method for the filter.

Why is this important for DPF ash cleaning?

Understanding the design and operation of the DPF to collect and trap particles, whether in the pores or on the surface, has a large impact on how easily the particles can later be removed. Soot is fundamentally different from ash in that the soot can be oxidized and removed through regeneration, while the ash is incombustible and remains in the DPF until the DPF is serviced for ash cleaning.  

Ash Accumulation in Diesel Particulate Filters

The accumulation of ash in diesel particulate filters is one of the most important factors limiting the filter’s service life and has been described as one of the most important problems facing diesel engine manufacturers [Sachdev 1983][Konstandopoulos 2000]. Despite considerable emphasis and work in understanding and optimizing DPF performance for soot accumulation alone, the reality is quite different. Unlike these idealized cases, the DPF always contains some amount of ash in real-world operation. In fact, more often than not, the amount of ash in the filter can significantly exceed the amount of soot the DPF was initially designed to trap. Figure 1 best illustrates the magnitude of the problem, as it presents the ash fraction of the total mass of material accumulated in the DPF (ash and soot), assuming a 6 g/L maximum soot load limit.

Based on Figure 1, after only 33,000 miles (53,000 km) of on-road use, approximately 50% of the material accumulated in the DPF is ash. In other words, the amount of ash equals the amount of soot at the maximum allowable soot load limit of 6 g/L. Further, after 150,000 miles (241,000 km) of operation—equivalent to the minimum EPA ash cleaning interval—ash comprises over 80% of the material trapped in the DPF, with the minority being soot.

Conceptual Description. Ash accumulates in the DPF over extended use, as the incombustible material left behind following filter regeneration and soot oxidation. The ash consists of various metallic compounds originating from lubricant additives, trace elements in the fuel, and engine wear and corrosion products. Ash accumulation in the DPF alters the filter geometry as shown in Figure 2, which illustrates the differences between a filter containing no ash and a filter containing significant amounts of ash.

As shown in Figure 2, the ash can occupy a large portion of the filter volume, as it may accumulate in a thin layer along the channel walls or pack in plugs towards the back of the filter channels. One effect of the ash is to decrease the effective filter volume or filtration area and reduce the filter’s soot storage capacity. The ash deposition also alters the distribution of the accumulated soot, generally shifting it toward the front of the filter. These combined effects serve to restrict the channel diameter and reduce the effective filter length. As a result, the ash contributes to increased exhaust flow restriction.

In addition, the reduction in channel diameter and filter length, due to the ash accumulation, result in an increase in the DPF channel and wall velocities, which may further alter the properties of the accumulated soot and affect the filter’s pressure drop sensitivity. Given the reliance on filter pressure drop measurements in filter soot load estimation, a thorough understanding of these ash effects is required to compensate for ash-induced variations in the filter’s pressure drop response over time.

Figure 2 also shows the ash layer forming a barrier, physically separating the soot from the channel walls. This is important for two reasons. First, after extended aging and with some level of ash accumulation, it is the ash that is doing the majority, if not all, of the soot filtering. In this sense, the filter substrate is acting as a support for the “new” filter medium, which is essentially composed of the ash. Given the small pore size of the ash layer, an increase in filtration efficiency is generally observed in particulate filters with even a low level (< 2 g/L) of ash loading [Viswanathan 2012]. Second, the ash layer also physically separates the accumulated soot from the catalyst which may be deposited on the surface of a catalyzed DPF. This not only prevents any contact between the soot and catalyst particles, but further increases the required diffusion length for NO2 assisted soot oxidation.

Impact on Performance. Due to the long time scales over which the ash builds up in the DPF, several thousand hours and tens-to-hundreds of thousands of miles, significant progress in understanding ash impacts on filter performance was limited prior to the widespread introduction of DPFs in 2007. Much of the early research into ash effects prior to 2007 utilized various approaches to accelerate filter aging and ash build up in an effort to identify the various ash sources and means by which ash may affect diesel aftertreatment system performance. This initial work resulted in the following generally-accepted observations and conclusions:

• Ash accumulation in the DPF increases with oil consumption and lubricant ash content, as lubricant additives are generally the largest source of ash [Givens 2003][Bardasz 2005][Sutton 2004].
• Ash derived from lubricant additives is composed primarily of zinc, calcium, and magnesium in the form of sulfates, phosphates, and oxides [MECA 2005][Givens 2003][Bardasz 2005][Manni 2006].
• Prediction of engine-out ash emissions based solely on bulk oil consumption and lubricant sulfated ash levels results in an over-estimate of ash emissions due to lubricant volatility and differences in speciated oil consumption rates [Sutton 2004][Manni 2006][Aravelli 2007].
• Pressure drop across the particulate filter is not indicative of total ash levels [Kimura 2006][Bardasz 2005][Aravelli 2007].
• Catalyst performance may be negatively impacted by specific ash-related elements, primarily sulfur and phosphorous [Nemoto 2004][Bunting 2005][Bardasz 2004].
• Ash distribution within the DPF, whether along the walls or in the channel end plugs may be influenced by filter operating conditions and regeneration strategy [Gaiser 2004][Piesche 2003].

A detailed review of the literature in 2007 was conducted by Bodek which provides additional details of the impact of ash on diesel aftertreatment system components including DOC, SCR, and LNT technologies, in addition to DPFs [Bodek 2007]. Figure 3 presents a summary of the heretofore known impact of ash accumulation in the DPF on exhaust backpressure increase for various lubricants, filter technologies, and drive cycles. More recent results show lubricant-derived ash from CJ-4 specification oils, containing no more than 1.0% sulfated ash, resulting in an approximately doubling of the DPF pressure drop after 4,680 hours or 188,000 miles (303,000 km) of equivalent on-road use [Sappok 2009].

figure 3. Impact of Ash on Measured Backpressure Increase As a Function of Simulated Driving Distance 

Fuel Economy Effects. Ash build up in the DPF directly affects fuel consumption through two pathways: (1) increased exhaust flow restriction and backpressure, and (2) decreased filter regeneration intervals (increased regeneration frequency) through a reduction in filter soot storage capacity. Furthermore, the ash may also reduce the regeneration efficiency in catalyzed systems, requiring an increased reliance on active regeneration or higher temperature operation for successful passive soot oxidation.

While several studies have quantified the increase in vehicle fuel consumption attributed to the DPF, most consider only the effects of soot accumulation on exhaust backpressure and regeneration intervals. Depending on the regeneration frequency and soot level, the DPF-related increase in fuel consumption has been reported to range from 4.5% to 7.0% [Singh 2009]. In reality, however, the DPF-related increase in fuel consumption may be greater, as all of these studies fail to consider the additional increase in exhaust flow restriction and regeneration frequency due to the build-up of ash over the life of the filter.

The contribution of the ash-related backpressure increase to an increase in the overall fuel consumption is estimated from 2% to 3%, which includes the compounding impact of the ash to increase the filter’s pressure drop sensitivity to soot accumulation . Specific to the increase in regeneration frequency, other studies have shown an increase in regeneration frequency by up to a factor of two following approximately 240,000 miles of ash accumulation, if ash effects are not properly accounted for in pressure-based regeneration control schemes. However, even assuming perfect knowledge of the amount and distribution of the ash in the DPF, an increase in regeneration frequency by a factor of 1.6 over 240,000 miles is unavoidable, in the best case, due to the significant filter volume occupied by the ash and reduction in the DPF’s soot storage capacity

Before and After DPF cleaning