A flow-through oxidation catalytic converter installed on a vehicle can reduce the soluble organic fraction (SOF) of the particulate by as much as 90 percent and total particulate by as much as approximately 25 to 50 percent depending on the composition of the particulate being emitted.
Smoke emissions from older vehicles can be reduced by over 50 percent and a catalyst can virtually eliminate the obnoxious odor of diesel exhaust.
Furthermore, reductions of 60 percent to 90 percent of CO and HC emissions can be achieved. As a result, the diesel oxidation catalyst has become a leading retrofit control strategy in both the on road and nonroad sectors throughout the world. Using a flow-through oxidation converter on diesel-powered vehicles is not a new concept. Oxidation converters have been installed on off-highway vehicles around the world for over 20 years and have been installed on many urban buses in the US and Europe.
A. Operating Characteristics and Control Capabilities
The concept behind an oxidation catalyst is that it causes chemical reactions without being changed or consumed.
An oxidation catalytic converter consists of a stainless steel canister that typically contains a honeycomb-like structure called a substrate or catalyst support. There are no moving parts, just acres of interior surfaces on the substrate coated with catalytic precious metals such as platinum or palladium.
In the case of diesel exhaust, the catalyst oxidizes carbon monoxide (CO), gaseous hydrocarbons (HCs) and the liquid hydrocarbons adsorbed on the carbon particles. The liquid hydrocarbons are referred to as the soluble organic fraction (SOF) and can make up a significant part of the total particulate matter.
The level of total particulate reduction is influenced in part by the percentage of SOF in the particulate. For example, it has been reported that oxidation catalysts could reduce the SOF of the particulate by 90 percent under certain operating conditions, and could reduce total particulate emissions by 40 to 50 percent. Destruction of the SOF is important since this portion of the particulate emissions contains numerous chemical pollutants that are of particular concern to health experts.
Oxidation catalysts are also effective in reducing particulate and smoke emissions on older vehicles. Under the US EPA's urban bus rebuild/retrofit program, several manufacturers have certified diesel oxidation catalysts as providing at least a 25 percent reduction in PM emissions (see below). The certification data also indicates substantial reductions in CO and HC emissions.
Combining an oxidation catalyst with engine management techniques can reduce both NOx and PM emissions from diesel engines. This is achieved by adjusting the engine for low NOx emissions which is typically accompanied by increased CO, HC, and particulate emissions and then using an oxidation catalyst to offset these increases, thereby lowering the exhaust levels for all of the pollutants.
Often, the increases in CO, HC, and particulate can be reduced to levels lower than otherwise could be achieved. In fact, a system which uses an oxidation catalyst combined with proprietary ceramic engine coatings and injection timing retard to provide over a 40 percent NOx reduction while maintaining low particulate emissions has been approved under EPA's urban bus rebuild/retrofit program.
This same system has also been approved as reducing PM emissions to below 0.1 g/bhp-hr. Also, two systems employing catalysts and modified engine components, e.g. camshafts and turbochargers, have also been submitted for approval as providing less than 0.1 g/bhp-hr PM emissions.
B. Impact of Sulfur in Diesel Fuel on Catalyst Technologies
The sulfur content of diesel fuel is critical to applying catalyst technology. Catalysts used to oxidize the SOF of the particulate can also oxidize sulfur dioxide to form sulfates, which is part of the particulate.
This reaction is not only dependent on the level of sulfur in the fuel, but also the temperature of the exhaust gases. Catalyst formulations have been developed which selectively oxidize the SOF while minimizing oxidation of the sulfur dioxide.
However, the lower the sulfur content in the fuel, the greater the opportunity to maximize the effectiveness of oxidation catalyst technology. The low sulfur fuel (0.05% wt), which was introduced in 1993 throughout the US and in 1995 throughout Europe has facilitated the application of catalyst technology to diesel-powered vehicles.
Furthermore, the very low fuel sulfur content (<0.005% wt) available in several European countries has further enhanced catalyst performance. Several Asian countries including Hong Kong have also lowered diesel fuel sulfur levels to a maximum of 0.05% and this level is increasingly becoming the global maximum norm. Hong Kong has recently demonstrated global leadership by adopting a tax policy which has resulted in most if not all commercially available diesel fuel meeting a standard of 50 PPM or less.
Catalysts have also been effectively retrofitted to vehicles that run on fuel containing sulfur levels above 0.05% wt. Typical nonroad retrofit applications reduce PM, HC, and CO emissions when fuel containing 0.25% wt sulfur is used. In some instances, CO and HC emissions have been effectively controlled with sulfur levels as high as 0.5% wt.
However, these elevated sulfur levels make it difficult to control particulate emissions and depending on the catalyst formulation and exhaust temperature, may actually increase particulate emissions and other hazardous substances such as sulfates.
Diesel particulate trap systems have also been retrofitted to existing vehicles. In some vehicles such as forklift trucks, simplified higher energy external heat addition systems have been found to be useful and are sold commercially in Europe. Second-generation regeneration systems are emerging for this application that are less complex.
These systems rely on catalyst fuel additives like cerium and copper, or platinum catalysts placed in front of the filter, or catalysts coated directly on the filter to initiate the regeneration process. These systems have also been retrofitted on both a commercial and demonstration basis in different areas of the world.
Diesel particulate trap oxidizers or diesel particulate filters can achieve up to, and in some cases exceed, a 90 percent reduction in particulate. The trap is extremely effective in controlling the carbon core of the particulate and recent evidence indicates it can be very effective in reducing ultrafine PM emissions, which are likely to be the most hazardous to health.
A. Operating Characteristics and Control Capabilities
The trap oxidizer system consists of a filter positioned in the exhaust stream designed to collect a significant fraction of the particulate emissions while allowing the exhaust gases to pass through the system.
Since the volume of particulate matter generated by a diesel engine is sufficient to fill up and plug a reasonably sized filter over time, some means of disposing of this trapped particulate must be provided. The most common means of disposal is to burn or oxidize the particulate in the trap, thus regenerating, or cleansing, the filter.
A complete trap oxidizer system consists of the filter and the means to facilitate the regeneration.
B. Filter Material
A number of filter materials have been tested, including ceramic monoliths and woven fibers, woven silica fiber coils, ceramic foam, wire mesh, sintered metal substrates, and temperature resistant paper in the case of disposable filters. Currently, the ceramic monoliths, woven fibers, and paper filters have been used commercially.
All of the technologies function in a similar manner; that is, forcing particulate-laden exhaust gases through a porous media and trapping the particulate matter on the intake side. Excellent filter efficiency has rarely been a problem with the various filter materials listed above, but work has continued with the materials, for example, to: (1) optimize high filter efficiency with accompanying low back pressure, (2) improve the radial flow of oxidation through the filter during regeneration, and (3) improve the mechanical strength of the filter designs.
Particulate-laden diesel exhaust enters the filter, but because the cell of the filter is blocked at the opposite end, the exhaust cannot exit out the cell. Instead the exhaust gases pass through the porous walls of the cell. The particulate is trapped on the cell wall. The exhaust gases exit the filter through the adjacent cell.
Impressive results with an improved cordierite ceramic monolith filter have been reported. The newly designed filter achieved over a 90 percent particulate control efficiency while improving the coefficient of thermal expansion by 60 percent and the predicted thermal shock resistance by 200 percent over current filter designs. These significant improvements enable the filters to withstand the rigorous operating conditions during planned, as well as unplanned, regenerations.
C. Regeneration
The exhaust temperature of diesels is not always sufficient to initiate regeneration in the trap. Therefore, a number of techniques have been developed to facilitate combustion of the trapped particulate.
Some of these methods include
1. Using a catalyst-coated trap. The application of a base or precious metal coating applied to the surface of the filter reduces the ignition temperature necessary for oxidation of the particulate;
2. Using a catalyst to oxidize NO to NO2, which adsorbs on the collected particulate substantially reducing the temperature required to regenerate the filter;
3. Using catalytic fuel additives to reduce the temperature required for ignition of the accumulated material;
4. Throttling the air intake to one or more of the cylinders, thereby increasing the exhaust temperature;
5. Using fuel burners, electrical heaters, or combustion of atomized fuel by catalyst to heat the incoming exhaust gas to a temperature sufficient to ignite the particulate;
6. Using periodically compressed air flowing in the opposite direction of the particulate from the filter into a collection bag which is periodically discarded or burned; and
7. Throttling the exhaust gas downstream of the trap. This method consists of a butterfly valve with a small orifice in it. The valve restricts the exhaust gas flow, adding backpressure to the engine, thereby causing the temperature of the exhaust gas to rise and initiating combustion.
Some trap systems, to protect the filter from overheating and possibly being damaged, incorporate a by-pass for exhaust gases that is triggered and used only when exhaust temperatures reach critical levels in order to slow the regeneration process. The period during which the by-pass is operated is very short and relatively infrequent. Some systems are also designed with dual filters in which one filter collects while the other is being regenerated.
Optimizing a trap oxidizer system to a particular application has as a prime engineering goal the elimination (or minimization) of any adverse effects of the system on engine or vehicle performance. Evaluations with trap oxidizer development suggest these goals are attainable.
Non-catalyzed trap systems appear to have little or no effect on NOx, CO, or HC emissions. Experience with the catalyzed trap system indicates that HC and CO emissions have also been reduced to a considerable degree (in the range of 60-90 percent) with no adverse impact on NOx emissions.
Though difficult to quantify, one manufacturer has found that ceramic traps significantly reduce gas phase aromatics and noise. The experience with catalyzed traps indicates that there is a virtually complete elimination of odor and the soluble organic fraction of the particulate.
Trap systems, which replace mufflers in retrofit applications, have achieved sound attenuation equal to a standard muffler.
A fuel economy penalty has been experienced with trap oxidizer technology, which is attributable to the backpressure of the system. Some forms of regeneration involve the use of diesel fuel burners, and to the extent those methods are used, there will be an additional consumption of fuel. It is expected that the systems can be optimized to minimize, or in some cases eliminate, any noticeable fuel economy penalty. For example, in a demonstration program in Athens, no noticeable fuel penalty was recorded when the trap was regenerated with a cerium fuel additive.
Trap systems do not appear to cause any additional engine wear or affect vehicle maintenance. Concerning maintenance of the trap system itself, manufacturers are designing systems to minimize maintenance requirements during the useful life of the vehicle.
There have also been several successful retrofit programs on buses and trucks that can provide useful insights into effective programs. Some of these will be discussed below.
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