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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.
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.