3.1 Operation of wall flow and deep-bed filters
3.1.3 Regeneration
All kinds of filters must be periodically regenerated, that is, the particulate matter that is accumulated in the filter must be pediodically removed. The filter regenera- tion process depends on the particular application, especially on the filter type and the collected matter. In automotive applications, the soot deposit is removed from the filter by burning it. On-site soot oxidation is the only feasible way to regenerate automotive DPFs because the filter is mounted on the exhaust line, thus it may not be easily removed to be cleaned. Oxidation is caused by exhaust gas species such as O2 or NO2 and any catalyst possibly present in the DPF or the soot itself.
The products of the oxidation are gases (primarily CO and CO2) that escape to the
outlet of the filter.
Even before clogging, highly loaded filters exhibit unfavorable operation charac- teristics [14]. The presence of the filter in the exhaust line induces some pressure drop which is felt by the engine as additional load. Induced pressure drop (and the corresponding engine load) increases rapidly as a function of the particulate matter collected in the filter. Therefore, the engine does not operate neither smoothly nor economically. Consequently, DPF induced pressure drop must be kept in the lowest levels possible.
A second reason that high filter loads are undesirable is that their filtration efficiency may deteriorate with loading. This is true only for the fiber and foam filters. After some point, the filter is unable to retain the particles. Clusters of accumulated matter are detached by the filter and escape to the outlet [9].
Finally, when regeneration of a highly loaded filter finally occurs, it may endanger the integrity of the filter itself. Since soot combustion is accompanied by significant exothermy, regeneration always results in filter heating. Heavily loaded filters are difficult to regenerate in a controlled fashion. Onset of regeneration will probably lead to excessive heat release in certain locations within the filter, which is usually followed by cracks due to thermal stresses [15] or, in extreme cases, local filter melting.
The regeneration process If no special regeneration technique is used, thermal regeneration begins in a DPF when two conditions are fulfilled: (a) the temperature of the soot is high enough (above 550◦C) to enable its oxidation by the exhaust gas oxygen and (b) the necessary oxygen is provided by the gas flow. Both depend on the operating point of the engine.
Since neither the temperature nor the exhaust gas flow is uniform within the filter, regeneration does not start simultaneously everywhere, but in certain points where the conditions are favorable. From these points, regeneration propagates in the filter, enhanced by the heat release due to regeneration itself. Propagation depends on the exhaust gas flow (which is continuously changing in real-world driving conditions), and on the distribution of temperatures and accumulated soot in the filter.
Bearing in mind that, in urban driving conditions, the engine usually operates in low loads, there is a high probability that no regeneration occurs until the filter is highly loaded. Since this should be avoided for the reasons mentioned above, several forced regeneration techniques have been devised, to regenerate the filter under all driving modes. The principles, advantages and disadvantages of the most promising forced regeneration techniques are discussed below.
Forced regeneration techniques A straightforward way to regenerate a DPF under low engine loads is to increase the temperature of the exhaust gas after it leaves the combustion chamber of the engine. This can be done with the use of electric resistances or a fuel burner [16]. Whatever the heating device, it does not operate continuously; rather, the pressure drop along the DPF is continuously monitored and the heating device operates if the DPF pressure drop exceeds some limit. Such systems are called active regeneration systems.
The main advantage of this approach is that the onset of regeneration is guaran- teed regardless of the DPF’s operating conditions. Nevertheless, the disadvantages are multiple. The installation of the heating device and the pressure drop sensors increase the initial as well as the running cost of the vehicle, because of increased fuel consumption. Finally, the installation of such a complicated system also raises the issue of its reliability and cost of service.
To circumvent the above problems, automotive manufacturers have also consid- ered passive regeneration systems, which do not introduce any new subsystems in the exhaust line. Instead, passive subsystem are designed so that the conditions for regeneration onset in the DPF are favorable under all driving modes. Passive regeneration systems usually seek to lower the temperature at which soot begins to get oxidized. This implies the use of a catalyst; the term catalytic regeneration is then used and is to be contrasted with the thermal regeneration of the DPF when no catalyst is present.
There are currently two approaches to catalytic regeneration [16]. The first, the washcoated DPF, is inspired by the success of the catalytic converters. The idea is to apply a catalytically active washcoat on the interior surface of a DPF (the walls of the wall-flow DPF or the ceramic mesh of a foam of fiber filter). The deposited soot that is in contact with the precious metal catalyst is then oxidized in lower temperatures than without catalyst. The second approach uses a catalyst in liquid form that is added in the fuel before it is burned. The catalyst is a liquid solution of some metal oxide, usually Cerium (Ce) or Ferrocine (Fe). In this way, the catalyst enters the engine’s combustion chamber with the fuel and the soot particles that are
Sec. 3.2 Operation of wall flow and deep-bed filters 55 formed contain the catalyst. As a result, the soot deposit in the filter contains the catalyst and it is oxidized in lower temperatures.
In principle, the washcoated DPF is much more attractive than any other regen- eration method. This stems from its simplicity, low initial cost, no operating cost and no need for service. Its primary disadvantage, though, is the low catalyst–soot contact area, especially in the case of the wall-flow DPF, since only the soot de- posited within the pores of the washcoat is in contact with the catalyst. Deep-bed filters such as foam filters are better in this regard, because they have much higher surface area and more soot is in contact with the catalyst [9]. Research is ongoing in improving washcoated DPF technology, and combine it with advanced catalyst technologies such as the NOx-storage catalyst [29, 30]
A promising regeneration technology based on the washcoated filter is the CRT (Continuously Regenerating Trap), covered by patents by Johnson-Matthey [31], which is basen on positioning a oxidation catalyst upstream the DPF that oxidizes NO to NO2. Subsequently, NO2 reacts with the particulate matter of the DPF.
The additive-assisted regeneration resulting in uniform distribution of the cata- lyst in the particulate deposit. This addresses the problem of soot–catalyst contact area in the wall-flow DPF but reintroduces the complexity and increased cost needed by the additive dosimetry device. Moreover, after every regeneration, catalyst ash remains in the filter and gradually increases the induced pressure drop and clogs the filter. This reduces the life span of the filter, which must be removed and cleaned after 30000–50000 Km of continuous operation.
A certain manufacturer ([2]) has already produced about 400,000 passenger cars is equipped with a diesel particulate filter that uses a Cerium based fuel additive and a wall-flow DPF [1, 2, 32]. The system is coupled with the fuel post-injection technique, which enriches the exhaust gas with fuel injected in the cylinder after the combustion phase, in order to increase the VOF content of the soot. This is essentially an active regeneration system, since the addititve dosimetry and fuel post-injection is controlled by a computerized engine management system, based on the monitoring of the DPF pressure drop as an indication of DPF soot loading. The system is highly innovative and is covered by a number of patents [33, 34, 35].
The effect of the VOF Owing to the very close contact with the catalyst metal oxides, the VOF content of the particles may be oxidized at temperatures below 250◦C. Thus, a slow regeneration procedure may start in specific points in the DPF where the local soot loading, composition, and temperatures are favorable. Under certain circumstances the heat release from the oxidized VOF leads to further carbon oxidation by the additive oxides at temperatures higher than 350◦C. The resulting additional exothermic reactions may locally increase the temperature to above 500◦C, thus allowing thermal oxidation of carbon by exhaust gas oxygen. This process is called erratic regeneration and was first systematically reported by Lepperhoff et al. [22, 36].
The presence of VOF may have assist positive or negative impact on the DPF operation: On the one hand, it may assist the onset of a regeneration in low tem- peratures, but on the other hand it may promote an uncontrolled regeneration in a heavily loaded filter. The post-injection method is an attempt to benefit from the presence of the VOF to regenerate the filter in lower temperatures. Again, modeling is needed in this regard.