A process for regenerating ceramic honeycomb filters by combusting

Abstract: A process for regenerating ceramic honecycomb filters by removing soot heaped thereon through combusting, which comprises the step of controlling the combusting of the soot such that a time t (minute) required for raising a temperature of an exhaust gas from a soot-ignition temperature, Tig, to Tig + 200°C satisfies the following inequality. in which: Q: a total flow rate of the waste gas per unit volume of the filter in initial stage (Nm³/min/ℓ) V: the volume of the filter (ℓ); and S: a heaped soot per unit volume of the filter (g/ℓ).

Description: The present invention relates to a process for regenerating soot-collecting ceramic honeycomb filters by removing soot heaped on partition walls thereof through combustion. In ceramic honeycomb filters used in diesel engines or the like, in order to remove soot filtered off from exhaust gases and heaped on partition walls, it is necessary that the soot is removed by combustion after use for a given time period to regenerate the ceramic honeycomb filters. According to a conventional process for combusting such soot, amounts of a fuel and combustion air are set such that temperatures of waste gases are made sufficiently higher than an ignition temperature of the soot (soot-ignition temperature) so as to enable complete combustion thereof, and these amounts are set at constant values during soot combustion, respectively. However, in the above regenerating process, since the waste gas having the temperature well higher than the soot-ignition temperature is at one time fed to the ceramic honeycomb filter from the beginning of the soot combustion, soot heaped on the partition walls is rapidly combusted at once. Consequently, there is a problem in that the ceramic honeycomb filter is broken or melts. JP-A-59-101518 discloses a process for regenerating a ceramic trap by removing particulates thereon by combustion. To avoid melting of the trap, its temperature is controlled during regeneration. It is an object of the present invention to solve the above-mentioned problem, and to provide a process for regenerating ceramic honeycomb filters by combustion while preventing rapid temperature increase due to the soot combustion during regenerating of the ceramic honeycomb filters. The process for regenerating a soot-collecting ceramic honeycomb filter according to the present invention is set out in claim 1. In the process, the rapid temperature rise due to combusting-removal of soot during regeneration of the ceramic honeycomb filter is prevented by controlling the temperature of the exhaust gas to be fed to the ceramic honeycomb filter such that the time required to raise the temperature of the exhaust gas over the specified temperature range may be set at not less than a lower limit value determined by the above inequality. These and other optional features, and advantages of the invention will be appreciated upon reading of following description of the invention when taken in conjunction with the attached drawings, with the understanding that some modifications, variations and changes of the same could be made by the skilled person in the art to which the invention pertains. For a better understanding of the invention, reference is made to the attached drawings, wherein: Fig. 1 is a graph showing the relationship between the time (t) required for raising the temperature of the exhaust gas from Tig to Tig + 200°C and the amount of soot heaped; Fig. 2 is a graph showing the relationship between the temperature rising time (t) and the soot-ignition temperature (Tig); Fig. 3 is a graph showing the relationship between the temperature rising time (t) and the reciprocal of the total flow rate (Q) of the exhaust gas in the initial stage; and Fig. 4 is a graph showing the relationship between the temperature rising time (t) and (1/V) 0.2 . In the present invention, it was found that the ceramic honeycomb filter can be regenerated without being broken or melted by raising the temperature of the exhaust gas under heating in a time period satisfying the above inequality over the specified temperature range. Preferably, the temperature of the gas is rapidly raised to the soot-ignition temperature Tig in 1 minute by heating, because a time required for regeneration is shortened in that case. Further, it is also preferable to raise the temperature of the exhaust gas such that heat curve is straight (That the heat curve is straight means that heating is effected such that the temperature linearly increases by heating with respect to lapse of time). The exhaust gas is intended to mean a waste gas coming from an internal combustion engine such as a diesel engine, etc. The time (t) required for raising the temperature from Tig to Tig + 200°C may be controlled by, for instance, mixing cooling air into the waste gas. The present invention will be explained in more detail with reference to the following specific Examples. Cylindrical ceramic honeycomb filters made of cordierite were prepared. Those cylindrical ceramic honeycomb filters had various soots possessing different soot-ignition temperatures, filter volumes, and heaped soot amounts as shown in the following Table 1, respectively. Each of the honeycomb filters was regenerated by combusting the soot while amounts of combustion air and cooling air and a time required to raise the temperature from the soot-ignition temperature, Tig, to Tig + 200°C were varied. First, diesel oil was combusted with the combustion air, and the time was controlled by mixing the cooling air into the waste gas obtained by combustion before the waste gas entered the filter. The temperature of the thus controlled waste gas was measured 20 mm upstream the filter. The heating was carried out such that the temperature was raised up to the Tig in a minute and that the heat curve in a range from Tig to Tig + 200°C was straight. At that time, the maximum temperature inside the filter was measured during regeneration by combusting, and damages of the filters after the combustion-regenerating were visually examined. Results are shown in Table 1. In Table 1, filters not damaged or those damaged as visually judged are indicated by ○ and X, respectively. As is seen from the results in Table 1, the filters having shorter heating times required for raising Tig to Tig + 200°C were conspicuously damaged. Based on the above results, in order that the minimum heating time (t) which is required for raising Tig to Tig + 200°C and during which no damage is produced may be determined by the total flow rate (Q) of the waste gas per unit volume of the filter in the initial stage, the filter volume (V), the amount (S) of soot heaped per unit volume of the filter, and the soot-ignition temperature (Tig), the relations between the heating time and the above factors were examined as shown in Figs. 1 through 4. As is seen from the results in Figs. 1 through 4, the heating time (t) is in proportion to each of S, Tig, 1/Q, and (1/V) 0.2 . By calculating a proportional constant thereof, the following relationship was found: As a matter of course, the present invention is not limited to the above-mentioned examples, but many modifications, variations, and changes may be made. For instance, in the above examples, the temperature was raised to Tig in one minute and the heat curves in the range from Tig to Tig + 200°C were straight, as preferred embodiments. However, the present invention is exclusively characterized in that the minimum time required for raising Tig to Tig + 200°C is specified, and thus the heating time required to raise the temperature of the exhaust gas to Tig and the profile of the heat curve are arbitrary. In addition, in the above Examples, although the temperature of the waste gases was controlled by adding the cooling air to the waste gas, but this is not restrictive. For example, a feed amount of fuel or an amount of air may be controlled, or the concentration of oxygen in the exhaust gases may be controlled. As is clear from the aforegoing detailed description, according to the process for burning the ceramic honeycomb filters in the present invention, the ceramic honeycomb filters can be regenerated by heating in the shortened time without being broken or melted by determining the minimum time required for raising the exhaust gas from the soot-ignition temperature, Tig, to Tig + 200°C.

Claims: A process for regenerating a soot-collecting ceramic honeycomb filter by removing soot collected thereon by burning the soot, characterised in that the soot is burned by means of an exhaust gas produced by burning of fuel, the process comprising the step of controlling the burning of the soot in such a manner that the time t (minutes) required for raising the temperature of the exhaust gas from a soot-ignition temperature (Tig) to Tig + 200°C satisfies the following inequality. in which: Q: a total flow rate of the exhaust gas per unit volume of the filter at the time the exhaust gas temperature reaches Tig (Nm³/min/ l ) V: the volume of the filter ( l ); and S: the amount of collected soot per unit volume of the filter (g/ l ). A regenerating process according to claim 1, wherein the temperature of the exhaust gas is raised to the soot-ignition temperature (Tig) in one minute. A regenerating process according to claim 1 or 2, wherein the exhaust gas temperature is raised in a manner such that the heating curve in a temperature range from the soot-ignition temperature (Tig) to Tig + 200°C is straight.

Inventors: HIGUCHI TOSHIAKI, HIJIKATA TOSHIHIKO, HIGUCHI, TOSHIAKI, HIJIKATA, TOSHIHIKO
Applicants: NGK INSULATORS LTD, NGK INSULATORS, LTD.
Application year: 1988
Main classification: F01N3
IPC codes: F01N3, B01D46, F01N9