Flow reversal in exhaust system including filter

Abstract

A process for treating an exhaust gas on a lean-burn reciprocating internal combustion engine includes the steps of collecting particulate matter (PM) on a first filter and combusting a carbon fraction thereof, periodically reversing the direction of gas flow through the first filter thereby displacing uncombusted PM from the upstream side of the first filter and collecting at least part of the displaced PM on a second filter.

Description

This invention relates to engine exhaust gas treatment when the exhaust gas contains particulate matter (PM), and in particular such treatment including intermittent flow reversal in a particulate filter.

Lean-burn reciprocating engines produce exhaust gas containing PM comprising carbon (including carbon compounds), and also other materials summarised as ‘ash’. A major decrease in emission of carbon at moderate exhaust temps has been provided by the CRT® process described inter alia in U.S. Pat. No. 4,902,487, SAE paper 890404 (Cooper et al.) and Platinum Metals Review 1995, 39, 2-8 (Hawker), the contents of all of which are incorporated herein by reference.

Recent advances in engine design have decreased emission of carbon PM to the extent but ash accumulation on filters presents a significant problem. It has been proposed to deal with this by reversing the direction of flow through the PM filter when PM has accumulated beyond a design level. This discharges ash to atmosphere, where it may be injurious to health, and is at least inconvenient. It also risks discharge of carbon PM not oxidised before such flow reversal.

The problem is likely to persist while lubricating oils contain inorganic atoms that react to produce involatile oxides in the engine and if soot-combustion catalyst additives are present in the fuel. A further problem recently recognised is that very fine PM is not collected by available filters.

According to a first aspect, the invention provides a process for treating an exhaust gas of a lean-burn reciprocating internal combustion engine comprising, in downstreamward order, the steps of:

(i) collecting PM on a first, porous filter and combusting a carbon fraction thereof, and

(ii) periodically reversing the direction of gas flow through the first filter, thereby displacing uncombusted PM from the erstwhile upstream side thereof, characterised by collecting at least part of the displaced PM on a second filter.

According to a second aspect, the invention provides an exhaust gas treatment system for a lean-burn reciprocating internal combustion engine comprising, in downstreamward order:

(i) a first, porous filter effective to collect PM for combustion of carbon fraction thereof, and

(ii) means for periodically reversing the direction of gas flow through the first filter, whereby to displace uncombusted PM, characterised in that a second filter for collecting at least part of said displaced PM is disposed downstream of said first filter.

The second filter may have pores smaller than in the first filter, whereby to collect particles too small to be collected by the first filter. In such a combination, the first filter can have larger pores than those of established systems using a single filtering stage.

We have realised that, as a result of lower exhaust temperature in modern engines of high thermodynamic efficiency and of the cooling due to remoteness from the engine, the second filter can be an inexpensive, readily replaceable unit using organic material such as woven or non-woven textile e.g. paper; this is preferably mounted in a snap-on or screw-on housing, suitably made by injection-moulding. Suitable filter material includes papers resistant to heat and moisture. They typically comprise cellulose with hydrophobic fibres.

A further characteristic of modern engines is that engine out NO_x can be too low for passive filter regeneration using NO₂ generated catalytically by oxidation of NO as described in U.S. Pat. No. 4,902,487. This problem can be exacerbated by the above-mentioned lower exhaust temperatures, so that, in practice, active techniques for filter regeneration may be required to maintain the back pressure across the filter within acceptable design tolerances. Such active regeneration techniques are described in EP 0758713, for example.

An advantage of the present invention to the practical application of the CRT® process is that flow reversal in the first filter can be used to clear PM that is not readily combusted in NO₂ over a drive cycle. The uncombusted PM can be collected on the second filter for removal at suitable intervals. Thus, the arrangement has particular use to the retrofit market, providing a practical and economic alternative to fitting expensive and fuel-costly active regeneration apparatus.

Preferably the gas is subjected to catalytic nitrogen monoxide (NO) oxidation upstream of the first filter. Instead or additionally, the first filter may be catalysed. Suitable filter catalysts include base metals for promoting PM combustion, such as La/Cs/V₂O₅, or one or more platinum group metals (PGM), particularly platinum. The PGM can be supported on the filter material per se, or on washcoat components formulated to prevent clogging of the filter pores. Such washcoat components can include alumina, ceria and ceria/zirconia mixed or composite oxides, for example. Advantageously, the filter catalyst can include a NO_x absorber in addition to PGM-based components, in which case the filter can be referred to as a 4-way catalyst. These measures lead to carbon combustion at moderate temperatures, and are thus readily adaptable to second filter temperatures tolerated by disposable filter media.

The material of the second filter may be treated externally to recover catalyst dust if the first filter is catalysed.

In the process or system, filter flow reversal is by means of a pipe junction: receiving exhaust gas inlet from an engine possibly via preceding treatment(s); feeding said gas to one end of the filter; intermittently switching said gas feed to the other end of the filter; and receiving filter outlet gas, delivering it to said second filter and thence to atmosphere, possibly via further treatment.

Preferably the flow switching step is operable via a condition in which all connections are open and excluding a condition in which all are closed.

connections to the valve may be duplicated thus: for feeding gas to the filter, a lagged pipe; and for receiving gas from the filter, a finned pipe.

To switch the direction of gas flow through the filter, the pipe junction suitably includes a 4-way valve. Such a valve typically comprises: an outer cylindrical or frusto-conical casing formed with angularly spaced apertures each leading to external flow connections; and deflector means effective to: select alternatingly one from two incoming streams; or to direct a single incoming stream alternatingly to either of two outlets; or to direct two incoming streams into a single stream. The deflector means is preferably operable over an arcuate path between the two extreme positions at which selected gas flow is required. The deflector means may comprise a ‘butterfly’. The valve casing may be formed with a wall-region of greater diameter corresponding to the intended traverse of the butterfly, and the change to the lesser diameter at the extremities of the traverse is formed as a step conformed to the profile of the butterfly and effective as a seal against gas leakage. The traverse of the butterfly is typically 10-20% of the circumference of the casing. If intermediate non-selective gas flow is required, this is provided by the actuator means.

The deflector means may be provided by a barrel fitting fluid-tightly within the casing and rotatable on an axis transverse to the main direction of fluid flow; formed along each of two or more radial planes of the barrel at least one fluid tight dividing member; and formed in each division at least one passage open at mutually angled positions about the circumference of the barrel, said positions corresponding to the apertures.

The barrel (if used) can be provided by uniting sheet material to define its outer shape and internal passages or by shaping solid material and forming the passages by boring thereinto, so that the residues between bores constitute the dividing members. Each passage normally has an outlet angled to its inlet, for example perpendicularly in a 4-way valve with one inlet connection and two or three outlet connections. In a 4-way valve having two inlet connections and two outlet connections, each passage may have one inlet and two outlets.

The valve may include means limiting angular movement of the barrel to positions in which selected flow connections or all flow connections are open.

Other features of the process and system may include: upstream of the junction, at least one of:

• • i) catalytically oxidising NO to nitrogen dioxide (NO₂) as already mentioned; and/or
  • ii) actively increasing the gas temperature when required; downstream of the junction, at least one of:
  • i. a nitrogen oxides (NO_x) removal step such as a lean-NO_x catalyst, a regenerable NO_x sorber or selective catalytic reduction (SCR);
  • ii. a diesel oxidation catalyst (DOC); and optionally on the first filter and/or (if heat-stable) the second filter:
  • a catalyst effective for combustion of the carbon fraction of PM with NO₂ and/or oxygen.

The catalyst of i), ii), i or ii is suitably on a flowthrough monolithic substrate composed of ceramic, wound corrugated metal, metal as foam or sinter or orderly or random wire or flat wire. The first and possibly the second filter may use substrate material similar to those of sorbents and catalysts, but in ‘filter grade’ permeable to gas but with limited permeability to PM.

The invention is illustrated by the accompanying drawings in which:

FIG. 1 shows a system in which flow-reversal is effected by four separate valves;

FIG. 2 shows a system in which flow reversal is effected by a single 4-way valve; and

FIGS. 3A and 3B show enlarged plan views of a valve as used in FIG. 2 representing the two extreme positions of the valve.

Referring to FIG. 1, the system, referred to generally as 10, comprises reactor 20 connected to a diesel engine exhaust manifold (not shown) and containing oxidation catalyst 22 consisting of a ceramic honeycomb carrying a washcoat and platinum (Pt). From the outlet end of reactor 20, pipes lead via valves 24A and 24B respectively to the header spaces of one end (shown LHS) and the other (shown RHS) of PM filter 26 consisting of a filter-grade ceramic honeycomb of the wall-flow type, the passages of which are alternatingly open and closed at the inlet end and, corresponding to the inlet open passages, alternatingly closed at the outlet end. From the LHS and RHS header spaces respectively, pipes lead via valves 24C and 24D to the header space of second filter vessel 30, which is formed with a ready-opening end-cover indicated generally by flanges 32 and contains disposable filter unit 34. In use the filtered exhaust gas is discharged to atmosphere at 36.

The system is operated under the control of a computer programmed to: open valves 24A and 24C and close valves 24B and 24D (as shown); receive a signal as to the pressure drop across filter 26; and close valves 24A and 24C and open valves 24B and 24D when the pressure drop reaches a pre-determined level. Such opening and closing is timed to avoid blockage or bypassing.

Referring to FIG. 2, items 10, 20 and 22 are the same as in FIG. 1 but first filter 26 is in vessel 29 having only 2 pipe connections. Reversal of flow is effected by 4-way valve 25, having outlets X and Y to the first filter and a connection to second filter vessel 31. Valve 25 is operable in 3 positions: open to X, open to Y, and intermediately to all four ways, so that the changeover takes place without possibility of blockage. The time period of 4-way opening is controlled to be minimal, to limit passing to filter 34 gas containing PM at the full engine-out concentration.

Referring to FIGS. 3A and 3B, each view is numbered as in FIG. 2. The plan views shown in these figures relate to an essentially cylindrical valve casing 50 formed internally with circumferential regions 52 of greater diameter, defining the range of traverse of rectangular butterfly deflector 54 having pivoted operating shaft 56 extending out of the valve casing via a seal to an actuator (not shown). The extremities of the range of traverse are defined by steps 58 between the regions differing in diameter, such steps limiting gas leakage out of its intended path.

Referring to FIG. 2, in normal operation of the engine the exhaust gas, comprising steam (H₂O_(g)), nitrogen (N₂), oxygen (O₂), carbon dioxide (CO₂), unburned hydrocarbon fuel (HC), carbon monoxide (CO), NO_x and PM, at e.g. 300° C. contacts catalyst 22 over which NO is oxidised to NO₂ and some of the HC and CO are oxidised to steam and CO₂. It then enters filter 26 on which most of the PM is collected and the carbon content thereof is combusted by reaction with the NO₂ formed in catalyst 22 and possibly with O₂. The PM-depleted gas then passes to vessel 31 holding filter 34, which collects PM released from filter 26. Such PM is typically ash, in which event filter 34 may be disposable, such as fibre or paper. Another duty of filter 34 can be to collect any ultra-fine combustible PM not collected by filter 26.

Claims

1. A process for treating an exhaust gas of a lean-burn reciprocating internal combustion engine, which process comprising the steps of: (i) collecting particulate matter (PM) on a first, porous filter and combusting a carbon fraction of the collected PM; (ii) contacting exhaust gas exiting the first filter with a second filter; (iii) periodically reversing the direction of gas flow through the first filter thereby displacing uncombusted PM from an upstream side of the first filter, and collecting at least part of the displaced PM on the second filter; and (iv) periodically removing the second filter containing the collected PM and replacing the second filter with a fresh second filter.

2. A process according to claim 1, further comprising upstream of step (i) the step of catalytically oxidising NO in the exhaust gas to NO2 and wherein the step of combusting the carbon fraction of the collected PM comprises reacting the collected PM with the NO2 on the first filter.

3. A process according to claim 1, further comprising collecting PM on the second filter which is more fine than the PM collected on the first filter.

4. An exhaust gas treatment system for a lean-burn reciprocating internal combustion engine comprising: (i) a first, porous filter for collecting PM for combusting a carbon fraction of the collected PM; (ii) means for periodically reversing the direction of gas flow through the first filter to displace uncombusted PM; and (iii) a second porous filter for receiving exhaust gas exiting the first filter and for collecting at least part of said displaced PM, which second filter is adapted for periodic removal and replacement.

5. A system according to claim 4, further comprising a catalyst for oxidising NO in the exhaust gas to NO2 is disposed upstream of the first filter.

6. A system according to claim 4, wherein the pores of the second filter are smaller than the pores of the first filter.

7. A system according to claim 4, wherein the second filter comprises an organic medium material comprising a woven or non-woven textile.

8. A system according to claim 7, wherein the textile is a paper material.

9. A system according to claim 7, wherein the filter medium is mounted in an injection-moulded snap-on or screw-on housing.

10. A system according to claim 4, wherein the gas flow reversal means comprises a 4-way fluid flowpath switching valve comprising: an outer cylindrical or frusto-conical casing formed with angularly spaced apertures each leading to external flow connections; and deflector means effective to: select one from two incoming streams, or to direct a single incoming stream to either of two outlets: X and Y, or to direct two incoming streams furcately, whereby the switching valve is operable in 3 positions: open to X, open to Y, and intermediately to open all four ways.

11. A system according to claim 10, wherein the deflector means is operable over an arcuate path between the two extreme positions at which selected or furcate gas flow is obtained.

12. A system according to claim 10, wherein the deflector means comprises a ‘butterfly’ and the cylindrical or frusto-conical valve casing is formed with a wall-region of greater diameter corresponding to the intended traverse of the butterfly, and the change to the lesser diameter at the extremities of the traverse is formed as a step conformed to the profile of the butterfly and effective as a seal against gas leakage.

13. A system according to claim 10, wherein the connections to the valve are duplicated for feeding gas to the first filter via a lagged pipe, and for receiving gas from the first filter via a finned pipe.

14. A system according to claim 4, further comprising control means for selecting between periodic reversal of gas flow through the first filter and intermediate non-selective gas flow, which control means comprising actuator means.