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.

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 NO 2  and wherein the step of combusting the carbon fraction of the collected PM comprises reacting the collected PM with the NO 2  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 NO 2  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.