Gasification of soot trapped in a particulate filter under reducing conditions
A power generation system comprises a diesel engine, a diesel particulate filter and a LNT configured to receive the exhaust from the DPF. The DPF is provided with a catalyst coating that is functional to catalyze methanation of trapped soot with H 2 contained in the exhaust. Preferably, the catalyst has little or no oxygen storage capacity. The system is configured to regenerate the LNT by providing syn gas to the exhaust in rich regeneration phases. The syn gas-containing exhaust passes through the DPF and then the LNT. Within the DPF, the syn gas-containing exhaust removes soot be methanation and other soot gasification reactions, thus reducing or eliminating the need to reduce the DPF by other means. Soot gasification is preferred over soot combustion in that soot gasification avoids the destructive high DPF temperatures associated with soot combustion.
1. A method of operating a power generation system, comprising:
operating a diesel engine to produce power and exhaust;
passing the exhaust through a particulate filter to trap soot from the exhaust;
passing the exhaust through a LNT, whereby the LNT adsorbs and stores NO x and SO x from the exhaust;
from time-to-time initiating a rich phase in which a fuel reformer is used to produce reformate that is provided to the exhaust upstream of the DPF, whereby the DPF receives a rich exhaust mixture containing reformate;
in the rich phase, catalyzing at temperatures of 400° C. or less soot gasification reactions selected from the group consisting of hydrogen-methanation, steam gasification, and the reverse-Boudouard reaction, by which soot is removed from the DPF and the DPF produces a rich DPF exhaust; and
passing the rich DPF exhaust through the LNT, whereby the LNT is regenerated.
2. The method of claim 1 , wherein at least about 50% of the trapped soot is removed by the soot gasification reactions.
3. The method of claim 1 , wherein at least about 50% of the trapped soot is removed by the soot gasification reactions during LNT denitrations.
4. The method of claim 1 , wherein at least about 50% of the trapped soot is removed by the soot gasification reactions during LNT desulfations.
5. The method of claim 1 , wherein the fuel reformer is configured to receive the engine exhaust.
6. The method of claim 5 , further comprising, immediately preceding the rich phase, a lean phase in which fuel is injected into the exhaust upstream of the reformer at a rate that leaves the exhaust lean, whereby the fuel reformer heats by combustion of the injected fuel.
7. The method of claim 1 , wherein the DPF catalyzes soot gasification with a catalyst that consists essentially of materials other than alkali metals.
8. The method of claim 1 , wherein the DPF catalyzes soot gasification with a catalyst that consists essentially of precious metal.
9. The method of claim 1 , wherein the DPF removes the soot primarily by depth filtration.