A Theoretical and Experimental Study of the Regeneration Process in a Silicon Carbide Particulate Trap Using a Copper Fuel Additive 970188

The purpose of this study was to investigate the pressure drop and regeneration characteristics of a silicon carbide (SiC) wall-flow diesel particulate filter. The performance of a 25 μm mean pore size SiC dual trap system (DTS) consisting of two 12 liter traps connected in parallel in conjunction with a copper (Cu) fuel additive was evaluated. A comparison between the 25 μm DTS and a 15 μm DTS was performed, in order to show the effect of trap material mean pore size on trap loading and regeneration behavior. A 1988 Cummins LTA 10-300 diesel engine was used to evaluate the performance of the 15 and 25 μm DTS. A mathematical model was developed to better understand the thermal and catalytic oxidation of the particulate matter.

For all the trap steady-state loading tests, the engine was run at EPA mode 11 for 10 hours. Raw exhaust samples were taken upstream and downstream of the trap system in order to determine the DTS filtration efficiency. The overall filtration efficiency of the 25 μm DTS was greater than 97% and was similar to that of the 15 μm DTS (typically > 98%). The trap wall permeability was estimated using experimental clean trap pressure drop data and Darcy's law. The estimated wall permeability for the 25 μm DTS (8.2 x 10^-13 m²) was about 10% higher than that of the 15 μm DTS (7.5 x 10^-13 m²). It was observed that the trap pressure drop profiles at 60 ppm Cu concentration were markedly lower than those without the additive. The results also showed that the 25 μm DTS had less particulate mass stored in the trap and, in turn, lower pressure drop than the 15 μm DTS for identical trap loading conditions.

Two types of regeneration tests were performed on the 25 μm DTS, the ideal case “controlled regeneration” and the worst or attempted induced failure case “uncontrolled regeneration”. Experimental results indicated that the controlled regeneration test yielded a higher regeneration efficiency(83%) over the uncontrolled regeneration test (36%). This was attributed to the higher flow rate and temperature of the exhaust gases at full-load. The Cu additive had a significant effect on the regeneration efficiency. Both uncontrolled and controlled regeneration efficiencies for the 25 μm DTS with 60 ppm Cu additive (36% and 83%, respectively) were higher than those with no additive fuel (7% and 25%, respectively). This was due to the higher reaction rate resulting from using the Cu additive. The 15 μm DTS had a better regeneration efficiency than the 25 μm DTS. This was caused by the lower trap mass loading of the 25 μm DTS prior to regeneration.

A mathematical model was developed from the fundamental equations, namely the conservation of mass law and the chemical reaction kinetics. The model was used to develop a time-dependent relationship that determines the particulate mass accumulated in the trap, based on engine operating condition (e.g., exhaust temperature, oxygen concentration, and exhaust flow rate) and fuel additive concentration. The model was used in conjunction with an empirical pressure drop model that was derived from Darcy's generalized law. The particulate layer permeability was determined using the particulate mass in the trap and the pressure drop models. The activation energy and frequency factor for the chemical reaction kinetics were calculated from engine and trap data collected at modes 10 and 11 loading tests. The activation energy and frequency factor (for non-catalytic combustion) were determined to be 118 kJ/g-mol and 300 m³/gs, respectively. The Cu additive markedly reduced the activation energy to 102 kJ/g-mol and slightly changed the frequency factor to 290 m³/gs.