Diesel Filter Regeneration

W. Addy Majewski

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Abstract: The regeneration of diesel filters is characterized by a dynamic equilibrium between the soot being captured and the soot being oxidized in the filter. Soot oxidation rates depend on the filter temperature, soot load in the filter, and a number of other factors. Continuously regenerating filters operate at a balance temperature, which can be determined through a laboratory measurement. To facilitate filter regeneration on diesel engines in real operation the exhaust gas temperature has to be increased or the soot ignition temperature has to be lowered using a catalyst.

Oxidation of Diesel Soot

Introduction

Thermal regeneration of diesel particulate filters involves the oxidation of solid particulates to gaseous products, preferably to CO2. Depending on the DPF system design and on the engine operating conditions (exhaust gas temperature), the oxidation of carbon in diesel soot can occur via reactions with oxygen or nitrogen dioxide:

Other, even more active oxidants are also possible. Ozone was suggested [2344] as a diesel soot oxidant that could enable regeneration at temperatures even lower than those possible with NO2. The use of ozone regeneration would require an onboard ozone generator.

Oxidation by Oxygen

The oxidation of diesel soot, represented here as carbon, by oxygen can be described by one of the following reactions:

(1)C + O2 → CO2

(2)C + ½O2 → CO

where C (carbon) represents the combustible portion of diesel PM.

Carbon dioxide is the preferred product, but increased carbon monoxide emission may occur if oxygen is depleted in the reaction zone.

The rate of thermal regeneration is limited by the kinetics of soot oxidation. The rate of soot oxidation is significantly lower than that of the oxidation of gaseous exhaust pollutants, such as carbon monoxide. This is one of the reasons why carbonaceous particulates are not oxidized in flow-through catalysts which provide relatively short particle residence times. The oxidation of soot requires diesel filters which extend the residence time by physically capturing the particulates.

[chart]

Figure 1. Diesel Soot Oxidation

Laboratory measured oxidation rates of diesel soot by oxygen in air are shown in Figure 1 [184]. The mass fraction of soot which was oxidized is plotted against the oxidation time for different temperatures. At low temperatures (360°C, 400°C) the oxidation rates are very slow and the regeneration is incomplete. Only at temperatures of about 600°C the soot could be oxidized quickly and completely. At any given temperature, the oxidation rate changes with the amount of soot in the sample. Fast oxidation rates (i.e., steep curves in Figure 1) are observed at the beginning of each experiment. As the mass of soot in the sample decreases, so does its oxidation rate. This behavior is also seen in diesel filters, which regenerate faster when the soot loading is higher.

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