Diesel Filter Regeneration

W. Addy Majewski

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• Oxidation of Diesel Soot
• Regeneration as Equilibrium Process
• Passive Regeneration
• Active Regeneration
• Emissions During Regeneration

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:

• Oxygen is present in diesel exhaust at sufficient concentrations at nearly all operating conditions. However, relatively high temperatures are necessary to achieve appreciable regeneration rates with O₂. Oxygen based regeneration is mostly seen in active DPF systems.
• Nitrogen dioxide based regeneration can be conducted at lower temperatures than oxygen regeneration. However, the NO₂ concentrations must be increased, which is typically realized via catalytic oxidation of NO (i.e., by increasing the NO₂:NO ratio in NOx). Oxidation of soot by NO₂ is the dominant mechanism of passive DPF regeneration.

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 by oxygen can be described by one of the following reactions:

C + O₂ → CO₂(1)

C + ½O₂ → CO(2)

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 that provide relatively short particle residence times. The oxidation of soot requires diesel filters that extend the residence time by physically capturing the particulates.

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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