Microwave Regenerated Filters

W. Addy Majewski

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• Introduction
• Microwave Heating
• Diesel Filter Experience

Introduction

Microwave heating has been evaluated as a means of supplying heat to diesel particulate filters to trigger DPF regeneration. Microwave regeneration was proposed already in the early 1990s [321], but the concept has never reached technical maturity and has not been used in commercial DPF systems.

The main advantage of microwave heating is its capability to deposit energy directly into diesel particulates which are collected throughout the volume of the filter. This is very different from the electric or fuel burner regeneration, where heat is normally supplied with the regenerating gas, starting from the inlet face of the soot-laden filter. If a satisfactory degree of control over the RF energy deposition was achieved, microwave regeneration could offer more flexibility in designing and controlling the regeneration than, for example, resistive electric heaters.

Some studies have also suggested that a regeneration enhancement effect can be triggered by microwaves [323]. Soot burn-off temperatures measured with microwaves were up to 200°C lower than those needed with electric heating, indicating that diesel soot is particularly susceptible to microwave-assisted incineration. More research would be required to explain the nature of this phenomenon.

On the other hand, the application of microwave heating for DPF regeneration faces a number of challenges:

• Microwave heating produces uneven energy deposition patterns that depend of the geometry of the system, and are challenging to control. As a result, microwave regenerated filters are prone to irregular regeneration patterns—areas of incomplete regeneration may be present along with spots of excessive exothermal heat release leading to substrate damage.
• Microwave leakage was encountered in some studies. Leaks of microwave energy can cause interference with external electronic circuits and decrease energy efficiency. Faraday cages were build around experimental systems in order to arrest microwave leaks.
• The efficiency of microwave heating is lower than that of resistive heating, because of the extra step of converting electrical energy into electromagnetic waves. On a vehicle, where electricity is generated from fuel energy, microwave regenerated systems would likely show a higher fuel economy penalty.

Microwave Heating

Process Principle

Microwaves, also known as the radio frequency (RF) waves, are electromagnetic waves with frequencies in the range of 500 MHz to 100 GHz. Microwaves have been used primarily in the field of telecommunications. After the invention of the cavity magnetron, an efficient high frequency microwave generator, microwaves have also been used for heating applications with the first microwave ovens introduced to the market in the late 1960s.

Microwave heating is based on absorption of RF energy by dielectric materials. Molecules of polar components, called susceptors, vibrate when placed in electromagnetic fields. This vibration causes an increase in the molecule's kinetic energy which is dissipated as heat.

The dielectric properties of a material are characterized by its complex permittivity ε [326]:

ε = ε' - i ε''(1)

where the real part (ε') is the dielectric constant and the imaginary part (ε'') is the dielectric loss factor. The dielectric constant is an indication of the amount of energy that can be stored in a material in the form of an electric field, while the dielectric loss factor is a direct measure of how much energy a material can dissipate in the form of heat.

When a material is heated by microwave irradiation, the amount of RF power converted to heat per unit volume of a susceptor is a function of the electric field frequency, its intensity, and the dielectric loss factor, as given by the following equation:

P = 2 π f ε₀ ε'' E²(2)

where:
P - power, W/m³
f - field frequency, Hz
ε₀ - absolute permittivity, 8.854·10^-12 F/m
ε'' - dielectric loss factor, dimensionless
E - electric field intensity, V/m

Materials that exhibit magnetic properties, e.g. ferrite, can be heated by both the electric (E) and magnetic (H) components of an electromagnetic field. RF power associated with the H-field component can be expressed by a relationship similar to Equation (2), in which the dielectric loss factor (ε'') and the electric field intensity (E) are replaced by magnetic loss factor (µ'') and magnetic field intensity (H), respectively.

Microwave heating differs significantly from conventional heating. During conventional heating of a solid material, the heat is first transferred to its surface, typically by a combination of convection and radiation mechanisms. The inside of the material is then heated through conduction from the surface. In microwave heating, however, the energy deposition is concentrated in the material itself, resulting in heating from the inside out.

Another characteristic feature of microwave heating is its selectivity. Some materials are strong absorbers of RF energy, while others are not. Materials with high dielectric constants and high dielectric loss factors are most effectively heated by microwaves. Examples of such materials include water, carbon black, as well as diesel soot. Most ceramic materials, due to their low dielectric constant and loss factor, are practically transparent to RF energy. This class of materials includes cordierite, a ceramic used in many diesel particulate filter substrates, which is virtually transparent to microwaves. Metals, on the other hand, are nearly perfect reflectors of microwave energy. Therefore, metal ducts can be used as waveguides for RF energy.

In many cases, specific data on complex permittivities of materials is difficult to find in the published literature. The data can also be inconsistent, presumably due to the use of different measurement techniques and parameters by different authors. Dielectric properties of some selected materials, including diesel soot, are listed in Table 1. This data, despite some discrepancies, confirms that diesel particulates are an RF absorber, while cordierite is not (this characteristic can be also used for the measurement of soot loading in ceramic diesel particulate filters by microwaves [327]).

The deposition of microwave energy within a heated medium is dependent on the medium geometry, sometimes called an RF cavity (in the case of a diesel filter, the cavity would be defined by the metallic filter container). If RF energy launched down a metal waveguide strikes a metal wall, the energy will be reflected off the wall, setting up a pattern of E- and H-field standing waves. The E-field distribution in a microwave oven can be pictured as a two-dimensional array of E-field peaks of high electrical field intensity and valleys where the field intensity is low. Materials with dielectric loss properties will be heated only when placed in an E-field standing wave peak and not if located in a valley. Consumer microwave oven manufacturers attempt to compensate for this uneven heating by moving the food through the electrical field on a rotating platform and/or by utilizing rotating metal vanes to periodically redistribute the field.

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