Microwave heating applications in environmental engineering—a review

Abstract

This paper presents a review of microwave heating applications in environmental engineering. A number of areas are assessed, including contaminated soil remediation, waste processing, minerals processing and activated carbon regeneration. Conclusions are presented, which identify the areas of potential commercial development as contaminated soil vitrification, volatile organic compounds (VOC) treatment and recovery, waste sludge processing, mineral ore grinding and carbon in pulp gold recovery. Reasons are detailed why other areas have not seen investment into and implementation of microwave heating technology. These include difficulties associated with the scaling up of laboratory units to industrial capacities and a lack of fundamental data on material dielectric properties. This has meant that commercialisation of microwave heating processes for environmental engineering applications has so far been slow. In fact, commercialisation is only deemed viable when microwave heating offers additional process-specific advantages over conventional methods of heating.

Introduction

Since World War II, there have been major developments in the use of microwaves for heating applications. After this time it was realised that microwaves had the potential to provide rapid, energy-efficient heating of materials. The main applications of microwave heating today include food processing, wood drying, plastic and rubber treating as well as curing and preheating of ceramics. Broadly speaking, microwave radiation is the term associated with any electromagnetic radiation in the microwave frequency range of 300 MHz–300 GHz. Domestic and industrial microwave ovens generally operate at a frequency of 2.45 GHz corresponding to a wavelength of 12.2 cm and energy of 1.02×10⁻⁵ eV (Jacob et al., 1995). However, not all materials can be heated rapidly by microwaves. Materials may be classified into three groups, i.e.conductors, insulators and absorbers (Church, 1993). This classification is illustrated in Fig. 1. Materials that absorb microwave radiation are called dielectrics, thus, microwave heating is also referred to as dielectric heating. Dielectrics have two important properties (Oespchuck, 1984).

• They have very few free charge carriers. When an external electrical field is applied there is very little charge carried through the material matrix.

• The molecules or atoms comprising the dielectric exhibit a dipole movement.

A dipole is essentially two equal and opposite charges separated by a finite distance. An example of this is the stereochemistry of covalent bonds in a water molecule, giving the water molecule a dipole movement. Water is the typical case of a non-symmetric molecule. Dipoles may be a natural feature of the dielectric or they may be induced (Kelly and Rowson, 1995). Distortion of the electron cloud around non-polar molecules or atoms through the presence of an external electric field can induce a temporary dipole movement. This movement generates friction inside the dielectric and the energy is dissipated subsequently as heat.

The interaction of dielectric materials with electromagnetic radiation in the microwave range results in energy absorbance. The ability of a material to absorb energy while in a microwave cavity is related to the loss tangent of the material. This depends on the relaxation times of the molecules in the material, which, in turn, depends on the nature of the functional groups and the volume of the molecule (Gabriel et al., 1998). Generally, the dielectric properties of a material are related to temperature, moisture content, density and material geometry (Metaxas and Meredith, 1993).

An important characteristic of microwave heating is the phenomenon of ‘hotspot’ formation, whereby regions of very high temperature form due to non-uniform heating (Hill and Marchant, 1996). This thermal instability arises because of the non-linear dependence of the electromagnetic and thermal properties of the material on temperature (Reimbert et al., 1996). The formation of standing waves within the microwave cavity results in some regions being exposed to higher energy than others. This results in an increased rate of heating in these higher energy areas due to the non-linear dependence. Cavity design is an important factor in the control, or the utilisation of this hotspot phenomenon.

Microwave energy is extremely efficient in the selective heating of materials as no energy is wasted in ‘bulk heating’ the sample. This is a clear advantage that microwave heating has over conventional methods (bulk heating in furnaces).

Microwave heating processes are currently undergoing investigation for application in a number of fields where the advantages of microwave energy may lead to significant savings in energy consumption, process time and environmental remediation.

Compared with conventional heating techniques, microwave heating has the following additional advantages (Ontario Hydro Technologies website, 2001).

• Higher heating rates;

• no direct contact between the heating source and the heated material;

• selective heating may be achieved;

• greater control of the heating or drying process;

• reduced equipment size and waste.

The principal areas of study reviewed in this paper are minerals processing, waste treatment, contaminated soil remediation, recycling of rubber tyres, activated carbon applications and the treatment of volatile organic compounds (VOCs).