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Applied Energy 81 (2005) 85–113

APPLIED ENERGY
www.elsevier.com/locate/apenergy

Microwave technology for energy-efficient processing of waste
T.J. Appleton, R.I. Colder, S.W. Kingman, I.S. Lowndes *, A.G. Read
Nottingham Mining and Minerals Centre, School of Chemical, Environmental and Mining Engineering, University of Nottingham, University Park, Nottingham NG7 2RD, UK Available online 11September 2004

Abstract This paper discusses the potential use of microwave technology as an energy-efficient alternative to current heating technologies employed in the processing and treatment of waste. The process applications considered are the treatment and control of specific and often problematic waste-streams, including scrap tyres and plastics, and the remediation of contaminated landand groundwater. Where appropriate, the paper highlights the technical and economic factors that promote or inhibit the development and application of a specific technology. It is concluded that there is significant potential for microwave technology to be employed as an alternative heating source in the treatment of waste streams and environmental remediation. However, several major limitationsprevent these technologies from being widely employed. These include the absence of sufficient data to quantify the dielectric properties of the treated waste streams, and technical difficulties encountered when upgrading successful laboratory or pilot-scale processes to the industrial scale. Ó 2004 Elsevier Ltd. All rights reserved.
Keywords: Microwaves; Waste treatment; Tyres; Plastics; Contaminatedland; Emissions

*

Corresponding author. Tel.: +44 115 951 4086. E-mail address: ian.lowndes@nottingham.ac.uk (I.S. Lowndes).

0306-2619/$ - see front matter Ó 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.apenergy.2004.07.002

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T.J. Appleton et al. / Applied Energy 81 (2005) 85–113

1. Introduction Heating is one of the most common processes employed by the process andmanufacturing industries. It is widely used for drying or for promoting chemical or physical change within the food, chemical, textile and engineering sectors. There are several electro-heat technologies such as induction, radio frequency, direct resistance, infrared and microwave heating, which all utilise specific parts of the electromagnetic spectrum. Microwave heating has long been employed in thefood and rubber industries, but there is a growing interest for its potential use in other industrial processes, such as the treatment of various waste-streams and the processing of mineral ores. Microwave heating is a potentially attractive technique as it provides a volumetric heating process at improved heating efficiencies as compared with conventional techniques. If the microwave process iscontrolled correctly, uniform heating within the material can be obtained. Heat is generated volumetrically within the material rather than from an external source. However, materials differ in their response to microwave heating. Materials possess different optimum frequencies that can be measured and not all materials absorb microwaves. Some materials reflect or appear transparent to microwaves and arethus less responsive to heating. Materials that absorb microwaves are known as dielectrics and possess two important properties [1]:  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. The microwave frequency range liesbetween 300 MHz and 300 GHz, with most microwave applications falling between 3 and 30 GHz. In industry, microwave heating is performed at either a frequency close to 900 MHz or at 2450 MHz [2]. A materialÕs ability to couple with (i.e., absorb) microwave energy is determined by its loss factor. Insulating materials do not store microwave energy in the form of heat, as the incident waves travel...