Microwave Heating Assisted Biorefinery of Biomass

Microwave Heating Assisted Biorefinery of Biomass

Sherif Farag (École Polytechnique de Montréal, Canada) and Jamal Chaouki (École Polytechnique de Montréal, Canada)
DOI: 10.4018/978-1-4666-8711-0.ch005
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Abstract

This chapter debates the potential of the biorefinery of biomass using microwave heating. First, the essential information regarding electromagnetic radiation is explained and the pros and cons of microwave heating versus conventional heating, especially in the thermochemical treatment of biomass, are discussed. Different methodologies for predicting and measuring the temperature gradient within a material subjected to electromagnetic waves are demonstrated. The chapter summarizes the key conclusions of various investigations regarding the effects of microwave heating on chemical reactions and presents how electromagnetic radiation can assist the biorefinery of biomass. Finally, the issues and limitations regarding scaling-up microwave heating are elucidated, along with possible solutions to these problems.
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Introduction

Fundamentals of Electromagnetic Waves

Electromagnetic radiation behaves like waves moving at the speed of light and photons carrying radiated energy. Electromagnetic waves are comprised of both alternating electric and magnetic fields that are orthogonal to each other and propagate in the direction of oscillation, as shown in Figure 1.

The history of electromagnetism dates back to 1831, when Michael Faraday discovered electromagnetic induction. Then, James Clerk Maxwell commenced working on Faraday’s concept, and in 1864 presented a mathematical framing to explain the combined impact of electric and magnetic fields that later became known as the electromagnetic theory. In Maxwell’s original paper, entitled “A Dynamical Theory of The Electromagnetic Field,” published in 1865, electromagnetic theory was presented through 20 equations. Thereafter, these equations were simplified into the forms known today as “Maxwell’s four equations”. The differential and integral forms of these equations are expressed in Equations (1) to (4). Equation (1) is Gauss’s law for an electric field, Equation (2) is Gauss’s law for a magnetic field, Equation (3) is Faraday’s law of induction, and Equation (4) is Ampere’s law (Gupta & Eugene, 2007):

Differential form:

Integral form: ∇∙D = ρ

(1)

Differential form:

Integral form: ∇∙B = 0

(2)

Differential form:

Integral form: ∇×E = ˗B/∂t

(3)

Differential form:

Integral form: ∇×H = J+D/∂t

(4) where E refers to the electric field strength [V/m], H the magnetic field strength [A/m], D the electric flux density [C/m2], B the magnetic flux density [Wb/m2], J the current density [A/m2], v the velocity [m/s], and ρ the charge density [C/m3].

An electromagnetic spectrum covers a wide range of frequencies and a corresponding range of wavelengths. Each of the electromagnetic applications (radio waves, microwaves, infrared, visible light, ultraviolet radiation, X-ray, and gamma ray) holds a specific frequency to avoid overlapping each other. As illustrated in Figure 2, microwaves take a place in the electromagnetic spectrum, from 300 MHz to 300 GHz of frequency and from 1 m to 1 mm of corresponding wavelength. Throughout the 20th century, the rapid development of electromagnetic waves’ technology has established microwaves in a number of implementations, such as communications, navigation, radar detection, power transmission, and microwave heating. Nowadays, microwave heating (MWH) is being widely implemented in both small scale applications, such as domestic microwave ovens, and large scale ones, as is evident in a number of industrial sectors. This is a consequence of the increased attention in research on microwave heating, which is manifest in the potential of publications in scientific literature. This chapter briefly presents several aspects, which are the result of replacing classical heating by electromagnetic radiation. If further details are needed, kindly refer to the additional readings at the end of this chapter.

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