Microwave heating is an energy transfer process in which an electromagnetic field, or microwaves, which are generated from a magnetron (similar to the one in a home microwave), excites molecules within feedstock, causing the particles to become excited and physically move, generating friction and heat. This heating process is volumetric in that heat is generated from within the feedstock, selective in that only the feedstock is heated and not the airspace or reactor body, instantaneous in that microwaves transfer energy to the feedstock at the speed of light, and rapid in that several hundred degrees can be obtained in seconds.
A magnetron produces microwaves. Today’s magnetrons are a result of inventions and improvements by several people including the original invention by Albert Hull while at General Electric in the 1920s, later Arthur Samuel at Bell Laboratories, John Randall and Harry Boot at The University of Birmingham, and Percy Spencer who modernized magnetrons while at Raytheon, the US Navy, MIT, and who famously invented the microwave oven for home cooking by investigating why a chocolate bar melted in his pocket.
Technically a diode, a cavity magnetron is a high-powered self-excited microwave oscillator vacuum tube. In essence, electrical energy pushes electrons through a magnetic field which generates electromagnetic radiation - microwaves. Energy is lost pushing the electrons through the magnetic field, energy loss increases proportional to the frequency of the magnetron while it decreases proportional to the power capacity of the magnetron.
Microwaves are electromagnetic energy located between infrared and radio waves on the electromagnetic spectrum with a frequency between 300 and 300,000MHz and a wavelength measuring between 1cm to 1 meter. The majority of commercial and industrial microwaves operate at either 2450 MHz (2.45GHz) or 915 MHz. Puraloop’s 100D system used 2.45GHz with a 12.2cm wavelength, identical to a microwave oven, whereas the 5C commercial system uses 915MHz with a 32.8cm wavelength. 915MHz microwaves are more complicated to use due to their increased sensitivity to interference by reflective materials such as metal, but are more energy efficient (85-95% electrical to microwave energy conversion efficiency, compared to 65-75% for 2450MHz magnetrons) and can be much larger – a single 915MHz magnetron is 20kW to 100kW, compared to a 2450MHz magnetron which is often 10kW or less. Below, an electromagnetic wave.
ENERGY TRANSFER & MICROWAVE HEATING PROCESS
Microwave heating, also called dielectric heating, is an energy transfer process, unlike conventional thermal heating which is a heat transfer process. In this process, a magnetron first converts electricity into microwaves, which then travel through a waveguide into a reaction chamber. A waveguide is a metal box in which microwaves bounce of the surface, being formed into an ideal electromagnetic field as they move through the box towards the reactor. When feedstock, such as sludge, is exposed to this electromagnetic field inside the reactor, two processes occur which produce heat: ionic conduction and dipolar polarization.
Ionic conduction, which generates the most heat, occurs when charged particles (ions, atoms with missing or extra electrons) within the feedstock are moved though the feedstock by the electric component of the electromagnetic field, similar to how an electric field causes an electric motor to rotate. As these charged particles move around they physically collide with other moving particles, causing friction and heat. Rate of heat generation from ionic conduction increases as the feedstock’s temperature increases.
The second process, dipolar polarization, shown in the image to the right, occurs when molecules with strong north/ south poles, such as water, attempt to align with the rapidly oscillating electromagnetic field. A 2.45GHz field oscillates at the same rate water molecules takes to rotate 180 degrees, causing water molecules to constantly rotate while minimizing energy loss. The peaks and valleys of an electromagnetic field’s “waves” are actually the field’s positive/negative charge oscillating, or flipping back and forth. Water molecules rotate 4.9 billion times per second under microwave irradiation.
ENERGY TRANSFER EFFICIENCY
The term absorb microwaves is used to describe the two energy transfer processes defined above. A feedstock’s ability to absorb microwaves is determined by its Loss Tangent, shown to the right, where ε’ is the dielectric constant of the material, which is the ability of the material to store electromagnetic energy, and ε”, the dielectric loss factor, which is the ability of the material to convert stored electromagnetic energy into thermal heat. A feedstock’s Loss Tangent is essentially a function of temperature, moisture content, feedstock density and physical properties, and electric field direction and frequency.
The material’s resulting Loss Tangent is either high (tan δ = >0.5), medium (tan δ = 0.1 to 0.5) or low (tan δ = <0.1)107. For example, water has a Loss Tangent of 0.118 (2450MHz at 25 °C) while carbon materials such as activated carbon and charcoal are 0.1 to 0.8 on average. Fused quarts and silica materials such as quartz glass have extremely low Loss Tangents (0.0002 to 0.00006)108 and as a result are essentially invisible to microwaves, making them ideal for structural components inside a reactor.
The lower the material’s loss tangent, the more microwave energy is needed to achieve the same temperature, as the material is less efficient at absorbing microwave energy and converting it into thermal heat energy. Loss tangent is dynamic and changes as the feedstock’s temperature, moisture, and volatile content changes throughout the pyrolysis process.
A higher loss factor is better as more microwave energy can be converted into thermal energy. The loss factor is highly positively correlated with moisture content, dry sludge is less efficient at converting microwave energy into thermal energy than wet sludge. For example, sludge with 60% DS, thus 40% moisture content, will be over 1,000 times more efficient at converting microwave energy into thermal energy than dry sludge, shown in the diagram below.
Microwave to thermal conversion efficiency can be defined by the following expression:
Where p is the power conversion per unit of volume (W/cm3)
f is the microwave frequency in GHz
ε” is the relative dielectric loss factor
E is the dielectric field strength (V/cm).
Generally, only materials with a high loss tangent are suitable for microwave pyrolysis applications, otherwise an extremely high electric field would be necessary to provide enough microwave energy for the poorly receptive feedstock to absorb. Adding a microwave receptor is described below.
DYNAMIC LOSS TANGENT
As the pyrolysis process begins moisture is evaporated from the feedstock, leading to increased penetration depth of the microwaves into the feedstock but a dramatically lower (1,000x) Loss Tangent (ability to convert microwave energy into thermal energy). As volatiles are released from the feedstock char begins to form, leading to a rapid recovery in Loss Tangent. When a sufficient temperature is attained during the drying and initial pyrolysis phase, complete pyrolysis will occur within a short amount of time, as the newly produced char takes up the energy conversion duties which were lost when the moisture was evaporated. If a sufficient temperature to continue pyrolysis is not attained quickly, then either 1) complete pyrolysis is not attainable, 2) the residency time of the feedstock will increase until complete pyrolysis occurs, or 3), a microwave absorber is added to the feedstock. This microwave absorber will convert microwave energy into thermal energy which is then transferred via convection and conduction to the feedstock. Absorbers include as SiC, char (resulting from pyrolysis of sludge or plastics), activated carbon, charcoal, or glycol. Pyrolysis char is a superior microwave absorber relative to Activated Carbon.
The relationship between moisture content, tangent loss, and penetration depth is shown below. For a given material at a given temperature, as moisture content reduces Loss Tangent also reduces while penetration depth increases.
MIXING LOW & HIGH LOSS TANGENT MATERIAL
Pyrolysis of low loss tangent material is still viable when it is mixed at a proper ratio with high loss tangent material, a process which also increases the heating uniformity and energy conversion ratio of the high loss tangent material110. For example, a mixture of 1:5 activated carbon (very high loss tangent) to quartz sand (very low loss tangent, effectively transparent) had superior heating characteristics compared to pure activated carbon. This is because decreasing the effective bulk loss tangent increases the microwave penetration depth while power dissipation capacity is decreased – meaning microwaves penetrate deeper into the material while this energy cannot be dissipated, leading to higher thermal temperatures i.e. a higher energy efficiency.
CHARACTERISTICS OF MICROWAVE HEATING IN A PYROLYSIS REACTOR
Microwave heating occurs from within the feedstock, described as volumetric heating, the inverse of thermal conventional heating (e.g. an oven) which occurs from the outside into the feedstock, shown in the images below. Microwave heating is also selective heating in that only the feedstock material is heated, unlike conventional thermal heating in which heating coils are heated followed by the reactor body, the airspace, and finally the feedstock.
The heating rate of feedstock, and changes to the heating rate, are near instantaneous due to heat being generated by microwaves which travel near the speed of light. Whereas in conventional pyrolysis, which uses thermal heating, a time gap is experienced between changes in input energy and changes in feedstock heating rate, during microwave heating changes in microwave power instantly affect the feedstock’s heating rate, enabling high controllability of microwave pyrolysis’s heating rate.
Microwave heating can be very rapid, in flash pyrolysis the residency time of feedstock in a reactor can be as little as a fraction of a second, up to several seconds - within this time the temperature of feedstock can increase by several hundred degrees. Importantly, because of the speed at which microwaves interacts with the feedstock’s molecules, they do not have time to relax, consequently, heat generated within the feedstock can be much higher than the feedstock’s external temperature when using an IR or physical (e.g. k-type thermocouple) temperature sensor, a phenomenon known as instantaneous localized superheating.
PYROLYSIS AS A RESULT OF MICROWAVE HEATING IN A REACTOR
The complex chemical and physical reactions which occur as a result of microwave heating are described as a physiochemical decomposition, reforming, melting, or thermal degradation process, among other terms. Due to these reactions occurring within an anaerobic environment (no oxygen), the thermal energy cannot lead to burning of the feedstock, instead, the thermal energy is consumed via the physical separation (decomposition, reforming, etc.) of the feedstock’s compounds, via reduction reactions, into small compounds such as H2 and CO. This separation, or cleaving, of the feedstock’s molecular bonds occurs because the thermal energy is greater than the strength of the molecular bonds. An important note is that pyrolysis is a chemical process, it is not simply a phase change, the feedstock does not “evaporate” into a gas.