MICROWAVE-BASED PYROLYSIS REACTOR AND ASSOCIATED METHODS

A method for processing waste using pyrolysis reactor include generating a microwave signal; amplifying the microwave signal into a plurality of drive signals; measuring temperature of the waste; determining, based on measured temperature, at least one phase-control signal; and outputting the at least one phase-control signal to maximize power transfer to the waste.

BACKGROUND

Pyrolysis is the process of heating waste in an inert environment to induce thermal decomposition of the waste. This thermal decomposition changes the chemical composition of the waste, resulting in by-products with a host of industrial and commercial uses. When the waste contains organic matter, any resulting solid residue is typically high in carbon, taking the form of char. However, pyrolysis may also generate gaseous and liquid by-products, depending on the composition of the waste. Applications of pyrolysis include the conversion of biowaste to syngas and biochar, the production of coke from coal, the production of charcoal from wood, the conversion of ethane to ethylene (i.e., “cracking”), and waste management.

SUMMARY

Pyrolysis has been investigated as a feasible approach to processing municipal solid waste. However, such waste typically has a high moisture content (often in excess of 70%), which slows the heating of the waste. One way to improve the heating rate of pyrolysis is to use microwaves (i.e., electromagnetic waves in the microwave region of the electromagnetic spectrum). Water is an excellent absorber of microwaves, converting the absorbed microwaves into heat. This same principle is used in microwave ovens to heat food (through its water content) much more rapidly than through conventional ovens. Some waste materials (e.g., plastic) are fairly transparent to microwaves, allowing the microwave energy to penetrate the waste and heat it from within. Other waste materials (e.g., food) are not as transparent to microwaves, yet may become increasingly transparent as they are heated and broken down.

In the military, current methods for waste management depend on the type of waste and platform. For example, the United States Navy is permitted to throw certain materials overboard ships (e.g., paper rags, glass, metal, food when further than twelve nautical miles from land, ground-up food waste when further than three nautical miles from shore). An incinerator using JP-5 jet fuel is used for paper, rags and wood, wasting weight and cost related to burning jet fuel to dispose of garbage. In the case of US Navy submarines, all possible waste is eliminated prior to deployment, while on-board waste is pumped or blown out. Other solutions include storing waste in frozen food lockers, burial in the field, and pyrolysis.

Thermochemical processes, pyrolysis, incineration, gasification, etc., have been used for waste management. Unlike incineration and gasification, pyrolysis does not directly process the waste for energy production, but converts it into intermediary compounds that can be used for up-conversion, fuel, or environmentally safe products that can be used for fertilization. Another process for upconverting waste materials is hydrothermal carbonization. Near-supercritical and supercritical fluid have been successfully used for carbonization, gasification, liquefaction, and upgrading of hydrocarbon resources including crude oils, microalgae, lignocellulosic biomasses, food wastes, and plastics. Combined hydrocarbon and nitrogen compounds can be successfully recovered from human biowaste.

Most of the work in the waste sector uses convection-based heating methods. However, microwave heating offers improved thermal efficiency and reaction speed. Microwave heating uses Joule losses of the electric field, combined with electrical conductivity of the waste material at GHz frequencies, to increase waste temperature. Because of the high penetration depth of electromagnetic waves in the radio-frequency spectrum, this type of heating may be confined within a volume possibly without damage or overheating of the surface, which is difficult to achieve with convection heating. This localized heating also improves system efficiency by directly producing heat inside the intended material, and therefore the heating rate is not limited due to thermal conductivity to an external heat source and the heat capacity of the material.

The present embodiments feature pyrolysis reactors that use microwaves to rapidly heat waste materials. At the heart of these pyrolysis reactors are microwave cavities within which the waste materials are located. Electromagnetic modes of the cavities are excited by electromagnetic fields that are coupled into the cavity by one or more microwave probes (or launchers) driven by an external microwave source. The present embodiments advantageously use a number of techniques to ensure appropriate distribution of the electromagnetic energy throughout the cavity, thereby ensuring that the waste materials are rapidly and more uniformly heated (as compared to operation without these techniques). As described in more detail below, these techniques include overmoding of the cavity, outphasing, amplitude modulation, and frequency modulation. Unlike many prior-art pyrolysis reactors, the present embodiments are not adversely affected by the moisture content of the waste, and may even perform better when water is intentionally added to the waste.

The pyrolysis reactors of the present embodiments feature spatial power combining, within the cavity, of electromagnetic power from several solid-state sources. By comparison, magnetron sources (e.g., as typically used in conventional microwave ovens) are usually not power combined since it is difficult to control the relative frequency and phase. Solid-state sources do not achieve as high power as magnetrons, but with spatial power combining, high microwave power levels (e.g., up to tens of kilowatts) can be achieved inside the cavity using solid-state amplifiers. To increase efficiency, the amplifiers can be operated nonlinearly. The microwave cavity can be excited using multiple probes that are all driven by solid-state power amplifiers, which are excited by one or more microwave oscillators. Dynamic relative phase shifting of the power amplifier outputs can be used to improve heating efficiency of the waste in the cavity.

In some embodiments, a radio frequency (RF) applicator delivers electromagnetic energy to a treatment volume containing an emulsion of a hydrocarbon and a substrate, which facilitates separation of the hydrocarbon and substrate molecules into layers. Hydrocarbons and other products can then be recovered from their respective layers (e.g., see U.S. Pat. No. 7,486,248). Microwave heating has also been shown to be much faster than conventional resistance heating in hydrothermal treatments.

The present embodiments can be used to process both large and small quantities of waste mass, and are therefore scalable while maintaining high energy conversion efficiency. This scaling can be implemented via (1) selection of the cavity size and frequency, (2) microwave power density by power combining the outputs of multiple solid-state amplifiers, (3) overall power using solid-state amplifiers, (4) time-domain profile when the power is on, and (5) mode-mixing through one or more of mechanical means, frequency modulation, and varying the cavity boundary conditions.

DETAILED DESCRIPTION

FIG.1is a perspective view of a microwave-based pyrolysis reactor100that uses electromagnetic energy to induce thermal breakdown of a waste mass102. Waste mass102may include one or more of several types of waste, such as biowaste (e.g., food scraps, vegetation, animal waste), wood-based products (e.g., paper, wood), metals (e.g., aluminum), Styrofoam, plastic, crude oils, microalgae, and lignocellulosic biomasses. Advantageously, waste mass102may contain liquids, such as water, that rapidly absorb electromagnetic energy in the microwave region of the electromagnetic spectrum, and increase the speed with which waste mass102is heated. An output chute104may be used to collect solid and liquid byproducts as they form and drop to the bottom of pyrolysis reactor100. Gaseous byproducts may be collected with a pump142that evacuates an inner volume112of pyrolysis reactor100via a pump port106. Examples of gaseous byproducts include particulate matter, volatile organic compounds, polynuclear aromatic hydrocarbons, methane, ammonia, carbon monoxide, hydrogen sulfide, hydrogen cyanide, and sulfur oxides. While waste mass102is shown inFIG.1as a rectangular prism, it may alternatively have a different shape, provided that it fits within inner volume112.

Pyrolysis reactor100includes a microwave cavity110formed by electrically conductive walls114that surround inner volume112. Electrically conductive walls114may be made of metal (e.g., copper, silver, gold, aluminum, etc.). Microwave cavity110supports a plurality of electromagnetic modes that store electromagnetic energy when excited. InFIG.1, microwave cavity110is a cylinder in which the conductive walls114form a cylindrical shell having a length in the z direction (see right-handed coordinate system150) and a radius in the transverse x-y plane. However, microwave cavity110may have a different geometry without departing from scope hereof. For example, microwave cavity110may be shaped as a rectangular cuboid, a square cuboid, or another type of hollow three-dimensional prism.

Microwaves are coupled into the microwave cavity110through one or more probes122, each driven by a corresponding amplifier120. In the example ofFIG.1, first and second probes122(1),122(2) couple electromagnetic energy into the microwave cavity110, where waste mass102absorbs the electromagnetic energy, converting it to heat that increases the temperature of waste mass102and results in breakdown of waste mass102. As described in more detail below, power to each amplifier120may be optimized to increase the amount of electromagnetic power delivered to waste mass102, in turn increasing the speed at which waste mass102increases in temperature and therefore breaks down. More than two probes122(e.g., four, ten, twenty-eight, fifty or other number) may be used without departing from the scope hereof. Furthermore, whileFIG.1shows first and second probes122(1),122(2) located on opposite sides of microwave cavity110(in the x direction) and at the same z position, probes122may be located at different positions without departing from the scope hereof.

Probes122are driven with an electrical microwave signal131that is generated by a frequency source130and split with a power splitter132. Microwave signal131may have a single frequency selected to resonate with one of the modes of microwave cavity110. For example, the frequency may be selected to coincide with the lowest-frequency fundamental resonance of microwave cavity110(when the cavity is empty, i.e., not loaded). However, to increase the spatial uniformity of the electric field within microwave cavity110, microwave cavity110may be overmoded, i.e., driven at a frequency higher than that of the fundamental resonance to excite several modes. In some embodiments, microwave signal131contains a plurality of frequencies.

Since amplifier linearity is not a relevant specification for microwave-based pyrolysis, each amplifier120(1),120(2) may be a high-efficiency solid-state amplifier based on GaN or LDMOS power transistors. Each amplifier120(1),120(2) may output several watts of power, or more, depending on the volume and contents of waste mass102.

The frequency of microwave signal131may be selected near 2.45 GHz or 900 MHz, where the electromagnetic energy has sufficient penetration depth given the electrical conductivity of most waste materials. Therefore, the use of these frequencies may help ensure uniform heating and even thermal breakdown throughout all of waste mass102. In some embodiments, the frequency of microwave signal131lies within the Industrial, Scientific, and Medical (ISM) band between 2.4 and 2.5 GHz. In these embodiments, each amplifier120(1),120(2) may be based on GaN power transistors. In other embodiments, the frequency of microwave signal131lies within the ISM band between 915 and 928 MHz. In these embodiments, each amplifier120(1),120(2) may be based on either GaN power transistors or LDMOS power transistors However, in general, the frequency of microwave signal131is not limited to only these bands, provided that the cavity is shielded.

Given that pyrolysis reactor100may operate with powers up to tens of kilowatts, or more, operation at frequencies within ISM bands advantageously minimizes the likelihood of generating interference that can disrupt telecommunications. In some embodiments, pyrolysis reactor100operates at multiple frequencies simultaneously. For example, first probe122(1) may be driven near the ISM band at 915 MHz, while second probe122(2) is driven near the ISM band at 2.45 GHz. The use of multiple frequencies can help ensure uniform heating given the different absorption properties of materials within waste mass102, and how these absorption properties change as the waste mass102breaks down. Additional ISM bands are centered at 5.8 GHz, 24.125 GHz, 61.25 GHz, 433.92 MHz, 40.68 MHz, etc. Additional probes122may be driven at frequencies within these additional ISM bands to further increase the frequency diversity of the electric field within microwave cavity110. Since the number of transverse-electric (TE) and transverse-magnetic (TM) modes within the microwave cavity110scales as f3, where f is the frequency of the microwave signal131, operating microwave cavity110at higher frequencies can advantageously facilitate overmoding. However, a higher frequency usually results in a lower efficiency of solid-state amplifiers120, and less penetration of the waste due to a smaller skin depth. Thus, there is a trade-off between overmoding and efficiency that can be evaluated based on the design requirements.

FIG.2is a perspective view of a microwave-based pyrolysis reactor200that uses time-varying outphasing to increase the efficiency with which power is coupled out of amplifiers120(1),120(2) and into waste mass102. Pyrolysis reactor200is similar to pyrolysis reactor100ofFIG.1except that it includes a signal processing module210that controls a variable phase shifter214with a phase-control signal208to change the relative phase Δϕ between first and second drive signals206(1),206(2) that are fed to first and second amplifiers120(1),120(2), respectively. The relative phase Δϕ is selected to maximize constructive interference between the electromagnetic fields near waste mass102, which in turn maximizes the power density and thus absorption by waste mass102. Additionally, outphasing can improve amplifier efficiency when the load is varying.

To determine the relative phase Δϕ, signal processing module210monitors waste mass102to determine how microwave properties (e.g., real and imaginary components of the permittivity) change as waste mass102heats and breaks down. Thus, waste mass102may be thought of as a microwave load that varies spatially and temporally. To determine changes in the microwave load, signal processing module210may use a plurality of thermocouples212to record temperatures of waste mass102at various positions, both at the surface of waste mass102and inside waste mass102. Signal processing module210may additionally or alternatively use one or more infrared cameras (e.g., infrared camera207as shown) to record a temperature distribution of waste mass102. Infrared camera207may view waste mass102via a viewport204. Signal processing module210processes this temperature data to determine the optimum phase shift AO, and then outputs corresponding phase-control signal208. Signal processing module210may operate as part of a feedback loop.

In one embodiment, pyrolysis reactor200uses radiometry at microwave frequencies to measure the properties of waste mass102at depths of several centimeters. This embodiment advantageously overcomes limitations of infrared camera207, which can only detect the temperature of waste mass102to within a few millimeters of its surface. Radiometry may also provide a greater density of spatial temperature measurements than can be implemented with the plurality of thermocouples212. In this embodiment, pyrolysis reactor200additionally includes a radiometer209for receiving the emitted black-body radiation in the microwave region of the spectrum where the skin depth in waste is comparable to the waste dimensions.

AlthoughFIG.2shows pyrolysis reactor200with two probes122(1),122(2), pyrolysis reactor200may include a plurality of n probes122, where n is an integer greater than or equal to 2. In general, the output of frequency source130is split into n drive signals206that are amplified by n corresponding amplifiers120to drive the n probes122. Signal processing module210processes the temperature data to determine n−1 optimum phase shifts Δϕ1, . . . , ϕn-1for maximizing power absorbed by waste mass102. Signal processing module210then outputs n−1 corresponding phase-control signals208that control corresponding n−1 variable phase shifters214. Since only relative shifts need to be controlled for outphasing, one of the n drive signals206does not need to be phase shifted.

FIG.3illustrates how microwave-based pyrolysis reactor200ofFIG.2can be additionally configured to modulate the electromagnetic power coupled to each of probes122. For clarity of illustration, thermocouples212, infrared camera207, and variable phase shifter214are not shown inFIG.3. Signal processing module210processes received temperature data to determine n amplitudes, or power levels, at which each the n probes122should be driven to control the power absorbed by waste mass102in addition to outphasing. Advantageously, variation of the power levels throughout the pyrolysis process can be used to maintain high efficiency as the electrical properties of waste mass102changes as it breaks down. Such a variation in power levels is difficult to implement in a conventional magnetron-based microwave reactor.

As shown inFIG.3, each amplifier120(1),120(2) includes a power amplifier320(1),320(2) driven by a corresponding driver amplifier322(1),322(2). The main purpose of each driver amplifier322is gain, not efficiency. However, if the gain of the corresponding power amplifier320is low, then the efficiency of driver amplifier322affects overall efficiency. Each power amplifier320and driver amplifier322is powered by a DC power supply326via a corresponding modulator324. Each modulator324is controlled via a corresponding modulation signal326to change the power outputted by corresponding amplifier320,322. In some embodiments, only power amplifiers320(1),320(2) are modulated. In other embodiments, both driver amplifiers322(1),322(2) and power amplifiers320(1),320(2) are modulated. Each amplifier120(1),120(2) may include additional stages than shown inFIG.3, and each of these additional stages may be similarly modulated. WhileFIG.3shows pyrolysis reactor200with n=2 probes122and 2n=4 modulators324, pyrolysis reactor200may have a different integer number n of probes122and k*n modulators (where k is the number of stages used for each amplifier120) without departing from the scope hereof.

To improve electric field uniformity inside microwave cavity110, some embodiments use frequency modulation to implement mode mixing in microwave cavity110. When microwave cavity110is overmoded, a small change in the microwave frequency can greatly change the electric field distribution within the microwave cavity110.FIG.2shows one example of frequency modulation, wherein frequency source130is a voltage-controlled oscillator driven with a frequency-control signal218that changes the frequency of microwave signal131. Frequency-control signal218may be generated by signal-processing module210, as shown inFIG.2, in response to certain detected temperatures, temperature gradients, or temperature distributions. Alternatively, frequency-control signal218may be generated by a circuit separate from signal-processing module210. Frequency-control signal218may be a periodic signal (e.g., sine wave, triangle wave, sawtooth) such that the frequency modulation is periodic. In this case, waste mass102responds to a time-averaged electric field at each spatial point within microwave cavity110, where the time averaging is over one period of frequency-control signal218. To further enhance spatial uniformity, frequency-control signal218may be random (e.g., white noise) or include a random component (e.g., a triangle wave superimposed with white noise).

FIG.4is a side view of a microwave-based pyrolysis reactor400that is similar to pyrolysis reactors100,200ofFIGS.1and2, except that it uses a coaxial microwave cavity410instead of a hollow microwave cavity. A mixing auger422serves as a linear electrical conductor (hereinafter a “center” conductor) of the coaxial microwave cavity410, and may be made of metal with high electrical conductivity (e.g., copper, silver, aluminum, etc.). Mixing auger422includes one or more helical paddles426attached to a spindle424that is rotated by a motor412. As mixing auger422rotates, paddles426mix waste mass102to help insure thermal uniformity of waste mass110. For clarity of illustration, waste mass102is not shown inFIG.4.

In some embodiments, spindle424is made of metal and paddles426are made of a non-electrically conductive material, such as ceramic, that can withstand the high temperatures achieved within pyrolysis reactor400. Therefore, in these embodiments, only spindle424, and not paddles426, serves as the center conductor of coaxial microwave cavity410. In other embodiments, paddles426are also made of metal and are in electrical contact with spindle424. In these embodiments, both spindle424and paddles426serve as the center conductor of coaxial microwave cavity410. WhileFIG.4shows pyrolysis reactor400with two probes122, pyrolysis reactor400may have more than two probes122(e.g., 4, 10, 28, 50, etc.) without departing from the scope hereof.

FIGS.5and6show a microwave-based pyrolysis reactor500that is similar to pyrolysis reactor100ofFIG.1except that it includes a frequency-selective surface502. Frequency-selective surface502is mounted in front of electrically conductive wall514, which may be electrically grounded, to form microwave cavity510(similar to microwave cavity110in earlier figures). As described below, frequency-selective surface502may be configured to appear as an inductive, capacitive, or resonant surface. As a result, the tangential electric field of the cavity's modes do not need to equal zero at frequency-selective surface502(unlike electrically conductive wall514, where the tangential electric field always equal zero). Maxwell's equations can be solved for the electric and magnetic field distributions of the modes satisfying this modified boundary condition. In general, the field distributions of these modes will differ from those for microwave cavity510without the frequency-selective surface502. Thus, frequency-selective surface502provides a mechanism to further alter the energy distribution within inner volume512of cavity510. For example, frequency-selective surface502may be used to create an energy distribution that is spatially more uniform, as compared to the energy distribution of the modes that are obtained when cavity510excludes frequency-selective surface502.

Frequency-selective surface502is shown inFIG.5as a two-dimensional square grid of elements504formed on a dielectric substrate506. In the example ofFIG.5, each element504is a Jerusalem cross formed from metal (e.g., copper). To prevent electrical shorting across elements504(e.g., due to waste mass102, which may be electrically conductive and splatter on frequency-selective surface502), a dielectric overlayer may be placed on top of elements504. Dielectric substrate506(and the dielectric overlayer, when included) may be flexible so that frequency-selective surface502can conform to the curvature of electrically conductive wall514.

Frequency-selective surface502may be configured differently than shown inFIGS.5and6. For example, each element504may be a different type of N-pole element (e.g., straight dipole, tripole, square spiral, etc.), loop-type element (e.g., three-legged loaded element, four-legged loaded element, hexagon element), plate-type element with solid interior (e.g., square, hexagon, circle, etc.), or a combination thereof. Similarly, elements504may be arranged in a different type of grid (e.g., hexagonal, triangular, rectangular, interlaced, closely-packed, etc.). Other design parameters (e.g., element size, spacings between neighboring elements, thickness of dielectric substrate506, etc.) may be selected to achieve a particular boundary condition that gives a desired electric field distribution.

InFIGS.5and6, frequency-selective surface502subtends an azimuthal angle ϕ with respect to a cylindrical axis602of microwave cavity510, and is located a radial distance504in front of electrically conductive wall514(i.e., toward cylindrical axis602). However, frequency-selective surface502may subtend a different azimuthal angle than shown. Furthermore, frequency-selective surface502may have a height, in the z direction, of any value less than or equal to the height of electrically conductive wall514. In one embodiment, frequency-selective surface502azimuthally subtends 360° and has a height equal to that of wall514, thereby fully covering wall514. In some embodiments, an additional frequency-selective surface is placed over one or both of the flat circular top and bottom walls514of cylindrical microwave cavity510. Similar frequency-selective surfaces may be placed in front of electrically conductive walls of a microwave cavity having a different shape. For example, when microwave cavity510is rectangular, one or more planar frequency-selective surfaces may be placed in front of these flat walls of the rectangular cavity510.

In some embodiments, multiple frequency-selective panels502are placed in front of walls514of microwave cavity510. In some embodiments, frequency-selective panels502with different electrical properties are used in different regions of microwave cavity510to account for spatial variations of waste mass102. For example, a first frequency-selective panel502may be used at the top of microwave cavity510(in the +z direction) to create a first electric field distribution that is matched to that portion of waste mass102at the top of the inner volume512. Similar, a second frequency-selective panel502may be used at the middle of microwave cavity510to create a second electric field distribution that is matched to that portion of waste mass102at the middle of the inner volume512. Finally, a third frequency-selective panel502may be used at the bottom of microwave cavity510to create a third electric field distribution that is matched to that portion of waste mass102at the bottom of inner volume512. As an example of how these embodiments may be used, consider waste mass102as it undergoes chemical changes: the resulting char and ash may fall to the bottom of inner volume512(i.e., in the −z direction), where it accumulates. In this case, the third frequency-selective panel502may be designed to optimally heat the char and ash, while the first and/or second frequency-selective panels502may be designed to optimally heat that portion of waste mass102that has not yet been converted to char and ash.

In any of the above embodiments, water may be added to waste mass102within the inner volume112,512to facilitate thermal conduction of heat within waste mass102. The water may be salt water. Alternatively or additionally, small metallic particles (e.g., nanoparticles) may be added to waste mass102(either with or without additional water added) to enhance scattering of the electromagnetic fields throughout waste mass102, thereby improving the uniformity with which the electromagnetic energy is absorbed by waste mass102.

In any of the above embodiments, the microwave cavity110,510may be externally surrounded with thermal shielding, for example in space553, to reduce heat loss, thereby improving the energy efficiency of the pyrolysis reactor. For example, this thermal shielding may be made of a material having low thermal conductivity (e.g., fiberglass, ceramic, Styrofoam, plastic, etc.) to minimize convective heat loss from the microwave cavity to the surrounding environment. For similar purposes, a heat shield (e.g., aluminum foil, steel panels, ceramic thermal barriers) may be placed around the microwave cavity to reduce radiative heat loss. In some embodiments, the microwave cavity is placed within a larger vacuum system that may be evacuated to reduce convective heat loss from the microwave cavity. For example, wall551confines vacuum space553, in which microwave cavity510is placed. Wall551of the larger vacuum system may be made of metal to act as heat shield.

The features described above with respect to the pyrolysis reactors100,200,400, and500may be combined in any way without departing from the scope hereof. Specifically, the present embodiments include all combinations of: the outphasing shown inFIG.2, the frequency modulation shown inFIG.2, the amplifier modulation shown inFIG.3, the use of frequency-selective surfaces shown inFIG.5, and overmoding. Any of these combinations may be further implemented with the mixing auger422to form the coaxial microwave cavity410ofFIG.4.

FIG.7is a flowchart illustrating a method700for processing a waste mixture using a pyrolysis reactor. Method700may be implemented by part or all of any of pyrolysis reactors100,200,400, and500. Method700includes steps740and760. However, in other embodiments method700may further include one or more of steps710,720,730, and750. Method700may be implemented with a pyrolysis reactor storing a waste mass (used interchangeably with “waste mixture” herein) within an internal space of a microwave cavity. In one embodiment, method700includes placing the waste mass (e.g., waste mass102ofFIG.1) into the microwave cavity (e.g., microwave cavity110). The waste mass may include one or more types of waste, such as biowaste (e.g., food scraps, vegetation, animal waste), wood-based products (e.g., paper, wood), metals (e.g., aluminum), Styrofoam, plastic, crude oils, microalgae, and lignocellulosic biomasses. In certain embodiments, the heating profile, as described in steps below, may be configured specifically for the types of waste present in the waste mass. For example, waste mass containing liquid, such as water, absorbs electromagnetic energy in the microwave region of the electromagnetic spectrum more rapidly and, therefore heats up faster, than waste mass with less liquid, and may require a different heating profile. In addition, to facilitate thermal conduction within the waste mass, additives may be added to the waste mass, such as one or more of water, salt water, and metal or dielectric nanoparticles.

In step710, a microwave signal is generated with an oscillator. In one example of step710, probes122ofFIG.1are driven with microwave signal131that is generated by frequency source130. In step720, the generated microwave signal is split into a plurality of drive signals. In general, one oscillator may generate drive signals for all probes in the microwave cavity. For example, inFIG.2, microwave signal131from oscillator130is split with power splitter132into the n drive signals206, where n is an integer greater than 1. Microwave signal131may have a single frequency selected to resonate with one of the modes of microwave cavity110. To increase the spatial uniformity of the electric field within microwave cavity110, microwave cavity110may be overmoded, i.e., driven at a frequency higher than that of the fundamental resonance to excite multiple modes.

Step730includes amplifying the plurality of drive signals into a corresponding one of a plurality of amplified signals. This amplification is for example achieved using a solid-state power amplifier for each probe. In step740, the amplified drive signals are coupled into the microwave cavity to excite a plurality of cavity modes. In one example of step740, one or more probes122, each driven by a corresponding amplifier120, excites microwave modes in microwave cavity110(seeFIG.2). In this example, probes122(1) and122(2) couple electromagnetic energy into microwave cavity110, where waste mass102absorbs the electromagnetic energy, converting it to thermal energy that increases the temperature of waste mass102and results in breakdown of waste mass102. Multiple probes and amplifiers122increase the electromagnetic power delivered to waste mass102, in turn increasing the speed at which waste mass102increases in temperature and therefore breaks down.

Step750includes measuring one or more temperatures of the waste mass. Step750is, for example, performed with thermocouples212inFIG.2. In step760, the temperature measurements are used to determine the relative phases for the variable phase shifters, thereby phase-shifting one or more of the plurality of drive signals. In one example of step760, signal processing module210monitors waste mass102to determine how its microwave properties (e.g., real and imaginary components of the permittivity) change as it heats and breaks down. Thus, waste mass102may be thought of as a microwave load that varies spatially and temporally. To determine changes in the microwave load, signal processing module210may use thermocouples212to record temperatures of waste mass102at various positions, both at the surface and inside waste mass102.

Step750may also include recording a thermal image stream of the waste mixture using an infrared camera. For example, infrared camera207records a temperature distribution of waste mass102through viewport204. Infrared camera, however, penetrates only a few millimeters into the water volume due to the limited skin depth. For this reason, the image stream from the infrared camera may be used in conjunction with thermocouples212placed at various locations and depth of waste mass102.

In certain embodiments, step750includes, instead of or in addition to the infrared camera, a microwave radiometer (e.g., radiometer209inFIG.2) that measures thermally-emitted electromagnetic radiation from waste mass102.

In step760, at least one phase-control signal is determined based on the measured temperatures. By way of example, to determine the phase-control signal, or relative phase Δϕ, processing module210monitors the measured temperatures at various locations including at the surface and inside of waste mass102, to determine how its microwave properties (e.g., real and imaginary components of the permittivity) change as waste mass102heats and breaks down. As waste mass102undergoes changes, the thermal load changes as a function of space and time. As a result, the relative phase is continuously updated to maintain thermal efficiency and uniformity. This process requires accurate measurements of temperature at as many locations in the cavity as possible. In certain embodiments, one or both of an infrared camera (e.g., infrared camera207) and a microwave radiometer (e.g., radiometer209) may be used in conjunction with the plurality of thermocouples to provide better spatial and temporal resolution of the measured temperature profile.

Additionally, in step760, the one or more phase-control signals are outputted to a one or more corresponding variable phase-shifters. In one example of step760, after having determined the relative phase between two probes120(1) and120(2) needed to maintain thermal uniformity and efficiency in cavity110in step740, signal processing module210outputs the relative phase208to variable phase shifter214for probe122(2). For reactors having more than two probes, only the relative phase differences among the plurality of probes are determined. Accordingly, the number of variable phase shifters, and therefore the number of phase-control signals, may be one less than the number of probes. However, a different number of phase shifters (and corresponding phase control signals) may be used without departing from the scope hereof.

From step760, the phase-adjusted drive signal from each phase-shifter is fed into a corresponding power amplifier of step730. Accordingly, steps730,740,750and760may be iterated until the waste is fully converted into one or more intermediary compounds that can be used for up-conversion, fuel, or environmentally safe products that can be used for, for example, fertilization.

Method700may include additional steps without departing from the scope hereof. As part of maintaining thermal uniformity of the waste mass throughout the thermochemical process, method700may include, for example, stirring the waste mixture with an auger located at least partially within the internal space of the microwave cavity. For example, mixing auger422may be rotated by motor412to mix waste mass110, thereby improving thermal uniformity of waste mass110.

Method700may also include additional or alternate steps to maintain an optimal level of power transfer, or heating efficiency, for efficient breakdown of waste. One such step includes improving electric field uniformity inside the microwave cavity by implementing mode mixing using frequency modulation. When the microwave cavity is overmoded, a small change in the frequency of the microwave signal can greatly change the electric field distribution within the microwave cavity. For example, inFIG.2, frequency source130is a voltage-controlled oscillator driven with frequency-control signal218that changes the frequency of microwave signal131. In this example, the frequency-modulation control signal218may be generated by signal-processing module210in response to certain detected temperatures, temperature gradients, or temperature distributions. The frequency-modulation control signal may also be random (e.g., white noise) or include a random component (e.g., a triangle wave superimposed with white noise) to further enhance spatial uniformity.

Additionally, to achieve and maintain a high level of field efficiency and therefore the power transfer, the steps in method700may be modified to accommodate design choices in hardware. For example, the cavity design may include frequency selective surfaces, as shown inFIGS.5and6, to modify boundary conditions near the inner walls of the cavity. In another example, time-domain pulse shaping may be used with a DC power source to enable superposition of modes.

The pyrolysis reactor and methods described herein may be used in a system that can be scaled to process waste of any size including, for example, municipal solid waste. In such a system, any microwave pyrolysis reactor, including magnetron-based microwave reactor, has advantages over a conventional convection-based reactor, with those advantages being (1) improved thermal efficiency, leading to more rapid thermochemical process to take place and (2) improved thermal uniformity (e.g., not overheating certain parts, such as the surface of waste mass) due to high penetration depth of electromagnetic waves in the radio-frequency spectrum. However, solid-state-based microwave pyrolysis reactor disclosed herein has additional advantages over a conventional magnetron-based microwave reactor with the most important advantage being that the solid-state-based microwave pyrolysis reactor can be scaled in terms of (1) waste mass by cavity size and frequency choice, (2) microwave power density by power combining a plurality of solid-state amplifiers, and (3) overall power by mode-mixing through mechanical, frequency and/or cavity boundary condition design. Additionally, in such a system, byproducts of the pyrolysis process can be further processed to produce liquid fuel, which in turn may at least partially power pyrolysis reactor.

More specifically, in waste processing system800a waste mass802is placed in the microwave cavity of pyrolysis reactor810and is processed following method700. Dirty bio-oil870from the pyrolysis process has characteristics that vary according to the initial composition of waste mass802. However, there are some contaminants that negatively influence the quality of the final product of the system. For example, liquid fuel890, as the final product in system800, may contain contaminants including nitrogen, sulfur, and metal-containing compounds. In some embodiments, waste processing system800includes a co-product recovery module874that analyzes dirty bio-oil870for these contaminants, and removes any such contaminants that are found. Some of the analysis techniques that may be used in co-product recovery module874include gas chromatography-mass spectrometry (GC-MS) and liquid chromatography-mass spectrometry (LC-MS) for compound identification and quantitation. Removal of the contaminants from dirty bio-oil870may involve one of more compound separation techniques, such as counter current chromatography.

In addition, depending on the biosolids pyrolyzed, dirty bio-oil can harm industrial equipment, produce hazardous emissions upon combustion, or both. Accordingly, some embodiments of the waste processing system800includes a microbial upgrading module872that uses a sequence of one or more microbial bioreactors to remove these harmful compounds, especially sulfur and nitrogen. An example bioreactor may have an established mixed culture dominated by strains ofPseudomonas, Klebsiella, or other genera that metabolize contaminating nitrogen compounds. In certain embodiments, the sequence of bioreactors is optimized, such that the broadest and the highest toxic compounds in dirty bio-oil870are removed first. This type of optimization may prolong and enhance the stability of the biological treatment processes.

After the removal of harmful contaminants, the product of system800is clean biofuel. For example, dirty bio-oil870, after having toxic and harmful contaminants removed by co-product recovery module874and microbial upgrading module872, becomes the final product of system800, i.e., liquid fuel890. Liquid fuel890may be sold as clean biofuel886.

Additionally, system800may recycle all or a portion of the final product to at least partially power the pyrolysis reactor. More specifically, in some embodiments, system800includes an electric generator892. Electric generator892may use all or portion of liquid fuel890to generate electricity884that can be combined with external power source882and supply electricity to pyrolysis reactor810. Accordingly, pyrolysis reactor810can be powered fully or partially by an external power source882, by electric generator892using liquid fuel890or by a combination of both sources.

(1) Efficiency of Waste Breakdown

Commercial ashing machines typically use approximately 1200 W of power in a small cavity (e.g., 22 cm×23 cm×10 cm), giving about 0.24 W/cm3of volume power density compared to a standard microwave oven (which results in approximately 0.01 W/cm3). A full calculation requires many assumptions since waste content varies and results in varied heating. For example, carbon content is converted to coke (highly caloric fuel), and waste with more carbon results in more coke mass. By-products include particulate matter, volatile organic compounds, polynuclear aromatic hydrocarbons, methane, ammonia, carbon monoxide, hydrogen sulfide, hydrogen cyanide, and sulfur oxides that can be either pumped out or used to continue the coking process. Generally, a reasonable approximation for the temperature change dT/dt is given by

where P is the power in watts, C the specific heat in cal/g, and ρ is the volume mass density in g/cm3.

The efficiency of incineration methods is informally quantified by the amount of diesel energy required to produce carbon-rich products from waste. In the case of microwave breakdown, the input is electrical power and the total efficiency will depend on (1) the primary electrical power source efficiency, (2) the microwave generation chain efficiency, and (3) the efficiency of power delivery to the waste mass over time. For the purposes of waste breakdown, a rate efficiency can be defined as:

where V is the volume of the mixed waste (making it difficult to define mass), P is the power in watts required to heat the waste, and τ is the time required to reach an average temperature T. We control power and time separately, but one can also quantify it in terms of energy (e.g., joules). This definition of rate efficiency scales in volume.

(2) Heat Delivery to Waste Mass

The input power in the rate efficiency η is the electrical power delivered to the microwave source. The heat energy dissipated in the waste mass can be expressed as:

where σ(x,y,z,t,T) is the non-uniform waste conductivity, which is a function of temperature and therefore time. The total electric field magnitude is also a function of position within the waste volume and changes with time, since the waste mass electrical properties change, therefore modifying the loading of the microwave cavity. Some relevant values that can be used in initial simulations for possible non-metallic waste are given in Table 1.

An example of the approach for predicting the thermal power is shown next. The thermal power produced in the cavity depends on the filling material properties, relative permittivity (εr), conductivity (σ), geometry of the cavity and type of excitation, and is calculated from:

It is desirable to keep the power density of Joule losses uniform through the heating process because that will usually result in the best time-average efficiency. Several rectangular and cylindrical cavities with volumes ranging from 0.0008 to 0.15 m3were compared using a full-wave eigenmode solver (Ansys HFSS) to determine the modal content around the operating frequency of 2.4 GHz. With a single waveguide port excitation used for the simulation there is a tradeoff between large volumes which can have a high level of field uniformity and small volumes which are effectively heated with a single feed. The cavity has the shape of an actual trash-can, cylindrical in shape and with one basis larger, as shown inFIG.9.FIG.9shows the simulated magnitude of the electric field for single-waveguide-port excitation of a cavity completely filled with high and low permittivity and loss materials.FIG.9Ashows the simulated cavity with dimensions labeled.FIG.9Bshows the calculated electric field magnitude for low-density polyethylene (LDPE) and polystyrene with εr=2.25 and tan δ=0.0003, andFIG.9Cshows the calculated electric field magnitude for a food mixture with εr=42 and tan δ=0.3, where the field value is normalized to a 1 W incident power at 2.45 GHz at the waveguide port. The simulated results are shown for a cavity completely filled with an effective food waste material (high loss) and for lower electrical loss plastic materials. For the simulations, the radii of the bases are chosen to be R1=12.7 cm and R2=11.2 cm, and the height is chosen to be H=28.2 cm, as shown inFIG.9A. The feed is an S-band (WR340) waveguide port with a lower edge 13 cm above the bottom of the cavity. FromFIG.9, it can be seen that the higher permittivity lossy loading has poor penetration depth (seeFIG.9C), while the loading with lower permittivity loss results in much higher field values and more penetration (seeFIG.9B). This indicates that, for an efficient process, the system will have to adapt to the different feed stocks.

The equation above for thermal power can be simplified if the waste volume is assumed to consist of sub-volumes that have uniform conductivities, where the integral becomes a sum:

The fields can be found using a full-wave electromagnetic (EM) solver given a specific geometry, electrical properties, and excitation. It is worth noting that the magnetic component of the electromagnetic field has a small effect on heating since the magnetic properties/loss of the waste load are usually negligible compared to the dielectric ones/loss.

Hydrothermal carbonization (HTC) is carried out mainly to produce a solid product similar to coal starting from wet biomass feedstock. Though it is most effective with wet biomass, HTC has been successfully applied to municipal solid waste and plastics. Removing carboxyl and hydroxy (—OH) groups reduces the oxygen-to-carbon ratio significantly to make the final product more energy dense. The solid yield from this process roughly varies between 35% and 65% of the initial dry feedstock with a higher heating value (HHV) around 13-30 MJ/kg depending on the initial energy content of the feedstock. A feedstock with 75-90% moisture content is considered ideal for this process. HTC consists of three processes, namely, dehydration, decarboxylation, and decarbonylation, for which pretreatment or drying of the feedstock waste is not needed.

The idea remains the same, while process conditions (temperatures of 180-220° C. and pressures of 20-25 bar) are intensified to speed up the process. The reaction time can vary from 1 to 72 hours based on the feedstock and desired output products. This is one of the most promising processes for waste treatment as it can handle large amount of water content. Notice how the pressure requirement is rather moderate for this process that keeps the material cost low.

(4) Microwave Cavity and Probe Design

To maximize efficiency, thermal insulation may be used, in certain embodiments. The electrical and chemical processes that occur when there is an electromagnetic field inside a material are linked through the heating rate caused by Joule losses as:

where ρ is the mass density of the filling material, c is the specific heat of the material, T is the temperature at a specific point within the material, and K is the thermal conductivity. An analytical solution for the heating rate is complicated since the temperature and electric field are functions of position. Therefore, multi-physics simulations were used to determine the temperature increase due to Joule losses and account for the anticipated energy leakage through the cavity walls with specific thermal insulation as shown inFIG.11.FIG.11shows the image of thermal simulation setup and a side cut of the insulation. The cavity walls are 1 mm thick tin-plated steel, the reflective surrounding has a reflectivity of 94%, and the foam insulation is 1-inch thick expanded polystyrene (XPS). Although not shown inFIG.11, the setup may be placed inside a second metal enclosure for safety and additional thermal insulation. Note that the waste feed stock is subjected to pressure on the order of 20 bar. An example of a coaxial cavity is shown inFIG.4. The center conductor is implemented as an auger which has three functions: to form the cavity with many EM modes, to mix the modes mechanically, and to mix the electric field distribution.

(4) High-Efficiency Power Amplifiers and Power Combining

The RF power is first generated using an efficient oscillator, then amplified through efficient saturated gain stages and delivered to the output power stage. Since linearity is not a specification for the heating application, solid-state GaN circuits can be designed with efficiencies exceeding 80% at the lower power levels. Multiple-level power combining is investigated as a means to increase overall efficiency. Circuit-level combining is limited by combiner loss for a larger number of devices. To maximize efficiency in the present embodiments, power may be combined at the circuit level and via spatial combining within the cavity. Spatial combining has been demonstrated to maintain combining efficiency in plane-wave combiners and over-moded waveguide horn combiners. At the cavity level, the waste mass heating will result in a variable load to the probe, and thus the output-state power amplifiers (PA), which can result in reduced efficiency and instabilities.

The standard approach to dealing with load mismatch is to use isolators or impedance tuners at the output of the power amplifier. These devices can also be controlled adaptively in a closed loop. The performance of the tuners, such as insertion loss and adaptation speed, typically degrades as the range of load reflection coefficient increases. The type of tuning device has been limited to lossy p-i-n diodes at higher power levels. For a larger tuning range, the number of tuning elements increases, therefore increasing insertion loss. In the present embodiments, out-phasing of two or more PAs is used. Outphasing is traditionally used to maintain efficiency for high peak-to-average power ratio (PAPR) signals by feeding two amplifiers with a constant-envelope signal, varying their relative phase to change the combined amplitude at the output. In the case of a non-isolated Chireix combiner, the reactive impedances presented to the two PAs are complex conjugates. A tunable Chireix-type combiner can be designed to address load impedance variations instead of signal amplitude variations (PAPR). This architecture requires a very simple tuner with only two tunable elements which handle half of the output power with a small required tuning range.

In addition to varying the phase for load-modulation, the DC supply to the PA transistor can be varied to produce RF power variation, which is not possibly with, for example, magnetron sources. This method is referred to as “supply modulation” and is used effectively for efficiency enhancement when communication signals are amplified. In this case, however, the supply modulation does not need to be fast (e.g., 100 MHz in communications systems), but can be on the order of the heating time constants, and therefore the dynamic supply (supply modulator) can be extremely efficient and not degrade the overall system efficiency while providing a means to vary the power level. Variation of the power level is crucial as waste changes properties during breakdown, and from one loading to another where materials might have dramatically different electrical properties.

(6) Sensing and Controlling the RF Field Distribution

The key to efficient waste heating is maintaining the electric field uniformity inside the cavity. The present embodiments combine several approaches to ensure field homogeneity for scalable pyrolysis. First, the temperature of the waste mixture may be measured throughout the volume, such as with an infrared (IR) camera and several thermocouples (seeFIG.2). The measured temperatures are collected as inputs to the processing unit, which then controls the phase and power delivered to the probes. An IR camera only penetrates a few millimeters into the waste volume due to the limited skin depth at 2.45 GHz combined with the relatively high average waste conductivity. Therefore several thermocouples may be positioned at various depths in the waste. Should this not prove sufficient for proper feedback, radiometry at microwave frequencies may be used which enables temperature measurements at depths of several centimeters, or tens of centimeters. Noteworthy, the presence of liquid water throughout the entire HTC process will strongly favor temperature homogenization.

Other approaches to maintain electric field uniformity include: (a) the use of multiple probes and variable/adaptive probe excitation phasing, which has been shown to improve field uniformity, especially as the cavity loading changes with heating; (b) various types of mode mixing such as frequency modulation (FM) and load stirring; (c) cavity design with modified wall boundary conditions through frequency selective surfaces (FSS), which can also be electrically or mechanically variable; and (d) time-domain pulse shaping can enable superposition of modes over time thus improving overall uniformity averaged over time. These approaches may be used in any combination without departing from the scope hereof.

Frequency modulation can be done at the low-power voltage controlled oscillator.FIG.12illustrates the effect of a small change in the frequency on the fields in a simulated overmoded cavity. The simulations inFIG.12show that a small change in frequency (6 MHz) has a large effect on the field distribution (cross-sectional view) in a 2.45 GHz cavity. The simulation in this example is for an empty cavity. Note that frequency modulation, power modulation or power combining are straightforward with magnetron sources, while SSPAs lend themselves easily to both frequency variation and power variation.

Load stirring is another effective means of mode mixing. The statistics are different and therefore combining different methods will give improved uniformity. A statistical approach to modeling this type of problem also showed agreement with simulation.

For cavity design and modified boundary conditions, metallic walls are fixed boundary conditions which enforce the tangential electric field to be zero on the walls, limiting the type of modes that exist. It has been shown that varying boundary conditions with periodic structures can improve uniformity of the B-field in high-field travelling-wave magnetic resonance imaging (MRI) bores (e.g., at 16 T and 10.5 T). This can also be applied to the electric field for volume power density uniformity, through frequency selective surfaces (FSS). In fact, these can also be electrically or mechanically variable for adaptive boundary conditions.

For time-domain pulse shaping, this concept is borrowed from MRI time-interleaved acquisition of modes method, which enables superposition of modes over time thus improving overall uniformity. With supply modulation, combined with FM, time-domain pulse shaping is straightforward.

Combination of Features

Features described above as well as those claimed below may be combined in various ways without departing from the scope hereof. The following examples illustrate possible, non-limiting combinations of features and embodiments described above. It should be clear that other changes and modifications may be made to the present embodiments without departing from the spirit and scope of this invention:

(A) A pyrolysis reactor includes a microwave cavity configured to contain a waste mixture within an internal space and a plurality of probes that couple a corresponding plurality of drive signals into the microwave cavity to excite a plurality of cavity modes and heat the waste mixture.

(B) In the pyrolysis reactor denoted (A), the microwave cavity has walls shaped as a cylindrical shell surrounding the internal space.

(C) In the pyrolysis reactor denoted (B), the pyrolysis reactor further includes a linear electrical conductor surrounded by the cylindrical shell.

(D) In the pyrolysis reactor denoted (C), the linear electrical conductor is a spindle for an auger located at least partially within the internal space, to stir the waste mixture.

(E) In any one of the pyrolysis reactors denoted (A) to (D), the pyrolysis reactor further includes thermal insulation surrounding the microwave cavity.

(F) In the pyrolysis reactor denoted (E), the pyrolysis reactor further includes a heat shield between the microwave cavity and the thermal insulation.

(G) In any one of the pyrolysis reactors denoted (A) to (F), the pyrolysis reactor further includes at least one frequency selective surface disposed in front of at least a part of the microwave cavity and in the internal space, to surround the waste mixture.

(H) In any one of the pyrolysis reactors denoted (A) to (G), the pyrolysis reactor further includes an oscillator for generating a microwave signal, a plurality of solid-state power amplifiers for amplifying the microwave signal into the plurality of drive signals, and one or more variable phase shifters electrically connected to the oscillator, each of the variable phase shifters being configured to phase-shift one of the microwave signals prior to amplification in a corresponding one of the plurality of solid-state power amplifiers.

(I) In the pyrolysis reactor denoted (H), the pyrolysis reactor further includes a plurality of sensors for measuring temperatures of the waste mixture, and a processing unit configured to (i) determine, based on measured temperatures, one or more phase-control signals and (ii) output the phase-control signals to the one or more phase-shifters to maximize power transfer to the waste mixture.

(J) In the pyrolysis reactor denoted (I), the plurality of sensors includes a plurality of thermocouples and an infrared camera for recording a thermal image stream of the waste mixture.

(K) In the pyrolysis reactor denoted (I), the plurality of sensors includes one or more microwave radiometers for measuring thermally-emitted electromagnetic radiation from the waste mixture.

(L) A method for processing a waste mixture includes coupling a plurality of drive signals into a microwave cavity to excite a plurality of cavity modes, the microwave cavity containing the waste mixture. The method further includes phase-shifting, based on a temperature of the waste mixture, one or more of the plurality of drive signals.

(M) In the method denoted (L), the method further includes generating a microwave signal with an oscillator, splitting the microwave signal into the plurality of drive signals, and amplifying each of the plurality of drive signals into a corresponding one of a plurality of amplified signals. Said coupling includes coupling the plurality of amplified signals into the microwave cavity, and said phase-shifting occurs prior to said amplifying.

(N) In either one of the methods denoted (L) and (M), the method further includes measuring the temperature of the waste mixture with a thermocouple.

(O) In any one of the methods denoted (L) to (N), the method further includes recording a thermal image stream of the waste mixture using an infrared camera.

(P) In any one of the methods denoted (L) to (O), the method further includes measuring thermally-emitted electromagnetic radiation from the waste mixture using a microwave radiometer.

(Q) In any of the methods for processing a waste mixture denoted (L)-(P), the method further includes stirring the waste mixture with an auger.

(R) In any of the methods for processing a waste mixture denoted (L)-(Q), the method further includes adding one or more additives to the waste mixture to increase thermal conductivity of the waste mixture.

(S) A system for processing a waste mixture includes a microwave power generator, a pyrolysis reactor having a plurality of microwave probes connected to the microwave power generator and configured to drive electromagnetic energy that heats the waste mixture inside the pyrolysis reactor and produce bio-oil. The system also includes a bio-oil upgrade element for producing a liquid fuel from the bio-oil.

(T) In the system denoted (S), the system further includes an electric generator for at least partially powering the microwave power generator using the liquid fuel.